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.

Varying Structure and Physical Properties of the Lithosphere Subducting Beneath Indonesia, Consequences on the Subduction

NASA Astrophysics Data System (ADS)

Jacob, J.; Dyment, J.

2013-12-01

We make inferences on the structure, age and physical properties of the subducting northern Wharton Basin lithosphere by (1) modeling the structure and age of the lithosphere subducted under the Sumatra trench through three-plate reconstructions involving Australia, Antarctica, and India, and (2) superimposing the resulting fracture zones and magnetic isochrons to the geometry of the subducting plate as imaged by seismic tomography. The model of Pesicek et al. (2010) was digitized and smoothed in order to get a realistic topography of the subducting plate. The fracture zone and magnetic isochron geometry was draped on this topography assuming a N18°E direction of subduction. This model provides an effective means to study the effect of varying physical properties of the subducting lithosphere on the subduction along the Sumatra trench. 1) The age of the oceanic lithosphere determines its thickness and buoyancy, then its ability to comply with or resist subduction. We define the "subductability" of the lithosphere as the extra weight applied on the asthenosphere by the part of the bulk lithospheric density exceeding the asthenospheric density. A negative subductability means that the bulk lithospheric density is lower than the asthenospheric density, i.e. the plate will resist subduction, which is the case for lithosphere less than ~23 Ma. The area off Sumatra corresponds to oceanic lithosphere formed between 80 and 38 Ma, with a lower subductability than other areas along the Sunda Trench. 2) The spreading rate at which the oceanic lithosphere was formed has implications of the structure and composition of the oceanic crust, and therefore on its rheology. In a subduction zone, the contact between the subducting and overriding plates is often considered to be the top of the oceanic crust and the overlying sediments. The roughness of this interface and the rheology of its constitutive material are essential parameters constraining the slip of the down going plate in the seismogenic zone, and therefore the characteristics of the resulting earthquakes. Indeed the rough topography of a slow crust may offer more asperities, and therefore a more irregular slip, than the smooth topography of a fast crust. Conversely, the weak rheology of serpentines present in a slow crust would favor a regular slip, unlike the brittle magmatic rocks of the fast crust and the underlying dry olivine mantle. 3) Local features, including fracture zones and seamounts, may affect the seismic segmentation of the subduction zone. Many seamounts have been mapped in the Wharton Basin between 10°S and 15°S., their age decreasing from 136 Ma to the East to 47 Ma to the West, with anomalously younger ages in Christmas Island. Similar seamounts belonging to the same province may have existed further north and subducted in the Sunda Trench from southern Sumatra to Java and eastward. Conversely, the Roo Rise, a larger plateau located south of Eastern Java, may have more difficulty to enter the subduction, as suggested by the geometry of the Sunda Trench in this area, diverting from the regular arc by a maximum of 60 km. References Pesicek, J.D., C.H. Thurber, S. Widiyantoro, H. Zhang, H.R. DeShon, and E.R. Engdahl (2010), Sharpening the tomographic image of the subducting slab below Sumatra, the Andaman Islands and Burma, Geophys. J. Int., 182, 433-453.

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.

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.

Matching Lithosphere velocity changes to the GOCE gravity signal

NASA Astrophysics Data System (ADS)

Braitenberg, Carla

2016-07-01

Authors: Carla Braitenberg, Patrizia Mariani, Alberto Pastorutti Department of Mathematics and Geosciences, University of Trieste Via Weiss 1, 34100 Trieste Seismic tomography models result in 3D velocity models of lithosphere and sublithospheric mantle, which are due to mineralogic compositional changes and variations in the thermal gradient. The assignment of density is non-univocal and can lead to inverted density changes with respect to velocity changes, depending on composition and temperature. Velocity changes due to temperature result in a proportional density change, whereas changes due to compositional changes and age of the lithosphere can lead to density changes of inverted sign. The relation between velocity and density implies changes in the lithosphere rigidity. We analyze the GOCE gradient fields and the velocity models jointly, making simulations on thermal and compositional density changes, using the velocity models as constraint on lithosphere geometry. The correlations are enhanced by applying geodynamic plate reconstructions to the GOCE gravity field and the tomography models which places today's observed fields at the Gondwana pre-breakup position. We find that the lithosphere geometry is a controlling factor on the overlying geologic elements, defining the regions where rifting and collision alternate and repeat through time. The study is carried out globally, with focus on the conjugate margins of the African and South American continents. The background for the study can be found in the following publications where the techniques which have been used are described: Braitenberg, C., Mariani, P. and De Min, A. (2013). The European Alps and nearby orogenic belts sensed by GOCE, Boll. Bollettino di Geofisica Teorica ed Applicata, 54(4), 321-334. doi:10.4430/bgta0105---- Braitenberg, C. and Mariani, P. (2015). Geological implications from complete Gondwana GOCE-products reconstructions and link to lithospheric roots. Proceedings of 5th International GOCE User Workshop, 25 - 28 November 2014.---- Braitenberg, C. (2015). Exploration of tectonic structures with GOCE in Africa and across-continents. Int. J.Appl. Earth Observ. Geoinf. 35, 88-95. http://dx.doi.org/10.1016/j.jag.2014.01.013------ Braitenberg, C. (2015). A grip on geological units with GOCE, IAG Symp. 141

Gravity signals from the lithosphere in the Central European Basin System

NASA Astrophysics Data System (ADS)

Yegorova, T.; Bayer, U.; Thybo, H.; Maystrenko, Y.; Scheck-Wenderoth, M.; Lyngsie, S. B.

2007-01-01

We study the gravity signals from different depth levels in the lithosphere of the Central European Basin System (CEBS). The major elements of the CEBS are the Northern and Southern Permian Basins which include the Norwegian-Danish Basin (NDB), the North-German Basin (NGB) and the Polish Trough (PT). An up to 10 km thick sedimentary cover of Mesozoic-Cenozoic sediments, hides the gravity signal from below the basin and masks the heterogeneous structure of the consolidated crust, which is assumed to be composed of domains that were accreted during the Paleozoic amalgamation of Europe. We performed a three-dimensional (3D) gravity backstripping to investigate the structure of the lithosphere below the CEBS. Residual anomalies are derived by removing the effect of sediments down to the base of Permian from the observed field. In order to correct for the influence of large salt structures, lateral density variations are incorporated. These sediment-free anomalies are interpreted to reflect Moho relief and density heterogeneities in the crystalline crust and uppermost mantle. The gravity effect of the Moho relief compensates to a large extent the effect of the sediments in the CEBS and in the North Sea. Removal of the effects of large-scale crustal inhomogeneities shows a clear expression of the Variscan arc system at the southern part of the study area and the old crust of Baltica further north-east. The remaining residual anomalies (after stripping off the effects of sediments, Moho topography and large-scale crustal heterogeneities) reveal long wavelength anomalies, which are caused mainly by density variations in the upper mantle, though gravity influence from the lower crust cannot be ruled out. They indicate that the three main subbasins of the CEBS originated on different lithospheric domains. The PT originated on a thick, strong and dense lithosphere of the Baltica type. The NDB was formed on a weakened Baltica low-density lithosphere formed during the Sveco-Norwegian orogeny. The major part of the NGB is characterized by high-density lithosphere, which includes a high-velocity lower crust (relict of Baltica passive margin) overthrusted by the Avalonian terrane. The short wavelength pattern of the final residuals shows several north-west trending gravity highs between the Tornquist Zone and the Elbe Fault System. The NDB is separated by a gravity low at the Ringkøbing-Fyn high from a chain of positive anomalies in the NGB and the PT. In the NGB these anomalies correspond to the Prignitz (Rheinsberg anomaly), the Glueckstadt and Horn Graben, and they continue further west into the Central Graben, to join with the gravity high of the Central North Sea.

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

Density structure of the lithosphere in the southwestern United States and its tectonic significance

USGS Publications Warehouse

Kaban, M.K.; Mooney, W.D.

2001-01-01

We calculate a density model of the lithosphere of the southwestern United States through an integrated analysis of gravity, seismic refraction, drill hole, and geological data. Deviations from the average upper mantle density are as much as ?? 3%. A comparison with tomographic images of seismic velocities indicates that a substantial part (>50%) of these density variations is due to changes in composition rather than temperature. Pronounced mass deficits are found in the upper mantle under the Basin and Range Province and the northern part of the California Coast Ranges and adjacent ocean. The density structure of the northern and central/southern Sierra Nevada is remarkably different. The central/southern part is anomalous and is characterized by a relatively light crust underlain by a higher-density upper mantle that may be associated with a cold, stalled subducted plate. High densities are also determined within the uppermost mantle beneath the central Transverse Ranges and adjoining continental slope. The average density of the crystalline crust under the Great Valley and western Sierra Nevada is estimated to be up to 200 kg m~3 higher than the regional average, consistent with tectonic models for the obduction of oceanic crust and uppermost mantle in this region.

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.

Combined constraints on the structure and physical properties of the East Antarctic lithosphere from geology and geophysics.

NASA Astrophysics Data System (ADS)

Reading, A. M.; Staal, T.; Halpin, J.; Whittaker, J. M.; Morse, P. E.

2017-12-01

The lithosphere of East Antarctica is one of the least explored regions of the planet, yet it is gaining in importance in global scientific research. Continental heat flux density and 3D glacial isostatic adjustment studies, for example, rely on a good knowledge of the deep structure in constraining model inputs.In this contribution, we use a multidisciplinary approach to constrain lithospheric domains. To seismic tomography models, we add constraints from magnetic studies and also new geological constraints. Geological knowledge exists around the periphery of East Antarctica and is reinforced in the knowledge of plate tectonic reconstructions. The subglacial geology of the Antarctic hinterland is largely unknown but the plate reconstructions allow the well-posed extrapolation of major terranes into the interior of the continent, guided by the seismic tomography and magnetic images. We find that the northern boundary of the lithospheric domain centred on the Gamburtsev Subglacial Mountains has a possible trend that runs south of the Lambert Glacier region, turning coastward through Wilkes Land. Other periphery-to-interior connections are less well constrained and the possibility of lithospheric domains that are entirely sub-glacial is high. We develop this framework to include a probabilistic method of handling alternate models and quantifiable uncertainties. We also show first results in using a Bayesian approach to predicting lithospheric boundaries from multivariate data.Within the newly constrained domains, we constrain heat flux (density) as the sum of basal heat flux and upper crustal heat flux. The basal heat flux is constrained by geophysical methods while the upper crustal heat flux is constrained by geology or predicted geology. In addition to heat flux constraints, we also consider the variations in friction experienced by moving ice sheets due to varying geology.

Lithospheric structure in the Pacific geoid

NASA Technical Reports Server (NTRS)

Marsh, B. D.

1984-01-01

In order that sub-lithospheric density variations be revealed with the geoid, the regional geoid anomalies associated with bathymetric variations must first be removed. Spectral techniques were used to generate a synthetic geoid by filtering the residual bathymetry assuming an Airy-type isostatic compensation model. An unbiased estimated of the admittances show that for region under study, no single compensation mechanism will explain all of the power in the geoid. Nevertheless, because topographic features are mainly coherent with the geoid, to first order an isostationally compensated lithosphere cut by major E-W fracture zones accounts for most of the power in the high degree and other SEASAT geoid in the Pacific.

Structure of the Lithosphere in Central Europe: Integrated Density Modelling

NASA Astrophysics Data System (ADS)

Bielik, M.; Grinč, M.; Zeyen, H. J.; Plašienka, D.; Pasteka, R.; Krajňák, M.; Bošanský, M.; Mikuška, J.

2014-12-01

Firstly, we present new results related to the lithospheric structure and tectonics of the Central Europe and the Western Carpathians. For geophysical study of the lithosphere in Central Europe we calculated four original 2D lithosphere-scales transects crossing this area from the West European Platform in the North to the Aegean Sea in the South and from the Adriatic Sea in the West to the East European Platform in the East. Modelling is based on the joint interpretation of gravity, geoid, topography and surface heat flow data with temperature-dependent density. Wherever possible, crustal structure is constrained by seismic data. The thickness of the lithosphere decreases from the older and colder platforms to the younger and hotter Pannonian Basin with a maximum thickness under the Eastern and Southern Carpathians. The thickness of the Carpathian arc lithosphere varies between 150 km in the North (the Western Carpathians) and about 300 km in the Vrancea zone (the Eastern and Southern Carpathian junction). In the Platform areas it is between 120 and 150 km and in the Pannonian Basin it is about 70 km. The models show that the Moesian Platform is overthrust from the North by the Southern Carpathians and from the South by the Balkanides and characterized by bending of this platform. In all transects, the thickest crust is found underneath the Carpathian Mountains or, as in the case of the Vrancea area, under their immediate foreland. The thickest crust outside the orogens is modelled for the Moesian Platform with Moho depths of up to 45 km. The thinnest crust is located under the Pannonian Basin with about 26-27 km. Secondly, our presentation deals with construction of the stripped gravity map in the Turiec Basin, which represents typical intramontane Neogene depression of the Western Carpathians. Based on this new and original gravity map corrected by regional gravity effect we were able to interpret the geological structure and tectonics of this sedimentary basin. This work was supported by the Slovak Grant Agency VEGA (grants No. 1/0095/12, 2/0067/12) and Slovak Research and Development Agency (grants No. APVV-0827-12, APVV-0194-10).

Deep structure of the Mid-Norwegian continental margin (the Vøring and Møre basins) according to 3-D density and magnetic modelling

NASA Astrophysics Data System (ADS)

Maystrenko, Yuriy Petrovich; Gernigon, Laurent; Nasuti, Aziz; Olesen, Odleiv

2018-03-01

A lithosphere-scale 3-D density/magnetic structural model of the Møre and Vøring segments of the Mid-Norwegian continental margin and the adjacent areas of the Norwegian mainland has been constructed by using both published, publically available data sets and confidential data, validated by the 3-D density and magnetic modelling. The obtained Moho topography clearly correlates with the major tectonic units of the study area where a deep Moho corresponds to the base of the Precambrian continental crust and the shallower one is located in close proximity to the younger oceanic lithospheric domain. The 3-D density modelling agrees with previous studies which indicate the presence of a high-density/high-velocity lower-crustal layer beneath the Mid-Norwegian continental margin. The broad Jan Mayen Corridor gravity low is partially related to the decreasing density of the sedimentary layers within the Jan Mayen Corridor and also has to be considered in relation to a possible low-density composition- and/or temperature-related zone in the lithospheric mantle. According to the results of the 3-D magnetic modelling, the absence of a strong magnetic anomaly over the Utgard High indicates that the uplifted crystalline rocks are not so magnetic there, supporting a suggestion that the entire crystalline crust has a low magnetization beneath the greater part of the Vøring Basin and the northern part of the Møre Basin. On the contrary, the crystalline crust is much more magnetic beneath the Trøndelag Platform, the southern part of the Møre Basin and within the mainland, reaching a culmination at the Frøya High where the most intensive magnetic anomaly is observed within the study area.

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.

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.

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.

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.

Crustal and mantle structure of the greater Jan Mayen-East Greenland region (NE Atlantic) from combined 3D structural, S-wave velocity, and gravity modeling

NASA Astrophysics Data System (ADS)

Tan, P.; Sippel, J.; Scheck-Wenderoth, M.; Meeßen, C.; Breivik, A. J.

2016-12-01

The study area is located between the Jan Mayen Ridge and the east coast of Greenland. It has a complex geological setting with the ultraslow Kolbeinsey and Mohn's spreading ridges, the anomalously shallow Eggvin Bank, the Jan Mayen Microcontinent (JMMC), and the tectonically active West Jan Mayen Fracture Zone (WJMFZ). In this study, we present the results of forward 3D structural, S-wave velocity, and gravity modeling which provide new insights into the deep crust and mantle structure and the wide-ranging influence of the Iceland Plume. The crustal parts of the presented 3D structural model are mainly constrained by local seismic refraction and reflection data. Accordingly, greatest crustal thicknesses (24 km) are observed on the northern boundary of the JMMC, while the average crustal thickness is 8.5 km and 4 km in the Kolbeinsey and Mohn's Ridge, respectively. The densities of the crustal parts are from previous studies. Additionally, the mantle density is derived from S-wave velocity data (between 50 and 250 km depth), while densities of the lithospheric mantle between the Moho and 50 km are calculated assuming isostatic equilibrium at 250 km depth. This is used as a starting density model which is further developed to obtain a reasonable fit between the calculated and measured (free-air) gravity fields. The observed S-wave tomographic data and the gravity modeling prove that the Iceland plume anomaly in the asthenosphere affects the lithospheric thickness and temperature, from the strongly influenced Middle Kolbeinsey Ridge, to the less affected North Kolbeinsey Ridge (Eggvin Bank), and to the little impacted Mohn's Ridge. Thus, the age-temperature relations of the different mid-ocean ridges of the study area are perturbed to different degrees controlled by the distance from the Iceland Plume. Furthermore, we find that the upper 50 km of lithospheric mantle are thermally affected by the plume only in the southwestern parts of the study area.

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.

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.

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.

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.

Geophysical Age Dating of Seamounts using Dense Core Flexure Model

NASA Astrophysics Data System (ADS)

Hwang, Gyuha; Kim, Seung-Sep

2016-04-01

Lithospheric flexure of oceanic plate is thermo-mechanical response of an elastic plate to the given volcanic construct (e.g., seamounts and ocean islands). If the shape and mass of such volcanic loads are known, the flexural response is governed by the thickness of elastic plate, Te. As the age of oceanic plate increases, the elastic thickness of oceanic lithosphere becomes thicker. Thus, we can relate Te with the age of plate at the time of loading. To estimate the amount of the driving force due to seamounts on elastic plate, one needs to approximate their density structure. The most common choice is uniform density model, which utilizes constant density value for a seamount. This approach simplifies computational processes for gravity prediction and error estimates. However, the uniform density model tends to overestimate the total mass of the seamount and hence produces more positive gravitational contributions from the load. Minimization of gravity misfits using uniform density, therefore, favors thinner Te in order to increase negative contributions from the lithospheric flexure, which can compensate for the excessive positives from the seamount. An alternative approach is dense core model, which approximate the heterogeneity nature of seamount density as three bodies of infill sediment, edifice, and dense core. In this study, we apply the dense core model to the Louisville Seamount Chain for constraining flexural deformation. We compare Te estimates with the loading time of the examined seamounts to redefine empirical geophysical age dating of seamounts.

Lithospheric density structure beneath the Tarim basin and surroundings, northwestern China, from the joint inversion of gravity and topography

USGS Publications Warehouse

Deng, Yangfan; Levandowski, William Brower; Kusky, Tim

2017-01-01

Intraplate strain generally focuses in discrete zones, but despite the profound impact of this partitioning on global tectonics, geodynamics, and seismic hazard, the processes by which deformation becomes localized are not well understood. Such heterogeneous intraplate strain is exemplified in central Asia, where the Indo-Eurasian collision has caused widespread deformation while the Tarim block has experienced minimal Cenozoic shortening. The apparent stability of Tarim may arise either because strain is dominantly accommodated by pre-existing faults in the continental suture zones that bound it—essentially discretizing Eurasia into microplates—or because the lithospheric-scale strength (i.e., viscosity) of the Tarim block is greater than its surroundings. Here, we jointly analyze seismic velocity, gravity, topography, and temperature to develop a 3-D density model of the crust and upper mantle in this region. The Tarim crust is characterized by high density, vs, vp, and vp/vs, consistent with a dominantly mafic composition and with the presence of an oceanic plateau beneath Tarim. Low-density but high-velocity mantle lithosphere beneath southern (southwestern) Tarim underlies a suite of Permian plume-related mafic intrusions and A-type granites sourced in previously depleted mantle lithosphere; we posit that this region was further depleted, dehydrated, and strengthened by Permian plume magmatism. The actively deforming western and southern margins of Tarim—the Tien Shan, Kunlun Shan, and Altyn Tagh fault—are underlain by buoyant upper mantle with low velocity; we hypothesize that this material has been hydrated by mantle-derived fluids that have preferentially migrated along Paleozoic continental sutures. Such hydrous material should be weak, and herein strain focuses there because of lithospheric-scale variations in rheology rather than the pre-existence of faults in the brittle crust. Thus this world-class example of strain partitioning arises not simply from the pre-existence of brittle faults but from the thermo-chemical and therefore rheological variations inherited from prior tectonism.

Lithosphere-asthenosphere interaction beneath the western United States from the joint inversion of body-wave traveltimes and surface-wave phase velocities

USGS Publications Warehouse

Obrebski, M.; Allen, R.M.; Pollitz, F.; Hung, S.-H.

2011-01-01

The relation between the complex geological history of the western margin of the North American plate and the processes in the mantle is still not fully documented and understood. Several pre-USArray local seismic studies showed how the characteristics of key geological features such as the Colorado Plateau and the Yellowstone Snake River Plains are linked to their deep mantle structure. Recent body-wave models based on the deployment of the high density, large aperture USArray have provided far more details on the mantle structure while surface-wave tomography (ballistic waves and noise correlations) informs us on the shallow structure. Here we combine constraints from these two data sets to image and study the link between the geology of the western United States, the shallow structure of the Earth and the convective processes in mantle. Our multiphase DNA10-S model provides new constraints on the extent of the Archean lithosphere imaged as a large, deeply rooted fast body that encompasses the stable Great Plains and a large portion of the Northern and Central Rocky Mountains. Widespread slow anomalies are found in the lower crust and upper mantle, suggesting that low-density rocks isostatically sustain part of the high topography of the western United States. The Yellowstone anomaly is imaged as a large slow body rising from the lower mantle, intruding the overlying lithosphere and controlling locally the seismicity and the topography. The large E-W extent of the USArray used in this study allows imaging the 'slab graveyard', a sequence of Farallon fragments aligned with the currently subducting Juan de Fuca Slab, north of the Mendocino Triple Junction. The lithospheric root of the Colorado Plateau has apparently been weakened and partly removed through dripping. The distribution of the slower regions around the Colorado Plateau and other rigid blocks follows closely the trend of Cenozoic volcanic fields and ancient lithospheric sutures, suggesting that the later exert a control on the locus of magmato-tectonic activity today. The DNA velocity models are available for download and slicing at http://dna.berkeley.edu. ?? 2011 The Authors Geophysical Journal International ?? 2011 RAS.

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.

Lithospheric structure of the Western Alps as seen by full-waveform inversion of CIFALPS teleseismic data

NASA Astrophysics Data System (ADS)

Beller, Stephen; Monteiller, Vadim; Operto, Stéphane; Nolet, Guust; Paul, Anne; Zhao, Liang

2017-04-01

Full-waveform inversion (FWI) is a powerful but constitutionally intensive technique that aims to recover 3D multiparameter images of the subsurface by minimising the waveform difference between the full recorded and modelled seismograms. This method has recently been adapted and successfully applied in lithospheric settings by tackling teleseismic waveform modelling with hybrid methods. For each event, a global scale simulation is performed once and for all to store the wavefield solutions on the edges of the lithospheric target. Then, for each modelling involved in the FWI process, these global scale solutions are injected within the lithospheric medium from the boundaries. We present the results of the application of teleseismic FWI to the data acquired by the CIFALPS experiment that was conducted in the Western Alps to gain new insights its lithospheric structure and geodynamic evolution of the alpine range. Nine teleseismic events were inverted to infer 3D models of density, P-wave velocity and S-wave velocity of the crust and the upper-mantle down to 200 km depth. Our models show clear evidences of continental subduction during the alpine orogeny. They outline a dipping European Moho down to 75 km depth and finely delineate the geometry of the Ivrea body at the suture between European and Adriatic plates. Deeper, in the mantle a slow S-wave velocity anomaly might indicate the location of the European slab detachment. Overall, FWI models give access to new seismic images that fill the resolution gap between smooth tomographic model and sharp receiver function images of the lithosphere and enable integrated interpretations of crustal and upper-mantle structures.

Magmatism and Dynamic Topography of Libya and Tibesti, North Africa

NASA Astrophysics Data System (ADS)

Ball, P.; White, N. J.; Maclennan, J.; Stuart, F. M.

2016-12-01

In the continents, dynamic topography is difficult to determine because the density structure of the lithosphere is poorly known. It is generally agreed that hot upwelling mantle produces dynamic uplift whilst cold downwelling mantle causes regional subsidence. Calculating asthenospheric potential temperatures from basalts provides one important constraint on dynamic uplift at the present day and in the geologic record. The spatial and temporal distribution of eruptive products together with the compositional variation of lavas allows the origin of continental volcanic events to be interpreted. The Cenozoic Libyan volcanic field is characterized by a series of long wavelength topographic swells that may reflect sub-lithospheric dynamic processes. Admittance analysis of gravity and topographic data as well as seismic tomographic imaging suggest that a low density anomaly sits beneath the lithospheric plate. A new regional basaltic database of 188 XRF and ICP-MS analyses together with 39 40Ar-39Ar dates has been assembled. The Libyan volcanic field has been active from at least 17 Ma until the present day. Inverse modeling of rare earth elemental distributions shows that Libyan basalts were generated by melting of a predominantly anhydrous mixed peridotitic mantle source with an asthenospheric potential temperature of 1400 oC. Our results suggest that the existence and distribution of volcanism is caused by the combination of warm, upwelling asthenospheric mantle and thinner (< 100 km) lithosphere beneath Libya whereby melts may ascend to the surface through metasomatized lithospheric channels.

Isostatic Model and Isostatic Gravity Anomalies of the Arabian Plate and Surroundings

NASA Astrophysics Data System (ADS)

Kaban, Mikhail K.; El Khrepy, Sami; Al-Arifi, Nassir

2016-04-01

The isostatic modeling represents one of the most useful "geological" reduction methods of the gravity field. With the isostatic correction, it is possible to remove a significant part of the effect of deep density heterogeneity, which dominates in the Bouguer gravity anomalies. Although there exist several isostatic compensation schemes, it is usually supposed that a choice of the model is not an important factor to first order, since the total weight of compensating masses remains the same. We compare two alternative models for the Arabian plate and surrounding area. The Airy model gives very significant regional isostatic anomalies, which cannot be explained by the upper crust structure or disturbances of the isostatic equilibrium. Also, the predicted "isostatic" Moho is very different from existing seismic observations. The second isostatic model includes the Moho, which is based on seismic determinations. Additional compensation is provided by density variations within the lithosphere (chiefly in the upper mantle). According to this model, the upper mantle under the Arabian Shield is less dense than under the Platform. In the Arabian platform, the maximum density coincides with the Rub' al Khali, one of the richest oil basin in the world. This finding agrees with previous studies, showing that such basins are often underlain by dense mantle, possibly related to an eclogite layer that has caused their subsidence. The mantle density variations might be also a result of variations of the lithosphere thickness. With the combined isostatic model, it is possible to minimize regional anomalies over the Arabian plate. The residual local anomalies correspond well to tectonic structure of the plate. Still very significant anomalies, showing isostatic disturbances of the lithosphere, are associated with the Zagros fold belt, the collision zone of the Arabian and Eurasian plates.

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.

Crust-mantle density distribution in the eastern Qinghai-Tibet Plateau revealed by satellite-derived gravity gradients

NASA Astrophysics Data System (ADS)

LI, Honglei; Fang, Jian; Braitenberg, Carla; Wang, Xinsheng

2015-04-01

As the highest, largest and most active plateau on Earth, the Qinghai-Tibet Plateau has a complex crust-mantle structure, especially in its eastern part. In response to the subduction of the lithospheric mantle of the Indian plate, large-scale crustal motion occurs in this area. Despite the many previous studies, geodynamic processes at depth remain unclear. Knowledge of crust and upper mantle density distribution allows a better definition of the deeper geological structure and thus provides critically needed information for understanding of the underlying geodynamic processes. With an unprecedented precision of 1-2 mGal and a spatial resolution better than 100 km, GOCE (Gravity field and steady-state Ocean Circulation Explorer) mission products can be used to constrain the crust-mantle density distribution. Here we used GOCE gravitational gradients at an altitude of 10km after reducing the effects of terrain, sediment thickness variations, and Moho undulations to image the density structures of eastern Tibet up to 200 km depths. We inverted the residual satellite gravitational gradients using a least square approach. The initial density model for the inversion is based on seismic velocities from the tomography. The model is composed of rectangular blocks, having a uniform density, with widths of about 100 km and variable thickness and depths. The thickness of the rectangular cells changes from10 to 60km in accordance with the seismic model. Our results reveal some large-scale, structurally controlled density variations at depths. The lithospheric root defined by higher-density contrast features from southwest to northeast, with shallowing in the central part: base of lithosphere reaches a depth of180 km, less than 100km, and 200 km underneath the Lhasa, Songpan-Ganzi, and Ordos crustal blocks, respectively. However, these depth values only represent a first-order parameterization because they depend on model discretization inherited from the original seismic tomography model. For example, the thickness of the uniform density blocks centered at140 km depth is as large as 60 km. Low-density crustal anomalies beneath the southern Lhasa and Songpan-Ganzi blocks in our model support the idea of weak lower crust and possible crustal flow, as a result of the thermal anomalies caused by the upwelling of hot deep materials. The weak lower crust may cause the decoupling of the upper crust and the mantle. These results are consistent with many other geophysical studies, confirming the effectiveness of the GOCE gravitational gradient data. Using these data in combination with other geodynamic constraints (e.g., gravity and seismic structure and preliminary reference Earth model), an improved dynamic model can be derived.

Crustal and lithospheric structure of the Alborz Mountains (Iran) and surrounding areas from integrated geophysical modeling

NASA Astrophysics Data System (ADS)

Motavallianbaran, S.; Zeyen, H. J.; Brunet, M.; Ardestani, V. E.

2010-12-01

The tectonic evolution of Alborz Mountains (northern Iran) and the South Caspian Basin as well as its transition into the Scythian and Turan platforms are yet an unsolved and debated problem. 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 found thin lithosphere (100-120 km) underneath Central Iran, whereas thick lithosphere (up to 260 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 58 km) and under the Kopet-Dagh Mountains (48 km), whereas the thin crust under the southern Caspian Sea is interpreted as oceanic crust. Modeling result of Profile I is shown below with the crust in gray scale (darker gray: higher density) and the lithospheric mantle with color-coded temperatures. Since some previous studies argued for the absence of a root under the Alborz, we tested different models to see whether it is possible to explain the data without a root beneath the Alborz and finally we found that it is impossible to fit the calculated data to the measured ones with a geologically reasonable model. Below the South Caspian Sea, the form of the crust-mantle interface and the base of the lithosphere indicate a subduction of the South Caspian block towards the N-NW. Further east, under the Kopet-Dagh, no evidence for active subduction is visible. Based on the temperature distribution, we calculated the vertically integrated rock rigidity along the profiles. It shows that a rheologically very strong South Caspian block is surrounded by weaker continental lithosphere which may explain the rigid-block subduction of the South Caspian block on the one hand and internal deformation of the lithosphere under the Kopet-Dagh on the other hand.

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.

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.

Continental collision with a sandwiched accreted terrane: Insights into Himalayan-Tibetan lithospheric mantle tectonics?

NASA Astrophysics Data System (ADS)

Kelly, Sean; Butler, Jared P.; Beaumont, Christopher

2016-12-01

Many collisional orogens contain exotic terranes that were accreted to either the subducting or overriding plate prior to terminal continent-continent collision. The ways in which the physical properties of these terranes influence collision remain poorly understood. We use 2D thermomechanical finite element models to examine the effects of prior 'soft' terrane accretion to a continental upper plate (retro-lithosphere) on the ensuing continent-continent collision. The experiments explore how the style of collision changes in response to variations in the density and viscosity of the accreted terrane lithospheric mantle, as well as the density of the pro-lithospheric mantle, which determines its propensity to subduct or compress the accreted terrane and retro-lithosphere. The models evolve self-consistently through several emergent phases: breakoff of subducted oceanic lithosphere; pro-continent subduction; shortening of the retro-lithosphere accreted terrane, sometimes accompanied by lithospheric delamination; and, terminal underthrusting of pro-lithospheric mantle beneath the accreted terrane crust or mantle. The modeled variations in the properties of the accreted terrane lithospheric mantle can be interpreted to reflect metasomatism during earlier oceanic subduction beneath the terrane. Strongly metasomatized (i.e., dense and weak) mantle is easily removed by delamination or entrainment by the subducting pro-lithosphere, and facilitates later flat-slab underthrusting. The models are a prototype representation of the Himalayan-Tibetan orogeny in which there is only one accreted terrane, representing the Lhasa terrane, but they nonetheless exhibit processes like those inferred for the more complex Himalayan-Tibetan system. Present-day underthrusting of the Tibetan Plateau crust by Indian mantle lithosphere requires that the Lhasa terrane lithospheric mantle has been removed. Some of the model results support previous conceptual interpretations that Tibetan lithospheric mantle was removed by convective coupling to the pro-lithosphere. They can also be interpreted to suggest that delamination beneath Tibet was facilitated by densification and weakening of the plateau lithosphere, perhaps owing to long-lived pre- to syn-collisional subduction-related metasomatism beneath the Asian margin.

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.

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.

Geophysical, petrological and mineral physics constraints on Earth's surface topography

NASA Astrophysics Data System (ADS)

Guerri, Mattia; Cammarano, Fabio; Tackley, Paul J.

2015-04-01

Earth's surface topography is controlled by isostatically compensated density variations within the lithosphere, but dynamic topography - i.e. the topography due to adjustment of surface to mantle convection - is an important component, specially at a global scale. In order to separate these two components it is fundamental to estimate crustal and mantle density structure and rheological properties. Usually, crustal density is constrained from interpretation of available seismic data (mostly VP profiles) based on empirical relationships such those in Brocher [2005]. Mantle density structure is inferred from seismic tomography models. Constant coefficients are used to interpret seismic velocity anomalies in density anomalies. These simplified methods are unable to model the effects that pressure and temperature variations have on mineralogical assemblage and physical properties. Our approach is based on a multidisciplinary method that involves geophysical observables, mineral physics constraints, and petrological data. Mantle density is based on the thermal interpretation of global seismic tomography models assuming various compositional structures, as in Cammarano et al. [2011]. We further constrain the top 150 km by including heat-flow data and considering the thermal evolution of the oceanic lithosphere. Crustal density is calculated as in Guerri and Cammarano [2015] performing thermodynamic modeling of various average chemical compositions proposed for the crust. The modeling, performed with the code PerpleX [Connolly, 2005], relies on the thermodynamic dataset from Holland and Powell [1998]. Compressional waves velocity and crustal layers thickness from the model CRUST 1.0 [Laske et al., 2013] offer additional constrains. The resulting lithospheric density models are tested against gravity (GOCE) data. Various crustal and mantle density models have been tested in order to ascertain the effects that uncertainties in the estimate of those features have on the modeled topography. We also test several viscosity models, either radially symmetric, the V1 profile from Mitrovica and Forte [2004], or more complex laterally varying structures. All the property fields are expanded in spherical harmonics, until degree 24, and implemented in the code StagYY [Tackley, 2008] to perform mantle instantaneous flow modeling and compute surface topography and gravitational field. Our results show the importance of constraining the crustal and mantle density structure relying on a multidisciplinary approach that involves experimentally robust thermodynamic datasets. Crustal density field has a strong effect on the isostatic component of topography. The models that we test, CRUST 1.0 and those in Guerri and Cammarano [2015], produce strong differences in the computed isostatic topography, in the range ±600 m. For the lithospheric mantle, relying on experimentally robust material properties constraints is necessary to infer a reliable density model that takes into account chemical heterogeneities. This approach is also fundamental to correctly interpret seismic models in temperature, a crucial parameter, necessary to determine the lithosphere-asthenosphere boundary, where static effects on topography leave place to dynamic ones. The comparison between results obtained with different viscosity fields, either radially symmetric or vertically and laterally varying, shows how lateral viscosity variations affect the results, in particular the modeled geoid, at different wavelengths. References: Brocher, T. M. (2005), Empirical Relations between Elastic Wavespeeds and Density in the Earth's Crust, Bulletin of the Seismological Society of America, 95(6), 2081-2092. Cammarano, F., P. J. Tackley, and L. Boschi (2011), Seismic, petrological and geodynamical constraints on thermal and compositional structure of the upper mantle: global thermochemical models, Geophys. J. Int. Connolly, J. A. D. (2005), Computation of phase equilibria by linear programming: A tool for geodynamic modeling and its application to subduction zone decarbonation, Earth and Planetary Science Letters (236), 524-541. Guerri, M., and F. Cammarano (2015), On the effects of chemical composition, water and temperature on physical properties of the Earth's continental crust, submitted to Geochemistry, Geophysics, Geosystem. Holland, T. J. B., and R. Powell (1998), An internally consistent thermodynamic data set for phases of petrological interest, J. metamorphic Geol., 16(309-343). Laske, G., G. Masters, Z. Ma, and M. E. Pasyanos (2013), CRUST1.0: An updated global model of Earth's crust, in EGU General Assembly 2013, edited, Geophysical Research Abstracts, Vienna. Mitrovica, J. X., and A. M. Forte (2004), A new inference of mantle viscosity based upon joint inversion of convection and glacial isostatic adjustment data, Earth and Planetary Science Letters, 225, 177-189. Tackley, P. J. (2008), Modelling compressible mantle convection with large viscosity contrasts in a three-dimensional spherical shell using the yin-yang grid, Phys. Earth Planet. Int.

Crustal Structure of the Andean Foreland in Northern Argentina: Results From Data-Integrative Three-Dimensional Density Modeling

NASA Astrophysics Data System (ADS)

Meeßen, C.; Sippel, J.; Scheck-Wenderoth, M.; Heine, C.; Strecker, M. R.

2018-02-01

Previous thermomechanical modeling studies indicated that variations in the temperature and strength of the crystalline crust might be responsible for the juxtaposition of domains with thin-skinned and thick-skinned crustal deformation along strike the foreland of the central Andes. However, there is no evidence supporting this hypothesis from data-integrative models. We aim to derive the density structure of the lithosphere by means of integrated 3-D density modeling, in order to provide a new basis for discussions of compositional variations within the crust and for future thermal and rheological modeling studies. Therefore, we utilize available geological and geophysical data to obtain a structural and density model of the uppermost 200 km of the Earth. The derived model is consistent with the observed Bouguer gravity field. Our results indicate that the crystalline crust in northern Argentina can be represented by a lighter upper crust (2,800 kg/m3) and a denser lower crust (3,100 kg/m3). We find new evidence for high bulk crustal densities >3,000 kg/m3 in the northern Pampia terrane. These could originate from subducted Puncoviscana wackes or pelites that ponded to the base of the crystalline crust in the late Proterozoic or indicate increasing bulk content of mafic material. The precise composition of the northern foreland crust, whether mafic or felsic, has significant implications for further thermomechanical models and the rheological behavior of the lithosphere. A detailed sensitivity analysis of the input parameters indicates that the model results are robust with respect to the given uncertainties of the input data.

Thermodynamic, geophysical and rheological modeling of the lithosphere underneath the North Atlantic Porcupine Basin (Ireland).

NASA Astrophysics Data System (ADS)

Botter, C. D.; Prada, M.; Fullea, J.

2017-12-01

The Porcupine is a North-South oriented basin located southwest of Ireland, along the North Atlantic continental margin, formed by several rifting episodes during Late Carboniferous to Early Cretaceous. The sedimentary cover is underlined by a very thin continental crust in the center of the basin (<5 km) that has been generally associated with hyperextension and mantle serpentinization. From North to South lithospheric stretching factors increase drastically from 2 in the North to >10 in the South. In spite of the abundant literature, most of the oil and gas exploration in the Porcupine Basin has been targeting its northern part and is mostly restricted to relatively shallow depths, giving a restrained overview of the basin structure. Therefore, studying the thermodynamic and composition of the deep and broader structures is needed to understand the processes linked to the formation and the symmetry signature of the basin. Here, we model the present-day thermal and compositional structure of the continental crust and lithospheric mantle underneath the Porcupine basin using gravity, seismic, heat flow and elevation data. We use an integrated geophysical-petrological framework where most relevant rock properties (density, seismic velocities) are determined as a function of temperature, pressure and composition. Our modelling approach solves simultaneously the heat transfer, thermodynamic, geopotential, seismic and isostasy equations, and fit the results to all available geophysical and petrological observables (LitMod software). In this work we have implemented a module to compute self-consistently a laterally variable lithospheric elastic thickness based on mineral physics rheological laws (yield strength envelopes over the 3D volume). An appropriate understanding of local and flexural isostatic behavior of the basin is essential to unravel its tectonic history (i.e. stretching factors, subsidence etc.). Our Porcupine basin 3D model is defined by four lithological layers, representing properties from post- and syn-rift sequences to the lithospheric mantle. The computed yield strength envelopes are representative of hyperextended lithosphere and reveal the sensitivity of the lithospheric strength to the geotherm, as well as to the thickness and composition of the crust.

Velocity and Density Heterogeneities of the Tien-Shan Lithosphere

NASA Astrophysics Data System (ADS)

Sabitova, T. M.; Lesik, O. M.; Adamova, A. A.

The Tien-Shan orogene is a region in which the earth's crust undergoes considerable thickening and tangential compression. Under these conditions the lithosphere heterogeneities (composi tion, rheological) create the prerequisites for the development of various phenomena of tectonic layering (lateral shearing, different deformation of layers). To study the distribution of velocity, density and other elastic parameters, the results from a seismic tomography study on P-wave as well as S-wave velocities were used. Using empirical as well as theoretical formulas on the relationship between velocity, density and silica content in rocks, their distribution in the Tien-Shan's lithosphere has been calculated. In addition, other elastic parameters, such as Young's modulus, shear modulus, Poisson's ratio and coefficient of general compressions have been determined. Zoning of different types of crust was carried out for the region investigated. The characteristics of the "crust-mantle" transition have been investi gated. Large blocks with different types of the earth's crust were distinguished. Layers with inverse values of velocity, density and shear and Young modulus are revealed in the Tien-Shan lithosphere. All of the above described features open new ways to solve geodynamics problems.

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.

Structure and Evolution of the Central Appalachians from the Mantle to the Surface: Results from the MAGIC Project

NASA Astrophysics Data System (ADS)

Long, M. D.; Benoit, M. H.; Evans, R. L.; King, S. D.; Kirby, E.; Aragon, J. C.; Miller, S. R.; Liu, S.; Elsenbeck, J.

2017-12-01

The eastern margin of North America has undergone multiple episodes of orogenesis and rifting, yielding the surface geology and topography visible today. It is poorly known, however, how the crust and mantle lithosphere have responded to these tectonic forces, and how geologic units preserved at the surface relate to deeper structures. Furthermore, the evolution of Appalachian topography through time, which reflects a complex interplay among erosion, lithology, and mantle flow, remains a major outstanding problem. The MAGIC project involves a multidisciplinary, collaborative effort to understand the structure and evolution of the central Appalachians, from the mantle to the surface. New images of the lithosphere derived from a passive broadband seismic array and a magnetotelluric deployment demonstrate significant along-strike lateral variability across the MAGIC transect. We observe a sharp change in crustal thickness across the eastern edge of the Appalachians, with a deeper Moho beneath the mountains than suggested by simple isostatic models. We find evidence for a relatively shallow lithosphere-asthenosphere boundary (LAB) beneath the Appalachians, with the thinnest LAB coinciding with the location of Eocene volcanism in and around Harrisonburg, VA. This observation is consistent with lithospheric loss as a mechanism for Eocene volcanic activity. Observations of seismic anisotropy suggest deformation of the mantle lithosphere associated with both Appalachian orogenesis and later Mesozoic rifting, with an observable component of anisotropy due to present-day mantle flow. Geodynamic models of mantle flow using a variety of tomographic models and density scaling relationships are being used to generate predictions of dynamic topography and plate motions for comparison with observations, and are currently being refined to incorporate realistic lithospheric morphology based on imaging results. Models of present-day erosion rates throughout the Appalachians from stream profile analysis show particularly fast erosion rates just to the west of Harrisonburg. Integration of results from the MAGIC project is yielding new insight into the structure and evolution of the central Appalachians and into the processes associated with orogenesis, rifting, and post-rift evolution of the passive margin.

Seismic evidence for depth-dependent metasomatism in cratons

NASA Astrophysics Data System (ADS)

Eeken, Thomas; Goes, Saskia; Pedersen, Helle A.; Arndt, Nicholas T.; Bouilhol, Pierre

2018-06-01

The long-term stability of cratons has been attributed to low temperatures and depletion in iron and water, which decrease density and increase viscosity. However, steady-state thermal models based on heat flow and xenolith constraints systematically overpredict the seismic velocity-depth gradients in cratonic lithospheric mantle. Here we invert for the 1-D thermal structure and a depth distribution of metasomatic minerals that fit average Rayleigh-wave dispersion curves for the Archean Kaapvaal, Yilgarn and Slave cratons and the Proterozoic Baltic Shield below Finland. To match the seismic profiles, we need a significant amount of hydrous and/or carbonate minerals in the shallow lithospheric mantle, starting between the Moho and 70 km depth and extending down to at least 100-150 km. The metasomatic component can consist of 0.5-1 wt% water bound in amphibole, antigorite and chlorite, ∼0.2 wt% water plus potassium to form phlogopite, or ∼5 wt% CO2 plus Ca for carbonate, or a combination of these. Lithospheric temperatures that fit the seismic data are consistent with heat flow constraints, but most are lower than those inferred from xenolith geothermobarometry. The dispersion data require differences in Moho heat flux between individual cratons, and sublithospheric mantle temperatures that are 100-200 °C less beneath Yilgarn, Slave and Finland than beneath Kaapvaal. Significant upward-increasing metasomatism by water and CO2-rich fluids is not only a plausible mechanism to explain the average seismic structure of cratonic lithosphere but such metasomatism may also lead to the formation of mid-lithospheric discontinuities and would contribute to the positive chemical buoyancy of cratonic roots.

Seismic Velocity Structure and Depth-Dependence of Anisotropy in the Red Sea and Arabian Shield from Surface Wave Analysis

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

Hansen, S; Gaherty, J; Schwartz, S

2007-07-25

We investigate the lithospheric and upper mantle structure as well as the depth-dependence of anisotropy along the Red Sea and beneath the Arabian Peninsula using receiver function constraints and phase velocities of surface waves traversing two transects of stations from the Saudi Arabian National Digital Seismic Network. Frequency-dependent phase delays of fundamental-mode Love and Rayleigh waves, measured using a cross-correlation procedure, require very slow shear velocities and the presence of anisotropy throughout the upper mantle. Linearized inversion of these data produce path-averaged 1D radially anisotropic models with about 4% anisotropy in the lithosphere, increasing to about 4.8% anisotropy across themore » lithosphere-asthenosphere boundary (LAB). Models with reasonable crustal velocities in which the mantle lithosphere is isotropic cannot satisfy the data. The lithospheric lid, which ranges in thickness from about 70 km near the Red Sea coast to about 90 km beneath the Arabian Shield, is underlain by a pronounced low-velocity zone with shear velocities as low as 4.1 km/s. Forward models, which are constructed from previously determined shear-wave splitting estimates, can reconcile surface and body wave observations of anisotropy. The low shear velocity values are similar to many other continental rift and oceanic ridge environments. These low velocities combined with the sharp velocity contrast across the LAB may indicate the presence of partial melt beneath Arabia. The anisotropic signature primarily reflects a combination of plate- and density-driven flow associated with active rifting processes in the Red Sea.« less

Sediment basin modeling through GOCE gradients controlled by thermo-isostatic constraints

NASA Astrophysics Data System (ADS)

Pivetta, Tommaso; Braitenberg, Carla

2015-04-01

Exploration of geodynamic and tectonic structures through gravity methods has experienced an increased interest in the recent years thank's to the possibilities offered by satellite gravimetry (e.g. GOCE). The main problem with potential field methods is the non-uniqueness of the underground density distributions that satisfy the observed gravity field. In terrestrial areas with scarce geological and geophysical information, valid constraints to the density model could be obtained from the application of geodynamic models. In this contribution we present the study of the gravity signals associated to the thermo-isostatic McKenzie-model (McKenzie, 1978) that predicts the development of sedimentary basins from the stretching of lithosphere. This model seems to be particularly intriguing for gravity studies as we could obtain estimates of densities and thicknesses of crust and mantle before and after a rifting event and gain important information about the time evolution of the sedimentary basin. The McKenzie-model distinguishes the rifting process into two distinct phases: a syn-rift phase that occurs instantly and is responsible of the basin formation, the thinning of lithosphere and the upwelling of hot asthenosphere. Then a second phase (post-rift), that is time dependent, and predicts further subsidence caused by the cooling of mantle and asthenosphere and subsequently increase in rock density. From the application of the McKenzie-model we have derived density underground distributions for two scenarios: the first scenario involves the lithosphere density distribution immediately after the stretching event; the second refers to the density model when thermal equilibrium between stretched and unstretched lithospheres is achieved. Calculations of gravity anomalies and gravity gradient anomalies are performed at 5km height and at the GOCE mean orbit quota (250km). We have found different gravity signals for syn-rift (gravimetric maximum) and post-rift (gravimetric minimum) scenarios and that satellite measurements are sufficiently precise to discriminate between them. The McKenzie-model is then applied to a real basin in Africa, the Benue Trough, which is an aborted rift that seems to be particularly adapt to be studied with satellite gravity techniques. McKenzie D., 1978, Some remarks on the development of sedimentary basins, Earth and Planetary Science Letters, 40, 25-32

Utopia and Hellas basins, Mars: Twins separated at birth

NASA Astrophysics Data System (ADS)

Searls, Mindi L.; Banerdt, W. Bruce; Phillips, Roger J.

2006-08-01

Using topography and gravity data as constraints, we formulate spherical harmonic thin elastic-shell models to determine the subsurface structure of the Hellas and Utopia basins. For Hellas, we show that our model is consistent with the elastic thickness results of McGovern et al. (2002, 2004). The thin elastic lithosphere at the time of formation implies that Hellas is close to isostatic. Since Utopia formed earlier, we argue that an isostatic assumption is justified for the Utopia basin before it was filled. From this supposition, we derive a system of equations that allows us to solve for the amount of fill, the prefill topography, and the amount of flexure due to the fill within the Utopia basin. An analysis of the parameter space shows that the fill density and the amount of fill is strongly dependent on the elastic thickness at the time of infilling. A thinner elastic lithosphere favors a denser fill, while a thicker lithosphere will allow for less dense material. Likewise, larger crustal thickness values lead to smaller fill density values. The presence of quasi-circular depressions, interpreted as impact craters, within the Utopia basin indicates that the majority of the material within Utopia was deposited prior to 4.04-4.11 Ga. The early timing for the deposition combined with the heat imparted by the basin forming event argues for a thinner lithosphere which could, in turn, suggest fill densities that are more consistent with a volcanic load than with pure sediment or ice-rich material. These results are supported using an alternative method of determining the amount of fill and flexure within Utopia. This model assumes that Hellas and Utopia were initially identical and that the only difference in their subsequent evolution was the addition of material in the Utopia basin. The volume of material needed to fill Utopia is immense (on the order of 50 million km3 or more). The high density obtained for the fill requires that it contain a large igneous component, the source of which is problematic. Relaxing the isostatic assumption to a reasonable degree perturbs the density bound only slightly.

Slab dehydration in Cascadia and its relationship to volcanism, seismicity, and non-volcanic tremor

NASA Astrophysics Data System (ADS)

Delph, J. R.; Levander, A.; Niu, F.

2017-12-01

The characteristics of subduction beneath the Pacific Northwest (Cascadia) are variable along strike, leading to the segmentation of Cascadia into 3 general zones: Klamath, Siletzia, and Wrangelia. These zones show marked differences in tremor density, earthquake density, seismicity rates, and the locus and amount of volcanism in the subduction-related volcanic arc. To better understand what controls these variations, we have constructed a 3D shear-wave velocity model of the upper 80 km along the Cascadia margin from the joint inversion of CCP-derived receiver functions and ambient noise surface wave data using 900 temporary and permanent broadband seismic stations. With this model, we can investigate variations in the seismic structure of the downgoing oceanic lithosphere and overlying mantle wedge, the character of the crust-mantle transition beneath the volcanic arc, and local to regional variations in crustal structure. From these results, we infer the presence and distribution of fluids released from the subducting slab and how they affect the seismic structure of the overriding lithosphere. In the Klamath and Wrangelia zones, high seismicity rates in the subducting plate and high tremor density correlate with low shear velocities in the overriding plate's forearc and relatively little arc volcanism. While the cause of tremor is debated, intermediate depth earthquakes are generally thought to be due to metamorphic dehydration reactions resulting from the dewatering of the downgoing slab. Thus, the seismic characteristics of these zones combined with rather sparse arc volcanism may indicate that the slab has largely dewatered by the time it reaches sub-arc depths. Some of the water released during earthquakes (and possibly tremor) may percolate into the overriding plate, leading to slow seismic velocities in the forearc. In contrast, Siletzia shows relatively low seismicity rates and tremor density, with relatively higher shear velocities in the forearc. Siletzia also contains most of the young arc volcanoes in the Cascades, indicating that water is retained in the slab to depths where it can feed arc volcanism. Thus, the along strike variations in volcanic activity and seismic activity in Cascadia appear to be related to variations in depth of dewatering of the downgoing oceanic lithosphere.

The Importance of Lower Mantle Structure to Plate Stresses and Plate Motions

NASA Astrophysics Data System (ADS)

Holt, W. E.; Wang, X.; Ghosh, A.

2016-12-01

Plate motions and plate stresses are widely assumed as the surface expression of mantle convection. The generation of plate tectonics from mantle convection has been studied for many years. Lithospheric thickening (or ridge push) and slab pull forces are commonly accepted as the major driving forces for the plate motions. However, the importance of the lower mantle to plate stresses and plate motions remains less clear. Here, we use the joint modeling of lithosphere and mantle dynamics approach of Wang et al. (2015) to compute the tractions originating from deeper mantle convection and follow the method of Ghosh et al. (2013) to calculate gravitational potential energy per unit area (GPE) based on Crust 1.0 (Laske et al., 2013). Absolute values of deviatoric stresses are determined by the body force distributions (GPE gradients and traction magnitudes applied at the base of the lithosphere). We use the same relative viscosity model that Ghosh et al. (2013) used, and we solve for one single adjustable scaling factor that multiplies the entire relative viscosity field to provide absolute values of viscosity throughout the lithosphere. This distribution of absolute values of lithosphere viscosities defines the magnitudes of surface motions. In this procedure, the dynamic model first satisfies the internal constraint of no-net-rotation of motions. The model viscosity field is then scaled by the single factor until we achieve a root mean square (RMS) minimum between computed surface motions and the kinematic no-net-rotation (NNR) model of Kreemer et al. (2006). We compute plate stresses and plate motions from recently published global tomography models (over 70 based on Wang et al., 2015). We find that RMS misfits are significantly reduced when details of lower mantle structure from the latest tomography models are added to models that contain only upper and mid-mantle density distributions. One of the key reasons is that active upwelling from the Large Low Shear Velocity Provinces (LLSVPs) in the lower mantle in Pacific (Frost and Rost, 2014) provides important components of mantle flow affecting plate stresses and motions. We demonstrate in this paper how lower mantle density heterogeneity has a marked influence on plate stresses and plate motions.

