Complex N-S variations in Moho depth and Vp/Vs ratio beneath the western Tibetan Plateau as revealed by receiver function analysis

NASA Astrophysics Data System (ADS)

Murodov, Davlatkhudzha; Zhao, Junmeng; Xu, Qiang; Liu, Hongbing; Pei, Shunping

2018-04-01

We present herein detailed images of the Moho depth and Vp/Vs ratio along ANTILOPE-1 profile beneath the western Tibetan Plateau derived from receiver function analysis. Along the ANTILOPE -1 profile, a rapidly northward dipping Moho extends from ˜50 km below the Himalaya to ˜80 km across the Indus-Yarlung suture (IYS), shallowing to ˜66 km under the central Lhasa terrane. The Moho depth shows a dramatic increase from ˜66 km north of the Bangong-Nujiang suture (BNS) to ˜93 km beneath central Qiangtang terrane where it reaches the maximum depth observed along this profile before steeply rising to ˜73 km. We interpret both the 15 km and 20 km offsets of Moho depth occurring beneath the central Lhasa and central Qiangtang terranes as being related to the northern frontiers of the decoupled underthrusting Indian lower crust and lithospheric mantle, respectively. The Moho remains at a depth of ˜70 km with a slight undulation beneath the northern Qiangtang and Songpan-Ganzi terranes, and then abruptly shallows to ˜45 km near the Altyn Tagh Fault. The ˜25 km Moho offset observed at the conjunction of the Tarim Basin and the Altyn Tagh mountain range suggests that the crustal shortening is achieved by pure shear thickening without much underthrusting. The average crustal Vp/Vs ratio changes from 1.66 to 1.80 beneath the Himalaya, the Lhasa terrane and the Tarim Basin indicating a felsic-to-intermediate composition. However, higher Vp/Vs ratios between 1.76 and 1.83 (except for a few outlying low values) are found beneath the Qiangtang and Songpan-Ganzi terranes, which could be attributed to the joint effects of the more mafic composition and partial melt within the crust. The Moho depth and Vp/Vs ratio exhibit complex N-S variations along this profile, which can be attributed to the joint effects of Indian lower crust underthrusting, the low velocity zone of the mid-upper crust, crustal shortening and thickening and other involved dynamic mechanisms.

Rapid conversion of an oceanic spreading center to a subduction zone inferred from high-precision geochronology.

PubMed

Keenan, Timothy E; Encarnación, John; Buchwaldt, Robert; Fernandez, Dan; Mattinson, James; Rasoazanamparany, Christine; Luetkemeyer, P Benjamin

2016-11-22

Where and how subduction zones initiate is a fundamental tectonic problem, yet there are few well-constrained geologic tests that address the tectonic settings and dynamics of the process. Numerical modeling has shown that oceanic spreading centers are some of the weakest parts of the plate tectonic system [Gurnis M, Hall C, Lavier L (2004) Geochem Geophys Geosys 5:Q07001], but previous studies have not favored them for subduction initiation because of the positive buoyancy of young lithosphere. Instead, other weak zones, such as fracture zones, have been invoked. Because these models differ in terms of the ages of crust that are juxtaposed at the site of subduction initiation, they can be tested by dating the protoliths of metamorphosed oceanic crust that is formed by underthrusting at the beginning of subduction and comparing that age with the age of the overlying lithosphere and the timing of subduction initiation itself. In the western Philippines, we find that oceanic crust was less than ∼1 My old when it was underthrust and metamorphosed at the onset of subduction in Palawan, Philippines, implying forced subduction initiation at a spreading center. This result shows that young and positively buoyant, but weak, lithosphere was the preferred site for subduction nucleation despite the proximity of other potential weak zones with older, denser lithosphere and that plate motion rapidly changed from divergence to convergence.

Rapid conversion of an oceanic spreading center to a subduction zone inferred from high-precision geochronology

PubMed Central

Keenan, Timothy E.; Encarnación, John; Buchwaldt, Robert; Fernandez, Dan; Mattinson, James; Rasoazanamparany, Christine; Luetkemeyer, P. Benjamin

2016-01-01

Where and how subduction zones initiate is a fundamental tectonic problem, yet there are few well-constrained geologic tests that address the tectonic settings and dynamics of the process. Numerical modeling has shown that oceanic spreading centers are some of the weakest parts of the plate tectonic system [Gurnis M, Hall C, Lavier L (2004) Geochem Geophys Geosys 5:Q07001], but previous studies have not favored them for subduction initiation because of the positive buoyancy of young lithosphere. Instead, other weak zones, such as fracture zones, have been invoked. Because these models differ in terms of the ages of crust that are juxtaposed at the site of subduction initiation, they can be tested by dating the protoliths of metamorphosed oceanic crust that is formed by underthrusting at the beginning of subduction and comparing that age with the age of the overlying lithosphere and the timing of subduction initiation itself. In the western Philippines, we find that oceanic crust was less than ∼1 My old when it was underthrust and metamorphosed at the onset of subduction in Palawan, Philippines, implying forced subduction initiation at a spreading center. This result shows that young and positively buoyant, but weak, lithosphere was the preferred site for subduction nucleation despite the proximity of other potential weak zones with older, denser lithosphere and that plate motion rapidly changed from divergence to convergence. PMID:27821756

The 2015 Mw7.2 Sarez Strike-Slip Earthquake in the Pamir Interior: Response to the Underthrusting of India's Western Promontory

NASA Astrophysics Data System (ADS)

Metzger, Sabrina; Schurr, Bernd; Ratschbacher, Lothar; Sudhaus, Henriette; Kufner, Sofia-Katerina; Schöne, Tilo; Zhang, Yong; Perry, Mason; Bendick, Rebecca

2017-11-01

The Pamir orogen, Central Asia, is the result of the ongoing northward advance of the Indian continent causing shortening inside Asia. Geodetic and seismic data place the most intense deformation along the northern rim of the Pamir, but the recent 7 December 2015, Mw7.2 Sarez earthquake occurred in the Pamir's interior. We present a distributed slip model of this earthquake using coseismic geodetic data and postseismic field observations. The earthquake ruptured an ˜80 km long, subvertical, sinistral fault consisting of three right-stepping segments from the surface to ˜30 km depth with a maximum slip of three meters in the upper 10 km of the crust. The coseismic slip model agrees well with en échelon secondary surface breaks that are partly influenced by liquefaction-induced mass movements. These structures reveal up to 2 m of sinistral offset along the northern, low-offset segment of modeled rupture. The 2015 event initiated close to the presumed epicenter of the 1911 Mw˜7.3 Lake Sarez earthquake, which had a similar strike-slip mechanism. These earthquakes highlight the importance of NE trending sinistral faults in the active tectonics of the Pamir. Strike-slip deformation accommodates shear between the rapidly northward moving eastern Pamir and the Tajik basin in the west and is part of the westward (lateral) extrusion of thickened Pamir plateau crust into the Tajik basin. The Sarez-Karakul fault system and the two large Sarez earthquakes likely are crustal expressions of the underthrusting of the northwestern leading edge of the Indian mantle lithosphere beneath the Pamir.

Insights from 3D numerical simulations on the dynamics of the India-Asia collision zone

NASA Astrophysics Data System (ADS)

Pusok, A. E.; Kaus, B.; Popov, A.

2013-12-01

The dynamics of the India-Asia collision zone remains one of the most remarkable topics of the current research interest: the transition from subduction to collision and uplift, followed by the rise of the abnormally thick Tibetan plateau, and the deformation at its Eastern and Western syntaxes, are processes still not fully understood. Models that have addressed this topic include wholescale underthrusting of Indian lithospheric mantle under Tibet, distributed homogeneous shortening or the thin-sheet model, slip-line field model for lateral extrusion or lower crustal flow models for the exhumation of the Himalayan units and lateral spreading of the Tibetan plateau. Of these, the thin-sheet model has successfully illustrated some of the basic physics of continental collision and has the advantage of a 3D model being reduced to 2D, but one of its major shortcomings is that it cannot simultaneously represent channel flow and gravitational collapse of the mantle lithosphere, since these mechanisms require the lithosphere to interact with the underlying mantle, or to have a vertically non-homogeneous rheology. As a consequence, 3D models are emerging as powerful tools to understand the dynamics of coupled systems. However, because of yet recent developments and various complexities, the current 3D models simulating the dynamics of continent collision zones have relied on certain explicit assumptions, such as replacing part of the asthenosphere with various types of boundary conditions that mimic the effect of mantle flow, in order to focus on the lithospheric/crustal deformation. Here, we employ the parallel 3D code LaMEM (Lithosphere and Mantle Evolution Model), with a finite difference staggered grid solver, which is capable of simulating lithospheric deformation while simultaneously taking mantle flow and a free surface into account. We present qualitative results on lithospheric and upper-mantle scale simulations in which the Indian lithosphere is subducted and/or indented into Asia. We investigate the way deep processes affect continental tectonics at convergent margins, addressing the role the continent subduction and indentation plays on the development of continental tectonics during convergence and we discuss the implications these offer for the Asian tectonics. Acknowledgements: Funding was provided by the European Research Council under the European Community's Seventh Framework Program (FP7/2007-2013) / ERC Grant agreement #258830. Numerical computations have been performed on MOGON (ZDV Mainz computing center) and JUQUEEN (Jülich high-performance computing center).

An assessment of the crustal remelting hypothesis for volcanism in the Freyja Montes deformation zone

NASA Astrophysics Data System (ADS)

Namiki, Noriyuki; Solomon, Sean C.

1991-06-01

The linear mountain belts of Ishtar Terra on Venus are notable for their topographic relief and slope and for the intensity of surface deformation. The mountains surround the highland plain Lakshmi Planum, the site of two major paterae and numerous other volcanic features and deposits, and evidence is widespread for volcanism within the mountains and in terrain immediately outward of the mountain belt units. While two hypotheses for magmatism can be distinguished on the basis of the chemistry of the melts, chemical data are presently lacking for the Ishtar region. The competing hypotheses for magmatism in Western Ishtar Terra can also be tested with thermal models, given a kinematic or dynamic model for the evolution of the region. The crustal remelting hypothesis is assessed, using the kinematic scenario of Head for the evolution of Freyja Montes. In that scenario, Freyja Montes formed by a sequence of large scale underthrusts of the lithosphere of the North Polar Plains beneath Ishtar Terra, with successive blocks of underthrust crust sutured in imbricate fashion onto the thickened crust of Lakshmi Planum and the mantle portion of underthrusting lithosphere episodically detached. The numerical experiments thus show that volcanic activity associated with the formation of the Frejya Montes deformation zone can be explained by crustal melting, due either to direct contact of crustal material with the hot asthenosphere or to heat generation in a thickened crustal layer.

3D numerical modeling of India-Asia-like collision

NASA Astrophysics Data System (ADS)

-Erika Püsök, Adina; Kaus, Boris; Popov, Anton

2013-04-01

One of the most striking features of plate tectonics and lithospheric deformation is the India-Asia collision zone, which formed when the Indian continent collided with Eurasia, around 50 million years ago. The rise of the abnormally thick Tibetan plateau, the deformation at its Eastern and Western syntaxes, the transition from subduction to collision and uplift and the interaction of tectonics and climate are processes not fully understood. Though various geophysical methods have been employed to shed light on the present structure of the Indian-Asian lithosphere, the driving mechanisms that uplifted the Tibetan plateau remain highly controversial and different hypotheses imply contradictory scenarios. Models for double crustal thickness include: wholescale underthrusting of Indian lithospheric mantle under Tibet (Argand model), distributed homogeneous shortening or the thin-sheet model (England and Houseman, 1986), slip-line field model to also explain extrusion of Eastern side of Tibet away from Indian indentor (Tapponier and Molnar, 1976) or lower crustal flow models for the exhumation of the Himalayan units and lateral spreading of the Tibetan plateau (Royden et al., 1998, Beaumont et al., 2004). The thin-sheet model has emerged as a more successful (or at least more widely used) model, but one of its major shortcomings is that it cannot simultaneously represent channel flow and gravitational collapse of the mantle lithosphere (Lechmann et al., 2011), since these mechanisms require the lithosphere to interact with the underlying mantle, or to have a vertically non-homogeneous rheology. Of those who favour a layered structure of the lithosphere beneath Tibet, some attribute the lack of substantial seismicity underneath the Moho as evidence that all the strength of the lithosphere resides in the upper crust and the mantle is weak - the crème brulée model (Jackson, 2002), while others point out that some processes can be well explained if the crust resides above a strong mantle lithosphere - the jelly sandwich model (Burov and Watts, 2006). 3D models are thus needed to investigate these hypotheses. However, fully 3D models of the dynamics of continent collision zones have only been developed very recently, and presently most research groups have relied on certain explicit assumptions for their codes. Here, we employ the parallel 3D code LaMEM (Lithosphere and Mantle Evolution Model), with a finite difference staggered grid solver, which is capable of simulating lithospheric deformation while simultaneously taking mantle flow and a free surface into account. We here report on first lithospheric and upper-mantle scale simulations in which the Indian lithosphere is indented into Asia. Acknowledgements. Funding was provided by the European Research Council under the European Community's Seventh Framework Program (FP7/2007-2013) / ERC Grant agreement #258830. Numerical computations have been performed on JUQUEEN of the Jülich high-performance computing center. • Beaumont, C., Jamieson, R.A., Nguyen, M.H., Medvedev, S.E., 2004. Crustal channel flows: 1. Numerical models with applications to the tectonics of the Himalayan-Tibetan orogeny. J. Geophys. Res. 109, B06406. • Burov, E. & Watts, W.S., 2006. The long-term strength of continental lithosphere: "jelly sandwich" or "crème brûlée"?. GSA Today, 16, doi: 10.1130/1052-5173(2006)1016<1134:TLTSOC>1132.1130.CO;1132. • England P., Houseman, G., 1986. Finite strain calculations of continental deformation. 2. Comparison with the India-Asia collision zone. J. Geophys. Res.- Solid Earth and Planets 91 (B3), 3664-3676. • Jackson, J., 2002. Strength of the continental lithosphere: time to abandon the jelly sandwich?. GSA Today, September, 4-10. • Lechmann, S.M., May, D.A., Kaus, B.J.P., Schmalholz, S.M., 2011. Comparing thin-sheet models with 3D multilayer models for continental collision. Geophy. Int. J. doi: 10.1111/j.1365-246X.2011.05164.x • Royden, L.H., Burchfiel, B.C., King, R.W., Wang, E., Chen, Z.L., Shen, F., Liu, Y.P., 1997. Surface deformation and lower crustal flow in eartern Tibet. Science 276 (5313), 788-790. • Tapponier, P., Molnar, P., 1976. Slip-line field-theory and large-scale continental tectonics. Nature 264 (5584), 319-324.

A Comparative Study of the Electrical Structure of Circum Tibetan Plateau Orogenic Belts and its Tectonic Implications

NASA Astrophysics Data System (ADS)

Jin, Sheng; Zhang, Letian; Wei, Wenbo; Ye, Gaofeng; Jing, Jianen; Dong, Hao; Xie, Chengliang; Yin, Yaotian

2017-04-01

The Tibetan Plateau, as known as "roof of the world", was created through the on-going continent-continent collision between the Indian and Eurasian plates since 55 Ma. As the process continues, the plateau is growing both vertically and horizontally. The horizontal expansion of the plateau is blocked by the Yangtze block in the east, the Tarim block in the north, and the Ordos block in the northeast, and consequently lead to the formation of the circum Tibetan plateau orogenic belts. To better understand the mechanism behind this process, we conducted a comparative study by collecting 7 magnetotelluric (MT) profiles over the margins of the Tibetan plateau, namely, the INDEPTH 100, 700 and 800 lines in the southern Tibet, the INDEPTH 4000 and 5000 lines across the Altyn Tagh fault on the northern margin of the plateau, as well as other two profiles across the Haiyuan fault and the Longmenshan fault on the northeastern and eastern margins of the plateau deployed under the framework of project SinoProbe. The electrical features of the stable blocks surrounding the Tibetan plateau are generally resistive, while crustal conductive layers are found to be wide spread within the plateau. The southern margin of the Tibetan plateau is characterized by large scale underthrust of the Indian lithosphere beneath the plateau. This intense converging process created the thrust fault system distributed along the southern margin of the Tibetan plateau over 1000 km. Crustal conductive layers discovered in southern Tibet are generally associated with the southward crustal flow that originated from the lower crust within the plateau and exhumed along the thrust belts in the Himalayas. On the eastern margin of the Tibetan plateau, the electrical structures suggest that the Yangtze block wedged into the Tibetan lithosphere and caused decoupling between the crust and upper mantel. Large scale conductors discovered beneath the Songpan-Ganze block reflect that the eastward crustal flow was blocked and piled up along the eastern margin of the plateau due to the block of the Sichuan Basin, which further result in the uplift and expansion of the eastern Tibetan plateau. The northeastern and northern margins of the Tibetan plateau is bounded by large scale left-lateral strike-slip Haiyuan and Altyn Tagh faults. In these regions, the plateau interacts with the surrounding stable blocks in a way of oblique strike-slip. The deformation of the northern Tibetan lithosphere is dominated by crustal thickening, where no features of decoupling or large scale underthrusting were seen. Crustal conductors in these regions are generally not very well connected, which suggest the absence of crustal flow. Deep metamorphism fluids could be an alternative interpretation of the crustal conductors in these regions. * This work was jointly supported by the grants from Project SinoProbe-02-04 and National Natural Science Foundation of China (41404060).

Underthrusting of passive margin strata into deep crustal hot zones associated with Cretaceous arc magmatism in North America: links and timescales of magmatic vs. tectonic thickening

NASA Astrophysics Data System (ADS)

Chin, E. J.; Lee, C.; Tollstrup, D. L.; Xie, L.; Wimpenny, J.; Yin, Q.

2011-12-01

The North American Cordillera experienced lithospheric thickening during the Cretaceous as a result of subduction-induced magmatism and tectonic shortening. Several studies suggest correlations between increased plate convergence rates and crustal underthrusting with apparent magmatic flux and evolved isotopic excursions, yet questions still remain regarding causality between tectonic and magmatic thickening. Here, we use lower crustal garnet-bearing metaquartzite (80% SiO2) xenoliths hosted in late Miocene basalts in the central Sierra Nevada Batholith, California to constrain the P-T-t (pressure-temperature-time) history of crustal thickening. The xenoliths are equigranular in texture and are comprised of >50% quartz, ~10% metamorphic garnet, <40% plagioclase, and trace rutile, kyanite, and biotite. High quartz mode, abundant well-rounded detrital zircons, and oriented graphite laths demonstrating sedimentary or metamorphic layering point to a supracrustal sedimentary protolith. However, final equilibration temperatures using titanium-in-quartz thermometry are 700 - 800 °C, and final equilibration pressures using the GASP barometer yield 0.9 - 1.3 GPa, indicating the metaquartzites equilibrated within a hot lower crust (18 - 45 km). Low whole-rock REE totals, lack of whole-rock HREE enrichment relative to LREE and MREE, and absence of positive Eu anomalies suggest that significant melting in the garnet or plagioclase fields did not occur. The whole-rock trace element geochemistry is also consistent with an initially garnet-free protolith. Simultaneous LA-ICP-MS measurements of U-Pb and Hf isotopes in detrital zircons show that all zircons have discordant U-Pb with variable upper intercept ages (1.7, 2.7, 3.3 Ga; consistent with Hf model ages), but common lower intercept ages (100 Ma). The above indicate that protoliths of the metaquartzites were North American Proterozoic to Paleozoic passive margin sediments which were simultaneously emplaced into the lower crust at ~100 Ma, during the peak of Cretaceous arc magmatism. We envision underthrusting of N. American lithosphere beneath the active Sierran arc as the mechanism for transporting these sediments to high P, T conditions, but underthrusting cold continental lithosphere alone cannot explain the xenoliths' high final temperatures. An additional heat source, derived from deep crustal magmatic "hot zones", seems required. We are currently exploring diffusion modeling in garnet porphyroblasts as a way to estimate rates of thickening. Because the protoliths were initially garnet-free, growth of metamorphic garnet can potentially record the length of time it took the metaquartzites to achieve their high P, T conditions. We will also use Ti zonation in detrital zircons as an added constraint on timescales involved in thickening. So far, our results indicate firsthand that tectonic underthrusting of continental supracrustal rocks extends all the way into deep magmatic zones beneath arcs, implying that magmatic differentiation alone is not the only mechanism by which continental crust achieves its felsic composition.

Greater India

NASA Astrophysics Data System (ADS)

Ali, Jason R.; Aitchison, Jonathan C.

2005-10-01

"Greater India" is an 80-yr-old concept that has been used by geoscientists in plate tectonic models of the India-Asia collision system. Numerous authors working on the orogen and/or plate models of the broader region have added various sized chunks of continental lithosphere to the now northern edge of their reconstructed Indian plate. Prior to plate tectonic theory, Emile Argand (1924) [Argand, E., 1924. La tectonique de l' Asie. Proc. 13th Int. Geol. Cong. 7 (1924), 171-372.] and Arthur Holmes (1965) [Holmes, A., 1965. Principles of Physical Geology, Second Edition. The Ronald Press Company, New York, 1128.] thought that the Himalayan Mountains and Tibetan Plateau had been raised due to the northern edge of the Indian craton under-thrusting the entire region. Since the advent of plate tectonic theory, Greater India proposals have been based principally on three lines of logic. One group of workers has added various amounts of continental lithosphere to India as part of their Mesozoic Gondwana models. A second form of reconstruction is based on Himalayan crustal-shortening estimates. A third body of researchers has used India continent extensions as means of allowing initial contact between the block and the Eurasian backstop plate in southern Tibet to take place at various times between the Late Cretaceous and late Eocene in what we call "fill-the-gap" solutions. The Indian craton and the southern edge of Eurasia were almost invariably some distance from one another when the collision was supposed to have started; extensions to the sub-continent were used to circumvent the problem. Occasionally, Greater India extensions have been based on a combination of fill-the-gap and shortening estimate arguments. In this paper, we exhume and re-examine the key Greater India proposals. From our analysis, it is clear that many proponents have ignored key information regarding the sub-continent's pre break-up position within Gondwana and the bathymetry of the Indian Ocean west of Australia, in particular the Wallaby-Zenith Plateau Ridge and the Wallaby-Zenith Fracture Zone. We suggest that the Indian continent probably extended no more than 950 km in the central portion of the Main Boundary Thrust, up to the Wallaby-Zenith Fracture Zone. At the Western Syntaxis, the extension was about 600 km. These estimates are broadly compatible with some of the geophysically-derived models depicting subducted Indian lithosphere beneath Tibet, as well as estimates of Himalayan shortening. Models requiring sub-continent extensions > 9° ahead of the craton are probably wrong. We also suggest that northern India did not have a thinned rifted passive margin due to the earlier rifting of blocks away from it when it formed part of Gondwana. Instead, the boundary developed as a transform fault and probably had a very narrow ocean-continent transition zone (5-10 km wide), similar to the Romanche Fracture Zone offshore of Ghana, West Africa.

Estimation of Water Within the Lithospheric Mantle of Central Tibet from Petrological-Geophysical Investigations

NASA Astrophysics Data System (ADS)

Vozar, J.; Fullea, J.; Jones, A. G.

2013-12-01

Investigations of the lithosphere and sub-lithospheric upper mantle by integrated petrological-geophysical modeling of magnetotelluric (MT) and seismic surface-wave data, which are differently sensitive to temperature and composition, allows us to reduce the uncertainties associated with modeling these two data sets independently, as commonly undertaken. We use selected INDEPTH MT data, which have appropriate dimensionality and large penetration depths, across central Tibet for 1D modeling. Our deep resistivity models from the data can be classified into two different and distinct groups: (i) the Lhasa Terrane and (ii) the Qiangtang Terrane. For the Lhasa Terrane group, the models show the existence of upper mantle conductive layer localized at depths of 200 km, whereas for the Qiangtang Terrane, this conductive layer is shallower at depths of 120 km. We perform the integrated geophysical-petrological modeling of the MT and surface-wave data using the software package LitMod. The program facilitates definition of realistic temperature and pressure distributions within the upper mantle for given thermal structure and oxide chemistry in the CFMAS system. This allows us to define a bulk geoelectric and seismic model of the upper mantle based on laboratory and xenolith data for the most relevant mantle minerals, and to compute synthetic geophysical observables. Our results suggest an 80-120 km-thick, dry lithosphere in the central part of the Qiangtang Terrane. In contrast, in the central Lhasa Terrane the predicted MT responses are too resistive for a dry lithosphere regardless its thickness; according to seismic and topography data the expected lithospheric thickness is about 200 km. The presence of small amounts of water significantly decreases the electrical resistivity of mantle rocks and is required to fit the MT responses. We test the hypothesis of small amounts of water (ppm scale) in the nominally anhydrous minerals of the lithospheric mantle. Such a small amount of water dramatically affects the resistivity but has no influence on the seismic velocities (and therefore, the calculated surface wave's dispersion curves are unaffected too). Three different proton conduction models for olivine conductivity (1 - Wang et al., 2006; 2 - Yoshino et al., 2009; 3 -Jones et al., 2012) and two water partition coefficients are tested. The presence of water in lithospheric mantle is decreased from 170 km to the LAB depth at 200 km. If we move this water-presentbottom boundary to shallower depth, the lithospheric mantle becomes too resistive. Our results favour a moderately wet (<0.01 wt%) mantle above the underthrusted Indian lithosphere, probably as a result of the dehydration processes. The presence of percolating water-rich fluids has the additional effect of lowering the solidus, and therefore facilitating partial melting in the warm lower crust of Lhasa.

Detachments of the subducted Indian continental lithosphere based on 3D finite-frequency tomographic images

NASA Astrophysics Data System (ADS)

Liang, X.; Tian, X.; Wang, M.

2017-12-01

Indian plate collided with Eurasian plate at 60 Ma and there are about 3000 km crustal shortening since the continental-continental collision. At least one third of the total amount of crustal shortening between Indian and Eurasian plates could not be accounted by thickened Tibetan crust and surface erosion. It will need a combination of possible transfer of lower crust to the mantle by eclogitization and lateral extrusion. Based on the lithosphere-asthenosphere boundary images beneath the Tibetan plateau, there is also at least the same amount deficit for lithospheric mantle subducted into upper/lower mantle or lateral extrusion with the crust. We have to recover a detailed Indian continental lithosphere image beneath the plateau in order to explain this deficit of mass budget. Combining the new teleseismic body waves recorded by SANDWICH passive seismic array with waveforms from several previous temporary seismic arrays, we carried out finite-frequency tomographic inversions to image three-dimensional velocity structures beneath southern and central Tibetan plateau to examine the possible image of subducted Indian lithosphere in the Tibetan upper mantle. We have recovered a continuous high velocity body in upper mantle and piece-wised high velocity anomalies in the mantle transition zone. Based on their geometry and relative locations, we interpreted these high velocity anomalies as the subducted and detached Indian lithosphere at different episodes of the plateau evolution. Detachments of the subducted Indian lithosphere should have a crucial impact on the volcanism activities and uplift history of the plateau.

Using natural laboratories and modeling to decipher lithospheric rheology

NASA Astrophysics Data System (ADS)

Sobolev, Stephan

2013-04-01

Rheology is obviously important for geodynamic modeling but at the same time rheological parameters appear to be least constrained. Laboratory experiments give rather large ranges of rheological parameters and their scaling to nature is not entirely clear. Therefore finding rheological proxies in nature is very important. One way to do that is finding appropriate values of rheological parameter by fitting models to the lithospheric structure in the highly deformed regions where lithospheric structure and geologic evolution is well constrained. Here I will present two examples of such studies at plate boundaries. One case is the Dead Sea Transform (DST) that comprises a boundary between African and Arabian plates. During the last 15- 20 Myr more than 100 km of left lateral transform displacement has been accumulated on the DST and about 10 km thick Dead Sea Basin (DSB) was formed in the central part of the DST. Lithospheric structure and geological evolution of DST and DSB is rather well constrained by a number of interdisciplinary projects including DESERT and DESIRE projects leaded by the GFZ Potsdam. Detailed observations reveal apparently contradictory picture. From one hand widespread igneous activity, especially in the last 5 Myr, thin (60-80 km) lithosphere constrained from seismic data and absence of seismicity below the Moho, seem to be quite natural for this tectonically active plate boundary. However, surface heat flow of less than 50-60mW/m2 and deep seismicity in the lower crust ( deeper than 20 km) reported for this region are apparently inconsistent with the tectonic settings specific for an active continental plate boundary and with the crustal structure of the DSB. To address these inconsistencies which comprise what I call the "DST heat-flow paradox", a 3D numerical thermo-mechanical model was developed operating with non-linear elasto-visco-plastic rheology of the lithosphere. Results of the numerical experiments show that the entire set of observations for the DSB can be explained within the classical pull-apart model assuming that (1) the lithosphere has been thermally eroded at about 20 Ma, just before the active faulting at the DST, and (2) the uppermost mantle in the region have relatively weak rheology consistent with the experimental data for wet olivine or pyroxenite. Another example is modeling of the collision of India and Eurasia in Tibet. Our recent thermo-mechanical model (see abstract by Tympel et al) reproduce well many important features of this orogeny, including observed convergence and distance of underthrusting of Indian lithosphere beneath Tibet, if long-term friction at India-Eurasia interface is about 0.04- 0.05, which is typical for oceanic subduction zones, but is unexpected low for continental setting.

Widespread ground motion distribution caused by rupture directivity during the 2015 Gorkha, Nepal earthquake

NASA Astrophysics Data System (ADS)

Koketsu, Kazuki; Miyake, Hiroe; Guo, Yujia; Kobayashi, Hiroaki; Masuda, Tetsu; Davuluri, Srinagesh; Bhattarai, Mukunda; Adhikari, Lok Bijaya; Sapkota, Soma Nath

2016-06-01

The ground motion and damage caused by the 2015 Gorkha, Nepal earthquake can be characterized by their widespread distributions to the east. Evidence from strong ground motions, regional acceleration duration, and teleseismic waveforms indicate that rupture directivity contributed significantly to these distributions. This phenomenon has been thought to occur only if a strike-slip or dip-slip rupture propagates to a site in the along-strike or updip direction, respectively. However, even though the earthquake was a dip-slip faulting event and its source fault strike was nearly eastward, evidence for rupture directivity is found in the eastward direction. Here, we explore the reasons for this apparent inconsistency by performing a joint source inversion of seismic and geodetic datasets, and conducting ground motion simulations. The results indicate that the earthquake occurred on the underthrusting Indian lithosphere, with a low dip angle, and that the fault rupture propagated in the along-strike direction at a velocity just slightly below the S-wave velocity. This low dip angle and fast rupture velocity produced rupture directivity in the along-strike direction, which caused widespread ground motion distribution and significant damage extending far eastwards, from central Nepal to Mount Everest.

Three Dimensional Lithospheric Electrical Structure of the Tibetan Plateau as Revealed by SinoProbe Long Period Magnetotelluric Array Data

NASA Astrophysics Data System (ADS)

Wei, Wenbo; Zhang, Letian; Jin, Sheng; Ye, Gaofeng; Jing, Jianen; Dong, Hao; Xie, Chengliang; Yin, Yaotian

2017-04-01

The on-going continent-continent collision between the Indian and Eurasian plates since 55 Ma has created the spectacular topography of the Tibetan plateau. However, many first order questions remain to be answered as to the mechanisms behind this young orogenic process. Under the auspices of the SinoProbe Project, a three dimensional (3-D) Magnetotelluric (MT) array have been deployed on the Tibetan Plateau from 2010 to 2013 to better understand this orogeny. By the end of 2013, 1099 MT stations have been completed, including 102 combined broadband MT (BBMT) and long period MT (LMT) stations. In this study, MT data of these 102 combined stations have been used to investigate the deep lithospheric electrical structure of the Tibetan Plateau. MT impedances within the period range of 10 - 50000 s were extracted to be used for 3-D inversions with the ModEM code using the standard NLCG algorithm. The resulting lithospheric electrical structure of the Tibetan Plateau shows a distinct pattern of strong variation not only vertically, but also horizontally. Conductors are found to be widespread in the middle to lower crust. But their geometries are quite complex, and not obviously consistent with the hypothesis of continuous eastward channel flow. Instead, most crustal conductors in central and southern Tibet display a pattern of N-S extension. In the depth range of the upper mantle, two more conductive regions can be identified in the southern Qiangtang Terrane and in the central Lhasa Terrane. Resistor associated with the underthrust Inidan plate can be traced beneath the Bangong-Nujiang suture in western Tibet, but only beneath the central Lhasa terrane in central Tibet. * This work was jointly supported by the grants from Project SinoProbe-01 and National Natural Science Foundation of China (41404060).

Paleolatitudes of the Tibetan Himalaya from primary and secondary magnetizations of Jurassic to Lower Cretaceous sedimentary rocks

NASA Astrophysics Data System (ADS)

Huang, Wentao; van Hinsbergen, Douwe J. J.; Dekkers, Mark J.; Garzanti, Eduardo; Dupont-Nivet, Guillaume; Lippert, Peter C.; Li, Xiaochun; Maffione, Marco; Langereis, Cor G.; Hu, Xiumian; Guo, Zhaojie; Kapp, Paul

2015-01-01

The Tibetan Himalaya represents the northernmost continental unit of the Indian plate that collided with Asia in the Cenozoic. Paleomagnetic studies on the Tibetan Himalaya can help constrain the dimension and paleogeography of "Greater India," the Indian plate lithosphere that subducted and underthrusted below Asia after initial collision. Here we present a paleomagnetic investigation of a Jurassic (limestones) and Lower Cretaceous (volcaniclastic sandstones) section of the Tibetan Himalaya. The limestones yielded positive fold test, showing a prefolding origin of the isolated remanent magnetizations. Detailed paleomagnetic analyses, rock magnetic tests, end-member modeling of acquisition curves of isothermal remanent magnetization, and petrographic investigation reveal that the magnetic carrier of the Jurassic limestones is authigenic magnetite, whereas the dominant magnetic carrier of the Lower Cretaceous volcaniclastic sandstones is detrital magnetite. Our observations lead us to conclude that the Jurassic limestones record a prefolding remagnetization, whereas the Lower Cretaceous volcaniclastic sandstones retain a primary remanence. The volcaniclastic sandstones yield an Early Cretaceous paleolatitude of 55.5°S [52.5°S, 58.6°S] for the Tibetan Himalaya, suggesting it was part of the Indian continent at that time. The size of "Greater India" during Jurassic time cannot be estimated from these limestones. Instead, a paleolatitude of the Tibetan Himalaya of 23.8°S [21.8°S, 26.1°S] during the remagnetization process is suggested. It is likely that the remagnetization, caused by the oxidation of early diagenetic pyrite to magnetite, was induced during 103-83 or 77-67 Ma. The inferred paleolatitudes at these two time intervals imply very different tectonic consequences for the Tibetan Himalaya.

The rapid drift of the Indian tectonic plate.

PubMed

Kumar, Prakash; Yuan, Xiaohui; Kumar, M Ravi; Kind, Rainer; Li, Xueqing; Chadha, R K

2007-10-18

The breakup of the supercontinent Gondwanaland into Africa, Antarctica, Australia and India about 140 million years ago, and consequently the opening of the Indian Ocean, is thought to have been caused by heating of the lithosphere from below by a large plume whose relicts are now the Marion, Kerguelen and Réunion plumes. Plate reconstructions based on palaeomagnetic data suggest that the Indian plate attained a very high speed (18-20 cm yr(-1) during the late Cretaceous period) subsequent to its breakup from Gondwanaland, and then slowed to approximately 5 cm yr(-1) after the continental collision with Asia approximately 50 Myr ago. The Australian and African plates moved comparatively less distance and at much lower speeds of 2-4 cm yr(-1) (refs 3-5). Antarctica remained almost stationary. This mobility makes India unique among the fragments of Gondwanaland. Here we propose that when the fragments of Gondwanaland were separated by the plume, the penetration of their lithospheric roots into the asthenosphere were important in determining their speed. We estimated the thickness of the lithospheric plates of the different fragments of Gondwanaland around the Indian Ocean by using the shear-wave receiver function technique. We found that the fragment of Gondwanaland with clearly the thinnest lithosphere is India. The lithospheric roots in South Africa, Australia and Antarctica are between 180 and 300 km deep, whereas the Indian lithosphere extends only about 100 km deep. We infer that the plume that partitioned Gondwanaland may have also melted the lower half of the Indian lithosphere, thus permitting faster motion due to ridge push or slab pull.

The Rapid Drift of the Indian Tectonic Plate

NASA Astrophysics Data System (ADS)

Kumar, P.; Yuan, X.; Kumar, R.; Kind, R.; Li, X.; Chadha, R.

2007-12-01

The breakup of the supercontinent Gondwanaland into Africa, Antarctica, Australia and India about 140 million years ago and consequently the opening of the Indian Ocean was caused by heating of the lithosphere from below by a large plume whose relicts are the Marion, Kerguelen and Reunion plumes. Plate reconstructions based on paleomagnetic data suggest that the Indian plate attained a very high speed (18-20 cm/yr during late Cretaceous) subsequent to its breakup from the Gondwanaland and slowed down to ~5 cm/yr since the continental collision with Asia during the last ~50 Ma. The Australian and African plates moved comparatively lesser distances and at much lesser speed of 2-4 cm/yr. Antarctica remained almost stationary. This super mobility makes India unique compared to the other fragments of Gondwanaland. We propose that when the parts of Gondwanaland were separated by the plume, the penetration of their lithospheric roots into the asthenosphere played an important role in determining their speed. We estimated the thickness of the lithospheric plates of the different parts of Gondwanaland around the Indian Ocean using the S-receiver function technique. We found that the part of Gondwanaland with clearly the thinnest lithosphere has travelled with the highest speed - India. The lithospheric root in South Africa, Australia and Antarctica is between 180 and 300 km deep. The Indian lithosphere is in contrast only about 100 km thick. Our interpretation is that the plume that partitioned Gondwanaland has also melted the lower half of the Indian lithosphere thus permitting faster motion due to the ridge push or slab pull.

Is there uniformitarian or catastrophic tectonics on Venus?

NASA Technical Reports Server (NTRS)

Turcotte, Donald L.

1993-01-01

The distribution and modification of craters on Venus favors a near global, volcanic resurfacing event about 500 Myrs ago. Such an event indicates that the tectonic evolution of Venus was catastrophic rather than uniformitarian. The creation of a global, single-plate lithosphere on Venus about 500 Myrs ago can explain a variety of tectonic features on Venus that are not consistent with the thin lithosphere required by a uniformitarian hypothesis. A lithosphere on Venus that has thickened for 500 Myrs has a present thickness of about 300 km whereas steady-state heat loss from Venus requires a mean lithospheric thickness near 40 km. A thick lithosphere on Venus can support the high plateaus (elevations of 3-4 km) and mountain belts (up to 9 km) using the same isostatic compensation concepts applicable to the earth. If a thick lithosphere is thinned by a mantle plume, elevation is caused by thermal isostasy. The elevation due to the thinning of a 300 km thick lithosphere is about 3 km. Thus the domal elevation of Beta Regio can be explained by the same mechanism responsible for the elevation of the Hawaiian Swell. While the broad highland plateaus on Venus may be associated with thermal isostasy, the mountain belts in Ishtar Terra clearly cannot be. The high topography of Freyja Montes is almost certainly associated with underthrusting and the likely compensation mechanism is Airy isostasy associated with a thickened crust. With a density contrast delta, of 500 kg m(exp -3) an elevation of 9 km requires a crustal thickening of about 70 km. With a thick lithosphere there is no difficulty in supporting such a thick crust.

Crustal thickness and images of the lithospheric discontinuities in the Gibraltar arc and surrounding areas

NASA Astrophysics Data System (ADS)

Mancilla, Flor de Lis; Stich, Daniel; Morales, José; Martín, Rosa; Diaz, Jordi; Pazos, Antonio; Córdoba, Diego; Pulgar, Javier A.; Ibarra, Pedro; Harnafi, Mimoun; Gonzalez-Lodeiro, Francisco

2015-12-01

The Gibraltar arc and surrounding areas are a complex tectonic region and its tectonic evolution since Miocene is still under debate. Knowledge of its lithospheric structure will help to understand the mechanisms that produced extension and westward motion of the Alboran domain, simultaneously with NW-SE compression driven by Africa-Europe plates convergence. We perform a P-wave receiver function analysis in which we analyse new data recorded at 83 permanent and temporary seismic broad-band stations located in the South of the Iberian peninsula. These data are stacked and combined with data from a previous study in northern Morocco to build maps of thickness and average vP/vS ratio for the crust, and cross-sections to image the lithospheric discontinuities beneath the Gibraltar arc, the Betic and Rif Ranges and their Iberian and Moroccan forelands. Crustal thickness values show strong lateral variations in the southern Iberia peninsula, ranging from ˜19 to ˜46 km. The Variscan foreland is characterized by a relatively flat Moho at ˜31 km depth, and an average vP/vS ratio of ˜1.72, similar to other Variscan terranes, which may indicate that part of the lower crustal orogenic root was lost. The thickest crust is found at the contact between the Alboran domain and the External Zones of the Betic Range, while crustal thinning is observed southeastern Iberia (down to 19 km) and in the Guadalquivir basin where the thinning at the Iberian paleomargin could be still preserved. In the cross-sections, we see a strong change between the eastern Betics, where the Iberian crust underthrusts and couples to the Alboran crust, and the western Betics, where the underthrusting Iberian crust becomes partially delaminated and enters into the mantle. The structures largely mirror those on the Moroccan side where a similar detachment was observed in northern Morocco. We attribute a relatively shallow strong negative-polarity discontinuity to the lithosphere-asthenosphere boundary. This means relatively thin lithosphere ranging from ˜50 km thickness in southeastern Iberia and northeastern Morocco to ˜90-100 km beneath the western Betics and the Rif, with abrupt changes of ˜30 km under the central Betics and northern Morocco. Our observations support a geodynamic scenario where in western Betics oceanic subduction has developed into ongoing continental subduction/delamination while in eastern Betics this process is inactive.

A Review of Recent Developments in the Study of Regional Lithospheric Electrical Structure of the Asian Continent

NASA Astrophysics Data System (ADS)

Zhang, Letian

2017-09-01

The Asian continent was formed through the amalgamation of several major continental blocks that were formerly separated by the Paleo-Asian and Tethyan Oceans. During this process, the Asian continent underwent a long period of continental crustal growth and tectonic deformation, making it the largest and youngest continent on Earth. This paper presents a review of the application of geophysical electromagnetic methods, mainly the magnetotelluric (MT) method, in recent investigations of the diverse tectonic features across the Asian continent. The case studies cover the major continental blocks of Asia, the Central Asian orogenic system, the Tethyan orogenic system, as well as the western Pacific subduction system. In summary, most of the major continental blocks of Asia exhibit a three-layer structure with a resistive upper crust and upper mantle and a relatively conductive mid-lower crust. Large-scale conductors in the upper mantle were interpreted as an indication of lithospheric modification at the craton margins. The electrical structure of the Central Asian orogenic system is generally more resistive than the bordering continental blocks, whereas the Tethyan orogenic system displays more conductive, with pervasive conductors in the lower crust and upper mantle. The western Pacific subduction system shows increasing complexity in its electrical structure from its northern extent to its southern extent. In general, the following areas of the Asian continent have increasingly conductive lithospheric electrical structures, which correspond to a transition from the most stable areas to the most active tectonic areas of Asia: the major continental blocks, the accretionary Central Asian orogenic system, the collisional Tethyan orogenic system, and the western Pacific subduction system. As a key part of this review, a three-dimensional (3-D) model of the lithospheric electrical structure of a large portion of the Tibetan Plateau is presented and discussed in detail; the model indicates tearing of the underthrusting Indian slab as well as complex crustal conductor geometries, which are not obviously consistent with the hypothesis of a continuous, eastward channel flow. These studies have greatly enhanced our knowledge of the formation and deformation processes of the Asian continent. Lastly, future research to expand field data coverage, improve related techniques, and integrate data from other disciplines is suggested.

Construction and destruction of some North American cratons

NASA Astrophysics Data System (ADS)

Snyder, D. B.; Humphreys, G.

2015-12-01

Construction histories of Archean cratons remain poorly understood; their destruction is even less clear because of, by definition, its rarity. By assembling geophysical and geochemical data in 3-D lithosphere models, a clearer understanding of the geometry of major structures within the Rae, Slave and Wyoming cratons of central North America is now possible. Little evidence exists of subducted slabs similar to modern oceanic lithosphere in these construction histories whereas underthrusting and wedging of proto-continental lithosphere is inferred from multiple dipping discontinuities. Archean continental building blocks may resemble the modern lithosphere of Ontong-Java-Hikurangi oceanic plateau. Radiometric dating of xenoliths provides estimates of rock types and ages at depth beneath sparse kimberlite occurrences. These ages can be correlated to surface rocks. The 3.6-2.6 Ga Rae, Slave and Wyoming cratons comprise smaller continental terranes that 'cratonized' during a granitic bloom at 2.61-2.55 ga. Cratonization probably represents the final differentiation of early crust into a relatively homogeneous, uniformly thin (35-42 km), tonalite-trondhjemite-granodiorite crust with pyroxenite layers near the Moho atop depleted lithospheric mantle. Peak thermo-tectonic events at 1.86-1.7 Ga broadly metasomatized, mineralized and recrystallized mantle and lower crustal rocks, apparently making mantle peridotite more 'fertile' and conductive by introducing or concentrating sulfides or graphite throughout the lithosphere at 80-120 km depths. This metasomatism may have also weakened the lithosphere or made it more susceptible to tectonic or chemical erosion. The arrival of the subducted Shatsky Rise conjugate at the Wyoming craton at 65-75 Ma appears to have eroded and displaced the thus weakened base of the craton below 140-160 km. This replaced old refertilized continental mantle with new depleted oceanic mantle. Is this the same craton?

Imaging Lithospheric Structure beneath the Indian continent

NASA Astrophysics Data System (ADS)

Maurya, S.; Montagner, J. P.; Mangalampally, R. K.; Stutzmann, E.; Burgos, G.; Kumar, P.; Davuluri, S.

2015-12-01

The lithospheric structure and thickness to the LAB are the most debated issues, especially beneath continents. In this context, the structure and thickness of the Indian lithosphere has been controversial. Paleomagnetic data reveals that the Indian continent moved northwards at exceptionally high speeds (18-20 cm/year) and subsequently slowed down to 4-5 cm/year after its collision with Asia ≈40 Myr ago. This super mobility has been explained by an unusually thin Indian lithosphere (≈100 km; Kumar et al., 2007) in contradiction with the thick lithosphere that commonly underlies old cratonic nuclei. It is pertinent to note that the thermobarometric estimates on the ultramafic xenoliths from 65 Myr kimberlites of the Central India (Babu et al. 2009) suggest an approximately 175 km thick lithosphere. Also, recent results of P and S wave travel time tomography of India suggest that the lithospheric roots are not uniformly thick on a regional scale. Although high velocity roots typical of Precambrian shields are preserved beneath a few cratons of the Indian shield, they seem to have suffered attrition, in the plume ravaged regions like the NDVP and the Southern SGT (Singh et al., 2014). We assembled a new massive surface wave database towards obtaining 3D isotropic and anisotropic models for the Indian sub-continent, using surface waves. This necessitated processing of data from more than 500 seismic broadband stations across India and surrounding regions. Surface waves group and phase dispersion measurements are performed in a broad frequency range (16-250s). Our phase velocity anomaly maps recover most of the known geological structures. The cratons are associated with high velocity (4-6%) anomalies till 200 sec, with the WDC being faster than the EDC. Slow velocities in NW India and very high velocity anomalies (6-8%) beneath the central part of the Indo-Gangetic plains are possibly associated with the subducting Indian lithosphere. The LAB depths inferred from isotropic depth inversion reveal a very large variability around 120 km to 210 km thick beneath craton. Depth inversion incorporating anisotropy, is in progress. We observe that the best resolved parameters are the transversal shear wave velocities, radial anisotropy and fast axis directions. These play a key role in mapping the LAB

A STEP fault in Central Betics, associated with lateral lithospheric tearing at the northern edge of the Gibraltar arc subduction system

NASA Astrophysics Data System (ADS)

Mancilla, Flor de Lis; Heit, Benjamin; Morales, Jose; Yuan, Xiaohui; Stich, Daniel; Molina-Aguilera, Antonio; Azañon, Jose Miguel; Martín, Rosa

2018-03-01

We study the crustal and lithospheric mantle structure under central Betics in the westernmost Mediterranean region by migrating P-receiver functions along a dense seismic profile (∼2 km interstation distance). The profile, North-South oriented, probes the crustal structure of different geological units, from the Alboran domain in the south with metamorphic rocks, through the External Zones with sedimentary rocks to the Variscan terrains of the Iberian Massif in the north. From north to south, the Moho depth increases from ∼30 km to ∼46 km underneath the Guadix basin, due to the underthrusting of the Iberian crust below the Alboran crust, and suddenly shallows to ∼30 km underneath the Internal Zones with a step of 17 km. This sharp Moho step correlates well with a lithospheric step of ∼40 km, where the thickness of the lithosphere changes abruptly from ∼100 km in the north to ∼50 km in the south. We interpret this sharp and prominent lithospheric step as the termination of the Iberian lithosphere caused by a near-vertical STEP (Subduction-Transform-Edge-Propagator) fault that continues towards the surface as a positive flower tectonic structure of crustal scale. This STEP fault is located at the northern edge of the narrow Westernmost Mediterranean subduction system facilitating the slab rollback motion towards the west. The sharp termination of the Iberian lithosphere occurs under the contact between the Alpujarride and the Nevado-Filabride complexes of the Alboran domain in an ENE-WSW right-lateral transpressive shear zone. The thickest crust and lithosphere do not correlate with the highest topography along the profile suggesting that this high topography is a combined effect of the positive flower structure, and the push up of the asthenosphere produced by the removal of the Iberian lithosphere.

Interpretation of the northern boundary of Ishtar Terra from Magellan images and altimetry

NASA Technical Reports Server (NTRS)

Mueller, S.; Grimm, Robert E.; Phillips, Roger J.

1991-01-01

Part of the controversy on the origin of western Ishtar Terra (IT) concerns the nature of Uorsar Rupes (UR), the northern boundary of IT. In the hypothesis of lithospheric convergence and underthrusting, UR is held to be the main boundary thrust fault at the toe of an accretionary wedge. A topographic rise parallel to the scarp was interpreted as a flexural bulge similar to those of terrestrial subduction zones, and quantitative models of this feature seemed broadly consistent with the expected lithospheric structure of Venus. In the alternative mantle upwelling hypothesis for western IT, the outer margins of the highland are thought to be collapsing, and UR has been interpreted as a normal fault. Herein, Magellan images and altimetry are interpreted for this region and the hypothesis that a flexural signature can be distinguished is reassessed. The Magellan images of IT show evidence of crustal shortening adjacent to UR, but extension and burial dominate northwards. Altimetric profiles display the same long wavelength trends visible in Venera data, but no clear evidence of the lithospheric flexure. A model of regional extension and burial is herein favored, but regional compression cannot be ruled out.

Contrast of lithospheric dynamics across the southern and eastern margins of the Tibetan Plateau: a numerical study

NASA Astrophysics Data System (ADS)

Sun, Yujun; Fan, Taoyuan; Wu, Zhonghai

2018-05-01

Both of the southern and eastern margins of the Tibetan Plateau are bounded by the cratonic blocks (Indian plate and Sichuan basin). However, there are many differences in tectonic deformation, lithospheric structure and surface heat flow between these two margins. What dynamics cause these differences? With the constraints of the lithospheric structure and surface heat flow across the southern and eastern margins of Tibetan Plateau, we constructed 2-D thermal-mechanical finite-element models to investigate the dynamics across these two margins. The results show that the delamination of mantle lithosphere beneath the Lhasa terrane in Oligocene and the rheological contrast between the Indian and Tibetan crust are the two main factors that control the subduction of the Indian plate. The dynamics across the eastern margin of the Tibetan Plateau are different from the southern margin. During the lateral expansion of the Tibetan Plateau, pure shear thickening is the main deformation characteristic for the Songpan-Ganzi lithosphere. This thickening results in the reduction of geothermal gradient and surface heat flow. From this study, it can be seen that the delamination of the mantle lithosphere and the rheological contrast between the Tibetan Plateau and its bounding blocks are the two main factors that control the lithospheric deformation and surface heat flow.

The Himalayan Seismogenic Zone: A New Frontier for Earthquake Research

NASA Astrophysics Data System (ADS)

Brown, Larry; Hubbard, Judith; Karplus, Marianne; Klemperer, Simon; Sato, Hiroshi

2016-04-01

The Mw 7.8 Gorkha, Nepal, earthquake that occurred on April 25 of this year was a dramatic reminder that great earthquakes are not restricted to the large seismogenic zones associated with subduction of oceanic lithosphere. Not only does Himalayan seismogenesis represents important scientific and societal issues in its own right, it constitutes a reference for evaluating general models of the earthquake cycle derived from the studies of the oceanic subduction systems. This presentation reports results of a Mini-Workshop sponsored by the GeoPrisms project that was held in conjunction with the American Geophysical Union on December 15, 2015, designed to organize a new initiative to study the great Himalaya earthquake machine. The Himalayan seismogenic zone shares with its oceanic counterparts a number of fundamental questions, including: a) What controls the updip and downdip limits of rupture? b) What controls the lateral segmentation of rupture zones (and hence magnitude)? c) What is the role of fluids in facilitating slip and or rupture? d) What nucleates rupture (e..g. asperities?)? e) What physical properties can be monitored as precursors to future events? f) How effectively can the radiation pattern of future events be modeled? g) How can a better understanding of Himalayan rupture be translated into more cost effective preparations for the next major event in this region? However the underthrusting of continental, as opposed to oceanic, lithosphere in the Himalayas frames these questions in a very different context: h) How does the greater thickness and weaker rheology of continental crust/lithosphere affect locking of the seismogenic zone? i) How does the different thermal structure of continental vs oceanic crust affect earthquake geodynamics? j) Are fluids a significant factor in intercontinental thrusting? k) How does the basement morphology of underthrust continental crust affect locking/creep, and how does it differ from the oceanic case? l) What is the significance of blind splay faulting in accommodating slip? m) Do lithologic contrasts juxtaposed across the continental seismogenic zone play a role in the rheological behavior of the SZ in the same manner as proposed for the ocean SZ? Major differences in the study of the continental vs oceanic seismogenic zone include the fact that Himalaya structures are open to: a) direct geological observation via field mapping b) dense and wide aperture monitoring of surface strain via GPS and INSAR c) extensive sampling of geofluids via surface flows and shallow drill holes d) cost effective deployment of long term geophysical arrays (e.g. seismic and MT) designed to detect subtle variations if physical properties within the seismogenic zone, and ultimately, e) a fixed platform for deep drilling of past and future rupture zones It remains to be established whether the Himalayan seismogenic zone has the potential for earthquakes of the greatest magnitudes (e.g. 9.0+). However, there is no question that future ruptures in this system represent a serious threat to major population centers (megacities) in the Indian subcontinent. For this reason alone the HSZ is deserving of a major new international, multidisciplinary effort.

The lithosphere-asthenosphere boundary beneath the South Island of New Zealand

NASA Astrophysics Data System (ADS)

Hua, Junlin; Fischer, Karen M.; Savage, Martha K.

2018-02-01

Lithosphere-asthenosphere boundary (LAB) properties beneath the South Island of New Zealand have been imaged by Sp receiver function common-conversion point stacking. In this transpressional boundary between the Australian and Pacific plates, dextral offset on the Alpine fault and convergence have occurred for the past 20 My, with the Alpine fault now bounded by Australian plate subduction to the south and Pacific plate subduction to the north. Using data from onland seismometers, especially the 29 broadband stations of the New Zealand permanent seismic network (GeoNet), we obtained 24,971 individual receiver functions by extended-time multi-taper deconvolution, and mapped them to three-dimensional space using a Fresnel zone approximation. Pervasive strong positive Sp phases are observed in the LAB depth range indicated by surface wave tomography. These phases are interpreted as conversions from a velocity decrease across the LAB. In the central South Island, the LAB is observed to be deeper and broader to the northwest of the Alpine fault. The deeper LAB to the northwest of the Alpine fault is consistent with models in which oceanic lithosphere attached to the Australian plate was partially subducted, or models in which the Pacific lithosphere has been underthrust northwest past the Alpine fault. Further north, a zone of thin lithosphere with a strong and vertically localized LAB velocity gradient occurs to the northwest of the fault, juxtaposed against a region of anomalously weak LAB conversions to the southeast of the fault. This structure could be explained by lithospheric blocks with contrasting LAB properties that meet beneath the Alpine fault, or by the effects of Pacific plate subduction. The observed variations in LAB properties indicate strong modification of the LAB by the interplay of convergence and strike-slip deformation along and across this transpressional plate boundary.

Was Late Cretaceous Magmatism in the Northern Rocky Mountains Really Arc-Related?

NASA Astrophysics Data System (ADS)

Farmer, G.

2011-12-01

Calc-alkaline, Cretaceous magmatism affected much of the northern Rocky Mountain region in the western U.S. and is generally interpreted as continental arc magmatism despite the fact that it occurred as far east into the continental interior as the Late Cretaceous (75 Ma to 78 Ma) Sliderock Mountain volcanoplutonic complex in south-central Montana. Magmatism may have migrated so far inboard as a response to shallowing of the dip angle of underthrust oceanic lithosphere, but the exact sources, tectonic setting and trigger mechanisms for the Late Cretaceous igneous activity remain unclear. In this study, new trace element and Nd and Sr isotopic data, combined with existing age and major element data (duBray et al., 1998, USGS Prof. Paper 1602), from the most mafic lavas present at the Sliderock Mountain Volcano were used to further define the source regions of the Late Cretaceous magmatism. The most mafic lava flows are high K (~2-3 wt. % K2O), low Ti (< 1 wt. % TiO2), low Ni (< 20 ppm) basaltic andesites. Major element oxide contents for these rocks are only weakly correlated with increasing wt. % SiO2 on conventional Harker diagrams. All of the rocks are characterized by high LILE/HFSE ratios and high Pb contents (17-20 ppm), as expected for arc-related magmatism. The rocks also have high (La/Yb)N (7-20) but show decreasing (Dy/Yb)N with increasing wt.% SiO2, suggesting a cryptic role for amphibole fractionation during evolution of their parental magmas. Initial ɛNd values range from -19 to -29 but do not covary with rock bulk composition and as a result are unlikely to represent the result of interaction with local Archean continental crust. Initial 87Sr/86Sr, in contrast, vary over a restricted range from 0.7045 to 0.7065. The lowest 87Sr/86Sr correspond to samples with the highest Sr/Y (120-190). The low ɛNd values for the basaltic andesites suggest that if these volcanic rocks were ultimately derived from ultramafic mantle sources, melting must have occurred in Archean mantle lithosphere. Given the correlation between increasing Sr/Y and decreasing 87Sr/86Sr in the basaltic andesites, one possible trigger mechanism for lithospheric mantle melting is the influx into the thick Archean mantle keel of slab fluids (possibly including high Sr/Y slab melts) derived from oceanic lithosphere underthrust beneath this region in the Late Cretaceous. In this case, the Sliderock Mountain Volcano could, in fact, represent an example of continental interior "arc" magmatism.

Construction and destruction of some North American cratons

NASA Astrophysics Data System (ADS)

Snyder, David B.; Humphreys, Eugene; Pearson, D. Graham

2017-01-01

Construction histories of Archean cratons remain poorly understood; their destruction is even less clear because of its rarity, but metasomatic weakening is an essential precursor. By assembling geophysical and geochemical data in 3-D lithosphere models, a clearer understanding of the geometry of major structures within the Rae, Slave and Wyoming cratons of central North America is now possible. Little evidence exists of subducted slab-like geometries similar to modern oceanic lithosphere in these construction histories. Underthrusting and wedging of proto-continental lithosphere is inferred from multiple dipping discontinuities, emphasizing the role of lateral accretion. Archean continental building blocks may resemble the modern lithosphere of oceanic plateau, but they better match the sort of refractory crust expected to have formed at Archean ocean spreading centres. Radiometric dating of mantle xenoliths provides estimates of rock types and ages at depth beneath sparse kimberlite occurrences, and these ages can be correlated to surface rocks. The 3.6-2.6 Ga Rae, Slave and Wyoming cratons stabilized during a granitic bloom at 2.61-2.55 Ga. This stabilization probably represents the final differentiation of early crust into a relatively homogeneous, uniformly thin (35-42 km), tonalite-trondhjemite-granodiorite crust with pyroxenite layers near the Moho atop depleted lithospheric mantle. Peak thermo-tectonic events at 1.86-1.7 Ga broadly metasomatized, mineralized and recrystallized mantle and lower crustal rocks, apparently making mantle peridotite more 'fertile' and more conductive by introducing or concentrating sulfides or graphite at 80-120 km depths. This metasomatism may have also weakened the lithosphere or made it more susceptible to tectonic or chemical erosion. Late Cretaceous flattening of Farallon lithosphere that included the Shatsky Rise conjugate appears to have weakened, eroded and displaced the base of the Wyoming craton below 140-160 km. This process replaced the old re-fertilized continental mantle with relatively young depleted oceanic mantle.

Tectonic Evolution of the Careón Ophiolite (Northwest Spain): A Remnant of Oceanic Lithosphere in the Variscan Belt.

PubMed

Díaz García F; Arenas; Martínez Catalán JR; González del Tánago J; Dunning

1999-09-01

Analysis of the Careón Unit in the Ordenes Complex (northwest Iberian Massif) has supplied relevant data concerning the existence of a Paleozoic oceanic lithosphere, probably related to the Rheic realm, and the early subduction-related events that were obscured along much of the Variscan belt by subsequent collision tectonics. The ophiolite consists of serpentinized harzburgite and dunite in the lower section and a crustal section made up of coarse-grained and pegmatitic gabbros. An Early Devonian zircon age (395+/-2 Ma, U-Pb) was obtained in a leucocratic gabbro. The whole section was intruded by numerous diabasic gabbro dikes. Convergence processes took place shortly afterward, giving rise to a mantle-rooted synthetic thrust system, with some coeval igneous activity. Garnet amphibolite, developed in metamorphic soles, was found discontinuously attached to the thrust fault. The soles graded downward to epidote-amphibolite facies metabasite and were partially retrogressed to greenschist facies conditions. Thermobarometric estimations carried out at a metamorphic sole (T approximately 650 degrees C; P approximately 11.5 kbar) suggested that imbrications developed in a subduction setting, and regional geology places this subduction in the context of an early Variscan accretionary wedge. Subduction and imbrication of oceanic lithosphere was followed by underthrusting of the Gondwana continental margin.

The formation of Laurentia: Evidence from shear wave splitting

NASA Astrophysics Data System (ADS)

Liddell, Mitch V.; Bastow, Ian; Darbyshire, Fiona; Gilligan, Amy; Pugh, Stephen

2017-12-01

The northern Hudson Bay region in Canada comprises several Archean cratonic nuclei, assembled by a number of Paleoproterozoic orogenies including the Trans-Hudson Orogen (THO) and the Rinkian-Nagssugtoqidian Orogen. Recent debate has focused on the extent to which these orogens have modern analogues such as the Himalayan-Karakoram-Tibet Orogen. Further, the structure of the lithospheric mantle beneath the Hudson Strait and southern Baffin Island is potentially indicative of Paleoproterozoic underthrusting of the Superior plate beneath the Churchill collage. Also in question is whether the Laurentian cratonic root is stratified, with a fast, depleted, Archean core underlain by a slower, younger, thermally-accreted layer. Plate-scale process that create structures such as these are expected to manifest as measurable fossil seismic anisotropic fabrics. We investigate these problems via shear wave splitting, and present the most comprehensive study to date of mantle seismic anisotropy in northern Laurentia. Strong evidence is presented for multiple layers of anisotropy beneath Archean zones, consistent with the episodic development model of stratified cratonic keels. We also show that southern Baffin Island is underlain by dipping anisotropic fabric, where underthrusting of the Superior plate beneath the Churchill has previously been interpreted. This provides direct evidence of subduction-related deformation at 1.8 Ga, implying that the THO developed with modern plate-tectonic style interactions.

Underplating along the northern portion of the Zagros suture zone, Iran

NASA Astrophysics Data System (ADS)

Motaghi, K.; Shabanian, E.; Kalvandi, F.

2017-07-01

A 2-D absolute shear wave velocity model has been resolved beneath a seismic profile across the northeastern margin of the Arabian Plate-Central Iran by simultaneously inverting data from P receiver functions and fundamental mode Rayleigh wave phase velocity. The data were gathered by a linear seismic array crossing the Zagros fold and thrust belt, Urmia-Dokhtar magmatic arc and Central Iran block assemblage as three major structural components of the Arabia-Eurasia collision. Our model shows a low-velocity tongue protruding from upper to lower crust which, north of the Zagros suture, indicates the signature of an intracontinent low-strength shear zone between the underthrusting and overriding continents. The velocity model confirms the presence of a significant crustal root as well as a thick high-velocity lithosphere in footwall of the suture, continuing northwards beneath the overriding continent for at least 200 km. These features are interpreted as underthrusting of Arabia beneath Central Iran. Time to depth migration of P receiver functions reveals an intracrustal flat interface at ∼17 km depth south of the suture; we interpret it as a significant decoupling within the upper crust. All these crustal scale structural features coherently explain different styles and kinematics of deformation in northern Zagros (Lorestan zone) with respect to its southern part (Fars zone).

A two-dimensional hybrid method for modeling seismic waves propagation in laterally-varying anisotropic media and its application to central Tibet

NASA Astrophysics Data System (ADS)

Zhao, L.; Wen, L.

2009-12-01

The shear wave splitting measurements provide important information on mantle flow, deformation and mineralogy. They are now routinely made using the method developed by Silver and Chan (1994). More and more dense regional observations also begin to reveal sharp spatial variations of seismic anisotropy which could not be explained by simplified horizontal homogeneous anisotropic structures. To better constrain the mantle anisotropy beneath those regions, we developed a two-dimensional hybrid method for simulating seismic wave propagation in laterally-varying anisotropic media [Zhao et al., 2008]. In this presentation, we apply the method to study anisotropic structures beneath central Tibet by waveform modeling the teleseismic SKS phases recorded in the International Deep Profiling of Tibet and the Himalayas project (INDEPTH) III. Using data from two events that were selected such that the stations and sources can be approximated as a two-dimensional profile, we derived an optimal model for the anisotropic structures of the upper mantle beneath the study region: a 50-70 km thick anisotropic layer with a fast direction trending N95°E beneath the Qiangtang block, a 150 km thick and 60 km wide anisotropic segment with an axis trending N95°E beneath the northernmost Lhasa block, and a ~30 km wide transition zone in between within which the fast direction trends N45°E and the depth extent of anisotropy decreases northward sharply. Synthetic waveform modeling further suggests that an anisotropic model with a horizontal symmetry axis can explain the observations better than that with a dipping symmetry, and a low velocity zone possibly underlies or mixes with the anisotropic structures in the northern portion of the region. The optimal model yields synthetic seismograms that are in good agreement with the observations in both amplitudes and relative arrival times of SKS phases. Synthetic tests also indicate that different elastic constants, source parameters and depth extents of anisotropy adopted in the calculations do not affect the general conclusions, although trade-offs exist between the model parameters. Our modeling results suggest that, if the complex seismic structures in central Tibet are associated with the underthrusting of the Indian lithosphere beneath the Asian lithosphere, the inferred horizontal symmetry of anisotropy was likely generated during the collision because an inherited anisotropy would have a dipping angle of symmetry axis that is parallel to the underthrusting direction. References Silver, P. G., and M. K. Savage (1994), The interpretation of shear-wave splitting parameters in the presence of two anisotropic layers, Geophys. J. Int., 119, 949-963. Zhao L., L.X. Wen, L. Chen, T.Y. Zheng (2008). A two-dimensional hybrid method for modeling seismic wave propagation in anisotropic media, J. Geophys. Res., 113, B12307, doi:10.1029/2008JB005733.

Subduction of the Indian lithosphere beneath Tibet and deformation of the Tibetan crust and mantle, imaged with broad-band surface waves

NASA Astrophysics Data System (ADS)

Agius, Matthew R.; Lebedev, Sergei

2013-04-01

Seismic deployments over the last two decades have produced dense broadband data coverage across the Tibetan Plateau. Yet, the lithospheric dynamics of Tibet remains enigmatic, with even its basic features debated and with very different end-member models still advocated today. Most body-wave tomographic models do not resolve any high-velocity anomalies in the upper mantle beneath central and northern Tibet, which motivated the inference that the Indian lithosphere may sink into deep mantle beneath the Himalayas in the south, with parts of it possibly extruded laterally eastward. In contrast, surface-wave tomographic models all show pronounced high-velocity anomalies beneath much of Tibet at depths around 200 km. Uncertainties over the shapes and amplitudes of the anomalies, however, contribute to the uncertainty of their interpretations, ranging from the subduction of India or Asia to the extreme viscous thickening of the Tibetan lithosphere. Within the lithosphere itself, a low-viscosity layer in the mid-lower crust is evidenced by many observations. It is still unclear, however, whether this layer accommodates a large-scale channel flow (which may have uplifted eastern Tibet, according to one model) or if, instead, deformation within it is similar to that observed at the surface. Broad-band surface waves provide resolving power from the upper crust down to the asthenosphere, for both the isotropic-average shear-wave speeds (characterising the composition and thermal state of the lithosphere) and the radial and azimuthal shear-wave anisotropy (indicative, in an actively deforming region, of the current and recent flow). We measured highly accurate Love- and Rayleigh-wave phase-velocity curves in broad period ranges (up to 5-200 s) for a few tens of pairs and groups of stations across Tibet, combining, in each case, hundreds to thousands of inter-station measurements made with cross-correlation and waveform-inversion methods. Robust shear-velocity profiles were then determined by extensive series of non-linear inversions, designed to constrain the depth-dependent ranges of isotropic-average shear speeds and radial anisotropy consistent with the data. Temperature anomalies in the upper mantle were estimated from shear-velocity using pre-computed petro-physical relationships. Azimuthal anisotropy in the crust and upper mantle was determined by surface-wave tomography and, also, by sub-array analysis targeting the anisotropy amplitude. Our results show that the prominent high-velocity anomalies in the upper mantle are most consistent with the presence of subducted Indian lithosphere beneath much of Tibet. The large estimated thermal anomalies within the high-velocity features match those to be expected within subducted India. The morphology of India's subduction beneath Tibet is complex and shows pronounced west-east variations. Beneath eastern and northeastern Tibet, in particular, the subducted Indian lithosphere appears to have subducted, at a shallow angle, hundreds of km NNE-wards. Azimuthal anisotropy beneath Tibet is distributed in multiple layers with different fast-propagations directions, which accounts for the complexity of published shear-wave splitting observations. The fast directions within the mid-lower crust are parallel to the extensional components of the current strain rate field at the surface, consistent with similar deformation through the entire ­crust, rather than channel flow. Anisotropy within the asthenosphere beneath northeastern Tibet (sandwiched between the Tibetan lithosphere above and the subducted Indian lithosphere below) indicates SSW-NNE flow, parallel to the direction of motion of the Indian Plate, including its subducted leading edge.

Interference of lithospheric folding in western Central Asia by simultaneous Indian and Arabian plate indentation

NASA Astrophysics Data System (ADS)

Smit, J. H. W.; Cloetingh, S. A. P. L.; Burov, E.; Tesauro, M.; Sokoutis, D.; Kaban, M.

2013-08-01

Large-scale intraplate deformation of the crust and the lithosphere in Central Asia as a result of the indentation of India has been extensively documented. In contrast, the impact of continental collision between Arabia and Eurasia on lithosphere tectonics in front of the main suture zone, has received much less attention. The resulting Neogene shortening and uplift of the external Zagros, Alborz, Kopeh Dagh and Caucasus Mountain belts in Iran and surrounding areas is characterised by a simultaneous onset of major topography growth at ca. 5 Ma. At the same time, subsidence accelerated in the adjacent Caspian, Turan and Amu Darya basins. We present evidence for interference of lithospheric folding patterns induced by the Arabian and Indian collision with Eurasia. Wavelengths and spatial patterns are inferred from satellite-derived topography and gravity models. The observed interference of the patterns of folding appears to be primarily the result of spatial orientation of the two indenters, differences in their convergence velocities and the thermo-mechanical structure of the lithosphere west and east of the Kugitang-Tunka Line.

Sediment underthrusting within a continental magmatic arc: Coast Mountains batholith, British Columbia

NASA Astrophysics Data System (ADS)

Pearson, David M.; MacLeod, Douglas R.; Ducea, Mihai N.; Gehrels, George E.; Jonathan Patchett, P.

2017-10-01

Though continental magmatic arcs are factories for new continental crust, a significant proportion of continental arc magmas are recycled from supracrustal material. To evaluate the relative contributions of retroarc underthrusting and trench side partial sediment subduction for introducing supracrustal rocks to the middle and lower crust of continental magmatic arcs, we present results from the deeply exposed country rocks of the Coast Mountains batholith of western British Columbia. Prior work demonstrates that these rocks underwent widespread partial melting that contributed to the Coast Mountains batholith. We utilize U-Pb zircon geochronology, Sm-Nd thermochronology, and field-based studies to document the protoliths and early burial history of amphibolite and granulite-facies metasedimentary rocks in the Central Gneiss Complex. U-Pb detrital zircon data from the structurally highest sample localities yielded 190 Ma unimodal age peaks and suggest that retroarc rocks of the Stikine terrane constitute a substantial portion of the Central Gneiss Complex. These supracrustal rocks underwent thrust-related burial and metamorphism at >25 km depths prior to 80 Ma. These rocks may also be underlain at the deepest exposed structural levels by Upper Cretaceous metasedimentary rocks, which may have been emplaced as a result of trench side underplating or intraarc burial. These results further our understanding of the mechanisms of material transport within the continental lithosphere along Cordilleran subduction margins.

Cascade Mountain Range in Oregon

USGS Publications Warehouse

Sherrod, David R.

2016-01-01

Along its Oregon segment, the Cascade Range is almost entirely volcanic in origin. The volcanoes and their eroded remnants are the visible magmatic expression of the Cascadia subduction zone, where the offshore Juan de Fuca tectonic plate is subducted beneath North America. Subduction occurs as two lithospheric plates collide, and an underthrusted oceanic plate is commonly dragged into the mantle by the pull of gravity, carrying ocean-bottom rock and sediment down to where heat and pressure expel water. As this water rises, it lowers the melting temperature in the overlying hot mantle rocks, thereby promoting melting. The molten rock supplies the volcanic arcs with heat and magma. Cascade Range volcanoes are part of the Ring of Fire, a popular term for the numerous volcanic arcs that encircle the Pacific Ocean.

The Lithosphere-asthenosphere Boundary beneath the South Island of New Zealand

NASA Astrophysics Data System (ADS)

Hua, J.; Fischer, K. M.; Savage, M. K.

2017-12-01

Lithosphere-asthenosphere boundary (LAB) properties beneath the South Island of New Zealand have been imaged by Sp receiver function common-conversion point stacking. In this transpressional boundary between the Australian and Pacific plates, dextral offset on the Alpine fault and convergence have occurred for the past 20 My, with the Alpine fault now bounded by Australian plate subduction to the south and Pacific plate subduction to the north. This study takes advantage of the long-duration and high-density seismometer networks deployed on or near the South Island, especially 29 broadband stations of the New Zealand permanent seismic network (GeoNet). We obtained 24,980 individual receiver functions by extended-time multi-taper deconvolution, mapping to three-dimensional space using a Fresnel zone approximation. Pervasive strong positive Sp phases are observed in the LAB depth range indicated by surface wave tomography (Ball et al., 2015) and geochemical studies. These phases are interpreted as conversions from a velocity decrease across the LAB. In the central South Island, the LAB is observed to be deeper and broader to the west of the Alpine fault. The deeper LAB to the west of the Alpine fault is consistent with oceanic lithosphere attached to the Australian plate that was partially subducted while also translating parallel to the Alpine fault (e.g. Sutherland, 2000). However, models in which the Pacific lithosphere has been underthrust to the west past the Alpine fault cannot be ruled out. Further north, a zone of thin lithosphere with a strong and vertically localized LAB velocity gradient occurs to the west of the fault, juxtaposed against a region of anomalously weak LAB conversions to the east of the fault. This structure, similar to results of Sp imaging beneath the central segment of the San Andreas fault (Ford et al., 2014), also suggests that lithospheric blocks with contrasting LAB properties meet beneath the Alpine fault. The observed variations in LAB properties indicate strong modification of the LAB by the interplay of convergence and strike-slip deformation along and across this transpressional plate boundary.

Interference of lithospheric folding in Central Asia by simultaneous Indian and Arabian plate indentation

NASA Astrophysics Data System (ADS)

Smit, J. H. W.; Cloetingh, S. A. P. L.; Burov, E.; Sokoutis, D.; Kaban, M.; Tesauro, M.; Burg, J.-P.

2012-04-01

Although large-scale folding of the crust and the lithosphere in Central Asia as a result of the indentation of India has been extensively documented, the impact of continental collision between Arabia and Eurasia has been largely overlooked. The resulting Neogene shortening and uplift of the Zagros, Albors, Kopet Dagh and Kaukasus mountain belts in Iran and surrounding areas is characterised by a simultaneous onset of major topography growth at ca. 5 Ma. At the same time, the adjacent Caspian, Turan and Amu Darya basins underwent an acceleration in subsidence. It is common knowledge that waves with different orientations will interfere with each other. Folding, by its nature similar to a standing wave, is not likely to be an exception. We demonstrate that collision of the Eurasian plate with the Arabian and Indian plates generates folding of the Eurasian lithosphere in two different directions and that interaction between both generates characteristic interference patterns that can be recognised from the regional gravity signal. We present evidence for interference of lithospheric folding patterns induced by Arabian and Indian collision with Eurasia. Wavelengths (from 50 to 250 km) and spatial patterns are inferred from satellite-derived topography and gravity models and attest for rheologically stratified lithosphere with relatively strong mantle rheology (thickness of strong mechanical core on the order of 40-50 km) and less competent crust (thickness of the mechanical core on the order of 10-15 km). The observations are compared with inferences from numerical and analogue tectonic experiments for a quantitative assessment of factors such as lithosphere rheology and stratification, lateral variations in lithosphere strength, thermo-mechanical age and distance to the plate boundary on the activity of folding as a mechanism of intra-plate deformation in this area. The observed interference of the patterns of folding appears to be primarily the result of spatial orientation of the two indenters, differences in their convergence velocities and the thermo-mechanical structure of the lithosphere west and east of the Kugitang-Tunka line.

Geothermal areas in Pakistan

DOE Office of Scientific and Technical Information (OSTI.GOV)

Shuja, T.A.

1986-01-01

In this paper an attempt has been made to correlate the tectonic and geologic features with surface manifestations of geothermal activity in Pakistan to delineate prospective areas for exploration and development of geothermal energy. Underthrusting of the Arabian plate beneath the Eurasian plate has resulted in the formation of Chagai volcanic arc which extends into Iran. Quaternary volcanics in this environment, along with the presence of thermal springs, is an important geotectonic feature revealing the possible existence of geothermal fields. Geothermal activity in the northern areas of Pakistan, as evidenced by thermal springs, is the likely result of collision andmore » underthrusting of the Indian plate beneath the Eurasian plate. Numerous hot springs are found along the Main Mantle thrust and the Main Karakorum thrust in Chilas and Hunza areas respectively. The concentration of hot springs in Sind Province is also indicative of geothermal activity. A string of thermal seepages and springs following the alignment of the Syntaxial Bend in Punjab Province is also noteworthy from the geothermal viewpoint. In Baluchistan Province (southwest Pakistan), Hamun-e-Mushkel, a graben structure, also shows geothermal prospects on the basis of aeromagnetic studies.« less

Seismic anisotropy of western Mexico and northeastern Tibet

NASA Astrophysics Data System (ADS)

Leon-Soto, Gerardo

In this dissertation, characteristics of upper mantle anisotropy, using shear wave splitting techniques, for two distinct tectonic provinces are presented. In the first part, in western Mexico, the Rivera and Cocos plates subduct beneath the North America plate constituting a young subduction setting where plate fragmentation and capture is occurring today. We characterize the upper mantle anisotropy from SKS and local S phases from the data collected by the MARS experiment (MApping the Rivera Subduction zone) and by two stations of the Mexican Servicio Sismologico National. SKS shear-wave splitting parameters indicate that the fast directions of the split SKS waves for the stations that lie on the central and southern Jalisco block are approximately trench normal. Fast polarizations of these phases also follow the convergence direction between the Rivera Plate and Jalisco block with respect to the North America plate. S-wave splitting from slab events show a small averaged delay time of about 0.2 sec for the upper 60 km of the crust and mantle. Therefore, the main source of anisotropy must reside on the entrained mantle below the young and thin Rivera Plate. Trench-oblique fast SKS split directions are observed in the western edge of the Rivera Plate and western parts of the Cocos slab. The curved pattern of fast SKS split directions in the western Jalisco block and the Rivera-Cocos gap indicate 3-D toroidal mantle flow, around the northwestern edge of the Rivera slab and Rivera- Cocos gap. This behavior profoundly affects finite strain field in the northwestern edge of the Rivera slab and the mantle wedge. The shear wave splitting results support the idea that the Rivera and Cocos plates not only moved in a down-dip direction but also have recently rolled back towards the trench and the Colima rift is intimately related to the tearing between the Rivera and Cocos plates. In the second study, the tectonic enviroment of the northeastern Tibetan plateau is considered. Shear wave splitting measurements using teleseismic SKS and SKKS phases recorded by the ASCENT (A Seismic Collaborative Experiment in Northeastern Tibet) and INDEPTH-IV (International Deep Profiling of Tibet and the Himalaya, Phase IV) experiments reveal significant anisotropy in north-eastern Tibet with a large delay time of up 2.2 sec, indicating that anisotropy exists in both the lithospheric and asthenospheric mantle. The coherence between fast polarization directions of split core phases and the left-lateral slip on eastern-striking, southeastern-striking and southern-striking faults in eastern Tibet as well as the surface velocity calculated from GPS data support the idea that left-lateral shear strain is the predominant cause of the orientation of the upper mantle petrofabrics. The left-lateral motion can be best understood as a manifestation of north-striking right-lateral simple shear exerted by the eastern edge of the underthrusting Indian plate as it penetrates into Eurasia, as well as the bending of the Eastern Himalayan Syntaxis (EHS) by the foundering Burma-Andaman-Sumatra slab. Two plausible competing models are proposed for the flow of asthenosphere. In the first, the deforming lithosphere gliding over the passive asthenosphere induces flow of the asthenosphere. In the second, the asthenosphere beneath northeastern Tibet is flowing eastward in an asthenosphere channel that lies between the Ordos plateau and Sichuan basin, and around the EHS as it is being compressed between the advancing Indian continental lithosphere and the thick Tarim and Qaidam lithospheres to the north. Delay times from stations in the EHS have a maximum of 1.3 sec suggesting that although most anisotropy is residing in the lithosphere, some may be associated with flow of the asthenosphere. The retreating Burma slab induces flow that is toroidal and located exclusively around the northern edge of the slab. The curved fast directions of split shear waves for stations in the EHS are consistent with the toroidal flow pattern as well as the rotational deformation of the overlying lithosphere. It is suggested that the foundering Burma plate may also play an important role in bending the EHS in the late Cenozoic time.

Localization instability and the origin of regularly- spaced faults in planetary lithospheres

NASA Astrophysics Data System (ADS)

Montesi, Laurent Gilbert Joseph

2002-10-01

Brittle deformation is not distributed uniformly in planetary lithospheres but is instead localized on faults and ductile shear zones. In some regions such as the Central Indian Basin or martian ridged plains, localized shear zones display a characteristic spacing. This pattern can constrain the mechanical structure of the lithosphere if a model that includes the development of localized shear zones and their interaction with the non- localizing levels of the lithosphere is available. I construct such a model by modifying the buckling analysis of a mechanically-stratified lithosphere idealization, by allowing for rheologies that have a tendency to localize. The stability of a rheological system against localization is indicated by its effective stress exponent, ne. That quantity must be negative for the material to have a tendency to localize. I show that a material deforming brittly or by frictional sliding has ne < 0. Localization by shear heating or grain size feedback in the ductile field requires significant deviations from non-localized deformation conditions. The buckling analysis idealizes the lithosphere as a series of horizontal layers of different mechanical properties. When this model is subjected to horizontal extension or compression, infinitesimal perturbation of its interfaces grow at a rate that depends on their wavelength. Two superposed instabilities develop if ne < 0 in a layer overlying a non-localizing substratum. One is the classical buckling/necking instability. The other gives rise to regularly-spaced localized shear zones, with a spacing proportional to the thickness of the localizing layer, and dependent on n e. I call that second instability the localization instability. Using the localization instability, the depth to which fault penetrate in the Indian Ocean and in martian ridged plains can be constrained from the ridge spacing. The result are consistent with earthquake data in the Indian Ocean and radiogenic heat production on Mars. It is therefore possible that the localization instability exerts a certain control on the formation of fault patterns in planetary lithospheres. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253- 1690.)

Continental lithospheric subduction and intermediate-depth seismicity: Constraints from S-wave velocity structures in the Pamir and Hindu Kush

NASA Astrophysics Data System (ADS)

Li, Wei; Chen, Yun; Yuan, Xiaohui; Schurr, Bernd; Mechie, James; Oimahmadov, Ilhomjon; Fu, Bihong

2018-01-01

The Pamir has experienced more intense deformation and shortening than Tibet, although it has a similar history of terrane accretion. Subduction as a primary way to accommodate lithospheric shortening beneath the Pamir has induced the intermediate-depth seismicity, which is rare in Tibet. Here we construct a 3D S-wave velocity model of the lithosphere beneath the Pamir by surface wave tomography using data of the TIPAGE (Tien Shan-Pamir Geodynamic program) and other seismic networks in the area. We imaged a large-scale low velocity anomaly in the crust at 20-50 km depth in the Pamir overlain by a high velocity anomaly at a depth shallower than 15 km. The high velocity anomalies colocate with exposed gneiss domes, which may imply a similar history of crustal deformation, partial melting and exhumation in the hinterland, as has occurred in the Himalaya/Tibet system. At mantle depths, where the intermediate-depth earthquakes are located, a low velocity zone is clearly observed extending to about 180 km and 150 km depth in the Hindu Kush and eastern Pamir, respectively. Moreover, the geometry of the low-velocity anomaly suggests that lower crustal material has been pulled down into the mantle by the subducting Asian and Indian lithospheric mantle beneath the Pamir and Hindu Kush, respectively. Metamorphic processes in the subducting lower crust may cause the intermediate-depth seismicity down to 150-180 km depth beneath the Pamir and Hindu Kush. We inverted focal mechanisms in the seismic zone for the stress field. Differences in the stress field between the upper and lower parts of the Indian slab imply that subduction and detachment of the Indian lithosphere might cause intense seismicity associated with the thermal shear instability in the deep Hindu Kush.

The 3-dimensional construction of the Rae craton, central Canada

NASA Astrophysics Data System (ADS)

Snyder, David B.; Craven, James A.; Pilkington, Mark; Hillier, Michael J.

2015-10-01

Reconstruction of the 3-dimensional tectonic assembly of early continents, first as Archean cratons and then Proterozoic shields, remains poorly understood. In this paper, all readily available geophysical and geochemical data are assembled in a 3-D model with the most accurate bedrock geology in order to understand better the geometry of major structures within the Rae craton of central Canada. Analysis of geophysical observations of gravity and seismic wave speed variations revealed several lithospheric-scale discontinuities in physical properties. Where these discontinuities project upward to correlate with mapped upper crustal geological structures, the discontinuities can be interpreted as shear zones. Radiometric dating of xenoliths provides estimates of rock types and ages at depth beneath sparse kimberlite occurrences. These ages can also be correlated to surface rocks. The 3.6-2.6 Ga Rae craton comprises at least three smaller continental terranes, which "cratonized" during a granitic bloom. Cratonization probably represents final differentiation of early crust into a relatively homogeneous, uniformly thin (35-42 km), tonalite-trondhjemite-granodiorite crust with pyroxenite layers near the Moho. The peak thermotectonic event at 1.86-1.7 Ga was associated with the Hudsonian orogeny that assembled several cratons and lesser continental blocks into the Canadian Shield using a number of southeast-dipping megathrusts. This orogeny metasomatized, mineralized, and recrystallized mantle and lower crustal rocks, apparently making them more conductive by introducing or concentrating sulfides or graphite. Little evidence exists of thin slabs similar to modern oceanic lithosphere in this Precambrian construction history whereas underthrusting and wedging of continental lithosphere is inferred from multiple dipping discontinuities.

Continental underplating after slab break-off

NASA Astrophysics Data System (ADS)

Magni, V.; Allen, M. B.; van Hunen, J.; Bouilhol, P.

2017-09-01

We present three-dimensional numerical models to investigate the dynamics of continental collision, and in particular what happens to the subducted continental lithosphere after oceanic slab break-off. We find that in some scenarios the subducting continental lithosphere underthrusts the overriding plate not immediately after it enters the trench, but after oceanic slab break-off. In this case, the continental plate first subducts with a steep angle and then, after the slab breaks off at depth, it rises back towards the surface and flattens below the overriding plate, forming a thick horizontal layer of continental crust that extends for about 200 km beyond the suture. This type of behaviour depends on the width of the oceanic plate marginal to the collision zone: wide oceanic margins promote continental underplating and marginal back-arc basins; narrow margins do not show such underplating unless a far field force is applied. Our models show that, as the subducted continental lithosphere rises, the mantle wedge progressively migrates away from the suture and the continental crust heats up, reaching temperatures >900 °C. This heating might lead to crustal melting, and resultant magmatism. We observe a sharp peak in the overriding plate rock uplift right after the occurrence of slab break-off. Afterwards, during underplating, the maximum rock uplift is smaller, but the affected area is much wider (up to 350 km). These results can be used to explain the dynamics that led to the present-day crustal configuration of the India-Eurasia collision zone and its consequences for the regional tectonic and magmatic evolution.

Geodynamic Models of Plume-Ridge Interaction in the Indian Ocean and its Effect on the Crustal Thickness of the Réunion Hotspot Track

NASA Astrophysics Data System (ADS)

Bredow, E.; Gassmöller, R.; Dannberg, J.; Steinberger, B.

2016-12-01

The Réunion mantle plume had a first impact on the Earth's surface when the plume head approached the base of the lithosphere around 67 million years ago and the first vigorous volcanic eruptions created the Deccan Traps in India, one of the largest flood basalt provinces in the world. During this period, the Indian plate may have been accelerated by the uprising plume head, leading to a northeastward plate motion with a unique velocity of up to 18 cm/year. The hotspot track, generally considered to be created by the plume tail impinging on the moving plates, comprises the volcanic chains of the Laccadives, Maldives and Chagos on the Indian plate and the Southern Mascarene Plateau on the African plate. It has been divided by seafloor spreading, since the Central Indian Ridge has passed over the plume approximately 50 million years ago, leading to intensive and continuing plume-ridge interaction. Considering the whole geodynamic history of the plume up to its currently active position underneath the island of Réunion, we set up three-dimensional regional convection models of the upper mantle using the mantle convection code ASPECT (Advanced Solver for Problems in Earth's ConvecTion). In order to study this specific plume, we prescribe the global flow field from a coarser global model at the side boundaries and bottom of the box model and the reconstructed tectonic plate velocities at the uppermost 200 km while a plume inflow is enforced at the bottom. Furthermore, we extended the code to import varying lithosphere thickness values at the side boundaries to compare realistic lithosphere models with simple constant lithosphere thickness models. Finally, we compare the amount and pattern of the resulting crustal thickness produced by the plume with present-day topographic maps to constrain plume properties such as the excess temperature and buoyancy flux. Special focus is placed on how the ridge geometry helps generating the distinctive gap in the hotspot track between Chagos and the Maldives and on the origin of the Rodriguez Ridge, a volcanic lineament between the plume and the Central Indian Ridge, which has been suggested to be a sub-lithosphere melt channel by Morgan already in 1978 and can be observed in our models.

Obduction of old oceanic lithosphere due to reheating and plate reorganization: Insights from numerical modelling and the NE Anatolia - Lesser Caucasus case example

NASA Astrophysics Data System (ADS)

Hässig, Marc; Duretz, Thibault; Rolland, Yann; Sosson, Marc

2016-05-01

The ophiolites of NE Anatolia and of the Lesser Caucasus (NALC) evidence an obduction over ∼200 km of oceanic lithosphere of Middle Jurassic age (c. 175-165 Ma) along an entire tectonic boundary (>1000 km) at around 90 Ma. The obduction process is characterized by four first order geological constraints: Ophiolites represent remnants of a single ophiolite nappe currently of only a few kilometres thick and 200 km long. The oceanic crust was old (∼80 Ma) at the time of its obduction. The presence of OIB-type magmatism emplaced up to 10 Ma prior to obduction preserved on top of the ophiolites is indicative of mantle upwelling processes (hotspot). The leading edge of the Taurides-Anatolides, represented by the South Armenian Block, did not experience pressures exceeding 0.8 GPa nor temperatures greater than ∼300 °C during underthrusting below the obducting oceanic lithosphere. An oceanic domain of a maximum 1000 km (from north to south) remained between Taurides-Anatolides and Pontides-Southern Eurasian Margin after the obduction. We employ two-dimensional thermo-mechanical numerical modelling in order to investigate obduction dynamics of a re-heated oceanic lithosphere. Our results suggest that thermal rejuvenation (i.e. reheating) of the oceanic domain, tectonic compression, and the structure of the passive margin are essential ingredients for enabling obduction. Afterwards, extension induced by far-field plate kinematics (subduction below Southern Eurasian Margin), facilitates the thinning of the ophiolite, the transport of the ophiolite on the continental domain, and the exhumation of continental basement through the ophiolite. The combined action of thermal rejuvenation and compression are ascribed to a major change in tectonic motions occurring at 110-90 Ma, which led to simultaneous obductions in the Oman (Arabia) and NALC regions.

Constraints on the shear speed, crust thickness and residual topography of western Tibet from surface wave tomography and virtual deep seismic sounding

NASA Astrophysics Data System (ADS)

Matchette-Downes, H.; van der Hilst, R. D.; Priestley, K. F.

2017-12-01

We have estimated the thickness of the crust in western Tibet by measuring the time delays between the direct S and the SsPmp seismic phases. We find that the thickness of the crust increases from around 50 km beneath the Tethyan Himalayas to around 80 km beneath the Lhasa block, and then decreases to around 70 km beneath the Qiangtang terrane.This method, virtual deep seismic sounding (VDSS), also yields robust estimates of the contribution of crust buoyancy to elevation. By subtracting the predicted elevation from the real topography, we find there is no observable deviation from hydrostatic topography beneath the Tethyan Himalaya, but there is negative residual topography of 1.5 to 2.0 km beneath the Lhasa and Qiangtang terranes. It is also known that the interior of the Plateau is isostatically compensated, as it has small free air gravity anomalies.Additionally, we have estimated the 3D shear speed structure of the crust and upper mantle. This model is derived from maps of the fundamental mode Rayleigh wave phase speed dispersion in the period range from 20 to 140 s, obtained from a standard two-plane-wave inversion constrained with receiver functions and group speeds from ambient noise. The observations agree with previous observations of a low-wavespeed zone in the mid-crust and a gradual Moho. Furthermore, the long-period Rayleigh waves detect a high-wavespeed upper mantle.Together, the observations of high upper mantle wavespeeds, negative residual topography, and small free air gravity anomalies support the hypothesis that cold, dense Indian lithosphere has underthrust the Plateau in this region. However, in the presentation we also consider contributions to residual topography from plate flexure, lower crustal flow, or deeper mantle flow (dynamic topography).

Plate convergence, transcurrent faults and internal deformation adjacent to Southeast Asia and the western Pacific

NASA Technical Reports Server (NTRS)

Fitch, T. J.

1971-01-01

A model for oblique convergence between plates of lithosphere is proposed in which at least a fraction of slip parallel to the plate margin results in transcurrent movements on a nearly vertical fault which is located on the continental side of a zone of plate consumption. In an extreme case of complete decoupling only the component of slip normal to the plate margin can be inferred from underthrusting. Recent movements in the western Sunda region provide the most convincing evidence for decoupling of slip, which in this region is thought to be oblique to the plate margin. A speculative model for convergence along the margins of the Philippine Sea is constructed from an inferred direction of oblique slip in the Philippine region. This model requires that the triple point formed by the junction of the Japanese and Izu-Bonin trenches and the Nankai trough migrate along the Sagami trough.

Western Ishtar Terra and Lakshmi Planum, Venus - Models of formation and evolution

NASA Astrophysics Data System (ADS)

Roberts, Kari M.; Head, James W.

1990-08-01

Regional geologic mapping and gravity data reveal a variety of characteristics that must be accounted for in models for the formation and evolution of Western Ishtar Terra and Lakshmi Planum, including: (1) high elevation, (2) plateau-shaped profile, (3) abnormally steep bounding slopes, (4) foredeeps, (5) polygonal outline, (6) adjacent orogenic belts, (7) volcanic plains, (8) plains emplaced synchronously with orogenic belts, (9) paterae, (10) variable topography of Lakshmi, (11) tessera-like material underlying Lakshmi, and (12) a large apparent depth of compensation. A tessera/peripheral deformation model, in which a preexisting block of tessera is the locus of convergence of adjacent thinner crust and lithosphere, underthrusting, mountain building, subsurface melting, and plateau uplift, is interpreted to account for most of the characteristics. The aparent depth of compensation is not simply explained by this model and appears to require a second, deeper mantle anomaly component, such as broad mantle upwelling or a hot spot.

Double subduction of continental lithosphere, a key to form wide plateau

NASA Astrophysics Data System (ADS)

Replumaz, Anne; Funiciello, Francesca; Reitano, Riccardo; Faccenna, Claudio; Balon, Marie

2016-04-01

The mechanisms involved in the creation of the high and wide topography, like the Tibetan Plateau, are still controversial. In particular, the behaviour of the indian and asian lower continental lithosphere during the collision is a matter of debate, either thickening, densifying and delaminating, or keeping its rigidity and subducting. But since several decades seismicity, seismic profiles and global tomography highlight the lithospheric structure of the Tibetan Plateau, and make the hypotheses sustaining the models more precise. In particular, in the western syntaxis, it is now clear that the indian lithosphere subducts northward beneath the Hindu Kush down to the transition zone, while the asian one subducts southward beneath Pamir (e.g. Negredo et al., 2007; Kufner et al., 2015). Such double subduction of continental lithospheres with opposite vergence has also been inferred in the early collision time. Cenozoic volcanic rocks between 50 and 30 Ma in the Qiangtang block have been interpreted as related to an asian subduction beneath Qiangtang at that time (De Celles et al., 2011; Guillot and Replumaz, 2013). We present here analogue experiments silicone/honey to explore the subduction of continental lithosphere, using a piston as analogue of far field forces. We explore the parameters that control the subductions dynamics of the 2 continental lithospheres and the thickening of the plates at the surface, and compare with the Tibetan Plateau evolution. We show that a continental lithosphere is able to subduct in a collision context, even lighter than the mantle, if the plate is rigid enough. In that case the horizontal force due to the collision context, modelled by the piston push transmitted by the indenter, is the driving force, not the slab pull which is negative. It is not a subduction driving by the weight of the slab, but a subduction induced by the collision, that we could call "collisional subduction".

Shear wave anisotropy in northwestern South America and its link to the Caribbean and Nazca subduction geodynamics

NASA Astrophysics Data System (ADS)

Idárraga-García, J.; Kendall, J.-M.; Vargas, C. A.

2016-09-01

To investigate the subduction dynamics in northwestern South America, we measured SKS and slab-related local S splitting at 38 seismic stations. Comparison between the delay times of both phases shows that most of the SKS splitting is due to entrained mantle flow beneath the subducting Nazca and Caribbean slabs. On the other hand, the fast polarizations of local S-waves are consistently aligned with regional faults, which implies the existence of a lithosphere-confined anisotropy in the overriding plate, and that the mantle wedge is not contributing significantly to the splitting. Also, we identified a clear change in SKS fast directions at the trace of the Caldas Tear (˜5°N), which represents a variation in the subduction style. To the north of ˜5°N, fast directions are consistently parallel to the flat subduction of the Caribbean plate-Panama arc beneath South America, while to the south fast polarizations are subparallel to the Nazca-South America subduction direction. A new change in the SKS splitting pattern is detected at ˜2.8°N, which is related to another variation in the subduction geometry marked by the presence of a lithosphere-scale tearing structure, named here as Malpelo Tear; in this region, NE-SW-oriented SKS fast directions are consistent with the general dip direction of the underthrusting of the Carnegie Ridge beneath South America. Further inland, this NE-SW-trending mantle flow continues beneath the Eastern Cordillera of Colombia and Merida Andes of Venezuela. Finally, our results suggest that the subslab mantle flow in northwestern South America is strongly controlled by the presence of lithospheric tearing structures.

Crust and Upper Mantle Structure Beneath Tibet and SW China From Seismic Tomography and Array Analysis

NASA Astrophysics Data System (ADS)

van der Hilst, R. D.; Li, C.; Yao, H.; Sun, R.; Meltzer, A. S.

2007-12-01

We will present a summary of the results of our seismological studies of crust and upper mantle heterogeneity and anisotropy beneath Tibet and SW China with data from temporary (PASSCAL) arrays as well as other regional, national, and global networks. In 2003 and 2004 MIT and CIGMR (Chengdu Institute of Geology and Mineral Resources) operated a 25 station array (3-component, broad band seismometers) in Sichuan and Yunnan provinces, SW China; during the same period Lehigh University (also in collaboration with CIGMR) operated a 75 station array in east Tibet. Data from these arrays allow delineation of mantle structure in unprecedented detail. We focus our presentation on results of two lines of seismological study. Travel time tomography (Li et al., PEPI, 2006; EPSL, 2007) with hand-picked phase arrivals from recordings at regional arrays, and combined with data from over 1,000 stations in China and with the global data base due to Engdahl et al. (BSSA, 1998), reveals substantial the structural complexity of the upper mantle beneath SE Asia. In particular, structures associated with subduction of the Indian plate beneath the Himalayas vary significantly from west Tibet (where the plate seems to have underthrusted the entire plateau) to east Tibet (where P-wave tomography provides no evidence for the presence of fast lithosphere beneath the Plateau proper). Further east, fast structures appear in the upper mantle transition zone, presumably related to stagnation of slab fragments associated with subduction of the Pacific plate. (2) Surface wave array tomography (Yao et al., GJI, 2006, 2007), using ambient noise interferometry and traditional (inter station) dispersion analysis, is used to delineate the 3-D structure of the crust and lithospheric mantle at length scales as small as 100 km beneath the MIT and Lehigh arrays. This analysis reveals a complex spatial distribution of intra-crustal low velocity zones (which may imply that crustal-scale faults influence the pattern of middle/lower crustal flow). We will also show preliminary results of surface wave inversion for azimuthal anisotropy, which - combined with previous results from shear wave splitting (Lev et al., EPSL, 2006) - give insight into the deformation of the upper mantle beneath the area under study.

Anisotropic tomography of the Indian continent and the geodynamic role of its keel

NASA Astrophysics Data System (ADS)

Montagner, J. P.; Maurya, S.; Sibrant, A.; Davaille, A.; Stutzmann, E.; Kumar, R.; Jean, B.

2017-12-01

The Indian moved at an exceptional high rate (18-20cm/year) after the birth of La Réunion hotspot, ≈65Ma ago and the Deccan volcanic province before the collision with the Asian continent. Other older plumes Marion, Kerguelen located in the Indian ocean are also associated with very fast plate motion. We present a high-resolution 3D anisotropic model of the Indian plate region down to 300 km depth, obtained by inverting a new massive database of surface-wave observations. The Rayleigh and Love wave dispersion measurements along 14,000 paths are made in a broad frequency range (16-250s). Our estimates of the depth to the Lithosphere-Asthenosphere Boundary (LAB) derived from seismic velocity Vsv variations at depth reveal large variations (120-250 km) beneath the different cratonic blocks. A low velocity layer associated with the Mid-lithospheric discontinuity is present when the root of the lithosphere is deep. This extensive anisotropic tomographic investigation of the Indian continent displays an almost north-south keel, 600km long and 300km wide, down to 250km depth. The keel is characterized by fast velocities, smaller than average radial and azimuthal anisotropies. The distribution of azimuthal anisotropy defines the flow lines around the keel, and, at the LAB, coincides with the APM direction of the Indian plate. The fast axis azimuths at 250 km depth are also in accordance with the results from SK(K)S splitting. Such a keel could probably perturb plume-induced flow in the asthenosphere. To determine the influence of such a keel on the interaction of India with several mantle plumes (Marion, Crozet, Kerguelen, La Réunion), we used laboratory experiments. Some preliminary results will be presented on the comparison between the laboratory flowlines and the direction of seismic anisotropy.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Pasyanos, M E

The behavior of surface waves at long periods is indicative of subcrustal velocity structure. Using recently published dispersion models, we invert surface wave group velocities for lithospheric structure, including lithospheric thickness, over much of the Eastern Hemisphere, encompassing Eurasia, Africa, and the Indian Ocean. Thicker lithosphere under Precambrian shields and platforms are clearly observed, not only under the large cratons (West Africa, Congo, Baltic, Russia, Siberia, India), but also under smaller blocks like the Tarim Basin and Yangtze craton. In contrast, it is found that remobilized Precambrian structures like the Saharan Shield and Sino-Korean Paraplatform do not have well-established lithosphericmore » keels. The thinnest lithospheric thickness is found under oceanic and continental rifts, as well as along convergence zones. We compare our results to thermal models of continental lithosphere, lithospheric cooling models of oceanic lithosphere, lithosphere-asthenosphere boundary (LAB) estimates from S-wave receiver functions, and velocity variations of global tomography models. In addition to comparing results for the broad region, we examine in detail the regions of Central Africa, Siberia, and Tibet. While there are clear differences in the various estimates, overall the results are generally consistent. Inconsistencies between the estimates may be due to a variety of reasons including lateral and depth resolution differences and the comparison of what may be different lithospheric features.« less

Sedimentary masses and concepts about tectonic processes at underthrust ocean margins ( subduction).

USGS Publications Warehouse

Scholl, D. W.; von Huene, Roland E.; Vallier, T.L.; Howell, D.G.

1980-01-01

Tectonic processes associated with subduction of oceanic crust, but unrelated to the collision of thick crustal masses or microplates, are presumed by many geologists to significantly affect the formation and deformation of large sedimentary bodies at underthrust ocean margins. More geologists are familiar with the concept of subduction accretion than with other noncollision processes - for example, sediment subduction, subduction erosion, and subduction kneading. In our opinion, no single subduction-related tectonic process is the dominant or typical one that forges the geologic framework of modern underthrust ocean margins. It is likely, therefore, that the rock records of ancient underthrust margins are preserved in a multitude of structural and stratigraphic forms.-from Authors

Deformation of Tibetan Crust and Mantle and the Uplift of the Plateau: Insights from Broadband Surface Waves

NASA Astrophysics Data System (ADS)

Agius, M. R.; Lebedev, S.

2013-12-01

Seismic deployments over the last two decades have produced dense broadband data coverage across the Tibetan Plateau. Yet, the lithospheric dynamics of Tibet is still debated, with very different end-member models advocated to this day. Uncertainties over the anomalies in seismic tomography models contribute to the uncertainty of their interpretations, ranging from the subduction of India as far as northern Tibet to subduction of Asia there and to extreme viscous thickening of the entire Tibetan lithosphere. Within the lithosphere itself, a low-viscosity layer in the mid-lower crust is evidenced by many observations. It is still unclear, however, whether this layer accommodates a large-scale channel flow (which may have uplifted northern and eastern Tibet, according to one model) or if, instead, deformation within it is similar to that observed at the surface (which implies different uplift mechanisms). Broad-band surface waves provide resolving power from the upper crust down to the asthenosphere, for both isotropic-average shear-wave speeds (proxies for composition and temperature) and the radial and azimuthal shear-wave anisotropy (indicative of the patterns of deformation and flow). We measured highly accurate Love- and Rayleigh-wave phase-velocity curves in broad period ranges (up to 5-200 s) for a few tens of pairs and groups of stations across Tibet, combining, in each case, hundreds to thousands of inter-station measurements, made with cross-correlation and waveform-inversion methods. Robust shear-velocity profiles were then determined by series of non-linear inversions, yielding depth-dependent ranges of shear speeds and radial anisotropy consistent with the data. Temperature anomalies in the upper mantle were estimated from shear-velocity ones using accurate petro-physical relationships. Azimuthal anisotropy in the crust and upper mantle was determined by surface-wave tomography and, also, by sub-array analysis targeting the anisotropy amplitude. Our results show that the prominent high-velocity anomalies in the upper mantle are most consistent with the presence of subducted Indian lithosphere beneath large portions of Tibet. Estimated thermal anomalies within the high-velocity features match those expected for subducted India. The morphology of India's subduction beneath Tibet is complex and shows pronounced west-east variations. Beneath eastern and northeastern Tibet, in particular, the subducted Indian lithosphere appears to have subducted, at a shallow angle, hundreds of km NNE-wards. Azimuthal anisotropy beneath Tibet is distributed in multiple layers with different fast-propagations directions, which accounts for the complexity of published shear-wave splitting observations. The fast directions within the mid-lower crust are parallel to the extensional components of the current strain rate field at the surface, consistent with similar deformation through the entire crust, rather than channel flow. Anisotropy within the asthenosphere beneath northeastern Tibet (sandwiched between the Tibetan lithosphere above and the subducted Indian lithosphere below) indicates SSW-NNE flow, parallel to the direction of motion of the Indian Plate, including its subducted leading edge.

Along-Strike Differences of the Main Himalayan Thrust and Deformation within the Indian Crust: Insights from Seismicity and Seismic Velocities in Bhutan and its Foreland

NASA Astrophysics Data System (ADS)

Diehl, T.; Singer, J.; Hetényi, G.; Kissling, E. H.; Clinton, J. F.