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.

A random-walk algorithm for modeling lithospheric density and the role of body forces in the evolution of the Midcontinent Rift

USGS Publications Warehouse

Levandowski, William Brower; Boyd, Oliver; Briggs, Richard; Gold, Ryan D.

2015-01-01

We test this algorithm on the Proterozoic Midcontinent Rift (MCR), north-central U.S. The MCR provides a challenge because it hosts a gravity high overlying low shear-wave velocity crust in a generally flat region. Our initial density estimates are derived from a seismic velocity/crustal thickness model based on joint inversion of surface-wave dispersion and receiver functions. By adjusting these estimates to reproduce gravity and topography, we generate a lithospheric-scale model that reveals dense middle crust and eclogitized lowermost crust within the rift. Mantle lithospheric density beneath the MCR is not anomalous, consistent with geochemical evidence that lithospheric mantle was not the primary source of rift-related magmas and suggesting that extension occurred in response to far-field stress rather than a hot mantle plume. Similarly, the subsequent inversion of normal faults resulted from changing far-field stress that exploited not only warm, recently faulted crust but also a gravitational potential energy low in the MCR. The success of this density modeling algorithm in the face of such apparently contradictory geophysical properties suggests that it may be applicable to a variety of tectonic and geodynamic problems. 

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.

The lithospheric Structure of the Sahara Metacraton From Joint Analysis of Satellite Gravity Gradients and Seismological Data

NASA Astrophysics Data System (ADS)

Sobh, M.; Ebbing, J.; Goetze, H. J.; Abdelsalam, M. G.

2016-12-01

For the Saharan Metacraton in northern Africa only a few geophysical results exists, which can be used to characterize its deep structure. We combine recent seismological models with satellite gravity gradients to build a 3D lithospheric density model of the metacraton and its surrounding regions. Due to the sparse distribution of seismic data, we estimate the Moho boundary by non-linear gravity inversion in spherical coordinates. The model is constrained by some wide angle refraction seismic profiles and receiver function Moho depths. Despite the high topography of the Darfur and Tibisti Cenozoic volcanic provinces, we estimate thin crust which indicates an upper mantle contribution to the isostatic balance. In combination with seismic tomography models, we found that the lithospheric thickness in the western part of the Metacraton is thicker than in the eastern part. This indicates that the western resembles the remnants of the pre-Neoproterozoic Sahara craton (e.g. the Marzuk craton which escaped the metacratonization process). In order to explain the partial loss of the expected cratonic root beneath the Metacraton, we present different petrological-geophysical scenario testing for different upper mantle compositions.

Structure and State of Stress of the Chilean Subduction Zone from Terrestrial and Satellite-Derived Gravity and Gravity Gradient Data

NASA Astrophysics Data System (ADS)

Gutknecht, B. D.; Götze, H.-J.; Jahr, T.; Jentzsch, G.; Mahatsente, R.; Zeumann, St.

2014-11-01

It is well known that the quality of gravity modelling of the Earth's lithosphere is heavily dependent on the limited number of available terrestrial gravity data. More recently, however, interest has grown within the geoscientific community to utilise the homogeneously measured satellite gravity and gravity gradient data for lithospheric scale modelling. Here, we present an interdisciplinary approach to determine the state of stress and rate of deformation in the Central Andean subduction system. We employed gravity data from terrestrial, satellite-based and combined sources using multiple methods to constrain stress, strain and gravitational potential energy (GPE). Well-constrained 3D density models, which were partly optimised using the combined regional gravity model IMOSAGA01C (Hosse et al. in Surv Geophys, 2014, this issue), were used as bases for the computation of stress anomalies on the top of the subducting oceanic Nazca plate and GPE relative to the base of the lithosphere. The geometries and physical parameters of the 3D density models were used for the computation of stresses and uplift rates in the dynamic modelling. The stress distributions, as derived from the static and dynamic modelling, reveal distinct positive anomalies of up to 80 MPa along the coastal Jurassic batholith belt. The anomalies correlate well with major seismicity in the shallow parts of the subduction system. Moreover, the pattern of stress distributions in the Andean convergent zone varies both along the north-south and west-east directions, suggesting that the continental fore-arc is highly segmented. Estimates of GPE show that the high Central Andes might be in a state of horizontal deviatoric tension. Models of gravity gradients from the Gravity field and steady-state Ocean Circulation Explorer (GOCE) satellite mission were used to compute Bouguer-like gradient anomalies at 8 km above sea level. The analysis suggests that data from GOCE add significant value to the interpretation of lithospheric structures, given that the appropriate topographic correction is applied.

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.

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.

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.

Thick shell tectonics on one-plate planets - Applications to Mars

NASA Technical Reports Server (NTRS)

Banerdt, W. B.; Saunders, R. S.; Phillips, R. J.; Sleep, N. H.

1982-01-01

Using the zero frequency equations of a self-gravitating elastic spherical shell overlying a strengthless fluid, a theory for stress distribution in thick lithospheric shells on one-plate planets is developed. For both the compensated and flexural modes, stress distributions in lithospheres are reviewed. For compensated modes, surface stresses depend only on surface topography, whereas for flexural modes it is shown that, for long wavelengths, stress trajectories are mainly dependent on the lithospheric lateral density distribution and not on elastic properties. Computational analyses are performed for Mars, and it is found that isostatically compensated models correctly predict the graben structure in the immediate Tharsis region and a flexural loading model is satisfactory in explaining the graben in the regions surrounding Tharsis. A three-stage model for the evolution of Tharsis is hypothesized: isostasy with north-south graben formation on Tharsis, followed by flexural loading and radial graben formation on the perimeter of Tharsis, followed by a last stage of loading with little or no regional deformation.

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.

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.

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.

Lithospheric structure and deformation of the North American continent

NASA Astrophysics Data System (ADS)

Tesauro, Magdala; Kaban, Mikhail; Cloetingh, Sierd; Mooney, Walter

2013-04-01

We estimate the integrated strength and elastic thickness (Te) of the North American lithosphere based on thermal, density and structural (seismic) models of the crust and upper mantle. The temperature distribution in the lithosphere is estimated considering for the first time the effect of composition as a result of the integrative approach based on a joint analysis of seismic and gravity data. We do this via an iterative adjustment of the model. The upper mantle temperatures are initially estimated from the NA07 tomography model of Bedle and Van der Lee (2009) using mineral physics equations. This thermal model, obtained for a uniform composition, is used to estimate the gravity effect and to remove it from the total mantle gravity anomalies, which are controlled by both temperature and compositional variations. Therefore, we can predict compositional variations from the residual gravity anomalies and use them to correct the initial thermal model. The corrected thermal model is employed again in the gravity calculations. The loop is repeated until convergence is reached. The results demonstrate that the lithospheric mantle is characterized by strong compositional heterogeneity, which is consistent with xenolith data. Seismic data from the USGS database allow to define P-wave velocity and thickness of each crustal layer of the North American geological provinces. The use of these seismic data and of the new compositional and thermal models gives us the chance to estimate lateral variation of rheology of the main lithospheric layers and to evaluate coupling-decoupling conditions at the layers' boundaries. In the North American Cordillera the strength is mainly localized in the crust, which is decoupled from the mantle lithosphere. In the cratons the strength is chiefly controlled by the mantle lithosphere and all the layers are generally coupled. These results contribute to the long debates on applicability of the "crème brulée" or "jelly-sandwich" models for the lithosphere structure. Intraplate earthquakes (USGS database) occur mainly in the weak regions, such as the Appalachians, and in the transition zones from low to high strength surrounding the craton. The obtained 3D strength model is used to compute Te of the North American lithosphere. This parameter is derived from the thermo-rheological model using new equations that consider variations of the Young's Modulus in the lithosphere. It shows large variability within the cratons, ranging from 70 km to >100km, while it drops to <30 km in the young Phanerozoic regions. The new crustal model is also used to compute the lateral pressure gradient (LPG) that can initiate horizontal ductile flow in the crust. In general, the crustal flow is directed away from the orogens towards adjacent weaker areas. The results show that the effects of the channel flow superimposed with the regional tectonic forces might result in additional significant horizontal and vertical movements associated with zones of compression or extension.

Model Order Reduction for the fast solution of 3D Stokes problems and its application in geophysical inversion

NASA Astrophysics Data System (ADS)

Ortega Gelabert, Olga; Zlotnik, Sergio; Afonso, Juan Carlos; Díez, Pedro

2017-04-01

The determination of the present-day physical state of the thermal and compositional structure of the Earth's lithosphere and sub-lithospheric mantle is one of the main goals in modern lithospheric research. All this data is essential to build Earth's evolution models and to reproduce many geophysical observables (e.g. elevation, gravity anomalies, travel time data, heat flow, etc) together with understanding the relationship between them. Determining the lithospheric state involves the solution of high-resolution inverse problems and, consequently, the solution of many direct models is required. The main objective of this work is to contribute to the existing inversion techniques in terms of improving the estimation of the elevation (topography) by including a dynamic component arising from sub-lithospheric mantle flow. In order to do so, we implement an efficient Reduced Order Method (ROM) built upon classic Finite Elements. ROM allows to reduce significantly the computational cost of solving a family of problems, for example all the direct models that are required in the solution of the inverse problem. The strategy of the method consists in creating a (reduced) basis of solutions, so that when a new problem has to be solved, its solution is sought within the basis instead of attempting to solve the problem itself. In order to check the Reduced Basis approach, we implemented the method in a 3D domain reproducing a portion of Earth that covers up to 400 km depth. Within the domain the Stokes equation is solved with realistic viscosities and densities. The different realizations (the family of problems) is created by varying viscosities and densities in a similar way as it would happen in an inversion problem. The Reduced Basis method is shown to be an extremely efficiently solver for the Stokes equation in this context.

Electrical structure of the lithosphere across the Western Paraná suture zone: the role of a Neoproterozoic-Cambrian subduction in generating the Paraná Magmatic Province

NASA Astrophysics Data System (ADS)

Dragone, G. N.; Bologna, M.; Gimenez, M. E.; Alvarez, O.; Lince Klinger, F. G.; Correa-Otto, S.; Ussami, N.

2017-12-01

The Paraná Magmatic Province (PMP) together with the Etendeka Province (EP) in Africa is one of the Earth's largest igneous provinces originated prior to the Western Gondwanaland break-up and the inception of the South Atlantic Ocean in the Lower Cretaceous. Geochemical data of PMP-EP basalts collected since late 1980's indicate the origin of PMP-EP by melting of a heterogeneous and enriched subcontinental lithospheric mantle with fast rate of eruption (< 3 My). The geodynamical cause of this magmatism is still a matter of debate (deep mantle plume x plate model). New isotopic geochemical data from Re-Os systematics (Rocha-Jr et al., 2012, EPSL) of PMP basalts indicate metasomatized asthenospheric mantle component probably generated at the mantle wedge between the PMP-EP lithosphere and the subducting oceanic plate. A combined seismic velocity and density model of PMP by Chaves et al. (2016, G3) indicates high velocity and a density increase of PMP ancient lithosphere interpreted as due to a long-term mantle refertilization process. To investigate the role of the subduction zones in the development of both the Paraná basin subsidence and the magmatic province we present the results of regional scale broad-band MT-magnetotelluric soundings across the western and southern borders of the PMP, the Western Paraná suture zone (WPS in Fig. 1). We discuss the electrical properties of the lithosphere along three MT profiles across the WPS. MT-A profile (Padilha et al., 2015, JGR) extends from Rio Apa craton towards the center of PMP (high-TiO2 basalts). Profile MT-B extends from Tebicuary craton towards the center of PMP (low-TiO2) and profile MT-C extends from Rio de la Plata craton towards the southern PMP (low- and high-TiO2). All profiles show a resistive ( 104 ohm m) and thick (> 150 km) lithosphere in the cratonic areas whereas the electrical lithosphere is thinner (<100 km) with alternating high and low resistivities within PMP. Vertically elongated and high electrical conductivity anomalies ( 10 ohm m) centered at 40 km depth occur along the -30 mGal contour line in the three profiles, and are interpreted as the location of the suture and former subduction zone. We will discuss the correlation between geochemical and petrological characteristics of basalts and the electrical properties of the lithospheric mantle underneath.

Counter-intuitive features of the dynamic topography unveiled by tectonically realistic 3D numerical models of mantle-lithosphere interactions

NASA Astrophysics Data System (ADS)

Burov, Evgueni; Gerya, Taras

2013-04-01

It has been long assumed that the dynamic topography associated with mantle-lithosphere interactions should be characterized by long-wavelength features (> 1000 km) correlating with morphology of mantle flow and expanding beyond the scale of tectonic processes. For example, debates on the existence of mantle plumes largely originate from interpretations of expected signatures of plume-induced topography that are compared to the predictions of analytical and numerical models of plume- or mantle-lithosphere interactions (MLI). Yet, most of the large-scale models treat the lithosphere as a homogeneous stagnant layer. We show that in continents, the dynamic topography is strongly affected by rheological properties and layered structure of the lithosphere. For that we reconcile mantle- and tectonic-scale models by introducing a tectonically realistic continental plate model in 3D large-scale plume-mantle-lithosphere interaction context. This model accounts for stratified structure of continental lithosphere, ductile and frictional (Mohr-Coulomb) plastic properties and thermodynamically consistent density variations. The experiments reveal a number of important differences from the predictions of the conventional models. In particular, plate bending, mechanical decoupling of crustal and mantle layers and intra-plate tension-compression instabilities result in transient topographic signatures such as alternating small-scale surface features that could be misinterpreted in terms of regional tectonics. Actually thick ductile lower crustal layer absorbs most of the "direct" dynamic topography and the features produced at surface are mostly controlled by the mechanical instabilities in the upper and intermediate crustal layers produced by MLI-induced shear and bending at Moho and LAB. Moreover, the 3D models predict anisotropic response of the lithosphere even in case of isotropic solicitations by axisymmetric mantle upwellings such as plumes. In particular, in presence of small (i.e. insufficient to produce solely any significant deformation) uniaxial extensional tectonic stress field, the plume-produced surface and LAB features have anisotropic linear shapes perpendicular to the far-field tectonic forces, typical for continental rifts. Compressional field results in singular sub-linear folds above the plume head, perpendicular to the direction of compression. Small bi-axial tectonic stress fields (compression in one direction and extension in the orthogonal direction) result in oblique, almost linear segmented normal or inverse faults with strike-slip components (or visa verse , strike-slip faults with normal or inverse components)

Mechanisms of crustal shortening in the foreland of the central Andes, Argentina: insights from data-driven three-dimensional gravity, thermal and rheological modelling

NASA Astrophysics Data System (ADS)

Meeßen, Christian; Scheck-Wenderoth, Magdalena; Sippel, Judith; Strecker, Manfred

2017-04-01

Thin- and thick-skinned deformation styles in the foreland of the central Andes are the result of ongoing crustal shortening since the early Neogene. The mechanisms proposed for these different styles range from variations in subduction angle of the Nazca plate, lithospheric thickening to variations in temperature and strength of the crystalline crust. The latter hypothesis states a cold and strong lithosphere in the foreland of the Altiplano Plateau, facilitating thin-skinned shortening. In contrast, the foreland of the Puna plateau is proposed to be characterized by a warm lithosphere and strong upper crust, resulting in thick-skinned deformation. Whilst this hypothesis has been confirmed in numerical thermomechanical experiments, there is no evidence for this mechanism from data integrative modelling. We test this hypothesis by means of three-dimensional data integrative gravity, thermal and rheological modelling. Therefore, we constructed a lithospheric-scale density model of the foreland of northern Argentina and southern Bolivia using gravity forward modelling and inversion techniques. Into this density model we implemented sediment isopachs, data from receiver functions and densities from shear-wave velocities of the upper mantle. The model was verified using the observed Bouguer gravity anomaly. By assigning thermal and rheological properties to the modelled units we are able to quantify the strength of the lithosphere and test the predictions by the thermomechanical models.

Variation of the subsidence parameters, effective thermal conductivity, and mantle dynamics

NASA Astrophysics Data System (ADS)

Adam, C.; King, S. D.; Vidal, V.; Rabinowicz, M.; Jalobeanu, A.; Yoshida, M.

2015-09-01

The subsidence of young seafloor is generally considered to be a passive phenomenon related to the conductive cooling of the lithosphere after its creation at mid-oceanic ridges. Recent alternative theories suggest that the mantle dynamics plays an important role in the structure and depth of the oceanic lithosphere. However, the link between mantle dynamics and seafloor subsidence has still to be quantitatively assessed. Here we provide a statistical study of the subsidence parameters (subsidence rate and ridge depth) for all the oceans. These parameters are retrieved through two independent methods, the positive outliers method, a classical method used in signal processing, and through the MiFil method. From the subsidence rate, we compute the effective thermal conductivity, keff, which ranges between 1 and 7 W m-1 K-1. We also model the mantle flow pattern from the S40RTS tomography model. The density anomalies derived from S40RTS are used to compute the instantaneous flow in a global 3D spherical geometry. We show that departures from the keff = 3 Wm-1K-1 standard value are systematically related to mantle processes and not to lithospheric structure. Regions characterized by keff > 3 Wm-1K-1 are associated with mantle uplifts (mantle plumes or other local anomalies). Regions characterized by keff < 3 Wm-1K-1 are related to large-scale mantle downwellings such as the Australia-Antarctic Discordance (AAD) or the return flow from the South Pacific Superswell to the East Pacific Rise. This demonstrates that mantle dynamics plays a major role in the shaping of the oceanic seafloor. In particular, the parameters generally considered to quantify the lithosphere structure, such as the thermal conductivity, are not only representative of this structure but also incorporate signals from the mantle convection occurring beneath the lithosphere. The dynamic topography computed from the S40RTS tomography model reproduces the subsidence pattern observed in the bathymetry. Overall we find a good correlation between the subsidence parameters derived from the bathymetry and the dynamic topography. This demonstrates that these parameters are strongly dependent on mantle dynamics.

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.

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.

Mechanical behaviour of the lithosphere beneath the Adamawa uplift (Cameroon, West Africa) based on gravity data

NASA Astrophysics Data System (ADS)

Poudjom Djomani, Y. H.; Diament, M.; Albouy, Y.

1992-07-01

The Adamawa massif in Central Cameroon is one of the African domal uplifts of volcanic origin. It is an elongated feature, 200 km wide. The gravity anomalies over the Adamawa uplift were studied to determine the mechanical behaviour of the lithosphere. Two approaches were used to analyse six gravity profiles that are 600 km long and that run perpendicular to the Adamawa trend. Firstly, the coherence function between topography and gravity was interpreted; secondly, source depth estimations by spectral analysis of the gravity data was performed. To get significant information for the interpretation of the experimental coherence function, the length of the profiles was varied from 320 km to 600 km. This treatment allows one to obtain numerical estimates of the coherence function. The coherence function analysis points out that the lithosphere is deflected and thin beneath the Adamawa uplift, and the Effective Elastic Thickness is of about 20 km. To fit the coherence, a load from below needs to be taken into account. This result on the Adamawa massif is of the same order of magnitude as those obtained on other African uplifts such as Hoggar, Darfur and Kenya domes. For the depth estimation, three major density contrasts were found: the shallowest depth (4-15 km) can be correlated to shear zone structures and the associated sedimentary basins beneath the uplift; the second density contrast (18-38 km) corresponds to the Moho; and finally, the last depth (70-90 km) would be the top of the upper mantle and demotes the low density zone beneath the Adamawa uplift.

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

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.

High-Resolution Gravity Field Modeling for Mercury to Estimate Crust and Lithospheric Properties

NASA Astrophysics Data System (ADS)

Goossens, S.; Mazarico, E.; Genova, A.; James, P. B.

2018-05-01

We estimate a gravity field model for Mercury using line-of-sight data to improve the gravity field model at short wavelengths. This can be used to infer crustal density and infer the support mechanism of the lithosphere.

Geological Implications From Complete Gondwana GOCE- Products Reconstructions and Link to Lithospheric Roots

NASA Astrophysics Data System (ADS)

Braitenberg, Carla; Mariani, Patrizia

2015-03-01

The GOCE gravity field is globally homogeneous at the resolution of about 80km or better allowing for the first time to analyze tectonic structures at continental scale. Geologic correlation studies propose to continue the tectonic lineaments across continents to the pre-breakup position. Tectonic events that induce density changes, as metamorphic events and magmatic events, should then show up in the gravity field. Applying geodynamic plate reconstructions to the GOCE gravity field places today’s observed field at the pre-breakup position. The same reconstruction can be applied to the seismic velocity models, to allow a joint gravity-velocity analysis. The geophysical fields allow to control the likeliness of the hypothesized continuation of lineations based on sparse surface outcrops. Total absence of a signal, makes the cross-continental continuation of the lineament improbable, as continental-wide lineaments are controlled by rheologic and compositional differences of lithospheric mantle. It is found that the deep lithospheric roots as those found below cratons control the position of the positive gravity values. The explanation is that the deep lithospheric roots focus asthenospheric upwelling outboard of the root protecting the overlying craton from magmatic intrusions. The study is carried out over the African and South American continents.

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.

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.

A numerical investigation of continental collision styles

NASA Astrophysics Data System (ADS)

Ghazian, Reza Khabbaz; Buiter, Susanne J. H.

2013-06-01

Continental collision after closure of an ocean can lead to different deformation styles: subduction of continental crust and lithosphere, lithospheric thickening, folding of the unsubducted continents, Rayleigh-Taylor (RT) instabilities and/or slab break-off. We use 2-D thermomechanical models of oceanic subduction followed by continental collision to investigate the sensitivity of these collision styles to driving velocity, crustal and lithospheric temperature, continental rheology and the initial density difference between the oceanic lithosphere and the asthenosphere. We find that these parameters influence the collision system, but that driving velocity, rheology and lithospheric (rather than Moho and mantle) temperature can be classified as important controls, whereas reasonable variations in the initial density contrast between oceanic lithosphere and asthenosphere are not necessarily important. Stable continental subduction occurs over a relatively large range of values of driving velocity and lithospheric temperature. Fast and cold systems are more likely to show folding, whereas slow and warm systems can experience RT-type dripping. Our results show that a continent with a strong upper crust can experience subduction of the entire crust and is more likely to fold. Accretion of the upper crust at the trench is feasible when the upper crust has a moderate to weak strength, whereas the entire crust can be scraped-off in the case of a weak lower crust. We also illustrate that weakening of the lithospheric mantle promotes RT-type of dripping in a collision system. We use a dynamic collision model, in which collision is driven by slab pull only, to illustrate that adjacent plates can play an important role in continental collision systems. In dynamic collision models, exhumation of subducted continental material and sediments is triggered by slab retreat and opening of a subduction channel, which allows upward flow of buoyant materials. Exhumation continues after slab break-off by reverse motion of the subducting plate (`eduction') caused by the reduced slab pull. We illustrate how a simple force balance of slab pull, slab push, slab bending, viscous resistance and buoyancy can explain the different collision styles caused by variations in velocity, temperature, rheology, density differences and the interaction with adjacent plates.

A global reference model of Moho depths based on WGM2012

NASA Astrophysics Data System (ADS)

Zhou, D.; Li, C.

2017-12-01

The crust-mantle boundary (Moho discontinuity) represents the largest density contrast in the lithosphere, which can be detected by Bouguer gravity anomaly. We present our recent inversion of global Moho depths from World Gravity Map 2012. Because oceanic lithospheres increase in density as they cool, we perform thermal correction based on the plate cooling model. We adopt a temperature Tm=1300°C at the bottom of lithosphere. The plate thickness is tested by varying by 5 km from 90 to 140 km, and taken as 130 km that gives a best-fit crustal thickness constrained by seismic crustal thickness profiles. We obtain the residual Bouguer gravity anomalies by subtracting the thermal correction from WGM2012, and then estimate Moho depths based on the Parker-Oldenburg algorithm. Taking the global model Crust1.0 as a priori constraint, we adopt Moho density contrasts of 0.43 and 0.4 g/cm3 , and initial mean Moho depths of 37 and 20 km in the continental and oceanic domains, respectively. The number of iterations in the inversion is set to be 150, which is large enough to obtain an error lower than a pre-assigned convergence criterion. The estimated Moho depths range between 0 76 km, and are averaged at 36 and 15 km in continental and oceanic domain, respectively. Our results correlate very well with Crust1.0 with differences mostly within ±5.0 km. Compared to the low resolution of Crust1.0 in oceanic domain, our results have a much larger depth range reflecting diverse structures such as ridges, seamounts, volcanic chains and subduction zones. Base on this model, we find that young(<5 Ma) oceanic crust thicknesses show dependence on spreading rates: (1) From ultraslow (<4mm/yr) to slow (4 45mm/yr) spreading ridges, the thicknesses increase dramatically; (2)From slow to fast (45 95mm/yr) spreading ridges , the thickness decreases slightly; (3) For the super-fast ridges (>95mm/yr) we observe relatively thicker crust. Conductive cooling of lithosphere may constrain the melting of the mantle at ultraslow spreading centers. Lower mantle temperatures indicated by deeper Curie depths at slow and fast spreading ridges may decrease the volume of magmatism and crustal thickness. This new global model of gravity-derived Moho depth, combined with geochemical and Curie point depth, can be used to investigate thermal evolution of lithosphere.

Deep structure of the Tristan-Gough plume revealed by geoid anomalies

NASA Astrophysics Data System (ADS)

Maia, M.; Flamme, J.; Cadio, C.; Lalancette, M. F.; Metivier, L.; Pajot-Métivier, G.; Diament, M.

2017-12-01

The origin of the hotspot Tristan da Cunha located at the southwestern end of Walvis Ridge in the Atlantic Ocean is still a controversial topic. We especially question on the nature of the involved geodynamical processes and on their origin depth. The latest results based on local seismic and magnetic data (Schlömer et al., 2016; Baba et al., 2016; Geissler et al., 2016) suggest the existence of a plume coming from the mid-mantle in the southwest of the archipelago. Here we give a regional view of mantle dynamics patterns in the area by using the high-quality satellite geoid data. To extract the mantle signature, we estimate the crustal and lithospheric signals of the ocean basin and South American and African continents, which contribute to mid- and long-wavelengths in the total geoid. We pay particular attention to the modeling of continental margins and their effects on the residual geoid signal. In addition, we explore a large density values set derived from petrological and geochemical studies in the calculation of the lithospheric geoid model. After subtracting the lithospheric signature to the EGM2008 geoid, we apply a multi-scale analysis, which unfolds the different components of the geoid residual signal. The analysis underlines a set of positive anomalies at 200-400 km in the study area, notably in north and west of Tristan de Cunha, and a positive anomaly at 700-1100 km scale in the southwest of the archipelago. These patterns do not change by using different lithospheric geoid models, which allow us to evaluate the reliability of the residual geoid anomalies. These results indicate the existence of small-scale density anomalies in the upper mantle and a larger scale density anomaly in the mid-mantle. Our study suggests that a large dome toped by plume clusters could be a good candidate to explain the volcanism of Tristan da Cunha.Schlömer et al., 2016 Hunting for the Tristan mantle plume..., EPSL, http://dx.doi.org/10.1016/j.epsl.2016.12.028Baba et al., 2016 Marine magnetotellurics imaged no distinct plume..., EPSL, http://dx.doi.org/10.1016/j.tecto.2016.09.033Geissler et al., 2016 Thickness of the oceanic crust, the lithosphere, and..., EPSL, http://dx.doi.org/10.1016/j.tecto.2016.12.013

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

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.

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.

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.

Lithospheric Structure In Asia Based On Pp-, P-waves Data

NASA Astrophysics Data System (ADS)

Bushenkova, N.

There is the RR-R method of tomographic inversion, which is based on joint use of teleseismic P or S refracted rays and corresponding PP or SS rays with bounce points located within a study region. This scheme allows imaging the deep seismic structure beneath "blank" areas where there are neither recording stations nor earthquakes. Uti- lization of differential travel times makes it possible to avoid the difficulty of source and station corrections, which cause problems in teleseismic tomography. The RR-R scheme has been applied to more than 10000 ray pairs from the ISC database to in- vestigate a large region from the North Arctic Ocean to the northern part of China and Mongolia. Velocity anomalies were computed in grid nodes distributed in the study 3D area according to ray density. Relatively high velocities beneath the Siberian cra- ton (positive velocity anomalies under 3 % ) observed down to the depth of 350 km are consistent with the geothermal model of Artemieva and Mooney (2001) and with global seismic tomography and may correspond to traces of thick lithosphere. A local low-velocity anomaly is imaged in the northern part of the Siberian craton at sub- lithospheric depths below 300 km. Its centre coincides with the swell of the contem- porary Putorana plateau. This isolated anomaly may be accounted for by the presence of a limited volume of hot and light mantle material that, by its buoyancy, provides a dynamic uplift of the surface. Upper mantle beneath West Siberia is dominated by low velocities which can be interpreted as a result of relatively thin lithosphere. The veloc- ity jump at the base of the lithosphere (~150 km) is smooth due to its essentially ther- mal nature. The velocity anomalies in the southern part of the study area have NW-SE trend, which corresponds to the strike of major lithospheric structures formed by suc- cessive accretion of terranes to the Siberian craton through the Paleozoic (Molnar and Tapponnier, 1975; Sengör et al., 1993; Dobretsov et al., 1996). Negative veloc- ity anomalies are related to the thin lithosphere of the Altai Mountains and the Hangai plateau (negative velocity anomalies under 3 % in the depth range 30 to 430 km). The low-velocity seismic anomaly beneath the eastern Hangai plateau can be interpreted as a mantle plume, which is consistent with other geophysical and geological data. The strong negative velocity anomaly beneath Altai reflects the present-day weak rhe- ology most likely related to conductive heating of the thinned continental lithosphere in the Late Cenozoic. The positive velocity anomaly beneath and the western Hangai plateau and Tuva corresponds to the Precambrian Mongolia-Tuva microcontinent.

Lithospheric architecture of the Levant Basin (Eastern Mediterranean region): A 2D modeling approach

NASA Astrophysics Data System (ADS)

Inati, Lama; Zeyen, Hermann; Nader, Fadi Henri; Adelinet, Mathilde; Sursock, Alexandre; Rahhal, Muhsin Elie; Roure, François

2016-12-01

This paper discusses the deep structure of the lithosphere underlying the easternmost Mediterranean region, in particular the Levant Basin and its margins, where the nature of the crust, continental versus oceanic, remains debated. Crustal thickness and the depth of the lithosphere-asthenosphere boundary (LAB) as well as the crustal density distribution were calculated by integrating surface heat flow data, free-air gravity anomaly, geoid and topography. Accordingly, two-dimensional, lithospheric models of the study area are discussed, demonstrating the presence of a progressively attenuated crystalline crust from E to W (average thickness from 35 to 8 km). The crystalline crust is best interpreted as a strongly thinned continental crust under the Levant Basin, represented by two distinct components, an upper and a lower crust. Further to the west, the Herodotus Basin is believed to be underlain by an oceanic crust, with a thickness between 6 and 10 km. The Moho under the Arabian Plate is 35-40 km deep and becomes shallower towards the Mediterranean coast. It appears to be situated at depths ranging between 20 and 23 km below the Levant Basin and 26 km beneath the Herodotus Basin, based on our proposed models. At the Levantine margin, the thinning of the crust in the transitional domain between the onshore and the offshore is gradual, indicating successive extensional regimes that did not reach the beak up stage. In addition, the depth to LAB is around 120 km under the Arabian and the Eurasian Plates, 150 km under the Levant Basin, and it plunges to 180 km under the Herodotus Basin. This study shows that detailed 2D lithosphere modeling using integrated geophysical data can help understand the mechanisms responsible for the modelled lithospheric architecture when constrained with geological findings.

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 structure of the Eastern Iranian plateau from integrated geophysical modeling: A transect from Makran to the Turan platform

NASA Astrophysics Data System (ADS)

Entezar-Saadat, Vahid; Motavalli-Anbaran, Seyed-Hani; Zeyen, Hermann

2017-05-01

We present a 2D profile of density and temperature distribution in the lithosphere across Iran along a more than 1600 km long profile extending from the Oman Gulf in the South to the Kopeh-Dagh and the Turan platform in the North. Gravity, geoid, topography and surface heat flow data were used for modeling, assuming local isostatic equilibrium. As much as possible, crustal structure has been constrained by seismic data. Crustal thickening is found under the Taftan-Bazman volcanic-arc (up to 47 km), under the Binalud foreland (∼54 km) and beneath the Kopeh-Dagh mountains (up to 50 km) whereas thin crust has been obtained under the Oman Gulf (20 km). Moho depth under the Lut block and the Turan platform is about 40 km. The lithospheric thickness is ∼90 km under the Oman Gulf and increases slightly until the Jazmourian depression. Then the lithospheric-asthenospheric boundary (LAB) bends significantly and sinks to ∼260 km under the Taftan-Bazman volcanic-arc. The LAB depth is about 190 km beneath the Lut block. A slight increase of LAB depth under the Binalud foreland towards the North may indicate a suture zone. Under the Turan platform, the LAB depth reaches ∼210 km. We also modeled two possible positions of the deep suture zone in NE Iran (along the main Kopeh-Dagh fault or along the Atrak River) and concluded that, when the suture zone is along the Atrak River, we obtained the better fit between calculated and measured data.

Lithospheric Structure of Central Europe: Puzzle Pieces from Pannonian Basin to Trans-European Suture Zone Resolved by Geophysical-Petrological Modeling

NASA Astrophysics Data System (ADS)

Bielik, M.; Tašárová, Z. A.; Fullea, J.; Sroda, P.

2017-12-01

We have analysed the thermochemical structure of the mantle in Central Europe comprising the Western Carpathians, Pannonian Basin and parts of the European Platform, Bohemian Massif and easternmost Eastern Alps. This area is very complex and characterized by a highly heterogeneous lithospheric structure reflecting the interplay of contraction, strike slip, subduction, and extension tectonics. Our modelling is based on an integrative 3-D approach (LitMod) that combines in a self-consistent manner concepts and data from thermodynamics, mineral physics, geochemistry, petrology, and solid Earth geophysics. This approach minimizes uncertainties of the estimates derived from modelling of various data sets separately. To further constrain our 3-D model we have made use of the vast geophysical and geological data (2-D and 3-D, shallow/crustal versus deep lithospheric experiments) based on experiments performed in Central Europe in the past decades. Given the amount and the different nature/resolution of the available constraints, one of the most challenging tasks of this study was to consistently combine them. Our goal was to find a trade-off between all local and regional data sets available in a way that preserves as many structural details as possible and summarizes those data sets into a single robust regional model. The resulting P/T-dependent mantle densities are in LitMod 3-D calculated based on a given mineralogical composition. Therefore, they provide more reliable estimates compared to pure gravity models and enhance modelling of the crustal structures. Our results clearly indicate presence of several lithospheric domains characterized by distinct features, Pannonian Basin being one of the most outstanding ones. It has the thinnest crust and lithosphere in the area modelled, characterized by relatively fertile composition. Z. Alasonati Tašárová's research project was financed by Deutsche Forschungsgemeinschaft (project TA553/1-2); M. Bielik was funded by the Slovak Grant Agency VEGA (grants 1/0141/15 and 2/0042/15), and APVV-16-0146, APVV-16-0482; and J. Fullea was supported by Spanish Ministry of Economy and Competitiveness grants CGL2014-58821-C2-1-R, CGL2009-13103, and CGL2012-37222, as well as a Marie Skłodowska-Curie Individual Fellowship (grant 657357).

3D interactive forward and inversion gravity modelling at different scales: From subduction zone modelling to cavity detection.

NASA Astrophysics Data System (ADS)

Götze, Hans-Jürgen; Schmidt, Sabine

2014-05-01

Modern geophysical interpretation requires an interdisciplinary approach, particularly when considering the available amount of 'state of the art' information. A combination of different geophysical surveys employing seismic, gravity and EM, together with geological and petrological studies, can provide new insights into the structures and tectonic evolution of the lithosphere, natural deposits and underground cavities. Interdisciplinary interpretation is essential for any numerical modelling of these structures and the processes acting on them Interactive gravity and magnetic modeling can play an important role in the depth imaging workflow of complex projects. The integration of the workflow and the tools is important to meet the needs of today's more interactive and interpretative depth imaging workflows. For the integration of gravity and magnetic models the software IGMAS+ can play an important role in this workflow. For simplicity the focus is on gravity modeling, but all methods can be applied to the modeling of magnetic data as well. Currently there are three common ways to define a 3D gravity model. Grid based models: Grids define the different geological units. The densities of the geological units are constant. Additional grids can be introduced to subdivide the geological units, making it possible to represent density depth relations. Polyhedral models: The interfaces between different geological units are defined by polyhedral, typically triangles. Voxel models: Each voxel in a regular cube has a density assigned. Spherical Earth modeling: Geophysical investigations may cover huge areas of several thousand square kilometers. The depression of the earth's surface due to the curvature of the Earth is 3 km at a distance of 200 km and 20 km at a distance of 500 km. Interactive inversion: Inversion is typically done in batch where constraints are defined beforehand and then after a few minutes or hours a model fitting the data and constraints is generated. As examples I show results from the Central Andes and the North Sea. Both gravity and geoid of the two areas were investigated with regard to their isostatic state, the crustal density structure and rigidity of the Lithosphere. Modern satellite measurements of the recent ESA campaigns are compared to ground observations in the region. Estimates of stress and GPE (gravitational potential energy) at the western South American margin have been derived from an existing 3D density model. Here, sensitivity studies of gravity and gravity gradients indicate that short wavelength lithospheric structures are more pronounced in the gravity gradient tensor than in the gravity field. A medium size example of the North Sea underground demonstrates how interdisciplinary data sets can support aero gravity investigations. At the micro scale an example from the detection of a crypt (Alversdorf, Northern Germany) is shown.

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.

Lithospheric structure of Taiwan from gravity modelling and sequential inversion of seismological and gravity data

NASA Astrophysics Data System (ADS)

Masson, F.; Mouyen, M.; Hwang, C.; Wu, Y.-M.; Ponton, F.; Lehujeur, M.; Dorbath, C.

2012-11-01

Using a Bouguer anomaly map and a dense seismic data set, we have performed two studies in order to improve our knowledge of the deep structure of Taiwan. First, we model the Bouguer anomaly along a profile crossing the island using simple forward modelling. The modelling is 2D, with the hypothesis of cylindrical symmetry. Second we present a joint analysis of gravity anomaly and seismic arrival time data recorded in Taiwan. An initial velocity model has been obtained by local earthquake tomography (LET) of the seismological data. The LET velocity model was used to construct an initial 3D gravity model, using a linear velocity-density relationship (Birch's law). The synthetic Bouguer anomaly calculated for this model has the same shape and wavelength as the observed anomaly. However some characteristics of the anomaly map are not retrieved. To derive a crustal velocity/density model which accounts for both types of observations, we performed a sequential inversion of seismological and gravity data. The variance reduction of the arrival time data for the final sequential model was comparable to the variance reduction obtained by simple LET. Moreover, the sequential model explained about 80% of the observed gravity anomaly. New 3D model of Taiwan lithosphere is presented.

GOCE gravity gradient data for lithospheric modeling - From well surveyed to frontier areas

NASA Astrophysics Data System (ADS)

Bouman, J.; Ebbing, J.; Gradmann, S.; Fuchs, M.; Fattah, R. Abdul; Meekes, S.; Schmidt, M.; Lieb, V.; Haagmans, R.

2012-04-01

We explore how GOCE gravity gradient data can improve modeling of the Earth's lithosphere and thereby contribute to a better understanding of the Earth's dynamic processes. The idea is to invert satellite gravity gradients and terrestrial gravity data in the well explored and understood North-East Atlantic Margin and to compare the results of this inversion, providing improved information about the lithosphere and upper mantle, with results obtained by means of models based upon other sources like seismics and magnetic field information. Transfer of the obtained knowledge to the less explored Rub' al Khali desert is foreseen. We present a case study for the North-East Atlantic margin, where we analyze the use of satellite gravity gradients by comparison with a well-constrained 3D density model that provides a detailed picture from the upper mantle to the top basement (base of sediments). The latter horizon is well resolved from gravity and especially magnetic data, whereas sedimentary layers are mainly constrained from seismic studies, but do in general not show a prominent effect in the gravity and magnetic field. We analyze how gravity gradients can increase confidence in the modeled structures by calculating a sensitivity matrix for the existing 3D model. This sensitivity matrix describes the relation between calculated gravity gradient data and geological structures with respect to their depth, extent and relative density contrast. As the sensitivity of the modeled bodies varies for different tensor components, we can use this matrix for a weighted inversion of gradient data to optimize the model. This sensitivity analysis will be used as input to study the Rub' al Khali desert in Saudi Arabia. In terms of modeling and data availability this is a frontier area. Here gravity gradient data will be used to better identify the extent of anomalous structures within the basin, with the goal to improve the modeling for hydrocarbon exploration purposes.

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.

Heterogeneity of the North Atlantic oceanic lithosphere based on integrated analysis of GOCE satellite gravity and geological data

NASA Astrophysics Data System (ADS)

Barantseva, Olga; Artemieva, Irina; Thybo, Hans; Herceg, Matija

2015-04-01

We present the results from modelling the gravity and density structure of the upper mantle for the off-shore area of the North Atlantic region. The crust and upper mantle of the region is expected to be anomalous: Part of the region affected by the Icelandic plume has an anomalously shallow bathymetry, whereas the northern part of the region is characterized by ultraslow spreading. In order to understand the links between deep geodynamical processes that control the spreading rate, on one hand, and their manifestations such as oceanic floor bathymetry and heat flow, on the other hand, we model the gravity and density structure of the upper mantle from satellite gravity data. The calculations are based on interpretation of GOCE gravity satellite data for the North Atlantics. To separate the gravity signal responsible for density anomalies within the crust and upper mantle, we subtract the lower harmonics caused by deep density structure of the Earth (the core and the lower mantle). The gravity effect of the upper mantle is calculated by subtracting the gravity effect of the crust for two crustal models. We use a recent regional seismic model for the crustal structure (Artemieva and Thybo, 2013) based om seismic data together with borehole data for sediments. For comparison, similar results are presented for the global CRUST 1.0 model as well (Laske, 2013). The conversion of seismic velocity data for the crustal structure to crustal density structure is crucial for the final results. We use a combination of Vp-to-density conversion based on published laboratory measurements for the crystalline basement (Ludwig, Nafe, Drake, 1970; Christensen and Mooney, 1995) and for oceanic sediments and oceanic crust based on laboratory measurements for serpentinites and gabbros from the Mid-Atlantic Ridge (Kelemen et al., 2004). Also, to overcome the high degree of uncertainty in Vp-to-density conversion, we account for regional tectonic variations in the Northern Atlantics as constrained by numerous published seismic profiles and potential-field models across the Norwegian off-shore crust (e.g. Breivik et al., 2005, 2007). The results demonstrate the presence of strong gravity and density heterogeneity of the upper mantle in the North Atlantic region. In particular, there is a sharp contrast at the continent-ocean transition, which also allows for recognising mantle gravity anomalies associated with continental fragments and with anomalous oceanic lithosphere.

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.

Global Mapping of Oceanic and Continental Shelf Crustal Thickness and Ocean-Continent Transition Structure

NASA Astrophysics Data System (ADS)

Kusznir, Nick; Alvey, Andy; Roberts, Alan

2017-04-01

The 3D mapping of crustal thickness for continental shelves and oceanic crust, and the determination of ocean-continent transition (OCT) structure and continent-ocean boundary (COB) location, represents a substantial challenge. Geophysical inversion of satellite derived free-air gravity anomaly data incorporating a lithosphere thermal anomaly correction (Chappell & Kusznir, 2008) now provides a useful and reliable methodology for mapping crustal thickness in the marine domain. Using this we have produced the first comprehensive maps of global crustal thickness for oceanic and continental shelf regions. Maps of crustal thickness and continental lithosphere thinning factor from gravity inversion may be used to determine the distribution of oceanic lithosphere, micro-continents and oceanic plateaux including for the inaccessible polar regions (e.g. Arctic Ocean, Alvey et al.,2008). The gravity inversion method provides a prediction of continent-ocean boundary location which is independent of ocean magnetic anomaly and isochron interpretation. Using crustal thickness and continental lithosphere thinning factor maps with superimposed shaded-relief free-air gravity anomaly, we can improve the determination of pre-breakup rifted margin conjugacy and sea-floor spreading trajectory during ocean basin formation. By restoring crustal thickness & continental lithosphere thinning to their initial post-breakup configuration we show the geometry and segmentation of the rifted continental margins at their time of breakup, together with the location of highly-stretched failed breakup basins and rifted micro-continents. For detailed analysis to constrain OCT structure, margin type (i.e. magma poor, "normal" or magma rich) and COB location, a suite of quantitative analytical methods may be used which include: (i) Crustal cross-sections showing Moho depth and crustal basement thickness from gravity inversion. (ii) Residual depth anomaly (RDA) analysis which is used to investigate OCT bathymetric anomalies with respect to expected oceanic values. This includes flexural backstripping to produce bathymetry corrected for sediment loading. (iii) Subsidence analysis which is used to determine the distribution of continental lithosphere thinning. (iv) Joint inversion of time-domain deep seismic reflection and gravity anomaly data which is used to determine lateral variations in crustal basement density and velocity across the OCT, and to validate deep seismic reflection interpretations of Moho depth. The combined interpretation of these independent quantitative measurements is used to determine crustal thickness and composition across the ocean-continent-transition. This integrated approach has been validated on the Iberian margin where ODP drilling provides ground-truth of ocean-continent-transition crustal structure, continent-ocean-boundary location and magmatic type.

Isostatic models and isostatic gravity anomalies of the Arabian plate and surroundings

NASA Astrophysics Data System (ADS)

Kaban, Mikhail K.; El Khrepy, Sami; Al-Arifi, Nassir

2015-04-01

Isostaic anomalies represent one of the most useful "geological" reduction of the gravity field. With the isostatic correction it is possible to remove a significant part of the effect of deep density heterogeneity, which dominates in the Bouguer gravity anomalies. This correction is based on the fact that a major part of the near-surface load is compensated by variations of the lithosphere boundaries (chiefly the Moho and LAB) and by density variations within the crust and upper mantle. It is usually supposed that it is less important to a first order, what is the actual compensation model when reducing the effect of compensating masses, since their total weight is exactly opposite to the near-surface load. We compare several compensating models for the Arabian plate and surrounding area. The Airy model gives very significant regional isostatic anomalies, which can not be explained by the upper crust structure or disturbances of the isostatic equilibrium. Also the predicted "isostatic" Moho is very different from the existing observations. The second group of the isostatic models includes the Moho, which is based on existing seismic determinations. Additional compensation is provided by density variations within the lithosphere (chiefly in the upper mantle). In this way we minimize regional anomalies over the Arabian plate. The residual local anomalies well correspond to tectonic structure of the plate. Still very significant anomalies are associated with the Zagros fold belt, the collision zone of the Arabian and Eurasian plates.

Upper crustal structure of the Hawaiian Swell from seafloor compliance measurements

NASA Astrophysics Data System (ADS)

Doran, A. K.; Laske, G.

2017-12-01

We present new constraints on elastic properties of the marine sediments and crust surrounding the Hawaiian Islands derived from seafloor compliance measurements. We analyze long-period seismic and pressure data collected during the Plume-Lithosphere Undersea Mantle Experiment [Laske et al, 2009], a deployment consisting of nearly 70 broadband ocean-bottom seismometers with an array aperture of over 1000 kilometers. Our results are supported by previous reflection & refraction studies and by direct sampling of the crust from regional drilling logs. We demonstrate the importance of simultaneously modeling density, compressional velocity, and shear velocity, the former two of which are often ignored during compliance investigations. We find variable sediment thickness and composition across the Hawaiian Swell, with the thickest sediments located within the Hawaiian Moat. Improved resolution of near-surface structure of the Hawaiian Swell is crucially important to improve tomographic images of the underlying lithosphere and asthenosphere and to address outstanding questions regarding the size, source, and location of the hypothesized mantle plume.

Variations in Crustal Structure, Lithospheric Flexural Strength, and Isostatic Compensation Mechanisms of Mars

NASA Astrophysics Data System (ADS)

Ding, M.; Lin, J.; Zuber, M. T.

2014-12-01

We analyze gravity and topography of Mars to investigate the spatial variations in crustal thickness, lithospheric strength, and mechanisms of support of prominent topographic features on Mars. The latest gravity model JGMRO110c (released in 2012) from the Mars Reconnaissance Orbiter mission has a spatial block size resolution of ~97 km (corresponding to degree-110), enabling us to resolve crustal structures at higher spatial resolution than those determined from previous degree-80 and 85 gravity models [Zuber et al., 2000; McGovern et al., 2002, 2004; Neumann et al., 2004; Belleguic et al., 2005]. Using the latest gravity data, we first inverted for a new version of crustal thickness model of Mars assuming homogeneous crust and mantle densities of 2.9 and 3.5 g/cm3. We calculated "isostatic" topography for the Airy local isostatic compensation mechanism, and "non-isostatic" topography after removing the isostatic part. We find that about 92% of the Martian surface is in relatively isostatic state, indicating either relatively small lithospheric strength and/or small vertical loading. Relatively isostatic regions include the hemispheric dichotomy, Hellas and Argyre Planitia, Noachis and Arabia Terra, and Terra Cimmeria. In contrast, regions with significant amount of non-isostatic topography include the Olympus, Ascraeus, Arsia, Pavonis, Alba, and Elysium Mons, Isidis Planitia and Valles Marineris. Their relatively large "non-isostatc topography" implies relatively strong lithospheric strength and large vertical loading. Spectral analysis of the admittance and correlation relationship between gravity and topography were conducted for the non-isostatic regions using the localized spectra method [Wieczorek and Simons, 2005, 2007] and thin-shell lithospheric flexural approximation [Forsyth, 1985; McGovern et al., 2002, 2004]. The best-fitting models reveal significant variations in the effective lithospheric thickness with the greatest values for the Olympus Mon, Valles Marineris, and Isidis Planitia; reduced values for the Ascraeus, Arsis, and Pavonis Mons; and smallest values for the Alba and Elysium Mons. Our models also suggest that there could be significant sub-surface loading underneath the Olympus, Ascraeus, Arsia, and Pavonis Mons, and Isidis Planitia.

3D isotropic shear wave velocity structure of the lithosphere-asthenosphere system underneath the Alpine-Mediterranean Mobile belt

NASA Astrophysics Data System (ADS)

El-Sharkawy, Amr; Weidle, Christian; Christiano, Luigia; Lebedev, Sergei; Meier, Thomas

2017-04-01

The Alpine-Mediterranean mobile belt is, tectonically, one of the most complicated and active regions in the world. Since the Mesozoic, collisions between Gondwana-derived continental blocks and Eurasia, due to the closure of a number of rather small ocean basins, have shaped the Mediterranean geology. During the late Mesozoic, it was dominated by subduction zones (e.g., in Anatolia, the Dinarides, the Carpathians, the Alps, the Apennines, and the Betics), which inverted the extensional regime, consuming the previously formed oceanic lithosphere, the adjacent passive continental margins and presumably partly also continental lithosphere. The location, distribution, and evolution of these subduction zones were mainly controlled by the continental or oceanic nature, density, and thickness of the lithosphere inherited from the Mesozoic rift after the European Variscan Orogeny. Despite the numerous studies that have attempted to characterize the lithosphere-asthenosphere structure in that area, details of the lithospheric structure and dynamics, as well as flow in the asthenosphere are, however, poorly known. A 3D shear-wave velocity structure of the lithosphere-asthenosphere system in the Mediterranean is investigated using new tomographic images obtained from surface wave tomography. An automated algorithm for inter-station phase velocity measurements is applied here to obtain both Rayleigh and Love fundamental mode phase velocities. We utilize a database consisting of more than 4000 seismic events recorded by more than 2000 broadband seismic stations within the area, provided by the European Integrated Data Archive (WebDc/EIDA) and IRIS. Moreover, for the first time, data from the Egyptian National Seismological Network (ENSN), recorded by up to 25 broad band seismic stations, are also included in the analysis. For each station pair, approximately located on the same great circle path, the recorded waveforms are cross correlated and the dispersion curves of fundamental modes are calculated from the phase of the cross correlation functions weighted in the time-frequency plane. Path average dispersion curves are obtained by averaging the smooth parts of single-event dispersion curves. A careful quality control of the resulting phase velocities is performed. We calculate maps of Love and Rayleigh phase velocity at more than 100 different periods. The phase-velocity maps provide the local phase-velocity dispersion curve for each geographical grid node of the map. Each of these local dispersion curves is inverted individually for 1D shear wave velocity model using a newly implemented Particle Swarm Optimization (PSO) algorithm. The resulted 1D velocity models are then combined to construct the 3D shear-velocity model. Horizontal and vertical cross sections through the 3D isotropic model reveal significant variations in shear wave velocity with depth, and lateral changes in the crust and upper mantle structure emphasizing the processes associated with the convergence of the Eurasian and African plates. Key words: seismic tomography, Mediterranean, surface waves, particle swarm optimization.

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.

Seismological Constraints on Lithospheric Evolution in the Appalachian Orogen

NASA Astrophysics Data System (ADS)

Fischer, K. M.; Hopper, E.; Hawman, R. B.; Wagner, L. S.

2017-12-01

Crust and mantle structures beneath the Appalachian orogen, recently resolved by seismic data from the EarthScope SESAME Flexible Array and Transportable Array, provide new constraints on the scale and style of the Appalachian collision and subsequent lithospheric evolution. In the southern Appalachians, imaging with Sp and Ps phases reveals the final (Alleghanian) suture between the crusts of Laurentia and the Gondwanan Suwannee terrane as a low angle (<15°) southward-dipping interface that soles into a flat-lying mid-crustal detachment. The suture location near the top of the crust coincides closely with the northern limit of the Suwannee terrane reconstructed from its lower Paleozoic shelf strata (Boote and Knapp, 2016). The observed suture geometry implies over 300 km of head-on shortening across a plate boundary structure similar in scale to the Himalayan mid-crustal detachment. While the suture and other structures from the Alleghanian collision are preserved in the upper and mid-crust, the lower crust and mantle lithosphere beneath this region have been significantly modified by later processes. Ps receiver functions, wavefield migration and SsPmp modeling reveal that crustal thickness reaches a maximum of 58 km (beneath high elevations in the Blue Ridge terrane) and decreases to 29-35 km (beneath lower elevations in the Carolina and Suwannee terranes). Given metamorphic estimates of unroofing (Duff and Kellogg, 2017) isostatic arguments indicate crustal thicknesses were 15-25 km larger at the end of the orogeny, indicating a thick crustal root across the region. The present-day residual crustal root beneath the Blue Ridge mountains is estimated to have a density contrast with the mantle of only 104±20 kg/m3. This value is comparable to other old orogens but lower than values typical of young or active orogens, indicating a loss of lower crustal buoyancy over time. At mantle depths, the negative shear velocity gradient that marks the transition from lithosphere to asthenosphere, as illuminated by Sp phases, varies across the Appalachian orogen. This boundary is shallow beneath the northeastern U.S. and in the zone of Eocene volcanism in Virginia, where low velocity anomalies occur in the upper mantle. These correlations suggest recent active lithosphere-asthenosphere interaction.