2015-12-01

The seismicity of Bhutan is characterized by the apparent lack of great earthquakes and a significantly lower activity compared to most other parts of the Himalayan arc. To better understand the underlying mechanisms of this anomalously low activity and to relate it with possible along-strike differences in the structure of the orogenic belt, a temporary network with up to 38 broadband seismometers was installed in Bhutan between January 2013 and November 2014. In this work we present a catalog of local and regional earthquakes detected and located with the GANSSER network complemented by regional stations in India, Bangladesh, and China. State-of-the-art data analysis and earthquake location procedures were applied to derive a high-precision earthquake catalog of Bhutan and surrounding regions. Focal mechanisms from regional moment tensor inversions and first-motion polarities complement the earthquake catalog. In the vicinity of the Shumar-Kuru Chu Spur in East Bhutan, seismicity forms a moderately dipping structure at about 12 km depth, which we associate with the Main Himalayan Thrust (MHT). North of 27.6°N the dip of the structure steepens, which can be interpreted as a ramp along the MHT. In West Bhutan seismicity occurs at depths of 20 to 40 km and receiver function images indicate that seismicity occurs in the underthrusting Indian crust rather than on the MHT. The highest seismic activity is clustered along the Goalpara Lineament, a dextral NE-SW striking shear zone in southwest Bhutan, which appears to connect to the western edge of the Shillong Plateau in the foreland. Focal depths indicate that this shear zone is located at depths of 20-30 km and therefore in the underthrusting Indian crust. Preliminary results of a 3D local earthquake tomography show substantial differences in the uppermost crust between east and west Bhutan. Consistent with our receiver function images, the results also indicate a thinning of the crustal root towards eastern Bhutan.

Deformation of the Pacific/North America plate boundary at Queen Charlotte Fault: The possible role of rheology

USGS Publications Warehouse

ten Brink, Uri S.; Miller, Nathaniel; Andrews, Brian; Brothers, Daniel; Haeussler, Peter J.

2018-01-01

The Pacific/North America (PA/NA) plate boundary between Vancouver Island and Alaska is similar to the PA/NA boundary in California in its kinematic history and the rate and azimuth of current relative motion, yet their deformation styles are distinct. The California plate boundary shows a broad zone of parallel strike slip and thrust faults and folds, whereas the 49‐mm/yr PA/NA relative plate motion in Canada and Alaska is centered on a single, narrow, continuous ~900‐km‐long fault, the Queen Charlotte Fault (QCF). Using gravity analysis, we propose that this plate boundary is centered on the continent/ocean boundary (COB), an unusual location for continental transform faults because plate boundaries typically localize within the continental lithosphere, which is weaker. Because the COB is a boundary between materials of contrasting elastic properties, once a fault is established there, it will probably remain stable. We propose that deformation progressively shifted to the COB in the wake of Yakutat terrane's northward motion along the margin. Minor convergence across the plate boundary is probably accommodated by fault reactivation on Pacific crust and by an eastward dipping QCF. Underthrusting of Pacific slab under Haida Gwaii occurs at convergence angles >14°–15° and may have been responsible for the emergence of the archipelago. The calculated slab entry dip (5°–8°) suggests that the slab probably does not extend into the asthenosphere. The PA/NA plate boundary at the QCF can serve as a structurally simple site to investigate the impact of rheology and composition on crustal deformation and the initiation of slab underthrusting.

The role of lithospheric strength heterogeneities in the dynamics of Tienshan and neighbouring regions

NASA Astrophysics Data System (ADS)

Wang, K.; Xiong, X.; Hao, X.; Li, J.

2017-12-01

Tienshan mountain is located about 1500 km away from the plate boundary, but it absorbs approximately 30% of the total effect of the Indian-Eurasian collision. As its rapid shortening and distinct deformation, Tienshan is considered as a good laboratory for studying the dynamics of intra-plate compressional deformation. However, a better understanding of the mechanics of Tienshan mountain building processes demands a detailed knowledge of the rheological structure of the lithosphere in Tienshan region.Here we take advantages of the new data sets from the geothermal, seismology and geodesy to re-estimate the strength of lithosphere in the Tienshan mountain and neighbouring region. We have developed two numerical deformation models (two-dimension profile) along the eastern and western Tienshan Mountain in order to investigate the effects of lateral strength heterogeneities on mountain building.We find that (1) the lithospheric strength of Tienshan mountain has significant difference with adjacent area, and its strength is significantly lower than that of Tarim Basin and Junggar Basin; (2) the strength also shows difference between the eastern and western of Tienshan Mountain, the eastern is strong and the western is weak. Our numerical results reveal that (3) the presence of strong Tarim Basin caused the Indian-Eurasian collision effect to be transferred to the Tienshan Mountains beyond 1500km, while the Tarim Basin shows little internal deformation; (4) the Tienshan region with weak lithosphere contributes to its horizontal shortening and vertical uplift; (5) the existence of high strength Junggar Basin is advantageous to the deformation and orogenic of Tienshan, and also prevents the orogenic range from spreading further northward.

Depth variations of P-wave azimuthal anisotropy beneath Mainland China

PubMed Central

Wei, Wei; Zhao, Dapeng; Xu, Jiandong; Zhou, Bengang; Shi, Yaolin

2016-01-01

A high-resolution model of P-wave anisotropic tomography beneath Mainland China and surrounding regions is determined using a large number of arrival-time data recorded by the China seismic network, the International Seismological Centre (ISC) and temporary seismic arrays deployed on the Tibetan Plateau. Our results provide important new insights into the subducted Indian plate and mantle dynamics in East Asia. Our tomographic images show that the northern limit of the subducting Indian plate has reached the Jinsha River suture in eastern Tibet. A striking variation of P-wave azimuthal anisotropy is revealed in the Indian lithosphere: the fast velocity direction (FVD) is NE-SW beneath the Indian continent, whereas the FVD is arc parallel beneath the Himalaya and Tibetan Plateau, which may reflect re-orientation of minerals due to lithospheric extension, in response to the India-Eurasia collision. There are multiple anisotropic layers with variable FVDs in some parts of the Tibetan Plateau, which may be the cause of the dominant null splitting measurements in these regions. A circular pattern of FVDs is revealed around the Philippine Sea slab beneath SE China, which reflects asthenospheric strain caused by toroidal mantle flow around the edge of the subducting slab. PMID:27432744

Depth variations of P-wave azimuthal anisotropy beneath Mainland China

NASA Astrophysics Data System (ADS)

Wei, Wei; Zhao, Dapeng; Xu, Jiandong; Zhou, Bengang; Shi, Yaolin

2016-07-01

A high-resolution model of P-wave anisotropic tomography beneath Mainland China and surrounding regions is determined using a large number of arrival-time data recorded by the China seismic network, the International Seismological Centre (ISC) and temporary seismic arrays deployed on the Tibetan Plateau. Our results provide important new insights into the subducted Indian plate and mantle dynamics in East Asia. Our tomographic images show that the northern limit of the subducting Indian plate has reached the Jinsha River suture in eastern Tibet. A striking variation of P-wave azimuthal anisotropy is revealed in the Indian lithosphere: the fast velocity direction (FVD) is NE-SW beneath the Indian continent, whereas the FVD is arc parallel beneath the Himalaya and Tibetan Plateau, which may reflect re-orientation of minerals due to lithospheric extension, in response to the India-Eurasia collision. There are multiple anisotropic layers with variable FVDs in some parts of the Tibetan Plateau, which may be the cause of the dominant null splitting measurements in these regions. A circular pattern of FVDs is revealed around the Philippine Sea slab beneath SE China, which reflects asthenospheric strain caused by toroidal mantle flow around the edge of the subducting slab.

Transition from Subduction to Strike-Slip in the Southeast Caribbean: Effects on Lithospheric Structures and Overlying Basin Evolution

NASA Astrophysics Data System (ADS)

Alvarez, T.; Mann, P.; Wood, L. J.; Vargas, C. A.; Latchman, J. L.

2013-12-01

Topography, basin structures and geomorphology of the southeast Caribbean-northeast South American margin are controlled by a 200-km-long transition from westward-directed subduction of South American lithosphere beneath the Caribbean plate, to east-west strike-slip motion of the Caribbean and South American plates. Our study of structures and basins present in the transitional area integrates a tomographic study of the lithospheric structures associated with lateral variations in the subduction of the South American lithosphere and orientation of the slab beneath the Caribbean plate as well as the evolution of overlying sedimentary basins imaged with deep-penetration seismic data kindly provided by the oil industry and Trinidad & Tobago government agencies. We use an earthquake dataset containing more than 700 events recorded by the eastern Caribbean regional seismograph network to build travel-time and attenuation tomography models used to image the mantle to depths of 100 km beneath transition zone. Approximately 10,000 km of 2D seismic reflection lines which are recorded to depths > 12 seconds TWT are used to interpret basin scale structures including tectono-stratigraphic sequences and structures which deform and displace sedimentary sequences. We use the observed satellite gravity to generate a gravity model for key sections traversing the tectonic transitional zone and to determine depth to basement in basins with sedimentary fill > 12 km. Within the study area, the dip of subducted South American oceanic lithosphere imaged on tomographic images is variable from ~44 to ~24 degrees. There is a distinct low gravity, low velocity, high attenuation, northwest - southeast trending lineation located east of Trinidad which defines the location of a Mesozoic oceanic fracture zone which accommodated the opening of the Central Atlantic during the Jurassic to Middle Cretaceous. This feature is also coincident with the present-day continent-ocean boundary and acts as a lithospheric weakness during subduction. We propose that this fracture zone is a key transition point between the subduction of South American/Atlantic oceanic lithosphere; which descends into the mantle, to the northeast, and the under-thrusting of transitional to continental South American lithosphere which resists subduction to the southwest. Maps of South American basement and its overlying Cretaceous passive margin illustrates a northwesterly basement dip with a distinct change in angle of the northwest dip across the paleo-fracture zone consistent with our tomographic model. We propose that flexure of the subducting South American plate at this location exerts a critical control on the formation and evolution of the basins and the lateral distribution of Cretaceous through Pleistocene stratigraphic fill. East of the fracture zone, the overlying strata is deformed by active subduction and accretionary prism processes with a wider zone of shortening with lower overall topography, while to the west of the fracture zone there is active oblique collision with a narrower zone of shortening and greater uplift.

Is Ishtar Terra a thickened basaltic crust?

NASA Technical Reports Server (NTRS)

Arkani-Hamed, Jafar

1992-01-01

The mountain belts of Ishtar Terra and the surrounding tesserae are interpreted as compressional regions. The gravity and surface topography of western Ishtar Terra suggest a thick crust of 60-110 km that results from crustal thickening through tectonic processes. Underthrusting was proposed for the regions along Danu Montes and Itzpapalotl Tessera. Crustal thickening was suggested for the entire Ishtar Terra. In this study, three lithospheric models with total thicknesses of 40.75 and 120 km and initial crustal thicknesses of 3.9 and 18 km are examined. These models could be produced by partial melting and chemical differentiation in the upper mantle of a colder, an Earth-like, and a hotter Venus having temperatures of respectively 1300 C, 1400 C, and 1500 C at the base of their thermal boundary layers associated with mantle convection. The effects of basalt-granulite-eclogite transformation (BGET) on the surface topography of a thickening basaltic crust is investigated adopting the experimental phase diagram and density variations through the phase transformation.

Do faults trigger folding in the lithosphere?

NASA Astrophysics Data System (ADS)

Gerbault, Muriel; Burov, Eugenii B.; Poliakov, Alexei N. B.; Daignières, Marc

A number of observations reveal large periodic undulations within the oceanic and continental lithospheres. The question if these observations are the result of large-scale compressive instabilities, i.e. buckling, remains open. In this study, we support the buckling hypothesis by direct numerical modeling. We compare our results with the data on three most proeminent cases of the oceanic and continental folding-like deformation (Indian Ocean, Western Gobi (Central Asia) and Central Australia). We demonstrate that under reasonable tectonic stresses, folds can develop from brittle faults cutting through the brittle parts of a lithosphere. The predicted wavelengths and finite growth rates are in agreement with observations. We also show that within a continental lithosphere with thermal age greater than 400 My, either a bi-harmonic mode (two superimposed wavelengths, crustal and mantle one) or a coupled mode (mono-layer deformation) of inelastic folding can develop, depending on the strength and thickness of the lower crust.

Rayleigh-wave dispersion reveals crust-mantle decoupling beneath eastern Tibet.

PubMed

Legendre, Cédric P; Deschamps, Frédéric; Zhao, Li; Chen, Qi-Fu

2015-11-09

The Tibetan Plateau results from the collision of the Indian and Eurasian Plates during the Cenozoic, which produced at least 2,000 km of convergence. Its tectonics is dominated by an eastward extrusion of crustal material that has been explained by models implying either a mechanical decoupling between the crust and the lithosphere, or lithospheric deformation. Discriminating between these end-member models requires constraints on crustal and lithospheric mantle deformations. Distribution of seismic anisotropy may be inferred from the mapping of azimuthal anisotropy of surface waves. Here, we use data from the CNSN to map Rayleigh-wave azimuthal anisotropy in the crust and lithospheric mantle beneath eastern Tibet. Beneath Tibet, the anisotropic patterns at periods sampling the crust support an eastward flow up to 100°E in longitude, and a southward bend between 100°E and 104°E. At longer periods, sampling the lithospheric mantle, the anisotropic structures are consistent with the absolute plate motion. By contrast, in the Sino-Korean and Yangtze cratons, the direction of fast propagation remains unchanged throughout the period range sampling the crust and lithospheric mantle. These observations suggest that the crust and lithospheric mantle are mechanically decoupled beneath eastern Tibet, and coupled beneath the Sino-Korean and Yangtze cratons.

A resolved mantle anomaly as the cause of the Australian-Antarctic Discordance

NASA Astrophysics Data System (ADS)

Ritzwoller, M. H.; Shapiro, N. M.; Leahy, G. M.

2003-12-01

We present evidence for the existence of an Australian-Antarctic Mantle Anomaly (AAMA), which trends northwest-southeast (NW-SE) through the Australian-Antarctic Discordance (AAD) on the Southeast Indian Ridge (SEIR), is confined to the upper 120 km of the mantle beneath the AAD, and dips shallowly to the west so that it extends to a depth of about 150 km west of the AAD. Average temperatures within the AAMA are depressed about 100°C relative to surrounding lithosphere and suggest very rapid cooling of newly formed lithosphere at the AAD to an effective thermal age between 20 and 30 Ma. A convective down welling beneath the AAD is not consistent with the confinement of the AAMA in the uppermost mantle. In substantial agreement with the model of [1998], we argue that the AAMA is the suspended remnant of a slab that subducted at the Gondwanaland-Pacific convergent margin more than 100 Myr ago, foundered in the deeper mantle, and then ascended into the shallow mantle within the past 30 Myr, cutting any ties to deeper roots. The stability of the AAMA and its poor correlation with residual topography and gravity imply that it is approximately neutrally buoyant. The thermally induced density anomaly can be balanced by bulk iron depletion of less than 0.8%, consistent with the warmer conditions of formation for the Pacific than Indian lithosphere. We hypothesize that the low temperatures in the AAMA inhibit crustal formation and the AAD depth anomaly is formed at the intersection of the SEIR and the AAMA. The northward migration of the SEIR overriding the cold NW-SE trending AAMA therefore presents a simple kinematic explanation for both the V-shaped residual depth anomaly in the southeast Indian Ocean and the western migration of the AAD along the SEIR. Neither explanation requires the Pacific asthenospheric mantle to push westward and displace Indian asthenosphere. The AAMA may also act as a barrier to large-scale flows in the shallow asthenosphere and may therefore define a boundary for mantle convection and between the Indian and Pacific isotopic provinces. The westward dip of the AAMA would also favor along-axis flow from the Indian Ocean asthenosphere to the AAD that may contribute to the penetration of Indian Ocean mid-ocean ridge basalts into the AAD.

The Rise of the Hindu Kush and its Role in the South Asian Monsoon

NASA Astrophysics Data System (ADS)

Molnar, P. H.; Bendick, R. O.; Boos, W. R.

2017-12-01

The emergence of the Hindu Kush to its mean elevation of 3000 m since 10 Ma may have profoundly affected summer rainfall over the Indian subcontinent - the South Asian Monsoon. General Circulation Model runs of climate with different surface heights suggest that the Hindu Kush in Afghanistan may be the most important high terrain that affects the timing and strength of the South Asian monsoon [Chakraborty, Nanjundiah, and Srinivasan, 2002, 2006]. That high terrain, more than the Tibetan Plateau and the Himalaya, blocks warm dry air, with low moist static energy, from mixing with warm moist air from the Bay of Bengal and over India, and therefore enables a moist static energy maximum to be generated by local sources in the northern edges of the Indian subcontinent, facilitating a strong monsoon circulation. Boos and Hurley [2013] showed that if the Hindu Kush is smoothed too much, so that its maximum height is only 1000 m, nearly all General Circulation Model runs yield atmospheric temperature profiles inconsistent with those of monsoons. Fault plane solutions of earthquakes show underthrusting of the Tajik Depression beneath the Hindu Kush, and GPS velocities require 30-35 mm/yr of convergence between India and the Depression. Some of that convergence might be absorbed by subduction of lithosphere with thin crust, but GPS measurements suggest at least 10 and more likely 20 mm/yr of shortening across the Hindu Kush. For a belt 300 km wide with a mean elevation of its crest of 3 km, isostatic balance of 900 km^2 of excess elevation calls for 6000 km^2 of excess crust in a transect across the Hindu Kush. If crust 30-40 km in thickness were shortened horizontally at 20 (10) km/Myr, then 10-7.5 (20-15) Myr would be needed to build the entire range. If a range 1000 m high would have had little effect on the South Asian Monsoon, and if a height of 2000 m were necessary, at current rates of convergence only a few million years would be needed to raise the Hindu Kush from a height of 1000 m to 2000 (or 3000) m. If the South Asian monsoon strengthened at 10 Ma, a rise of the Hindu Kush since 10 Ma may have played a key role in that event.

Subduction initiation, recycling of Alboran lower crust, and intracrustal emplacement of subcontinental lithospheric mantle in the Westernmost Mediterranean

NASA Astrophysics Data System (ADS)

Varas-Reus, María Isabel; Garrido, Carlos J.; Bosch, Delphine; Marchesi, Claudio; Hidas, Károly; Booth-Rea, Guillermo; Acosta-Vigil, Antonio

2015-04-01

Unraveling the tectonic settings and processes involved in the annihilation of subcontinental mantle lithosphere is of paramount importance for our understanding of the endurance of continents through Earth history. Unlike ophiolites -- their oceanic mantle lithosphere counterparts -- the mechanisms of emplacement of the subcontinental mantle lithosphere in orogens is still poorly known. The emplacement of subcontinental lithospheric mantle peridotites is often attributed to extension in rifted passive margins or continental backarc basins, accretionary processes in subduction zones, or some combination of these processes. One of the most prominent features of the westernmost Mediterranean Alpine orogenic arcs is the presence of the largest outcrops worldwide of diamond facies, subcontinental mantle peridotite massifs; unveiling the mechanisms of emplacement of these massifs may provide important clues on processes involved in the destruction of continents. The western Mediterranean underwent a complex Alpine evolution of subduction initiation, slab fragmentation, and rollback within a context of slow convergence of Africa and Europe In the westernmost Mediterranean, the alpine orogeny ends in the Gibraltar tight arc, which is bounded by the Betic, Rif and Tell belts that surround the Alboran and Algero-Balearic basins. The internal units of these belts are mostly constituted of an allochthonous lithospheric domain that collided and overthrusted Mesozoic and Tertiary sedimentary rocks of the Mesozoic-Paleogene, South Iberian and Maghrebian rifted continental paleomargins. Subcontinental lithospheric peridotite massifs are intercalated between polymetamorphic internal units of the Betic (Ronda, Ojen and Carratraca massifs), Rif (Beni Bousera), and Tell belts. In the Betic chain, the internal zones of the allochthonous Alboran domain include, from bottom to top, polymetamorphic rock of the Alpujarride and Malaguide complexes. The Ronda peridotite massif -- the largest outcrop (> 300 km2) of subcontinental lithospheric mantle peridotite in westernmost Mediterranean -- occurs at the basal units of the western Alpujarride. Late, intrusive mantle, high-Mg pyroxenite dykes in the Ronda peridotite (Betic Cordillera, S. Spain) show geochemical signature akin to high-pressure (> 1 GPa) segregates of high-Mg andesite and boninite found in island arc terrains and ophiolite, where they usually witness nascent subduction and/or oceanic accretion in a forearc setting. These pyroxenites point to a suprasubduction environment prior to the intracrustal emplacement of subcontinental peridotites drawing some parallels between the crustal emplacement environment of some ophiolites and that of sublithospheric mantle in the westernmost Mediterranean. Here, we present new Sr-Nd-Pb-isotopic data from a variety of crustal rocks that might account for the crustal components seen in high-Mg Ronda pyroxenites. This data allows the origin of this crustal component to be unveiled, providing fundamentally constraints on the processes involved in the emplacement of large massifs of subcontinental mantle lithosphere in the westernmost Mediterranean. In order to test the hypothesis that the crustal component in Ronda high-Mg pyroxenites was acquired during the Alpine evolution of the Betic-Rif orogen, we selected samples from crustal sections that might have been underthrusted beneath the Alboran lithospheric mantle before the putative Miocene intra-crustal emplacement of peridotites. Samples are from the western Betics and comprise sediments from the Gibraltar Arc Flysch Trough units, which forms a fold-and-thrust belt between the Iberian paleomargin and the allochthonous Alboran domain, and metasedimentary rocks from the Jubrique and Blanca units of the Alpujarride complex, which underlie and overlie the Ronda peridotite and constitute the crustal section of the Alboran lithosphere domain to which the Ronda peridotite pertains. Sr-Nd-Pb systematic of sediments strongly support Alboran geodynamic models that envisage slab roll-back as the tectonic mechanism responsible for Miocene lithospheric thinning, and consistent with a scenario where back-arc inversion leading to subduction initiation of crustal units at the front of the Alboran wedge

Thermal buoyancy on Venus - Underthrusting vs subduction

NASA Technical Reports Server (NTRS)

Burt, Jeffrey D.; Head, James W.

1992-01-01

The thermal and buoyancy consequences of the subduction endmember are modeled in an attempt to evaluate the conditions distinguishing underthrusting and subduction. Thermal changes in slabs subducting into the Venusian mantle with a range of initial geotherms are used to predict density changes and, thus, slab buoyancy. Based on a model for subduction-induced mantle flow, it is then argued that the angle of the slab dip helps differentiate between underthrusting and subduction. Mantle flow applies torques to the slab which, in combination with torques due to slab buoyancy, act to change the angle of slab dip.

Transient rheology of the oceanic asthenosphere following the 2012 Indian Ocean Earthquake inferred from geodetic data

NASA Astrophysics Data System (ADS)

Pratama, Cecep; Ito, Takeo; Sasajima, Ryohei; Tabei, Takao; Kimata, Fumiaki; Gunawan, Endra; Ohta, Yusaku; Yamashina, Tadashi; Ismail, Nazli; Nurdin, Irwandi; Sugiyanto, Didik; Muksin, Umar; Meilano, Irwan

2017-10-01

Postseismic motion in the middle-field (100-500 km from the epicenter) geodetic data resulting from the 2012 Indian Ocean earthquake exhibited rapid change during the two months following the rupture. This pattern probably indicates multiple postseismic deformation mechanisms and might have been controlled by transient rheology. Therefore, the relative contribution of transient rheology in the oceanic asthenosphere and afterslip in the oceanic lithosphere should be incorporated to explain short- and long-term transitional features of postseismic signals. In this study, using two years of post-earthquake geodetic data from northern Sumatra, a three-dimensional spherical-earth finite-element model was constructed based on a heterogeneous structure and incorporating transient rheology. A rheology model combined with stress-driven afterslip was estimated. Our best-fit model suggests an oceanic lithosphere thickness of 75 km with oceanic asthenosphere viscosity values of 1 × 1017 Pa s and 2 × 1018 Pa s for the Kelvin and Maxwell viscosity models, respectively. The model results indicate that horizontal landward motion and vertical uplift in northern Sumatra require viscoelastic relaxation of the oceanic asthenosphere coupled with afterslip in the lithosphere. The present study demonstrates that transient rheology is essential for reproducing the rapidly changing motion of postseismic deformation in the middle-field area.

Exhumation history of the NW Indian Himalaya revealed by fission track and 40Ar/39Ar ages

USGS Publications Warehouse

Schlup, M.; Steck, A.; Carter, A.; Cosca, M.; Epard, J.-L.; Hunziker, J.

2011-01-01

New fission track and Ar/Ar geochronological data provide time constraints on the exhumation history of the Himalayan nappes in the Mandi (Beas valley) - Tso Morari transect of the NW Indian Himalaya. Results from this and previous studies suggest that the SW-directed North Himalayan nappes were emplaced by detachment from the underthrusted upper Indian crust by 55. Ma and metamorphosed by ca. 48-40. Ma. The nappe stack was subsequently exhumed to shallow upper crustal depths (<10. km) by 40-30. Ma in the Tso Morari dome (northern section of the transect) and by 30-20. Ma close to frontal thrusts in the Baralacha La region. From the Oligocene to the present, exhumation continued slowly.Metamorphism started in the High Himalayan nappe prior to the Late Oligocene. High temperatures and anatexis of the subducting upper Indian crust engendered the buoyancy-driven ductile detachment and extrusion of the High Himalayan nappe in the zone of continental collision. Late extrusion of the High Himalayan nappe started about 26. Ma ago, accompanied by ductile extensional shearing in the Zanskar shear zone in its roof between 22 and 19. Ma concomitant with thrusting along the basal Main Central Thrust to the south. The northern part of the nappe was then rapidly exhumed to shallow depth (<10. km) between 20 and 6. Ma, while its southern front reached this depth at 10-5. Ma. ?? 2010 Elsevier Ltd.

The contemporary North Pangea supercontinent and the geodynamic causes of its formation

NASA Astrophysics Data System (ADS)

Kovalenko, V. I.; Yarmolyuk, V. V.; Bogatikov, O. A.

2010-11-01

The supercontinental status of the contemporary aggregation of continents called North Pangea is substantiated. This supercontinent comprises all continents with the probable exception of Antarctica. In addition to the spatial contiguity of continents, the supercontinent is characterized by the prevalence of the continental crust that combines North America and Eurasia, Eurasia and Africa, and Eurasia and Australia. Over the course of the 300-250-Ma evolution from Wegener's Pangea to contemporary North Pangea, the aggregation of continents has not lost its supercontinental status, despite modification of the supercontinent shape and opening and closure of the newly formed Paleotethys, Tethys, Atlantic, and Indian oceans. Over the last 250-300 Ma, all movements of the lithospheric plates have most likely occurred within the Indo-Atlantic segment of the Earth, whereas the Pacific segment has remained oceanic. In short, the formation of the North Pangea supercontinent can be outlined in the following terms. The long and deep subduction of the lithospheric plates beneath Eurasia and North America gave rise to the stabilization of the continents and accumulation of huge bodies of the cold lithosphere commensurable in volume with the upper mantle at the deeper mantle levels. This brought about compensation ascent of hot mantle (mantle plumes) near the convergent plate boundaries and far from them. A special geodynamic setting develops beneath the supercontinent. Due to encircling subduction of the lithospheric plates and related squeezing of the hot mantle, an ascending flow, or plume (superplume) formed beneath the central part of the supercontinent. In our view, the African superplume broke up Wegener's Pangea in the Atlantic region, caused the opening of the Atlantic and Indian oceans, and migrated to the Arctic Region 53 Ma ago.

Depth-variant azimuthal anisotropy in Tibet revealed by surface wave tomography

NASA Astrophysics Data System (ADS)

Pandey, Shantanu; Yuan, Xiaohui; Debayle, Eric; Tilmann, Frederik; Priestley, Keith; Li, Xueqing

2015-06-01

Azimuthal anisotropy derived from multimode Rayleigh wave tomography in China exhibits depth-dependent variations in Tibet, which can be explained as induced by the Cenozoic India-Eurasian collision. In west Tibet, the E-W fast polarization direction at depths <100 km is consistent with the accumulated shear strain in the Tibetan lithosphere, whereas the N-S fast direction at greater depths is aligned with Indian Plate motion. In northeast Tibet, depth-consistent NW-SE directions imply coupled deformation throughout the whole lithosphere, possibly also involving the underlying asthenosphere. Significant anisotropy at depths of 225 km in southeast Tibet reflects sublithospheric deformation induced by northward and eastward lithospheric subduction beneath the Himalaya and Burma, respectively. The multilayer anisotropic surface wave model can explain some features of SKS splitting measurements in Tibet, with differences probably attributable to the limited back azimuthal coverage of most SKS studies in Tibet and the limited horizontal resolution of the surface wave results.

The tectonic and volcanic evolution of Venus: Catastrophic or gradual?

NASA Technical Reports Server (NTRS)

Solomon, Sean C.

1993-01-01

Radar imaging and altimetry data from the Magellan mission have yielded important new constraints on the tectonic and volcanic history of Venus and on its internal dynamics. The planet lacks global plate tectonics, but a number of chasm systems and corona moat structures have arcuate planforms, asymmetric topogrpahic profiles, and relief analogous to deep-sea trenches on Earth and may be products of limited lithospheric underthrusting or subduction. Several lines of evidence point to a crust and upper mantle stronger than would be predicted by simple extrapolation from Earth and the 450 K greater surface temperature; these include the unrelaxed depths of impact craters, apparently large values of elastic lithosphere thickness, and large ratios of gravity to topography. The density of impact craters indicates an averate crater retention age of about 500 My, but not more than 5% of the recognized craters have been volcanically embayed. This last observation has led to the proposal that Venus has been subjected to one or more global resurfacing events, the latest about 500 My ago, and that the volcanic flux during intervals between such events has been low. That more recent tectonic activity has been widespread, however, is indicated by the high relief and slopes of mountains, chasm walls, and plateau margins; the significant fraction (0.3) of impact craters deformed by younger faults; and the postformational vertical deformation of long channels. Interior dynamical scenarios advanced to account for episodic volcanic resurfacing include catastrophic overturn of a global lithosphere thickened by cooling or compositional buoyancy and strongly time-dependent mantle convective heat flux. Outgassing considerations and analogy with Earth and other terrestrial planets, however, suggests that such catastrophic models are unlikely. If the mantle of Venus cooled more efficiently than that of Earth because of, say, different boundary conditions, a different flow law, or a different degree of layering, then the planet may in the last 500 My have attained lesser mantle temperatures, lower mantle heat flux, and a significantly lesser rate of magma production than Earth yet still display evidence for ongoing convection and active tectonics. Such a 'cold Venus' scenario would be broadly consistent with observations yet be characterized by a gradual volcanic and tectonic evolution.

Observations on the extended tectonic history of the southern Sierra Nevada

DOE Office of Scientific and Technical Information (OSTI.GOV)

Silver, L.T.

1993-04-01

The crust of the southern Sierra Nevada has been the site of repeated major tectonic dislocations in keeping with its Mesozoic-Cenzoic positions near active plate boundaries. The several Mesozoic magmatic arc which invaded it show evidence of pre- and inter-batholithic juxtapositions of different lithospheres as far back as the Jurassic. This has been noted in mapping strontium, neodymium and lead initial ratios and [delta][sup 18]O variations. The Cretaceous arc carries isotopic zonations consistent with a major lithospheric dislocation extending SE from the Melones-Bear Mountain fault systems through the southern Sierra Nevada into the Mojave desert (restoring the Garlock fault). Thismore » is a candidate site for the postulated late Jurassic Mojave-Sonora megashear. During Cretaceous arc evolution major plate changes have taken place at [approximately]104[+-]2 ma and [approximately]80--85 ma. A broad (100( )km) wedge of accreted deepwater sediments and oceanic crust was partly subducted eastward under the Cretaceous arc, producing the Rand, Pelona, Orocopia and Chocolate Mountain schists of southern California. The southern Sierra Nevada saw the northern part of this event. The underlying subduction zone was not disrupted; arc magmatism was quickly renewed in the northern part of the wedge (Rand Mountains). Eastern underthrusting was accompanied and followed by a succession of major westward-vergent low angle faults in the interval 80--60( ) ma with net displacements well in excess of 150 km, and shallow crustal surface rotations in the southern Sierra Nevada and adjacent regions. The southern Sierra Nevada is now clearly detached from its plutonic roots by several generations of low-angle faulting.« less

Formation of plate boundaries: The role of mantle volatilization

NASA Astrophysics Data System (ADS)

Seno, Tetsuzo; Kirby, Stephen H.

2014-02-01

In the early Earth, convection occurred with the accumulation of thick crust over a weak boundary layer downwelling into the mantle (Davies, G.F., 1992. On the emergence of plate tectonics. Geology 20, 963-966.). This would have transitioned to stagnant-lid convection as the mantle cooled (Solomatov, V.S., Moresi, L.-N., 1997. Three regimes of mantle convection with non-Newtonian viscosity and stagnant lid convection on the terrestrial planets. Geophys. Res. Lett. 24, 1907-1910.) or back to a magma ocean as the mantle heated (Sleep, N., 2000. Evolution of the mode of convection within terrestrial planets. J. Geophys. Res. 105(E7): 17563-17578). Because plate tectonics began operating on the Earth, subduction must have been initiated, thus avoiding these shifts. Based on an analogy with the continental crust subducted beneath Hindu Kush and Burma, we propose that the lithosphere was hydrated and/or carbonated by H2O-CO2 vapors released from magmas generated in upwelling plumes and subsequently volatilized during underthrusting, resulting in lubrication of the thrust above, and subduction of the lithosphere along with the overlying thick crust. Once subduction had been initiated, serpentinized forearc mantle may have formed in a wedge-shaped body above a dehydrating slab. In relict arcs, suture zones, or rifted margins, any agent that warms and dehydrates the wedge would weaken the region surrounding it, and form various types of plate boundaries depending on the operating tectonic stress. Thus, once subduction is initiated, formation of plate boundaries might be facilitated by a major fundamental process: weakening due to the release of pressurized water from the warming serpentinized forearc mantle.

Subduction and Slab Advance at Orogen Syntaxes: Predicting Exhumation Rates and Thermochronometric Ages with Numerical Modeling

NASA Astrophysics Data System (ADS)

Nettesheim, Matthias; Ehlers, Todd A.; Whipp, David M.

2017-04-01

The change in plate boundary orientation and subducting plate geometry along orogen syntaxes may have major control on the subduction and exhumation dynamics at these locations. Previous work documents that the curvature of subducting plates in 3D at orogen syntaxes forces a buckling and flexural stiffening of the downgoing plate. The geometry of this stiffened plate region, also called indenter, can be observed in various subduction zones around the world (e.g. St. Elias Range, Alaska; Cascadia, USA; Andean syntaxis, South America). The development of a subducting, flexurally stiffened indenter beneath orogen syntaxes influences deformation in the overriding plate and can lead to accelerated and focused rock uplift above its apex. Moreover, the style of deformation in the overriding plate is influenced by the amount of trench or slab advance, which is the amount of overall shortening not accommodated by underthrusting. While many subduction zones exhibit little to no slab advance, the Nazca-South America subduction and especially the early stages of the India-Eurasia collision provide end-member examples. Here, we use a transient, lithospheric-scale, thermomechanical 3D model of an orogen syntaxis to investigate the effects of subducting a flexurally stiffened plate geometry and slab advance on upper plate deformation. A visco-plastic upper-plate rheology is used, along with a buckled, rigid subducting plate. The free surface of the thermomechanical model is coupled to a landscape evolution model that accounts for erosion by fluvial and hillslope processes. The cooling histories of exhumed rocks are used to predict the evolution of low-temperature thermochronometer ages on the surface. With a constant overall shortening for all simulations, the magnitude of slab advance is varied stepwise from no advance, with all shortening accommodated by underthrusting, to full slab advance, i.e. no motion on the megathrust. We show that in models where most shortening is accommodated by subduction, the uplift is highly localized and focused in a shape resembling the geometry of the subducting plate. Strong erosion of the growing orogen can shift the center of uplift towards the orogen flanks facing the trench. In contrast, large amounts of slab advance lead to a less focused uplift with lower maximum velocities and the uplift peak located farther away from the trench. The observed thermochronometric ages follow the uplift pattern, but indicate a significantly deeper and more rapid exhumation for models with a higher underthrusting component. These variations in amount and style of upper plate deformation may help to deepen the understanding of the different types of orogeny observed at plate corners around the world.

Pore pressure development and progressive dewatering in underthrust sediments at the Costa Rican subduction margin: Comparison with northern Barbados and Nankai

NASA Astrophysics Data System (ADS)

Saffer, Demian M.

2003-05-01

At subduction zones, pore pressure affects fault strength, deformation style, structural development, and potentially the updip limit of seismogenic faulting behavior through its control on effective stress and consolidation state. Despite its importance for a wide range of subduction zone processes, few detailed measurements or estimates of pore pressure at subduction zones exist. In this paper, I combine logging-while-drilling (LWD) data, downhole physical properties data, and laboratory consolidation tests from the Costa Rican, Nankai, and Barbados subduction zones, to document the development and downsection variability of effective stress and pore pressure within underthrust sediments as they are progressively loaded by subduction. At Costa Rica, my results suggest that the lower portion of the underthrust section remains nearly undrained, whereas the upper portion is partially drained. An inferred minimum in effective stress developed within the section ˜1.5 km landward of the trench is consistent with core and seismic observations of faulting, and illustrates the important effects of heterogeneous drainage on structural development. Inferred pore pressures at the Nankai and northern Barbados subduction zones indicate nearly undrained conditions throughout the studied intervals, and are consistent with existing direct measurements and consolidation test results. Slower dewatering at Nankai and Barbados than at Costa Rica can be attributed to higher permeability and larger compressibility of near-surface sediments underthrust at Costa Rica. Results for the three margins indicate that the pore pressure ratio (λ) in poorly drained underthrust sediments should increase systematically with distance landward of the trench, and may vary with depth.

Crustal and uppermost mantle structure and deformation in east-central China

NASA Astrophysics Data System (ADS)

Li, H.; Yang, X.; Ouyang, L.; Li, J.

2017-12-01

We conduct a non-linear joint inversion of receiver functions and Rayleigh wave dispersions to obtain the crustal and upper mantle velocity structure in east-central China. In the meanwhile, the lithosphere and upper mantle deformation beneath east-central China is also evaluated with teleseismic shear wave splitting measurements. The resulting velocity model reveals that to the east of the North-South Gravity Lineament, the crust and the lithosphere are significantly thinned. Furthermore, three extensive crustal/lithospheric thinning sub-regions are clearly identified within the study area. This indicates that the modification of the crust and lithosphere in central-eastern China is non-uniform due to the heterogeneity of the lithospheric strength. Extensive crustal and lithospheric thinning could occur in some weak zones such as the basin-range junction belts and large faults. The structure beneath the Dabie orogenic belt is complex due to the collision between the North and South China Blocks during the Late Paleozoic-Triassic. The Dabie orogenic belt is generally delineated by a thick crust with a mid-crust low-velocity zone and a two-directional convergence in the lithospheric scale. Obvious velocity contrast exhibits in the crust and upper mantle at both sides of the Tanlu fault, which suggests the deep penetration of this lithospheric-scale fault. Most of our splitting measurements show nearly E-W trending fast polarization direction which is slightly deviating from the direction of plate motion. The similar present-day lithosphere structure and upper mantle deformation may imply that the eastern NCC and the eastern SCB were dominated by a common dynamic process after late Mesozoic, i.e., the westward subduction of Pacific plate and the retreat of the subduction plate. The westward subduction of the Philippine plate and the long-range effects of the collision between the Indian plate and Eurasia plate during Cenozoic may have also contributed to the present velocity structure and stress environment of eastern China.

Seismicity and the nature of plate movement along the Himalayan arc, Northeast India and Arakan-Yoma: a review

NASA Astrophysics Data System (ADS)

Verma, R. K.; Kumar, G. V. R. Krishna

1987-03-01

The Himalaya together with Arakan-Yoma form a well defined seismic belt to the north and east of the Indian Peninsula. The Seismicity along this belt is attributed mostly to collision between the Indian and the Eurasian plates. However, the exact nature of activity along the major thrusts and faults is not well understood. The seismicity along the entire Himalaya and Northern Burma has been studied in detail. It has been found that besides the Main Boundary Fault and the Main Central Thrust several transverse features are also very active. Some of these behave like steeply dipping fracture zones. Along the Arakan-Yoma most of the seismicity appears to be due to subduction of the Indian lithosphere to the east. Analysis of focal mechanism solutions for the Himalaya shows that although thrust movements are predominant, normal and strike-slip faulting is taking place along some of the transverse features. In addition to thrusting, strike-slip faulting is also taking place along the Arakan-Yoma. Orientation of P-axes for all thrust solutions show a sharp change from predominantly east-west along the Burmese arc to N-S and NE-SW along the Himalaya. The direction further changes to NW-SE along the Baluchistan arc. It appears that the Indian lithosphere is under compression from practically all sides. The present day seismicity of Northeast India and Northern Burma can be explained in terms of a plate tectonics model after Nandy (1976). No simple model appears to be applicable for the entire Himalaya.

Seismic Reflection Images of Deep Lithospheric Faults and Thin Crust at the Actively Deforming Indo-Australian Plate Boundary in the Indian Ocean

NASA Astrophysics Data System (ADS)

Singh, S. C.; Carton, H.; Chauhan, A.; Dyment, J.; Cannat, M.; Hananto, N.; Hartoyo, D.; Tapponnier, P.; Davaille, A.

2007-12-01

Recently, we acquired deep seismic reflection data using a state-of-the-art technology of Schlumberger having a powerful source (10,000 cubic inch) and a 12 km long streamer along a 250 km long trench parallel line offshore Sumatra in the Indian Ocean deformation zone that provides seismic reflection image down to 40 km depth over the old oceanic lithosphere formed at Wharton spreading centre about 55-57 Ma ago. We observe deep penetrating faults that go down to 37 km depth (~24 km in the oceanic mantle), providing the first direct evidence for full lithospheric-scale deformation in an intra-plate oceanic domain. These faults dip NE and have dips between 25 and 40 degrees. The majority of faults are present in the mantle and are spaced at about 5 km, and do not seem cut through the Moho. We have also imaged active strike-slip fault zones that seem to be associated with the re-activation of ancient fracture zones, which is consistent with previous seismological and seafloor observations. The geometries of the deep penetrating faults neither seem to correspond to faulting associated with the plate bending at the subduction front nor with the re-activation of fracture zone that initiated about 7.5 Ma ago, and therefore, we suggest that these deep mantle faults were formed due to compressive stress at the beginning of the hard collision between India and Eurasia, soon after the cessation of seafloor spreading in the Wharton basin. We also find that the crust generated at the fast Wharton spreading centre 55-57 Ma ago is only 3.5-4.5 km thick, the thinnest crust ever observed in a fast spreading environment. We suggest that this extremely thin crust is due to 40-50°C lower than normal mantle temperature in this part of the Indian Ocean during its formation.

Modulation of Crustal Faulting in the Crescent Terrane by the Volume of Underthrust Accretionary Complex Along the Washington Cascadia Forearc

NASA Astrophysics Data System (ADS)

Brocher, T. M.

2017-12-01

Amphibious seismic experiments reveal widespread underthrusting of Cascadia accretionary rocks beneath basalts of the Crescent terrane, a large igneous province in the Washington forearc. Along margin variations in the volumes of the underthrust accretionary rocks appear to modulate the faulting within the overlying Crescent terrane, which hosts nearly all of the seismicity in the Washington forearc: the underlying accretionary rocks appear to deform aseismically. The underthrusting and underplating of large volumes of accretionary rocks on the Olympic Peninsula have uplifted and completely eroded a significant volume of the Crescent terrane, affecting the load-bearing strength of the forearc. I propose that as a consequence, the remnant Crescent terrane is actively deforming, as evidenced by the concentrated seismicity within it beneath Puget Lowland. This seismicity, focal mechanisms, fault geometries, and seismic tomography indicate that clockwise rotation and north-south compression of the forearc crust inferred from GPS data are accommodated by numerous thrust and strike slip faults in the remnant Crescent terrane. In addition to the spatial association between the erosion of the Crescent terrane on the Olympic Peninsula and the crustal faulting beneath Puget Lowland, support for the interpretation that the two are related also derives from the temporal coincidence between the mid to late Miocene uplift of the Crescent terrane on the peninsula and the mid-Miocene initiation of the thrust faulting in the lowland. In contrast, the underthrusting and underplating of lower volumes of accretionary rocks in the Washington forearc south of the Olympic Peninsula correlate with lower rates of crustal seismicity. These lower volumes of accretionary rocks have not caused the removal of a significant fraction of the Crescent terrane, resulting in a stronger, more structurally coherent Crescent terrane that deforms at lower rate than to the north.

Peculiarities of the tectonic and magma evolution of the southwestern Indian middle-ocean crust within the range of 51°-67° eastern longitude

NASA Astrophysics Data System (ADS)

Shreider, A. A.; Kashintsev, G. L.

2010-02-01

The comparative estimation of the parameters of the lithosphere of the Mid-Ocean Southwestern Indian range in the areas westwards and eastwards of the Atlantis II transform fault zone shows that, within this zone, an alteration in the basalt composition occurred. Eastwards of this zone, a decrease of the anomaly of the magnetic field occurred and increased average depths of the axial part (4.7 km) and thinning (up to 4-5 km) of the ocean crust with increased rates of seismic waves in the upper mantle were observed. This, first of all, indicates an anomalously cold mantle below the oceanic crust. The changes that occurred in the location of the Euler pole within the last millions of years resulted in slanting spreading in the area of the investigation with rates of opening lower than 1.8 cm/year probably accompanied by the phenomena of transtension in the active parts of the transform faults. The interaction between the Landly and Somali lithosphere plates occurred along the diffusion boundary and was accompanied by problems with tracing the chrones between the neighboring profiles of geomagnetic observations. Consequently, the more detailed investigation of the configuration of the diffusion boundary will contribute to the more accurate reconstruction of the paleogeodynamics of the central part of the Indian Ocean.

Lithospheric Shear Stresses Over And Around Africa

NASA Astrophysics Data System (ADS)

Greff-Lefftz, M.; Jean, B.; Vicente De Gouveia, S.

2017-12-01

We use a simple model for mantle dynamics combining contributions of subducted lithosphere, domes at the bottom of the mantle and upwelling plumes. A dominant feature of plate tectonics is the quasi permanence of a girdle of subductions around the Pacific ocean (or its ancestor), which creates large-wavelength positive topography anomaly within the ring they form. The superimposition of the resultant extension with the one induced by the dome leads to a permanent extensional regime over Africa and the future Indian ocean which creates faults with azimuth directions depending on the direction of the most active part of the ring of subductions. We thus obtain fractures with NW-SE azimuth during the period 275-165 Ma parallel to the strike of the subduction zone of the West South American active margin, which appears to be very active during this period. Between 155-95 Ma, subduction became more active along the Eastern Australian coast involving a change in the direction of the faults toward an E-W direction, in agreement with the observed fault systems between Africa and India, Antartica and Australia. During the Mesozoic and the Cenozoic, we correlate the permanent extensional regime over Africa and Indian ocean with the observed rift systems.Finally we emphasize the role of three primary hotspots as local additional contributors to the stress field imposed by our proposed subduction-doming system, which help in the opening of Indian and South Atlantic oceans.

Lithospheric electrical structure of the middle Lhasa terrane in the south Tibetan plateau

NASA Astrophysics Data System (ADS)

Liang, Hongda; Jin, Sheng; Wei, Wenbo; Gao, Rui; Ye, Gaofeng; Zhang, Letian; Yin, Yaotian; Lu, Zhanwu

2018-04-01

The Lhasa terrane in southern Tibetan plateau is a huge tectono-magmatic belt and an important metallogenic belt. Its formation evolution process and mineralization are affected by the subduction of oceanic plate and subsequent continental collision. However, the evolution of Lhasa terrane has been a subject of much debate for a long time. The Lithospheric structure records the deep processes of the subduction of oceanic plate and continental collision. The magnetotelluric (MT) method can probe the sub-surface electrical conductivity, newly dense broadband and long period magnetotelluric data were collected along a south-north trending profile that across the Lhasa terrane at 88°-89°E. Dimensionality analyses demonstrated that the MT data can be interpreted using two-dimensional approaches, and the regional strike direction was determined as N110°E.Based on data analysis results, a two-dimensional (2-D) resistivity model of crust and upper mantle was derived from inversion of the transverse electric mode, transverse magnetic mode and vertical magnetic field data. Inversion model shows a large north-dipping resistor that extended from the upper crust to upper mantle beneath the Himalaya and the south of Lhasa Terrane, which may represent the subducting Indian continental lithosphere. The 31°N may be an important boundary in the Lhasa Terrane, the south performs a prominent high-conductivity anomaly from the lower crust to upper mantle which indicates the existence of asthenosphere upwelling, while the north performs a higher resistivity and may have a reworking ancient basement. The formation of the ore deposits in the study area may be related to the upwelling of the mantle material triggered by slab tearing and/or breaking off of the Indian lithosphere, and the mantle material input also contributed the total thickness of the present-day Tibetan crust. The results provide helpful constrains to understand the mechanism of the continent-continent collision and the regional exploratory prospect of the deep resources.

Subduction and Plate Edge Tectonics in the Southern Caribbean

NASA Astrophysics Data System (ADS)

Levander, A.; Schmitz, M.; Niu, F.; Bezada, M. J.; Miller, M. S.; Masy, J.; Ave Lallemant, H. G.; Pindell, J. L.; Bolivar Working Group

2013-05-01

The southern Caribbean plate boundary consists of a subduction zone at at either end of a complex strike-slip fault system: In the east at the Lesser Antilles subduction zone, the Atlantic part of the South American plate subducts beneath the Caribbean. In the north and west in the Colombia basin, the Caribbean subducts under South America. In a manner of speaking, the two plates subduct beneath each other. Finite-frequency teleseismic P-wave tomography confirms this, imaging the Atlantic and the Caribbean plates subducting steeply in opposite directions to transition zone depths under northern South America (Bezada et al, 2010). The two subduction zones are connected by the El Pilar-San Sebastian strike-slip fault system, a San Andreas scale system that has been cut off at the Bocono fault, the southeastern boundary fault of the Maracaibo block. A variety of seismic probes identify subduction features at either end of the system (Niu et al, 2007; Clark et al., 2008; Miller et al. 2009; Growdon et al., 2009; Huang et al., 2010; Masy et al, 2011). The El Pilar system forms at the southeastern corner of the Antilles subduction zone with the Atlantic plate tearing from South America. The deforming plate edges control mountain building and basin formation at the eastern end of the strike-slip system. Tearing the Atlantic plate from the rest of South America appears to cause further lithospheric instability continentward. In northwestern South America the Caribbean plate very likely also tears, as its southernmost element subducts at shallow angles under northernmost Colombia but then rapidly descends to the transition zone under Lake Maracaibo (Bezada et al., 2010). We believe that the flat slab controls the tectonics of the Neogene Merida Andes, Perija, and Santa Marta ranges. The nonsubducting part of the Caribbean plate also underthrusts northern Venezuela to about the width of the coastal mountains (Miller et al., 2009). We infer that the edge of the underthrust Caribbean plate supports the elevations of the coastal mountains and controls continuing deformation.

A Review of Geophysical Constraints on the Deep Structure of the Tibetan Plateau, the Himalaya and the Karakoram, and their Tectonic Implications

NASA Astrophysics Data System (ADS)

Molnar, P.

1988-09-01

The Tibetan Plateau, the Himalaya and the Karakoram are the most spectacular consequences of the collision of the Indian subcontinent with the rest of Eurasia in Cainozoic time. Accordingly, the deep structures beneath them provide constraints on both the tectonic history of the region and on the dynamic processes that have created these structures. The dispersion of seismic surface waves requires that the crust beneath Tibet be thick: nowhere less than 50 km, at least 65 km, in most areas, but less than 80 km in all areas that have been studied. Wide-angle reflections of P-waves from explosive sources in southern Tibet corroborate the existence of a thick crust but also imply the existence of marked lateral variations in that thickness, or in the velocity structure of the crust. Thus isostatic compensation occurs largely by an Airy-type mechanism, unlike that, for instance, of the Basin and Range Province of western North America where a hot upper mantle buoys up a thin crust. The P-wave and S-wave velocities in the uppermost mantle of most of Tibet are relatively high and typical of those of Precambrian shields and stable platforms: Vp = 8.1 km s-1 or higher, and Vs≈ 4.7 km s-1. Travel times and waveforms of S-waves passing through the uppermost mantle of much of Tibet, however, require a much lower average velocity in the uppermost mantle than that of the Indian, or other, shields. They indicate a thick low-velocity zone in the upper mantle beneath Tibet, reminiscent of tectonically active regions. These data rule out a shield structure beneath northern Tibet and suggest that if such a structure does underlie part of the plateau, it does so only beneath the southern part. Lateral variations in the upper-mantle structure of Tibet are apparent from differences in travel times of S-waves from earthquakes in different parts of Tibet, in the attenuation of short-period phases, Pn and Sn, that propagate through the uppermost mantle of Tibet, and in surface-wave dispersion for different paths. The notably lower velocities and the greater attenuation in the mantle of north--central Tibet than elsewhere imply higher temperatures there and are consistent with the occurrence of active and young volcanism in roughly the same area. Surface-wave dispersion across north--central Tibet also requires a thinner crust in that area than in most of the plateau. Consequently the relatively uniform height of the plateau implies that isostatic compensation in the north--central part of Tibet occurs partly because the density of the relatively hot material in the upper mantle is lower than that elsewhere beneath Tibet, the mechanism envisioned by Pratt. Several seismological studies provide evidence consistent with a continuity of the Indian Shield, and its cold thick lithosphere, beneath the Himalaya. Fault-plane solutions and focal depths of the majority of moderate earthquakes in the Himalaya are consistent with their occurring on the top surface of the gently flexed, intact Indian plate that has underthrust the Lesser Himalaya roughly 80-100 km or more. P-waves from explosions in southern Tibet and recorded in Nepal can be interpreted as wide-angle reflections from this fault zone. P-wave delays across the Tarbela network in Pakistan from distant earthquakes indicate a gentle dip of the Moho beneath the array without pronounced later variations in upper-mantle structure. High Pn and Sn velocities beneath the Himalaya and normal to early S-wave arrival times from Himalayan earthquakes recorded at teleseismic distances are consistent with Himalaya being underlain by the same structure that underlies India. Results from explosion seismology indicate an increase in crustal thickness from the Indo--Gangetic Plain across the Himalaya to southern Tibet, but Hirn, Lepine, Sapin and their co-workers inferred that the depth of the Moho does not increase smoothly northward, as it would if the Indian Shield had been underthrust coherently beneath the Himalaya. They interpreted wide-angle reflections as evidence for steps in the Moho displaced from one another on southward-dipping faults. Although I cannot disprove this interpretation, I think that one can recognize a sequence of signals on their wide-angle reflection profiles that could be wide-angle reflections from a northward-dipping Moho. Gravity anomalies across the Himalaya show that both the Indo--Gangetic Plain and the Himalaya are not in local isostatic equilibrium. A mass deficit beneath the plain is apparently caused by the flexure of the Indian Shield and by the low density of the sedimentary rock in the basin formed by the flexure. The mass excess in the Himalaya seems to be partly supported by the strength of the Indian plate, for which the flexural rigidity is particularly large. An increase in the Bouguer gravity gradient from about 1 mGal km-1 (1 mGal = 10-3 cm s-2) over the Indo--Gangetic Plain to 2 mGal km-1 over the Himalaya implies a marked steepening of the Moho, and therefore a greater flexure of the Indian plate, beneath the Himalaya. This implies a northward decrease in the flexural rigidity of the part of the Indian plate underlying the range. Nevertheless, calculations of deflections of elastic plates with different flexural rigidities and flexed by the weight of the Himalaya show larger deflections and yield more negative gravity anomalies than are observed. Thus, some other force, besides the flexural strength of the plate, must contribute to the support of the range. A bending moment applied to the end of the Indian plate could flex the plate up beneath the range and provide the needed support. The source of this moment might be gravity acting on the mantle portion of the subducting Indian continental lithosphere with much or all of the crust detached from it. Seismological studies of the Karakoram are consistent with its being underlain by particularly cold material in the upper mantle. Intermediate-depth earthquakes occur between depths of 70 and 100 km but apparently do not define a zone of subducted oceanic lithosphere. Rayleigh-wave phase velocities are particularly high for paths across this area and imply high shear wave velocities in the upper mantle. Isostatic gravity anomalies indicate a marked low of 70 mGal over the Karakoram, which could result from a slightly thickened crust pulled down by the sinking of cold material beneath it. Geophysical constraints on the structure of Tibet, the Himalaya and the Karakoram are consistent with a dynamic uppermost mantle that includes first, the plunging of cold material into the asthenosphere beneath southern Tibet and the Karakoram, as the Indian plate slides beneath the Himalaya, and second, an upwelling of hot material beneath north--central Tibet. The structure is too poorly resolved to require such dynamic flow, but the existence for both a hot uppermost mantle beneath north--central Tibet and a relatively cold uppermost mantle beneath southern Tibet and the Karakoram seem to be required. Both group and phase velocities of Rayleigh waves and Love waves are delayed along paths crossing Tibet. The low velocities require a crustal thickness in excess of 50 km, and for most regions in excess of 60 km. Crustal thicknesses in excess of 80 km can be ruled out for all paths studied, and for most of Tibet, a crustal thickness of 65-70 km seems required. Clear evidence for lateral heterogeneity beneath Tibet is provided not only by body waves (discussed below) but also by surface waves (Brandon & Romanowicz 1986), which show an area of lower uppermost shear-wave velocity and thinner crust in north--central Tibet than elsewhere in the plateau. These variations might explain the differences in group velocities measured by different workers, and the different structures that they deduced, but if so, they also render the regionalization of surface-wave dispersion into arbitrary tectonic provinces risky. Although Rayleigh-wave phase velocities can resolve large differences in upper-mantle velocities for regions the size of Tibet, constraints on these velocities are best derived from body waves. Thus, with the exceptions of Brandon & Romanowicz's (1986) detailed investigation of north--central Tibet, the study of southernmost Tibet by Jobert et al. (1985) and that of Romanowicz (1982) for the northeasternmost part of the plateau, I do not think that surface waves have placed an important bound on the velocity in the upper mantle beneath Tibet. The seismic data are broadly consistent with partial melting of the uppermost mantle of north--central Tibet, where recent volcanism has been observed. Correspondingly, there is no suggestion of such low velocities, and such high temperatures, in the mantle elsewhere beneath Tibet, for which late-Cainozoic volcanism has not been reported. The results are also consistent with a slightly thinner crust in north--central Tibet than farther south, suggesting that both Airy and Pratt isostasy share compensation for north--central Tibet's great height. Finally, the average shear-wave velocity in the upper mantle of southern Tibet seems to be higher than that in northern Tibet, but neither is the degree of difference well determined, nor is the location of the transition from one to the other well mapped.

Numerical modeling of the Indo-Australian intraplate deformation

NASA Astrophysics Data System (ADS)

Brandon, Vincent; Royer, Jean-Yves

2014-05-01

The Indo-Australian plate is perhaps the best example of wide intraplate deformation within an oceanic plate. The deformation is expressed by an unusual level of intraplate seismicity, including magnitude Mw > 8 events, large-scale folding and deep faulting of the oceanic lithosphere and reactivation of extinct fracture zones. The deformation pattern and kinematic data inversions suggest that the Indo-Australian plate can be viewed as a composite plate made of three rigid component plates - India, Capricorn, Australia - separated by wide and diffuse boundaries undergoing either extensional or compressional deformation. We tested this model using the SHELLS numerical code (Kong & Bird, 1995). The Indo-Australian plate is modeled by a mesh of 5281 spherical triangular finite elements. Mesh edges parallel the major extinct fracture zones so that they can be reactivated by reducing their friction rates. Strength of the plate is defined by the age of the lithosphere and seafloor topography. Model boundary conditions are only defined by the plate velocities predicted by the rotation vectors between rigid components of the Indo-Australian plate and their neighboring plates. Since the mesh limits all belong to rigid plates with fully defined Euler vectors, no conditions are imposed on the location, extent and limits of the diffuse and deforming zones. Using MORVEL plate velocities (DeMets et al., 2010), predicted deformation patterns are very consistent with that observed. Pre-existing structures of the lithosphere play an important role in the intraplate deformation and its distribution. The Chagos Bank focuses most of the extensional deformation between the Indian and Capricorn plates. Agreement between models and observation improves by weakening fossil fracture zones relative to the surrounding crust; however only limited sections of FZ's accommodate deformation. The reactivation of the Eocene FZ's in the Central Indian Basin (CIB) and Wharton Basin (WB) explains the drastic change in the deformation style between these basins across the Ninetyeast ridge. The highest slip rates along the WB FZ's are predicted where two major strike-slip faulting earthquakes occurred in April 2012 (Mw=8.6 and 8.2). The best model is obtained when adding a local HF anomaly in the center of the CIB (proxy for weakening the lithospheric strength), consistent with evidence of mantle serpentinization in the CIB where deep seismics image a series of N-S dipping thrust faults reaching Moho depths. The rates of extension or shortening, inferred from the predicted strain rates, are consistent with previous estimates based on different approaches. This finite element modeling confirms that oceanic lithosphere, like the continental lithosphere, can slowly deform over very broad areas (> 1000 x 1000 km).

Upper-mantle water stratification inferred from observations of the 2012 Indian Ocean earthquake.

PubMed

Masuti, Sagar; Barbot, Sylvain D; Karato, Shun-Ichiro; Feng, Lujia; Banerjee, Paramesh

2016-10-20

Water, the most abundant volatile in Earth's interior, preserves the young surface of our planet by catalysing mantle convection, lubricating plate tectonics and feeding arc volcanism. Since planetary accretion, water has been exchanged between the hydrosphere and the geosphere, but its depth distribution in the mantle remains elusive. Water drastically reduces the strength of olivine and this effect can be exploited to estimate the water content of olivine from the mechanical response of the asthenosphere to stress perturbations such as the ones following large earthquakes. Here, we exploit the sensitivity to water of the strength of olivine, the weakest and most abundant mineral in the upper mantle, and observations of the exceptionally large (moment magnitude 8.6) 2012 Indian Ocean earthquake to constrain the stratification of water content in the upper mantle. Taking into account a wide range of temperature conditions and the transient creep of olivine, we explain the transient deformation in the aftermath of the earthquake that was recorded by continuous geodetic stations along Sumatra as the result of water- and stress-activated creep of olivine. This implies a minimum water content of about 0.01 per cent by weight-or 1,600 H atoms per million Si atoms-in the asthenosphere (the part of the upper mantle below the lithosphere). The earthquake ruptured conjugate faults down to great depths, compatible with dry olivine in the oceanic lithosphere. We attribute the steep rheological contrast to dehydration across the lithosphere-asthenosphere boundary, presumably by buoyant melt migration to form the oceanic crust.

Block structure and geodynamics of the continental lithosphere on plate boundaries

NASA Astrophysics Data System (ADS)

Gatinsky, Yu. G.; Prokhorova, T. V.; Romanyuk, T. V.; Vladova, G. L.

2009-04-01

Division of the Earth lithosphere on large plates must be considered only as the first and most general approximation in its structure hierarchy. Some transit zones or difuuse boundaries after other authors take place in lithosphere plate boundaries. The tectonic tension of plate interaction is transferred and relaxed within these zones, which consist of blocks limited by seismoactive faults. Vectors of block horizontal displacements often don't coincide with vectors of main plates and change together with changing block rigidity. As a rule the intensity the seismic energy at plate and transit zone boundaries decreases linearly with distancing from these boundaries and correlates with decreasing of velocities of block horizontal displacements. But sometimes the maximum of the energy manifestation takes place in inner parts of transit zones. Some relatively tight interblock zones established in central and east Asia are the most seismically active. They limited such blocks as Pamir, Tien Shan, Bayanhar, Shan, Japanese-Korean, as well as the north boundary of the Indian Plate. A seismic energy intensity of these zones can be compared with the energy of Pacific subduction zones. It is worthy to note that the majority catastrophic earthquakes took place in Central Asia just within interblock zones. A level of block displacement is situated mainly in the bottom or inside the Earth crust, more rare in the lithosphere mantle. Blocks with the most thick lithosphere roots (SE China, Amurian) are the most rigid and weakly deformed.

Oceanic Lithosphere/Asthenosphere Boundary from surface wave dispersion data

NASA Astrophysics Data System (ADS)

Burgos, G.; Montagner, J.; Beucler, E.; Capdeville, Y.; Mocquet, A.

2013-12-01

The nature of Lithosphere-Asthenosphere boundary (LAB) is controversial according to different types of observations. Using a massive dataset of surface wave dispersions in a broad frequency range (15-300s), we have developed a 3-D tomographic model (1st order perturbation theory) of the upper-mantle at the global scale. It is used to derive maps of LAB from the resolved elastic parameters. The key effects of shallow layers and anisotropy are taken into account in the inversion process. We investigate LAB distributions primarily below oceans according to three different proxies which corresponds to the base of the lithosphere from the vertically polarized shear velocity variation at depth, the top of the radial anisotropy positive anomaly and from the changes in orientation of the fast axis of azimuthal anisotropy. The LAB depth determinations of the different proxies are basically consistent for each oceanic region. The estimations of the LAB depth based on the shear velocity proxy increase from thin (20 km) lithosphere in the ridges to thick (120--130 km) old ocean lithosphere. The radial anisotropy proxy presents a very fast increase of the LAB depth from the ridges, from 50 km to older ocean where it reaches a remarkable monotonic sub-horizontal profile (70--80 km). LAB depths inferred from azimuthal anisotropy proxy show deeper values for the increasing oceanic lithosphere (130--135 km). The results present two types of pattern of the age of oceanic lithosphere evolution with the LAB depth. The shear velocity and azimuthal anisotropy proxies show age-dependent profiles in agreement with thermal plate models while the LAB based on radial anisotropy is characterized by a shallower depth, defining a sub-horizontal interface with a very small age dependence for all three main oceans (Pacific, Atlantic and Indian). These different patterns raise questions about the nature of the LAB in the oceanic regions, and of the formation of oceanic plates.

Evolution of crustal stress, pressure and temperature around shear zones during orogenic wedge formation: a 2D thermo-mechanical numerical study

NASA Astrophysics Data System (ADS)

Markus Schmalholz, Stefan; Jaquet, Yoann

2016-04-01

We study the formation of an orogenic wedge during lithospheric shortening with 2D numerical simulations. We consider a viscoelastoplastic rheology, thermo-mechanical coupling by shear heating and temperature-dependent viscosities, gravity and erosion. In the initial model configuration there is either a lateral temperature variation at the model base or a lateral variation in crustal thickness to generate slight stress variations during lithospheric shortening. These stress variations can trigger the formation of shear zones which are caused by thermal softening associated with shear heating. We do not apply any kind of strain softening, such as reduction of friction angle with progressive plastic strain. The first major shear zone that appears during shortening crosscuts the entire crust and initiates the asymmetric subduction/underthrusting of mainly the mechanically strong lower crust. After some deformation, the first shear zone in the upper crust is abandoned, the deformation propagates towards the foreland and a new shear zone forms only in the upper crust. The shear zone propagation occurs several times where new shear zones form in the upper crust and the mechanically strong top of the lower crust acts as detachment horizon. We calculate the magnitudes of the maximal and minimal principal stresses and of the mean stress (or dynamic pressure), and we record also the temperature for several marker points in the upper and lower crust. We analyse the evolution of stresses and temperature with burial depth and time. Deviatoric stresses (half the differential stress) in the upper crust are up to 200 MPa and associated shear heating in shear zones ranges between 40 - 80 °C. Lower crustal rocks remain either at the base of the orogenic wedge at depths of around 50 km or are subducted to depths of up to 120 km, depending on their position when the first shear zone formed. Largest deviatotric stresses in the strong part of the lower crust are about 1000 MPa and maximal shear heating in shear zones is approximately 200 °C. Marker points can migrate through the main shear zone in the lower crust which remains active throughout lithospheric shortening. Some pressure-temperature paths show an anti-clockwise evolution. The impact of various model parameters on the results is discussed as well as applications of the results to geological data.

The Large Scale Tectonic Framework of SE Asia and the Deformation of the lithosphere Beneath Tibet and SW China (Invited)

NASA Astrophysics Data System (ADS)

van der Hilst, R. D.; Huang, H.; Yao, H.

2010-12-01

We summarize results of our seismological studies of the lithosphere beneath Tibet and SW China. Joint analysis of geological, geodetic, and seismological data suggests that the Tibetan plateau formed through interplay between continental collision between India and Asia in the west and ocean floor subduction along the western Pacific island arcs and marginal basins in the east. These dynamic systems combine to facilitate the eastward extrusion of lithospheric material away from central Tibet. Located near the transition of these systems, SE Tibet is a key area for understanding regional seismicity as well as eastward plateau expansion. For a detailed regional study MIT installed an array of 25 three-component, broad band seismometers in Sichuan and Yunnan provinces, SW China. During the same 1-year period Lehigh University operated a 75 station array in east Tibet. Data from these and other nearby arrays have been used in a range of studies of crust and mantle heterogeneity and anisotropy. We focus our presentation on results of two lines of seismological study. First, travel time tomography (Li et al., PEPI 2006, EPSL 2008, JGR 2010) - with hand-picked phase arrivals from recordings at regional arrays, data from over 1,000 stations in China, and the global data base due to Engdahl et al. (BSSA, 1998) - has revealed that structures associated with subduction of the Indian plate beneath the Himalayas vary significantly from west Tibet (where the plate seems to underlie the entire plateau) to east Tibet (where Indian lithosphere seems to have plunged deeper into the mantle). Further east, fast structures appear in the upper mantle transition zone, presumably related to stagnation of slab fragments from westward subduction along Asia’s eastern sea board. Second, surface wave array tomography (Yao et al., GJI 2006, GJI 2008, JGR 2010; Huang et al., GRL 2010), based on ambient noise interferometry and traditional (inter station) dispersion analysis, is used to delineate the 3-D anisotropic structure of the crust and lithospheric mantle at length scales as small as 100 km beneath SE Asia. These inversions revealed (i) the presence of intra-crustal low velocity zones (perhaps bounded by major faults), (ii) a strong correlation between these low velocity zones and radial anisotropy (Vsh faster than Vsv), and (iii) that the pattern of crustal (azimuthal) anisotropy is quite different from that in the deep crust and mantle lithosphere. Furthermore, the spatial relationship with high heat flow, high (electrical) conductivity, and high Poisson’s ratio’s suggests that the crustal zones of low shear velocity are mechanically weak. Collectively, these inferences suggest that deformation is generally not vertically coherent and that (horizontal) ductile flow occurs (at least locally) in the deep crust of SE Tibet. Deformation of the lithosphere in SE Tibet may thus occur through interaction of geological units with and without crustal flow that are separated by major faults.

Geodynamic inversion to constrain the non-linear rheology of the lithosphere

NASA Astrophysics Data System (ADS)

Baumann, T. S.; Kaus, Boris J. P.

2015-08-01

One of the main methods to determine the strength of the lithosphere is by estimating it's effective elastic thickness. This method assumes that the lithosphere is a thin elastic plate that floats on the mantle and uses both topography and gravity anomalies to estimate the plate thickness. Whereas this seems to work well for oceanic plates, it has given controversial results in continental collision zones. For most of these locations, additional geophysical data sets such as receiver functions and seismic tomography exist that constrain the geometry of the lithosphere and often show that it is rather complex. Yet, lithospheric geometry by itself is insufficient to understand the dynamics of the lithosphere as this also requires knowledge of the rheology of the lithosphere. Laboratory experiments suggest that rocks deform in a viscous manner if temperatures are high and stresses low, or in a plastic/brittle manner if the yield stress is exceeded. Yet, the experimental results show significant variability between various rock types and there are large uncertainties in extrapolating laboratory values to nature, which leaves room for speculation. An independent method is thus required to better understand the rheology and dynamics of the lithosphere in collision zones. The goal of this paper is to discuss such an approach. Our method relies on performing numerical thermomechanical forward models of the present-day lithosphere with an initial geometry that is constructed from geophysical data sets. We employ experimentally determined creep-laws for the various parts of the lithosphere, but assume that the parameters of these creep-laws as well as the temperature structure of the lithosphere are uncertain. This is used as a priori information to formulate a Bayesian inverse problem that employs topography, gravity, horizontal and vertical surface velocities to invert for the unknown material parameters and temperature structure. In order to test the general methodology, we first perform a geodynamic inversion of a synthetic forward model of intraoceanic subduction with known parameters. This requires solving an inverse problem with 14-16 parameters, depending on whether temperature is assumed to be known or not. With the help of a massively parallel direct-search combined with a Markov Chain Monte Carlo method, solving the inverse problem becomes feasible. Results show that the rheological parameters and particularly the effective viscosity structure of the lithosphere can be reconstructed in a probabilistic sense. This also holds, with somewhat larger uncertainties, for the case where the temperature distribution is parametrized. Finally, we apply the method to a cross-section of the India-Asia collision system. In this case, the number of parameters is larger, which requires solving around 1.9 × 106 forward models. The resulting models fit the data within their respective uncertainty bounds, and show that the Indian mantle lithosphere must have a high viscosity. Results for the Tibetan plateau are less clear, and both models with a weak Asian mantle lithosphere and with a weak Asian lower crust fit the data nearly equally well.

Anatomy of the western Java plate interface from depth-migrated seismic images

NASA Astrophysics Data System (ADS)

Kopp, H.; Hindle, D.; Klaeschen, D.; Oncken, O.; Reichert, C.; Scholl, D.

2009-11-01

Newly pre-stack depth-migrated seismic images resolve the structural details of the western Java forearc and plate interface. The structural segmentation of the forearc into discrete mechanical domains correlates with distinct deformation styles. Approximately 2/3 of the trench sediment fill is detached and incorporated into frontal prism imbricates, while the floor sequence is underthrust beneath the décollement. Western Java, however, differs markedly from margins such as Nankai or Barbados, where a uniform, continuous décollement reflector has been imaged. In our study area, the plate interface reveals a spatially irregular, nonlinear pattern characterized by the morphological relief of subducted seamounts and thicker than average patches of underthrust sediment. The underthrust sediment is associated with a low velocity zone as determined from wide-angle data. Active underplating is not resolved, but likely contributes to the uplift of the large bivergent wedge that constitutes the forearc high. Our profile is located 100 km west of the 2006 Java tsunami earthquake. The heterogeneous décollement zone regulates the friction behavior of the shallow subduction environment where the earthquake occurred. The alternating pattern of enhanced frictional contact zones associated with oceanic basement relief and weak material patches of underthrust sediment influences seismic coupling and possibly contributed to the heterogeneous slip distribution. Our seismic images resolve a steeply dipping splay fault, which originates at the décollement and terminates at the sea floor and which potentially contributes to tsunami generation during co-seismic activity.

Anatomy of the western Java plate interface from depth-migrated seismic images

USGS Publications Warehouse

Kopp, H.; Hindle, D.; Klaeschen, D.; Oncken, O.; Reichert, C.; Scholl, D.

2009-01-01

Newly pre-stack depth-migrated seismic images resolve the structural details of the western Java forearc and plate interface. The structural segmentation of the forearc into discrete mechanical domains correlates with distinct deformation styles. Approximately 2/3 of the trench sediment fill is detached and incorporated into frontal prism imbricates, while the floor sequence is underthrust beneath the d??collement. Western Java, however, differs markedly from margins such as Nankai or Barbados, where a uniform, continuous d??collement reflector has been imaged. In our study area, the plate interface reveals a spatially irregular, nonlinear pattern characterized by the morphological relief of subducted seamounts and thicker than average patches of underthrust sediment. The underthrust sediment is associated with a low velocity zone as determined from wide-angle data. Active underplating is not resolved, but likely contributes to the uplift of the large bivergent wedge that constitutes the forearc high. Our profile is located 100 km west of the 2006 Java tsunami earthquake. The heterogeneous d??collement zone regulates the friction behavior of the shallow subduction environment where the earthquake occurred. The alternating pattern of enhanced frictional contact zones associated with oceanic basement relief and weak material patches of underthrust sediment influences seismic coupling and possibly contributed to the heterogeneous slip distribution. Our seismic images resolve a steeply dipping splay fault, which originates at the d??collement and terminates at the sea floor and which potentially contributes to tsunami generation during co-seismic activity. ?? 2009 Elsevier B.V.

Density contrast across the Moho beneath the Indian shield: Implications for isostasy

NASA Astrophysics Data System (ADS)

Paul, Himangshu; Mangalampally, Ravi Kumar; Tiwari, Virendra Mani; Singh, Arun; Chadha, Rajender Kumar; Davuluri, Srinagesh

2018-04-01

Knowledge of isostasy provides insights into how excess (or deficit) of mass on and within the lithosphere is maintained over different time scales, and also helps decipher the vertical dynamics. In continental regions, isostasy is primarily manifested as a crustal root, the extent of which is defined by the lithospheric strength and the density contrast at the Moho. In this study, we briefly review the methodology for extracting the density contrast across the Moho using the amplitudes of the P-to-s converted and free-surface reverberating phases in a receiver function (RF). We test the efficacy of this technique by applying it on synthetic and real data from 10 broadband seismic stations sited on diverse tectonic provinces in the Indian shield. We determine the density contrast after parameterizing the shear-wave velocity structure beneath the stations using the nearest neighbourhood algorithm. We find considerable variation in the density contrast across the Moho beneath the stations (0.4-0.65 gm/cc). This is explained in terms of isostatic compensation, incorporating the existing estimates of lithospheric strength (Te). Crustal roots computed using the estimated Te and the deduced density contrast substantiate the crustal thickness values inferred through RF analysis, and vice versa. This illustrates isostasy as a combination of variation in density contrast and Te. The density contrasts and crustal thicknesses inferred from RF analysis explain well the isostatic compensation mechanism in different regions. However, unusually large density contrasts (∼0.6 gm/cc) corresponding to elevated regions are intriguing and warrant further investigations. Our observation of varied density contrasts at the Moho in a Precambrian continental setting is interesting and raises a question about the existence of such situations in other parts of the world.

Eastern Indian Ocean microcontinent formation driven by plate motion changes

NASA Astrophysics Data System (ADS)

Whittaker, J. M.; Williams, S. E.; Halpin, J. A.; Wild, T. J.; Stilwell, J. D.; Jourdan, F.; Daczko, N. R.

2016-11-01

The roles of plate tectonic or mantle dynamic forces in rupturing continental lithosphere remain controversial. Particularly enigmatic is the rifting of microcontinents from mature continental rifted margins, with plume-driven thermal weakening commonly inferred to facilitate calving. However, a role for plate tectonic reorganisations has also been suggested. Here, we show that a combination of plate tectonic reorganisation and plume-driven thermal weakening were required to calve the Batavia and Gulden Draak microcontinents in the Cretaceous Indian Ocean. We reconstruct the evolution of these two microcontinents using constraints from new paleontological samples, 40Ar/39Ar ages, and geophysical data. Calving from India occurred at 101-104 Ma, coinciding with the onset of a dramatic change in Indian plate motion. Critically, Kerguelen plume volcanism does not appear to have directly triggered calving. Rather, it is likely that plume-related thermal weakening of the Indian passive margin preconditioned it for microcontinent formation but calving was triggered by changes in plate tectonic boundary forces.

New seismic observation on the lithosphere and slab subduction beneath the Indo-Myanmar block: Implications for continent oblique subduction and transition to oceanic slab subduction

NASA Astrophysics Data System (ADS)

Jiang, M.; He, Y.; Zheng, T.; Mon, C. T.; Thant, M.; Hou, G.; Ai, Y.; Chen, Q. F.; Sein, K.

2017-12-01

The Indo-Myanmar block locates to the southern and southeastern of the Eastern Himalayan Syntax (EHS) and marks a torsional boundary of the collision between the Indian and Eurasian plates. There are two fundamental questions concerned on the tectonics of Indo-Myanmar block since the Cenozoic time. One is whether and how the oblique subduction is active in the deep; the other is where and how the transition from oceanic subduction and continental subduction operates. However, the two problems are still under heated debate mainly because the image of deep structure beneath this region is still blurring. Since June, 2016, we have executed the China-Myanmar Geophysical Survey in the Myanmar Orogen (CMGSMO) and deployed the first portable seismic array in Myanmar in cooperation with Myanmar Geosciences Society (MGS). This array contains 70 stations with a dense-deployed main profile across the Indo-Myanmar Range, Central Basin and Shan State Plateau along latitude of 22° and a 2-D network covering the Indo-Myanmar Range and the western part of the Central Basin. Based on the seismic data collected by the new array, we conducted the studies on the lithospheric structure using the routine surface wave tomography and receiver function CCP stacking. The preliminary results of surface wave tomography displayed a remarkable high seismic velocity fabric in the uppermost of mantle beneath the Indo-Myanmar Range and Central Basin, which was interpreted as the subducted slab eastward. Particularly, we found a low velocity bulk within the high-velocity slab, which was likely to be a slab window due to the slab tearing. The preliminary results of receiver function CCP stacking showed the obvious variations of the lithospheric structures from the Indo-Myanmar Range to the Central Basin and Shan State Plateau. The lithospheric structure beneath the Indo-Myanmar Range is more complex than that beneath the Central Basin and Shan State Plateau. Our resultant high-resolution images will provide important constrains for establishing the tectonic framework of Indian plate eastward subduction. This study is supported by the National Natural Science Foundation of China (grants 41490612, 41274002).

A New Paradigm for New Oceans

NASA Astrophysics Data System (ADS)

Foulger, G. R.; Doré, A. G.; Franke, D.; Geoffroy, L.; Gernigon, L.; Hole, M.; Hoskuldsson, A.; Julian, B. R.; Kusznir, N.; Martinez, F.; Natland, J. H.; Peace, A.; Petersen, K. D.; Schiffer, C.; Stephenson, R.; Stoker, M. S.

2017-12-01

The original simple theory of plate tectonics had to be refined to accommodate second-order geological features such as back-arc basins and continental deformation zones. We propose an additional refinement that is required by complexities that form and persist in new oceans when inhomogeneous continental lithosphere/tectosphere disintegrates. Such complexities include continual plate-boundary reorganizations and migrations, distributed continental material in the ocean, propagating and dying ridges, and sagging, flexing and tilting in the oceans and at continent-ocean boundary zones. Reorganizations of stress and motion persist, resulting in variable orientations over short distances, tectonic reactivations, complex plate boundary configurations including multiple triple junctions, and the formation and abandonment of oceanic microplates. Resulting local compressions and extensions are manifest as bathymetric anomalies, vertical motions, and distributed volcanism at various times and places as the new ocean grows. Examples of regions that exhibit some or all of these features include the North Atlantic, the Rio Grande Rise/Walvis Ridge region of the South Atlantic, and the Seychelles-Mauritius region in the Indian Ocean. We suggest that these complexities arise as a result of the formation of new spreading plate boundaries by rifts propagating through continental lithosphere/tectosphere that is anisotropic as a result of inherited structure/composition and/or a sub-lithospheric mantle destabilized by lithospheric-controlled processes. Such scenarios result in complicated disintegration of continents and local persistent dynamic instability in the new ocean.

Depths of Intraplate Indian Ocean Earthquakes from Waveform Modeling

NASA Astrophysics Data System (ADS)

Baca, A. J.; Polet, J.

2014-12-01

The Indian Ocean is a region of complex tectonics and anomalous seismicity. The ocean floor in this region exhibits many bathymetric features, most notably the multiple inactive fracture zones within the Wharton Basin and the Ninetyeast Ridge. The 11 April 2012 MW 8.7 and 8.2 strike-slip events that took place in this area are unique because their rupture appears to have extended to a depth where brittle failure, and thus seismic activity, was considered to be impossible. We analyze multiple intraplate earthquakes that have occurred throughout the Indian Ocean to better constrain their focal depths in order to enhance our understanding of how deep intraplate events are occurring and more importantly determine if the ruptures are originating within a ductile regime. Selected events are located within the Indian Ocean away from major plate boundaries. A majority are within the deforming Indo-Australian tectonic plate. Events primarily display thrust mechanisms with some strike-slip or a combination of the two. All events are between MW5.5-6.5. Event selections were handled this way in order to facilitate the analysis of teleseismic waveforms using a point source approximation. From these criteria we gathered a suite of 15 intraplate events. Synthetic seismograms of direct P-waves and depth phases are computed using a 1-D propagator matrix approach and compared with global teleseismic waveform data to determine a best depth for each event. To generate our synthetic seismograms we utilized the CRUST1.0 software, a global crustal model that generates velocity values at the hypocenter of our events. Our waveform analysis results reveal that our depths diverge from the Global Centroid Moment Tensor (GCMT) depths, which underestimate our deep lithosphere events and overestimate our shallow depths by as much as 17 km. We determined a depth of 45km for our deepest event. We will show a comparison of our final earthquake depths with the lithospheric thickness based on halfspace cooling models and the local plate age.

Depth variations of P-wave azimuthal anisotropy beneath East Asia

NASA Astrophysics Data System (ADS)

Wei, W.; Zhao, D.; Xu, J.

2017-12-01

We present a new P-wave anisotropic tomographic model beneath East Asia by inverting a total of 1,488,531 P wave arrival-time data recorded by the regional seismic networks in East Asia and temporary seismic arrays deployed on the Tibetan Plateau. Our results provide important new insights into the subducting Indian, Pacific and Philippine Sea plates and mantle dynamics in East Asia. Our tomographic images show that the northern limit of the subducting Indian plate has reached the Jinsha River suture in eastern Tibet. A striking variation of P-wave azimuthal anisotropy is revealed in the Indian lithosphere: the fast velocity direction (FVD) is NE-SW beneath the Indian continent, whereas the FVD is arc parallel beneath the Himalaya and Tibetan Plateau, which may reflect re-orientation of minerals due to lithospheric extension, in response to the India-Eurasia collision. The FVD in the subducting Philippine Sea plate beneath the Ryukyu arc is NE-SW(trench parallel), which is consistent with the spreading direction of the West Philippine Basin during its initial opening stage, suggesting that it may reflect the fossil anisotropy. A circular pattern of FVDs is revealed around the Philippine Sea slab beneath SE China. We suggest that it reflects asthenospheric strain caused by toroidal mantle flow around the edge of the subducting slab. We find a striking variation of the FVD with depth in the subducting Pacific slab beneath the Northeast Japan arc. It may be caused by slab dehydration that changed elastic properties of the slab with depth. The FVD in the mantle wedge beneath the Northeast Japan and Ryukyu arcs is trench normal, which reflects subduction-induced convection. Beneath the Kuril and Izu-Bonin arcs where oblique subduction occurs, the FVD in the mantle wedge is nearly normal to the moving direction of the downgoing Pacific plate, suggesting that the oblique subduction together with the complex slab morphology have disturbed the mantle flow.

Continent-arc collision in the Banda Arc imaged by ambient noise tomography

NASA Astrophysics Data System (ADS)

Porritt, Robert W.; Miller, Meghan S.; O'Driscoll, Leland J.; Harris, Cooper W.; Roosmawati, Nova; Teofilo da Costa, Luis

2016-09-01

The tectonic configuration of the Banda region in southeast Asia captures the spatial transition from subduction of Indian Ocean lithosphere to subduction and collision of the Australian continental lithosphere beneath the Banda Arc. An ongoing broadband seismic deployment funded by NSF is aimed at better understanding the mantle and lithospheric structure in the region and the relationship of the arc-continent collision to orogenesis. Here, we present results from ambient noise tomography in the region utilizing this temporary deployment of 30 broadband instruments and 39 permanent stations in Indonesia, Timor Leste, and Australia. We measure dispersion curves for over 21,000 inter-station paths resulting in good recovery of the velocity structure of the crust and upper mantle beneath the Savu Sea, Timor Leste, and the Nusa Tenggara Timur (NTT) region of Indonesia. The resulting three dimensional model indicates up to ∼25% variation in shear velocity throughout the plate boundary region; first-order velocity anomalies are associated with the subducting oceanic lithosphere, subducted Australian continental lithosphere, obducted oceanic sediments forming the core of the island of Timor, and high velocity anomalies in the Savu Sea and Sumba. The structure in Sumba and the Savu Sea is consistent with an uplifting forearc sliver. Beneath the island of Timor, we confirm earlier inferences of pervasive crustal duplexing from surface mapping, and establish a link to underlying structural features in the lowermost crust and uppermost mantle that drive upper crustal shortening. Finally, our images of the volcanic arc under Flores, Wetar, and Alor show high velocity structures of the Banda Terrane, but also a clear low velocity anomaly at the transition between subduction of oceanic and continental lithosphere. Given that the footprint of the Banda Terrane has previously been poorly defined, this model provides important constraints on tectonic reconstructions that formerly have lacked information on the lower crust and uppermost mantle.

Moho depth variations over the Maldive Ridge and adjoining Arabian and Central Indian Basins, Western Indian Ocean, from three dimensional inversion of gravity anomalies

NASA Astrophysics Data System (ADS)

Kunnummal, Priyesh; Anand, S. P.; Haritha, C.; Rama Rao, P.

2018-05-01

Analysis of high resolution satellite derived free air gravity data has been undertaken in the Greater Maldive Ridge (GMR) (Maldive Ridge, Deep Sea Channel, northern limit of Chagos Bank) segment of the Chagos Laccadive Ridge and the adjoining Arabian and Central Indian Basins. A Complete Bouguer Anomaly (CBA) map was generated from the Indian Ocean Geoidal Low removed Free Air Gravity (hereinafter referred to as "FAG-IOGL") data by incorporating Bullard A, B and C corrections. Using the Parker method, Moho topography was initially computed by inverting the CBA data. From the CBA the Mantle Residual Gravity Anomalies (MRGA) were computed by incorporating gravity effects of sediments and lithospheric temperature and pressure induced anomalies. Further, the MRGA was inverted to get Moho undulations from which the crustal thickness was also estimated. It was found that incorporating the lithospheric thermal and pressure anomaly correction has provided substantial improvement in the computed Moho depths especially in the oceanic areas. But along the GMR, there was not much variation in the Moho thickness computed with and without the thermal and pressure gravity correction implying that the crustal thickness of the ridge does not depend on the oceanic isochrones used for the thermal corrections. The estimated Moho depths in the study area ranges from 7 km to 28 km and the crustal thickness from 2 km to 27 km. The Moho depths are shallower in regions closer to Central Indian Ridge in the Arabian Basin i.e., the region to the west of the GMR is thinner compared to the region in the east (Central Indian Basin). The thickest crust and the deepest Moho are found below the N-S trending GMR segment of the Chagos-Laccadive Ridge. Along the GMR the crustal thickness decreases from north to south with thickness of 27 km below the Maldives Ridge reducing to ∼9 km at 3°S and further increasing towards Chagos Bank. Even though there are similarities in crustal thickness between Maldive Ridge and other regions like Mascarene Plateau which was recently interpreted as underlain by continental crust, much more geoscientific work including drilling has to be undertaken to finally confirm the exact nature of the ridge.

Gravity anomalies and the structure of western Tibet and the southern Tarim Basin

NASA Technical Reports Server (NTRS)

Lyon-Caen, H.; Molnar, P.

1984-01-01

Gravity anomalies across the western part of the Tarim Basin and the Kunlun mountain belt show that this area is not in local isostatic equilibrium. These data can be explained if a strong plate underlying the Tarim Basin extends southwestward beneath the belt at least 80 km and supports part of the topography of northwest Tibet. This corroborates Norin's inference that late Tertiary crustal shortening has occurred in this area by southward underthrusting of the Tarim Basin beneath the Kunlun. This study places a lower bound on the amount of underthrusting.

Why does near ridge extensional seismicity occur primarily in the Indian Ocean?

NASA Technical Reports Server (NTRS)

Stein, Seth; Cloetingh, Sierd; Wortel, Rinus; Wiens, Douglas A.

1987-01-01

It is argued that though thermoelastic stresses provide a low level background in all plates, the data favoring their contributing significantly to the stress field and seismicity in the young oceanic lithosphere may be interpreted in terms of stresses resulting from individual plate geometry and local boundary effects. The dramatic concentration of extensional seismicity in the Central Indian Ocean region is shown to be consistent with finite element results for the intraplate stress incorporating the effects of the Himalayan collision and the various subduction zones. Most of the data for both ridge-parallel extension and depth stratification are provided by earthquakes in this area, and it is suggested that these effects may be due more to the regional stress.

Lithospheric structure of the southeastern margin of the Tibetan Plateau from Rayleigh wave tomography

NASA Astrophysics Data System (ADS)

Fu, Yuanyuan V.; Gao, Yuan; Li, Aibing; Li, Lun; Chen, Anguo

2017-06-01

Lithospheric shear wave velocity beneath the southeastern margin of the Tibetan Plateau is obtained from Rayleigh wave tomography using earthquake data recorded by the temporary ChinArray and permanent China Digital Seismic Array. Fundamental mode Rayleigh wave phase velocities at periods of 20-100 s are determined and used to construct the 3-D shear wave velocity model. Low-velocity anomalies appear along or close to the major faults in the middle crust and become a broad zone in the lower crust, suggesting block extrusion in the shallow crust and diffuse deformation in the lower crust, both of which play important roles in accommodating the collision between the Indian and Eurasian plates. A vertical low-velocity column beneath the Tengchong Volcano is observed, which could be caused by upwelling of warm mantle due to the lithosphere extension in the Thailand rift basin to the south or by fluid-induced partial melting due to the subduction of the Burma slab. The western Yangtze Craton is characterized by low velocity in the crust and uppermost mantle above the fast mantle lithosphere, indicating possible thermal erosion at the western craton edge resulted from the extrusion of the Tibetan Plateau. A low-velocity zone is imaged at the depths of 70-150 km beneath the eastern part of the Yangtze Craton, which could be caused by small-scale mantle convection associated with the subduction of the Burma microplate and/or the opening of the South China Sea.

Electromagnetic, seismic and petro-physical investigations of the lithosphere-asthenosphere boundary in central Tibet

NASA Astrophysics Data System (ADS)

Vozar, J.; Fullea, J.; Jones, A. G.; Agius, M. R.; Lebedev, S.

2011-12-01

Combined seismological and electromagnetic investigations of the lithosphere and underlying asthenosphere have the potential to yield superior inferences than using either one on its own. Central Tibet offers an excellent natural laboratory for testing such approaches, given the high quality seismological and magnetotelluric (MT) data available as a consequence of INDEPTH studies. In particular, the presence and lateral and vertical extent of the Indian lithosphere beneath Tibet is highly debated. Integrated petrological-geophysical modeling of MT and surface-wave data, which are differently sensitive to temperature and composition, allows us to reduce the uncertainties associated with modeling these two data sets independently, as commonly undertaken. For the MT data, we use selected distortion-corrected MT transfer functions, from INDEPTH Phase III line 500 across central Tibet for 1D modeling. The selected data fit well the 1D assumption and exhibit large penetration depth. Our deep resistivity models can be classified into two different groups: i) the Lhasa Terrane and ii) the Qiangtang Terrane. For the Lhasa Terrane group, the models show the existence of two high conductive layers localized at depths of 60-80 km and more than 200 km, whereas for the Qiangtang Terrane these conductive layers appears to be occur at shallower depths, namely 30-50 km and 120 km depth respectively. Our dispersion curves for Rayleigh and Love surface waves were measured using seismograms recorded by stations of INDEPTH and PASSCAL experiments. Dispersion curves for central Lhasa and Qiangtang terranes show similarly low phase velocities at periods sampling the thick crust beneath the regions, but differ at periods sampling the mantle. Inverting the dispersion data for 1D, radially-anisotropic Vs profiles, we find that beneath central Qiangtang terrane shear velocity is lower than the global average down to 75 km below the Moho, indicating relatively high temperatures, whereas beneath Central Lhasa terrane S-velocities are close to global-average values. We perform the integrated petro-physical modeling of MT and surface-wave data using the software package LitMod. The program facilitates definition of realistic temperature and pressure distributions within the upper mantle, and characterizes the mineral assemblages given bulk chemical compositions as well as water content. This allows us to firstly define a bulk geoelectric and seismic model of the upper mantle based on laboratory and xenolith data for the most relevant mantle minerals, and secondly to compute synthetic geophysical observables that are compared with measured data (i.e., MT responses, surface-wave dispersion curves, topography, and surface heat flow). Our preliminary results suggest an 80-120 km-thick, dry lithosphere in the central part of the Qiangtang Terrane. In the central Lhasa Terrane the data can be explained by a relatively warm 100-120 km-thick Tibetian lithosphere underlain by an 80-120-km-thick Indian lithosphere. The mid-lower crust in Lhasa shows strong seismic and electric anisotropy, with a predominant E-W oriented high velocity/conductivity axis.

Greater India Basin hypothesis and a two-stage Cenozoic collision between India and Asia

PubMed Central

van Hinsbergen, Douwe J. J.; Lippert, Peter C.; Dupont-Nivet, Guillaume; McQuarrie, Nadine; Doubrovine, Pavel V.; Spakman, Wim; Torsvik, Trond H.

2012-01-01

Cenozoic convergence between the Indian and Asian plates produced the archetypical continental collision zone comprising the Himalaya mountain belt and the Tibetan Plateau. How and where India–Asia convergence was accommodated after collision at or before 52 Ma remains a long-standing controversy. Since 52 Ma, the two plates have converged up to 3,600 ± 35 km, yet the upper crustal shortening documented from the geological record of Asia and the Himalaya is up to approximately 2,350-km less. Here we show that the discrepancy between the convergence and the shortening can be explained by subduction of highly extended continental and oceanic Indian lithosphere within the Himalaya between approximately 50 and 25 Ma. Paleomagnetic data show that this extended continental and oceanic “Greater India” promontory resulted from 2,675 ± 700 km of North–South extension between 120 and 70 Ma, accommodated between the Tibetan Himalaya and cratonic India. We suggest that the approximately 50 Ma “India”–Asia collision was a collision of a Tibetan-Himalayan microcontinent with Asia, followed by subduction of the largely oceanic Greater India Basin along a subduction zone at the location of the Greater Himalaya. The “hard” India–Asia collision with thicker and contiguous Indian continental lithosphere occurred around 25–20 Ma. This hard collision is coincident with far-field deformation in central Asia and rapid exhumation of Greater Himalaya crystalline rocks, and may be linked to intensification of the Asian monsoon system. This two-stage collision between India and Asia is also reflected in the deep mantle remnants of subduction imaged with seismic tomography. PMID:22547792

Asthenosphere rheology inferred from observations of the 2012 Indian Ocean earthquake.

PubMed

Hu, Yan; Bürgmann, Roland; Banerjee, Paramesh; Feng, Lujia; Hill, Emma M; Ito, Takeo; Tabei, Takao; Wang, Kelin

2016-10-20

The concept of a weak asthenospheric layer underlying Earth's mobile tectonic plates is fundamental to our understanding of mantle convection and plate tectonics. However, little is known about the mechanical properties of the asthenosphere (the part of the upper mantle below the lithosphere) underlying the oceanic crust, which covers about 60 per cent of Earth's surface. Great earthquakes cause large coseismic crustal deformation in areas hundreds of kilometres away from and below the rupture area. Subsequent relaxation of the earthquake-induced stresses in the viscoelastic upper mantle leads to prolonged postseismic crustal deformation that may last several decades and can be recorded with geodetic methods. The observed postseismic deformation helps us to understand the rheological properties of the upper mantle, but so far such measurements have been limited to continental-plate boundary zones. Here we consider the postseismic deformation of the very large (moment magnitude 8.6) 2012 Indian Ocean earthquake to provide by far the most direct constraint on the structure of oceanic mantle rheology. In the first three years after the Indian Ocean earthquake, 37 continuous Global Navigation Satellite Systems stations in the region underwent horizontal northeastward displacements of up to 17 centimetres in a direction similar to that of the coseismic offsets. However, a few stations close to the rupture area that had experienced subsidence of up to about 4 centimetres during the earthquake rose by nearly 7 centimetres after the earthquake. Our three-dimensional viscoelastic finite-element models of the post-earthquake deformation show that a thin (30-200 kilometres), low-viscosity (having a steady-state Maxwell viscosity of (0.5-10) × 10 18 pascal seconds) asthenospheric layer beneath the elastic oceanic lithosphere is required to produce the observed postseismic uplift.

Investigating the magnitude of lower crustal flow and impact on surface deformation patterns in Tibet using 3-D geodynamic models

NASA Astrophysics Data System (ADS)

Bischoff, S. H.; Flesch, L. M.

2016-12-01

Differential flow in the lower crust of Tibet has been invoked to explain features in the region, including uniform plateau elevation, crustal thickness/topographic gradients, and uplift without observed shortening. Here, we use 3-D finite element modeling to test impacts of assumed lower crustal viscosities on deformation patterns in the India-Eurasia collision zone. We simulate instantaneous lithospheric deformation with Stokes flow using COMSOL Multiphysics (www.comsol.com). Our model geometry ranges eastward from the Pamir to Sichuan, northward from the southern tip of India to the Tien Shan, and vertically downward from the Earth's surface to 100 km below sea level. We divide model geometry into four domains: Indian lithosphere, Eurasian upper crust, lower crust, and upper mantle. Seismic and magnetotelluric study results guide inclusion of subducted Indian and Burma slabs along with our targeted weak lower crust. Within the larger Eurasian lower crust domain, weak lower crust is restricted to a zone bounded clockwise by the Himalayan Frontal Thrust, Karakorum, Altyn-Tagh, Kunlun, Longmen Shan, and onset of lower elevations along the plateau's southeastern margin. From top to bottom, vertical bounds of the zone are constrained by a constant 20 km below sea level and the shallower of either the top of the Indian slab or Moho. Strength is approximated via 3-D maps of effective viscosity constrained by the vertically-averaged lithospheric estimates of Flesch et al. [2001]. We forward model lower crust effective viscosities on the order of 1018 to 1022 Pa•s and inspect resulting horizontal and vertical deformation patterns. Results suggest that effective viscosities of less than 1020 Pa•s are required for both appreciable differential mass flux through lower crustal flow as well as decoupled lower crustal flow from the upper crust or mantle. Movement of the lower crust is partitioned within weaker fault zones. Effective viscosities of 1020 Pa•s or less produce pronounced patterns of surface subsidence in Qiangtang and uplift in eastern Lhasa and Longmen Shan inconsistent with observations. Solutions show lower crust strength impacts surface stress style with weaker strengths leading to regions of dominant extension separated by compression in the east central Tibetan Plateau.

An Andean-type retro-arc foreland system beneath northwest South China revealed by SINOPROBE profiling

NASA Astrophysics Data System (ADS)

Li, Jianhua; Dong, Shuwen; Cawood, Peter A.; Zhao, Guochun; Johnston, Stephen T.; Zhang, Yueqiao; Xin, Yujia

2018-05-01

In the Mesozoic, South China was situated along the convergent margin between the Asian and Pacific plates, providing an excellent laboratory to understand the interactions between deformation, sedimentation and magmatism in a retroarc environment. The crustal architecture of northwest South China is displayed along the ∼600-km-long SINOPROBE deep seismic reflection profiles and reveals from east to west: (1) highly folded and truncated reflectors in the upper crust of the Yangtze Fold Zone, which correspond to thin- and thick-skinned thrust systems, and document large-scale intraplate structural imbrication and shortening; (2) a crustal-scale flat-ramp-flat structure, termed the Main Yangtze decollement, which forms a weak, viscous layer to accommodate strain decoupling and material transport in the thin- and thick-skinned systems; and (3) nearly flat-lying reflectors in the Sichuan Basin, which support interpretation of the basin as a weakly deformed depocentre. The Yangtze Fold Zone and the Sichuan Basin represent a retro-arc foreland basin system that is >800 km away from the continental-margin magmatic arc. We suggest that tectonic processes across the arc and retro-arc systems, including arc magma flare-up, basin sedimentation, retroarc thrust propagation, lithosphere underthrusting, root foundering, and extension-related magmatism were interrelated and governed mass transfer. Age data and geological relations link the tectonic processes to evolving geodynamics of the subducting Paleo-Pacific plate.

Zircon Zoning, Trace Elements and U-Pb Dates Reveal Crustal Foundering Beneath the Pamir

NASA Astrophysics Data System (ADS)

Hacker, B. R.; Shaffer, M. E. F.; Ratschbacher, L.; Kylander-Clark, A. R.

2017-12-01

Xenoliths that erupted in the SE Pamir of Tajikistan at 11.2 Ma from 1000-1050°C and 90 km depth illuminate what happens when crust founders into the mantle. The xenoliths are a broad range of crustal rock types and contain abundant xenoliths whose U-Pb isotopic ratios and trace-element contents were examined by laser-ablation split stream inductively coupled plasma mass spectrometry. Cathodoluminescence imaging of the grains shows igneous cores with oscillatory zoning overprinted by substantial recrystallization. The bulk of the U-Pb dates are concordant and range from 160 Ma to 11 Ma. The range of dates suggest that the xenoliths were likely derived from the Jurassic-Cretaceous Andean-style magmatic arc and its Proterozoic-Mesozoic host rocks along the southern margin of Asia. The zircons show distinct changes in Eu anomaly, Lu/Gd ratio, and Ti concentrations that are interpreted to indicate garnet growth and minimal heating at 22-20 Ma, and then 200-300°C of heating, 25 km of burial, and alkali-carbonate melt injection at 14-11 Ma. These changes are interpreted to coincide with: i) heat input due to Indian slab breakoff at 22‒20 Ma; ii) rapid thickening and foundering of the Pamir lithosphere at 14‒11 Ma, prior to and synchronous with collision between deep Indian and Asian lithospheres beneath the Pamir.

Foundering Triggered by the Collision of India and Asia Captured in Xenoliths

NASA Astrophysics Data System (ADS)

Shaffer, Madeline; Hacker, Bradley R.; Ratschbacher, Lothar; Kylander-Clark, Andrew R. C.

2017-10-01

Xenoliths that erupted in the SE Pamir of Tajikistan from 1000 to 1050°C and 90 km depth illuminate what happens when crust founders into the mantle. 40Ar/39Ar dating of minerals from the xenoliths and volcanic host rocks of the shoshonitic Dunkeldik pipe and dike field indicates eruption at 11.2 ± 0.2 Ma. U-Pb and trace element laser-ablation split stream inductively coupled plasma mass spectrometry of zircon shows that the igneous and metasedimentary xenoliths were likely derived from the crustal section into which they were intruded: the Jurassic-Cretaceous Andean-style magmatic arc and its Proterozoic-Mesozoic host rocks along the southern margin of Asia. Recrystallization of these zircons was extensive, yielding a range of dates down to 11 Ma. The zircons show distinct changes in Eu anomaly, Lu/Gd ratio, and Ti concentrations compatible with garnet growth and minimal heating at 22-20 Ma and then 200-300°C of heating, 25 km of burial, and alkali-carbonate melt injection at 14-11 Ma. These changes are interpreted to coincide with (i) heat input due to Indian slab breakoff at 22-20 Ma and (ii) rapid thickening and foundering of the Pamir lithosphere at 14-11 Ma, prior to and synchronous with collision between deep Indian and Asian lithospheres beneath the Pamir.

Oceanic magmatic evolution during ocean opening under influence of mantle plume

NASA Astrophysics Data System (ADS)

Sushchevskaya, Nadezhda; Melanholina, Elena; Belyatsky, Boris; Krymsky, Robert; Migdisova, Natalya

2015-04-01

Petrology, geochemistry and geophysics as well as numerical simulation of spreading processes in plume impact environments on examples of Atlantic Ocean Iceland and the Central Atlantic plumes and Kerguelen plume in the Indian Ocean reveal: - under interaction of large plume and continental landmass the plume can contribute to splitting off individual lithosphere blocks, and their subsequent movement into the emergent ocean. At the same time enriched plume components often have geochemical characteristics of the intact continental lithosphere by early plume exposure. This is typical for trap magmatism in Antarctica, and for magmatism of North and Central Atlantic margins; - in the course of the geodynamic reconstruction under the whole region of the South Atlantic was formed (not in one step) metasomatized enriched sub-oceanic mantle with pyroxenite mantle geochemical characteristics and isotopic composition of enriched HIMU and EM-2 sources. That is typical for most of the islands in the West Antarctic. This mantle through spreading axes jumping involved in different proportions in the melting under the influence of higher-temperature rising asthenospheric lherzolite mantle; - CAP activity was brief enough (200 ± 2 Ma), but Karoo-Maud plume worked for a longer time and continued from 180 to 170 Ma ago in the main phase. Plume impact within Antarctica distributed to the South and to the East, leading to the formation of extended igneous provinces along the Transantarctic Mountains and along the east coast (Queen Maud Land province and Schirmacher Oasis). Moreover, this plume activity may be continued later on, after about 40 million years cessation, as Kerguelen plume within the newly-formed Indian Ocean, significantly affects the nature of the rift magmatism; - a large extended uplift in the eastern part of the Indian Ocean - Southeastern Indian Ridge (SEIR) was formed on the ancient spreading Wharton ridge near active Kerguelen plume. The strongest plume influence on the SEIR formation occurred 70-50 mln years ago, when the process of primary magma generation happened at high degrees of melting (up to 30%), which is not typical for spreading ridges of the Atlantic and Pacific Oceans. According to geochemical characteristics of the Kerguelen Plateau and SEIR magma sources close to each other, and have an enriched source of more typical for Kerguelen plume magmas and diluted by depleted substance for SEIR melts. Appearance of magmatism on the Antarctic margin about 56 thousand years ago, in the form of a stratovolcano Gaussberg indicates sublithospheric Kerguelen plume distribution in the south-west direction. The source of primary magmas (lamproite composition) is an ancient Gondwana lithosphere, has undergone repeated changes in the early stages of evolution during which it was significantly enriched in volatile and lithophile elements, and radiogenic Sr and Pb.

In-situ seismic record of potential sill intrusion at the ultraslow spreading Southwest Indian Ridge

NASA Astrophysics Data System (ADS)

Meier, M.; Schlindwein, V. S. N.

2017-12-01

Ultraslow spreading mid-ocean ridges with full spreading rates up to 15 mm/yr are described as the melt poor endmember of the entire mid-ocean ridge system. The melt supply along ultraslow spreading ridges is uneven resulting in the formation of volcanic centres and amagmatic segments. Amagmatic segments show thicker brittle lithosphere of up to 30 km, whereas magmatic segments have much thinner lithosphere of up to less than 15 km. It is supposed that melt travels along the lithosphere asthenosphere boundary from amagmatic segments to magmatic segments, where it can reach the seafloor and erupt. These spreading events are rare at ultraslow spreading ridges compared to faster spreading ridges and insitu observations hardly exist. During an ocean bottom seismometer (OBS) experiment at the eastern Southwest Indian Ridge two earthquake swarms were accidentally recorded. The swarms occurred in January and April 2013 and both lasted for a few days. The events of the earthquake swarms were relatively located with HypoDD for better spatial resolution. This unique dataset allowed for studying active spreading processes at an ultraslow spreading ridge. The earthquakes occurred in depths, where the magma chamber of the nearby Segment-8 volcano is located. This magma chamber potentially fed a sill intrusion, which was recorded as earthquake swarms. During the first hours of the first earthquake swarm a migration pattern was identified. The hypocentres migrated away from the Segment-8 volcanic centre and slightly downwards. Later events occurred more randomly in the active area. Simultaneously seismic tremor was recorded at the station closest to the swarm locations. The tremor lasted longer for the shorter earthquake swarm in April. During both tremor phases the signal was modulated with a 12 hour period. We speculate that a hydrothermal system was affected by the intrusion and fluid flow modulated by the tides produced the tremor signal.

Thoughts on Multi-sphere Study in the Indo-Pacific Convergent Zone

NASA Astrophysics Data System (ADS)

Wang, F.

2016-12-01

Interactions of the ocean with other components of the earth system, such as atmosphere, lithosphere, and biosphere are the front and hotspot of the ocean and earth sciences. In the Indonesian Archipelago and adjacent western Pacific and eastern Indian Oceans, both the upper oceanic circulation and lower atmospheric circulation convergent and consequently enhance the fresh water and heat fluxes, affecting the East Asian and global climate. This region is considered as the world's center of marine bio-diversity and sediment discharge, as well as the collision center of the Eurasian, Indian and Pacific plates. Why and how the energy and material of multiple spheres convergent toward the region are important scientific issues on the front of earth system science and marine sciences, and need to be investigated through international cooperation.

Plate motion changes drive Eastern Indian Ocean microcontinent formation

NASA Astrophysics Data System (ADS)

Whittaker, J. M.; Williams, S.; Halpin, J.; Wild, T.; Stilwell, J.; Jourdan, F.; Daczko, N. R.

2016-12-01

The roles of plate tectonic or mantle dynamic forces in rupturing continental lithosphere remain controversial. Particularly enigmatic is the rifting of microcontinents from mature continental rifted margin - several well-studied microcontinent calving events coincide in space and time with mantle plume activity, but the significance of plumes in driving microcontinent formation remains controversial, and a role for plate-driven processes has also been suggested. In 2011, our team discovered two new microcontinents in the eastern Indian Ocean, the Batavia and Gulden Draak microcontinents. These microcontinents are unique as they are the only surviving remnants of the now-destroyed or highly deformed Greater Indian margin and provide us with an opportunity to test existing models of microcontinent formation against new observations. Here, we explore models for microcontinent formation using our new data from the Eastern Indian Ocean in a plate tectonic reconstruction framework. We use Argon dating and paleontology results to constrain calving from greater India at 101-104 Ma. This region had been proximal to the active Kerguelen plume for 30 Myrs but we demonstrate that calving did not correspond with a burst of volcanic activity. Rather, it is likely that plume-related thermal weakening of the Indian passive margin preconditioned it for microcontinent formation but calving was triggered by changes in plate tectonic boundary forces. Changes in the relative motions between Indian and Australia led to increasing compressive forces along the long-offset Wallaby-Zenith Fracture Zone, which was eventually abandoned during the jump of the spreading ridge into the Indian continental margin.

An Alternative Estimate of the Motion of the Capricorn Plate

NASA Astrophysics Data System (ADS)

Burris, S. G.; Gordon, R. G.

2013-12-01

Diffuse plate boundaries cover ~15% of Earth's surface and can exceed 1000 km in across-strike width. Deforming oceanic lithosphere in the equatorial Indian Ocean accommodates the motion between the India and Capricorn plates and serves as their mutual diffuse plate boundary. This deforming lithosphere lies between the Central Indian Ridge to the west and the Sumatra trench to the east; the plates diverge to the west of ≈74°E and converge to the east of it. Many data have shown that the pole of rotation between the India and Capricorn plates lies within this diffuse plate boundary [1,2]. Surprisingly, however, the recently estimated angular velocity in the MORVEL global set of angular velocities [3] places this pole of rotation north of prior poles by several degrees, and north of the diffuse plate boundary. The motion between the India and Capricorn plates can only be estimated indirectly by differencing the motion of the India plate relative to the Somalia plate, on the one hand, and the motion of the Capricorn plate relative to Somalia plate, on the other. While the MORVEL India-Somalia angular velocity is similar to prior estimates, the MORVEL Capricorn-Somalia pole of rotation lies northwest of its predecessors. The difference is not caused by new transform azimuth data incorporated into MORVEL or by the new application of a correction to spreading rates for outward displacement. Instead the difference appears to be caused by a few anomalous spreading rates near the northern end of the Capricorn-Somalia plate boundary along the Central Indian Ridge. Rejecting these data leads to consistency with prior results. Implications for the motion of the Capricorn plate relative to Australia will be discussed. [1] DeMets, C., R. G. Gordon, and J.-Y. Royer, 2005. Motion between the Indian, Capricorn, and Somalian plates since 20 Ma: implications for the timing and magnitude of distributed deformation in the equatorial Indian ocean, Geophys. J. Int., 161, 445-468. [2] Gordon, R. G., Royer, J.-Y., and D. F. Argus, 2008. Space geodetic test of kinematic models for the Indo-Australian composite plate, Geology, 36, 827-830, doi: 10.1130/G25089A.1. [3] DeMets, C., Gordon, R. G., & Argus, D. F., 2010. Geologically current plate motions, Geophys. J. Int., 181, 1-80, doi: 10.1111/j.1365-246X.2009.04491.x.

Marine magnetic anomalies in the NE Indian Ocean: the Wharton and Central Indian basins revisited

NASA Astrophysics Data System (ADS)

Jacob, J.; Dyment, J.; Yatheesh, V.; Bhattacharya, G. C.

2009-04-01

The North-eastern Indian Ocean has recently received a renewed interest. The disastrous earthquakes and tsunamis of Dec. 2004 off Sumatra have triggered a large international effort including several oceanographic cruises. The Ninetyeast Ridge, a long submarine ridge which extends NS on more than 4000 km, has been the focus of a recent cruise aiming to study the interaction of a hotspot with the oceanic lithosphere and spreading centres. Both the study of the seismogenic zone under Sumatra and the Ninetyeast Ridge formation require accurate determination of the age and structure of the oceanic lithosphere in the Wharton and Central Indian Basins. First we delineate tectonic elements such as the Sunda Trench, the Ninetyeast Ridge, and the fracture zones of the Wharton and Central Indian basins from a recent version of the free-air gravity anomaly deduced from satellite altimetry and available multibeam bathymetric data. We use all available magnetic data to identify magnetic anomalies and depict seafloor spreading isochrons in order to build a tectonic map of the Wharton Basin. To do so, we apply the analytic signal method to unambiguously determine the location of the magnetic picks. The new tectonic map shows more refinements than previous ones, as expected from a larger data set. The fossil ridge in the Wharton Basin is clearly defined; spreading ceased at anomaly 18 young (38.5 Ma), and, perhaps, as late as anomaly 15 (35 Ma). Symmetric anomalies are observed on both flanks of the fossil ridge up to anomaly 24 (54 Ma), preceded by a slight reorganization of the spreading compartments between anomalies 28 and 25 (64 - 56 Ma) and a more stable phase of spreading between anomalies 34 and 29 (83 - 64 Ma). Earlier, a major change of spreading direction is clearly seen in the bending fracture zones; interpolating in the Cretaceous Quiet Zone between anomaly 34 in the Wharton Basin and anomaly M0 off Australia leads to an age of ~100 Ma for this reorganization. Anomalies 20 to 34 are clearly identified in the western part of the Central Indian Basin. The interpretation is more difficult in the compartments located immediately west of the Ninetyeast Ridge, where multiple ridge jumps have been proposed to explain complex anomaly patterns. In a different way, we recognize a continuous sequence of anomalies 20 to 34, although the anomalies 25 to 29 seem to be wider and display complex boundaries.

3-dimensional structure of the Indian Ocean inferred from long period surface waves

NASA Astrophysics Data System (ADS)

Montagner, Jean-Paul

1986-04-01

To improve the lateral resolution of the first global 3 - dimensional models of seismic wave velocities, regional studies have to be undertaken. The dispersion of Rayleigh waves along 86 paths across the Indian Ocean and surrounding regions is investigated in the period range 40 - 300 s. The regionalization of group velocity according to the age of the sea floor shows an increase of velocity with age up to 150 s only, similar to the results in the Pacific Ocean. But here, this relationship vanishes more quickly at long period. Therefore the correlation of the deep structure with surface tectonics seems to be shallower in the Indian Ocean than in the Pacific Ocean. A tomographic method is applied to compute the geographical distributions of group velocity and azimuthal anisotropy and then the 3-D structure of S-wave velocity. Horizontal wavelengths of 2000 km for velocity and 3000 km for azimuthal anisotropy distribution can be resolved. Except for the central part of the South East Indian ridge which displays high velocities at all depths, the inversion corroborates a good correlation between lithospheric structure down to 120 km and surface tectonics: low velocities along the central and southeast Indian ridges, velocity increasing with the age of the sea floor, high velocities under African, Indian and Australian shields. At greater depths, the low velocity zones under the Gulf of Aden and the western part of the Southeast Indian ridges hold but the low velocity anomaly of the Central Indian ridge is offset eastward. The low velocity anomalies suggest uprising material and complex plate boundary.

Indian Ocean floor deformation induced by the Reunion plume rather than the Tibetan Plateau

NASA Astrophysics Data System (ADS)

Iaffaldano, G.; Davies, D. R.; DeMets, C.

2018-05-01

The central Indian Ocean is considered the archetypal diffuse oceanic plate boundary. Data from seismic stratigraphy and deep-sea drilling indicate that the contractional deformation of the Indian Ocean lithosphere commenced at 15.4-13.9 Ma, but experienced a sharp increase at 8-7.5 Ma. This has been maintained through to the present day, with over 80% of the shortening accrued over the past 8 Myr. Here we build on previous efforts to refine the form, timing and magnitude of the regional plate-motion changes by mitigating the noise in reconstructed Indian and Capricorn plate motions relative to Somalia. Our noise-mitigated reconstructions tightly constrain the significant speed up of the Capricorn plate over the past 8 Myr and demonstrate that the history of the Indian Ocean floor deformation cannot be explained without this plate-motion change. We propose that the Capricorn plate-motion change is driven by an increase in the eastward-directed asthenospheric flow associated with the adjacent Reunion plume, and quantitatively demonstrate the viability of this hypothesis. Our inference is supported by volcanic age distributions along the Reunion hotspot track, the anomalously high residual bathymetry of the Central Indian Ridge, full-waveform seismic tomography of the underlying asthenosphere and geochemical observations from the Central Indian Ridge. These findings challenge the commonly accepted link between the deformation of the Indian Ocean floor and the Tibetan Plateau's orogenic evolution and demonstrate that temporal variations in upwelling mantle flow can drive major tectonic events at the Earth's surface.

New structural and stratigraphic insights for northwestern Pakistan from field and Landsat Thematic Mapper data

USGS Publications Warehouse

Robinson, J.; Beck, R.; Gnos, E.; Vincent, R.K.

2000-01-01

The remote Waziristan region of northwestern Pakistan includes outcrops of the India-Asia suture zone. The excellent exposure of the Waziristan ophiolite and associated sedimentary lithosomes and their inaccessibility made the use of Landsat Thematic Mapper (TM) data desirable in this study. Landsat TM data were used to create a spectral ratio image of bands 3/4, 5/4, and 7/5, displayed as red, green, and blue, respectively, and a principal component analysis image of bands 4, 5, and 7 (RGB). These images were interpreted in the context of available geologic maps, limited field work, and biostratigraphic, lithostratigraphic, and radiometric data. They were used to create a coherent geologic map of Waziristan and cross section of the area that document five tectonic units in the region and provide a new and more detailed tectonic history for the region. The lowest unit is comprised of Indian shelf sediments that were thrust under the Waziristan ophiolite. The ophiolite has been tectonically shuffled and consists of two separate tectonic units. The top thrust sheet is a nappe comprised of distal Triassic to Lower Cretaceous Neotethyan sediments that were underthrust during the Late Cretaceous by the ophiolite riding on Indian shelf strata. The uppermost unit contains unconformable Tertiary and younger strata. The thrust sheets show that the Waziristan ophiolite was obducted during Late Cretaceous time and imply that the Paleocene and Eocene deformation represents collision of India with the Kabul block and/or Asia.

The Formation of Laurentia: Evidence from Shear Wave Splitting and Seismic Tomography

NASA Astrophysics Data System (ADS)

Liddell, M. V.; Bastow, I. D.; Rawlinson, N.; Darbyshire, F. A.; Gilligan, A.

2017-12-01

The northern Hudson Bay region of Canada comprises several Archean cratonic nuclei, assembled by Paleoproterozoic orogenies including the 1.8 Ga Trans-Hudson Orogen (THO) and Rinkian-Nagssugtoqidian Orogen (NO). Questions remain about how similar in scale and nature these orogens were compared to modern orogens like the Himalayas. Also in question is whether the thick Laurentian cratonic root below Hudson Bay is stratified, with a seismically-fast Archean core underlain by a lower, younger, thermal layer. We investigate these problems via shear-wave splitting and teleseismic tomography using up to 25 years of data from 65 broadband seismic stations across northern Hudson Bay. The results of the complementary studies comprise the most comprehensive study to date of mantle seismic velocity and anisotropy in northern Laurentia. Splitting parameter patterns are used to interpret multiple layers, lithospheric boundaries, dipping anisotropy, and deformation zone limits for the THO and NO. Source-side waveguide effects from Japan and the Aleutian trench are observed despite the tomographic data being exclusively relative arrival time. Mitigating steps to ensure data quality are explained and enforced. In the Hudson Strait, anisotropic fast directions (φ) generally parallel the THO, which appears in tomographic images as a strong low velocity feature relative to the neighbouring Archean cratons. Several islands in northern Hudson Bay show short length-scale changes in φ coincident with strong velocity contrasts. These are interpreted as distinct lithospheric blocks with unique deformational histories, and point to a complex, rather than simple 2-plate, collisional history for the THO. Strong evidence is presented for multiple anisotropic layers beneath Archean zones, consistent with the episodic development model of cratonic keels (e.g., Yuan & Romanowicz 2010). We show via both tomographic inversion models and SKS splitting patterns that southern Baffin Island was underthrust by the Superior plate; slow wavespeed material underlies this region, and modelling of SKS splitting patterns indicates a dipping anisotropic layer. This aligns our most up-to-date geophysical results with recent geological evidence (Weller et al., 2017) that the THO developed with modern plate-tectonic style interactions.

Transform Faults and Lithospheric Structure: Insights from Numerical Models and Shipboard and Geodetic Observations

NASA Astrophysics Data System (ADS)

Takeuchi, Christopher S.

In this dissertation, I study the influence of transform faults on the structure and deformation of the lithosphere, using shipboard and geodetic observations as well as numerical experiments. I use marine topography, gravity, and magnetics to examine the effects of the large age-offset Andrew Bain transform fault on accretionary processes within two adjacent segments of the Southwest Indian Ridge. I infer from morphology, high gravity, and low magnetization that the extremely cold and thick lithosphere associated with the Andrew Bain strongly suppresses melt production and crustal emplacement to the west of the transform fault. These effects are counteracted by enhanced temperature and melt production near the Marion Hotspot, east of the transform fault. I use numerical models to study the development of lithospheric shear zones underneath continental transform faults (e.g. the San Andreas Fault in California), with a particular focus on thermomechanical coupling and shear heating produced by long-term fault slip. I find that these processes may give rise to long-lived localized shear zones, and that such shear zones may in part control the magnitude of stress in the lithosphere. Localized ductile shear participates in both interseismic loading and postseismic relaxation, and predictions of models including shear zones are within observational constraints provided by geodetic and surface heat flow data. I numerically investigate the effects of shear zones on three-dimensional postseismic deformation. I conclude that the presence of a thermally-activated shear zone minimally impacts postseismic deformation, and that thermomechanical coupling alone is unable to generate sufficient localization for postseismic relaxation within a ductile shear zone to kinematically resemble that by aseismic fault creep (afterslip). I find that the current record geodetic observations of postseismic deformation do not provide robust discriminating power between candidate linear and power-law rheologies for the sub-Mojave Desert mantle, but longer observations may potentially allow such discrimination.

Re-appraisal of the Magma-rich versus Magma-poor Paradigm at Rifted Margins: consequences for breakup processes

NASA Astrophysics Data System (ADS)

Tugend, J.; Gillard, M.; Manatschal, G.; Nirrengarten, M.; Harkin, C. J.; Epin, M. E.; Sauter, D.; Autin, J.; Kusznir, N. J.; McDermott, K.

2017-12-01

Rifted margins are often classified based on their magmatic budget only. Magma-rich margins are commonly considered to have excess decompression melting at lithospheric breakup compared with steady state seafloor spreading while magma-poor margins have suppressed melting. New observations derived from high quality geophysical data sets and drill-hole data have revealed the diversity of rifted margin architecture and variable distribution of magmatism. Recent studies suggest, however, that rifted margins have more complex and polyphase tectono-magmatic evolutions than previously assumed and cannot be characterized based on the observed volume of magma alone. We compare the magmatic budget related to lithospheric breakup along two high-resolution long-offset deep reflection seismic profiles across the SE-Indian (magma-poor) and Uruguayan (magma-rich) rifted margins. Resolving the volume of magmatic additions is difficult. Interpretations are non-unique and several of them appear plausible for each case involving variable magmatic volumes and mechanisms to achieve lithospheric breakup. A supposedly 'magma-poor' rifted margin (SE-India) may show a 'magma-rich' lithospheric breakup whereas a 'magma-rich' rifted margin (Uruguay) does not necessarily show excess magmatism at lithospheric breakup compared with steady-state seafloor spreading. This questions the paradigm that rifted margins can be subdivided in either magma-poor or magma-rich margins. The Uruguayan and other magma-rich rifted margins appear characterized by an early onset of decompression melting relative to crustal breakup. For the converse, where the onset of decompression melting is late compared with the timing of crustal breakup, mantle exhumation can occur (e.g. SE-India). Our work highlights the difficulty in determining a magmatic budget at rifted margins based on seismic reflection data alone, showing the limitations of margin classification based solely on magmatic volumes. The timing of decompression melting onset and melting rates (magmatic processes) relative to crustal thinning (tectonic processes) appear equally, if not more important, than the magmatic budget for unravelling the evolution of rifted margins.

Miocene burial and exhumation of the India-Asia collision zone in southern Tibet: response to slab dynamics and erosion

USGS Publications Warehouse

Carrapa, Barbara; Orme, D.A.; DeCelles, Peter G.; Kapp, Paul; Cosca, Michael A.; Waldrip, R.

2014-01-01

The India-Asia collision zone in southern Tibet preserves a record of geodynamic and erosional processes following intercontinental collision. Apatite fission-track and zircon and apatite (U-Th)/He data from the Oligocene–Miocene Kailas Formation, within the India-Asia collision zone, show a synchronous cooling signal at 17 ± 1 Ma, which is younger than the ca. 26–21 Ma depositional age of the Kailas Formation, constrained by U-Pb and 40Ar/39Ar geochronology, and requires heating (burial) after ca. 21 Ma and subsequent rapid exhumation. Data from the Gangdese batholith underlying the Kailas Formation also indicate Miocene exhumation. The thermal history of the Kailas Formation is consistent with rapid subsidence during a short-lived phase of early Miocene extension followed by uplift and exhumation driven by rollback and northward underthrusting of the Indian plate, respectively. Significant removal of material from the India-Asia collision zone was likely facilitated by efficient incision of the paleo–Indus River and paleo–Yarlung River in response to drainage reorganization and/or intensification of the Asian monsoon.

Group velocity dispersion characteristics and one-dimensional regional shear velocity structure of the eastern Indian craton

NASA Astrophysics Data System (ADS)

Mandal, Prantik

2017-02-01

In the past three years, a semi-permanent network of fifteen 3-component broadband seismographs has become operational in the eastern Indian shield region occupying the Archean (∼2.5-3.6 Ga) Singhbhum-Odisha craton (SOC) and the Proterozoic (∼1.0-2.5 Ga) Chotanagpur Granitic Gneissic terrane (CGGT). The reliable and accurate broadband data for the recent 2015 Nepal earthquake sequence from 10 broadband stations of this network enabled us to estimate the group velocity dispersion characteristics and one-dimensional regional shear velocity structure of the region. First, we measure fundamental mode Rayleigh- and Love-wave group velocity dispersion curves in the period range of 7-70 s and then invert these curves to estimate the crustal and upper mantle structure below the eastern Indian craton (EIC). We observe that group velocities of Rayleigh and Love waves in SOC are relatively high in comparison to those of CGGT. This could be attributed to a relatively mafic-rich crust-mantle structure in SOC resulting from two episodes of magmatism associated with the 1.6 Ga Dalma and ∼117 Ma Rajmahal volcanisms. The best model for the EIC from the present study is found to be a two-layered crust, with a 14-km thick upper-crust (UC) of average shear velocity (Vs) of 3.0 km/s and a 26-km thick lower-crust (LC) of average Vs of 3.6 km/s. The present study detects a sharp drop in Vs (∼-2 to 3%) at 120-260 km depths, underlying the EIC, representing the probable seismic lithosphere-asthenosphere boundary (LAB) at 120 km depth. Such sharp fall in Vs below the LAB indicates a partially molten layer. Further, a geothermal gradient extrapolated from the surface heat flow shows that such a gradient would intercept the wet basalt solidus at 88-103 km depths, suggesting a 88-103 km thick thermal lithosphere below the EIC. This could also signal the presence of small amounts of partial melts. Thus, this 2-3% drop in Vs could be attributed to the presence of partial melts in the upper mantle related to the earlier volcanic episodes viz. back-arc volcanism associated with the Archean/Proterozoic subduction, 1.6 Ga Dalma volcanism, and ∼117 Ma Rajmahal volcanism. The main result of our modeling provides evidences for the absence of Keel or thick lithosphere below the EIC.

The Under-side of the Andes: Using Receiver Functions to Map the North Central Andean Subsurface

NASA Astrophysics Data System (ADS)

Ryan, J. C.; Beck, S. L.; Zandt, G.; Wagner, L. S.; Minaya, E.; Tavera, H.

2012-12-01

The Central Andean Uplift and Geodynamics of High Topography (CAUGHT) project is an interdisciplinary project to investigate connections between lithospheric removal, crustal shortening and surface uplift in the northern Bolivia and southern Peru region of the South American Andean orogen. The central Andes are defined by six major tectonomorphic provinces; the forearc, the volcanically active Western Cordillera (WC, ~6 km elevation), the internally drained Altiplano (~4 km elevation), an inactive fold and thrust belt in the Eastern Cordillera (EC, ~6 km elevation), a lower elevation active fold and thrust belt in the Subandean (SA) zone and the Beni, a foreland basin. Forty seismic stations installed for the CAUGHT project were deployed between 13° and 18° S latitude, covering the transition zone where the Altiplano region pinches out in southern Peru, in an effort to better constrain the changing character of the crust and mantle lithosphere. Geologic studies across the northern Bolivian portion of the eastern Andean margin (15-17° S) have documented a total of 275 km of upper crustal shortening (McQuarrie et al, Tectonics, v27, 2008), which may be associated with crustal thickening and/or the removal of lithospheric material as a thickened lithosphere root becomes unstable. For this receiver function (converted wave) study, we have little coverage in the forearc and foreland, ~75 km spacing in most of the array, and a relatively dense ~20 km spaced profile along the Charaña-La Paz-Yucumo transect, the eastern portion of which is nearly coincident with the balanced cross-section of McQuarrie et al. (2008). Using the first year of available data, more than 1200 receiver functions have been calculated using an iterative deconvolution method, and stacked using the common conversion point (CCP) method, along profiles parallel to and nearly coincident to those used for the geologic shortening estimates. We identified arrivals for the Moho and generated a 3D map of crustal thickness underneath the array that reveals distinct characteristics for the 4 major tectonomorphic provinces. The SA crust thickens westward from 40 km adjacent to the foreland to 50 km adjacent to the EC. The crust beneath the EC varies between 50-65 km, generally thickening westward. The crustal thickness beneath the Altiplano is variable along strike between 60-70 km, with a rapid change within the southern part of the eastern Altiplano with a northerly strike crossing into the EC east of Lake Titicaca. The crust under the WC is equally variable between 60-70 km, with numerous intracrustal interfaces. Comparisons with results from teleseismic tomography (Scire et al., this mtg.) and ambient noise tomography (Ward et al., this mtg.) show that this rapid Moho change is associated with changes in lower crustal and upper mantle velocities, suggesting it is an important deep-seated structure. We consider two possible interpretations of this structure; underthrusting of the Brazilian cratonic crust or a delamination structure. With a second year of data and enhanced processing we expect to improve the resolution of our receiver function study.

Lithospheric Structure of Central Asia

NASA Astrophysics Data System (ADS)

Martin, R. J.; Gulen, L.; Sun, Y.; Toksoz, M. N.

2009-12-01

The active tectonics of Central Asia is the result of ongoing, active continental collision between the Indian and the Eurasian plates. This geologically and tectonically complex area is also one of the most seismically active regions in the world. Previous studies in this region mostly depended on teleseismic data and the local and regional data from the stations located in China. In this study we used the local travel time data from Kazakhstan, Kyrgyzstan, Tajikistan, and Uzbekistan to study the lithospheric structure in this region. We selected the events and stations between 32°N-65°E and 45°N-85°E and focused on the areas of Pamir and Tien Shan. In this data set, there are more than 3000 P and S arrivals received at 68 stations from about 220 events. The double difference tomography is applied to relocate events and to invert for seismic structure simultaneously. Our results provide accurate locations of earthquakes and high resolution lithospheric structure in this region. We use both; the local travel times, and also the regional travel times to invert the 3D lithospheric velocity structure in this region. More than 2200 P wave phase picks were used in the inversion. The average grid spacing is 100km and the inverted grids lay on six layers. Then we use the Double Difference Tomography method developed by Zhang and Thurber (2003, 2006) to invert the 3-D P-wave velocity structure. Our tomographic results show highly heterogeneous crust and upper mantle structure for Central Asia. Some crustal low velocity zones are found beneath Tien Shan, the northern Pamir, and the Tajik depression, while high velocity anomalies are found beneath the Kazakh shield, the southern Pamir, and the Tarim basin.

Geochemical and Isotopic Variations Along the Southeast Indian Ridge (126°-140°E) Related to Mantle Flow Originating from Beneath Antarctica

NASA Astrophysics Data System (ADS)

Hanan, B. B.; Graham, D. W.; Hemond, C.; Dufour, F.; Briais, A.; Ceuleneer, G.; Maia, M.; Park, S. H.; Revillon, S.; Yang, Y. S.

2017-12-01

We present data for glassy basalts from 37 localities along the spreading axis of the Southeast Indian Ridge (SEIR) between 126°-140°E, eastward of the Australian-Antarctic Discordance (AAD). Each of the five ridge segments (A1 to A5, west to east) show well-defined major element trends. An isotopic and negative axial depth anomaly is present, centered on the overlapping tips of segments A3 and A4 at 135°E. Segment A4 basalts have distinct radiogenic Pb and He isotopes plus enriched MORB-like ɛHf, relative to segments to the west and east. Crystal fractionation is more extensive at the A3 and A5 overlapping segment tips adjacent to A4, and decreases both to the west and east. The along axis pattern suggests a mantle heterogeneity located beneath the A3-A4 segments. Pb-Pb isotopic co-variations for the 5 segments define two linear arrays, with a western trend (A1-A3) and an eastern trend (A4-A5) that intersects it at the composition of the anomalous A4 segment, at a 206Pb/204Pb 19. The western trend has higher 208Pb/204Pb for a given 206Pb/204Pb, revealing a gradient in the asthenosphere, with Δ208Pb/204Pb decreasing to the east away from the AAD. Overall, 206,207,208Pb/204Pb and 4He/3He of the A4 anomaly define trends that vector toward the fields for Cenozoic lavas from west Antarctica (Marie Byrd Land and Balleny Islands). West Antarctica has a history of mantle plume underplating and lithosphere modification by subduction [1,2], and there is a broad seismic anomaly below 250 km underlying the West Antarctic Rift system [3]. Our data supports a model in which flow of underplated material plus lithosphere may be guided by the underside topography of the lithosphere beneath the Transantarctic mountains. This flow emerges from beneath east Antarctica, where it leads to volcanism in the Balleny Islands [4]. The material apparently continues to flow northward to the SEIR at 135°E. The geochemical anomaly beneath Zone A is potentially explained by the presence of this residual plume/mobilized lithospheric material in the subridge mantle of the SEIR. [1] Hart et al., 1997, Chem Geol 139; [2] Aviado et al. 2015, G3 16; [3] Moelli and Danesi, 2004, GPC 42; [4] Sleep, 2006, ESR 77.

Mean age of oceanic lithosphere drives eustatic sea-level change since Pangea breakup

NASA Astrophysics Data System (ADS)

Cogné, Jean-Pascal; Humler, Eric; Courtillot, Vincent

2006-05-01

The Atlantic and Indian Oceans and the oceanic part of the Antarctic plate have formed at the expense of Panthalassa as a result of Pangea breakup over the last 180 Myr. This major plate reorganization has changed the age vs. surface distribution of oceanic lithosphere and has been a likely driver of sea-level change. Assuming that the age/surface structure of Panthalassa has remained similar to the present-day global distribution from 180 Ma to Present, and using the isochron patterns preserved in the newly formed oceans, we model resulting relative sea-level change. We find a first (slower) phase of sea-level rise (by 90 to 110 m), culminating between 120 and 50 Ma, followed by a (faster) phase of sea-level drop. We show that this result is not strongly sensitive to our hypothesis of constant mean age of Panthalassa, for which much of the information is now erased due to subduction. When the effects of oceanic plateau formation and ice cap development are added, the predicted sea-level curve fits remarkably well the first-order variations of observed sea-level change. We conclude that the changes in mean age of the oceanic lithosphere (varying between 56 and 62 ± 0.2 Myr), which are simply the expression of the Wilson cycle following Pangea breakup, are the main control, accounting for ˜ 70%, of first-order changes in sea-level.

Looking at the roots of the highest mountains: the lithospheric structure of the Himalaya-Tibet and the Zagros orogens. Results from a geophysical-petrological study

NASA Astrophysics Data System (ADS)

Tunini, L.; Jimenez-Munt, I.; Fernandez, M.; Villasenor, A.; Afonso, J. C.; Verges, J.

2013-12-01

The Himalaya-Tibet and Zagros orogens are the two most prominent mountain belts built by continental collision. They are part of a huge belt of Cenozoic age which runs from the Pyrenees to Burma. In its central sector, the collision with the southern margin of the Eurasian plate has resulted not only in the building of mountain ranges over the north-eastern edges of the Arabian and Indian plates but also in widespread deformation 1000-3000 km from the suture zones. Zagros and Himalaya-Tibet orogens share many geodynamic processes but at different rates, amount of convergence and stage of development. The study of their present-day structures provides new insights into their quasi coeval collisional event pointing out differences and similarities in the mountain building processes. We present 2D crust and upper mantle cross-sections down to 400 km depth, along four SW-NE trending profiles. Two profiles cross the Zagros Mountains, running from the Mesopotamian Foreland Basin up to the Alborz and Central Iran. Two other profiles run through the Himalaya-Tibetan orogen: the western transect crosses the western Himalaya, Tarim Basin, Tian Shan Mountains and Junggar Basin; the eastern transect runs from the Indian shield to the Beishan Basin, crossing the eastern Himalaya, Tibetan Plateau, Qaidam Basin and Qilian Mountains. We apply the LitMod-2D code which integrates potential fields (gravity and geoid), isostasy (elevation) and thermal (heat flow and temperature distribution) equations, and mantle petrology. The resulting crust and upper mantle structure is constrained by available data on elevation, Bouguer anomaly, geoid height, surface heat flow and seismic data including P- and S-wave tomography models. Our results show distinct deformation patterns between the crust and the lithospheric mantle beneath the Zagros and Himalaya-Tibetan orogens, indicating a strong strain partitioning in both areas. At crustal level, we found a thickening beneath the Zagros and the Alborz ranges, more pronounced in the southern profile. At sub-crustal level, a lithospheric mantle thinning affects the whole area beneath the Zagros range extending to the north through the zone below the Alborz and the central Iran. In the Himalaya-Tibet region our results show stronger strain partitioning in the horizontal (east-west) direction than in the vertical (depth) direction. At crustal level, the Tibetan Plateau extends more than 1000 km in the eastern profile, whereas it is squeezed between the Himalayan Mountains and the Tarim Basin along the western profile (~600 km). At sub-crustal level, the lithospheric mantle is more homogeneous in thickness and mineral composition along the western profile than the eastern one. Finally, our results on mineral composition show that both collisional regions are characterised by a predominant lherzolitic lithospheric mantle, whereas we observe compositional variations around the suture zones, probably related to subduction and mantle delamination processes.

Ar-Ar and Rb-Sr dating of very low-/low-grade metamorphism along the main Iapetus suture, Newfoundland

NASA Astrophysics Data System (ADS)

Willner, Arne P.; Glodny, Johannes; Massonne, Hans-Joachim; Romer, Rolf L.; Sudo, Masafumi; Van Staal, Cees R.; Zagorevski, Alexandre

2013-04-01

The Late Ordovician closure of the main tract of Iapetus resulted in juxtaposition of the leading edge of the peri-Gondwanan microplate Ganderia and the composite Laurentian margin. The suture is the Red Indian Line, which separates the Iapetan realm into peri-Gondwanan and peri-Laurentian arc-backarc complexes. The discrete Red Indian Line forms part of a wider collision zone that has a protracted and complicated tectonic history starting with underplating of oceanic terranes beneath the composite margin of Laurentia at ca. 471 Ma during the early stages of the Taconic orogeny. Final collision along the Red Indian Line and closure of the Iapetus ocean occurred at 455 Ma with the underthrusting of the peri-Gondwanan Victoria arc and its Ganderian basement beneath the composite Laurentian margin. The accreted Iapetan realm terranes were progressively deformed during the closure of a remaining Iapetan marginal basin, resulting locally in significant overprint and reactivation during the Silurian (Salinic orogeny). Metamorphic overprint in the deformed Laurentia margin (Notre Dame Zone) above the Red Indian Line is mainly of very low grade to low grade and very heterogeneous. PT-conditions cluster at 3-5 kbar, 300-400°C and 6-7 kbar, 270-330°C. Medium grade conditions are related to local contact metamorphism. Ages of the local peak metamorphism in the peri-Laurentian Iapetan realm were determined by dating white mica with the Ar-Ar system and white mica-bearing assemblages with the Rb-Sr mineral isochron method. Both methods yielded ages that postdate the closure of the main tract of Iapetus. These generally belong to two age ranges: 418-430 Ma (Salinic events) and 350-390 Ma Neoacadian events). Partly two overprints can be detected in one and the same sample. Metamorphism is related to (1) reactivation of deformation in shear zones which partly cause further crustal thickening or strike slip-related deformation, (2) to external fluid influx, (3) to advective heating by synkinematic intrusions or (4) to a combination of these effects. Salinic to Neoacadian postcollisional processes in the collision zone apparently are much more widespread than formerly recognized.

Non-linear 3-D Born shear waveform tomography in Southeast Asia

NASA Astrophysics Data System (ADS)

Panning, Mark P.; Cao, Aimin; Kim, Ahyi; Romanowicz, Barbara A.

2012-07-01

Southeast (SE) Asia is a tectonically complex region surrounded by many active source regions, thus an ideal test bed for developments in seismic tomography. Much recent development in tomography has been based on 3-D sensitivity kernels based on the first-order Born approximation, but there are potential problems with this approach when applied to waveform data. In this study, we develop a radially anisotropic model of SE Asia using long-period multimode waveforms. We use a theoretical 'cascade' approach, starting with a large-scale Eurasian model developed using 2-D Non-linear Asymptotic Coupling Theory (NACT) sensitivity kernels, and then using a modified Born approximation (nBorn), shown to be more accurate at modelling waveforms, to invert a subset of the data for structure in a subregion (longitude 75°-150° and latitude 0°-45°). In this subregion, the model is parametrized at a spherical spline level 6 (˜200 km). The data set is also inverted using NACT and purely linear 3-D Born kernels. All three final models fit the data well, with just under 80 per cent variance reduction as calculated using the corresponding theory, but the nBorn model shows more detailed structure than the NACT model throughout and has much better resolution at depths greater than 250 km. Based on variance analysis, the purely linear Born kernels do not provide as good a fit to the data due to deviations from linearity for the waveform data set used in this modelling. The nBorn isotropic model shows a stronger fast velocity anomaly beneath the Tibetan Plateau in the depth range of 150-250 km, which disappears at greater depth, consistent with other studies. It also indicates moderate thinning of the high-velocity plate in the middle of Tibet, consistent with a model where Tibet is underplated by Indian lithosphere from the south and Eurasian lithosphere from the north, in contrast to a model with continuous underplating by Indian lithosphere across the entire plateau. The nBorn anisotropic model detects negative ξ anomalies suggestive of vertical deformation associated with subducted slabs and convergent zones at the Himalayan front and Tien Shan at depths near 150 km.

New insights on the subsidence of the Ganges-Brahmaputra Delta Plain by using 2D multichannel seismic data, gravity and flexural modeling, BanglaPIRE Project

NASA Astrophysics Data System (ADS)

Grall, C.; Pickering, J.; Steckler, M. S.; Spiess, V.; Seeber, L.; Paola, C.; Goodbred, S. L., Jr.; Palamenghi, L.; Schwenk, T.

2015-12-01

Deltas can subside very fast, yet many deltas remain emergent over geologic time. A large sediment input is often enough to compensate for subsidence and rising sea level to keep many deltas at sea level. This implies a balance between subsidence and sedimentation, both of which may, however, be controlled by independent factors such as sediment supply, tectonic loads and sea-level change. We here examine the subsidence of the Ganges-Brahmaputra Delta (GBD). Located in the NE boundary of the Indian-Eurasian collision zone, the GBD is surrounded by active uplifts (Indo-Burma Fold Belt and the Shillong Massif). The pattern of subsidence from these tectonic loads can strongly vary depending on both loads and lithospheric flexural rigidity, both of which can vary in space and time. Sediment cover changes both the lithostatic pressure and the thermal properties and thus the rigidity of the lithosphere. While sediments are deposited cold, they also insulate the lithosphere, acting as a thermal blanket to increase lower crustal temperatures. These effects are a function of sedimentation rates and may be more important where the lithosphere is thin. At the massive GBD the impact of sedimentation should be considered for properly constraining flexural subsidence. The flexural rigidity of the lithosphere is here modeled by using a yield-stress envelope based on a thermomechanic model that includes geothermal changes associated with sedimentation. Models are constrained by using two different data sets, multichannel seismic data correlated to borehole stratigraphy, and gravity data. This approach allows us to determine the Holocene regional distribution of subsidence from the Hinge Zone to the Bengal Fan and the mass-anomalies associated with the flexural loading. Different end-member scenarios are explored for reproducing the observed land tilting and gravity anomalies. For all scenarios considered, data can be reproduced only if we consider an extremely weak lithosphere and we will quantify the extent that this weakness is influenced by the extreme sediment thickness of the delta. While the distribution of the present-day subsidence suggests that sediment compaction plays a major role on the current subsidence over the delta, its role over a geological time frame is probably minor.

The Indian Ocean gravity low - Evidence for an isostatically uncompensated depression in the upper mantle

NASA Technical Reports Server (NTRS)

Ihnen, S. M.; Whitcomb, J. H.

1983-01-01

The broad gravity low in the equatorial Indian Ocean south of Sri Lanka is the largest and most striking feature in the gravitational field of the earth. The most negative long-wavelength free-air gravity anomalies are found there and the sea surface (geoid) lies more than 100 meters below the best fitting ellipsoid. A model of the lithosphere and upper mantle is proposed which accurately predicts the observed free-air gravity and geoid elevation. This model is consistent with bathymetry and sediment thickness data and suggests that the crust south of India currently floats as much as 600 meters lower than would be expected if the region were isostatically compensated. This residual depression of the crust is apparently confirmed by observations of ocean depth. An uncompensated depression is consistent with the presence of a mechanical wake left in the upper mantle behind India as it traveled toward Asia.

Mechanism of underthrusting in southwest Japan: A model of convergent plate interactions

NASA Technical Reports Server (NTRS)

Fitch, T. J.; Scholz, C. H.

1971-01-01

An elastic rebound mechanism consistent with underthrusting at the time of the magnitude 8.2 Nankaido earthquake of 21 December, 1946 accounts for a reversal in sense between seismic and preseismic changes in elevation throughout a large portion of southwest Japan. It is shown that the seismic movements during the earthquake were generated by slip on a complex thrust fault that is inferred to intersect the surface near the base of the continental slope. The preseismic deformation is explained by strain accumulation equivalent to a virtual dislocation with the same orientation as the fault surface but a sense opposite to that of the real dislocation that occurred at the time of the earthquake. Adjustments by slip along the fault surface and extensions of that surface can account for postseismic movements.

Thermal contraints on high-pressure granulite metamorphism of supracrustal rocks

NASA Technical Reports Server (NTRS)

Ashwal, L. D.; Morgan, P.; Leslie, W. W.

1983-01-01

The circumstances leading to the formation and exposure at the Earth's surface of supracrustal granulites are examined. These are defined as sediments, volcanics, and other rock units which originally formed at the surface of the Earth, were metamorphosed to high-pressure granulite facies (T = 700-900 C, P = 5-10 kbar), and reexposed at the Earth's surface, in many cases underlain by normal thicknesses of continental crust (30-40 km). Five possible heating mechanisms to account for granulite metamorphism of supracrustal rocks are discussed: magnetic heating, thermal relaxation of perturbed temperature profiles following underthrusting of the continental crust, thermal relaxation after underthrusting of thin slivers of supracrustal rocks below continental crust of normal thickness, major preheating of the upper plate, and shear heating caused by frictional stress along the thrust plane.

Scaly fabrics and veins of tectonic mélanges in the Shimanto Belt, SW Japan

NASA Astrophysics Data System (ADS)

Ramirez, G. E.; Fisher, D. M.; Smye, A.; Hashimoto, Y.; Yamaguchi, A.

2017-12-01

Mélanges in ancient subduction fault zones provide a microstructural record of the plate boundary deformation associated with underthrusting. These rocks exhibit many of the characteristics associated with exposed ancient subduction fault zones worldwide, including: 1) σ1 is near orthogonal to the deformation fabric, 2) microstructurally pervasive quartz and calcite filled veins concentrated in coarser blocks and along extensional jogs on slip surfaces, 3) evidence for local diffusion of silica sourced from web-like arrays of slip surfaces (i.e., scaly fabrics), and 4) repeated cracking and sealing that record cyclic variations in stress. We present XRD, XRF, and EPMA observations of scaly fabrics from five ancient subduction-related shear zones (Yokonami, Mugi, Kure, Okitsu, and Makimine mélanges) from the Shimanto Belt in Japan that exemplify these characteristics and represent the full temperature range of the seismogenic zone ( 150-340 °C). The scaly fabrics associated with these shear zones display significantly different microstructural and geochemical characteristics. Individual slip surfaces in the scaly fabrics of Mugi mélange, underplated at the updip limit of the seismogenic zone, are characterized by broader (50-300 µm) anastomosing shear zones while the Makimine mélange, underplated at the downdip limit of the seismogenic zone, exhibits thinner (10-20 µm) anastomosing shear zones. XRD analyses also imply geochemical differences such as a decrease in albite concentration and an increase in illite concentration with increasing temperature/depth of underthrusting. Scaly fabrics are sites of silica redistribution in which silica is depleted on the slip surfaces and precipitated as mostly quartz in crack-seal veins. The time to seal, or heal, fractures is mainly temperature-dependent but can also be significantly quickened by fluid salinity, degree of fluid-rock interactions, and geochemical reactions (i.e. incongruent pressure solution). Microstructural and geochemical characteristics that show differences with temperature/depth of underthrusting highlight the importance of establishing the geochemical processes and activation energies that contribute to slip, fracturing, and healing of rocks that underthrust the subduction interface.

Evidence for Patchy Sediment Underthrusting and a Strong, Drained Outer Accretionary Wedge in Central Cascadia: Implications for Dynamic Slip Conditions

NASA Astrophysics Data System (ADS)

Tobin, H. J.; Webb, S. I.

2017-12-01

The central Cascadia subduction zone forearc in the region offshore Washington, where a hot, young incoming plate is covered by a 2-3 km thick sedimentary sequence, features a wide, very narrowly-tapered outer accretionary wedge composed of landward vergent thrust sheets. Longstanding questions for this region include the position and host-rock environment of the plate boundary décollement fault, the thickness of sedimentary strata underthrust beneath the wedge with the downgoing plate, and the effective stress or pore fluid pressure condition in the wedge and along its base. We have analyzed nine multichannel seismic lines of the 2012 COAST multi-channel seismic reflection survey using both time- and depth- migrated seismic sections for structural interpretation. Results show that there is evidence for two parallel décollement levels, with up to 200 - 500 meters thickness of a mostly-underthrust sequence in places, but which is absent entirely in others. This patchy distribution is mapped and related to features of the overlying wedge structure. We also analyzed the seismic interval velocity distribution produced during pre-stack depth migration imaging, and used it to compute estimated porosity, pore fluid pressure, and effective stress via empirical physical properties transforms. We find that the wedge shows evidence for at most only modest, localized excess pore pressure, and instead most of the wedge appears to be at near-hydrostatic, drained condition. Modest overpressure ratios of up to only 0.15 are detected, localized in the footwalls of thrust splays. We find no evidence for overpressure zones in the underthrust sequence below the upper décollement, in contrast to findings from several other wedges worldwide. Taken together, the accretionary wedge structure and apparent low pore pressure condition here is consistent with a mechanically strong wedge overlying a base that is very weak, at least transiently. By analogy with recent work from Sumatra, Tohoku, and elsewhere, we speculate that this is potentially conducive to efficient propagation of megathrust slip to the deformation front in large earthquakes.

Deformation Processes In SE Tibet: How Coupled Are The Surface And The Deeper Lithosphere? (Invited)

NASA Astrophysics Data System (ADS)

Zeitler, P. K.; Meltzer, A.

2010-12-01

We all like to cite the Himalayan collision as a type example of continent-continent collision, and the region has been used as a natural laboratory by a considerable number of diverse investigations. Southeastern Tibet and the Lhasa Block provide an interesting case to consider in this context. Surrounding portions of the Himalayan-Tibet system have been and are being intensely deformed, whereas the Andean-arc lithosphere of the Lhasa Block has remained enigmatically unscathed. High elevations throughout much of the terrane are fairly uniform but the eastern and western portions of block have experienced very different degrees of exhumation. Regions that experienced more exhumation have thinner crustal thicknesses, with the results that that Moho is warped up with respect to the surface. Thicker, less-exhumed portions of the Lhasa Block currently are underlain by what is inferred to be eclogitized lower crust, but this eclogitization is not seen where exhumation is significant. Beneath SE Tibet, subduction of the Indian lithosphere has been complicated, with tomographic imaging showing variations in mantle structure that do not register with the strike of surface features. Adjacent to the Lhasa Block, the Namche Barwa-Gyala metamorphic massif demonstrates a strong coupling between shallower crustal flow and localized erosion that is significant for the evolution of the Lhasa Block in the way that this feature controls base level for the upper Tsangpo drainage and thus the erosional driver for the system. More broadly, a weak lower crust and lower-crustal flow have been invoked by many workers to explain aspects of the region’s deformation patterns and topography. Thus it would seem that in SE Tibet, mid-to-upper crustal, lower-crustal, and whole-lithosphere processes all have the potential to either impact Earth-surface dynamics or be impacted by them. This leads to a number of questions about the 4D nature and scale of compensation, controls on the evolution of topography, and the degree to which feedbacks might exist between the surface and the deeper lithosphere.

Three-dimensional imaging of the S-velocity structure for the crust and the upper mantle beneath the Arabian Sea from Rayleigh wave analysis

NASA Astrophysics Data System (ADS)

Corchete, V.

2017-04-01

A 3D imaging of S-velocity for the Arabian Sea crust and upper mantle structure is presented in this paper, determined by means of Rayleigh wave analysis, for depths ranging from zero to 300 km. The crust and upper mantle structure of this region of the earth never has been the subject of a surface wave tomography survey. The Moho map performed in the present study is a new result, in which a crustal thickening beneath the Arabian Fan sediments can be observed. This crustal thickening can be interpreted as a quasi-continental oceanic transitional structure. A crustal thickness of up to 20 km also can be observed for the Murray Ridge system in this Moho map. This crustal thickening can be due to that the Murray Ridge System consists of Indian continental crust. This continental crust is extremely thinned to the southwest of this region, as shown in this Moho map. This area can be interpreted as oceanic in origin. In the depth range from 30 to 60 km, the S-velocity presents its lower values at the Carlsberg Ridge region, because it is the younger region of the study area. In the depth range from 60 to 105 km of depth, the S-velocity pattern is very similar to that shown for the previous depth range, except for the regions in which the asthenosphere is reached, for these regions appear a low S-velocity pattern. The lithosphere-asthenosphere boundary (LAB), or equivalently the lithosphere thickness, determined in the present study is also a new result, in which the lithosphere thickness for the Arabian Fan can be estimated in 60-70 km. The lower lithospheric thickness observed in the LAB map, for the Arabian Fan, shows that this region may be in the transition zone between continental and oceanic structure. Finally, a low-velocity zone (LVZ) has been determined, for the whole study area, located between the LAB and the boundary of the asthenosphere base (or equivalently the lithosphere-asthenosphere system thickness). The asthenosphere-base map calculated in the present study is also a new result.

Oceanic broad multifault transform plate boundaries

NASA Astrophysics Data System (ADS)

Ligi, Marco; Bonatti, Enrico; Gasperini, Luca; Poliakov, Alexei N. B.

2002-01-01

Oceanic transform plate boundaries consist of a single, narrow (a few kilometers wide) strike-slip seismic zone offsetting two mid-ocean ridge segments. However, we define here a new class of oceanic transform boundaries, with broad complex multifault zones of deformation, similar to some continental strike-slip systems. Examples are the 750-km- long, 120-km-wide Andrew Bain transform on the Southwest Indian Ridge, and the Romanche transform, where the Mid-Atlantic Ridge is offset by a lens-shaped, ˜900-km- long, ˜100-km-wide sliver of deformed lithosphere bound by two major transform valleys. One of the valleys is seismically highly active and constitutes the present-day principal transform boundary. However, strike-slip seismic events also occur in the second valley and elsewhere in the deformed zone. Some of these events may be triggered by earthquakes from the principal boundary. Numerical modeling predicts the development of wide multiple transform boundaries when the age offset is above a threshold value of ˜30 m.y., i.e., in extra-long (>500 km) slow-slip transforms. Multiple boundaries develop so that strike-slip ruptures avoid very thick and strong lithosphere.

Shear fabrics reveal orogen-parallel deformations, NW Lesser Garhwal Himalaya, Uttarakhand, India

NASA Astrophysics Data System (ADS)

Biswas, T.; Bose, N.; Mukherjee, S.

2017-12-01

Shear deformation along the Himalayan belt is poorly understood unlike that across the orogen. Field observations and structural analysis along Bhagirathi river section along the National Highway 34 reveals NW Lesser Himalaya (Garhwal region, India) suffered both compression and extension parallel to the orogenic belt and thus forms a unique venue of great structural and tectonic interest. Meso-scale ductile- and brittle shear fabrics, such as S-C, C-P, Y-P, Y-S; are emphasized describing such deformations. Extensional shear fabric strikes N43oE and compressional shear fabrics N39.5oE, which are at a low-angle with the orogenic trend. Our study reviews orogen parallel deformation, both extension as well as compression, taking examples from other part of the world (e.g., Central Andes, N Apennines and SW Alps) and from other terrains in the Himalaya. Proposed models are evaluated and compared with the study area. The results shows that the pre-existing remnant structures (e.g., the Delhi-Haridwar ridge) on the under-thrusting Indian shield/plate plays a vital role in modifying thin-skinned tectonics along with migration of the eastward extrusion of the Tibetian plateau (hinterland deformation) into the Himalayan foreland.

Why does continental convergence stop

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hynes, A.

1985-01-01

Convergence between India and Asia slowed at 45 Ma when they collided, but continues today. This requires that substantial proportions of the Indian and/or Asian lithospheric mantle are still being subducted. The resulting slab-pull is probably comparable with that from complete lithospheric slabs and may promote continued continental convergence even after collision. Since descending lithospheric slabs are present at all collision zones at the time of collision such continued convergence may be general after continental collisions. It may cease only when there is a major (global) plate reorganization which results in new forces on the convergent continents that may counteractmore » the slab-pull. These inferences may be tested on the late Paleozoic collision between Gondwanaland and Laurasia. This is generally considered to have been complete by mid-Permian time (250 Ma). However, this may be only the time of docking of Gondwanaland with North America, not that of the cessation of convergence. Paleomagnetic polar-wander paths for the Gondwanide continents exhibit consistently greater latitudinal shifts from 250 Ma to 200 Ma than those of Laurasia when corrected for post-Triassic drift, suggesting that convergence continued through late Permian well into the Triassic. It may have been accommodated by crustal thickening under what is now the US Coastal Plain, or by strike-slip faulting. Convergence may have ceased only when Pangea began to fragment again, in which case the cause for its cessation may be related to the cause of continental fragmentation.« less

Deep Seismic Reflection Images across a Major Reactivated Fracture Zone in the Wharton Basin: Implications for the Location of the Plate Boundary between India and Australia

NASA Astrophysics Data System (ADS)

Carton, H. D.; Singh, S. C.; Hananto, N. D.; Martin, J.; Djajadihardja, Y. S.; Udrekh, U.; Franke, D.; Gaedicke, C.

2012-12-01

The equatorial Indian Ocean has long been recognized to be hosting extensive "intra-plate" deformation. To west of the Ninety-East Ridge (NER), The Central Indian Ocean Basin is characterized by N-S compression in a broad region with E-W trending folds and high-angle reverse faulting. To the east of NER in the Wharton Basin, deformation mainly occurs along reactivated N5°E-trending oceanic fracture zones with left-lateral strike-slip motion. Near longitude 93°E in the Wharton Basin runs a major reactivated fracture zone, along which the epicenters of the two recent Mw=8.6 and Mw=8.2 strike-slip earthquakes of April 11, 2012, and an Mw=7.2 foreshock that occurred in January 2012 are aligned. The April 11 events are the largest known oceanic events occurring away from the main plate boundaries. They ruptured a 20-40 km thick section of the oceanic lithosphere, i.e. down to depths at which no direct images of fault zones have been obtained so far. Deep seismic reflection data acquired in the Mw=8.6 earthquake rupture zone ~100 km north of the epicenter shows the presence of sub-Moho reflectivity down to 37 km depth in the oceanic mantle. We interpret these events as reflections off the earthquake-generating fault plane in the oceanic mantle, in accordance with results suggesting that brittle deformation of the oceanic lithosphere extends well into the mantle down to the 600°C isotherm. The fracture zone near 93°E separates lithospheres of contrasting crustal thicknesses (3.5-4.5 km versus 6 km) with a 10 Ma age difference, and therefore seems to act as a rheological boundary. We find that the deep reflections could originate from either a plane trending approximately N105°E, at high angle to the fracture zone, or from the fracture zone itself if the dip of the fault surface decreases from nearly vertical in the sediments to about 45° in the oceanic mantle. We propose that this fracture zone is a major tectonic boundary in the Wharton Basin, and that the three 2012 earthquakes ruptured a large section of it as part of a poorly-defined diffuse plate boundary between the Indian and Australian plates, with slip occurring on this re-activated N-S fracture zone and on fossil E-W spreading-related faults. Over 1000 km of this plate boundary could have ruptured since the great 2004 Sumatra earthquake.

The Role of the Mantle on Structural Reactivation at the Plate Tectonics Scale (Invited)

NASA Astrophysics Data System (ADS)

Vauchez, A. R.; Tommasi, A.

2009-12-01

During orogeny, rifting, and in major strike-slip faults, the lithospheric mantle undergoes solid-state flow to accommodate the imposed strain. This deformation occurs mostly through crystal plasticity processes, like dislocation creep, and results in the development of a crystallographic preferred orientation (CPO) of olivine and pyroxene. Because these minerals, especially olivine, display strongly anisotropic physical properties, their preferred orientation confers anisotropic properties at the scale of the rock. When the deformation event comes to its end, the CPO are "frozen" and remain stable for millions or even billions years if no other deformation subsequently affects the lithospheric mantle. This means that anisotropic properties preserving a memory of previous deformation events may subsist in the continental mantle over very long periods of time. One of the main consequences of a well-developed olivine CPO is an anisotropic mantle viscosity and hence a deformation dependant on the orientation of the tectonic solicitations relative to the orientation of the olivine CPO inherited from the past orogenic events. The most obvious expression of this anisotropic mechanical behaviour is the influence of the inherited tectonic fabric on continental rifting. Most continental rifts that lead to successful continental breakup, like in the early Atlantic or the western Indian systems, formed parallel to ancient collisional belts. Moreover, the early stages of deformation in these systems are characterized by a transtensional strain regime involving a large component of strike-slip shearing parallel to the inherited fabric. The link between the lithospheric mantle fabric and the rift structure is further supported by seismic anisotropy measurements in major rifts (e.g., the East-African Rift) or at passive continental margins (e.g., the Atlantic Ocean) that show fast split S-waves polarized in a direction parallel to both the inherited fabric and the trend of the rift, and by the analysis of the CPO in mantle xenoliths collected in such areas. These observations are consistent with recent multi-scale numerical models showing that olivine CPO frozen in the lithospheric mantle result in an anisotropic mechanical behaviour. In a plate submitted to extension, CPO-induced anisotropy favours the reactivation in transtension of lithospheric-scale strike slip faults that are oblique to the imposed tensional stresses. Further investigation is needed to constrain the role of an inherited mechanical anisotropy of the lithosphere during compressional events and the possible feedbacks between an anisotropic viscous deformation of the lithospheric mantle and the seismic cycle. In both cases, crust-mantle coupling is likely for large-scale structures and mantle CPO may influence the kinematics of tectonic systems, at least during the initial stages of their evolution.

The Cooling Oceanic Lithosphere as Constrained by Surface Wave Dispersion Data

NASA Astrophysics Data System (ADS)

Hogg, C.; Laske, G.

2003-12-01

The tremendous improvement in resolution capabilities of global surface wave phase velocity maps now encourage us to search for anomalies that are caused by mantle plumes. On the other hand, the implications of even large--scale anomalies in such maps are still not well understood. One such anomaly is caused by the cooling oceanic lithosphere. Some studies investigate the cooling effects by fitting thermal models to the 3--dimensional mantle models resulting from tomographic inversions. The inversion of surface wave data for structure at depth is nonunique and the model often depends on the techniques applied. We prefer to compare the dispersion data directly with predictions from thermal models. Simple cooling models produce a signal that is roughly proportional to the square root of age. This signal is typically much smaller than the one caused by other lateral heterogeneity within the Earth's crust and upper mantle. In a careful analysis we are able to extract clear, roughly linear trends, in all major oceans. We explore the parameter space by fitting cooling half space as well as cooling plate models to the data. In the Pacific ocean, our data are inconsistent with standard parameters that are used to fit the observed bathymetry, and perhaps surface heat flux data. Instead of an initial temperature of 1350~deg C in the cooling half space model our data require a lower temperature (around 1200~deg C) to be well fit, especially the Love wave data. Regarding the cooling plate model, our data seem to require a thicker lithosphere to be well fit (135~km instead of the 'standard' 100 ~m). We observe similar trends for the other oceans investigated: the Indian ocean, the South and the North Atlantic oceans. For the Indian ocean in particular, a crust correction (removing the predictions caused by crustal structure including water depth and sediment thickness) is crucial to obtain an internally consistent dataset. For the Atlantic ocean, a large signal remains unexplained. An age--dependent signal is also apparent in the SS-S and PP-P body wave datasets. However, a comprehensive analysis is somewhat hampered for two reasons: 1) the uneven sampling of the data does not allow us to analyze trends in some oceans (e.g. South Atlantic Ocean); 2) the signal in the oldest parts of the oceans appear ''too fast''. We suspect that we observe effects that are deeper--rooted than the lithosphere--asthenosphere system (e.g. subducting slabs). The surface wave dispersion maps contain an intriguing oscillating signal that is particularly strong for Rayleigh waves in the Pacific ocean. This signal is symmetric to the EPR and we speculate that this is caused by current convective processes or by processes at the time when the plates were formed.

Plate break-up geometry in SE-Afar

NASA Astrophysics Data System (ADS)

Geoffroy, Laurent; Le Gall, Bernard; Daoud, Mohamed

2014-05-01

New structural data acquired in Djibouti strongly support the view of a magma-rich to magma-poor pair of conjugate margins developed in SE Afar since at least 9 Ma. Our model is illustrated by a crustal-scale transect that emphasizes the role of a two-stage extensional detachment fault system, with opposing senses of motion through time. The geometry and kinematics of this detachment fault pattern are mainly documented from lavas and fault dip data extracted from remote sensing imagery (Landsat ETM+, and corresponding DEM), further calibrated by field observations. Although expressed by opposite fault geometries, the two successive extensional events evidenced here are part of a two-stage continental extensional tear-system associated with the ongoing propagation of the Aden-Tadjoura oceanic axis to the NW. A flip-flop evolution of detachment faults accommodating lithosphere divergence has recently been proposed for the development of the Indian Ocean and continental margins (Sauter et al., 2013). However, the SE Afar evolution further suggests a radical and sudden change in lithosphere behavior during extension, from a long-term and widespread magmatic stage to a syn-sedimentary break-up stage where mantle melting concentrates along the future oceanic axis. Of special interest is the fact that a late and rapid stage of non-magmatic extension led to break-up, whose geometry triggered the location of the break-up axis and earliest oceanic accretion. New structural data acquired in Djibouti strongly support the view of a magma-rich to magma-poor pair of conjugate margins developed in SE Afar since at least 9 Ma. Our model is illustrated by a crustal-scale transect that emphasizes the role of a two-stage extensional detachment fault system, with opposing senses of motion through time. The geometry and kinematics of this detachment fault pattern are mainly documented from lavas and fault dip data extracted from remote sensing imagery (Landsat ETM+, and corresponding DEM), further calibrated by field observations. Although expressed by opposite fault geometries, the two successive extensional events evidenced here are part of a two-stage continental extensional tear-system associated with the ongoing propagation of the Aden-Tadjoura oceanic axis to the NW. A flip-flop evolution of detachment faults accommodating lithosphere divergence has recently been proposed for the development of the Indian Ocean and continental margins (Sauter et al., 2013). However, the SE Afar evolution further suggests a radical and sudden change in lithosphere behavior during extension, from a long-term and widespread magmatic stage to a syn-sedimentary break-up stage where mantle melting concentrates along the future oceanic axis. Of special interest is the fact that a late and rapid stage of non-magmatic extension led to break-up, whose geometry triggered the location of the break-up axis and earliest oceanic accretion.

Reconciling the geological history of western Turkey with plate circuits and mantle tomography

NASA Astrophysics Data System (ADS)

Kaymakci, N.; van Hinsbergen, D. J.; Spakman, W.; Torsvik, T. H.

2010-12-01

We place the geological history since Cretaceous times in western Turkey in a context of convergence, subduction, collision and slab break-off. To this end, we compare the west Anatolian geological history with amounts of Africa-Europe convergence calculated from the Atlantic plate circuit, and the seismic tomography images of the west Anatolian mantle structure. Western Turkish geology reflects the convergence between the Sakarya continent (here treated as Eurasia) in the north and Africa in the south, with the Anatolide-Tauride Block (ATB) between two strands of the Neotethyan ocean. Convergence between the Sakarya and the ATB started at least ~95-90Myr ago, marked by ages of metamorphic soles of ophiolites that form the highest structural unit below Sakarya. These are underlain by high-pressure, low-temperature metamorphic rocks of the Tavsanli and Afyon zones, and the Ören Unit, which in turn are underlain by the Menderes Massif derived from the ATB. Underthrusting of the ATB below Sakarya was since ~50Ma, associated with high-temperature metamorphism and widespread granitic magmatism. Thrusting in the Menderes Massif continued until 35 Ma, after which there is no record of accretion in western Turkey. Plate circuits show that since 90 Ma, ~1400 km of Africa-Europe convergence occurred, of which ~700 km since 50 Ma and ~450 km since 35Ma. Seismic tomography shows that the African slab under western Turkey is decoupled from the African Plate. This detached slab is a single, coherent body, representing the lithosphere consumed since 90 Ma. There was no subduction re-initiation after slab break-off. ATB collision with Europe therefore did not immediately lead to slab break-off but instead to delamination of subducting lithospheric mantle from accreting ATB crust, while staying attached to the African Plate. This led to asthenospheric inflow below the ATB crust, high-temperature metamorphism and felsic magmatism. Slab break-off in western Turkey probably occurred ~15 Myr ago, after which overriding plate compression and rotation accommodated ongoing Africa-Europe convergence. Slab break-off was accommodated along a vertical NE trending subduction transform edge propagator (STEP) fault zone, accelerating southwestward slab retreat of the Aegean slab. The SE Aegean slab edge may have existed already since early Miocene times or before, but started to rapidly roll back along the southeastern Aegean STEP in middle Miocene times, penetrating the Aegean region in the Pliocene.

Seismicity of the Earth 1900-2013 offshore British Columbia-southeastern Alaska and vicinity

USGS Publications Warehouse

Hayes, Gavin P.; Smoczyk, Gregory M.; Ooms, Jonathan G.; McNamara, Daniel E.; Furlong, Kevin P.; Benz, Harley M.; Villaseñor, Antonio

2014-01-01

The tectonics of the Pacific margin of North America between Vancouver Island and south-central Alaska are dominated by the northwest motion of the Pacific plate with respect to the North America plate at a velocity of approximately 50 mm/yr. In the south of this mapped region, convergence between the northern extent of the Juan de Fuca plate (also known as the Explorer microplate) and North America plate dominate. North from the Explorer, Pacific, and North America plate triple junction, Pacific:North America motion is accommodated along the ~650-km-long Queen Charlotte fault system. Offshore of Haida Gwaii and to the southwest, the obliquity of the Pacific:North America plate motion vector creates a transpressional regime, and a complex mixture of strike-slip and convergent (underthrusting) tectonics. North of the Haida Gwaii islands, plate motion is roughly parallel to the plate boundary, resulting in almost pure dextral strike-slip motion along the Queen Charlotte fault. To the north, the Queen Charlotte fault splits into multiple structures, continuing offshore of southwestern Alaska as the Fairweather fault, and branching east into the Chatham Strait and Denali faults through the interior of Alaska. The plate boundary north and west of the Fairweather fault ultimately continues as the Alaska-Aleutians subduction zone, where Pacific plate lithosphere subducts beneath the North America plate at the Aleutians Trench. The transition is complex, and involves intraplate structures such as the Transition fault. The Pacific margin offshore British Columbia is one of the most active seismic zones in North America and has hosted a number of large earthquakes historically.

Raising the Gangdese Mountains in southern Tibet

NASA Astrophysics Data System (ADS)

Zhu, Di-Cheng; Wang, Qing; Cawood, Peter A.; Zhao, Zhi-Dan; Mo, Xuan-Xue

2017-01-01

The surface uplift of mountain belts is in large part controlled by the effects of crustal thickening and mantle dynamic processes (e.g., lithospheric delamination or slab breakoff). Understanding the history and driving mechanism of uplift of the southern Tibetan Plateau requires accurate knowledge on crustal thickening over time. Here we determine spatial and temporal variations in crustal thickness using whole-rock La/Yb ratios of intermediate intrusive rocks from the Gangdese arc. Our results show that the crust was likely of normal thickness prior to approximately 70 Ma ( 37 km) but began to thicken locally at approximately 70-60 Ma. The crust reached (58-50) ± 10 km at 55-45 Ma extending over 400 km along the strike of the arc. This thickening was likely due to magmatic underplating as a consequence of rollback and then breakoff of the subducting Neo-Tethyan slab. The crust attained a thickness of 68 ± 12 km at approximately 20-10 Ma, as a consequence of underthrusting of India and associated thrust faulting. The Gangdese Mountains in southern Tibet broadly attained an elevation of >4000 m at approximately 55-45 Ma as a result of isostatic surface uplift driven by crustal thickening and slab breakoff and reached their present-day elevation by 20-10 Ma. Our paleoelevation estimates are consistent not only with the C-O isotope-based paleoaltimetry but also with the carbonate-clumped isotope paleothermometer, exemplifying the promise of reconstructing paleoelevation in time and space for ancient orogens through a combination of magmatic composition and Airy isostatic compensation.

Geophysical Characterization of Some Terranes and the Geophysical Modeling of Candidate Suture Zones

NASA Technical Reports Server (NTRS)

Ravat, D.

1997-01-01

Indian participation in this project was terminated during the last year by a sudden withdrawal of support by the Department of Science and Technology, India, to the Indian Institute of Geomagnetism, Bombay. As a result, significant changes in the project focus had to be undertaken. Much of the work carried out at Southern Illinois University at Carbondale during the first year of the project anticipated the Indian participation and included development of computer programs to be used on gravity and magnetic data from the Indian subcontinent and preparations for fieldwork, tutorials, and workshops in India. Despite these setbacks, which were beyond our control, a number of significant tasks have been accomplished during the project period. These include: (1) Completion of digitization of the regional Bouguer gravity anomaly map of India and the regional ground total intensity magnetic anomaly map of India at an overdetermined spacing of 0.05 degrees. (2) We investigated and assessed the limitations of the Euler method using environmental examples because detailed aeromagnetic maps of parts of India were not available for interpretation by this method. (3) We also undertook an assessment of a suture zone between the Nyaza Craton (Archean) and the Mozambique Belt (Pan African) in the Kenya Rift, Africa, using gravity anomalies and the lithospheric seismological models. (4) We studied Magsat and high-altitude (approx. 4 km) aeromagnetic data over Canada.

Structure, mechanical properties and evolution of the lithosphere below the northwest continental margin of India

NASA Astrophysics Data System (ADS)

Rao, G. Srinivasa; Kumar, Manish; Radhakrishna, M.

2018-02-01

The continental breakup history at the northwest continental margin of India remained conjectural due to lack of clearly discernable magnetic anomaly identifications and the presence of several enigmatic structural/basement features whose structure was partly obscured by the Late Cretaceous Deccan magmatic event. In this study, a detailed analysis of the existing seismic and seismological data covering both onshore and offshore areas of the northwest Indian margin along with 3-D/2-D constrained potential field (gravity, magnetic and geoid) modeling has been carried out. The crustal structure and lithosphere-asthenosphere boundary (LAB) delineated across the margin provided valuable insights on the mechanism of continental extension. An analysis of the residual geoid anomaly (degree-10) map and the modeled LAB below Deccan volcanic province (DVP) revealed significant variation in upper mantle characteristics between the northwest (NW) and south central (SC) parts of DVP having thinner lithosphere in the NW part. The depth to LAB ranges 80-130 km at the margin with gradual thinning towards the western offshore having sharp gradient in the south (SC part of DVP) and gentle gradient in the north (NW part of DVP). The Moho configuration obtained from seismically constrained 3-D gravity inversion reveals that Moho depths vary 34-42 km below DVP and gradually thins to 16-20 km in the western offshore. The effective elastic thickness (Te) map computed through 3-D flexural modeling indicates that the Te values are in general lower in the region and range 12-25 km. Such lower Te values could be ascribed to the combined effect of the lithosphere stretching during Gondwana fragmentation in the Mesozoic and subsequent thermal influence of the Reunion plume. Based on the crustal stretching factors (β), Te estimates and the modeled lithosphere geometry at the margin in this study, we propose that the lithosphere below Laxmi-Gop basin region (β > 3.0) had undergone continuous stretching since India-Madagascar rifting ( 88 Ma) /much prior to this event. However, this continuous stretching did not lead to breakup. Due to syn-rift cooling, the developed necking zone (brittle-ductile deformation) got ceased and led to the development of a new necking zone between Seychelles and Laxmi Ridge. Subsequent stretching between Seychelles and the Laxmi Ridge contemporaneous with the Deccan flood basalts eruption led to the seafloor spreading in the Western Basin (anomaly C28n). Thus, the Laxmi Ridge became a continental sliver.

The lithosphere of the Antarctic continent: new insights from satellite gravity gradient data

NASA Astrophysics Data System (ADS)

Ferraccioli, Fausto; Ebbing, Jorg; Pappa, Folker; Kern, Michael; Forsberg, Rene

2017-04-01

The GOCE+Antarctica project, part of the Support to Science (STSE) program of the European Space Agency (ESA) is a new polar geosciences research initiative that aims to investigate the thermal and compositional structure of the Antarctic lithosphere by combing satellite gravity gradients (Bouman et al., 2016), airborne gravity data compilations (Scheinert et al., 2016), seismological (e.g. An et al., 2015) and petrological models in a forward and inverse manner. This approach promises to shed new light into the fundamental interplays between Antarctic lithospheric architecture, bedrock topography, ice sheet dynamics, and also its dynamic relations with Glacial Isostatic Adjustment (GIA). Here we focus on the satellite gravity gradient signatures and superimpose these on major known tectonic and bedrock topography elements, as well as independent seismically-derived estimates of crustal thickness for the Antarctic continent. An ad hoc India up reference system was used to facilitate the initial interpretation of the satellite gravity gradient data images. The GIU component clearly reveals the marked contrast between the thinner crust and lithosphere underlying the West Antarctic Rift System and also the Weddell Sea Rift System and the thicker lithosphere of East Antarctica. Notably, the new images suggests that more distributed wide-mode lithospheric and crustal extension affects the Ross Sea Embayment and continues under the Ross Ice Shelf, but this pattern is less clear towards the Bellingshousen Embayment. This suggests that the rift system narrows considerably as it reaches the southern edge of the Antarctic Peninsula, perhaps also in response to the relatively thicker crust and potentially relatively more rigid Precambrian lithosphere of the displaced Haag-Ellsworth block, which was originally located closer to East Antarctica, prior to distributed Jurassic lithospheric and crustal extension in the Weddell Sea Rift System. In East Antarctica, the satellite gravity data arguably provides one the clearest large-scale views to date of the potential extent of the Archean to Mesoproterozoic Terre Adelie Craton, and clearly shows the contrast wrt to the crust and lithosphere underlying both the Wilkes Subglacial Basin to the east and the Sabrina Subglacial Basin to the west. This finding corroborates and also augments recent independent interpretations of aeromagnetic and airborne gravity data over the region, suggesting that the Mawson Continent is a composite lithospheric-scale entity, which was affected by several Paleoproterozoic and Mesoproterozoic orogenic events (Aitken et al., 2016). Thick crust is clearly imaged beneath the Transantarctic Mountains, the Terre Adelie Craton, the Gamburtsev Subglacial Mountains and also Eastern Dronning Maud Land, in particular beneath the recently proposed region of the Tonian Oceanic Arc Superterrane (Jacobs et al., 2015). The GIA and GIU components help delineate the edges of several of these lithospheric provinces, both in West and East Antarctica. One of the largest and previously unknown lithospheric-scale features discovered in East Antarctica from the satellite gravity gradient images is a linear feature that appears to cut across East Antarctica, potentially extending from the area of the Lutzow Holm Complex on the Indian side of East Antarctica right across the continent to South Pole. We name this feature the Trans East Antarctic Shear Zone and propose that it represents a major lithospheric scale shear zone and possibly a major suture zone that separates the Gamburtsev Province from the Eastern Dronning Maud Land Province and also appears to form the southern boundary of the composite Recovery Province. We infer based on geological data in the Lutzow Holm Complex region and formerly adjacent segments of India and Madagascar and eastern Africa that it may represent a major hitherto unrecongnised Pan-African age suture zone related to the assembly of the Gondwana supercontinent. New aerogeophysical surveys in interior East Antarctica, between the Recovery region, the Gamburtsev Province and the southern edge of the Eastern Dronning Maud Province are however required to investigate the detailed crustal architecture, evolution and also kinematics of this newly proposed shear/suture zone and to help understand its relationships with the previously proposed Gamburtsev and Shackleton Range suture zones.

On causal links between flood basalts and continental breakup

NASA Astrophysics Data System (ADS)

Courtillot, V.; Jaupart, C.; Manighetti, I.; Tapponnier, P.; Besse, J.

1999-03-01

Temporal coincidence between continental flood basalts and breakup has been noted for almost three decades. Eight major continental flood basalts have been produced over the last 300 Ma. The most recent, the Ethiopian traps, erupted in about 1 Myr at 30 Ma. Rifting in the Red Sea and Gulf of Aden, and possibly East African rift started at about the same time. A second trap-like episode occurred around 2 Ma and formation of true oceanic crust is due in the next few Myr. We find similar relationships for the 60 Ma Greenland traps and opening of the North Atlantic, 65 Ma Deccan traps and opening of the NW Indian Ocean, 132 Ma Parana traps and South Atlantic, 184 Ma Karoo traps and SW Indian Ocean, and 200 Ma Central Atlantic Margin flood basalts and opening of the Central Atlantic Ocean. The 250 Ma Siberian and 258 Ma Emeishan traps seem to correlate with major, if aborted, phases of rifting. Rifting asymmetry, apparent triple junctions and rift propagation (towards the flood basalt area) are common features that may, together with the relative timings of flood basalt, seaward dipping reflector and oceanic crust production, depend on a number of plume- and lithosphere- related factors. We propose a mixed scenario of `active/passive' rifting to account for these observations. In all cases, an active component (a plume and resulting flood basalt) is a pre-requisite for the breakup of a major oceanic basin. But rifting must be allowed by plate-boundary forces and is influenced by pre-existing heterogeneities in lithospheric structure. The best example is the Atlantic Ocean, whose large-scale geometry with three large basins was imposed by the impact points of three mantle plumes.

Miocene exhumation of the Indus-Yarlung Suture Zone in NW India: An insight into the controls of tectonics and climate

NASA Astrophysics Data System (ADS)

Bhattacharya, G.; Robinson, D. M.; Orme, D. A.; Olree, E.; Bosu, S.

2016-12-01

Detritus from the India-Asia collision and subsequent Cenozoic tectonic events is preserved in sedimentary basins along the 2500 km long Indus-Yarlung Suture Zone (IYSZ) in India and Tibet. In northwest India, these Eocene-Miocene synorogenic sedimentary rocks are preserved in the Tar and Indus Groups. We use (U-Th)/He dating of detrital zircons from units within these sedimentary basins, including the Temesgam Formation at Temesgam, the Lower Nimu Formation and the Sumdo Formation in the Zanskar Gorge, and the Artsa Formation and the Miru Formation in the Upshi-Lato region. These analyses indicate a phase of rapid exhumation from 19-8 Ma. Possible explanations for these data include a combination of tectonic events and the influence of climate. Regional back-thrusting initiated at 20 Ma along the Great Counter Thrust, which buried the IYSZ footwall with the Lamayuru slope deposits of the Indian passive margin. In south Tibet, previous studies identify underthrusting of the Indian plate as a key factor for basin exhumation in the IYSZ, which may also be a driver in northwest India. The flow of the paleo-Indus river through the IYSZ in Early Miocene time might have been triggered by the onset of Asian monsoon at 24 Ma and its intensification between 18-10 Ma. Our data demonstrate a phase of rapid exhumation in northwest India from 19-8 Ma, which may be linked to all of these tectonic and climate influences. Data in this study are similar to the data of Carrapa et al. (2014) from south Tibet that show peak exhumation at 17 Ma, and suggest that a regional cooling episode, driven by tectonics and climate, might have prevailed in the Miocene along the IYSZ.

Thermobarometric and fluid expulsion history of subduction zones

NASA Astrophysics Data System (ADS)

Ernst, W. G.

1990-06-01

Phanerozoic, unmetamorphosed, weathered, and altered lithotectonic complexes subjected to subduction exhibit the prograde metamorphic facies sequence: zeolite → prehnite-pumpellyite → glaucophane schist → eclogite. Parageneses reflect relatively high-P trajectories, accompanied by semicontinuous devolatilization. The thermal evolution of convergent plate junctions results in early production of high-rank blueschists, high-P amphibolites, and eclogues at depth within narrow subduction zones while the hanging wall lithosphere is still hot. Protracted underflow drains heat from the nonsubducted plate and, even at profound depths, generates very low-T/high-P parageneses. Inclusion studies suggest that two-phase immiscible volatiles (liquid H2O, and gaseous high-hydrocarbons, CH4 and CO2) are evolved in turn during progressive metamorphism of the subducted sections. Expulsion of pore fluids and transitions from weathered and altered supracrustal rocks to zeolite facies assemblages release far more fluid than the better understood higher-grade transformations. Many blueschist parageneses, such as those of the internal Western Alps, have been partially overprinted by later greenschist and/or epidote-amphibolite facies assemblages. Alpine-type postblueschist metamorphic paths involved fairly rapid, nearly adiabatic decompression; some terranes even underwent modest continued heating and fluid evolution during early stages of ascent. Uplift probably occurred as a consequence of the underthrusting of low-density island arc or microcontinental crust along the convergent plate junction, resulting in marked deceleration or cessation of lithospheric underflow, decoupling, and nearly isothermal rise of the recrystallized subduction complex. Other, less common blueschist terranes, such as the eastern Franciscan belt of western California, preserve metamorphic aragonite and other high-P minerals, and lack a low-pressure overprint; physical conditions during retrogression approximately retraced the prograde path or, for early formed high-grade blocks, reflect somewhat higher pressures and lower temperatures. Subducted sections constituting portions of the Franciscan-type of metamorphic belt evidently moved slowly back up the inclined lithospheric plate junction during continued convergence and sustained refrigeration. Upward motion due to isostatic forces was produced by tectonic imbrication of fault suces, laminar return flow in melange zones, and lateral extension of the underplated accretionary prism. The ease with which volatiles are expelled from a subduction complex and migrate upward along the plate junction zone is roughly proportional to the sandstone/shale ratio: low-permeability mudstones tend to maintain fluid values approaching lithostatic, lose strength, and deform chaotically (forming melange belts), whereas permeable sandstone-rich sections retain structural/stratigraphic coherence and fail brittlely (forming coherent terranes). Because of substantial updip expulsion of volatiles during prograde recrystallization, only small amounts of H2O and CO2 are available to support hydration and carbonation of the accretionary complex during its return toward the surface; thus limited back reaction takes place and occurs at low Pfluid/Plithostatic ratios, unless an abundance of volatiles is introduced during uplift.

Long wavelength gravity anomalies over India: Crustal and lithospheric structures and its flexure

NASA Astrophysics Data System (ADS)

Tiwari, V. M.; Ravi Kumar, M.; Mishra, D. C.

2013-07-01

Long wavelength gravity anomalies over India were obtained from terrestrial gravity data through two independent methods: (i) wavelength filtering and (ii) removing crustal effects. The gravity fields due to the lithospheric mantle obtained from two methods were quite comparable. The long wavelength gravity anomalies were interpreted in terms of variations in the depth of the lithosphere-asthenosphere boundary (LAB) and the Moho with appropriate densities, that are constrained from seismic results at certain points. Modeling of the long wavelength gravity anomaly along a N-S profile (77°E) suggest that the thickness of the lithosphere for a density contrast of 0.05 g/cm3 with the asthenosphere is maximum of ˜190 km along the Himalayan front that reduces to ˜155 km under the southern part of the Ganga and the Vindhyan basins increasing to ˜175 km south of the Satpura Mobile belt, reducing to ˜155-140 km under the Eastern Dharwar craton (EDC) and from there consistently decreasing south wards to ˜120 km under the southernmost part of India, known as Southern Granulite Terrain (SGT). The crustal model clearly shows three distinct terrains of different bulk densities, and thicknesses, north of the SMB under the Ganga and the Vindhyan basins, and south of it the Eastern Dharwar Craton (EDC) and the Southern Granulite Terrain (SGT) of bulk densities 2.87, 2.90 and 2.96 g/cm3, respectively. It is confirmed from the exposed rock types as the SGT is composed of high bulk density lower crustal rocks and mafic/ultramafic intrusives while the EDC represent typical granite/gneisses rocks and the basement under the Vindhyan and Ganga basins towards the north are composed of Bundelkhand granite massif of the lower density. The crustal thickness along this profile varies from ˜37-38 km under the EDC, increasing to ˜40-45 km under the SGT and ˜40-42 km under the northern part of the Ganga basin with a bulge up to ˜36 km under its southern part. Reduced lithospheric and crustal thicknesses under the Vindhyan and the Ganga basins are attributed to the lithospheric flexure of the Indian plate due to Himalaya. Crustal bulge due to lithospheric flexure is well reflected in isostatic Moho based on flexural model of average effective elastic thickness of ˜40 km. Lithospheric flexure causes high heat flow that is aided by large crustal scale fault system of mobile belts and their extensions northwards in this section, which may be responsible for lower crustal bulk density in the northern part. A low density and high thermal regime in north India north of the SMB compared to south India, however does not conform to the high S-wave velocity in the northern part and thus it is attributed to changes in composition between the northern and the southern parts indicating a reworked lithosphere. Some of the long wavelength gravity anomalies along the east and the west coasts of India are attributed to the intrusives that caused the breakup of India from Antarctica, and Africa, Madagascar and Seychelles along the east and the west coasts of India, respectively.

Subduction initiation and recycling of Alboran domain derived crustal components prior to the intra-crustal emplacement of mantle peridotites in the Westernmost Mediterranean: isotopic evidence from the Ronda peridotite

NASA Astrophysics Data System (ADS)

Varas-Reus, María Isabel; Garrido, Carlos J.; Bosch, Delphine; Marchesi, Claudio Claudio; Acosta-Vigil, Antonio; Hidas, Károly; Barich, Amel

2014-05-01

During Late Oligocene-Early Miocene different domains formed in the region between Iberia and Africa in the westernmost Mediterranean, including thinned continental crust and a Flysch Trough turbiditic deposits likely floored by oceanic crust [1]. At this time, the Ronda peridotite likely constituted the subcontinental lithospheric mantle of the Alboran domain, which mantle lithosphere was undergoing strong thinning and melting [2] [3] coevally with Early Miocene extension in the overlying Alpujárride-Maláguide stacked crust [4, 5]. Intrusive Cr- rich pyroxenites in the Ronda massif records the geochemical processes occurring in the subcontinental mantle of the Alboran domain during the Late Oligocene [6]. Recent isotopic studies of these pyroxenites indicate that their mantle source was contaminated by a subduction component released by detrital crustal sediments [6]. This new data is consistent with a subduction setting for the late evolution of the Alboran lithospheric mantle just prior to its final intracrustal emplacement in the early Miocene Further detailed structural studies of the Ronda plagioclase peridotites-related to the initial stages of ductile emplacement of the peridotite-have led to Hidas et al. [7] to propose a geodynamic model where folding and shearing of an attenuated mantle lithosphere occurred by backarc basin inversion followed by failed subduction initiation that ended into the intracrustal emplacement of peridotite into the Alboran wedge in the earliest Miocene. This hypothesis implies that the crustal component recorded in late, Cr-rich websterite dykes might come from underthrusted crustal rocks from the Flysch and/or Alpujárrides units that might have been involved in the earliest stages of this subduction initiation stage. To investigate the origin of crustal component in the mantle source of this late magmatic event recorded by Cr-pyroxenites, we have carried out a detail Sr-Nd-Pb-Hf isotopic study of a variety of Betic-Rif cordillera crustal rocks that might have been potentially subducted beneath the Alborán domain before the emplacement of Ronda peridotites. Isotopic data rules out potential crustal sources coming from pre-early Miocene Flysch Trough sediments and crustal rocks from the Blanca Unit currently underlying peridotite. Crustal rocks from the Jubrique Unit overlying the Ronda peridotite are the only crustal samples that may account for the relatively high 207Pb-208Pb/204Pb and low 206Pb/204Pb characteristic of the crustal contaminant added to the mantle source of late Cr-pyroxenites. These data strongly support Alboran geodynamic models that envisage slab roll-back as the tectonic mechanism responsible for Miocene lithospheric thinning, and provides a scenario where back-arc inversion leading to self-subduction of crustal units at the front of the Alboran wedge. REFERENCES 1. Durand-Delga, M., P. Rossi, P. Olivier, and D. Puglisi, Situation structurale et nature ophiolitique de roches basiques jurassiques associées aux flyschs maghrébins du Rif (Maroc) et de Sicile (Italie). Comptes Rendus de l'Académie des Sciences - Series IIA - Earth and Planetary Science, 2000. 331(1): p. 29-38. 2. Lenoir, X., C. Garrido, J.L. Bodinier, J.M. Dautria, and F. Gervilla, The Recrystallization Front of the Ronda Peridotite: Evidence for Melting and Thermal Erosion of Subcontinental Lithospheric Mantle beneath the Alboran Basin. Journal of Petrology, 2001. 42(1): p. 141-158. 3. Garrido, C.J., F. Gueydan, G. Booth-Rea, J. Precigout, K. Hidas, J.A. Padrón-Navarta, and C. Marchesi, Garnet lherzolite and garnet-spinel mylonite in the Ronda peridotite: Vestiges of Oligocene backarc mantle lithospheric extension in the western Mediterranean. Geology, 2011. 4. Balanyá, J.C., V. García-Dueñas, J.M. Azañón, and M. Sánchez-Gómez, Alternating contractional and extensional events in the Alpujarride nappes of the Alboran Domain (Betics, Gibraltar Arc). Tectonics, 1997. 16(2): p. 226-238. 5. Platt, J.P., M.J. Whitehouse, S.P. Kelley, A. Carter, and L. Hollick, Simultaneous extensional exhumation across the Alboran Basin: Implications for the causes of late orogenic extension. Geology, 2003. 31(3): p. 251-254. 6. Marchesi, C., C.J. Garrido, D. Bosch, J.-L. Bodinier, K. Hidas, J.A. Padrón-Navarta, and F. Gervilla, A Late Oligocene Suprasubduction Setting in the Westernmost Mediterranean Revealed by Intrusive Pyroxenite Dikes in the Ronda Peridotite (Southern Spain). The Journal of Geology, 2012. 120(2): p. 237-247. 7. Hidas, K., G. Booth-Rea, C.J. Garrido, J.M. Martínez-Martínez, J.A. Padrón-Navarta, Z. Konc, F. Giaconia, E. Frets, and C. Marchesi, Backarc basin inversion and subcontinental mantle emplacement in the crust: kilometre-scale folding and shearing at the base of the proto-Alborán lithospheric mantle (Betic Cordillera, southern Spain). Journal of the Geological Society, 2013. 170(1): p. 47-55.

Fluid Pressure in the Shallow Plate Interface at the Nankai Trough Subduction Zone

NASA Astrophysics Data System (ADS)

Tobin, H. J.; Saffer, D.

2003-12-01

The factors controlling the occurrence, magnitude, and other characteristics of great earthquakes is a fundamental outstanding question in fault physics. Pore fluid pressure is perhaps the most critical yet poorly known parameter governing the strength and seismogenic character of plate boundary faults, but unfortunately cannot be directly inferred through available geophysical sensing methods. Moreover, true in situ fluid pressure has proven difficult to measure even in boreholes. At the Nankai Trough, several hundred meters of sediment are subducted beneath the frontal portion of the accretionary prism. The up-dip portion of the plate interface is therefore hosted in these fine-grained marine sedimentary rocks. ODP Leg 190 and 196 showed that these rapidly-loaded underthrust sediments are significantly overpressured near the deformation front. Here, we attempt to quantitatively infer porosity, pore pressure, and effective normal stress at the plate interface at depths currently inaccessible to drilling. Using seismic reflection interval velocity calibrated at the boreholes to porosity, we quantitatively infer pore pressure to ˜ 20 km down-dip of the deformation front, to a plate interface depth of ˜ 6 km. We have developed a Nankai-specific velocity-porosity transform using ODP cores and logs. We use this function to derive a porosity profile for each of two down-dip seismic sections extracted from a 3-D dataset from the Cape Muroto region. We then calculate pore fluid pressure and effective vertical (fault-normal) stress for the underthrust sediment section using a compaction disequilibrium approach and core-based consolidation test data. Because the pore fluid pressure at the fault interface is likely controlled by that of the top of the underthrust section, this calculation represents a quantitative profile of effective stress and pore pressure at the plate interface. Results show that seismic velocity and porosity increase systematically downdip in the underthrust section, but the increase is suppressed relative to that expected from normally consolidating sediments. The computed pore pressure increases landward from an overpressure ratio (λ * = hydrostatic pressure divided by the lithostatic overburden) of ˜ 0.6 at the deformation front to ˜ 0.77 where sediments have been subducted 15 km. The results of this preliminary analysis suggest that a 3-dimensional mapping of predicted effective normal stress in the seismic data volume is possible.

Kinematics and dynamics of Nubia-Somalia divergence along the East African rift

NASA Astrophysics Data System (ADS)

Stamps, Dorothy Sarah

Continental rifting is fundamental to the theory of plate tectonics, yet the force balance driving Earth's largest continental rift system, the East African Rift (EAR), remains debated. The EAR actively diverges the Nubian and Somalian plates spanning ˜5000 km N-S from the Red Sea to the Southwest Indian Ridge and ˜3000 km NW-SE from eastern Congo to eastern Madagascar. Previous studies suggest either lithospheric buoyancy forces or horizontal tractions dominate the force balance acting to rupture East Africa. In this work, we investigate the large-scale dynamics of Nubia-Somalia divergence along the EAR driving present-day kinematics. Because Africa is largely surrounded by spreading ridges, we assume plate-plate interactions are minimal and that the major driving forces are gradients in gravitational potential energy (GPE), which includes the effect of vertical mantle tractions, and horizontal basal tractions arising from viscous coupling to horizontal mantle flow. We quantify a continuous strain rate and velocity field based on kinematic models, an updated GPS velocity solution, and the style of earthquake focal mechanisms, which we use as an observational constraint on surface deformation. We solve the 3D force balance equations and calculate vertically averaged deviatoric stress for a 100 km thick lithosphere constrained by the CRUST2.0 crustal density and thickness model. By comparing vertically integrated deviatoric stress with integrated lithospheric strength we demonstrate forces arising from gradients in gravitational potential energy are insufficient to rupture strong lithosphere, hence weakening mechanisms are required to initiate continental rupture. The next step involves inverting for a stress field boundary condition that is the long-wavelength minimum energy deviatoric stress field required to best-fit the style of our continuous strain rate field in addition to deviatoric stress from gradients in GPE. We infer the stress field boundary condition is an estimate of basal shear stress from viscous coupling to horizontal mantle flow. The stress field boundary condition is small (˜1.6 MPa) compared to deviatoric stress from GPE gradients (8-20 MPa) and does not improve the fit to surface deformation indicators more than 8% when combined with deviatoric stress from GPE gradients. Hence we suggest the style of deformation across the EAR can be explained by forces derived from gradients in GPE. We then calculate dynamic velocities using two types of forward models to solve the instantaneous momentum equations. One method is regional and requires vertically averaged effective viscosity to define lithospheric structure with velocity boundary conditions and a free-slip basal boundary condition. The second is a global model that accounts for a brittle upper crust and viscous mantle lithosphere with velocity boundary conditions imposed at the base of the lithosphere from 5 mantle flow models. With both methods we find deformation driven by internal lithospheric buoyancy forces provides the best-fit to GPS observations of surface velocities on the Somalian plate. We find that any additional contribution from horizontal tractions results in overpredicting surface velocities. This work indicates horizontal mantle flow plays a minimal role in Nubia-Somalia divergence and the EAR is driven largely by gradients in GPE.

Cretaceous potassic intrusives with affinities to aillikites from Jharia area: Magmatic expression of metasomatically veined and thinned lithospheric mantle beneath Singhbhum Craton, Eastern India

NASA Astrophysics Data System (ADS)

Srivastava, Rajesh K.; Chalapathi Rao, N. V.; Sinha, Anup K.

2009-11-01

Cretaceous potassic dykes and sills at the Jharia area intrude the Permo-carboniferous coal-bearing Gondwana sediments of the Eastern Damodar Valley, Singhbhum craton. These intrusives are widely regarded as a part of the Mesozoic alkaline and Rajmahal flood basalt magmatism in the Eastern Indian shield. Jharia intrusives display a wide petrographic diversity; olivine, phlogopite and carbonate are the predominant phases whereas apatite and rutile constitute important accessories. Impoverishment in sodium, silica and alumina and enrichment in potassium, titanium and phosphorous are the hallmark of these rocks and in this aspect they are strikingly similar to the rift-related aillikites (ultramafic lamprophyres) of Aillik Bay, Labrador. Crustal contamination of the Jharia magmas is minimal and the incompatible trace element ratios demonstrate (i) their generation by greater degrees of partial melting of a sub-continental lithospheric mantle (SCLM) source similar to that of the kimberlites of Dharwar craton, southern India, and (ii) retention of long-term memories of ancient (Archaean) subduction experienced by their source regions. We infer that a metasomatically veined and thinned lithosphere located at the margin of the Singhbhum craton and the inheritance of an ancient (Archaean) subducted component has played a significant role in deciding the diverging petrological and geochemical characters displayed by the Jharia potassic intrusives: those of kimberlites (orangeites) and lamproites (cratonic signature) and those of aillikites (rift-related signature). A substantial melt component of Jharia potassic intrusives was derived from the SCLM and the melt contribution of the Kerguelen plume is inferred to be minimal.

Dating the growth of oceanic crust at a slow-spreading ridge

USGS Publications Warehouse

Schwartz, J.J.; John, Barbara E.; Cheadle, Michael J.; Miranda, E.A.; Grimes, Craig B.; Wooden, J.L.; Dick, H.J.B.

2005-01-01

Nineteen uranium-lead zircon ages of lower crustal gabbros from Atlantis Bank, Southwest Indian Ridge, constrain the growth and construction of oceanic crust at this slow-spreading midocean ridge. Approximately 75% of the gabbros accreted within error of the predicted seafloor magnetic age, whereas ???25% are significantly older. These anomalously old samples suggest either spatially varying stochastic intrusion at the ridge axis or, more likely, crystallization of older gabbros at depths of ???5 to 18 kilometers below the base of crust in the cold, axial lithosphere, which were uplifted and intruded by shallow-level magmas during the creation of Atlantis Bank.

Structure and Tectonics of the Andaman Subduction Zone from Modeling of Seismological and Gravity Data

NASA Astrophysics Data System (ADS)

Nemalikanti, P. R.; Rao, N.; Hazarika, P.; Tiwari, V. M.; Mangalampally, R.; Singh, A.

2012-12-01

The 10 August 2009 Andaman earthquake of Mw 7.5 occurred to the north of the Andaman and Nicobar Islands at 14o N and 93o E which interestingly, coincides with the northern periphery of the rupture of the Sumatra-Andaman giant mega-thrust earthquake of Mw 9.1 that occurred on 26 December 2004. The event was followed by aftershocks with a peculiar vertical distribution at the same location which was earlier devoid of any significant seismicity. Waveform modeling of five of these events recorded by ISLANDS - the broadband seismic network deployed along the Andaman and Nicobar Islands, indicates that the main shock and two of its aftershocks have a normal fault mechanism with shallow focal depths within 18 km while two others have a strike-slip mechanism occurring deeper, down to 26 km. The computed Bouger gravity anomalies in this region indicate the steepest gradient of 1.5 mgal/km exactly centered over this zone of vertical seismic distribution that characterizes a region of lithospheric split or tear which is devoid of a subducting slab. This is in contrast to a clear subduction trend visible in the southern Andaman and Sunda arcs further south, as evidenced by tomographic images. Joint inversion of waveforms of these five events simultaneously, provides the best fitting P wave velocity structure of this region, given by a Moho at a depth of 30 km and a high crustal Vp/Vs ratio of 1.81. We infer an oceanic double crustal column corresponding to a thickness of about 21 km of Burmese crust including a 5 km thick sedimentary column, underlain by a thinner Indian crust which apparently has a thickness of about 9 km, a model that is also confirmed independently by gravity modeling. We interpret the mechanism of shallow normal fault earthquakes as an intra-plate relaxation phenomenon following the buckling of the overriding Burmese plate in the accretionary wedge of the fore-arc basin, in response to the 2004 mega-thrust subduction event. The deeper strike slip events correspond to an intra-plate phenomenon within the subducting Indian lithospheric plate representing left-lateral faulting across the Andaman arc, due to uneven convergence along the subduction front. Such strike-slip movements are seen all over the Indian Ocean diffuse deformation zone and represent strain accommodation in the Indian crust in response to a grosser mechanism of wrench fault tectonics of the Indo-Australian subduction beneath the Burma-Sunda plate.

Crustal scale detachment in the Himalayas: a reappraisal

NASA Astrophysics Data System (ADS)

Mukhopadhyay, S.; Sharma, J.

2010-11-01

According to the most popular tectonic model of the Himalayas proposed by a number of scientists the Indian crustal material underthrusts the Himalayas at a low angle and is relatively free of deformation compared to the overlying accreted material that makes up the Himalayan mountain chain. In this work we have carried out local earthquake tomography for the Garwhal-Kumaun Himalayas to estimate P- and S-wave velocity variations (Vp and Vs, respectively) and variation in their ratio (Vp/Vs) that would indicate the structure of the Himalayas and the underlying Indian crust in this part of the Himalayas. The results indicate that there is crustal level folding and faulting in this region indicating that the underlying Indian crustal material has also undergone deformation unlike what was postulated for the entire Himalayas by some workers before. By comparing our tomographic result with that for the eastern Nepal-southern Tibet region, it is concluded that there is variation in mode of deformation along the trend of the Himalayas. This observation matches well with the observed velocity variation in the upper mantle of these two regions reported by others. The area under investigation falls within a region where there is more oblique convergence between India and Eurasia compared to the Nepal Himalayas region. This may explain why such variation in mode of deformation is observed. The ratio Vp/Vs gets affected by strength of material. Presence or absence of fluid filled fractures or molten material affects it most strongly in the crustal region. The variation in Vp/Vs in the study area shows that almost the entire crust here have enough rheological strength such that it can store strain energy that can be released through earthquakes. A zone of low Vp/Vs beginning at the higher Himalayas and dipping towards SW is observed. This zone also has high Vp and Vs and is observed even when inversion is carried out with very high damping value. These observations do not suggest that seismic hazard will be less in the study area.

Spatial and temporal distribution of the seismicity along two mid-oceanic ridges with contrasted spreading rates in the Indian Ocean

NASA Astrophysics Data System (ADS)

Tsang-Hin-Sun, E.; Perrot, J.; Royer, J. Y.

2015-12-01

The seismicity of the ultra-slow spreading Southwest (14 mm/y) and intermediate spreading Southeast (60 mm/y) Indian ridges was monitored from February 2012 to March 2013 by the OHASISBIO array of 7 autonomous hydrophones. A total of 1471 events were located with 4 instruments or more, inside the array, with a median location uncertainty < 5 km and a completeness magnitude of mb = 3. Both ridges display similar average rates of seismicity, suggesting that there is no systematic relationship between seismicity and spreading rates. Accretion modes do differ, however, by the along-axis distribution of the seismic events. Along the ultra-slow Southwest Indian Ridge, events are sparse but regularly spaced and scattered up to 50 km off-axis. Along the fast Southeast Indian Ridge, events are irregularly distributed, focusing in narrow regions near the ridge axis at segment ends and along transform faults, whereas ridge-segment centers generally appear as seismic gaps (at the level of completeness of the array). Only two clusters, 6 months apart, are identified in a segment-center at 29°S. From the temporal distribution of the clustered events and comparisons with observations in similar mid-oceanic ridge setting, both clusters seem to have a volcanic origin and to be related to a dike emplacement or a possible eruption on the seafloor. Their onset time and migration rate are comparable to volcanic swarms recorded along the Juan de Fuca Ridge. Overall, the rate of seismicity along the two Indian spreading ridges correlates with the large-scale variations in the bathymetry and shear-wave velocity anomaly in the upper mantle, suggesting that the distribution of the low-magnitude seismicity is mainly controlled by along-axis variations in the lithosphere rheology and temperature.

Crustal Density Variation Along the San Andreas Fault Controls Its Secondary Faults Distribution and Dip Direction

NASA Astrophysics Data System (ADS)

Yang, H.; Moresi, L. N.

2017-12-01

The San Andreas fault forms a dominant component of the transform boundary between the Pacific and the North American plate. The density and strength of the complex accretionary margin is very heterogeneous. Based on the density structure of the lithosphere in the SW United States, we utilize the 3D finite element thermomechanical, viscoplastic model (Underworld2) to simulate deformation in the San Andreas Fault system. The purpose of the model is to examine the role of a big bend in the existing geometry. In particular, the big bend of the fault is an initial condition of in our model. We first test the strength of the fault by comparing the surface principle stresses from our numerical model with the in situ tectonic stress. The best fit model indicates the model with extremely weak fault (friction coefficient < 0.1) is requisite. To the first order, there is significant density difference between the Great Valley and the adjacent Mojave block. The Great Valley block is much colder and of larger density (>200 kg/m3) than surrounding blocks. In contrast, the Mojave block is detected to find that it has lost its mafic lower crust by other geophysical surveys. Our model indicates strong strain localization at the jointer boundary between two blocks, which is an analogue for the Garlock fault. High density lower crust material of the Great Valley tends to under-thrust beneath the Transverse Range near the big bend. This motion is likely to rotate the fault plane from the initial vertical direction to dip to the southwest. For the straight section, north to the big bend, the fault is nearly vertical. The geometry of the fault plane is consistent with field observations.

Upper-Mantel Earthquakes in the Australia-Pacific Plate Boundary Zone and the Roots of the Alpine Fault

NASA Astrophysics Data System (ADS)

Boese, C. M.; Warren-Smith, E.; Townend, J.; Stern, T. A.; Lamb, S. H.

2016-12-01

Seismicity in the upper mantle in continental collision zones is relatively rare, but observed around the world. Temporary seismometer deployments have repeatedly detected mantle earthquakes at depths of 40-100 km within the Australia-Pacific plate boundary zone beneath the South Island of New Zealand. Here, the transpressive Alpine Fault constitutes the primary plate boundary structure linking subduction zones of opposite polarity farther north and south. The Southern Alps Microearthquake Borehole Array (SAMBA) has been operating continuously since November 2008 along a 50 km-long section of the central Alpine Fault, where the rate of uplift of the Southern Alps is highest. To date it has detected more than 40 small to moderate-sized mantle events (1≤ML≤3.9). The Central Otago Seismic Array (COSA) has been in operation since late 2012 and detected 15 upper mantle events along the sub-vertical southern Alpine Fault. Various mechanisms have been proposed to explain the occurrence of upper mantle seismicity in the South Island, including intra-continental subduction (Reyners 1987, Geology); high shear-strain gradients due to depressed geotherms and viscous deformation of mantle lithosphere (Kohler and Eberhart-Phillips 2003, BSSA); high strain rates resulting from plate bending (Boese et al. 2013, EPSL), and underthrusting of the Australian plate (Lamb et al. 2015, G3). Focal mechanism analysis reveals a variety of mechanisms for the upper mantle events but predominantly strike-slip and reverse faulting. In this study, we apply spectral analysis to better constrain source parameters for these mantle events. These results are interpreted in conjunction with new information about crustal structure and low-frequency earthquakes near the Moho and in light of existing velocity, attenuation and resistivity models.

Epithermal gold-siver deposits in the western United States: time-space products of evolving plutonic, volcanic and tectonic environments

USGS Publications Warehouse

Berger, Byron R.; Bonham, Harold F.

1990-01-01

The western United States has been the locus of considerable subaerial volcanic and plutonic igneous activity since the mid-Mesozoic. After the destruction of the Jurassic-Cretaceous magmatic arc-trench system, subduction was re-established in the Late Mesozoic with low-angle underthrusting of the oceanic plate beneath western North America. This resulted in crustal shortening during the Late Cretaceous to Early Tertiary and removal of the mantle lithosphere west of the Rocky Mountains. Commencing in the Eocene, flat subduction ceased, the volcanic arc began to re-establish itself along the continental margin, and the hingeline along the steepening subducting plate migrated from east to west. The crust east of the migrating hingeline was exposed to hot asthenosphere, and widespread tectonics and volcanic activity resulted. Hydrothermal activity accompanied the volcanism resulting in numerous epithermal gold-silver deposits. The temporal and spatial distributions of epithermal deposits in the region are therefore systematic and can be subdivided into discrete time intervals which are related to widespread changes in magmatic activity. Time intervals selected for discussion are Pre-Cenozoic, 66-55 Ma, 54-43 Ma, 42-34 Ma, 33-24 Ma, 23-17 Ma, and <17 Ma. Many of these intervals contain both sedimentary-rock and two varieties of volcanic-rock hosted deposits (adularia-sericite and alunite-kaolinite ± pyrophyllite). Continental rifting is important to the formation of deposits, and, within any given region, it is at the initiation of deep rifting that alunite-kaolinite ± pyrophyllite type epithermal deposits are formed. Adularia-sericite type deposits are most common, being related to all compositions and styles of volcanic activity. Therefore, the volcano-tectonic context of the western United States provides a unified framework in which to understand and explore for epithermal type deposits.

Seismotectonics of the central segment of the Indonesian Arc

NASA Astrophysics Data System (ADS)

Eva, C.; Cattaneo, M.; Merlanti, F.