Anomalous Lower Crustal and Surface Features as a Result of Plume-induced Continental Break-up: Inferences from Numerical Models

NASA Astrophysics Data System (ADS)

Beniest, A.; Koptev, A.; Leroy, S. D.

2016-12-01

Anomalous features along the South American and African rifted margins at depth and at the surface have been recognised with gravity and magnetic modelling. They include high velocity/high density bodies at lower crustal level and topography variations that are usually interpreted as aborted rifts. We present fully-coupled lithosphere-scale numerical models that permit us to explain both features in a relatively simple framework of an interaction between rheologically stratified continental lithosphere and an active mantle plume. We used 2D and 3D numerical models to investigate the impact of thermo-rheological structure of the continental lithosphere and initial plume position on continental rifting and breakup processes. Based on the results of our 2D experiments, three main types of continental break-up are revealed: A) mantle plume-induced break-up, directly located above the centre of the mantle anomaly, B) mantle plume-induced break-up, 50 to 250 km displaced from the initial plume location and C) self-induced break-up due to convection and/or slab-subduction/delamination, considerably shifted (300 to 800 km) from the initial plume position. With our 3D, laterally homogenous initial setup, we show that a complex system, with the axis of continental break-up 100's of km's shifted from the original plume location, can arise spontaneously from simple and perfectly symmetric preliminary settings. Our modelling demonstrates that fragments of a laterally migrating plume head become glued to the base of the lithosphere and remain at both sides of the newly-formed oceanic basin after continental break-up. Underplated plume material soldered into lower parts of lithosphere can be interpreted as the high-velocity/high density magmatic bodies at lower crustal levels. In the very early stages of rifting, first impingement of the vertically upwelled mantle plume to the lithospheric base leads to surface topographic variations. Given the shifted position of the final spreading centre with respect to initial plume position, these topographic variations resemble aborted rifts that are observed on passive margins. Lastly, after continuous extension and transition to the spreading state, strain rate relocalizations develop that can be interpreted as ridge jumps that are commonly observed in nature.

Possible origin of the Bighorn uplift, WY, by lithospheric buckling during the Laramide orogeny

NASA Astrophysics Data System (ADS)

Tikoff, B.; Siddoway, C. S.; Worthington, L. L.; Anderson, M. L.

2017-12-01

The EarthScope Bighorn Project investigated the Bighorn uplift, Wyoming, a foreland structure developed during the 75-55 Ma Laramide orogeny. Any model for the Bighorn uplift must account for several geological and geophysical results from the EarthScope broadband and passive-active seismic study, the broader context provided by USArray, and legacy datasets: 1) The Moho is bulged up below portions of the surface exposure of the basement arch; 2) a high-velocity, high-density material (the "7.x layer") is absent in the lower crust beneath the arch culmination; 3) Shear wave splitting analysis shows distinct mantle fabrics on either side of the uplift; 4) Crustal thicknesses varied widely prior to the Laramide-age deformation; 5) A lack of reflectors associated with a regional decollement; 6) The Bighorn arch forms one in an array of low-amplitude, large-wavelength folds throughout the High Plains region. The uplift borders a NNW-trending (E-dipping?) geophysical anomaly inferred to be Proterozoic suture. A lithospheric buckling model offers a framework that accommodates most of the geological and geophysical data. Lithospheric buckling is the concept of low-amplitude, large-wavelength (150-350 km) lithospheric folding developed in response to an end-load, replicated in scaled physical models. A buckling instability focuses initial deformation, with faults developed in layered media/crustal section as shortening progresses. The strength/age of the mantle controls the fold wavelength, based on examples from multiple orogens (e.g. Urals, central Asia). Rarely does the geometry of the upward Moho deflection identically mirror the surface uplift in scaled models, nor does it in the Bighorn uplift, where fold localization is likely controlled by a pre-existing Proterozoic suture and/or mantle asperity. Indicated by shear wave SKS splitting data, distinct mantle fabrics on either side of the uplift extend into the lithospheric mantle, indicated the presence of a deep-rooted structure of a type that has not been incorporated in physical models.

Three-dimensional crustal structure of the southern Sierra Nevada from seismic fan profiles and gravity modeling

USGS Publications Warehouse

Fliedner, M.M.; Ruppert, S.; Malin, P.E.; Park, S.K.; Jiracek, G.; Phinney, R.A.; Saleeby, J.B.; Wernicke, B.; Clayton, R.; Keller, Rebecca Hylton; Miller, K.; Jones, C.; Luetgert, J.H.; Mooney, W.D.; Oliver, H.; Klemperer, S.L.; Thompson, G.A.

1996-01-01

Traveltime data from the 1993 Southern Sierra Nevada Continental Dynamics seismic refraction experiment reveal low crustal velocities in the southern Sierra Nevada and Basin and Range province of California (6.0 to 6.6 km/s), as well as low upper mantle velocities (7.6 to 7.8 km/s). The crust thickens from southeast to northwest along the axis of the Sierra Nevada from 27 km in the Mojave Desert to 43 km near Fresno, California. A crustal welt is present beneath the Sierra Nevada, but the deepest Moho is found under the western slopes, not beneath the highest topography. A density model directly derived from the crustal velocity model but with constant mantle density satisfies the pronounced negative Bouguer anomaly associated with the Sierra Nevada, but shows large discrepancies of >50 mgal in the Great Valley and in the Basin and Range province. Matching the observed gravity with anomalies in the crust alone is not possible with geologically reasonable densities; we require a contribution from the upper mantle, either by lateral density variations or by a thinning of the lithosphere under the Sierra Nevada and the Basin and Range province. Such a model is consistent with the interpretation that the uplift of the present Sierra Nevada is caused and dynamically supported by asthenospheric upwelling or lithospheric thinning under the Basin and Range province and eastern Sierra Nevada.

Lithospheric Structure of Antarctica and Implications for Geological and Cryospheric Evolution

NASA Astrophysics Data System (ADS)

Wiens, Douglas; Heeszel, David; Sun, Xinlei; Lloyd, Andrew; Nyblade, Andrew; Anandakrishnan, Sridhar; Aster, Richard; Chaput, Julien; Huerta, Audrey; Hansen, Samantha; Wilson, Terry

2013-04-01

Recent broadband seismic deployments, including the AGAP/GAMSEIS array of 24 broadband seismographs over the Gamburtsev Subglacial Mountains (GSM) in East Antarctica and the POLENET/ANET deployment of 33 seismographs across much of West Antarctica, reveal the detailed crust and upper mantle structure of Antarctica for the first time. The seismographs operate year-around even in the coldest parts of Antarctica, due to novel insulated boxes, power systems, and modified instrumentation developed in collaboration with the IRIS PASSCAL Instrument Center. We analyze the data using several different techniques to develop high-resolution models of Antarctic seismic structure. We use Rayleigh wave phase velocities at periods of 20-180 s determined using a modified two-plane wave decomposition of teleseismic Rayleigh waves to invert for the three dimensional shear velocity structure. In addition, Rayleigh wave group and phase velocities obtained by ambient seismic noise correlation methods provide constraints at shorter periods and shallower depths. Receiver functions provide precise estimates of crustal structure beneath the stations, and P and S wave tomography provides models of upper mantle structure down to ~ 500 km depth along transects of greater seismic station density. The new seismic results show that the high elevations of the GSM are supported by thick crust (~ 55 km), and are underlain by thick Precambrian continental lithosphere that initially formed during Archean to mid-Proterozoic times. The absence of lithospheric thermal anomalies suggests that the mountains were formed by a compressional orogeny during the Paleozoic, thus providing a locus for ice sheet nucleation throughout a long period of geological time. Within West Antarctica, the crust and lithosphere are extremely thin near the Transantarctic Mountain Front and topographic lows such as the Bentley Trench and Byrd Basin, which represent currently inactive Cenozoic rift systems. Slow seismic velocities beneath Marie Byrd Land at asthenospheric depths suggest a major thermal anomaly, possibly due to a mantle plume. Volcanic earthquakes detected in this region indicate the presence of currently active magma systems. The results suggest large lateral changes in parameters needed for glaciological models, including lithospheric thickness, mantle viscosity, and heat flow. Extremely high heat flow is predicted for much of West Antarctica, consistent with recent results from the WAIS ice drilling. Using the seismic results to estimate mantle viscosity, we find several orders of magnitude difference in viscosity between East and West Antarctica, with lowest viscosities found beneath Marie Byrd Land and the West Antarctic Rift System. Realistic glacial isostatic adjustment models must take these large lateral variations into account.

Lunar Gravity Studies from the Lunar Prospector Line-of-Sight Acceleration Data: Isostatic Compensation of Medium Sized Craters

NASA Astrophysics Data System (ADS)

Sugano, T.; Heki, K.

2002-12-01

Direct estimation of mass distribution on the lunar nearside surface using the Lunar Prospector (LP) line-of-sight (LOS) acceleration data has several merits over conventional methods to estimate Stokes' coefficients of the lunar gravity field, such as (1) high resolution gravity anomaly recovery without introducing Kaula's constraint, (2) fast inversion calculation by stepwise estimation of parameter sets enabled by small correlation between parameters sets. Resolution of the lunar free-air gravity anomaly map obtained here, is as high as a gravity model complete to degree/order 225, and yet less noisy than the recent models. Next we performed terrain correction for the raw LOS acceleration data using lunar topography model from the Clementine laser altimetry data and the average crustal density of 2.9 g/cm3. By conducting the same inversion for the data after the correction, we obtained the map of Bouguer gravity anomaly that mainly reflects the MOHO topography. By comparing maps we notice that signatures of medium-sized (80-300 km in diameter) craters visible as topographic depression and negative free air anomaly, disappear in the Bouguer anomaly. The absence of mass deficits in the Bouguer anomaly suggests that the MOHO beneath them is flat. Generally speaking, longer wavelength topographic features have to be supported by MOHO topography (Airy isostatic compensation) while small scale topographic features are supported by lithospheric strength. The boundary between these two modes constrains the lithosphere thickness, and hence thermal structure near the surface. Larger craters are known to have become Mascons; mantle plugs and high-density mare basalts cause positive gravity anomalies there. The smallest Mascon has diameters a little larger than 300 km (e.g. Schiller-Zuccius), and the boundary between the two compensation status seems to lie around 300 km. Thermal evolution history of the Moon suggests temporally increasing thickness of lithosphere over its entire history, and the lithosphere as thick as 50-100 km around 4.0 Ga. This is consistent with the isostatic compensation status of the craters studied here, and a model describing the degree of lithospheric supports for various wavelength topographies.

Geophysical Exploration Technologies for the Deep Lithosphere Research: An Education Materials for High School Students

NASA Astrophysics Data System (ADS)

Xu, H.; Xu, C.; Luo, S.; Chen, H.; Qin, R.

2012-12-01

The science of Geophysics applies the principles of physics to study of the earth. Geophysical exploration technologies include the earthquake seismology, the seismic reflection and refraction methods, the gravity method, the magnetic method and the magnetotelluric method, which are used to measure the interior material distribution, their structure and the tectonics in the lithosphere of the earth. Part of the research project in SinoProbe-02-06 is to develop suitable education materials for carton movies targeting the high school students and public. The carton movies include five parts. The first part includes the structures of the earth's interior and variation in their physical properties that include density, p-wave, s-wave and so on, which are the fundamentals of the geophysical exploration technologies. The second part includes the seismology that uses the propagation of elastic waves through the earth to study the structure and the material distribution of the earth interior. It can be divided into earthquake seismology and artifice seismics commonly using reflection and refraction. The third part includes the magnetic method. Earth's magnetic field (also known as the geomagnetic field)extends from the Earth's inner core to where it meets the solar wind, a stream of energetic particles emanating from the Sun. The aim of magnetic survey is to investigate subsurface geology on the basis of anomalies in the Earth's magnetic field resulting from the magnetic properties of the underlying rocks. The magnetic method in the lithosphere attempts to use magnetic disturbance to analyse the regional geological structure and the magnetic boundaries of the crust. The fourth part includes the gravity method. A gravity anomaly results from the inhomogeneous distribution of density of the Earth. Usually gravity anomalies contain superposed anomalies from several sources. The long wave length anomalies due to deep density contrasts are called regional anomalies. They are important for understanding the large-scale structure of the earth's crust under major geographic features, such as mountain ranges, oceanic ridges and subduction zones. Short wave length residual anomalies are due to shallow anomalous masses that may be of interest for commercial exploitation. The last part is the magnetotellurics (MT), which is an electromagnetic geophysical method of imaging the earth's subsurface by measuring natural variations of electrical and magnetic fields at the Earth's surface. The long-period MT technique is used to exploration deep crustal. MT has been used to investigate the distribution of silicate melts in the Earth's mantle and crust and to better understand the plate-tectonic processes.

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).

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.

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

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.

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.

Origin of the lithospheric stress field

NASA Astrophysics Data System (ADS)

Lithgow-Bertelloni, Carolina; Guynn, Jerome H.

2004-01-01

An understanding of the tectonic stress field is geologically important because it is the agent that preserves in the crust a memory of dynamical processes. In an effort to elucidate the origin of the present state of stress of the lithosphere we use a finite element model of the Earth's lithosphere to calculate stresses induced by mantle flow, crustal heterogeneity, and topography and compare these to observations of intraplate stresses as given by the World Stress Map. We explore two models of lithospheric heterogeneity, one based directly on seismic and other observational constraints (Crust 2.0), and another that assumes isostatic compensation. Mantle tractions are computed from two models of mantle density heterogeneity: a model based on the history of subduction of the last 180 Myr, which has proved successful at accurately reproducing the present-day geoid and Cenozoic plate velocities, and a model inferred from seismic tomography. We explore the effects of varying assumptions for the viscosity structure of the mantle, and the effects of lateral variations in viscosity in the form of weak plate boundaries. We find that a combined model that includes both mantle and lithospheric sources of stress yields the best match to the observed stress field (˜60% variance reduction), although there are many regions where agreement between observed and predicted stresses is poor. The stress field produced by mantle tractions alone shows a greater degree of long-wavelength structure than is apparent in the stress observations but agrees very well with observations in some areas where radial mantle tractions are particularly strong such as in southeast Asia and the western Pacific. The stress field produced by lithospheric heterogeneity alone depends strongly on the assumed crustal model: Whereas the isostatically compensated model yields very poor agreement with observations, the model based on Crust 2.0 matches the observations about as well as mantle tractions alone and matches very well in certain areas where the influence of high topography is very important (e.g., Andes, East Africa). A possible interpretation of our results is that the stress field is significantly influenced by lateral variations in the viscosity of the mantle, which leads to variable amounts of decoupling between lithosphere and mantle, allowing the mantle signature to dominate in some areas and the crustal signature to dominate in others. The poor fit between the isostatically compensated crustal model and observations and the large differences between the two crustal models point toward the importance of dynamic topography and remaining uncertainties in crustal structure and rheology. We also consider the possibility that observations of stress from the shallow crust may not reflect the state of stress of the entire plate; stresses in the upper plate may be at least partially decoupled from broader-scale plate driving forces by lateral and vertical variations in lithospheric rheology.

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.

Two-dimensional Coupled Petrological-tectonic Modelling of Extensional Basins

NASA Astrophysics Data System (ADS)

Kaus, B. J. P.; Podladchikov, Y. Y.; Connolly, J. A. D.

Most numerical codes that simulate the deformation of a lithosphere assume the den- sity of the lithosphere to be either constant or depend only on temperature and pres- sure. It is, however, well known that rocks undergo phase transformations in response to changes in pressure and temperature. Such phase transformations may substantially alter the bulk properties of the rock (i.e., density, thermal conductivity, thermal ex- pansivity and elastic moduli). Several previous studies demonstrated that the density effects due to phase transitions are indeed large enough to have an impact on the litho- sphere dynamics. These studies were however oversimplified in that they accounted for only one or two schematic discontinuous phase transitions. The current study there- fore takes into account all the reactions that occur for a realistic lithospheric composi- tion. Calculation of the phase diagram and bulk physical properties of the stable phase assemblages for the crust and mantle within the continental lithosphere was done ac- counting for mineral solution behaviour using a free energy minimization program for natural rock compositions. The results of these calculations provide maps of the varia- tions in rock properties as a function of pressure and temperature that are easily incor- porated in any dynamic model computations. In this contribution we implemented a density map in the two-dimensional basin code TECMOD2D. We compare the results of the model with metamorphic reactions with a model without reactions and define some effective parameters that allow the use of a simpler model that still mimics most of the density effects of the metamorphic reactions.

Using uplift rates and lithosphere stress pattern for the past 200 Ma to quantify deep and shallow mantle contributions to the present-day southern African topography

NASA Astrophysics Data System (ADS)

Osei Tutu, A.; Webb, S. J.; Steinberger, B. M.; Rogozhina, I.

2017-12-01

The debate about the origin of the highlands in southern African has generated varying hypothesis, since the nominal processes for mountain building such as evidence of orogeny is not observed here at present-day. For example, some studies have suggested a pre-Paleozoic subduction under the southern Africa plate, might have caused the high topography, whiles other have proposed a large-scale buoyant flow coming from the mid-mantle over the African Large Low Share Velocity Province (LLSVP) as the source. A different school of thought is centered on a probable plume-lithosphere interaction in the early Miocene to late Pliocene. Using joint analysis of geodynamics and geophysical models with geological records; we seek to quantify both shallow and deep mantle density heterogeneities and viscosity structure to understand the tectonics of the southern Africa regional topography. We estimate uplift rates and change in lithosphere stress field for the past 200 Ma and compare with geological records considering first only shallow and deep contributions and their combined effect using a thermo-mechanical model with a free surface.

Geophysical investigations of the area between the Mid-Atlantic Ridge and the Barents Sea: From water to the lithosphere-asthenosphere system

NASA Astrophysics Data System (ADS)

Grad, Marek; Mjelde, Rolf; Krysiński, Lech; Czuba, Wojciech; Libak, Audun; Guterch, Aleksander

2015-03-01

As a part of the large international panel "IPY Plate Tectonics and Polar Gateways" within the "4th International Polar Year" framework, extensive geophysical studies were performed in the area of southern Svalbard, between the Mid-Atlantic Ridge and the Barents Sea. Seismic investigations were performed along three refraction and wide-angle reflection seismic lines. Integrated with gravity data the seismic data were used to determine the structure of the oceanic crust, the transition between continent and ocean (COT), and the continental structures down to the lithosphere-asthenosphere system (LAB). We demonstrate how modeling of multiple water waves can be used to determine the sound velocity in oceanic water along a seismic refraction profile. Our 2D seismic and density models documents 4-9 km thick oceanic crust formed at the Knipovich Ridge, a distinct and narrow continent-ocean transition (COT), the Caledonian suture zone between Laurentia and Barentsia, and 30-35 km thick continental crust beneath the Barents Sea. The COT west of southern Spitsbergen expresses significant excess density (more than 0.1 g/cm3 in average), which is characteristic for mafic/ultramafic and high-grade metamorphic rocks. The results of the gravity modeling show relatively weak correlation of the density with seismic velocity in the upper mantle, which suggests that the horizontal differences between oceanic and continental mantle are dominated by mineralogical changes, although a thermal effect is also present. The seismic velocity change with depth suggests lherzolite composition of the uppermost oceanic mantle, and dunite composition beneath the continental crust.

Three-dimensional, thermo-mechanical and dynamical analogue experiments of subduction: first results

NASA Astrophysics Data System (ADS)

Boutelier, D.; Oncken, O.

2008-12-01

We present a new analogue modeling technique developed to investigate the mechanics of the subduction process and the build-up of subduction orogenies. The model consists of a tank filled with water representing the asthenosphere and two lithospheric plates made of temperature-sensitive hydrocarbon compositional systems. These materials possess elasto-plastic properties allowing the scaling of thermal and mechanical processes. A conductive thermal gradient is imposed in the lithosphere prior to deformation. The temperature of the asthenosphere and model surface are imposed and controlled with an electric heater, two infrared ceramic heat emitters, two thermocouples and a thermo-regulator. This system allows an unobstructed view of the model surface, which is monitored using a stereoscopic particle image technique. This monitoring technique provides a precise quantification of the horizontal deformation and variations of elevation in the three-dimensional model. Convergence is imposed with a piston moving at a constant rate or pushing at a constant stress. The velocity is scaled using the dimensionless ratio of thermal conduction over advection. The experiments are first produced at a constant rate and the stress in the horizontal direction of the convergence is recorded. Then the experiment is reproduced with a constant stress boundary condition where the stress value is set to the averaged value obtained in the previous experiment. Therefore, an initial velocity allowing proper scaling of heat exchanges is obtained, but deformation in the model and spatial variations of parameters such as density or friction coefficient can produce variations of plate convergence velocity. This in turn impacts the strength of the model lithosphere because it changes the model thermal structure. In the first presented experiments the model lithosphere is one layer and the plate boundary is linear. The effects of variations of the subducting plate thickness, density and the lubrication of the interface between the plates are investigated.

Importance of the Decompensative Correction of the Gravity Field for Study of the Upper Crust: Application to the Arabian Plate and Surroundings

NASA Astrophysics Data System (ADS)

Kaban, Mikhail K.; El Khrepy, Sami; Al-Arifi, Nassir

2017-01-01

The isostatic correction represents one of the most useful "geological" reduction methods of the gravity field. With this correction it is possible to remove a significant part of the effect of deep density heterogeneity, which dominates in the Bouguer gravity anomalies. However, even this reduction does not show the full gravity effect of unknown anomalies in the upper crust since their impact is substantially reduced by the isostatic compensation. We analyze a so-called decompensative correction of the isostatic anomalies, which provides a possibility to separate these effects. It was demonstrated that this correction is very significant at the mid-range wavelengths and may exceed 100 m/s2 (mGal), therefore ignoring this effect would lead to wrong conclusions about the upper crust structure. At the same time, the decompensative correction is very sensitive to the compensation depth and effective elastic thickness of the lithosphere. Therefore, these parameters should be properly determined based on other studies. Based on this technique, we estimate the decompensative correction for the Arabian plate and surrounding regions. The amplitude of the decompensative anomalies reaches ±250 m/s2 10-5 (mGal), evidencing for both, large density anomalies of the upper crust (including sediments) and strong isostatic disturbances of the lithosphere. These results improve the knowledge about the crustal structure in the Middle East.

GOCE gravity gradient data for lithospheric modeling and geophysical exploration research

NASA Astrophysics Data System (ADS)

Bouman, Johannes; Ebbing, Jörg; Meekes, Sjef; Lieb, Verena; Fuchs, Martin; Schmidt, Michael; Fattah, Rader Abdul; Gradmann, Sofie; Haagmans, Roger

2013-04-01

GOCE gravity gradient data can improve modeling of the Earth's lithosphere and upper mantle, contributing to a better understanding of the Earth's dynamic processes. We present a method to compute user-friendly GOCE gravity gradient grids at mean satellite altitude, which are easier to use than the original GOCE gradients that are given in a rotating instrument frame. In addition, the GOCE gradients are combined with terrestrial gravity data to obtain high resolution grids of gravity field information close to the Earth's surface. We also present a case study for the North-East Atlantic margin, where we analyze the use of satellite gravity gradients by comparison with a well-constrained 3D density model that provides a detailed picture from the upper mantle to the top basement (base of sediments). We demonstrate how gravity gradients can increase confidence in the modeled structures by calculating the sensitvity of model geometry and applied densities at different observation heights; e.g. satellite height and near surface. Finally, this sensitivity analysis is used as input to study the Rub' al Khali desert in Saudi Arabia. In terms of modeling and data availability this is a frontier area. Here gravity gradient data help especially to set up the regional crustal structure, which in turn allows to refine sedimentary thickness estimates and the regional heat-flow pattern. This can have implications for hydrocarbon exploration in the region.

Dynamic Topography at Earth's Surface: Fact or Fiction? (Invited)

NASA Astrophysics Data System (ADS)

Lithgow-Bertelloni, C. R.; Silver, P. G.

2009-12-01

Contributions to Earth’s surface topography range from short-wavelength uncompensated features due to tectonic activity, to variations in crustal structure and long-wavelength deflections of the lithosphere caused by mantle dynamics. The latter we call dynamic topography. Dynamic topography elevates or depresses the surface even if the density anomaly giving rise to flow is deep in the mantle. Dynamic topography is also a major contributor to Earth’s gravitational potential and to surface deformation. However, direct observations of dynamic topography are elusive, because signals are obscured by the isostatic contribution due to crustal and lithospheric structure. The only seemingly unequivocal signals of dynamically supported topography have been found over mantle upwellings on both continents (Africa [Lithgow-Bertelloni and Silver, 1998] and Arabia [Daradich et al., 2004]) and oceanic basins (North-Atlantic [Conrad et al., 2004]). Recent work on Africa’s geomorphic history [Moore et al., 2009] and North Atlantic gravity and topography have called even these results into questions. In downwelling regions (near slabs) no clear signals have been found. I will explore why this dichotomy may exist and relate it to the need for dynamic topography in mantle flow models, with an eye towards the effects of phase transitions, lateral variations in viscosity and layered convection. I will also present recent results on dynamic topography over flat slab segments that overturn the conventional wisdom and explain basin topography in the Andean foreland. Along with the new models I will discuss a recent global lithospheric structure model with which to compute the residual topography, i.e. the “observed” dynamic topography.

Mantle plume influence on the Neogene uplift and extension of the US western Cordillera?

USGS Publications Warehouse

Parsons, T.; Thompson, G.A.; Sleep, Norman H.

1994-01-01

Despite its highly extended and thinned crust, much of the western Cordillera in the United States is elevated more than 1km above sea level. Therefore, this region cannot be thought of as thick crust floating isostatically in a uniform mantle; rather, the lithospheric mantle and/or the upper asthenosphere must vary in thickness or density across the region. Utilizing crustal thickness and density constraints, the residual mass defcicit that must occur in the mantle lithosphere and asthenosphere beneath the western Cordillera was modelled. A major hot spot broke out during a complex series of Cenozoic tectonic events that included lithospheric thickening, back-arc extension, and transition from subduction to a transform plate boundary. It is suggested that many of the characteristics that make the western Cordillera unique among extensional provinces can be attributed to the mantle plume that created the Yellowstone hot spot. -Authors

Evidence of lower-mantle slab penetration phases in plate motions.

PubMed

Goes, Saskia; Capitanio, Fabio A; Morra, Gabriele

2008-02-21

It is well accepted that subduction of the cold lithosphere is a crucial component of the Earth's plate tectonic style of mantle convection. But whether and how subducting plates penetrate into the lower mantle is the subject of continuing debate, which has substantial implications for the chemical and thermal evolution of the mantle. Here we identify lower-mantle slab penetration events by comparing Cenozoic plate motions at the Earth's main subduction zones with motions predicted by fully dynamic models of the upper-mantle phase of subduction, driven solely by downgoing plate density. Whereas subduction of older, intrinsically denser, lithosphere occurs at rates consistent with the model, younger lithosphere (of ages less than about 60 Myr) often subducts up to two times faster, while trench motions are very low. We conclude that the most likely explanation is that older lithosphere, subducting under significant trench retreat, tends to lie down flat above the transition to the high-viscosity lower mantle, whereas younger lithosphere, which is less able to drive trench retreat and deforms more readily, buckles and thickens. Slab thickening enhances buoyancy (volume times density) and thereby Stokes sinking velocity, thus facilitating fast lower-mantle penetration. Such an interpretation is consistent with seismic images of the distribution of subducted material in upper and lower mantle. Thus we identify a direct expression of time-dependent flow between the upper and lower mantle.

Numerical investigation of deep-crust behavior under lithospheric extension

NASA Astrophysics Data System (ADS)

Korchinski, Megan; Rey, Patrice F.; Mondy, Luke; Teyssier, Christian; Whitney, Donna L.

2018-02-01

What are the conditions under which lithospheric extension drives exhumation of the deep orogenic crust during the formation of gneiss domes? The mechanical link between extension of shallow crust and flow of deep crust is investigated using two-dimensional numerical experiments of lithospheric extension in which the crust is 60 km thick and the deep-crust viscosity and density parameter space is explored. Results indicate that the style of extension of the shallow crust and the path, magnitude, and rate of flow of deep crust are dynamically linked through the deep-crust viscosity, with density playing an important role in experiments with a high-viscosity deep crust. Three main groups of domes are defined based on their mechanisms of exhumation across the viscosity-density parameter space. In the first group (low-viscosity, low-density deep crust), domes develop by lateral and upward flow of the deep crust at km m.y-1 velocity rates (i.e. rate of experiment boundary extension). In this case, extension in the shallow crust is localized on a single interface, and the deep crust traverses the entire thickness of the crust to the Earth's near-surface in 5 m.y. This high exhuming power relies on the dynamic feedback between the flow of deep crust and the localization of extension in the shallow crust. The second group (intermediate-viscosity, low-density deep crust) has less exhuming power because the stronger deep crust flows less readily and instead accommodates more uniform extension, which imparts distributed extension to the shallow crust. The third group represents the upper limits of viscosity and density for the deep crust; in this case the low buoyancy of the deep crust results in localized thinning of the crust with large upward motion of the Moho and lithosphere-asthenosphere boundary. These numerical experiments test the exhuming power of the deep crust in the formation of extensional gneiss domes.

Heat Flow, Regional Geophysics and Lithosphere Structure In The Czech Republic

NASA Astrophysics Data System (ADS)

Safanda, J.; Cermak, V.; Kresl, M.; Dedecek, P.

Paper summarises and critically revises heat flow data that have been collected in the Czech Republic to date. The regional heat flow density map was prepared in view of all existing heat flow data completed with the similar in the surrounding countries and taking into consideration also temperature measurements in deep boreholes. Crustal temperature profiles were calculated by using the available geological information, results of deep seismic sounding and the laboratory data on radiogenic heat produc- tion and thermal conductivity. Special attention was paid to numerous temperature logs in two sedimentary basins, namely in the Cheb and Ostrava-Karvina coal basins, for which detailed heat flow patterns were proposed. Relationships between heat flow distribution and the crustal/lithosphere evolution, between heat flow and the heat pro- duction of the crustal rocks, heat flow and crustal thickness and the steady-state vs. transient heat transport are discussed.

Subsurface Structure and the Stress State of the Utopia Basin, Mars

NASA Astrophysics Data System (ADS)

Searls, M. L.; Phillips, R. J.

2005-12-01

A great deal of work has been done in determining the resurfacing history of the northern lowlands; however, most of the previous research has focused on the depth and characteristics of the Hesperian and Amazonian plains units that cover an older, heavily cratered Noachian surface (e.g. Tanaka et. al. 2003). An analysis of the amount and density of fill within the Utopia Basin could provide valuable insight to the depositional environment of the northern lowlands during the earliest epoch of martian history. In the present study we use the topography and gravity data from recent Mars' missions to analyze the subsurface structure of the Utopia basin, focusing on the volume and density of fill that causes the shallowness of the basin. Using the assumption that the initial isostatic state of Utopia was similar to that of the Hellas basin allows us to construct a model for Utopia that facilitates investigation of its interior configuration. Based on the spherical harmonic, thin-shell elastic model of Banerdt (1986), we developed a system of equations that allows us to solve for the original basin shape, the amount of fill within Utopia basin, the amount of flexure due to the fill material, the total vertical load and the horizontal load potential. The presence of quasi-circular depressions within the Utopia basin (Frey 2004) indicates that the majority of the material within Utopia was deposited early in the Noachian when the elastic lithosphere of Mars was (presumably) relatively thin (<50 km). Given this constraint along with constraints placed on the system due to the pre-fill isostatic assumption, we can place a lower bound on the density of the fill within Utopia basin of 2800 kg/m3. This indicates that the amount of fill within the Utopia basin is >15 km, with a corresponding lithospheric flexure/membrane deformation of >14 km. The high density obtained for the fill requires that it contain a large igneous component, the source of which is problematic. Relaxing the isostatic assumption to a reasonable degree perturbs the density bound only slightly. This thin-shell model also allows us to calculate the stress field due to the flexure/membrane strains. The stress results show that the circumferential and radial tectonic features seen in the Utopia region (Thomson and Head 2001) are not due solely to deformation of the elastic lithosphere, so the tectonic features observed are the result, at least in part, of processes that occur within the load itself.

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.

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.

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.

Understanding the physics of the Yellowstone magmatic system with geodynamic inverse modelling

NASA Astrophysics Data System (ADS)

Reuber, Georg; Kaus, Boris

2017-04-01

The Yellowstone magmatic system is one of the largest magmatic systems on Earth. Thus, it is important to understand the geodynamic processes that drive this very complex system on a larger scale ranging from the mantle plume up to the shallow magma chamber in the upper crust. Recent geophysical results suggest that two distinct magma chambers exist: a shallow, presumably felsic chamber and a deeper and partially molten chamber above the Moho [1]. 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 puzzling. Therefore, we employ lithospheric-scale 2D and 3D geodynamic models to test the influence of different model parameters, such as the geometry of the magma chamber, the melt fraction, the rheological flow law, the densities and the thermal structure on their influence on the dynamics of the lithosphere. The melt content and the rock densities are obtained by consistent thermodynamic modelling of whole rock data of the Yellowstone stratigraphy. We present derivations in the stress field around the Yellowstone plume, diking areas and different melt accumulations. Our model predictions can be tested with available geophysical data (uplift rates, melt fractions, stress states, seismicity). By framing it in an inverse modelling approach we can constrain which parameters (melt fractions, viscosities, geometries) are consistent with the data and which are not. [1] Huang, Hsin-Hua, et al. "The Yellowstone magmatic system from the mantle plume to the upper crust." Science 348.6236 (2015): 773-776.

Developing Regionalized Models of Lithospheric Thickness and Velocity Structure Across Eurasia and the Middle East from Jointly Inverting P-Wave and S-Wave Receiver Functions with Rayleigh Wave Group and Phase Velocities

DTIC Science & Technology

2010-09-01

lithospheric velocity structure for a wide variety of tectonic regions throughout Eurasia and the Middle East. We expect the regionalized models will improve...constructed by combining the 1D joint inversion models within each tectonic region and validated through regional waveform modeling. The velocity models thus...important differences in lithospheric structure between the cratonic regions of Eastern Europe and the tectonic regions of Western Europe and the

Convection-driven tectonics on Venus

NASA Astrophysics Data System (ADS)

Phillips, R. J.

1990-02-01

An analysis is presented of convective stress coupling to an elastic lithosphere as applied to Venus. Theoretical solutions are introduced for the response of a mathematically thick elastic plate overlying a Newtonian viscous medium with an exponential depth dependence of viscosity, and a Green's function solution is obtained for the viscous flow driven by a harmonic density distribution at a specified depth. An elastic-plastic analysis is carried out for the deformation of a model Venus lithosphere. The results predict that dynamic uplift of Venusian topography must be accompanied by extensive brittle failure and viscous flow in the lithosphere.

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.

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.

Processes of lithosphere evolution: New evidence on the structure of the continental crust and uppermost mantle

USGS Publications Warehouse

Artemieva, I.M.; Mooney, W.D.; Perchuc, E.; Thybo, H.

2002-01-01

We discuss the structure of the continental lithosphere, its physical properties, and the mechanisms that formed and modified it since the early Archean. The structure of the upper mantle and the crust is derived primarily from global and regional seismic tomography studies of Eurasia and from global and regional data on seismic anisotropy. These data as documented in the papers of this special issue of Tectonophysics are used to illustrate the role of different tectonic processes in the lithospheric evolution since Archean to present. These include, but are not limited to, cratonization, terrane accretion and collision, continental rifting (both passive and active), subduction, and lithospheric basal erosion due to a relative motion of cratonic keels and the convective mantle. ?? 2002 Elsevier Science B.V. All rights reserved.

Crustal and upper-mantle structure of South China from Rayleigh wave tomography

NASA Astrophysics Data System (ADS)

Shan, B.; Xiong, X.; Zhao, K. F.; Xie, Z. J.; Zheng, Y.; Zhou, L.

2017-03-01

In this study, we image the crust and upper-mantle seismic velocity structures in South China using teleseismic Rayleigh waves recorded at 354 stations from the Chinese provincial networks (CEArray). We process Rayleigh wave data from 1087 teleseismic events and construct phase velocity maps at periods of 40-150 s. By combining dispersion curves at 6-70 s from Zhou et al. and at 40-150 s from the teleseismic surface wave tomography of this study, we construct a 3-D shear velocity model of the crust and upper mantle of South China. Distinct seismic structures are revealed from the eastern part of South China (including the South China Fold System and the eastern Yangtze Craton) to the western Yangtze Craton. The South China Fold System and eastern Yangtze Craton are characterized by lower velocities and shallow lithosphere-asthenosphere boundary (∼90 km), which are similar to the lithospheric thermal and seismic velocity structures of the North China basin. These observations may imply that the lithospheric destruction and thinning occurred not only beneath the North China Craton, but also beneath the eastern part of South China. The western Yangtze Craton, including the Sichuan Basin and Jiangnan Orogen, is underlain by a thicker and colder lithosphere with high velocities. The contrast of the lithosphere structure between the western Yangtze Craton and other parts of South China indicates that the lithospheric destruction and thinning of the east and southeast parts of South China may terminate at the boundary of the Jiangnan Orogen.

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.

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.

Buoyant subduction on Venus: Implications for subduction around coronae

NASA Astrophysics Data System (ADS)

Burt, J. D.; Head, J. W.

1993-03-01

Potentially low lithospheric densities, caused by high Venus surface and perhaps mantle temperatures, could inhibit the development of negative buoyancy-driven subduction and a global system of plate tectonics/crustal recycling on that planet. No evidence for a global plate tectonic system was found so far, however, specific features strongly resembling terrestrial subduction zones in planform and topographic cross-section were described, including trenches around large coronae and chasmata in eastern Aphrodite Terra. The cause for the absence, or an altered expression, of plate tectonics on Venus remains to be found. Slab buoyancy may play a role in this difference, with higher lithospheric temperatures and a tendency toward positive buoyancy acting to oppose the descent of slabs and favoring under thrusting instead. The effect of slab buoyancy on subduction was explored and the conditions which would lead to under thrusting versus those allowing the formation of trenches and self-perpetuating subduction were defined. Applying a finite element code to assess the effects of buoyant forces on slabs subducting into a viscous mantle, it was found that mantle flow induced by horizontal motion of the convergent lithosphere greatly influences subduction angle, while buoyancy forces produce a lesser effect. Induced mantle flow tends to decrease subduction angle to near an under thrusting position when the subducting lithosphere converges on a stationary overriding lithosphere. When the overriding lithosphere is in motion, as in the case of an expanding corona, subduction angles are expected to increase. An initial stage involved estimating the changes in slab buoyancy due to slab healing and pressurization over the course of subduction. Modeling a slab, descending at a fixed angle and heated by conduction, radioactivity, and the heat released in phase changes, slab material density changes due to changing temperature, phase, and pressure were derived.

Isostatic and Decompensative Gravity Anomalies of the Arabian Plate and Surrounding Regions: a Key for the Crustal Structure

NASA Astrophysics Data System (ADS)

Kaban, M. K.; El Khrepy, S.; Al-Arifi, N. S.

2016-12-01

The isostatic anomalies are often considered as one of the most useful correction of the gravity field for investigation of the upper crust structure in many practical applications. By applying this correction, a substantial part of the effect of deep density heterogeneity, which dominates in the Bouguer gravity anomaly, can be removed. With this approach, it is not even necessary to know the deep density structure of the crust and upper mantle in details; it is sufficient to prescribe some type of compensation (regional vs. local) and a compensation depth. However, even when all the parameters are chosen correctly, this reduction of the gravity field does not show the full gravity effect of unknown anomalies in the crust. The last ones should be also compensated to some extent; therefore their impact is substantially reduced by the isostatic compensation. Long ago (Cordell et al., 1991), it was suggested a so-called decompensative correction of the isostatic anomalies, which provides a possibility to separate these effects. However, the decompensative correction is very sensitive to the parameters of the compensation scheme. In the present study we analyse the ways to choose these parameters and extend this approach by assuming a possibility for the regional compensation via elastic deformations of the lithosphere. Based on this technique, we estimate the isostatic and decompensative anomalies for the Arabian plate and surrounding regions. The parameters of the isostatic model are chosen based on previous studies. It was demonstrated that the decompensative correction is very significant at the mid-range wavelengths and may exceed 100 mGal, therefore ignoring this effect would lead to wrong conclusions about the upper crust structure. The total amplitude of the decompensative anomalies reaches ±250 mGal, evidencing for both, large density anomalies of the upper crust (including sediments) and strong isostatic disturbances of the lithosphere. These results improve the knowledge about the crustal structure in the Middle East. Cordell, L., Zorin, Y. A., & Keller, G. R. (1991). The decompensative gravity anomaly and deep structure of the region of the Rio Grande rift. Journal of Geophysical Research: Solid Earth (1978-2012), 96(B4), 6557-6568.

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.

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.

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

Internal processes affecting surfaces of low-density satellites - Ganymede and Callisto

NASA Technical Reports Server (NTRS)

Parmentier, E. M.; Head, J. W.

1979-01-01

Possible significant physical processes on low-density (icy) satellites, particularly Ganymede and Callisto, are outlined, and the relations of these interior processes to the formation and evolution of satellite surfaces are discussed. A variety of mechanisms is shown to lead to interior melting in early satellite history and a configuration characterized by a predominantly water ice lithosphere overlying a mantle containing liquid water. Physical processes capable of affecting the lithosphere of an ice-silicate body and thus creating observable surface features are assessed, including tectonic stresses from tidal deformation and volume changes, gravitational effects on density differences and water volcanism. The residence time of surface features on icy bodies produced by the outlined processes and by impact cratering is considered, and a tentative outline of the geologic history of Ganymede and Callisto is presented. Observations from Voyager and Galileo are expected to provide evidence on the evolution and geologic history of low-density satellites.

The role of lithospheric stress in the support of the Tharsis rise

NASA Technical Reports Server (NTRS)

Willemann, R. J.; Turcotte, D. L.

1982-01-01

It is hypothesized that the Tharsis rise can be approximated as an axisymmetrical igneous construct. Linear theory for the deflection of planetary lithospheres is used to demonstrate that the lithospheric stresses required partially to support the construct are reasonable and consistent with the observed radial grabens around Tharsis. The computed thickness of the elastic lithosphere is between 110 and 260 km, depending of the values assumed for crustal thickness and crustal density. The computed thickness of the Tharsis load ranges from 40 to 70 km. Since in this model the height of the geoid is not specified a priori, the agreement between the observed and computed geoid is evidence for the validity of the model. The tectonics of the Tharsis region are briefly reviewed, and it is contended that all observations are consistent with the loading model.

Mantle viscosity structure constrained by joint inversions of seismic velocities and density

NASA Astrophysics Data System (ADS)

Rudolph, M. L.; Moulik, P.; Lekic, V.

2017-12-01

The viscosity structure of Earth's deep mantle affects the thermal evolution of Earth, the ascent of mantle upwellings, sinking of subducted oceanic lithosphere, and the mixing of compositional heterogeneities in the mantle. Modeling the long-wavelength dynamic geoid allows us to constrain the radial viscosity profile of the mantle. Typically, in inversions for the mantle viscosity structure, wavespeed variations are mapped into density variations using a constant- or depth-dependent scaling factor. Here, we use a newly developed joint model of anisotropic Vs, Vp, density and transition zone topographies to generate a suite of solutions for the mantle viscosity structure directly from the seismologically constrained density structure. The density structure used to drive our forward models includes contributions from both thermal and compositional variations, including important contributions from compositionally dense material in the Large Low Velocity Provinces at the base of the mantle. These compositional variations have been neglected in the forward models used in most previous inversions and have the potential to significantly affect large-scale flow and thus the inferred viscosity structure. We use a transdimensional, hierarchical, Bayesian approach to solve the inverse problem, and our solutions for viscosity structure include an increase in viscosity below the base of the transition zone, in the shallow lower mantle. Using geoid dynamic response functions and an analysis of the correlation between the observed geoid and mantle structure, we demonstrate the underlying reason for this inference. Finally, we present a new family of solutions in which the data uncertainty is accounted for using covariance matrices associated with the mantle structure models.

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.

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.

Electromagnetic study of lithospheric structure in Trans-European Suture Zone in Poland

NASA Astrophysics Data System (ADS)

Jóźwiak, Waldemar; Ślęzak, Katarzyna; Nowożyński, Krzysztof; Neska, Anne

2016-04-01

The area covered by magnetotelluric surveys in Poland is mostly related to the Trans-European Suture Zone (TESZ), the largest tectonic boundary in Europe. Numerous 1D, 2D, and pseudo-3D and 3D models of the electrical resistivity distribution were constructed, and a new interpretation method based on Horizontal Magnetic Tensor analysis has been applied recently. The results indicate that the TESZ is a lithospheric discontinuity and there are noticeable differences in geoelectric structures between the East European Craton (EEC), the transitional zone (TESZ), and the Paleozoic Platform (PP). The electromagnetic sounding is a very efficient tool for recognizing the lithospheric structure especially it helps in identification of important horizontal (or lateral) inhomogeneities in the crust. Due to our study we can clearly determine the areas of the East European Craton of high resistivity, Paleozoic Platform of somewhat lower resistivity value, and transitional TESZ of complicated structure. At the East European Craton, we observe very highly resistive lithosphere, reaching 220-240 km depth. Underneath, there is distinctly greater conductivity values, most probably resulting from partial melting of rocks; this layer may represent the asthenosphere. The resistivity of the lithosphere under the Paleozoic Platform is somewhat lower, and its thickness does not exceed 150 km. The properties of the lithosphere in the transition zone, under the TESZ, differ significantly. The presented models include prominent, NW-SE striking conductive lineaments. These structures, that related with the TESZ, lie at a depth of 10-30 km. They are located in a mid-crustal level and they reach the boundary of the EEC. The structures we initially connect to the Variscan Deformation Front (VDF) and the Caledonian Deformation Front (CDF). The differentiation of conductivity visible in the crust continues in the upper mantle.

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.

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.

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.

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.

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.

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.

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.

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.

3-D lithospheric structure and regional/residual Bouguer anomalies in the Arabia-Eurasia collision (Iran)

NASA Astrophysics Data System (ADS)

Jiménez-Munt, I.; Fernãndez, M.; Saura, E.; Vergés, J.; Garcia-Castellanos, D.

2012-09-01

The aim of this work is to propose a first-order estimate of the crustal and lithospheric mantle geometry of the Arabia-Eurasia collision zone and to separate the measured Bouguer anomaly into its regional and local components. The crustal and lithospheric mantle structure is calculated from the geoid height and elevation data combined with thermal analysis. Our results show that Moho depth varies from ˜42 km at the Mesopotamian-Persian Gulf foreland basin to ˜60 km below the High Zagros. The lithosphere is thicker beneath the foreland basin (˜200 km) and thinner underneath the High Zagros and Central Iran (˜140 km). Most of this lithospheric mantle thinning is accommodated under the Zagros mountain belt coinciding with the suture between two different mantle domains on the Sanandaj-Sirjan Zone. The regional gravity field is obtained by calculating the gravimetric response of the 3-D crustal and lithospheric mantle structure obtained by combining elevation and geoid data. The calculated regional Bouguer anomaly differs noticeably from those obtained by filtering or just isostatic methods. The residual gravity anomaly, obtained by subtraction of the regional components to the measured field, is analyzed in terms of the dominating upper crustal structures. Deep basins and areas with salt deposits are characterized by negative values (˜-20 mGal), whereas the positive values are related to igneous and ophiolite complexes and shallow basement depths (˜20 mGal).

Thermal structure of oceanic transform faults

USGS Publications Warehouse

Behn, M.D.; Boettcher, M.S.; Hirth, G.

2007-01-01

We use three-dimensional finite element simulations to investigate the temperature structure beneath oceanic transform faults. We show that using a rheology that incorporates brittle weakening of the lithosphere generates a region of enhanced mantle upwelling and elevated temperatures along the transform; the warmest temperatures and thinnest lithosphere are predicted to be near the center of the transform. Previous studies predicted that the mantle beneath oceanic transform faults is anomalously cold relative to adjacent intraplate regions, with the thickest lithosphere located at the center of the transform. These earlier studies used simplified rheologic laws to simulate the behavior of the lithosphere and underlying asthenosphere. We show that the warmer thermal structure predicted by our calculations is directly attributed to the inclusion of a more realistic brittle rheology. This temperature structure is consistent with a wide range of observations from ridge-transform environments, including the depth of seismicity, geochemical anomalies along adjacent ridge segments, and the tendency for long transforms to break into small intratransform spreading centers during changes in plate motion. ?? 2007 Geological Society of America.

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.

Upper mantle and crustal structure of southwestern Scandinavia: Results of the TopoScandiaDeep project

NASA Astrophysics Data System (ADS)

Köhler, A.; Balling, N.; Ebbing, J.; England, R.; Frassetto, A.; Gradmann, S.; Jacobsen, B. H.; Kvarven, T.; Maupin, V.; Medhus, A. Bondo; Mjelde, R.; Ritter, J.; Schweizer, J.; Stratford, W.; Thybo, H.; Wawerzinek, B.; Weidle, C.

2012-04-01

The origin of the Scandinavian mountains, located far away from any presently active plate margin, is still not well understood. In particular, it is not clear if the mountains are sustained isostatically either by crustal thickening or by light upper mantle material. Within the TopoScandiaDeep project (a collaborative research project within the ESF TOPO-EUROPE programme), we therefore analyse recently collected passive seismological and active seismic data in the southern Scandes and surrounding regions. We infer crustal and upper mantle (velocity) structures and relate them to results of gravity and temperature-composition modelling. The Moho under the high topography of southern Norway appears from controlled source seismic refraction and Receiver Functions as relatively shallow (<= 45 km) compared to the deeper conversion (>55 km) imaged beneath the low topography in Sweden and elsewhere in the Baltic Shield area outside Norway. The Receiver Function modeling as well as the active seismic results suggest that the differences in the observed Moho response may represent the transition between tectonically reworked Moho under southern Norway and an intact, cratonic crust-mantle boundary beneath the Baltic Shield. Furthermore, the 410km-discontinuity and the LAB is imaged, the latter one suggesting a lithospheric thickening in NE direction. Upper mantle P-wave and S-wave velocities in southern Sweden and southern Norway east of the Oslo Graben are correspondingly relatively high while lower velocities are observed in the southwestern part of Norway and northern Denmark. The lateral velocity gradient, interpreted as the southwestern boundary of thick Baltic Shield lithosphere, is remarkably sharp. Differences in upper mantle velocities are found at depths of 100-400 km and amount to ± 2-3%. S-to-P wave conversions, interpreted to originate from the lithosphere-asthenosphere boundary, are preliminary estimated to 90-120 km depth. Inversion of Rayleigh and Love surface wave phase velocity dispersion curves from observations of ambient noise and earthquakes yield another independent model of the crust and upper mantle structure below southern Norway. Inverted crustal velocities and Moho depths are consistent with the results of seismic refraction and receiver functions. Additionally, indications for radial crustal anisotropy of up to about 3% are found. The inferred upper mantle S-wave velocities show that the lithosphere under southern Norway has characteristics usually found under continental platforms and changes towards a cratonic-like velocity structure in the East, in agreement with the body wave tomography. All in all, these separate investigations give a very consistent and stable picture of the crust and upper mantle configuration. Integrated geophysical modeling of the results shows that a lateral transition from thinner, warmer lithosphere under southern Norway towards thicker, colder lithosphere under Sweden results in a density distribution that significantly adds to the isostatic support of Norway's high topography.

Deep structure of the Alborz Mountains by joint inversion of P receiver functions and dispersion curves

NASA Astrophysics Data System (ADS)

Rastgoo, Mehdi; Rahimi, Habib; Motaghi, Khalil; Shabanian, Esmaeil; Romanelli, Fabio; Panza, Giuliano F.

2018-04-01

The Alborz Mountains represent a tectonically and seismically active convergent boundary in the Arabia - Eurasia collision zone, in western Asia. The orogenic belt has undergone a long-lasted tectono-magmatic history since the Cretaceous. The relationship between shallow and deep structures in this complex tectonic domain is not straightforward. We present a 2D velocity model constructed by the assemblage of 1D shear wave velocity (Vs) models from 26 seismic stations, mainly distributed along the southern flank of the Alborz Mountains. The shear wave velocity structure has been estimated beneath each station using joint inversion of P-waves receiver functions and Rayleigh wave dispersion curves. A substantiation of the Vs inversion results sits on the modeling of Bouguer gravity anomaly data. Our velocity and density models show low velocity/density anomalies in uppermost mantle of western and central Alborz at a depth range of ∼50-100 km. In deeper parts of the uppermost mantle (depth range of 100-150 km), a high velocity/density anomaly is located beneath most of the Mountain range. The spatial pattern of these low and high velocity/density structures in the upper mantle is interpreted as the result of post collisional delamination of lower part of the western and central Alborz lithosphere.

Model Estimates of Non-Hydrostatic Stresses in the Martian Crust and Mantle: 1—Two-Level Model

NASA Astrophysics Data System (ADS)

Gudkova, T. V.; Batov, A. V.; Zharkov, V. N.

2017-11-01

Regions of maximum shear and tension-compression stresses in the Martian interior have been revealed using two types of models: the elastic model and the model with an elastic lithosphere of varied thickness (150-500 km) positioned on a weak layer that has partially lost its elastic properties. The weakening is simulated by a ten-fold lower value of the shear modulus down to the core boundary. The numerical simulation applies Green's functions (load number method) with the step of 1 × 1 grade along latitude and longitude down to a depth of 1000 km. The boundary condition is the expansion of the latest data on Martian topography and the gravitational field (model MRO120D) in spherical harmonics up to the degree and order of 90 in relation to the reference surface that is assumed an equilibrium spheroid. The considered two-level compensation model assumes nonequilibrium relief and density anomalies at the crust-mantle boundary to be the sources of the anomalous gravitational field. Calculations are performed for two test models of Martian internal structure with the crust mean thicknesses of 50 to 100 km and mean density of 2900 kg/m3. Considerable tangential and simultaneously compressive stresses occur under the Tharsis region. The main regions of high shear and simultaneously extentional stresses are located in the Hellas region crust and in the lithosphere of the following regions: Argyre Planitia, Mare Acidalium, Arcadia Planitia and Valles Marineris. The zone of high maximum shear and extentional stresses has been found at the base of the lithosphere under the Olympus volcano and that under the Elysium rise.

The Role of Water in the Stability of Cratonic Keels

NASA Technical Reports Server (NTRS)

Peslier, Anne H.; Woodland, Alan B.; Bell, David R.; Lazarov, Marina

2011-01-01

Cratons are typically underlain by large, deep, and old lithospheric keels (to greater than 200 km depth, greater than 2.5 Ga old) projecting into the asthenosphere (e.g., Jordan, 1978; Richardson et al., 1984). This has mystified Earth scientists as the dynamic and relatively hot asthenosphere should have eroded away these keels over time (e.g., Sleep, 2003; O'Neill et al., 2008; Karato, 2010). Three key factors have been invoked to explain cratonic root survival: 1) Low density makes the cratonic mantle buoyant (e.g., Poudjom Djomani et al., 2001). 2) Low temperatures (e.g., Pollack, 1986; Boyd, 1987), and 3) low water contents (e.g., Pollack, 1986), would make cratonic roots mechanically strong. Here we address the mechanism of the longevity of continental mantle lithosphere by focusing on the water parameter. Although nominally anhydrous , olivine, pyroxene and garnet can accommodate trace amounts of water in the form of H bonded to structural O in mineral defects (e.g., Bell and Rossman, 1992). Olivine softens by orders of magnitude if water (1-1000 ppm H2O) is added to its structure (e.g., Mackwell et al., 1985). Our recent work has placed constraints on the distribution of water measured in peridotite minerals in the cratonic root beneath the Kaapvaal in southern Africa (Peslier et al., 2010). At P greater than 5 GPa, the water contents of pyroxene remain relatively constant while those of olivine systematically decrease from 50 to less than 10 ppm H2O at 6.4 GPa. We hypothesized that at P greater than 6.4 GPa, i.e. at the bottom of the cratonic lithosphere, olivines are essentially dry (greater than 10 ppm H2O). As olivine likely controls the rheology of the mantle, we calculated that the dry olivines could be responsible for a contrast in viscosity between cratonic lithosphere and surrounding asthenosphere large enough to explain the resistance of cratonic root to asthenospheric delamination.

Lithospheric Structure of the Yamato Basin Inferred from Trans-dimensional Inversion of Receiver Functions

NASA Astrophysics Data System (ADS)

Akuhara, T.; Nakahigashi, K.; Shinohara, M.; Yamada, T.; Yamashita, Y.; Shiobara, H.; Mochizuki, K.

2017-12-01

The Yamato Basin, located at the southeast of the Japan Sea, has been formed by the back-arc opening of the Japan Sea. Wide-angle reflection surveys have revealed that the basin has anomalously thickened crust compared with a normal oceanic crust [e.g., Nakahigashi et al., 2013] while deeper lithospheric structure has not known so far. Revealing the lithospheric structure of the Yamato Basin will lead to better understanding of the formation process of the Japan Sea and thus the Japanese island. In this study, as a first step toward understanding the lithospheric structure, we aim to detect the lithosphere-asthenosphere boundary (LAB) using receiver functions (RFs). We use teleseismic P waveforms recorded by broad-band ocean-bottom seismometers (BBOBS) deployed at the Yamato Basin. We calculated radial-component RFs using the data with the removal of water reverberations from the vertical-component records [Akuhara et al., 2016]. The resultant RFs are more complicated than those calculated at an on-land station, most likely due to sediment-related reverberations. This complexity does not allow either direct detection of a Ps conversion from the LAB or forward modeling by a simple structure composed of a handful number of layers. To overcome this difficulty, we conducted trans-dimensional Markov Chain Monte Carlo inversion of RFs, where we do not need to assume the number of layers in advance [e.g., Bodin et al., 2012; Sambridge et al., 2014]. Our preliminary results show abrupt velocity reduction at 70 km depth, far greater depth than the expected LAB depth from the age of the lithosphere ( 20 Ma, although still debated). If this low-velocity jump truly reflects the LAB, the anomalously thickened lithosphere will provide a new constraint on the complex formation history of the Japan Sea. Further study, however, is required to deny the possibility that the obtained velocity jump is an artificial brought by the overfitting of noisy data.

BASE Flexible Array Preliminary Lithospheric Structure Analysis

NASA Astrophysics Data System (ADS)

Yeck, W. L.; Sheehan, A. F.; Anderson, M. L.; Siddoway, C. S.; Erslev, E.; Harder, S. H.; Miller, K. C.

2009-12-01

The Bighorns Arch Seismic Experiment (BASE) is a Flexible Array experiment integrated with EarthScope. The goal of BASE is to develop a better understanding of how basement-involved foreland arches form and what their link is to plate tectonic processes. To achieve this goal, the crustal structure under the Bighorn Mountain range, Bighorn Basin, and Powder River Basin of northern Wyoming and southern Montana are investigated through the deployment of 35 broadband seismometers, 200 short period seismometers, 1600 “Texan” instruments using active sources and 800 “Texan” instruments monitoring passive sources, together with field structural analysis of brittle structures. The novel combination of these approaches and anticipated simultaneous data inversion will give a detailed structural crustal image of the Bighorn region at all levels of the crust. Four models have been proposed for the formation of the Bighorn foreland arch: subhorizontal detachment within the crust, lithospheric buckling, pure shear lithospheric thickening, and fault blocks defined by lithosphere-penetrating thrust faults. During the summer of 2009, we deployed 35 broadband instruments, which have already recorded several magnitude 7+ teleseismic events. Through P wave receiver function analysis of these 35 stations folded in with many EarthScope Transportable Array stations in the region, we present a preliminary map of the Mohorovicic discontinuity. This crustal map is our first test of how the unique Moho geometries predicted by the four hypothesized models of basement involved arches fit seismic observations for the Bighorn Mountains. In addition, shear-wave splitting analysis for our first few recorded teleseisms helps us determine if strong lithospheric deformation is preserved under the range. These analyses help lead us to our final goal, a complete 4D (3D spatial plus temporal) lithospheric-scale model of arch formation which will advance our understanding of the mechanisms accommodating and driving basement-involved arch formation as well as continental lithospheric rheology.

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.

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.

Structure of the Lithosphere and Upper Mantle Across the Arabian Peninsula

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

Al-Amri, A; Rodgers, A

2007-01-05

Analysis of modern broadband (BB) waveform data allows for the inference of seismic velocity structure of the crust and upper mantle using a variety of techniques. This presentation will report inferences of seismic structure of the Arabian Plate using BB data from various networks. Most data were recorded by the Saudi Arabian National Digital Seismic Network (SANDSN) which consists of 38 (26 BB, 11 SP) stations, mostly located on the Arabian Shield. Additional data were taken from the 1995-7 Saudi Arabian IRIS-PASSCAL Deployment (9 BB stations) and other stations across the Peninsula. Crustal structure, inferred from teleseismic P-wave receiver functions,more » reveals thicker crust in the Arabian Platform (40-45 km) and the interior of the Arabian Shield (35-40 km) and thinner crust along the Red Sea coast. Lithospheric thickness inferred from teleseismic S-wave receiver functions reveals very thin lithosphere (40-80 km) along the Red Sea coast which thickens rapidly toward the interior of the Arabian Shield (100-120 km). We also observe a step of 20-40 km in lithospheric thickness across the Shield-Platform boundary. Seismic velocity structure of the upper mantle inferred from teleseismic P- and S-wave travel time tomography reveals large differences between the Shield and Platform, with the Shield being underlain by slower velocities, {+-}3% for P-waves and {+-}6% for S-waves. Seismic anisotropy was inferred from shear-wave splitting, using teleseismic SKS waveforms. Results reveal a splitting time of approximately 1.4 seconds, with the fast axis slightly east of north. The shear-wave splitting results are consistent across the Peninsula, with a slight clockwise rotation parallel for stations near the Gulf of Aqaba. In summary, these results allow us to make several conclusions about the tectonic evolution and current state of the Arabian Plate. Lithospheric thickness implies that thinning near the Red Sea has accompanied the rupturing of the Arabian-Nubian continental lithosphere. The step in the lithospheric thickness across the Shield-Platform boundary likely reveals a pre-existing difference in the lithospheric structure prior to accretion of the terranes composing the eastern Arabian Shield. Tomographic imaging of upper mantle velocities implies a single large-scale thermal anomaly underlies the Arabian Shield and is associated with Cenozoic uplift and volcanism.« less

The magma ocean as an impediment to lunar plate tectonics

NASA Technical Reports Server (NTRS)

Warren, Paul H.

1993-01-01

The primary impediment to plate tectonics on the moon was probably the great thickness of its crust and particularly its high crust/lithosphere thickness ratio. This in turn can be attributed to the preponderance of low-density feldspar over all other Al-compatible phases in the lunar interior. During the magma ocean epoch, the moon's crust/lithosphere thickness ratio was at the maximum theoretical value, approximately 1, and it remained high for a long time afterwards. A few large regions of thin crust were produced by basin-scale cratering approximately contemporaneous with the demise of the magma ocean. However, these regions probably also tend to have uncommonly thin lithosphere, since they were directly heated and indirectly enriched in K, Th, and U by the same cratering process. Thus, plate tectonics on the moon in the form of systematic lithosphere subduction was impeded by the magma ocean.

Tomography images of the Alpine roots and surrounding upper mantle

NASA Astrophysics Data System (ADS)

Plomerova, Jaroslava; Babuska, Vladislav

2017-04-01

Teleseismic body-wave tomography represents powerful tool to study regional velocity structure of the upper mantle and to image velocity anomalies, such as subducted lithosphere plates in collisional zones. In this contribution, we recapitulate 3D models of the upper mantle beneath the Alps, which developed at a collision zone of the Eurasian and African plates. Seismic tomography studies indicate a leading role of the rigid mantle lithosphere that functioned as a major stress guide during the plate collisions. Interactions of the European lithosphere with several micro-plates in the south resulted in an arcuate shape of this mountain range on the surface and in a complicated geometry of the Alpine subductions in the mantle. Early models with one bended lithosphere root have been replaced with more advanced models showing two separate lithosphere roots beneath the Western and Eastern Alps (Babuska et al., Tectonophysics 1990; Lippitsch et al., JGR 2003). The standard isotropic velocity tomography, based on pre-AlpArray data (the currently performed passive seismic experiment in the Alps and surroundings) images the south-eastward dipping curved slab of the Eurasian lithosphere in the Western Alps. On the contrary, beneath the Eastern Alps the results indicate a very steep northward dipping root that resulted from the collision of the European plate with the Adriatic microplate. Dando et al. (2011) interpret high-velocity heterogeneities at the bottom of their regional tomographic model as a graveyard of old subducted lithospheres. High density of stations, large amount of rays and dense ray-coverage of the volume studied are not the only essential pre-requisites for reliable tomography results. A compromise between the amount of pre-processed data and the high-quality of the tomography input (travel-time residuals) is of the high importance as well. For the first time, the existence of two separate roots beneath the Alps has been revealed from carefully pre-processed, mostly the ISC-bulletin data (Babuska et al., Tectonophysics 1990). Calculated relative travel-time residuals have been assigned to source clusters and filtered relative to the residual mean of each cluster of events. We expect that future 3D studies of the mantle velocities and mantle fabrics with the use of body-wave anisotropic parameters from the AlpArray data will shed a new light on tectonic development of the complex Alpine region and its surroundings.

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.

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.

Mars Express 10 years at Mars: Observations by the Mars Express Radio Science Experiment (MaRS)

NASA Astrophysics Data System (ADS)

Pätzold, M.; Häusler, B.; Tyler, G. L.; Andert, T.; Asmar, S. W.; Bird, M. K.; Dehant, V.; Hinson, D. P.; Rosenblatt, P.; Simpson, R. A.; Tellmann, S.; Withers, P.; Beuthe, M.; Efimov, A. I.; Hahn, M.; Kahan, D.; Le Maistre, S.; Oschlisniok, J.; Peter, K.; Remus, S.

2016-08-01

The Mars Express spacecraft is operating in Mars orbit since early 2004. The Mars Express Radio Science Experiment (MaRS) employs the spacecraft and ground station radio systems (i) to conduct radio occultations of the atmosphere and ionosphere to obtain vertical profiles of temperature, pressure, neutral number densities and electron density, (ii) to conduct bistatic radar experiments to obtain information on the dielectric and scattering properties of the surface, (iii) to investigate the structure and variation of the crust and lithosphere in selected target areas, (iv) to determine the mass, bulk and internal structure of the moon Phobos, and (v) to track the MEX radio signals during superior solar conjunction to study the morphology of coronal mass ejections (CMEs). Here we report observations, results and discoveries made in the Mars environment between 2004 and 2014 over almost an entire solar cycle.

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.

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.

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.

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.

A New Comprehensive Model for Crustal and Upper Mantle Structure of the European Plate

NASA Astrophysics Data System (ADS)

Morelli, A.; Danecek, P.; Molinari, I.; Postpischl, L.; Schivardi, R.; Serretti, P.; Tondi, M. R.

2009-12-01

We present a new comprehensive model of crustal and upper mantle structure of the whole European Plate — from the North Atlantic ridge to Urals, and from North Africa to the North Pole — describing seismic speeds (P and S) and density. Our description of crustal structure merges information from previous studies: large-scale compilations, seismic prospection, receiver functions, inversion of surface wave dispersion measurements and Green functions from noise correlation. We use a simple description of crustal structure, with laterally-varying sediment and cristalline layers thickness and seismic parameters. Most original information refers to P-wave speed, from which we derive S speed and density from scaling relations. This a priori crustal model by itself improves the overall fit to observed Bouguer anomaly maps, as derived from GRACE satellite data, over CRUST2.0. The new crustal model is then used as a constraint in the inversion for mantle shear wave speed, based on fitting Love and Rayleigh surface wave dispersion. In the inversion for transversely isotropic mantle structure, we use group speed measurements made on European event-to-station paths, and use a global a priori model (S20RTS) to ensure fair rendition of earth structure at depth and in border areas with little coverage from our data. The new mantle model sensibly improves over global S models in the imaging of shallow asthenospheric (slow) anomalies beneath the Alpine mobile belt, and fast lithospheric signatures under the two main Mediterranean subduction systems (Aegean and Tyrrhenian). We map compressional wave speed inverting ISC travel times (reprocessed by Engdahl et al.) with a non linear inversion scheme making use of finite-difference travel time calculation. The inversion is based on an a priori model obtained by scaling the 3D mantle S-wave speed to P. The new model substantially confirms images of descending lithospheric slabs and back-arc shallow asthenospheric regions, shown in other more local high-resolution tomographic studies, but covers the whole range of the European Plate. We also obtain three-dimensional mantle density structure by inversion of GRACE Bouguer anomalies locally adjusting density and the scaling relation between seismic wave speeds and density. We validate the new comprehensive model through comparison of recorded seismograms with numerical simulations based on SPECFEM3D. This work is a contribution towards the definition of a reference earth model for Europe. To this extent, in order to improve model dissemination and comparison, we propose the adoption of a common exchange format for tomographic earth models based on JSON, a lightweight data-interchange format supported by most high-level programming languages. We provide tools for manipulating and visualising models, described in this standard format, in Google Earth and GEON IDV.

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.

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.

Hemispheric dichotomy in lithosphere growth on Mars caused by differences in crustal composition

NASA Astrophysics Data System (ADS)

Thiriet, M.; Michaut, C.; Breuer, D.

2016-12-01

The surface dichotomy is the most striking feature of Mars. The Northern hemisphere is covered by extensive lava plains and is lower in altitude than the South which has higher and sharper reliefs and is more craterized and older than the North. Recent studies have suggested that this bimodal distribution of altitudes could be due to the existence of a buried felsic component similar to the terrestrial continental crust in the Southern hemisphere. The presence of a large buried component of evolved composition might imply an enrichment in incompatible radioactive elements. The thermal surface properties of the two hemispheres also seem to differ; the South shows fine-particulate materials probably resulting from explosive volcanism, while the Northern lava flows are more consolidated and characterized by a higher thermal conductivity. Using a parameterized convection model with a stagnant lid, we computed the thermal evolution and lithosphere growth of Mars accounting for potential differences in the thermal parameters characterizing the Northern and Southern crusts. We find that a stronger enrichment in radioactive elements and a lower surface conductivity in the South can cause a significant difference in elastic thickness of the lithosphere in between both hemispheres, with an elastic lithosphere thicker in the North by several tens of kilometers today. This result might explain the large and still unexplained difference in lithosphere elastic thickness estimated below the two polar caps, which is about 300 km in the North and only 140 km in the South. Assuming a crust in the Northern hemisphere with a thickness of 40 km, a density of 3000 kg/m3 and an enrichment factor in radioactive elements of 5 relative to the primitive mantle, Monte Carlo inversions show that the Southern crust requires a thickness of >60 km, a density between 2700 and 3000 kg/m3 and an enrichment factor of 13-20 to explain such a difference in lithosphere elastic thickness.

3D Rheological Modeling of NW Intraplate Europe, Deciphering Spatial Integrated strength patterns, Mechanical Strong Layering and EET

NASA Astrophysics Data System (ADS)

Beekman, F.; Hardebol, N.; Cloetingh, S.; Tesauro, M.

2006-12-01

Better understanding of 3D rheological heterogeneity of the European Lithosphere provide the key to tie the recorded intraplate deformation pattern to stress fields transmitted into plate interior from plate boundary forces. The first order strain patterns result from stresses transmitted through the European lithosphere that is marked by a patchwork of high strength variability from inherited structural and compositional heterogeneities and upper mantle thermal perturbations. As the lithospheric rheology depends primarily on its spatial structure, composition and thermal estate, the 3D strength model for the European lithosphere relies on a 3D compositional model that yields the compositional heterogeneities and an iteratively calculated thermal cube using Fouriers law for heat conduction. The accurate appraisal of spatial strength variability results from proper mapping and integration of the geophysical compositional and thermal input parameters. Therefore, much attention has been paid to a proper description of first order structural and tectonic features that facilitate compilation of the compositional and thermal input models. As such, the 3D strength model reflects the thermo-mechanical structure inherited from the Europeans polyphase deformation history. Major 3D spatial mechanical strength variability has been revealed. The East-European and Fennoscandian Craton to the NE exhibit high strength (30-50 1012 N/m) from low mantle temperatures and surface heatflow of 35-60 mW/m2 while central and western Europe reflect a polyphase Phanerozoic thermo- tectonic history. Here, regions with high rigidity are formed primarily by patches of thermally stabilized Variscan Massifs (e.g. Rhenish, Armorican, Bohemian, and Iberian Massif) with low heatflow and lithospheric thickness values (50-65 mW/m2; 110-150 km) yielding strengths of ~15-25 1012 N/m. In contrast, major axis of weakened lithosphere coincides with Cenozoic Rift System (e.g. Upper and Lower Rhine Grabens, Pannonian Basin and Massif Central) attributed to the presence of tomographically imaged plumes. This study has elucidated the memory of the present-days Europeans lithosphere induced by compositional and thermal heterogeneities. The resulting lateral strength variations has a clear signature of the pst lithospheres polyphase deformation and also entails active tectonics, tectonically induced topography and surface processes.

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.

The crust and upper mantle of central East Greenland - implications for continental accretion and rift evolution

NASA Astrophysics Data System (ADS)

Schiffer, Christian; Balling, Niels; Ebbing, Jörg; Holm Jacobsen, Bo; Bom Nielsen, Søren

2016-04-01

The geological evolution of the North Atlantic Realm during the past 450 Myr, which has shaped the present-day topographic, crustal and upper mantle features, was dominated by the Caledonian orogeny and the formation of the North Atlantic and associated igneous activity. The distinct high altitude-low relief landscapes that accompany the North Atlantic rifted passive margins are the focus of a discussion of whether they are remnant and modified Caledonian features or, alternatively, recently uplifted peneplains. Teleseismic receiver function analysis of 11 broadband seismometers in the Central Fjord Region in East Greenland indicates the presence of a fossil subduction complex, including a slab of eclogitised mafic crust and an overlying wedge of hydrated mantle peridotite. This model is generally consistent with gravity and topography. It is shown that the entire structure including crustal thickness variations and sub-Moho heterogeneity gives a superior gravity and isostatic topographic fit compared to a model with a homogeneous lithospheric layer (1). The high topography of >1000 m in the western part of the area is supported by the c. 40 km thick crust. The eastern part requires buoyancy from the low velocity/low density mantle wedge. The geometry, velocities and densities are consistent with structures associated with a fossil subduction zone. The spatial relations with Caledonian structures suggest a Caledonian origin. The results indicate that topography is isostatically compensated by density variations within the lithosphere and that significant present-day dynamic topography seems not to be required. Further, this structure is suggested to be geophysically very similar to the Flannan reflector imaged north of Scotland, and that these are the remnants of the same fossil subduction zone, broken apart and separated during the formation of the North Atlantic in the early Cenozoic (2). 1) Schiffer, C., Jacobsen, B.H., Balling, N., Ebbing, J. and Nielsen, S.B., 2015. The East Greenland Caledonides - teleseismic signature, gravity and isostasy. Geophysical Journal International, 203, 1400-1418. 2) Schiffer, C., Stephenson, R.A., Petersen, K.D., Nielsen, S.B., Jacobsen, B.H., Balling, N. and Macdonald, D.I.M., 2015. A sub-crustal piercing point for North Atlantic reconstructions and tectonic implications. Geology, 43, 1087-1090.

Lithospheric structure of east Asia from ambient noise and two-station Rayleigh wave tomography

NASA Astrophysics Data System (ADS)

Li, M.; Song, X.; Li, J.; Bao, X.

2017-12-01

The complex tectonic background of east Asia makes it an ideal region to investigate the evolution of continental lithosphere. High-resolution lithospheric structure models are essential in this endeavor. Surface-wave tomography has been an important technique for constructing 3D lithospheric structure in global and regional scales. In this study, using event data recorded by more than 1000 seismic stations from multiple national and international networks in and surrounding China (CEArray, PASSCAL, GSN), we systematically measured Rayleigh-wave phase-velocity dispersion curves at periods 10-120 s and group-velocity dispersion curves at periods 10-140 s based on the traditional two-station method. The dispersion curves were extracted from the cross-correlation functions of the earthquake data at the two stations near the great circle path using frequency-time analysis method. The new measurements extend the phase and group dispersion data to longer periods (i.e. >70 s), which are difficult to extract from ambient noise cross-correlation. The longer-period data allow us to image deeper lithospheric velocity structure. We combined the new dispersion measurements with two previously obtained data sets: (1) data set from Bao et al. (2015) across the Chinese continent that includes group and phase dispersion measurements from ambient noise correlations and group velocity measurements from earthquakes, and (2) data set from Wang et al. (2017) across the marginal seas in east Asia from ambient noise correlations. We used the combined data set to invert for the phase velocity maps up to 120 s and group velocity maps up to 140 s at a grid spacing of 0.5°×0.5°and then invert for the 1D shear-wave velocity structure at each grid to obtain the new 3D shear-wave velocity model. The new model is generally consistent with that of Bao et al. (2015) but with improved resolution particularly in greater depths and in east-Asia marginal seas. We also derived crustal thickness and lithospheric thickness models. The lithospheric thickness model shows strong spatial heterogeneity and thinning trend from west to east in our study region. These models reveal important lithospheric features beneath east Asia and provide a fundamental data set for understanding continental dynamics and evolution.

Control of deep lithospheric roots on crustal scale GOCE gravity and gradient fields evident in Gondwana reconstructions

NASA Astrophysics Data System (ADS)

Braitenberg, Carla; Mariani, Patrizia

2015-04-01

The GOCE gravity field is globally homogeneous at the resolution of about 80km or better allowing for the first time to analyze tectonic structures at continental scale. Geologic correlation studies based on age determination and mineral composition of rock samples propose to continue the tectonic lineaments across continents to the pre-breakup position. Tectonic events which induce density changes, as metamorphic events and magmatic events, should then show up in the gravity field. Therefore gravity can be used as a globally available supportive tool for interpolation of isolated samples. Applying geodynamic plate reconstructions to the GOCE gravity field places today's observed field at the pre-breakup position. In order to test the possible deep control of the crustal features, the same reconstruction is applied to the seismic velocity models, and a joint gravity-velocity analysis is performed. The geophysical fields allow to control the likeliness of the hypothesized continuation of lineations based on sparse surface outcrops. Total absence of a signal, makes the cross-continental continuation of the lineament improbable, as continental-wide lineaments are controlled by rheologic and compositional differences of lithospheric mantle. It is found that the deep lithospheric roots as those found below cratons control the position of the positive gravity values. The explanation is that the deep lithospheric roots focus asthenospheric upwelling outboard of the root protecting the overlying craton from magmatic intrusions. The study is carried out over the African and South American continents. The background for the study can be found in the following publications where the techniques which have been used are described: Braitenberg, C., Mariani, P. and De Min, A. (2013). The European Alps and nearby orogenic belts sensed by GOCE, Boll. Bollettino di Geofisica Teorica ed Applicata, 54(4), 321-334. doi:10.4430/bgta0105 Braitenberg, C. and Mariani, P. (2015). Geological implications from complete Gondwana GOCE-products reconstructions and link to lithospheric roots. Proceedings of 5th International GOCE User Workshop, 25 - 28 November 2014. Braitenberg, C. (2015). Exploration of tectonic structures with GOCE in Africa and across-continents. Int. J.Appl. Earth Observ. Geoinf. 35, 88-95. http://dx.doi.org/10.1016/j.jag.2014.01.013 Braitenberg, C. (2015). A grip on geological units with GOCE, IAG Symp. 141, in press.

Evolution of Meso-Cenozoic lithospheric thermal-rheological structure in the Jiyang sub-basin, Bohai Bay Basin, eastern North China Craton

NASA Astrophysics Data System (ADS)

Xu, Wei; Qiu, Nansheng; Wang, Ye; Chang, Jian

2018-01-01

The Meso-Cenozoic lithospheric thermal-rheological structure and lithospheric strength evolution of the Jiyang sub-basin were modeled using thermal history, crustal structure, and rheological parameter data. Results indicate that the thermal-rheological structure of the Jiyang sub-basin has exhibited obvious rheological stratification and changes over time. During the Early Mesozoic, the uppermost portion of the upper crust, middle crust, and the top part of the upper mantle had a thick brittle layer. During the early Early Cretaceous, the top of the middle crust's brittle layer thinned because of lithosphere thinning and temperature increase, and the uppermost portion of the upper mantle was almost occupied by a ductile layer. During the late Early Cretaceous, the brittle layer of the middle crust and the upper mantle changed to a ductile one. Then, the uppermost portion of the middle crust changed to a thin brittle layer in the late Cretaceous. During the early Paleogene, the thin brittle layer of the middle crust became even thinner and shallower under the condition of crustal extension. Currently, with the decrease in lithospheric temperature, the top of the upper crust, middle crust, and the uppermost portion of the upper mantle are of a brittle layer. The total lithospheric strength and the effective elastic thickness ( T e) in Meso-Cenozoic indicate that the Jiyang sub-basin experienced two weakened stages: during the late Early Cretaceous and the early Paleogene. The total lithospheric strength (approximately 4-5 × 1013 N m-1) and T e (approximately 50-60 km) during the Early Mesozoic was larger than that after the Late Jurassic (2-7 × 1012 N m-1 and 19-39 km, respectively). The results also reflect the subduction, and rollback of Pacific plate is the geodynamic mechanism of the destruction of the eastern North China Craton.

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.

Re-thinking the Laramide: Investigating the role of fluids in producing surface uplift using xenolith mineralogy and geochronology

NASA Astrophysics Data System (ADS)

Butcher, Lesley Ann

High-temperature, high-pressure mineral assemblages preserved in much of the North American lithosphere owe their origins to Archean and Proterozoic tectonic processes. Whether subsequent mechanical, thermal, or chemical modification of ancient lithosphere affects overlying crust and the extent to which such processes contribute to anomalous deformation and topography is the interior of continents is poorly understood. This study addresses the occurrence and effects of hydration on continental crust in producing regionally elevated topography in the Colorado Plateau since the Late Cretaceous. Mineralogical characteristics of two deep crustal xenoliths (GR-11 and RM-21) from the Four Corners Volcanic field record varying degrees of hydrous alteration including extensive replacement of garnet by hornblende, secondary albite and phengite growth at the expense of primary plagioclase, and secondary monazite growth in association with fluid-related allanite and plagioclase breakdown. Results from forward petrological modeling for both deep crustal xenoliths are consistent with hydration at greater than 20 km depth prior to exhumation in the ~20 Ma volcanic host. In situ Th/Pb dating provides evidence for a finite period of fluid-related monazite crystallization in xenolith RM-21 from 91 +/- 2.8 Ma to 58 +/- 4 Ma, concurrent with timing estimates of low-angle subduction of the Farallon slab. Hydration-related reactions at depth lead to a net density decrease as low-density hydrous phases (hbl+/-ab+/-phg) grow at the expense of high-density, anhydrous minerals (gt+/-pl) abundant in unaltered Proterozoic crust. If these reactions are sufficiently pervasive and widespread, reductions in lower crustal density would provide a significant and quantifiable source of lithospheric buoyancy. Calculations for density decreases associated with extensive hydration recorded in xenolith GR-11 for an ~25 km thick crustal layer yield uplift estimates on the order of hundreds of meters associated with phase changes at depth. The results of this study substantiate the hypothesis that chemical alteration of lower continental crust by slab-derived fluids played a role in producing Laramide-related surface uplift of the Colorado Plateau and establishes chemical modification of continental lithosphere as a credible possibility for producing elevated regional topography in continental interiors.

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.

Estimates of the topographic uplift of the Southern African Plateau from the African Superswell through petrologically-consistent thermo-chemical modelling of the geoid, SHF, Rayleigh and Love dispersion curves and MT data

NASA Astrophysics Data System (ADS)

Jones, Alan G.; Afonso, Juan Carlos; Fullea, Javier

2015-04-01

The deep mantle African Superswell is thought to cause up to 500 m of the uplift of the Southern African Plateau. We investigate this phenomenon through stochastic thermo-chemical inversion modelling of the geoid, surface heat flow, Rayleigh and Love dispersion curves and MT data, in a manner that is fully petrologically-consistent. We invert for a three layer crustal velocity, density and thermal structure, but assume the resistivity layering (based on prior inversion of the MT data alone). Inversions are performed using an improved Delayed Rejection and Adaptive Metropolis (DRAM) type Markov chain Monte Carlo (MCMC) algorithm. We demonstrate that a single layer lithosphere can fit most of the data, but not the MT responses. We further demonstrate that modelling the seismic data alone, without the constraint of requiring reasonable oxide chemistry or of fitting the geoid, permits wildly acceptable elevations and with very poorly defined lithosphere-asthenosphere boundary (LAB). We parameterise the lithosphere into three layers, and bound the permitted oxide chemistry of each layer consistent with known chemical layering. We find acceptable models, from 5 million tested in each case, that fit all responses and yield a posteriori elevation distributions centred on 900-950 m, suggesting dynamic support from the lower mantle of some 400 m.

Dynamics and the Wilson Cycle: An EarthScope vision

NASA Astrophysics Data System (ADS)

Ebinger, Cynthia; Humphreys, Eugene; Williams, Michael; van der Lee, Suzan; Levin, Vadim; Webb, Laura; Becker, Thorsten

2017-04-01

Wilson's model has two major components, each with distinctive observables. Initial subduction of ocean lithosphere collides continents across a closing ocean basin, creating a mountain range; rifting then initiates within the collisional orogeny and progresses to create oceanic spreading and creation of a new ocean basin. Subduction eventually initiates near the old, cold, and heavily sedimented continental margin, leading to subduction, and repeating the cycle. This model is largely kinematic in nature, and predictive in application. We re-evaluate the Wilson Cycle in light of process-oriented perspectives afforded by the surface to mantle Earthscope results. Repeating episodes of mountain building by means of continental collisions remains clear, but new observations augment or diverge from Wilson's concepts. A 'new' component stems from observations from both the East and West coasts: translational fault systems played critical roles in continental accretion, collision, and rifting. Earthscope data sets also have enabled imaging of the structure of western U.S. lithosphere with unprecedented detail. From new and existing data sets, we conclude that collision occurs in 'ribbons' in large part linked to the shapes of the landmasses colliding landmasses, and deformation includes a major component of transform tectonics. Post-orogenic gravitational collapse may occur far inboard of the site of collision. A third 'new' feature is that plate coupling with the mantle leads to deformation outside the classic Wilson Cycle. For example, the passive margin of eastern N. America shows tectonic activity, uplift, and magmatism long after the onset of seafloor spreading, demonstrating the dynamic nature of lithosphere-asthenosphere coupling. A 'fourth' observation is that lateral density contrasts and volatile migration during subduction and collision effectively refertilize mantle lithosphere, and pre-condition later tectonic cycles.

3D Thermomechanical Modeling of Rifted Margins with Coupled Surface Processes: the North West Shelf, Australia

NASA Astrophysics Data System (ADS)

Moresi, L. N.; Beucher, R.; Morón, S.; Rey, P. F.; Salles, T.; Brocard, G. Y.; Farrington, R.; Giordani, J.; Mansour, J.

2017-12-01

Thermo-mechanical numerical models and analogue experiments with a layered lithosphere have emphasised the role played by the composition and thermal state of the lithosphere on the style of extension. The variation in rheological properties and the coupling between lithospheric layers promote depth-dependent extension with the potential for complex rift evolution over space and time. Local changes in the stress field due to loading / unloading of the lithosphere can perturb the syn and post-rift stability of the margins. We investigate how erosion of the margins and sedimentation within the basins integrate with the thermo-mechanical processes involved in the structural and stratigraphic evolution of the North West Shelf (NWS), one of the most productive and prospective hydrocarbon provinces in Australia. The complex structural characteristics of the NWS include large-scale extensional detachments, difference between amounts of crustal and lithospheric extension and prolonged episodes of thermal sagging after rifting episodes. It has been proposed that the succession of different extensional styles mechanisms (Cambrian detachment faulting, broadly distributed Permo-Carboniferous extension and Late Triassic to Early Cretaceous localised rift development) is best described in terms of variation in deformation response of a lithosphere that has strengthened from one extensional episode to the next. However, previous models invoking large-scale detachments fail to explain changes in extensional styles and overestimate the structural importance of relatively local detachments. Here, we hypothesize that an initially weak lithosphere would distribute deformation by ductile flow within the lower crust and that the interaction between crustal flow, thermal-evolution and sediment loading/unloading could explain some of the structural complexities recorded by the NWS. We run a series of fully coupled 3D thermo-mechanical numerical experiments that include realistic thermal and mechanical properties, as well as surface processes (erosion, sediments transport and sedimentation). This modeling approach aims to provide insights into the thermal and structural history of the NWS, and a better understanding of the complex interactions between tectonics and surface processes at rifted margins.

Structure and tectonics of the northwestern United States from EarthScope USArray magnetotelluric data

USGS Publications Warehouse

Bedrosian, Paul A.; Feucht, Daniel W.

2014-01-01

The magnetotelluric component of the EarthScope USArray program has covered over 35% of the continental United States. Resistivity tomography models derived from these data image lithospheric structure and provide constraints on the distribution of fluids and melt within the lithosphere. We present a three-dimensional resistivity model of the northwestern United States which provides new insight into the tectonic assembly of western North America from the Archean to present. Comparison with seismic tomography models reveals regions of correlated and anti-correlated resistivity and velocity that help identify thermal and compositional variations within the lithosphere. Recent (Neogene) tectonic features reflected in the model include the subducting Juan de Fuca–Gorda plate which can be traced beneath the forearc to more than 100 km depth, high lithospheric conductivity along the Snake River Plain, and pronounced lower-crustal and upper-mantle conductivity beneath the Basin and Range. The latter is abruptly terminated to the northwest by the Klamath–Blue Mountains Lineament, which we interpret as an important structure during and since the Mesozoic assembly of the region. This boundary is interpreted to separate hot extended lithosphere from colder, less extended lithosphere. The western edge of Proterozoic North America, as indicated by the Cretaceous initial 87Sr/86Sr = 0.706 contour, is clearly reflected in the resistivity model. We further image an Archean crustal block (“Pend Oreille block”) straddling the Washington/Idaho border, which we speculate separated from the Archean Medicine Hat block in the Proterozoic. Finally, in the modern Cascades forearc, the geometry and internal structure of the Eocene Siletz terrane is reflected in the resistivity model. The apparent eastern edge of the Siletz terrane under the Cascades arc suggests that pre-Tertiary rocks fill the Washington and Oregon back-arc.

Lithospheric architecture of NE China from joint Inversions of receiver functions and surface wave dispersion through Bayesian optimisation

NASA Astrophysics Data System (ADS)

Sebastian, Nita; Kim, Seongryong; Tkalčić, Hrvoje; Sippl, Christian

2017-04-01

The purpose of this study is to develop an integrated inference on the lithospheric structure of NE China using three passive seismic networks comprised of 92 stations. The NE China plain consists of complex lithospheric domains characterised by the co-existence of complex geodynamic processes such as crustal thinning, active intraplate cenozoic volcanism and low velocity anomalies. To estimate lithospheric structures with greater detail, we chose to perform the joint inversion of independent data sets such as receiver functions and surface wave dispersion curves (group and phase velocity). We perform a joint inversion based on principles of Bayesian transdimensional optimisation techniques (Kim etal., 2016). Unlike in the previous studies of NE China, the complexity of the model is determined from the data in the first stage of the inversion, and the data uncertainty is computed based on Bayesian statistics in the second stage of the inversion. The computed crustal properties are retrieved from an ensemble of probable models. We obtain major structural inferences with well constrained absolute velocity estimates, which are vital for inferring properties of the lithosphere and bulk crustal Vp/Vs ratio. The Vp/Vs estimate obtained from joint inversions confirms the high Vp/Vs ratio ( 1.98) obtained using the H-Kappa method beneath some stations. Moreover, we could confirm the existence of a lower crustal velocity beneath several stations (eg: station SHS) within the NE China plain. Based on these findings we attempt to identify a plausible origin for structural complexity. We compile a high-resolution 3D image of the lithospheric architecture of the NE China plain.

The lithospheric-scale 3D structural configuration of the North Alpine Foreland Basin constrained by gravity modelling and the calculation of the 3D load distribution

NASA Astrophysics Data System (ADS)

Przybycin, Anna M.; Scheck-Wenderoth, Magdalena; Schneider, Michael

2014-05-01

The North Alpine Foreland Basin is situated in the northern front of the European Alps and extends over parts of France, Switzerland, Germany and Austria. It formed as a wedge shaped depression since the Tertiary in consequence of the Euro - Adriatic continental collision and the Alpine orogeny. The basin is filled with clastic sediments, the Molasse, originating from erosional processes of the Alps and underlain by Mesozoic sedimentary successions and a Paleozoic crystalline crust. For our study we have focused on the German part of the basin. To investigate the deep structure, the isostatic state and the load distribution of this region we have constructed a 3D structural model of the basin and the Alpine area using available depth and thickness maps, regional scale 3D structural models as well as seismic and well data for the sedimentary part. The crust (from the top Paleozoic down to the Moho (Grad et al. 2008)) has been considered as two-parted with a lighter upper crust and a denser lower crust; the partition has been calculated following the approach of isostatic equilibrium of Pratt (1855). By implementing a seismic Lithosphere-Asthenosphere-Boundary (LAB) (Tesauro 2009) the crustal scale model has been extended to the lithospheric-scale. The layer geometry and the assigned bulk densities of this starting model have been constrained by means of 3D gravity modelling (BGI, 2012). Afterwards the 3D load distribution has been calculated using a 3D finite element method. Our results show that the North Alpine Foreland Basin is not isostatically balanced and that the configuration of the crystalline crust strongly controls the gravity field in this area. Furthermore, our results show that the basin area is influenced by varying lateral load differences down to a depth of more than 150 km what allows a first order statement of the required compensating horizontal stress needed to prevent gravitational collapse of the system. BGI (2012). The International Gravimetric Bureau. IAG Geodesist's Handbook, 2012 - Journal of Geodesy, 86(10) Springer Grad, M., Tiira, T. and ESC Working Group (2009). The Moho depth map of 1 the European Plate. Geophysical Journal International 176(1): 279-292. Tesauro, M. (2009). An integrated study of the structure and thermomechanical properties of the European lithosphere. Department of Tectonics Faculty of Earth & Life Sciences. Amsterdam, Vrije Universiteit, Dissertation

Inherited structure and coupled crust-mantle lithosphere evolution: Numerical models of Central Australia

NASA Astrophysics Data System (ADS)

Heron, Philip J.; Pysklywec, Russell N.

2016-05-01

Continents have a rich tectonic history that have left lasting crustal impressions. In analyzing Central Australian intraplate orogenesis, complex continental features make it difficult to identify the controls of inherited structure. Here the tectonics of two types of inherited structures (e.g., a thermally enhanced or a rheologically strengthened region) are compared in numerical simulations of continental compression with and without "glacial buzzsaw" erosion. We find that although both inherited structures produce deformation in the upper crust that is confined to areas where material contrasts, patterns of deformation in the deep lithosphere differ significantly. Furthermore, our models infer that glacial buzzsaw erosion has little impact at depth. This tectonic isolation of the mantle lithosphere from glacial processes may further assist in the identification of a controlling inherited structure in intraplate orogenesis. Our models are interpreted in the context of Central Australian tectonics (specifically the Petermann and Alice Springs orogenies).

Application of MAGSAT to lithospheric modeling in South America

NASA Technical Reports Server (NTRS)

Keller, G. R.; Lidiak, E. G. (Principal Investigator)

1983-01-01

Progress in the determination of relations of MAGSAT anomalies to lithospheric structures is reported. The prime emphasis was on a Rayleigh wave study and the determination of both group and phase velocity dispersion.

3-D electrical structure across the Yadong-Gulu rift revealed by magnetotelluric data: New insights on the extension of the upper crust and the geometry of the underthrusting Indian lithospheric slab in southern Tibet

NASA Astrophysics Data System (ADS)

Wang, Gang; Wei, Wenbo; Ye, Gaofeng; Jin, Sheng; Jing, Jianen; Zhang, Letian; Dong, Hao; Xie, Chengliang; Omisore, Busayo O.; Guo, Zeqiu

2017-09-01

The approximately north-south trending Cenozoic Yadong-Gulu rift (YGR) in the eastern Lhasa block is an ideal location to investigate the extensional kinematic mechanism of the upper crust and the deformation characteristics of the Indian lithospheric slab in southern Tibet. The magnetotelluric (MT) method has been widely used in probing subsurface structures at lithospheric scale and is sensitive to high electrically conductive body (conductor). A three-dimensional (3-D) inversion of MT data was conducted to derive the east-west electrical structures across the northern segment of the YGR. The result reveals that the conductors in the middle crust are not continuous in the east-west direction. The deep conductor underneath the YGR is interpreted to result from the tearing of the Indian lithospheric slab. The upper crust to the east of the YGR is significantly intruded by underlying conductors. Based on the features of the 3-D inversion result from this study and other geophysical observations, the formation of the YGR is most likely caused by tearing of the Indian lithospheric slab through the pull of mid-lower crustal conductors that have locally weak strength beneath the YGR.

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

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.

OCT structure, COB location and magmatic type of the S Angolan & SE Brazilian margins from integrated quantitative analysis of deep seismic reflection and gravity anomaly data

NASA Astrophysics Data System (ADS)

Cowie, Leanne; Kusznir, Nick; Horn, Brian

2014-05-01

Integrated quantitative analysis using deep seismic reflection data and gravity inversion have been applied to the S Angolan and SE Brazilian margins to determine OCT structure, COB location and magmatic type. Knowledge of these margin parameters are of critical importance for understanding rifted continental margin formation processes and in evaluating petroleum systems in deep-water frontier oil and gas exploration. The OCT structure, COB location and magmatic type of the S Angolan and SE Brazilian rifted continental margins are much debated; exhumed and serpentinised mantle have been reported at these margins. Gravity anomaly inversion, incorporating a lithosphere thermal gravity anomaly correction, has been used to determine Moho depth, crustal basement thickness and continental lithosphere thinning. Residual Depth Anomaly (RDA) analysis has been used to investigate OCT bathymetric anomalies with respect to expected oceanic bathymetries and subsidence analysis has been used to determine the distribution of continental lithosphere thinning. These techniques have been validated for profiles Lusigal 12 and ISE-01 on the Iberian margin. In addition a joint inversion technique using deep seismic reflection and gravity anomaly data has been applied to the ION-GXT BS1-575 SE Brazil and ION-GXT CS1-2400 S Angola deep seismic reflection lines. The joint inversion method solves for coincident seismic and gravity Moho in the time domain and calculates the lateral variations in crustal basement densities and velocities along the seismic profiles. Gravity inversion, RDA and subsidence analysis along the ION-GXT BS1-575 profile, which crosses the Sao Paulo Plateau and Florianopolis Ridge of the SE Brazilian margin, predict the COB to be located SE of the Florianopolis Ridge. Integrated quantitative analysis shows no evidence for exhumed mantle on this margin profile. The joint inversion technique predicts oceanic crustal thicknesses of between 7 and 8 km thickness with normal oceanic basement seismic velocities and densities. Beneath the Sao Paulo Plateau and Florianopolis Ridge, joint inversion predicts crustal basement thicknesses between 10-15km with high values of basement density and seismic velocities under the Sao Paulo Plateau which are interpreted as indicating a significant magmatic component within the crustal basement. The Sao Paulo Plateau and Florianopolis Ridge are separated by a thin region of crustal basement beneath the salt interpreted as a regional transtensional structure. Sediment corrected RDAs and gravity derived "synthetic" RDAs are of a similar magnitude on oceanic crust, implying negligible mantle dynamic topography. Gravity inversion, RDA and subsidence analysis along the S Angolan ION-GXT CS1-2400 profile suggests that exhumed mantle, corresponding to a magma poor margin, is absent..The thickness of earliest oceanic crust, derived from gravity and deep seismic reflection data, is approximately 7km consistent with the global average oceanic crustal thicknesses. The joint inversion predicts a small difference between oceanic and continental crustal basement density and seismic velocity, with the change in basement density and velocity corresponding to the COB independently determined from RDA and subsidence analysis. The difference between the sediment corrected RDA and that predicted from gravity inversion crustal thickness variation implies that this margin is experiencing approximately 500m of anomalous uplift attributed to mantle dynamic uplift.

Gravity tectonics and seismic gaps in the mantle

NASA Technical Reports Server (NTRS)

Liu, H. S.

1974-01-01

The concept of gravity tectonics is applied to reveal the major clue as to the conditions which result in the correspondence of seismic and tectonic gaps in the mantle. An asymptotic theory is developed for the calculation of the thrust and moment when a descending lithospheric plate encounters resistance to its downward motion in the mesosphere. Dynamic analysis falls into two parts: (1) deriving equations for forces in the descending lithosphere, (2) deducing moment distribution which causes the detachment of lithosphere. For the analysis of forces a mathematical theory of shells is given. In order to determine the detachment mechanism, solutions of equations are obtained by asymptotic integration. It is found that a thrust N sub phi coupled with a moment M sub phi due to gravitational forces generated by density contrast may play a key role in the initial detachment of a piece of descending lithosphere. The results are in agreement with the observed seismic gaps beneath South America, Toga-Fiji, New Zealand and New Hebrides regions.

Constraining the near-surface response to lithospheric reorientation: Structural thermochronology along the TRANSALP geophysical transect

NASA Astrophysics Data System (ADS)

Glotzbach, Christoph; Büttner, Lukas; Ehlers, Todd

2017-04-01

Tomographic analyses of the lithosphere structure underneath the Alps suggest a complex geodynamic history (e.g. Lippitsch et al. 2003), indicating, among other things, switches in the direction of subduction. A subduction polarity switch is proposed to have occurred in Miocene times between the Central and Eastern Alps (e.g. Lippitsch et al. 2003; Handy et al. 2015). In the Western and Central Alps SE-directed subduction of European continental lithosphere occurs, whereas NW-directed subduction of Adriatic lithosphere occurs further east (e.g. Kissling et al. 2006). The subducted slab steepens at the transition to the Eastern Alps, roughly at the position of the TRANSALP geophysical profile (S. Germany to N. Italy). This lithospheric reorientation was pre-dated by slab breakoff and also involves the delamination of the lower lithosphere, both processes producing distinct long-wavelength deformation (e.g. Gerya et al. 2004). Thermochronological data can be used to study the surface response to such a long-wavelength deformation. We present new apatite and zircon (U-Th)/He ages of 23 samples collected along 210 km of the TRANSALP profile. The samples were collected along a balanced cross section the TRANSALP profile (e.g. Lüschen et al. 2004) across individual structures that can be tied to deeper, seismically imaged, structures. The thermochronometer ages provide a record of exhumation related to both crustal shortening and post deformation erosional exhumation. Interpretation of the data is in progress and being used to discriminate between competing kinematic/geometric models, and the timing of major fault activity. Variations in exhumation along the section will also unravel the timing and shape of possible long-wavelength rock uplift event(s). References Gerya, T.V., Yuen, D.A., Maresch, W.V. 2004. Thermomechanical modelling of slab detachment. Earth Planet. Sci. Lett. 226, 101-116. Handy, M.R., Ustaszewski, K., Kissling, E. 2015. Reconstructing the Alps-Carpathians-Dinarides as a key to understanding switches in subduction polarity, slab gaps and surface motion. Int. J. Earth Sci. 104, 1-26. Kissling, E., Schmid, S.M., Lippitsch, R., Ansorge, J., Fügenschuh, B. 2006. Lithosphere structure and tectonic evolution of the Alpine arc: new evidence from high-resolution teleseismic tomography. In: Gee, D.G., Stephenson, R.A. (eds) European Lithosphere Dynamics. Geol. Soc. London Mem. 32, 129-145. Lippitsch, R., Kissling, E., Ansorge, J. 2003. Upper mantle structure beneath the Alpine orogen from high-resolution teleseismic tomography. J. Geophys. Res. 108, 2376, doi:10.1029/2002JB002016. Lüschen, E., Lammerer, B., Gebrande, H., Millahn, K., Nicolich, R., TRANSALP Working Group 2004. Orogenic structure of the Eastern Alps, Europe, from TRANSALP deep seismic reflection profiling. Tectonophysics 388, 85-102.

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.

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.

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.

Three-dimensional lithospheric electrical structure of Southern Granulite Terrain, India and its tectonic implications

NASA Astrophysics Data System (ADS)

Patro, Prasanta K.; Sarma, S. V. S.; Naganjaneyulu, K.

2014-01-01

crustal as well as the upper mantle lithospheric electrical structure of the Southern Granulite Terrain (SGT) is evaluated, using the magnetotelluric (MT) data from two parallel traverses: one is an 500 km long N-S trending traverse across SGT and another a 200 km long traverse. Data space Occam 3-D inversion was used to invert the MT data. The electrical characterization of lithospheric structure in SGT shows basically a highly resistive (several thousands of Ohm meters) upper crustal layer overlying a moderately resistive (a few hundred Ohm meters) lower crustal layer which in turn is underlain by the upper mantle lithosphere whose resistivity shows significant changes along the traverse. The highly resistive upper crustal layer is interspersed with four major conductive features with three of them cutting across the crustal column, bringing out a well-defined crustal block structure in SGT with individual highly resistive blocks showing correspondence to the geologically demarcated Salem, Madurai, and Trivandrum blocks. The 3-D model also brought out a well-defined major crustal conductor located in the northern half of the Madurai block. The electrical characteristics of this south dipping conductor and its close spatial correlation with two of the major structural elements, viz., Karur-Oddanchatram-Kodaikanal Shear Zone and Karur-Kamban-Painavu-Trichur Shear Zone, suggest that this conductive feature is closely linked to the subduction-collision tectonic processes in the SGT, and it is inferred that the Archean Dharwar craton/neoproterozoic SGT terrain boundary lies south of the Palghat-Cauvery shear zone. The results also showed that the Achankovil shear zone is characterized by a well-defined north dipping conductive feature. The resistive block adjoining this conductor on the southern side, representing the Trivandrum block, is shown to be downthrown along this north dipping crustal conductor relative to the Madurai block, suggesting a northward movement of Trivandrum block colliding against the Madurai block. The lithospheric upper mantle electrical structure of the SGT up to a depth of 100 km may be broadly divided into two distinctly different segments, viz., northern and southern segments. The northern lithospheric segment, over a major part, is characterized by a thick resistive upper mantle, while the southern one is characterized by a dominantly conductive medium suggesting a relatively thinned lithosphere in the southern segment.

Deep seismic reflection evidence for ancient subduction and collision zones within the continental lithosphere of northwestern Europe

NASA Astrophysics Data System (ADS)

Balling, N.

2000-12-01

Deep seismic profiling experiments in the region of NW Europe (including BABEL in the Gulf of Bothnia and the Baltic Sea, Mobil Search in the Skagerrak and MONA LISA in the North Sea) have demonstrated the existence of seismic reflectors in the mantle lithosphere beneath the Baltic Shield, the Tornquist Zone and the North Sea basins. Different sets of reflectors are observed, notably dipping and sub-horizontal. Dipping, distinct reflectivity, which may be followed from Moho/Moho offsets into the deeper parts of the continental lithosphere, is of special interest because of its tectonic and geodynamic significance. Such reflectivity, observed in several places, dipping 15-35° and covering a depth range of 30-90 km, constrained by surface geological information and radiometric age data, is interpreted to represent fossil, ancient subduction and collison zones. Subduction slabs with remnant oceanic basaltic crust transformed into eclogite is assumed, in particular, to generate deep seismic reflectivity. Deep seismic evidence is presented for subduction, crustal accretion and collision processes with inferred ages from 1.9 to 1.1 Ga from the main structural provinces within the Baltic Shield including Svecofennian, Transscandinavian Igneous Belt, Gothian and Sveconorwegian. Along the southwestern border of Baltica (in the southeastern North Sea) south-dipping crustal and sub-crustal reflectivity is observed down to a depth of about 90 km, close to the lithosphere-asthenosphere boundary. These structures are interpreted to reveal a lithosphere-scale Caledonian (ca. 440 Ma) suture zone resulting from the closure of the Tornquist Sea/Thor Ocean and the amalgamation of Baltica and Eastern Avalonia. These results demonstrate that deep structures within the continental lithosphere, originating from early crust-forming plate tectonic processes, may survive for a very long time and form seismic marker reflectivity of great value in geotectonic interpretation and reconstructions. Furthermore, the depth of dipping reflectivity from ancient structures, such as subduction slabs, significantly contributes information about the thickness of the coherent lithosphere. The seismic observations and our interpretations support plate tectonic and structural models, suggesting crustal growth and amalgamation of tectonic units in the Baltic Shield and along its southwestern margin generally from the northeast (in present-day orientation) towards the southwest and west, likely to result in regional deep structural and tectonic age zonations.