1988-01-01

In this paper, a revision of seismicity affecting the central segment of the Indonesian island arc ranging between 110° and 126° E is presented. Using the areal and in-depth distribution of seismic activity, strain release maps and focal mechanisms, lateral changes in the Wadati-Benioff zone have been analyzed to determine possible boundaries between portions of lithosphere with different subduction geometries. The seismicity pattern indicates that the Sumbawa-Flores-Wetar sector shows different forms of behaviour with respect to the adjacent sectors. These include driving mechanism, inclination and continuity of the subducting slab and subduction features. This area therefore seems to be isolated from the Sunda and Banda arcs by two principal boundaries, these having a nearly N-S trend in the Bali region and with a nearly E-W trend in the region ranging between Wetar-Northern Timor and Tanibar. The first boundary, characterized by an absolute minimum of seismic activity at all ranges of depth, has been interpreted in terms of subduction of the Roo Rise aseismic bathymetric ridge. For the second boundary, dividing a northwardly steeply-dipping slab from an E-W subducting slab dipping with an angle of 30 ° -40 °, a tear in the upper part (depth less than 300 km) and a hinge fault system in the deepest part of the lithosphere, have been proposed. From the analysis of focal mechanisms of shallow earthquakes, it was inferred that the central part of the Indonesian Arc is subject to a vortex-shaped stress field centred on the Savu Basin. In this model, the compressive axes appear to rotate counterclockwise (from SW to NNE) in the Sumba-Sumbawa-Western Flores region and clockwise (from W to NNW) in the Timor-Eastern Flores zone. To interpret these features, on the basis of seismological evidence, a lateral discontinuity in the arc-trench system close to Sumba, a collision between Sumba and Sumbawa and a rotation towards the north-northeast of Sumba have been suggested. The proposed structural discontinuity, trending NW-SE, may represent a major transcurrent fault zone through which the Australian continental lithosphere comes into contact with the Indian oceanic lithosphere.

Thermal anomalies and magmatism due to lithospheric doubling and shifting

NASA Astrophysics Data System (ADS)

Vlaar, N. J.

1983-11-01

We present some thermal and magmatic consequences of the processes of lithospheric doubling and lithospheric shifting. Lithospheric doubling concerns the obduction of a cold continental or old oceanic lithospheric plate over a young and hot oceanic lithosphere/upper mantle system, including an oceanic ridge. Lithospheric shifting concerns the translation and rotation of a lithospheric plate relative to the upper mantle. In both cases the resulting thermal state of the upper mantle below the obducting or shifting lithosphere may be perturbed relative to a "normal" continental or oceanic geothermal situation. The perturbed geothermal state gives rise to a density inversion at depth and thus induces a vertical gravitational instability which favours magmatism. We speculate about the magmatic consequences of this situation and infer that in the case of lithospheric doubling our model may account for the petrology and geochemistry of the resulting magma. The original layering and composition of the overridden young oceanic lithosphere may strongly influence magmatic processes. We dwell shortly on the genesis of kimberlites within the framework of our lithospheric doubling model and on magmatism in general. Lithospheric recycling is inherent to the mechanism of lithospheric doubling.

Reconciling Geodetic Deformation and Long-term Exhumation Rates Across the Western Greater Caucasus

NASA Astrophysics Data System (ADS)

Avdeev, B.; Niemi, N. A.

2011-12-01

Low modern geodetic strain rates and minimal instrumentally recorded seismicity in the western Greater Caucasus contradict the the high topography, deep exhumation, and young low-temperature thermochronometric ages indicative of active tectonic deformation in this mountain range. We use new and existing low-temperature thermochronometric data to show that the rate of present-day convergence across the range is sufficient to sustain observed rates of long-term exhumation and topographic growth. Thus, it is possible that the western Greater Caucasus has existed in an erosional steady state since shortly after the onset of exhumation of the range in Pliocene. We employ a Markov chain Monte Carlo algorithm to estimate the parameters of a thermokinematic model constrained by thermochronometric data and a focal mechanism solution from the 1991 Racha earthquake. We find that the thermochronometric data are best fit by exhumation commencing at ~4 Ma and driven by 3-5 mm/y of overthrusting on the Main Caucasus thrust dipping 40-45° at the surface and becoming flat at a depth of 15-20 km. This long-term exhumation model was compared with active rates of convergence in the western Greater Caucasus using an elastic half-space deformation model to estimate the geometry and rate of slip on a buried dislocation that best fits the observed geodetic velocity field. The estimated active slip of 4-7 mm/y is comparable to the long-term rate of overthrusting and is, therefore, sufficient to produce the observed rock uplift. Up to 4 mm/y excess of active convergence may potentially be consumed by underthrusting of the Transcaucasus or on faults south of the Main Caucasus thrust. We conclude that high rates of rock uplift observed in the western Greater Caucasus are the result of focused shortening occurring on a single fault. This differs from the deformation style of the eastern Greater Caucasus, where a larger amount of shortening is distributed across the width of the range with slip occurring on numerous north and south-verging thrusts, resulting in lower uplift rates, lower relief, and lower exhumation. We hypothesize that this difference in tectonic styles is a result of differences in the lithosphere of the eastern and western Transcaucasus. To the west, the Greater Caucasus are juxtaposed against an ancient strong and buoyant micro-continent, while on the east, they are faulted against a Jurassic island arc that is likely to have a thinner, less competent lithosphere.

Reconciling the geological history of western Turkey with plate circuits and mantle tomography

NASA Astrophysics Data System (ADS)

van Hinsbergen, Douwe J. J.; Kaymakci, Nuretdin; Spakman, Wim; Torsvik, Trond H.

2010-09-01

We place the geological history since Cretaceous times in western Turkey in a context of convergence, subduction, collision and slab break-off. To this end, we compare the west Anatolian geological history with amounts of Africa-Europe convergence calculated from the Atlantic plate circuit, and the seismic tomography images of the west Anatolian mantle structure. Western Turkish geology reflects the convergence between the Sakarya continent (here treated as Eurasia) in the north and Africa in the south, with the Anatolide-Tauride Block (ATB) between two strands of the Neotethyan ocean. Convergence between the Sakarya and the ATB started at least ~ 95-90 Myr ago, marked by ages of metamorphic soles of ophiolites that form the highest structural unit below Sakarya. These are underlain by high-pressure, low-temperature metamorphic rocks of the Tavşanlı and Afyon zones, and the Ören Unit, which in turn are underlain by the Menderes Massif derived from the ATB. Underthrusting of the ATB below Sakarya was since ~ 50 Ma, associated with high-temperature metamorphism and widespread granitic magmatism. Thrusting in the Menderes Massif continued until 35 Ma, after which there is no record of accretion in western Turkey. Plate circuits show that since 90 Ma, ~ 1400 km of Africa-Europe convergence occurred, of which ~ 700 km since 50 Ma and ~ 450 km since 35 Ma. Seismic tomography shows that the African slab under western Turkey is decoupled from the African Plate. This detached slab is a single, coherent body, representing the lithosphere consumed since 90 Ma. There was no subduction re-initiation after slab break-off. ATB collision with Europe therefore did not immediately lead to slab break-off but instead to delamination of subducting lithospheric mantle from accreting ATB crust, while staying attached to the African Plate. This led to asthenospheric inflow below the ATB crust, high-temperature metamorphism and felsic magmatism. Slab break-off in western Turkey probably occurred ~ 15 Myr ago, after which overriding plate compression and rotation accommodated ongoing Africa-Europe convergence. Slab break-off was accommodated along a vertical NE trending subduction transform edge propagator (STEP) fault zone, accelerating southwestward slab retreat of the Aegean slab. The SE Aegean slab edge may have existed already since early Miocene times or before, but started to rapidly roll back along the southeastern Aegean STEP in middle Miocene times, penetrating the Aegean region in the Pliocene.

Evolution of the lithosphere in the Indian Ocean from combined earthquake and ambient noise tomography

NASA Astrophysics Data System (ADS)

Ma, Zhitu; Dalton, Colleen A.

2017-01-01

Rayleigh wave dispersion extracted from ambient seismic noise has been widely used to image crustal and uppermost mantle structure. Applications of this approach in continental settings are abundant, but there have been relatively few studies within ocean basins. By cross-correlating ambient noise recorded at broadband seismic stations around the Indian Ocean, we demonstrate the feasibility of extracting high-quality, long-period (10-30 mHz) Rayleigh waves that traverse the entire ocean basin. High-quality Rayleigh wave cross-correlation functions can be obtained from stacking waveforms over less than 2 years at land stations and less than 4 years at island stations. We show that adding the dispersion information extracted from ambient noise to a global earthquake data set can improve the resolution of phase velocity maps by about 20% in the northern Indian Ocean, where the station distribution is the best. We find that a plate cooling model with a potential temperature of 1450°C and plate thickness of 125 km can fit both the seismic observations and seafloor topography. The Seychelles-Mascarene Plateau is characterized by anomalously slow velocity at 30 mHz. The inclusion of ambient noise data in the tomographic inversion shifts the slow velocity anomaly into better agreement with the topographic relief, allowing us to estimate its crustal thickness and confirm that the plateau's elevation is supported by thick crust. The 10 and 20 mHz phase velocity maps show a strong asymmetry across the Central Indian Ridge that is best explained by eastward asthenospheric flow emanating from nearby hot spots.

Destruction of the North China Craton: Lithosphere folding-induced removal of lithospheric mantle?

NASA Astrophysics Data System (ADS)

Zhang, Kai-Jun

2012-01-01

High heat flow, high surface topography, and widespread volcanism indicate that the lithospheric mantle of typical cratonic character of the North China Craton has been seriously destroyed in its eastern half. However, the mechanism of this process remains open to intense debate. Here lithosphere folding-induced lithospheric mantle removal is proposed as a new mechanism for the destruction of the craton. Four main NNE-SSW-striking lithospheric-scale anticlines and synclines are recognized within North China east of the Helan fold-and-thrust belt. The lithosphere folding occurred possibly during the Late Triassic through Jurassic when the Yangzi Craton collided with the North China Craton. It was accompanied or followed by lithospheric dripping, and could have possibly induced the lithosphere foundering of the North China Craton. The lithosphere folding would have modified the lithosphere morphology, creating significant undulation in the lithospheric base and thus causing variations of the patterns of the small-scale convection. It also could have provoked the formation of new shear zones liable to impregnation of magma, producing linear incisions at the cratonic base and resulting in foundering of lithospheric mantle blocks. Furthermore, it generated thickening of the lithosphere or the lower crust and initiated the destabilization and subsequent removal of the lithospheric mantle.

Toward a Greater Kerguelen large igneous province: Evolving mantle source contributions in and around the Indian Ocean

NASA Astrophysics Data System (ADS)

Olierook, Hugo K. H.; Merle, Renaud E.; Jourdan, Fred

2017-06-01

The link between the Kerguelen large igneous province and several moderately-voluminous magmatic domains emplaced on continental crust near the relict triple junction of eastern Gondwana remains tentative. In particular, linking Sr-Nd-Pb isotopic ratios of the 90,000 km2 submerged Naturaliste Plateau at the relict triple junction of eastern Gondwana to the Kerguelen LIP were difficult due to previous age estimates of ca. 100 Ma. Sericite hydrothermal plateau ages as old as 127.6 ± 0.6 Ma indicate that the volcanism on the plateau began at or prior to ca. 128 Ma, which is > 25 m.y. older than previous estimations. These ages are closely matched by the then-nearby ca. 140-130 Ma Comei, 137-130 Ma Bunbury, 124 Ma Wallaby Plateau and 118-117 Ma Rajmahal-Bengal-Sylhet magmatic provinces. The Sr-Nd-Pb isotopic characteristics of the majority of these ca. 140-117 Ma circum-eastern Gondwana magmatic provinces display only source contributions from the depleted asthenosphere and lithosphere with negligible contribution from the Kerguelen mantle plume. The Comei Province shows a direct plume-related melt signature, probably because it sits directly in the center of the modeled plume head position at 140-130 Ma. We suggest that the Kerguelen mantle plume provided the additional heat necessary to melt the asthenosphere and lithosphere of the circum-eastern Gondwanan magmatic provinces. Only after the motion of the Kerguelen plume head into the nascent Indian Ocean at ca. 100-95 Ma does a significant melt contribution from the Kerguelen mantle plume become evident in the isotopic signature, a signal that persists until the present-day. Despite differences in source contributions over time, it is clear that the Kerguelen mantle plume is necessary for the production of all the circum-eastern Gondwana magmatic domains, which we propose should be referred to as the Greater Kerguelen Large Igneous Province.

3D geodynamic models for the development of opposing continental subduction zones: The Hindu Kush-Pamir example

NASA Astrophysics Data System (ADS)

Liao, Jie; Gerya, Taras; Thielmann, Marcel; Webb, A. Alexander G.; Kufner, Sofia-Katerina; Yin, An

2017-12-01

The development of opposing continental subduction zones remains scantly explored in three dimensions. The Hindu Kush-Pamir orogenic system at the western end of the Himalayan orogen provides a rare example of continental collision linked to two opposing intra-continental subduction zones. The subducted plates feature a peculiar 3D geometry consisting of two distinct lithospheric fragments with different polarities, subduction angles and slab-curvatures beneath the Hindu Kush and Pamir, respectively. Using 3D geodynamic modeling, we simulate possible development of two opposing continental subduction zones to understand the dynamic evolution of the Hindu Kush-Pamir orogenic system. Our geodynamic model reproduces the major tectonic elements observed: (1) the deeper subduction depth, the steeper dip angle and the southward offset of the Hindu Kush subduction zone relative to the Pamir naturally occur if convergence direction of the subducting Indian plate and dip-direction of the Hindu Kush subduction zone match. (2) The formation of the highly asymmetrically curved Pamir region and the south-dipping subduction is promoted by the initial geometry of the indenting Indian lithosphere together with the existence of a major strike-slip fault on the eastern margin of the Pamir region. (3) Subduction of only the lower continental crust during continental collision can occur if the coupling between upper and lower crusts is weak enough to allow a separation of these two components, and that (4) the subduction of mainly lower crust then facilitates that conditions for intermediate-depth seismicity can be reached. (5) The secondary tectonic features modeled here such as strike-slip-fault growth, north-northwest striking extension zone, and lateral flow of the thickened ductile upper crust are comparable to the current tectonics of the region. (6) Model results are further compared to the potentially similar orogenic system, i.e., the Alpine orogen, in terms of the curved Western Alpine arc and the two opposing subducted slabs beneath the Alps and the Dinarides.

How plume-ridge interaction shapes the crustal thickness pattern of the Réunion hotspot track

NASA Astrophysics Data System (ADS)

Bredow, Eva; Steinberger, Bernhard; Gassmöller, Rene; Dannberg, Juliane

2017-08-01

The Réunion mantle plume has shaped a large area of the Earth's surface over the past 65 million years: from the Deccan Traps in India along the hotspot track comprising the island chains of the Laccadives, Maldives, and Chagos Bank on the Indian plate and the Mascarene Plateau on the African plate up to the currently active volcanism at La Réunion Island. This study addresses the question how the Réunion plume, especially in interaction with the Central Indian Ridge, created the complex crustal thickness pattern of the hotspot track. For this purpose, the mantle convection code ASPECT was used to design three-dimensional numerical models, which consider the specific location of the plume underneath moving plates and surrounded by large-scale mantle flow. The results show the crustal thickness pattern produced by the plume, which altogether agrees well with topographic maps. Especially two features are consistently reproduced by the models: the distinctive gap in the hotspot track between the Maldives and Chagos is created by the combination of the ridge geometry and plume-ridge interaction; and the Rodrigues Ridge, a narrow crustal structure which connects the hotspot track and the Central Indian Ridge, appears as the surface expression of a long-distance sublithospheric flow channel. This study therefore provides further insight how small-scale surface features are generated by the complex interplay between mantle and lithospheric processes.

The history and fate of three families of lithosphere on Earth

NASA Astrophysics Data System (ADS)

Lee, C. T.

2016-12-01

Based on compilations of surface heat flux to constrain the thermal boundary layer thickness, lithosphere thickness can be shown to have a trimodal distribution. In ocean basins, lithosphere thickness ranges from thin (<10 km) beneath young ocean basins, which dominate, to thick (<100 km) beneath old ocean basins, which are rare due to subduction. Continents have thicker lithospheres and define two additional peaks: 30%, reflecting most of the Archean cratons, are 180-220 km thick and 60% are 90-140 km thick. While ocean basins subduct after their lithospheres grow thick, continents do not, despite their thicker lithospheres. The insubductibility of continents is because the buoyancy of thick crust compensates for the thick cold lithosphere and because continental thermal boundary layers do not grow indefinitely. Lithospheric growth is understood to be limited by the onset of small-scale convective instabilities, but why then do continental lithospheres have two different critical thicknesses? Initial thickness, at the time of formation, is critical. Continental lithospheres less than 120 km thick are subject to magmatic modification (refertilization) in the form of thermo-chemical erosion, which gradually thins the lithosphere. Lithospheres greater than 120 km appear to be relatively immune to significant lithospheric thinning. This may in part be because refertilization-driven destabilization does not occur since deep melting is suppressed beneath thick lithosphere. To resist thermal thinning, it seems necessary that anomalously thick lithospheres were born with intrinsic strength, widely hypothesized to have been imparted by the unusual petrogenesis of cratonic mantle, wherein high degrees of melting early in Earth's history resulted in the formation of a dehydrated and strong chemical boundary layer. Another possibility is that cratonic mantle is characterized by the strengthening effects of larger grain size, owing to the high degrees of melting that decrease the number of clinopyroxene pinning points. In summary, a lithosphere's fate depends on the nature of its origin. Continental lithospheres born thick will have long, boring lives, continental lithospheres born thin will be forever tormented, and oceanic lithospheres are fated to have calm but brief lives at the Earth's surface.

Mapping the megathrust beneath the northern Gulf of Alaska using wide-angle seismic reflection/refraction profiles

DOE Office of Scientific and Technical Information (OSTI.GOV)

Brocher, T.M.; Fuis, G.S.; Fisher, M.A.

1993-04-01

In the northern Gulf of Alaska and Prince William Sound, wide-angle seismic reflection/refraction profiling, earthquake studies, and laboratory measurements of physical properties are used to determine the geometry of the Prince William and Yakutat terranes, and the subducting Pacific plate. In this complex region, the Yakutat terrane is underthrust beneath the Prince William terrane, and both terranes are interpreted to be underlain by the Pacific plate. Wide-angle seismic reflection/refraction profiles recorded along 5 seismic lines are used to unravel this terrane geometry. Modeled velocities in the upper crust of the Prince William terrane (to 18-km depth) agree closely with laboratorymore » velocity measurements of Orca Group phyllites and quartzofeldspathic graywackes (the chief components of the Prince William terrane) to hydrostatic pressures as high as 600 MPa (6 KBAR). An interpretation consistent with these data extends the Prince William terrane to at least 18-km depth. A landward dipping reflection at depths of 16--24 km is interpreted as the base of the Prince William terrane. This reflector corresponds to the top of the Wadati-Benioff zone seismicity and is interpreted as the megathrust. Beneath this reflector is a 6.9-km/s refractor, that is strongly reflective and magnetic, and is interpreted to be gabbro in Eocene age oceanic crust of the underthrust Yakutat terrane. Both wide-angle seismic and magnetic anomaly data indicate that the Yakutat terrane has been underthrust beneath the Prince William terrane for at least a few hundred kilometers. Wide-angle seismic data are consistent with a 9 to 10[degree] landward dip of the subducting Pacific plate, distinctly different from the inferred average 3 to 4[degree] dip of the overlying 6.9-km/s refractor and Wadati-Benioff seismic zone. The preferred interpretation of the geophysical data is that one composite plate, composed of the Pacific and Yakutat plates, is subducting beneath southern Alaska.« less

Structural characteristics of the décollement zone and underthrust sediments in the Nankai accretionary prism: Geologic architectures in the Site C0023, IODP Expedition 370

NASA Astrophysics Data System (ADS)

Yamamoto, Y.; Okutsu, N.; Yamada, Y.; Bowden, S.; Tonai, S.; Yang, K.; Tsang, M. Y.; Hirose, T.; Kamiya, N.

2017-12-01

Expedition 370 penetrated the accretionary prism, plate boundary décollement zone, and underthrust sediment and touched the basement basalt on the Philippine Sea Plate. The drilling site (C0023) is located 4 km NE from the legacy sites, Sites 808 and 1174. Compared to the legacy sites, the décollement zone is characterized by weak and intermittent negative reflectors in the seismic profile. Onboard physical properties, e.g. porosity and P-wave velocity data, indeed show the smaller gaps at the top of the décollement zone. The nature of the deformation along the décollement zone represented 40 m thick phacoidal deformation zone composed of fragmented mudstone with slickenlines on the surfaces in the Sites 808 and 1174. Compare with this, décollement zone in Site C0023 represented the weaker and non-localized deformation zone comprised of alternating zone of 1 m thick phacoidal deformation zones and a few 10 m of intact intervals in the Site C0023. Many normal faults striking parallel to the trench were identified just below the décollement zone, which is indicative of non-localized deformations along the décollement zone. Many of these faults were accompanied with calcite and sulphate mineral veins (anhydrite and barite), indicative of high-temperature fluid migration just above the ridge-spreading center. Based on the paleomagnetic restoration of structure to the geologic coordinate, attitudes of the bedding and fault planes in the Site C0023 are controlled by two factors: 1) subduction/accretion producing the trench-parallel bedding strikes and trench-perpendicular principal stress and 2) ridge spreading that produces ridge-parallel bedding and vein strikes. The former developed in the accretionary prism and the upper part of the underthrust sediment (<900 mbsf), whereas the latter occurs in the lower part (>900 mbsf). These tectonic variations might affect fluid migration pathways.

Lithospheric thinning beneath rifted regions of Southern California.

PubMed

Lekic, Vedran; French, Scott W; Fischer, Karen M

2011-11-11

The stretching and break-up of tectonic plates by rifting control the evolution of continents and oceans, but the processes by which lithosphere deforms and accommodates strain during rifting remain enigmatic. Using scattering of teleseismic shear waves beneath rifted zones and adjacent areas in Southern California, we resolve the lithosphere-asthenosphere boundary and lithospheric thickness variations to directly constrain this deformation. Substantial and laterally abrupt lithospheric thinning beneath rifted regions suggests efficient strain localization. In the Salton Trough, either the mantle lithosphere has experienced more thinning than the crust, or large volumes of new lithosphere have been created. Lack of a systematic offset between surface and deep lithospheric deformation rules out simple shear along throughgoing unidirectional shallow-dipping shear zones, but is consistent with symmetric extension of the lithosphere.

Lithosphere mantle density of the North China Craton based on gravity data

NASA Astrophysics Data System (ADS)

Xia, B.; Artemieva, I. M.; Thybo, H.

2017-12-01

Based on gravity, seismic and thermal data we constrained the lithospheric mantle density at in-situ and STP condition. The gravity effect of topography, sedimentary cover, Moho and Lithosphere-Asthenosphere Boundary variation were removed from free-air gravity anomaly model. The sedimentary covers with density range from 1.80 g/cm3 with soft sediments to 2.40 g/cm3 with sandstone and limestone sediments. The average crustal density with values of 2.70 - 2.78 g/cm3 which corresponds the thickness and density of the sedimentary cover. Based on the new thermal model, the surface heat flow in original the North China Craton including western block is > 60 mW/m2. Moho temperature ranges from 450 - 600 OC in the eastern block and in the western block is 550 - 650 OC. The thermal lithosphere is 100 -140 km thick where have the surface heat flow of 60 - 70 mW/m2. The gravity effect of surface topography, sedimentary cover, Moho depth are 0 to +150 mGal, - 20 to -120 mGal and +50 to -200 mGal, respectively. By driving the thermal lithosphere, the gravity effect of the lithosphere-asthenosphere boundary ranges from 20 mGal to +200 mGal which shows strong correction with the thickness of the lithosphere. The relationship between the gravity effect of the lithosphere-asthenosphere boundary and the lithosphere thickness also for the seismic lithosphere, and the value of gravity effect is 0 to +220 mGal. The lithospheric mantle residual gravity which caused by lithospheric density variation range from -200 to +50 mGal by using the thermal lithosphere and from -250 to +100 mGal by driving the seismic lithosphere. For thermal lithosphere, the lithospheric mantle density with values of 3.21- 3.26 g/cm3 at in-situ condition and 3.33 - 3.38 g/cm3 at STP condition. Using seismic lithosphere, density of lithosphere ranges from 3.20 - 3.26 g/cm3 at in-situ condition and 3.31 - 3.41 g/cm3 at STP condition. The subcontinental lithosphere of the North China Craton is highly heterogeneous with Archean lithosphere at the southwestern of the Eastern Block, major the Trans-North China Orogen and western part of the Western Block. The lithospheric mantle beneath the northern part of the Eastern Block, central segment of the Trans-North China Craton and the eastern margin of the Western Block have experienced modification and replacement.

Southern hemisphere craton modification by plume-lithosphere interaction

NASA Astrophysics Data System (ADS)

Hu, J.; Liu, L.; Faccenda, M.; Zhou, Q.; Fischer, K. M.; Marshak, S.; Lundstrom, C.

2017-12-01

The longevity of cratons is generally attributed to neutrally-to-positively buoyant and mechanically strong lithosphere that shields the cratonic crust from underlying mantle dynamics. Large portions of the cratonic lithospheres in South America and Africa, however, have experienced significant modification since the Mesozoic, as demonstrated by widespread Cretaceous uplift and volcanism, present-day high topography, thin crust, and the presence of seismically fast but neutrally buoyant upper-mantle anomalies. We show that these observations reflect a permanent increase in lithospheric buoyancy due to plume-triggered lithosphere deformation and deep lithospheric loss during Late Cretaceous to early Tertiary, as further evidenced by positive lithosphere residual topography, negative lithosphere residual gravity and the realignment of seismic anisotropy in the cratonic roots. Lithosphere in these regions has been thermally reestablished since then, as confirmed by its present-day low heat flow and high seismic velocities. We conclude that lowermost cratonic lithospheres is compositionally denser than the asthenospheric mantle and can be episodically removed when perturbed by underlying mantle dynamics, while the shallower buoyant lithosphere helps to stabilize cratonic crust over billions of years. We further propose that zones where lithosphere was lost would take tens of millions of years to recover thermally, but the density of the new thermal root would remain less than that of the intact root.

Viscoelastic Lithosphere Response and Stress Memory of Tectonic Force History (Invited)

NASA Astrophysics Data System (ADS)

Kusznir, N. J.

2009-12-01

While great attention is often paid to the details of creep deformation mechanisms, brittle failure and their compositional controls when predicting the response of lithosphere to tectonic forces, the lithosphere’s elastic properties are usually neglected; a viscous rheology alone is often used to predict the resulting distribution of stress with depth or to determine lithosphere strength. While this may simplify geodynamic modelling of lithosphere response to tectonic processes, the omission of the elastic properties can often give misleading or false predictions. The addition of the elastic properties of lithosphere material in the form of a visco-elastic rheology results is a fundamentally different lithosphere response. This difference can be illustrated by examining the application of horizontal tectonic force to a section of lithosphere incorporating the brittle-visco-elastic response of each infinitesimal lithosphere layer with temperature and stress dependent viscous rheology. The transient response of a visco-elastic lithosphere to a constant applied tectonic force and the resulting distribution of stress with depth are substantially different from that predicted by a viscous lithosphere model, with the same lithosphere composition and temperature structure, subjected to a constant lateral strain rate. For visco-elastic lithosphere subject to an applied horizontal tectonic force, viscous creep in the lower crust and mantle leads to stress decay in these regions and to stress amplification in the upper lithosphere through stress redistribution. Cooling of lithosphere with a visco-elastic rheology results in thermal stresses which, as a consequence of stress dissipation by creep and brittle failure, results in a complex and sometimes counter-intuitive distribution of stress with depth. This can be most clearly illustrated for the cooling of oceanic lithosphere, however similar or more complex behaviour can be expected to occur for continental lithosphere. The application of changes in applied tectonic force with time to a visco-elastic lithosphere model results in reversals in the sign of stress with depth as a consequence of the “memory” of past stress dissipation by creep and brittle deformation. Because of this “memory”, locally stress polarity may be opposite to that of the current applied tectonic force. A lithosphere with viscous rheology displays no such “memory” of the applied tectonic stress history. The stress “memory” of lithosphere with visco-elastic rheology to its history of applied tectonic force, heating and cooling adds to its effective rheological complexity, particularly for continental lithosphere.

Identifying mantle lithosphere inheritance in controlling intraplate orogenesis

NASA Astrophysics Data System (ADS)

Heron, Philip J.; Pysklywec, Russell N.; Stephenson, Randell

2016-09-01

Crustal inheritance is often considered important in the tectonic evolution of the Wilson Cycle. However, the role of the mantle lithosphere is usually overlooked due to its difficulty to image and uncertainty in rheological makeup. Recently, increased resolution in lithosphere imaging has shown potential scarring in continental mantle lithosphere to be ubiquitous. In our study, we analyze intraplate deformation driven by mantle lithosphere heterogeneities from ancient Wilson Cycle processes and compare this to crustal inheritance deformation. We present 2-D numerical experiments of continental convergence to generate intraplate deformation, exploring the limits of continental rheology to understand the dominant lithosphere layer across a broad range of geological settings. By implementing a "jelly sandwich" rheology, common in stable continental lithosphere, we find that during compression the strength of the mantle lithosphere is integral in generating deformation from a structural anomaly. We posit that if the continental mantle is the strongest layer within the lithosphere, then such inheritance may have important implications for the Wilson Cycle. Furthermore, our models show that deformation driven by mantle lithosphere scarring can produce tectonic patterns related to intraplate orogenesis originating from crustal sources, highlighting the need for a more formal discussion of the role of the mantle lithosphere in plate tectonics.

Finding the last 200Ma of subducted lithosphere in tomography and incorporating it into plate reconstructions

NASA Astrophysics Data System (ADS)

Suppe, J.; Wu, J.; Chen, Y. W.

2016-12-01

Precise plate-tectonic reconstruction of the Earth has been constrained largely by the seafloor magnetic-anomaly record of the present oceans formed during the dispersal of the last supercontinent since 200Ma. The corresponding world that was lost to subduction has been only sketchily known. We have developed methodologies to map in 3D these subducted slabs of lithosphere in seismic tomography and unfold them to the Earth surface, constraining their initial size, shapes and locations. Slab edges are commonly formed at times of plate reorganization (for example bottom edges typically record initiation of subduction) such that unfolded slabs fit together at times of reorganization, as we illustrate for the Nazca slab at 80Ma and the western Pacific slabs between Kamchatka and New Zealand at 50Ma. Mapping to date suggests that a relatively complete and decipherable record of lithosphere subducted over the last 200Ma may exist in the mantle today, providing a storehouse for new discoveries. We briefly illustrate our procedure for obtaining slab-constrained plate-tectonic models from tomography with our recent study of the Philippine Sea plate, whose motions and tectonic history have been the least known of the major plates because it has been isolated from the global plate and hotspot circuit by trenches. We mapped and unfolded 28 subducted slabs in the mantle under East Asia and Australia/Oceania to depths of 1200km, with a subducted area of 25% of present-day global oceanic lithosphere, and incorporated them as constraints into a new globally-consistent plate reconstruction of the Philippine Sea and surrounding East Asia, leading to a number of new insights, including: [1] discovery of a major (8000 km x 2500 km) set of vanished oceans that we call the East Asia Sea that existed between the Pacific and Indian Oceans, now represented by flat slabs in the lower mantle under present-day Philippine Sea, eastern Sundaland and northern Australia and [2] the Philippine Sea nucleated as a small trench back-arc system along the East Asian Sea/Pacific boundary, adjacent to the Manus plume, somewhat analogous to the more recent nucleation of the Bismark Sea at the same Manus plume.

Density of the continental roots: Compositional and thermal contributions

USGS Publications Warehouse

Kaban, M.K.; Schwintzer, P.; Artemieva, I.M.; Mooney, W.D.

2003-01-01

The origin and evolution of cratonic roots has been debated for many years. Precambrian cratons are underlain by cold lithospheric roots that are chemically depleted. Thermal and petrologic data indicate that Archean roots are colder and more chemically depleted than Proterozoic roots. This observation has led to the hypothesis that the degree of depletion in a lithospheric root depends mostly on its age. Here we test this hypothesis using gravity, thermal, petrologic, and seismic data to quantify differences in the density of cratonic roots globally. In the first step in our analysis we use a global crustal model to remove the crustal contribution to the observed gravity. The result is the mantle gravity anomaly field, which varies over cratonic areas from -100 to +100 mGal. Positive mantle gravity anomalies are observed for cratons in the northern hemisphere: the Baltic shield, East European Platform, and the Siberian Platform. Negative anomalies are observed over cratons in the southern hemisphere: Western Australia, South America, the Indian shield, and Southern Africa. This indicates that there are significant differences in the density of cratonic roots, even for those of similar age. Root density depends on temperature and chemical depletion. In order to separate these effects we apply a lithospheric temperature correction using thermal estimates from a combination of geothermal modeling and global seismic tomography models. Gravity anomalies induced by temperature variations in the uppermost mantle range from -200 to +300 mGal, with the strongest negative anomalies associated with mid-ocean ridges and the strongest positive anomalies associated with cratons. After correcting for thermal effects, we obtain a map of density variations due to lithospheric compositional variations. These maps indicate that the average density decrease due to the chemical depletion within cratonic roots varies from 1.1% to 1.5%, assuming the chemical boundary layer has the same thickness as the thermal boundary layer. The maximal values of the density drop are in the range 1.7-2.5%, and correspond to the Archean portion of each craton. Temperatures within cratonic roots vary strongly, and our analysis indicates that density variations in the roots due to temperature are larger than the variations due to chemical differences. ?? 2003 Elsevier Science B.V. All rights reserved.

Magmatic record of India-Asia collision

PubMed Central

Zhu, Di-Cheng; Wang, Qing; Zhao, Zhi-Dan; Chung, Sun-Lin; Cawood, Peter A.; Niu, Yaoling; Liu, Sheng-Ao; Wu, Fu-Yuan; Mo, Xuan-Xue

2015-01-01

New geochronological and geochemical data on magmatic activity from the India-Asia collision zone enables recognition of a distinct magmatic flare-up event that we ascribe to slab breakoff. This tie-point in the collisional record can be used to back-date to the time of initial impingement of the Indian continent with the Asian margin. Continental arc magmatism in southern Tibet during 80–40 Ma migrated from south to north and then back to south with significant mantle input at 70–43 Ma. A pronounced flare up in magmatic intensity (including ignimbrite and mafic rock) at ca. 52–51 Ma corresponds to a sudden decrease in the India-Asia convergence rate. Geological and geochemical data are consistent with mantle input controlled by slab rollback from ca. 70 Ma and slab breakoff at ca. 53 Ma. We propose that the slowdown of the Indian plate at ca. 51 Ma is largely the consequence of slab breakoff of the subducting Neo-Tethyan oceanic lithosphere, rather than the onset of the India-Asia collision as traditionally interpreted, implying that the initial India-Asia collision commenced earlier, likely at ca. 55 Ma. PMID:26395973

Crustal structure of the Murray Ridge, northwest Indian Ocean, from wide-angle seismic data

NASA Astrophysics Data System (ADS)

Minshull, T. A.; Edwards, R. A.; Flueh, E. R.

2015-07-01

The Murray Ridge/Dalrymple Trough system forms the boundary between the Indian and Arabian plates in the northern Arabian Sea. Geodetic constraints from the surrounding continents suggest that this plate boundary is undergoing oblique extension at a rate of a few millimetres per year. We present wide-angle seismic data that constrains the composition of the Ridge and of adjacent lithosphere beneath the Indus Fan. We infer that Murray Ridge, like the adjacent Dalrymple Trough, is underlain by continental crust, while a thin crustal section beneath the Indus Fan represents thinned continental crust or exhumed serpentinized mantle that forms part of a magma-poor rifted margin. Changes in crustal structure across the Murray Ridge and Dalrymple Trough can explain short-wavelength gravity anomalies, but a long-wavelength anomaly must be attributed to deeper density contrasts that may result from a large age contrast across the plate boundary. The origin of this fragment of continental crust remains enigmatic, but the presence of basement fabrics to the south that are roughly parallel to Murray Ridge suggests that it separated from the India/Seychelles/Madagascar block by extension during early breakup of Gondwana.

Cretaceous evolution of the Indian Plate and consequences for the formation, deformation and obduction of adjacent oceanic crust

NASA Astrophysics Data System (ADS)

Gaina, C.; Van Hinsbergen, D. J.; Spakman, W.

2012-12-01

As part of the gradual Gondwana dispersion that started in the Jurassic, the Indian tectonic block was rifted away from the Antarctica-Australian margins, probably in the Early-Mid Cretaceous and started its long journey to the north until it collided with Eurasia in the Tertiary. In this contribution first we will revise geophysical and geological evidences for the formation of oceanic crust between India and Antarctica, India and Madagascar, and India and Somali/Arabian margins. This information and possible oceanic basin age interpretation are placed into regional kinematic models. Three important compressional events NW and W of the Indian plate are the result of the opening of the Enderby Basin from 132 to 124 Ma, the first phase of seafloor spreading in the Mascarene basin approximately from 84 to 80 Ma, and the incipient opening of the Arabian Sea and the Seychelles microplate formation around 65 to 60 Ma. Based on retrodeformation of the Afghan-Pakistan part of the India-Asia collision zone and the eastern Oman margin, the ages of regional ophiolite emplacement and crystallization of its oceanic crust, as well as the plate tectonic setting of these ophiolites inferred from its geochemistry, we evaluate possible scenarios for the formation of intra-oceanic subduction zones and their evolution until ophiolite emplacement time. Our kinematic scenarios are constructed for several regional models and are discussed in the light of global tomographic models that may image some of the subducted Cretaceous oceanic lithosphere.

On the role of heat flow, lithosphere thickness and lithosphere density on gravitational potential stresses

NASA Astrophysics Data System (ADS)

Pascal, Christophe

2006-10-01

Gravitational potential stresses (GPSt) are known to play a first-order role in the state of stress of the Earth's lithosphere. Previous studies focussed mainly on crust elevation and structure and little attention has been paid to modelling GPSt using realistic lithospheric structures. The aim of the present contribution is to quantify gravitational potential energies and stresses associated with stable lithospheric domains. In order to model realistic lithosphere structures, a wide variety of data are considered: surface heat flow, chemical depletion of mantle lithosphere, crustal thickness and elevation. A numerical method is presented which involves classical steady-state heat equations to derive lithosphere thickness, geotherm and density distribution, but additionally requires the studied lithosphere to be isostatically compensated at its base. The impact of varying surface and crustal heat flow, topography, Moho depth and crust density on the signs and magnitudes of predicted GPSt is systematically explored. In clear contrast with what is assumed in most previous studies, modelling results show that the density structure of the mantle lithosphere has a significant impact on the value of the predicted GPSt, in particular in the case of thick lithospheres. Using independent information from the literature, the method was applied to get insights in the state of stress of continental domains with contrasting tectono-thermal ages. The modelling results suggest that in the absence of tectonic stresses Phanerozoic and Proterozoic lithospheres are spontaneously submitted to compression whereas Archean lithospheres are in a neutral to slightly tensile stress state. These findings are in general in good agreement with global stress measurements and observed geoid undulations.

Magma-derived CO2 emissions in the Tengchong volcanic field, SE Tibet: Implications for deep carbon cycle at intra-continent subduction zone

NASA Astrophysics Data System (ADS)

Zhang, Maoliang; Guo, Zhengfu; Sano, Yuji; Zhang, Lihong; Sun, Yutao; Cheng, Zhihui; Yang, Tsanyao Frank

2016-09-01

Active volcanoes at oceanic subduction zone have long been regard as important pathways for deep carbon degassed from Earth's interior, whereas those at continental subduction zone remain poorly constrained. Large-scale active volcanoes, together with significant modern hydrothermal activities, are widely distributed in the Tengchong volcanic field (TVF) on convergent boundary between the Indian and Eurasian plates. They provide an important opportunity for studying deep carbon cycle at the ongoing intra-continent subduction zone. Soil microseepage survey based on accumulation chamber method reveals an average soil CO2 flux of ca. 280 g m-2 d-1 in wet season for the Rehai geothermal park (RGP). Combined with average soil CO2 flux in dry season (ca. 875 g m-2 d-1), total soil CO2 output of the RGP and adjacent region (ca. 3 km2) would be about 6.30 × 105 t a-1. Additionally, we conclude that total flux of outgassing CO2 from the TVF would range in (4.48-7.05) × 106 t a-1, if CO2 fluxes from hot springs and soil in literature are taken into account. Both hot spring and soil gases from the TVF exhibit enrichment in CO2 (>85%) and remarkable contribution from mantle components, as indicated by their elevated 3He/4He ratios (1.85-5.30 RA) and δ13C-CO2 values (-9.00‰ to -2.07‰). He-C isotope coupling model suggests involvement of recycled organic metasediments and limestones from subducted Indian continental lithosphere in formation of the enriched mantle wedge (EMW), which has been recognized as source region of the TVF parental magmas. Contamination by crustal limestone is the first-order control on variations in He-CO2 systematics of volatiles released by the EMW-derived melts. Depleted mantle and recycled crustal materials from subducted Indian continental lithosphere contribute about 45-85% of the total carbon inventory, while the rest carbon (about 15-55%) is accounted by limestones in continental crust. As indicated by origin and evolution of the TVF volatiles, mantle-derived magmas at continental subduction zone can act as important triggers for liberation of carbon stored in crustal carbonate rocks, which has the potential to be a complement to volatile recycling mechanism at subduction zones. Variations in He-Nd-Sr isotopes of magmas and volatiles from different types of plate boundaries suggest higher amounts of recycled materials for mantle wedge enrichment of continental subduction zone relative to that of oceanic subduction zone.

Formation and evolution of a metasomatized lithospheric root at the motionless Antarctic plate: the case of East Island, Crozet Archipelago (Indian Ocean)

NASA Astrophysics Data System (ADS)

Meyzen, Christine; Marzoli, Andrea; Bellieni, Giuliano; Levresse, Gilles

2016-04-01

Sitting atop the nearly stagnant Antarctic plate (ca. 6.46 mm/yr), the Crozet archipelago midway between Madagascar and Antarctica constitutes a region of unusually shallow (1543-1756 m below sea level) and thickened oceanic crust (10-16.5 km), high geoid height, and deep low-velocity zone, which may reflect the surface expression of a mantle plume. Here, we present new major and trace element data for Quaternary sub-aerial alkali basalts from East Island, the easterly and oldest island (ca. 9 Ma) of the Crozet archipelago. Crystallization at uppermost mantle depth and phenocryst accumulation have strongly affected their parental magma compositions. Their trace element patterns show a large negative K anomaly relative to Ta-La, moderate depletions in Rb and Ba with respect to Th-U, and heavy rare earth element (HREE) depletions relative to light REE. These characteristics allow limits to be placed upon the composition and mineralogy of their mantle source. The average trace element spectrum of East Island basalts can be matched by melting of about 2 % of a garnet-phlogopite-bearing peridotite source. The stability field of phlogopite restricts melting depth to lithospheric levels. The modelled source composition requires a multistage evolution, where the mantle has been depleted by melt extraction before having been metasomatized by alkali-rich plume melts. The depleted mantle component may be sourced by residual mantle plume remnants stagnated at the melting locus due to a weak lateral flow velocity inside the melting regime, whose accumulation progressively edifies a depleted lithospheric root above the plume core. Low-degree alkali-rich melts are likely derived from the plume source. Such a mantle source evolution may be general to both terrestrial and extraterrestrial environments where the lateral component velocity of the mantle flow field is extremely slow.

Structure and sediment budget of Yinggehai-Song Hong basin, South China Sea: Implications for Cenozoic tectonics and river basin reorganization in Southeast Asia

NASA Astrophysics Data System (ADS)

Lei, Chao; Ren, Jianye; Sternai, Pietro; Fox, Matthew; Willett, Sean; Xie, Xinong; Clift, Peter D.; Liao, Jihua; Wang, Zhengfeng

2015-08-01

The temporal link between offshore stratigraphy and onshore topography is of key importance for understanding the long-term surface evolution of continental margins. Here we present a grid of regional, high-quality reflection seismic and well data to characterize the basin structure. We identify fast subsidence of the basin basement and a lack of brittle faulting of the offshore Red River fault in the Yinggehai-Song Hong basin since 5.5 Ma, despite dextral strike-slip movement on the onshore Red River fault. We calculate the upper-crustal, whole-crustal, and whole-lithospheric stretching factors for the Yinggehai-Song Hong basin, which show that the overall extension observed in the upper crust is substantially less than that observed for the whole crust or whole lithosphere. We suggest that fast basement subsidence after 5.5 Ma may arise from crustal to lithospheric stretching by the regional dynamic lower crustal/mantle flow originated by collision between India-Eurasia and Indian oceanic subduction below the Eurasian margin. In addition, we present a basin wide sediment budget in the Yinggehai-Song Hong basin to reconstruct the sedimentary flux from the Red River drainage constrained by high-resolution age and seismic stratigraphic data. The sediment accumulation rates show a sharp increase at 5.5 Ma, which suggests enhanced onshore erosion rates despite a slowing of tectonic processes. This high sediment supply filled the accommodation space produced by the fast subsidence since 5.5 Ma. Our data further highlight two prominent sharp decreases of the sediment accumulation at 23.3 Ma and 12.5 Ma, which could reflect a loss of drainage area following headwater capture from the Paleo-Red River. However, the low accumulation rate at 12.5 Ma also correlates with drier and therefore less erosive climatic conditions.

The mantle lithosphere and the Wilson Cycle

NASA Astrophysics Data System (ADS)

Heron, Philip; Pysklywec, Russell; Stephenson, Randell

2017-04-01

In the view of the conventional theory of plate tectonics (e.g., the Wilson Cycle), crustal inheritance is often considered important in tectonic evolution. However, the role of the mantle lithosphere is usually overlooked due to its difficulty to image and uncertainty in rheological makeup. Deep seismic imaging has shown potential scarring in continental mantle lithosphere to be ubiquitous. Recent studies have interpreted mantle lithosphere heterogeneities to be pre-existing structures, and as such linked to the Wilson Cycle and inheritance. In our study, we analyze intraplate deformation driven by mantle lithosphere heterogeneities from ancient Wilson Cycle processes and compare this to crustal inheritance deformation. We present 2-D numerical experiments of continental convergence to generate intraplate deformation, exploring the limits of continental rheology to understand the dominant lithosphere layer across a broad range of geological settings. By implementing a "jelly sandwich" rheology, characteristic of stable continental lithosphere, we find that during compression the strength of the mantle lithosphere is integral in controlling deformation from a structural anomaly. We posit that if the continental mantle is the strongest layer within the lithosphere, then such inheritance may have important implications for the Wilson Cycle. Furthermore, our models show that deformation driven by mantle lithosphere scarring can produce tectonic patterns related to intraplate orogenesis originating from crustal sources, highlighting the need for a more formal discussion of the role of the mantle lithosphere in plate tectonics. We outline the difficulty in unravelling the causes of tectonic deformation, alongside discussing the role of deep lithosphere processes in plate tectonics.

Lithospheric Stress Tensor from Gravity and Lithospheric Structure Models

NASA Astrophysics Data System (ADS)

Eshagh, Mehdi; Tenzer, Robert

2017-07-01

In this study we investigate the lithospheric stresses computed from the gravity and lithospheric structure models. The functional relation between the lithospheric stress tensor and the gravity field parameters is formulated based on solving the boundary-value problem of elasticity in order to determine the propagation of stresses inside the lithosphere, while assuming the horizontal shear stress components (computed at the base of the lithosphere) as lower boundary values for solving this problem. We further suppress the signature of global mantle flow in the stress spectrum by subtracting the long-wavelength harmonics (below the degree of 13). This numerical scheme is applied to compute the normal and shear stress tensor components globally at the Moho interface. The results reveal that most of the lithospheric stresses are accumulated along active convergent tectonic margins of oceanic subductions and along continent-to-continent tectonic plate collisions. These results indicate that, aside from a frictional drag caused by mantle convection, the largest stresses within the lithosphere are induced by subduction slab pull forces on the side of subducted lithosphere, which are coupled by slightly less pronounced stresses (on the side of overriding lithospheric plate) possibly attributed to trench suction. Our results also show the presence of (intra-plate) lithospheric loading stresses along Hawaii islands. The signature of ridge push (along divergent tectonic margins) and basal shear traction resistive forces is not clearly manifested at the investigated stress spectrum (between the degrees from 13 to 180).

Preface to "Insights into the Earth's Deep Lithosphere"

DOE Office of Scientific and Technical Information (OSTI.GOV)

Pasyanos, M E

Dear Readers: I am pleased to present a special issue of Tectonophysics entitled 'Insights into the Earth's Deep Lithosphere.' This compilation sought to capture the flavor of the increasing number of studies that are emerging to investigate the complex lithospheric structure of the earth. This issue evolved out of a Fall 2007 AGU special session entitled 'Understanding the Earth's Deep Lithosphere' that I organized with Irina Artemieva from the University of Copenhagen. For that session, we solicited talks that discussed the increasing number of methods that have surfaced to study various aspects of the earth's deep lithosphere. These methods includemore » seismic, gravity, thermal, geochemical, and various combinations of these methods. The quality of the presentations (2 oral sessions with 16 talks and 23 associated poster presentations) was such that we felt that the emerging topic deserved a dedicated forum to address these questions in greater detail. The availability of new data sets has also improved the number and quality of lithospheric studies. With many new studies and methodologies, a better understanding of both continental and oceanic lithospheres is starting to emerge. Questions remain about the thickness and evolution of the lithosphere, the presence of lithospheric keels, the density and anisotropy of lithospheric roots, mechanisms of lithospheric thinning, and differences between mechanical, thermal and chemical boundary layers. While we did not get contributions on the full gamut of methods and regions, a lot of ground was covered in this issue's manuscripts. Like any collection of papers on the deep lithosphere, the topics are quite varied in methodology, geographic location, and what aspect of the lithosphere being studied. Still, the results highlight the rewarding aspects of earth structure, history, and evolution that can be gleaned. A brief synopsis of the papers contained in this issue is given.« less

Deformation of island-arc lithosphere due to steady plate subduction

NASA Astrophysics Data System (ADS)

Fukahata, Yukitoshi; Matsu'ura, Mitsuhiro

2016-02-01

Steady plate subduction elastically brings about permanent lithospheric deformation in island arcs, though this effect has been neglected in most studies based on elastic dislocation theory. We investigate the characteristics of the permanent lithospheric deformation using a kinematic model, in which steady slip motion is given along a plate interface in the elastic lithosphere overlying the viscoelastic asthenosphere under gravity. As a rule of thumb, long-term lithospheric deformation can be understood as a bending of an elastic plate floating on non-viscous fluid, because the asthenosphere behaves like water on the long term. The steady slip below the lithosphere-asthenosphere boundary does not contribute to long-term lithospheric deformation. Hence, the key parameters that control the lithospheric deformation are only the thickness of the lithosphere and the geometry of the plate interface. Slip on a plate interface generally causes substantial vertical displacement, and gravity always tries to retrieve the original gravitational equilibrium. For a curved plate interface gravity causes convex upward bending of the island-arc lithosphere, while for a planar plate interface gravity causes convex downward bending. Larger curvature and thicker lithosphere generally results in larger deformation. When the curvature changes along the plate interface, internal deformation is also involved intrinsically, which modifies the deformation field due to gravity. Because the plate interface generally has some curvature, at least near the trench, convex upward bending of the island-arc lithosphere, which involves uplift of island-arc and subsidence around the trench, is always realized. On the other hand, the deformation field of the island-arc lithosphere sensitively depends on lithospheric thickness and plate interface geometry. These characteristics obtained by the numerical simulation are consistent with observed topography and free-air gravity anomalies in subduction zones: a pair of topography and gravity anomalies, high in the arc and low around the trench, is observed without exceptions all over the world, while there are large variety in the amplitude and horizontal scale of the topography and gravity anomalies.

Crustal seismicity and the earthquake catalog maximum moment magnitudes (Mcmax) in stable continental regions (SCRs): correlation with the seismic velocity of the lithosphere

USGS Publications Warehouse

Mooney, Walter D.; Ritsema, Jeroen; Hwang, Yong Keun

2012-01-01

A joint analysis of global seismicity and seismic tomography indicates that the seismic potential of continental intraplate regions is correlated with the seismic properties of the lithosphere. Archean and Early Proterozoic cratons with cold, stable continental lithospheric roots have fewer crustal earthquakes and a lower maximum earthquake catalog moment magnitude (Mcmax). The geographic distribution of thick lithospheric roots is inferred from the global seismic model S40RTS that displays shear-velocity perturbations (δVS) relative to the Preliminary Reference Earth Model (PREM). We compare δVS at a depth of 175 km with the locations and moment magnitudes (Mw) of intraplate earthquakes in the crust (Schulte and Mooney, 2005). Many intraplate earthquakes concentrate around the pronounced lateral gradients in lithospheric thickness that surround the cratons and few earthquakes occur within cratonic interiors. Globally, 27% of stable continental lithosphere is underlain by δVS≥3.0%, yet only 6.5% of crustal earthquakes with Mw>4.5 occur above these regions with thick lithosphere. No earthquakes in our catalog with Mw>6 have occurred above mantle lithosphere with δVS>3.5%, although such lithosphere comprises 19% of stable continental regions. Thus, for cratonic interiors with seismically determined thick lithosphere (1) there is a significant decrease in the number of crustal earthquakes, and (2) the maximum moment magnitude found in the earthquake catalog is Mcmax=6.0. We attribute these observations to higher lithospheric strength beneath cratonic interiors due to lower temperatures and dehydration in both the lower crust and the highly depleted lithospheric root.

Origin of the high plateau in the Central Andes, Bolivia, South America

NASA Astrophysics Data System (ADS)

Lamb, Simon; Hoke, Leonore

1997-08-01

The Bolivian Altiplano, in the Central Andes of South America, is part of the second largest high plateau on Earth. It is an elongate region of subdued relief, ˜1.2 × 105 km2 and ˜4 km above sea level, bounded by the Eastern Cordillera and volcanic arc (Western Cordillera). Here the crust is up to ˜75 km thick. We describe the Cenozoic geological evolution of this region, using a revised chronostratigraphy and an analysis of the crustal and lithospheric structure. Crustal shortening and magmatic addition and, locally, sedimentation are the main mechanisms of Cenozoic crustal thickening, leading to nearly 4 km of surface uplift since the Paleocene. Addition of mafic melts appears to be a first-order mechanism of Cenozoic crustal growth, contributing ˜40% of the crustal thickening beneath the volcanic arc. Removal of the basal part of the lithosphere may have caused two episodes of widespread arc and behind-arc mafic volcanism, at ˜23 Ma and 0 - ˜5 Ma, contributing to the surface uplift. The Altiplano originated as a sedimentary basin, several hundred kilometers wide, between the proto-Western Cordillera and a narrow zone of uplift (proto-Eastern Cordillera) farther east. The latter zone formed by inversion of the center of a wide lacustrine or marine Cretaceous - Paleocene basin close to sea-level at ˜45 Ma. A thickness of 2-4 km of Paleogene continental elastics accumulated in the proto-Altiplano basin. Subsequently, in the Oligocene, we estimate that this region and the western margin of the Eastern Cordillera were technically shortened ˜22% (˜65 km), resulting in ˜9 km of average crustal thickening. The Altiplano basin was rejuvenated at ˜25 Ma and subsequently flooded with up to 8 km thickness of detritus eroded from the uplifting Eastern and Western Cordilleras. Between ˜25 and 5 Ma, folding and thrusting in the western margin of the Eastern Cordillera migrated westward into the center of the Altiplano basin, essentially terminating deposition, except in local subbasins, and accommodating ˜13% (˜30 km) of shortening and an estimated ˜7 km of average crustal thickening. Subsequently, there has been strike-slip deformation and limited local thrusting (< 5 km of shortening). Geomorphological and geochronological evidence for 1.5-2 km of surface uplift of this region since the Late Miocene suggests ˜14 km of lower crustal thickening beneath an essentially rigid "lid", and can be explained by ˜100-150 km of underthrusting of the Brazilian shield and adjacent regions beneath the eastern margin of the Central Andes. The present subdued relief in the Altiplano may be a result of ductile flow in the lower crust and sedimentation and erosion in an internal drainage basin.

Lithospheric structure, composition, and thermal regime of the East European Craton: Implications for the subsidence of the Russian platform

USGS Publications Warehouse

Artemieva, I.M.

2003-01-01

A new mechanism for Paleozoic subsidence of the Russian, or East European, platform is suggested, since a model of lithosphere tilting during the Uralian subduction does not explain the post-Uralian sedimentation record. Alternatively, I propose that the Proterozoic and Paleozoic rifting (when a platform-scale Central Russia rift system and a set of Paleozoic rifts were formed) modified the structure and composition of cratonic lithosphere, and these tectono-magmatic events are responsible for the post-Uralian subsidence of the Russian platform. To support this hypothesis, (a) the thermal regime and the thickness of the lithosphere are analyzed, and (b) lithospheric density variations of non-thermal origin are calculated from free-board constraints. The results indicate that Proterozoic and Paleozoic rifting had different effects on the lithospheric structure and composition. (1) Proterozoic rifting is not reflected in the present thermal regime and did not cause significant lithosphere thinning (most of the Russian platform has lithospheric thickness of 150-180 km and the lithosphere of the NE Baltic Shield is 250-300 km thick). Paleozoic rifting resulted in pronounced lithospheric thinning (to 120-140 km) in the southern parts of the Russian platform. (2) Lithospheric density anomalies suggest that Proterozoic-Paleozoic rifting played an important role in the platform subsidence. The lithospheric mantle of the Archean-early Proterozoic part of the Baltic Shield is ??? 1.4 ?? 0.2% less dense than the typical Phanerozoic upper mantle. However, the density deficit in the subcrustal lithosphere of most of the Russian platform is only about (0.4-0.8) ?? 0.2% and decreases southwards to ???0%. Increased densities (likely associated with low depletion values) in the Russian platform suggest strong metasomatism of the cratonic lithosphere during rifting events, which led to its subsidence. It is proposed that only the lower part of the cratonic lithosphere was metasomatized as a result of Proterozoic rifting; the boundary between a depleted upper and more fertile lower layers can be at ca. 90-150 km depth and can produce a seismic pattern similar to the top of a seismic low-velocity zone. Paleozoic rifting has modified the entire lithospheric column and the regions affected are still subsiding. Published by Elsevier B.V.

Tracking the India-Arabia Transform Plate Boundary during Paleogene Times.

NASA Astrophysics Data System (ADS)

Rodriguez, M.; Huchon, P.; Chamot-Rooke, N. R. A.; Fournier, M.; Delescluse, M.

2014-12-01

The Zagros and Himalaya mountain belts are the most prominent reliefs built by continental collision. They respectively result from Arabia and India collision with Eurasia. Convergence motions at mountain belts induced most of plate reorganization events in the Indian Ocean during the Cenozoic. Although critical for paleogeographic reconstructions, the way relative motion between Arabia and India was accommodated prior to the formation of the Sheba ridge in the Gulf of Aden remains poorly understood. The India-Arabia plate-boundary belongs to the category of long-lived (~90-Ma) oceanic transform faults, thus providing a good case study to investigate the role of major kinematic events over the structural evolution of a long-lived transform system. A seismic dataset crossing the Owen Fracture Zone, the Owen Basin, and the Oman Margin was acquired to track the past locations of the India-Arabia plate boundary. We highlight the composite age of the Owen Basin basement, made of Paleocene oceanic crust drilled on its eastern part, and composed of pre-Maastrichtian continental crust overlaid by Early Paleocene ophiolites on its western side. A major transform fault system crossing the Owen Basin juxtaposed these two slivers of lithosphere of different ages, and controlled the uplift of marginal ridges along the Oman Margin. This transform system deactivated ~40 Ma ago, coeval with the onset of ultra-slow spreading at the Carlsberg Ridge. The transform boundary then jumped to the edge of the present-day Owen Ridge during the Late Eocene-Oligocene period, before seafloor spreading began at the Sheba Ridge. This migration of the plate boundary involved the transfer of a part of the Indian oceanic lithosphere accreted at the Carlsberg Ridge to the Arabian plate. The episode of plate transfer at the India-Arabia plate boundary during the Late Eocene-Oligocene interval is synchronous with a global plate reorganization event corresponding to geological events at the Zagros and Himalaya belts. The Owen Ridge uplifted later, in Late Miocene times, and is unrelated to any major migration of the India-Arabia boundary.

Lithospheric Structure in Eastern Africa and the Arabian Plate from Joint Inversion of Surface Wave Dispersion Data and Receiver Functions

NASA Astrophysics Data System (ADS)

Dugda, M. T.; Nyblade, A. A.; Rodgers, A.; Al-Amri, A.; Julia, J.

2006-12-01

Lithospheric structure beneath Eastern Africa (Ethiopia, Kenya, and Djibouti) and the Arabian Shield and Platform has been investigated using a joint inversion of receiver functions and surface wave dispersion measurements from 10 to 175 s. Our models help to constrain the extent of modification made to the lithosphere in the region by hotspot tectonism. Most of the data for this study come from three major sources: the Ethiopia and Kenya Broadband Seismic Experiments which were carried out between 2000-2002 and 2001-2002, respectively, and data from the Saudi Arabia National Digital Seismic Network (KACST). We find that there is little or no seismic lid under the Main Ethiopian Rift and Afar. The results for the Ethiopian Plateau show that there has been thinning of the lithosphere by about ~30 40 km from typical Mozambique Belt lithosphere under Tanzania, which was reported to be up to 120 km thick, and that there has been a reduction in maximum shear wave velocity of the lid by about 7%. Replacement of the bottom of the former Mozambique Belt lithosphere by warm plume material with a partial erosion of the lithosphere can explain both the thinning of the lithosphere and the reduction of maximum velocity. Preliminary results suggest similar lithospheric structure beneath the Arabian Shield. The results from Kenya are similar to that of the results for the Mozambique Belt Lithosphere in Tanzania, showing 100-120 km thick lithosphere.

Insights on the seismotectonics of the western part of northern Calabria (southern Italy) by integrated geological and geophysical data: Coexistence of shallow extensional and deep strike-slip kinematics

NASA Astrophysics Data System (ADS)

Ferranti, L.; Milano, G.; Pierro, M.

2017-11-01

We assess the seismotectonics of the western part of the border area between the Southern Apennines and Calabrian Arc, centered on the Mercure extensional basin, by integrating recent seismicity with a reconstruction of the structural frame from surface to deep crust. The analysis of low-magnitude (ML ≤ 3.5) events occurred in the area during 2013-2017, when evaluated in the context of the structural model, has revealed an unexpected complexity of seismotectonics processes. Hypocentral distribution and kinematics allow separating these events into three groups. Focal mechanisms of the shallower (< 9 km) set of events show extensional kinematics. These results are consistent with the last kinematic event recorded on outcropping faults, and with the typical depth and kinematics of normal faulting earthquakes in the axial part of southern Italy. By contrast, intermediate ( 9-17 km) and deep ( 17-23 km) events have fault plane solutions characterized by strike- to reverse-oblique slip, but they differ from each other in the orientation of the principal axes. The intermediate events have P axes with a NE-SW trend, which is at odds with the NW-SE trend recorded by strike-slip earthquakes affecting the Apulia foreland plate in the eastern part of southern Italy. The intermediate events are interpreted to reflect reactivation of faults in the Apulia unit involved in thrust uplift, and appears aligned along an WNW-ESE trending deep crustal, possibly lithospheric boundary. Instead, deep events beneath the basin, which have P-axis with a NW-SE trend, hint to the activity of a deep overthrust of the Tyrrhenian back-arc basin crust over the continental crust of the Apulia margin, or alternatively, to a tear fault in the underthrust Apulia plate. Results of this work suggest that extensional faulting, as believed so far, does not solely characterizes the seismotectonics of the axial part of the Southern Apennines.

Age and provenance constraints on seismically-determined crustal layers beneath the Paleozoic southern Central Asian Orogen, Inner Mongolia, China

NASA Astrophysics Data System (ADS)

Jian, Ping; Kröner, Alfred; Shi, Yuruo; Zhang, Wei; Liu, Yaran; Windley, Brian F.; Jahn, Bor-ming; Zhang, Liqao; Liu, Dunyi

2016-06-01

We present 110 ages and 51 in-situ δ18O values for zircon xenocrysts from a post-99 Ma intraplate basaltic rock suite hosted in a subduction-accretion complex of the southern Central Asian Orogenic Belt in order to constrain a seismic profile across the Paleozoic Southern Orogen of Inner Mongolia and the northern margin of the North China Craton. Two zircon populations are recognized, namely a Phanerozoic group of 70 zircons comprising granitoid-derived (ca. 431-99 Ma; n = 31; peak at 256 Ma), meta-granitoid-derived (ca. 449-113 Ma; n = 24; peak at 251 Ma) and gabbro-derived (436-242 Ma; n = 15; peaks at 264 and 244 Ma) grains. Each textural type is characterized by a distinct zircon oxygen isotope composition and is thus endowed with a genetic connotation. The Precambrian population (2605-741 Ma; n = 40) exhibits a prominent age peak at 2520 Ma (granulite-facies metamorphism) and four small peaks at ca. 1900, 1600, and 800 Ma. Our new data, together with literature zircon ages, significantly constrain models of three seismically-determined deep crustal layers beneath the fossil subduction zone-forearc along the active northern margin of the North China Craton, namely: (1) an upper arc crust of early to mid-Paleozoic age, intruded by a major Permian-Triassic composite granitoid-gabbroic pluton (8-20 km depth); (2) a middle crust, predominantly consisting of mid-Meso- to Neoproterozoic felsic and mafic gneisses; and (3) a lower crust composed predominantly of late Archean granulite-facies rocks. We conclude that the Paleozoic orogenic crust is limited to the upper crustal level, and the middle to lower crust has a North China Craton affinity. Furthermore, integrating our data with surface geological, petrological and geochronological constraints, we present a new conceptual model of orogenic uplift, lithospheric delamination and crustal underthrusting for this key ocean-continent convergent margin.

Synchronous formation of the metamorphic sole and igneous crust of the Semail ophiolite: New constraints on the tectonic evolution during ophiolite formation from high-precision U-Pb zircon geochronology

NASA Astrophysics Data System (ADS)

Rioux, Matthew; Garber, Joshua; Bauer, Ann; Bowring, Samuel; Searle, Michael; Kelemen, Peter; Hacker, Bradley

2016-10-01

The Semail (Oman-United Arab Emirates) and other Tethyan-type ophiolites are underlain by a sole consisting of greenschist- to granulite-facies metamorphic rocks. As preserved remnants of the underthrust plate, sole exposures can be used to better understand the formation and obduction of ophiolites. Early models envisioned that the metamorphic sole of the Semail ophiolite formed as a result of thrusting of the hot ophiolite lithosphere over adjacent oceanic crust during initial emplacement; however, calculated pressures from granulite-facies mineral assemblages in the sole suggest the metamorphic rocks formed at >35 km depth, and are too high to be explained by the currently preserved thickness of ophiolite crust and mantle (up to 15-20 km). We have used high-precision U-Pb zircon dating to study the formation and evolution of the metamorphic sole at two well-studied localities. Our previous research and new results show that the ophiolite crust formed from 96.12-95.50 Ma. Our new dates from the Sumeini and Wadi Tayin sole localities indicate peak metamorphism at 96.16 and 94.82 Ma (±0.022 to 0.035 Ma), respectively. The dates from the Sumeini sole locality show for the first time that the metamorphic rocks formed either prior to or during formation of the ophiolite crust, and were later juxtaposed with the base of the ophiolite. These data, combined with existing geochemical constraints, are best explained by formation of the ophiolite in a supra-subduction zone setting, with metamorphism of the sole rocks occurring in a subducted slab. The 1.3 Ma difference between the Wadi Tayin and Sumeini dates indicates that, in contrast to current models, the highest-grade rocks at different sole localities underwent metamorphism, and may have returned up the subduction channel, at different times.

Os and HSE of the hot upper mantle beneath southern Tibet: Indian mantle affinity?

NASA Astrophysics Data System (ADS)

Zhao, Z.; Dale, C. W.; Pearson, D. G.; Niu, Y.; Zhu, D.; Mo, X.

2011-12-01

The subduction of the Indian plate (including cratonic continental crust and/or upper mantle) beneath southern Tibet is widely accepted from both geological and geophysical studies. Mantle-derived xenoliths from this region provide a means of directly investigating the mantle underlying the southern part of the plateau. Studies of xenoliths hosted in the Sailipu ultrapotassic volcanic rocks, erupted at ~17 Ma, have indicated that the subcontinental mantle of southern Tibetan Plateau is hot and strongly influenced by metasomatism (Zhao et al., 2008a, b; Liu et al., 2011). Here we report comprehensive EPMA and LA-ICP-MS major and trace element data for the Sailipu xenoliths and also whole rock Os isotope and HSE data in order to constrain the depletion history of the mantle and to identify the presence of any potential Indian cratonic mantle. The xenoliths, ranging in size from 0.5cm to 1.5cm in diameter, are mostly peridotites. The calculated temperatures are 1010-1230°C at the given pressures of ~1.6-2.0 GPa (n=47). These P-T conditions are similar to rift-related upper mantle regimes (e.g. Kenya), indicating the influence of regional extension beneath southern Tibet in the Miocene. A series of compositional discriminations for minerals (Cpx, Opx, Ol, and Phl), e.g. Fo<90, suggest that the xenoliths are non-cratonic spinel-peridotite (cratonic peridotite olivine Fo> ~91), with a clear metasomatic signature We obtained Os isotope data and abundances of highly siderophile elements (HSE, including Os, Ir, Ru, Pt, Pd and Re) on a set of six olivine-dominated peridotite samples from Sailipu volcanics, less than 1 cm in dimension. They allow us to further constrain the nature and state of the upper mantle beneath the southern Tibet. Sailipu samples display low total HSE abundances (Os+Ir+Ru+Pt+Pd+Re) ranging from 8.7 to 25 ppb, with nearly constant Os, Ir , and Ru, but rather varied Pt (2-13), Pd (0.4-5.2), and Re (0.01-0.5). Chondrite-normalised Pd/Ir ratios range from 0.2 to 2.4 reflecting significant metasomatism of some samples. The xenoliths exhibit 187Os/188Os ratios of 0.12213-0.12696, corresponding to γOs ranging from -4.2 to -0.4 - much higher than ancient cratonic mantle. Thus, on the basis of mineral chemistry and whole rock Os isotopes, Indian cratonic mantle is absent from our suite of xenoliths. Therefore, assuming the presence of cratonic mantle, it seems likely that the xenoliths do not sample the deep basal section of the lithosphere where cratonic Indian lithosphere is thought to be present under southern Tibet. In which case, testing of the seismic and tectonic models may not be possible without garnet-facies peridotites. More work need to be done to further reveal the mantle compostion and mantle dynamics beneath Tibet. [Financially supported by the National Key Project for Basic Research of China (Project 2011CB403102 and 2009CB421002)]. [1] Zhao Z, et al., 2008a. Acta Petrologica Sinica, 24 (2): 193-202 [2] Zhao Z, et al., 2008b. Geochimica et Cosmochimica Acta, 72, 12 (Supp.): A1095 [3] Liu C-Z, et al., 2011, Geology, in press

Constraints on Lithosphere Rheology from Observations of Volcano-induced Deformation

NASA Astrophysics Data System (ADS)

Zhong, S.; Watts, A. B.

2011-12-01

Mantle rheology at lithospheric conditions (i.e., temperature < 1200 oC) is important for understanding fundamental geodynamic problems including the dynamics of plate tectonics, subducted slabs, and lithosphere-mantle interaction. Laboratory studies suggest that the rheology at lithospheric conditions can be approximately divided into three different regimes: brittle or frictional sliding, semi-brittle, and plastic flow. In this study, we seek to constrain lithospheric rheology, using observations of deformation at seamounts and oceanic islands caused by volcanic loading. Volcano-induced surface deformation depends critically on lithospheric rheology at the time of seamount and oceanic island emplacement and while it changes rapidly on short time-scales it does not change significantly on long time-scales. In an earlier study [Watts and Zhong, 2000], we used the effective elastic thickness at seamounts and oceanic islands inferred from the observations of deformation and gravity to determine an effective activation energy of 120 KJ/mol for lithospheric mantle with Newtonian rheology. We have now expanded this study to incorporate non-Newtonian power-law and frictional sliding rheologies, and more importantly, to include realistic 3-D volcanic load geometries. We use the Hawaiian Islands as an example. We construct 3-D loads for the Hawaiian Islands by applying an appropriate median filter to remove Hawaiian swell topography and correcting for lithospheric age effect on the bathymetry. The loads are then used in 3-D finite element loading models with viscoelastic, non-Newtonian and frictional sliding rheologies to determine the lithospheric response including surface vertical motions and lithospheric stresses. Comparisons of our new model predictions to observations suggest that the activation energy of lithospheric mantle is significantly smaller than most experimentally determined values for olivine at high temperatures, but may be consistent with more recent experimental results at lithospheric temperatures. Seamounts and oceanic islands are therefore a 'natural laboratory', we believe, to study lithospheric rheology on both short and long time scales.

Lithospheric thickness jumps at the S-Atlantic continental margins from satellite gravity data and modelled isostatic anomalies

NASA Astrophysics Data System (ADS)

Shahraki, Meysam; Schmeling, Harro; Haas, Peter

2018-01-01

Isostatic equilibrium is a good approximation for passive continental margins. In these regions, geoid anomalies are proportional to the local dipole moment of density-depth distributions, which can be used to constrain the amount of oceanic to continental lithospheric thickening (lithospheric jumps). We consider a five- or three-layer 1D model for the oceanic and continental lithosphere, respectively, composed of water, a sediment layer (both for the oceanic case), the crust, the mantle lithosphere and the asthenosphere. The mantle lithosphere is defined by a mantle density, which is a function of temperature and composition, due to melt depletion. In addition, a depth-dependent sediment density associated with compaction and ocean floor variation is adopted. We analyzed satellite derived geoid data and, after filtering, extracted typical averaged profiles across the Western and Eastern passive margins of the South Atlantic. They show geoid jumps of 8.1 m and 7.0 m for the Argentinian and African sides, respectively. Together with topography data and an averaged crustal density at the conjugate margins these jumps are interpreted as isostatic geoid anomalies and yield best-fitting crustal and lithospheric thicknesses. In a grid search approach five parameters are systematically varied, namely the thicknesses of the sediment layer, the oceanic and continental crusts and the oceanic and the continental mantle lithosphere. The set of successful models reveals a clear asymmetry between the South Africa and Argentine lithospheres by 15 km. Preferred models predict a sediment layer at the Argentine margin of 3-6 km and at the South Africa margin of 1-2.5 km. Moreover, we derived a linear relationship between, oceanic lithosphere, sediment thickness and lithospheric jumps at the South Atlantic margins. It suggests that the continental lithospheres on the western and eastern South Atlantic are thicker by 45-70 and 60-80 km than the oceanic lithospheres, respectively.

Modification of the Western Gondwana craton by plume-lithosphere interaction

NASA Astrophysics Data System (ADS)

Hu, Jiashun; Liu, Lijun; Faccenda, Manuele; Zhou, Quan; Fischer, Karen M.; Marshak, Stephen; Lundstrom, Craig

2018-03-01

The longevity of cratons is generally attributed to persistence of neutrally-to-positively buoyant and mechanically strong lithosphere that shields the cratonic crust from underlying mantle dynamics. Here we show that large portions of the cratonic lithosphere in South America and Africa, however, experienced significant modification during and since the Mesozoic era, as demonstrated by widespread Cretaceous uplift and volcanism, present-day high topography, thin crust, and the presence of seismically fast but neutrally buoyant upper-mantle anomalies. We suggest that these observations reflect a permanent increase in lithospheric buoyancy due to plume-triggered delamination of deep lithospheric roots during the Late Cretaceous and early Cenozoic periods. Lithosphere in these regions has been thermally reestablished since then, as confirmed by its present-day low heat flow, high seismic velocities and realigned seismic anisotropy. We conclude that the original lowermost cratonic lithosphere is compositionally denser than the asthenospheric mantle and can be removed when perturbed by underlying mantle upwelling. Therefore, it is the buoyancy of the upper lithosphere that perpetuates stabilization of cratons.

Comparison of the Lithospheric Structure Beneath Kenya and Ethiopia From Joint Inversion of Receiver Functions and Rayleigh Wave Dispersion Velocities

NASA Astrophysics Data System (ADS)

Dugda, M. T.; Nyblade, A. A.; Julia, J.

2007-12-01

Shear-wave velocity structure of the crust and upper mantle beneath Kenya has been investigated using joint inversion of receiver functions, and Rayleigh wave group and phase velocities. Most of the data for this study come from the Kenya broadband seismic experiment, conducted between 2001 and 2002. Shear velocity models obtained from the joint inversion show crustal thicknesses of 37 to 42 km beneath the East African Plateau in Kenya and near the edge of the Kenya Rift, and a crustal thickness of about 30 km beneath the Kenya Rift. These crustal parameters are consistent with crustal thicknesses published previously by different authors. A comparison has been made between the lithosphere under Kenya and other parts of the East African Plateau in Tanzania. A comparison between the lithosphere under Kenya and that under Ethiopia has also been made, specifically between the lithosphere under the Ethiopian Plateau and the Kenya Plateau, and between the lithosphere beneath the Main Ethiopian Rift (MER) and the Kenya (Gregory) Rift. The lithospheric mantle beneath the East African Plateau in Kenya has a maximum shear wave velocity of about 4.6 km/s, similar to the value obtained under the East African Plateau in Tanzania. Beneath the Kenya Rift, the lithosphere extends to a depth of at most ~75 km. The average velocity of the mantle lithosphere under the East African Plateau in Kenya appears to be similar to the lithosphere under Tanzania away from the East African Rift System. The lithosphere under the Kenya Plateau is not perturbed as compared to the highly perturbed lithosphere beneath the Ethiopian Plateau. The lithosphere under the Kenya Rift is perturbed as compared to the rest of the region but is not as perturbed as that under the Main Ethiopian Rift or the Afar. Though Kenya and Ethiopia have similar uplift, volcanism and rifting at the surface, they have different lithospheric structures at the bottom. The Afar Flood Basalt Volcanism (AFB) may be the cause of this striking difference in the two lithosphere.

Plume-lithosphere interaction: Effects on the seismic anisotropy of the lithospheric mantle

NASA Astrophysics Data System (ADS)

Vauchez, A.; Tommasi, A.

2003-04-01

Interaction between a hot asthenospheric mantle and the base of the lithosphere above a mantle plume involves heat and mass transfer through melting and fluids percolation. These processes alter the mineralogy, microstructure and geochemical signature of the lithospheric mantle; altogether they lead to an asthenospherization, and thus to erosion of the lithosphere. Does this evolution modify or even erase the seismic anisotropy of the initial lithospheric mantle? In other words, is the structural memory of the lithospheric mantle preserved in such geodynamic situations? Insights on this process are provided by the measurement of the Lattice Preferred Orientation of rock-forming minerals and the computation of seismic properties of mantle rocks from the Ronda Peridotite Massif, and of xenoliths from Tanzania and Polynesia volcanoes. The Ronda massif displays clear microstructural and geochemical variations characterizing the limit between an ancient lithospheric mantle and its asthenospherized counterpart that has undergone partial melting and magmas percolation. The LPO measured in peridotites from both domains is quite similar and so are seismic properties, suggesting that the tectonic fabric inherited from previous deformation and the resulting seismic anisotropy are only slightly modified by asthenospherization. The Labait volcano in Tanzania sampled the Tanzania craton lithospheric mantle at depths between 150 km and less than 70 km. Although significant annealing and exaggerated grain growth of olivine occur between 70 km and 120 km the olivine LPO does not vary significantly, suggesting that the initial anisotropy of the lithospheric was preserved. Xenoliths from several Polynesian volcanoes display composition and geochemistry that suggest percolation of variable amounts of melt in the lithospheric mantle up to relatively shallow depths. Samples that have underwent the most percolation display very weak olivine LPO, and are almost seismically isotropic. Altogether the results of these studies suggest that asthenospherization does not necessarily erase the inherited seismic anisotropy of the older, previously structured, lithosphere. As far as melting and melt-rock interaction remain moderate the LPO of olivine, and thus the seismic anisotropy of the lithospheric mantle are largely preserved. However, when melt-rock interactions become large enough, then the lithospheric seismic anisotropy signature of the mantle may be erased.

Research in Seismology

DTIC Science & Technology

1975-01-01

edge of the Persian Gulf, isolatina the Arabian continental shelf from the subduction process. Slippage along this fault is Pleistocene and probably...Fault-plane solutions for these events (Fitch, 1970; Nowroozi, 1972) confirm the mechanisms to be shallow underthrusting. The Bouguer gravity anomaly...at 40 km behind the subduction zone. A Bouguer gravity anomaly has also been calculated for this model by assuming that the structure is flat and

Piecewise delamination of Moroccan lithosphere from beneath the Atlas Mountains

NASA Astrophysics Data System (ADS)

Bezada, M. J.; Humphreys, E. D.; Davila, J. M.; Carbonell, R.; Harnafi, M.; Palomeras, I.; Levander, A.

2014-04-01

The elevation of the intracontinental Atlas Mountains of Morocco and surrounding regions requires a mantle component of buoyancy, and there is consensus that this buoyancy results from an abnormally thin lithosphere. Lithospheric delamination under the Atlas Mountains and thermal erosion caused by upwelling mantle have each been suggested as thinning mechanisms. We use seismic tomography to image the upper mantle of Morocco. Our imaging resolves the location and shape of lithospheric cavities and of delaminated lithosphere ˜400 km beneath the Middle Atlas. We propose discontinuous delamination of an intrinsically unstable Atlas lithosphere, enabled by the presence of anomalously hot mantle, as a mechanism for producing the imaged structures. The Atlas lithosphere was made unstable by a combination of tectonic shortening and eclogite loading during Mesozoic rifting and Cenozoic magmatism. The presence of hot mantle sourced from regional upwellings in northern Africa or the Canary Islands enhanced the instability of this lithosphere. Flow around the retreating Alboran slab focused upwelling mantle under the Middle Atlas, which we infer to be the site of the most recent delamination. The Atlas Mountains of Morocco stand as an example of large-scale lithospheric loss in a mildly contractional orogen.

Episodic fluid flow in the Nankai accretionary complex: Timescale, geochemistry, flow rates, and fluid budget

USGS Publications Warehouse

Saffer, D.M.; Bekins, B.A.

1998-01-01

Down-hole geochemical anomalies encountered in active accretionary systems can be used to constrain the timing, rates, and localization of fluid flow. Here we combine a coupled flow and solute transport model with a kinetic model for smectite dehydration to better understand and quantify fluid flow in the Nankai accretionary complex offshore of Japan. Compaction of sediments and clay dehydration provide fluid sources which drive the model flow system. We explicitly include the consolidation rate of underthrust sediments in our calculations to evaluate the impact that variations in this unknown quantity have on pressure and chloride distribution. Sensitivity analysis of steady state pressure solutions constrains bulk and flow conduit permeabilities. Steady state simulations with 30% smectite in the incoming sedimentary sequence result in minimum chloride concentrations at site 808 of 550 mM, but measured chlorinity is as low as 447 mM. We simulate the transient effects of hydrofracture or a strain event by assuming an instantaneous permeability increase of 3-4 orders of magnitude along a flow conduit (in this case the de??collement), using steady state results as initial conditions. Transient results with an increase in de??collement permeability from 10-16 m2 to 10-13 m2 and 20% smectite reproduce the observed chloride profile at site 808 after 80-160 kyr. Modeled chloride concentrations are highly sensitive to the consolidation rate of underthrust sediments, such that rapid compaction of underthrust material leads to increased freshening. Pressures within the de??collement during transient simulations rise rapidly to a significant fraction of lithostatic and remain high for at least 160 kyr, providing a mechanism for maintaining high permeability. Flow rates at the deformation front for transient simulations are in good agreement with direct measurements, but steady state flow rates are 2-3 orders of magnitude smaller than observed. Fluid budget calculations indicate that nearly 71% of the incoming water in the sediments leaves the accretionary wedge via diffuse flow out the seafloor, 0-5% escapes by focused flow along the de??collement, and roughly 1% is subducted. Copyright 1998 by the American Geophysical Union.

Lithosphere-asthenosphere interaction beneath Ireland from joint inversion of teleseismic P-wave delay times and GRACE gravity

NASA Astrophysics Data System (ADS)

O'Donnell, J. P.; Daly, E.; Tiberi, C.; Bastow, I. D.; O'Reilly, B. M.; Readman, P. W.; Hauser, F.

2011-03-01

The nature and extent of the regional lithosphere-asthenosphere interaction beneath Ireland and Britain remains unclear. Although it has been established that ancient Caledonian signatures pervade the lithosphere, tertiary structure related to the Iceland plume has been inferred to dominate the asthenosphere. To address this apparent contradiction in the literature, we image the 3-D lithospheric and deeper upper-mantle structure beneath Ireland via non-linear, iterative joint teleseismic-gravity inversion using data from the ISLE (Irish Seismic Lithospheric Experiment), ISUME (Irish Seismic Upper Mantle Experiment) and GRACE (Gravity Recovery and Climate Experiment) experiments. The inversion combines teleseismic relative arrival time residuals with the GRACE long wavelength satellite derived gravity anomaly by assuming a depth-dependent quasilinear velocity-density relationship. We argue that anomalies imaged at lithospheric depths probably reflect compositional contrasts, either due to terrane accretion associated with Iapetus Ocean closure, frozen decompressional melt that was generated by plate stretching during the opening of the north Atlantic Ocean, frozen Iceland plume related magmatic intrusions, or a combination thereof. The continuation of the anomalous structure across the lithosphere-asthenosphere boundary is interpreted as possibly reflecting sub-lithospheric small-scale convection initiated by the lithospheric compositional contrasts. Our hypothesis thus reconciles the disparity which exists between lithospheric and asthenospheric structure beneath this region of the north Atlantic rifted margin.

Insights Into Layering in the Cratonic Lithosphere Beneath Western Australia

NASA Astrophysics Data System (ADS)

Sun, Weijia; Fu, Li-Yun; Saygin, Erdinc; Zhao, Liang

2018-02-01

The characteristics of internal lithospheric discontinuities carry crucial information regarding the origin and evolution of the lithosphere. However, the formation and mechanisms of the midlithosphere discontinuity (MLD) are still enigmatic and controversial. We investigate the midlithospheric discontinuities beneath the Archean Western Australian Craton, which represents one of the oldest continents on the globe, using a novel receiver-based reflectivity approach combined with other geophysical information comprising tomographic P and S wave velocity, radial anisotropy, electrical resistivity, and heat flow data. The MLD is rather shallow with a depth of 68-82 km. Multiple prominent discontinuities are observed in the lithospheric mantle using constructed high-frequency (0.5-4 Hz) P wave reflectivities. These multiple discontinuities coincide well with the broad-scale reduction of relative P and SV wave velocities at the top of the graded transition zone from the lithosphere to the asthenosphere. Strong radial anisotropy in the upper lithosphere mantle tends to be weak across the MLD, which might reflect quasi-laminar lithospheric heterogeneity behavior with a horizontal correlation length that is greater than its vertical correlation length. Broad-scale electrical resistivity variations show little coherence with the MLD. Given these various geophysical observations, the upper lithosphere exhibits rigid and elastic properties above the MLD, while the lower lithosphere tends to be ductile and rheological or viscous. A model comprising quasi-laminar lithospheric heterogeneity could effectively represent the MLD characteristics beneath the Archean continent.

Lithospheric flexural strength and effective elastic thicknesses of the Eastern Anatolia (Turkey) and surrounding region

NASA Astrophysics Data System (ADS)

Oruç, Bülent; Gomez-Ortiz, David; Petit, Carole

2017-12-01

The Lithospheric structure of Eastern Anatolia and the surrounding region, including the northern part of the Arabian platform is investigated via the analysis and modeling of Bouguer anomalies from the Earth Gravitational Model EGM08. The effective elastic thickness of the lithosphere (EET) that corresponds to the mechanical cores of the crust and lithospheric mantle is determined from the spectral coherence between Bouguer anomalies and surface elevation data. Its average value is 18.7 km. From the logarithmic amplitude spectra of Bouguer anomalies, average depths of the lithosphere-asthenosphere boundary (LAB), Moho, Conrad and basement in the study area are constrained at 84 km, 39 km, 16 km and 7 km, respectively. The geometries of the LAB and Moho are then estimated using the Parker-Oldenburg inversion algorithm. We also present a lithospheric strength map obtained from the spatial variations of EET determined by Yield Stress Envelopes (YSE). The EET varies in the range of 12-23 km, which is in good agreement with the average value obtained from spectral analysis. Low EET values are interpreted as resulting from thermal and flexural lithospheric weakening. According to the lithospheric strength of the Eastern Anatolian region, the rheology model consists of a strong but brittle upper crust, a weak and ductile lower crust, and a weak lower part of the lithosphere. On the other hand, lithosphere strength corresponds to weak and ductile lower crust, a strong upper crust and a strong uppermost lithospheric mantle for the northern part of the Arabian platform.

The Cotoncello Shear Zone (Elba Island, Italy): The deep root of a fossil oceanic detachment fault in the Ligurian ophiolites

NASA Astrophysics Data System (ADS)

Frassi, Chiara; Musumeci, Giovanni; Zucali, Michele; Mazzarini, Francesco; Rebay, Gisella; Langone, Antonio

2017-05-01

The ophiolite sequences in the western Elba Island are classically interpreted as a well-exposed ocean-floor section emplaced during the Apennines orogeny at the top of the tectonic nappe-stack. Stratigraphic, petrological and geochemical features indicate that these ophiolite sequences are remnants of slow-ultraslow spreading oceanic lithosphere analogous to the present-day Mid-Atlantic Ridge and Southwest Indian Ridge. Within the oceanward section of Tethyan lithosphere exposed in the Elba Island, we investigated for the first time a ​10s of meters-thick structure, the Cotoncello Shear Zone (CSZ), that records high-temperature ductile deformation. We used a multidisciplinary approach to document the tectono-metamorphic evolution of the shear zone and its role during spreading of the western Tethys. In addition, we used zircon U-Pb ages to date formation of the gabbroic lower crust in this sector of the Apennines. Our results indicate that the CSZ rooted below the brittle-ductile transition at temperature above 800 °C. A high-temperature ductile fabric was overprinted by fabrics recorded during progressive exhumation up to shallower levers under temperature < 500 °C. We suggest that the CSZ may represent the deep root of a detachment fault that accomplished exhumation of an ancient oceanic core complex (OCC) in between two stages of magmatic accretion. We suggest that the CSZ represents an excellent on-land example enabling to assess relationships between magmatism and deformation when extensional oceanic detachments are at work.

VLF Science at Indian Centre for Space Physics

NASA Astrophysics Data System (ADS)

Chakrabarti, Sandip Kumar

2016-07-01

Indian Centre for Space Physics has been monitoring VLF signals from stations around the world at its laboratories at Kolkata and Sitapur (Ionospheric and Earthquake Research Centre) as well as at several places throughout India when in a campaign mode. We have been interested to study high energy events from space, such as solar flares and gamma ray bursts. We have made studies during multiple solar eclipses and most importantly made substantial progress in the problem of lithosphere-ionosphere coupling while understanding various types of anomalies prior to major earthquakes. Other effects such as AGWs and LEPs are being studied. We have experience of two antarctic expedition and obtained VLF data from both Maitri and Bharati stations of India, which revealed, among other things, how the signal attenuation can indicate the extent of ice mass in Antarctica. We have been able to reproduce various VLF perturbation events using Atmospheric Chemical evolution model coupled with LWPC code. For instance we have reproduced solar flare induced VLF amplitude perturbation pattern by completely ab initio calculation. We also targeted the inverse problem, namely, deduction of the injected radiation spectra from space from the VLF signal alone, thereby establishing that the Earth can be used as a gigantic detector. These interesting results would be presented in my review talk.

Lithosphere thickness in the Gulf of California region

NASA Astrophysics Data System (ADS)

Fernández, Alejandra; Pérez-Campos, Xyoli

2017-11-01

The Gulf of California has a long tectonic history. Before the subduction of the Guadalupe and Magdalena plates ceased, extension of the Gulf began to the east, at the Basin and Range province. Later, it was focused west of the Sierra Madre Occidental and the opening of the Gulf started. Currently, the Gulf rifting has different characteristics to the north than to the south. In this study, we analyze the lithosphere thickness in the Gulf of California region by means of P-wave and S-wave receiver functions. We grouped our lithosphere-thickness estimates into five froups: 1) North of the Gulf, with a thin lithosphere ( 50 km) related to the extension observed in the Salton Through region; 2) the northwestern part of Baja California, with a thicker lithosphere ( 80 km), thinning towards the Gulf due to the extension and opening processes ( 65 km); 3) central Baja California, with no converted phase corresponding to the lithosphere-asthenosphere boundary but evidence of the presence of a slab remnant; 4) the southern Baja California peninsula, showing a shallow lithosphere-astenosphere boundary (LAB) (< 55 km) and a lithosphere thinning towards the Gulf; and 5) the eastern Gulf margin with lithosphere thinning towards the south. These groups can be further assembled into three regions: A) The northernmost Gulf, where both margins of the Gulf show a relatively constant lithosphere thickness, consistent with an old basement in Sonora and the presence of the Peninsular Ranges batholith in northern Baja California, thinning up towards the axis of the rift in the northernmost Gulf. B) Central and southern Gulf, where the lithosphere thickness in this region ranges from 40 to 55 km, which is consistent with the presence of a younger crust. C) Central Baja California peninsula, where LAB is not detected; but there is evidence of a slab remnant.

Subduction-driven recycling of continental margin lithosphere.

PubMed

Levander, A; Bezada, M J; Niu, F; Humphreys, E D; Palomeras, I; Thurner, S M; Masy, J; Schmitz, M; Gallart, J; Carbonell, R; Miller, M S

2014-11-13

Whereas subduction recycling of oceanic lithosphere is one of the central themes of plate tectonics, the recycling of continental lithosphere appears to be far more complicated and less well understood. Delamination and convective downwelling are two widely recognized processes invoked to explain the removal of lithospheric mantle under or adjacent to orogenic belts. Here we relate oceanic plate subduction to removal of adjacent continental lithosphere in certain plate tectonic settings. We have developed teleseismic body wave images from dense broadband seismic experiments that show higher than expected volumes of anomalously fast mantle associated with the subducted Atlantic slab under northeastern South America and the Alboran slab beneath the Gibraltar arc region; the anomalies are under, and are aligned with, the continental margins at depths greater than 200 kilometres. Rayleigh wave analysis finds that the lithospheric mantle under the continental margins is significantly thinner than expected, and that thin lithosphere extends from the orogens adjacent to the subduction zones inland to the edges of nearby cratonic cores. Taking these data together, here we describe a process that can lead to the loss of continental lithosphere adjacent to a subduction zone. Subducting oceanic plates can viscously entrain and remove the bottom of the continental thermal boundary layer lithosphere from adjacent continental margins. This drives surface tectonics and pre-conditions the margins for further deformation by creating topography along the lithosphere-asthenosphere boundary. This can lead to development of secondary downwellings under the continental interior, probably under both South America and the Gibraltar arc, and to delamination of the entire lithospheric mantle, as around the Gibraltar arc. This process reconciles numerous, sometimes mutually exclusive, geodynamic models proposed to explain the complex oceanic-continental tectonics of these subduction zones.

Observational Constraints on Lithospheric Rheology and Their Implications for Lithospheric Dynamics and Plate Tectonics

NASA Astrophysics Data System (ADS)

Zhong, S.; Watts, A. B.

2014-12-01

Lithospheric rheology and strength are important for understanding crust and lithosphere dynamics, and the conditions for plate tectonics. Laboratory studies suggest that lithospheric rheology is controlled by frictional sliding, semi-brittle, low-temperature plasticity, and high-temperature creep deformation mechanisms as pressure and temperature increase from shallow to large depths. Although rheological equations for these deformation mechanisms have been determined in laboratory settings, it is necessary to validate them using field observations. Here we present an overview of lithospheric rheology constrained by observations of seismic structure and load-induced flexure. Together with mantle dynamic modeling, rheological equations for high-temperature creep derived from laboratory studies (Hirth and Kohlstedt, 2003; Karato and Jung, 2003) satisfactorily explain the seismic structure of the Pacific upper mantle (Hunen et al., 2005) and Hawaiian swell topography (Asaadi et al., 2011). In a recent study that compared modeled surface flexure and stress induced by volcano loads in the Hawaiian Islands region with the observed flexure and seismicity, Zhong and Watts (2013) showed that the coefficient of friction is between 0.25 and 0.7, and is consistent with laboratory studies and also in-situ borehole measurements. However, this study indicated that the rheological equation for the low-temperature plasticity from laboratory studies (e.g., Mei et al., 2010) significantly over-predicts lithospheric strength and viscosity. Zhong and Watts (2013) also showed that the maximum lithospheric stress beneath Hawaiian volcano loads is about 100-200 MPa, which may be viewed as the largest lithospheric stress in the Earth's lithosphere. We show that the relatively weak lithospheric strength in the low-temperature plasticity regime is consistent with seismic observation of reactivated mantle lithosphere in the western US and the eastern North China. We discuss here the causes of this weakening in the context of the potential effects on laboratory studies of reduced grain size and Peierls stress on the low-temperature deformation regime.

Tear geometry at active STEPs: an analogue model approach

NASA Astrophysics Data System (ADS)

Broerse, Taco; Sokoutis, Dimitrios; Willingshofer, Ernst; Govers, Rob

2017-04-01

At the lateral end of a subduction zone, tearing of lithosphere is the result of subduction of oceanic lithosphere while adjacent buoyant continental lithosphere stays at the surface. The location of lithospheric tearing is called a Subduction-Transform-Edge-Propagator (STEP), which continuously extends the plate boundary between overriding plate and continental lithosphere. One of our areas of interest is the southern Caribbean where Atlantic lithosphere subducts below the Caribbean plate. Mantle tomography suggests a clear southern edge of the Lesser Antilles slab, which makes the boundary between the Caribbean and South America a clear STEP candidate. At the surface, the San Sebastián/El Pilar fault zone forms the plate boundary between the Caribbean and South America and the active STEP is located near Trinidad. For the deeper part of the damage/shear zone, some information is available from a recent 3D gravity study: significant lateral variability in densities of the lithospheric mantle to the south of the STEP fault zone. The low-density zone may result from higher sub-crustal temperatures, such as would arise from an asthenospheric window resulting from detachment. Interpreted in this way, the mantle part of the damage zone may be 200-250 km wide. So, while the location of the plate boundary at the surface is relatively well resolved, little is known about the deeper continuation of the active STEP in the mantle lithosphere. We study the evolution of the tearing process at a STEP using analogue models. In our models we use silicone putty (lithosphere) and glucose (asthenosphere). Solely gravitational forces resulting from density differences between oceanic lithosphere and asthenosphere drive our model. Lithospheric tearing commences after subduction has initiated. The geometry of the tear varies with the rheology of the lithosphere and asthenosphere, particularly Newtonian versus power-law. We investigate the dependence on model parameters of the width of the tearing zone and the depth at which tearing occurs.

Thinning of heterogeneous lithosphere: insights from field observations and numerical modelling

NASA Astrophysics Data System (ADS)

Petri, B.; Duretz, T.; Mohn, G.; Schmalholz, S. M.

2017-12-01

The nature and mechanisms of formation of extremely thinned continental crust (< 10 km) and lithosphere during rifting remain debated. Observations from present-day and fossil continental passive margins document the heterogeneous nature of the lithosphere characterized, among others, by lithological variations and structural inheritance. This contribution aims at investigating the mechanisms of extreme lithospheric thinning by exploring in particular the role of initial heterogeneities by coupling field observations from fossil passive margins and numerical models of lithospheric extension. Two field examples from the Alpine Tethys margins outcropping in the Eastern Alps (E Switzerland and N Italy) and in the Southern Alps (N Italy) were selected for their exceptional level of preservation of rift-related structures. This situation enables us to characterize (1) the pre-rift architecture of the continental lithosphere, (2) the localization of rift-related deformation in distinct portion of the lithosphere and (3) the interaction between initial heterogeneities of the lithosphere and rift-related structures. In a second stage, these observations are integrated in high-resolution, two-dimensional thermo-mechanical models taking into account various patterns of initial mechanical heterogeneities. Our results show the importance of initial pre-rift architecture of the continental lithosphere during rifting. Key roles are given to high-angle and low-angle normal faults, anastomosing shear-zones and decoupling horizons. We propose that during the first stages of thinning, deformation is strongly controlled by the complex pre-rift architecture of the lithosphere, localized along major structures responsible for the lateral extrusion of mid to lower crustal levels. This extrusion juxtaposes mechanically stronger levels in the hyper-thinned continental crust, being exhumed by subsequent low-angle normal faults. Altogether, these results highlight the critical role of the extraction of mechanically strong layers of the lithosphere during the extreme thinning of the continental lithosphere and allows to propose a new model for the formation of continental passive margins.

Building and Modification of the Continental Lithosphere: the History of the Contiguous U.S. as told by MLDs and LABs

NASA Astrophysics Data System (ADS)

Hopper, E.; Fischer, K. M.

2016-12-01

The lithosphere preserves a record of past and present tectonic processes in its internal structures and its boundary with the underlying asthenosphere. We use common conversion point stacked Sp converted waves recorded by EarthScope's Transportable Array, as well as other available permanent and temporary broadband stations, to image such structures in the lithospheric mantle of the contiguous U.S. In the tectonically youngest western U.S., a shallow, sharp velocity gradient at the base of the lithosphere suggests a boundary defined by ponded melt. The lithosphere thickens with age of volcanism, implying the lithosphere is a melt-mitigated, conductively cooling thermal boundary layer. Beneath older, colder lithosphere where melt fractions are likely much lower, the velocity gradient at the base of such a layer should be a more diffuse, primarily thermal boundary. This is consistent with observations in the eastern U.S. where the lithosphere-asthenosphere boundary (LAB) is locally sharp and shallower only in areas of inferred enhanced upwelling - such as ancient hot spot tracks and areas of inferred delamination. In the cratonic interior, the LAB is even more gradual in depth, and is transparent to Sp waves with dominant periods of 10 s. Although seismic imaging only provides a snapshot of the lithosphere as it is today, preserved internal structures extend the utility of this imaging back into deep geological time. Ancient accretion within the cratonic lithospheric mantle is preserved as dipping structures associated with relict subducted slabs from Paleoproterozoic continental accretion, suggesting that lateral accretion was integral to the cratonic mantle root formation process. Metasomatism, melt migration and ponding below a carbonated peridotite solidus explain a sub-horizontal mid-lithospheric discontinuity (MLD) commonly observed at 70-100 km depth. This type of MLD is strongest in Mesoproterozoic and older lithosphere, suggesting that it formed more vigorously in the deep past, that a billion years or more are required to build up an observable volatile-rich layer, or that strong, ancient lithosphere is required to support an inherently weak, volatilized layer.

Ridge Jumps Associated with Plume-Ridge Interaction 1: Off-axis Heating due to Lithospheric Magma Penetration

NASA Astrophysics Data System (ADS)

Mittelstaedt, E.; Ito, G.

2005-12-01

In many hot spot-ridge systems, changes in the ridge axis geometry occur between the hot spot centers and nearby mid-ocean ridges in the form of ridge jumps. Such ridge jumps likely occur as a result of anomalous lithospheric stresses associated with mantle plume-lithosphere interaction, as well as weakening of the hot spot lithosphere due to physical and thermal thinning caused by rising buoyant asthenosphere and magma transport through the lithosphere. In this study, we use numerical models to quantify the effects of excess magmatism through the near-ridge lithosphere. Hot spot magmatism can weaken the lithosphere both mechanically through fracturing and thermally through conduction and advection of heat into the plate. Here we focus on the effects of thermal weakening. Using a plane-strain approximation, we examine deformation in a 2-D cross section of a visco-elastic-plastic lithosphere with the finite element code FLAC. The model has isothermal top and bottom boundaries and a prescribed velocity equal to the half spreading rate is imposed on the sides to drive seafloor spreading. The initial condition, as predicted for normal mid-ocean ridges, is a square root of lithospheric age cooling curve with a corner flow velocity field symmetric about the ridge axis. A range of heat inputs are introduced at various plate ages and spreading rates to simulate off-axis magma transport. To reveal the physical conditions that allow for a ridge jump and control its timing, we vary 4 parameters: spreading rate, lithospheric age, crustal thickness and heat input. Results indicate that the heating rate required to produce a ridge jump increases as a function of lithospheric age at the location of magma intrusion. The time necessary for a ridge jump to develop in lithosphere of a particular age decreases with increasing crustal thicknesses. For magma fluxes comparable to those estimated for Galapagos and Iceland, lithospheric heating by the penetrating magma alone is sufficient to cause a ridge jump, even without the other effects.

Lithospheric structure beneath Mainland China from ambient noise tomography

NASA Astrophysics Data System (ADS)

Huang, J.; Peng, J.; Liu, Z.

2017-12-01

The Chinese continent is composed of several Precambrian craton blocks and Phanerozoic orogenic belts. To better understand the complex geological structure and tectonic evolution, it is important to develop a high-resolution shear velocity model of the lithosphere. In this study, we try to use ambient noise tomography to image the lithospheric structure beneath mainland China. However, in contrast with most of the existing ambient noise tomography studies which focus on the surface wave at periods shorter than 60 s, we apply the technique of phase-weighted stack (PWS) (Schimmel et al., 2011) when stacking the cross-correlations of ambient noise. We could extract long-period ( 125 s) dispersions to image the high-resolution lithospheric structure. We collected continuous seismic records from the broadband stations of China Regional Seismic Networks and NECESSArray between Sept., 2009 and Aug., 2011. We constructed Rayleigh wave group and phase velocity maps on 0.25 ×0.25 degree grids, and then inverted a high-resolution lithospheric 3D shear velocity model up to 150 km depth. The results exhibited pronounced lateral heterogeneity of the lithospheric structure of the study area. It is obvious that the high velocities beneath the Ordos and Sichuan Basin exceeds 150 km, representing the strong and thick lithosphere. The lithospheric thickness gradually thins from west to east for the North China Craton (NCC) and the Yangtze Craton (YZC). The lithospheric thickness of the eastern NCC is about 80-90 km, and which beneath the Bohai Bay is thinnest, only 60-80 km. For the lower YZC and the Cathaysia block, the lithospheric thickness is about 70-80 km, slightly thinner than the eastern NCC. The observed thin lithosphere (about 60-80 km) beneath the eastern Northeast China is likely to be associated with the Tanlu fault and the Quaternary Changbaishan and Jingpohu volcano. The lithosphere thickness beneath the Tanlu fault is thin or absent, which possibly be related to the upwelling of the hot asthenosphere, and the fault provides channels. *This work was supported by National Key R&D Plan (Grant No. 2017YFC0601406). KEYWORDS: Ambient noise, Phase-weighted stack, Lithosphere, Shear velocity

Lithospheric discontinuities beneath the U.S. Midcontinent - signatures of Proterozoic terrane accretion and failed rifting

NASA Astrophysics Data System (ADS)

Chen, Chen; Gilbert, Hersh; Fischer, Karen M.; Andronicos, Christopher L.; Pavlis, Gary L.; Hamburger, Michael W.; Marshak, Stephen; Larson, Timothy; Yang, Xiaotao

2018-01-01

Seismic discontinuities between the Moho and the inferred lithosphere-asthenosphere boundary (LAB) are known as mid-lithospheric discontinuities (MLDs) and have been ascribed to a variety of phenomena that are critical to understanding lithospheric growth and evolution. In this study, we used S-to-P converted waves recorded by the USArray Transportable Array and the OIINK (Ozarks-Illinois-Indiana-Kentucky) Flexible Array to investigate lithospheric structure beneath the central U.S. This region, a portion of North America's cratonic platform, provides an opportunity to explore how terrane accretion, cratonization, and subsequent rifting may have influenced lithospheric structure. The 3D common conversion point (CCP) volume produced by stacking back-projected Sp receiver functions reveals a general absence of negative converted phases at the depths of the LAB across much of the central U.S. This observation suggests a gradual velocity decrease between the lithosphere and asthenosphere. Within the lithosphere, the CCP stacks display negative arrivals at depths between 65 km and 125 km. We interpret these as MLDs resulting from the top of a layer of crystallized melts (sill-like igneous intrusions) or otherwise chemically modified lithosphere that is enriched in water and/or hydrous minerals. Chemical modification in this manner would cause a weak layer in the lithosphere that marks the MLDs. The depth and amplitude of negative MLD phases vary significantly both within and between the physiographic provinces of the midcontinent. Double, or overlapping, MLDs can be seen along Precambrian terrane boundaries and appear to result from stacked or imbricated lithospheric blocks. A prominent negative Sp phase can be clearly identified at 80 km depth within the Reelfoot Rift. This arrival aligns with the top of a zone of low shear-wave velocities, which suggests that it marks an unusually shallow seismic LAB for the midcontinent. This boundary would correspond to the top of a region of mechanically and chemically rejuvenated mantle that was likely emplaced during late Precambrian/early Cambrian rifting. These observations suggest that the lithospheric structure beneath the Reelfoot Rift may be an example of a global phenomenon in which MLDs act as weak zones that facilitate the removal of cratonic lithosphere that lies beneath.

Ridge jumps associated with plume-ridge interaction: Mantle plume-lithosphere interaction and hotspot magmatism

NASA Astrophysics Data System (ADS)

Mittelstaedt, E.; Ito, G.

2007-12-01

Interaction of mantle plumes and young lithosphere near mid-ocean ridges can lead to changes in spreading geometry by shifts of the ridge-axis toward the plume as seen at various hotspots, notably Iceland and the Galapagos. Previous work has shown that, with a sufficient magma flux, heating of the lithosphere by magmatism can significantly weaken the plate and, in some cases, could cause ridge jumps. Upwelling hot asthenosphere can also weaken the plate through thermal and mechanical thinning of the lithosphere. Using the finite element code CITCOM, we solve the equations of continuity, momentum and energy to examine deformation in near-ridge lithosphere associated with relatively hot upwelling asthenosphere and seafloor spreading. The mantle and lithosphere obey a non-Newtonian viscous rheology with plastic failure in the cold part of the lithosphere simulated by imposing an effective yield stress. Temperatures of the lithospheric thermal boundary region are initially given a square-root of age thermal profile while a hot patch is placed at the bottom to initiate a mantle-plume like upwelling. The effect of upwelling asthenosphere on ridge jumps is evaluated by varying three parameters: the plume excess temperature, the spreading rate and the distance of the plume from the ridge axis. Preliminary results show plume related thinning and weakening of the lithosphere over a wide area (100's of km's) with the rate of thinning increasing with the excess temperature of the plume. Initially, thinning occurs as the plume approaches the lithosphere and asthenospheric material is forced out of the way. As the plume material comes into contact with the lithosphere, thinning occurs through heating and mechanical removal of the thermal boundary layer. Thinning of the lithosphere is one of the primary factors in achieving a ridge jump. Another is large tensile stresses which can facilitate the initiation of rifting at this weakened location. Model stresses induced by the buoyant asthenosphere are significant fractions of the lithospheric yield strength near the plume and reach a maximum at the center of plume upwelling. Models predict that ridge jumps are not likely to occur by lithosphere interaction with the hot upwelling plume alone but require the added effects of magmatic weakening at the hotspot.

On the relations between cratonic lithosphere thickness, plate motions, and basal drag

USGS Publications Warehouse

Artemieva, I.M.; Mooney, W.D.

2002-01-01

An overview of seismic, thermal, and petrological evidence on the structure of Precambrian lithosphere suggests that its local maximum thickness is highly variable (140-350 km), with a bimodal distribution for Archean cratons (200-220 km and 300-350 km). We discuss the origin of such large differences in lithospheric thickness, and propose that the lithospheric base can have large depth variations over short distances. The topography of Bryce Canyon (western USA) is proposed as an inverted analog of the base of the lithosphere. The horizontal and vertical dimensions of Archean cratons are strongly correlated: larger cratons have thicker lithosphere. Analysis of the bimodal distribution of lithospheric thickness in Archean cratons shows that the "critical" surface area for cratons to have thick (>300 km) keels is >6-8 ?? 106 km2 . Extrapolation of the linear trend between Archean lithospheric thickness and cratonic area to zero area yields a thickness of 180 km. This implies that the reworking of Archean crust should be accompanied by thinning and reworking of the entire lithospheric column to a thickness of 180 km in accord with thickness estimates for Proterozoic lithosphere. Likewise, extrapolation of the same trend to the size equal to the total area of all Archean cratons implies that the lithospheric thickness of a hypothesized early Archean supercontinent could have been 350-450 km decreasing to 280-400 km for Gondwanaland. We evaluate the basal drag model as a possible mechanism that may thin the cratonic lithosphere. Inverse correlations are found between lithospheric thickness and (a) fractional subduction length and (b) the effective ridge length. In agreement with theoretical predictions, lithospheric thickness of Archean keels is proportional to the square root of the ratio of the craton length (along the direction of plate motion) to the plate velocity. Large cratons with thick keels and low plate velocities are less eroded by basal drag than small fast-moving cratons. Basal drag may have varied in magnitude over the past 4 Ga. Higher mantle temperatures in the Archean would have resulted in lower mantle viscosity. This in turn would have reduced basal drag and basal erosion, and promoted the preservation of thick (>300 km) Archean keels, even if plate velocities were high during the Archean. ?? 2002 Elsevier Science B.V. All rights reserved.

Impact of the rheological layering of the lithosphere on the topography generated by sublithospheric density anomalies: Insights from analog modeling

NASA Astrophysics Data System (ADS)

Sembroni, A.; Globig, J.; Rozel, A.; Faccenna, C.; Funiciello, F.; Fernandez, M.

2013-12-01

Density anomalies located beneath the lithosphere are thought to generate dynamic topography at the surface of the Earth. Tomographic models are often used to infer the later variations of the density field in the mantle. Surface topography can then be computed using analytical solutions or numerical simulations of mantle convection. It has been shown that the viscosity profile of the upper mantle has a strong influence on the magnitude and spectral signature of surface topography and uplift rate. Here we present results from analogue modeling of the interaction between a rising ball-shaped density anomaly and the lithosphere in an isoviscous, isothermal Newtonian mantle system. Preliminary data show that surface topography is strongly influenced not only by mantle viscosity but also by density and viscosity profiles of the lithosphere. Our apparatus consists of a plexiglass square box (40x40x50 cm3) filled with glucose syrup. From the bottom a silicon ball was free to rise up until impinging a silicon plate floating on top of the syrup, mimicking the lithosphere. In order to investigate the role of lithospheric thickness and layered continental crust on stress partitioning, maximum dynamic topography, uplift rate and signal wavelength, two different configurations were tested: homogeneous lithosphere and stratified lithosphere including a low-viscosity lower crust. The topographic evolution of the surface was tracked using a laser scanning the top of the apparatus. The rise of the density anomaly was recorded by a side camera. We observe that a thick and then more resistant lithosphere makes up to 2 times lower and laterally wider topographic signatures. Layered lithospheres including a decoupling lower crust decrease the equilibrium topography and its lateral extend by ~30% to 40%. Most importantly, the uplift rate is strongly affected by the choice of lithosphere model. Both lithosphere width and the presence of a decoupling lower crust may modify the uplift rate by a factor 3. Thus, depending on the lithosphere rheology, we show that uplift rate may vary by one order of magnitude, for the same density anomaly and mantle viscosity. This result shows that surface uplift rate can be used to infer the viscosity of the upper mantle in specific Earth regions only if the rheology of the lithosphere is well constrained. With respect to previous approaches, whether numerical or analog modeling of dynamic topography, our experiments represent a new attempt to investigate the propagation of normal stresses generated by mantle flow through a rheologically stratified lithosphere and its resulting topographic signal.

Rayleigh wave tomography in South China from ambient seismic noise cross-correlation

NASA Astrophysics Data System (ADS)

Xu, S.; Song, X.; Wang, L.; Xu, M.

2012-12-01

South China is a composite of continental blocks with relics of Proterozoic and late-Paleozoic sutures, and underwent complex transformations in geological history. The connection of Yangtze Block (YZB) and Cathaysian Block (CTB) around 1 Ga led to Neoproterozoic rift systems in the central South China. Subsequently, strong folding and thrusting with intracontinental shortening occurred in the central South China during Mesozoic as a response to the collision of China-Indochina with Philippine Plate. Post orogenesis during Jurassic and Cretaceous, accompanied by extension and delamination, led to the thinning of lithosphere and granite intrusion, which becomes the main mineralization process in South China. The mechanism of the intracontinental collision and boundary of YZB with CTB have been debated for a long time. Detailed lithospheric structures are important for us to understand characteristics of the continental blocks, their boundaries, and the evolution processes. We use ambient noise cross-correlation method to obtain high resolution 3-D velocity images of South China. Continuous records of 298 stations from January 2010 to June 2011 are used to retrieve Rayleigh wave Green functions between possible station pairs. Waveform and dispersion curve comparisons with earthquake-emitted Rayleigh wave indicate that the ambient noise method is efficient and reliable. Both group and phase velocity images of 0.5°×0.5° grid from 8 to 40 s are estimated. Then, shear velocities are inversed in each grid, using a recent regional S-velocity model as a constraint for deeper structures. The surface wave maps in short periods clearly delineate basins and mountains. Deeper Moho in west of South China is observed, which can be interpreted as a result of strata folding and shortening in Mesozoic or long-range effects from Indian-Asian collision in Cenozoic. The collision mode between YZB and CTB is different for Lower-YZB (L-YZB) and Upper-YZB (U-YZB): the L-YZB is a simple attachment while the CTB overthrusts over U-YZB. Strong extension and delamination in the eastern part of South China result in thinner and weaker lithosphere than in the west. Remnant materials of partial melting induced by Paleo-Pacific plate subduction form the origin of Late-Mesozoic Volcanic Belt in the Southeast China. Low velocity along the fault implies that Tanlu is a lithosphere-scale structure. We also observe a vertical low velocity zone under the previously-claimed "Hainan Island mantle plume".

Structure of the subduction system in southern Peru from seismic array data

NASA Astrophysics Data System (ADS)

Phillips, Kristin; Clayton, Robert W.; Davis, Paul; Tavera, Hernando; Guy, Richard; Skinner, Steven; Stubailo, Igor; Audin, Laurence; Aguilar, Victor

2012-11-01

The subduction zone in southern Peru is imaged using converted phases from teleseismic P, PP, and PKP waves and Pwave tomography using local and teleseismic events with a linear array of 50 broadband seismic stations spanning 300 km from the coast to near Lake Titicaca. The slab dips at 30° and can be observed to a depth of over 200 km. The Moho is seen as a continuous interface along the profile, and the crustal thickness in the back-arc region (the Altiplano) is 75 km thick, which is sufficient to isostatically support the Andes, as evidenced by the gravity. The shallow crust has zones of negative impedance at a depth of 20 km, which is likely the result of volcanism. At the midcrustal level of 40 km, there is a continuous structure with a positive impedance contrast, which we interpret as the western extent of the Brazilian Craton as it underthrusts to the west.Vp/Vs ratios estimated from receiver function stacks show average values for this region with a few areas of elevated Vp/Vs near the volcanic arc and at a few points in the Altiplano. The results support a model of crustal thickening in which the margin crust is underthrust by the Brazilian Shield.

Oceanic microplate formation records the onset of India-Eurasia collision

NASA Astrophysics Data System (ADS)

Matthews, Kara J.; Dietmar Müller, R.; Sandwell, David T.

2016-01-01

Mapping of seafloor tectonic fabric in the Indian Ocean, using high-resolution satellite-derived vertical gravity gradient data, reveals an extinct Pacific-style oceanic microplate ('Mammerickx Microplate') west of the Ninetyeast Ridge. It is one of the first Pacific-style microplates to be mapped outside the Pacific basin, suggesting that geophysical conditions during formation probably resembled those that have dominated at eastern Pacific ridges. The microplate formed at the Indian-Antarctic ridge and is bordered by an extinct ridge in the north and pseudofault in the south, whose conjugate is located north of the Kerguelen Plateau. Independent microplate rotation is indicated by asymmetric pseudofaults and rotated abyssal hill fabric, also seen in multibeam data. Magnetic anomaly picks and age estimates calculated from published spreading rates suggest formation during chron 21o (∼47.3 Ma). Plate reorganizations can trigger ridge propagation and microplate development, and we propose that Mammerickx Microplate formation is linked with the India-Eurasia collision (initial 'soft' collision). The collision altered the stress regime at the Indian-Antarctic ridge, leading to a change in segmentation and ridge propagation from an establishing transform. Fast Indian-Antarctic spreading that preceded microplate formation, and Kerguelen Plume activity, may have facilitated ridge propagation via the production of thin and weak lithosphere; however both factors had been present for tens of millions of years and are therefore unlikely to have triggered the event. Prior to the collision, the combination of fast spreading and plume activity was responsible for the production of a wide region of undulate seafloor to the north of the extinct ridge and 'W' shaped lineations that record back and forth ridge propagation. Microplate formation provides a precise means of dating the onset of the India-Eurasia collision, and is completely independent of and complementary to timing constraints derived from continental geology or convergence histories.

India-Eurasia collision triggers formation of an oceanic microplate

NASA Astrophysics Data System (ADS)

Matthews, Kara; Müller, Dietmar; Sandwell, David

2016-04-01

Detailed mapping of seafloor tectonic fabric in the Indian Ocean, using high-resolution satellite-derived vertical gravity gradient data, reveals an extinct Pacific-style oceanic microplate - the Mammerickx Microplate - west of the Ninetyeast Ridge. It is one of the first Pacific-style microplates to be mapped outside the Pacific basin, suggesting that geophysical conditions during formation probably resembled those that have dominated at eastern Pacific ridges. The microplate formed at the Indian-Antarctic ridge and is bordered by an extinct ridge in the north and pseudofault in the south, whose conjugate is located north of the Kerguelen Plateau. Independent microplate rotation is indicated by asymmetric pseudofaults and rotated abyssal hill fabric, also identified in multibeam data. Magnetic anomaly picks and age estimates calculated from published spreading rates suggest formation during chron 21o (~47.3 Ma). Plate reorganizations can trigger ridge propagation and microplate development, and we propose that formation of the Mammerickx Microplate is linked with the initial 'soft' stage of the India-Eurasia collision. The collision altered the stress regime at the Indian-Antarctic ridge, leading to a change in segmentation and ridge propagation from an establishing transform fault. Fast Indian-Antarctic spreading that preceded microplate formation, and Kerguelen Plume activity may have facilitated ridge propagation via the production of thin and weak lithosphere. However, both factors had been present for tens of millions of years and are therefore unlikely to have triggered the event. Prior to the collision, this combination of fast spreading and plume activity was responsible for the production of a wide region of undulate seafloor to the north of the extinct ridge and 'W' shaped lineations that record back and forth ridge propagation. Microplate formation provides a means of dating the onset of the India-Eurasia collision, and is completely independent of and complementary to timing constraints derived from continental geology or convergence histories.

Late Cenozoic deformation of the Eurasian and Burma Plates due to subduction of the Indian Plate beneath SE Tibetan Plateau and Myanmar

NASA Astrophysics Data System (ADS)

Ni, J.; Holt, W. E.; Flesch, L. M.; Sandvol, E. A.; Hearn, T. M.; Schmerr, N. C.

2015-12-01

The late Cenozoic tectonics of the southeastern Tibetan Plateau and surrounding regions needs to be evaluated within the context of a larger dynamic system related to the subduction of the Indian oceanic lithosphere beneath Myanmar and Yunnan. The details of the geodynamic processes involve mantle flows associated with rollback and tears (which probably occur both laterally and horizontally) of the Indian slab with consequent effects on the geology of the overriding plate. These effects include: 1) volcanism in Tongchong, Yunnan Province, 2) clockwise rotational deformation of the overriding plate and 3) Burma Plate capture. In this talk we will present the strain rate throughout the region with a moment tensor summation of earthquake data. The deformation of SE Tibet, Yunnan and western Sichuan constitutes a distributed N-S oriented dextral shear zone with clockwise rotations up to 1.7° per million years. It is the clockwise vorticity relative to south China that accounts for the relative northward motion of India at a rate of 38±12 mm/yr at the Himalayas. Relative to south China, there is no southeastward extrusion of crustal material beyond the eastern margin of the Tibetan Plateau. Studies on earthquake seismic moment data, fault-slip data, and GPS measurements all show a clockwise rotational motion of SE Tibet, Yunnan, western Sichuan and eastern Myanmar around the EHS. The mirror image of this situation that is occurring today is the counterclockwise rotation of Anatolia, which is associated with the rollback of the Hellenic and Cyprian Trenches. In this talk we will also discuss the extreme oblique convergence between Indian and Burma plates and one of the effects of the oblique subducation is the transfer of right-lateral strike-slip faulting to the Indo Burma Range, one of the largest GeoPRISMs on Earth.

Different stages of collision zones on examples of Gujarat province (India) and Caucasus

NASA Astrophysics Data System (ADS)

Zabelina, Irina; Koulakov, Ivan; Ranjan Kayal, Jnana; Pratap Singh, Ajay; Kumar, Santosh; Kukarina, Ekaterina; Amanatashvili, Iason

2016-04-01

In this study we present seismic structures of the crust and upper mantle beneath two regions: Kachchh Gujarat region (India), and Caucasus that may represent different stages of the collisional processes. In both cases, the 3D seismic models were obtained based on tomography inversion of arrival times of P and S seismic waves from local and regional earthquakes. Collisional processes in the Caucasus region began 35 million years ago with the closure of the Tethys Ocean, and continues to this day. The rate of shortening between the Scythian and the Arabian plate is currently 1-2.2 mm/year. The tomography inversion used the dataset provided by several seismic agencies of the Caucasus region that contained 23,071 P- and 21,598 S-picks from 1374 events. The obtained P and S velocity models clearly delineate major tectonic units in the study area. A high velocity anomaly in Transcaucasian separating the Great and Lesser Caucasus possibly represents a rigid crustal block corresponding to the remnant oceanic lithosphere of Tethys. Another high-velocity pattern coincides with the southern edge of the Scythian Plate. Strongly deformed areas of Great and Lesser Caucasus are mostly associated with low-velocity patterns representing thickened felsic part of the crust and strong fracturing of rocks. Most Cenozoic volcanic centers of Caucasus match to the low-velocity seismic anomalies in the crust. We propose that the mantle part of the Arabian and Eurasian Plates has been delaminated due to the continental collision in the Caucasus region. As a result, overheated asthenosphere appeared nearly the bottom of the crust and facilitated melting of the crustal material that caused the origin of recent volcanism in Great and Lesser Caucasus. The Kachchh province, in contrast to the Caucasus, is far from any boundaries of major lithospheric plates. However, this area is one of the most seismically active in India. It is suggested that it may be a site of the lithosphere rupture and initiation of a new collision zone. For the tomography inversion of the Kachchh region we selected the data of 4105 earthquakes with arrival times 29660 P and 30278 S waves. Based on the obtained seismic anomalies, we identify the left-lateral displacement to approximately 70 km along a hidden fault. We suggest that this fault can be associated with a series of ridges having the SW-NE direction, which are clearly seen on the bathymetry of the Indian Ocean bottom. Northwards displacement of the Indo-Australian Plate and contraction with Asia causes strong compression deformations in the broad areas of the Indian Plate. The curved geometry of the western boundary of the Indo-Australian plate and orientations of the fracture zones presume both shear and compressional displacements along faults. The presence of both thrust and strike-slip mechanisms of earthquakes in the Kachchh province may support the existence of such combined deformations leading to initiation of a new collision belt.

High-Resolution Lithosphere Viscosity and Dynamics Revealed by Magnetotelluric Imaging

NASA Astrophysics Data System (ADS)

Liu, L.; Hasterok, D. P.

2016-12-01

An accurate viscosity structure is critical to truthfully modeling continental lithosphere dynamics, especially at spatial scales of <200 km where active tectonic deformation and volcanism occur. However, the effective viscosity structure of the lithosphere remains a key challenge in geodynamics due to the intimate involvement of viscosity with time and its dependence on many factors including strain rate, plastic failure, composition, and grain size. Current efforts on inferring the detailed lithosphere viscosity structure are sparse and large uncertainties and discrepancies still exist. Here we report an attempt to infer the effective lithospheric viscosity from a high-resolution magnetotelluric (MT) survey across the western United States. The high sensitivity of MT fields to the presence of electrically conductive fluids makes it a promising proxy for determining mechanical strength variations throughout the lithosphere. We demonstrate how a viscosity structure, approximated from electrical resistivity, results in a geodynamic model that successfully predicts short-wavelength surface topography, lithospheric deformation, and mantle upwelling beneath recent volcanism. The results indicate that lithosphere viscosity structure rather than the buoyancy structure is the dominant controlling factor for short-wavelength topography and intra-plate deformation in tectonically active regions. We further show that this viscosity is consistent with and more effective than that derived from laboratory-based rheology. We therefore propose that MT imaging provides a practical observational constraint for quantifying the dynamic evolution of the continental lithosphere.

Mid-ocean-ridge seismicity reveals extreme types of ocean lithosphere.

PubMed

Schlindwein, Vera; Schmid, Florian

2016-07-14

Along ultraslow-spreading ridges, where oceanic tectonic plates drift very slowly apart, conductive cooling is thought to limit mantle melting and melt production has been inferred to be highly discontinuous. Along such spreading centres, long ridge sections without any igneous crust alternate with magmatic sections that host massive volcanoes capable of strong earthquakes. Hence melt supply, lithospheric composition and tectonic structure seem to vary considerably along the axis of the slowest-spreading ridges. However, owing to the lack of seismic data, the lithospheric structure of ultraslow ridges is poorly constrained. Here we describe the structure and accretion modes of two end-member types of oceanic lithosphere using a detailed seismicity survey along 390 kilometres of ultraslow-spreading ridge axis. We observe that amagmatic sections lack shallow seismicity in the upper 15 kilometres of the lithosphere, but unusually contain earthquakes down to depths of 35 kilometres. This observation implies a cold, thick lithosphere, with an upper aseismic zone that probably reflects substantial serpentinization. We find that regions of magmatic lithosphere thin dramatically under volcanic centres, and infer that the resulting topography of the lithosphere-asthenosphere boundary could allow along-axis melt flow, explaining the uneven crustal production at ultraslow-spreading ridges. The seismicity data indicate that alteration in ocean lithosphere may reach far deeper than previously thought, with important implications towards seafloor deformation and fluid circulation.

Deep Europe today: Geophysical synthesis of the upper mantle structure and lithospheric processes over 3.5 Ga

USGS Publications Warehouse

Artemieva, I.M.; Thybo, H.; Kaban, M.K.; ,

2006-01-01

We present a summary of geophysical models of the subcrustal lithosphere of Europe. This includes the results from seismic (reflection and refraction profiles, P- and S-wave tomography, mantle anisotropy), gravity, thermal, electromagnetic, elastic and petrological studies of the lithospheric mantle. We discuss major tectonic processes as reflected in the lithospheric structure of Europe, from Precambrian terrane accretion and subduction to Phanerozoic rifting, volcanism, subduction and continent-continent collision. The differences in the lithospheric structure of Precambrian and Phanerozoic Europe, as illustrated by a comparative analysis of different geophysical data, are shown to have both a compositional and a thermal origin. We propose an integrated model of physical properties of the European subcrustal lithosphere, with emphasis on the depth intervals around 150 and 250 km. At these depths, seismic velocity models, constrained by body-and surface-wave continent-scale tomography, are compared with mantle temperatures and mantle gravity anomalies. This comparison provides a framework for discussion of the physical or chemical origin of the major lithospheric anomalies and their relation to large-scale tectonic processes, which have formed the present lithosphere of Europe. ?? The Geological Society of London 2006.

Lithospheric thickness controlled compositional variations in potassic basalts of Northeast China by melt-rock interactions

NASA Astrophysics Data System (ADS)

Liu, Jian-Qiang; Chen, Li-Hui; Zeng, Gang; Wang, Xiao-Jun; Zhong, Yuan; Yu, Xun

2016-03-01

Melt-rock interaction is a common mantle process; however, it remains unclear how this process affects the composition of potassic basalt. Here we present a case study to highlight the link between compositional variations in the potassic basalts and melt-rock interaction in cold lithosphere. Cenozoic potassic basalts in Northeast China are strongly enriched in incompatible elements and show EM1-type Sr-Nd-Pb isotopes, suggesting an enriched mantle source. These rocks show good correlations between 87Sr/86Sr and K2O/Na2O and Rb/Nb. Notably, these ratios decrease with increasing lithospheric thickness, which may reflect melt-lithosphere interaction. Phlogopite precipitated when potassic melts passed through the lithospheric mantle, and K and Rb contents of the residual melts decreased over time. The thicker the lithosphere, the greater the loss of K and Rb from the magma. Therefore, the compositions of potassic basalts were controlled by both their enriched sources and reactions with lithospheric mantle.

Convective thinning of the lithosphere - A mechanism for the initiation of continental rifting

NASA Technical Reports Server (NTRS)

Spohn, T.; Schubert, G.

1982-01-01

A model of lithospheric thinning, in which heat is convected to the base and conducted within the lithosphere, is presented. An analytical equation for determinining the amount of thinning attainable on increasing the heat flux from the asthenosphere is derived, and a formula for lithosphere thickness approximations as a function of time is given. Initial and final equilibrium thicknesses, thermal diffusivity, transition temperature profile, and plume temperature profile are all factors considered for performing rate of thinning determinations. In addition, between initial and final equilibrium states, lithospheric thinning occurs at a rate which is inversely proportional to the square root of the time. Finally, uplift resulting from thermal expansion upon lithospheric thinning is on the order of 10 to the 2nd to 10 to the 3rd m.

Geodynamic inversion to constrain the rheology of the lithosphere: What is the effect of elasticity?

NASA Astrophysics Data System (ADS)

Baumann, Tobias; Kaus, Boris; Thielmann, Marcel

2016-04-01

The concept of elastic thickness (T_e) is one of the main methods to describe the integrated strength of oceanic lithosphere (e.g. Watts, 2001). Observations of the Te are in general agreement with yield strength envelopes estimated from laboratory experiments (Burov, 2007, Goetze & Evans 1979). Yet, applying the same concept to the continental lithosphere has proven to be more difficult (Burov & Diament, 1995), which resulted in an ongoing discussion on the rheological structure of the lithosphere (e.g. Burov & Watts, 2006, Jackson, 2002; Maggi et al., 2000). Recently, we proposed a new approach, which constrains rheological properties of the lithosphere directly from geophysical observations such as GPS-velocity, topography and gravity (Baumann & Kaus, 2015). This approach has the advantage that available data sets (such as Moho depth) can be directly taken into account without making the a-priori assumption that the lithosphere is thin elastic plate floating on the mantle. Our results show that a Bayesian inversion method combined with numerical thermo-mechanical models can be used as independent tool to constrain non-linear viscous and plastic parameters of the lithosphere. As the rheology of the lithosphere is strongly temperature dependent, it is even possible to add a temperature parameterisation to the inversion method and constrain the thermal structure of the lithosphere in this manner. Results for the India-Asia collision zone show that existing geophysical data require India to have a quite high effective viscosity. Yet, the rheological structure of Tibet less well constrained and a number of scenarios give a nearly equally good fit to the data. Yet, one of the assumptions that we make while doing this geodynamic inversion is that the rheology is viscoplastic, and that elastic effects do not significantly alter the large-scale dynamics of the lithosphere. Here, we test the validity of this assumption by performing synthetic forward models and retrieving the rheological parameters of these models with viscoplastic geodynamic inversions. We focus on a typical intra-oceanic subduction system as well as a typical scenario of subduction of an oceanic plate underneath a continental arc. Baumann, T. S. & Kaus, B. J. P., 2015. Geodynamic inversion to constrain thenon-linear rheology of the lithosphere, Geophys. J. Int., 202(2), 1289-1316. Burov, E. B. & Diament, M., 1995. The effective elastic thickness (Te) of continental lithosphere: What does it really mean?, J. Geophys. Res., 100, 3905-3927. Burov, E. B. & Watts, A. B., 2006. The long-term strength of continental lithosphere : jelly sandwich or crème brûlée?, GSA today, 16(1), 4-10. Burov, E. B., 2007. Crust and Lithosphere Dynamics: Plate Rheology and Mechanics, in Treatise Geophys., vol. 6, chap. 3, pp. 99-151, ed. Watts, A. B., Elsevier. Goetze, C. & Evans, B., 1979. Stress and temperature in the bending lithosphere as constrained by experimental rock mechanics, Geophys. J. Int., 59(3), 463-478. Jackson, J., 2002. Strength of the continental lithosphere: Time to abandon the jelly sandwich?, GSA today, 12(9), 4-9. Maggi, A., Jackson, J. A., McKenzie, D., & Priestley, K., 2000a. Earthquake focal depths, effective elastic thickness, and the strength of the continental lithosphere, Geology, 28, 495-498. Watts, A. B., 2001. Isostasy and Flexure of the Lithosphere, Cambridge University Press.

Continental growth by successive accretion of oceanic lithosphere: Evidence from tilted seismic anisotropy

NASA Astrophysics Data System (ADS)

Babuska, V.; Plomerova, J.; Karato, S. I.

2012-04-01

Although many studies indicate that subduction-related accretion, subduction-driven magmatism and tectonic stacking are major crustal-growth mechanisms, how the mantle lithosphere forms remains enigmatic. Cook (AGU Geod. Series 1986) published a model of continental 'shingling' based on seismic reflection data indicating dipping structures in the deep crust of accreted terranes. Helmstaedt and Gurney (J. Geoch. Explor. 1995) and Hart et al. (Geology 1997) suggest that the Archean continental lithosphere consists of alternating layers of basalt and peridotite derived from subducted and obducted Archean oceanic lithosphere. Peridotite xenoliths from the Mojavian mantle lithosphere (Luffi et al., JGR 2009), as well as xenoliths of eclogites underlying the Sierra Nevada batholith in California (Horodynskij et al., EPSL 2007), are representative for oceanic slab fragments successively attached to the continent. Recent seismological findings also seem to support a model of continental lithosphere built from systems of paleosubductions of plates of ancient oceanic lithosphere (Babuska and Plomerova, AGU Geoph. Monograph 1989), or by stacking of the plates (Helmstaedt and Schulze, Geol. Soc. Aust. Spec. Publ. 1989). Seismic anisotropy in the oceanic mantle lithosphere, explained mainly by the olivine A- (or D-) type fabric (Karato et al., Annu. Rev. Earth Planet. Sci. 2008), was discovered almost a half century ago (Hess, Nature 1964). Though it is difficult to determine seismic anisotropy within an active subducting slab (e.g., Healy et al., EPSL 2009; Eberhart-Phillips and Reyners, JGR 2009), field observations and laboratory experiments indicate the oceanic olivine fabric might be preserved there to a depth of at least 200-300 km. Dipping anisotropic fabrics in domains of the European mantle lithosphere were interpreted as systems of 'frozen' paleosubductions (Babuska and Plomerova, PEPI 2006), and the lithosphere base as a boundary between a fossil anisotropy in the lithospheric mantle and an underlying seismic anisotropy related to present-day flow in the asthenosphere (Plomerova and Babuska, Lithos 2010). Deep dipping reflectors in the Slave Craton were modelled as tops of a fossil oceanic lithosphere (Bostock, Lithos 1999). Using S-wave receiver functions, Miller and Eaton (GRL 2010) also interpreted mid-lithosphere discontinuities beneath British Columbia as remnant oceanic slabs. Strong radial anisotropy from global surface-wave data (Babuska et al., PAGEOPH 1998; Khan et al., JGR 2011), as well as differences between body-wave tomography images from SH and SV waves (Eken et al., Tectonophys. 2010), both showing strong anisotropy only down to ~200 km, are in agreement with the models of inclined olivine fabrics found in Phanerozoic and Precambrian mantle lithosphere (Plomerova et al., Solid Earth 2011). Models of assemblages of microplates with their own inclined fossil fabrics do not support a lithosphere growth by simple cooling processes, which should result in horizontal fabrics. The models with dipping fabrics also contribute to mapping boundaries of individual blocks building the continental lithosphere.

Venus Chasmata: A Lithospheric Stretching Model

NASA Technical Reports Server (NTRS)

Solomon, S. C.; Head, J. W.

1985-01-01

An outstanding problem for Venus is the characterization of its style of global tectonics, an issue intimately related to the dominant mechanism of lithospheric heat loss. Among the most spectacular and extensive of the major tectonic features on Venus are the chasmata, deep linear valleys generally interpreted to be the products of lithospheric extension and rifting. Systems of chasmata and related features can be traced along several tectonic zones up to 20,000 km in linear extent. A lithospheric stretching model was developed to explain the topographic characteristics of Venus chasmata and to constrain the physical properties of the Venus crust and lithosphere.

Mantle source volumes and the origin of the mid-Tertiary ignimbrite flare-up in the southern Rocky Mountains, western U.S.

NASA Astrophysics Data System (ADS)

Farmer, G. Lang; Bailley, Treasure; Elkins-Tanton, Linda T.

2008-04-01

Voluminous intermediate to silicic composition volcanic rocks were generated throughout the southern Rocky Mountains, western U.S., during the mid-Tertiary "ignimbrite flare-up", principally at the San Juan and Mogollon-Datil volcanic fields. At both volcanic centers, radiogenic isotope data have been interpreted as evidence that 50% or more of the volcanic rocks (by mass) were derived from mantle-derived, mafic parental magmas, but no consensus exists as to whether melting was largely of lithospheric or sub-lithospheric mantle. Recent xenolith studies, however, have revealed that thick (> 100 km), fertile, and hydrated continental lithosphere was present beneath at least portions of the southern Rocky Mountains during the mid-Tertiary. The presence of such thick mantle lithosphere, combined with an apparent lack of syn-magmatic extension, leaves conductive heating of lithospheric mantle as a plausible method of generating the mafic magmas that fueled the ignimbrite flare-up in this inland region. To further assess this possibility, we estimated the minimum volume of mantle needed to generate the mafic magmas parental to the preserved mid-Tertiary igneous rocks. Conservative estimates of the mantle source volumes that supplied the Mogollon-Datil and San Juan volcanic fields are ˜ 2 M km 3 and ˜ 7 M km 3, respectively. These volumes could have comprised only lithospheric mantle if at least the lower ˜ 20 km of the mantle lithosphere beneath the entire southern Rocky Mountains region underwent partial melting during the mid-Tertiary and if the resulting mafic magmas were drawn laterally for distances of up to ˜ 300 km into each center. Such widespread melting of lithospheric mantle requires that the lithospheric mantle have been uniformly fertile and primed for melting in the mid-Tertiary, a possibility if the lithospheric mantle had experienced widespread hydration and refrigeration during early Tertiary low angle subduction. Exposure of the mantle lithosphere to hot, upwelling sub-lithospheric mantle during mid-Tertiary slab roll back could have then triggered the mantle melting. While a plausible source for mid-Tertiary basaltic magmas in the southern Rocky Mountains, lithospheric mantle could not have been the sole source for mafic magmas generated to the south in that portion of the ignimbrite flare-up now preserved in the Sierra Madre Occidental of northern Mexico. The large mantle source volumes (> 45 M km 3) required to fuel the voluminous silicic ignimbrites deposited in this region (> 400 K km 3) are too large to have been accommodated within the lithospheric mantle alone, implying that melting in sub-lithospheric mantle must have played a significant role in generating this mid-Tertiary magmatic event.

Using crustal thickness and subsidence history on the Iberia-Newfoundland margins to constrain lithosphere deformation modes during continental breakup

NASA Astrophysics Data System (ADS)

Jeanniot, Ludovic; Kusznir, Nick; Manatschal, Gianreto; Mohn, Geoffroy

2014-05-01

Observations at magma-poor rifted margins such as Iberia-Newfoundland show a complex lithosphere deformation history during continental breakup and seafloor spreading initiation leading to complex OCT architecture with hyper-extended continental crust and lithosphere, exhumed mantle and scattered embryonic oceanic crust and continental slivers. Initiation of seafloor spreading requires both the rupture of the continental crust and lithospheric mantle, and the onset of decompressional melting. Their relative timing controls when mantle exhumation may occur; the presence or absence of exhumed mantle provides useful information on the timing of these events and constraints on lithosphere deformation modes. A single lithosphere deformation mode leading to continental breakup and sea-floor spreading cannot explain observations. We have determined the sequence of lithosphere deformation events for two profiles across the present-day conjugate Iberia-Newfoundland margins, using forward modelling of continental breakup and seafloor spreading initiation calibrated against observations of crustal basement thickness and subsidence. Flow fields, representing a sequence of lithosphere deformation modes, are generated by a 2D finite element viscous flow model (FeMargin), and used to advect lithosphere and asthenosphere temperature and material. FeMargin is kinematically driven by divergent deformation in the upper 15-20 km of the lithosphere inducing passive upwelling beneath that layer; extensional faulting and magmatic intrusions deform the topmost upper lithosphere, consistent with observations of deformation processes occurring at slow spreading ocean ridges (Cannat, 1996). Buoyancy enhanced upwelling, as predicted by Braun et al. (2000) is also kinematically included in the lithosphere deformation model. Melt generation by decompressional melting is predicted using the parameterization and methodology of Katz et al. (2003). The distribution of lithosphere deformation, the contribution of buoyancy driven upwelling and their spatial and temporal evolution including lateral migration are determined by using a series of numerical experiments, tested and calibrated against observations of crustal thicknesses and water-loaded subsidence. Pure-shear widths exert a strong control on the timing of crustal rupture and melt initiation; to satisfy OCT architecture, subsidence and mantle exhumation, we need to focus the deformation from a broad to a narrow region. The lateral migration of the deformation flow axis has an important control on the rupture of continental crust and lithosphere, melt initiation, their relative timing, the resulting OCT architecture and conjugate margin asymmetry. The numerical models are used to predict margin isostatic response and subsidence history.

Using crustal thickness, subsidence and P-T-t history on the Iberia-Newfoundland & Alpine Tethys margins to constrain lithosphere deformation modes during continental breakup

NASA Astrophysics Data System (ADS)

Jeanniot, L.; Kusznir, N. J.; Manatschal, G.; Mohn, G.; Beltrando, M.

2013-12-01

Observations at magma-poor rifted margins such as Iberia-Newfoundland show a complex lithosphere deformation history and OCT architecture, resulting in hyper-extended continental crust and lithosphere, exhumed mantle and scattered embryonic oceanic crust before continental breakup and seafloor spreading. Initiation of seafloor spreading requires both the rupture of the continental crust and lithospheric mantle, and the onset of decompressional melting. Their relative timing controls when mantle exhumation may occur; the presence or absence of exhumed mantle provides useful information on the timing of these events and constraints on lithosphere deformation modes. A single kinematic lithosphere deformation mode leading to continental breakup and sea-floor spreading cannot explain observations. We have determined the sequence of lithosphere deformation events, using forward modelling of crustal thickness, subsidence and P-T-t history calibrated against observations on the present-day Iberia-Newfoundland and the fossil analogue Alpine Tethys margins. Lithosphere deformation modes, represented by flow fields, are generated by a 2D finite element viscous flow model (FeMargin), and used to advect lithosphere and asthenosphere temperature and material. FeMargin is kinematically driven by divergent deformation in the topmost upper lithosphere inducing passive upwelling beneath that layer; the upper lithosphere is assumed to deform by extensional faulting and magmatic intrusions, consistent with observations of deformation processes occurring at slow spreading ocean ridges (Cannat, 1996). Buoyancy enhanced upwelling is also included in the kinematic model as predicted by Braun et al (2000). We predict melt generation by decompressional melting using the parameterization and methodology of Katz et al., 2003. We use a series of numerical experiments, tested and calibrated against crustal thicknesses and subsidence observations, to determine the distribution of lithosphere deformation, the contribution of buoyancy driven upwelling and their spatial and temporal evolution including lateral migration. Particle tracking is used to predict P-T-t histories for both Iberia-Newfoundland and the Alpine Tethys conjugate margin transects. The lateral migration of the deformation flow axis has an important control on the rupture of continental crust and lithosphere, melt initiation, their relative timing, the resulting OCT architecture and conjugate margin asymmetry. Initial continental crust thickness and lithosphere temperature structure are important in controlling initial elevation and subsequent subsidence and depositional histories. Numerical models are used to examine the possible isostatic responses of the present-day and fossil analogue rifted margins.

The impact of lateral variations in lithospheric thickness on glacial isostatic adjustment in West Antarctica

NASA Astrophysics Data System (ADS)

Nield, Grace A.; Whitehouse, Pippa L.; van der Wal, Wouter; Blank, Bas; O'Donnell, John Paul; Stuart, Graham W.

2018-04-01

Differences in predictions of Glacial Isostatic Adjustment (GIA) for Antarctica persist due to uncertainties in deglacial history and Earth rheology. The Earth models adopted in many GIA studies are defined by parameters that vary in the radial direction only and represent a global average Earth structure (referred to as 1D Earth models). Over-simplifying actual Earth structure leads to bias in model predictions in regions where Earth parameters differ significantly from the global average, such as West Antarctica. We investigate the impact of lateral variations in lithospheric thickness on GIA in Antarctica by carrying out two experiments that use different rheological approaches to define 3D Earth models that include spatial variations in lithospheric thickness. The first experiment defines an elastic lithosphere with spatial variations in thickness inferred from seismic studies. We compare the results from this 3D model with results derived from a 1D Earth model that has a uniform lithospheric thickness defined as the average of the 3D lithospheric thickness. Irrespective of deglacial history and sub-lithospheric mantle viscosity, we find higher gradients of present-day uplift rates (i.e. higher amplitude and shorter wavelength) in West Antarctica when using the 3D models, due to the thinner-than-1D-average lithosphere prevalent in this region. The second experiment uses seismically-inferred temperature as input to a power-law rheology thereby allowing the lithosphere to have a viscosity structure. Modelling the lithosphere with a power-law rheology results in behaviour that is equivalent to a thinner-lithosphere model, and it leads to higher amplitude and shorter wavelength deformation compared with the first experiment. We conclude that neglecting spatial variations in lithospheric thickness in GIA models will result in predictions of peak uplift and subsidence that are biased low in West Antarctica. This has important implications for ice-sheet modelling studies as the steeper gradients of uplift predicted from the more realistic 3D model may promote stability in marine-grounded regions of West Antarctica. Including lateral variations in lithospheric thickness, at least to the level of considering West and East Antarctica separately, is important for capturing short wavelength deformation and it has the potential to provide a better fit to GPS observations as well as an improved GIA correction for GRACE data.

Global map of lithosphere thermal thickness on a 1 deg x 1 deg grid - digitally available

NASA Astrophysics Data System (ADS)

Artemieva, Irina

2014-05-01

This presentation reports a 1 deg ×1 deg global thermal model for the continental lithosphere (TC1). The model is digitally available from the author's web-site: www.lithosphere.info. Geotherms for continental terranes of different ages (early Archean to present) are constrained by reliable data on borehole heat flow measurements (Artemieva and Mooney, 2001), checked with the original publications for data quality, and corrected for paleo-temperature effects where needed. These data are supplemented by cratonic geotherms based on xenolith data. Since heat flow measurements cover not more than half of the continents, the remaining areas (ca. 60% of the continents) are filled by the statistical numbers derived from the thermal model constrained by borehole data. Continental geotherms are statistically analyzed as a function of age and are used to estimate lithospheric temperatures in continental regions with no or low quality heat flow data. This analysis requires knowledge of lithosphere age globally. A compilation of tectono-thermal ages of lithospheric terranes on a 1 deg × 1 deg grid forms the basis for the statistical analysis. It shows that, statistically, lithospheric thermal thickness z (in km) depends on tectono-thermal age t (in Ma) as: z=0.04t+93.6. This relationship formed the basis for a global thermal model of the continental lithosphere (TC1). Statistical analysis of continental geotherms also reveals that this relationship holds for the Archean cratons in general, but not in detail. Particularly, thick (more than 250 km) lithosphere is restricted solely to young Archean terranes (3.0-2.6 Ga), while in old Archean cratons (3.6-3.0 Ga) lithospheric roots do not extend deeper than 200-220 km. The TC1 model is presented by a set of maps, which show significant thermal heterogeneity within continental upper mantle. The strongest lateral temperature variations (as large as 800 deg C) are typical of the shallow mantle (depth less than 100 km). A map of the depth to a 600 deg C isotherm in continental upper mantle is presented as a proxy to the elastic thickness of the cratonic lithosphere, in which flexural rigidity is dominated by olivine rheology of the mantle. The TC1 model of the lithosphere thickness is used to calculate the growth and preservation rates of the lithosphere since the Archean.

Controls of Lithospheric Mechanical Strength on the Deformation Pattern of Tien Shan

NASA Astrophysics Data System (ADS)

Li, Y.; Xiong, X.; Zheng, Y.; Hu, X.; Zhang, Y.

2015-12-01

The Tien Shan is an outstanding example of intracontinental mountain belt, which was built rapidly and formed far away from plate boundaries. It exhibits 300~500 km in width and extends ~2000 km EW, located in central Asia. The Tien Shan is a key area for solution of the problems relating to intracontinental geodynamics. During last decades, despite a large amount of results based on various geological, geophysical and geodetic data about the Tien Shan, however, deformation mechanism remains controversial and other several principal problems related to its structure and evolution also have not been completely resolved. As for patterns of continental deformation, they are always controlled by both the forces applied to the lithosphere and by lithospheric resistance to the forces. The latter is often measured by the mechanical strength of lithosphere. The lateral variation of strength of lithosphere has been recognized to be an important factor controlling the spatial construction and temporal evolution of continent. In this study, we investigate the mechanical strength (Te) of lithosphere in the Tien Shan using wavelet coherency between Bouguer anomaly and topography. The patterns of Te variations are closely related to major tectonic boundaries and blocks. Mechanical strength exhibits a weak zone (Te~5-20km) beneath the Tien Shan while its surrounding blocks including Tarim Basin, Junggar Basin and Kazakh platform are characterized by a strong lithosphere (Te>40km). The lateral variations in mechanical strength and velocity field of horizontal movement with GPS demonstrate that strain localization appears at the margins of Tarim Basin, which is also the strong lithospheric domain. It is suggested that the weak lithosphere allows the crustal stress accumulation and the strong lithosphere helps to stress transfer. There is also a good agreement between mechanical strength and shear wave velocity structure in upper mantle. It indicates a strong domain located in the lower crust and lithospheric mantle. Combined with results of analog models, the location and style of deformation are preliminary determined and thus the related topography evolution in the Tien Shan is mainly controlled by the lateral and depth variation in lithospheric mechanical strength of surrounding areas.

Lithospheric thermal evolution and dynamic mechanism of destruction of the North China Craton

NASA Astrophysics Data System (ADS)

Li, Zian; Zhang, Lu; Lin, Ge; Zhao, Chongbin; Liang, Yingjie

2018-06-01

The dynamic mechanism for destruction of the North China Craton (NCC) has been extensively discussed. Numerical simulation is used in this paper to discuss the effect of mantle upward throughflow (MUT) on the lithospheric heat flux of the NCC. Our results yield a three-stage destruction of the NCC lithosphere as a consequence of MUT variation. (1) In Late Paleozoic, the elevation of MUT, which was probably caused by southward and northward subduction of the paleo-Asian and paleo-Tethyan oceans, respectively, became a prelude to the NCC destruction. The geological consequences include a limited decrease of the lithospheric thickness, an increase of heat flux, and a gradual enhancement of the crustal activity. But the tectonic attribute of the NCC maintained a stable craton. (2) During Late Jurassic-Early Cretaceous, the initial velocity of the MUT became much faster probably in response to subduction of the Pacific Ocean; the conductive heat flux at the base of the NCC lithosphere gradually increased from west to east; and the lithospheric thickness was significantly decreased. During this stage, the heat flux distribution was characterized by zonation and partition, with nearly horizontal layering in the lithosphere and vertical layering in the underlying asthenosphere. Continuous destruction of the NCC lithosphere was associated with the intense tectono-magmatic activity. (3) From Late Cretaceous to Paleogene, the velocity of MUT became slower due to the retreat of the subducting Pacific slab; the conductive heat flux at the base of lithosphere was increased from west to east; the distribution of heat flux was no longer layered. The crust of the western NCC is relatively hotter than the mantle, so-called as a `hot crust but cold mantle' structure. At the eastern NCC, the crust and the mantle characterized by a `cold crust but hot mantle.' The western NCC (e.g., the Ordos Basin) had a tectonically stable crust with low thermal gradients in the lithosphere; whereas the eastern NCC was active with a hot lithosphere. The numerical results show that the MUT is the main driving force for the NCC destruction, whereas the complex interaction of surrounding plates lit a fuse for the lithospheric thinning.

Lithospheric thermal evolution and dynamic mechanism of destruction of the North China Craton

NASA Astrophysics Data System (ADS)

Li, Zian; Zhang, Lu; Lin, Ge; Zhao, Chongbin; Liang, Yingjie

2017-09-01

The dynamic mechanism for destruction of the North China Craton (NCC) has been extensively discussed. Numerical simulation is used in this paper to discuss the effect of mantle upward throughflow (MUT) on the lithospheric heat flux of the NCC. Our results yield a three-stage destruction of the NCC lithosphere as a consequence of MUT variation. (1) In Late Paleozoic, the elevation of MUT, which was probably caused by southward and northward subduction of the paleo-Asian and paleo-Tethyan oceans, respectively, became a prelude to the NCC destruction. The geological consequences include a limited decrease of the lithospheric thickness, an increase of heat flux, and a gradual enhancement of the crustal activity. But the tectonic attribute of the NCC maintained a stable craton. (2) During Late Jurassic-Early Cretaceous, the initial velocity of the MUT became much faster probably in response to subduction of the Pacific Ocean; the conductive heat flux at the base of the NCC lithosphere gradually increased from west to east; and the lithospheric thickness was significantly decreased. During this stage, the heat flux distribution was characterized by zonation and partition, with nearly horizontal layering in the lithosphere and vertical layering in the underlying asthenosphere. Continuous destruction of the NCC lithosphere was associated with the intense tectono-magmatic activity. (3) From Late Cretaceous to Paleogene, the velocity of MUT became slower due to the retreat of the subducting Pacific slab; the conductive heat flux at the base of lithosphere was increased from west to east; the distribution of heat flux was no longer layered. The crust of the western NCC is relatively hotter than the mantle, so-called as a `hot crust but cold mantle' structure. At the eastern NCC, the crust and the mantle characterized by a `cold crust but hot mantle.' The western NCC (e.g., the Ordos Basin) had a tectonically stable crust with low thermal gradients in the lithosphere; whereas the eastern NCC was active with a hot lithosphere. The numerical results show that the MUT is the main driving force for the NCC destruction, whereas the complex interaction of surrounding plates lit a fuse for the lithospheric thinning.

Collapse of passive margins by lithospheric damage and plunging grain size

NASA Astrophysics Data System (ADS)

Mulyukova, Elvira; Bercovici, David

2018-02-01

The collapse of passive margins has been proposed as a possible mechanism for the spontaneous initiation of subduction. In order for a new trench to form at the junction between oceanic and continental plates, the cold and stiff oceanic lithosphere must be weakened sufficiently to deform at tectonic rates. Such rates are especially hard to attain in the cold ductile portion of the lithosphere, at which the mantle lithosphere reaches peak strength. The amount of weakening required for the lithosphere to deform in this tectonic setting is dictated by the available stress. Stress in a cooling passive margin increases with time (e.g., due to ridge push), and is augmented by stresses present in the lithosphere at the onset of rifting (e.g., due to drag from underlying mantle flow). Increasing stress has the potential to weaken the ductile portion of the lithosphere by dislocation creep, or by decreasing grain size in conjunction with a grain-size sensitive rheology like diffusion creep. While the increasing stress acts to weaken the lithosphere, the decreasing temperature acts to stiffen it, and the dominance of one effect or the other determines whether the margin might weaken and collapse. Here, we present a model of the thermal and mechanical evolution of a passive margin, wherein we predict formation of a weak shear zone that spans a significant depth-range of the ductile portion of the lithosphere. Stiffening due to cooling is offset by weakening due to grain size reduction, driven by the combination of imposed stresses and grain damage. Weakening via grain damage is modest when ridge push is the only source of stress in the lithosphere, making the collapse of a passive margin unlikely in this scenario. However, adding even a small stress-contribution from mantle drag results in damage and weakening of a significantly larger portion of the lithosphere. We posit that rapid grain size reduction in the ductile portion of the lithosphere can enable, or at least significantly facilitate, the collapse of a passive margin and initiate a new subduction zone. We use this model to estimate the conditions for passive margin collapse for modern and ancient Earth, as well as for Venus.

Channeling at the base of the lithosphere during the lateral flow of plume material beneath flow line hot spots

NASA Astrophysics Data System (ADS)

Sleep, Norman H.

2008-08-01

Chains of volcanic edifices lie along flow lines between plume-fed hot spots and the thin lithosphere at ridge axes. Discovery and Euterpe/Musicians Seamounts are two examples. An attractive hypothesis is that buoyant plume material flows along the base of the lithosphere perpendicular to isochrons. The plume material may conceivably flow in a broad front or flow within channels convectively eroded into the base to the lithosphere. A necessary but not sufficient condition for convective channeling is that the expected stagnant-lid heat flow for the maximum temperature of the plume material is comparable to the half-space surface heat flow of the oceanic lithosphere. Two-dimensional and three-dimensional numerical calculations confirm this inference. A second criterion for significant convective erosion is that it needs to occur before the plume material thins by lateral spreading. Scaling relationships indicate spreading and convection are closely related. Mathematically, the Nusselt number (ratio of convective to conductive heat flow in the plume material) scales with the flux (volume per time per length of flow front) of the plume material. A blob of unconfined plume material thus spreads before the lithosphere thins much and evolves to a slowly spreading and slowly convecting warm region in equilibrium with conduction into the base of the overlying lithosphere. Three-dimensional calculations illustrate this long-lasting (and hence observable) state of plume material away from its plume source. A different flow domain occurs around a stationary hot plume that continuously supplies hot material. The plume convectively erodes the overlying lithosphere, trapping the plume material near its orifice. The region of lithosphere underlain by plume material grows toward the ridge axis and laterally by convective thinning of the lithosphere at its edges. The hottest plume material channels along flow lines. Geologically, the regions of lithosphere underlain by either warm or hot plume material are likely to extend laterally away from the volcanic edifices whether or not channeling occurs.

Dipping fossil fabrics of continental mantle lithosphere as tectonic heritage of oceanic paleosubductions

NASA Astrophysics Data System (ADS)

Babuska, Vladislav; Plomerova, Jaroslava; Vecsey, Ludek; Munzarova, Helena

2016-04-01

Subduction and orogenesis require a strong mantle layer (Burov, Tectonophys. 2010) and our findings confirm the leading role of the mantle lithosphere. We have examined seismic anisotropy of Archean, Proterozoic and Phanerozoic provinces of Europe by means of shear-wave splitting and P-wave travel-time deviations of teleseismic waves observed at dense arrays of seismic stations (e.g., Vecsey et al., Tectonophys. 2007). Lateral variations of seismic-velocity anisotropy delimit domains of the mantle lithosphere, each of them having its own consistent fabric. The domains, modeled in 3D by olivine aggregates with dipping lineation a, or foliation (a,c), represent microplates or their fragments that preserved their pre-assembly fossil fabrics. Evaluating seismic anisotropy in 3D, as well as mapping boundaries of the domains helps to decipher processes of the lithosphere formation. Systematically dipping mantle fabrics and other seismological findings seem to support a model of continental lithosphere built from systems of paleosubductions of plates of ancient oceanic lithosphere (Babuska and Plomerova, AGU Geoph. Monograph 1989), or from stacking of the plates (Helmstaedt and Schulze, Geol. Soc. Spec. Publ. 1989). Seismic anisotropy in the oceanic mantle lithosphere, explained mainly by the olivine A- or D-type fabric (Karato et al., Annu. Rev. Earth Planet. Sci. 2008), was discovered a half century ago (Hess, Nature 1964). Field observations and laboratory experiments indicate the oceanic olivine fabric might be preserved in the subducting lithosphere to a depth of at least 200-300 km. We thus interpret the dipping anisotropic fabrics in domains of the European mantle lithosphere as systems of "frozen" paleosubductions (Babuska and Plomerova, PEPI 2006) and the lithosphere base as a boundary between the fossil anisotropy in the lithospheric mantle and an underlying seismic anisotropy related to present-day flow in the asthenosphere (Plomerova and Babuska, Lithos 2010).

In situ rheology of the oceanic lithosphere along the Hawaiian ridge

NASA Astrophysics Data System (ADS)

Pleus, A.; Ito, G.; Wessel, P.; Frazer, L. N.

2017-12-01

Much of our quantitative understanding of lithospheric rheology is based on rock deformation experiments carried out in the laboratory. The accuracy of the relationships between stress and lithosphere deformation, however, are subject to large extrapolations, given that laboratory strain rates (10-7 s-1) are much greater than geologic rates (10-15 to 10-12 s-1). In situ deformation experiments provide independent constraints and are therefore needed to improve our understanding of natural rheology. Zhong and Watts [2013] presented such a study around the main Hawaiian Islands and concluded that the lithosphere flexure requires a much weaker rheology than predicted by laboratory experiments. We build upon this study by investigating flexure around the older volcanoes of the Hawaiian ridge. The ridge is composed of a diversity of volcano sizes that loaded seafloor of nearly constant age (85+/-8 Ma); this fortunate situation allows for an analysis of flexural responses to large variations in applied loads at nearly constant age-dependent lithosphere thermal structure. Our dataset includes new marine gravity and multi-beam bathymetry data collected onboard the Schmidt Ocean Institute's R/V Falkor. These data, along with forward models of lithospheric flexure, are used to obtain a joint posterior probability density function for model parameters that control the lithosphere's flexural response to a given load. These parameters include the frictional coefficient constraining brittle failure in the shallow lithosphere, the activation energy for the low-temperature plasticity regime, and the geothermal gradient of the Hawaiian lithosphere. The resulting in situ rheological parameters may be used to verify or update those derived in the lab. Attaining accurate lithospheric rheological properties is important to our knowledge, not only of the evolution of the Hawaiian lithosphere, but also of other solid-earth geophysical problems, such as oceanic earthquakes, subduction dynamics, and coastal topographic response to sea level rise.

Synthetic Analysis of the Effective Elastic Thickness of the Lithosphere in China

NASA Astrophysics Data System (ADS)

Lu, Z.; Li, C.

2017-12-01

Effective elastic thickness (Te) represents the response of the lithosphere to a long-term (larger than 105 years) geological loading and reflects the deformation mechanism of plate and its thermodynamic state. Temperature and composition of the lithosphere, coupling between crust and lithospheric mantle, and lithospheric structures affect Te. Regional geology in China is quite complex, influenced by the subduction of the Pacific and Philippine Sea plates in the east and the collision of the Eurasia plate with the India-Australia plate in the southwest. Te can help understand the evolution and strength of the lithospheres in different areas and tectonic units. Here we apply the multitaper coherence method to estimate Te in China using the topography (ETOPO1) and Bouguer gravity anomalies (WGM2012) , at different window sizes (600km*600km, 800km*800km, 1000km*1000km) and moving steps. The lateral variation of Te in China coincides well with the geology. The old stable cratons or basins always correspond to larger Te, whereas the oceanic lithosphere or active orogen blocks tend to get smaller Te. We further correlate Te to curie-point depths (Zb) and heat flow to understand how temperature influences the strength of the lithosphere. Despite of a complex correlation between Te and Zb, good positive correlations are found in the North China Block, Tarim Basin, and Lower Yangtze, showing strong influence of temperature on lithospheric strength. Conversely, the Tibetan Plateau, Upper and Middle Yangtze, and East China Sea Basin even show negative correlation, suggesting that lithospheric structures and compositions play more important roles than temperature in these blocks. We also find that earthquakes tend to occur preferably in a certain range of Te. Deeper earthquakes are more likely to occur where the lithosphere is stronger with larger Te. Crust with a larger Te may also have a deeper ductile-brittle boundary, along which deep large earthquakes tend to cluster.

Enriched continental flood basalts from depleted mantle melts: modeling the lithospheric contamination of Karoo lavas from Antarctica

NASA Astrophysics Data System (ADS)

Heinonen, Jussi S.; Luttinen, Arto V.; Bohrson, Wendy A.

2016-01-01

Continental flood basalts (CFBs) represent large-scale melting events in the Earth's upper mantle and show considerable geochemical heterogeneity that is typically linked to substantial contribution from underlying continental lithosphere. Large-scale partial melting of the cold subcontinental lithospheric mantle and the large amounts of crustal contamination suggested by traditional binary mixing or assimilation-fractional crystallization models are difficult to reconcile with the thermal and compositional characteristics of continental lithosphere, however. The well-exposed CFBs of Vestfjella, western Dronning Maud Land, Antarctica, belong to the Jurassic Karoo large igneous province and provide a prime locality to quantify mass contributions of lithospheric and sublithospheric sources for two reasons: (1) recently discovered CFB dikes show isotopic characteristics akin to mid-ocean ridge basalts, and thus help to constrain asthenospheric parental melt compositions and (2) the well-exposed basaltic lavas have been divided into four different geochemical magma types that exhibit considerable trace element and radiogenic isotope heterogeneity (e.g., initial ɛ Nd from -16 to +2 at 180 Ma). We simulate the geochemical evolution of Vestfjella CFBs using (1) energy-constrained assimilation-fractional crystallization equations that account for heating and partial melting of crustal wall rock and (2) assimilation-fractional crystallization equations for lithospheric mantle contamination by using highly alkaline continental volcanic rocks (i.e., partial melts of mantle lithosphere) as contaminants. Calculations indicate that the different magma types can be produced by just minor (1-15 wt%) contamination of asthenospheric parental magmas by melts from variable lithospheric reservoirs. Our models imply that the role of continental lithosphere as a CFB source component or contaminant may have been overestimated in many cases. Thus, CFBs may represent major juvenile crustal growth events rather than just recycling of old lithospheric materials.

Seismic and Thermal Structure of the Arctic Lithosphere, From Waveform Tomography and Thermodynamic Modelling

NASA Astrophysics Data System (ADS)

Lebedev, S.; Schaeffer, A. J.; Fullea, J.; Pease, V.

2015-12-01

Thermal structure of the lithosphere is reflected in the values of seismic velocities within it. Our new tomographic models of the crust and upper mantle of the Arctic are constrained by an unprecedentedly large global waveform dataset and provide substantially improved resolution, compared to previous models. The new tomography reveals lateral variations in the temperature and thickness of the lithosphere and defines deep boundaries between tectonic blocks with different lithospheric properties and age. The shape and evolution of the geotherm beneath a tectonic unit depends on both crustal and mantle-lithosphere structure beneath it: the lithospheric thickness and its changes with time (these determine the supply of heat from the deep Earth), the crustal thickness and heat production (the supply of heat from within the crust), and the thickness and thermal conductivity of the sedimentary cover (the insulation). Detailed thermal structure of the basins can be modelled by combining seismic velocities from tomography with data on the crustal structure and heat production, in the framework of computational petrological modelling. The most prominent lateral contrasts across the Arctic are between the cold, thick lithospheres of the cratons (in North America, Greenland and Eurasia) and the warmer, non-cratonic blocks. The lithosphere of the Canada Basin is cold and thick, similar to old oceanic lithosphere elsewhere around the world; its thermal structure offers evidence on its lithospheric age and formation mechanism. At 150-250 km depth, the central Arctic region shows a moderate low-velocity anomaly, cooler than that beneath Iceland and N Atlantic. An extension of N Atlantic low-velocity anomaly into the Arctic through the Fram Strait may indicate an influx of N Atlantic asthenosphere under the currently opening Eurasia Basin.

Lithospheric structure of northwest Africa: Insights into the tectonic history and influence of mantle flow on large-scale deformation

NASA Astrophysics Data System (ADS)

Miller, Meghan S.; Becker, Thorsten

2014-05-01

Northwest Africa is affected by late stage convergence of Africa with Eurasia, the Canary Island hotspot, and bounded by the Proterozoic-age West African craton. We present seismological evidence from receiver functions and shear-wave splitting along with geodynamic modeling to show how the interactions of these tectonic features resulted in dramatic deformation of the lithosphere. We interpret seismic discontinuities from the receiver functions and find evidence for localized, near vertical-offset deformation of both crust-mantle and lithosphere-asthenosphere interfaces at the flanks of the High Atlas. These offsets coincide with the locations of Jurassic-aged normal faults that have been reactivated during the Cenozoic, further suggesting that inherited, lithospheric-scale zones of weakness were involved in the formation of the Atlas. Another significant step in lithospheric thickness is inferred within the Middle Atlas. Its location corresponds to the source of regional Quaternary alkali volcanism, where the influx of melt induced by the shallow asthenosphere appears restricted to a lithospheric-scale fault on the northern side of the mountain belt. Inferred stretching axes from shear-wave splitting are aligned with the topographic grain in the High Atlas, suggesting along-strike asthenospheric shearing in a mantle channel guided by the lithospheric topography. Isostatic modeling based on our improved lithospheric constraints indicates that lithospheric thinning alone does not explain the anomalous Atlas topography. Instead, an mantle upwelling induced by a hot asthenospheric anomaly appears required, likely guided by the West African craton and perhaps sucked northward by subducted lithosphere beneath the Alboran. This dynamic support scenario for the Atlas also suggests that the timing of uplift is contemporaneous with the recent volcanismin the Middle Atlas.

Imaging Canary Island hotspot material beneath the lithosphere of Morocco and southern Spain

NASA Astrophysics Data System (ADS)

Miller, Meghan S.; O'Driscoll, Leland J.; Butcher, Amber J.; Thomas, Christine

2015-12-01

The westernmost Mediterranean has developed into its present day tectonic configuration as a result of complex interactions between late stage subduction of the Neo-Tethys Ocean, continental collision of Africa and Eurasia, and the Canary Island mantle plume. This study utilizes S receiver functions (SRFs) from over 360 broadband seismic stations to seismically image the lithosphere and uppermost mantle from southern Spain through Morocco and the Canary Islands. The lithospheric thickness ranges from ∼65 km beneath the Atlas Mountains and the active volcanic islands to over ∼210 km beneath the cratonic lithosphere in southern Morocco. The common conversion point (CCP) volume of the SRFs indicates that thinned lithosphere extends from beneath the Canary Islands offshore southwestern Morocco, to beneath the continental lithosphere of the Atlas Mountains, and then thickens abruptly at the West African craton. Beneath thin lithosphere between the Canary hot spot and southern Spain, including below the Atlas Mountains and the Alboran Sea, there are distinct pockets of low velocity material, as inferred from high amplitude positive, sub-lithospheric conversions in the SRFs. These regions of low seismic velocity at the base of the lithosphere extend beneath the areas of Pliocene-Quaternary magmatism, which has been linked to a Canary hotspot source via geochemical signatures. However, we find that this volume of low velocity material is discontinuous along strike and occurs only in areas of recent volcanism and where asthenospheric mantle flow is identified with shear wave splitting analyses. We propose that the low velocity structure beneath the lithosphere is material flowing sub-horizontally northeastwards beneath Morocco from the tilted Canary Island plume, and the small, localized volcanoes are the result of small-scale upwellings from this material.

Thermal erosion of cratonic lithosphere as a potential trigger for mass-extinction

NASA Astrophysics Data System (ADS)

Pilet, S.; Müntener, O.; Jean, G.; Schoene, B.; Schaltegger, U.

2016-12-01

The temporal coincidence between LIPs and mass extinctions has led many to pose a causal relationship between the two. However, there is still no consensus on a mechanistic model that explains how magmatism leads to the turnover of terrestrial and marine plants, invertebrates and vertebrates. Here, we present a synthesis of stratigraphic constraints on the Triassic-Jurassic and Pliensbachian-Toarcian boundaries combined with geochronological data demonstrating that these biotic crises are both associated with rapid change from an initial cool period to greenhouse conditions. As current hypothesis for LIPs seems unable to produce these successive climatic changes, we evaluate an alternative suggesting that the initial cooling could be due to gas release during the initial thermal erosion of the cratonic lithosphere due to emplacement of the CAMP and Karoo-Ferrar volcanic provinces. Karoo and CAMP areas were underlain by thick lithosphere (>200 km) prior to continental break up. Even in presence of abnormal potential mantle temperature, the presence of thick lithosphere excludes significant melting of the asthenospheric mantle without initial stage of thermal erosion of the cratonic lithosphere. Various studies on Kaapvaal craton have shown that sulfide minerals are enclosed in the basal part of the cratonic lithosphere. We argue that initial gas emission was dominated by sulfur liberated from sulfide-bearing cratonic lithosphere causing global cooling and eustatic regression, which was followed by warming/transgression associated with the progressive increase of CO2 in the atmosphere associated to LIPs emission. We suggest that the nature of the underlying lithosphere during large LIP eruption exerts an important control on the consequences at the Earth's surface. This model offers an explanation for why LIPs erupted through oceanic lithosphere are not associated with climatic and biotic crises comparable to LIPs emitted through cratonic lithosphere.

The MOZART Project - MOZAmbique Rift Tomography

NASA Astrophysics Data System (ADS)

Fonseca, J. F.; Chamussa, J. R.; Domingues, A.; Helffrich, G. R.; Fishwick, S.; Ferreira, A. M.; Custodio, S.; Brisbourne, A. M.; Grobbelaar, M.

2012-12-01

Project MOZART (MOZAmbique Rift Tomography) is an ongoing joint effort of Portuguese, Mozambican and British research groups to investigate the geological structure and current tectonic activity of the southernmost tip of the East African Rift System (EARS) through the deployment of a network of 30 broad band seismic stations in Central and Southern Mozambique. In contrast with other stretches of the EARS to the North and with the Kapvaal craton to the West and South, the lithosphere of Mozambique was not previously studied with a dense seismographic deployment on account of past political instability, and many questions remain unanswered with respect to the location and characteristics of the EARS to the south of Tanzania. In recent years, space geodesy revealed the existence of three microplates in and off Mozambique - Victoria, Rovuma, Lwandle - whose borders provide a connection of the EARS to the South West Indian Ridge as required by plate tectonics. However, the picture is still coarse concerning the location of the rift structures. The 2006 M7 Machaze earthquake in Central Mozambique highlighted the current tectonic activity of the region and added a further clue to the location of the continental rift, prompting the MOZART deployment. Besides helping unravel the current tectonics, the project is expected to shed light on the poorly known Mesoproterozoic structure described by Arthur Holmes in 1951 as the Mozambique Belt, and on the mechanisms of transition from stable craton to rifted continental crust, through the development of a tomographic model for the lithosphere. The MOZART network is distributed South of the Zambezi river at average inter-station spaces of the order of 100 km and includes four stations across the border in South Africa. Data exchange was agreed with AfricaArray. The deployment proceeded in two phases in March 2011, and November and December 2011. Decommissioning is foreseen for August 2013. We report preliminary results for this previously unexplored region concerning the seismicity and ambient noise (see also Domingues et al, this conference), receiver function analysis, surface wave dispersion and SEM forward modelling. These preliminary results will pave the way for a tomographic model of the lithosphere, to be developed in the next stage of the project.

Tomotectonic constraints on deformation of Cordilleran North America since Late Jurassic

NASA Astrophysics Data System (ADS)

Mihalynuk, M. G.; Sigloch, K.

2017-12-01

Seismic tomography reveals detailed mantle structure beneath North America, largely thanks to USArray. TWO massive composite slabs are recognized down to 2000 km depth and their topologies are combined with quantitative plate reconstructions back to the breakup of Pangea using Atlantic and Pacific magnetic isochrons. This tomotectonic analysis reveals evolving arc/trench-plate geometries of a vast archipelago/microcontinent and ocean plateau that were overridden by North America, and an explanation for Cordilleran deformation episodes. As Pangea fragmented, subduction reconfigured from EAST-directed beneath the continent (during final growth of the Intermontane Superterrane, IMS, or "AltaBC"), to WEST-directed beneath an intraoceanic, massive arc chevron (MAC). MAC trenches were stationary within a mantle reference frame, as indicated by near-vertical slab walls 4-7x as thick as mature ocean lithosphere, and its trenches were >10,000 km long. East-pointing MAC apex was located 2000-4000 km off Pangea's west coast where MAC arc was built atop the Insular superterrane (INS, or "BajaBC"), a microcontinent extending >2600 km southwards from the apex. Ocean lithosphere between the MAC apex and west-drifting North America was consumed by 155 Ma. INS, comparable in length to the Indian subcontinent, initially collided with the leading edge of North America/IMS and generated "Nevadan" deformation. Diachronous Sevier deformation followed as MAC was driven farther into the continental margin and raked southward (sinistral offsets w.r.t. North America). By 130 Ma, with large segments accreted and MAC geometry breaking down, subduction was forced to jump outboard (westward) of MAC. The Franciscan accretionary complex marks a return to eastward/Andean-style subduction (of the Farallon plate). A remarkably complete analogue for collision at 130 Ma is found in modern Australia's override of arcs to its north. Rapid northward transport of BajaBC w.r.t. North America 90-50 Ma is attributed to arrival of the buoyant Shatsky conjugate plateau on the Farallon plate 90 Ma, which coupled with BajaBC lithosphere, as recorded by slab truncation, paleomagnetic measurements, an extinguished Sierra Nevada arc (80 Ma), subducted sediments underplated far inboard of the margin, and Laramide deformation.

Gravity Maps of Antarctic Lithospheric Structure from Remote-Sensing and Seismic Data

NASA Astrophysics Data System (ADS)

Tenzer, Robert; Chen, Wenjin; Baranov, Alexey; Bagherbandi, Mohammad

2018-02-01

Remote-sensing data from altimetry and gravity satellite missions combined with seismic information have been used to investigate the Earth's interior, particularly focusing on the lithospheric structure. In this study, we use the subglacial bedrock relief BEDMAP2, the global gravitational model GOCO05S, and the ETOPO1 topographic/bathymetric data, together with a newly developed (continental-scale) seismic crustal model for Antarctica to compile the free-air, Bouguer, and mantle gravity maps over this continent and surrounding oceanic areas. We then use these gravity maps to interpret the Antarctic crustal and uppermost mantle structure. We demonstrate that most of the gravity features seen in gravity maps could be explained by known lithospheric structures. The Bouguer gravity map reveals a contrast between the oceanic and continental crust which marks the extension of the Antarctic continental margins. The isostatic signature in this gravity map confirms deep and compact orogenic roots under the Gamburtsev Subglacial Mountains and more complex orogenic structures under Dronning Maud Land in East Antarctica. Whereas the Bouguer gravity map exhibits features which are closely spatially correlated with the crustal thickness, the mantle gravity map reveals mainly the gravitational signature of the uppermost mantle, which is superposed over a weaker (long-wavelength) signature of density heterogeneities distributed deeper in the mantle. In contrast to a relatively complex and segmented uppermost mantle structure of West Antarctica, the mantle gravity map confirmed a more uniform structure of the East Antarctic Craton. The most pronounced features in this gravity map are divergent tectonic margins along mid-oceanic ridges and continental rifts. Gravity lows at these locations indicate that a broad region of the West Antarctic Rift System continuously extends between the Atlantic-Indian and Pacific-Antarctic mid-oceanic ridges and it is possibly formed by two major fault segments. Gravity lows over the Transantarctic Mountains confirms their non-collisional origin. Additionally, more localized gravity lows closely coincide with known locations of hotspots and volcanic regions (Marie Byrd Land, Balleny Islands, Mt. Erebus). Gravity lows also suggest a possible hotspot under the South Orkney Islands. However, this finding has to be further verified.

Melt distribution along the axis of ultraslow spreading mid-ocean ridges

NASA Astrophysics Data System (ADS)

Schlindwein, V. S. N.; Schmid, F.; Meier, M.

2017-12-01

Ultraslow spreading mid-ocean ridges (<15 mm/y full spreading rate) differ from faster spreading ridges by their uneven melt distribution. Crustal thickness varies along axis from zero to more than 8 km at volcanic centers. These volcanic centers receive more melt than the regional average and may be sustained for millions of years. The segmentation pattern and active volcanism at ultraslow spreading ridges greatly differs from faster spreading ridges. Using networks of ocean bottom seismometers at three differing ridge segments, we could show that the maximum depth of brittle faulting, equivalent approximately to temperatures of 600-700°C, varies drastically along axis. Ridge sections that lack an igneous crust exhibit a thick lithosphere as evidenced by the deepest mid-ocean ridge earthquakes observed so far at more than 30 km depth. Beneath areas of basalt exposure, in particular beneath pronounced volcanic centers, the axial lithosphere may be more than 15 km thinner allowing for melt flow at the base of the lithosphere towards the volcanoes, a process that has been postulated to explain the uneven along-axis melt distribution. Spreading events at ultraslow spreading ridges are unusual as we found from two spreading episodes at 85°E Gakkel Ridge and Segment 8 volcano on the Southwest Indian Ridge. These eruptions were preceded or accompanied by large (M>5) and long-lasting earthquake swarms and active magmatism lasted over 3-16 years. A massive hydrothermal event plume and sounds from deep submarine explosive volcanism were observed at Gakkel Ridge. At the Segment 8 volcano, we imaged a melt reservoir extending to about 8 km depth below the volcano that potentially fed a sill intrusion recorded by an ocean bottom seismometers about 30 km away at a neighboring subordinate volcanic center. To better understand the segmentation and melt transport at ultraslow spreading rigdes, we recently conducted a segment-scale seismicity survey of Knipovich Ridge in the Norwegian-Greenland Sea. Here we deployed 28 ocean bottom seismometers along 160 km of ridge axis for one year, the currently largest mid-ocean ridge microseismicity experiment.

Lithosphere structure of the west Qinling orogenic belt revealed by deep seismic reflection profile

NASA Astrophysics Data System (ADS)

Wang, H.

2009-12-01

The west Qinling orogen located in the northeastern margin of the Qinghai-Tibet plateau, is transformation zone between the N-S-trending and E-W-trending tectonics in the Chinese continent. Further study of the fine crust structure of the west Qinling orogen and its relationships with surrounding basins have very important significance for understanding tectonic response of the northeastern margin of the plateau about collision convergence of the Indian block and Asian block and learning formation and evolution of the plateau. In 2009, we reprocessed the data of the Tangke-Hezuo deep seismic reflection profiles collected in 2004 across the west Qinling orogen and the northern Songpan block. The new results show the lithosphere fine structure of the west Qinling orogen. Reflection features indicate that an interface at 6.0-7.0s (TWT) divided the crust into the upper and lower crust, whose structural style and deformation are totally different. Integrating geological data, we deduce that the interface at 6.0-7.0s (depth with 18-21 km) was the basement detachment, which made deformation decoupled of the upper and lower crust. The multi-layered reflections in the upper crust reveal the sedimentary covers of the west Qinling orogen, disclose the thickness of the various structure layer and deformation degree, and provide a basis for the prospective evaluation of a multi-metallic mineral and energy exploration. The north dipping strong reflection characteristics of the lower crust in the west Qinling orogen constituted imbricate structure, such imbricate structural features provide seismology evidence for researching the west Qinling thrusting toward the northern Songpan block, and have great significance for studying formation and evolution of the Songpan-Garze structure. Moho reflections are observed around 17.0-17.2s, characterized by nearly horizontal reflections, which implies the west Qinling orogen underwent an intense extension post orogeny caused the lithosphere extensional thinning formed a nearly level Moho reflections. The study was financed by National Natural Science Foundation of china (No. 40830316 and 40604010),the Basic outlay of scientific research work from Ministry of Science and Technology of the People’s Republic of China and SINOPPROBE-02.

Numerical modeling of mountain formation on Io

NASA Astrophysics Data System (ADS)

Turtle, E. P.; Jaeger, W. L.; McEwen, A. S.; Keszthelyi, L.

2000-10-01

Io has ~ 100 mountains [1] that, although often associated with patera [2], do not appear to be volcanic structures. The mountains are up to 16 km high [3] and are generally isolated from each other. We have performed finite-element simulations of the formation of these mountains, investigating several mountain building scenarios: (1) a volcanic construct due to heterogeneous resurfacing on a coherent, homogeneous lithosphere; (2) a volcanic construct on a faulted, homogeneous lithosphere; (3) a volcanic construct on a faulted, homogeneous lithosphere under compression induced by subsidence due to Io's high resurfacing rate; (4) a faulted, homogeneous lithosphere under subsidence-induced compression; (5) a faulted, heterogeneous lithosphere under subsidence-induced compression; and (6) a mantle upwelling beneath a coherent, homogeneous lithosphere under subsidence-induced compression. The models of volcanic constructs do not produce mountains similar to those observed on Io. Neither do those of pervasively faulted lithospheres under compression; these predict a series of tilted lithospheric blocks or plateaus, as opposed to the isolated structures that are observed. Our models show that rising mantle material impinging on the base of the lithosphere can focus the compressional stresses to localize thrust faulting and mountain building. Such faults could also provide conduits along which magma could reach the surface as is observed near several mountains. [1] Carr et al., Icarus 135, pp. 146-165, 1998. [2] McEwen et al., Science 288, pp. 1193-1198, 2000. [3] Schenk and Bulmer, Science 279, pp. 1514-1517, 1998.

Lithospheric structure of the South China Sea and adjacent regions: Results from potential field modelling

NASA Astrophysics Data System (ADS)

Chen, Ming; Fang, Jian; Cui, Ronghua

2018-02-01

This work aims to investigate the crustal and lithospheric mantle thickness of the South China Sea (SCS) and adjacent regions. The crust-mantle interface, average crustal density, and lithospheric mantle base are calculated from free-air gravity anomaly and topographic data using an iterative inversion method. We construct a three-dimensional lithospheric model with different hierarchical layers. The satellite-derived gravity is used to invert the average crustal density and Moho (crust-mantle interface) undulations. The average crustal density and LAB (lithosphere-asthenosphere boundary) depths are further adjusted by topographic data under the assumption of local isostasy. The average difference in Moho depths between this study and the seismic measurement results is <1.5 km. The results show that in oceanic regions, the Moho depths are 7.5-30 km and the LAB depths are 65-120 km. The lithospheric thickness of the SCS basin and the adjacent regions increases from the sea basin to the continental margin with a large gradient in the ocean-continent transition zones. The Moho depths of conjugate plots during the opening of SCS, Zhongsha Islands and Reed Bank, reveal the asymmetric spreading pattern of SCS seafloor spreading. The lithospheric thinning pattern indicate two different spreading directions during seafloor spreading, which changed from N-S to NW-SE after the southward transition of the spreading axis. The lithosphere of the SCS basin and adjacent regions indicate that the SCS basin is a young basin with a stable interior lithosphere.

Rheology of the lithosphere and the folding caused by horizontal compression

NASA Astrophysics Data System (ADS)

Birger, B. I.

2015-05-01

The laboratory tests of rock specimens show that transient creep, at which deformations increase with time whereas strain rate decreases occurs when creep strains are sufficiently small. Since plate tectonics only permits small deformations in the lithospheric plates, the creep of the lithosphere is transient (non-steady-state). In this work, we study how the rheology of the lithosphere that possesses elasticity, brittleness (pseudo-plasticity), and creep affects the folding in the Earth's crust. Folding is caused by horizontal compression that results from the collision between the lithospheric plates. The effective viscosity characterizing the transient creep is lower than in the case of a steady-state creep and depends on the characteristic time of the considered process. The allowance for transient creep gives the distribution of the rheological properties of the horizontally compressed lithosphere in which the upper crust is brittle, whereas the lower crust and mantle lithosphere are dominated by transient creep. It is shown that the flows that arise in the lithosphere due to the instability under horizontal compression and cause folding are small-scale. These flows are concentrated in the upper brittle crust, they determine the short-wave Earth's surface topography, penetrate into the lower, creep-dominated crust to a shallow depth, and do not penetrate into the mantle. Therefore, these flows do not deform the Moho.

Evidence for frozen melts in the mid-lithosphere detected from active-source seismic data.

PubMed

Ohira, Akane; Kodaira, Shuichi; Nakamura, Yasuyuki; Fujie, Gou; Arai, Ryuta; Miura, Seiichi

2017-11-17

The interactions of the lithospheric plates that form the Earth's outer shell provide much of the evidentiary basis for modern plate tectonic theory. Seismic discontinuities in the lithosphere arising from mantle convection and plate motion provide constraints on the physical and chemical properties of the mantle that contribute to the processes of formation and evolution of tectonic plates. Seismological studies during the past two decades have detected seismic discontinuities within the oceanic lithosphere in addition to that at the lithosphere-asthenosphere boundary (LAB). However, the depth, distribution, and physical properties of these discontinuities are not well constrained, which makes it difficult to use seismological data to examine their origin. Here we present new active-source seismic data acquired along a 1,130 km profile across an old Pacific plate (148-128 Ma) that show oceanic mid-lithosphere discontinuities (oceanic MLDs) distributed 37-59 km below the seafloor. The presence of the oceanic MLDs suggests that frozen melts that accumulated at past LABs have been preserved as low-velocity layers within the current mature lithosphere. These observations show that long-offset, high-frequency, active-source seismic data can be used to image mid-lithospheric structure, which is fundamental to understanding the formation and evolution of tectonic plates.

River history and tectonics.

PubMed

Vita-Finzi, C

2012-05-13

The analysis of crustal deformation by tectonic processes has gained much from the clues offered by drainage geometry and river behaviour, while the interpretation of channel patterns and sequences benefits from information on Earth movements before or during their development. The interplay between the two strands operates at many scales: themes which have already benefited from it include the possible role of mantle plumes in the breakup of Gondwana, the Cenozoic development of drainage systems in Africa and Australia, Himalayan uplift in response to erosion, alternating episodes of uplift and subsidence in the Mississippi delta, buckling of the Indian lithospheric plate, and changes in stream pattern and sinuosity along individual alluvial channels subject to localized deformation. Developments in remote sensing, isotopic dating and numerical modelling are starting to yield quantitative analyses of such effects, to the benefit of geodymamics as well as fluvial hydrology. This journal is © 2012 The Royal Society

Limitations on Inferring 3D Architecture and Dynamics From Surface Velocities in the India-Eurasia Collision Zone

NASA Astrophysics Data System (ADS)

Flesch, L.; Bendick, R.; Bischoff, S.

2018-02-01

Surface velocities derived from Global Positioning System observations and Quaternary fault slip rates measured throughout an extended region of high topography in South Asia vary smoothly over thousands of kilometers and are broadly symmetrical, with components of both north-south shortening and east-west extension relative to stable Eurasia. The observed velocity field does not contain discontinuities or steep gradients attributable to along-strike differences in collision architecture, despite the well-documented presence of a lithospheric slab beneath the Pamir but not the Tibetan Plateau. We use a modified Akaike information criterion (AICc) to show that surface velocities do not efficiently constrain 3D rheology, geometry, or force balance. Therefore, although other geophysical and geological observations may indicate the presence of mechanical or dynamic heterogeneities within the Indian-Asian collision, the surface Global Positioning System velocities contain little or no usable information about them.

Mid-lithospheric discontinuity and its roles in the dynamic evolution of the craton-example from the North China Craton

NASA Astrophysics Data System (ADS)

Chen, Ling; Wei, Zigen; Jiang, Mingming; Ling, Yuan

2016-04-01

Mid-lithospheric discontinuity and its roles in the dynamic evolution of the craton - example from the North China Craton Ling Chen1,2, Zigen Wei3, Mingming Jiang1, Yuan Ling1 1. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China 2. CAS Center for Excellence in Tibetan Plateau Earth Sciences, Beijing100101, China 3. State Key Laboratory of Geodesy and Earth's Dynamics, Institute of Geodesy and Geophysics, Chinese Academy of Sciences, Wuhan 430077, China Detailed knowledge of lithospheric structure is essential for understanding the long-term evolution and dynamics of continents. We present an integrated lithospheric structural image along an E-W profile across the North China Craton (NCC) derived from the teleseismic data recorded at two dense seismic arrays in combination with other geophysical and geological observations. Our S- and P-receiver function images show substantial undulations of the lithosphere-asthenosphere boundary (LAB), from 60-100 km in the eastern NCC to ~160-200 km in the central-western NCC, and <150-km in the Qilian orogenic belt further to the west, accompanying marked lithospheric structural variations. This agrees with previous studies that suggest the occurrence of fundamental destruction in the eastern NCC but localized lithospheric thinning and modifications in the central-western NCC. A negative velocity discontinuity is identified at the depth of ~80-100 km within the thick lithosphere of the central-western NCC, spatially coincident with the top interface of a relatively low velocity layer in the overall high velocity mantle root imaged by surface wave tomography. Detailed data analyses show that this mid- or intra-lithospheric discontinuity has considerably larger S-to-P and P-to-S conversion amplitudes than the LAB below, which provides observational constraints to further decipher the origin of the discontinuity. Our imaging results corroborate recent seismic studies that reveal similar discontinuities at ~100 km depth under stable continental regions worldwide, suggesting the common presence of vertical heterogeneities and layering in the sub-continental lithospheric mantle (SCLM). The ~100-km depth discontinuity and the corresponding velocity decrease in the SCLM may indicate an ancient, mechanically weak layer within the overall strong cratonic lithosphere, which probably also existed beneath the eastern NCC before its Mesozoic destruction. The presence of such a weak layer could have facilitated simultaneous lithospheric modification at the base and in the middle of the lithosphere in the eastern NCC, especially under the strong influence of the Mesozoic Pacific subduction, eventually leading to the severe lithospheric thinning and destruction recorded in this part of the craton. The weak layer probably did not strongly affect the stability and evolution of the central and western NCC and other cratonic regions where effects from plate boundary processes were weak. Our seismic images, integrated with geological data, provide new insights into structural heterogeneities in the subcontinental lithospheric mantle and their roles in the dynamic evolution of continents.

Asthenospheric outflow from the shrinking Philippine Sea Plate: Evidence from Hf-Nd isotopes of southern Mariana lavas

NASA Astrophysics Data System (ADS)

Ribeiro, Julia M.; Stern, Robert J.; Martinez, Fernando; Woodhead, Jon; Chen, Min; Ohara, Yasuhiko

2017-11-01

At subduction zones, sinking of the downgoing lithosphere is thought to enable a return flow of asthenospheric mantle around the slab edges, so that the asthenosphere from underneath the slab invades the ambient mantle flowing underneath the volcanic arc and the backarc basin. For instance at the northern end of the Lau Basin, trench retreat and slab rollback enable toroidal return flow of Samoan mantle beneath a transform margin to provide a supply of fresh, undepleted Indian mantle that feeds the backarc spreading center. Questions, however, arise about the sense of mantle flow when plate kinematics predict that the trench is advancing, as seen in the Mariana convergent margin. Does the mantle flow in or does it escape outward through slab tears or gaps? Here, we address the origin and sense of asthenospheric mantle flow supplying the southern Mariana convergent margin, a region of strong extension occurring above the subducting Pacific plate. Does the asthenosphere flow northward, from underneath the Pacific plate and Caroline hotspot through a slab tear or gap, or does it flow outward from the Mariana Trough, which possesses a characteristic Indian Ocean isotopic signature? To address these questions, we integrate geodetic data along with new Hf-Nd isotopic data for fresh basaltic lavas from three tectonic provinces in the southernmost Marianas: the Fina Nagu volcanic complex, the Malaguana-Gadao backarc spreading ridge and the SE Mariana forearc rift. Our results indicate that Indian mantle flows outward and likely escapes through slab tears or gaps to accommodate shrinking of the Philippine Sea plate. We thus predict that asthenospheric flow around the Pacific slab at the southern Mariana Trench is opposite to that predicted by most subduction-driven mantle flow models.

Three-dimensional Gravity Modeling of Ocean Core Complexes at the Central Indian Ridge

NASA Astrophysics Data System (ADS)

Kim, S. S.; Chandler, M. T.; Pak, S. J.; Son, S. K.

2017-12-01

The spatial distribution of ocean core complexes (OCCs) on mid-ocean ridge flanks can indicate the variation of magmatism and tectonic extension at a given spreading center. A recent study revealed 11 prominent OCCs developed along the middle portion of the Central Indian Ridge (CIR) based on the high-resolution shipboard bathymetry. The CIR is located between the Carlsberg Ridge and the Indian Ocean triple junction. The detailed morphotectonic interpretations from the recent study suggested that the middle ridge segments of the CIR were mainly developed through tectonic extension with little magmatism. Furthermore, the OCCs exposed by detachment faults appear to the main host for active off-axis hydrothermal circulations. Here we form a three-dimensional gravity model to investigate the crustal structures of OCCs developed between 12oS and 14oS at the CIR. These OCCs exhibit domal topographic highs with corrugated surface. The rock samples from these areas include deep-seated rocks such as serpentinized harzburgite and gabbro. A typical gravity study on mid-ocean ridges assumes a constant density contrast along the water-crust interface and constant crustal thickness and removes its gravitational contributions and thermal effects of lithospheric cooling from the free-air gravity anomaly. This approach is effective to distinguish anomalous regions that deviate from the applied crustal and thermal models. The oceanic crust around the OCCs, however, tends to be thinned due to detachment faulting and tectonic extension. In this study, we include multi-layers with different density contrast and variable thickness to approximate gravity anomalies resulting from the OCCs. In addition, we aim to differentiate the geophysical characteristics of the OCCs from the nearby ridge segments and infer tectonic relationship between the OCCs and ridges.

Mantle heterogeneities beneath the Northeast Indian Ocean as sampled by intra-plate volcanism at Christmas Island

NASA Astrophysics Data System (ADS)

Taneja, Rajat; Rushmer, Tracy; Blichert-Toft, Janne; Turner, Simon; O'Neill, Craig

2016-10-01

The intra-plate region of the Northeast Indian Ocean, located between the Ninetyeast Ridge and the North West Shelf of Australia, contains numerous submerged seamounts and two sub-aerially exposed volcanic island groups. While the Cocos (Keeling) Archipelago is a coral atoll, Christmas Island is the only sub-aerially exposed volcanic island and contains Late Cretaceous, Eocene and Pliocene lavas. The lavas are predominantly basaltic in composition, except for one sampled flow that is trachytic. Although the evolution of the western margin of Australia, and the seismicity in the intra-plate region, has received considerable attention, the origin of the seamount province in the Northeast Indian Ocean is still a matter of debate. In order to constrain the origin of volcanism on Christmas Island and the associated Seamount Province we analysed 14 Christmas Island samples for major and trace element abundances and 12 of these for Nd, Hf and Pb isotope compositions. The trace element patterns of the lavas are similar to many ocean island basalts, while high 208Pb/204Pb and 207Pb/204Pb at a given 206Pb/204Pb suggest affiliation with the DUPAL anomaly. The reconstructed position of Christmas Island during the Eocene (44-37 Ma) places the island in close proximity to the (present-day) upper mantle low-seismic velocity anomalies. Moreover, an enriched mantle (EM-2) type component in addition to the DUPAL anomaly is observed in the Eocene volcanic phase. The younger Pliocene ( 4 Ma) sequences at Christmas Island are inferred to be the product of partial melting of existing material induced by lithospheric flexure.

Preferential rifting of continents - A source of displaced terranes

NASA Technical Reports Server (NTRS)

Vink, G. E.; Morgan, W. J.; Zhao, W.-L.

1984-01-01

Lithospheric rifting, while prevalent in the continents, rarely occurs in oceanic regions. To explain this preferential rifting of continents, the total strength of different lithospheres is compared by integrating the limits of lithospheric stress with depth. Comparisons of total strength indicate that continental lithosphere is weaker than oceanic lithosphere by about a factor of three. Also, a thickened crust can halve the total strength of normal continental lithosphere. Because the weakest area acts as a stress guide, any rifting close to an ocean-continent boundary would prefer a continental pathway. This results in the formation of small continental fragments or microplates that, once accreted back to a continent during subduction, are seen as displaced terranes. In addition, the large crustal thicknesses associated with suture zones would make such areas likely locations for future rifting episodes. This results in the tendency of new oceans to open along the suture where a former ocean had closed.

Reconciling Electromagnetic and Seismic Constraints on Lithospheric Thickness and Composition of the Kaapvaal Craton, South Africa

NASA Astrophysics Data System (ADS)

Muller, M. R.; Fullea, J.; Jones, A. G.

2010-12-01

Much of the long-running debate regarding the depth extent of the continental lithosphere beneath Archean shield areas has focussed on the Kaapvaal Craton of South Africa. Our recent magnetotelluric surveys across the Kaapvaal Craton, as part of the Southern African Magnetotelluric Experiment (SAMTEX), indicate a lithospheric thickness of the order of 220 km or greater for the central core of the craton. In contrast, a recently published S-wave receiver function study and several surface wave studies suggest that the Kaapvaal lithosphere is characterized by an approximately 160 km thick high-velocity “lid” underlain by a low-velocity layer that is between 65 - 150 km thick, with the base of the high-velocity lid inferred to represent the “lithosphere-asthenosphere boundary”. Other body-wave, surface wave and S-wave receiver function studies in the area suggest that the (high-velocity) lithosphere is substantially thicker, in excess of 250 km for the most part. Evidence from mantle xenolith pressure-temperature arrays derived from Mesozoic kimberlites found across the Kaapvaal Craton requires that the base of the lithosphere (i.e., the base of the thermal boundary layer above which a conductive geotherm is maintained) be at least 220 km deep, if observed mantle geotherms in the range 35 - 38 mWm-2 are to be accounted for. The presence of richly diamondiferous kimberlites across the Kaapvaal Craton is also impossible to reconcile with a 160 km lithospheric thickness: the top of the diamond (pressure-temperature) stability field is deeper than 160 km for the mantle geotherm associated with a 160 km lithospheric thickness. In the work presented here, we use the recently developed LitMOD software package to derive both seismic velocity and electrical resistivity models for the lithosphere that are fully chemically, petrologically and thermodynamically consistent, and assess whether these apparently disparate views of the Kaapvaal lithosphere - provided by seismic, magnetotelluric and xenolith studies - can be reconciled. We address directly several key issues: (i) whether a 160 km lithospheric thickness (and its associated temperature and pressure variation with depth) is “internally” consistent with the high (> 4.7 km/s) S-wave velocities predicted for the seismic high-velocity lid, given typical Kaapvaal geochemical compositions from xenolith analyses, (ii) whether a 160 km lithospheric thickness and its associated electrical resistivity variation with depth is consistent with observed magnetotelluric responses, and (iii) whether the observed (negative) mantle conversion event at 160 km depth in one S-wave receiver function study can be explained by compositional layering within the Kaapvaal Craton, given that the geochemistry of xenoliths from younger Group I kimberlites provides evidence for chemical refertilization of the lithosphere in the depth range 160 - 200 km.

Magma explains low estimates of lithospheric strength based on flexure of ocean island loads

NASA Astrophysics Data System (ADS)

Buck, W. Roger; Lavier, Luc L.; Choi, Eunseo

2015-04-01

One of the best ways to constrain the strength of the Earth's lithosphere is to measure the deformation caused by large, well-defined loads. The largest, simple vertical load is that of the Hawaiian volcanic island chain. An impressively detailed recent analysis of the 3D response to that load by Zhong and Watts (2013) considers the depth range of seismicity below Hawaii and the seismically determined geometry of lithospheric deflection. These authors find that the friction coefficient for the lithosphere must be in the normal range measured for rocks, but conclude that the ductile flow strength has to be far weaker than laboratory measurements suggest. Specifically, Zhong and Watts (2013) find that stress differences in the mantle lithosphere below the island chain are less than about 200 MPa. Standard rheologic models suggest that for the ~50 km thick lithosphere inferred to exist below Hawaii yielding will occur at stress differences of about 1 GPa. Here we suggest that magmatic accommodation of flexural extension may explain Hawaiian lithospheric deflection even with standard mantle flow laws. Flexural stresses are extensional in the deeper part of the lithosphere below a linear island load (i.e. horizontal stresses orthogonal to the line load are lower than vertical stresses). Magma can accommodate lithospheric extension at smaller stress differences than brittle and ductile rock yielding. Dikes opening parallel to an island chain would allow easier downflexing than a continuous plate, but wound not produce a freely broken plate. The extensional stress needed to open dikes at depth depends on the density contrast between magma and lithosphere, assuming magma has an open pathway to the surface. For a uniform lithospheric density ρL and magma density ρM the stress difference to allow dikes to accommodate extension is: Δσxx (z) = g z (ρM - gρL), where g is the acceleration of gravity and z is depth below the surface. For reasonable density values (i.e. ρL = 3300 Kg/m3 and ρM = 2800 kg/m3) this 'magmatic yield stress' is 250 MPa at 50 km depth. Dikes accommodating flexing below Hawaii would be at most about 2 km wide. This amount of intrusion would significantly heat the lithosphere, leading to lower stress differences below the islands. Since Hawaii marks the highest magma flux on Earth today it seems that 'magma assisted flexure' offers a viable alternative to extremely weak lithospheric rheology as an explanation for low stresses below this load.

Metasomatism and the Weakening of Cratons: A Mechanism to Rift Cratons

NASA Astrophysics Data System (ADS)

Wenker, Stefanie; Beaumont, Christopher

2016-04-01

The preservation of cratons is a demonstration of their strength and resistance to deformation. However, several cratons are rifting now (e.g. Tanzania and North China Craton) or have rifted in the past (e.g. North Atlantic Craton). To explain this paradox, we suggest that widespread metasomatism of the originally cold depleted dehydrated craton mantle lithosphere root can act as a potential weakening mechanism. This process, particularly melt metasomatism, increases root density through a melt-peridotite reaction, and reduces root viscosity by increasing the temperature and rehydrating the cratonic mantle lithosphere. Using 2D numerical models, we model silicate-melt metasomatism and rehydration of cold cratonic mantle lithosphere that is positioned beside standard Phanerozoic lithosphere. The models are designed to investigate when a craton is sufficiently weakened to undergo rifting and is no longer protected by the initially weaker adjacent standard Phanerozoic lithosphere. Melt is added to specified layers in the cratonic mantle lithosphere at a uniform volumetric rate determined by the duration of metasomatism (3 Myr, 10 Myr or 30 Myr), until a total of ~30% by volume of melt has been added. During melt addition heat and mass are properly conserved and the density and volume increase by the respective amounts required by the reaction with the peridotite. No extensional boundary conditions are applied to the models during the metasomatism process. As expected, significant refertilization leads to removal and thinning of progressively more gravitationally unstable cratonic mantle lithosphere. We show that the duration of metasomatism dictates the final temperature in the cratonic upper mantle lithosphere. Consequently, when extensional boundary conditions are applied in our rifting tests in most cases the Phanerozoic lithosphere rifts. The craton rifts only in the models with the hottest cratonic upper mantle lithosphere. Our results indicate rifting of cratons depends on the timing of extension, with respect to metasomatism. The key effect is the associated increase in temperature which must have time to reach peak values in the initially cold and strongest, uppermost mantle lithosphere. However, it remains true that the model cratons mostly remain strong and only rift when subjected to intensive metasomatism. This may explain why so many cratons have survived and only a few have rifted. An additional effect is that the craton surface subsides isostatically to balance the increasing density of craton mantle lithosphere where it is moderately metasomatized. We suggest that this is the mechanism that forms intracratonic basins. If correct, subsidence and subsequent uplift of intracratonic basins, and cratonic rifting constitute evidence of progressive metasomatism of cratonic mantle lithosphere.

Lithosphere erosion and continental breakup: Interaction of extension, plume upwelling and melting

NASA Astrophysics Data System (ADS)

Lavecchia, Alessio; Thieulot, Cedric; Beekman, Fred; Cloetingh, Sierd; Clark, Stuart

2017-06-01

We present the results of thermo-mechanical modelling of extension and breakup of a heterogeneous continental lithosphere, subjected to plume impingement in presence of intraplate stress field. We incorporate partial melting of the extending lithosphere, underlying upper mantle and plume, caused by pressure-temperature variations during the thermo-mechanical evolution of the conjugate passive margin system. Effects of melting included in the model account for thermal effects, causing viscosity reduction due to host rock heating, and mechanical effects, due to cohesion loss. Our study provides better understanding on how presence of melts can influence the evolution of rifting. Here we focus particularly on the role of melting for the temporal and spatial evolution of passive margin geometry and rift migration. Depending on the lithospheric structure, melt presence may have a significant impact on the characteristics of areas affected by lithospheric extension. Pre-existing lithosphere heterogeneities determine the location of initial breakup, but in presence of plumes the subsequent evolution is more difficult to predict. For small distances between plume and area of initial rifting, the development of symmetric passive margins is favored, whereas increasing the distance promotes asymmetry. For a plume-rifting distance large enough to prevent interaction, the effect of plumes on the overlying lithosphere is negligible and the rift persists at the location of the initial lithospheric weakness. When the melt effect is included, the development of asymmetric passive continental margins is fostered. In this case, melt-induced lithospheric weakening may be strong enough to cause rift jumps toward the plume location.

Characterizing Lithospheric Thickness in Australia using Ps and Sp Scattered Waves

NASA Astrophysics Data System (ADS)

Ford, H. A.; Fischer, K. M.; Rychert, C. A.

2008-12-01

The purpose of this study is to constrain the morphology of the lithosphere-asthenosphere boundary throughout Australia using scattered waves. Prior surface wave studies have shown a correlation between lithospheric thickness and the three primary geologic provinces of Australia, with the shallowest lithosphere located beneath the Phanerozoic province to the east, and the thicker lithosphere located beneath the Proterozoic and Archean regions. To determine lithospheric thickness, waveform data from twenty permanent broadband stations spanning mainland Australia and the island of Tasmania were analyzed using Ps and Sp migration techniques. Waveform selection for each station was based on epicentral distance (35° to 80° for Ps and 55° to 80° for Sp), and event depth (no greater than 300 km for Sp). For both Ps and Sp a simultaneous deconvolution was performed on the data for each of the twenty stations, and the resulting receiver function for each station was migrated to depth. Data were binned with epicentral distance to differentiate direct discontinuity phases from crustal reverberations (for Ps) and other teleseismic arrivals (for Sp). Early results in both Ps and Sp show a clear Moho discontinuity at most stations in addition to sharp, strong crustal reverberations seen in many of the Ps images. In the eastern Phanerozoic province, a strong negative phase at 100-105 km is evident in Ps for stations CAN and EIDS. The negative phase lies within a depth range that corresponds to the negative velocity gradient between fast lithosphere and slow asthenosphere imaged by surface waves. We therefore think that it is the lithosphere- asthenosphere boundary. On the island of Tasmania, a negative phase at 70-75 km in Ps images at stations TAU and MOO also appears to be the lithosphere-asthenosphere boundary. In the Proterozoic and Archean regions of the Australian continent, initial results for both Ps and Sp migration indicate clear crustal phases, but significantly more complicated signals at mantle depths. However, at some stations along the southern edge of the thick sub-cratonic lithosphere (previously imaged by surface waves) phases exist which may represent a lithosphere-asthenosphere boundary at depths of 110-115 km. Constraining the relationship of lithospheric thickness to the age and tectonic history of the overlying crust in Australia is important for better understanding the long term evolution of the continent.

The growth of a mountain belt forced by base-level fall: Tectonics and surface processes during the evolution of the Alborz Mountains, N Iran

NASA Astrophysics Data System (ADS)

Ballato, Paolo; Landgraf, Angela; Schildgen, Taylor F.; Stockli, Daniel F.; Fox, Matthew; Ghassemi, Mohammad R.; Kirby, Eric; Strecker, Manfred R.

2015-09-01

The idea that climatically modulated erosion may impact orogenic processes has challenged geoscientists for decades. Although modeling studies and physical calculations have provided a solid theoretical basis supporting this interaction, to date, field-based work has produced inconclusive results. The central-western Alborz Mountains in the northern sectors of the Arabia-Eurasia collision zone constitute a promising area to explore these potential feedbacks. This region is characterized by asymmetric precipitation superimposed on an orogen with a history of spatiotemporal changes in exhumation rates, deformation patterns, and prolonged, km-scale base-level changes. Our analysis suggests that despite the existence of a strong climatic gradient at least since 17.5 Ma, the early orogenic evolution (from ∼36 to 9-6 Ma) was characterized by decoupled orographic precipitation and tectonics. In particular, faster exhumation and sedimentation along the more arid southern orogenic flank point to a north-directed accretionary flux and underthrusting of Central Iran. Conversely, from ∼6 to 3 Ma, erosion rates along the northern orogenic flank became higher than those in the south, where they dropped to minimum values. This change occurred during a ∼3-Myr-long, km-scale base-level lowering event in the Caspian Sea. We speculate that mass redistribution processes along the northern flank of the Alborz and presumably across all mountain belts adjacent to the South Caspian Basin and more stable areas of the Eurasian plate increased the sediment load in the basin and ultimately led to the underthrusting of the Caspian Basin beneath the Alborz Mountains. This underthrusting in turn triggered a new phase of northward orogenic expansion, transformed the wetter northern flank into a new pro-wedge, and led to the establishment of apparent steady-state conditions along the northern orogenic flank (i.e., rock uplift equal to erosion rates). Conversely, the southern mountain front became the retro-wedge and experienced limited tectonic activity. These observations overall raise the possibility that mass-distribution processes during a pronounced erosion phase driven by base-level changes may have contributed to the inferred regional plate-tectonic reorganization of the northern Arabia-Eurasia collision during the last ∼5 Ma.

Lithospheric evolution of the Northern Arabian Shield: Chemical and isotopic evidence from basalts, xenoliths and granites

NASA Technical Reports Server (NTRS)

Stein, M.

1988-01-01

The evolution of the upper-mantle and the lower-crust (the conteinental lithosphere), is the area of Israel and Sinai was studied, using the chemical composition and the Nd-Sr isotopic systematics from mantle and crustal nodules, their host basalts, and granites. The magmatism and the metasomatism making the lithosphere are related to uprise of mantle diapirs in the uppermost mantle of the area. These diapirs heated the base of the lithosphere, eroded, and replaced it with new hot material. It caused a domal uplift of the lithosphere (and the crust). The doming resulted in tensional stresses that in turn might develop transport channels for the basalt.

Lithospheric structure beneath Eastern Africa from joint inversion of receiver functions and Rayleigh wave velocities

NASA Astrophysics Data System (ADS)

Dugda, Mulugeta Tuji

Crust and upper mantle structure beneath eastern Africa has been investigated using receiver functions and surface wave dispersion measurements to understand the impact of the hotspot tectonism found there on the lithospheric structure of the region. In the first part of this thesis, I applied H-kappa stacking of receiver functions, and a joint inversion of receiver functions and Rayleigh wave group velocities to determine the crustal parameters under Djibouti. The two methods give consistent results. The crust beneath the GEOSCOPE station ATD has a thickness of 23+/-1.5 km and a Poisson's ratio of 0.31+/-0.02. Previous studies give crustal thickness beneath Djibouti to be between 8 and 10 km. I found it necessary to reinterprete refraction profiles for Djibouti from a previous study. The crustal structure obtained for ATD is similar to adjacent crustal structure in many other parts of central and eastern Afar. The high Poisson's ratio and Vp throughout most of the crust indicate a mafic composition, suggesting that the crust in Afar consists predominantly of new igneous rock emplaced during the late synrift stage where extension is accommodated within magmatic segments by diking. In the second part of this thesis, the seismic velocity structure of the crust and upper mantle beneath Ethiopia and Djibouti has been investigated by jointly inverting receiver functions and Rayleigh wave group velocities to obtain new constraints on the thermal structure of the lithosphere. Crustal structure from the joint inversion for Ethiopia and Djibouti is similar to previously published models. Beneath the Main Ethiopian Rift (MER) and Afar, the lithospheric mantle has a maximum shear wave velocity of 4.1-4.2 km/s and extends to a depth of at most 50 km. In comparison to the lithosphere away from the East African Rift System in Tanzania, where the lid extends to depths of ˜100-125 km and has a maximum shear velocity of 4.6 km/s, the mantle lithosphere under the Ethiopian Plateau appears to have been thinned by ˜30-50 km and the maximum shear wave velocity reduced by ˜0.3 km/s. Results from a 1D conductive thermal model suggest that the shear velocity structure of the lithosphere beneath the Ethiopian Plateau can be explained by a plume model, if a plume rapidly thinned the lithosphere by ˜30--50 km at the time of the flood basalt volcanism (c. 30 Ma), and if warm plume material has remained beneath the lithosphere since then. About 45-65% of the 1-1.5 km of plateau uplift in Ethiopia can be attributed to the thermally perturbed lithospheric structure. In the final part of this thesis, the shear-wave velocity structure of the crust and upper mantle beneath Kenya has been obtained from a joint inversion of receiver functions, and Rayleigh wave group and phase velocities. The crustal structure from the joint inversion is consistent with crustal structure published previously by different authors. The lithospheric mantle beneath the East African Plateau in Kenya is similar to the lithosphere under the East African Plateau in Tanzania. Beneath the Kenya Rift, the lithosphere extends to a depth of at most ˜75 km. The lithosphere under the Kenya Plateau is not perturbed when compared to the highly perturbed lithosphere beneath the Ethiopian Plateau. On the other hand, the lithosphere under the Kenya Rift is perturbed as compared to the Kenya Plateau or the rest of the East African Plateau, but is not as perturbed as the lithosphere beneath the Main Ethiopian Rift or the Afar. Although Kenya and Ethiopia have similar uplift and rifting histories, they have different volcanic histories. Much of Ethiopia has been affected by the Afar Flood Basalt volcanism, which may be the cause of this difference in lithospheric structure between these two regions.