Constraints on Composition, Structure and Evolution of the Lithosphere

NASA Astrophysics Data System (ADS)

Bianchini, Gianluca; Bonadiman, Costanza; Aulbach, Sonja; Schutt, Derek

2015-05-01

The idea for this special issue was triggered at the Goldschmidt Conference held in Florence (August 25-30, 2013), where we convened a session titled "Integrated Geophysical-Geochemical Constraints on Composition and Structure of the Lithosphere". The invitation to contribute was extended not only to the session participants but also to a wider spectrum of colleagues working on related topics. Consequently, a diverse group of Earth scientists encompassing geophysicists, geodynamicists, geochemists and petrologists contributed to this Volume, providing a comprehensive overview on the nature and evolution of lithospheric mantle by combining studies that exploit different types of data and interpretative approaches. The integration of geochemical and geodynamic datasets and their interpretation represents the state of the art in our knowledge of the lithosphere and beyond, and could serve as a blueprint for future strategies in concept and methodology to advance our knowledge of this and other terrestrial reservoirs.

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.

Lithospheric Velocity Structure of the Anatolain plateau-Caucasus-Caspian Regions

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

Gok, R; Mellors, R J; Sandvol, E

Anatolian Plateau-Caucasus-Caspian region is an area of complex structure accompanied by large variations in seismic wave velocities. Despite the complexity of the region little is known about the detailed lithospheric structure. Using data from 29 new broadband seismic stations in the region, a unified velocity structure is developed using teleseismic receiver functions and surface waves. Love and Rayleigh surface waves dispersion curves have been derived from event-based analysis and ambient-noise correlation. We jointly inverted the receiver functions with the surface wave dispersion curves to determine absolute shear wave velocity and important discontinuities such as sedimentary layer, Moho, lithospheric-asthenospheric boundary. Wemore » combined these new station results with Eastern Turkey Seismic Experiment results (29 stations). Caspian Sea and Kura basin underlained by one of the thickest sediments in the world. Therefore, short-period surface waves are observed to be very slow. The strong crustal multiples in receiver functions and the slow velocities in upper crust indicate the presence of thick sedimentary unit (up to 20 km). Crustal thickness varies from 34 to 52 km in the region. The thickest crust is in Lesser Caucasus and the thinnest is in the Arabian Plate. The lithospheric mantle in the Greater Caucasus and the Kura depression is faster than the Anatolian Plateau and Lesser Caucasus. This possibly indicates the presence of cold lithosphere. The lower crust is slowest in the northeastern part of the Anatolian Plateau where Holocene volcanoes are located.« less

Sutures and an Anomalous Deep Lithospheric Electrical Resistor in the Southeastern United States as Revealed by EarthScope Magnetotelluric Data

NASA Astrophysics Data System (ADS)

Murphy, B. S.; Egbert, G. D.

2016-12-01

We use newly acquired long-period magnetotelluric data to examine lithospheric structure beneath the modern Southern Appalachian Mountains and the adjacent Piedmont. The New York-Alabama Lineament is clearly visible both in inverse models and in the data themselves as a major Appalachian-parallel, mid- to lower-crustal conductive feature. This observation supports geologically-based interpretations of the NY-AL Lineament as a major Grenville suture. We also discern several other suture zones in our inverse models, including the Central Piedmont Suture. Interestingly, we do not observe any geoelectric signature of the Suwannee Suture. Most strikingly, we find a zone of exceptionally high resistivity (>1000 μm) that extends to a depth of more than 200 km beneath the modern Piedmont. This resistive block abuts more conductive lithosphere ( 100 μm, as would be expected for Phanerozoic lithosphere) to the northwest. The boundary between these two distinct domains coincides with the modern Appalachian topographic escarpment to within our resolution. The high resistivity values would seem to require completely dry, highly depleted lithosphere at anomalously cold temperatures; however, corresponding seismically fast lithospheric mantle that would be expected for such a structure has not been observed in any previous studies. The exact nature of this feature therefore remains uncertain at present. Regardless, as it is a persistent feature in inversions and it is also readily apparent in the impedance data, this geoelectric structure likely holds important implications for the past, present, and future tectonic evolution of the Southeastern United States.

Satellite tidal magnetic signals constrain oceanic lithosphere-asthenosphere boundary.

PubMed

Grayver, Alexander V; Schnepf, Neesha R; Kuvshinov, Alexey V; Sabaka, Terence J; Manoj, Chandrasekharan; Olsen, Nils

2016-09-01

The tidal flow of electrically conductive oceans through the geomagnetic field results in the generation of secondary magnetic signals, which provide information on the subsurface structure. Data from the new generation of satellites were shown to contain magnetic signals due to tidal flow; however, there are no reports that these signals have been used to infer subsurface structure. We use satellite-detected tidal magnetic fields to image the global electrical structure of the oceanic lithosphere and upper mantle down to a depth of about 250 km. The model derived from more than 12 years of satellite data reveals a ≈72-km-thick upper resistive layer followed by a sharp increase in electrical conductivity likely associated with the lithosphere-asthenosphere boundary, which separates colder rigid oceanic plates from the ductile and hotter asthenosphere.

Seismic imaging of the lithosphere beneath Hudson Bay: Episodic growth of the Laurentian mantle keel

NASA Astrophysics Data System (ADS)

Darbyshire, Fiona A.; Eaton, David W.; Bastow, Ian D.

2013-07-01

The Hudson Bay basin in northern Canada conceals one of the major collisional zones of the Canadian Shield, the Trans-Hudson Orogen (THO), which marks the Paleoproterozoic collision between the Archean Superior and Western Churchill cratons at ˜1.9-1.8Ga. Improved knowledge of upper mantle structure beneath the region is essential to establish the nature of the THO, specifically whether Himalayan-style plate tectonics operated in Paleoproterozoic times. Detailed seismological constraints on lithospheric architecture are also required to advance our understanding of the mechanism and timing of keel formation. We use surface wave tomography to illuminate new details of the lithospheric architecture of the Hudson Bay region, resolving both seismic wavespeed and azimuthal anisotropy. Phase velocity maps are calculated from fundamental-mode Rayleigh wave dispersion curves, then used to construct a 3D model exploring upper mantle structure to depths of ˜300km. Fast shear wavespeeds suggest a lithospheric thickness varying from ˜180km to almost 280 km beneath the Hudson Bay region. The new study confirms previous inferences that there is no correlation between crustal ages and lithospheric thickness. Patterns of shear wavespeed and azimuthal anisotropy indicate a layered lithosphere. In the uppermost mantle, both the highest velocities and the anisotropic fast directions wrap around the Bay. This structure is likely related to the formation processes of the Paleozoic intracratonic basin. At greater depth (˜70-150km) we resolve two high-wavespeed cores separated by a relatively narrow near-vertical lower-velocity curtain. This internal architecture is suggested to result from the terminal phase of a modern-style plate-tectonic collision between the Archean Superior and Churchill cratons during the Trans-Hudson orogeny, entrapping juvenile Proterozoic material. The lower lithosphere (≥160km depth) has a relatively homogeneous wavespeed structure across the region, with distinct patterns of anisotropy closely resembling the subsurface geometry of the THO. We interpret this basal layer as juvenile or reworked material accreted to the base of the existing cratonic lithosphere during or soon after the Trans-Hudson orogeny. The formation of the Laurentian keel thus likely occurred in multiple phases, with a basal layer developing in post-Archean times, during the THO.

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

Oceanic lithosphere and asthenosphere - Thermal and mechanical structure

NASA Technical Reports Server (NTRS)

Schubert, G.; Yuen, D. A.; Froidevaux, C.

1976-01-01

A coupled thermomechanical subsolidus model of the oceanic lithosphere and asthenosphere is developed which includes vertical heat conduction, a temperature-dependent thermal conductivity, heat advection by a horizontal and vertical mass flow that depends on depth and age, contributions of viscous dissipation or shear heating, a linear or nonlinear deformation law relating shear stress and strain rate, as well as a temperature- and pressure-dependent viscosity. The model requires a constant horizontal velocity and temperature at the surface, but zero horizontal velocity and constant temperature at great depths. The depth- and age-dependent temperature, horizontal and vertical velocities, and viscosity structure of the lithosphere and asthenosphere are determined along with the age-dependent shear stress in those two zones. The ocean-floor topography, oceanic heat flow, and lithosphere thickness are deduced as functions of ocean-floor age; seismic velocity profiles which exhibit a marked low-velocity zone are constructed from the age-dependent geotherms and assumed values of the elastic parameters. It is found that simple boundary-layer cooling determines the thermal structure at young ages, while effects of viscous dissipation become more important at older ages.

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.

Mountains and Calderas on Io: Possible Implications for Lithosphere Structure and Magma Generation

USGS Publications Warehouse

Carr, M.H.; McEwen, A.S.; Howard, K.A.; Chuang, F.C.; Thomas, P.; Schuster, P.; Oberst, J.; Neukum, G.; Schubert, G.

1998-01-01

The combination of Voyager images and newly acquired Galileo images with low illumination and resolutions ranging from 2 to 6 km/pixel now allows determination of the global distribution of mountains and volcanic centers on Io. The mountains generally do not have characteristics typical of terrestrial volcanic landforms, they are evenly distributed across the surface and show no obvious correlation with known hot spots or plumes. Relative elevations, determined by shadow measurements and stereoscopy, indicate that mountains in the newly imaged area range in elevation up to at least 7.6 km. The origin of the mountains remains uncertain. Some appear to be multitiered volcanic constructs; others enclosing the partial remains of large circular depressions appear to be remnants of old volcanoes; yet others show extensive tectonic disruption. Volcanic centers also appear to be distributed evenly across the surface except for an apparently somewhat lower density at high latitudes. The low latitudes have one volcanic center per 7 ?? 104km2, and, on average, the centers are spaced roughly 250 km apart. The global distribution of high mountains suggests that the lithosphere over most of Io is thick. Although the thickness cannot be calculated, the previously suggested 30 km appears reasonable as a lower limit. The high rates of resurfacing combined with the likely dissipation of most of the tidal energy in the asthenosphere and underlying mantle implies a very low temperature gradient in the upper part of the lithosphere and steep gradients in the lower lithosphere. The slow rate of separation of melt from host rock in the magma source regions as a consequence of the low gravity on Io, coupled with the high rate of magma production, will likely result in larger melt fractions than is typical for source regions on Earth. The variety of volcanic landforms suggests that volcanic products with a range of compositions are deposited on the surface. This mixture will be carried downward through the lithosphere as a consequence of the 0.5-1.5 cm/yr resurfacing rates. During descent, the more volatile components will tend to be driven off early, but complete or near-complete melting at the base of the lithosphere may result in rehomogenization of the silicate mixture that remains. ?? 1998 Academic Press.

Numerical modeling of continental lithospheric weak zone over plume

NASA Astrophysics Data System (ADS)

Perepechko, Y. V.; Sorokin, K. E.

2011-12-01

The work is devoted to the development of magmatic systems in the continental lithosphere over diffluent mantle plumes. The areas of tension originating over them are accompanied by appearance of fault zones, and the formation of permeable channels, which are distributed magmatic melts. The numerical simulation of the dynamics of deformation fields in the lithosphere due to convection currents in the upper mantle, and the formation of weakened zones that extend up to the upper crust and create the necessary conditions for the formation of intermediate magma chambers has been carried out. Thermodynamically consistent non-isothermal model simulates the processes of heat and mass transfer of a wide class of magmatic systems, as well as the process of strain localization in the lithosphere and their influence on the formation of high permeability zones in the lower crust. The substance of the lithosphere is a rheologic heterophase medium, which is described by a two-velocity hydrodynamics. This makes it possible to take into account the process of penetration of the melt from the asthenosphere into the weakened zone. The energy dissipation occurs mainly due to interfacial friction and inelastic relaxation of shear stresses. The results of calculation reveal a nonlinear process of the formation of porous channels and demonstrate the diversity of emerging dissipative structures which are determined by properties of both heterogeneous lithosphere and overlying crust. Mutual effect of a permeable channel and the corresponding filtration process of the melt on the mantle convection and the dynamics of the asthenosphere have been studied. The formation of dissipative structures in heterogeneous lithosphere above mantle plumes occurs in accordance with the following scenario: initially, the elastic behavior of heterophase lithosphere leads to the formation of the narrow weakened zone, though sufficiently extensive, with higher porosity. Further, the increase in the width of the weakened area with a small decrease in porosity occurs due to the increase of inelastic stresses. The longitudinal scale of the structure remain unchanged. The evolution of intraplate magmatic systems associated with weakened zones is accompanied by the formation of intermediate intracrustal magma chambers. This work was financially supported by the project #24.1.2, the program of RAS #24.

Intracontinental Deformation in the NW Iranian Plateau and Comparisons with the Northern Margin of the Tibetan Plateau

NASA Astrophysics Data System (ADS)

Chen, L.; Jiang, M.; Talebian, M.; Wan, B.; Ai, Y.; Ghods, A.; Sobouti, F.; Xiao, W.; Zhu, R.

2017-12-01

This study investigates the intracontinental deformation and its relationship with the structure of the crust and uppermost mantle in the NW Iranian plateau by combining new seismic and geological observations, to understand how this part of the plateau deformed to accommodate the Arabia-Eurasia plate collision and how the property of the lithosphere controls the deformation pattern. In contrast to the adjacent Anatolian block that exhibits westward large-scale extrusion, the northwesternmost part of the Iranian plateau shows dispersed intracontinental deformations with the development of numerous small-scale and discontinuous right-lateral strike-slip faults. The dispersed surface structures and deformation pattern correspond well to the active volcanism and seismically slow crust and uppermost mantle, and hence a weak lithosphere of the area. Further to the southeast are the western part of the Alborz Mountains and the southern Caspian Sea, both of which are characterized by stronger and more rigid lithosphere with relatively fast crust and uppermost mantle and absence of Quaternary volcanoes. A sharp Moho offset of 18 km has been imaged at the border of the Alborz and southern Caspian Sea using teleseismic receiver function data from a dense seismic array deployed under a collaborative project named "China-Iran Geological and Geophysical Survey in the Iranian Plateau (CIGSIP)". The sharp Moho offset and the minor undulations of the Moho on both sides indicate insignificant intracrustal deformation but mainly relative crustal movements between the Alborz Mountains and southern Caspian Sea, a behavior consistent with the relatively rigid nature of the lithosphere. Similar Moho offsets and lithospheric structures have been reported at the borders between the Kunlun Mountains and Qaidam or Tarim Basins in the northern margin of the Tibetan plateau, suggesting the occurrence of relative crustal movements with the effects of rigid continental lithosphere in the region. The new observations in the NW Iranian plateau combined with those in the Tibetan plateau thus provide solid evidence that intracontinental deformation is primarily controlled by the structure and properties of the continental lithosphere that may or may not have been severely altered by the collisional processes at plate margins.

Investigation of lunar crustal structure and isostasy

NASA Technical Reports Server (NTRS)

Thurber, Clifford H.

1987-01-01

The lunar mascon basins have strongly free air gravity anomalies, generally exceeding 100 milligals at an elevation of 100 km. The source of the anomalies is a combination of mantle uplift beneath the impact basins and subsequent infilling by high-density mare basalts. The relative contribution of these two components is still somewhat uncertain, although it is generally accepted that the amount of mantle uplift greatly exceeds the thickness of the basalts. Extensive studies have been carried out of the crustal structure of mare basins, based on gravity data, and their tectonic evolution, based on compressive and extensional tectonic features. The present study endeavored to develop a unified, self-consistent model of the lunar crust and lithosphere incorporating both gravity and tectonic constraints.

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.

Crustal layering and gravity highs in the Midcontinent of North America - implications for the formation of the Illinois Basin

NASA Astrophysics Data System (ADS)

Gilbert, H. J.; Boschelli, J.; Pavlis, G. L.; Hamburger, M. W.; Marshak, S.; Chen, C.; Yang, X.; DeLucia, M. S.; Larson, T. H.; Rupp, J.

2017-12-01

The emerging picture of crustal and lithospheric structure beneath the North American cratonic platform resulting from recent increases in the resolution of seismic studies is revealing a scale of complexity and heterogeneity not previously recognized. Examples of novel images of the lithosphere allowed by this increased sampling come from the results of the OIINK project, an EarthScope FlexArray experiment. OIINK data provides new insight into tectonic relationships among the Reelfoot Rift, Ozark Plateau, Rough Creek Graben, and Illinois Basin. Making use of ambient-noise tomography from data recorded by the OIINK Array and surrounding stations we produced a new shear-wave velocity model of the region. This model indicates detailed variations in crustal wavespeeds align with the regional tectonic features. Beyond corroborating previous observations of high-speed material in the mid- to lower crust of the southern Illinois Basin, this new model demonstrates that these anomalous velocities extend continuously from the Reelfoot, beneath the Mississippi Embayment, into southern Indiana. This model also includes a separate area characterized by a similarly thickened layer of increased velocities in the middle and lower crust beneath the LaSalle Deformation Belt, a north-south band of faults and folds that runs along the axis of the Illinois Basin. At depths of about 20 km, the top of these areas of thickened high-velocity crust align with a midcrustal discontinuity identified by receiver functions. Additionally, the lateral extent of these structures correlates with regions of increased Bouguer gravity. If the high-velocity structures contain high-density material, this configuration provides an explanation for the source of these positive gravity anomalies. These observations support a model in which Late Proterozoic rifting beneath the region of the Illinois Basin provided an opportunity for high-density material to enter the crust as residuum from melt extraction. In turn, the negative buoyancy forces resulting from this high-density material could then contribute to subsidence in the Illinois Basin, emphasizing the potential for intracrationic basins to originate from failed rifts.

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.

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.

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.

Lithospheric structure of a nascent spreading ridge inferred from gravity data: The western Gulf of Aden

NASA Astrophysics Data System (ADS)

HéBert, HéLèNe; Deplus, Christine; Huchon, Philippe; Khanbari, Khaled; Audin, Laurence

2001-11-01

The Aden spreading ridge (Somalia/Arabia plate boundary) does not connect directly to the Red Sea spreading ridge. It propagates toward the East African Rift through the Afar depression, where the presence of a hot spot has been postulated from seismological and geochemical evidence. The spreading direction (N37°E) is highly oblique to the overall trend (N90°E) of the ridge. We present and interpret new geophysical data gathered during the Tadjouraden cruise (R/V L'Atalante, 1995) in the Gulf of Aden west of 46°E. These data allow us to study the propagation of the ridge toward the Afar and to discuss the processes of the seafloor spreading initiation. We determine the lithospheric structure of the ridge using gravity data gathered during the cruise with the constraint of available refraction data. A striking Bouguer anomaly gradient together with the identification of magnetic anomalies defines the geographical extent of oceanic crust. The inversion of the Bouguer anomaly is performed in terms of variations of crustal thickness only and then discussed with respect to the expected thermal structure of the mantle lithosphere, which should depend not only on the seafloor spreading but also on the hot spot beneath East Africa. Our results allow us to define three distinct lithospheric domains in the western Gulf of Aden. East of 44°45'E the lithosphere displays an oceanic character (thermal subsidence recorded for the last 10 Ma and constant crustal thickness). Between 43°30'E and 44°10'E the lithosphere is of continental type but locally thinned beneath the axial valley. The central domain defined between 44°10'E and 44°45'E is characterized by a transitional lithosphere which can be seen as a stretched continental crust where thick blocks are mixed with thinned crust; it displays en echelon basins that are better interpreted as extension cells rather than accretion cells.

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.

Dynamic Topography Revisited

NASA Astrophysics Data System (ADS)

Moresi, Louis

2015-04-01

Dynamic Topography Revisited Dynamic topography is usually considered to be one of the trinity of contributing causes to the Earth's non-hydrostatic topography along with the long-term elastic strength of the lithosphere and isostatic responses to density anomalies within the lithosphere. Dynamic topography, thought of this way, is what is left over when other sources of support have been eliminated. An alternate and explicit definition of dynamic topography is that deflection of the surface which is attributable to creeping viscous flow. The problem with the first definition of dynamic topography is 1) that the lithosphere is almost certainly a visco-elastic / brittle layer with no absolute boundary between flowing and static regions, and 2) the lithosphere is, a thermal / compositional boundary layer in which some buoyancy is attributable to immutable, intrinsic density variations and some is due to thermal anomalies which are coupled to the flow. In each case, it is difficult to draw a sharp line between each contribution to the overall topography. The second definition of dynamic topography does seem cleaner / more precise but it suffers from the problem that it is not measurable in practice. On the other hand, this approach has resulted in a rich literature concerning the analysis of large scale geoid and topography and the relation to buoyancy and mechanical properties of the Earth [e.g. refs 1,2,3] In convection models with viscous, elastic, brittle rheology and compositional buoyancy, however, it is possible to examine how the surface topography (and geoid) are supported and how different ways of interpreting the "observable" fields introduce different biases. This is what we will do. References (a.k.a. homework) [1] Hager, B. H., R. W. Clayton, M. A. Richards, R. P. Comer, and A. M. Dziewonski (1985), Lower mantle heterogeneity, dynamic topography and the geoid, Nature, 313(6003), 541-545, doi:10.1038/313541a0. [2] Parsons, B., and S. Daly (1983), The relationship between surface topography, gravity anomalies, and temperature structure of convection, Journal of Geophysical Research: Solid Earth (1978-2012), 88(B2), 1129-1144, doi:10.1029/JB088iB02p01129. [3] Robinson, E. M., B. Parsons, and S. F. Daly (1987), The effect of a shallow low viscosity zone on the apparent compensation of mid-plate swells, Earth and Planetary Science Letters, 82(3-4), 335-348, doi:10.1016/0012-821X(87)90207-X.

Lithospheric Structure Beneath Taiwan From Sp Converted Waves

NASA Astrophysics Data System (ADS)

Glasgow, D.; McGlashan, N.; Brown, L.

2006-12-01

Taiwan is the product of three dimensionally complex interaction between the Eurasian Plate (EP) and the Philippine Sea plate (PSP), with the EP subducting eastward beneath the PSP in southern Taiwan while the PSP subducts northward beneath the EP in northern Taiwan. The structural emplacement of Philippine Arc lithosphere onto Chinese passive margin lithosphere is an exemplar of continental amalgamation, yet there are relatively few contraints on the geometry of lithosphere involved at depth. We have used teleseismic data recorded by the Broadband Array for Taiwan Seismology (BATS) to compute S-to-p wave receiver functions for the Taiwan region to provide new constraints on deep geometries. Moho conversions provide independent new estimates of crustal thickness, which vary from 35 to 55 km across the island in agreement with previous P to S conversion studies and local tomography. More significantly, our results suggest that the lithosphere- asthenosphere boundary (LAB) varies in depth from ca 140 km beneath northeastern Taiwan to ca 120 km beneath central Taiwan to perhaps less than 80 km beneath southern Taiwan. We attribute this along strike variation to the depression and decapitation of the Eurasian plate in the transition to northward subduction of the PSP.

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.

The Impacts of 3-D Earth Structure on GIA-Induced Crustal Deformation and Future Sea-Level Change in the Antarctic

NASA Astrophysics Data System (ADS)

Powell, E. M.; Hay, C.; Latychev, K.; Gomez, N. A.; Mitrovica, J. X.

2016-12-01

Glacial Isostatic Adjustment (GIA) models used to constrain the extent of past ice sheets and viscoelastic Earth structure, or to correct geodetic and geological observables for ice age effects, generally only consider depth-dependent variations in Earth viscosity and lithospheric structure. A et al. [2013] argued that 3-D Earth structure could impact GIA observables in Antarctica, but concluded that the presence of such structure contributes less to GIA uncertainty than do differences in Antarctic deglaciation histories. New seismic and geological evidence, however, indicates the Antarctic is underlain by complex, high amplitude variability in viscoelastic structure, including a low viscosity zone (LVZ) under West Antarctica. Hay et al. [2016] showed that sea-level fingerprints of modern melting calculated using such Earth models differ from those based on elastic or 1-D viscoelastic Earth models within decades of melting. Our investigation is motivated by two questions: (1) How does 3-D Earth structure, especially this LVZ, impact observations of GIA-induced crustal deformation associated with the last deglaciation? (2) How will 3-D Earth structure affect predictions of future sea-level rise in Antarctica? We compute the gravitationally self-consistent sea level, uplift, and gravity changes using the finite volume treatment of Latychev et al. [2005]. We consider four viscoelastic Earth models: a global 1-D model; a regional, West Antarctic-like 1-D model; a 3-D model where the lithospheric thickness varies laterally; and a 3-D model where both viscosity and lithospheric thickness vary laterally. For our Last Glacial Maximum to present investigations we employ ICE6g [Peltier et al., 2015]. For our present-future investigations we consider a melt scenario consistent with GRACE satellite gravity derived solutions [Harig et al., 2015]. Our calculations indicate that predictions of crustal deformations due to both GIA and ongoing melting are strongly influenced by 3-D lithospheric thickness and viscosity structure. Future sea level change due to ongoing melting is primarily influenced by 3-D viscosity structure. We show that 1-D Earth models built using regional inferences of viscosity and lithospheric thickness do not accurately capture the variability introduced by 3-D Earth structure.

The Impacts of 3-D Earth Structure on GIA-Induced Crustal Deformation and Future Sea-Level Change in the Antarctic

NASA Astrophysics Data System (ADS)

Powell, E. M.; Hay, C.; Latychev, K.; Gomez, N. A.; Mitrovica, J. X.

2017-12-01

Glacial Isostatic Adjustment (GIA) models used to constrain the extent of past ice sheets and viscoelastic Earth structure, or to correct geodetic and geological observables for ice age effects, generally only consider depth-dependent variations in Earth viscosity and lithospheric structure. A et al. [2013] argued that 3-D Earth structure could impact GIA observables in Antarctica, but concluded that the presence of such structure contributes less to GIA uncertainty than do differences in Antarctic deglaciation histories. New seismic and geological evidence, however, indicates the Antarctic is underlain by complex, high amplitude variability in viscoelastic structure, including a low viscosity zone (LVZ) under West Antarctica. Hay et al. [2016] showed that sea-level fingerprints of modern melting calculated using such Earth models differ from those based on elastic or 1-D viscoelastic Earth models within decades of melting. Our investigation is motivated by two questions: (1) How does 3-D Earth structure, especially this LVZ, impact observations of GIA-induced crustal deformation associated with the last deglaciation? (2) How will 3-D Earth structure affect predictions of future sea-level rise in Antarctica? We compute the gravitationally self-consistent sea level, uplift, and gravity changes using the finite volume treatment of Latychev et al. [2005]. We consider four viscoelastic Earth models: a global 1-D model; a regional, West Antarctic-like 1-D model; a 3-D model where the lithospheric thickness varies laterally; and a 3-D model where both viscosity and lithospheric thickness vary laterally. For our Last Glacial Maximum to present investigations we employ ICE6g [Peltier et al., 2015]. For our present-future investigations we consider a melt scenario consistent with GRACE satellite gravity derived solutions [Harig et al., 2015]. Our calculations indicate that predictions of crustal deformations due to both GIA and ongoing melting are strongly influenced by 3-D lithospheric thickness and viscosity structure. Future sea level change due to ongoing melting is primarily influenced by 3-D viscosity structure. We show that 1-D Earth models built using regional inferences of viscosity and lithospheric thickness do not accurately capture the variability introduced by 3-D Earth structure.

Influence of the Structural Dichotomy of Antarctic Lithosphere on Regional Glacial-Isostatic Adjustment

NASA Astrophysics Data System (ADS)

Klemann, V.; Rau, D.; Martinec, Z.; Wolf, D.

2009-05-01

The strong structural dichotomy between East and West Antarctica is related to the West Antarctic Rift. The rheological implications are a reduction of the elastic-lithosphere thickness by a factor of more than 2 from East to West Antarctica as well as a strongly reduced mantle viscosity below West Antarctica and the Antarctic Peninsula. For modelling glacial-isostatic adjustment, we use a global viscoelastic earth model and apply the spectral finite-element method for the solution of the field equations. Ice models ICE-5G and IJ05 are used for parameterizing the last Pleistocene deglaciation. Lateral viscosity variations in the upper mantle are derived from variations in seismic velocity by applying scaling laws. Considering also lateral variations in the lithosphere structure, we study the implications of lateral variability on the glacial-isostatic adjustment of Antarctica.

Satellite Tidal Magnetic Signals Constrain Oceanic Lithosphere-Asthenosphere Boundary Earth Tomography with Tidal Magnetic Signals

NASA Technical Reports Server (NTRS)

Grayver, Alexander V.; Schnepf, Neesha R.; Kuvshinov, Alexey V.; Sabaka, Terence J.; Chandrasekharan, Manoj; Olsen, Niles

2016-01-01

The tidal flow of electrically conductive oceans through the geomagnetic field results in the generation of secondary magnetic signals, which provide information on the subsurface structure. Data from the new generation of satellites were shown to contain magnetic signals due to tidal flow; however, there are no reports that these signals have been used to infer subsurface structure. Here we use satellite-detected tidal magnetic fields to image the global electrical structure of the oceanic lithosphere and upper mantle down to a depth of about 250 km. The model derived from more than 12 years of satellite data reveals an Approximately 72 km thick upper resistive layer followed by a sharp increase in electrical conductivity likely associated with the lithosphere-asthenosphere boundary, which separates colder rigid oceanic plates from the ductile and hotter asthenosphere.

Satellite tidal magnetic signals constrain oceanic lithosphere-asthenosphere boundary

PubMed Central

Grayver, Alexander V.; Schnepf, Neesha R.; Kuvshinov, Alexey V.; Sabaka, Terence J.; Manoj, Chandrasekharan; Olsen, Nils

2016-01-01

The tidal flow of electrically conductive oceans through the geomagnetic field results in the generation of secondary magnetic signals, which provide information on the subsurface structure. Data from the new generation of satellites were shown to contain magnetic signals due to tidal flow; however, there are no reports that these signals have been used to infer subsurface structure. We use satellite-detected tidal magnetic fields to image the global electrical structure of the oceanic lithosphere and upper mantle down to a depth of about 250 km. The model derived from more than 12 years of satellite data reveals a ≈72-km-thick upper resistive layer followed by a sharp increase in electrical conductivity likely associated with the lithosphere-asthenosphere boundary, which separates colder rigid oceanic plates from the ductile and hotter asthenosphere. PMID:27704045

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.

Lithosphere deformation methods and models constrained by surface fault data on Mars

NASA Astrophysics Data System (ADS)

Dimitrova, Lada L.

Models of lithospheric deformation tie observed field measurements of gravity and topography with surface observations of tectonic features. An understanding of the sources of stress, and the expected style, orientation, and magnitudes of stress and associated elastic strain is important for understanding the evolution of faulting on Mars and its relationship to loading. At the same time, theoretical models of deformation mechanisms and forces, when tied to tectonic observations, can be interpreted in terms of major tectonic events and allow insights into the planet's history and evolution as well as its internal structure and processes. This is particularly important for understanding solid planetary bodies other than Earth where the seismic data is either sparse, e.g. the Moon, or non-existent, e.g. Mars. This kind of research has implications for, and benefits from, an understanding of the petrology and surface processes. In this work, I use MGS MOLA and Radio Science data products (topography and gravity) to systematically test new geodynamic models and evaluate lithosphere dynamics on Mars as a function of time, while satisfying geologic surface observations (surface features) that have been and are being catalogued and studied from Viking, MOLA, MOC, and THEMIS IR images. I investigate (1) the role of internal loads (internal body force effects), (2) loading from the surface and base of lithosphere, and the effects of this loading on membrane and flexural strains and stresses, and (3) the role of global contraction, all viewed in the context of how the surface elastic layer has changed as the planet has evolved. I show that deviatoric stresses associated with gravitational potential differences do a good job at matching the normal faults; however, fitting all the surface-breaking faults is more difficult. I argue that global planetary contraction is an unlikely source of significant deformation. Instead, the simplest inverse models show that small lateral variations (1¡6%) in crust and mantle density in conjunction with small vertical displacement, O(100m), provide sufficient additional GPE and membrane stress to fit the majority of the data. These inverse models are consistent with lithosphere modification by erosion from running water.

Upper mantle structure across the Trans-European Suture Zone imaged by S-receiver functions

NASA Astrophysics Data System (ADS)

Knapmeyer-Endrun, Brigitte; Krüger, Frank; Geissler, Wolfram H.; Passeq Working Group

2017-01-01

We present a high-resolution study of the upper mantle structure of Central Europe, including the western part of the East European Platform, based on S-receiver functions of 345 stations. A distinct contrast is found between Phanerozoic Europe and the East European Craton across the Trans-European Suture Zone. To the west, a pronounced velocity reduction with depth interpreted as lithosphere-asthenosphere boundary (LAB) is found at an average depth of 90 km. Beneath the craton, no strong and continuous LAB conversion is observed. Instead we find a distinct velocity reduction within the lithosphere, at 80-120 km depth. This mid-lithospheric discontinuity (MLD) is attributed to a compositional boundary between depleted and more fertile lithosphere created by late Proterozoic metasomatism. A potential LAB phase beneath the craton is very weak and varies in depth between 180 and 250 km, consistent with a reduced velocity contrast between the lower lithosphere and the asthenosphere. Within the Trans-European Suture Zone, lithospheric structure is characterized by strong heterogeneity. A dipping or step-wise increase to LAB depth of 150 km is imaged from Phanerozoic Europe to 20-22° E, whereas no direct connection to the cratonic LAB or MLD to the east is apparent. At larger depths, a positive conversion associated with the lower boundary of the asthenosphere is imaged at 210-250 km depth beneath Phanerozoic Europe, continuing down to 300 km depth beneath the craton. Conversions from both 410 km and 660 km discontinuities are found at their nominal depth beneath Phanerozoic Europe, and the discontinuity at 410 km depth can also be traced into the craton. A potential negative conversion on top of the 410 km discontinuity found in migrated images is analyzed by modeling and attributed to interference with other converted phases.

Finite Frequency Traveltime Tomography of Lithospheric and Upper Mantle Structures beneath the Cordillera-Craton Transition in Southwestern Canada

NASA Astrophysics Data System (ADS)

Chen, Y.; Gu, Y. J.; Hung, S. H.

2014-12-01

Based on finite-frequency theory and cross-correlation teleseismic relative traveltime data from the USArray, Canadian National Seismograph Network (CNSN) and Canadian Rockies and Alberta Network (CRANE), we present a new tomographic model of P-wave velocity perturbations for the lithosphere and upper mantle beneath the Cordillera-cration transition region in southwestern Canada. The inversion procedure properly accounts for the finite-volume sensitivities of measured travel time residuals, and the resulting model shows a greater resolution of upper mantle velocity heterogeneity beneath the study area than earlier approaches based on the classical ray-theoretical approach. Our model reveals a lateral change of P velocities from -0.5% to 0.5% down to ~200-km depth in a 50-km wide zone between the Alberta Basin and the foothills of the Rocky Mountains, which suggests a sharp structural gradient along the Cordillera deformation front. The stable cratonic lithosphere, delineated by positive P-velocity perturbations of 0.5% and greater, extends down to a maximum depth of ~180 km beneath the Archean Loverna Block (LB). In comparison, the mantle beneath the controversial Medicine Hat Block (MHB) exhibits significantly higher velocities in the uppermost mantle and a shallower (130-150 km depth) root, generally consistent with the average depth of the lithosphere-asthenosphere boundary beneath Southwest Western Canada Sedimentary Basin (WCSB). The complex shape of the lithospheric velocities under the MHB may be evidence of extensive erosion or a partial detachment of the Precambrian lithospheric root. Furthermore, distinct high velocity anomalies in LB and MHB, which are separated by 'normal' mantle block beneath the Vulcan structure (VS), suggest different Archean assembly and collision histories between these two tectonic blocks.

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.

Interactions between propagating rifts and pre-existing linear rheological heterogeneities: insights from 3D analogue experiments of rotational extension

NASA Astrophysics Data System (ADS)

Molnar, Nicolas; Cruden, Alexander

2017-04-01

Propagating rifts are a natural consequence of lithospheric plates that diverge with respect to each other about a pole of rotation. This process of "unzipping" is common in the geological record, but how rifts interact with pre-existing structures (i.e., with a non-homogeneous lithosphere) as they propagate is poorly understood. Here we report on a series of lithospheric-scale three-dimensional analogue experiments of rotational extension with in-built, variably oriented linear weak zones in the lithospheric mantle, designed to investigate the role that inherited structural or thermal weaknesses play in the localisation of strain and rifting. Surface strain and dynamic topography in the analogue models are quantified by high-resolution particle imaging velocimetry and digital photogrammetry, which allows us to characterise the spatio-temporal evolution of deformation as a function of the orientation of the linear heterogeneities in great detail. The results show that the presence of a linear zone of weakness oriented at low angles with respect to the rift axis (i.e., favourably oriented) produces strain localisation in narrow domains, which enhances the "unzipping" process prior to continental break up. Strong strain partitioning is observed when the linear heterogeneity is oriented at high angles with respect to the rift axis (i.e., unfavourably oriented). In these experiments, early sub-parallel V-shaped basins propagate towards the pole of rotation until they are abandoned and strain is transferred entirely to structures developed in the vicinity of the strongly oblique weak lithosphere zone boundary. The modelling also provides insights on how propagating rift branches that penetrate the weak linear zone boundary are aborted when strain is relayed onto structures that develop in rheologically weaker areas. The experimental results are summarised in terms of their evolution, patterns of strain localisation, and dynamic topography as a function of the lithospheric heterogeneity obliquity angle, and compared to ancient and modern examples in nature.

North American Crust and Upper Mantle Structure Imaged Using an Adaptive Bayesian Inversion

NASA Astrophysics Data System (ADS)

Eilon, Z.; Fischer, K. M.; Dalton, C. A.

2017-12-01

We present a methodology for imaging upper mantle structure using a Bayesian approach that incorporates a novel combination of seismic data types and an adaptive parameterization based on piecewise discontinuous splines. Our inversion algorithm lays the groundwork for improved seismic velocity models of the lithosphere and asthenosphere by harnessing increased computing power alongside sophisticated data analysis, with the flexibility to include multiple datatypes with complementary resolution. Our new method has been designed to simultaneously fit P-s and S-p converted phases and Rayleigh wave phase velocities measured from ambient noise (periods 6-40 s) and earthquake sources (periods 30-170s). Careful processing of the body wave data isolates the signals from velocity gradients between the mid-crust and 250 km depth. We jointly invert the body and surface wave data to obtain detailed 1-D velocity models that include robustly imaged mantle discontinuities. Synthetic tests demonstrate that S-p phases are particularly important for resolving mantle structure, while surface waves capture absolute velocities with resolution better than 0.1 km/s. By treating data noise as an unknown parameter, and by generating posterior parameter distributions, model trade offs and uncertainties are fully captured by the inversion. We apply the method to stations across the northwest and north-central United States, finding that the imaged structure improves upon existing models by sharpening the vertical resolution of absolute velocity profiles and offering robust uncertainty estimates. In the tectonically active northwestern US, a strong velocity drop immediately beneath the Moho connotes thin (<70 km) lithosphere and a sharp lithosphere-asthenosphere transition; the asthenospheric velocity profile here matches observations at mid-ocean ridges. Within the Wyoming and Superior cratons, our models reveal mid-lithospheric velocity gradients indicative of thermochemical cratonic layering, but the lithosphere-asthenosphere boundary is relatively gradual. This flexible method holds promise for increasingly detailed understanding of the lithosphere-asthenosphere system.

The North American upper mantle: density, composition, and evolution

USGS Publications Warehouse

Mooney, Walter D.; Kaban, Mikhail K.

2010-01-01

The upper mantle of North America has been well studied using various seismic methods. Here we investigate the density structure of the North American (NA) upper mantle based on the integrative use of the gravity field and seismic data. The basis of our study is the removal of the gravitational effect of the crust to determine the mantle gravity anomalies. The effect of the crust is removed in three steps by subtracting the gravitational contributions of (1) topography and bathymetry, (2) low-density sedimentary accumulations, and (3) the three-dimensional density structure of the crystalline crust as determined by seismic observations. Information regarding sedimentary accumulations, including thickness and density, are taken from published maps and summaries of borehole measurements of densities; the seismic structure of the crust is based on a recent compilation, with layer densities estimated from P-wave velocities. The resultant mantle gravity anomaly map shows a pronounced negative anomaly (−50 to −400 mGal) beneath western North America and the adjacent oceanic region and positive anomalies (+50 to +350 mGal) east of the NA Cordillera. This pattern reflects the well-known division of North America into the stable eastern region and the tectonically active western region. The close correlation of large-scale features of the mantle anomaly map with those of the topographic map indicates that a significant amount of the topographic uplift in western NA is due to buoyancy in the hot upper mantle, a conclusion supported by previous investigations. To separate the contributions of mantle temperature anomalies from mantle compositional anomalies, we apply an additional correction to the mantle anomaly map for the thermal structure of the uppermost mantle. The thermal model is based on the conversion of seismic shear-wave velocities to temperature and is consistent with mantle temperatures that are independently estimated from heat flow and heat production data. The thermally corrected mantle density map reveals density anomalies that are chiefly due to compositional variations. These compositional density anomalies cause gravitational anomalies that reach ~250 mGal. A pronounced negative anomaly (−50 to −200 mGal) is found over the Canadian shield, which is consistent with chemical depletion and a corresponding low density of the lithospheric mantle, also referred to as the mantle tectosphere. The strongest positive anomaly is coincident with the Gulf of Mexico and indicates a positive density anomaly in the upper mantle, possibly an eclogite layer that has caused subsidence in the Gulf. Two linear positive anomalies are also seen south of 40°N: one with a NE-SW trend in the eastern United States, roughly coincident with the Grenville-Appalachians, and a second with a NW-SE trend beneath the states of Texas, New Mexico, and Colorado. These anomalies are interpreted as being due to (1) the presence of remnants of an oceanic slab in the upper mantle beneath the Grenville-Appalachian suture and (2) mantle thickening caused by a period of shallow, flat subduction during the Laramie orogeny, respectively. Based on these geophysical results, the evolution of the NA upper mantle is depicted in a series of maps and cartoons that display the primary processes that have formed and modified the NA crust and lithospheric upper mantle.

Intraplate deformation, stress in the lithosphere and the driving mechanism for plate motions

NASA Technical Reports Server (NTRS)

Hager, Bradford H.

1988-01-01

During this period work was carried out on three fronts relevant to the understanding of intraplate deformation, stress in the lithosphere, and the driving mechanisms for plate motions: (1) observational constraints, using GPS geodesy on the deformation in the region of the boundry between the Pacific and North American plates in central and southern California; (2) numerical modeling of the effects of temperature dependent lithospheric viscosity on the stress and strain history of extensional regimes; and (3) improvement of estimates of mantle viscosity variation, the long-wave-length density variations in the mantle, and the topography of the core-mantel boundary from modeling of geoid anomalies, nutation, and changes in length of day. These projects are described in more detail, followed by a discussion of meetings attended and a list of abstracts and papers submitted and/or published.

Lithospheric Structure from Mars Global Surveyor Topography and Gravity and Implications for the Early Thermal Evolution of Mars

NASA Technical Reports Server (NTRS)

Solomon, Sean C.; Zuber, Maria T.; Phillips, Roger J.; Smith, David E.; Tyler, G. Leonard; Aharonson, Oded; Balmino, Georges; Banerdt, W. B.; Head, James W.; Johnson, Catherine L.

2000-01-01

Regional variations in the thickness of the elastic lithosphere on Mars derived from a combined analysis of topography and gravity anomalies determined by Mars Global Surveyor provide new insight into the planet's thermal history.

Oceanic lithosphere and asthenosphere: The thermal and mechanical structure

NASA Technical Reports Server (NTRS)

Schubert, G.; Froidevaux, C.; Yuen, D. A.

1976-01-01

A coupled thermal and mechanical solid state model of the oceanic lithosphere and asthenosphere is presented. The model includes vertical conduction of heat with a temperature dependent thermal conductivity, horizontal and vertical advection of heat, viscous dissipation or shear heating, and linear or nonlinear deformation mechanisms with temperature and pressure dependent constitutive relations between shear stress and strain rate. A constant horizontal velocity u sub 0 and temperature t sub 0 at the surface and zero horizontal velocity and constant temperature t sub infinity at great depth are required. In addition to numerical values of the thermal and mechanical properties of the medium, only the values of u sub 0, t sub 0 and t sub infinity are specified. The model determines the depth and age dependent temperature horizontal and vertical velocity, and viscosity structures of the lithosphere and asthenosphere. In particular, ocean floor topography, oceanic heat flow, and lithosphere thickness are deduced as functions of the age of the ocean floor.

Mountain building long after plate collision. Possible mechanisms

NASA Astrophysics Data System (ADS)

Artyushkov, Eugene; Chekhovich, Peter; Korikovsky, Sergei; Massonne, Hans-Joachim

2016-04-01

It is commonly believed that mountain building occurs synchronously to plate collision. However, it was well known long ago that in most cases mountain building began 10-100 Ma later. For example, in the Middle and Southern Urals collision occurred from the Late Devonian and until the Early Permian. The shortened regions remained covered by a shallow sea. High mountains began to form rapidly 10 Ma after the termination of collision. The Verkhoyansk Range in Northeastern Asia was strongly shortened at mid-Cretaceous time. It remained at a low altitude for 100 Ma and rose by 2 km in the Pleistocene. Compressive stresses most probably were acting in the Urals during all the epoch of collision. Strong shortening however occurred only as several impulses 1-2 Ma long. This can be explained by temporary weakening of the lithosphere due to a change in the mechanism of creep under infiltration of fluids from the mantle. To sustain a thickened crust at a low altitude, a density increase in the lithosphere was necessary. A possible cause could be metamorphism in crustal rocks, both mafic and felsic, under a pressure increase during collision. Rapid uplift of the shortened crust long after collision and establishment of a new temperature distribution indicates a density decrease in the lithosphere. Thus, on the Precambrian cratons which cover about 70% of continental areas collision terminated ≥ 500 Ma ago. However, during the last several Ma most of them underwent the uplift ranging from 100-200 m to 1000-1500 m. This occurred on the African continent, in central and eastern Australia, East Siberia, East Antarctica and in many other regions. Preservation of thick mantle roots precluded delamination of the lowermost lithosphere as a mechanism for the uplift. Due to a strong denudation of cratons deeply metamorphosed rocks of the lower crust emerged to a shallow depth. Under dry conditions for a long time they remained metastable. Recent inflow of fluid from the mantle ensured a new phase of metamorphism with the formation of hydrous minerals and rock expansion. Possible density changes at different levels and the corresponding crustal uplift are calculated using phase diagrams for the main types of crustal rocks. Expansion of rocks within the crust is also indicated by numerous slopes tens of kilometers wide that formed during the recent uplift. Strong thinning of mantle lithosphere occurs under many Phanerozoic mountain ranges, e.g., in the Alpine Belt which underwent intense recent uplift. This can be attributed to rapid replacement by the asthenosphere of the lower mantle lithosphere. Its strong weakening by infiltration of mantle fluid was necessary to ensure such a replacement with the isostatic crustal uplift after a long period of relative stability. Strong lateral variations of the uplift indicate a large input in it of metamorphism within the crustal layer. Thus, infiltration of mantle fluids into the continental lithosphere appears to be a trigger for strong shortening of the crust and mountain building.

Elastic energy distribution in bi-material lithosphere: implications for shear zone formation

NASA Astrophysics Data System (ADS)

So, B.; Yuen, D. A.

2013-12-01

Shear instability in the lithosphere can cause mechanical rupturing such as slab detachment and deep focus earthquake. Recent studies reported that bi-material interface, which refers to sharp elastic modulus contrast, plays an important role in triggering the instability [So and Yuen et al., 2012, GJI]. In present study, we performed two-dimensional numerical simulations to investigate the distribution of thermal-mechanical energy within the bi-material lithosphere. Under the far-field constant compression exerted on the domain, a larger elastic energy is accumulated into the compliant part than stiff medium. For instance, the compliant part has two times greater elastic energy density than surrounding stiff part, when the elastic modulus contrast between two different parts is five. Although these elastic energies in both parts are conversed into thermal energies after plastic yielding, denser elastic energy in the compliant is released more efficiently. This leads to efficient strength weakening and the subsequent ductile shear zone in the compliant part. We propose that strong shear heating occurs in lithosphere with the bi-material interface due to locally non-uniform distribution of the energy around the interface.

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.

Secular rotational motions and the mechanical structure of a dynamical viscoelastic earth

NASA Technical Reports Server (NTRS)

Yuen, D. A.; Sabadini, R.

1984-01-01

A survey is presented of analytical methods for computing the linear responses of the rotational axis of a layered viscoelastic earth to surface loading. Theoretical research in this area is first summarized, and the differences between the mechanical boundary conditions to be applied at the interface separating the upper and lower mantles for an adiabatically and chemically stratified mantle are discussed. Some examples of polar wander and secular variation of the spin rate from glacial excitation are presented for various types of chemical and viscosity stratifications. The effects of an artificial density jump at the base of the lithosphere in models are examined, and certain issues concerning the fluid tidal Love number for different types of density stratification are addressed. The meaning of effective plate thickness over geological time scales for rotational dynamics is discussed.

First heat flow density determinations from Southeastern Zaïre (Central Africa)

NASA Astrophysics Data System (ADS)

Sebagenzi, M. N.; Vasseur, G.; Louis, P.

1993-05-01

First heat flow density determinations from southeastern Zaïre are presented. Sites are located in the late Proterozoic metasedimentary cover of the Pan-African belt (600 Ma.). For each individual boreholes, heat flow ranges between 48 and 72 mWm -2. The average value of 62 mW m -2 for the sites is similar to that of 66 mW m -2 observed in Zambia. Both values are higher than what is expected for Pan-African terranes. These heat flow determinations in Shaba province of southeastern Zaïre, together with gravity and seismological observations, support the hypothesis of lithospheric thinning for this area. As already suggested for Zambia, this lithospheric thinning may be associated with a southwestern extension of the East African Rift System from Tanganyika across the central African plateau.

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.

Gravity anomalies and flexure of the lithosphere at the Middle Amazon Basin, Brazil

NASA Astrophysics Data System (ADS)

Nunn, Jeffrey A.; Aires, Jose R.

1988-01-01

The Middle Amazon Basin is a large Paleozoic sedimentary basin on the Amazonian craton in South America. It contains up to 7 km of mainly shallow water sediments. A chain of Bouguer gravity highs of approximately +40 to +90 mGals transects the basin roughly coincident with the axis of maximum thickness of sediment. The gravity highs are flanked on either side by gravity lows of approximately -40 mGals. The observed gravity anomalies can be explained by a steeply sided zone of high density in the lower crust varying in width from 100 to 200 km wide. Within this region, the continental crust has been intruded/replaced by more dense material to more than half its original thickness of 45-50 km. The much wider sedimentary basin results from regional compensation of the subsurface load and the subsequent load of accumulated sediments by flexure of the lithosphere. The observed geometry of the basin is consistent with an elastic lithosphere model with a mechanical thickness of 15-20 km. Although this value is lower than expected for a stable cratonic region of Early Proterozoic age, it is within the accepted range of effective elastic thicknesses for the earth. Rapid subsidence during the late Paleozoic may be evidence of a second tectonic event or lithospheric relaxation which could lower the effective mechanical thickness of the lithosphere. The high-density zone in the lower crust, as delineated by gravity and flexural modeling, has a complex sinuous geometry which is narrow and south of the axis of maximum sediment thickness on the east and west margins and wide and offset to the north in the center of the basin. The linear trough geometry of the basin itself is a result of smoothing by regional compensation of the load in the lower crust.

Lithospheric bending at subduction zones based on depth soundings and satellite gravity

NASA Technical Reports Server (NTRS)

Levitt, Daniel A.; Sandwell, David T.

1995-01-01

A global study of trench flexure was performed by simultaneously modeling 117 bathymetric profiles (original depth soundings) and satellite-derived gravity profiles. A thin, elastic plate flexure model was fit to each bathymetry/gravity profile by minimization of the L(sub 1) norm. The six model parameters were regional depth, regional gravity, trench axis location, flexural wavelength, flexural amplitude, and lithospheric density. A regional tilt parameter was not required after correcting for age-related trend using a new high-resolution age map. Estimates of the density parameter confirm that most outer rises are uncompensated. We find that flexural wavelength is not an accurate estimate of plate thickness because of the high curvatures observed at a majority of trenches. As in previous studies, we find that the gravity data favor a longer-wavelength flexure than the bathymetry data. A joint topography-gravity modeling scheme and fit criteria are used to limit acceptable parameter values to models for which topography and gravity yield consistent results. Even after the elastic thicknesses are converted to mechanical thicknesses using the yield strength envelope model, residual scatter obscures the systematic increase of mechanical thickness with age; perhaps this reflects the combination of uncertainties inherent in estimating flexural wavelength, such as extreme inelastic bending and accumulated thermoelastic stress. The bending moment needed to support the trench and outer rise topography increases by a factor of 10 as lithospheric age increases from 20 to 150 Ma; this reflects the increase in saturation bending moment that the lithosphere can maintain. Using a stiff, dry-olivine rheology, we find that the lithosphere of the GDH1 thermal model (Stein and Stein, 1992) is too hot and thin to maintain the observed bending moments. Moreover, the regional depth seaward of the oldest trenches (approximately 150 Ma) exceeds the GDH1 model depths by about 400 m.

Eastern US crustal thickness estimates from spectral analysis and inversion of onshore Bouguer gravity anaomalies

NASA Astrophysics Data System (ADS)

Dybus, W.; Benoit, M. H.; Ebinger, C. J.

2011-12-01

The crustal thickness beneath much of the eastern half of the US is largely unconstrained. Though there have been several controlled source seismic surveys of the region, many of these studies suffer from rays that turn in the crust above the Moho, resulting in somewhat ambiguous crustal thickness values. Furthermore, the broadband seismic station coverage east of the Mississippi has been limited, and most of the region remains largely understudied. In this study, we estimated the depth to the Moho using both spectral analysis and inversion of Bouguer gravity anomalies. We systematically estimated depths to lithospheric density contrasts from radial power spectra of Bouguer gravity within 100 km X 100 km windows eastward from the Mississippi River to the Atlantic Coast, and northward from North Carolina to Maine. The slopes and slope breaks in the radial power spectra were computed using an automated algorithm. The slope values for each window were visually inspected and then used to estimate the depth to the Moho and other lithospheric density contrasts beneath each windowed region. Additionally, we performed a standard Oldenburg-Parker inversion for lithospheric density contrasts using various reference depths and density contrasts that are realistic for the different physiographic provinces in the Eastern US. Our preliminary results suggest that the gravity-derived Moho depths are similar to those found using seismic data, and that the crust is relatively thinner (~28-33 km) than expected in beneath the Piedmont region (~35-40 km). Given the relative paucity of seismic data in the eastern US, analysis of onshore gravity data is a valuable tool for interpolating between seismic stations.