What major faults look like, and why this matters for lithospheric dynamics

NASA Astrophysics Data System (ADS)

Fagereng, Ake

2016-04-01

Earthquakes involve seconds to minutes of frictional sliding on a discontinuity, likely of sub-cm thickness, within a damage zone. Earthquakes are separated by an interseismic period of hundreds to thousands of years, during which a number of healing and weakening processes occur within the fault zone. The next earthquake occurs as shear stress exceeds frictional resistance, on the same or a different discontinuity as the previous event, embedded within the fault damage zone. After incremental damage and healing in multiple earthquake cycles, the fault zone rock assemblage evolves to a structure and composition distinctly different from the host rock(s). This presentation presents field geology evidence from a range of settings, to discuss the interplay between the earthquake cycle, long-term deformation, and lithospheric rheology. Classic fault zone models are based on continental transforms, which generally form discrete faults in the upper crust, and wide, anastomosing shear zones in the lower crust. In oceanic crust, transforms are considered frictionally weak, and appear to exploit dyke margins and joint surfaces, but also locally cross-cut these structures in anastomosing networks. In the oceanic lower crust and upper mantle, serpentinisation significantly alters fault structure. In old continental crust, previous deformation events leave a heterogeneous geology affecting active faulting. For example, the amagmatic, southern East African Rift has long been thought to exploit weak Proterozoic 'mobile belts'. However, detailed look at the Bilila-Mtakataka border fault in Malawi indicates that this fault locally exploits weak foliation in existing deformed zones, but also locally forms a new set of anastomosing fault surfaces cross-cutting existing weak foliation. In exhumed lower crust, the Antarctic Maud Belt provides an example of multiple phases of plastic deformation, where the second event is only visible in localised shear zones, likely inherited from the first event. The subduction thrust interface provides an example of fault evolution in underthrust sediments as they deform and dewater. At shallow levels, distributed shear leads to development of scaly cleavage, which in places provides weak, clay surfaces on which earthquakes can propagate to the sea floor. With further deformation, a melange is progressively developed, with increasingly dismembered, sheared lenses of higher viscosity sedimentary rock and slivers of oceanic crust, in a low viscosity, cleaved matrix. The range of examples presented here illustrate how long-term deformation results in weak structures that likely control future deformation. Yet, the rheology of these structures is modulated by strength fluctuations during the earthquake cycle, illustrated by common evidence of episodic fault healing. The take home message from these field studies of fault zones is therefore the heterogeneity of the Earth's crust, the importance of long-term weak zones as a first order control on crustal deformation, and short-term strength fluctuations within these zones as a consequence of, and reason for, the earthquake cycle.

Pn tomography with Moho depth correction from eastern Europe to western China

NASA Astrophysics Data System (ADS)

Lü, Yan; Ni, Sidao; Chen, Ling; Chen, Qi-Fu

2017-02-01

We proposed a modified Pn velocity and anisotropy tomography method by considering the Moho depth variations using the Crust 1.0 model and obtained high-resolution images of the uppermost mantle Pn velocity and anisotropy structure from eastern Europe to western China. The tomography results indicate that the average Pn velocities are approximately 8.0 and 8.1 km/s under the western and eastern parts of the study area, respectively, with maximum velocity perturbations of 3%-4%. We observed high Pn velocities under the Adriatic Sea, Black Sea, Caspian Sea, Arabian Plate, Indian Plate, and in the Tarim and Sichuan Basins but low Pn velocities under the Apennine Peninsula, Dead Sea fault zone, Anatolia, Caucasus, Iranian Plateau, Hindu Kush, and in the Yunnan and Myanmar regions. Generally, regions with stable structures and low lithospheric temperatures exhibit high Pn velocities. Low Pn velocities provide evidence for the upwelling of hot material, which is associated with plate subduction and continental collision processes. Our Pn velocity and anisotropy imaging results indicate that the Adriatic microplate dives to the east and west, the hot material upwelling caused by subduction beneath the Tibetan Plateau is not as significant as that in the Caucasus and Myanmar regions, the lithosphere exhibits coupled rotational movement around the Eastern Himalayan syntaxes, and the areas to the north and south of 26°N in the Yunnan region are affected by different geodynamic processes. Our newly captured images of the uppermost mantle velocity and anisotropy structure provide further information about continental collision processes and associated dynamic mechanisms.

Convective thinning of the lithosphere: A mechanism for rifting and mid-plate volcanism on Earth, Venus, and Mars

NASA Technical Reports Server (NTRS)

Spohn, T.; Schubert, G.

1982-01-01

Thinning of the Earth's lithosphere by heat advected to its base is a possible mechanism for continental rifting and continental and oceanic mid-plate volcanism. It might also account for continental rifting-like processes and volcanism on Venus and Mars. Earth's continental lithosphere can be thinned to the crust in a few tens of million years by heat advected at a rate of 5 to 10 times the normal basal heat flux. This much heat is easily carried to the lithosphere by mantle plumes. The continent is not required to rest over the mantle hot spot but may move at tens of millimeters per year. Because of the constant level of crustal radioactive heat production, the ratio of the final to the initial surface heat flow increases much less than the ratio of the final to initial basal heat flow. For large increases in asthenospheric heat flow, the lithosphere is almost thinned to the crust before any significant change in surface heat flow occurs. Uplift due to thermal expansion upon thinning is a few kilometers. The oceanic lithosphere can be thinned to the crust in less than 10 million years if the heat advection is at a rate around 5 or more times the basal heat flow into 100 Ma old lithosphere. Uplift upon thinning can compensate the subsidence of spreading and cooling lithosphere.

Craton Heterogeneity in the South American Lithosphere

NASA Astrophysics Data System (ADS)

Lloyd, S.; Van der Lee, S.; Assumpcao, M.; Feng, M.; Franca, G. S.

2012-04-01

We investigate structure of the lithosphere beneath South America using receiver functions, surface wave dispersion analysis, and seismic tomography. The data used include recordings from 20 temporary broadband seismic stations deployed across eastern Brazil (BLSP02) and from the Chile Ridge Subduction Project seismic array in southern Chile (CRSP). By jointly inverting Moho point constraints, Rayleigh wave group velocities, and regional S and Rayleigh wave forms we obtain a continuous map of Moho depth. The new tomographic Moho map suggests that Moho depth and Moho relief vary slightly with age within the Precambrian crust. Whether or not a correlation between crustal thickness and geologic age can be derived from the pre-interpolation point constraints depends strongly on the selected subset of receiver functions. This implies that using only pre-interpolation point constraints (receiver functions) inadequately samples the spatial variation in geologic age. We also invert for S velocity structure and estimate the depth of the lithosphere-asthenosphere boundary (LAB) in Precambrian South America. The new model reveals a relatively thin lithosphere throughout most of Precambrian South America (< 140 km). Comparing LAB depth with lithospheric age shows they are overall positively correlated, whereby the thickest lithosphere occurs in the relatively small Saõ Francisco craton (200 km). However, within the larger Amazonian craton the younger lithosphere is thicker, indicating that locally even larger cratons are not protected from erosion or reworking of the lithosphere.

Rifting the continental lithosphere: case studies of the lithosphere-asthenosphere system in rifted settings across the western U.S. and in the southern East African Rift

NASA Astrophysics Data System (ADS)

Hopper, E.; Gaherty, J. B.; Shillington, D. J.

2016-12-01

Continental extension comes in many guises, often described in terms of two endmembers. Narrow rifting is typified by a rift valley narrower than lithospheric thickness (50-100 km), presumed to result in steep lateral changes in crustal and lithospheric topography; wide rifting by a broad zone (<1000 km) of normal faulting associated with much smaller topographic gradients. A type example for the former is the East African Rift Valley; for the latter, the Basin and Range in the western U.S.A. An important control on rift development is the state of the lithosphere: for example, its strength and thickness. We analyse common conversion point stacked Sp converted wave images of the lithosphere beneath rift systems in the contiguous U.S., both the wide Basin and Range, and narrow rift systems such as the Rio Grande Rift and Salton Trough. We use Sp waves recorded by EarthScope's Transportable Array and other available permanent and temporary broadband stations. Beneath the Basin and Range, we observe a very strong, shallow velocity decrease (the lithosphere-asthenosphere boundary, or LAB) that is relatively uniform over 100s of km. The strength of this feature indicates melt has ponded at this transition. We have not observed a clear relationship between lithospheric thickness beneath the Basin and Range, and total degree of extension, current extension rate, or age since surface volcanism. Beneath narrow rifts in the western U.S., however, more localised thinning of the lithosphere has been observed. We also compare these observations with seismic images of the Malawi Rift, at the southern end of the Western Branch of the East African Rift System, using broadband data acquired as part of the Study of Extension and MaGmatism in Malawi aNd Tanzania (SEGMeNT) experiment. The Malawi Rift is extending slowly in a magma-poor region of relatively strong lithosphere. We constrain the pattern of plate-scale extension by observations of crustal thinning, and image complex variations in deeper lithospheric structure.

Rifting the continental lithosphere: case studies of the lithosphere-asthenosphere system in rifted settings across the western U.S. and in the southern East African Rift

NASA Astrophysics Data System (ADS)

Hopper, E.; Gaherty, J. B.; Shillington, D. J.

2017-12-01

Continental extension comes in many guises, often described in terms of two endmembers. Narrow rifting is typified by a rift valley narrower than lithospheric thickness (50-100 km), presumed to result in steep lateral changes in crustal and lithospheric topography; wide rifting by a broad zone (<1000 km) of normal faulting associated with much smaller topographic gradients. A type example for the former is the East African Rift Valley; for the latter, the Basin and Range in the western U.S.A. An important control on rift development is the state of the lithosphere: for example, its strength and thickness. We analyse common conversion point stacked Sp converted wave images of the lithosphere beneath rift systems in the contiguous U.S., both the wide Basin and Range, and narrow rift systems such as the Rio Grande Rift and Salton Trough. We use Sp waves recorded by EarthScope's Transportable Array and other available permanent and temporary broadband stations. Beneath the Basin and Range, we observe a very strong, shallow velocity decrease (the lithosphere-asthenosphere boundary, or LAB) that is relatively uniform over 100s of km. The strength of this feature indicates melt has ponded at this transition. We have not observed a clear relationship between lithospheric thickness beneath the Basin and Range, and total degree of extension, current extension rate, or age since surface volcanism. Beneath narrow rifts in the western U.S., however, more localised thinning of the lithosphere has been observed. We also compare these observations with seismic images of the Malawi Rift, at the southern end of the Western Branch of the East African Rift System, using broadband data acquired as part of the Study of Extension and MaGmatism in Malawi aNd Tanzania (SEGMeNT) experiment. The Malawi Rift is extending slowly in a magma-poor region of relatively strong lithosphere. We constrain the pattern of plate-scale extension by observations of crustal thinning, and image complex variations in deeper lithospheric structure.

Sumatra Megathrust Earthquakes Trigger Intraplate Seismicity in the Indo-Australian Oceanic Lithosphere

NASA Astrophysics Data System (ADS)

Delescluse, M.; Chamot-Rooke, N.; Cattin, R.

2009-05-01

The present-day intraplate deformation between India and Australia started 9 Myrs ago. In the Central Indian Basin (CIB), this deformation is recorded in the thick sediments of the Bengal fan. The equatorial, dense E-W thrust fault network in this region is the result of a massive reverse reactivation of normal faults at the onset of deformation. The Wharton Basin (WB), separated from the CIB by the NinetyEast Ridge (NyR), shows a contrasting style of deformation with mainly left-lateral strike-slip seismicity. The WB finite deformation and seismicity also involve pre-existing faults, in this case the N-S paleo-transforms of the fossile Wharton spreading-ridge system. The oceanic plate seismicity after the December 2004 Aceh subduction earthquake shows strike-slip events with a clear intraplate P-axis. No thrust faults are detected. This indicates short-term reactivation of the transform faults near the trench. Spatial and temporal distribution of intraplate erthquakes, as well as their anomalous moment release suggests triggering by the Aceh megathrust earthquake, which appears to have acted as an "accelerator" for the oceanic intraplate deformation. In this study, we use Coulomb stress static variations to confirm our seismicity observations. We first assume that the reactivated transform and the neoformed thrust fault plane families are present in the oceanic lithosphere. We then compute the coseismic stresses in the vicinity of the trench from the Aceh and Nias earthquakes slip distributions. Finally, we derive the normal and shear stresses on the fault planes. The results show that the strike-slip events are all favored by the subduction earthquakes coseismic stresses. They also show that the normal fault earthquakes at oceanic bulges are supported by the modeled coseismic stresses, except offshore Myanmar. The particularly interesting result is that all the possible neoformed thrust faults perpendicular to the intraplate P-axis are inhibited by the same coseismic stresses. This suggests that the style of intraplate deformation favored near the Sumatra Trench in the short-term by subduction earthquakes is the same than the long-term style. Under the effect of northward slab pull forces, Australia tries to detach from its Indian "brake" along the WB's N-S transform faults.

Plume-induced subduction initiation at the Cretaceous India-Arabia transform plate boundary: paleomagnetic constraints from the Semail ophiolite, Oman

NASA Astrophysics Data System (ADS)

Van Hinsbergen, D. J. J.; Maffione, M.; Koornneef, L.; Guilmette, C.

2016-12-01

The Neotethyan realm hosts a prominent belt of Cretaceous supra-subduction zone ophiolites from Turkey and Cyprus in the west, to Oman in the east. Associated crustal and metamorphic sole ages tightly cluster at 95-90 Ma, interpreted to shortly post-date subduction initiation in an intra-oceanic setting along transform faults or ridge segments (or ridge-parallel oceanic detachments). This subduction episode ended when the Arabian-African continental lithosphere arrived in the trench in the late Cretaceous and the leading edge of the overriding oceanic lithosphere obducted as ophiolites, including the famous Semail ophiolite of Oman. This catastrophic subduction initiation phase is assumed to be as response to some far-field trigger. Here, we analyzed whether the Semail ophiolite was generated at an E-W trending Neotethyan ridge or at a N-S trending transform. Therefore we paleomagnetically analyzed 10 localities in sheeted dyke sections of the Semail ophiolite that trend parallel to the obduction front of the ophiolite taken to reflect the paleo-trench. We demonstrate that the sheeted dyke sections, and thus also the trench, had an initial N-S strike, indicating that subduction below the Semail ophiolite probably initiated along a N-S striking transform plate boundary between the Indian and Arabian plate rather than at a Neotethyan mid-ocean ridge. Sometime before 83 Ma, India broke away from Madagascar, and underwent a counterclockwise rotation relative to Africa/Arabia around an Euler pole just north of Madagascar, likely triggered by the arrival of the Morondova mantle plume, the associated large igneous province formed since at least 91 Ma. Numerical models have shown that plume push was a likely driver for the inception of India-Madagascar spreading and associated Indian rotation. North of the associated Euler pole, E-W convergence India-Arabia must have occurred during India-Madagascar break-up. This has already been related to 96-90 Ma subduction initiation below the Waziristan ophiolite of Pakistan. Our new results suggest that subduction initiation below the Semail ophiolite is directly related to this plume-triggered break-up. We speculate that also the synchronous subduction initiation farther west in the Neotethys, towards Cyprus and Turkey, may have been triggered by this mechanism.

Alkalic Basalt in Ridge Axis of 53˚E Amagmatic Segment Center, Southwest Indian Ridge

NASA Astrophysics Data System (ADS)

Zhou, H.; Wang, J.; Liu, Y.; Ji, F.; Dick, H. J.

2014-12-01

Mid-ocean ridge basalt (MORB) is key tracer of composition and process in the mantle. It is interesting to notice that some alkalic basalts occur in amagmatic spreading center of ultraslow spreading ridges, for examples, 9-16˚E of the Southwest Indian ridge (Standish et al., 2008) and Lena Trough of Arctic Ocean (Snow et al., 2011). The latter is interpreted as the result of the pre-existence of continental transform fault or the especially cold thermal structure of ancient continental lithosphere. 53˚E segment, east of the Gallieni transform fault, was discovered as an amagmatic segment (Zhou and Dick, 2013). On both sides of the ridge axis, peridotites with a little gabbro are exposed in an area more than 3200 km2. Basalts exist in the southern portion of 53˚E segment, indicating the transformation from magmatic to amagmatic spreading about 9.4 million years ago. In April of 2014, Leg 4 of the RV Dayang Yihao cruise 30, basaltic glasses was dredged at one location (3500 m water depth) in the ridge axis of 53˚E segment center. It is shown by electric probe analysis that the samples have extremely high sodium content (4.0-4.49 wt% Na­2O ), relative higher potassium content (0.27-0.32 wt% K2O) and silica (50.67-51.87 wt% SiO2), and lower MgO content (5.9-6.4 wt% MgO). Mg-number is 0.55-0.59. It is distinctly different from the N-MORB (2.42-2.68 wt% Na2O, 0.03-0.06 wt% K2O, 48.6-49.6 wt% Si2O, 8.8-9.0 wt% MgO, Mg-numbers 0.63) distributed in the 560-km-long supersegment, west of the Gallieni transform fault, where the active Dragon Flag hydrothermal field was discovered at 49.6˚E in 2007. The reasons for the alkalic basalt in the ridge axis of 53˚E amagmatic segment center, either by low melting degree of garnet stability field, by melting from an ancient subcontinental lithospheric mantle, or by sodium-metasomatism or even other mantle processes or their combination in the deep mantle, are under further studies.

The evolution of passive rifting: contributions from field and laboratory studies to the interpretation of modelling results

NASA Astrophysics Data System (ADS)

Piccardo, Giovanni; Ranalli, Giorgio

2015-04-01

Direct field/laboratory, structural/petrologic investigations of mantle lithosphere from orogenic peridotites in Alpine-Apennine ophiolites provide significant constraints to the rift evolution of the Jurassic Ligurian Tethys ocean (Piccardo et al., 2014, and references therein). These studies have shown that continental extension and passive rifting were characterized by an important syn-rift "hidden" magmatic event, pre-dating continental break-up and sea-floor spreading. Occurrence of km-scale bodies of reactive spinel-harzburgites and impregnated plagioclase-peridotites, formed by melt/peridotite interaction, and the lack of any extrusive counterpart, show that the percolating magmas remained stored inside the mantle lithosphere. Petrologic-geochemical data/modelling and mineral Sm/Nd age constraints evidence that the syn-rift melt infiltration and reactive porous-flow percolation through the lithosphere were induced by MORB-type parental liquids formed by decompression melting of the passively upwelling asthenosphere. Melt thermal advection through, and melt stagnation within the lithosphere, heated the mantle column to temperatures close to the dry peridotite solidus ("asthenospherization" of mantle lithosphere). Experimental results of numerical/analogue modelling of the Ligurian rifting, based on field/laboratory constraints, show that: (1) porous flow percolation of asthenospheric melts resulted in considerable softening of the mantle lithosphere, decreasing total strength TLS from 10 to 1 TN m-1 as orders of magnitude (Ranalli et al. 2007), and (2) the formation of an axial lithospheric mantle column, with softened rheological characteristics (Weakened Lithospheric Mantle - WLM), induced necking instability in the extending lithosphere and subsequent active upwelling of the asthenosphere inside the WLM zone (Corti et al., 2007). Therefore, the syn-rift hidden magmatism (melt thermo-chemical-mechanical erosion, melt thermal advection and melt storage) caused important compositional and rheological modifications in the mantle lithosphere and played a fundamental role in the evolution of rifting, favouring, in particular, faster divergence of future continental margins and active upwelling of deeper/hotter asthenosphere. Active divergent forces probably changed the extension regime from passive to active rifting (as envisaged by Huismans et al., 2001). Accordingly, melt thermal advection and melt storage, and the rheological modifications induced in the mantle lithosphere, had a fundamental role in the evolution of the Ligurian rifting (Piccardo, 2014; Piccardo et al., 2014). Observations from the natural laboratory are pivotal when interpreting modelling results on the formation of rifted continental margins by extension of continental lithosphere leading to seafloor spreading. The rheological characteristics of the melt-modified mantle lithosphere can provide natural constraints for the interpretation of variously termed components ("oceanic lithosphere, Huismans & Beaumont, 2014; "oceanic and syn-rift lithospheric mantle", Whitmarsh & Manatschal, 2012), located in some models at non-oceanic, sub-continental settings, either below the extending continental crust or between the sub-continental lithosphere and the upwelling asthenosphere. Corti, G., Piccardo, G.B., Ranalli, G., et al., 2007. J. Geodynamics, 43, 465-483. Huismans, R.S., Beaumont, C., 2014. EPSL, 407, 148-162. Huismans, R.S., Podladchikov, Y.Y., Cloetingh, S., 2001, J. Geophys. Res. 106(11), 271-291. Piccardo, G.B., 2014. Geol. Soc. London, Spec. Publ., online 413, http://dx.doi.org/10.1144/SP413.7. Piccardo, G.B., et al., 2014. Earth-Science Reviews, http://dx.doi.org/10.1016/j.earscirev.2014.07.002. Ranalli, G., Piccardo, G.B., Corona-Chavez, P., 2007. J. Geodynamics, 43, 450-464. Whitmarsh, R.B., Manatschal, G., 2012. Roberts & Bally (eds), http://eprints.soton.ac.uk/id/eprint/358832.

Lithosphere destabilization by melt percolation during pre-oceanic rifting: Evidence from Alpine-Apennine ophiolitic peridotites

NASA Astrophysics Data System (ADS)

Piccardo, Giovanni; Ranalli, Giorgio

2017-04-01

Orogenic peridotites from Alpine-Apennine ophiolite Massifs (Lanzo, Voltri, External and Internal Ligurides, - NW Italy, and Mt. Maggiore - Corsica) derive from the mantle lithosphere of the Ligurian Tethys. Field/structural and petrologic/geochemical studies provide constraints on the evolution of the lithospheric mantle during pre-oceanic passive rifting of the late Jurassic Ligurian Tethys ocean. Continental rifting by far-field tectonic forces induced extension of the lithosphere by means of km-scale extensional shear zones that developed before infiltration of melts from the asthenosphere (Piccardo and Vissers, 2007). After significant thinning of the lithosphere, the passively upwelling asthenosphere underwent spinel-facies decompression melting along the axial zone of the extensional system. Silica-undersaturated melt fractions percolated through the lithospheric mantle via diffuse/focused porous flow and interacted with the host peridotite through pyroxenes-dissolving/olivine-precipitating melt/rock reactions. Pyroxene dissolution and olivine precipitation modified the composition of the primary silica-undersaturated melts into derivative silica-saturated melts, while the host lithospheric spinel lherzolites were transformed into pyroxene-depleted/olivine-enriched reactive spinel harzburgites and dunites. The derivative liquids interacted through olivine-dissolving/orthopyroxene+plagioclase-crystallizing reactions with the host peridotites that were impregnated and refertilized (Piccardo et al., 2015). The saturated melts stagnated and crystallized in the shallow mantle lithosphere (as testified by diffuse interstitial crystallization of euhedral orthopyroxene and anhedral plagioclase) and locally ponded, forming orthopyroxene-rich/olivine-free gabbro-norite pods (Piccardo and Guarnieri, 2011). Reactive and impregnated peridotites are characterized by high equilibration temperatures (up to 1250 °C) even at low pressure, plagioclase-peridotite facies conditions. This indicates that thermal advection by percolation of hot asthenospheric melts significantly heated the lithospheric mantle column above the melting asthenosphere. Numerical and analogue models show that infiltration of melts results in considerable softening of mantle rocks. Total ithospheric strength can be decreased from 10 to 1 TN m-1 as orders of magnitude and the sin-rift thermo-mechanical erosion of the lithospheric mantle induces significant rheological softening along the axial zone of extension (Corti et al., 2007; Ranalli et al., 2007). Softening of the lithospheric mantle may lead to whole lithospheric failure and consequently to transition from continental extension to oceanic spreading. Therefore, rheological softening caused destabilization of the lithospheric mantle between the future continental margins (Piccardo et al., 2014; Piccardo, 2016) of the Ligurian Tethys. The wedge of destabilized lithosphere favored faster divergence of the continental blocks and enhanced doming and thermal buoyancy of deeper/hotter asthenosphere that rose between the future continental margins and originated aggregated MORB melts (i.e., the oceanic magmatism that formed olivine-gabbro intrusions and pillowed basalt extrusions). Lithosphere destabilization by melt percolation can play a fundamental role in the geodynamic evolution of lithosphere extension causing transition from continental extension to continental break-up to oceanic spreading. Corti, G., Bonini, M., Innocenti, F., Manetti, P., Piccardo, G.B., Ranalli, G., 2007. Journal of Geodynamics, 43, 465-483. Piccardo, G.B., Padovano, M., Guarnieri, L. 2014. Earth-Science Reviews, 138, 409-434. Piccardo, G.B., 2016. Gondwana Research, 39, 230-249. Piccardo, G.B., Vissers, R.L.M., 2007. Journal of Geodynamics, 43, 417-449. Piccardo, G.B., Guarnieri, L., 2011. Lithos, 124, 210-214. Ranalli, G., Piccardo, G.B., Corona-Chavez, P., 2007. Journal of Geodynamics, 43, 450-464.

Geodynamic Constraints on the Sources of Seismic Anisotropy Beneath Madagascar

NASA Astrophysics Data System (ADS)

Rajaonarison, T. A.; Stamps, D. S.; Fishwick, S.

2017-12-01

The rheological structure of the lithosphere-asthenosphere system controls the degree in which the mantle drives surface motions. Seismic anisotropy is a proxy to infer information about previous tectonic events imprinted in lithospheric structures and/or asthenospheric flow pattern in regions absent of active volcanism, however, distinguishing between the shallow and deeper sources, respectively, remains ambiguous. Madagascar is an ideal natural laboratory to study the sources of anisotropy and the rheological implications for lithosphere-asthenosphere system because 1) active volcanism is minimal or absent, 2) there are well-exposed tectonic fabrics for comparison, and 3) numerous geological and geophysical observations provides evidence of present-day tectonic activities. Recent studies suggest new seismic anisotropy observations in southern Madagascar are sourced from both fossilized lithospheric structure and asthenospheric flow driven by rigid lithospheric plate motion. In this work we compare geodynamic simulations of the lithosphere-asthenosphere system with seismic anisotropy data set that includes all of Madagascar. We use the numerical code Advanced Solver for Problems in Earth's ConvecTion (ASPECT) to calculate instantaneous deformation in the lithosphere and edge-driven convective flow in the asthenosphere accounting for variations in buoyancy forces and temperature dependent viscosity. The initial temperature conditions are based on interpretations from high resolution regional surface wave tomography. We assume visco-plastic rheology for a uniform crust, dislocation creep for a laterally varying mantle lithospheric structure, and diffusion creep for the asthenosphere. To test for the source of anisotropy we compare our velocity solution azimuths with azimuths of anisotropy at 25 km depth intervals. Calculated asthenospheric flow aligns with measured seismic anisotropy with a 15° WRMS at 175 km depth and possibly down to 250 km suggesting the majority of the seismic anisotropy are due to sub-lithospheric asthenospheric flow beneath Madagascar. Our results suggest the dislocation creep regime extends beneath the lithosphere, which implies the rheology of the upper asthenosphere deforms by dislocation creep rather than diffusion creep.

Lithospheric strucutre and relationship to seismicity beneath the Southeastern US using reciever functions

NASA Astrophysics Data System (ADS)

Cunningham, E.; Lekic, V.

2017-12-01

Despite being on a passive margin for millions of years, the Southeastern United States (SEUS) contains numerous seismogenic zones with the ability to produce damaging earthquakes. However, mechanisms controlling these intraplate earthquakes are poorly understood. Recently, Biryol et al. 2016 use P-wave tomography suggest that upper mantle structures beneath the SEUS correlate with areas of seismicity and seismic quiescence. Specifically, thick and fast velocity lithosphere beneath North Carolina is stable and indicative of areas of low seismicity. In contrast, thin and slow velocity lithosphere is weak, and the transition between the strong and weak lithosphere may be correlated with seismogenic zones found in the SEUS. (eg. Eastern Tennessee seismic zone and the Central Virginia seismic zone) Therefore, I systematically map the heterogeneity of the mantle lithosphere using converted seismic waves and quantify the spatial correlation between seismicity and lithospheric structure. The extensive network of seismometers that makes up the Earthscope USArray combined with the numerous seismic deployments in the Southeastern United States allows for unprecedented opportunity to map changes in lithospheric structure across seismogenic zones and seismic quiescent regions. To do so, I will use both P-to-s and S-to-p receiver functions (RFS). Since RFs are sensitive to seismic wavespeeds and density discontinuities with depth, they particularly useful for studying lithospheric structure. Ps receiver functions contain high frequency information allowing for high resolution, but can become contaminated by large sediment signals; therefore, I removed sediment multiples and correct for time delays of later phases using the method of Yu et. al 2015 which will allow us to see later arriving phases associated with lithospheric discontinuities. S-to-p receiver functions are not contaminated by shallow layers, making them ideal to study deep lithospheric structures but they can suffer from low signal-to-noise levels. I compensate for this difficulty by using high quality deployments and stacking these data at common conversion points to increase lateral resolution.

Thin Lithosphere Beneath the Ethiopian Plateau Revealed by a Joint Inversion of Rayleigh Wave Group Velocities and Receiver Functions

NASA Astrophysics Data System (ADS)

Dugda, Mulugeta T.; Nyblade, Andrew A.; Julia, Jordi

2007-08-01

The seismic velocity structure of the crust and upper mantle beneath Ethiopia and Djibouti has been investigated by jointly inverting receiver functions and Rayleigh wave group velocities to obtain new constraints on the thermal structure of the lithosphere. Most of the data for this study come from the Ethiopia broadband seismic experiment, conducted between 2000 and 2002. Shear velocity models obtained from the joint inversion show crustal structure that is similar to previously published models, with crustal thicknesses of 35 to 44 km beneath the Ethiopian Plateau, and 25 to 35 km beneath the Main Ethiopian Rift (MER) and the Afar. The lithospheric mantle beneath the Ethiopian Plateau has a maximum shear wave velocity of about 4.3 km/s and extends to a depth of ˜70-80 km. Beneath the MER and Afar, the lithospheric mantle has a maximum shear wave velocity of 4.1-4.2 km/s and extends to a depth of at most 50 km. In comparison to the lithosphere away from the East African Rift System in Tanzania, where the lid extends to depths of ˜100-125 km and has a maximum shear velocity of 4.6 km/s, the mantle lithosphere under the Ethiopian Plateau appears to have been thinned by ˜30-50 km and the maximum shear wave velocity reduced by ˜0.3 km/s. Results from a 1D conductive thermal model suggest that the shear velocity structure of the Ethiopian Plateau lithosphere can be explained by a plume model, if a plume rapidly thinned the lithosphere by ˜30-50 km at the time of the flood basalt volcanism (c. 30 Ma), and if warm plume material has remained beneath the lithosphere since then. About 45-65% of the 1-1.5 km of plateau uplift in Ethiopia can be attributed to the thermally perturbed lithospheric structure.

Interaction Between Downwelling Flow and the Laterally-Varying Thickness of the North American Lithosphere Inferred from Seismic Anisotropy

NASA Astrophysics Data System (ADS)

Behn, M. D.; Conrad, C. P.; Silver, P. G.

2005-12-01

Shear flow in the asthenosphere tends to align olivine crystals in the direction of shear, producing a seismically anisotropic asthenosphere that can be detected using a number of seismic techniques (e.g., shear-wave splitting (SWS) and surface waves). In the ocean basins, where the asthenosphere has a relatively uniform thickness and lithospheric anisotropy appears to be small, observed azimuthal anisotropy is well fit by asthenospheric shear flow in global flow models driven by a combination of plate motions and mantle density heterogeneity. In contrast, beneath the continents both the lithospheric ceiling and asthenospheric thickness may vary considerably across cratonic regions and ocean-continent boundaries. To examine the influence of a continental lithosphere with variable thickness on predictions of continental seismic anisotropy, we impose lateral variations in lithospheric viscosity in global models of mantle flow driven by plate motions and mantle density heterogeneity. For the North American continent, the Farallon slab descends beneath a deep cratonic root, producing downwelling flow in the upper mantle and convergent flow beneath the cratonic lithosphere. We evaluate both the orientation of the predicted azimuthal anisotropy and the depth dependence of radial anisotropy for this downwelling flow and find that the inclusion of a strong continental root provides an improved fit to observed SWS observations beneath the North American craton. Thus, we hypothesize that at least some continental anisotropy is associated with sub-lithospheric viscous shear, although fossil anisotropy in the lithospheric layer may also contribute significantly. Although we do not observe significant variations in the direction of predicted anisotropy with depth, we do find that the inclusion of deep continental roots pushes the depth of the anisotropy layer deeper into the upper mantle. We test several different models of laterally-varying lithosphere and asthenosphere viscosity. These models can be used to separate the contributions of asthenospheric flow and lithospheric fossil fabric in observations of continental anisotropy.

Numerical simulations of the mantle lithosphere delamination

NASA Astrophysics Data System (ADS)

Morency, C.; Doin, M.-P.

2004-03-01

Sudden uplift, extension, and increased igneous activity are often explained by rapid mechanical thinning of the lithospheric mantle. Two main thinning mechanisms have been proposed, convective removal of a thickened lithospheric root and delamination of the mantle lithosphere along the Moho. In the latter case, the whole mantle lithosphere peels away from the crust by the propagation of a localized shear zone and sinks into the mantle. To study this mechanism, we perform two-dimensional (2-D) numerical simulations of convection using a viscoplastic rheology with an effective viscosity depending strongly on temperature, depth, composition (crust/mantle), and stress. The simulations develop in four steps. (1) We first obtain "classical" sublithospheric convection for a long time period (˜300 Myr), yielding a slightly heterogeneous lithospheric temperature structure. (2) At some time, in some simulations, a strong thinning of the mantle occurs progressively in a small area (˜100 km wide). This process puts the asthenosphere in direct contact with the lower crust. (3) Large pieces of mantle lithosphere then quickly sink into the mantle by the horizontal propagation of a detachment level away from the "asthenospheric conduit" or by progressive erosion on the flanks of the delaminated area. (4) Delamination pauses or stops when the lithospheric mantle part detaches or when small-scale convection on the flanks of the delaminated area is counterbalanced by heat diffusion. We determine the parameters (crustal thicknesses, activation energies, and friction coefficients) leading to delamination initiation (step 2). We find that delamination initiates where the Moho temperature is the highest, as soon as the crust and mantle viscosities are sufficiently low. Delamination should occur on Earth when the Moho temperature exceeds ˜800°C. This condition can be reached by thermal relaxation in a thickened crust in orogenic setting or by corner flow lithospheric erosion in the overriding lithosphere of subduction zones.

The rheological structure of the lithosphere in the Eastern Marmara region, Turkey

NASA Astrophysics Data System (ADS)

Oruç, Bülent; Sönmez, Tuba

2017-05-01

The aim of this work is to propose the geometries of the crustal-lithospheric mantle boundary (Moho) and lithosphere-asthenosphere boundary (LAB) and the 1D thermal structure of the lithosphere, in order to establish a rheological model of the Eastern Marmara region. The average depths of Moho and LAB are respectively 35 km and 51 km from radially averaged amplitude spectra of EGM08 Bouguer anomalies. The geometries of Moho and LAB interfaces are estimated from the Parker-Oldenburg gravity inversion algorithm. Our results show the Moho depth varies from 31 km at the northern part of North Anatolian Fault Zone (NAFZ) to 39 km below the mountain belt in the southern part of the NAFZ. The depth to the LAB beneath the same parts of the region ranges from 45 km to 55 km. Having lithospheric strength and thermal boundary layer structure, we analyzed the conditions of development of lithosphere thinning. A two-dimensional strength profile has been estimated for rheology model of the study area. Thus we suggest that the rheological structure consists of a strong upper crust, a weak lower crust, and a partly molten upper lithospheric mantle.

Pre-subduction metasomatic enrichment of the oceanic lithosphere induced by plate flexure

NASA Astrophysics Data System (ADS)

Pilet, S.; Abe, N.; Rochat, L.; Kaczmarek, M.-A.; Hirano, N.; Machida, S.; Buchs, D. M.; Baumgartner, P. O.; Müntener, O.

2016-12-01

Oceanic lithospheric mantle is generally interpreted as depleted mantle residue after mid-ocean ridge basalt extraction. Several models have suggested that metasomatic processes can refertilize portions of the lithospheric mantle before subduction. Here, we report mantle xenocrysts and xenoliths in petit-spot lavas that provide direct evidence that the lower oceanic lithosphere is affected by metasomatic processes. We find a chemical similarity between clinopyroxene observed in petit-spot mantle xenoliths and clinopyroxene from melt-metasomatized garnet or spinel peridotites, which are sampled by kimberlites and intracontinental basalts respectively. We suggest that extensional stresses in oceanic lithosphere, such as plate bending in front of subduction zones, allow low-degree melts from the seismic low-velocity zone to percolate, interact and weaken the oceanic lithospheric mantle. Thus, metasomatism is not limited to mantle upwelling zones such as mid-ocean ridges or mantle plumes, but could be initiated by tectonic processes. Since plate flexure is a global mechanism in subduction zones, a significant portion of oceanic lithospheric mantle is likely to be metasomatized. Recycling of metasomatic domains into the convecting mantle is fundamental to understanding the generation of small-scale mantle isotopic and volatile heterogeneities sampled by oceanic island and mid-ocean ridge basalts.

Magmatic controls on axial relief and faulting at mid-ocean ridges

NASA Astrophysics Data System (ADS)

Liu, Zhonglan; Buck, W. Roger

2018-06-01

Previous models do not simultaneously reproduce the observed range of axial relief and fault patterns at plate spreading centers. We suggest that this failure is due to the approximation that magmatic dikes open continuously rather than in discrete events. During short - lived events, dikes open not only in the strong axial lithosphere but also some distance into the underlying weaker asthenosphere. Axial valley relief affects the partitioning of magma between the lithosphere and asthenosphere during diking events. The deeper the valley, the more magma goes into lithospheric dikes in each event and so the greater the average opening rate of those dikes. The long-term rate of lithospheric dike opening controls faulting rate and axial depth. The feedback between axial valley depth D and lithospheric dike opening rate allows us to analytically relate steady-state values of D to lithospheric thickness HL and crustal thickness HC. A two-dimensional model numerical model with a fixed axial lithospheric structure illustrates the analytic model implications for axial faulting. The predictions of this new model are broadly consistent with global and segment-scale trends of axial depth and fault patterns with HL and HC.

A Precambrian microcontinent in the Indian Ocean

NASA Astrophysics Data System (ADS)

Torsvik, Trond H.; Amundsen, Hans; Hartz, Ebbe H.; Corfu, Fernando; Kusznir, Nick; Gaina, Carmen; Doubrovine, Pavel V.; Steinberger, Bernhard; Ashwal, Lewis D.; Jamtveit, Bjørn

2013-03-01

The Laccadive-Chagos Ridge and Southern Mascarene Plateau in the north-central and western Indian Ocean, respectively, are thought to be volcanic chains formed above the Réunion mantle plume over the past 65.5 million years. Here we use U-Pb dating to analyse the ages of zircon xenocrysts found within young lavas on the island of Mauritius, part of the Southern Mascarene Plateau. We find that the zircons are either Palaeoproterozoic (more than 1,971 million years old) or Neoproterozoic (between 660 and 840 million years old). We propose that the zircons were assimilated from ancient fragments of continental lithosphere beneath Mauritius, and were brought to the surface by plume-related lavas. We use gravity data inversion to map crustal thickness and find that Mauritius forms part of a contiguous block of anomalously thick crust that extends in an arc northwards to the Seychelles. Using plate tectonic reconstructions, we show that Mauritius and the adjacent Mascarene Plateau may overlie a Precambrian microcontinent that we call Mauritia. On the basis of reinterpretation of marine geophysical data, we propose that Mauritia was separated from Madagascar and fragmented into a ribbon-like configuration by a series of mid-ocean ridge jumps during the opening of the Mascarene ocean basin between 83.5 and 61 million years ago. We suggest that the plume-related magmatic deposits have since covered Mauritia and potentially other continental fragments.

Lithospheric strength of Ganymede: Clues to early thermal profiles from extensional tectonic features

NASA Technical Reports Server (NTRS)

Golombek, M. P.; Banerdt, W. B.

1985-01-01

While it is generally agreed that the strength of a planet's lithosphere is controlled by a combination of brittle sliding and ductile flow laws, predicting the geometry and initial characteristics of faults due to failure from stresses imposed on the lithospheric strength envelope has not been thoroughly explored. Researchers used lithospheric strength envelopes to analyze the extensional features found on Ganymede. This application provides a quantitative means of estimating early thermal profiles on Ganymede, thereby constraining its early thermal evolution.

Rheology of the lithosphere: selected topics.

USGS Publications Warehouse

Kirby, S.H.; Kronenberg, A.K.

1987-01-01

Reviews recent results concerning the rheology of the lithosphere with special attention to the following topics: 1) the flexure of the oceanic lithosphere, 2) deformation of the continental lithosphere resulting from vertical surface loads and forces applied at plate margins, 3) the rheological stratification of the continents, 4) strain localization and shear zone development, and 5) strain-induced crystallographic preferred orientations and anisotropies in body-wave velocities. We conclude with a section citing the 1983-1986 rock mechanics literature by category.-Authors

Analysis of Lithospheric Stresses Using Satellite Gravimetry: Hypotheses and Applications to North Atlantic

NASA Astrophysics Data System (ADS)

Minakov, A.; Medvedev, S.

2017-12-01

Analysis of lithospheric stresses is necessary to gain understanding of the forces that drive plate tectonics and intraplate deformations and the structure and strength of the lithosphere. A major source of lithospheric stresses is believed to be in variations of surface topography and lithospheric density. The traditional approach to stress estimation is based on direct calculations of the Gravitational Potential Energy (GPE), the depth integrated density moment of the lithosphere column. GPE is highly sensitive to density structure which, however, is often poorly constrained. Density structure of the lithosphere may be refined using methods of gravity modeling. However, the resulted density models suffer from non-uniqueness of the inverse problem. An alternative approach is to directly estimate lithospheric stresses (depth integrated) from satellite gravimetry data. Satellite gravity gradient measurements by the ESA GOCE mission ensures a wealth of data for mapping lithospheric stresses if a link between data and stresses or GPE can be established theoretically. The non-uniqueness of interpretation of sources of the gravity signal holds in this case as well. Therefore, the data analysis was tested for the North Atlantic region where reliable additional constraints are supplied by both controlled-source and earthquake seismology. The study involves comparison of three methods of stress modeling: (1) the traditional modeling approach using a thin sheet approximation; (2) the filtered geoid approach; and (3) the direct utilization of the gravity gradient tensor. Whereas the first two approaches (1)-(2) calculate GPE and utilize a computationally expensive finite element mechanical modeling to calculate stresses, the approach (3) uses a much simpler numerical treatment but requires simplifying assumptions that yet to be tested. The modeled orientation of principal stresses and stress magnitudes by each of the three methods are compared with the World Stress Map.

A rapid method to map the crustal and lithospheric thickness using elevation, geoid anomaly and thermal analysis. Application to the Gibraltar Arc System, Atlas Mountains and adjacent zones

NASA Astrophysics Data System (ADS)

Fullea, J.; Fernàndez, M.; Zeyen, H.; Vergés, J.

2007-02-01

We present a method based on the combination of elevation and geoid anomaly data together with thermal field to map crustal and lithospheric thickness. The main assumptions are local isostasy and a four-layered model composed of crust, lithospheric mantle, sea water and the asthenosphere. We consider a linear density gradient for the crust and a temperature dependent density for the lithospheric mantle. We perform sensitivity tests to evaluate the effect of the variation of the model parameters and the influence of RMS error of elevation and geoid anomaly databases. The application of this method to the Gibraltar Arc System, Atlas Mountains and adjacent zones reveals the presence of a lithospheric thinning zone, SW-NE oriented. This zone affects the High and Middle Atlas and extends from the Canary Islands to the eastern Alboran Basin and is probably linked with a similarly trending zone of thick lithosphere constituting the western Betics, eastern Rif, Rharb Basin, and Gulf of Cadiz. A number of different, even mutually opposite, geodynamic models have been proposed to explain the origin and evolution of the study area. Our results suggest that a plausible slab-retreating model should incorporate tear and asymmetric roll-back of the subducting slab to fit the present-day observed lithosphere geometry. In this context, the lithospheric thinning would be caused by lateral asthenospheric flow. An alternative mechanism responsible for lithospheric thinning is the presence of a hot magmatic reservoir derived from a deep ancient plume centred in the Canary Island, and extending as far as Central Europe.

The plume head-continental lithosphere interaction using a tectonically realistic formulation for the lithosphere

NASA Astrophysics Data System (ADS)

Burov, E.; Guillou-Frottier, L.

2005-05-01

Current debates on the existence of mantle plumes largely originate from interpretations of supposed signatures of plume-induced surface topography that are compared with predictions of geodynamic models of plume-lithosphere interactions. These models often inaccurately predict surface evolution: in general, they assume a fixed upper surface and consider the lithosphere as a single viscous layer. In nature, the surface evolution is affected by the elastic-brittle-ductile deformation, by a free upper surface and by the layered structure of the lithosphere. We make a step towards reconciling mantle- and tectonic-scale studies by introducing a tectonically realistic continental plate model in large-scale plume-lithosphere interaction. This model includes (i) a natural free surface boundary condition, (ii) an explicit elastic-viscous(ductile)-plastic(brittle) rheology and (iii) a stratified structure of continental lithosphere. The numerical experiments demonstrate a number of important differences from predictions of conventional models. In particular, this relates to plate bending, mechanical decoupling of crustal and mantle layers and tension-compression instabilities, which produce transient topographic signatures such as uplift and subsidence at large (>500 km) and small scale (300-400, 200-300 and 50-100 km). The mantle plumes do not necessarily produce detectable large-scale topographic highs but often generate only alternating small-scale surface features that could otherwise be attributed to regional tectonics. A single large-wavelength deformation, predicted by conventional models, develops only for a very cold and thick lithosphere. Distinct topographic wavelengths or temporarily spaced events observed in the East African rift system, as well as over French Massif Central, can be explained by a single plume impinging at the base of the continental lithosphere, without evoking complex asthenospheric upwelling.

Tracing the thermal evolution of continental lithosphere through depth-dependent extension

NASA Astrophysics Data System (ADS)

Smye, A.; Lavier, L. L.; Stockli, D. F.; Zack, T.

2015-12-01

Rifting of continental lithosphere requires a mechanism to reduce lithospheric thickness from 100-150 kilometers to close to zero kilometers at the point of rupture. At magma-poor continental margins, this has long-thought to be caused by uniform stretching and thinning of the lithosphere accompanied by passive upwelling of the asthenosphere [1]. For the last thirty years depth-dependent thinning has been proposed as an alternative to this model to explain the anomalously shallow environment of deposition along many continental margins [2, 3]. A critical prediction of this modification is that the lower crust and sub-continental lithospheric mantle undergo a phase of increased heat flow, potentially accompanied by heating, during thinning of the lithospheric mantle. Here, we test this prediction by applying recently developed U-Pb age depth profiling techniques [4] to lower crustal accessory minerals from the exhumed Alpine Tethys and Pyrenean margins. Inversion of diffusion-controlled U-Pb age profiles in rutile affords the opportunity to trace the thermal evolution of the lower crust through the rifting process. Resultant thermal histories are used to calculate thinning factors of the crust and lithospheric mantle by 2D thermo-kinematic models of extending lithosphere. Combined, we use the measured and modeled thermal histories to propose a mechanism to explain the initiation and growth of lithospheric instabilities that lead to depth-dependent thinning at magma-poor continental margins. [1] McKenzie, D. (1978) EPSL 40, 25-32; [2] Royden, L. & Keen, C. (1980) EPSL 51, 343-361; [3] Huismans, R. & Beaumont, C. (2014) EPSL, 407, 148-162; [4] Smye, A. and Stockli, D. (2014) EPSL, 408, 171-182.

The Pyrenean Hercynian Keirogen and the Cantabrian Orocline as genetically coupled structures

NASA Astrophysics Data System (ADS)

Şengör, A. M. Celâl

2013-04-01

The two most enigmatic structures of the western European Hercynian orogenic system are the (late Palaeozoic parts of the) Pyrenean mountain range and the extremely tight (in some estimates more than 180°) Cantabrian Orocline, the innermost section of the Ibero-Armorican Arc. They have developed coevally in close proximity to one another and any hypothesis proposed to explain the evolution of the western European Hercynides must explain both in terms of a single model. The Pyrenees have both a peculiar position in the Hercynian architecture and a strange evolution if viewed from the viewpoint of the development of orogens. They sit on the 'hinterland', south of the main orogenic body, the 'innermost' edge of which is marked by the south-vergent Montaigne Noire Helvetic-type nappes. They have a possible minor and local early shortening (D1) accompanied by Barrovian metamorphism followed by a major ESE-WNW-directed stretching that created a pervasive flat foliation and recumbent gneissic nappes of the first genre (D2) accompanied by a major Buchan-type metamorphism. This latter phase also displays porphyroblasts indicating dextral motion along the range. It was followed by later shortening nearly 90° to the stretching creating mostly upright structures with an E-W striking foliation and a following crenulation cleavage also with indication of continuing right-handed slip along the entire system (D3 and its subphases). The Visean to Bashkirian 'flysches' developed from north to south across the entire mountain range plus its bounding plains to the north and south and no flysch production accompanied any later deformation. All the major structures of the late Palaeozoic range face upwards. Such an architecture and sequence of events create a picture that is odd in terms of an orogenic development leading one to suspect the presence here of what John F. Dewey called 'spoof orogeny', but seems in accord with a keirogen that created, in addition to extension, a high geothermal gradient and associated Buchan-type HT/LP metamorphism. It was during the later episodes of the strike-slip in the Pyrenees that the Cantabrian Orocline formed. It seems clear that a westerly escaping fragment between the Pyrenean keirogen and the left-later shear systems of inner Iberia indented the formerly N-S Iberian segment of the Hercynides and produced the extreme curvature of the orogen similar to the tectonic evolution of the Appenninic/Alpine/Dinaride Orocline around the Apulian indenter (Argand's African promontory), the Carpathian/Balkan Orocline around the Moesian indenter, Hazara and the Assam Oroclines around the Indian indenter (Argand's Indian promontory). Such a model is not in itself inconsistent with models involving lithospheric delamination, but I find it hard to see how the highest supracrustal sedimentary rocks in the core of the Cantabrian Orocline could have been preserved if a lithosphere-detaching thickening had taken place here. Contrary to the earlier models of lithospheric detachment by thickening, even much of Tibet still preserves its thick lithospheric root. The Hazara and the Assam oroclines are so tight that they have been classified also as syntaxes by Suess. But not all syntaxes are also oroclines: For example, the NE Greenland syntaxis joins two branches of early Palaeozoic orogens that formed along margins that meet at a right angle. The active Aleutian and the Kuril/Kamchatka arcs also meet at a syntaxis that is not an orocline. Oroclines are also different from simple deflections (Bucher, 1933, fig. 18; note here, however, that the fixist Bucher used at least four oroclines, namely those of Calabria, western Alps, Carpathians and the Balkans, and only one true deflection, the Alpine-Carpathian passage, to illustrate the concept of deflection in this figure; also in the same figure his concept of syntaxis is entirely wrong) that form around curved margins. Oroclines must show strain indicating bending synchronously with or postdating orogeny.

Thinning Factors and Crustal Thicknesses at the Propagating Tip of Sea-floor Spreading in the Woodlark Basin

NASA Astrophysics Data System (ADS)

Gozzard, S. P.; Kusznir, N.; Goodliffe, A.; Manatschal, G.

2007-12-01

Understanding how the continental crust and lithosphere thins at the propagating tip of sea-floor spreading is the key to understanding the continental breakup process. The Woodlark Basin, a young ocean basin located in the Western Pacific to the east of Papua New Guinea, commenced formation at approximately 8.4Ma and is propagating westwards at a rate of approximately 140km/Myr. Immediately to the west of the most recent segment of sea-floor spreading propagation, in the vicinity of the Moresby Seamount, evidence from bathymetry, subsidence and seismic Moho depth suggests that continental lithosphere is being thinned. In this study we have determined lithosphere thinning in the vicinity of the Moresby Seamount at the level of the whole lithosphere, the whole crust and the upper crust. Whole lithosphere thinning factors have been determined from subsidence analysis; whole continental crustal thinning factors have been determined from gravity inversions and upper crustal thinning factors have been determined from fault analysis. Three 2D seismic profiles surrounding the Moresby Seamount have been flexurally backstripped to the base of the syn-rift sediments to determine the water loaded subsidence. Using the McKenzie lithosphere extension model, modified to include volcanic addition at high thinning factors, whole thinning factors for the lithosphere have been determined from the water loaded subsidence. Results show that thermal subsidence alone cannot account for the observed subsidence, and that an additional initial subsidence is needed. Whole lithosphere thinning factors increase from an average of 0.5 to 0.8 across the Moresby Seamount eastwards towards the propagating tip. A satellite gravity inversion incorporating a lithosphere thermal gravity anomaly correction has been used to determine Moho depth, crustal thickness and thinning factors for the propagating tip in the Woodlark Basin. Moho depths are consistent with depths obtained from receiver function analysis (Ferris et al. 2006). Crustal thickness estimates do not include a correction for sediment thickness and are upper bounds. Crustal thinning factors in the vicinity of the Moresby Seamount are similar to those observed for the whole lithosphere. Fault analysis of the three 2D profiles have been used to determine upper crustal thinning factors. Upper crustal thinning factors between 0.1 to 0.2 are observed for the vicinity of the Moresby Seamount, substantially lower than thinning factors predicted for the whole lithosphere and continental crust, suggesting depth-dependent lithosphere thinning. Crustal thicknesses predicted from gravity inversion immediately to the east of the Moresby Seamount are substantially greater than would be expected for oceanic lithosphere in this region, while highly thinned, has not completely ruptured.

Seismic Constraints on the Lithosphere-Asthenosphere Boundary Beneath the Izu-Bonin Area: Implications for the Oceanic Lithospheric Thinning

NASA Astrophysics Data System (ADS)

Cui, Qinghui; Wei, Rongqiang; Zhou, Yuanze; Gao, Yajian; Li, Wenlan

2018-01-01

The lithosphere-asthenosphere boundary (LAB) is the seismic discontinuity with negative velocity contrasts in the upper mantle. Seismic detections on the LAB are of great significance in understanding the plate tectonics, mantle convection and lithospheric evolution. In this paper, we study the LAB in the Izu-Bonin subduction zone using four deep earthquakes recorded by the permanent and temporary seismic networks of the USArray. The LAB is clearly revealed with sP precursors (sdP) through the linear slant stacking. As illustrated by reflected points of the identified sdP phases, the depth of LAB beneath the Izu-Bonin Arc (IBA) is about 65 km with a range of 60-68 km. The identified sdP phases with opposite polarities relative to sP phases have the average relative amplitude of 0.21, which means a 3.7% velocity drop and implies partial melting in the asthenosphere. On the basis of the crustal age data, the lithosphere beneath the IBA is located at the 1100 °C isotherm calculated with the GDH1 model. Compared to tectonically stable areas, such as the West Philippine Basin (WPB) and Parece Vela Basin (PVB) in the Philippine Sea, the lithosphere beneath the Izu-Bonin area shows the obvious lithospheric thinning. According to the geodynamic and petrological studies, the oceanic lithospheric thinning phenomenon can be attributed to the strong erosion of the small-scale convection in the mantle wedge enriched in volatiles and melts.

Variations in Temperature at the Base of the Lithosphere Beneath the Archean Superior Province, Canada

NASA Astrophysics Data System (ADS)

Mareschal, J.; Jaupart, C. P.

2013-12-01

Most of the variations in surface heat flux in stable continents are caused by variations in crustal heat production, with an almost uniform heat flux at the base of the crust ( 15+/-3 mW/m2). Such relatively small differences in Moho heat flux cannot be resolved by heat flow data alone, but they lead to important lateral variations in lithospheric temperatures and thicknesses. In order to better constrain temperatures in the lower lithosphere, we have combined surface heat flow and heat production data from the southern Superior Province in Canada with vertical shear wave velocity profiles obtained from surface wave inversion. We use the Monte-Carlo method to generate lithospheric temperature profiles from which shear wave velocity can be calculated for a given mantle composition. We eliminate thermal models which yield lithospheric and sub-lithospheric velocities that do not fit the shear wave velocity profile. Surface heat flux being constrained, the free parameters of the thermal model are: the mantle heat flux, the mantle heat production, the crustal differentiation index (ratio of surface to bulk crustal heat production) and the temperature of the mantle isentrope. Two conclusions emerge from this study. One is that, for some profiles, the vertical variations in shear wave velocities cannot be accounted for by temperature alone but also require compositional changes within the lithosphere. The second is that there are long wavelength horizontal variations in mantle temperatures (~80-100K) at the base of the lithosphere and in the mantle below

Formation of cratonic lithosphere during the initiation of plate tectonics

NASA Astrophysics Data System (ADS)

Moresi, L. N.; Beall, A.; Cooper, C. M.

2017-12-01

The Earth's oldest near-surface material, the cratonic crust, is typically underlain by unusually thick Archean lithosphere (<300 km). This cratonic lithosphere likely thickened in a high compressional stress environment. Mantle convection in the hotter Archean Earth would have imparted relatively low stresses on the lithosphere, whether or not tectonics was operating, so a high stress signal from the early Earth is paradoxical. We propose that a rapid transition, from a stagnant lid Earth to the onset of plate tectonics, generated the high stresses required to thicken the cratonic lithosphere. Numerical calculations are used to demonstrate that an existing buoyant and strong layer, representing harzburgite and felsic crust, can thicken and stabilize during the lid-breaking event. The peak compressional stress experienced by lithosphere is 3-4 higher than for the stagnant lid or mobile lid regimes immediately before and after. It is plausible that the cratonic lithosphere has still not returned to this high stress-state, explaining its stability. The lid-breaking thickening event reproduces craton features previously attributed to subduction: thrust structures, assembled crustal fragments and transport of basaltic upper crust to depths required to generate felsic melt. Palaeoarchean `pre-tectonic' structures can also survive the lid-breaking event, acting as strong crustal rafts. Together, the results indicate that the signature of a catastrophic switch, from a stagnant lid Earth to the initiation of plate tectonics, has been captured and preserved in the unusual characteristics of cratonic crust and lithosphere.

Piecewise Delamination Drives Uplift in the Atlas Mountains Region of Morocco

NASA Astrophysics Data System (ADS)

Bezada, M. J.; Humphreys, E.; Martin Davila, J.; mimoun, H.; Josep, G.; Palomeras, I.

2013-12-01

The elevation of the intra-continental Atlas Mountains of Morocco and surrounding regions requires a mantle component of buoyancy, and there is consensus that this buoyancy results from an abnormally thin lithosphere. Lithospheric delamination under the Atlas Mountains and thermal erosion caused by upwelling mantle have each been suggested as thinning mechanisms. We use seismic tomography to image the upper mantle of Morocco by inverting teleseimic p-wave delay times, complemented with local delays, recorded on a dense array of stations in the Iberian peninsula and Morocco. A surface wave model provides constraint on the shallower layers. We determine the geometry of lithospheric cavities and mantle upwelling beneath the Middle Atlas and central High Atlas, and image delaminated lithosphere at ~400 km beneath the Middle Atlas. We propose discontinuous delamination of an intrinsically unstable Atlas lithosphere, enabled by the presence of anomalously hot mantle, as a mechanism for producing the imaged structures. The Atlas lithosphere was made unstable by a combination of tectonic shortening and eclogite loading during Mesozoic rifting and Cenozoic magmatism. The presence of hot mantle, sourced from regional upwellings in northern Africa or the Canary Islands, enabled the mobilization of this lithosphere. Flow around the retreating Alboran slab focused upwelling mantle under the Middle Atlas, where we image the most recent delamination. The Atlas Mountains of Morocco stand as an example of mantle-generated uplift and large-scale lithospheric loss in a mildly contractional orogen.

Physical properties and Consolidation behavior of sediments from the N. Japan subduction zone

NASA Astrophysics Data System (ADS)

Valdez, R. D., II; Lauer, R. M.; Ikari, M.; Kitajima, H.; Saffer, D. M.

2013-12-01

Sediment hydraulic properties, consolidation state, and ambient pore pressure development are key parameters that affect fluid migration, deformation, and the slip behavior and mechanical strength of subduction zone megathrusts. In order to better understand the dynamics and mechanisms of large subduction earthquakes, Integrated Oceanic Drilling Program (IODP) Expedition 343, drilled into the toe of the Japan Trench subduction zone in a region of large shallow slip in the M 9.0 Tohoku earthquake, as part of the Japan Trench Fast Drilling Project (J-FAST). Here, we report on two constant rate of strain (CRS) uniaxial consolidation experiments and two triaxial deformation experiments on bedded claystone and clayey mudstone core samples collected from the frontal prism and subducted sediment section cored at Site C0019, 2.5 km landward of the Japan Trench, from depths of 697.18 and 831.45 mbsf. The goals of our experiments were: (1) to define the hydraulic and acoustic properties of sediments that host the subduction megathrust fault that slipped in the M 9.0 Tohoku earthquake; and (2) to constrain in-situ consolidation state and its implications for in-situ stress. The permeability-porosity trends are similar for the two samples, and both exhibit permeability that decreases systematically with increasing effective stress and decreasing porosity, and which varies log-linearly with porosity. Permeabilities of material from the frontal prism decrease from 5×10-18 m2 at 5 MPa effective stress, to 3.0×10-19 m2 at 70 MPa, and porosities decrease from 51% to 29%, while permeabilities of the subducted sediment sample decrease from 5×10-18 m2 at 5 MPa to 3.6×10-19 m2 at 90 MPa, and porosities decrease from 49% to 36%. In-situ permeabilities for the prism and underthrust sediment samples, estimated using laboratory defined permeability-porosity relationships, are 4.9×10-18 m2 and 3.7×10-18 m2, respectively. Elastic wavespeeds increase systematically with increasing effective stress. P-wave velocities (Vp) in the frontal prism sample increase from 2.1 km/s at 8 MPa to 2.7 km/s at 55 MPa effective stress, and velocities in the underthrust sediment sample increase from 2.3 km/s at 6 MPa to 3.0 km/s at 76.5 MPa. Estimated in-situ Vp for the frontal prism and underthrust sediment sample are 2.1 km/s and 2.4 km/s, respectively. This is slightly higher than both the logging while drilling (LWD) measurements and shipboard velocity measurements on discrete samples. We also estimated pre-consolidation pressures (Pc) for each sample using the work-stress method. Comparing Pc with the present day in-situ vertical stress calculated from shipboard bulk density data, we find that both samples are severely overconsolidated. We report this in terms of overconsolidation ratio (OCR), defined as the ratio of Pc to the in-situ stress expected for the case of normal consolidation. Values of OCR for the prism and underthrust samples are 3.95 and 4.28, respectively. This overconsolidation is broadly consistent with fully drained (non-overpressured) conditions, and may reflect uplift and unroofing of the sediments following peak burial greater than their current depth, a significant contribution from lateral tectonic stresses leading to an effective stress far greater than expected for the case of uniaxial burial, or cementation that leads to apparent overconsolidation.

Structure and rheology of the lithosphere below southeastern margin of India and Sri Lanka, and its conjugate segment of the east Antarctica: Implications on early breakup history and margin formation

NASA Astrophysics Data System (ADS)

Rao Gangumalla, Srinivasa; Radhakrishna, Munukutla

2014-05-01

The eastern continental margin of India has evolved as a consequence of rifting and breakup between India and east Antarctica during the early Cretaceous. Plate reconstruction models for the breakup of eastern Gondwanaland by many earlier workers have unambiguously placed the southeast margin of Sri Lanka and India together as a conjugate segment with the east Antarctica margin that extends from Gunnerus Ridge in the west to western Enderby basin in the east. In this study, we present results of integrated analysis of gravity, geoid, magnetic and seismic data from these two conjugate portions in order to examine the lithosphere structure and early seafloor spreading, style of breakup, continent-ocean boundary (COB) and rheological characteristics at these margins. The interpreted COB lies at a distance of 55-140 km on the side of southeast margin of Sri Lanka and India, whereas, it lies at a distance of 190-550 km on the side of east Antarctica margin. The seismic profiles and the constrained potential field models across these two segments do not show the existence of seaward dipping reflector sequences or magmatic underplating suggesting that these segments have not encountered major magmatic activity. While, significant crustal thinning/stretching is observed at the east Antarctic margin, the Cauvery offshore had experienced limited stretching with faulted Moho interface. Further, the conspicuous residual geoid low in the Cauvery offshore basin is inferred to be due to a continental crustal block. The modelled Lithosphere-Astenosphere Boundary (LAB) in these two margins is located around 110-120 km depth with slightly thicker lithosphere at the east Antarctica margin. In addition, the interpretation of magnetic anomalies provided structure of the oceanic crust generated through seafloor spreading processes with age and magnetization data constrained from the identified magnetic anomalies in the respective margins. Using the Bouguer coherence method, we computed spatial variations in effective elastic thickness (Te) at these margin segments. The estimated Te values at the Indian margin ranges between 5-8 km in the southeast of Sri Lanka to around 10-12 km in the Cauvery offshore which decrease further north to < 5 km in the Cauvery-Palar basin. Along the east Antarctic margin, the Te values ranges between 5-10 km in the Gunnerus ridge region, 35-40 km in the western Enderby basin which decrease further towards the central Enderby basin up to 20 km. In this study, the above results have been analyzed in terms of early breakup mechanism and subsequent evolution of these two conjugate segments.

Evolution of planetary lithospheres - Evidence from multiringed structures on Ganymede and Callisto

NASA Technical Reports Server (NTRS)

Mckinnon, W. B.; Melosh, H. J.

1980-01-01

The thickness and viscosity of a planetary lithosphere increase with time as the mantle cools, with a thicker lithosphere leading to the formation of one (or very few) irregular normal faults concentric to the crater. Since a gravity wave or tsunami induced by impact into a liquid mantle would result in both radial and concentric extension features, which are not observed in the case of the large impact structures on Ganymede and Callisto, an alternative mechanism is proposed in which the varying ice/silicate ratios, tectonic histories, and erosional mechanisms of the two bodies are considered to explain the subtle differences in thin lithosphere ring morphology between Ganymede and Callisto. It is concluded that the present lithosphere thickness of Ganymede is too great to permit the development of any rings.

Impact of lithospheric rheology on surface topography

NASA Astrophysics Data System (ADS)

Liao, K.; Becker, T. W.

2017-12-01

The expression of mantle flow such as due to a buoyant plume as surface topography is a classical problem, yet the role of rheological complexities could benefit from further exploration. Here, we investigate the topographic expressions of mantle flow by means of numerical and analytical approaches. In numerical modeling, both conventional, free-slip and more realistic, stress-free boundary conditions are applied. For purely viscous rheology, a high viscosity lithosphere will lead to slight overestimates of topography for certain settings, which can be understood by effectively modified boundary conditions. Under stress-free conditions, numerical and analytical results show that the magnitude of dynamic topography decreases with increasing lithosphere thickness (L) and viscosity (ηL), as L-1 and ηL-3. The wavelength of dynamic topography increases linearly with L and (ηL/ ηM) 1/3. We also explore the time-dependent interactions of a rising plume with the lithosphere. For a layered lithosphere with a decoupling weak lower crust embedded between stronger upper crust and lithospheric mantle, dynamic topography increases with a thinner and weaker lower crust. The dynamic topography saturates when the decoupling viscosity is 3-4 orders lower than the viscosity of upper crust and lithospheric mantle. We further explore the role of visco-elastic and visco-elasto-plastic rheologies.

On the Yield Strength of Oceanic Lithosphere

NASA Astrophysics Data System (ADS)

Jain, C.; Korenaga, J.; Karato, S. I.

2017-12-01

The origin of plate tectonic convection on Earth is intrinsically linked to the reduction in the strength of oceanic lithosphere at plate boundaries. A few mechanisms, such as deep thermal cracking [Korenaga, 2007] and strain localization due to grain-size reduction [e.g., Ricard and Bercovici, 2009], have been proposed to explain this reduction in lithospheric strength, but the significance of these mechanisms can be assessed only if we have accurate estimates on the strength of the undamaged oceanic lithosphere. The Peierls mechanism is likely to govern the rheology of old oceanic lithosphere [Kohlstedt et al., 1995], but the flow-law parameters for the Peierls mechanism suggested by previous studies do not agree with each other. We thus reanalyze the relevant experimental deformation data of olivine aggregates using Markov chain Monte Carlo inversion, which can handle the highly nonlinear constitutive equation of the Peierls mechanism [Korenaga and Karato, 2008; Mullet et al., 2015]. Our inversion results indicate nontrivial nonuniqueness in every flow-law parameter for the Peierls mechanism. Moreover, the resultant flow laws, all of which are consistent with the same experimental data, predict substantially different yield stresses under lithospheric conditions and could therefore have different implications for the origin of plate tectonics. We discuss some future directions to improve our constraints on lithospheric yield strength.

Density Of The Continental Roots: Compositional And Thermal Effects

NASA Astrophysics Data System (ADS)

Kaban, M. K.; Schwintzer, P.; Artemieva, I.; Mooney, W. D.

We use gravity, thermal, and seismic data to examine how the density and composi- tion of lithospheric roots vary beneath the cratons. Our interpretation is based on the gravity anomalies calculated by subtracting the gravitational effects of bathymetry, to- pography, and the crust from the observed gravity field, and the residual topography that characterizes the isostatic state of the lithosphere. We distinguish the effects of temperature and compositional variations in producing lithospheric density anomalies using two independent temperature constrains: based on interpretation of the surface heat flow data and estimated from global seismic tomography data. We find that in situ lithospheric density differs significantly between individual cratons, with the most dense values found beneath Eurasia and the least dense values beneath South Africa. This demonstrates that there is not a simple compensation of thermal and composition effects. We present a new gravity anomaly map that was corrected for crustal density structure and lithospheric temperatures. This map reveals differences in lithospheric composition, that are the result of the petrologic processes that have formed and mod- ified the lithosphere. All significant negative gravity anomalies are found in cratonic regions. In contrast, positive gravity anomalies are found in two distinct regions: near ocean-continent and continent-continent subduction zones, and within some continen- tal interiors. The origin of the latter positive anomalies is uncertain.

A shear-wave velocity model of the European upper mantle from automated inversion of seismic shear and surface waveforms

NASA Astrophysics Data System (ADS)

Legendre, C.; Meier, T.; Lebedev, S.; Friederich, W.; Viereck-Götte, L.

2012-04-01

Broadband waveforms recorded at stations in Europe and surrounding regions were inverted for shear-wave velocity of the European upper mantle. For events between 1995 and 2007 seismograms were collected from all permanent stations for which data are available via the data centers ORFEUS, GEOFON, ReNaSs and IRIS. In addition, we incorporated data from temporary experiments, including SVEKALAPKO, TOR, Eifel Plume, EGELADOS and other projects. Automated Multimode Inversion of surface and S-wave forms was applied to extract structural information from the seismograms, in the form of linear equations with uncorrelated uncertainties. Successful waveform fits for about 70,000 seismograms yielded over 300,000 independent linear equations that were solved together for a three-dimensional tomographic model. Resolution of the imaging is particularly high in the mantle lithosphere and asthenosphere. The highest velocities in the mantle lithosphere of the East European Craton are found at about 150 km depth. There are no indications for a large scale deep cratonic root below about 330 km depth. Lateral variations within the cratonic mantle lithosphere are resolved by our model as well. The locations of diamond bearing kimberlites correlate with reduced S-wave velocities in the cratonic mantle lithosphere. This anomaly is present in regions of both Proterozoic and Archean crust, pointing to an alteration of the mantle lithosphere after the formation of the craton. Strong lateral changes in S-wave velocity are found at the western margin of the East European Craton and hint to erosion of cratonic mantle lithosphere beneath the Scandes by hot asthenosphere. The mantle lithosphere beneath Western Europe and between the Tornquist-Teyissere Zone and the Elbe Line shows moderately high velocities and is of an intermediate character, between cratonic lithosphere and the thin lithosphere of central Europe. In central Europe, Caledonian and Variscian sutures are not associated with strong lateral changes in the lithosphere-asthenosphere system. Cenozoic anorogenic intraplate volcanism in central Europe and the Circum Mediterranean is found in regions of shallow asthenosphere and close to sharp gradients in the depth of the lithosphere-asthenosphere boundary. Low-velocity anomalies extending vertically from shallow upper mantle down to the transition zone are found beneath the Massive Central, Sinai, Canary Islands and Iceland.

Thermal thickness and evolution of Precambrian lithosphere: A global study

USGS Publications Warehouse

Artemieva, I.M.; Mooney, W.D.

2001-01-01

The thermal thickness of Precambrian lithosphere is modeled and compared with estimates from seismic tomography and xenolith data. We use the steady state thermal conductivity equation with the same geothermal constraints for all of the Precambrian cratons (except Antarctica) to calculate the temperature distribution in the stable continental lithosphere. The modeling is based on the global compilation of heat flow data by Pollack et al. [1993] and more recent data. The depth distribution of heat-producing elements is estimated using regional models for ???300 blocks with sizes varying from 1?? ?? 1?? to about 5?? ?? 5?? in latitude and longitude and is constrained by laboratory, seismic and petrologic data and, where applicable, empirical heat flow/heat production relationships. Maps of the lateral temperature distribution at depths 50, 100, and 150 km are presented for all continents except Antarctica. The thermal thickness of the lithosphere is calculated assuming a conductive layer overlying the mantle with an adiabat of 1300??C. The Archean and early Proterozoic lithosphere is found to have two typical thicknesses, 200-220 km and 300-350 km. In general, thin (???220 km) roots are found for Archean and early Proterozoic cratons in the Southern Hemisphere (South Africa, Western Australia, South America, and India) and thicker (>300 km) roots are found in the Northern Hemisphere (Baltic Shield, Siberian Platform, West Africa, and possibly the Canadian Shield). We find that the thickness of continental lithosphere generally decreases with age from >200 km beneath Archean cratons to intermediate values of 200 ?? 50 km in early Proterozoic lithosphere, to about 140 ?? 50 km in middle and late Proterozoic cratons. Using known crustal thickness, our calculated geotherms, and assuming that isostatic balance is achieved at the base of the lithosphere, we find that Archean and early Proterozoic mantle lithosphere is 1.5% less dense (chemically depleted) than the underlying asthenosphere, while middle and late Proterozoic subcrustal lithosphere should be depleted by ???0.6-0.7%. Our results suggest three contrasting stages of lithosphere formation at the following ages: >2.5 Ga, 2.5-1.8 Ga, and <1.8 Ga. Ages of komatiites, greenstone belts, and giant dike swarms broadly define similar stages and apparently reflect secular changes in mantle temperature and, possibly, convection patterns.

Numerical modeling of Farallon Plate flat-slab subduction: Influence of lithosphere structure and rheology on slab dynamics

NASA Astrophysics Data System (ADS)

Liu, X.; Currie, C. A.

2017-12-01

The subducted Farallon plate is believed to have evolved to a flat geometry underneath North America plate during Late Cretaceous, triggering Laramide deformation within the continental interior. However, the mechanism that caused the oceanic slab to flatten and the factors that control the flat-slab depth remain uncertain. In this work, we use 2D thermal-mechanical models using the SOPALE code to study the subduction dynamics from 90 Ma to 50 Ma. During this period, an oceanic plateau (Shatsky Conjugate) is inferred to have subducted beneath western North America and interacted with the continental lithosphere, including areas of thicker lithosphere such as the Colorado Plateau and Wyoming Craton. Based on seismic tomography and plate reconstruction data sets, we built a set of models to examine the influence of the structure and rheology of the oceanic and continental plates on slab dynamics. Models include a 600 km wide oceanic plateau consisting of 18 km thick crust and a 36 km thick underlying harzburgite layer, and we ran a series of model experiments to test different continental thicknesses (80 km, 120 km, & 180 km) and continental mantle lithosphere strengths (approximating conditions from wet olivine to dry olivine). Consistent with earlier studies, we find that creation of a long flat slab requires a buoyant oceanic plateau (i.e., non-eclogitized crust) and trenchward motion of the continent. In addition, our models demonstrate the upper plate has an important control on slab dynamics. A flat slab requires either a thin continent or, if the continent is thick, its mantle lithosphere must be relatively weak so that it can be displaced by the flattening slab. The depth of the flat slab is mainly controlled by two factors: (1) the continental thickness and (2) the strength of the continental mantle lithosphere. For the same initial lithosphere thickness (120 km), a shallower flat slab ( 90 km depth) occurs for the weakest mantle lithosphere ( wet olivine) compared to 120 km depth for strong ( dry) mantle lithosphere because the flat slab removes the lowermost weak lithosphere. Moreover, an even deeper slab ( 130 km) can be found underneath the weakest but thicker continental lithosphere (180 km). Future models will focus on how the flat slab may induce hydration and deformation for the overriding continental plate.

Effect of Upper Mantle Heterogeneities on Lithosphere Stresses and Topography

NASA Astrophysics Data System (ADS)

Osei Tutu, A.; Steinberger, B.; Rogozhina, I.; Sobolev, S. V.

2016-12-01

The orientation and magnitude of lithosphere stresses give us knowledge about most of the processes within the Earth that are not easy to observe. It has been established (Steinberger, Schmeling, and Marquart 2001) that large contribution of the forces producing lithosphere stresses have their source origination from the buoyancies of both the upper and lower mantle acting beneath the lithosphere. The contribution of the crustal thickness to the stresses has been estimated to be less than 10% (Steinberger et al. 2001) in most region and increases in areas with high gravitational potential energy like the Himalayas. In most of these studies, the effect of the crust was determined separately by computing the gravitational potential energy from the crust (Ghosh et al. 2013) and applied as correction. (Artyushkov 1973) showed that the inhomogeneous nature of the crust contribute to the stresses observed as against using constant lithosphere thickness in most studies, due to the complexities for implementing a variable lithosphere. We seek extend the approach of Ghosh et al. (2013) by coupling the Crust 1.0 (Laske et al. 2013) to a varaible lithosphere thickness in our numerical method. Using a 3D global lithosphere-asthenosphere model (Popov and Sobolev 2008) with visco-elasto-plastic rheology, coupled at 300 km depth to a mantle modeled with a spectral technique (Hager and O'Connell, 1981), we compute lithosphere stresses and topography. we compare our model with observations; the World Stress Map, Global Strain Rate Map and the observed topgraphy. We use S40RTS seismic tomography below 300 km depth, with radial viscosity distribution (Steinberger et al 2006). To account for all the heterogeneities in the upper mantle (300 km) we used different 3D temperatures models setups. The first model is the thermal lithosphere model (Artemieva and Mooney, 2001) in continental regions and assumes half-space cooling of sea floor with age (Müller et al. 2008) for oceans. For the second model we inferred temperatures from seismic tomography SL2013sv (Schaeffer and Lebedev 2013) for separate stress predictions. We investigate the effect of Newtonian and power law rheology on stresse and also look at different deformation mechanisms; diffusion and dislocation creeps in the upper mantle on lithosphere stresses.

Evidence for refertilization of the Pacific plate: implications for the seismic and geochemical properties of the oceanic lithosphere

NASA Astrophysics Data System (ADS)

Pilet, S.; Müntener, O.; Duretz, T.; Hetényi, G.

2017-12-01

Garnet xenocryst sampled by petit-spot lavas offshore Japan provides evidence for the formation of gabbroic cumulates within the Pacific lithosphere. The trace element signature indicates that garnet probably formed subsolidus from plagioclase-bearing cumulates during off-axis cooling of the oceanic lithosphere. The specific P-T conditions required for garnet subsolidus formation (0.7 - 1.2 GPa) indicate that melt percolation to produce plagioclase-bearing cumulate occurs at more than 150 km off-axis. Although mantle refertilization in periphery of mid-ocean ridge has been previously shown for (ultra-) slow spreading ridges, our finding indicates that similar processes also occur in portions of the Pacific lithospheric mantle formed at intermediate spreading rates. Recent numerical simulations of melting and melt transport at mid-ocean ridges in presence of volatiles1 support our hypothesis. These simulations suggest that volatile extraction at mid ocean ridges is limited and up to 50% of deep, volatile-rich melt is not focused to the axis but percolated along the LAB. Magma evolution at lithospheric pressure2 predicts that these distal volatile-rich melts will cool and crystallize producing anhydrous and hydrous metasomatic cumulates within the base of the lithosphere. As the lithosphere cools, the hydrous metasomatic cumulates will stay close to their solidus temperature. Any thermo-mechanical perturbation at the base of the lithosphere could potentially reactivate melts and remobilize hydrous phases, which may explain the formation of small-scale seamounts characterized by alkaline magma composition. The presence of hydrous phases and residual CO2 -rich melt at depths around 40 to 70 km could also explain the seismic and electric anomalies observed within the Pacific lithosphere4. Addition of 1-2% volatile-rich melt to the base of the lithosphere predicted by the geochemical simulation3 is sufficient to modify the composition of the oceanic lithospheric mantle and produce, after recycling into the convecting mantle, enriched isotopic signature such as E-DMM or even HIMU. 1 Keller et al. 2017, EPSL 464, 55-68; 2 Pilet et al. 2010, CMP 159, 621-643; 3 Pilet et al. 2011 JPet 52, 1415-1442; 4 Tharimena et al. 2017, JGR Solid Earth 122, 2131-2152.

Venus orogenic belt environments - Architecture and origin

NASA Astrophysics Data System (ADS)

Head, James W.; Vorder Bruegge, Richard W.; Crumpler, L. S.

1990-08-01

Orogenic belt environments (Danu, Akna, Freyja, and Maxwell Montes) in Western Ishtar Terra, Venus, display a range of architectural elements, including (from the center of Western Ishtar outward) an inboard plateau (Lakshmi Planum), the linear orogenic belts themselves, outboard plateaus, steep scarps bounding Ishtar, adjacent linear foredeeps and outboard rises, and outboard low-lying volcanic plains. The main elements of the architecture are interpreted to be due to the convergence, underthrusting, and possible subduction of lowland plains at the margins of a preexisting tessera plateau of thicker crust.

Proceedings of West Coast Regional Coastal Design Conference Held on 7-8 November 1985 at Oakland, California.

DTIC Science & Technology

1986-04-01

Lon- don, 53 2p. Yancey, T.E. and Lee, J.W. 1972. Major heavy mineral assemblages and heavy mineral provinces of the central California Coast Region...sites have been collected. A data base and a user-friendly retrieval system have been de - veloped and are presently undergoing evaluation before they are...Continental Margin The tectonic evolution of the Oregon coastline appears to be controlled at present by the underthrusting of the Juan de Fuca plate beneath

Plate tectonic reconstruction of South and East Asia since 43 Ma using seismic tomographic constraints: role of the subducted ';East Asia Sea' (Invited)

NASA Astrophysics Data System (ADS)

Wu, J. E.; Suppe, J.; Renqi, L.; Kanda, R. V.

2013-12-01

Lithosphere that subducts at convergent plate boundaries provides a potentially decipherable plate tectonic record. In this study we use global seismic tomography to map subducted slabs in the upper and lower mantle under South and East Asia to constrain plate reconstructions. The mapped slabs include the Pacific, the Indian Ocean and Banda Sea, the Molucca Sea, Celebes Sea, the Philippine Sea and Eurasia, New Guinea and other lower mantle detached slabs. The mapped slabs were restored to the earth surface and used with Gplates software to constrain a globally-consistent, fully animated plate reconstruction of South and East Asia. Three principal slab elements dominate possible plate reconstructions: [1] The mapped Pacific slabs near the Izu-Bonin and the Marianas trenches form a subvertical slab curtain or wall extending down to 1500 km in the lower mantle. The ';slab curtain' geometry and restored slabs lengths indicate that the Pacific subduction zone has remained fixed within +/- 250 km of its present position since ~43 Ma. In contrast, the Tonga Pacific slab curtain records at least 1000 km trench rollback associated with expansion of back-arc basins. [2] West of the Pacific slab curtain, a set of flat slabs exist in the lower mantle and record a major 8000km by 2500-3000km ocean that existed at ~43 Ma. This now-subducted ocean, which we call the ';East Asian Sea', existed between the Ryukyu Asian margin and the Lord Howe hotspot, present-day eastern Australia, and fills a major gap in Cenozoic plate reconstructions between Indo-Australia, the Pacific Ocean and Asia. [3] An observed ';picture puzzle' fit between the restored edges of the Philippine Sea, Molucca Sea and Indian Ocean slabs suggests that the Philippine Sea was once part of a larger Indo-Australian Ocean. Previous models of Philippine Sea plate motions are in conflict with the location of the East Asian Sea lithosphere. Using the mapped slab constraints, we propose the following 43 Ma to 0 plate tectonic reconstruction. At ~43 Ma a major plate reorganization occurred in South and East Asia marked by Indian Ocean Wharton ridge extinction, initiation of Pacific Ocean WNW motions and the rapid northward motion of the Australian plate. The Philippine Sea and Molucca Sea were clustered at the northern margin of Australia, northwest of New Guinea. During the mid-Cenozoic these plates moved NNE with Australia, accommodated by N-S transforms at the eastern margin of Sundaland. The East Asian Sea was subducted under the northward-moving Philippine Sea and Australia plates, and the expanding Melanesian and Shikoku-Parece Vela backarc basins. At ~20 to 25 Ma the Philippine Sea and Molucca Sea were fragmented from Indo-Australia and began to have a westward component of motion due to partial Pacific capture. Around 1-2 Ma the Philippine Sea was more fully captured by the Pacific and now has rapid Pacific-like northwestward motions.

Effective Elastic Thickness of the Lithosphere in Continental China from Heat Flow: Implications for the Lithospheric Rheology

NASA Astrophysics Data System (ADS)

Liu, S.; Wang, L.

2006-12-01

The effective elastic thickness (Te) of lithosphere is one parameter describing the responses of the lithosphere to long term forces, and is still controversial in estimation by different methods. Here we present the effective elastic thickness of the lithosphere in continental China from heat flow data by the method proposed by Burov et al, J.G.R., 1995,100(B3):3905-3927. Our results show that Te varies much in different sub-areas in continental China due to different geological evolution and associated thermal regimes. Te is much greater than the crustal thickness in the area where the heat flow is really low and the lithosphere is really thick, indicating much more contribution from the lithospheric mantle and the dominative control of the mantle with olivine on the rheology of the lithosphere, and the major basins (Tarim, Junggar, Ordos and Sichuan basins) in central-western China share this characteristic. For instance, the Te of the Tarim basin is 66km with crustal thickness of 45km. Te is less than the crustal thickness in the region where the heat flow is relatively high, and approximates to the crustal brittle-ductile transition depth, suggesting more contribution from the crust and the dominative control of the felsic crust on the rheology of the lithosphere, and this phenomenon is obvious in the SE coastal China, eastern North China and the orogenic belts. Compared the estimated Te with the seismogenic layer thickness (Ts) available in China, it is also found that the Te is much greater than Ts in the major basins with low heat flow, and is similar to Ts in the active zones with high heat flow, which is inconsistent with that Te is usually smaller than Ts proposed by Maggi et al., Geology,2000,28(6):495-498. Generally, two end elements rheological modes for continental lithosphere of the strong crust-weak mantle and the weak crust-strong mantle are all available in continental China considering different thermal regime, composition and geological evolution.

Lithospheric structure beneath the central and western North China Craton and adjacent regions from S-receiver function imaging

NASA Astrophysics Data System (ADS)

Yinshuang, A.; Zhang, Y.; Chen, L.

2016-12-01

The central and western NCC(CWNCC) only experienced localized lithospheric modification and has remained relatively stable since the Pre-Cambrian in contrast to the fundamental destruction in the east. For better unraveling the tectonic evolution and dynamics of CWNCC, detailed knowledge of lithospheric structure is thus important. However, most of the available seismological observations are dominated by regional seismic tomography and the resolutions are rather low due to the limited data coverage or intrinsic limitation of the methods. S receiver function(RF) contains information from deep velocity discontinuities and is free from the interference of crustal multiples, so it is widely used in subcontinental lithospheric structural studies. We collected teleseismic data from 340 broadband stations in CWNCC, and adopted 2-D wave equation-based poststack migration method to do S-receiver function CCP imaging. Finally, we get 8 migrated profile images in CWNCC and adjacent areas and integrate them for an overview. The most prominent feature of the LAB beneath central NCC is an sudden subsidence to 160km in the southern portion, and the dimension and extension of this deep anomaly is correlated to the lithosphere in Ordos, so we interpret it as a remnant cratonic mantle root. The LAB beneath western NCC can extend to the depth of 150-180 km but appears laterally variable. Western Ordos becomes shallower than its eastern counterpart and there are two obvious deep anomalies beneath the eastern Ordos, divided by a geological boundary at 37°N, which reflects that the lithosphere of Ordos is not so homogeneous or rigid as people thought before. Furthermore, a negative velocity discontinuity is widely identified at the depth of 80- 110 km within the thick lithosphere of CWNCC, and the location is spatially coincide with the modified LAB in ENCC. Although the cause of this mid-lithospheric discontinuity(MLD) is still controversial, mechanically, it may indicate an ancient, weak layer within the overall strong cratonic lithosphere. Our result is broadly consistent with the previous tomography studies, but shows more detailed information of lithospheric variations,. Moreover, it corroborates the existence of the similar discontinuities at 100 km depth under stable continental regions worldwide.

3D Integrated geophysical-petrological modelling of the Iranian lithosphere

NASA Astrophysics Data System (ADS)

Mousavi, Naeim; Ardestani, Vahid E.; Ebbing, Jörg; Fullea, Javier

2016-04-01

The present-day Iranian Plateau is the result of complex tectonic processes associated with the Arabia-Eurasia Plate convergence at a lithospheric scale. In spite of previous mostly 2D geophysical studies, fundamental questions regarding the deep lithospheric and sub-lithospheric structure beneath Iran remain open. A robust 3D model of the thermochemical lithospheric structure in Iran is an important step toward a better understanding of the geological history and tectonic events in the area. Here, we apply a combined geophysical-petrological methodology (LitMod3D) to investigate the present-day thermal and compositional structure in the crust and upper mantle beneath the Arabia-Eurasia collision zone using a comprehensive variety of constraining data: elevation, surface heat flow, gravity potential fields, satellite gravity gradients, xenoliths and seismic tomography. Different mantle compositions were tested in our model based on local xenolith samples and global data base averages for different tectonothermal ages. A uniform mantle composition fails to explain the observed gravity field, gravity gradients and surface topography. A tectonically regionalized lithospheric mantle compositional model is able to explain all data sets including seismic tomography models. Our preliminary thermochemical lithospheric study constrains the depth to Moho discontinuity and intra crustal geometries including depth to sediments. We also determine the depth to Curie isotherm which is known as the base of magnetized crustal/uppermost mantle bodies. Discrepancies with respect to previous studies include mantle composition and the geometry of Moho and Lithosphere-Asthenosphere Boundary (LAB). Synthetic seismic Vs and Vp velocities match existing seismic tomography models in the area. In this study, depleted mantle compositions are modelled beneath cold and thick lithosphere in Arabian and Turan platforms. A more fertile mantle composition is found in collision zones. Based on our 3D thermochemical model we propose a new scenario to interpret the geodynamical history of area. In this context the present-day central Iran block would be as remain of the older and larger Iranian block present before the onset of Turan platform subduction beneath the Iranian Plateau. Further analysis of sub-lithospheric density anomalies (e.g., subducted slabs) is required to fully understand the geodynamics of the area.

Contributions to the geodynamics of western Canada

NASA Astrophysics Data System (ADS)

Fluck, Paul

Western Canada exhibits a large variation in continental lithosphere from very old rocks in the Canadian Shield across the younger Cordillera to the current accretion of the Yakutat Terrane in the Gulf of Alaska. The geodynamics are driven by the Pacific-North America plate motion resulting in deformation, seismicity, and mountain building across the Canadian Cordillera. The way the lithosphere reacts to deformation or loading depends on its thickness and strength. The effective elastic thickness of the lithosphere, Te , has been estimated in this thesis study using a coherence analysis of Bouguer gravity and topography. There is very thick and strong lithosphere in the old Canadian Shield (Te > 100 km) and thin and weak lithosphere in the Cordillera (Te = 20--30 km). Lithospheric temperature, derived from surface heat flow and upper crust radioactive heat generation, is the most important control on the strength of the lithosphere. Calculated temperatures at the base of the crust are high in the young and hot Cordillera (˜900--1000°C) and very low in the old and cold Craton (˜400--450°C). The depths to the thermally controlled brittle-ductile transition are in general agreement with the Te estimates. The high temperatures in the lower crust and upper mantle of the Cordillera reduce the density by thermal expansion. This thermal isostasy explains the surprising observation of high topography over thin crust. The estimated lithospheric temperatures are used to calculate lithospheric strength profiles. In agreement with the Te estimates, the Cordillera has a weak zone in the lower crust facilitating detachment of the upper crust. Analysis of GPS continuous and campaign data show that the Northern Cordillera is moving at ˜5--10 mm/y in a northward direction driven by the collision of the Yakutat Block in the Gulf of Alaska and is overthrusting the strong lithosphere of the Canadian Shield.* *This dissertation is multimedia (contains text and other applications not available in printed format). The CD requires the following system applications: Internet Browser; Adobe Acrobat; Microsoft Office.

Interaction between mantle and crustal detachments: a non-linear system controlling lithospheric extension

NASA Astrophysics Data System (ADS)

Rosenbaum, G.; Regenauer-Lieb, K.; Weinberg, R. F.

2009-12-01

We use numerical modelling to investigate the development of crustal and mantle detachment faults during lithospheric extension. Our models simulate a wide range of rift systems with varying values of crustal thickness and heat flow, showing how strain localization in the mantle interacts with localization in the upper crust and controls the evolution of extensional systems. Model results reveal a richness of structures and deformation styles, which grow in response to a self-organized mechanism that minimizes the internal stored energy of the system by localizing deformation at different levels of the lithosphere. Crustal detachment faults are well developed during extension of overthickened (60 km) continental crust, even when the initial heat flow is relatively low (50 mW/m2). In contrast, localized mantle deformation is most pronounced when the extended lithosphere has a normal crustal thickness (30-40 km) and an intermediate (60-70 mW/m2) heat flow. Results show a non-linear response to subtle changes in crustal thickness or heat flow, characterized by abrupt and sometime unexpected switches in extension modes (e.g. from diffuse rifting to effective lithospheric-scale rupturing) or from mantle- to crust-dominated strain localization. We interpret this non-linearity to result from the interference of doming wavelengths. Disharmony of crust and mantle doming wavelengths results in efficient communication between shear zones at different lithospheric levels, leading to rupturing of the whole lithosphere. In contrast, harmonious crust and mantle doming inhibits interaction of shear zones across the lithosphere and results in a prolonged rifting history prior to continental breakup.

Lithospheric processes that enhance melting at rifts

NASA Astrophysics Data System (ADS)

Elkins-Tanton, L. T.; Furman, T.

2008-12-01

Continental rifts are commonly sites for mantle melting, whether in the form of ridge melting to create new oceanic crust, or as the locus of flood basalt activity, or in the long initial period of rifting before lavas evolve fully into MORBs. The high topography in the lithosphere-asthenosphere boundary under a rift creates mantle upwelling and adiabatic melting even in the absence of a plume. This geometry itself, however, is conducive to lithospheric instability on the sides of the rifts. Unstable lithosphere may founder into the mantle, producing more complex aesthenospheric convective patterns and additional opportunities to produce melt. Lithospheric instabilities can produce additional adiabatic melting in convection produced as they sink, and they may also devolatilize as they sink, introducing the possibility of flux melting to the rift environment. We call this process upside-down melting, since devolatilization and melting proceed as the foundering lithosphere sinks, rather than while rising, as in the more familiar adiabatic decompression melting. Both adiabatic melting and flux melting would take place along the edges of the rift and may even move magmatism outside the rift, as has been seen in Ethiopia. In volcanism postdating the flood basalts on and adjacent to the Ethiopian Plateau there is evidence for both lithospheric thinning and volatile enrichment in the magmas, potentially consistent with the upside-down melting model. Here we present a physical model for the conjunction of adiabatic decompression melting to produce new oceanic crust in the rift, while lithospheric gravitational instabilities drive both adiabatic and flux melting at its margins.

Evolution of the East African rift: Drip magmatism, lithospheric thinning and mafic volcanism

NASA Astrophysics Data System (ADS)

Furman, Tanya; Nelson, Wendy R.; Elkins-Tanton, Linda T.

2016-07-01

The origin of the Ethiopian-Yemeni Oligocene flood basalt province is widely interpreted as representing mafic volcanism associated with the Afar mantle plume head, with minor contributions from the lithospheric mantle. We reinterpret the geochemical compositions of primitive Oligocene basalts and picrites as requiring a far more significant contribution from the metasomatized subcontinental lithospheric mantle than has been recognized previously. This region displays the fingerprints of mantle plume and lithospheric drip magmatism as predicted from numerical models. Metasomatized mantle lithosphere is not dynamically stable, and heating above the upwelling Afar plume caused metasomatized lithosphere with a significant pyroxenite component to drip into the asthenosphere and melt. This process generated the HT2 lavas observed today in restricted portions of Ethiopia and Yemen now separated by the Red Sea, suggesting a fundamental link between drip magmatism and the onset of rifting. Coeval HT1 and LT lavas, in contrast, were not generated by drip melting but instead originated from shallower, dominantly anhydrous peridotite. Looking more broadly across the East African Rift System in time and space, geochemical data support small volume volcanic events in Turkana (N. Kenya), Chyulu Hills (S. Kenya) and the Virunga province (Western Rift) to be derived ultimately from drip melting. The removal of the gravitationally unstable, metasomatized portion of the subcontinental lithospheric mantle via dripping is correlated in each case with periods of rapid uplift. The combined influence of thermo-mechanically thinned lithosphere and the Afar plume together thus controlled the locus of continental rift initiation between Africa and Arabia and provide dynamic support for the Ethiopian plateau.

Lithospheric controls on the formation of provinces hosting giant orogenic gold deposits

USGS Publications Warehouse

Bierlein, F.P.; Groves, D.I.; Goldfarb, R.J.; Dube, B.

2006-01-01

Ages of giant gold systems (>500 t gold) cluster within well-defined periods of lithospheric growth at continental margins, and it is the orogen-scale processes during these mainly Late Archaean, Palaeoproterozoic and Phanerozoic times that ultimately determine gold endowment of a province in an orogen. A critical factor for giant orogenic gold provinces appears to be thickness of the subcontinental lithospheric mantle (SCLM) beneath a province at the time of gold mineralisation, as giant gold deposits are much more likely to develop in orogens with subducted oceanic or thin continental lithosphere. A proxy for the latter is a short pre-mineralisation crustal history such that thick SCLM was not developed before gold deposition. In constrast, orogens with protracted pre-mineralisation crustal histories are more likely to be characterised by a thick SCLM that is difficult to delaminate, and hence, such provinces will normally be poorly endowed. The nature of the lithosphere also influences the intrinsic gold concentrations of potential source rocks, with back-arc basalts, transitional basalts and basanites enriched in gold relative to other rock sequences. Thus, segments of orogens with thin lithosphere may enjoy the conjunction of giant-scale fluid flux through gold-enriched sequences. Although the nature of the lithosphere plays the crucial role in dictating which orogenic gold provinces will contain one or more giant deposits, the precise siting of those giants depends on the critical conjunction of a number of province-scale factors. Such features control plumbing systems, traps and seals in tectonically and lithospherically suitable terranes within orogens. ?? Springer-Verlag 2006.

The delineation and interpretation of the earth's gravity field

NASA Technical Reports Server (NTRS)

Marsh, Bruce D.

1989-01-01

In an attempt to understand the mechanical interaction of a growing lithosphere containing fracture zones with small and large scale mantle convection, which gives rise to geoid anomalies in oceanic regions, a series of fluid dynamical experiments is in progress to investigate: (1) the influence of lithosphere structure, fluid depth and viscosity field on the onset, scale, and evolution of sublithospheric convection; (2) the role of this convection in determining the rate of growth of lithosphere, especially in light of the flattening of the lithosphere bathymetry and heat flow at late times; and (3) combining the results of both numerical and laboratory experiments to decide the dominate factors in producing geoid anomalies in oceanic regions through the thermo-mechanical interaction of the lithosphere and subjacent mantle. The clear existence of small scale convection associated with a downward propagating solidification front (i.e., the lithosphere) and a larger scale flow associated with a discontinuous upward heat flux (i.e., a fracture zone) has been shown. The flows exist simultaneously and each may have a significant role in deciding the thermal evolution of the lithosphere and in understanding the relation of shallow mantle convection to deep mantle convection. This overall process is reflected in the geoid, gravity, and topographic anomalies in the north-central Pacific. These highly correlated fields of intermediate wavelength (approx. 200 to 2000 km) show isostatic compensation by a thin lithosphere for shorter (less than or equal to approx. 500 km), but not the longer, wavelengths. The ultimate, dynamic origin of this class of anomalies is being investigated.

Lithospheric-Mantle Structure of the Kaapvaal Craton, South Africa, Derived From Thermodynamically Self-Consistent Modelling of Seismic Surface-Wave and S-wave Receiver Function, Heat-flow, Elevation, Xenolith and Magnetotelluric Observations

NASA Astrophysics Data System (ADS)

Muller, M. R.; Fullea, J.; Jones, A. G.; Adam, J.; Lebedev, S.; Piana Agostinetti, N.

2012-12-01

Results from recent geophysical and mantle-xenolith geochemistry studies of the Kaapvaal Craton appear, at times, to provide disparate views of the physical, chemical and thermal structure of the lithosphere. Models from our recent SAMTEX magnetotelluric (MT) surveys across the Kaapvaal Craton indicate a resistive, 220-240 km thick lithosphere for the central core of the craton. One published S-wave receiver function (SRF) study and other surface-wave studies suggest a thinner lithosphere characterised by a ~160 km thick high-velocity "lid" underlain by a low-velocity zone (LVZ) of between 65-150 km in thickness. Other seismic studies suggest that the (high-velocity) lithosphere is thicker, in excess of 220 km. Mantle xenolith pressure-temperature arrays from Mesozoic kimberlites require that the base of the "thermal" lithosphere (i.e., the depth above which a conductive geotherm is maintained) is at least 220 km deep, to account for mantle geotherms in the range 35-38 mWm-2. Richly diamondiferous kimberlites across the Kaapvaal Craton require a lithospheric thickness substantially greater than 160 km - the depth of the top of the diamond stability field. In this paper we use the recently developed LitMod software code to derive, thermodynamically consistently, a range of 1-D seismic velocity, density, electrical resistivity and temperature models from layered geochemical models of the lithosphere based on mantle xenolith compositions. In our work, the "petrological" lithosphere-asthenosphere boundary (pLAB) (i.e., the top of the fertile asthenospheric-mantle) and the "thermal" LAB (tLAB as defined above) are coincident. Lithospheric-mantle models are found simultaneously satisfying all geophysical observables: new surface-wave dispersion data, published SRFs, MT responses, surface elevation and heat-flow. Our results show: 1. All lithospheric-mantle models are characterised by a seismic LVZ with a minimum velocity at the depth of the petrological/thermal LAB. The top of the LVZ does not correspond with the LAB. 2. Thin (~160 km-thick) lithospheric-mantle models are consistent with surface elevation and heat-flow observations only for unreasonably low average crustal heat production values (~0.4 μWm-3). However, such models are inconsistent both with the surface-wave dispersion data and youngest (Group I) palaeo-geotherms defined by xenolith P-T arrays. 3. A three-layered geochemical model (consistent with mantle xenoliths), with lithospheric thickness in excess of 220 km, is required to match all geophysical constraints. 4. The chemical transition from a depleted harzburgitic composition (above) to a refertilised high-T lherzolitic composition (below) at 160 km depth produces a sharp onset of the seismic LVZ and a sharp increase in density. Synthetic SRFs will assess whether this chemical transition may account for the reported S-to-P conversion event at 160 km depth. However, in this this instance the SRF conversion event would not represent the petrological/thermal LAB.

Lithospheric-Mantle Structure of the Kaapvaal Craton, South Africa, Derived from Thermodynamically Self-Consistent Modelling of Magnetotelluric, Surface-Wave Dispersion, S-wave Receiver Function, Heat-flow, Elevation and Xenolith Observations

NASA Astrophysics Data System (ADS)

Muller, Mark; Fullea, Javier; Jones, Alan G.; Adam, Joanne; Lebedev, Sergei; Piana Agostinetti, Nicola

2013-04-01

Results from recent geophysical and mantle-xenolith geochemistry studies of the Kaapvaal Craton appear, at times, to provide disparate views of the physical, chemical and thermal structure of the lithosphere. Models from our recent SAMTEX magnetotelluric (MT) surveys across the Kaapvaal Craton indicate a resistive, 220-240 km thick lithosphere for the central core of the craton. One published S-wave receiver function (SRF) study and other surface-wave studies suggest a thinner lithosphere characterised by a ~160 km thick high-velocity "lid" underlain by a low-velocity zone (LVZ) of between 65-150 km in thickness. Other seismic studies suggest that the (high-velocity) lithosphere is thicker, in excess of 220 km. Mantle xenolith pressure-temperature arrays from Mesozoic kimberlites require that the base of the "thermal" lithosphere (i.e., the depth above which a conductive geotherm is maintained - the tLAB) is at least 220 km deep, to account for mantle geotherms in the range 35-38 mWm-2. Richly diamondiferous kimberlites across the Kaapvaal Craton require a lithospheric thickness substantially greater than 160 km - the depth of the top of the diamond stability field. In this paper we use the recently developed LitMod software code to derive, thermodynamically consistently, a range of 1-D electrical resistivity, seismic velocity, density and temperature models from layered geochemical models of the lithosphere based on mantle xenolith compositions. In our work, the "petrological" lithosphere-asthenosphere boundary (pLAB) (i.e., the top of the fertile asthenospheric-mantle) and the "thermal" LAB (tLAB) are coincident. Lithospheric-mantle models are found simultaneously satisfying all geophysical observables: MT responses, new surface-wave dispersion data, published SRFs, surface elevation and heat-flow. Our results show: 1. All lithospheric-mantle models are characterised by a seismic LVZ with a minimum velocity at the depth of the petrological/thermal LAB. The top of the LVZ does not correspond with the LAB. 2. Thin (~160 km-thick) lithospheric-mantle models are consistent with surface elevation and heat-flow observations only for unreasonably low average crustal heat production values (~0.4 µWm-3). However, such models are inconsistent both with the surface-wave dispersion data and youngest (Group I) palaeo-geotherms defined by xenolith P-T arrays. 3. A three-layered geochemical model, with lithospheric thickness in excess of 230 km, is required to match all geophysical and xenolith constraints. 4. The chemical transition from a depleted harzburgitic composition (above) to a refertilised high-T lherzolitic composition (below) at 160 km depth produces a sharp onset of the seismic LVZ and a sharp increase in density. Synthetic SRFs indicate that this chemical transition is able to account for the reported S-to-P conversion event at 160 km depth. In this this instance the 160 km deep SRF event does not represent the petrological/thermal LAB.

Thermal erosion of cratonic lithosphere as a potential trigger for mass-extinction

PubMed Central

Guex, Jean; Pilet, Sebastien; Müntener, Othmar; Bartolini, Annachiara; Spangenberg, Jorge; Schoene, Blair; Sell, Bryan; Schaltegger, Urs

2016-01-01

The temporal coincidence between large igneous provinces (LIPs) and mass extinctions has led many to pose a causal relationship between the two. However, there is still no consensus on a mechanistic model that explains how magmatism leads to the turnover of terrestrial and marine plants, invertebrates and vertebrates. Here we present a synthesis of ammonite biostratigraphy, isotopic data and high precision U-Pb zircon dates from the Triassic-Jurassic (T-J) and Pliensbachian-Toarcian (Pl-To) boundaries demonstrating that these biotic crises are both associated with rapid change from an initial cool period to greenhouse conditions. We explain these transitions as a result of changing gas species emitted during the progressive thermal erosion of cratonic lithosphere by plume activity or internal heating of the lithosphere. Our petrological model for LIP magmatism argues that initial gas emission was dominated by sulfur liberated from sulfide-bearing cratonic lithosphere before CO2 became the dominant gas. This model offers an explanation of why LIPs erupted through oceanic lithosphere are not associated with climatic and biotic crises comparable to LIPs emitted through cratonic lithosphere. PMID:27009463

Crustal and lithospheric structure of the Alborz Mountains, Iran, and surrounding areas from integrated geophysical modeling

NASA Astrophysics Data System (ADS)

Motavalli-Anbaran, Seyed-Hani; Zeyen, Hermann; Brunet, Marie-FrançOise; Ardestani, Vahid Ebrahimzadeh

2011-10-01

Using gravity, geoid, topography and surface heat flow data, we have modeled the density and temperature distribution in the lithosphere along three profiles crossing Iran in SW-NE direction from the Arabian foreland in the SW to the South Caspian Basin and the Turan Platform to the NE. We find thin lithosphere (100-120 km) underneath central Iran, whereas thick lithosphere (up to 240 km) is found underneath Arabia, the South Caspian Basin and the Turan Platform. Crustal thickening is found under the Zagros and Alborz mountains (up to 60 km) and under the Kopet-Dagh Mountains (48 km), whereas the thin crust under the southern Caspian Sea is either an oceanic crust or a highly thinned continental one. Below the South Caspian Sea, the shape of the crust-mantle interface and the base of the lithosphere indicate a subduction of the South Caspian block toward the N-NW. Further east, under the Kopet-Dagh, no evidence for active subduction is visible. This can be explained by a rheologically very strong South Caspian block, surrounded by weaker continental lithosphere.