Three-dimensional flexure modelling of seamounts near the Ogasawara Fracture Zone in the western Pacific

NASA Astrophysics Data System (ADS)

Lee, Tae-Gook; Moon, Jai-Woon; Jung, Mee-Sook

2009-04-01

The geophysical data were obtained in 2000-2003 during a survey of seamounts near the Ogasawara Fracture Zone (OFZ) to the northwest of the Marshall Islands in the western Pacific. The OFZ is unique in that it is a wide rift zone showing 600-km-long right-lateral movement between the Pigafetta Basin (PB) and East Mariana Basin (EMB), and contains many seamounts (e.g. the Magellan Seamounts and the seamounts on the Dutton Ridge). Most seamounts in this study are newly mapped using modern multibeam echosounder (Seabeam 2000) and denoted sequentially by Korea Ocean Research and Development Institute (KORDI). OSM2, OSM4, OSM7, OSM8-1 and OSM8-2 seamounts of the study area are located in the OFZ which formed by the spreading ridge between the Izanagi and Pacific plates, and OSM5-1, Seascan, OSM6-1 and OSM6-2 seamounts in the PB which is a part of the oldest oceanic crust in the Pacific. In this study, the densities of seamounts and the elastic thickness values of lithosphere are estimated by using 3-D flexure and gravity modelling by considering several boundary conditions and a constant sediment layer. The infinite model with two different elastic thickness values is the best-fitting model and it indicates that the OFZ was mechanically coupled with plate of different elastic thickness values, probably after the reorganization of Izanagi-Pacific spreading zone. Very low elastic thickness values (5-10 km), relatively young seamounts, and old lithosphere in the east study area suggest the possibility of the rejuvenation of the lithosphere by widespread volcanism pulses, whereas higher elastic thickness values (15-20 km), relatively younger lithosphere, and old seamounts of the west study area are comparable with a simple cooling plate model. It implies that the west study area is outside the rejuvenation range of the lithosphere. In the flexure and gravity modelling, the different residual pattern of OSM6-1 and OSM6-2, which are joined, suggests that they have different load densities or elastic thickness values. OSM2 and OSM7 may be close to a basaltic volcano with low viscosity because they have high densities and ratios of the basal diameter to the height, whereas OSM4, OSM5-1 and Seascan may be close to an andesitic volcano.

Orogenic structural inheritance and rifted passive margin formation

NASA Astrophysics Data System (ADS)

Salazar Mora, Claudio A.; Huismans, Ritske S.

2016-04-01

Structural inheritance is related to mechanical weaknesses in the lithosphere due to previous tectonic events, e.g. rifting, subduction and collision. The North and South Atlantic rifted passive margins that formed during the breakup of Western Gondwana, are parallel to the older Caledonide and the Brasiliano-Pan-African orogenic belts. In the South Atlantic, 'old' mantle lithospheric fabric resulting from crystallographic preferred orientation of olivine is suggested to play a role during rifted margin formation (Tommasi and Vauchez, 2001). Magnetometric and gravimetric mapping of onshore structures in the Camamu and Almada basins suggest that extensional faults are controlled by two different directions of inherited older Brasiliano structures in the upper lithosphere (Ferreira et al., 2009). In the South Atlantic Campos Basin, 3D seismic data indicate that inherited basement structures provide a first order control on basin structure (Fetter, 2009). Here we investigate the role of structural inheritance on the formation of rifted passive margins with high-resolution 2D thermo-mechanical numerical experiments. The numerical domain is 1200 km long and 600 km deep and represents the lithosphere and the sublithospheric mantle. Model experiments were carried out by creating self-consistent orogenic inheritance where a first phase of orogen formation is followed by extension. We focus in particular on the role of varying amount of orogenic shortening, crustal rheology, contrasting styles of orogen formation on rifted margin style, and the time delay between orogeny and subsequent rifted passive formation. Model results are compared to contrasting structural styles of rifted passive margin formation as observed in the South Atlantic. Ferreira, T.S., Caixeta, J.M., Lima, F.D., 2009. Basement control in Camamu and Almada rift basins. Boletim de Geociências da Petrobrás 17, 69-88. Fetter, M., 2009. The role of basement tectonic reactivation on the structural evolution of Campos Basin, offshore Brazil: Evidence from 3D seismic analysis and section restoration. Marine and Petroleum Geology 26, 873-886. Tommasi, A., Vauchez, A., 2001. Continental rifting parallel to ancient collisional belts: An effect of the mechanical anisotropy of the lithospheric mantle. Earth and Planetary Science Letters 185, 199-210.

The Central Eurasia collision zone: insights from a neotectonic study

NASA Astrophysics Data System (ADS)

Tunini, Lavinia; Jiménez-Munt, Ivone; Fernandez, Manel; Vergés, Jaume

2017-04-01

In this study, we explore the neotectonic deformation in the whole Central Eurasia, including both the India-Eurasia and the Arabia-Eurasia collision zones, by using the thin-sheet approach in which the lithosphere strength is calculated from the lithosphere structure and thermal regime. We investigate the relative contributions of the lithospheric structure, rheology, boundary conditions, and friction coefficient on faults on the predicted velocity and stress fields. The resulting models have been evaluated by comparing the predictions with available data on seismic deformation, stress directions and GPS velocities. A first order approximation of the velocity and stress directions is obtained, reproducing the counter-clockwise rotation of Arabia and Iran, the westward escape of Anatolia, and the eastward extrusion of the northern Tibetan Plateau. To simulate the observed extensional faults within Tibet a weaker lithosphere is required, provided by a change in the rheological parameters or a reduction of the lithosphere thickness in NE-Tibet. The temperature increase generated by the lithospheric thinning below the Tibetan Plateau would also allow reconciling the model with the high heat flow and low mantle seismic velocities observed in the area. Besides the large scale, this study offers a coherent result in regions with little or no data coverage, as in the case of the Arabia-India inter-collision zone, over large areas of Pakistan and entire Afghanistan. The study is supported by MITE (CGL2014-59516-P) and WE-ME (PIE-CSIC-201330E111) projects.

Two-stage magmatism during the evolution of the transitional Ethiopian rift

NASA Astrophysics Data System (ADS)

Cornwell, D. G.; England, R. W.; Maguire, P. K.; Kendall, M.; Stuart, G. W.

2008-12-01

The Ethiopian rift marks the transition between continental rifting and incipient seafloor spreading. The Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE) included a 400 km-long cross-rift profile with 97 broadband passive seismometers with the aim to investigate the change from mechanical to magmatic extension by defining the lithospheric structure and extent of magmatism beneath the rift. Complimentary studies of P-wave receiver functions, shear-wave splitting and teleseismic earthquake arrival times show that the lithospheric structure is inherently different beneath the north-western rift flank, rift valley and south- eastern rift flank, with contrasting crustal thickness and composition, upper mantle velocity and lithospheric anisotropy. Two stages of magmatic addition are interpreted: 1) a 6--18 km-thick underplate lens at the base of the crust, which probably formed synchronous with an Oligocene flood basalt event (and therefore pre-dates the adjacent rifting by ~20 Myr); and 2) a 20--30 km-wide zone of intense dyking and partial melt, which most likely pervades the entire crust beneath the rift valley and marks the locus of current rift extension. Furthermore, Precambrian collision-related lithospheric fabric is proposed to be the main source of the strong anisotropy that is observed along the entire cross-rift profile, which may be augmented by magmatism beneath the rift. An active, followed by a passive magma-assisted rifting model that is controlled by a combination of far-field plate stresses, the pre-existing lithospheric framework and magmatism is invoked to explain the rift evolution.

Mercury's lithospheric thickness and crustal density, as inferred from MESSENGER observations

NASA Astrophysics Data System (ADS)

James, P. B.; Mazarico, E.; Genova, A.; Smith, D. E.; Neumann, G. A.; Solomon, S. C.

2015-12-01

The gravity field and topography of Mercury measured by the MESSENGER spacecraft have provided insights into the thickness of the planet's elastic lithosphere, Te. We localized the HgM006 free-air gravity anomaly and gtmes_125v03 shape datasets to search for theoretical elastic thickness solutions that best fit a variety of localized coherence spectra between Bouguer gravity anomaly and topography. We adopted a crustal density of ρcrust =2700 kg m-3 for the Bouguer gravity correction, but density uncertainty did not markedly affect the elastic thickness estimates. A best-fit solution in the northern smooth plains (NSP) gives an elastic thickness of Te =30-60 km at the time of formation of topography for a range of ratios of top to bottom loading from 1 to 5. For a mechanical lithosphere with a thickness of ~2Te and a temperature of 1600 °C at the base, this solution is consistent with a geothermal gradient of 9-18 K km-1. A similar coherence analysis exterior to the NSP produces an elastic thickness estimate of Te =20-50 km, albeit with a poorer fit. Coherence in the northern hemisphere as a whole does not approach zero at any wavelength, because of the presence of variations in crustal thickness that are unassociated with elastic loading. The ratios and correlations of gravity and topography at intermediate wavelengths (harmonic degree l between 30 and 50) also constrain regional crustal densities. We localized gravity and topography with a moving Slepian taper and calculated regionally averaged crustal densities with the approximation ρcrust=Zl/(2πG), where Zl is the localized admittance and G is the gravitational constant. The only regional density estimates greater than 2000 kg m-3 for l=30 correspond to the NSP. Density estimates outside of the NSP were unreasonably low, even for highly porous crust. We attribute these low densities to the confounding effects of crustal thickness variations and Kaula filtering of the gravity dataset at the highest harmonic degrees, both of which tend to introduce a downward bias to crustal density estimation. An alternative analysis—which corrected for crustal thickness variability and was restricted to regions with gravity/topography coherence greater than 0.6—yielded an aggregate crustal density of ρcrust=2602 ± 470 kg m-3 for Mercury's high northern latitudes.

Inheritance vs ongoing evolution of the passive margin lithosphere in the southeastern United States: A comparison of <50Ma tectonism with tomographically imaged lithospheric structures.

NASA Astrophysics Data System (ADS)

Wagner, L. S.; Fischer, K. M.; Hawman, R. B.; Hopper, E.; Howell, D.

2017-12-01

The southeastern United States is an archetypical passive margin, and yet significant evidence exists that this region, separated from the nearest plate boundary by thousands of kilometers and over 170 Ma, has experienced significant tectonism since the Eocene. This tectonism includes volcanism, uplift/deformation, and ongoing seismicity such as the 2011 Mw = 5.8 Mineral, VA earthquake and the 1886 M=7 Charleston, SC event. For each of these examples, numerous theories exist on their respective causes. However, there are two common themes that span all of these types of events: first, their proximity to regional terrane boundaries whose inherited structures could play a role; second, the nature of the mantle lithosphere underlying them. We present a recently completed inversion of seismic Rayleigh waves for the shear wave velocity structure of the uppermost 150 - 200 km beneath the southeastern United States. This inversion includes not only EarthScope Transportable Array data, but also the data from the 85 broadband stations installed as part of the Flex Array SouthEastern Suture of the Appalachian Mountains Experiment (SESAME). We find some evidence for structures inherited from previous episodes of rifting, accretion, and orogenesis. However, we also find several examples of mantle lithospheric structures that spatially correlate strongly with Eocene to recent tectonic activity, but do not correlate to any known inherited geometries. These examples include a small but pronounced sub-crustal low velocity anomaly beneath the Eocene volcanoes in western Virginia and eastern West Virginia, as well as evidence for mantle delamination beneath the Cape Fear Arch and uplifted portions of the Orangeburg Escarpment. We will discuss these, along with instances of recent tectonism in our study area that do not bear any obvious relationship to lithospheric structures, in order to shed light on the causes of ongoing tectonic activity in this supposedly "passive" margin setting.

The Western Edge of Cratonic North America and Topography of the Northern U.S. Rocky Mountains

NASA Astrophysics Data System (ADS)

Foster, D. A.; Russo, R. M.; van der Lee, S.; Mueller, P. A.

2009-12-01

We used seismic structure of the upper mantle determined via waveform inversions of surface and regional shear waves (Beadle and van der Lee, 2007) to examine the 3-D geometry of the base of North American lithosphere at the junction between thick, stable cratonic eastern North America and the thinner, recently tectonized western part of the continent. This boundary has been affected by long-term subduction beneath North America. Variability in convergence rates and directions, and especially in slab dip, have been postulated as important controls on the configuration of the transition from thick to thin lithosphere, and on the distribution and degree of crustal deformation and volcanism in the western U.S. We show that the lithospheric thickness transition at depths of 70-130 km - defined as contours of zero shear velocity anomaly - correlates strongly with the high topography of Laramide uplifts in the northern Rockies, which lie west of this seismically defined craton edge. The transition from thick to thin lithosphere also includes an embayment symmetrically centered on the Yellowstone hotspot, offset cratonward from the surface position of the hotspot by ca. 140-180 km at depths of 130-150 km. We interpret this structure as a reduction of cratonic seismic velocities reflecting the thermal halo around the hotspot, and perhaps associated with the separation of the lower lithosphere. The steep velocity gradient (boundary) east of the hotspot occurs along the Big Horn Mountains, and distributed mountain ranges of southwestern Montana. The steep transition between thin and thick lithosphere turns sharply west along the northern margin of the Helena thrust salient-Lewis and Clark fault zone, where it may reflect the edge of the Archean Medicine Hat Block and/or the northern termination of the influence shallow Farallon slab subduction the during Laramide time. Laramide-style basement uplifts are absent north of this zone and the eastern front ranges of the Rockies in northern Montana and Alberta are located further west. South of the Yellowstone lithospheric embayment, a westward salient of high seismic velocities at 70-130 km depths coincides with near surface structures along the Cheyenne Belt, possibly representing an accreted relict subduction margin. Relationships between anomalously hot asthenosphere and thin lithosphere are widely supported for the Basin and Range Province. East of this region, the location of the western edge of thick cratonic North American lithosphere and associated transitions from high to lower topography suggests a strong relationship that dates back to Laramide subduction erosion, and subsequent influence on the thermal/chemical modifications to the lithosphere during Cenozoic and Recent times.

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.

Tectonics and volcanism of Eastern Aphrodite Terra: No subduction, no spreading

NASA Technical Reports Server (NTRS)

Hansen, Vicki L.; Keep, Myra; Herrick, Robert R.; Phillips, Roger J.

1992-01-01

Eastern Aphrodite Terra is approximately equal in size to the western North American Cordillera, from Mexico to Alaska. Its size and unique landforms make it an important area for understanding the tectonics of Venus, yet models for its formation are diametrically opposed. This region is part of the Equatorial Highlands, which was proposed as a region of lithospheric thinning, isostatic uplift, and attendant volcanism. Eastern Aphrodite Terra is dominated by circular structures within which deformation and volcanism are intimately related. These structures are marked by radial and concentric fractures, and volcanic flows that emanate from a central vent, as well as from concentric fracture sets. Cross-cutting relations between flows and concentric fracture sets indicate that outer concentric fracture sets are younger than inner fracture sets. The circular structures are joined by regional northeast- to east-trending fractures that dominantly postdate formation of the circular structures. We propose that the circular structures 'grow' outward with time. Although these structures probably represent addition of crust to the lithosphere, they do not represent significant lithospheric spreading or convergence, and the region does not mark the boundary between two distinct tectonic plates. This region is not easily explained by analogy with either terrestrial midocean rifts or subduction zones. It is perhaps best explained by upwelling of magma diapirs that blister the surface, but do not cause significant lithospheric spreading. Further study of the structural and volcanic evolution of this region using Magellan altimetry and SAR data should lead to better understanding of the tectonic evolution of this region.

Petrophysical Properties (Density and Magnetization) of Rocks from the Suhbaatar-Ulaanbaatar-Dalandzadgad Geophysical Profile in Mongolia and Their Implications

PubMed Central

Gao, Jintian; Gu, Zuowen; Dagva, Baatarkhuu; Tserenpil, Batsaikhan

2013-01-01

Petrophysical properties of 585 rock samples from the Suhbaatar-Ulaanbaatar-Dalandzadgad geophysical profile in Mongolia are presented. Based on the rock classifications and tectonic units, petrophysical parameters (bulk density, magnetic susceptibility, intensity of natural remanent magnetization, and Köenigsberger ratio) of these rocks are summarized. Results indicate that (1) significant density contrast of different rocks would result in variable gravity anomalies along the profile; (2) magnetic susceptibility and natural remanent magnetization of all rocks are variable, covering 5-6 orders of magnitude, which would make a variable induced magnetization and further links to complex magnetic anomalies in ground surface; (3) the distribution of rocks with different lithologies controls the pattern of lithospheric magnetic anomaly along the profile. The petrophysical database thus provides not only one of the keys to understand the geological history and structure of the profile, but also essential information for analysis and interpretation of the geophysical (e.g., magnetic and gravity) survey data. PMID:24324382

Petrophysical properties (density and magnetization) of rocks from the Suhbaatar-Ulaanbaatar-Dalandzadgad geophysical profile in Mongolia and their implications.

PubMed

Yang, Tao; Gao, Jintian; Gu, Zuowen; Dagva, Baatarkhuu; Tserenpil, Batsaikhan

2013-01-01

Petrophysical properties of 585 rock samples from the Suhbaatar-Ulaanbaatar-Dalandzadgad geophysical profile in Mongolia are presented. Based on the rock classifications and tectonic units, petrophysical parameters (bulk density, magnetic susceptibility, intensity of natural remanent magnetization, and Köenigsberger ratio) of these rocks are summarized. Results indicate that (1) significant density contrast of different rocks would result in variable gravity anomalies along the profile; (2) magnetic susceptibility and natural remanent magnetization of all rocks are variable, covering 5-6 orders of magnitude, which would make a variable induced magnetization and further links to complex magnetic anomalies in ground surface; (3) the distribution of rocks with different lithologies controls the pattern of lithospheric magnetic anomaly along the profile. The petrophysical database thus provides not only one of the keys to understand the geological history and structure of the profile, but also essential information for analysis and interpretation of the geophysical (e.g., magnetic and gravity) survey data.

Estimation of the Lithospheric Component Share in the Earth Natural Pulsed Electromagnetic Field Structure

NASA Astrophysics Data System (ADS)

Malyshkov, S. Y.; Gordeev, V. F.; Polyvach, V. I.; Shtalin, S. G.; Pustovalov, K. N.

2017-04-01

Article describes the results of the atmosphere and Earth’s crust climatic and ecological parameters integrated monitoring. The estimation is made for lithospheric component share in the Earth natural pulsed electromagnetic field structure. To estimate lithospheric component we performed a round-the-clock monitoring of the Earth natural pulsed electromagnetic field background variations at the experiment location and measured the Earth natural pulsed electromagnetic field under electric shields. Natural materials in a natural environment were used for shielding, specifically lakes with varying parameters of water conductivity. Skin effect was used in the experiment - it is the tendency of electromagnetic waves amplitude to decrease with greater depths in the conductor. Atmospheric and lithospheric component the Earth natural pulsed electromagnetic field data recorded on terrain was compared against the recorded data with atmosphere component decayed by an electric shield. In summary we have demonstrated in the experiment that thunderstorm discharge originating electromagnetic field decay corresponds to the decay calculated using Maxwell equations. In the absence of close lightning strikes the ratio of field intensity recorded on terrain to shielded field intensity is inconsistent with the ratio calculated for atmospheric sources, that confirms there is a lithospheric component present to the Earth natural pulsed electromagnetic field.

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.

Factors controlling the structures of magma chambers in basaltic volcanoes

NASA Technical Reports Server (NTRS)

Wilson, L.; Head, James W.

1991-01-01

The depths, vertical extents, and lateral extents of magma chambers and their formation are discussed. The depth to the center of a magma chamber is most probably determined by the density structure of the lithosphere; this process is explained. It is commonly assumed that magma chambers grow until the stress on the roof, floor, and side-wall boundaries exceed the strength of the wall rocks. Attempts to grow further lead to dike propagation events which reduce the stresses below the critical values of rock failure. The tensile or compressive failure of the walls is discussed with respect to magma migration. The later growth of magma chambers is accomplished by lateral dike injection into the country rocks. The factors controlling the patterns of growth and cooling of such dikes are briefly mentioned.

Investigation of lunar crustal structure and isostasy. Final technical report

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

Thurber, C.H.

1987-07-01

The lunar mascon basins have strongly free air gravity anomalies, generally exceeding 100 milligals at an elevation of 100 km. The source of the anomalies is a combination of mantle uplift beneath the impact basins and subsequent infilling by high-density mare basalts. The relative contribution of these two components is still somewhat uncertain, although it is generally accepted that the amount of mantle uplift greatly exceeds the thickness of the basalts. Extensive studies have been carried out of the crustal structure of mare basins, based on gravity data, and their tectonic evolution, based on compressive and extensional tectonic features. Themore » present study endeavored to develop a unified, self-consistent model of the lunar crust and lithosphere incorporating both gravity and tectonic constraints.« less

Flexural bending of the Zagros foreland basin

NASA Astrophysics Data System (ADS)

Pirouz, Mortaza; Avouac, Jean-Philippe; Gualandi, Adriano; Hassanzadeh, Jamshid; Sternai, Pietro

2017-09-01

We constrain and model the geometry of the Zagros foreland to assess the equivalent elastic thickness of the northern edge of the Arabian plate and the loads that have originated due to the Arabia-Eurasia collision. The Oligo-Miocene Asmari formation, and its equivalents in Iraq and Syria, is used to estimate the post-collisional subsidence as they separate passive margin sediments from the younger foreland deposits. The depth to these formations is obtained by synthesizing a large database of well logs, seismic profiles and structural sections from the Mesopotamian basin and the Persian Gulf. The foreland depth varies along strike of the Zagros wedge between 1 and 6 km. The foreland is deepest beneath the Dezful embayment, in southwest Iran, and becomes shallower towards both ends. We investigate how the geometry of the foreland relates to the range topography loading based on simple flexural models. Deflection of the Arabian plate is modelled using point load distribution and convolution technique. The results show that the foreland depth is well predicted with a flexural model which assumes loading by the basin sedimentary fill, and thickened crust of the Zagros. The model also predicts a Moho depth consistent with Free-Air anomalies over the foreland and Zagros wedge. The equivalent elastic thickness of the flexed Arabian lithosphere is estimated to be ca. 50 km. We conclude that other sources of loading of the lithosphere, either related to the density variations (e.g. due to a possible lithospheric root) or dynamic origin (e.g. due to sublithospheric mantle flow or lithospheric buckling) have a negligible influence on the foreland geometry, Moho depth and topography of the Zagros. We calculate the shortening across the Zagros assuming conservation of crustal mass during deformation, trapping of all the sediments eroded from the range in the foreland, and an initial crustal thickness of 38 km. This calculation implies a minimum of 126 ± 18 km of crustal shortening due to ophiolite obduction and post-collisional shortening.

Continental and oceanic magnetic anomalies: Enhancement through GRM

NASA Technical Reports Server (NTRS)

Vonfrese, R. R. B.; Hinze, W. J.

1985-01-01

In contrast to the POGO and MAGSAT satellites, the Geopotential Research Mission (GRM) satellite system will orbit at a minimum elevation to provide significantly better resolved lithospheric magnetic anomalies for more detailed and improved geologic analysis. In addition, GRM will measure corresponding gravity anomalies to enhance our understanding of the gravity field for vast regions of the Earth which are largely inaccessible to more conventional surface mapping. Crustal studies will greatly benefit from the dual data sets as modeling has shown that lithospheric sources of long wavelength magnetic anomalies frequently involve density variations which may produce detectable gravity anomalies at satellite elevations. Furthermore, GRM will provide an important replication of lithospheric magnetic anomalies as an aid to identifying and extracting these anomalies from satellite magnetic measurements. The potential benefits to the study of the origin and characterization of the continents and oceans, that may result from the increased GRM resolution are examined.

Mars tectonics and volcanism

NASA Technical Reports Server (NTRS)

Solomon, Sean C.

1990-01-01

The focus of this research was on three broad areas: (1) the relation between lithospheric stress in the vicinity of a growing volcano and the evolution of eruption characteristics and tectonic faulting; (2) the relation between elastic lithosphere thickness and thermal structure; and (3) a synthesis of constraints on heat flow and internal dynamics on Mars. The two reports presented are: (1) Heterogeneities in the Thickness of the Elastic Lithosphere of Mars--Constraints on Heat Flow and Internal Dynamics; and (2) State of Stress, Faulting, and Eruption Characteristics of Large Volcanoes on Mars.

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.

Geodynamic processes and deformation in orogenic belts

NASA Astrophysics Data System (ADS)

Dennis, John G.; Jacoby, Wolfgang R.

1980-03-01

The development of geosynclines and orogenic belts is related to lithosphere convergence. Initial sediment accumulation implying subsidence, and volcanic activity implying extension and rise of geotherms, are in most cases followed by folding and thrusting suggesting compression and by uplift. In terms of recent analogs, sediment accumulation and crustal extension are characteristic of back-arc spreading; subsequent compression would indicate continent—continent collision; and rise of geotherms most likely requires localized thermal flow (convection) in the asthenosphere. These events are here shown to agree with Andrews and Sleep's (1974) numerical model of asthenosphere flow at converging plate margins. Orthogeosynclinal subsidence appears to be a consequence of subcrustal ablation and lithosphere extension and thinning in active marginal basins. Arc and Andean type magmatism mark the reappearance of ablated and transported, relatively low-density subcrustal material. Collision slows and eventually stops the local convection cell, resulting in local heat accumulation and hence high- T, low- P metamorphism and granitization while marginal basin (orthogeosynclinal) deposits are being compressed into Alpine style orogenic structures. Moreover, closing of the marginal basin leads to subsidiary subduction, which in turn may be responsible for some Alpine style structures. Oceanic trench deposits may become incorporated in orogenic zones, as high- P, low- T metamorphic belts (thalassogeosynclines). Dynamic uplift is a fundamental characteristic of orogeny. Most rising and sinking in orogenic zones can be linked to those asthenosphere processes which are a consequence of Andrews-Sleep convection.

Using seismically constrained magnetotelluric inversion to recover velocity structure in the shallow lithosphere

NASA Astrophysics Data System (ADS)

Moorkamp, M.; Fishwick, S.; Jones, A. G.

2015-12-01

Typical surface wave tomography can recover well the velocity structure of the upper mantle in the depth range between 70-200km. For a successful inversion, we have to constrain the crustal structure and assess the impact on the resulting models. In addition,we often observe potentially interesting features in the uppermost lithosphere which are poorly resolved and thus their interpretationhas to be approached with great care.We are currently developing a seismically constrained magnetotelluric (MT) inversion approach with the aim of better recovering the lithospheric properties (and thus seismic velocities) in these problematic areas. We perform a 3D MT inversion constrained by a fixed seismic velocity model from surface wave tomography. In order to avoid strong bias, we only utilize information on structural boundaries to combine these two methods. Within the region that is well resolved by both methods, we can then extract a velocity-conductivity relationship. By translating the conductivitiesretrieved from MT into velocities in areas where the velocity model is poorly resolved, we can generate an updated velocity model and test what impactthe updated velocities have on the predicted data.We test this new approach using a MT dataset acquired in central Botswana over the Okwa terrane and the adjacent Kaapvaal and Zimbabwe Cratons togetherwith a tomographic models for the region. Here, both datasets have previously been used to constrain lithospheric structure and show some similarities.We carefully asses the validity of our results by comparing with observations and petrophysical predictions for the conductivity-velocity relationship.

Contrasts in lithospheric structure within the Australian craton—insights from surface wave tomography

NASA Astrophysics Data System (ADS)

Fishwick, S.; Kennett, B. L. N.; Reading, A. M.

2005-03-01

Contrasts in the seismic structure of the lithosphere within and between elements of the Australian Craton are imaged using surface wave tomography. New data from the WACRATON and TIGGER experiments are integrated with re-processed data from previous temporary deployments of broad-band seismometers and permanent seismic stations. The much improved path coverage in critical regions allows an interpretation of structures in the west of Australia, and a detailed comparison between different cratonic regions. Improvements to the waveform inversion procedure and a new multi-scale tomographic method increase the reliability of the tomographic images. In the shallowest part of the model (75 km) a region of lowered velocity is imaged beneath central Australia, and confirmed by the delayed arrival times of body waves for short paths. Within the cratonic lithosphere there is clearly structure at scale lengths of a few hundred kilometres; resolution tests indicate that path coverage within the continent is sufficient to reveal features of this size in the upper part of our model. In Western Australia, differences are seen beneath and within the Archaean cratons: at depths greater than 150 km faster velocities are imaged beneath the Yilgarn Craton than beneath the Pilbara Craton. In the complex North Australian Craton a fast wavespeed anomaly continuing to at least 250 km is observed below parts of the craton, suggesting the possibility of Archaean lithosphere underlying areas of dominantly Proterozoic surface geology.

Surface-Wave Tomographic Studies of the Hudson Bay Lithosphere: Implications for Paleoproterozoic Tectonic Processes and the Assembly of the Canadian Shield

NASA Astrophysics Data System (ADS)

Darbyshire, F. A.

2015-12-01

Hudson Bay is a shallow intracratonic basin that partially conceals the Trans-Hudson Orogen (THO) in northern Canada. The THO is thought to be a Himalayan-scale Paleoproterozoic orogenic event that was an important component of assembly of the Canadian Shield, marking the collision of the Archean Superior and Western Churchill plates. Until recently, only global and continental-scale seismic tomographic models had imaged the upper-mantle structure of the region, giving a broad but relatively low-resolution picture of the thick lithospheric keel. The Hudson Bay Lithospheric Experiment (HuBLE) investigated the present-day seismic structure beneath Hudson Bay and its surroundings, using a distributed broadband seismograph network installed around the periphery of the Bay and complemented by existing permanent and temporary seismographs further afield. This configuration, though not optimal for body-wave studies which use subvertical arrivals, is well-suited to surface wave tomographic techniques, with many paths crossing the Bay. As there is little seismicity in the region around the Canadian Shield, two-station measurements of teleseismic Rayleigh wave phase velocity formed the principal data set for lithospheric studies. The interstation measurements were combined in a linearized tomographic inversion for maps of phase velocity and azimuthal anisotropy at periods of 20-200 s; these maps were then used to calculate a pseudo-3D anisotropic upper-mantle shear-wavespeed model of the region. The model shows thick (~180-260 km), seismically fast lithosphere across the Hudson Bay region, with a near-vertical 'curtain' of lower wavespeeds trending NE-SW across the Bay, likely associated with more juvenile material trapped between the Archean Superior and Churchill continental cores during the THO. The lithosphere is layered, suggesting a 2-stage formation process. Seismic anisotropy patterns vary with depth; a circular pattern in the uppermost mantle wrapping around the Hudson Bay basin is superseded in the lower lithosphere by a pattern that mirrors THO-related structures within the crust; the lower layer thus likely formed when stress patterns related to the THO were still active.

Shear wave velocity structure in the lithosphere and asthenosphere across the Southern California continent and Pacific plate margin using inversion of Rayleigh wave data from the ALBACORE project.

NASA Astrophysics Data System (ADS)

Price, A. C.; Weeraratne, D. S.; Kohler, M. D.; Rathnayaka, S.; Escobar, L., Sr.

2015-12-01

The North American and Pacific plate boundary is a unique example of past subduction of an oceanic spreading center which has involved oceanic plate capture and inception of a continental transform boundary that juxtaposes continental and oceanic lithosphere on a single plate. The amphibious ALBACORE seismic project (Asthenospheric and Lithospheric Broadband Architecture from the California Offshore Region Experiment) deployed 34 ocean bottom seismometers (OBS) on 15-35 Ma seafloor and offers a unique opportunity to compare the LAB in continental and oceanic lithosphere in one seismic study. Rayleigh waves were recorded simultaneously by our offshore array and 82 CISN network land stations from 2010-2011. Here we predict phase velocities for a starting shear wave velocity model for each of 5 regions in our study area and compare to observed phase velocities from our array in a least-squares sense that produces the best fit 1-D shear wave velocity structure for each region. Preliminary results for the deep ocean (seafloor 25-32 Ma) indicates high velocities reaching 4.5 km/s at depths of 50 km associated with the lithosphere for seafloor 25-32 Ma. A negative velocity gradient is observed below this which reaches a minimum of 4.0 km/s at 160 km depth. The mid-ocean region (age 13-25 Ma) indicates a slightly lower magnitude and shallower LVZ. The Inner Borderland displays the highest lithospheric velocities offshore reaching 4.8 km/s at 40 km depth indicating underplating. The base of the LVZ in the Borderland increases sharply from 4.0 km/s to 4.5 km/s at 80-150 km depth indicating partial melt and compositional changes. The LVZ displays a very gradual positive velocity gradient in all other regions such as the deep seafloor and continent reaching 4.5 km/s at 300 km depth. The deep ocean, Borderlands, and continental region each have unique lithospheric velocities, LAB depths, and LVZ character that indicate stark differences in mantle structure that occur on a single plate as well as across the continental margin.

Coupled anisotropic and isotropic tomography of the upper mantle beneath northern Fennoscandia - Application of a novel code AniTomo

NASA Astrophysics Data System (ADS)

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

2017-04-01

Seismological investigations of the continental mantle lithosphere, particularly its anisotropic structure, advance our understanding of plate tectonics and formation of continents. Orientation of the anisotropic fabrics reflects stress fields during the lithosphere origin and its later deformations. To contribute to studies of the large-scale upper-mantle anisotropy, we have developed code AniTomo for regional anisotropic tomography. AniTomo allows a simultaneous inversion of relative travel time residuals of teleseismic P waves for 3D distribution of isotropic-velocity perturbations and anisotropy in the upper mantle. Weak hexagonal anisotropy with symmetry axis oriented generally in 3D is assumed. The code was successfully tested on a large series of synthetic datasets and synthetic structures. In this contribution we present results of the first application of novel code AniTomo to real data, i.e., relative travel-time residuals of teleseismic P waves recorded during passive seismic experiment LAPNET in the northern Fennoscandia between 2007 and 2009. The region of Fennoscandia is a suitable choice for the first application of the new code. This Precambrian region is tectonically stable and has a thick anisotropic mantle lithosphere (Plomerova and Babuska, Lithos 2010) without significant thermal heterogeneities. In the resulting anisotropic model of the upper mantle beneath the northern Fennoscandia, the strongest anisotropy and the largest velocity perturbations concentrate in the mantle lithosphere. We delimit regions of laterally and vertically consistent anisotropy in the mantle-lithospheric part of the model. In general, the identified anisotropic regions correspond to domains detected by joint interpretation of lateral variations of the P- and SKS-wave anisotropic parameters (Plomerova et al., Solid Earth 2011). Particularly, the mantle lithosphere in the western part of the volume studied exhibits a distinct and uniform fabric that is sharply separated from the surrounding regions. The eastern boundary of this region gradually shifts westward with increasing depth in the tomographic model. We connect the retrieved domain-like anisotropic structure of the mantle lithosphere in the northern Fennoscandia with preserved fossil fabrics of the Archean micro-plates, accreted during the Precambrian orogenic processes.

Strength and Deformation Rate of Plate Boundaries: The Rheological Effects of Grain Size Reduction, Structure, and Serpentinization.

NASA Astrophysics Data System (ADS)

Montesi, L.; Gueydan, F.

2016-12-01

Global strain rate maps reveal 1000-fold contrasts between plate interiors, oceanic or continental diffuse plate boundaries and narrow plate boundaries. Here, we show that rheological models based on the concepts of shear zone localization and the evolution of rock structure upon strain can explain these strain rate contrasts. Ductile shear zones constitute a mechanical paradox in the lithosphere. As every plastic deformation mechanism is strain-rate-hardening, ductile rocks are expected to deform at low strain rate and low stress (broad zone of deformation). Localized ductile shear zones require either a localized forcing (locally high stress) or a thermal or structural anomaly in the shear zone; either can be inherited or develop progressively as rocks deform. We previously identified the most effective process at each depth level of the lithosphere. In the upper crust and middle crust, rocks fabric controls localization. Grain size reduction is the most efficient mechanism in the uppermost mantle. This analysis can be generalized to consider a complete lithospheric section. We assume strain rate does not vary with depth and that the depth-integrated strength of the lithospheric does not change over time, as the total force is controlled by external process such as mantle convection and plate and slab buoyancy. Reducing grain size from a coarse value typical of undeformed peridotite to a value in agreement with the stress level (piezometer) while letting that stress vary from depth to depth (the integrated stress remains the same) increases the lithospheric strain rate by about a factor of 1000. This can explain the development of diffuse plate boundaries. The slightly higher strain rate of continental plate boundary may reflect development of a layered rock fabric in the middle crust. Narrow plate boundaries require additional weakening process. The high heat flux near mid-ocean ridge implies a thin lithosphere, which enhances stress (for constant integrated stress). While this can increase strain rate by another factor of 1000, another process must generate the lithospheric thickness variation in the first place. One possibility is serpentinization, which reduces the strength of the brittle crust, especially when coupled with the development of a fabric in brittle faults.

Propagation of the Hawaiian-Emperor volcano chain by Pacific plate cooling stress

USGS Publications Warehouse

Stuart, W.D.; Foulger, G.R.; Barall, M.

2007-01-01

The lithosphere crack model, the main alternative to the mantle plume model for age-progressive magma emplacement along the Hawaiian-Emperor volcano chain, requires the maximum horizontal tensile stress to be normal to the volcano chain. However, published stress fields calculated from Pacific lithosphere tractions and body forces (e.g., subduction pull, basal drag, lithosphere density) are not optimal for southeast propagation of a stress-free, vertical tensile crack coincident with the Hawaiian segment of the Hawaiian-Emperor chain. Here we calculate the thermoelastic stress rate for present-day cooling of the Pacific plate using a spherical shell finite element representation of the plate geometry. We use observed seafloor isochrons and a standard model for lithosphere cooling to specify the time dependence of vertical temperature profiles. The calculated stress rate multiplied by a time increment (e.g., 1 m.y.) then gives a thermoelastic stress increment for the evolving Pacific plate. Near the Hawaiian chain position, the calculated stress increment in the lower part of the shell is tensional, with maximum tension normal to the chain direction. Near the projection of the chain trend to the southeast beyond Hawaii, the stress increment is compressive. This incremental stress field has the form necessary to maintain and propagate a tensile crack or similar lithosphere flaw and is thus consistent with the crack model for the Hawaiian volcano chain.?? 2007 The Geological Society of America.

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.

Metamorphic density controls on early-stage subduction dynamics

NASA Astrophysics Data System (ADS)

Duesterhoeft, Erik; Oberhänsli, Roland; Bousquet, Romain

2013-04-01

Subduction is primarily driven by the densification of the downgoing oceanic slab, due to dynamic P-T-fields in subduction zones. It is crucial to unravel slab densification induced by metamorphic reactions to understand the influence on plate dynamics. By analyzing the density and metamorphic structure of subduction zones, we may gain knowledge about the driving, metamorphic processes in a subduction zone like the eclogitization (i.e., the transformation of a MORB to an eclogite), the breakdown of hydrous minerals and the release of fluid or the generation of partial melts. We have therefore developed a 2D subduction zone model down to 250 km that is based on thermodynamic equilibrium assemblage computations. Our model computes the "metamorphic density" of rocks as a function of pressure, temperature and chemical composition using the Theriak-Domino software package at different time stages. We have used this model to investigate how the hydration, dehydration, partial melting and fractionation processes of rocks all influence the metamorphic density and greatly depend on the temperature field within subduction systems. These processes are commonly neglected by other approaches (e.g., gravitational or thermomechanical in nature) reproducing the density distribution within this tectonic setting. The process of eclogitization is assumed as being important to subduction dynamics, based on the very high density (3.6 g/cm3) of eclogitic rocks. The eclogitization in a MORB-type crust is possible only if the rock reaches the garnet phase stability field. This process is primarily temperature driven. Our model demonstrates that the initiation of eclogitization of the slab is not the only significant process that makes the descending slab denser and is responsible for the slab pull force. Indeed, our results show that the densification of the downgoing lithospheric mantle (due to an increase of pressure) starts in the early subduction stage and makes a significant contribution to the slab pull, where eclogitization does not occur. Thus, the lithospheric mantle acts as additional ballast below the sinking slab shortly after the initiation of subduction. Our calculation shows that the dogma of eclogitized basaltic, oceanic crust as the driving force of slab pull is overestimated during the early stage of subduction. These results improve our understanding of the force budget for slab pull during the intial and early stage of subduction. Therefore, the complex metamorphic structure of a slab and mantle wedge has an important impact on the development and dynamics of subduction zones. Further Reading: Duesterhoeft, Oberhänsli & Bousquet (2013), submitted to Earth and Planetary Science Letters

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.

Seismic structure of the lithosphere and upper mantle beneath the ocean islands near mid-oceanic ridges

NASA Astrophysics Data System (ADS)

Haldar, C.; Kumar, P.; Kumar, M. Ravi

2014-05-01

Deciphering the seismic character of the young lithosphere near mid-oceanic ridges (MORs) is a challenging endeavor. In this study, we determine the seismic structure of the oceanic plate near the MORs using the P-to-S conversions isolated from quality data recorded at five broadband seismological stations situated on ocean islands in their vicinity. Estimates of the crustal and lithospheric thickness values from waveform inversion of the P-receiver function stacks at individual stations reveal that the Moho depth varies between ~ 10 ± 1 km and ~ 20 ± 1 km with the depths of the lithosphere-asthenosphere boundary (LAB) varying between ~ 40 ± 4 and ~ 65 ± 7 km. We found evidence for an additional low-velocity layer below the expected LAB depths at stations on Ascension, São Jorge and Easter islands. The layer probably relates to the presence of a hot spot corresponding to a magma chamber. Further, thinning of the upper mantle transition zone suggests a hotter mantle transition zone due to the possible presence of plumes in the mantle beneath the stations.

Formation of fold and thrust belts on Venus due to horizontal shortening of a laterally heterogeneous lithosphere

NASA Technical Reports Server (NTRS)

Zuber, M. T.; Parmentier, E. M.; Neumann, G. A.

1994-01-01

An outstanding question relevant to understanding the tectonics of Venus is the mechanism of formation of fold and thrust belts, such as the mountain belts that surround Lakshmi Planum in western Ishtar Terra. These structures are typically long (hundreds of km) and narrow (many tens of km), and are often located at the margins of relatively high (km-scale) topographic rises. Previous studies have attempted to explain fold and thrust belts in various areas of Venus in the context of viscous and brittle wedge theory. However, while wedge theory can explain the change in elevation from the rise to the adjacent lowland, it fails to account for a fundamental aspect of the deformation, i.e., the topographic high at the edge of the rise. In this study we quantitatively explore the hypothesis that fold and thrust belt morphology on Venus can alternatively be explained by horizontal shortening of a lithosphere that is laterally heterogeneous, due either to a change in thickness of the lithosphere or the crust. Lateral heterogeneities in lithosphere structure may arise in response to thermal thinning or extensive faulting, while variations in crustal thickness may arise due to either spatially variable melting of mantle material or by horizontal shortening of the crust. In a variable thickness lithosphere or crust that is horizontally shortened, deformation will tend to localize in the vicinity of thickness heterogeneity, resulting in a higher component of dynamic topography there as compared to elsewhere in the shortening lithosphere. This mechanism may thus provide a simple explanation for the topographic high at the edge of the rise.

Formation of fold and thrust belts on Venus due to horizontal shortening of a laterally heterogeneous lithosphere

NASA Astrophysics Data System (ADS)

Zuber, M. T.; Parmentier, E. M.; Neumann, G. A.

1994-03-01

An outstanding question relevant to understanding the tectonics of Venus is the mechanism of formation of fold and thrust belts, such as the mountain belts that surround Lakshmi Planum in western Ishtar Terra. These structures are typically long (hundreds of km) and narrow (many tens of km), and are often located at the margins of relatively high (km-scale) topographic rises. Previous studies have attempted to explain fold and thrust belts in various areas of Venus in the context of viscous and brittle wedge theory. However, while wedge theory can explain the change in elevation from the rise to the adjacent lowland, it fails to account for a fundamental aspect of the deformation, i.e., the topographic high at the edge of the rise. In this study we quantitatively explore the hypothesis that fold and thrust belt morphology on Venus can alternatively be explained by horizontal shortening of a lithosphere that is laterally heterogeneous, due either to a change in thickness of the lithosphere or the crust. Lateral heterogeneities in lithosphere structure may arise in response to thermal thinning or extensive faulting, while variations in crustal thickness may arise due to either spatially variable melting of mantle material or by horizontal shortening of the crust. In a variable thickness lithosphere or crust that is horizontally shortened, deformation will tend to localize in the vicinity of thickness heterogeneity, resulting in a higher component of dynamic topography there as compared to elsewhere in the shortening lithosphere. This mechanism may thus provide a simple explanation for the topographic high at the edge of the rise.

The effect of dynamic topography and gravity on lithospheric effective elastic thickness estimation: a case study

NASA Astrophysics Data System (ADS)

Bai, Yongliang; Dong, Dongdong; Kirby, Jon F.; Williams, Simon E.; Wang, Zhenjie

2018-04-01

Lithospheric effective elastic thickness (Te), a proxy for plate strength, is helpful for the understanding of subduction characteristics. Affected by curvature, faulting and magma activity, lithospheric strength near trenches should be weakened but some regional inversion studies have shown much higher Te values along some trenches than in their surroundings. In order to improve Te estimation accuracy, here we discuss the long-wavelength effect of dynamic topography and gravity on Te estimation by taking the Izu-Bonin-Mariana (IBM) Trench as a case study area. We estimate the long-wavelength influence of the density and negative buoyancy of the subducting slab on observed gravity anomalies and seafloor topography. The residual topography and gravity are used to map Te using the fan-wavelet coherence method. Maps of Te, both with and without the effects of dynamic topography and slab gravity anomaly, contain a band of high-Te values along the IBM Trench, though these values and their errors are lower when slab effects are accounted for. Nevertheless, tests show that the Te map is relatively insensitive to the choice of slab-density modelling method, even though the dynamic topography and slab-induced gravity anomaly vary considerably when the slab density is modelled by different methods. The continued presence of a high-Te band along the trench after application of dynamic corrections shows that, before using 2D inversion methods to estimate Te variations in subduction zones, there are other factors that should be considered besides the slab dynamic effects on the overriding plate.

Buoyancy-driven, rapid exhumation of ultrahigh-pressure metamorphosed continental crust.

PubMed

Ernst, W G; Maruyama, S; Wallis, S

1997-09-02

Preservation of ultrahigh-pressure (UHP) minerals formed at depths of 90-125 km require unusual conditions. Our subduction model involves underflow of a salient (250 +/- 150 km wide, 90-125 km long) of continental crust embedded in cold, largely oceanic crust-capped lithosphere; loss of leading portions of the high-density oceanic lithosphere by slab break-off, as increasing volumes of microcontinental material enter the subduction zone; buoyancy-driven return toward midcrustal levels of a thin (2-15 km thick), low-density slice; finally, uplift, backfolding, normal faulting, and exposure of the UHP terrane. Sustained over approximately 20 million years, rapid ( approximately 5 mm/year) exhumation of the thin-aspect ratio UHP sialic sheet caught between cooler hanging-wall plate and refrigerating, downgoing lithosphere allows withdrawal of heat along both its upper and lower surfaces. The intracratonal position of most UHP complexes reflects consumption of an intervening ocean basin and introduction of a sialic promontory into the subduction zone. UHP metamorphic terranes consist chiefly of transformed, yet relatively low-density continental crust compared with displaced mantle material-otherwise such complexes could not return to shallow depths. Relatively rare metabasaltic, metagabbroic, and metacherty lithologies retain traces of phases characteristic of UHP conditions because they are massive, virtually impervious to fluids, and nearly anhydrous. In contrast, H2O-rich quartzofeldspathic, gneissose/schistose, more permeable metasedimentary and metagranitic units have backreacted thoroughly, so coesite and other UHP silicates are exceedingly rare. Because of the initial presence of biogenic carbon, and its especially sluggish transformation rate, UHP paragneisses contain the most abundantly preserved crustal diamonds.

Buoyancy-driven, rapid exhumation of ultrahigh-pressure metamorphosed continental crust

PubMed Central

Ernst, W. G.; Maruyama, S.; Wallis, S.

1997-01-01

Preservation of ultrahigh-pressure (UHP) minerals formed at depths of 90–125 km require unusual conditions. Our subduction model involves underflow of a salient (250 ± 150 km wide, 90–125 km long) of continental crust embedded in cold, largely oceanic crust-capped lithosphere; loss of leading portions of the high-density oceanic lithosphere by slab break-off, as increasing volumes of microcontinental material enter the subduction zone; buoyancy-driven return toward midcrustal levels of a thin (2–15 km thick), low-density slice; finally, uplift, backfolding, normal faulting, and exposure of the UHP terrane. Sustained over ≈20 million years, rapid (≈5 mm/year) exhumation of the thin-aspect ratio UHP sialic sheet caught between cooler hanging-wall plate and refrigerating, downgoing lithosphere allows withdrawal of heat along both its upper and lower surfaces. The intracratonal position of most UHP complexes reflects consumption of an intervening ocean basin and introduction of a sialic promontory into the subduction zone. UHP metamorphic terranes consist chiefly of transformed, yet relatively low-density continental crust compared with displaced mantle material—otherwise such complexes could not return to shallow depths. Relatively rare metabasaltic, metagabbroic, and metacherty lithologies retain traces of phases characteristic of UHP conditions because they are massive, virtually impervious to fluids, and nearly anhydrous. In contrast, H2O-rich quartzofeldspathic, gneissose/schistose, more permeable metasedimentary and metagranitic units have backreacted thoroughly, so coesite and other UHP silicates are exceedingly rare. Because of the initial presence of biogenic carbon, and its especially sluggish transformation rate, UHP paragneisses contain the most abundantly preserved crustal diamonds. PMID:11038569

The effect of dynamic topography and gravity on lithospheric effective elastic thickness estimation: a case study

NASA Astrophysics Data System (ADS)

Bai, Yongliang; Dong, Dongdong; Kirby, Jon F.; Williams, Simon E.; Wang, Zhenjie

2018-07-01

Lithospheric effective elastic thickness (Te), a proxy for plIate strength, is helpful for the understanding of subduction characteristics. Affected by curvature, faulting and magma activity, lithospheric strength near trenches should be weakened but some regional inversion studies have shown much higher Te values along some trenches than in their surroundings. In order to improve Te-estimation accuracy, here we discuss the long-wavelength effect of dynamic topography and gravity on Te estimation by taking the Izu-Bonin-Mariana (IBM) Trench as a case study area. We estimate the long-wavelength influence of the density and negative buoyancy of the subducting slab on observed gravity anomalies and seafloor topography. The residual topography and gravity are used to map Te using the fan-wavelet coherence method. Maps of Te, both with and without the effects of dynamic topography and slab gravity anomaly, contain a band of high-Te values along the IBM Trench, though these values and their errors are lower when slab effects are accounted for. Nevertheless, tests show that the Te map is relatively insensitive to the choice of slab-density modelling method, even though the dynamic topography and slab-induced gravity anomaly vary considerably when the slab density is modelled by different methods. The continued presence of a high-Te band along the trench after application of dynamic corrections shows that, before using 2-D inversion methods to estimate Te variations in subduction zones, there are other factors that should be considered besides the slab dynamic effects on the overriding plate.

Buoyancy-Driven, Rapid Exhumation of Ultrahigh-Pressure Metamorphosed Continental Crust

NASA Astrophysics Data System (ADS)

Ernst, W. G.; Maruyama, S.; Wallis, S.

1997-09-01

Preservation of ultrahigh-pressure (UHP) minerals formed at depths of 90-125 km require unusual conditions. Our subduction model involves underflow of a salient (250 ± 150 km wide, 90-125 km long) of continental crust embedded in cold, largely oceanic crust-capped lithosphere; loss of leading portions of the high-density oceanic lithosphere by slab break-off, as increasing volumes of microcontinental material enter the subduction zone; buoyancy-driven return toward midcrustal levels of a thin (2-15 km thick), low-density slice; finally, uplift, backfolding, normal faulting, and exposure of the UHP terrane. Sustained over ≈ 20 million years, rapid (≈ 5 mm/year) exhumation of the thin-aspect ratio UHP sialic sheet caught between cooler hanging-wall plate and refrigerating, downgoing lithosphere allows withdrawal of heat along both its upper and lower surfaces. The intracratonal position of most UHP complexes reflects consumption of an intervening ocean basin and introduction of a sialic promontory into the subduction zone. UHP metamorphic terranes consist chiefly of transformed, yet relatively low-density continental crust compared with displaced mantle material--otherwise such complexes could not return to shallow depths. Relatively rare metabasaltic, metagabbroic, and metacherty lithologies retain traces of phases characteristic of UHP conditions because they are massive, virtually impervious to fluids, and nearly anhydrous. In contrast, H2O-rich quartzofeldspathic, gneissose/schistose, more permeable metasedimentary and metagranitic units have backreacted thoroughly, so coesite and other UHP silicates are exceedingly rare. Because of the initial presence of biogenic carbon, and its especially sluggish transformation rate, UHP paragneisses contain the most abundantly preserved crustal diamonds.

Tectonic evolution and mantle structure of the Caribbean

NASA Astrophysics Data System (ADS)

van Benthem, Steven; Govers, Rob; Spakman, Wim; Wortel, Rinus

2013-06-01

investigate whether predictions of mantle structure from tectonic reconstructions are in agreement with a detailed tomographic image of seismic P wave velocity structure under the Caribbean region. In the upper mantle, positive seismic anomalies are imaged under the Lesser Antilles and Puerto Rico. These anomalies are interpreted as remnants of Atlantic lithosphere subduction and confirm tectonic reconstructions that suggest at least 1100 km of convergence at the Lesser Antilles island arc during the past 45 Myr. The imaged Lesser Antilles slab consists of a northern and southern anomaly, separated by a low-velocity anomaly across most of the upper mantle, which we interpret as the subducted North America-South America plate boundary. The southern edge of the imaged Lesser Antilles slab agrees with vertical tearing of South America lithosphere. The northern Lesser Antilles slab is continuous with the Puerto Rico slab along the northeastern plate boundary. This results in an amphitheater-shaped slab, and it is interpreted as westward subducting North America lithosphere that remained attached to the surface along the northeastern boundary of the Caribbean plate. At the Muertos Trough, however, material is imaged until a depth of only 100 km, suggesting a small amount of subduction. The location and length of the imaged South Caribbean slab agrees with proposed subduction of Caribbean lithosphere under the northern South America plate. An anomaly related to proposed Oligocene subduction at the Nicaragua rise is absent in the tomographic model. Beneath Panama, a subduction window exists across the upper mantle, which is related to the cessation of subduction of the Nazca plate under Panama since 9.5 Ma and possibly the preceding subduction of the extinct Cocos-Nazca spreading center. In the lower mantle, two large anomaly patterns are imaged. The westernmost anomaly agrees with the subduction of Farallon lithosphere. The second lower mantle anomaly is found east of the Farallon anomaly and is interpreted as a remnant of the late Mesozoic subduction of North and South America oceanic lithosphere at the Greater Antilles, Aves ridge, and Leeward Antilles. The imaged mantle structure does not allow us to discriminate between an "Intra-Americas origin" and a "Pacific origin" of the Caribbean plate.