Thermoelastic stress in oceanic lithosphere due to hotspot reheating

NASA Technical Reports Server (NTRS)

Zhu, Anning; Wiens, Douglas A.

1991-01-01

The effect of hotspot reheating on the intraplate stress field is investigated by modeling the three-dimensional thermal stress field produced by nonuniform temperature changes in an elastic plate. Temperature perturbations are calculated assuming that the lithosphere is heated by a source in the lower part of the thermal lithosphere. A thermal stress model for the elastic lithosphere is calculated by superposing the stress fields resulting from temperature changes in small individual elements. The stress in an elastic plate resulting from a temperature change in each small element is expressed as an infinite series, wherein each term is a source or an image modified from a closed-from half-space solution. The thermal stress solution is applied to midplate swells in oceanic lithosphere with various thermal structures and plate velocities. The results predict a stress field with a maximum deviatoric stress on the order of 100 MPa covering a broad area around the hotspot plume. The predicted principal stress orientations show a complicated geographical pattern, with horizontal extension perpendicular to the hotspot track at shallow depths and compression along the track near the bottom of the elastic lithosphere.

The Formation of Laurentia: Evidence from Shear Wave Splitting and Seismic Tomography

NASA Astrophysics Data System (ADS)

Liddell, M. V.; Bastow, I. D.; Gilligan, A.; Darbyshire, F. A.; Pugh, S.

2016-12-01

Earth conditions during the Precambrian, and their effect on plate tectonic processes during that era, are not fully understood; how the fast wave-speed cratonic roots, or keels, often found beneath these regions were formed is also debated. The geological record of northern Hudson Bay includes the 1.8Ga TransHudson Orogen (THO) terrane, a remnant of the Paleoproterozoic collision of the Archean Rae and Churchill domains that overlies one of Earth's largest cratonic keels. This region is thus an ideal natural laboratory for the study of Precambrian processes. We use broadband seismological data recorded at 65 stations in northern Hudson Bay to address questions regarding the manner and scale of plate deformation during Precambrian assembly of the region. To explore these questions, we conduct a study of mantle seismic anisotropy using SKS splitting. Our study constitutes the most complete shear wave splitting analysis of northern Canada to date utilising up to 11 years of data for some stations. Anisotropic fast directions (φ) and delay times (δt) are determined using a modified Silver and Chan (1991) method. In the Hudson Strait, φ directions parallel the THO, while the islands in northern Hudson Bay show changes in φ over length scales short enough to indicate lithospheric origin. Complex anisotropy patterns are observed in the central Rae craton and southern Baffin Island, suggesting multiple sources. Several possible sources of anisotropy are explored, including basal drag of the North American plate, mantle flow deflected by the Laurentian keel, and discontinuities associated with possible two-stage keel development P and S-wave relative arrival-time tomographic velocity models are also presented. Waveforms are aligned using the adaptive stacking routine of Rawlinson et al. (2004), and models are produced using the Fast Marching Tomography inversion code of Rawlinson et al. (2006). The THO is modeled as a low velocity feature relative to the neighbouring Archean cratons, corroborating previous studies that interpret Proterozoic under-thrusting of the Superior beneath southern Baffin Island (e.g. St. Onge et al., 2006). The interpretation of the mantle velocity structure and the SKS splitting results presented here contribute to a complex and ongoing story about the Precambrian assembly of Laurentia

Flat-slab subduction, whole crustal faulting, and geohazards in Alaska: Targets for Earthscope

NASA Astrophysics Data System (ADS)

Gulick, S. P.; Pavlis, T. L.; Bruhn, R. L.; Christeson, G. L.; Freymueller, J. T.; Hansen, R. A.; Koons, P. O.; Pavlis, G. L.; Roeske, S.; Reece, R.; van Avendonk, H. J.; Worthington, L. L.

2010-12-01

Crustal structure and evolution illuminated by the Continental Dynamics ST. Elias Erosion and tectonics Project (STEEP) highlights some fundamental questions about active tectonics processes in Alaska including: 1) what are the controls on far field deformation and lithospheric stabilization, 2) do strike slip faults extend through the entire crust and upper mantle and how does this influence mantle flow, and 3) how does the transition from “normal” subduction of the Pacific along the Aleutians to flat slab subduction of the Yakutat Terrane beneath southeast and central Alaska to translation of the Yakutat Terrane past North American in eastern Alaska affect geohazard assessment for the north Pacific? Active and passive seismic studies and geologic fieldwork focusing on the Yakutat Terrane show that the Terrane ranges from 15-35 km thick and is underthrusting the North American plate from the St. Elias Mountains to the Alaska Range (~500 km). Deformation of the upper plate occurs within the offshore Pamplona Zone fold and thrust belt, and onshore throughout the Robinson Mountains. Deformation patterns, structural evolution, and the sedimentary products of orogenesis are fundamentally influenced by feedbacks with glacial erosion. The Yakutat megathrust extends beneath Prince William Sound such that the 1964 Mw 9.2 great earthquake epicenter was on this plate boundary and jumped to the adjacent Aleutian megathrust coseismically; this event illuminates the potential for transitional tectonic systems to enhance geohazards. The northern, southern, and eastern limits of the Yakutat microplate are strike-slip faults that, where imaged, appear to cut the entire crustal section and may allow for crustal extrusion towards the Bering Sea. Yakutat Terrane effects on mantle flow, however, have been suggested to cross these crustal features to allow for far-field deformation in the Yukon, Brooks Range, and Amerasia Basin. From the STEEP results it is clear that the Yakutat Terrane is driving a range of tectonic and surface processes perturbing the Aleutian subduction system at its eastern extent and linking this system with Laramide style subduction and plate boundary strike-slip tectonics farther east. Targeted geodetic and seismic deployments as part of Earthscope could examine all of these features and seek to address fundamental questions about tectonic interactions.

Metamorphic sole formation reveals plate interface rheology during early subduction

NASA Astrophysics Data System (ADS)

Mathieu, S.; Agard, P.; Dubacq, B.; Plunder, A.; Prigent, C.

2015-12-01

Metamorphic soles are m to ~500m thick tectonic slices welded beneath most large ophiolites. They correspond to highly to mildly deformed portions of oceanic lithosphere metamorphosed at amphibolite to granulite facies peak conditions. Metamorphic soles are interpreted as formed ≤1-2Ma after intraoceanic subduction initiation by heat transfer from the hot, incipient mantle wegde to the underthrusting lower plate. Their early accretion and exhumation together with the future ophiolite implies at least one jump of the subduction plate interface from above to below the metamorphic sole. Metamorphic soles thus represent one of the few remnants of the very early evolution of the subduction plate interface and provide major constraints on the thermal structure and the effective rheology of the crust and mantle along the nascent slab interface.We herein present a structural and petrological detailed description of the Oman and Turkey metamorphic soles. Both soles present a steep inverted metamorphic structure, with isograds subparallel to the peridotite contact, in which the proportion of mafic rocks, pressure and temperature conditions increase upward. They comprise, as most metamorphic soles worldwide, two main units: (1) a high-grade unit adjacent to the overlying peridotite composed of granulitized to amphibolized metabasalts, with rare metasedimentary interlayers (~800±100ºC at 10±2kbar) and (2) a low-grade greenschist facies unit composed of metasedimentary rocks with rare metatuffs (~500±100ºC at 5±2kbar). We provide for the first time refined P-T peak condition estimations by means of pseudosection modelling and maximum temperature constraints for the Oman low-grade sole by RAMAN thermometry. In order to quantify micro-scale deformations trough the sole, we also present EBSD data on the Oman garnet-bearing and garnet-free high-grade sole.With these new constraints, we finally propose a new conceptual mechanical model for metamorphic sole formation. This model excludes the presence of a continuous inverted metamorphic gradient through the sole but implies the stacking of several homogeneous slivers to constitute the present structure of the sole. These successive thrusts are the result of rheological changes as the plate interface progressively cools.

Interseismic coupling, seismic potential and earthquake recurrence on the southern front of the Eastern Alps (NE Italy)

NASA Astrophysics Data System (ADS)

Cheloni, Daniele; D'Agostino, Nicola; Selvaggi, Giulio

2014-05-01

The interaction of the African, Arabian, and Eurasia plates in the "greater" Mediterranean region yields to a broad range of tectonic processes including active subduction, continental collision, major continental strike-slip faults and "intra-plate" mountain building. In this puzzling region the convergence between Adria microplate and Eurasia plate is partly or entirely absorbed within the South-Eastern Alps, where the Adriatic lithosphere underthrusts beneath the mountain belt. Historical seismicity and instrumentally recorded earthquakes show thrust faulting on north-dipping low-angle faults in agreement with geological observations of active mountain building and active fold growing at the foothills of the South-Eastern Alps. In this study, we use continuous GPS observations to document the geodetic strain accumulation across the South-Eastern Alps (NE Italy). We estimate the pattern of interseismic coupling on the intra-continental collision north-dipping thrust faults that separate the Eastern Alps and the Venetian-Friulian plain using the back-slip approach and discuss the seismic potential and earthquake recurrence. Comparison between the rigid-rotation predicted motion and the shortening observed across the studied area indicates that the South-Eastern Alpine thrust front absorbs about 80% of the total convergence rate between the Adria microplate and Eurasia plate. The modelled thrust fault is currently locked from the surface to a depth of approximately 10 km. The transition zone between locked and creeping portions of the fault roughly corresponds with the belt of microseismicity parallel and to the north of the mountain front. The estimated moment deficit rate is 1.27±0.14×10^17 Nm/yr. The comparison between the estimated moment deficit and that released historically by the earthquakes suggests that to account for the moment deficit the following two factors or their combination should be considered: (1) a significant part of the observed interseismic coupling is released aseismically by folding or creeping; (2) infrequent "large" events with long return period (>1000 years) and with magnitudes larger than the value assigned to the largest historical events (Mw≡6.7).

Limits on modes of lithospheric heat transport on Venus from impact crater density

NASA Technical Reports Server (NTRS)

Grimm, Robert E.; Solomon, Sean C.

1987-01-01

Based on the observed density of impact craters on the Venus surface obtained from Venera 15-16 radar images, a formalism to estimate the upper bounds on the contributions made to lithospheric heat transport by volcanism and lithospheric recycling is presented. The Venera 15-16 data, if representative of the entire planet, limit the average rate of volcanic resurfacing on Venus to less than 2 cu km/yr (corresponding to less than 1 percent of the global heat loss), and limit the rate of lithospheric recycling to less than 1.5 sq km/yr (and probably to less than 0.5 sq km/yr), corresponding to 25 percent (and to 9 percent) of the global heat loss. The present results indicate that heat loss at lithospheric levels in Venus is dominated by conduction.

Paleoproterozoic Collisional Structures in the Hudson Bay Lithosphere Constrained by Multi-Observable Probabilistic Inversion

NASA Astrophysics Data System (ADS)

Darbyshire, F. A.; Afonso, J. C.; Porritt, R. W.

2015-12-01

The Paleozoic Hudson Bay intracratonic basin conceals a Paleoproterozoic Himalayan-scale continental collision, the Trans-Hudson Orogen (THO), which marks an important milestone in the assembly of the Canadian Shield. The geometry of the THO is complex due to the double-indentor geometry of the collision between the Archean Superior and Western Churchill cratons. Seismic observations at regional scale show a thick, seismically fast lithospheric keel beneath the entire region; an intriguing feature of recent models is a 'curtain' of slightly lower wavespeeds trending NE-SW beneath the Bay, which may represent the remnants of more juvenile material trapped between the two Archean continental cores. The seismic models alone, however, cannot constrain the nature of this anomaly. We investigate the thermal and compositional structure of the Hudson Bay lithosphere using a multi-observable probabilistic inversion technique. This joint inversion uses Rayleigh wave phase velocity data from teleseismic earthquakes and ambient noise, geoid anomalies, surface elevation and heat flow to construct a pseudo-3D model of the crust and upper mantle. Initially a wide range of possible mantle compositions is permitted, and tests are carried out to ascertain whether the lithosphere is stratified with depth. Across the entire Hudson Bay region, low temperatures and a high degree of chemical depletion characterise the mantle lithosphere. Temperature anomalies within the lithosphere are modest, as may be expected from a tectonically-stable region. The base of the thermal lithosphere lies at depths of >250 km, reaching to ~300 km depth in the centre of the Bay. Lithospheric stratification, with a more-depleted upper layer, is best able to explain the geophysical data sets and surface observables. Some regions, where intermediate-period phase velocities are high, require stronger mid-lithospheric depletion. In addition, a narrow region of less-depleted material extends NE-SW across the Bay, likely associated with the trace of the THO collision and the entrapment of juvenile material between the highly-depleted Archean cores.

Repeat ridge jumps associated with plume-ridge interaction, melt transport, and ridge migration

NASA Astrophysics Data System (ADS)

Mittelstaedt, Eric; Ito, Garrett; van Hunen, Jeroen

2011-01-01

Repeated shifts, or jumps, of mid-ocean ridge segments toward nearby hot spots can produce large, long-term changes to the geometry and location of the tectonic plate boundaries. Ridge jumps associated with hot spot-ridge interaction are likely caused by several processes including shear on the base of the plate due to expanding plume material as well as reheating of lithosphere as magma passes through it to feed off-axis volcanism. To study how these processes influence ridge jumps, we use numerical models to simulate 2-D (in cross section) viscous flow of the mantle, viscoplastic deformation of the lithosphere, and melt migration upward from the asthenospheric melting zone, laterally along the base of the lithosphere, and vertically through the lithosphere. The locations and rates that magma penetrates and heats the lithosphere are controlled by the time-varying accumulation of melt beneath the plate and the depth-averaged lithospheric porosity. We examine the effect of four key parameters: magmatic heating rate of the lithosphere, plate spreading rate, age of the seafloor overlying the plume, and the plume-ridge migration rate. Results indicate that the minimum value of the magmatic heating rate needed to initiate a ridge jump increases with plate age and spreading rate. The time required to complete a ridge jump decreases with larger values of magmatic heating rate, younger plate age, and faster spreading rate. For cases with migrating ridges, models predict a range of behaviors including repeating ridge jumps, much like those exhibited on Earth. Repeating ridge jumps occur at moderate magmatic heating rates and are the result of changes in the hot spot magma flux in response to magma migration along the base of an evolving lithosphere. The tendency of slow spreading to promote ridge jumps could help explain the observed clustering of hot spots near the Mid-Atlantic Ridge. Model results also suggest that magmatic heating may significantly thin the lithosphere, as has been suggested at Hawaii and other hot spots.

Lithospheric structure of Africa: insights from its effective elastic thickness variations.

NASA Astrophysics Data System (ADS)

Pérez-Gussinyé, M.; Metois, M.; Fernández, M.; Vergés, J.; Fullea, J.

2009-04-01

Detailed images of lithospheric structure can help understand how surface deformation is related to Earth's deep structure. A proxy for lithospheric structure is its effective elastic thickness, Te, which mainly depends on its thermal state and composition. We present a new effective elastic thickness, Te, map of the African lithosphere estimated using the coherence function between topography and Bouguer anomaly. The Bouguer anomaly used in this study derives from the EGM 2008 model, which constitutes the highest resolution gravity database over Africa, allowing a significant improvement on lateral resolution in Te. Our map shows that Te is high > 100 km, in the West African, Congo, Kalahari and Tanzania cratons. Of these, the Kalahari presents the thinnest elastic thicknesses and, based on additional seismic and mineral physics studies, we suggest this may reflect modification of the lithosphere by anomalously hot mantle beneath the lithosphere. The effective elastic thickness is lowest beneath the Afar and Main Ethiopian rifts, where the maximum extension and thinnest lithosphere of Africa occur. The Tanzania craton appears as two rigid blocks separated by a relatively low Te area located southwest of lake Victoria. This coincides with the centre of seismic radial anisotropy beneath the craton, suggested to be the Victoria plume head by Weertrane et al. [2003]. Along the eastern branch of the East African rift Te is low and increases abruptly at 2 to 3 degrees South, coinciding with a deepening of earthquake depocenter and a change from narrow to wide rifting. These and other considerations suggest that the southern part of the eastern branch is underlain by thick, rigid cratonic lithosphere. Finally, the northern part of Africa is characterised by low Te on the Darfur, Tibesti, Hoggar and Cameroon line volcanic provinces, suggesting that the underlying lithospheric mantle has been thermally thinned. Corridors of low Te connect these volcanic provinces, supporting the idea that hot mantle flows between them. However, a thin corridor between these volcanic provinces and the Afar hot-spot is less obvious.

Geophysical Investigations of Crustal and Upper Mantle Structure of Oceanic Intraplate Volcanoes (OIVs)

NASA Astrophysics Data System (ADS)

Robinson, A. H.; Peirce, C.; Funnell, M.; Watts, A. B.; Grevemeyer, I.

2016-12-01

Oceanic intraplate volcanoes (OIVs) represent a record of the modification of the oceanic crust by volcanism related to a range of processes including hot-spots, small scale mantle convection, and localised lithospheric extension. Geophysical studies of OIVs show a diversity in crustal and upper mantle structures, proposed to exist on a spectrum between two end-members where the main control is the age of the lithosphere at the time of volcanism. This hypothesis states that where the lithosphere is older, colder, and thicker it is more resistant to vertical magmatism than younger, hotter, thinner lithosphere. It is suggested that the Moho acts as a density filter, permitting relatively buoyant magma to vertically intrude the crust, but preventing denser magma from ascending to shallow levels. A key control may therefore be the melting depth, known to affect magma composition, and itself related to lithosphere age. Combined geophysical approaches allow us to develop robust models for OIV crustal structures with quantifiable resolution and uncertainty. As a case study, we present results from a multi-approach geophysical experiment at the Louisville Ridge Seamount Chain, believed to have formed on young (<10 Ma) lithosphere, which aimed at characterising the along-ridge crustal structure. The wide-angle seismic crustal model, generated by independent forward and inverse travel-time modelling of picked arrivals, is tested against reflection and gravity data. We compare our observations with studies of other OIVs to test whether lithospheric age controls OIV structure. Comparisons are limited by the temporal and spatial distribution of lithosphere and volcano ages, but suggest the hypothesis does not hold for all OIV features. While age may be the main control on OIV structure, as it determines lithosphere thermal and mechanical properties, other factors such as thermal rejuvenation, mechanical weakening, and volcano load size and distribution, may also come into play.

Orogenic plateau magmatism of the Arabia-Eurasia collision zone

NASA Astrophysics Data System (ADS)

Allen, M. B.; Neill, I.; Kheirkhah, M.; van Hunen, J.; Davidson, J. P.; Meliksetian, Kh.; Emami, M. H.

2012-04-01

Magmatism is a common feature of high plateaux created during continental collision, but the causes remain enigmatic. Here we study Pliocene-Quaternary volcanics from the active Arabia-Eurasia collision zone, to determine the chemistry of these rocks and their relations to faulting and deeper lithospheric structure. The great majority of the centres lie within the overriding Eurasian plate in Iran, eastern Turkey and Armenia , implying that mantle fertilised by pre-collision subduction processes plays a significant role in magma generation. The composition of the Pliocene-Quaternary centres is extremely variable, ranging from OIB-like alkali basalts, to intermediate types resembling mature continental arc lavas, to potassic and even ultrapotassic lavas. These centres are erupted across a mosaic of pre-Cenozoic suture zones and heterogeneous lithospheric blocks. The chemical diversity implies a range of partial melting conditions operating on lithospheric and perhaps sub-lithospheric sources. Published data show a thick (>200 km) lithospheric keel beneath the Arabia-Eurasia suture, thinning to near normal thicknesses (~120 km) across much of central and northern Iran. Thin mantle lithosphere under eastern Turkey (max. ~30 km) may relate to the region's juvenile, accretionary lithosphere. These variable thicknesses are constraints on the cause of the melting in each area, and the degree of variation suggests that no one mechanism applies across the plateau. Various melting models have been suggested. Break-off of the subducted Neo-Tethyan oceanic slab is supported by tomographic data, which may have permitted melting related to adiabatic ascent of hot asthenosphere under areas where the lithosphere is thin. This seems a less plausible mechanism where the lithosphere is at normal or greater than normal thickness. The same problem applies to postulated lower lithosphere delamination. Isolated pull-aparts may account for the location of some centres, but are not generally applicable as melt triggers. Enigmatic lavas are erupted over the thick lithosphere of Kurdistan Province, Iran. These alkali basalts and basanites have the chemical characteristics of small degree (<1%) melts in the garnet stability field. Most possess supra-subduction zone chemistry (La/Nb = 1-3), but this signature is highly variable. Similar La/Nb variability occurs in the basic lavas of Damavand volcano in the Alborz Mountains of northern Iran. Modelling suggests the depletion of residual amphibole during the progression of partial melting can explain the observed La/Nb range. This melting may occur as the result of lithospheric thickening. At depths of ~90 km, amphibole-bearing peridotite crosses an experimentally-determined "backbend" in its solidus. Melting can continue while the source remains hydrated. Such "compression" melting may apply to parts of other orogenic plateaux, including Tibet.

The Stability of Tibetan Mantle Lithosphere

NASA Astrophysics Data System (ADS)

Houseman, Gregory; England, Philip

2017-04-01

The large area of thickened crust beneath the Tibetan Plateau is a consequence of sustained continental convergence between India and the Eurasian land mass during the last 50 m.y. Although the Tibetan crust has thickened, there has been much debate about the consequences for its sub-crustal mantle lithosphere. The onset of crustal thinning in the late Miocene appears to require an increase in the gravitational potential energy of the plateau at that time. One explanation for that increase depended on the idea that the mantle lithosphere beneath Tibet had been replaced by asthenosphere, either by some form of convective thinning or by a delamination process akin to retreating subduction acting on the unstable lithospheric mantle layer. Such ideas seem consistent with the history of magmatism and volcanism on the plateau. However, the dispersion of surface waves crossing the plateau implies that a relatively cold and fast layer of mantle remains beneath the plateau to depths of at least 250 km. Because the surface wave data appear inconsistent with the idea that mantle lithosphere has been removed, we investigate an alternative explanation that could explain the apparent increase in gravitational potential energy of the Tibetan lithosphere. If that mantle lithosphere has remained largely in place due to an intrinsic compositional buoyancy but, on thickening, has become unstable to an internal convective overturn, then: (1) mantle material at near asthenospheric temperatures would be emplaced below the crust, and (2) colder mantle from beneath the Moho could become stranded above about 250 km depth. This mechanism is feasible if the Tibetan sub-continental mantle lithosphere is depleted and intrinsically less dense than the underlying asthenosphere. The mechanism is broadly consistent with the surface wave analyses (which cannot resolve the short horizontal wavelengths on which overturn is likely to occur), and it predicts the kind of short-wavelength variations that are revealed by body-wave tomography. The thermal re-equilibration of the disturbed lithosphere may take 100s of m.y. but there is a rapid transient transfer of heat as the coldest parts of the mantle lithosphere are juxtaposed with the asthenosphere and the hotter parts juxtaposed with the base of the crust. Heat transfer at the base of the lithosphere could explain a short-term uplift of the surface ( 500 m in 10 m.y.). Heat transfer at the Moho could cause lower-crustal melting and volcanism, and could trigger retrograde metamorphic reactions in the lowermost crust that would contribute to further uplift. The increase in gravitational potential energy of the lithosphere associated with surface uplift thereby can explain the onset of extension in the plateau.

Lithospheric Architecture Beneath Hudson Bay

NASA Astrophysics Data System (ADS)

Porritt, R. W.; Miller, M. S.; Darbyshire, F. A.

2015-12-01

Hudson Bay overlies some of the thickest Precambrian lithosphere on Earth, whose internal structures contain important clues to the earliest workings of plate formation. The terminal collision, the Trans-Hudson Orogen, brought together the Western Churchill craton to the northwest and the Superior craton to the southeast. These two Archean cratons along with the Paleo-Proterozoic Trans-Hudson internides, form the core of the North American craton. We use S to P converted wave imaging and absolute shear velocity information from a joint inversion of P to S receiver functions, new ambient noise derived phase velocities, and teleseismic phase velocities to investigate this region and determine both the thickness of the lithosphere and the presence of internal discontinuities. The lithosphere under central Hudson Bay approaches 􏰂350 km thick but is thinner (􏰂200-250 km) around the periphery of the Bay. Furthermore, the amplitude of the lithosphere-asthenosphere boundary (LAB) conversion from the S receiver functions is unusually large for a craton, suggesting a large thermal contrast across the LAB, which we interpret as direct evidence of the thermal insulation effect of continents on the asthenosphere. Within the lithosphere, midlithospheric discontinuities, significantly shallower than the base of the lithosphere, are often imaged, suggesting the mechanisms that form these layers are common. Lacking time-history information, we infer that these discontinuities reflect reactivation of formation structures during deformation of the craton.

Impact of rheological layering on rift asymmetry

NASA Astrophysics Data System (ADS)

Jaquet, Yoann; Schmalholz, Stefan M.; Duretz, Thibault

2015-04-01

Although numerous models of rift formation have been proposed, what triggers asymmetry of rifted margins remains unclear. Parametrized material softening is often employed to induce asymmetric fault patterns in numerical models. Here, we use thermo-mechanical finite element models that allow softening via thermal weakening. We investigate the importance of lithosphere rheology and mechanical layering on rift morphology. The numerical code is based on the MILAMIN solver and uses the Triangle mesh generator. Our model configuration consists of a visco-elasto-platic layered lithosphere comprising either (1) only one brittle-ductile transition (in the mantle) or (2) three brittle-ductile transitions (one in the upper crust, one in the lower crust and one in the mantle). We perform then two sets of simulations characterized by low and high extensional strain rates (5*10-15 s-1, 2*10-14 s-1). The results show that the extension of a lithosphere comprising only one brittle-ductile transition produces a symmetric 'neck' type rift. The upper and lower crusts are thinned until the lithospheric mantle is exhumed to the seafloor. A lithosphere containing three brittle-ductile transitions favors strain localization. Shear zones at different horizontal locations and generated in the brittle levels of the lithosphere get connected by the weak ductile layers. The results suggest that rheological layering of the lithosphere can be a reason for the generation of asymmetric rifting and subsequent rift morphology.

The role of elastic stored energy in controlling the long term rheological behaviour of the lithosphere

NASA Astrophysics Data System (ADS)

Regenauer-Lieb, Klaus; Weinberg, Roberto F.; Rosenbaum, Gideon

2012-04-01

The traditional definition of lithospheric strength is derived from the differential stresses required to form brittle and ductile structures at a constant strain rate. This definition is based on dissipative brittle and ductile deformation and does not take into account the ability of the lithosphere to store elastic strain. Here we show the important role of elasticity in controlling the long-term behaviour of the lithosphere. This is particularly evident when describing deformation in a thermodynamic framework, which differentiates between stored (Helmholtz free energy) and dissipative (entropy) energy potentials. In our model calculations we stretch a continental lithosphere with a wide range of crustal thickness (30-60 km) and heat flow (50-80 mW/m2) at a constant velocity. We show that the Helmholtz free energy, which in our simple calculation describes the energy stored elastically, converges for all models within a 25% range, while the dissipated energy varies over an order of magnitude. This variation stems from complex patterns in the local strain distributions of the different models, which together operate to minimize the Helmholtz free energy. This energy minimization is a fundamental material behaviour of the lithosphere, which in our simple case is defined by its elastic properties. We conclude from this result that elasticity (more generally Helmholtz free energy) is an important regulator of the long-term geological strength of the lithosphere.

Density, temperature, and composition of the North American lithosphere—New insights from a joint analysis of seismic, gravity, and mineral physics data: 2. Thermal and compositional model of the upper mantle

NASA Astrophysics Data System (ADS)

Tesauro, Magdala; Kaban, Mikhail K.; Mooney, Walter D.; Cloetingh, Sierd A. P. L.

2014-12-01

Temperature and compositional variations of the North American (NA) lithospheric mantle are estimated using a new inversion technique introduced in Part 1, which allows us to jointly interpret seismic tomography and gravity data, taking into account depletion of the lithospheric mantle beneath the cratonic regions. The technique is tested using two tomography models (NA07 and SL2013sv) and different lithospheric density models. The first density model (Model I) reproduces the typical compositionally stratified lithospheric mantle, which is consistent with xenolith samples from the central Slave craton, while the second one (Model II) is based on the direct inversion of the residual gravity and residual topography. The results obtained, both in terms of temperature and composition, are more strongly influenced by the input models derived from seismic tomography, rather than by the choice of lithospheric density Model I versus Model II. The final temperatures estimated in the Archean lithospheric root are up to 150°C higher than in the initial thermal models obtained using a laterally and vertically uniform "fertile" compositional model and are in agreement with temperatures derived from xenolith data. Therefore, the effect of the compositional variations cannot be neglected when temperatures of the cratonic lithospheric mantle are estimated. Strong negative compositional density anomalies (<-0.03 g/cm3), corresponding to Mg # (100 × Mg/(Mg + Fe)) >92, characterize the lithospheric mantle of the northwestern part of the Superior craton and the central part of the Slave and Churchill craton, according to both tomographic models. The largest discrepancies between the results based on different tomography models are observed in the Proterozoic regions, such as the Trans Hudson Orogen (THO), Rocky Mountains, and Colorado Plateau, which appear weakly depleted (>-0.025 g/cm3 corresponding to Mg # ˜91) when model NA07 is used, or locally characterized by high-density bodies when model SL2013sv is used. The former results are in agreement with those based on the interpretation of xenolith data. The high-density bodies might be interpreted as fragments of subducted slabs or of the advection of the lithospheric mantle induced from the eastward-directed flat slab subduction. The selection of a seismic tomography model plays a significant role when estimating lithospheric density, temperature, and compositional heterogeneity. The consideration of the results of more than one model gives a more complete picture of the possible compositional variations within the NA lithospheric mantle.

Polar wander caused by the Quaternary glacial cycles and fluid Love number

NASA Astrophysics Data System (ADS)

Nakada, Masao

2002-06-01

Perturbations of the Earth's rotation caused by the Quaternary glacial cycles provide an important constraint on the viscosity of the deep mantle because they represent a long-wavelength response of the Earth to surface load redistribution. The predicted present-day polar wander speed (PWS) is, however, sensitive to both the lower mantle viscosity ( ηlm), the density jump at 670 km depth, and the lithospheric thickness and viscosity (e.g., Sabadini and Peltier, Geophys. J. R. Astron. Soc. 66 (1981) 553-578; Yuen et al., J. Geophys. Res. 87 (1982) 10745-10762; Peltier and Wu, Geophys. Res. Lett. 10 (1983) 181-184; Wu and Peltier, Geophys, J. R. Astron. Soc. 76 (1984) 753-791; Peltier, J. Geophys. Res. 89 (1984) 11303-11316; Vermeersen et al., J. Geophys. Res. 102 (1997) 27689-27702; Mitrovica and Milne, J. Geophys. Res. 103 (1998) 985-1005; Johnston and Lambeck, Geophys. J. Int. 136 (1999) 537-558; Nakada, Geophys. J. Int. 143 (2000) 230-238). For earth models with ηlm<5×10 21 Pa s and an elastic lithosphere, the present-day PWS is very sensitive to the M1 mode (buoyancy mode) related to the density jump at 670 km depth [Mitrovica and Milne, J. Geophys. Res. 103 (1998) 985-1005]. The contribution of the M1 mode, however, is less significant for earth models with a viscoelastic lithosphere [Nakada, Geophys. J. Int. 143 (2000) 230-238]. This is due to the fact that this contribution depends on the relative strength of the M1 mode, Δ k2T(M1)/ kfT, where Δ k2T(M1) is the magnitude of tidal Love number ( k2T) of the M1 mode and kfT is the value of k2T in the fluid limit (fluid Love number). The magnitude of kfT for earth models with a viscoelastic lithosphere is larger than that for an elastic lithosphere, and it is smaller for a thicker elastic lithosphere than for a thinner one. Thus, for earth models with a viscoelastic lithosphere, the PWS is mainly sensitive to the lower mantle viscosity regardless of the behavior of the 670 km density discontinuity. This relation also explains why the predicted PWS increases with increasing thickness of an elastic lithosphere. That is, since the value of Δ k2T(M1)/ kfT with a thicker elastic lithosphere is larger than that with a thinner elastic lithosphere, the M1 mode will have a higher contribution in the case of a thicker elastic lithosphere.

Eocene slab breakoff of Neotethys as suggested by dioritic dykes in the Gangdese magmatic belt, southern Tibet

NASA Astrophysics Data System (ADS)

Ma, Xuxuan; Xu, Zhiqin; Meert, Joseph G.

2016-04-01

The Gangdese magmatic belt in southern Tibet demarcates an important boundary between the Indian and Eurasian plates. Due to its location and magmatic evolutionary history, it is key to understanding both the history of Neotethys closure and the Indo-Asian collisional process. This study presents new geochronological and geochemical data for dioritic dykes in the southern Gangdese magmatic belt in southern Tibet. U-Pb geochronological results reveal that the dykes were emplaced at ca. 41 Ma and thus broadly coeval with the 40-38 Ma Dazi volcanics and the 42-40 Ma Gaoligong-Tengliang basaltic dykes. Geochemically, these dykes are characterized by alkaline signature, high Mg# (57-63) and low TiO2 contents ( 0.9-1.0), showing notable enrichment of light rare earth elements relative to the heavy rare earth elements, enrichment of incompatible elements (i.e. Cs, Rb, Ba, Th and U), and depletion of high field strength elements (i.e. Nb, Ta and Ti). In addition, a large variation of zircon εHf(t) values (- 10 to + 13) was shown, implying heterogeneity of magma sources. A heterogeneous source is also suggested by the occurrence of xenocrysts in the dykes. These observations suggest that the magma source of the dykes was dominated by partial melting of lithospheric mantle and then subsequently contaminated by crustal material during ascent. In combination with other geological data in the region, we suspect that the slab slicing of the Neotethys played a key role in the formation of the lithospheric mantle-derived dioritic dykes and adakitic granite, asthenosphere-derived volcanics, basaltic dykes, as well as the recently reported strongly fractionated granites.

Motions of Australia and surroundings since 43 Ma as recorded by subducted mantle lithosphere--evidence for a lost ocean between the Pacific and Indian Oceans

NASA Astrophysics Data System (ADS)

Renqi, L.; Wu, J. E.; Suppe, J.; Kanda, R. V.

2013-12-01

It is well known from seafloor spreading and hotspot data that the Australian plate has moved ~2500km northward in a mantle reference frame since 43Ma, during which time the Pacific plate moved approximately orthogonally ~3000km in a WNW direction. In addition the Australian plate has expanded up to 2000 km as a result of back arc spreading associated with evolving subduction systems on its northern and eastern margins. Here we attempt to account for this plate motion and subduction using new quantitative constraints of mapped slabs of subducted mantle lithosphere underlying the Australian plate and its surroundings. We have mapped a large swath of sub-horizontal slabs in the lower mantle under onshore and offshore NE Australia using global mantle seismic tomography. When restored together with other mapped slabs from the Asia Pacific region, these slabs reveal the existence of a major ocean between NE Australia, E. Asian, and the Pacific at 43 Ma, which we call the East Asian Sea. The southern half of this East Asian Sea was overrun and completely subducted by northward-moving Australia and the expanding Melanesian arcs, and the WNW-converging Pacific. This lost ocean fills a major gap in plate tectonic reconstructions and also constraints the possible motion of the Caroline Sea and New Guinea arcs. Slabs were mapped from MITP08 global P-wave seismic tomography data (Li and Hilst, 2008) and the TX2011 S-wave seismic tomography data (Grand and Simmons, 2011) using Gocad software. The mapped slabs were unfolded to the spherical Earth surface to assess their pre-subduction geometry. Gplates software was used to constrain plate tectonic reconstructions within a fully animated, globally consistent framework.

High resolution P-wave velocity structure beneath Northeastern Tibet from multiscale seismic tomography

NASA Astrophysics Data System (ADS)

Guo, B.; Gao, X.; Chen, J.; Liu, Q.; Li, S.

2016-12-01

The continuing collision of the northward advancing Indian continent with the Eurasia results in the high elevations and thickened Tibetan Plateau. Numerous geologic and geophysical studies engaged in the mechanics of the Tibetan Plateau deformation and uplift. Many seismic experiments were deployed in south and central Tibet, such as INDEPTH and Hi-climb, but very few in northeastern Tibet. Between 2013 and 2015, The China Seismic Array-experiment operated 670 broadband seismic stations with an average station spacing of 35km. This seismic array located in northeastern Tibet and covered the Qilian Mountains, Qaidam Basin, and part of Songpan-Ganzi, Gobi-Alashan, Yangzi, and Ordos. A new multiscale seismic traveltime tomography technique with sparsity constrains were used to map the upper mantle P-wave velocity structure beneath northeastern Tibet. The seismic tomography algorithm employs sparsity constrains on the wavelet representation velocity model via the L1-norm regularization. This algorithm can efficiently deal with the uneven-sampled volume, and give multiscale images of the model. Our preliminary results can be summarized as follows: 1) in the upper mantle down to 200km, significate low-velocity anomalies exist beneath the northeastern Tibet, and slight high-velocity anomalies beneath the Qaidam basin; 2) under Gobi-Alashan, Yangzi, and Ordos, high-velocity anomalies appear to extend to a depth of 250km, this high-velocity may correspond to the lithosphere; 3) there exist relative high-velocity anomalies at depth of 250km-350km underneath north Tibet, which suggests lithospheric delamination; 4) the strong velocity contrast between north Tibet and Yangzi, Gabi-Alashan is visible down to 200km, which implies the north Tibet boundary.

Global model for the lithospheric strength and effective elastic thickness

NASA Astrophysics Data System (ADS)

Tesauro, Magdala; Kaban, Mikhail K.; Cloetingh, Sierd A. P. L.

2013-08-01

Global distribution of the strength and effective elastic thickness (Te) of the lithosphere are estimated using physical parameters from recent crustal and lithospheric models. For the Te estimation we apply a new approach, which provides a possibility to take into account variations of Young modulus (E) within the lithosphere. In view of the large uncertainties affecting strength estimates, we evaluate global strength and Te distributions for possible end-member 'hard' (HRM) and a 'soft' (SRM) rheology models of the continental crust. Temperature within the lithosphere has been estimated using a recent tomography model of Ritsema et al. (2011), which has much higher horizontal resolution than previous global models. Most of the strength is localized in the crust for the HRM and in the mantle for the SRM. These results contribute to the long debates on applicability of the "crème brulée" or "jelly-sandwich" model for the lithosphere structure. Changing from the SRM to HRM turns most of the continental areas from the totally decoupled mode to the fully coupled mode of the lithospheric layers. However, in the areas characterized by a high thermal regime and thick crust, the layers remain decoupled even for the HRM. At the same time, for the inner part of the cratons the lithospheric layers are coupled in both models. Therefore, rheological variations lead to large changes in the integrated strength and Te distribution in the regions characterized by intermediate thermal conditions. In these areas temperature uncertainties have a greater effect, since this parameter principally determines rheological behavior. Comparison of the Te estimates for both models with those determined from the flexural loading and spectral analysis shows that the 'hard' rheology is likely applicable for cratonic areas, whereas the 'soft' rheology is more representative for young orogens.

3D lithospheric mapping of the Iberian Peninsula and surrounding Atlantic and Mediterranean margins from 3D joint inversion of potential field and elevation data.

NASA Astrophysics Data System (ADS)

Torne, Montserrat; Zeyen, Hermann; Jimenez-Munt, Ivone; Fernandez, Manel; Vergés, Jaume

2017-04-01

We investigate the lithospheric density structure of the Iberian Peninsula and the surrounding Atlantic and Mediterranean margins from a 3D joint inversion of free-air, geoid and elevation data, based on a Bayesian approach. In addition, the crustal structure has been further constrained by incorporating about 750 Moho values from DSS investigations and RF analysis covering the entire region. Our preliminary results shows a significant lithospheric deformation along the plate boundaries, the Bay of Biscay-Pyrenees to the North and the Azores-Gibraltar to the south, where the CMB and LAB are located at depths more than 45 and 150 km, respectively. Noteworthy is the arcuate lithospheric thickening located at the westernmost end of the Gibraltar Arc system showing the presence of the NW-to-Westward retreated Gibraltar Arc slab that has given rise to the formation of the Betics-Rif Alpine belt system and the back arc Alboran basin. To the west, the stable-slightly deformed Iberian massif shows a quasi-flat CMB and LAB topography (30 to 32 km and about 110 km, respectively). The crust and mantle lithosphere thin towards the Mediterranean and Atlantic margins, with the exception of its northern margin where lithospheric thickening extends offshore to the Gulf of Biscay. In the western Mediterranean the SE-Neogene slab retreat has resulted in a significant thinning of the crust and mantle lithosphere. Thin lithosphere is also observed in the Tagus-Horseshoe abyssal plain region where the LAB shallows to less than 90 km. This work has been funded by the Spanish projects MITE (CGL2014-59516-P) and WEME-CSIC project 201330E11.

Estimates of effective elastic thickness of oceanic lithosphere using model including surface and subsurface loads and effective elastic thickness of subduction zones

NASA Astrophysics Data System (ADS)

Yang, A.; Yongtao, F.

2016-12-01

The effective elastic thickness (Te) is an important parameter that characterizes the long term strength of the lithosphere, which has great significance on understanding the mechanical properties and evolution of the lithosphere. In contrast with many controversies regarding elastic thickness of continent lithosphere, the Te of oceanic lithosphere is thought to be in a simple way that is dependent on the age of the plate. However, rescent studies show that there is no simple relationship between Te and age at time of loading for both seamounts and subduction zones. As subsurface loading is very importand and has large influence in the estimate of Te for continent lithosphere, and many oceanic features such as subduction zones also have considerable subsurface loading. We introduce the method to estimate the effective elastic thickness of oceanic lithosphere using model including surface and subsurface loads by using free-air gravity anomaly and bathymetric data, together with a moving window admittance technique (MWAT). We use the multitaper spectral estimation method to calculate the power spectral density. Through tests with synthetic subduction zone like bathymetry and gravity data show that the Te can be recovered in an accurance similar to that in the continent and there is also a trade-off between spatial resolution and variance for different window sizes. We estimate Te of many subduction zones (Peru-Chile trench, Middle America trench, Caribbean trench, Kuril-Japan trench, Mariana trench, Tonga trench, Java trench, Ryukyu-Philippine trench) with an age range of 0-160 Myr to reassess the relationship between elastic thickness and the age of the lithosphere at the time of loading. The results do not show a simple relationship between Te and age.

Control of the Lithospheric Mantle on intracontinental Deformation: Revival of Eastern U.S. Tectonism

NASA Astrophysics Data System (ADS)

Biryol, C. B.; Wagner, L. S.; Fischer, K. M.; Hawman, R. B.

2016-12-01

The present tectonic configuration of the southeastern United States is a product of earlier episodes of arc accretion, continental collision and breakup. This region is located in the interior of the North American Plate, some 1500 km away from closest active plate margin. However, there is ongoing tectonism across the area with multiple zones of seismicity, rejuvenation of the Appalachians of North Carolina, Virginia, and Pennsylvania, and Cenozoic intraplate volcanism. The mechanisms controlling this activity and the modern-day state of stress remain enigmatic. Two factors often regarded as major contributors are plate strength and preexisting inherited structures. Recent improvements in broadband seismic data coverage in the region associated with the South Eastern Suture of the Appalachian Margin Experiment (SESAME) and EarthScope Transportable Array make it possible to obtain detailed information on the structure of the lithosphere in the region. Here we present new tomographic images of the upper mantle beneath the Southeastern United States, revealing large-scale structural variations in the upper mantle. Our results indicate fast seismic velocity patterns that can be interpreted as ongoing lithospheric foundering. We observe an agreement between the locations of these upper mantle anomalies and the location of major zones of tectonism, volcanism and seismicity, providing a viable explanation for modern-day activity in this plate interior setting long after it became a passive margin. Based on distinct variations in the geometry and thickness of the lithospheric mantle and foundered lithosphere, we propose that piecemeal delamination has occurred beneath the region throughout the Cenozoic, removing a significant amount of reworked/deformed mantle lithosphere. Ongoing lithospheric foundering beneath the eastern margin of stable North America explains significant variations in thickness of lithospheric mantle across the former Grenville deformation front.

Modeling Geodynamic Mobility of Anisotropic Lithosphere

NASA Astrophysics Data System (ADS)

Perry-Houts, J.; Karlstrom, L.

2016-12-01

The lithosphere is often idealized as a linear, or plastic layer overlying a Newtonian half-space. This approach has led to many insights into lithospheric foundering that include Rayligh-Taylor drips, slab-style delaminations, and small scale convection in the asthenosphere. More recent work has begun to quantify the effect of anisotropic lithosphere viscosity on these same phenomena. Anisotropic viscosity may come about due to stratigraphic deposition in the upper crust, dike/sill emplacement in the mid crust, or volcanic underplating at the Moho related to arcs or plumes. Anisotropic viscosity is also observed in the mantle, due to preferential orientation of olivine grains during flow. Here we extend the work of Lev & Hager (2008) on modeling anisotropic lithospheric foundering to investigate the effects of anisotropic regions which vary in size, magnitude, and orientation. We have extended Aspect, a modern geodynamic finite element code with a large developer and user base, to model exotic constitutive laws with an arbitrary fourth order tensor in place of the viscosity term. We further implement a material model to represent a transverse isotropic medium, such as is expected in a layered, or fractured lithosphere. We have validated our implementation against previous results, and analytic solutions, reproducing the result that horizontally oriented anisotropy tends to inhibit drips, and produce longer-wavelength instabilities. We expect that increased lateral extent of anisotropic regions will exaggerate this effect, to a limit at which the effect will plateau. Varying lithosphere thickness, and mantle anisotropy anisotropy may produce similar behavior. The implications of this effect are significant to lithospheric foundering beneath arcs and hotspots, possibly influencing the recycling of eclogite, production of silicic magmas, and dynamic topography.

Constraints from Xenoliths on Cenozoic Deformation and Rheology of the Western North American Mantle Lithosphere

NASA Astrophysics Data System (ADS)

Behr, W. M.; Smith, D.; Bernard, R. E.

2015-12-01

We investigate xenoliths from several volcanic centers in the western US Cordillera, including the Navajo Volcanic Field in the Four Corners region of the Colorado Plateau, the San Carlos Volcanic Field in Arizona, and the Cima and Dish Hill volcanic fields in the western Mojave. We use these xenolith suites to determine to what extent and by what mechanisms the western North American lithospheric mantle has deformed during Cenozoic tectonic events, including Laramide flat-slab subduction, Basin-and-Range extension, and Quaternary strike-slip faulting associated with the San Andreas Fault System. We find the following. 1) Laramide flat-slab subduction substantially and heterogeneously deformed the North American lithospheric mantle. Despite some serpentinization, deformation along the plate interface was accommodated primarily by olivine dislocation creep, and was cold enough that the mantle lithosphere was strong and could transmit basal shear tractions into the upper plate crust, generating high topography. 2) During B&R extension, the mantle lithosphere was thinned and heated, and Laramide-age shear zone foliations were obliterated by grain growth, even in mixed phase lithologies. Despite annealing, LPO in olivine is preserved in several samples. This fossil LPO may control present-day mantle lid seismic anisotropy in the Basin and Range and may also provide an important source of viscous anisotropy. 3) The mantle lithosphere is actively deforming in localized zones beneath faults of the San Andreas system, but high sub-Moho temperatures render it very weak such that most of the strength of the lithosphere resides in the crust. Because deformation is localized, mantle lid anisotropy in the Mojave region is likely controlled by a fossil LPO, despite present-day deformation in the lithospheric mantle.

Inelastic models of lithospheric stress - I. Theory and application to outer-rise plate deformation

USGS Publications Warehouse

Mueller, S.; Choy, G.L.; Spence, W.

1996-01-01

Outer-rise stress distributions determined in the manner that mechanical engineers evaluate inelastic stress distributions within conventional materials are contrasted with those predicted using simple elastic-plate models that are frequently encountered in studies of outer-rise seismicity. This comparison indicates that the latter are inherently inappropriate for studies of intraplate earthquakes, which are a direct manifestation of lithospheric inelasticity. We demonstrate that the common practice of truncating elastically superimposed stress profiles so that they are not permitted to exceed laboratory-based estimates of lithospheric yield strength will result in an accurate characterization of lithospheric stress only under relatively restrictive circumstances. In contrast to elastic-plate models, which predict that lithospheric stress distributions depend exclusively upon the current load, inelastic plate models predict that stress distributions are also significantly influenced by the plate-loading history, and, in many cases, this influence is the dominant factor in determining the style of potential seismicity (e.g. thrust versus normal faulting). Numerous 'intuitive' interpretations of outer-rise earthquakes have been founded upon the implicit assumption that a unique relationship exists between a specified combination of plate curvature and in-plane force, and the resulting lithospheric stress distribution. We demonstrate that the profound influence of deformation history often invalidates such interpretations. Finally, we examine the reliability of 'yield envelope' representations of lithospheric strength that are constructed on the basis of empirically determined frictional sliding relationships and silicate plastic-flow laws. Although representations of this nature underestimate the strength of some major interplate faults, such as the San Andreas, they appear to represent a reliable characterization of the strength of intraplate oceanic lithosphere.

Updated Reference Model for Heat Generation in the Lithosphere

NASA Astrophysics Data System (ADS)

Wipperfurth, S. A.; Sramek, O.; Roskovec, B.; Mantovani, F.; McDonough, W. F.

2017-12-01

Models integrating geophysics and geochemistry allow for characterization of the Earth's heat budget and geochemical evolution. Global lithospheric geophysical models are now constrained by surface and body wave data and are classified into several unique tectonic types. Global lithospheric geochemical models have evolved from petrological characterization of layers to a combination of petrologic and seismic constraints. Because of these advances regarding our knowledge of the lithosphere, it is necessary to create an updated chemical and physical reference model. We are developing a global lithospheric reference model based on LITHO1.0 (segmented into 1°lon x 1°lat x 9-layers) and seismological-geochemical relationships. Uncertainty assignments and correlations are assessed for its physical attributes, including layer thickness, Vp and Vs, and density. This approach yields uncertainties for the masses of the crust and lithospheric mantle. Heat producing element abundances (HPE: U, Th, and K) are ascribed to each volume element. These chemical attributes are based upon the composition of subducting sediment (sediment layers), composition of surface rocks (upper crust), a combination of petrologic and seismic correlations (middle and lower crust), and a compilation of xenolith data (lithospheric mantle). The HPE abundances are correlated within each voxel, but not vertically between layers. Efforts to provide correlation of abundances horizontally between each voxel are discussed. These models are used further to critically evaluate the bulk lithosphere heat production in the continents and the oceans. Cross-checks between our model and results from: 1) heat flux (Artemieva, 2006; Davies, 2013; Cammarano and Guerri, 2017), 2) gravity (Reguzzoni and Sampietro, 2015), and 3) geochemical and petrological models (Rudnick and Gao, 2014; Hacker et al. 2015) are performed.

Lithospheric controls on magma composition along Earth's longest continental hotspot track.

PubMed

Davies, D R; Rawlinson, N; Iaffaldano, G; Campbell, I H

2015-09-24

Hotspots are anomalous regions of volcanism at Earth's surface that show no obvious association with tectonic plate boundaries. Classic examples include the Hawaiian-Emperor chain and the Yellowstone-Snake River Plain province. The majority are believed to form as Earth's tectonic plates move over long-lived mantle plumes: buoyant upwellings that bring hot material from Earth's deep mantle to its surface. It has long been recognized that lithospheric thickness limits the rise height of plumes and, thereby, their minimum melting pressure. It should, therefore, have a controlling influence on the geochemistry of plume-related magmas, although unambiguous evidence of this has, so far, been lacking. Here we integrate observational constraints from surface geology, geochronology, plate-motion reconstructions, geochemistry and seismology to ascertain plume melting depths beneath Earth's longest continental hotspot track, a 2,000-kilometre-long track in eastern Australia that displays a record of volcanic activity between 33 and 9 million years ago, which we call the Cosgrove track. Our analyses highlight a strong correlation between lithospheric thickness and magma composition along this track, with: (1) standard basaltic compositions in regions where lithospheric thickness is less than 110 kilometres; (2) volcanic gaps in regions where lithospheric thickness exceeds 150 kilometres; and (3) low-volume, leucitite-bearing volcanism in regions of intermediate lithospheric thickness. Trace-element concentrations from samples along this track support the notion that these compositional variations result from different degrees of partial melting, which is controlled by the thickness of overlying lithosphere. Our results place the first observational constraints on the sub-continental melting depth of mantle plumes and provide direct evidence that lithospheric thickness has a dominant influence on the volume and chemical composition of plume-derived magmas.

Evolution of passive continental margins and initiation of subduction zones

NASA Astrophysics Data System (ADS)

Cloetingh, S. A. P. L.; Wortel, M. J. R.; Vlaar, N. J.

1982-05-01

Although the initiation of subduction is a key element in plate tectonic schemes for evolution of lithospheric plates, the underlying mechanisms are not well understood. Plate rupture is an important aspect of the process of creating a new subduction zone, as stresses of the order of kilobars are required to fracture oceanic lithosphere1. Therefore initiation of subduction could take place preferentially at pre-existing weakness zones or in regions where the lithosphere is prestressed. As such, transform faults2,3 and passive margins4,5 where the lithosphere is downflexed under the influence of sediment loading have been suggested. From a model study of passive margin evolution we found that ageing of passive margins alone does not make them more suitable sites for initiation of subduction. However, extensive sediment loading on young lithosphere might be an effective mechanism for closure of small ocean basins.

Lasting mantle scars lead to perennial plate tectonics.

PubMed

Heron, Philip J; Pysklywec, Russell N; Stephenson, Randell

2016-06-10

Mid-ocean ridges, transform faults, subduction and continental collisions form the conventional theory of plate tectonics to explain non-rigid behaviour at plate boundaries. However, the theory does not explain directly the processes involved in intraplate deformation and seismicity. Recently, damage structures in the lithosphere have been linked to the origin of plate tectonics. Despite seismological imaging suggesting that inherited mantle lithosphere heterogeneities are ubiquitous, their plate tectonic role is rarely considered. Here we show that deep lithospheric anomalies can dominate shallow geological features in activating tectonics in plate interiors. In numerical experiments, we found that structures frozen into the mantle lithosphere through plate tectonic processes can behave as quasi-plate boundaries reactivated under far-field compressional forcing. Intraplate locations where proto-lithospheric plates have been scarred by earlier suturing could be regions where latent plate boundaries remain, and where plate tectonics processes are expressed as a 'perennial' phenomenon.

Lasting mantle scars lead to perennial plate tectonics

PubMed Central

Heron, Philip J.; Pysklywec, Russell N.; Stephenson, Randell

2016-01-01

Mid-ocean ridges, transform faults, subduction and continental collisions form the conventional theory of plate tectonics to explain non-rigid behaviour at plate boundaries. However, the theory does not explain directly the processes involved in intraplate deformation and seismicity. Recently, damage structures in the lithosphere have been linked to the origin of plate tectonics. Despite seismological imaging suggesting that inherited mantle lithosphere heterogeneities are ubiquitous, their plate tectonic role is rarely considered. Here we show that deep lithospheric anomalies can dominate shallow geological features in activating tectonics in plate interiors. In numerical experiments, we found that structures frozen into the mantle lithosphere through plate tectonic processes can behave as quasi-plate boundaries reactivated under far-field compressional forcing. Intraplate locations where proto-lithospheric plates have been scarred by earlier suturing could be regions where latent plate boundaries remain, and where plate tectonics processes are expressed as a ‘perennial' phenomenon. PMID:27282541

A multilayer model of time dependent deformation following an earthquake on a strike-slip fault

NASA Technical Reports Server (NTRS)

Cohen, S. C.

1981-01-01

A multilayer model of the Earth to calculate finite element of time dependent deformation and stress following an earthquake on a strike slip fault is discussed. The model involves shear properties of an elastic upper lithosphere, a standard viscoelastic linear solid lower lithosphere, a Maxwell viscoelastic asthenosphere and an elastic mesosphere. Systematic variations of fault and layer depths and comparisons with simpler elastic lithosphere over viscoelastic asthenosphere calculations are analyzed. Both the creep of the lower lithosphere and astenosphere contribute to the postseismic deformation. The magnitude of the deformation is enhanced by a short distance between the bottom of the fault (slip zone) and the top of the creep region but is less sensitive to the thickness of the creeping layer. Postseismic restressing is increased as the lower lithosphere becomes more viscoelastic, but the tendency for the width of the restressed zone to growth with time is retarded.

Three-Dimensional Magnetotelluric Imaging of the Cascadia Subduction Zone with an Amphibious Array

NASA Astrophysics Data System (ADS)

Egbert, G. D.; Yang, B.; Bedrosian, P.; Kelbert, A.; Key, K.; Livelybrooks, D.; Parris, B. A.; Schultz, A.

2017-12-01

We present results from three-dimensional inversion of an amphibious magnetotelluric (MT) array consisting of 71 offshore and 75 onshore sites in the central part of Cascadia, to image down-dip and along strike variations of electrical conductivity, and to constrain the 3D distribution of fluids and melt in the subduction zone. The array is augmented by EarthScope TA MT data and legacy 2D profiles providing sparser coverage of western WA, OR, and northern CA. The prior model for the inversion includes ocean bathymetry, conductive marine sediments, and a resistive subducting plate, with geometry derived from the model of McCrory et al. (2012) and seismic tomography. Highly conductive features appear just above the interface with the a priori resistive plate in three zones. (1) In the area with marine MT data a conductive layer, which we associate with fluid-rich decollement and subduction channel sediments, extends eastward from the trench to underthrust the seaward edge of Siletzia, which is clearly seen as a thick crustal resistor. The downdip extent of the underthrust conductive layer is a remarkably uniform 35 km. (2) High conductivities, consistent with metamorphic fluids associated with eclogitization, occur near the forearc mantle corner. Conductivity is highly variable along strike, organized in a series of E-W to diagonal elongated conductive/resistive structures, whose significance remains enigmatic. (3) High conductivities associated with fluids and melts are found in the backarc, again exhibiting substantial along strike variability.

Progressive deformation and superposed fabrics related to Cretaceous crustal underthrusting in western Arizona, U.S.A.

USGS Publications Warehouse

Laubach, S.E.; Reynolds, S.J.; Spencer, J.E.; Marshak, S.

1989-01-01

In the Maria fold and thrust belt, a newly recognized E-trending Cretaceous orogenic belt in the southwestern United States, ductile thrusts, large folds and superposed cleavages record discordant emplacement of crystalline thrust sheets across previously tilted sections of crust. Style of deformation and direction of thrusting are in sharp contrast to those of the foreland fold-thrust belt in adjacent segments of the Cordillera. The net effect of polyphase deformation in the Maria belt was underthrusting of Paleozoic and Mesozoic metasedimentary rocks under the Proterozoic crystalline basement of North America. The structure of the Maria belt is illustrated by the Granite Wash Mountains in west-central Arizona, where at least four non-coaxial deformation events (D1-D4) occurred during the Cretaceous. SSE-facing D1 folds are associated with S-directed thrusts and a low-grade slaty cleavage. D1 structures are truncated by the gently-dipping Hercules thrust zone (D2), a regional SW-vergent shear zone that placed Proterozoic and Jurassic crystalline rocks over upturned Paleozoic and Mesozoic supracrustal rocks. Exposures across the footwall margin of the Hercules thrust zone show the progressive development of folds, cleavage and metamorphism related to thrusting. D3 and D4 structures include open folds and spaced cleavages that refold or transect D1 and D2 folds. The D2 Hercules thrust zone and a D3 shear zone are discordantly crosscut by late Cretaceous plutons. ?? 1989.

Crustal deformation rates in Assam Valley, Shillong Plateau, Eastern Himalaya, and Indo-Burmese region from 11 years (2002-2013) of GPS measurements

NASA Astrophysics Data System (ADS)

Barman, Prakash; Jade, Sridevi; Shrungeshwara, T. S.; Kumar, Ashok; Bhattacharyya, Sanjeev; Ray, Jagat Dwipendra; Jagannathan, Saigeetha; Jamir, Wangshi Menla

2017-09-01

The present study reports the contemporary deformation of the tectonically complex northeast India using 11 years (2002-2013) of GPS observations. The central Shillong Plateau and few sites north of Plateau located in Assam Valley behave like a rigid block with 7 mm/year India-fixed southward velocity. The Euler pole of rotation of this central Shillong Plateau-Assam Valley (SH-AS) block is estimated to be at -25.1° ± 0.2°N, -97.8° ± 1.8°E with an angular velocity of 0.533° ± 0.10° Myr-1 relative to India-fixed reference frame. Kopili fault located between Shillong Plateau and Mikir massif records a dextral slip of 4.7 ± 1.3 mm/year with a locking depth of 10.2 ± 1.4 km indicating the fragmentation of Assam Valley across the fault. Presently, western edge of Mikir massif appears to be locked to Assam block indicating strain accumulation in this region. First-order elastic dislocation modelling of the GPS velocities estimates a slip rate of 16 mm/year along the Main Himalayan Thrust in Eastern Himalaya which is locked over a width of 130 km from the surface to a depth of 17 km with underthrusting Indian plate. Around 9 mm/year arc-normal convergence is accommodated in Lesser Himalaya just south of Main Central Thrust indicating high strain accumulation. Out of 36 mm/year (SSE) India-Sunda plate motion, about 16 mm/year motion is accommodated in Indo-Burmese Fold and Thrust Belt, both as normal convergence ( 6 mm/year) and active slip ( 7-11 mm/year) in this region.

Seismotectonics of the trans-Himalaya, Eastern Ladakh, India: constraints from Moment Tensor Solutions of local earthquake data

NASA Astrophysics Data System (ADS)

Paul, A.

2017-12-01

The eastern Ladakh-Karakoram zone, the northwest part of the Trans-Himalayan belt, bears signature of this collisional process in the form of suture zones, exhumed blocks that underwent deeper subduction and also intra-continental fault zones. The seismotectonic scenario of northwest part of India-Asia collision zone is studied by analyzing the local earthquake data (M 1.4-4.3) recorded by a broadband seismological network consisting of 14 stations. Focal Mechanism Solution (FMS) of 13 selected earthquakes were computed through waveform inversion of three-component broadband records. Depth distribution of the earthquakes and FMS of local earthquakes obtained through waveform inversion reveal the kinematics of the major fault zones present in Eastern Ladakh. The most pronounced cluster of seismicity is observed in the Karakoram Fault (KF) zone up to a depth of 65 km (Fig.1). The FMS reveals transpressive environment with the strike of inferred fault plane roughly parallel to the KF. It is inferred that the KF at least penetrates up to the lower crust and is a manifestation of active under thrusting of Indian lower crust beneath Tibet. Two clusters of micro seismicity is observed at a depth range of 5-20 km at north western and southeastern fringe of the Tso Morari gneiss dome which can be correlated to the activities along the Zildat fault and Karzok fault respectively. The FMSs estimated for representative earthquakes show thrust fault solutions for the Karzok fault and normal fault solution for the Zildat fault. It is inferred that the Zildat fault is acting as detachment, facilitating the exhumation of the Tso Morari dome. On the other hand, the Tso Morari dome is underthrusting the Karzok ophiolite on its southern margin along the Karzok fault, due to gravity collapse.

Late Archean greenstone tectonics: Evidence for thermal and thrust-loading lithospheric subsidence from stratigraphic sections in the Slave Province, Canada

NASA Technical Reports Server (NTRS)

Kidd, W. S. F.; Kusky, T. M.; Bradley, D. C.

1988-01-01

How late Archean tectonics could be seen to have operated in the Slave Province is illustrated. Lithospheric thinning and stretching, with the formation of rifted margins (to continental or island arc fragments), and lithospheric flexural loading of the kind familiar in arcs and mountain belts could be discerned.

Volcano spacings and lithospheric attenuation in the Eastern Rift of Africa

NASA Technical Reports Server (NTRS)

Mohr, P. A.; Wood, C. A.

1976-01-01

The Eastern Rift of Africa runs the gamut of crustal and lithospheric attenuation from undeformed shield through attenuated rift margin to active neo-oceanic spreading zones. It is therefore peculiarly well suited to an examination of relationships between volcano spacings and crust/lithosphere thickness. Although lithospheric thickness is not well known in Eastern Africa, it appears to have direct expression in the surface spacing of volcanoes for any given tectonic regime. This applies whether the volcanoes are essentially basaltic, silicic, or alkaline-carbonatitic. No evidence is found for control of volcano sites by a pre-existing fracture grid in the crust.

Report of the panel on lithospheric structure and evolution, section 3

NASA Technical Reports Server (NTRS)

Chase, Clement G.; Lang, Harold; Mcnutt, Marcia K.; Paylor, Earnest D.; Sandwell, David T.; Stern, Robert J.

1991-01-01

The panel concluded that NASA can contribute to developing a refined understanding of the compositional, structural, and thermal differences between continental and oceanic lithosphere through a vigorous program in solid Earth science with the following objectives: determine the most fundamental geophysical property of the planet; determine the global gravity field to an accuracy of a few milliGals at wavelengths of 100 km or less; determine the global lithospheric magnetic field to a few nanoTeslas at a wavelength of 100 km; determine how the lithosphere has evolved to its present state via acquiring geologic remote sensing data over all the continents.

Global maps of the magnetic thickness and magnetization of the Earth's lithosphere

NASA Astrophysics Data System (ADS)

Vervelidou, Foteini; Thébault, Erwan

2015-10-01

We have constructed global maps of the large-scale magnetic thickness and magnetization of Earth's lithosphere. Deriving such large-scale maps based on lithospheric magnetic field measurements faces the challenge of the masking effect of the core field. In this study, the maps were obtained through analyses in the spectral domain by means of a new regional spatial power spectrum based on the Revised Spherical Cap Harmonic Analysis (R-SCHA) formalism. A series of regional spectral analyses were conducted covering the entire Earth. The R-SCHA surface power spectrum for each region was estimated using the NGDC-720 spherical harmonic (SH) model of the lithospheric magnetic field, which is based on satellite, aeromagnetic, and marine measurements. These observational regional spectra were fitted to a recently proposed statistical expression of the power spectrum of Earth's lithospheric magnetic field, whose free parameters include the thickness and magnetization of the magnetic sources. The resulting global magnetic thickness map is compared to other crustal and magnetic thickness maps based upon different geophysical data. We conclude that the large-scale magnetic thickness of the lithosphere is on average confined to a layer that does not exceed the Moho.

3D Numerical Examination of Continental Mantle Lithosphere Response to Lower Crust Eclogitization and Nearby Slab Subduction

NASA Astrophysics Data System (ADS)

Janbakhsh, P.; Pysklywec, R.

2017-12-01

2D numerical modeling techniques have made great contribution to understanding geodynamic processes involved in crustal and lithospheric scale deformations for the past 20 years. The aim of this presentation is to expand the scope covered by previous researchers to 3 dimensions to address out-of-plane intrusion and extrusion of mantle material in and out of model space, and toroidal mantle wedge flows. In addition, 3D velocity boundary conditions can create more realistic models to replicate real case scenarios. 3D numerical experiments that will be presented are designed to investigate the density and viscosity effects of lower crustal eclogitization on the decoupling process of continental mantle lithosphere from the crust and its delamination. In addition, these models examine near-field effects of a subducting ocean lithosphere and a lithospheric scale fault zone on the evolution of the processes. The model solutions and predictions will also be compared against the Anatolian geology where subduction of Aegean and Arabian slabs, and the northern boundary with the North Anatolian Fault Zone are considered as two main contributing factors to anomalous crustal uplift, missing mantle lithosphere, and anomalous surface heat flux.

Numerical modelling of lithospheric flexure at subduction zones: what controls the formation of petit-spot volcanoes?

NASA Astrophysics Data System (ADS)

Bessat, Annelore; Pilet, Sébastien; Duretz, Thibault; Schmalholz, Stefan M.

2017-04-01

Petit-spot volcanoes were discovered fifteen years ago by Japanese researchers at the top of the down going plate in front of Japan (1). The location of these small lava flows is unusual, and seems related to the plate flexure in front of the subduction zone. Their formation seems, therefore, not to correspond to any classical type of volcanism such as MORB generation at mid ocean ridges, arc volcanism in subduction zones or intraplate volcanoes classically associated to deep mantle plumes. The discovery of petit-spot volcanoes is of great significance as it demonstrates, for the first time, that tectonic processes could generate intraplate volcanism and supports the existence of small-degree melts at the base of the lithosphere. First models for the formation of petit-spot volcanoes suggest that plate bending produces extension at the base of the lithosphere, thus allowing large cracks to propagate across the lithosphere. These cracks promote the extraction of low degree melts from the base of the lithosphere (2). However, the study of petit-spot mantle xenoliths from Japan (3) demonstrates that low degree melts are not directly extracted to the surface, but percolate and metasomatize the oceanic lithosphere. The aim of this study is to better understand the physical processes associated with the formation of petit-spot volcanoes. These thermo-mechanical processes will be studied using upper-mantle scale numerical simulations based on a 2D finite difference code. The numerical model considers viscoelastoplastic deformation; combination of laboratory-derived flow laws (e.g. diffusion and dislocation creep, Peierls creep) and heat transfer. The first step is to quantify the deformation processes that occur in the lithosphere and at the Lithosphere-Asthenosphere Boundary (LAB). The aims are to investigate, in particular, extensional deformation at the base of the lithosphere which is induced by plate flexure in front of a subduction zone. This study focuses on quantifying stresses, strain rates, and viscosities to evaluate the thermo-mechanical conditions which are important for the percolation of melt initially stocked at the base of the lithosphere. References (1) Hirano et al., 2006. Science 313, 1426-1428. (2) Yamamoto et al., 2014, Geology 42, 967-970. (3) Pilet et al., 2016, Nature Geoscience 9, 898-903.

Lithospheric strength across the ocean-continent transition in the NW of the Iberian Peninsula

NASA Astrophysics Data System (ADS)

Martín-Velázquez, Silvia; Martín-González, Fidel

2014-05-01

The main objective of this work is to investigate the relation between the strength of the lithosphere and the observed pattern of seismicity across the ocean-continent transition in the NW margin of the Iberian Peninsula. The seismicity is diffuse in this intraplate area, far from the seismically active margin of the plate: the Eurasia-African plate boundary, where convergence occurs at a rate of 4-5mm/year. The earthquake epicentres are mainly limited to an E-W trending zone (onshore seismicity is more abundant than offshore), and most earthquakes occur at depths less than 30 km, however, offshore depths are up to 150 km). Moreover, one of the problems to unravel in this area is that the seismotectonic interpretations of the anomalous seismicity in the NW peninsular are contradictory. The temperature and strength profiles have been modelled in three domains along the non-volcanic rifted West Iberian Margin: 1) the oceanic lithosphere of the Iberian Abyssal Plain, 2) the oceanic lithosphere near the ocean-continent transition of the Galicia Bank, and 3) the continental lithosphere of the NW Iberian Massif. The average bathymetry and topography have been used to fit the thermal structures of the three types of lithospheres, given that the heat flow and heat production values show a varied range. The geotherms, together with the brittle and ductile rheological laws, have been used to calculate the strength envelopes in different stress regimes (compression, shear and tensile). The continental lithosphere-asthenosphere boundary is located at 123 km and several brittle-ductile transitions appear in the crust and the mantle. However, the oceanic lithospheres are thinner (110 km near the Galicia Bank and 87 km in the Iberian Abbysal Plain) and more simple (brittle behaviour in the crust and upper mantle). The earthquake distribution is best explained by lithospheres with dry compositions and shear or tensile stress regimes. These results are similar can be compared to those of the Gulf of Cadiz oceanic-continental transition near the Eurasia-African plate boundary (Neves and Neves, 2009), and they contribute to complete the knowledge about seismicity and lithospheric strength in the ocean-continent transition of the Iberian Peninsula. References Neves M.C., Neves, R.G.M., 2009. Flexure and seismicity across the ocean-continent transition in the Gulf of Cadiz. Journal of Geodynamics, 47, 119-129.

Plate tectonics beyond plate boundaries: the role of ancient structures in intraplate orogenesis

NASA Astrophysics Data System (ADS)

Heron, Philip; Pysklywec, Russell; Stephenson, Randell

2015-04-01

The development of orogens that occur at a distance from plate boundaries (i.e., `intraplate' deformation) cannot be adequately explained through conventional plate tectonic theory. Intraplate deformation infers a more complex argument for lithospheric and mantle interaction than plate tectonic theory allows. As a result, the origins of intraplate orogenesis are enigmatic. One hypothesis is the amalgamation of continental material (i.e., micro-plates) leaves inherent scars on the crust and mantle lithosphere. Previous studies into continent-continent collisions identify a number of scenarios from accretionary tectonics that affect the crust and mantle (namely, the development of a Rayleigh-Taylor instability, lithospheric underplating, lithospheric delamination, and lithospheric subduction). Any of these processes may weaken the lithosphere allowing episodic reactivation of faults within continental interiors. Hence, continental convergence (i.e., shortening) at a time after continental collision may cause the already weakened crust and mantle lithosphere to produce intraplate deformation. In order to better understand the processes involved in deformation away from plate boundaries, we present suites of continental shortening models (using the high-resolution thermal-mechanical modelling code SOPALE) to identify the preferred style of deformation. We model ancient structures by applying weak subduction scarring, changing the rheological conditions, and modifying the thermal structure within the lithosphere. To highlight the role of surface processes on plate and lithosphere deformation, the effect of climate-driven erosion and deposition on the tectonic structure of intraplate deformation is also addressed. We explore the relevance of the models to previously studied regions of intraplate orogenesis, including the Pyrenees in Europe, the Laramide orogen in North America, Tien Shan orogen in Central Asia, and Central Australia. The findings of the simulations with regards to past and future North American intraplate deformation are also discussed. Our results indicate that there exists a number of tectonic environments that can be produced relating to continental accretion, and that specific observational constraints to the local area (e.g., geological, geophysical, geodetic) are required to be integrated directly into the analyses for better interpretation. The models shown here find that although rheological changes to the lithosphere can produce a range of deformation during continental convergence (i.e., crustal thickening, thinning, and folding), mantle weak zones from ancient subduction can generate more localized deformation and topography.

Initiation of Subduction Zones: A Consequence of Lateral Compositional Buoyancy Contrast Within the Lithosphere

NASA Astrophysics Data System (ADS)

Niu, Y.; O'Hara, M. J.; Pearce, J. A.

2001-12-01

Subduction of oceanic lithosphere into deep mantle is one of the key aspects of plate tectonics. Pull by the subducting-slab due to its negative buoyancy is widely accepted as the major driving force for plate motion and plate tectonics. Hence, there would be no plate tectonics if there were no subduction zones. Yet how a subduction zone initiates remains poorly known. Here we show that lateral compositional (vs. thermal) buoyancy contrast within the lithosphere creates the favored and necessary condition for the initiation of a subduction zone by (1) comparing the compositional and density differences between normal oceanic lithosphere (NOL) represented by abyssal peridotites (AP) and subarc lithosphere (SAL) represented by forearc peridotites (FP), and (2) simple physical analysis. As the gravitational attraction is the principal driving force of the subducting slab, it would be optimal if one part of the lithosphere experiences a greater gravitational attraction than its adjacent neighbor prior to or during the initiation of a subduction. This requires the pre-existence of a density contrast within the lithosphere. If the lithosphere is thermally uniform as is often the case, then the density contrast must result from a compositional contrast. This hypothesis can be tested by examining the lithospheric materials on both sides of a subduction zone. Subduction of a dense NOL beneath a buoyant continental lithosphere is straightforward, but intra-oceanic subduction such as in the western Pacific requires a scrutiny. Our data show that FP of Mariana and Tonga - two of the most important intra-oceanic subduction zones on Earth - are compositionally more depleted than AP: Cr#-sp (mean+/- 1σ ) = 0.584+/-0.084(FP) vs. 0.307+/-0.134(AP); Mg#-ol = 0.915+/-0.006(FP) vs. 0.898+/-0.082(AP); Mg#-opx = 0.917+/-0.006(FP) vs. 0.908+/-0.006(AP); Mg#-cpx = 0.929+/-0.021(FP) vs. 0.917+/-0.011(AP). As a result, SAL is > 0.7% less dense than NOL. This density contrast due to compositional difference is equivalent to Δ T = ~230° C, which is similar to or greater than the postulated thermal buoyancy contrast between a hot mantle plume and its surroundings. While the depleted nature of FP has been interpreted to result from subducting-slab dehydration induced high extents of mantle wedge melting, evidence indicates that the depletion of these FP predates the inception of the subduction, thus these FP are not residues of present-day arc magmatism. Hence, the compositional buoyancy contrast already existed within the lithosphere before the inception of the subduction in the western Pacific. Much of the Mariana SAL may be fragments of old continental lithosphere, whereas the Tonga/Fiji plateau and Kamchatka lithosphere may be remnants of buoyant, hence unsubductable oceanic plateaus (mantle plume head materials) for the Louisville and Hawaiian hotspots respectively. Passive continental margins, where the largest compositional buoyancy contrast exists within the lithosphere, are the loci of future subduction zones. Geometrical analysis shows that the compositional buoyancy contrast within the lithosphere under compression (e.g., ridge push) induces transtensional planes. The weakest plane in the vicinity of the compositional buoyancy contrast develops into a reverse fault. The dense NOL (the foot-wall) tends to sink into the hot and less dense asthenosphere. Calculations show that this tendency to sink reduces both the normal stress to, and shear resistance along, the fault plane, thus easing the sinking and favoring the initiation of a subduction zone. This concept also explains other observations and makes testable predictions on important geodynamic problems.

Topography associated with crustal flow in continental collisions, with application to Tibet

NASA Astrophysics Data System (ADS)

Bendick, R.; McKenzie, D.; Etienne, J.

2008-10-01

Collision between an undeformable indenter and a viscous region generates isostatically compensated topography by solid-state flow. We model this process numerically, using a finite element scheme. The slope, amplitude and symmetry of the topographic signal depend on the indenter size and the Argand number of the viscous region, a dimensionless ratio of gravitational body forces to viscous forces. When applied to convergent continental settings, these scaling rules provide estimates of the position of an indenter at depth and the mechanical properties of the viscous region, especially effective viscosity. In Tibet, forward modelling suggests that some elevated, low relief topography within the northern plateau may be attributed to lower crustal flow, stimulated by a crustal indenter, possibly Indian lithosphere. The best-fit model constrains the northernmost limit of this indenter to 33.7°N and the maximum effective viscosity of Eurasian middle and lower crust to 1 × 1020 +/- 0.3 × 1020 Pa s.

Reconstructing plate-motion changes in the presence of finite-rotations noise.

PubMed

Iaffaldano, Giampiero; Bodin, Thomas; Sambridge, Malcolm

2012-01-01

Understanding lithospheric plate motions is of paramount importance to geodynamicists. Much effort is going into kinematic reconstructions featuring progressively finer temporal resolution. However, the challenge of precisely identifying ocean-floor magnetic lineations, and uncertainties in geomagnetic reversal timescales result in substantial finite-rotations noise. Unless some type of temporal smoothing is applied, the scenario arising at the native temporal resolution is puzzling, as plate motions vary erratically and significantly over short periods (<1 Myr). This undermines our ability to make geodynamic inferences, as the rates at which forces need to be built upon plates to explain these kinematics far exceed the most optimistic estimates. Here we show that the largest kinematic changes reconstructed across the Atlantic, Indian and South Pacific ridges arise from data noise. We overcome this limitation using a trans-dimensional hierarchical Bayesian framework. We find that plate-motion changes occur on timescales no shorter than a few million years, yielding simpler kinematic patterns and more plausible dynamics.

Vertical coherence of deformation in lithosphere in the eastern Himalayan syntaxis using GPS, Quaternary fault slip rates, and shear wave splitting data

NASA Astrophysics Data System (ADS)

Chang, Lijun; Flesch, Lucy M.; Wang, Chun-Yung; Ding, Zhifeng

2015-07-01

We present 59 new SKS/SKKS and combine them with 69 previously published data to infer the mantle deformation field in SE Tibet. The dense set of anisotropy measurements in the eastern Himalayan syntaxis (EHS) are oriented along a NE-SW azimuth and rotate clockwise in the surround regions. We use GPS measurements and geologic data to determine a continuous surface deformation field that is then used to predict shear wave spitting directions at each station. Comparison of splitting observations with predictions yields an average misfit of 11.7° illustrating that deformation is vertically coherent, consistent with previous studies. Within the central EHS in areas directly surrounding the Namche-Barwa metamorphic massif, the average misfit of 11 stations increases to 60.8°, and vertical coherence is no longer present. The complexity of the mantle anisotropy and surface observations argues for local alteration of the strain fields here associated with recent rapid exhumation of the Indian crust.

Peeling back the lithosphere: Controlling parameters, surface expressions and the future directions in delamination modeling

NASA Astrophysics Data System (ADS)

Göğüş, Oğuz H.; Ueda, Kosuke

2018-06-01

Geodynamical models investigate the rheological and physical properties of the lithosphere that peels back (delaminates) from the upper-middle crust. Meanwhile, model predictions are used to relate to a set of observations in the geological context to the test the validity of delamination. Here, we review numerical and analogue models of delamination from these perspectives and provide a number of first-order topics which future modeling studies may address. Models suggest that the presence of the weak lower crust that resides between the strong mantle lithosphere (at least 100 times more viscous/stronger) and the strong upper crust is necessary to develop delamination. Lower crustal weakening may be induced by melt infiltration, shear heating or it naturally occurs through the jelly sandwich type strength profile of the continental lithosphere. The negative buoyancy of the lithosphere required to facilitate the delamination is induced by the pre-existing ocean subduction and/or the lower crustal eclogitization. Surface expression of the peeling back lithosphere has a distinct transient and migratory imprint on the crust, resulting in rapid surface uplift/subsidence, magmatism, heating and shortening/extension. New generation of geodynamical experiments can explain how different types of melting (e.g hydrated, dry melting) occurs with delamination. Reformation of the lithosphere after removal, three dimensional aspects, and the termination of the process are key investigation areas for future research. The robust model predictions, as with other geodynamic modeling studies should be reconciled with observations.

The relationship between the instantaneous velocity field and the rate of moment release in the lithosphere

USGS Publications Warehouse

Pollitz, F.F.

2003-01-01

Instantaneous velocity gradients within the continental lithosphere are often related to the tectonic driving forces. This relationship is direct if the forces are secular, as for the case of loading of a locked section of a subduction interface by the downgoing plate. If the forces are static, as for the case of lateral variations in gravitational potential energy, then velocity gradients can be produced only if the lithosphere has, on average, zero strength. The static force model may be related to the long-term velocity field but not the instantaneous velocity field (typically measured geodetically over a period of several years) because over short time intervals the upper lithosphere behaves elastically. In order to describe both the short- and long-term behaviour of an (elastic) lithosphere-(viscoelastic) asthenosphere system in a self-consistent manner, I construct a deformation model termed the expected interseismic velocity (EIV) model. Assuming that the lithosphere is populated with faults that rupture continually, each with a definite mean recurrence time, and that the Earth is well approximated as a linear elastic-viscoelastic coupled system, I derive a simple relationship between the instantaneous velocity field and the average rate of moment release in the lithosphere. Examples with synthetic fault networks demonstrate that velocity gradients in actively deforming regions may to a large extent be the product of compounded viscoelastic relaxation from past earthquakes on hundreds of faults distributed over large ( ≥106 km2) areas.

Complex deformation in western Tibet revealed by anisotropic tomography

NASA Astrophysics Data System (ADS)

Zhang, Heng; Zhao, Junmeng; Zhao, Dapeng; Yu, Chunquan; Liu, Hongbing; Hu, Zhaoguo

2016-10-01

The mechanism and pattern of deformation beneath western Tibet are still an issue of debate. In this work we present 3-D P- and S-wave velocity tomography as well as P-wave radial and azimuthal anisotropy along the ANTILOPE-I profile and surrounding areas in western Tibet, which are determined by using a large number of P and S arrival-time data of local earthquakes and teleseismic events. Our results show that low-velocity (low-V) zones exist widely in the middle crust, whereas low-V zones are only visible in the lower crust beneath northwestern Tibet, indicating the existence of significant heterogeneities and complex flow there. In the upper mantle, a distinct low-V gap exists between the Indian and Asian plates. Considering the P- and S-wave tomography and P-wave azimuthal and radial anisotropy results, we interpret the gap to be caused mainly by shear heating. Depth-independent azimuthal anisotropy and high-velocity zones exist beneath the northern part of the study region, suggesting a vertically coherent deformation beneath the Tarim Basin. In contrast, tomographic and anisotropic features change with depth beneath the central and southern parts of the study region, which reflects depth-dependent (or decoupled) deformations there. At the northern edge of the Indian lithospheric mantle (ILM), P-wave azimuthal anisotropy shows a nearly east-west fast-velocity direction, suggesting that the ILM was re-built by mantle materials flowing to the north.

Thermally-driven subsidence of large platformal basins: linking growth of the lithosphere subsidence patterns on the surface

NASA Astrophysics Data System (ADS)

Holt, P.; Allen, M. B.; Van Hunen, J.

2012-04-01

A large number of areas which have experienced platformal subsidence during the Phanerozoic are located upon regions of juvenile accretionary crust. These include the Palaeozoic basins of North Africa, the Paraná and Parnaíba basins in South America, the Cape-Karoo basin in South Africa, the Mesozoic Scythian and Turan platforms in Central Asia and the Eastern Australian basins. We hypothesise that the juvenile accretionary crust is initially underlain by a thin mantle lithosphere. This is most likely inherited from the island arcs, accretionary prisms and microcontinents that collided to form this juvenile crust, although it could also be due to lithospheric delamination as a result of the collision. Once the crust has stabilised the lithosphere begins to cool and thicken, which drives the observed subsidence. To test this we constructed a simple 1D forward finite difference model which calculates heat conduction through a column of crust, mantle lithosphere and upper mantle as it cools. The model then isostatically calculates the water loaded subsidence produced by this process. This allows us to use subsidence curves calculated from the sedimentary record preserved within the basin to test whether the basins could be forming in response to growth of the lithosphere. The results from the model showed that the subsidence produced was most sensitive to variations in crustal thickness and plate thickness (final lithospheric thickness). The modelled subsidence curves were then compared to subsidence curves acquired by backstripping the sediments within the basins mentioned above. The parameters were varied iteratively to find the best fit between the modelled and the observed subsidence. This produced good fits and also provided another method to validate the model results. The crustal thickness and final lithospheric thickness from the models were then compared to measurements of these parameters from other sources such as deep seismic lines and tomographic imaging of the Low Velocity Zone. These generally agreed well with the values used in the model and were used to further constrain the model. However, subsidence of thin lithosphere is not necessarily limited to unmodified accretionary crust, as described above. For instance the subsidence of the West Siberian Basin, outside the rift system, is similar to the platformal basins mentioned above except that there is a delay of 50 - 90 Myrs between the rifting (and associated eruption of the Siberian flood basalts), and the onset of sedimentation. We used a variant of our model that incorporated an anomalously hot layer beneath a thinned lithosphere to represent a cooling mantle plume head. This produced a good match to the subsidence patterns from the West Siberian Basin. This coupling of deep processes with surface processes allows us to further understand how the basins form, but inversely the sedimentary record could be used to investigate the growth of the lithosphere and provide a prediction of present day lithospheric thickness independent of seismic methods.

Integrating shear velocity observations of the Hudson Bay

NASA Astrophysics Data System (ADS)

Porritt, R. W.; Miller, M. S.; Darbyshire, F. A.

2013-12-01

Hudson Bay is the core of the Laurentia craton of North America. This region contains some of the thickest lithosphere globally, reaching 250-300 km depth. Previous studies have shown that much of this region is composed of amalgamated proto-continents including the Western Churchill and Superior provinces and that much of the structure of these constituents has been retained since the Trans-Hudson Orogen at 1.8 Ga. Using the Hudson Bay Lithospheric Experiment (HuBLE) and other permanent and POLARIS broadband seismic data, we image the region with S to P receiver functions, joint inversion of P to S receiver functions with surface waves, and teleseismic S and P wave travel-times. The receiver function imaging reveals a persistent mid-lithospheric layer at ~80 km depth under all stations, but a variable lithospheric thickness. The teleseismic S delay times show a pattern of early arrivals around the center of the network, beneath Hudson Bay where the lithosphere is thickest, while the P delay times are early in the Superior province relative to the Western Churchill province. This suggests higher Vp/Vs ratios in the Superior province, which is evidence that stacked oceanic plates formed this province. The relatively flat Moho imaged by earlier receiver function studies and the lower mantle Vp/Vs of the Western Churchill province provides evidence of formation by plume head extraction. The joint inversion shows an LAB that is typically a broad discontinuity spanning ~20-30 km at ~220 km depth suggesting a primarily thermal boundary zone. The mid-lithospheric layer is composed of increasing velocity from the ~40 km depth Moho defined by H-k stacking of PRFs to a broad, constant velocity lithospheric lid spanning 80-200 km depth. We suggest this mid-lithospheric layer represents the mantle lithosphere of the proto-continents prior to collision and the lid formed due to post-collisional cooling. The integration of these seismic datasets furthers our understanding of plate tectonic and non-tectonic processes during the Archean formation of Laurentia craton.

Three-Dimensional Rheological Structure of North China Craton Determined by Integration of Multiple observations: Controlling Role for Lithospheric Rifting

NASA Astrophysics Data System (ADS)

Xiong, X.; Shan, B.; Li, Y.

2017-12-01

The North China Craton (NCC) has undergone significant lithospheric rejuvenation in late Mesozoic and Cenozoic, one feature of which is the widespread extension and rifting. The extension is distinct between the two parts of NCC: widespread rifting in the eastern NCC and localized narrow rifting in the west. The mechanism being responsible for this difference is uncertain and highly debated. Since lithospheric deformation can be regarded as the response of lithosphere to various dynamic actions, the rheological properties of lithosphere must have a fundamental influence on its tectonics and deformation behavior. In this study, we investigated the 3D thermal and rheological structure of NCC by developing a model integrating several geophysical observables (such as surface heatflow, regional elevation, gravity and geoid anomalies, and seismic tomography models). The results exhibit obvious lateral variation in rheological structure between the eastern and western NCC. The overall lithospheric strength is higher in the western NCC than in the east. Despite of such difference in rheology, both parts of NCC are characterized by mantle dominated strength regime, which facilitates the development of narrow rifting. Using ancient heatflow derived from mantle xenoliths studies, and taking the subduction-related dehydration reactions during Mesozoic into account, we constructed the thermal and rheological structure of NCC in Ordovician, early Cretaceous and early Cenozoic. Combining the evidence from numerical simulations, we proposed an evolution path of the rifting in NCC. The lithosphere of NCC in Ordovician was characterized by a normal craton features: low geotherm, high strength and mantle dominated regime. During Jurassic and Cretaceous, the mantle lithosphere in the eastern NCC was hydrated by fluid released by the suduction of the Pacific plate, resulting in weakening of the lithosphere and a transition from mantle dominated to crust dominated regime, which facilitated the development of metamorphic core complex extension. The rifting in eastern NCC experienced a further transition to the wide rifting style under a low strain rate environment during early Cenozoic. In contrast, the western NNC has been kept mantle dominated regime, leading to a localized narrow rifting.

The Lithospheric Fabric of Southern North America and the Wide Gulf of Mexico Rift

NASA Astrophysics Data System (ADS)

Stern, R. J.

2017-12-01

Rifting of Laurentia out of Greater Gondwana and Cuyania out of Laurentia in Cambrian time was associated with a strongly magmatic triple junction centered near modern Dallas, one arm of which is preserved as the S. Oklahoma Aulacogen. The position of this hotspot and the trend of its two successful arms (which opened to form the Iapetus/Rheic ocean in Early Paleozoic time) carved an irregular southern margin of Laurentia, which has since controlled the tectonic evolution of the region. This re-entrant margin was modified by Pennsylvanian collision of rigid indentor Laurentia with weak arc lithosphere of N. Gondwana, juxtaposing strong Laurentian lithosphere of the Texas craton in the west with weak (hydrated and partially molten) arc lithosphere of N. Gondwana to the east. The different strengths of the two lithospheres was remarkable, with strong Laurentia contrasted with weak N. Gondwana margin, and persisted for 150 m.y. to control Gulf of Mexico rifting. The Ouachita-Marathon foldbelt demarcates regions strongly affected by extension (lithosphere that originally was part of the N. Gondwanan arc and forearc) from unaffected regions (lithosphere that was originally part of Laurentia). Extensional strain to open the Gulf of Mexico in Jurassic time totally occurred in Gondwanan lithosphere and had little effect on Laurentia except for Triassic uplift in Texas (which shed large volumes of clastic sediments westwards, now preserved as Late Triassic Dockum and Chinle Groups) and rifting in Arkansas (to form Late Triassic Eagle Mills grabens) and farther east. Even Pennsylvanian foreland basins and Ancestral Rockies faults intersecting the Ouachita-Marathon orogen do not appear to have been rejuvenated by Triassic-Jurassic extension. Extension in weak Gondwanan lithosphere resulted in a broad rift zone that now buried beneath Mesozoic and younger sediments. Buried fragments of thicker continental crust - the Sabine and Monroe uplifts, the Wiggins Arch, and Florida - must be fragments of Gondwanan arc crust. Because they are buried, we know little about these "Gondwana orphans" and also the deeper basins associated with the buried broad region of distributed extension. It will require joint efforts by academia, industry, and government to probe this region.

Effective elastic thickness of Africa and its relationship to other proxies for lithospheric structure and surface tectonics

NASA Astrophysics Data System (ADS)

Pérez-Gussinyé, M.; Metois, M.; Fernández, M.; Vergés, J.; Fullea, J.; Lowry, A. R.

2009-09-01

Detailed information on lateral variations in lithospheric properties can aid in understanding how surface deformation relates to deep Earth processes. The effective elastic thickness, Te, of the lithosphere is a proxy for lithospheric strength. Here, we present a new Te map of the African lithosphere estimated from coherence analysis of topography and Bouguer anomaly data. The latter data set derives from the EGM 2008 model, the highest resolution gravity database over Africa, enabling a significant improvement in lateral resolution of Te. The methodology used for Te estimation improves upon earlier approaches by optimally combining estimates from several different window sizes and correcting for an estimation bias term. Our analysis finds that Te is high, ~ 100 km, in the West African, Congo, Kalahari and Tanzania cratons. Of these, the Kalahari exhibits the lowest Te. Based in part on published seismic and mineral physics constraints, we suggest this may reflect modification of Kalahari lithosphere by anomalously hot asthenospheric mantle. Similarly, the Tanzania craton exhibits relatively lower Te east of Lake Victoria, where a centre of seismic radial anisotropy beneath the craton has been located and identified with a plume head, thus suggesting that here too, low Te reflects modification of cratonic lithosphere by an underlying hot mantle. The lowest Te in Africa occurs in the Afar and Main Ethiopian rifts, where lithospheric extension is maximum. In the western Ethiopian plateau a local Te minimum coincides with published images of a low P and S seismic velocity anomaly extending to ~ 400 km depth. Finally, the Darfur, Tibesti, Hoggar and Cameroon line volcanic provinces are characterised by low Te and no deep-seated seismic anomalies in the mantle. Corridors of relatively low Te connect these volcanic provinces to the local Te minima within the western Ethiopian plateau. We interpret the low Te to indicate thinner lithosphere within the corridors than in the surrounding cratons. We speculate that these corridors may provide potential conduits for hot asthenospheric material to flow from the western Ethiopian plateau to the volcanic provinces of central and western Africa.

Effective elastic thickness of Africa and its relationship to other proxies for lithospheric structure and surface tectonics

NASA Astrophysics Data System (ADS)

Perez-Gussinye, M.; Metois, M.; Fernandez, M.; Verges, J.; Fullea, J.; Lowry, A. R.

2009-12-01

Detailed information on lateral variations in lithospheric properties can aid in understanding how surface deformation relates to deep Earth processes. The effective elastic thickness, Te, of the lithosphere is a proxy for lithospheric strength. Here, we present a new Te map of the African lithosphere estimated from coherence analysis of topography and Bouguer anomaly data. The latter data set derives from the EGM 2008 model, the highest resolution gravity database over Africa, enabling a significant improvement in lateral resolution of Te. The methodology used for Te estimation improves upon earlier approaches by optimally combining estimates from several different window sizes and correcting for an estimation bias term. Our analysis finds that Te is high, ~ 100 km, in the West African, Congo, Kalahari and Tanzania cratons. Of these, the Kalahari exhibits the lowest Te. Based in part on published seismic and mineral physics constraints, we suggest this may reflect modification of Kalahari lithosphere by anomalously hot asthenospheric mantle. Similarly, the Tanzania craton exhibits relatively lower Te east of Lake Victoria, where a centre of seismic radial anisotropy beneath the craton has been located and identified with a plume head, thus suggesting that here too, low Te reflects modification of cratonic lithosphere by an underlying hot mantle. The lowest Te in Africa occurs in the Afar and Main Ethiopian rifts, where lithospheric extension is maximum. In the western Ethiopian plateau a local Te minimum coincides with published images of a low P and S seismic velocity anomaly extending to ~400 km depth. Finally, the Darfur, Tibesti, Hoggar and Cameroon line vo provinces lcanic are characterised by low Te and no deep-seated seismic anomalies in the mantle. Corridors of relatively low Te connect these volcanic provinces to the local Te minima within the western Ethiopian plateau. We interpret the low Te to indicate thinner lithosphere within the corridors than in the surrounding cratons. We speculate that these corridors may provide potential conduits for hot asthenospheric material to flow from the western Ethiopian plateau to the volcanic provinces of central and western Africa.

The Main Ethiopian Rift: a Narrow Rift in a Hot Craton?

NASA Astrophysics Data System (ADS)

Gashawbeza, E.; Keranen, K.; Klemperer, S.; Lawrence, J.

2008-12-01

The Main Ethiopian Rift (MER) is a classic example of a narrow rift, but a synthesis of our results from the EAGLE (Ethiopia-Afar Geoscientific Lithospheric Experiment Phase I broadband experiment) and from the EBSE experiment (Ethiopia Broadband Seismic Experiment) suggests the MER formed in thin, hot, weak continental lithosphere, in strong contrast with predictions of the Buck model of modes of continental lithospheric extension. Our joint inversion of receiver functions and Rayleigh-wave group velocities yields shear-wave velocities of the lowermost crust and uppermost mantle across the MER and the Ethiopian Plateau that are significantly lower than the equivalent velocities in the Eastern and Western branches of the East African Rift System. The very low shear-wave velocities, high electrical conductivity in the lower-crust, and high shear-wave splitting delay times beneath a very broad region of the MER and the Ethiopian Plateau indicate that the lower-crust is hot and likely contains partial melt. Our S-receiver function data demonstrate shallowing of the lithosphere-asthenosphere boundary from 90 km beneath the northwestern Ethiopian Plateau to 60 km beneath the MER. Although we lack good spatial resolution on the lithosphere-asthenosphere boundary, the region of thinned lithosphere may be intermediate in width between the narrow surface rift (< 100 km) and the broader zone of strain in the lower crust (~ 300 km). The MER developed as a narrow rift at the surface, localized along the Neoproterozoic suture that joined East and West Gondwana. However, a far broader of lower crust and uppermost mantle remains thermally weakened since the Oligocene formation of the flood basalts by the Afar plume head. If the lithosphere- asthenosphere boundary is indeed a strain marker then lithospheric mantle deformation is localized beneath the surface rift. The development of both the Eastern/Western branches of the East African Rift System to the south and of the MER in the north as narrow rifts, despite vastly different lithospheric strength profiles, indicates that inherited structure, rather than rheological stratification, is the primary control on the mode of extension in these continental rifts.

Low lower crustal velocity across Ethiopia: Is the Main Ethiopian Rift a narrow rift in a hot craton?

NASA Astrophysics Data System (ADS)

Keranen, Katie M.; Klemperer, Simon L.; Julia, Jordi; Lawrence, Jesse F.; Nyblade, Andy A.

2009-05-01

The Main Ethiopian Rift (MER) is a classic narrow rift that developed in hot, weak lithosphere, not in the initially cold, thick, and strong lithosphere that would be predicted by common models of rift mode formation. Our new 1-D seismic velocity profiles from Rayleigh wave/receiver function joint inversion across the MER and the Ethiopian Plateau indicate that hot lower crust and upper mantle are present throughout the broad region affected by Oligocene flood basalt volcanism, including both the present rift and the adjacent Ethiopian Plateau hundreds of kilometers from the rift valley. The region of hot lithosphere closely corresponds to the region of flood basalt volcanism, and we interpret that the volcanism and thermal perturbation were jointly caused by impingement of the Afar plume head. Across the affected region, Vs is 3.6-3.8 km/s in the lowermost crust and ≤4.3 km/s in the uppermost mantle, both ˜0.3 km/s lower than in the eastern and western branches of the East African Rift System to the south. We interpret the low Vs in the lower crust and upper mantle as indicative of hot lithosphere with partial melt. Our results lead to a hybrid rift mode, in which the brittle upper crust has developed as a narrow rift along the Neoproterozoic suture between East and West Gondwana, while at depth lithospheric deformation is distributed over the broad region (˜400 km wide) thermally perturbed by the broad thermal upwelling associated with the Afar plume head. Development of both the East African Rift System to the south (in cold, strong lithosphere) and the MER to the north (in hot, weak lithosphere) as narrow rifts, despite their vastly different initial thermal states and depth-integrated lithospheric strength, indicates that common models of rift mode formation that focus only on temperature, thickness, and vertical strength profiles do not apply to these classic continental rifts. Instead, inherited structure and associated lithospheric weaknesses are the primary control on the mode of extension.

Geochemical evidence for pre- and syn-rifting lithospheric foundering in the East African Rift System

NASA Astrophysics Data System (ADS)

Nelson, W. R.; Furman, T.; Elkins-Tanton, L. T.

2015-12-01

The East African Rift System (EARS) is the archetypal active continental rift. The rift branches cut through the elevated Ethiopian and Kenyan domes and are accompanied by a >40 Myr volcanic record. This record is often used to understand changing mantle dynamics, but this approach is complicated by the diversity of spatio-temporally constrained, geochemically unique volcanic provinces. Various sources have been invoked to explain the geochemical variability across the EARS (e.g. mantle plume(s), both enriched and depleted mantle, metasomatized or pyroxenitic lithosphere, continental crust). Mantle contributions are often assessed assuming adiabatic melting of mostly peridotitic material due to extension or an upwelling thermal plume. However, metasomatized lithospheric mantle does not behave like fertile or depleted peridotite mantle, so this model must be modified. Metasomatic lithologies (e.g. pyroxenite) are unstable compared to neighboring peridotite and can founder into the underlying asthenosphere via ductile dripping. As such a drip descends, the easily fusible metasomatized lithospheric mantle heats conductively and melts at increasing T and P; the subsequent volcanic products in turn record this drip magmatism. We re-evaluated existing data of major mafic volcanic episodes throughout the EARS to investigate potential evidence for lithospheric drip foundering that may be an essential part of the rifting process. The data demonstrate clearly that lithospheric drip melting played an important role in pre-flood basalt volcanism in Turkana (>35 Ma), high-Ti "mantle plume-derived" flood basalts and picrites (HT2) from NW Ethiopia (~30 Ma), Miocene shield volcanism on the E Ethiopian Plateau and in Turkana (22-26 Ma), and Quaternary volcanism in Virunga (Western Rift) and Chyulu Hills (Eastern Rift). In contrast, there is no evidence for drip melting in "lithosphere-derived" flood basalts (LT) from NW Ethiopia, Miocene volcanism in S Ethiopia, or Quaternary within-rift lavas in Ethiopia, Turkana or Kivu. The evidence for widespread lithospheric removal across eastern Africa coincides with the timing of dome uplift (e.g. Gani et al., 2007; Wichura et al., 2015) and further demonstrates the controls of lithospheric mantle on volcano-tectonic processes throughout the evolving EARS.

Low lower crustal velocity across Ethiopia: Is the Main Ethiopian Rift a narrow rift in a hot craton?

USGS Publications Warehouse

Keranen, K.M.; Klemperer, S.L.; Julia, J.; Lawrence, J. F.; Nyblade, A.A.

2009-01-01

[1] The Main Ethiopian Rift (MER) is a classic narrow rift that developed in hot, weak lithosphere, not in the initially cold, thick, and strong lithosphere that would be predicted by common models of rift mode formation. Our new 1-D seismic velocity profiles from Rayleigh wave/receiver function joint inversion across the MER and the Ethiopian Plateau indicate that hot lower crust and upper mantle are present throughout the broad region affected by Oligocene flood basalt volcanism, including both the present rift and the adjacent Ethiopian Plateau hundreds of kilometers from the rift valley. The region of hot lithosphere closely corresponds to the region of flood basalt volcanism, and we interpret that the volcanism and thermal perturbation were jointly caused by impingement of the Afar plume head. Across the affected region, Vs is 3.6-3.8 km/s in the lowermost crust and ???4.3 km/s in the uppermost mantle, both ??0.3 km/s lower than in the eastern and western branches of the East African Rift System to the south. We interpret the low Vs in the lower crust and upper mantle as indicative of hot lithosphere with partial melt. Our results lead to a hybrid rift mode, in which the brittle upper crust has developed as a narrow rift along the Neoproterozoic suture between East and West Gondwana, while at depth lithospheric deformation is distributed over the broad region (??400 km wide) thermally perturbed by the broad thermal upwelling associated with the Afar plume head. Development of both the East African Rift System to the south (in cold, strong lithosphere) and the MER to the north (in hot, weak lithosphere) as narrow rifts, despite their vastly different initial thermal states and depth-integrated lithospheric strength, indicates that common models of rift mode formation that focus only on temperature, thickness, and vertical strength profiles do not apply to these classic continental rifts. Instead, inherited structure and associated lithospheric weaknesses are the primary control on the mode of extension. ?? 2009 by the American Geophysical Union.

Metasomatic Enrichment of Oceanic Lithospheric Mantle Documented by Petit-Spot Xenoliths

NASA Astrophysics Data System (ADS)

Pilet, S.; Abe, N.; Rochat, L.; Hirano, N.; Machida, S.; Kaczmarek, M. A.; Muntener, O.

2015-12-01

Oceanic lithosphere is generally interpreted as mantle residue after MORB extraction. It has been proposed, however, that metasomatism could take place at the interface between the low-velocity zone and the cooling and thickening oceanic lithosphere or by the percolation of low-degree melts produced in periphery of Mid Ocean Ridges. This later process is observed in slow spreading ridges and ophiolites where shallow oceanic lithospheric mantle could be metasomatized/refertilized during incomplete MORB melt extraction. Nevertheless, direct evidence for metasomatic refertilization of the deep part of the oceanic lithospheric mantle is still missing. Xenoliths and xenocrysts sampled by petit-spot volcanoes interpreted as low-degree melts extracted from the base of the lithosphere in response to plate flexure, provide important new information about the nature and the processes associated with the evolution of oceanic lithospheric mantle. Here, we report, first, the presence of a garnet xenocryst in petit-spot lavas from Japan characterized by low-Cr, low-Ti content and mostly flat MREE-HREE pattern. This garnet is interpreted as formed during subsolidus cooling of pyroxenitic or gabbroic cumulates formed at ~1 GPa during the incomplete melt extraction at the periphery of the Pacific mid-ocean ridge. It is the first time that such processes are documented in fast spreading context. Second, we report petit-spot mantle xenoliths with cpx trace element "signatures" characterized by high U, Th, relative depletion in Nb, Pb, Ti and high but variable LREE/HREE ratio suggesting equilibration depth closed to the Gt/Sp transition zone. Such "signatures" are unknown from oceanic settings and show unexpected similarity to melt-metasomatized gt-peridotites sampled by kimberlites. This similarity suggests that metasomatic processes are not restricted to continental setting, but could correspond to a global mechanism at the lithosphere-asthenosphere boundary. As plate flexure represents a global mechanism in subduction zone, a portion of oceanic lithospheric mantle is likely to be metasomatized; recycling of these enriched domains into the convecting mantle is fundamental to understand the generation of small scale mantle isotopic and volatile heterogeneities sampled by OIBs and MORBs.

Water in the Cratonic Mantle Lithosphere

NASA Technical Reports Server (NTRS)

Peslier, A. H.

2016-01-01

The fact that Archean and Proterozoic cratons are underlain by the thickest (>200 km) lithosphere on Earth has always puzzled scientists because the dynamic convection of the surrounding asthenosphere would be expected to delaminate and erode these mantle lithospheric "keels" over time. Although density and temperature of the cratonic lithosphere certainly play a role in its strength and longevity, the role of water has only been recently addressed with data on actual mantle samples. Water in mantle lithologies (primarily peridotites and pyroxenites) is mainly stored in nominally anhydrous minerals (olivine, pyroxene, garnet) where it is incorporated as hydrogen bonded to structural oxygen in lattice defects. The property of hydrolytic weakening of olivine [4] has generated the hypothesis that olivine, the main mineral of the upper mantle, may be dehydrated in cratonic mantle lithospheres, contributing to its strength. This presentation will review the distribution of water concentrations in four cratonic lithospheres. The distribution of water contents in olivine from peridotite xenoliths found in kimberlites is different in each craton (Figure 1). The range of water contents of olivine, pyroxene and garnet at each xenolith location appears linked to local metasomatic events, some of which occurred later then the Archean and Proterozoic when these peridotites initially formed via melting. Although the low olivine water contents (<10 ppm wt H2O) at > 6 GPa at the base of the Kaapvaal cratonic lithosphere may contribute to its strength, and prevent its delamination, the wide range of those from Siberian xenoliths is not compatible with providing a high enough viscosity contrast with the asthenophere. The water content in olivine inclusions from Siberian diamonds, on the other hand, have systematically low water contents (<20 ppm wt H2O). The xenoliths may represent a biased sample of the cratonic lithosphere with an over-­abundance of metasomatized peridotites with high water contents. The olivine inclusions, however, may have been protected from metasomatism by their host diamond and record the overall low olivine water content of the cratonic lithosphere. Water may thus still play a role in cratonic keel longevity.

Lu-Hf isotope constraints on plume-lithosphere interaction during emplacement of the Bushveld Large Igneous Province at 2.06 Ga: Implications for the structure and evolution of the Kaapvaal Craton's lithospheric mantle

NASA Astrophysics Data System (ADS)

Zirakparvar, N. A.; Mathez, E. A.; Rajesh, H.; Vervoort, J. D.; Choe, S.

2016-12-01

The Bushveld Large Igneous Province (B-LIP) comprises a diverse array of >30 magma bodies that intruded the Kaapvaal Craton at 2.06 Ga. In this talk we use zircon and bulk-rock Lu-Hf isotope data to show that the B-LIP formed in response to the arrival of a plume(s) from the deep mantle. New zircon Hf isotope compositions for four B-LIP bodies yield intrusion-specific average ɛHf (2.06 Ga) values that range from -20.7 ± 2.8 to -2.7 ± 2.8, largely consistent with literature zircon data for other B-LIP intrusions. Bulk-rock solution ɛHf (2.06 Ga) values for a variety of B-LIP intrusions range from -2.1 ± 0.2 to -10.6 ± 0.2. Because the most radiogenic Hf isotope compositions across the entire B-LIP are nearly primordial with an ɛHf (2.06 Ga) close to 0, it is likely that the heat source of the B-LIP was a plume(s) from deep mantle. The Hf isotope data further suggests that individual intrusions in the B-LIP can be grouped into four categories based on their ultimate sources: 1) melts generated in subduction and plume modified continental lithospheric mantle; 2) melts generated by melting of a mafic-ultramafic reservoir composed of older ( 2.7 Ga) plume-related material trapped in the Kaapvaal lithosphere; 3) melts generated in the mid- to upper crust; and 4) melts generated from the 2.06 Ga mantle plume itself. The presence of 2.7 Ga mafic-ultramafic material in the Kaapvaal lithosphere may have acted to strengthen the lithosphere so that it was able to resist being dispered by the arrival of the B-LIP plume at 2.06 Ga. Because the B-LIP extends into a 2.7 Ga aged suture zone between the Kaapvaal and Zimbabwe cratons, it is also possible to understand the role of the lithospheric mantle in producing the Lu-Hf signatures observed in the various B-LIP intrusions as a function of two different types of the continental lithosphere: The very old lithosphere comprising the Kaapvaal Craton and the somewhat younger lithosphere comprising the suture zone. A basic observation is that the Hf isotope signature of the plume source is only directly expressed in B-LIP bodies that intruded the suture zone, providing further evidence that the craton was already underlain by thick lithospheric mantle at the time of B-LIP magmatism.

Convective instability within the Tibetan Lithospheric Mantle (Invited)

NASA Astrophysics Data System (ADS)

Houseman, G. A.; Molnar, P. H.; Evans, L.; England, P. C.

2013-12-01

Studies of seismic surface waves in Asia show that shear-wave speeds at depths of ~120-250km beneath the Tibetan Plateau are higher than is generally observed for continents, other than beneath Archaean cratons. The high-speed layer has been interpreted as continental lithosphere that was thickened during the convergence between India and Asia. This interpretation contradicts conceptual models in which gravitational instabilities remove a significant fraction of the mantle lithosphere beneath Tibet during that convergence. In contrast, the suggestion of relatively recent (post-early-Miocene) surface uplift of the Plateau, inferred from the onset of normal faulting across the plateau, synchronous increased rates of compressional deformation in the surroundings of the the plateau, and widespread volcanism in the northern part of the plateau, implies action of a mechanism that increased the gravitational potential energy of, and temperatures within, the Tibetan lithosphere in a way that would not occur if the mantle lithosphere had simply thickened continually throughout the India-Asia convergence. A resolution to this paradox is suggested by the observation that, while shear-wave speeds are indeed high at depths of 120-250 km beneath the Tibetan plateau, they are anomalously low at shallower depths, implying a temperature inversion that is hard to reconcile with uninterrupted lithospheric thickening. We suggest that the ensemble of observations may be explained by the convective overturn of a lithospheric root that is depleted in iron such that it remains buoyant with respect to normal upper mantle. The increased rate of strain within the Tibetan lithosphere once convergence began reduced its effective viscosity, and continuing convergence thickened the lithospheric root. These conditions led to convective overturn, similar to the original conceptual models, with the difference that the overturn was confined within the root, which remains buoyant with respect to the deeper upper mantle. The intrinsic density difference between the root and underlying asthenosphere need only be as large as the density difference due to ~600 K temperature contrast (i.e., ~ 60 kg/m^3) in order that the lithospheric layer remains in place during convective overturn. Such an overturn can occur on a short geological time scale (~ 10 Myr), with the wavelength of the convective flow field likely to be a small multiple of the ~130 km thickness of the depleted lithospheric layer. The horizontal variations of density and temperature implied by this process would not be detectable using typical surface wave analyses, which lack the necessary horizontal resolution at such depths, but may be detectable using body wave tomography given a sufficiently dense ray coverage from a large aperture surface array. Internal convective overturn is a process that can explain the horizontally averaged depth variation of velocity and an abrupt but delayed heating event at the base of the crust.

Asthenosphere–lithosphere interactions in Western Saudi Arabia: Inferences from 3He/4He in xenoliths and lava flows from Harrat Hutaymah

USGS Publications Warehouse

Konrad, Kevin; Graham, David W; Thornber, Carl; Duncan, Robert A.; Kent, Adam J.R.; Al-Amri, Abdulla

2016-01-01

Elevated 3He/4He in the western harrats has been observed only at Rahat (up to 11.8 RA; Murcia et al., 2013), a volcanic field situated above thinned lithosphere beneath the Makkah-Medinah-Nafud volcanic lineament. Previous work established that spinel lherzolites at Hutaymah are sourced near the lithosphere-asthenosphere boundary (LAB), while other xenolith types there are derived from shallower depths within the lithosphere itself (Thornber, 1992). Helium isotopes are consistent with melts originating near the LAB beneath many of the Arabian harrats, and any magma derived from the Afar mantle plume currently appears to be of minor importance.

The Effect of Plume Impingement on Lithospheric Preservation Beneath the Kenya Rift, East Africa

NASA Astrophysics Data System (ADS)

Hamblock, J. M.; Anthony, E. Y.; Chesley, J. T.; Omenda, P. A.

2003-12-01

The Kenya Rift is located at the transition between Archean Tanzanian craton and Proterozoic mobile belt. Currently, discrepancies exist between geochemical and geophysical interpretations of lithospheric preservation in the Kenya Rift. Seismic data show a sharp vertical boundary between low velocity mantle in the axis and higher velocity mantle on the flanks, which is interpreted to reflect lithospheric erosion from the axis (Mechie et al., 1997; Prodehl et al., 1997). However, geochemical data suggest that the lithospheric mantle is intact beneath both the axis and the flanks. Different elemental groups are observed for rocks from Kenya (Hamblock et al., 2003). One group is characterized by elemental concentrations greater than ocean island basalts (OIB), negative K and Sr anomalies, and Lan and Cen greater than 100. These characteristics are found in silica-undersaturated rocks such as nephelinites, basanites, and some alkali basalts from the flank and the axis and are interpreted to represent melting of an enriched lithosphere. A second group is characterized by elemental concentrations less than OIB, a flat overall pattern, and Lan and Cen less than 100. This pattern is found in alkali basalts and hypersthene-normative rocks. The multi-element pattern varies minimally between axis and flank lavas, with axial lavas containing higher concentrations of Ba (Macdonald et al., 2001). Because rocks of both groups are present in the axis and the flanks, lithosphere appears to be intact across the Kenya Rift, and strong lateral contrasts in composition do not exist. Sr, Nd, and Pb isotopes also suggest that ancient lithospheric mantle is present in Kenya and Tanzania (Macdonald et al., 2001; Paslick et al., 1995). A consistent difference between axis and flank is lower La/Yb for axis lavas, indicating that they originate in the spinel stability field. Flank lavas, regardless of their silica saturation, have higher La/Yb and are interpreted to come from garnet peridotite. Discrepancies between geophysical and geochemical data exist for other parts of the East African Rift as well. In the axis of the rift in Tanzania, tomography suggests that upwelling asthenosphere has eroded the lithosphere (Nyblade, 2002). However, gravity models (Simiyu and Keller, 1997, 2001) and the presence of subchondritic 187Os/188Os in spinel and garnet-bearing xenoliths (TRD of 2.6 Ga) suggest that the lithosphere is intact (Chesley et al., 1999). In contrast, for the Tanzanian craton, Os isotopes, gravity, and tomography are consistent with 2.5-2.9 Ga lithosphere existing to depths of 140 km and a broad thermal and geochemical anomaly (plume?) below the lithosphere (Chesley et al., 1999; Owens et al., 2000; Nyblade et al., 2000; Simiyu and Keller, 1997, 2001). In the Sidamo region of Ethiopia, Os isotopes suggest that ancient depleted mantle is present and has been modified by recent melt percolation (Lorand et al., 2003; Reisberg et al., in press). Finally, for Ethiopian flood basalts, element chemistry, petrology, and 3He/4He (Marty et al., 1996; Scarsi and Craig, 1996) indicate a dominant role for plume. Os isotopes (Davies et al., 2003), however, are lower than PUM, indicating that an ancient lithospheric mantle reservoir is present. In order to help resolve the discrepancies between geophysical and geochemical interpretations, we will obtain petrologic and isotopic data for xenoliths and mafic lavas in both east-west and north-south directions. The lavas span a wide range of silica saturation and La/Yb ratios, and thus are intended to represent lithospheric as well as asthenospheric sources.

Nurture Versus Nature: Accounting for the Differences Between the Taiwan and Timor active arc-continent collisions

NASA Astrophysics Data System (ADS)

Harris, R. A.

2011-12-01

The active Banda arc/continent collision of the Timor region provides many important contrasts to what is observed in Taiwan, which is mostly a function of differences in the nature of the subducting plate. One of the most important differences is the thermal state of the respective continental margins: 30 Ma China passive margin versus 160 Ma NW Australian continental margin. The subduction of the cold and strong NW Australian passive margin beneath the Banda trench provides many new constraints for resolving longstanding issues about the formative stages of collision and accretion of continental crust. Some of these issues include evidence for slab rollback and subduction erosion, deep continental subduction, emplacement or demise of forearc basement, relative amounts of uplift from crustal vs. lithospheric processes, influence of inherited structure, partitioning of strain away from the thrust front, extent of mélange development, metamorphic conditions and exhumation mechanisms, continental contamination and accretion of volcanic arcs, does the slab tear, and does subduction polarity reverse? Most of these issues link to the profound control of lower plate crustal heterogeneity, thermal state and inherited structure. The thermomechanical characteristics of subducting an old continental margin allow for extensive underthrusting of lower plate cover units beneath the forearc and emplacement and uplift of extensive nappes of forearc basement. It also promotes subduction of continental crust to deep enough levels to experience high pressure metamorphism (not found in Taiwan) and extensive contamination of the volcanic arc. Seismic tomography confirms subduction of continental lithosphere beneath the Banda Arc to at least 400 km with no evidence for slab tear. Slab rollback during this process results in massive subduction erosion and extension of the upper plate. Other differences in the nature of the subducting plates in Taiwan in Timor are differences in the lateral continuity of the continental margins. The northern Australian continental margin is highly irregular with many rift basins subducting parallel to their axes. This feature gives rise to irregularities in the uplift pattern of the collision and its continental margin parallel structural grain. Another major difference between Taiwan and Timor is the mechanical stratigraphy entering the trench. The Australian continental margin bears a carbonate rich pre and post rift sequence that is separated by a 1000 m thick, over pressured mudstone unit that acts as major detachment and promotes extensive mud diapirism. The post breakup Australian Passive Margin Sequence is incorporated into the orogenic wedge by frontal accretion and forms a classic imbricate thrust stack near the front of the Banda forearc. The pre breakup Gondwana Sequence below the detachment continues at least to depth of 30 km in the subduction channel beneath the Banda forearc upper plate and stacks up into a duplex zone that forms structural culminations throughout Timor. The upper plate of both collisions is similar in nature but is deformed in different ways due to the strong influence of the lower plate. However, both have extensive subduction erosion and demise of the forearc and systematic accretion of the arc.

Subduction and exhumation of a continental margin in the Scandinavian Caledonides: Insights from ultrahigh pressure metamorphism, late orogenic basins and 3D numerical modelling

NASA Astrophysics Data System (ADS)

Cuthbert, Simon

2017-04-01

The Scandinavian Caledonides (SC) represents a plate collision zone of Himalayan style and scale. Three fundamental characteristics of this orogen are: (1) early foreland-directed, tectonic transport and stacking of nappes; (2) late, wholesale reversal of tectonic transport; (3) ultrahigh pressure metamorphism of felsic crust derived from the underthrusting plate at several levels in the orogenic wedge and below the main thrust surface, indicating subduction of continental crust into the mantle. The significance of this for crustal evolution is the profound remodeling of continental crust, direct geochemical interaction of such crust and the mantle and the opening of accommodation space trapping large volumes of clastic detritus within the orogen. The orogenic wedge of the SC was derived from the upper crust of the Baltica continental margin (a hyper-extended passive margin), plus terranes derived from an assemblage of outboard arcs and intra-oceanic basins and, at the highest structural level, elements of the Laurentian margin. Nappe emplacement was driven by Scandian ( 430Ma) collision of Baltica with Laurentia, but emerging Middle Ordovician ages for diamond-facies metamorphism for the most outboard (or rifted) elements of Baltica suggest prior collision with an arc or microcontinent. Nappes derived from Baltica continental crust were subducted, in some cases to depths sufficient to form diamond. These then detached from the upper part of the down-going plate along major thrust faults, at which time they ceased to descend and possibly rose along the subduction channel. Subduction of the remaining continental margin continued below these nappes, possibly driven by slab-pull of the previously subducted Iapetus oceanic lithosphere and metamorphic densification of subducted felsic continental margin. 3D numerical modelling based upon a Caledonide-like plate scenario shows that if a continental corner or promontory enters the subduction zone, the continental margin descends to greater depths than for a simple orthogonal collision and its modelled thermal evolution is consistent with UHP metamorphic assemblages recorded in the southern part of the SC. Furthermore, a tear initiates at the promontary tip along the ocean-continent junction and propagates rapidly along the orogen. The buoyant upthrust of the subducted margin can then lead to reversal of the motion vector of the entire subducting continent, which withdraws the subducted lithospheric margin out of the subduction channel ("eduction"). Because of the diachroneity of slab failure, the continent also rotates, which causes the eduction vector to change azimuth over time. These model behaviours are consistent with the late orogenic structural evolution of the southern SC. However, during the final exhumation stage the crust may not have acted entirely coherently, as some eduction models propose: There is evidence that some inboard Baltica crust experienced late, shallow subduction before detaching as giant "flakes" that carried the orogenic wedge piggyback, forelandwards. Eduction and flake-tectonics could have operated coevally; the model system does not preclude this. Finally, the traction of a large educting (or extruding) mass of continental margin against the overlying orogenic wedge may have stretched and ruptured the wedge, resulting in opening of the late-orogenic Old Red Sandstone molasse basins.

State of stress, faulting, and eruption characteristics of large volcanoes on Mars

NASA Technical Reports Server (NTRS)

Mcgovern, Patrick J.; Solomon, Sean C.

1993-01-01

The formation of a large volcano loads the underlying lithospheric plate and can lead to lithospheric flexure and faulting. In turn, lithospheric stresses affect the stress field beneath and within the volcanic edifice and can influence magma transport. Modeling the interaction of these processes is crucial to an understanding of the history of eruption characteristics and tectonic deformation of large volcanoes. We develop models of time-dependent stress and deformation of the Tharsis volcanoes on Mars. A finite element code is used that simulates viscoelastic flow in the mantle and elastic plate flexural behavior. We calculate stresses and displacements due to a volcano-shaped load emplaced on an elastic plate. Models variously incorporate growth of the volcanic load with time and a detachment between volcano and lithosphere. The models illustrate the manner in which time-dependent stresses induced by lithospheric plate flexure beneath the volcanic load may affect eruption histories, and the derived stress fields can be related to tectonic features on and surrounding martian volcanoes.

Earthquake rupture below the brittle-ductile transition in continental lithospheric mantle

PubMed Central

Prieto, Germán A.; Froment, Bérénice; Yu, Chunquan; Poli, Piero; Abercrombie, Rachel

2017-01-01

Earthquakes deep in the continental lithosphere are rare and hard to interpret in our current understanding of temperature control on brittle failure. The recent lithospheric mantle earthquake with a moment magnitude of 4.8 at a depth of ~75 km in the Wyoming Craton was exceptionally well recorded and thus enabled us to probe the cause of these unusual earthquakes. On the basis of complete earthquake energy balance estimates using broadband waveforms and temperature estimates using surface heat flow and shear wave velocities, we argue that this earthquake occurred in response to ductile deformation at temperatures above 750°C. The high stress drop, low rupture velocity, and low radiation efficiency are all consistent with a dissipative mechanism. Our results imply that earthquake nucleation in the lithospheric mantle is not exclusively limited to the brittle regime; weakening mechanisms in the ductile regime can allow earthquakes to initiate and propagate. This finding has significant implications for understanding deep earthquake rupture mechanics and rheology of the continental lithosphere. PMID:28345055

Earthquake rupture below the brittle-ductile transition in continental lithospheric mantle.

PubMed

Prieto, Germán A; Froment, Bérénice; Yu, Chunquan; Poli, Piero; Abercrombie, Rachel

2017-03-01

Earthquakes deep in the continental lithosphere are rare and hard to interpret in our current understanding of temperature control on brittle failure. The recent lithospheric mantle earthquake with a moment magnitude of 4.8 at a depth of ~75 km in the Wyoming Craton was exceptionally well recorded and thus enabled us to probe the cause of these unusual earthquakes. On the basis of complete earthquake energy balance estimates using broadband waveforms and temperature estimates using surface heat flow and shear wave velocities, we argue that this earthquake occurred in response to ductile deformation at temperatures above 750°C. The high stress drop, low rupture velocity, and low radiation efficiency are all consistent with a dissipative mechanism. Our results imply that earthquake nucleation in the lithospheric mantle is not exclusively limited to the brittle regime; weakening mechanisms in the ductile regime can allow earthquakes to initiate and propagate. This finding has significant implications for understanding deep earthquake rupture mechanics and rheology of the continental lithosphere.

Shield volcanism and lithospheric structure beneath the Tharsis plateau, Mars

NASA Technical Reports Server (NTRS)

Blasius, K. R.; Cutts, J. A.

1976-01-01

The heights of four great shield volcanoes, when interpreted as reflecting the local hydrostatic head on a common source of upwelling magma, provide significant constraints on models of lithospheric structure beneath the Tharsis plateau. If Bouguer gravity anomalies are modeled in terms of a variable thickness crust, and a two-component (crust/mantle) earth-like structure is assumed for the Martian lithosphere, the derived model lithosphere beneath the Tharsis plateau has the following properties: (1) the upper low-density 'crustal' component is thickened beneath the Tharsis plateau; (2) the lower high-density 'mantle' component is thinned beneath the Tharsis plateau; and (3) there is a net gradient on the base of the Martian lithosphere directed downward away from beneath the summit of the Tharsis plateau. A long history of magmatic intrusion is hypothesized to have been the cause of the updoming of the Tharsis plateau and the maintenance of the plateau in a state of only partial compensation.

The importance of structural softening for the evolution and architecture of passive margins

PubMed Central

Duretz, T.; Petri, B.; Mohn, G.; Schmalholz, S. M.; Schenker, F. L.; Müntener, O.

2016-01-01

Lithospheric extension can generate passive margins that bound oceans worldwide. Detailed geological and geophysical studies in present and fossil passive margins have highlighted the complexity of their architecture and their multi-stage deformation history. Previous modeling studies have shown the significant impact of coarse mechanical layering of the lithosphere (2 to 4 layer crust and mantle) on passive margin formation. We built upon these studies and design high-resolution (~100–300 m) thermo-mechanical numerical models that incorporate finer mechanical layering (kilometer scale) mimicking tectonically inherited heterogeneities. During lithospheric extension a variety of extensional structures arises naturally due to (1) structural softening caused by necking of mechanically strong layers and (2) the establishment of a network of weak layers across the deforming multi-layered lithosphere. We argue that structural softening in a multi-layered lithosphere is the main cause for the observed multi-stage evolution and architecture of magma-poor passive margins. PMID:27929057

Ancient Continental Lithosphere Dislocated Beneath Ocean Basins Along the Mid-Lithosphere Discontinuity: A Hypothesis

NASA Astrophysics Data System (ADS)

Wang, Zhensheng; Kusky, Timothy M.; Capitanio, Fabio A.

2017-09-01

The documented occurrence of ancient continental cratonic roots beneath several oceanic basins remains poorly explained by the plate tectonic paradigm. These roots are found beneath some ocean-continent boundaries, on the trailing sides of some continents, extending for hundreds of kilometers or farther into oceanic basins. We postulate that these cratonic roots were left behind during plate motion, by differential shearing along the seismically imaged mid-lithosphere discontinuity (MLD), and then emplaced beneath the ocean-continent boundary. Here we use numerical models of cratons with realistic crustal rheologies drifting at observed plate velocities to support the idea that the mid-lithosphere weak layer fostered the decoupling and offset of the African continent's buoyant cratonic root, which was left behind during Meso-Cenozoic continental drift and emplaced beneath the Atlantic Ocean. We show that in some cratonic areas, the MLD plays a similar role as the lithosphere-asthenosphere boundary for accommodating lateral plate tectonic displacements.

Tectonic evolution of the terrestrial planets.

PubMed

Head, J W; Solomon, S C

1981-07-03

The style and evolution of tectonics on the terrestrial planets differ substantially. The style is related to the thickness of the lithosphere and to whether the lithosphere is divided into distinct, mobile plates that can be recycled into the mantle, as on Earth, or is a single spherical shell, as on the moon, Mars, and Mercury. The evolution of a planetary lithosphere and the development of plate tectonics appear to be influenced by several factors, including planetary size, chemistry, and external and internal heat sources. Vertical tectonic movement due to lithospheric loading or uplift is similar on all of the terrestrial planets and is controlled by the local thickness and rheology of the lithosphere. The surface of Venus, although known only at low resolution, displays features both similar to those on Earth (mountain belts, high plateaus) and similar to those on the smaller planets (possible impact basins). Improved understanding of the tectonic evolution of Venus will permit an evaluation of the relative roles of planetary size and chemistry in determining evolutionary style.

Effect of 3-D heterogeneous-earth on rheology inference of postseismic model following the 2012 Indian Ocean earthquake

NASA Astrophysics Data System (ADS)

Pratama, C.; Ito, T.; Sasajima, R.; Tabei, T.; Kimata, F.; Gunawan, E.; Ohta, Y.; Yamashina, T.; Ismail, N.; Muksin, U.; Maulida, P.; Meilano, I.; Nurdin, I.; Sugiyanto, D.; Efendi, J.

2017-12-01

Postseismic deformation following the 2012 Indian Ocean earthquake has been modeled by several studies (Han et al. 2015, Hu et al. 2016, Masuti et al. 2016). Although each study used different method and dataset, the previous studies constructed a significant difference of earth structure. Han et al. (2015) ignored subducting slab beneath Sumatra while Masuti et al. (2016) neglect sphericity of the earth. Hu et al. (2016) incorporated elastic slab and spherical earth but used uniform rigidity in each layer of the model. As a result, Han et al. (2015) model estimated one order higher Maxwell viscosity than the Hu et al. (2016) and half order lower Kelvin viscosity than the Masuti et al. (2016) model predicted. In the present study, we conduct a quantitative analysis of each heterogeneous geometry and parameter effect on rheology inference. We develop heterogeneous three-dimensional spherical-earth finite element models. We investigate the effect of subducting slab, spherical earth, and three-dimensional earth rigidity on estimated lithosphere-asthenosphere rheology beneath the Indian Ocean. A wide range of viscosity structure from time constant rheology to time dependent rheology was chosen as previous studies have been modeled. In order to evaluate actual displacement, we compared the model to the Global Navigation Satellite System (GNSS) observation. We incorporate the GNSS data from previous studies and introduce new GNSS site as a part of the Indonesian Continuously Operating Reference Stations (InaCORS) located in Sumatra that has not been used in the last analysis. As a preliminary result, we obtained the effect of the spherical earth and elastic slab when we assumed burgers rheology. The model that incorporates the sphericity of the earth needs a one third order lower viscosity than the model that neglects earth curvature. The model that includes elastic slab needs half order lower viscosity than the model that excluding the elastic slab.

3D seismic reflection imaging of nearly amagmatic oceanic lithosphere at the ultra-slow spreading Southwest Indian Ridge

NASA Astrophysics Data System (ADS)

Momoh, E. I.; Cannat, M.; Singh, S. C.; Watremez, L.; Leroy, S. D.

2016-12-01

Ultra-slow spreading ridges (< 10 mm/yr half-spreading rate), are characterized by a variety of mode accretion, from purely magmatic to nearly amagmatic. With the prevalence of mantle-derived peridotites and sparse volcanism on the seafloor, the easternmost portion of the ultra-slow spreading Southwest Indian Ridge (SWIR) at 64°E represents a melt-poor end-member in the global ridge system. Mantle-derived peridotites there are proposed to have been exhumed along the footwall of detachment faults (Cannat et al, 2006; Sauter et al, 2013). However, the geometry and structural styles of detachments at depth are conjectural. We show the first 3D seismic reflection images of nearly amagmatic axial oceanic lithosphere in this region. The results are from 3D processing of 2D seismic data acquired during the SISMOSMOOTH 2014 cruise along 100 m-spaced profiles in a 1.8 km wide by 24 km long box spanning the axial valley and a part of its elevated northern wall. Wide-angle tomography results from Ocean bottom Seismometer (OBS) line are used to provide a velocity structure of the crust and correlate the MCS reflection images. We image 4 classes of reflectors. The first class occurs in 2 parts as south-dipping events and can be followed in the cross-line of the survey area. The upper part terminates on the northern slope of the massif. The lower part occurs as an isolated event until half of the width of the survey area after which it appears as a continuation of the upper part. This class of reflectors may be due to the damage zone of the active axial detachment fault. The second class of reflectors occurs as north-dipping events. They extend 1 km in the cross-line. They can be interpreted as fractured zones, zones of localized serpentinization or as dikes. The third class of reflectors occurs as sub-horizontal events at depth and seems to serve as the termination of the proposed dikes/fractured zones. On the OBS result, this reflector mimics the 7.5 km/s velocity contour in some areas and < 0.5 s TWT. It is unclear if the depths are coincident. The fourth class of reflectors occurs as shallow intra-crustal reflectors 0.2 - 1 s TWT below the seafloor which does not maintain a consistent character across the survey area. We envisage that they may be associated with zones of localized serpentinization, intrusive melt or fractured zones in the axial valley.

Olivine anisotropy suggests Gutenberg discontinuity is not the base of the lithosphere

PubMed Central

Qi, Chao; Warren, Jessica M.

2016-01-01

Tectonic plates are a key feature of Earth’s structure, and their behavior and dynamics are fundamental drivers in a wide range of large-scale processes. The operation of plate tectonics, in general, depends intimately on the manner in which lithospheric plates couple to the convecting interior. Current debate centers on whether the transition from rigid lithosphere to flowing asthenosphere relates to increases in temperature or to changes in composition such as the presence of a small amount of melt or an increase in water content below a specified depth. Thus, the manner in which the rigid lithosphere couples to the flowing asthenosphere is currently unclear. Here we present results from laboratory-based torsion experiments on olivine aggregates with and without melt, yielding an improved database describing the crystallographic alignment of olivine grains. We combine this database with a flow model for oceanic upper mantle to predict the structure of the seismic anisotropy beneath ocean basins. Agreement between our model and seismological observations supports the view that the base of the lithosphere is thermally controlled. This model additionally supports the idea that discontinuities in velocity and anisotropy, often assumed to be the base of the lithosphere, are, instead, intralithospheric features reflecting a compositional boundary established at midocean ridges, not a rheological boundary. PMID:27606485

3D joint inversion modeling of the lithospheric density structure based on gravity, geoid and topography data — Application to the Alborz Mountains (Iran) and South Caspian Basin region

NASA Astrophysics Data System (ADS)

Motavalli-Anbaran, Seyed-Hani; Zeyen, Hermann; Ebrahimzadeh Ardestani, Vahid

2013-02-01

We present a 3D algorithm to obtain the density structure of the lithosphere from joint inversion of free air gravity, geoid and topography data based on a Bayesian approach with Gaussian probability density functions. The algorithm delivers the crustal and lithospheric thicknesses and the average crustal density. Stabilization of the inversion process may be obtained through parameter damping and smoothing as well as use of a priori information like crustal thicknesses from seismic profiles. The algorithm is applied to synthetic models in order to demonstrate its usefulness. A real data application is presented for the area of northern Iran (with the Alborz Mountains as main target) and the South Caspian Basin. The resulting model shows an important crustal root (up to 55 km) under the Alborz Mountains and a thin crust (ca. 30 km) under the southernmost South Caspian Basin thickening northward to the Apsheron-Balkan Sill to 45 km. Central and NW Iran is underlain by a thin lithosphere (ca. 90-100 km). The lithosphere thickens under the South Caspian Basin until the Apsheron-Balkan Sill where it reaches more than 240 km. Under the stable Turan platform, we find a lithospheric thickness of 160-180 km.

Olivine anisotropy suggests Gutenberg discontinuity is not the base of the lithosphere.

PubMed

Hansen, Lars N; Qi, Chao; Warren, Jessica M

2016-09-20

Tectonic plates are a key feature of Earth's structure, and their behavior and dynamics are fundamental drivers in a wide range of large-scale processes. The operation of plate tectonics, in general, depends intimately on the manner in which lithospheric plates couple to the convecting interior. Current debate centers on whether the transition from rigid lithosphere to flowing asthenosphere relates to increases in temperature or to changes in composition such as the presence of a small amount of melt or an increase in water content below a specified depth. Thus, the manner in which the rigid lithosphere couples to the flowing asthenosphere is currently unclear. Here we present results from laboratory-based torsion experiments on olivine aggregates with and without melt, yielding an improved database describing the crystallographic alignment of olivine grains. We combine this database with a flow model for oceanic upper mantle to predict the structure of the seismic anisotropy beneath ocean basins. Agreement between our model and seismological observations supports the view that the base of the lithosphere is thermally controlled. This model additionally supports the idea that discontinuities in velocity and anisotropy, often assumed to be the base of the lithosphere, are, instead, intralithospheric features reflecting a compositional boundary established at midocean ridges, not a rheological boundary.

Interaction between mantle and crustal detachments: A nonlinear system controlling lithospheric extension

NASA Astrophysics Data System (ADS)

Rosenbaum, Gideon; Regenauer-Lieb, Klaus; Weinberg, Roberto F.

2010-11-01

We use numerical modeling to investigate the development of crustal and mantle detachments during lithospheric extension. Our models simulate a wide range of extensional systems with varying values of crustal thickness and heat flow, showing how strain localization in the mantle interacts with localization in the upper crust and controls the evolution of extensional systems. Model results reveal a richness of structures and deformation styles as a response to a self-organized mechanism that minimizes the internal stored energy of the system by localizing deformation. Crustal detachments, here referred as low-angle normal decoupling horizons, are well developed during extension of overthickened (60 km) continental crust, even when the initial heat flow is relatively low (50 mW m-2). In contrast, localized mantle deformation is most pronounced when the extended lithosphere has a normal crustal thickness (30-40 km) and an intermediate heat flow (60-70 mW m-2). Results show a nonlinear response to subtle changes in crustal thickness or heat flow, characterized by abrupt and sometimes unexpected switches in extension modes (e.g., from diffuse extensional deformation to effective lithospheric-scale rupturing) or from mantle- to crust-dominated strain localization. We interpret this nonlinearity to result from the interference of doming wavelengths in the presence of multiple necking instabilities. Disharmonic crust and mantle doming wavelengths results in efficient communication between shear zones at different lithospheric levels, leading to rupturing of the whole lithosphere. In contrast, harmonic crust and mantle doming inhibits interaction of shear zones across the lithosphere and results in a prolonged history of extension prior to continental breakup.

Current kinematics and dynamics of Africa and the East African Rift System

NASA Astrophysics Data System (ADS)

Stamps, D. S.; Flesch, L. M.; Calais, E.; Ghosh, A.

2014-06-01

Although the East African Rift System (EARS) is an archetype continental rift, the forces driving its evolution remain debated. Some contend buoyancy forces arising from gravitational potential energy (GPE) gradients within the lithosphere drive rifting. Others argue for a major role of the diverging mantle flow associated with the African Superplume. Here we quantify the forces driving present-day continental rifting in East Africa by (1) solving the depth averaged 3-D force balance equations for 3-D deviatoric stress associated with GPE, (2) inverting for a stress field boundary condition that we interpret as originating from large-scale mantle tractions, (3) calculating dynamic velocities due to lithospheric buoyancy forces, lateral viscosity variations, and velocity boundary conditions, and (4) calculating dynamic velocities that result from the stress response of horizontal mantle tractions acting on a viscous lithosphere in Africa and surroundings. We find deviatoric stress associated with lithospheric GPE gradients are ˜8-20 MPa in EARS, and the minimum deviatoric stress resulting from basal shear is ˜1.6 MPa along the EARS. Our dynamic velocity calculations confirm that a force contribution from GPE gradients alone is sufficient to drive Nubia-Somalia divergence and that additional forcing from horizontal mantle tractions overestimates surface kinematics. Stresses from GPE gradients appear sufficient to sustain present-day rifting in East Africa; however, they are lower than the vertically integrated strength of the lithosphere along most of the EARS. This indicates additional processes are required to initiate rupture of continental lithosphere, but once it is initiated, lithospheric buoyancy forces are enough to maintain rifting.

Mantle Lithosphere Rheology, Vertical Tectonics, and the Exhumation of (U)HP Rocks

NASA Astrophysics Data System (ADS)

Bodur, Ömer F.; Göǧüş, Oǧuz H.; Pysklywec, Russell N.; Okay, Aral I.

2018-02-01

Numerical modeling results indicate that mantle lithosphere rheology can influence the pressure-temperature-time (P-T-t) trajectories of continental crust subducted and exhumed during the onset of continental collision. Exhumation of ultrahigh-pressure ( 35 kbar)/high-temperature ( 750°C) metamorphic rocks is more prevalent in models with stronger continental mantle lithosphere (e.g., dry), whereas high-pressure ( 9-22 kbar)/low-temperature (350°C-630°C) metamorphic rocks occur in models with weaker rheology (e.g., hydrated) for the same layer. In the latter case, the buried crustal rocks can remain encased in ablatively subducting mantle lithosphere, reach only moderate temperatures, and exhume by dripping/detachment of the lithospheric root. In this transition from subduction to a dripping style of "vertical tectonics," burial and exhumation of crustal rocks are driven without imposed far-field plate convergence. The model results are compared against thermobarometric P-T estimates from major (ultra)high-pressure metamorphic terranes. We propose that the exhumation of high-pressure/low-temperature metamorphic rocks in Tavşanlı and Afyon zones in western Anatolia may be caused by viscous dripping of mantle lithosphere suggesting a weaker continental mantle lithosphere, whereas (ultra)high-pressure exhumation (e.g., Dabie Shan-eastern China and Dora Maira-western Alps) may be associated with plate-like subduction. In the latter case, the slab is much stronger and deformation is localized to the subduction interface along which rocks are buried to >100 km depth before they are exhumed to the near surface.

The interplay between rheology and pre-existing structures in the lithosphere and its influence on intraplate tectonics: Insights from scaled physical analogue models.

NASA Astrophysics Data System (ADS)

Santimano, T. N.; Adiban, P.; Pysklywec, R.

2017-12-01

The primary controls of deformation in the lithosphere are related to its rheological properties. In addition, recent work reveals that inherited zones of weakness in the deep lithosphere are prevalent and can also define tectonic activity. To understand how deformation is genetically related to rheology and/or pre-existing structures, we compare a set of physical analogue models with the presence and absence of a fault in the deep lithosphere. The layered lithosphere scaled models of a brittle upper crust, viscous lower crust and viscous mantle lithosphere are deformed in a convergent setting. Deformation of the model is recorded using high spatial and temporal stereoscopic cameras. We use Particle Image Velocimetry (PIV) to acquire a time-series dataset and study the velocity field and subsequently strain in the model. The finished model is also cut into cross-section revealing the finite internal structures that are then compared to the topography of the model. Preliminary results show that deformation in models with an inherited fault in the mantle lithosphere is accommodated by displacement along the fault plane that propagates into the overlying viscous lower crust and brittle upper crust. Here, the majority of the deformation is localized along the fault in a brittle manner. This is in contrast to the model absent of a fault that also displays significant amounts of deformation. In this setting, ductile deformation is accommodated by folding and thickening of the viscous layers and flexural shearing of the brittle upper crust. In these preliminary experiments, the difference in the strength profile between the mantle lithosphere and the lower crust is within the same order of magnitude. Future experiments will include models where the strength difference is an order of magnitude. This systematic study aids in understanding the role of rheology and deep structures particularly in transferring stress over time to the surface and is therefore fundamental in understanding intraplate tectonics and orogenesis.

Temporal evolution of continental lithospheric strength in actively deforming regions

USGS Publications Warehouse

Thatcher, W.; Pollitz, F.F.

2008-01-01

It has been agreed for nearly a century that a strong, load-bearing outer layer of earth is required to support mountain ranges, transmit stresses to deform active regions and store elastic strain to generate earthquakes. However the dept and extent of this strong layer remain controversial. Here we use a variety of observations to infer the distribution of lithospheric strength in the active western United States from seismic to steady-state time scales. We use evidence from post-seismic transient and earthquake cycle deformation reservoir loading glacio-isostatic adjustment, and lithosphere isostatic adjustment to large surface and subsurface loads. The nearly perfectly elastic behavior of Earth's crust and mantle at the time scale of seismic wave propagation evolves to that of a strong, elastic crust and weak, ductile upper mantle lithosphere at both earthquake cycle (EC, ???10?? to 103 yr) and glacio-isostatic adjustment (GIA, ???103 to 104 yr) time scales. Topography and gravity field correlations indicate that lithosphere isostatic adjustment (LIA) on ???106-107 yr time scales occurs with most lithospheric stress supported by an upper crust overlying a much weaker ductile subtrate. These comparisons suggest that the upper mantle lithosphere is weaker than the crust at all time scales longer than seismic. In contrast, the lower crust has a chameleon-like behavior, strong at EC and GIA time scales and weak for LIA and steady-state deformation processes. The lower crust might even take on a third identity in regions of rapid crustal extension or continental collision, where anomalously high temperatures may lead to large-scale ductile flow in a lower crustal layer that is locally weaker than the upper mantle. Modeling of lithospheric processes in active regions thus cannot use a one-size-fits-all prescription of rheological layering (relation between applied stress and deformation as a function of depth) but must be tailored to the time scale and tectonic setting of the process being investigated.

What Petit-Spot Volcanoes Tell us about the Lithosphere-Asthenosphere Boundary?

NASA Astrophysics Data System (ADS)

Pilet, S.; Abe, N.; Rochat, L.; Kaczmarek, M. A.; Bessat, A.; Duretz, T.; Muntener, O.

2015-12-01

The top of the low seismic velocity zone (LVZ) is frequently used to localize the lithosphere -asthenosphere boundary (LAB) which separates rigid oceanic plates from the underlying ductile asthenosphere. The seismic and electric properties of the LVZ are generally explained by the presence of low degree melts located at the base of the lithosphere, but the composition of these melts (silicate or carbonated melts) is still in debate. If most models for the LAB are based on geophysical or experimental studies, the discovery of petit-spot volcanoes on the top of the down-going Pacific plate (1) provides unique opportunities to obtain direct information on the LAB. Petit-spot volcanoes are interpreted as small-scale seamounts formed by the extraction of low-degree melts from the base of the lithosphere in response of plate flexure and/or crack propagation (2). The petrology of petit-spot lavas from Japan and Costa Rica demonstrates, first, that melts from the LVZ correspond to volatiles rich low degree silicate melts rather then to carbonatitic melts. Second, the discovery of lithospheric metasomatized mantle xenoliths and xenocrysts in the petit-spot lavas suggest that plate bending in front of subduction zones does not only produce petit-spot lavas at the surface, but allowed low degree melts from the LVZ to percolate and differentiate across the base of the oceanic lithosphere. This observation has important implication for the LAB because it demonstrates that deformed LAB does not represent a impermeable barrier for melt percolation as communally assumed, but deformation allows melts from the asthenosphere to percolate through peridotite matrix for significant distance (~10-20 km) modifying the rheology and the seismic properties of the base of the lithospheric mantle. This aspect needs to be taking into account in any model trying to simulate lithosphere asthenosphere deformation. (1) Hirano et al., 2006, Science 313, 1426-1428; (2) Valentine & Hirano, 2010, Geology 38, 55-58.

Low surface gravitational acceleration of Mars results in a thick and weak lithosphere: Implications for topography, volcanism, and hydrology

NASA Astrophysics Data System (ADS)

Heap, Michael J.; Byrne, Paul K.; Mikhail, Sami

2017-01-01

Surface gravitational acceleration (surface gravity) on Mars, the second-smallest planet in the Solar System, is much lower than that on Earth. A direct consequence of this low surface gravity is that lithostatic pressure is lower on Mars than on Earth at any given depth. Collated published data from deformation experiments on basalts suggest that, throughout its geological history (and thus thermal evolution), the Martian brittle lithosphere was much thicker but weaker than that of present-day Earth as a function solely of surface gravity. We also demonstrate, again as a consequence of its lower surface gravity, that the Martian lithosphere is more porous, that fractures on Mars remain open to greater depths and are wider at a given depth, and that the maximum penetration depth for opening-mode fractures (i.e., joints) is much deeper on Mars than on Earth. The result of a weak Martian lithosphere is that dykes-the primary mechanism for magma transport on both planets-can propagate more easily and can be much wider on Mars than on Earth. We suggest that this increased the efficiency of magma delivery to and towards the Martian surface during its volcanically active past, and therefore assisted the exogeneous and endogenous growth of the planet's enormous volcanoes (the heights of which are supported by the thick Martian lithosphere) as well as extensive flood-mode volcanism. The porous and pervasively fractured (and permeable) nature of the Martian lithosphere will have also greatly assisted the subsurface storage of and transport of fluids through the lithosphere throughout its geologically history. And so it is that surface gravity, influenced by the mass of a planetary body, can greatly modify the mechanical and hydraulic behaviour of its lithosphere with manifest differences in surface topography and geomorphology, volcanic character, and hydrology.

Effects of upper mantle heterogeneities on the lithospheric stress field and dynamic topography

NASA Astrophysics Data System (ADS)

Osei Tutu, Anthony; Steinberger, Bernhard; Sobolev, Stephan V.; Rogozhina, Irina; Popov, Anton A.

2018-05-01

The orientation and tectonic regime of the observed crustal/lithospheric stress field contribute to our knowledge of different deformation processes occurring within the Earth's crust and lithosphere. In this study, we analyze the influence of the thermal and density structure of the upper mantle on the lithospheric stress field and topography. We use a 3-D lithosphere-asthenosphere numerical model with power-law rheology, coupled to a spectral mantle flow code at 300 km depth. Our results are validated against the World Stress Map 2016 (WSM2016) and the observation-based residual topography. We derive the upper mantle thermal structure from either a heat flow model combined with a seafloor age model (TM1) or a global S-wave velocity model (TM2). We show that lateral density heterogeneities in the upper 300 km have a limited influence on the modeled horizontal stress field as opposed to the resulting dynamic topography that appears more sensitive to such heterogeneities. The modeled stress field directions, using only the mantle heterogeneities below 300 km, are not perturbed much when the effects of lithosphere and crust above 300 km are added. In contrast, modeled stress magnitudes and dynamic topography are to a greater extent controlled by the upper mantle density structure. After correction for the chemical depletion of continents, the TM2 model leads to a much better fit with the observed residual topography giving a good correlation of 0.51 in continents, but this correction leads to no significant improvement of the fit between the WSM2016 and the resulting lithosphere stresses. In continental regions with abundant heat flow data, TM1 results in relatively small angular misfits. For example, in western Europe the misfit between the modeled and observation-based stress is 18.3°. Our findings emphasize that the relative contributions coming from shallow and deep mantle dynamic forces are quite different for the lithospheric stress field and dynamic topography.

Lithospheric thermal-rheological structure of the Ordos Basin and its geodynamics

NASA Astrophysics Data System (ADS)

Pan, J.; Huang, F.; He, L.; Wu, Q.

2015-12-01

The study on the destruction of the North China Craton has always been one of the hottest issues in earth sciences.Both mechanism and spatial variation are debated fiercely, still unclear.However, geothermal research on the subject is relatively few. Ordos Basin, located in the west of the North China Craton, is a typical intraplate. Based on two-dimensional thermal modeling along a profile across Ordos Basin from east to west, obtained the lithospheric thermal structure and rheology. Mantle heat flow in different regions of Ordos Basin is from 21.2 to 24.5 mW/m2. In the east mantle heat flow is higher while heat flow in western region is relatively low. But mantle heat flow is smooth and low overall, showing a stable thermal background. Ratio of crustal and mantle heat flow is between 1.51 and 1.84, indicating that thermal contribution from shallow crust is lower than that from the mantle. Rheological characteristics along the profile are almost showed as "jelly sandwich" model and stable continental lithosphere structure,which is represent by a weak crust portion but a strong lithospheric mantle portion in vertical strength profile. Based on above , both thermal structure and lithospheric rheology of Ordos Basin illustrate that tectonic dynamics environment in the west of North China Craton is relatively stable. By the study on lithospheric thermal structure, we focus on the disparity in thickness between the thermal lithosphere and seismic lithosphere.The difference in western Ordos Basin is about 140km, which decreases gradually from Fenwei graben in the eastern Ordos Basin to the Bohai Bay Basin.That is to say the difference decreases gradually from the west to the east of North China Craton.The simulation results imply that viscosity of the asthenosphere under North China Craton also decreases gradually from west to east, confirming that dehydration of the Pacific subduction is likely to have great effect on the North China Craton.

The role of the metasomatized oceanic lithosphere on the composition of mid-ocean ridge basalts from the East Pacific Rise

NASA Astrophysics Data System (ADS)

Shimizu, K.; Saal, A. E.

2016-12-01

In the present study, we evaluate the effect of melting of a metasomatized oceanic lithosphere on the chemical composition of MORB using the East Pacific Rise (EPR) mid-ocean ridge basalts (MORB) from the Quebrada-Discovery-GoFar (QDG) transform fault system, Northern EPR seamounts, and Macquarie Island [1-3]. EMORB from the QDG have trace element and volatile-refractory element ratios different from those measured in NEPR seamounts and Macquarie EMORB. The unique chemical composition of the QDG EMORB might indicate contribution from the oceanic lithosphere during the formation of intra-transform spreading centers due to clockwise rotation in Pacific-Nazca plate relative motion. In addition, the compositions of some of the Petit-spot lavas recently erupted along lithospheric fractures in the Pacific Plate in response to its flexure near the Japan Trench [4] have geochemical signatures that might suggest melts derived from a metasomatized oceanic mantle lithosphere. We evaluate this hypothesis using a geochemical model assuming a two-component asthenospheric mantle (DDMM and EDMM) and formation of hydrous cumulates in the oceanic mantle lithosphere by crystallization of low degree melts of the EDMM [3, 5]. The model suggests that melting of the hydrous cumulates can reproduce the composition of EMORB from QDG transform fault and some of the Petit-spot lavas. The process of melting the metasomatized oceanic lithosphere may significantly affect the chemical composition of MORB, and the common assumption for the purely asthenosphere origin of MORB could lead to inaccurate estimates of the Earth's upper mantle composition. We also show that similar process might affect not only oceanic, but also off-craton sub continental mantle lithosphere. References: [1] Niu et al., 2002 EPSL 199. [2] Kamenetsky et al., 2002 J Petrol 43. [3] Shimizu et al., 2016 GCA 176. [4] Hirano et al., 2006 Science 313. [5] Pilet et al., 2011 J Petrol 52.

When mountain belts disrupt mantle flow: from natural evidences to numerical modelling

NASA Astrophysics Data System (ADS)

Yamato, Philippe; Husson, Laurent; Guillaume, Benjamin

2016-04-01

During the Cenozoic, the number of orogens on Earth increased. This observation readily indicates that in the same time, compression in the lithosphere became gradually more and more important. Here, we show that such mountain belts, at plate boundaries, increasingly obstruct plate tectonics, slowing down and reorienting their motions. In turn, it changes the dynamic and kinematic surface conditions of the underlying flowing mantle, which ultimately modifies the pattern of mantle flow. Such forcing could explain many first order features of Cenozoic plate tectonics and mantle flow. Among others, at lithospheric scale, one can cite the compression of passive margins, the important variations in the rates of spreading at oceanic ridges, the initiation of subductions, or the onset of obductions. In the mantle, such changes in boundary conditions redesign the flow pattern and, consequently, disturb the oceanic lithosphere cooling. In order to test this hypothesis we first present thermo-mechanical numerical models of mantle convection above which a lithosphere is resting on top. Our results show that when collision occurs, the mantle flow is strongly modified, which leads to (i) increasing shear stresses below the lithosphere and (ii) a modification of the convection style. In turn, the transition between a "free" convection (mobile lid) and a "disturbed" convection (stagnant - or sluggish - lid) highly impacts the dynamics of the lithosphere at the surface. Thereby, on the basis of these models and a variety of real examples, we show that on the other side of a lithosphere presenting a collision zone, passive margins become squeezed and can undergo compression, which may ultimately evolve into subduction initiation or obduction. We also show that much further, due to the blocking of the lithosphere, spreading rates decrease at the ridge, which may explain a variety of features such as the low magmatism of ultraslow spreading ridges or the departure of slow spreading ridges from the half-space cooling model.

The many impacts of building mountain belts on plate tectonics and mantle flow

NASA Astrophysics Data System (ADS)

Yamato, Philippe; Husson, Laurent

2015-04-01

During the Cenozoic, the number of orogens on Earth increased. This observation readily indicates that in the same time, compression in the lithosphere became gradually more and more important. Such an increase of stresses in the lithosphere can impact on plate tectonics and mantle dynamics. We show that mountain belts at plate boundaries increasingly obstruct plate tectonics, slowing down and reorienting their motions. In turn, this changes the dynamic and kinematic surface conditions of the underlying flowing mantle. Ultimately, this modifies the pattern of mantle flow. This forcing could explain many first order features of Cenozoic plate tectonics and mantle flow. Among these, one can cite the compression of passive margins, the important variations in the rates of spreading at oceanic ridges, or the initiation of subduction, the onset of obduction, for the lithosphere. In the mantle, such change in boundary condition redesigns the pattern of mantle flow and, consequently, the oceanic lithosphere cooling. In order to test this hypothesis we first present thermo-mechanical numerical models of mantle convection above which a lithosphere rests. Our results show that when collision occurs, the mantle flow is highly modified, which leads to (i) increasing shear stresses below the lithosphere and (ii) to a modification of the convection style. In turn, the transition between a 'free' convection (mobile lid) and an 'upset' convection (stagnant -or sluggish- lid) highly impacts the dynamics of the lithosphere at the surface of the Earth. Thereby, on the basis of these models and a variety of real examples, we show that on the other side of a collision zone, passive margins become squeezed and can undergo compression, which may ultimately evolve into subduction or obduction. We also show that much further, due to the blocking of the lithosphere, spreading rates decrease at the ridge, a fact that may explain a variety of features such as the low magmatism of ultraslow spreading ridges or the departure of slow spreading ridges from the half-space cooling model.

Lithospheric architecture of the South-Western Alps revealed by multiparameter teleseismic full-waveform inversion

NASA Astrophysics Data System (ADS)

Beller, S.; Monteiller, V.; Operto, S.; Nolet, G.; Paul, A.; Zhao, L.

2018-02-01

The Western Alps, although being intensively investigated, remains elusive when it comes to determining its lithospheric structure. New inferences on the latter are important for the understanding of processes and mechanisms of orogeny needed to unravel the dynamic evolution of the Alps. This situation led to the deployment of the CIFALPS temporary experiment, conducted to address the lack of seismological data amenable to high-resolution seismic imaging of the crust and the upper mantle. We perform a 3-D isotropic full-waveform inversion (FWI) of nine teleseismic events recorded by the CIFALPS experiment to infer 3-D models of both density and P- and S-wave velocities of the Alpine lithosphere. Here, by FWI is meant the inversion of the full seismograms including phase and amplitude effects within a time window following the first arrival up to a frequency of 0.2 Hz. We show that the application of the FWI at the lithospheric scale is able to generate images of the lithosphere with unprecedented resolution and can furnish a reliable density model of the upper lithosphere. In the shallowest part of the crust, we retrieve the shape of the fast/dense Ivrea body anomaly and detect the low velocities of the Po and SE France sedimentary basins. The geometry of the Ivrea body as revealed by our density model is consistent with the Bouguer anomaly. A sharp Moho transition is followed from the external part (30 km depth) to the internal part of the Alps (70-80 km depth), giving clear evidence of a continental subduction event during the formation of the Alpine Belt. A low-velocity zone in the lower lithosphere of the S-wave velocity model supports the hypothesis of a slab detachment in the western part of the Alps that is followed by asthenospheric upwelling. The application of FWI to teleseismic data helps to fill the gap of resolution between traditional imaging techniques, and enables integrated interpretations of both upper and lower lithospheric structures.

Lithospheric buoyancy and continental intraplate stresses

USGS Publications Warehouse

Zoback, M.L.; Mooney, W.D.

2003-01-01

Lithospheric buoyancy, the product of lithospheric density and thickness, is an important physical property that influences both the long-term stability of continents and their state of stress. We have determined lithospheric buoyancy by applying the simple isostatic model of Lachenbruch and Morgan (1990). We determine the crustal portion of lithospheric buoyancy using the USGS global database of more than 1700 crustal structure determinations (Mooney et al., 2002), which demonstrates that a simple relationship between crustal thickness and surface elevation does not exist. In fact, major regions of the crust at or near sea level (0-200 m elevation) have crustal thicknesses that vary between 25 and 55 km. Predicted elevations due to the crustal component of buoyancy in the model exceed observed elevations in nearly all cases (97% of the data), consistent with the existence of a cool lithospheric mantle lid that is denser than the asthenosphere on which it floats. The difference between the observed and predicted crustal elevation is assumed to be equal to the decrease in elevation produced by the negative buoyancy of the mantle lid. Mantle lid thickness was first estimated from the mantle buoyancy and a mean lid density computed using a basal crust temperature determined from extrapolation of surface heat flow, assuming a linear thermal gradient in the mantle lid. The resulting values of total lithosphere thickness are in good agreement with thicknesses estimated from seismic data, except beneath cratonic regions where they are only 40-60% of the typical estimates (200-350 km) derived from seismic data. This inconsistency is compatible with petrologic data and tomography and geoid analyses that have suggested that cratonic mantle lids are ??? 1% less dense than mantle lids elsewhere. By lowering the thermally determined mean mantle lid density in cratons by 1%, our model reproduces the observed 200-350+ km cratonic lithospheric thickness. We then computed gravitational potential energy by taking a vertical integral over the computed lithosphere density. Our computed values suggest that the thick roots beneath cratons lead to strong negative potential energy differences relative to surrounding regions, and hence exert compressive stresses superimposed on the intraplate stresses derived from plate boundary forces. Forces related to this lithosphere structure thus may explain the dominance of reverse-faulting earthquakes in cratons. Areas of high elevation and a thin mantle lid (e.g., western U.S. Basin and Range, East African rift, and Baikal rift) are predicted to be in extension, consistent with the observed stress regime in these areas.

Evaluation of the Lithospheric Contribution to Southern Rio Grande Rift Mafic Melts

NASA Astrophysics Data System (ADS)

Konter, J. G.; Crocker, L.; Anaya, L. M.; Rooney, T. O.

2011-12-01

As continental rifting proceeds, the accommodation of lithospheric thinning by mechanical extension and magmatic intrusion represents an important but poorly constrained tectonic process. Insight into role of the magmatic component may come from the composition of volcanic products, which can record magma-lithosphere interactions. The volcanic activity in continental rift environments is frequently characterized by bimodal associations of mafic and silicic volcanism with heterogenous lithospheric contributions. We present a new integrated data set from several mafic volcanic fields in the Rio Grande Rift, consisting of major and trace element compositions, as well as isotopes. This data set provides insight into asthenospheric melting processes and interactions with the overlying lithosphere. The melting processes and the related extensional volcanism is the result of foundering of the Farallon slab. Large volume silicic eruptions such as those in the Sierra Madre Occidental originate from a large contribution of lithospheric melting, with a subordinate asthenospheric contribution. In contrast, Late Tertiary and Quaternary basaltic volcanic fields in the Rio Grande Rift were likely sourced in the asthenosphere and did not reside in the lithosphere for substantial periods. As a result the region is the ideal natural laboratory to investigate the interaction of asthenospheric melts with the lithosphere. In particular the wide array of volcanic fields contain multiple xenolith localities, such as Kilbourne Hole, providing direct samples of lithosphere and crust. Although previous studies have focused on correlations between amount of extension related to Farallon slab foundering, volcanic compositions, and their mantle sources, we present data that suggest that some compositional signatures may pre-date current tectonic processes. Radiogenic isotope data from several volcanic fields in New Mexico show a converging pattern in Pb isotope compositions, focusing on the unradiogenic Pb isotope composition of lower crustal xenoliths from Kilbourne Hole. The opposite ends of the converging trends are more radiogenic for some volcanic fields than the (lithospheric) mantle xenoliths of the Potrillo, San Carlos and Geranimo volcanic fields. Combined Pb-Sr isotope compositions for these fields are consistent with a trend from lower crustal xenoliths to mantle xenoliths, but show more variability. This variability may be explained by a small upper crustal contribution, in agreement with the Pb isotope systematics. Therefore, a common unradiogenic lower crustal composition likely contributed to the asthenospheric melts, followed by upper crustal contamination. The unradiogenic character of the lower crust implies an ancient event created the required low U/Pb ratios that generated the present-day Pb isotope compositions.

In-Situ Lithospheric Rheology Measurement Using Isostatic Response and Geophysical State

NASA Astrophysics Data System (ADS)

Lowry, A. R.; Becker, T. W.; Buehler, J. S.; ma, X.; Miller, M. S.; Perez-Gussinye, M.; Ravat, D.; Schutt, D.

2013-12-01

Measurements of effective elastic thickness, Te, from flexural isostatic modeling are sensitive to flow rheology of the lithosphere. Nevertheless, Te has not been widely used to estimate in-situ rheology. Past methodological controversies regarding Te measurement are partly to blame for under-utilization of isostatic response in rheology studies, but these controversies are now largely resolved. The remaining hurdles include uncertainties in properties of geophysical state such as temperature, lithology, and water content. These are ambiguous in their relative contributions to total strength, and the unknown state-of-stress adds to ambiguity in the rheology. Dense seismic and other geophysical arrays such as EarthScope's USArray are providing a wealth of new information about physical state of the lithosphere, however, and these data promise new insights into rheology and deformation processes. For example, new estimates of subsurface mass distributions derived from seismic data enable us to examine controversial assumptions about the nature of lithospheric loads. Variations in crustal lithology evident in bulk crustal velocity ratio, vP/vS, contribute a surprisingly large fraction of total loading. Perhaps the most interesting new information on physical state derives from imaging of uppermost mantle velocities using refracted mantle phases, Pn and Sn, and depths to negative velocity gradients imaged as converted phases in receiver functions (so-called seismic lithosphere-asthenosphere boundary, 'LAB', and mid-lithosphere discontinuity, 'MLD'). Imaging of the ~580°C isotherm associated with the phase transition from alpha- to beta-quartz affords another exciting new avenue for investigation, in part because the transition closely matches the Curie temperature thought to control magnetic bottom in some continental crust. Reconciling seismic estimates of temperature variations with measurements of Te and upper-mantle negative velocity gradients in the US requires that we invoke variations in lithology, water concentrations, and/or membrane stress. In deforming lithosphere, Te and Pn are best-reconciled using a wet quartz crustal lithology, wet olivine mantle lithology, and large membrane stress. More stable lithosphere to the east is best-modeled with a dry feldspar or pyroxene crustal lithology and dry olivine in the mantle. Greater crustal quartz abundance in deforming lithosphere (and in ancient orogens further east) is observed independently in measurements of bulk-crustal vP/vS. Independent evidence also supports the inference of variable water concentrations. Taken together, these lines of evidence suggest that lithology and water abundance are at least as important as temperature variation in determining rheological behavior of the lithosphere.

Relationship between the upper mantle high velocity seismic lid and the continental lithosphere

NASA Astrophysics Data System (ADS)

Priestley, Keith; Tilmann, Frederik

2009-04-01

The lithosphere-asthenosphere boundary corresponds to the base of the "rigid" plates - the depth at which heat transport changes from advection in the convecting deeper upper mantle to conduction in the shallow upper mantle. Although this boundary is a fundamental feature of the Earth, mapping it has been difficult because it does not correspond to a sharp change in temperature or composition. Various definitions of the lithosphere and asthenosphere are based on the analysis of different types of geophysical and geological observations. The depth to the lithosphere-asthenosphere boundary determined from these different observations often shows little agreement when they are applied to the same region because the geophysical and geological observations (i.e., seismic velocity, strain rate, electrical resistivity, chemical depletion, etc.) are proxies for the change in rheological properties rather than a direct measure of the rheological properties. In this paper, we focus on the seismic mapping of the upper mantle high velocity lid and low velocity zone and its relationship to the lithosphere and asthenosphere. We have two goals: (a) to examine the differences in how teleseismic body-wave travel-time tomography and surface-wave tomography image upper mantle seismic structure; and (b) to summarise how upper mantle seismic velocity structure can be related to the structure of the lithosphere and asthenosphere. Surface-wave tomography provides reasonably good depth resolution, especially when higher modes are included in the analysis, but lateral resolution is limited by the horizontal wavelength of the long-period surface waves used to constrain upper mantle velocity structure. Teleseismic body-wave tomography has poor depth resolution in the upper mantle, particularly when no strong lateral contrasts are present. If station terms are used, features with large lateral extent and gradual boundaries are attenuated in the tomographic image. Body-wave models are not useful in mapping the thickness of the high velocity upper mantle lid because this type of analysis often determines wave speed perturbations from an unknown horizontal average and not absolute velocities. Thus, any feature which extends laterally across the whole region beneath a seismic network becomes invisible in the teleseismic body-wave tomographic image. We compare surface-wave and body-wave tomographic results using southern Africa as an example. Surface-wave tomographic images for southern Africa show a strong, high velocity upper mantle lid confined to depths shallower than ~ 200 km, whereas body-wave tomographic images show weak high velocity in the upper mantle extending to depths of ~ 300 km or more. However, synthetic tests show that these results are not contradictory. The absolute seismic velocity structure of the upper mantle provided by surface wave analysis can be used to map the thermal lithosphere. Priestley and McKenzie (Priestley, K., McKenzie, D., 2006. The thermal structure of the lithosphere from shear wave velocities. Earth and Planetary Science Letters 244, 285-301.) derive an empirical relationship between shear wave velocity and temperature. This relationship is used to obtain temperature profiles from the surface-wave tomographic models of the continental mantle. The base of the lithosphere is shown by a change in the gradient of the temperature profiles indicative of the depth where the mode of heat transport changes from conduction to advection. Comparisons of the geotherms determined from the conversion of surface-wave wave speeds to temperatures with upper mantle nodule-derived geotherms demonstrate that estimates of lithospheric thickness from Vs and from the nodule mineralogy agree to within about 25 km. The lithospheric thickness map for Africa derived from the surface-wave tomographic results shows that thick lithosphere underlies most of the Archean crust in Africa. The distribution of diamondiferous kimberlites provides an independent estimate of where thick lithosphere exists. Diamondiferous kimberlites generally occur where the lower part of the thermal lithosphere as indicated by seismology is in the diamond stability field.

Determination of intrinsic attenuation in the oceanic lithosphere-asthenosphere system

NASA Astrophysics Data System (ADS)

Takeuchi, Nozomu; Kawakatsu, Hitoshi; Shiobara, Hajime; Isse, Takehi; Sugioka, Hiroko; Ito, Aki; Utada, Hisashi

2017-12-01

We recorded P and S waves traveling through the oceanic lithosphere-asthenosphere system (LAS) using broadband ocean-bottom seismometers in the northwest Pacific, and we quantitatively separated the intrinsic (anelastic) and extrinsic (scattering) attenuation effects on seismic wave propagation to directly infer the thermomechanical properties of the oceanic LAS. The strong intrinsic attenuation in the asthenosphere obtained at higher frequency (~3 hertz) is comparable to that constrained at lower frequency (~100 seconds) by surface waves and suggests frequency-independent anelasticity, whereas the intrinsic attenuation in the lithosphere is frequency dependent. This difference in frequency dependence indicates that the strong and broad peak dissipation recently observed in the laboratory exists only in the asthenosphere and provides new insight into what distinguishes the asthenosphere from the lithosphere.

Stratification of Seismic Anisotropy Beneath Hudson Bay

NASA Astrophysics Data System (ADS)

Darbyshire, F. A.; Eaton, D. W.; Bastow, I. D.

2012-12-01

The Hudson Bay region has a complex tectonic history spanning ~4 Ga of Earth's evolution. During the ~1.8 Ga Trans-Hudson orogeny, the Archean Superior and Western Churchill cratons collided following the subduction of a Pacific-scale ocean. It is thought that a significant amount of juvenile material is preserved in the Trans-Hudson Orogen, in part due to the complex double-indentor geometry of the Superior-Churchill collision. In the region of interest, the orogen lies beneath a large but shallow Paleozoic intra-cratonic basin. Studies of the crust and upper mantle beneath this region have been enabled through the HuBLE (Hudson Bay Lithospheric Experiment) project, through the deployment of broadband seismographs around the Bay and across the islands to the north. A surface-wave tomography study has taken advantage of the data coverage, providing new information on phase velocity heterogeneity and anisotropy for wave periods of 25-200 seconds (equivalent to depths from the lower crust to ~300 km). On a large scale, our results show that the entire region is underlain by a seismically fast lithospheric lid corresponding to the continental keel. The lithospheric thickness ranges from ~180km in the northeast, beneath a zone of Paleozoic rifting, to ~280km beneath central Hudson Bay. Within the lithosphere, seismic velocities vary laterally, including high-velocity material wrapping around the Bay in the uppermost mantle. In the mid-lithosphere, two high-velocity cores are imaged, with a zone of lower velocity between them beneath the Bay. We interpret these high-velocity structures to represent the strongest central cores of the Superior and Churchill cratons, with more-juvenile material preserved between them. The near-vertical geometry of the lower-velocity zone suggests that it is only the effects of terminal collision of the cratonic cores, rather than any preceding subduction, that is preserved today. The lowermost lithosphere has a more uniform velocity, and may represent a pervasive zone of metasomatism or underplating. Anisotropy patterns across the region also vary with depth, suggesting ~3 layers of stratification of lithospheric fabric. At the shallowest depths, anisotropic fast directions wrap around the Bay in a similar fashion to the patterns of isotropic wavespeed. The upper lithospheric mantle below is characterized by relatively weak and incoherent anisotropy; however the mid-to-lower lithosphere shows stronger anisotropy, with a pattern of fast directions broadly consistent with the tectonics of the Superior-Churchill collision as inferred from potential-field data. This may suggest some degree of coherency of deformation between the crust, uppermost mantle and lower lithosphere. These models of seismic wavespeed variation beneath the Hudson Bay region reveal the preservation of a major collision zone during the assembly of the Laurentian continental mass, and also suggest that the Archean cratons can be subdivided into different lithospheric domains that reflect their tectonic history but do not necessarily correspond to surface geological boundaries.

South China Sea crustal thickness and lithosphere thinning from satellite gravity inversion incorporating a lithospheric thermal gravity anomaly correction

NASA Astrophysics Data System (ADS)

Kusznir, Nick; Gozzard, Simon; Alvey, Andy

2016-04-01

The distribution of ocean crust and lithosphere within the South China Sea (SCS) are controversial. Sea-floor spreading re-orientation and ridge jumps during the Oligocene-Miocene formation of the South China Sea led to the present complex distribution of oceanic crust, thinned continental crust, micro-continents and volcanic ridges. We determine Moho depth, crustal thickness and continental lithosphere thinning (1- 1/beta) for the South China Sea using a gravity inversion method which incorporates a lithosphere thermal gravity anomaly correction (Chappell & Kusznir, 2008). The gravity inversion method provides a prediction of ocean-continent transition structure and continent-ocean boundary location which is independent of ocean isochron information. A correction is required for the lithosphere thermal gravity anomaly in order to determine Moho depth accurately from gravity inversion; the elevated lithosphere geotherm of the young oceanic and rifted continental margin lithosphere of the South China Sea produces a large lithosphere thermal gravity anomaly which in places exceeds -150 mGal. The gravity anomaly inversion is carried out in the 3D spectral domain (using Parker 1972) to determine 3D Moho geometry and invokes Smith's uniqueness theorem. The gravity anomaly contribution from sediments assumes a compaction controlled sediment density increase with depth. The gravity inversion includes a parameterization of the decompression melting model of White & McKenzie (1999) to predict volcanic addition generated during continental breakup lithosphere thinning and seafloor spreading. Public domain free air gravity anomaly, bathymetry and sediment thickness data are used in this gravity inversion. Using crustal thickness and continental lithosphere thinning factor maps with superimposed shaded-relief free-air gravity anomaly, we improve the determination of pre-breakup rifted margin conjugacy, rift orientation and sea-floor spreading trajectory. SCS conjugate margins are highly asymmetric and have several striking features such as the Macclesfield Bank, Xisha Trough, Reed Bank and Dangerous Grounds. Thin continental crust is predicted extending westwards from thin oceanic crust north of Macclesfield Bank into the Quiondongnan (QDN) basin and is interpreted as being generated ahead of westward propagating sea-floor spreading most in the Oligocene. Further south, highly thinned continental crust or possibly serpentinised exhumed mantle is predicted in the Phu Khanh Basin. Ahead of the failed propagating tip of seafloor spreading, offshore southern Vietnam, thinned continental crust is predicted for the Cuu Long and Nam Con Son Basins. Crustal thicknesses from gravity inversion confirms that the southern margin of the SCS consists of fragmented blocks of thinned continental crust separated by thinner regions of continental crust that have undergone higher degrees of stretching and thinning. The Reed Bank is predicted to have a crustal thickness of 20 to 25km, similar to that of Macclesfield Bank. The Dangerous Grounds, west of the Reed Bank, are also predicted to consist of continental crust. This region has been thinned to a higher degree than the Reed Bank, with continental crustal thickness ranging between 10 and 20km thick.

Three-Dimensional Modeling of Fluid and Heat Transport in an Accretionary Complex

NASA Astrophysics Data System (ADS)

Paula, C. A.; Ge, S.; Screaton, E. J.

2001-12-01

As sediments are scraped off of the subducting oceanic crust and accreted to the overriding plate, the rapid loading causes pore pressures in the underthrust sediments to increase. The change in pore pressure drives fluid flow and heat transport within the accretionary complex. Fluid is channeled along higher permeability faults and fractures and expelled at the seafloor. In this investigation, we examined the effects of sediment loading on fluid flow and thermal transport in the decollement at the Barbados Ridge subduction zone. Both the width and thickness of the Barbados Ridge accretionary complex increase from north to south. The presence of mud diapers south of the Tiburon Rise and an observed southward decrease in heat flow measurements indicate that the increased thickness of the southern Barbados accretionary prism affects the transport of chemicals and heat by fluids. The three-dimensional geometry and physical properties of the accretionary complex were utilized to construct a three-dimensional fluid flow/heat transport model. We calculated the pore pressure change due to a period of sediment loading and added this to steady-state pressure conditions to generate initial conditions for transient simulations. We then examined the diffusion of pore pressure and possible perturbation of the thermal regime over time due to loading of the underthrust sediments. The model results show that the sediment-loading event was sufficient to create small temperature fluctuations in the decollement zone. The magnitude of temperature fluctuation in the decollement was greatest at the deformation front but did not vary significantly from north to south of the Tiburon Rise.

Birch Lecture : The Deep Roots of Continents

NASA Astrophysics Data System (ADS)

Jaupart, C.

2006-12-01

The roots of Archean continents are made of depleted and buoyant mantle and may extend to depths larger than 250 km. Such distinctive characteristics have key dynamical and geological consequences that we are only beginning to address. Thick roots provide large volume repositories for chemical elements that do not mix with Earth's convecting interior. Their large diffusive relaxation time implies long-term thermal disequilibrium with their radioactive heat sources and with the cooling of the mantle. Their negative thermal buoyancy may drive convective instabilities with implications for intracontinental deformation and magmatism as well as for continental growth. The dynamical behaviour of continental roots depends on the buoyancy ratio B, the ratio of the intrinsic (chemical) buoyancy of depleted lithospheric mantle and the density difference due to thermal expansion. The lithosphere can be mechanically stable and in thermal equilibrium with heat supplied by small-scale convection at the top of the asthenosphere. Sufficient cooling may result in an oscillatory convective instability whereby perturbations to the base of the lithosphere rise and fall periodically. The lithosphere seems to have developed in a state near that of instability with different thicknesses depending on its intrinsic buoyancy. It may have grown not only by chemical differentiation during melting, but also by oscillatory convection entraining chemically denser material from the asthenosphere. Mantle plumes have different effects on lithospheres of different thicknesses and compositions. For B values larger than about 0.6, plume material does not really penetrate into the lithosphere and spreads beneath it. In this case, the buoyancy force that is applied to the base of the lithosphere drives moderate thinning and extension over large horizontal distances. It takes values of B less than 0.6 to achieve true plume penetration with a significant vertical velocity component. In this case, thinning and extension get localized above the rising plume. In both cases, heated lithosphere material becomes convectively unstable after some time and entrains asthenospheric material as it rises. Temperatures in thick continental lithosphere do not adjust rapidly to secular changes of mantle temperature. Analysis of (P,T) data from xenolith studies indicates that the Earth's mantle has cooled at a rate of 80 K Ga-1 or less. Thick continental roots preserve a record of Archean processes and of Earth's evolution through geological ages. Deciphering this record may well be our next challenge.

Continental extension, magmatism and elevation; formal relations and rules of thumb

USGS Publications Warehouse

Lachenbruch, A.H.; Morgan, P.

1990-01-01

To investigate simplified relations between elevation and the extensional, magmatic and thermal processes that influence lithosphere buoyancy, we assume that the lithosphere floats on an asthenosphere of uniform density and has no flexural strength. A simple graph relating elevation to lithosphere density and thickness provides an overview of expectable conditions around the earth and a simple test for consistancy of continental and oceanic lithosphere models. The mass-balance relations yield simple general rules for estimating elevation changes caused by various tectonic, magmatic and thermal processes without referring to detailed models. The rules are general because they depend principally on buoyancy, which under our assumptions is specified by elevation, a known quantity; they do not generally require a knowledge of lithosphere thickness and density. The elevation of an extended terrain contains important information on its tectonic and magmatic history. In the Great Basin where Cenozoic extension is estimated to be 100%, the present high mean elevation ( ~ 1.75 km) probably requires substantial low-density magmatic contributions to the extending lithosphere. The elevation cannot be reasonably explained solely as the buoyant residue of a very high initial terrane, or of a lithosphere that was initially very thick and subsequently delaminated and heated. Even models with a high initial elevation typically call for 10 km or so of accumulated magmatic material of near-crustal density. To understand the evolution of the Great Basin, it is important to determine whether such intruded material is present; some could replenish the stretching crust by underplating and crustal intrusion and some might reside in the upper mantle. The elevation maintained or approached by an intruded extending lithosphere depends on the ratio B of how fast magma is supplied from the asthenosphere ( b km/Ma) to how fast the lithosphere spreads the magma out by extension (?? Ma-1). For a surface maintained 2 1 2km below sea level (e.g., an ocean ridge) B is about 5 km; for continental extension the ratio may be much greater. The frequent association of volcanism with continental extension, the high elevation (and buoyancy) of some appreciably extended terrains, and the oceanic spreading analog all suggest that magmatism may play an important role in continental extension. Better estimates of total extension and elevation change in extended regions can help to identify that role. ?? 1990.

Constraints on the Lithospheric Strength at Volcanic Rifted Margins from the Geometry of Seaward Dipping Reflectors Using Analytic and Numerical Models

NASA Astrophysics Data System (ADS)

Tian, X.; Buck, W. R.

2017-12-01

Seaward dipping reflectors (SDRs) are found at many rifted margins. Drilling indicates SDRs are interbedded layers of basalts and sediments. Multi-channel seismic reflection data show SDRs with various width (2 100 km), thickness (1 15 km) and dip angles (0 30). Recent studies use analytic thin plate models (AtPM) to describe plate deflections under volcanic loads. They reproduce a wide range of SDRs structures without detachment faulting. These models assume that the solidified dikes provide downward loads at the rifting center. Meanwhile, erupted lava flows and sediments fill in the flexural depression and further load the lithosphere. Because the strength of the lithosphere controls the amount and wavelength of bending, the geometries of SDRs provide a window into the strength of the lithosphere during continental rifting. We attempt to provide a quantitative mapping between the SDR geometry and the lithospheric strength and thickness during rifting. To do this, we first derive analytic solutions to two observables that are functions of effective elastic thickness (Te). One observable (Xf) is the horizontal distance for SDRs to evolve from flat layers to the maximum bent layers. Another observable is the ratio between the thickness and the tangent of the maximum slope of SDRs at Xf. We then extend the AtPM to numerical thin plate models (NtPM) with spatially restricted lava flows. AtPM and NtPM show a stable and small relative difference in terms of the two observables with different values of Te. This provides a mapping of Te between NtPM and AtPM models. We also employ a fully two-dimensional thermal-mechanical treatment with elasto-visco-plastic rheology to simulate SDRs formation. These models show that brittle yielding due to bending can reduce the Te of the lithosphere by as much as 50% of the actual brittle lithospheric thickness. Quantification of effects of plastic deformation on bending allow us to use Te to link SDRs geometries to brittle lithospheric thickness. From published seismic reflection data, we obtain a global map of Te at volcanic rifted margins that ranges from 2 12 km using the AtPM and NtPM mapping. The corresponding brittle lithospheric thickness ranges from 6 20 km. In addition, preliminary results show Te increases along a given margin with distance away from a Large Igneous Province.

Waveform Tomography of the South Atlantic Region

NASA Astrophysics Data System (ADS)

Celli, N. L.; Lebedev, S.; Schaeffer, A. J.; Gaina, C.

2016-12-01

The rapid growth in broadband seismic data, along with developments in waveform tomography techniques, allow us to greatly improve the data sampling in the southern hemisphere and resolve the upper-mantle structure beneath the South Atlantic region at a new level of detail. We have gathered a very large waveform dataset, including all publicly available data from permanent and temporary networks. Our S-velocity tomographic model is constrained by vertical-component waveform fits, computed using the Automated Multimode Inversion of surface, S and multiple S waves. Each seismogram fit provides a set of linear equations describing 1D average velocity perturbations within approximate sensitivity volumes, with respect to a 3D reference model. All the equations are then combined into a large linear system and inverted jointly for a model of shear- and compressional-wave speeds and azimuthal anisotropy within the lithosphere and underlying mantle. The isotropic-average shear speeds are proxies for temperature and composition at depth, while azimuthal anisotropy provides evidence on the past and present deformation in the lithosphere and asthenosphere beneath the region. We resolve the complex boundaries of the mantle roots of South America's and Africa's cratons and the deep low-velocity anomalies beneath volcanic areas in South America. Pronounced lithospheric high seismic velocity anomalies beneath the Argentine Basin suggest that its anomalously deep seafloor, previously attributed to dynamic topography, is mainly due to anomalously cold, thick lithosphere. Major hotspots show low-velocity anomalies extending substantially deeper than those beneath the mid-ocean ridge. The Vema Hotspot shows a major, hot asthenospheric anomaly beneath thick, cold oceanic lithosphere. The mantle lithosphere beneath the Walvis Ridge—a hotspot track—shows normal cooling. The volcanic Cameroon Line, in contrast, is characterized by thin lithosphere beneath the locations of recent volcanism.

Understanding the Yellowstone magmatic system using 3D geodynamic inverse models

NASA Astrophysics Data System (ADS)

Kaus, B. J. P.; Reuber, G. S.; Popov, A.; Baumann, T.

2017-12-01

The Yellowstone magmatic system is one of the largest magmatic systems on Earth. Recent seismic tomography suggest that two distinct magma chambers exist: a shallow, presumably felsic chamber and a deeper much larger, partially molten, chamber above the Moho. Why melt stalls at different depth levels above the Yellowstone plume, whereas dikes cross-cut the whole lithosphere in the nearby Snake River Plane is unclear. Partly this is caused by our incomplete understanding of lithospheric scale melt ascent processes from the upper mantle to the shallow crust, which requires better constraints on the mechanics and material properties of the lithosphere.Here, we employ lithospheric-scale 2D and 3D geodynamic models adapted to Yellowstone to better understand magmatic processes in active arcs. The models have a number of (uncertain) input parameters such as the temperature and viscosity structure of the lithosphere, geometry and melt fraction of the magmatic system, while the melt content and rock densities are obtained by consistent thermodynamic modelling of whole rock data of the Yellowstone stratigraphy. As all of these parameters affect the dynamics of the lithosphere, we use the simulations to derive testable model predictions such as gravity anomalies, surface deformation rates and lithospheric stresses and compare them with observations. We incorporated it within an inversion method and perform 3D geodynamic inverse models of the Yellowstone magmatic system. An adjoint based method is used to derive the key model parameters and the factors that affect the stress field around the Yellowstone plume, locations of enhanced diking and melt accumulations. Results suggest that the plume and the magma chambers are connected with each other and that magma chamber overpressure is required to explain the surface displacement in phases of high activity above the Yellowstone magmatic system.

Geophysical and petrological characterization of the lithospheric mantle in Iberia, Western Mediterranean and North Africa

NASA Astrophysics Data System (ADS)

Fernandez, M.; Torne, M.; Carballo, A.; Jiménez-Munt, I.; Verges, J.; Villasenor, A.; Garcia-Castellanos, D.; Diaz Cusi, J.

2015-12-01

We present a geophysical and petrological study that aims to define the lithosphere structure and the variations of the chemical composition of the lithospheric mantle along three geo-transects crossing Iberia, the westernmost Mediterranean and North Africa. The modeling is based on an integrated geophysical-petrological methodology that combines elevation, gravity, geoid, surface heat flow, seismic and geochemical data. Unlike previous models, where the density of the lithospheric mantle is only temperature-dependent, the applied methodology allows inferring seismic velocities and density in the mantle down to 400 km depth from its chemical composition through self-consistent thermodynamic calculations. The first geo-transect with a length of 1100 km runs from the NE-Iberian Peninsula to the Tell-Atlas Mountains in Algeria. The second profile crosses the entire Iberian Peninsula, from the Northern Iberian Margin to the Alboran Basin. The third runs from the Iberian Massif to the Sahara Platform crossing the Betic-Rif orogenic system through the Gibraltar Strait and the Atlas Mountains. Results are compared to available tomography models and Pn-velocity data. The obtained lithospheric structure shows large lateral variations in crustal and lithospheric mantle thicknesses and mantle chemical composition. Measured low Pn velocities in the Western Mediterranean basin can be explained either by serpentinization and/or seismic anisotropy and only partly by transient thermal effects. In the Bay of Biscay low Pn velocities are explained only by serpentinization. The negative sub-lithospheric velocity anomalies imaged by tomography models below the Iberian plate and the Atlas Mountains are interpreted in terms of high-temperature/low-density regions being responsible for the high mean topography.

Comparison of Oceanic and Continental Lithosphere, Asthenosphere, and the LAB Through Shear Velocity Inversion of Rayleigh Wave Data from the ALBACORE Amphibious Array in Southern California

NASA Astrophysics Data System (ADS)

Amodeo, K.; Rathnayaka, S.; Weeraratne, D. S.; Kohler, M. D.

2016-12-01

Continental and oceanic lithosphere, which form in different tectonic environments, are studied in a single amphibious seismic array across the Southern California continental margin. This provides a unique opportunity to directly compare oceanic and continental lithosphere, asthenosphere, and the LAB (Lithosphere-Asthenosphere Boundary) in a single data set. The complex history of the region, including spreading center subduction, block rotation, and Borderland extension, allows us to study limits in the rigidity and strength of the lithosphere. We study Rayleigh wave phase velocities obtained from the ALBACORE (Asthenospheric and Lithospheric Broadband Architecture from the California Offshore Region Experiment) offshore seismic array project and invert for shear wave velocity structure as a function of depth. We divide the study area into several regions: continent, inner Borderland, outer Borderland, and oceanic seafloor categorized by age. A unique starting Vs model is used for each case including layer thicknesses, densities, and P and S velocities which predicts Rayleigh phase velocities and are compared to observed phase velocities in each region. We solve for shear wave velocities with the best fit between observed and predicted phase velocity data in a least square sense. Preliminary results indicate that lithospheric velocities in the oceanic mantle are higher than the continental region by at least 2%. The LAB is observed at 50 ± 20 km beneath 15-35 Ma oceanic seafloor. Asthenospheric low velocities reach a minimum of 4.2 km/s in all regions, but have a steeper positive velocity gradient at the base of the oceanic asthenosphere compared to the continent. Seismic tomography images in two and three dimensions will be presented from each study region.

Global thermal models of the lithosphere

NASA Astrophysics Data System (ADS)

Cammarano, F.; Guerri, M.

2017-12-01

Unraveling the thermal structure of the outermost shell of our planet is key for understanding its evolution. We obtain temperatures from interpretation of global shear-velocity (VS) models. Long-wavelength thermal structure is well determined by seismic models and only slightly affected by compositional effects and uncertainties in mineral-physics properties. Absolute temperatures and gradients with depth, however, are not well constrained. Adding constraints from petrology, heat-flow observations and thermal evolution of oceanic lithosphere help to better estimate absolute temperatures in the top part of the lithosphere. We produce global thermal models of the lithosphere at different spatial resolution, up to spherical-harmonics degree 24, and provide estimated standard deviations. We provide purely seismic thermal (TS) model and hybrid models where temperatures are corrected with steady-state conductive geotherms on continents and cooling model temperatures on oceanic regions. All relevant physical properties, with the exception of thermal conductivity, are based on a self-consistent thermodynamical modelling approach. Our global thermal models also include density and compressional-wave velocities (VP) as obtained either assuming no lateral variations in composition or a simple reference 3-D compositional structure, which takes into account a chemically depleted continental lithosphere. We found that seismically-derived temperatures in continental lithosphere fit well, overall, with continental geotherms, but a large variation in radiogenic heat is required to reconcile them with heat flow (long wavelength) observations. Oceanic shallow lithosphere below mid-oceanic ridges and young oceans is colder than expected, confirming the possible presence of a dehydration boundary around 80 km depth already suggested in previous studies. The global thermal models should serve as the basis to move at a smaller spatial scale, where additional thermo-chemical variations required by geophysical observations can be included.

Implications for anomalous mantle pressure and dynamic topography from lithospheric stress patterns in the North Atlantic Realm

NASA Astrophysics Data System (ADS)

Schiffer, Christian; Nielsen, Søren Bom

2016-08-01

With convergent plate boundaries at some distance, the sources of the lithospheric stress field of the North Atlantic Realm are mainly mantle tractions at the base of the lithosphere, lithospheric density structure and topography. Given this, we estimate horizontal deviatoric stresses using a well-established thin sheet model in a global finite element representation. We adjust the lithospheric thickness and the sub-lithospheric pressure iteratively, comparing modelled in plane stress with the observations of the World Stress Map. We find that an anomalous mantle pressure associated with the Iceland and Azores melt anomalies, as well as topography are able to explain the general pattern of the principle horizontal stress directions. The Iceland melt anomaly overprints the classic ridge push perpendicular to the Mid Atlantic ridge and affects the conjugate passive margins in East Greenland more than in western Scandinavia. The dynamic support of topography shows a distinct maximum of c. 1000 m in Iceland and amounts <150 m along the coast of south-western Norway and 250-350 m along the coast of East Greenland. Considering that large areas of the North Atlantic Realm have been estimated to be sub-aerial during the time of break-up, two components of dynamic topography seem to have affected the area: a short-lived, which affected a wider area along the rift system and quickly dissipated after break-up, and a more durable in the close vicinity of Iceland. This is consistent with the appearance of a buoyancy anomaly at the base of the North Atlantic lithosphere at or slightly before continental breakup, relatively fast dissipation of the fringes of this, and continued melt generation below Iceland.

Understanding lithospheric stresses in Arctic: constraints and models

NASA Astrophysics Data System (ADS)

Medvedev, Sergei; Minakov, Alexander; Lebedeva-Ivanova, Nina; Gaina, Carmen

2016-04-01

This pilot project aims to model stress patterns and analyze factors controlling lithospheric stresses in Arctic. The project aims to understand the modern stresses in Arctic as well as to define the ways to test recent hypotheses about Cenozoic evolution of the region. The regions around Lomonosov Ridge and Barents Sea are of particular interest driven by recent acquisition of high-resolution potential field and seismic data. Naturally, the major contributor to the lithospheric stress distribution is the gravitational potential energy (GPE). The study tries to incorporate available geological and geophysical data to build reliable GPE. In particular, we use the recently developed integrated gravity inversion for crustal thickness which incorporates up-to-date compilations of gravity anomalies, bathymetry, and sedimentary thickness. The modelled lithosphere thermal structure assumes a pure shear extension and the ocean age model constrained by global plate kinematics for the last ca. 120 Ma. The results of this approach are juxtaposed with estimates of the density variation inferred from the upper mantle S-wave velocity models based on previous surface wave tomography studies. Although new data and interpretations of the Arctic lithosphere structure become available now, there are areas of low accuracy or even lack of data. To compensate for this, we compare two approaches to constrain GPE: (1) one that directly integrates density of modelled lithosphere and (2) one that uses geoid anomalies which are filtered to account for density variations down to the base of the lithosphere only. The two versions of GPE compared to each other and the stresses calculated numerically are compared with observations. That allows us to optimize GPE and understand density structure, stress pattern, and factors controlling the stresses in Arctic.

Seismic imaging of lithospheric discontinuities and continental evolution

NASA Astrophysics Data System (ADS)

Bostock, M. G.

1999-09-01

Discontinuities in physical properties within the continental lithosphere reflect a range of processes that have contributed to craton stabilization and evolution. A survey of recent seismological studies concerning lithospheric discontinuities is made in an attempt to document their essential characteristics. Results from long-period seismology are inconsistent with the presence of continuous, laterally invariant, isotropic boundaries within the upper mantle at the global scale. At regional scales, two well-defined interfaces termed H (˜60 km depth) and L (˜200 km depth) of continental affinity are identified, with the latter boundary generally exhibiting an anisotropic character. Long-range refraction profiles are frequently characterized by subcontinental mantle that exhibits a complex stratification within the top 200 km. The shallow layering of this package can behave as an imperfect waveguide giving rise to the so-called teleseismic Pn phase, while the L-discontinuity may define its lower base as the culmination of a low velocity zone. High-resolution, seismic reflection profiling provides sufficient detail in a number of cases to document the merging of mantle interfaces into lower continental crust below former collisional sutures and magmatic arcs, thus unambiguously identifying some lithospheric discontinuities with thrust faults and subducted oceanic lithosphere. Collectively, these and other seismic observations point to a continental lithosphere whose internal structure is dominated by a laterally variable, subhorizontal layering. This stratigraphy appears to be more pronounced at shallower lithospheric levels, includes dense, anisotropic layers of order 10 km in thickness, and exhibits horizontal correlation lengths comparable to the lateral dimensions of overlying crustal blocks. A model of craton evolution which relies on shallow subduction as a principal agent of craton stabilization is shown to be broadly compatible with these characteristics.

Persistence of mantle lithospheric Re-Os signature during asthenospherization of the subcontinental lithospheric mantle: insights from in situ isotopic analysis of sulfides from the Ronda peridotite (Southern Spain)

NASA Astrophysics Data System (ADS)

Marchesi, Claudio; Griffin, William L.; Garrido, Carlos J.; Bodinier, Jean-Louis; O'Reilly, Suzanne Y.; Pearson, Norman J.

2010-03-01

The western part of the Ronda peridotite massif (Southern Spain) consists mainly of highly foliated spinel-peridotite tectonites and undeformed granular peridotites that are separated by a recrystallization front. The spinel tectonites are interpreted as volumes of ancient subcontinental lithospheric mantle and the granular peridotites as a portion of subcontinental lithospheric mantle that underwent partial melting and pervasive percolation of basaltic melts induced by Cenozoic asthenospheric upwelling. The Re-Os isotopic signature of sulfides from the granular domain and the recrystallization front mostly coincides with that of grains in the spinel tectonites. This indicates that the Re-Os radiometric system in sulfides was highly resistant to partial melting and percolation of melts induced by Cenozoic lithospheric thermal erosion. The Re-Os isotopic systematics of sulfides in the Ronda peridotites thus mostly conserve the geochemical memory of ancient magmatic events in the subcontinental lithospheric mantle. Os model ages record two Proterozoic melting episodes at ~1.6 to 1.8 and 1.2-1.4 Ga, respectively. The emplacement of the massif into the subcontinental lithospheric mantle probably coincided with one of these depletion events. A later metasomatic episode caused the precipitation of a new generation of sulfides at ~0.7 to 0.9 Ga. These Proterozoic Os model ages are consistent with results obtained for several mantle suites in Central/Western Europe and Northern Africa as well as with the Nd model ages of the continental crust of these regions. This suggests that the events recorded in mantle sulfides of the Ronda peridotites reflect different stages of generation of the continental crust in the ancient Gondwana supercontinent.

Characteristics and habitat of deep vs. shallow slow slip events

NASA Astrophysics Data System (ADS)

Wipperfurth, S. A.; Sramek, O.; Roskovec, B.; Mantovani, F.; McDonough, W. F.

2016-12-01

Models integrating geophysics and geochemistry allow for characterization of the Earth's heat budget and geochemical evolution. Global lithospheric geophysical models are now constrained by surface and body wave data and are classified into several unique tectonic types. Global lithospheric geochemical models have evolved from petrological characterization of layers to a combination of petrologic and seismic constraints. Because of these advances regarding our knowledge of the lithosphere, it is necessary to create an updated chemical and physical reference model. We are developing a global lithospheric reference model based on LITHO1.0 (segmented into 1°lon x 1°lat x 9-layers) and seismological-geochemical relationships. Uncertainty assignments and correlations are assessed for its physical attributes, including layer thickness, Vp and Vs, and density. This approach yields uncertainties for the masses of the crust and lithospheric mantle. Heat producing element abundances (HPE: U, Th, and K) are ascribed to each volume element. These chemical attributes are based upon the composition of subducting sediment (sediment layers), composition of surface rocks (upper crust), a combination of petrologic and seismic correlations (middle and lower crust), and a compilation of xenolith data (lithospheric mantle). The HPE abundances are correlated within each voxel, but not vertically between layers. Efforts to provide correlation of abundances horizontally between each voxel are discussed. These models are used further to critically evaluate the bulk lithosphere heat production in the continents and the oceans. Cross-checks between our model and results from: 1) heat flux (Artemieva, 2006; Davies, 2013; Cammarano and Guerri, 2017), 2) gravity (Reguzzoni and Sampietro, 2015), and 3) geochemical and petrological models (Rudnick and Gao, 2014; Hacker et al. 2015) are performed.

Lithospheric buckling and intra-arc stresses: A mechanism for arc segmentation

NASA Technical Reports Server (NTRS)

Nelson, Kerri L.

1989-01-01

Comparison of segment development of a number of arcs has shown that consistent relationships between segmentation, volcanism and variable stresses exists. Researchers successfully modeled these relationships using the conceptual model of lithospheric buckling of Yamaoka et al. (1986; 1987). Lithosphere buckling (deformation) provides the needed mechanism to explain segmentation phenomenon; offsets in volcanic fronts, distribution of calderas within segments, variable segment stresses and the chemical diversity seen between segment boundary and segment interior magmas.

Constraints on the subsurface structure of Europa

NASA Astrophysics Data System (ADS)

Golombek, M. P.; Banerdt, W. B.

1990-02-01

The wedge-shaped bands appearing near the anti-Jovian point on Europa are tension cracks which, after formation on an intact lithosphere, have facilitated the rotation of ice-lithosphere sections decoupled from the silicate interior. Such factors as fluid pressure, surface temperature, silicate impurities in the ice, and strain rates, would have affected the processes in question. A minimum degree of differentiation is required for Europa to mechanically decouple the rotated ice lithosphere from the underlying, predominantly silicate mantle.

Changes in the earth's rotation by tectonic movements

NASA Astrophysics Data System (ADS)

Vermeersen, L. L. A.; Vlaar, N. J.

1993-01-01

We propose that lithospheric processes unrelated to postglacial rebound and taking place under nonisostatic conditions are able to induce nonnegligible influences on the earth's rotation. Examples of such processes are mountain building and erosion, foundering flexure of oceanic basins and lithospheric snapbacking resulting from detachment of subducting slabs. Lithospheric and crustal rheologies and intraplate stresses are the dominant factors in these mechanisms, contrary to the mantle rheologies which are assumed to dominate the process of postglacial rebound.

Lithospheric architecture beneath Hudson Bay

NASA Astrophysics Data System (ADS)

Porritt, Robert W.; Miller, Meghan S.; Darbyshire, Fiona A.

2015-07-01

Hudson Bay overlies some of the thickest Precambrian lithosphere on Earth, whose internal structures contain important clues to the earliest workings of plate formation. The terminal collision, the Trans-Hudson Orogen, brought together the Western Churchill craton to the northwest and the Superior craton to the southeast. These two Archean cratons along with the Paleo-Proterozoic Trans-Hudson internides, form the core of the North American craton. We use S to P converted wave imaging and absolute shear velocity information from a joint inversion of P to S receiver functions, new ambient noise derived phase velocities, and teleseismic phase velocities to investigate this region and determine both the thickness of the lithosphere and the presence of internal discontinuities. The lithosphere under central Hudson Bay approaches ˜350 km thick but is thinner (˜200-250 km) around the periphery of the Bay. Furthermore, the amplitude of the LAB conversion from the S receiver functions is unusually large for a craton, suggesting a large thermal contrast across the LAB, which we interpret as direct evidence of the thermal insulation effect of continents on the asthenosphere. Within the lithosphere, midlithospheric discontinuities, significantly shallower than the base of the lithosphere, are often imaged, suggesting the mechanisms that form these layers are common. Lacking time-history information, we infer that these discontinuities reflect reactivation of formation structures during deformation of the craton.

Three-dimensional estimate of the lithospheric effective elastic thickness of the Line ridge

NASA Astrophysics Data System (ADS)

Hu, Minzhang; Li, Jiancheng; Jin, Taoyong; Xu, Xinyu; Xing, Lelin; Shen, Chongyang; Li, Hui

2015-09-01

Using a new bathymetry grid formed with vertical gravity gradient anomalies and ship soundings (BAT_VGG), a 1° × 1° lithospheric effective elastic thickness (Te) grid of the Line ridge was calculated with the moving window admittance technique. As a comparison, both the GEBCO_08 and SIO V15.1 bathymetry datasets were used to calculate Te as well. The results show that BAT_VGG is suitable for the calculation of lithospheric effective elastic thickness. The lithospheric effective elastic thickness of the Line ridge is shown to be low, in the range of 5.5-13 km, with an average of 8 km and a standard deviation of 1.3 km. Using the plate cooling model as a reference, most of the effective elastic thicknesses are controlled by the 150-300 °C isotherm. Seamounts are primarily present in two zones, with lithospheric ages of 20-35 Ma and 40-60 Ma, at the time of loading. Unlike the Hawaiian-Emperor chain, the lithospheric effective elastic thickness of the Line ridge does not change monotonously. The tectonic setting of the Line ridge is discussed in detail based on our Te results and the seamount ages collected from the literature. The results show that thermal and fracture activities must have played an important role in the origin and evolution of the ridge.

Rheology Gradients at the Base of the Lithosphere and the Stabilization of Deep Mantle Plumes in Stagnant-Lid Planets

NASA Astrophysics Data System (ADS)

King, S. D.

2017-12-01

In high-Rayleigh-number, spherical-shell convection, such as one expects to find in the interiors of large silicate planetary bodies, plumes will migrate unless they are anchored to fixed structures. Within the Earth LLSVPs or core-mantle boundary topography have been proposed to anchor deep mantle plumes, fixing the location of hotspots. The relative stability of volcanic features on Mars and Venus, which are thought to be related to mantle plumes, have not be satisfactorily explained. Thus, it is surprising to see high-Rayleigh-number, stagnant-lid, spherical-shell convection calculations where plumes seeded by the structure of the initial condition persist in a stable configuration for more than 1 Gyr. By comparing calculations with a fixed lithospheric rheology structure with a lithosphere rheology determined by temperature and pressure, I show that in these calculations, topography on the base of the stagnant lid (i.e., the lithosphere-asthenosphere boundary) is responsible for the spatial stability of the plumes. If there is symmetry in the plume distribution, this symmetry can prevent the lithosphere becoming unstable and overturning, leading to a significantly over-thickened lithosphere relative to predictions based on scaling laws. This is confirmed by considering an identical calculation where the symmetry in the plume distribution is broken. I discuss geological and geophysical implications for planetary bodies resulting of long-lived, stable, mantle structures.

Differences in the lithosphere seismic structure along the Brazilian continental margin in the South Atlantic from travel time seismic tomography

NASA Astrophysics Data System (ADS)

Peres Rocha, M.; Azevedo, P. A. D.; Assumpcao, M.; Franca, G. S.; Marotta, G. S.

2016-12-01

Results of the P-wave travel-time seismic tomography method allowed observing differences in the seismic behavior of the lithosphere along the Brazilian continental margin in the South Atlantic. High velocity anomalies have predominance in the northern portion, which extends from the Rio de Janeiro to Alagoas States (between latitudes -22.5 and -8.5), and low velocity anomalies in the southern portion, which extends from Rio de Janeiro to Rio Grande do Sul States (between latitudes -30 and -22.5). Low velocities coincide spatially with the offshore high seismicity areas, as indicated by Assumpção (1998) and at the high velocities with low seismicity regions. The high velocity anomalies at northern portion are related to the cratonic and low-stretched lithosphere of San Francisco block that was connected to the Congo block before the opening of the Atlantic Ocean. Low velocities can be assigned to more weakened lithosphere, where it started the South Atlantic Ocean opening process. The oldest lithosphere in the South Atlantic, indicated by the magnetic anomalies of the oceanic floor, is higher in the southern part than in the northern part, suggesting that the continents in this region were separating, while the northern region was still connected to Africa, which could explain the lithospheric stretching process.

Cordilleran Longevity, Elevation and Heat Driven by Lithospheric Mantle Removal

NASA Astrophysics Data System (ADS)

Mackay-Hill, A.; Currie, C. A.; Audet, P.; Schaeffer, A. J.

2017-12-01

Cordilleran evolution is controlled by subduction zone back-arc processes that generate and maintain high topography due to elevated uppermost mantle temperatures. In the northern Canadian Cordillera (NCC), the persisting high mean elevation long after subduction has stopped (>50 Ma) requires a sustained source of heat either from small-scale mantle convection or lithospheric mantle removal; however direct structural constraints of these processes are sparse. We image the crust and uppermost mantle beneath the NCC using scattered teleseismic waves recorded on an array of broadband seismograph stations. We resolve two sharp and flat seismic discontinuities: a downward velocity increase at 35 km that we interpret as the Moho; and a deeper discontinuity with opposite velocity contrast at 50 km depth. Based on petrologic estimates, we interpret the deeper interface as the lithosphere-asthenosphere boundary (LAB), which implies an extremely thin ( 15 km) lithospheric mantle. We calculate the temperature at the Moho and the LAB in the range 800-900C and 1200-1300C, respectively. Below the LAB, we find west-dipping features far below the LAB beneath the eastern NCC that we associate with laminar downwelling of Cordilleran lithosphere. Whether these structures are fossilized or active, they suggest that lithospheric mantle removal near the Cordillera-Craton boundary may have provided the source of heat and elevation and therefore played a role in the longevity and stability of the Cordillera.

Formation of the bulge of Iapetus through long-wavelength folding of the lithosphere

NASA Astrophysics Data System (ADS)

Kay, Jonathan P.; Dombard, Andrew J.

2018-03-01

Previous models that attempted to explain the formation of the pronounced oblate shape of Iapetus suggested that it was a preserved rotational bulge. These models found that heating was provided by short-lived radioactive isotopes that decayed rapidly and allowed the excess flattening of the lithosphere to be locked in by a thickening lithosphere, but placed severe timing constraints on the formation of Iapetus and its bulge. Here, we show that finite element simulations with an elastic-viscous-plastic rheology indicate it is possible to form the bulge through long-wavelength folding of the lithosphere of Iapetus during an epoch of contraction combined with a latitudinal surface temperature gradient. In contrast to models of a frozen rotational bulge, heat generated by long-lived radioactive isotopes warms the interior, which causes porosity loss and forces Iapetus to compact by ∼10%. Our simulations are most successful when there is a 30 K temperature difference between the pole and the equator. Tectonic growth of the bulge is not sensitive to the time scale over which the moon contracts, and lithospheric thickness primarily controls whether a fold can form, not fold wavelength. In addition, long term simulations show that when no stress is applied, the mechanical lithosphere is strong enough to support the bulge, with negligible relaxation over billion year time scales.

Spreading continents kick-started plate tectonics.

PubMed

Rey, Patrice F; Coltice, Nicolas; Flament, Nicolas

2014-09-18

Stresses acting on cold, thick and negatively buoyant oceanic lithosphere are thought to be crucial to the initiation of subduction and the operation of plate tectonics, which characterizes the present-day geodynamics of the Earth. Because the Earth's interior was hotter in the Archaean eon, the oceanic crust may have been thicker, thereby making the oceanic lithosphere more buoyant than at present, and whether subduction and plate tectonics occurred during this time is ambiguous, both in the geological record and in geodynamic models. Here we show that because the oceanic crust was thick and buoyant, early continents may have produced intra-lithospheric gravitational stresses large enough to drive their gravitational spreading, to initiate subduction at their margins and to trigger episodes of subduction. Our model predicts the co-occurrence of deep to progressively shallower mafic volcanics and arc magmatism within continents in a self-consistent geodynamic framework, explaining the enigmatic multimodal volcanism and tectonic record of Archaean cratons. Moreover, our model predicts a petrological stratification and tectonic structure of the sub-continental lithospheric mantle, two predictions that are consistent with xenolith and seismic studies, respectively, and consistent with the existence of a mid-lithospheric seismic discontinuity. The slow gravitational collapse of early continents could have kick-started transient episodes of plate tectonics until, as the Earth's interior cooled and oceanic lithosphere became heavier, plate tectonics became self-sustaining.

Seismic constraints of thinning and fragmenting continental lithosphere beneath the Korean Peninsula

NASA Astrophysics Data System (ADS)

Kim, S.; Tauzin, B.; Tkalcic, H.; Rhie, J.

2017-12-01

Modification of the continental lithosphere is still an enigmatic process. The Korean Peninsula (KP) is one of ideal place to depict the process by interactions with subducting oceanic slabs. We detect a significant thickness change (>50 km) of the continental lithosphere beneath the KP that is confirmed by two independent approaches: (1) 3D imaging using ambient noise analysis and (2) receiver function CCP stacking. A series of transdimensional and hierarchical Bayesian joint inversions is performed to obtain a high-resolution 3D model from different types of surface wave dispersion data. For the stacking of receiver function waveforms, the coda waveforms of crustal multi-modes (PpPs and PpSs) are combined together to better image the lithosphere-asthenosphere boundary. We estimate the relatively deeper rooted lithosphere (>100 km) in the southwestern part of the KP compared to shallower surrounding regions. The lithospheric structure is underlain by lower velocity anomalies (Vs<4.1 km/s), which extends from back-arc regions near subducting slabs horizontally and connects to low velocity anomalies in the deeper upper mantle vertically. The imaged features clearly show that the effect of the oceanic slab subduction is a key factor controlling the modification process. We further examine the implication for the occurrence of intraplate volcanoes and the relationship to the mantle transition zone heterogeneities due to stagnant slabs in the northeast Asia.

Magnetic mineralogy of the Mercurian lithosphere

NASA Astrophysics Data System (ADS)

Strauss, B. E.; Feinberg, J. M.; Johnson, C. L.

2016-11-01

Mercury and Earth are the only inner solar system planets with active, internally generated dynamo magnetic fields. The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission recently detected magnetic fields on Mercury that are consistent with lithospheric magnetization. We investigate the physical and chemical environment of Mercury's lithosphere, past and present, to establish the conditions under which magnetization may have been acquired and modified. Three factors are particularly crucial to the determination of crustal composition and iron mineralogy: redox conditions in the planet's crust and mantle, the iron content of the lithosphere, and, for any remanent magnetization, the temperature profile of the lithosphere and its evolution over time. We explore potential mechanisms for remanence acquisition and alteration on Mercury, whose surface environment is both hot and highly reducing. The long-term thermal history of Mercury's crust plays an important role in the longevity of any remanent crustal magnetization, which may be subject to remagnetization through thermal, viscous, and shock mechanisms. This thermal and compositional framework is used both to constrain plausible candidate minerals that could carry magnetic remanence on Mercury and to evaluate their capacity to acquire and retain sufficient magnetization to be detectable from satellite orbit. We propose that iron metal and its alloys are likely to be the dominant contributors to induced and remanent magnetization in Mercury's lithosphere, with additional contributions from iron silicides, sulfides, and carbides.