Tectonic evolution and mantle structure of the Caribbean

NASA Astrophysics Data System (ADS)

Benthem, Steven; Govers, Rob; Spakman, Wim; Wortel, Rinus

2013-06-01

investigate whether predictions of mantle structure from tectonic reconstructions are in agreement with a detailed tomographic image of seismic P wave velocity structure under the Caribbean region. In the upper mantle, positive seismic anomalies are imaged under the Lesser Antilles and Puerto Rico. These anomalies are interpreted as remnants of Atlantic lithosphere subduction and confirm tectonic reconstructions that suggest at least 1100 km of convergence at the Lesser Antilles island arc during the past ~45 Myr. The imaged Lesser Antilles slab consists of a northern and southern anomaly, separated by a low-velocity anomaly across most of the upper mantle, which we interpret as the subducted North America-South America plate boundary. The southern edge of the imaged Lesser Antilles slab agrees with vertical tearing of South America lithosphere. The northern Lesser Antilles slab is continuous with the Puerto Rico slab along the northeastern plate boundary. This results in an amphitheater-shaped slab, and it is interpreted as westward subducting North America lithosphere that remained attached to the surface along the northeastern boundary of the Caribbean plate. At the Muertos Trough, however, material is imaged until a depth of only 100 km, suggesting a small amount of subduction. The location and length of the imaged South Caribbean slab agrees with proposed subduction of Caribbean lithosphere under the northern South America plate. An anomaly related to proposed Oligocene subduction at the Nicaragua rise is absent in the tomographic model. Beneath Panama, a subduction window exists across the upper mantle, which is related to the cessation of subduction of the Nazca plate under Panama since 9.5 Ma and possibly the preceding subduction of the extinct Cocos-Nazca spreading center. In the lower mantle, two large anomaly patterns are imaged. The westernmost anomaly agrees with the subduction of Farallon lithosphere. The second lower mantle anomaly is found east of the Farallon anomaly and is interpreted as a remnant of the late Mesozoic subduction of North and South America oceanic lithosphere at the Greater Antilles, Aves ridge, and Leeward Antilles. The imaged mantle structure does not allow us to discriminate between an "Intra-Americas origin" and a "Pacific origin" of the Caribbean plate.

Three dimensional lithospheric magnetization structures beneath Australia derived by inverse modeling of CHAMP satellite magnetic field model

NASA Astrophysics Data System (ADS)

Du, Jinsong; Chen, Chao; Lesur, Vincent; Li, Yaoguo; Lane, Richard; Liang, Qing; Wang, Haoran

2014-05-01

We present an inversion algorithm for magnetic anomaly data in spherical coordinates to image the three dimensional (3-D) susceptibility distributions in the lithosphere. The method assumes that remanent magnetization is absent and that the magnetic anomalies are solely the result of lateral variations in magnetic susceptibility. To take into account the curvature of the Earth, the 3-D model is comprised of a set of spherical prisms (referred to as tesseroids), each of which has a constant isotropic susceptibility. The inversion method is formulated with a specifically designed model objective function and radial weighting function in spherical coordinates. A Tikhonov regularization technique is used to obtain an optimal solution with data misfit consistent with the estimated error level. Results for regional synthetic models with different magnetized inclinations and declinations are presented to demonstrate the capability of the method to recover large scale lithospheric magnetic structures. We have applied the algoithm to study the lithospheric susceptibility structures in the Australia region using magnetic anomaly data from the GRIMM_L120v0.0 model, which is based on ten years of CHAMP satellite data. As a self-constrained inversion, the maximum depths variation of magnetization layer is estimated first and then incorporated to the three dimensional (3-D) inversion. Results showed that the susceptibility anomalies concentrate in the depth range from 25 km to 45 km, i.e., focused in the lower crust. In addition, the results showed that the susceptibilities in continental lithosphere are higher than those in oceanic lithosphere. The inverted 3-D susceptibility distribution in the region of Australia reveals significant features related to tectonics, surface heat-flux, crustal thickness and Curie isotherm depths. In general, the higher susceptibility anomalies correlate with Precambrian rocks, and the lower susceptibility anomalies correlate with younger orogenic belts, suture zones and modern uplifts. In details, the inverted susceptibility distribution shows differences in the magnetic structures between the eastern and western parts of the Yilgarn Craton, and three lower susceptibility belts from north to south in the Eromanga Basin and the Gawler Craton with high susceptibility that extend to the ocean and then to the west.

Crust and lithosphere structure in the eastern segment of the Xing-Meng orogenic belt from S-receiver function

NASA Astrophysics Data System (ADS)

Zhang, R.; Wu, Q.

2013-12-01

From 2009 to 2011, a 60 station broadband seismic array extending over 1200km was deployed in northeast China (NEC) by the Institute of geophysics, China Earthquake Administration (CEA). The recently linear deployment of seismic array in Northeast China (NEC) facilitated collection of more high-quality broadband data, thus provide us an opportunity to use S-wave receiver functions to investigate its crustal and mantle lithosphere structure with high resolution. Two distinct signals with large amplitude can be identified in our imaged S receiver functions. The strong positive one from the Moho can be observed continuously at depths from 40 km beneath Great Xing'an Range to less than 30 km beneath the Songliao Basin. The imaged Moho agrees with previous estimate of crustal thickness, and the lateral variations correlate to its surface tomography. The deep negative Sp phase interpreted as from the lithosphere-asthenosphere boundary (LAB) is as shallow as ~100km in the Songliao basin, down to 140-160km in the westward of Xingmeng block. The boundary is less prominent east of the Songliao Basin. The imaged Moho and LAB structure indicate the crust and lithosphere thinning in the Songliao Basin, and the vertical thinning of LAB is more obvious, evidence in a depth variation up to 50 km. The Songliao Basin is a continental rifting where a large amount of extension occurs, and the coupling of thinning between in the crust and underlying lithosphere indicated that the lithosphere stretching may be involved to the crustal rifting. The stretching can be more explained by the pure shear regime proposed in extensional tectonics. Acknowledgments. Seismic data were collected by the by the Institute of Geophysics, China Earthquake Administration. This work was supported by the NSF of China (grants 40974061, 90814013), the Chinese government's executive program (SinoProbe-02-03) and the international cooperation project of the Ministry of Science and Technology of China (2011DFB20120).

Comprehensive analysis of Curie-point depths and lithospheric effective elastic thickness at Arctic Region

NASA Astrophysics Data System (ADS)

Lu, Y.; Li, C. F.

2017-12-01

Arctic Ocean remains at the forefront of geological exploration. Here we investigate its deep geological structures and geodynamics on the basis of gravity, magnetic and bathymetric data. We estimate Curie-point depth and lithospheric effective elastic thickness to understand deep geothermal structures and Arctic lithospheric evolution. A fractal exponent of 3.0 for the 3D magnetization model is used in the Curie-point depth inversion. The result shows that Curie-point depths are between 5 and 50 km. Curie depths are mostly small near the active mid-ocean ridges, corresponding well to high heat flow and active shallow volcanism. Large curie depths are distributed mainly at continental marginal seas around the Arctic Ocean. We present a map of effective elastic thickness (Te) of the lithosphere using a multitaper coherence technique, and Te are between 5 and 110 km. Te primarily depends on geothermal gradient and composition, as well as structures in the lithosphere. We find that Te and Curie-point depths are often correlated. Large Te are distributed mainly at continental region and small Te are distributed at oceanic region. The Alpha-Mendeleyev Ridge (AMR) and The Svalbard Archipelago (SA) are symmetrical with the mid-ocean ridge. AMR and SA were formed before an early stage of Eurasian basin spreading, and they are considered as conjugate large igneous provinces, which show small Te and Curie-point depths. Novaya Zemlya region has large Curie-point depths and small Te. We consider that fault and fracture near the Novaya Zemlya orogenic belt cause small Te. A series of transform faults connect Arctic mid-ocean ridge with North Atlantic mid-ocean ridge. We can see large Te near transform faults, but small Curie-point depths. We consider that although temperature near transform faults is high, but mechanically the lithosphere near transform faults are strengthened.

Origin of Starting Earthquakes under Complete Coupling of the Lithosphere Plates and a Base

NASA Astrophysics Data System (ADS)

Babeshko, V. A.; Evdokimova, O. V.; Babeshko, O. M.; Zaretskaya, M. V.; Gorshkova, E. M.; Mukhin, A. S.; Gladskoi, I. B.

2018-02-01

The boundary problem of rigid coupling of lithospheric plates modeled by Kirchhoff plates with a base represented by a three-dimensional deformable layered medium is considered. The possibility of occurrence of a starting earthquake in such a block structure is investigated. For this purpose, two states of this medium in the static mode are considered. In the first case, the semi-infinite lithospheric plates in the form of half-planes are at a distance so that the distance between the end faces is different from zero. In the second case, the lithospheric plates come together to zero spacing between them. Calculations have shown that in this case more complex movements of the Earth's surface are possible. Among such movements are the cases described in our previous publications [1, 2].

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.

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.

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.

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.

Tectonic evolution and mantle structure of the Caribbean

NASA Astrophysics Data System (ADS)

van Benthem, Steven; Govers, Rob; Spakman, Wim; Wortel, Rinus

2013-04-01

In the broad context of investigating the relationship between deep structure & processes and surface expressions, we study the Caribbean plate and underlying mantle. We investigate whether predictions of mantle structure from tectonic reconstructions are in agreement with a detailed tomographic image of seismic P-wave velocity structure under the Caribbean region. In the upper mantle, positive seismic anomalies are imaged under the Lesser Antilles and Puerto Rico. These anomalies are interpreted as remnants of Atlantic lithosphere subduction and confirm tectonic reconstructions that suggest at least 1100 km of convergence at the Lesser Antilles island arc during the past ~45 Myr. The imaged Lesser-Antilles slab consists of a northern and southern anomaly, separated by a low velocity anomaly across most of the upper mantle, which we interpret as the subducted North-South America plate boundary. The southern edge of the imaged Lesser Antilles slab agrees with vertical tearing of South America lithosphere. The northern Lesser Antilles slab is continuous with the Puerto Rico slab along the northeastern plate boundary. This results in an amphitheater-shaped slab and it is interpreted as westward subducting North America lithosphere that remained attached to the surface along the northern boundary. At the Muertos Trough, however, material is imaged until a depth of only 100 km, suggesting a small amount of subduction. The location and length of the imaged South Caribbean slab agrees with proposed subduction of Caribbean lithosphere under the northern South America plate. An anomaly related to proposed Oligocene subduction at the Nicaragua rise is absent in the tomographic model. Beneath Panama, a subduction window exists across the upper mantle, which is related to the cessation of subduction of the Nazca plate under Panama since 9.5 Ma and possibly the preceding subduction of the extinct Cocos-Nazca spreading center. In the lower mantle two large anomaly patterns are imaged. The westernmost anomaly agrees with the subduction of Farallon lithosphere. The second lower mantle anomaly is found east of the Farallon anomaly and is interpreted as a remnant of the late Mesozoic subduction of North and South America oceanic lithosphere at the Greater Antilles, Aves ridge and Leeward Antilles. The imaged mantle structure does not allow us to discriminate between an 'Intra-Americas' origin and a 'Pacific origin' of the Caribbean plate.

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.

THE LITHOSPHERIC MAGNETIC FIELD ALONG THE NISHAN-GUYANG PROFILE, CHINA 6 datesYuan Jiehao1; Gu Zuowen1; Chen Bin1; Xu Rugang2; Wang Lie2; Wang Can1 1. Institute of Geophysics, China Earthquake Administration(IGP CEA), yjh810405@163.com 2. Anhui Earthquake Administration

NASA Astrophysics Data System (ADS)

Yuan, J.

2014-12-01

In order to research the lithospheric magnetic field and the magnetization structure, the geomagnetic field was surveyed along the Nishan-Guyang profile with 900 km long in the Neimenggu and Shanxi provinces in China. The distance between the adjacent geomagnetic sites is about 7 km along the profile. The geomagnetic data were analyzed, and the lithospheric magnetic field was obtained. Using the upward continuation method, the geomagnetic anomalies in different depths were obtained: the basement anomaly, the upper crust anomaly and the superficial anomaly. Basing on these geomagnetic anomalies, the magnetization structure was obtained by using the software of the ModelVision 11.0. The preliminary results show: (1) The geomagnetic anomalies have good relationship with the local geological structure. (2) The magnetization structure is related to the depth variation of the Curie interface, the values of the magnetic susceptibility, the seismic activity and the tectonic block.

An integrated geophysical study of the lithospheric structure beneath Libya

NASA Astrophysics Data System (ADS)

Brown, Wesley A.

This doctoral dissertation constitutes an integrated geophysical investigation of the lithospheric structure in the region of Libya. It is separated into three sections, each of which will be submitted to different scientific journals for publication. In the first part of the study, I utilized a seamless mosaicking approach based on the commercial Environment for Visualizing Images (ENVI) software package to create mosaics of two geologically interesting portions of Libya. In this study I present a step by step method of mosaicking Landsat 4 satellite images. Firstly, I performed histogram matching to give images the same color scale, then I used a cutline feathering technique to blend suture areas and finally I overlaid the images to form the two mosaics. The resulting mosaics were then combined with structural features and the seismicity map of the area. The resulting mosaics were proven to be useful in identifying recently active faults and shows great potential for verification of other faults and in natural hazard assessment. For the second portion of my research, I made use of over 6,000 free air corrected gravity data in conjunction with other geological and geophysical data to develop a 3D density model for northern Libya. I used a gravity modeling program (SURFGRAV) to develop the 3D density model by manipulating it to accurately predict large areas of Free Air anomaly shown in the data. The residual gravity anomaly values were calculated by subtracting predicted Free Air anomaly from the observed Free Air anomaly. The results were satisfactory for uplifted areas of Libya while there were significant mismatches in basin areas. The density model was iterated and used as a starting model for the final portion of the study. In the last part of this research, I used the Nafe-Drake relationship along with other geological data to convert the 3D density model to a 3D velocity model (LIBYA3D) for the region. Two earthquakes having source receiver paths sampling much of the modeled area were used to perform 1D and 1.5D validation tests, and the results were compared to those from previous studies. The results showed that the new 3D velocity model is valid and superior to the global model. However, until there is sufficient earthquake data acquired, and we are able to perform 2D and 3D modeling we may not be able to see the true improvement of LIBYA3D as compared to the other regional models.

From underplating to delamination-retreat in the northern Apennines

NASA Astrophysics Data System (ADS)

Chiarabba, C.; Giacomuzzi, G.; Bianchi, I.; Agostinetti, N. P.; Park, J.

2014-10-01

Recordings of teleseismic earthquakes from a dense set of temporary and permanent broadband seismic stations reveal the lithospheric structure of the northern Apennines and support the scenario of a retreating detachment within the mid-crust. Lithospheric delamination appears crucial to the formation and evolution of the Apennines orogen. Receiver-function (RF) stacks outline a continuous west-dipping Ps converted phase from a positive velocity jump that we interpret as the top of the lower crust and mantle of the Adria continental lithosphere, which is descending into the shallow mantle. The correlation of seismicity with two RF profiles across the northern Apennines suggests distinct stages of lithospheric delamination. Active penetration of the detachment into the Adria lithosphere seems evident in the south/east, with induced shallow-mantle flow facilitated by slab dehydration. Penetration of the detachment in the north/west seems to have arrested, and is possibly marked by crustal underplating. This layer atop the Apennines slab is visible only down to 80 km depth and suspends above an oppositely-dipping paired positive/negative Ps converted phase in stacked receiver functions. The break in the west-dipping Adria lithosphere conflicts with a westward-subduction scenario continuous from the Oligocene. Lateral changes of deep structure and seismicity along the northern Apennines suggest that underplating of crustal material and delamination-retreat are distinct mechanisms active today in the western and eastern sectors, respectively, of the northern Apennines. Negative Ps-pulses at 100-120 km depth help to define a seismic lithosphere-asthenosphere boundary (LAB), but cross-cut a volume of high-velocity mantle rock, as inferred from tomographic models. We hypothesize that this seismic LAB is a rheological discontinuity that affects the frequency band of seismic body waves, but not the long-term viscous response that governs the evolution and eventual detachment of the continental slab.

Quasi-quantitative analysis of the lithospheric rheology across an incipient continental rift based on 3-D magnetotelluric imaging of Linfen Basin within the North China Craton

NASA Astrophysics Data System (ADS)

Yin, Y.; Jin, S.; Wei, W.; Ye, G.; Dong, H.; Zhang, L.

2017-12-01

The Shanxi Rift being located within the interior of the North China Craton and far from any plate boundaries has undergone dramatic deformation and seismicity during the Cenozoic. In this study, we build 3-D lithospheric resistivity model by MT array data, across the Linfen Basin which is the most active segment of this intraplate rift. Accordingly, combined with previous rock physics experimental results, we estimate the fluid contents of lower crustal granulites and upper mantle peridotites and thereby the rough distribution of lithospheric rheological strength. On the two sides of Linfen Basin, lithosphere beneath the Precambrian terranes are of high strength. By contrast, a high-conductivity nearly upright lithosphere weak zone occurs beneath the eastern margin of the Linfen Basin and appears to be connected to the high-conductivity and therefore weak lower crust just beneath the basin, probably indicating a structure of asthenospheric upwelling causing the lower crustal decoupling through lateral drag forces. The distribution of lithospheric weak zones, brittle faults, ductile shear zones and detachment structures determined from our resistivity model is in good agreement with the 8-My stage model of a previous numerical geodynamic simulation for continental rift evolution by reconstruction of the South Atlantic plate. Accordingly, we suggest that the lithospheric weak zone could be a preexisting Precambrian shear zone and has reactivated as an asthenospheric upwelling conduit under the far-field effects of Indo- Asian collision or Pacific Plate subduction since the late Mesozoic. This process could have caused the upper crustal extension and rifting through the stress regulation by the plastic lower crust, which could be the mechanism of rift formation. In summary, we suggest the Linfen segment of the Shanxi Rift, is a simple shear mode rift in the incipient stage of rift evolution, rather than a mature pure shear mode one as determined by precious seismic imaging.

Modeling Archean Subduction Initiation from Continental Spreading with a Free-Surface

NASA Astrophysics Data System (ADS)

Adams, A.; Thielmann, M.; Golabek, G.

2017-12-01

Earth is the only planet known to have plate tectonics, however the onset of plate tectonics and Earth's early tectonic environment are highly uncertain. Modern plate tectonics are characterized by the sinking of dense lithosphere at subduction zones; however this process may not have been feasible if Earth's interior was hotter in the Archean, resulting in thicker and more buoyant oceanic lithosphere than observed at present [van Hunen and van den Berg, 2008]. Previous studies have proposed gravitational spreading of early continents at passive margins as a mechanism to trigger early episodes of plate subduction using numerical simulations with a free-slip upper boundary condition [Rey et al., 2014]. This study utilizes 2D thermo-mechanical numerical experiments using the finite element code MVEP2 [Kaus, 2010; Thielmann et al., 2014] to investigate the viability of this mechanism for subduction initiation in an Archean mantle for both free-slip and free-surface models. Radiogenic heating, strain weakening, and eclogitization were systematically implemented to determine critical factors for modeling subduction initiation. In free-slip models, results show episodes of continent spreading and subduction initiation of oceanic lithosphere for low limiting yield stresses (100-150 MPa) and increasing continent width with no dependency on radiogenic heating, strain weakening, or eclogitization. For models with a free-surface, subduction initiation was observed at low limiting yield stresses (100-225 MPa) with increasing continent width and only in models with eclogitization. Initial lithospheric stress states were studied as a function of density and viscosity ratios between continent and oceanic lithosphere, and results indicate the magnitude of lithospheric stresses increases with increasing continental buoyancy. This work suggests continent spreading may trigger episodes of subduction in models with a free-surface with critical factors being low limiting yield stresses and eclogitization.

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.

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.

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.

Parallel Extension Tectonics (PET): Early Cretaceous tectonic extension of the Eastern Eurasian continent

NASA Astrophysics Data System (ADS)

Liu, Junlai; Ji, Mo; Ni, Jinlong; Guan, Huimei; Shen, Liang

2017-04-01

The present study reports progress of our recent studies on the extensional structures in eastern North China craton and contiguous areas. We focus on characterizing and timing the formation/exhumation of the extensional structures, the Liaonan metamorphic core complex (mcc) and the Dayingzi basin from the Liaodong peninsula, the Queshan mcc, the Wulian mcc and the Zhucheng basin from the Jiaodong peninsula, and the Dashan magmatic dome within the Sulu orogenic belt. Magmatic rocks (either volcanic or plutonic) are ubiquitous in association with the tectonic extension (both syn- and post-kinematic). Evidence for crustal-mantle magma mixing are popular in many syn-kinematic intrusions. Geochemical analysis reveals that basaltic, andesitic to rhyolitic magmas were generated during the tectonic extension. Sr-Nd isotopes of the syn-kinematic magmatic rocks suggest that they were dominantly originated from ancient or juvenile crust partly with mantle signatures. Post-kinematic mafic intrusions with ages from ca. 121 Ma to Cenozoic, however, are of characteristic oceanic island basalts (OIB)-like trace element distribution patterns and relatively depleted radiogenic Sr-Nd isotope compositions. Integrated studies on the extensional structures, geochemical signatures of syn-kinematic magmatic rocks (mostly of granitic) and the tectono-magmatic relationships suggest that extension of the crust and the mantle lithosphere triggered the magmatisms from both the crust and the mantle. The Early Cretaceous tectono-magmatic evolution of the eastern Eurasian continent is governed by the PET in which the tectonic processes is subdivided into two stages, i.e. an early stage of tectonic extension, and a late stage of collapse of the extended lithosphere and transformation of lithospheric mantle. During the early stage, tectonic extension of the lithosphere led to detachment faulting in both the crust and mantle, resulted in the loss of some of the subcontinental roots, gave rise to the exhumation of the mccs, and triggered plutonic emplacement and volcanic eruptions of hybrid magmas. During the late stage, the nature of mantle lithosphere in North China was changed from the ancient SCLM to the juvenile SCLM. Extensional structures in eastern Eurasian continent provide a general architecture of the extensional tectonics of a rifted continent. Progressive extension resulted a sudden collaps of the crust (lithosphere) at ca. 130 to 120 Ma, associated with exhumation of mcc's and giant syn-kinematic magmatism, and post-kinematic magmatism. Parallel extension of both the crust and the mantle resulted in detachment faulting and magmatism, and also contributed to inhomogeneous thinning of the NCC lithosphere. Paleo-Pacific plate subduction and roll-back of the subducting oceanic plate contributed to the PET tectonic processes.

Improved Seismic Images of the Pacific Northwest Interior, With a Focus on the Region of the Columbia River Flood Basalts and Central Idaho

NASA Astrophysics Data System (ADS)

Stanciu, A. C.; Humphreys, E.; Clayton, R. W.

2017-12-01

We construct a P-wave model of the upper mantle based on new and previously acquired data from the USArray-TA stations and regional deployments, including the HLP, ID-OR, and the currently recording Wallowa stations. Our teleseismic arrival times are corrected for crustal structure (based on surface wave, receiver function, and controlled-source models from the region). Our modeling incorporates 3-D ray tracing and several simple considerations of radial anisotropy on travel time. As imaged previously, we find high P-wave velocity anomalies located beneath the Wallowa Mountains and beneath the Idaho Batholith in central west Idaho. Our improved imaging finds that these two anomalies are located down to 350 km depth, and are clearly separated from one another and from a shallower fast anomaly in the uppermost mantle beneath the westernmost Snake River Plain. Our preferred interpretation includes a combination of delamination and slab fragments in this region. As fast (and presumably cool) structures, these upper-mantle anomalies are thought to have a lithospheric origin. The anomaly beneath central Idaho is interpreted as the leading edge of the Farallon slab associated with the accretion of Siletzia terrane to North America. This anomaly may include some North American lithosphere that delaminated from the Laramide-thickened lithospheric mantle, perhaps related to Challis magmatism. The Wallowa anomaly is likely to represent Farallon lithosphere that delaminated during the Columbia River flood basalt event. The small anomaly between the two deeper fast anomalies, occurring at depths above 150km, could represent an isolated lithospheric fragment or a structure created by the Columbia River flood basalt event.

Lateral variations of thermo-rheological structure in SE Tibet

NASA Astrophysics Data System (ADS)

Jiang, X.; Gong, W.

2017-12-01

The structure and geodynamics in SE Tibet is important to developing a full understanding of tectonic evolution of the Tibetan plateau. To investigate the lithospheric structure and deformation, we present thermo-rheological models for two transects across SE Tibet. The thermal models are determined by the heat flow and P-wave velocity models. Based on thermal models, the rheological models are constructed in the weak and strong cases where the lower crust is felsic or mafic granulite and the lithospheric mantle is wet or dry peridotite. The thermal models show an obvious high-temperature anomaly within the lithosphere beneath the Chuandian block. Strong lateral heterogeneity is present in the rheological modeling and corresponds to variations of thermal models. The Chuandian block demonstrates a lower level of lithospheric strength than its neighboring regions, which is in accord with the seismogenic layer distribution. Combining with a joint analysis of SKS splitting and GPS data, the crust and mantle is decoupled at a depth below the topmost mantle in SE Tibet. The strong crust beneath the South China plate and Indochina block has two brittle load-bearing layers in the crust, indicating the system is mechanically coupled. The crust beneath the Emeishan igneous province also has two brittle load-bearing layers, but the brittle deformation is restricted to the topmost 10 km of the upper and lower crust. In contrast, only one brittle load-bearing layer resides in the upper crust with the lower crust contributing little to the lithospheric strength at the location where low-velocity-high-conductivity zones have been recognized within the crust in the Chuandian block. This indicates that the crust beneath the Chuandian block becomes decoupled, as evidenced by the crustal anisotropy pattern.

The non-hydrostatic figures of the terrestrial planets

NASA Technical Reports Server (NTRS)

Runcorn, S. K.

1985-01-01

Solid state creep being exponentially dependent on temperature must dominate the mechanical behavior of the mantles of terrestrial planets beneath their lithospheres. General arguments suggest that the lithospheres of the Moon and Mars are about 200 km thick; the Earth, Venus and Mercury much less. Short wavelength gravity anomalies are explained by the finite strength of the lithosphere: the lunar mascons being an example. The good correlation of the Venus and Mars gravity anomalies with topography up to spherical harmonics of degrees 10-15 is in striking contrast to the lack of correlation between the long wavelength components of the geoid and the continent-ocean distribution or even the plates. Attempts have been made to explain the former correlations by isostatic models but the depths of compensation seem implausible. Low degree harmonics of the gravity fields of the terrestrial planets as is certainly the case in the Earth must arise from the density variations driving solid state convection. In the case of Venus the less dense differentiated materials of the highlands seems to be positioned over the singular points of the convection pattern. Thus the correlated gravity field does not arise from the highlands but from the density difference in the convecting interior. In the Earth lack of correlation seems to arise from the fact that the plates have moved relative to the convection pattern the last 100 M yr.

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.

Modelling of Continental Lithosphere Breakup and Rifted Margin Formation in Response to an Upwelling Divergent Flow Field Incorporating a Temperature Dependent Rheology

NASA Astrophysics Data System (ADS)

Tymms, V. J.; Kusznir, N. J.

2005-05-01

We numerically model continental lithosphere deformation leading to breakup and sea floor spreading initiation in response to an imposed upwelling and divergent flow field applied to continental lithosphere and asthenosphere. The model is used to predict rifted continental margin lithosphere thinning and temperature structure. Model predictions are compared with observed rifted margin structure for four diverse case studies. Prior to application of the upwelling divergent flow field the continental lithosphere is undeformed with a uniform temperature gradient. The upwelling divergent flow field is defined kinematically using boundary conditions consisting of the upwelling velocity Vz at the divergence axis and the half divergence rate Vx . The resultant velocity field throughout the continuum is computed using finite element (FE) code incorporating a Newtonian temperature dependent rheology. The flow field is used to advect the continental lithosphere material and lithospheric and asthenospheric temperatures. Viscosity structure is hence modified and the velocities change correspondingly in a feedback loop. We find the kinematic boundary conditions Vz and Vx to be of first order importance. A high Vz/Vx (greater than10), corresponding to buoyancy assisted flow, leads to minimal mantle exhumation and a well defined continent ocean transition consistent with observations at volcanic margins. For Vz/Vx near unity, corresponding to plate boundary driven divergence, mantle exhumation over widths of up to 100 km is predicted which is consistent with observations at non-volcanic margins. The FE method allows the upwelling velocity Vz to be propagated upwards from the top of the asthenosphere to the Earth's surface without the requirement of imposing Vx. When continental breakup is achieved the half divergence velocity Vx can be applied at the lithosphere surface and the upwelling velocity Vz left free. We find this time and space dependent set of boundary conditions is more plausible than a constant corner flow type solution and predicts levels of depth dependent stretching and continent ocean transitions consistent with observation. Depth dependent lithosphere stretching, which is observed at rifted continental margins, is predicted to occur before continental breakup and sea-floor spreading initiation. The model may be used to predict surface heat flow and bathymetry, and to provide estimates of melt production rates and cumulative thickness. We compare model predictions with observed margin structure for four diverse rifted margins: the Lofoten Margin (a mature volcanic margin), Goban Spur (a mature non-volcanic margin), the Woodlark Basin (a neotectonic young ocean basin) and the Faroe-Shetland Basin (a failed attempt at continental breakup). This work forms part of the NERC Margins iSIMM project. iSIMM investigators are from Liverpool and Cambridge Universities, Badley Geoscience & Schlumberger Cambridge Research supported by the NERC, the DTI, Agip UK, BP, Amerada Hess Ltd, Anadarko, Conoco¬Phillips, Shell, Statoil and WesternGeco. The iSIMM team comprises NJ Kusznir, RS White, AM Roberts, PAF Christie, A Chappell, J Eccles, R Fletcher, D Healy, N Hurst, ZC Lunnon, CJ Parkin, AW Roberts, LK Smith, V Tymms & R Spitzer.

Imaging the density distributions at the regional scale using full waveform and gravity data inversion - Application to the Pyrenees

NASA Astrophysics Data System (ADS)

Martin, Roland; Chevrot, Sébastien; Wang, Yi; Spangenberg, Hannah; Goubet, Marie; Monteiller, Vadim; Komatitsch, Dimitri; Seoane, Lucia; Dufréchou, Grégory

2017-04-01

We present a hybrid inversion method that allows us to image density distributions at the regional scale using both seismic and gravity data. One main goal is to obtain densities and seismic wave velocities (P and S) in the lithosphere with a fine resolution to get important constraints on the mineralogic composition and thermal state of the lithosphere. In the context of the Pyrenees (located between Spain and France), accurate Vp and Vs seismic velocity models are computed first on a 3D spectral element grid at the scale of the Pyrenees by inverting teleseismic full waveforms. In a second step, Vp velocities are mapped to densities using empirical relations to build an a priori density model. BGI and BRGM Bouguer gravity anomaly data sets are then inverted on the same 3D spectral element grid as the Vp model at a resolution of 1-2 km by using high-order numerical integration formulae. Solutions are compared to those obtained using classical semi-analytical techniques. This procedure opens the possibility to invert both teleseismic and gravity data on the same finite-element grid. It can handle topography of the free surface in the same spectral-element distorted mesh that is used to solve the wave equation, without performing extra interpolations between different grids and models. WGS84 curvature, SRTM or ETOPO1 topographies are used.

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.

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.

Pn Tomography of the Central and Eastern United States

NASA Astrophysics Data System (ADS)

Zhang, Q.; Sandvol, E. A.; Liu, M.

2005-12-01

Approximately 44,000 Pn phase readings from the ISC and NEIC catalogs and 750 hand picked arrivals were inverted to map the velocity structure of mantle lithosphere in the Central and Eastern United States (CEUS). Overall we have a high density of ray paths within the active seismic zones in the eastern and southern parts of the CEUS, while ray coverage is relatively poor to the west of Great Lakes as well as along the eastern and southern coastlines of the U.S. The average Pn velocity in the CEUS is approximately 8.03 km/s. High Pn velocities (~8.18 km/s) within the northeastern part of the North American shield are reliable, while the resolution of the velocity image of the American shield around the mid-continent rift (MCR) is relatively low due to the poor ray coverage. Under the East Continent Rift (EC), the northern part of the Reelfoot Rift Zone (RRZ), and the South Oklahoma Aulacogen (SO), we also observe high velocity lithospheric mantle (~8.13-8.18 km/s). Typical Pn velocities (~7.98 km/s) are found between those three high velocity blocks. Low velocities are shown in the northern and southern Appalachians (~7.88-7.98 km/s) as well as the Rio Grande Rift (~7.88 km/s). In the portion of our model with the highest ray density, the Pn azimuthal anisotropy seems to be robust. These fast directions appear to mirror the boundaries of the low Pn velocity zone and parallel the Appalachians down to the southwest.

Horizontal stresses induced by vertical processes in planetary lithospheres

NASA Technical Reports Server (NTRS)

Banerdt, W. B.

1993-01-01

Understanding the state of stress in the elastic lithosphere is of fundamental importance for planetary geophysics, as it is the link between the observed geologic structures on the surface and the processes which form and modify these structures. As such, it can provide valuable constraints for the difficult problem of determining interior structure and processes. On the Earth, most large scale, organized deformation can be related to lateral tectonics associated with plate dynamics; however, the tectonics on many extraterrestrial bodies (such as the Moon, Mars, and most of the outer-planet satellites) appears to be primarily vertical in nature, and the horizontal stresses induced by vertical motions and loads are expected to dominate the deformation of their lithospheres. The largest stress contributions from vertical loading come from the flexure of the lithosphere, which induces both bending moments and membrane stresses. We are concerned here only with nonflexural changes in the state of stress induced by processes such as sedimentary and volcanic deposition, erosional denudation, and changes in the thermal gradient that induce uplift or subsidence. This analysis is important both for evaluating stresses for specific regions in which the vertical stress history can be estimated, as well as for applying the proper loading conditions to global stress models. It is also of interest for providing a reference state of stress for interpreting stress measurements in the crust of the Earth.

Upper mantle beneath foothills of the western Himalaya: subducted lithospheric slab or a keel of the Indian shield?

NASA Astrophysics Data System (ADS)

Vinnik, L.; Singh, A.; Kiselev, S.; Kumar, M. Ravi

2007-12-01

The fate of the mantle lithosphere of the Indian Plate in the India-Eurasia collision zone is not well understood. Tomographic studies reveal high P velocity in the uppermost mantle to the south of the western Himalaya, and these high velocities are sometimes interpreted as an image of subducting Indian lithosphere. We suggest that these high velocities are unrelated to the ongoing subduction but correspond to a near-horizontal mantle keel of the Indian shield. In the south of the Indian shield upper-mantle velocities are anomalously low, and relatively high velocities may signify a recovery of the normal shield structure in the north. Our analysis is based on the recordings of seismograph station NIL in the foothills of the western Himalaya. The T component of the P receiver functions is weak relative to the Q component, which is indicative of a subhorizontally layered structure. Joint inversion of the P and S receiver functions favours high uppermost mantle velocities, typical of the lithosphere of Archean cratons. The arrival of the Ps converted phase from 410 km discontinuity at NIL is 2.2 s earlier than in IASP91 global model. This can be an effect of remnants of Tethys subduction in the mantle transition zone and of high velocities in the keel of the Indian shield. Joint inversion of SKS particle motions and P receiver functions reveals a change in the fast direction of seismic azimuthal anisotropy from 60° at 80-160 km depths to 150° at 160-220 km. The fast direction in the lower layer is parallel to the trend of the Himalaya. The change of deformation regimes at a depth of 160 km suggests that this is the base of the lithosphere of the Indian shield. A similar boundary was found with similar techniques in central Europe and the Tien Shan region, but the base of the lithosphere in these regions is relatively shallow, in agreement with the higher upper-mantle temperatures. The ongoing continental collision is expressed in crustal structure: the crust beneath NIL is very thick (58 +/- 2 km), and the S velocity in the intermediate and lower crust is around 4.0 km s-1. This anomalously large velocity and thickness can be explained by scraping off the lower crust, when the Indian lithosphere underthrusts the Himalaya.

The Density of the North Polar Layered Deposit from Gravity and Topography

NASA Astrophysics Data System (ADS)

Ojha, L.; Lewis, K. W.

2017-12-01

The North Polar Layered Deposit (NPLD) of Mars is a vast reservoir of water ice with a volume of 1.14 million km3. Radar data indicates that the ice in the NPLD is extremely pure with dust content between 5 % to 10 %, however the radar data has not been able to put a direct constraint on the density of the NPLD. Here, we localize the gravity and topography signature of the NPLD and place a direct constraint on its density. We performed a grid search by generating admittance spectrum at each latitude and longitude between 75° N to 90° N and 0° E to 360° E, using a spherical cap of angular radius () of 7°, and a harmonic-bandwidth of the localization window Lwin of 37°. A region between Gemina Lingula and Planum Boreum was found to possesss an adequate correlation between gravity and topography. The estimated admittance spectra were compared with synthetic admittance spectra to constrain the load-density and the elastic thickness of the lithosphere. We constructed forward models by assuming that the lithosphere is a thin shell that deforms elastically in response to surface loads. We find that the bulk density of the NPLD ranges between 1000 to 1100 kg.m-3. Assuming a grain density of 3000 kg.m-3 for dust, the NPLD region within our localized window can contain dust content between 3 - 8 %, which is in an excellent agreement with the radar data.

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.

Extension of continental lithosphere - A model for two scales of basin and range deformation

NASA Technical Reports Server (NTRS)

Zuber, M. T.; Parmentier, E. M.; Fletcher, R. C.

1986-01-01

The development of a model for deformation in an extending continental lithosphere that is stratified in density and strength is described. The lithosphere model demonstrates that the necking instabilities at two wavelengths originate due to a strong upper crust, a mantle layer, and a weak lower crust. It is observed that the dominant wavelengths of necking are controlled by layer thickness and the strength of the layers control the amplitude of the instabilities. The model is applied to the Basin and Range Province of the western U.S. where deformations in ranges and tile domains are detected. The relation between the Bouguer gravity anomaly and the deformations is studied. The data reveal that the horizontal scale of short wavelength necking correlates with the spacings of individual basins and ranges, and the longer wavelength corresponds to the width of tilt domains. The control of the Basin and Range deformation by two scales of extensional instability is proposed.

Early impact basins and the onset of plate tectonics. Ph.D. Thesis - Maryland Univ.

NASA Technical Reports Server (NTRS)

Frey, H.

1977-01-01

The fundamental crustal dichotomy of the Earth (high and low density crust) was established nearly 4 billion years ago. Therefore, subductable crust was concentrated at the surface of the Earth very early in its history, making possible an early onset for plate tectonics. Simple thermal history calculations spanning 1 billion years show that the basin forming impact thins the lithosphere by at least 25%, and increases the sublithosphere thermal gradients by roughly 20%. The corresponding increase in convective heat transport, combined with the highly fractured nature of the thinned basin lithosphere, suggest that lithospheric breakup or rifting occurred shortly after the formation of the basins. Conditions appropriate for early rifting persisted from some 100,000,000 years following impact. We suggest a very early stage of high temperature, fast spreading "microplate" tectonics, originating before 3.5 billion years ago, and gradually stabilizing over the Archaean into more modern large plate or Wilson Cycle tectonics.

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.

A Geophysical Model for the Origin of Volcano Vent Clusters in a Colorado Plateau Volcanic Field

NASA Astrophysics Data System (ADS)

Deng, Fanghui; Connor, Charles B.; Malservisi, Rocco; Connor, Laura J.; White, Jeremy T.; Germa, Aurelie; Wetmore, Paul H.

2017-11-01

Variation in spatial density of Quaternary volcanic vents, and the occurrence of vent clusters, correlates with boundaries in Proterozoic crust in the Springerville volcanic field (SVF), Arizona, USA. Inverse modeling using 538 gravity measurements shows that vent clusters correlate with gradients in the gravity field due to lateral variation in crustal density. These lateral discontinuities in the crustal density can be explained by boundaries in the North American crust formed during Proterozoic accretion. Spatial density of volcanic vents is low in regions of high-density Proterozoic crust, high in areas of relatively low density Proterozoic crust, and is greatest adjacent to crustal boundaries. Vent alignments parallel these boundaries. We have developed 2-D and 3-D numerical models of magma ascent through the crust to simulate long-term, average magma migration that led to the development of vent clusters in the SVF, assuming that a viscous fluid flow through a porous media is statistically equivalent to magma migration averaged over geological time in the full field scale. The location and flux from the uniform magma source region are boundary conditions of the model. Changes in model diffusivity, associated with changes in the bulk properties of the lithosphere, can simulate preferential magma migration paths and alter estimated magma flux at the surface, implying that large-scale crustal structures, such as inherited tectonic block boundaries, influence magma ascent and clustering of volcanic vents. Probabilistic models of volcanic hazard for distributed volcanic fields can be improved by identifying crustal structures and assessing their impact on volcano distribution with the use of numerical models.

Crustal and Upper Mantle Investigations Using Receiver Functions and Tomographic Inversion in the Southern Puna Plateau Region of the Central Andes

NASA Astrophysics Data System (ADS)

Heit, B.; Yuan, X.; Bianchi, M.; Jakovlev, A.; Kumar, P.; Kay, S. M.; Sandvol, E. A.; Alonso, R.; Coira, B.; Comte, D.; Brown, L. D.; Kind, R.

2011-12-01

We present here the results obtained using the data form our passive seismic array in the southern Puna plateau between 25°S to 28°S latitude in Argentina and Chile. In first instance we have been able to calculate P and S receiver functions in order to investigate the Moho thickness and other seismic discontinuities in the study area. The RF data shows that the northern Puna plateau has a thicker crust and that the Moho topography is more irregular along strike. The seismic structure and thickness of the continental crust and the lithospheric mantle beneath the southern Puna plateau reveals that the LAB is deeper to the north of the array suggesting lithospheric removal towards the south. Later we performed a joint inversion of teleseismic and regional tomographic data in order to study the distribution of velocity anomalies that could help us to better understand the evolution of the Andean elevated plateau and the role of lithosphere-asthenosphere interactions in this region. Low velocities are observed in correlation with young volcanic centers (e.g. Ojos del Salado, Cerro Blanco, Galan) and agree very well with the position of crustal lineaments in the region. This is suggesting a close relationship between magmatism and lithospheric structures at crustal scale coniciding with the presence of hot asthenospheric material at the base of the crust probably induced by lithospheric foundering.

Asymmetric three-dimensional topography over mantle plumes.

PubMed

Burov, Evgueni; Gerya, Taras

2014-09-04

The role of mantle-lithosphere interactions in shaping surface topography has long been debated. In general, it is supposed that mantle plumes and vertical mantle flows result in axisymmetric, long-wavelength topography, which strongly differs from the generally asymmetric short-wavelength topography created by intraplate tectonic forces. However, identification of mantle-induced topography is difficult, especially in the continents. It can be argued therefore that complex brittle-ductile rheology and stratification of the continental lithosphere result in short-wavelength modulation and localization of deformation induced by mantle flow. This deformation should also be affected by far-field stresses and, hence, interplay with the 'tectonic' topography (for example, in the 'active/passive' rifting scenario). Testing these ideas requires fully coupled three-dimensional numerical modelling of mantle-lithosphere interactions, which so far has not been possible owing to the conceptual and technical limitations of earlier approaches. Here we present new, ultra-high-resolution, three-dimensional numerical experiments on topography over mantle plumes, incorporating a weakly pre-stressed (ultra-slow spreading), rheologically realistic lithosphere. The results show complex surface evolution, which is very different from the smooth, radially symmetric patterns usually assumed as the canonical surface signature of mantle upwellings. In particular, the topography exhibits strongly asymmetric, small-scale, three-dimensional features, which include narrow and wide rifts, flexural flank uplifts and fault structures. This suggests a dominant role for continental rheological structure and intra-plate stresses in controlling dynamic topography, mantle-lithosphere interactions, and continental break-up processes above mantle plumes.

Neotectonic Deformation in Central Eurasia: A Geodynamic Model Approach

NASA Astrophysics Data System (ADS)

Tunini, Lavinia; Jiménez-Munt, Ivone; Fernandez, Manel; Vergés, Jaume; Bird, Peter

2017-11-01

Central Eurasia hosts wide orogenic belts of collision between India and Arabia with Eurasia, with diffuse or localized deformation occurring up to hundreds of kilometers from the primary plate boundaries. Although numerous studies have investigated the neotectonic deformation in central Eurasia, most of them have focused on limited segments of the orogenic systems. Here we explore the neotectonic deformation of all of central Eurasia, including both collision zones and the links between them. We use a thin-spherical sheet approach in which lithosphere strength is calculated from lithosphere structure and its thermal regime. We investigate the contributions of variations in lithospheric structure, rheology, boundary conditions, and fault friction coefficients on the predicted velocity and stress fields. Results (deformation pattern, surface velocities, tectonic stresses, and slip rates on faults) are constrained by independent observations of tectonic regime, GPS, and stress data. Our model predictions reproduce the counterclockwise rotation of Arabia and Iran, the westward escape of Anatolia, and the eastward extrusion of the northern Tibetan Plateau. To simulate the observed extensional faults in the Tibetan Plateau, a weaker lithosphere is required, provided by a change in the rheological parameters. The southward movement of the SE Tibetan Plateau can be explained by the combined effects of the Sumatra trench retreat, a thinner lithospheric mantle, and strik-slip faults in the region. This study offers a comprehensive model for regions with little or no data coverage, like the Arabia-India intercollision zone, where the surface velocity is northward showing no deflection related to Arabia and India indentations.

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.

Horizontal stress in planetary lithospheres from vertical processes

NASA Technical Reports Server (NTRS)

Banerdt, W. B.

1991-01-01

Understanding the stress states in a lithosphere is of fundamental importance for planetary geophysics. It is closely linked to the processes which form and modify tectonic features on the surface and reflects the behavior of the planet's interior, providing a constraint for the difficult problem of determining interior structure and processes. The tectonics on many extraterrestrial bodies (Moon, Mars, and most of the outer planet satellites) appears to be mostly vertical, and the horizontal stresses induced by vertical motions and loads are expected to dominate the deformation of their lithospheres. Herein, only changes are examined in the state of stress induced by processes such as sedimentary and volcanic deposition, erosional denudation, and changes in the thermal gradient that induce uplift or subsidence. This analysis is important both for evaluating stresses for specific regions in which the vertical stress history can be estimated, as well as for applying the proper loading conditions to global stress models. All references to lithosphere herein should be understood to refer to the elastic lithosphere, that layer which deforms elastically or brittlely when subjected to geologically scaled stresses.

Boundary-layer mantle flow under the Dead Sea transform fault inferred from seismic anisotropy.

PubMed

Rümpker, Georg; Ryberg, Trond; Bock, Günter

2003-10-02

Lithospheric-scale transform faults play an important role in the dynamics of global plate motion. Near-surface deformation fields for such faults are relatively well documented by satellite geodesy, strain measurements and earthquake source studies, and deeper crustal structure has been imaged by seismic profiling. Relatively little is known, however, about deformation taking place in the subcrustal lithosphere--that is, the width and depth of the region associated with the deformation, the transition between deformed and undeformed lithosphere and the interaction between lithospheric and asthenospheric mantle flow at the plate boundary. Here we present evidence for a narrow, approximately 20-km-wide, subcrustal anisotropic zone of fault-parallel mineral alignment beneath the Dead Sea transform, obtained from an inversion of shear-wave splitting observations along a dense receiver profile. The geometry of this zone and the contrast between distinct anisotropic domains suggest subhorizontal mantle flow within a vertical boundary layer that extends through the entire lithosphere and accommodates the transform motion between the African and Arabian plates within this relatively narrow zone.

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.

Plate tectonics on the terrestrial planets

NASA Astrophysics Data System (ADS)

van Thienen, P.; Vlaar, N. J.; van den Berg, A. P.

2004-05-01

Plate tectonics is largely controlled by the buoyancy distribution in oceanic lithosphere, which correlates well with the lithospheric age. Buoyancy also depends on compositional layering resulting from pressure release partial melting under mid-ocean ridges, and this process is sensitive to pressure and temperature conditions which vary strongly between the terrestrial planets and also during the secular cooling histories of the planets. In our modelling experiments we have applied a range of values for the gravitational acceleration (representing different terrestrial planets), potential temperatures (representing different times in the history of the planets), and surface temperatures in order to investigate under which conditions plate tectonics is a viable mechanism for the cooling of the terrestrial planets. In our models we include the effects of mantle temperature on the composition and density of melt products and the thickness of the lithosphere. Our results show that the onset time of negative buoyancy for oceanic lithosphere is reasonable (less than a few hundred million years) for potential temperatures below ˜ 1500 ° C for the Earth and ˜ 1450 ° C for Venus. In the reduced gravity field of Mars a much thicker stratification is produced and our model indicates that plate tectonics could only operate on reasonable time scales at a potential mantle temperature below about 1300-1400 °C.

Drip tectonics and the enigmatic uplift of the Central Anatolian Plateau.

PubMed

Göğüş, Oğuz H; Pysklywec, Russell N; Şengör, A M C; Gün, Erkan

2017-11-16

Lithospheric drips have been interpreted for various regions around the globe to account for the recycling of the continental lithosphere and rapid plateau uplift. However, the validity of such hypothesis is not well documented in the context of geological, geophysical and petrological observations that are tested against geodynamical models. Here we propose that the folding of the Central Anatolian (Kırşehir) arc led to thickening of the lithosphere and onset of "dripping" of the arc root. Our geodynamic model explains the seismic data showing missing lithosphere and a remnant structure characteristic of a dripping arc root, as well as enigmatic >1 km uplift over the entire plateau, Cappadocia and Galatia volcanism at the southern and northern plateau margins since ~10 Ma, respectively. Models show that arc root removal yields initial surface subsidence that inverts >1 km of uplift as the vertical loading and crustal deformation change during drip evolution.

Micro-seismicity in the Gulf of Cadiz: Is there a link between micro-seismicity, high magnitude earthquakes and active faults?

NASA Astrophysics Data System (ADS)

Silva, Sónia; Terrinha, Pedro; Matias, Luis; Duarte, João C.; Roque, Cristina; Ranero, César R.; Geissler, Wolfram H.; Zitellini, Nevio

2017-10-01

The Gulf of Cadiz seismicity is characterized by persistent low to intermediate magnitude earthquakes, occasionally punctuated by high magnitude events such as the M 8.7 1755 Great Lisbon earthquake and the M = 7.9 event of February 28th, 1969. Micro-seismicity was recorded during 11 months by a temporary network of 25 ocean bottom seismometers (OBSs) in an area of high seismic activity, encompassing the potential source areas of the mentioned large magnitude earthquakes. We combined micro-seismicity analysis with processing and interpretation of deep crustal seismic reflection profiles and available refraction data to investigate the possible tectonic control of the seismicity in the Gulf of Cadiz area. Three controlling mechanisms are explored: i) active tectonic structures, ii) transitions between different lithospheric domains and inherited Mesozoic structures, and iii) fault weakening mechanisms. Our results show that micro-seismicity is mostly located in the upper mantle and is associated with tectonic inversion of extensional rift structures and to the transition between different lithospheric/rheological domains. Even though the crustal structure is well imaged in the seismic profiles and in the bathymetry, crustal faults show low to negligible seismic activity. A possible explanation for this is that the crustal thrusts are thin-skinned structures rooting in relatively shallow sub-horizontal décollements associated with (aseismic) serpentinization levels at the top of the lithospheric mantle. Therefore, co-seismic slip along crustal thrusts may only occur during large magnitude events, while for most of the inter-seismic cycle these thrusts remain locked, or slip aseismically. We further speculate that high magnitude earthquake's ruptures may only nucleate in the lithospheric mantle and then propagate into the crust across the serpentinized layers.

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.

Anisotropic Lithospheric layering in the North American craton, revealed by Bayesian inversion of short and long period data

NASA Astrophysics Data System (ADS)

Roy, C.; Calo, M.; Bodin, T.; Romanowicz, B. A.

2016-12-01

Competing hypotheses for the formation and evolution of continents are highly under debate, including the theory of underplating by hot plumes or accretion by shallow subduction in continental or arc settings. In order to support these hypotheses, documenting structural layering in the cratonic lithosphere becomes especially important. Studies of seismic-wave receiver function data have detected a structural boundary under continental cratons at 100-140 km depths, which is too shallow to be consistent with the lithosphere-asthenosphere boundary, as inferred from seismic tomography and other geophysical studies. This leads to the conclusion that 1) the cratonic lithosphere may be thinner than expected, contradicting tomographic and other geophysical or geochemical inferences, or 2) that the receiver function studies detect a mid-lithospheric discontinuity rather than the LAB. Recent studies (Bodin et al., 2015, Calo et al. 2016) confirmed the presence of a structural boundary under the north American craton at 100-140 km depths by taking advantage of the power of a trans-dimensional Monte Carlo Markov chain (TMCMC). They generated probabilistic 1D radially shear wave velocity profiles for selected stations in North America by jointly inverting 2 different data types (PS Receiver Functions, surface wave dispersion for Love and Rayleigh waves), which sample different volumes of the Earth and have different sensitivities to structure. The resulting 1D profiles include both isotropic and anisotropic discontinuities in the upper mantle (above 350 km depth). Here we extend this approach and include the vp/vs ratio as an unknown in the TMCMC algorithm to avoid artificial layers induced by multiples of the receiver functions. Additionally, we include SKS waveforms in the joint inversion and invert for azimuthal anisotropy to verify if layering in the anisotropic structure of the stable part of the North American continent involves significant changes in the direction of azimuthal anisotropy as suggested by Yuan and Romanowicz (2010). We recently demonstrated the power of this approach in the case of two stations located in different tectonic settings (Bodin et al., 2016. Here we extend this approach to a broader range of settings within the north American continent.

Oblique reactivation of lithosphere-scale lineaments controls rift physiography - the upper-crustal expression of the Sorgenfrei-Tornquist Zone, offshore southern Norway

NASA Astrophysics Data System (ADS)

Phillips, Thomas B.; Jackson, Christopher A.-L.; Bell, Rebecca E.; Duffy, Oliver B.

2018-04-01

Pre-existing structures within sub-crustal lithosphere may localise stresses during subsequent tectonic events, resulting in complex fault systems at upper-crustal levels. As these sub-crustal structures are difficult to resolve at great depths, the evolution of kinematically and perhaps geometrically linked upper-crustal fault populations can offer insights into their deformation history, including when and how they reactivate and accommodate stresses during later tectonic events. In this study, we use borehole-constrained 2-D and 3-D seismic reflection data to investigate the structural development of the Farsund Basin, offshore southern Norway. We use throw-length (T-x) analysis and fault displacement backstripping techniques to determine the geometric and kinematic evolution of N-S- and E-W-striking upper-crustal fault populations during the multiphase evolution of the Farsund Basin. N-S-striking faults were active during the Triassic, prior to a period of sinistral strike-slip activity along E-W-striking faults during the Early Jurassic, which represented a hitherto undocumented phase of activity in this area. These E-W-striking upper-crustal faults are later obliquely reactivated under a dextral stress regime during the Early Cretaceous, with new faults also propagating away from pre-existing ones, representing a switch to a predominantly dextral sense of motion. The E-W faults within the Farsund Basin are interpreted to extend through the crust to the Moho and link with the Sorgenfrei-Tornquist Zone, a lithosphere-scale lineament, identified within the sub-crustal lithosphere, that extends > 1000 km across central Europe. Based on this geometric linkage, we infer that the E-W-striking faults represent the upper-crustal component of the Sorgenfrei-Tornquist Zone and that the Sorgenfrei-Tornquist Zone represents a long-lived lithosphere-scale lineament that is periodically reactivated throughout its protracted geological history. The upper-crustal component of the lineament is reactivated in a range of tectonic styles, including both sinistral and dextral strike-slip motions, with the geometry and kinematics of these faults often inconsistent with what may otherwise be inferred from regional tectonics alone. Understanding these different styles of reactivation not only allows us to better understand the influence of sub-crustal lithospheric structure on rifting but also offers insights into the prevailing stress field during regional tectonic events.

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.

Interactions of multi-scale heterogeneity in the lithosphere: Australia

NASA Astrophysics Data System (ADS)

Kennett, B. L. N.; Yoshizawa, K.; Furumura, T.

2017-10-01

Understanding the complex heterogeneity of the continental lithosphere involves a wide variety of spatial scales and the synthesis of multiple classes of information. Seismic surface waves and multiply reflected body waves provide the main constraints on broad-scale structure, and bounds on the extent of the lithosphere-asthenosphere transition (LAT) can be found from the vertical gradients of S wavespeed. Information on finer-scale structures comes through body wave studies, including detailed seismic tomography and P-wave reflectivity extracted from stacked autocorrelograms of continuous component records. With the inclusion of deterministic large-scale structure and realistic medium-scale stochastic features fine-scale variations are subdued. The resulting multi-scale heterogeneity model for the Australian region gives a good representation of the character of observed seismograms and their geographic variations and matches the observations of P-wave reflectivity. P reflections in the 0.5-3.0 Hz band in the uppermost mantle suggest variations on vertical scales of a few hundred metres with amplitudes of the order of 1%. Interference of waves reflected or converted at sequences of such modest variations in physical properties produce relatively simple behaviour for lower frequencies, which can suggest simpler structures than are actually present. Vertical changes in the character of fine-scale heterogeneity can produce apparent discontinuities. In Central Australia a 'mid-lithospheric discontinuity' can be tracked via changes in frequency content of station reflectivity, with links to the broad-scale pattern of wavespeed gradients and, in particular, the gradients of radial anisotropy. Comparisons with xenolith results from southeastern Australia indicate a strong tie between geochemical stratification and P-wave reflectivity.

Modeling the Internal Structure of Mars Using Normal Mode Relaxation Theory

NASA Astrophysics Data System (ADS)

Pithawala, T. M.; Ghent, R. R.; Bills, B. G.

2010-12-01

We seek to resolve an apparent paradox between two sets of observations, which seem to suggest quite different thermal structures for the deep interior of Mars. The orbit of Phobos is observed to be accelerating along-track at a rate of (273.4 ± 1.2) 10^(-5) deg/yr^(2), which implies that the orbit is shrinking at (4.03 ± 0.03) cm/yr, and losing energy at a rate of 3.4 MW. The most likely sink for that energy is tidal dissipation within Mars, seemingly requiring a warm interior. However, static support of the gravity and topography of Mars requires a thick elastic lithosphere, indicating a relatively cool (and therefore stiff) mantle. Using normal mode relaxation theory we model the internal viscosity structure of Mars by analyzing its response to tidal forcing from Phobos. We investigate spherical axisymmetric layered viscoelastic models, seeking to satisfy what is known about planetary differentiation, to support large-scale topography via a thick elastic lithosphere, and to yield the observed tidal dissipation rate. We present a family of 4-layer models (core, mantle, lithosphere, and thin weak layer) that satisfy these constraints, and discuss the implications for Mars’ internal structure.

Seismic Attenuation Structure and Intraplate Deformation

NASA Astrophysics Data System (ADS)

Bezada, M.; Kowalke, S.; Smale, J.

2017-12-01

It has been suggested that intraplate deformation and seismicity is localized at weak zones in the lithosphere and at rheological boundaries. Comparisons of intraplate deformation regions with mantle seismic velocity structure suggest a correlation, but are not universally accepted as compelling evidence. We present P-wave attenuation models built from records of teleseismic deep-focus earthquakes in three different regions that show significant correlation between attenuation structure and intraplate seismicity and deformation. In the eastern United States, the New Madrid, Wabash Valley, Eastern Tennessee, Central Virginia, and Carolina seismic zones all occur at or near the edges of high-Q (low attenuation) regions. In Spain, intraplate seismicity is absent from high-Q regions but relatively abundant in surrounding low-Q regions where intraplate orogeny is also observed. In Australia, where our model resolution is relatively poor owing to sparse and uneven station coverage, the Petermann and Alice Springs intraplate orogens occur near the edge of a high-Q feature roughly coinciding with the undeformed Amadeus basin. Our results suggest that lithospheric structure exerts important controls on the localization of intraplate deformation and seismicity and that seismic attenuation is a useful proxy for lithospheric strength.

Lithospheric mantle structure beneath Northern Scotland: Pre-plume remnant or syn-plume signature?

NASA Astrophysics Data System (ADS)

Knapp, J.

2003-04-01

Upper mantle reflectors (Flannan and W) beneath the northwestern British Isles are some of the best-known and most-studied examples of preserved structure within the continental mantle lithosphere, and are spatially coincident with the surface location of early Iceland plume volcanism in the British Tertiary Province. First observed on BIRPS (British Institutions Reflection Profiling Syndicate) marine deep seismic reflection profiles in the early 1980's, these reflectors have subsequently been imaged and correlated on additional reflection and refraction profiles in the offshore area of northern and western Scotland. The age and tectonic significance of these reflectors remains a subject of wide debate, due in part to the absence of robust characterization of the upper mantle velocity structure in this tectonically complex area. Interpretations advanced over the past two decades for the dipping Flannan reflector range from fossilized subduction complex to large-scale extensional shear zone, and span ages from Proterozoic to early Mesozoic. Crustal geology of the region records early Paleozoic continental collision and late Paleozoic to Mesozoic extension. Significant modification of the British lithosphere in early Tertiary time, including dramatic thinning and extensive basaltic intrusion associated with initiation and development of the Iceland plume, suggests either (1) an early Tertiary age for the Flannan reflector or (2) preservation of ancient features within the mantle lithosphere despite such pervasive modification. Exisitng constraints are consistent with a model for early Tertiary origin of the Flannan reflector as the downdip continuation of the Rockall Trough extensional system of latest Cretaceous to earliest Tertiary age during opening of the northern Atlantic Ocean and initiation of the Iceland plume. Lithopsheric thinning beneath present-day northern Scotland could have served to focus the early expression of plume volcanism (British Tertiary Province), despite the inferred distant locus of the initial plume head. Alternatively, preservation of large-scale pre-plume fabric in the Scottish mantle would imply long-lived tectonic heredity in the continental lithospheric mantle, and place important constraints on the plume-related effects (or lack thereof) in the mantle lithosphere.

3D Thermo-Mechanical Models of Plume-Lithosphere Interactions: Implications for the Kenya rift

NASA Astrophysics Data System (ADS)

Scheck-Wenderoth, M.; Koptev, A.; Sippel, J.

2017-12-01

We present three-dimensional (3D) thermo-mechanical models aiming to explore the interaction of an active mantle plume with heterogeneous pre-stressed lithosphere in the Kenya rift region. As shown by the recent data-driven 3D gravity and thermal modeling (Sippel et al., 2017), the integrated strength of the lithosphere for the region of Kenya and northern Tanzania appears to be strongly controlled by the complex inherited crustal structure, which may have been decisive for the onset, localization and propagation of rifting. In order to test this hypothesis, we have performed a series of ultra-high resolution 3D numerical experiments that include a coupled mantle/lithosphere system in a dynamically and rheologically consistent framework. In contrast to our previous studies assuming a simple and quasi-symmetrical initial condition (Koptev et al., 2015, 2016, 2017), the complex 3D distribution of rock physical properties inferred from geological and geophysical observations (Sippel et al., 2017) has been incorporated into the model setup that comprises a stratified three-layer continental lithosphere composed of an upper and lower crust and lithospheric mantle overlaying the upper mantle. Following the evidence of the presence of a broad low-velocity seismic anomaly under the central parts of the East African Rift system (e.g. Nyblade et al, 2000; Chang et al., 2015), a 200-km radius mantle plume has been seeded at the bottom of a 635 km-depth model box representing a thermal anomaly of 300°C temperature excess. In all model runs, results show that the spatial distribution of surface deformation is indeed strongly controlled by crustal structure: within the southern part of the model box, a localized narrow zone stretched in NS direction (i.e. perpendicularly to applied far-field extension) is aligned along a structural boundary within the lower crust, whereas in the northern part of the model domain, deformation is more diffused and its eastern limit coincides with the eastern side of a weaker unit within the upper crustal layer. This northward transition from more localized to more distributed strain bears some general similarity to the distribution of major faults within the studied area (Chorowicz, 2005).

Structure of the Lithosphere-Asthenosphere Boundary Onshore and Offshore the California Continental Margin from Three-Dimensional Seismic Anisotropy

NASA Astrophysics Data System (ADS)

Gomez, C. D.; Escobar, L., Sr.; Rathnayaka, S.; Weeraratne, D. S.; Kohler, M. D.

2016-12-01

The California continental margin, a major transform plate boundary in continental North America, is the locus of complex tectonic stress fields that are important in interpreting both remnant and ongoing deformational strain. Ancient subduction of the East Pacific Rise spreading center, the rotation and translation of tectonic blocks and inception of the San Andreas fault all contribute to the dynamic stress fields located both onshore and offshore southern California. Data obtained by the ALBACORE (Asthenospheric and Lithospheric Broadband Architecture from the California Offshore Region Experiment) and the CISN (California Integrated Seismic Network) seismic array are analyzed for azimuthal anisotropy of Rayleigh waves from 80 teleseismic events at periods 16 - 78 s. Here we invert Rayleigh wave data for shear wave velocity structure and three-dimensional seismic anisotropy in the thee regions designated within the continental margin including the continent, seafloor and California Borderlands. Preliminary results show that seismic anisotropy is resolved in multiple layers and can be used to determine the lithosphere-asthenosphere boundary (LAB) in offshore and continental regions. The oldest seafloor in our study at age 25-35 Ma indicates that the anisotropic transition across the LAB occurs at 73 km +/- 25 km with the lithospheric fast direction oriented WNW-ESE, consistent with current Pacific plate motion direction. The continent region west of the San Andreas indicates similar WNW-ESE anisotropy and LAB depth. Regions east of the San Andreas fault indicate NW-SE anisotropy transitioning to a N-S alignment at 80 km depth north of the Garlock fault. The youngest seafloor (15 - 25 Ma) and outer Borderlands indicate a more complex three layer fabric where shallow lithospheric NE-SW fast directions are perpendicular with ancient Farallon subduction arc, a mid-layer with E-W fast directions are perpendicular to remnant fossil fabric, and the deepest layer indicates NW-SE fast directions below the LAB likely controlled by current Pacific plate motion. The inner Borderland indicates two layer anisotropic structure with a shallow NW-SE lithospheric fast direction that changes to NE-SW fast directions below the LAB, possibly consistent with the ancient subduction direction.

Crustal and upper mantle investigations of the Caribbean-South American plate boundary

NASA Astrophysics Data System (ADS)

Bezada, Maximiliano J.

The evolution of the Caribbean --- South America plate boundary has been a matter of vigorous debate for decades and many questions remain unresolved. In this work, and in the framework of the BOLIVAR project, we shed light on some aspects of the present state and the tectonic history of the margin by using different types of geophysical data sets and techniques. An analysis of controlled-source traveltime data collected along a boundary-normal profile at ˜65°W was used to build a 2D P-wave velocity model. The model shows that the Caribbean Large Igenous Province is present offshore eastern Venezuela and confirms the uniformity of the velocity structure along the Leeward Antilles volcanic belt. In contrast with neighboring profiles, at this longitude we see no change in velocity structure or crustal thickness across the San Sebastian - El Pilar fault system. A 2D gravity modeling methodology that uses seismically derived initial density models was developed as part of this research. The application of this new method to four of the BOLIVAR boundary-normal profiles suggests that the uppermost mantle is denser under the South American continental crust and the island arc terranes than under the Caribbean oceanic crust. Crustal rocks of the island arc and extended island arc terranes of the Leeward Antilles have a relatively low density, given their P-wave velocity. This may be caused by low iron content, relative to average magmatic arc rocks. Finally, an analysis of teleseismic traveltimes with frequency-dependent kernels produced a 3D P-wave velocity perturbation model. The model shows the structure of the mantle lithosphere under the study area and clearly images the subduction of the Atlantic slab and associated partial removal of the lower lithosphere under northern South America. We also image the subduction of a section of the Caribbean plate under South America with an east-southeast direction. Both the Atlantic and Caribbean subducting slabs penetrate the mantle transition zone, affecting the topography of the 410-km and 660-km discontinuities.

The Mechanics of Impact Basin Formation: Comparisons between Modeling and Geophysical Observations

NASA Astrophysics Data System (ADS)

Stewart, S. T.

2010-12-01

Impact basins are the largest geologic structures on planetary surfaces. Single or multiple ring-shaped scarps or arcuate chains of massifs typically surround basin-sized craters (e.g., larger than about 300 km diameter on the moon [1]). Impact basins also possess central mass anomalies related to ejection of a portion of the crust (and mantle) and uplift of the mantle. I will discuss insights into the mechanics of impact basin formation derived from numerical simulations and focus on features that may be compared with gravity and topography data. The simulations of basin formation use the method of [2] with an improved rheological model that includes dynamic weakening of faults and more accurate treatment of the mantle solidus. Two-dimensional simulations of vertical impacts onto spherical planets utilize a central gravity field, and three-dimensional simulations of oblique impacts include a self-gravity calculation. During the opening and collapse of the transient crater, localization of strain leads to deformation features that are interpreted as deep faults through the lithosphere. Based on simulations of mantle-excavating impacts onto the moon and Mars with thermal gradients that intersect the solidus in the asthenosphere, the final impact structure has three major features: (i) an inner basin filled with melt and bounded by the folded lithosphere, (ii) a broad shallow terrace of faulted and translated lithosphere with an ejecta deposit, and (iii) the surrounding autochthonous lithosphere with radially thinning ejecta. The folded lithosphere is a complex structure that experiences translation inward and then outward again during collapse of the transient cavity. The uplifted mantle within this structure is overlain by a thin layer of hot crustal material. In addition to asymmetry in the excavated material, 45-degree impact events produce an asymmetric terrace feature. The principal observations for comparison to the calculations are the inferred locations of major ring structures (derived from topography and geologic mapping) and the crustal thickness and mantle topography (derived from gravity and topography) [see also 3]. Preliminary comparisons indicate that the simulations produce the major features in the observations. I will present detailed comparisons between simulations and observations for major basins on the moon, including South Pole-Aitken, for different initial lithospheric thicknesses and thermal gradients. [1] Spudis, P.D. (1993) The Geology of Multi-Ring Impact basins: Cambridge University Press. [2] Senft, L.E. and S.T. Stewart (2009) Earth and Planetary Science Letters 287, 471-482. [3] Lillis, R.J., et al. (2010) AGU Fall Meeting.

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.

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.

Lithospheric structure of the Rio Grande rift.

PubMed

Wilson, David; Aster, Richard; West, Michael; Ni, James; Grand, Steve; Gao, Wei; Baldridge, W Scott; Semken, Steve; Patel, Paresh

2005-02-24

A high-resolution, regional passive seismic experiment in the Rio Grande rift region of the southwestern United States has produced new images of upper-mantle velocity structure and crust-mantle topography. Synthesizing these results with geochemical and other geophysical evidence reveals highly symmetric lower-crustal and upper-mantle lithosphere extensional deformation, suggesting a pure-shear rifting mechanism for the Rio Grande rift. Extension in the lower crust is distributed over a region four times the width of the rift's surface expression. Here we propose that the laterally distributed, pure shear extension is a combined effect of low strain rate and a regionally elevated geotherm, possibly abetted by pre-existing lithospheric structures, at the time of rift initiation. Distributed extension in the lower crust and mantle has induced less concentrated vertical mantle upwelling and less vigorous small-scale convection than would have arisen from more localized deformation. This lack of highly focused mantle upwelling may explain a deficit of rift-related volcanics in the Rio Grande rift compared to other major rift systems such as the Kenya rift.

Water-rich Martian mantle can account for the elastic thickness in Amazonian era

NASA Astrophysics Data System (ADS)

Katayama, I.; Matsuoka, Y.; Azuma, S.

2016-12-01

Although high water content in the Martian mantle is inferred from cosmochemistry, the direct measurements of water in the SNC meteorites are controversial, because hydrogen is a highly mobile element and the later terrestrial alteration can modify the primarily concentration in the Mars. On the one hand, water has a significant effect on the rock strength in both brittle and ductile fields; consequently, the presence of water can influence the elastic thickness that is primary controlled by stress distribution in the lithosphere. The Martian elastic lithosphere estimated from gravity and topography data indicates different thickness at the time of loading (e.g. McGovern et al. 2002). The increase of elastic thickness from Noachian to Hesperian is most likely related to the secular cooling in the Mars; however, the nearly constant elastic lithosphere in Amazonian cannot be explained by the thermal evolution alone. In this study, we applied recent rheological data to the Martian lithosphere and tested whether water can account for the elastic thickness seen in the Amazonian era. We incorporated the effect of pore fluid pressure in the brittle regime and Peierls mechanism in the ductile regime in the rheological model, which are not applied in the most previous calculation (e.g. Grott and Breuer 2008) but have a significant influence on the stress distribution in the lithosphere. Since the pore pressure reduces the effective normal stress on the fault plane, the maximum stress in the brittle regime is markedly decreased by the presence of pore fluid. The estimate of elastic lithosphere is dependent on thermal structure, and we used the heat production rate obtained from the Mars Odyssey spectrometry as thermal model (Hahn et al. 2011). Our results indicate the elastic thickness in Amazonian era of 120-170 km for dry condition and 80-110 km for wet condition. The thin elastic thickness calculated under wet environments is a result of significant reduction of flexure moment in the lithosphere. Our model indicates that water-rich Martian lithosphere can be responsible for the observed elastic thickness in Amazonian. However, the model is highly sensitive to the thermal structure and curvature, and more realistic data of heat flow targeted by the Insight mission would provide the robust water concentration in the Martian mantle.

Intraplate mafic magmatism: New insights from Africa and N. America

NASA Astrophysics Data System (ADS)

Ebinger, C. J.; van der Lee, S.; Tepp, G.; Pierre, S.

2017-12-01

Plate tectonic concepts consider that continental interiors are stable, with magmatism and strain localized to plate boundaries. We re-evaluate the role of pre-existing and evolving lithospheric heterogeneities in light of perspectives afforded by surface to mantle results from active and ancient rift zones in Africa and N. America. Our process-oriented approach addresses the localization of strain and magmatism and stability of continental plate interiors. In both Africa and N. America, geophysical imaging and xenolith studies reveal that thick, buoyant, and chemically distinct Archaean cratons with deep roots may deflect mantle flow, and localize magmatism and strain over many tectonic cycles. Studies of the Colorado Plateau and East African rift reveal widespread mantle metasomatism, and high levels of magma degassing along faults and at active volcanoes. The volcanoes and magmatic systems show a strong dependence on pre-existing heterogeneities in plate structure. Syntheses of the EarthScope program ishow that lateral density contrasts and migration of volatiles that accumulated during subduction can refertilize mantle lithosphere, and enable volatile-rich magmatism beneath relatively thick continental lithosphere. For example, the passive margin of eastern N. America shows uplift and magmatism long after the onset of seafloor spreading, demonstrating the dynamic nature of coupling between the lithosphere, asthenosphere, and deeper mantle. As demonstrated by the East African Rift, the Mid-Continent Rift, and other active and ancient rift zones, the interiors of continents, including thick, cold Archaean cratons are not immune to mafic magmatism and tectonism. Recent studies in N. America and Africa reveal ca. 1000 km-wide zones of dynamic uplift, low upper mantle velocities, and broadly distributed strain. The distribution of magmatism and volatile release, in combination with geophysical signals, indicates a potentially convective origin for widespread intraplate earthquakes and magmatism, across areas broader than the surface expression of rifting. Integrated geophysical, geological and geochemical studies reveal large volumes and rates of magmatism at rift zones, provoking re-evaluation of crustal accretion and carbon and water cycles, as well as earthquake and volcanic hazards.

Deep thermal structure of Southeast Asia constrained by S-velocity data

NASA Astrophysics Data System (ADS)

Yu, Chuanhai; Shi, Xiaobin; Yang, Xiaoqiu; Zhao, Junfeng; Chen, Mei; Tang, Qunshu

2017-12-01

Southeast Asia, located in the southeastern part of the Eurasian Plate, is surrounded by tectonically active margins, exhibiting intense seismicity and volcanism, contains complex geological units with a perplexing evolution history. Because tectonic evolution is closely related to the deep thermal structure, an accurate estimation of the lithosphere thermal structure and thickness is important in extracting information on tectonics and geodynamics. However, there are significant uncertainties in the calculated deep thermal state constrained only by the observed surface heat flow. In this study, in order to obtain a better-constrained deep thermal state, we first calculate the deep thermal structure of Southeast Asia by employing an empirical relation between S-velocity and temperature, and then we estimate the base of the thermal lithosphere from the calculated temperature-depth profiles. The results show that, in general, the temperature is higher than the dry mantle solidus below the top of the seismic low-velocity zone, possibly indicating the presence of partial melt in the asthenosphere, particularly beneath oceanic basins such as the South China Sea. The temperature at a depth of 80 km in rifted and oceanic basins such as the Thailand Rift Basin, Thailand Bay, Andaman Sea, and South China Sea is about 200 °C higher than in plateaus and subduction zones such as the Khorat Plateau, Sumatra Island, and Philippine Trench regions. We suggest that the relatively cold and thick lithosphere block of the Khorat Plateau has not experienced significant internal deformation and might be extruded and rotated as a rigid block in response to the Indo-Eurasia collision. Our results show that the surface heat flow in the South China Sea is mainly dominated by the deep thermal state. There is a thermal anomaly in the Leiqiong area and in the areas adjacent to the northern margin of the South China Sea, indicating the presence of a high-temperature and thin lithosphere in the area of the well-known and controversial Hainan plume. The thermal lithosphere-asthenosphere boundary uplift area along the Xisha and southeastern Vietnam margin, in the western margin of South China Sea, which corresponds to the volcanic belt around this area, might indicate upwelling of hot mantle materials. The temperature values at 100 and 120 km depths through most regions of Southeast Asia are about 1400-1500 and 1550-1600 °C, respectively, which are nearly uniform with a small temperature difference. Our results also show that the lithosphere becomes thinner from the continent blocks toward the oceanic basins, with the smaller thickness values of 65-70 km in the South China Sea. The estimated base of the lithosphere corresponds approximately to the 1400 °C isotherm and shows good correlation with the tectonic setting.

Rheology, tectonics, and the structure of the Venus lithosphere

NASA Technical Reports Server (NTRS)

Zuber, M. T.

1994-01-01

Given the absence of ground truth information on seismic structure, heat flow, and rock strength, or short wavelength gravity or magnetic data for Venus, information on the thermal, mechanical and compositional nature of the shallow interior must be obtained by indirect methods. Using pre-Magellan data, theoretical models constrained by the depths of impact craters and the length scales of tectonic features yielded estimates on the thickness of Venus' brittle-elastic lithosphere and the allowable range of crustal thickness and surface thermal gradient. The purpose of this study is to revisit the question of the shallow structure of Venus based on Magellan observations of the surface and recent experiments that address Venus' crustal rheology.

Evaluation of existing knowledge of the tectonic history and lithospheric structure of South America

NASA Technical Reports Server (NTRS)

Keller, G. R.; Lidiak, E. G. (Principal Investigator)

1980-01-01

While data is available on the lithospheric and crustal structure of the Andes region of South America, there is limited knowledge of these aspects of the eastern portion of the continent. For this reason, a surface wave dispersion study of the area was initiated. Long period seismograms were obtained for a tripartite analysis of dispersion. A flow chart of the analysis to be conducted is presented along with a preliminary geologic/tectonic map that was prepared. Efforts to characterize the provinces identified in terms of their geological and geophysical parameters continue.

Mathematical geophysics: A survey of recent developments in seismology and geodynamics

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

Vlaar, N.J.

1988-01-01

This survey deals with modern methods for the determination of the structure of the Earth and for the analysis and modeling of the dynamic processes in the Earth's interior. Seismology and the three-dimensional structure of the Earth are covered in chapters devoted to waves in the three-dimensional Earth and large-scale inversion, while the discussion of convection and lithospheric processes focuses on geomagnetism, mantle convection, post-glacial rebound, and thermomechanical processes in the lithosphere. The emphasis of the work is theoretical, but the reader will find a discussion of the pertinent observational evidence.

The Lithospheric Structure of the Solonker Suture Zone and Adjacent Areas: Crustal Structure Revealed by a High-Resolution Magnetotelluric Study

NASA Astrophysics Data System (ADS)

Ye, Gaofeng; Jin, Sheng; Wei, Wenbo; Jing, Jian'en

2017-04-01

The closure of the Paleo-Asian Ocean along the Solonker Suture Zone (SSZ) during the Late Permian and Triassic represented the final stage in the formation of the Central Asian Orogenic Belt between the Siberian Craton and the North China Craton. In order to better understand the structure and formation of this ancient subduction zone, a high-resolution magnetotelluric (MT) profile was collected with both broadband and long-period MT data. The high resolution mapping of the lithosphere achieved in this study is due to the closely spaced MT stations (2-3 km). With the 2-D resistivity model, a south-dipping conductor was detected and extends through the entire crust. The geometry of this feature provides evidence that a southward directed subduction zone formed the Solonker suture. The enhanced conductivity was interpreted to subducted sulfide-bearing graphitic sediments. The resistive body beneath the northern margin of the North China Craton indicates a thickened lithosphere caused by the southward subduction at this region, and the resistive body beneath the Solonker Suture Zone indicates the subducted oceanic lithosphere. North-dipping low resistivity features were also detected in the crust of both the North China Craton and Central Asian Orogenic Belt, and were interpreted as post-collisional thrust faults. Strong anisotropy was found beneath the suture zone, and can be explained if the high strain rate has rotated the fold axes into the dip direction.

Electromagnetic study of lithospheric structure in the marginal zone of East European Craton in NW Poland

NASA Astrophysics Data System (ADS)

Jóźwiak, Waldemar

2013-10-01

The marginal zone of the East European Platform, an area of key importance for our understanding of the geotectonic history of Europe, has been a challenge for geophysicists for many years. The basic research method is seismic survey, but many important data on physical properties and structure of the lithosphere may also be provided by the electromagnetic methods. In this paper, results of deep basement study by electromagnetic methods performed in Poland since the mid-1960s are presented. Over this time, several hundred long-period soundings have been executed providing an assessment of the electric conductivity distribution in the crust and upper mantle. Numerous 1D, 2D, and pseudo-3D electric conductivity models were constructed, and a new interpretation method based on Horizontal Magnetic Tensor analysis has been applied recently. The results show that the contact zone is of lithospheric discontinuity character and there are distinct differences in geoelectric structures between the Precambrian Platform, transitional zone (TESZ), and the Paleozoic Platform. The wide-spread conducting complexes in the crust with integral conductivity values reaching 10 000 S at 20-30 km depths are most spectacular. They are most likely consequences of geological processes related to Caledonian and Variscan orogenesis. The upper mantle conductivity is also variable, the thickness of high-resistive lithospheric plates ranging from 120-140 km under the Paleozoic Platform to 220-240 km under the East European Platform.

Bulldozing of Basal Continental Mantle Lithosphere During Flat-Slab Subduction

NASA Astrophysics Data System (ADS)

Axen, G. J.; van Wijk, J.; Currie, C. A.

2017-12-01

Flat-slab subduction occurs along 10% of subduction margins, forming magmatic gaps and causing inland migration of upper-plate deformation. We suggest that basal continental mantle lithosphere (CML) can be bulldozed ahead of the flat portion of horizontally-subducted oceanic lithosphere, forming a growing and advancing keel of thickened CML. This process fills the asthenospheric mantle wedge with CML, precluding melting. The bulldozed CML keel may transmit tectonic stresses ahead of the flat slab itself, causing upper-plate deformation ahead of the slab hinge. We designed 2-D numerical models after the North American Laramide orogeny, with subduction of a thick, buoyant oceanic plateau (conjugate Shatsky Rise) and with the continent advancing trenchward over the initial slab hinge. This results in slab-flattening, and removal of CML material. In our models, the thickness of the CML layer removed by this process depends on overriding plate rheology and is up to 25 km. The removed material is bulldozed ahead of the hinge and may fill up the asthenospheric wedge. Low-density (depleted) CML favors formation of bulldozed keels, which increase in width as CML strength decreases. Regular-density and/or stronger CML forms smaller bulldozed keels that are more likely to sink with the slab as eclogitization and densification proceed. When the flat slab rolls back, it leaves a step in the CML at the farthest extent of the slab. Relics of this step may remain below North America or may have dripped off. We interpret an upper-mantle fast-velocity anomaly below SE New Mexico and W Texas as a drip/keel, and the step in lithosphere thickness in southwestern Colorado as a fossil step, caused by the removal of the CML layer. Our model predicts that the Laramide bulldozed CML keel may have aided in stress transmission that caused basement uplifts as far as NE Wyoming and subsurface folds even farther N and E. Modern examples may exist in South American flat slab segments.

Lithospheric models of the North American continent

NASA Astrophysics Data System (ADS)

Tesauro, Magdala; Kaban, Mikhail; Mooney, Walter; Cloetingh, Sierd

2015-04-01

We constructed NACr14, a 3D model of the North American (NA) crust, based on the most recent seismic data from the USGS database. In comparison with the global crustal model CRUST 1.0, NACr14 is more heterogeneous, showing a larger spatial variability of the thickness and average velocities of the crustal layers. Velocities of the lower crust vary in a larger range than those of the other layers, while the thickness of all the three layers is on average between 11 and 13 km. The largest velocities of the crystalline crust (>6.6 km/s) reflect the presence of a 7.x layer (>7.0 km/s) in the lowermost part of the crust. Using NACr2014, a regional (NA07) and a global (SL201sv) tomography model, and gravity data, we apply an iterative technique, which jointly interprets seismic tomography and gravity data, to estimate temperature and compositional variations in the NA upper mantle. The results obtained demonstrate that temperature of the cratonic mantle is up to 150°C higher than when using a uniform compositional model. The differences between the two tomography models influence the results more strongly than possible changes of the depth distribution of compositional variations. Strong negative compositional density anomalies, corresponding to Mg # >92, characterize the upper mantle of the northwestern part of the Superior craton and the central part of the Slave and Churchill craton. The Proterozoic upper mantle of the western and more deformed part of the NA cratons, appears weakly depleted (Mg# ~91) when NA07 is used, in agreement with the results based on the interpretation of xenolith data. When we use SL2013sv, the same areas are locally characterized by high density bodies, which might be interpreted as the effect due to fragments of subducted slabs, as those close to the suture of the Appalachians and Grenville province. We used the two thermal models to estimate the integrated strength and the effective elastic thickness (Te) of the lithosphere. In the peripheral parts of the cratons, as the Proterozoic Canadian Platform and Grenville, the integrated strength for model NA07 is ten times larger than in model SL2013sv, due to a model-dependent temperature difference of >200˚C in the uppermost mantle. In both models, Proterozoic regions reactivated by Meso-Cenozoic tectonics (e.g., Rocky Mountains and the Mississippi Embayment) show a weak lithosphere due to the absence of the mechanically strong part of the mantle lithospheric layer. Intraplate earthquakes are distributed along the edges of the cratons, characterized by a weak lithosphere or pronounced variations in integrated lithospheric strength and Te. In addition, the sum of the seismic moments shows that most of the energy is released by the weak lithosphere. These results suggest that the edges of the cratons are more prone to accumulation of tectonic stress and subsequent release by earthquakes, in comparison with the stable cratonic regions which resist deformation.

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.

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.

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.

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.

The long-term strength of Europe and its implications for plate-forming processes.

PubMed

Pérez-Gussinyé, M; Watts, A B

2005-07-21

Field-based geological studies show that continental deformation preferentially occurs in young tectonic provinces rather than in old cratons. This partitioning of deformation suggests that the cratons are stronger than surrounding younger Phanerozoic provinces. However, although Archaean and Phanerozoic lithosphere differ in their thickness and composition, their relative strength is a matter of much debate. One proxy of strength is the effective elastic thickness of the lithosphere, Te. Unfortunately, spatial variations in Te are not well understood, as different methods yield different results. The differences are most apparent in cratons, where the 'Bouguer coherence' method yields large Te values (> 60 km) whereas the 'free-air admittance' method yields low values (< 25 km). Here we present estimates of the variability of Te in Europe using both methods. We show that when they are consistently formulated, both methods yield comparable Te values that correlate with geology, and that the strength of old lithosphere (> or = 1.5 Gyr old) is much larger (mean Te > 60 km) than that of younger lithosphere (mean Te < 30 km). We propose that this strength difference reflects changes in lithospheric plate structure (thickness, geothermal gradient and composition) that result from mantle temperature and volatile content decrease through Earth's history.

Magma-assisted rifting in Ethiopia.

PubMed

Kendall, J-M; Stuart, G W; Ebinger, C J; Bastow, I D; Keir, D

2005-01-13

The rifting of continents and evolution of ocean basins is a fundamental component of plate tectonics, yet the process of continental break-up remains controversial. Plate driving forces have been estimated to be as much as an order of magnitude smaller than those required to rupture thick continental lithosphere. However, Buck has proposed that lithospheric heating by mantle upwelling and related magma production could promote lithospheric rupture at much lower stresses. Such models of mechanical versus magma-assisted extension can be tested, because they predict different temporal and spatial patterns of crustal and upper-mantle structure. Changes in plate deformation produce strain-enhanced crystal alignment and increased melt production within the upper mantle, both of which can cause seismic anisotropy. The Northern Ethiopian Rift is an ideal place to test break-up models because it formed in cratonic lithosphere with minor far-field plate stresses. Here we present evidence of seismic anisotropy in the upper mantle of this rift zone using observations of shear-wave splitting. Our observations, together with recent geological data, indicate a strong component of melt-induced anisotropy with only minor crustal stretching, supporting the magma-assisted rifting model in this area of initially cold, thick continental lithosphere.

Lateral variation in upper mantle temperature and composition beneath mid-ocean ridges inferred from shear-wave propagation, geoid, and bathymetry. Ph.D. Thesis

NASA Technical Reports Server (NTRS)

Sheehan, Anne Francis

1991-01-01

Resolution of both the extent and mechanism of lateral heterogeneity in the upper mantle constraints the nature and scales of mantle convection. Oceanic regions are of particular interest as they are likely to provide the closest glimpse at the patterns of temperature anomalies and convective flow in the upper mantle because of their young age and simple crustal structure relative to continental regions. Lateral variations were determined in the seismic velocity and attenuation structure of the lithosphere and astenosphere beneath the oceans, and these seismological observations were combined with the data and theory of geoid and bathymetry anomalies in order to test and improve current models for seafloor spreading and mantle convection. Variations were determined in mantle properties on a scale of about 1000 km, comparable to the thickness of the upper mantle. Seismic velocity, geoid, and bathymetry anomalies are all sensitive to variations in upper mantle density, and inversions were formulated to combine quantitatively these different data and to search for a common origin. Variations in mantle density can be either of thermal or compositional origin and are related to mantle convection or differentiation.

Physical Processes Contributing To Small-scale Vertical Movements During Changing Inplane Stresses In Rift Basins and At Passive Continental Margins

NASA Astrophysics Data System (ADS)

Paulsen, G. E.; Nielsen, S. B.; Hansen, D. L.

The vertical movements during a regional stress reversal in a rifted basin or on a passive continental margin are examined using a numerical 2D thermo-mechanical finite element model with a visco-elastic-plastic rheology. Three different physical mechanisms are recognized in small-scale vertical movements at small inplane force variations: elastic dilatation, elastic flexure, and permanent deformation. Their rela- tive importance depend on the applied force, the duration of the force, and the thermal structure of the lithosphere. Elastic material dilatation occurs whenever the stress state changes. A reversal from extension to compression therefore immediately leads to elastic dilatation, and re- sults in an overall subsidence of the entire profile. Simultaneously with dilatation the lithosphere reacts with flexure. The significance of the flexural component strongly depends on the thermal structure of the lithosphere. The polarity and amplitude of the flexure depends on the initial (before compression) loading of the lithosphere. Gener- ally, the flexural effects lead to subsidence of the overdeep in the landward part of the basin and a small amount of uplift at the basin flanks. The amplitudes of the flexural response are small and comparable with the amplitudes of the elastic dilatation. With continuing compression permanent deformation and lithospheric thickening becomes increasingly important. Ultimately, the thickened part of the lithosphere stands out as an inverted zone. The amount of permanent deformation is directly connected with the size and duration of the applied force, but even a relatively small force leads to inversion tectonics in the landward part of the basin. The conclusions are: 1) small stress induced vertical movements in rift basins and at passive continental margins are the result of a complex interaction of at least three different processes, 2) the total sediment loaded amplitudes resulting from these pro- cesses are small (2-300 m) for inplane forces up to 1.5·1012 N/m.

Lithospheric Shear Velocity Structure of South Island, New Zealand from Rayleigh Wave Tomography of Amphibious Array Data

NASA Astrophysics Data System (ADS)

Ball, J. S.; Sheehan, A. F.; Stachnik, J. C.; Lin, F. C.; Collins, J. A.

2015-12-01

We present the first 3D shear velocity model extending well offshore of New Zealand's South Island, imaging the lithosphere beneath Campbell and Challenger plateaus. Our model is constructed via linearized inversion of both teleseismic (18 -70 s period) and ambient noise-based (8 - 25 s period) Rayleigh wave dispersion measurements. We augment an array of 29 ocean-bottom instruments deployed off the South Island's east and west coasts in 2009-2010 with 28 New Zealand land-based seismometers. The ocean-bottom seismometers and 4 of the land seismometers were part of the Marine Observations of Anisotropy Near Aotearoa (MOANA) experiment, and the remaining land seismometers are from New Zealand's permanent GeoNet array. Major features of our shear wave velocity (Vs) model include a low-velocity (Vs<4.3km/s) body extending to at least 75km depth beneath the Banks and Otago peninsulas, a high-velocity (Vs~4.7km/s) upper mantle anomaly underlying the Southern Alps to a depth of 100km, and discontinuous lithospheric velocity structure between eastern and western Challenger Plateau. Using the 4.5km/s contour as a proxy for the lithosphere-asthenosphere boundary, our model suggests that the lithospheric thickness of Challenger Plateau is substantially greater than that of Campbell Plateau. The high-velocity anomaly we resolve beneath the central South Island exhibits strong spatial correlation with subcrustal earthquake hypocenters along the Alpine Fault (Boese et al., 2013). The ~400km-long low velocity zone we image beneath eastern South Island underlies Cenozoic volcanics and mantle-derived helium observations (Hoke et al., 2000) on the surface. The NE-trending low-velocity zone dividing Challenger Plateau in our model underlies a prominent magnetic discontinuity (Sutherland et al., 1999). The latter feature has been interpreted to represent a pre-Cretaceous crustal boundary, which our results suggest may involve the entire mantle lithosphere.

Apparent Susceptibility Contrast Distribution of Continental Lithosphere in China and Its Surroundings: Implications to Regional Tectonics

NASA Astrophysics Data System (ADS)

Du, J.; Chen, C.; Sun, S.; Zhang, Y.; Liang, Q.

2015-12-01

Lithospheric magnetic field characterizes response of magnetic properties of rocks, which are mainly dependent on mineral and temperature variations. Hence, lithospheric magnetic structure brings important information to understand tectonic and thermal processes in the crust and uppermost mantle. In particular, the reliable global geomagnetic field models with large-scales based on satellite magnetic measurements provide regional view of the lithospheric magnetic structure. Here, with smallest and flattest constraints we use the inversion method based on the single layer model to calculate the spatial distribution of apparent susceptibility of continental lithosphere in China and its surroundings. It should be noted that: (1) magnetic anomaly data we used has removed the effect of global oceanic remanent magnetization, (2) the error of magnetic anomaly data is estimated from statistical analysis among MF7, GRIMM_L120, CHAOS5 and CM5 models, (3) the magnetic layer is bounded by the bottom of sediment and the Moho from CRUST1.0 model and is discretized into ellipsoidal prisms with equal angles in latitude and longitude, and (4) an adaptive subdivision & Gauss-Legendre quadrature with fixed order is adopted to solve the forward problem and IGRF11 is utilized as inducing field model. Since the missing longest wavelength components in the lithospheric magnetic field models and the so-called magnetic annihilators, the Apparent Susceptibility Contrast (ASC) distribution is obtained. The ASC distribution has obvious variations and illustrates the mosaic continent with old blocks, orogenic belts, rework fragments and also earthquake regions/zones. Moreover, the ASC distribution provides new insights and evidences of the destruction of North China Craton and geodynamic processes of Tibetan plateau and Baikal rift etc. This study is supported by China Postdoctoral Science Foundation (Grant No.: 2015M572217) and Natural Science Fund of Hubei Province (Grant No.: 2015CFB361).

Earth's structure and evolution inferred from topography, gravity, and seismicity.

NASA Astrophysics Data System (ADS)

Watkinson, A. J.; Menard, J.; Patton, R. L.

2016-12-01

Earth's wavelength-dependent response to loading, reflected in observed topography, gravity, and seismicity, can be interpreted in terms of a stack of layers under the assumption of transverse isotropy. The theory of plate tectonics holds that the outermost layers of this stack are mobile, produced at oceanic ridges, and consumed at subduction zones. Their toroidal motions are generally consistent with those of several rigid bodies, except in the world's active mountain belts where strains are partitioned and preserved in tectonite fabrics. Even portions of the oceanic lithosphere exhibit non-rigid behavior. Earth's gravity-topography cross-spectrum exhibits notable variations in signal amplitude and character at spherical harmonic degrees l=13, 116, 416, and 1389. Corresponding Cartesian wavelengths are approximately equal to the respective thicknesses of Earth's mantle, continental mantle lithosphere, oceanic thermal lithosphere, and continental crust, all known from seismology. Regional variations in seismic moment release with depth, derived from the global Centroid Moment Tensor catalog, are also evident in the crust and mantle lithosphere. Combined, these observations provide powerful constraints for the structure and evolution of the crust, mantle lithosphere, and mantle as a whole. All that is required is a dynamically consistent mechanism relating wavelength to layer thickness and shear-strain localization. A statistically-invariant 'diharmonic' relation exhibiting these properties appears as the leading order approximation to toroidal motions on a self-gravitating body of differential grade-2 material. We use this relation, specifically its predictions of weakness and rigidity, and of folding and shear banding response as a function of wavelength-to-thickness ratio, to interpret Earth's gravity, topography, and seismicity in four-dimensions. We find the mantle lithosphere to be about 255-km thick beneath the Himalaya and the Andes, and the long-wavelength (l < 14) low-amplitude portion of Earth's gravity field to be consistent with loading of the mesosphere by subducted slabs. The Earth that emerges from this work might be characterized as a self-gravitating, self-peeling onion.

Resolving Discrepancies Between Observed and Predicted Dynamic Topography on Earth

NASA Astrophysics Data System (ADS)

Richards, F. D.; Hoggard, M.; White, N. J.

2017-12-01

Compilations of well-resolved oceanic residual depth measurements suggest that present-day dynamic topography differs from that predicted by geodynamic simulations in two significant respects. At short wavelengths (λ ≤ 5,000 km), much larger amplitude variations are observed, whereas at long wavelengths (λ > 5,000 km), observed dynamic topography is substantially smaller. Explaining the cause of this discrepancy with a view to reconciling these different approaches is central to constraining the structure and dynamics of the deep Earth. Here, we first convert shear wave velocity to temperature using an experimentally-derived anelasticity model. This relationship is calibrated using a pressure and temperature-dependent plate model that satisfies age-depth subsidence, heat flow measurements, and seismological constraints on the depth to the lithosphere-asthenosphere boundary. In this way, we show that, at short-wavelengths, observed dynamic topography is consistent with ±150 ºC asthenospheric temperature anomalies. These inferred thermal buoyancy variations are independently verified by temperature measurements derived from geochemical analyses of mid-ocean ridge basalts. Viscosity profiles derived from the anelasticity model suggest that the asthenosphere has an average viscosity that is two orders of magnitude lower than that of the underlying upper mantle. The base of this low-viscosity layer coincides with a peak in azimuthal anisotropy observed in recent seismic experiments. This agreement implies that lateral asthenospheric flow is rapid with respect to the underlying upper mantle. We conclude that improved density and viscosity models of the uppermost mantle, which combine a more comprehensive physical description of the lithosphere-asthenosphere system with recent seismic tomographic models, can help to resolve spectral discrepancies between observed and predicted dynamic topography. Finally, we explore possible solutions to the long-wavelength discrepancy that exploit the velocity to density conversion described above combined with radial variation of mantle viscosity.

Lithospheric Structure and Seismotectonics of Central East Antarctica

NASA Astrophysics Data System (ADS)

Reading, A. M.

2006-12-01

The lithosphere of central East Antarctica, the sector of the continent between 30°E - 120°E, is investigated using seismic methods including receiver function and shear-wave splitting analysis. Data from the broadband stations of the temporary SSCUA deployment (in the continental interior) are used together with records from the permanent GSN stations (on the coast) to carry out the first studies of crustal depth and structure, and patterns of seismic anisotropy across this region. The depth of the Moho is found to be 42 km (+/- 2 km) beneath Mawson station with similar structures extending southward across the Rayner province as far south as Beaver Lake. The Fisher Terrane is characterised by a crustal shear wavespeed profile showing few discontinuties with the Moho at a similar depth to the Rayner. South of Fisher, the crust becomes much shallower, with the Moho at 32 km depth. This shallow crust extends across the Lambert glacier to the Prydz coast and the Lambert Terrane. The characteristic crustal wavespeed profiles provide baseline structure for mapping the extent of the terrance beneath the Antarctic Ice Sheet in future deployments. Observations of seismic anisotropy are less well- defined but, at a reconnaissance level, show fast directions parallel to the present day coastline. This may be controlled by rift-related influences on the lithosphere associated with the breakup of East Gondwana. The seismicity is confirmed to be extremely low. The only seismogenic forces on the Antarctic plate in this region are acting at the boundary between the continental and oceanic lithosphere west of 50°E and east of 100°E and represent a superposition of tectonic and glaciogenic controls. The Lambert Glacier region shows little or no seismotectonic activity in the continental interior or on the oceanic margin.

Can We Probe the Conductivity of the Lithosphere and Upper Mantle Using Satellite Tidal Magnetic Signals?

NASA Technical Reports Server (NTRS)

Schnepf, N. R.; Kuvshinov, A.; Sabaka, T.

2015-01-01

A few studies convincingly demonstrated that the magnetic fields induced by the lunar semidiurnal (M2) ocean flow can be identified in satellite observations. This result encourages using M2 satellite magnetic data to constrain subsurface electrical conductivity in oceanic regions. Traditional satellite-based induction studies using signals of magnetospheric origin are mostly sensitive to conducting structures because of the inductive coupling between primary and induced sources. In contrast, galvanic coupling from the oceanic tidal signal allows for studying less conductive, shallower structures. We perform global 3-D electromagnetic numerical simulations to investigate the sensitivity of M2 signals to conductivity distributions at different depths. The results of our sensitivity analysis suggest it will be promising to use M2 oceanic signals detected at satellite altitude for probing lithospheric and upper mantle conductivity. Our simulations also suggest that M2 seafloor electric and magnetic field data may provide complementary details to better constrain lithospheric conductivity.

Crustal rheology controls on the Tibetan plateau formation during India-Asia convergence

PubMed Central

Chen, Lin; Capitanio, Fabio A.; Liu, Lijun; Gerya, Taras V.

2017-01-01

The formation of the Tibetan plateau during the India-Asia collision remains an outstanding issue. Proposed models mostly focus on the different styles of Tibetan crustal deformation, yet these do not readily explain the observed variation of deformation and deep structures along the collisional zone. Here we use three-dimensional numerical models to evaluate the effects of crustal rheology on the formation of the Himalayan-Tibetan orogenic system. During convergence, a weaker Asian crust allows strain far north within the upper plate, where a wide continental plateau forms behind the orogeny. In contrast, a stronger Asian crust suppresses the plateau formation, while the orogeny accommodates most of the shortening. The stronger Asian lithosphere is also forced beneath the Indian lithosphere, forming a reversed-polarity underthrusting. Our results demonstrate that the observed variations in lithosphere deformation and structures along the India-Asia collision zone are primarily controlled by the strength heterogeneity of the Asian continental crust. PMID:28722008

Seismic constraints on the lithosphere-asthenosphere boundary

NASA Astrophysics Data System (ADS)

Rychert, Catherine A.

2014-05-01

The basic tenet of plate tectonics is that a rigid plate, or lithosphere, moves over a weaker asthenospheric layer. However, the exact location and defining mechanism of the boundary at the base of the plate, the lithosphere-asthenosphere boundary (LAB) is debated. The oceans should represent a simple scenario since the lithosphere is predicted to thicken with seafloor age if it thermally defined, whereas a constant plate thickness might indicate a compositional definition. However, the oceans are remote and difficult to constrain, and studies with different sensitivities and resolutions have come to different conclusions. Hotspot regions lend additional insight, since they are relatively well instrumented with seismic stations, and also since the effect of a thermal plume on the LAB should depend on the defining mechanism of the plate. Here I present new results using S-to-P receiver functions to image upper mantle discontinuity structure beneath volcanically active regions including Hawaii, Iceland, Galapagos, and Afar. In particular I focus on the lithosphere-asthenosphere boundary and discontinuities related to the base of melting, which can be used to highlight plume locations. I image a lithosphere-asthenosphere boundary in the 50 - 95 km depth range beneath Hawaii, Galapagos, and Iceland. Although LAB depth variations exist within these regions, significant thinning is not observed in the locations of hypothesized plume impingement from receiver functions (see below). Since a purely thermally defined lithosphere is expected to thin significantly in the presence of a thermal plume anomaly, a compositional component in the definition of the LAB is implied. Beneath Afar, an LAB is imaged at 75 km depth on the flank of the rift, but no LAB is imaged beneath the rift itself. The transition from flank of rift is relatively abrupt, again suggesting something other than a purely thermally defined lithosphere. Melt may also exist in the asthenosphere in these regions of hotpot volcanism. Indeed, S-to-P also images strong velocity increases that are likely related to the base of a melt-rich layer beneath the oceanic LAB. This discontinuity may highlight plume locations since melt is predicted deeper at thermal anomalies. For instance, beneath Hawaii the base of melting increases from 110 km to 155 km depth 100 km west of Hawaii, i.e., the likely location of plume impingement on the lithosphere. Beneath Galapagos the discontinuity is deeper in 3 sectors, all off the island axis, suggesting multiple plume diversions and complex plume-ridge interactions. Beneath Iceland deepening is imaged to the northeast of the island. Beneath the Afar rift a shallow melt discontinuity is imaged at ~75 km, suggesting that the plume is located outside the study region. Overall, the deepest realizations of the discontinuities agree with the slowest velocities from surface waves, but are not located directly beneath surface volcanoes. This suggests that either plumes approach the surface at an angle or that restite roots beneath hotspots divert plumes at shallow depths. In either case, mantle melts are likely guided from the location of impingement on the lithosphere to current day surface volcanoes by pre-existing structures of the lithosphere.

Rheological structure of the lithosphere in plate boundary strike-slip fault zones

NASA Astrophysics Data System (ADS)

Chatzaras, Vasileios; Tikoff, Basil; Kruckenberg, Seth C.; Newman, Julie; Titus, Sarah J.; Withers, Anthony C.; Drury, Martyn R.

2016-04-01

How well constrained is the rheological structure of the lithosphere in plate boundary strike-slip fault systems? Further, how do lithospheric layers, with rheologically distinct behaviors, interact within the strike-slip fault zones? To address these questions, we present rheological observations from the mantle sections of two lithospheric-scale, strike-slip fault zones. Xenoliths from ˜40 km depth (970-1100 ° C) beneath the San Andreas fault system (SAF) provide critical constraints on the mechanical stratification of the lithosphere in this continental transform fault. Samples from the Bogota Peninsula shear zone (BPSZ, New Caledonia), which is an exhumed oceanic transform fault, provide insights on lateral variations in mantle strength and viscosity across the fault zone at a depth corresponding to deformation temperatures of ˜900 ° C. Olivine recrystallized grain size piezometry suggests that the shear stress in the SAF upper mantle is 5-9 MPa and in the BPSZ is 4-10 MPa. Thus, the mantle strength in both fault zones is comparable to the crustal strength (˜10 MPa) of seismogenic strike-slip faults in the SAF system. Across the BPSZ, shear stress increases from 4 MPa in the surrounding rocks to 10 MPa in the mylonites, which comprise the core of the shear zone. Further, the BPSZ is characterized by at least one order of magnitude difference in the viscosity between the mylonites (1018 Paṡs) and the surrounding rocks (1019 Paṡs). Mantle viscosity in both the BPSZ mylonites and the SAF (7.0ṡ1018-3.1ṡ1020 Paṡs) is relatively low. To explain our observations from these two strike-slip fault zones, we propose the "lithospheric feedback" model in which the upper crust and lithospheric mantle act together as an integrated system. Mantle flow controls displacement and the upper crust controls the stress magnitude in the system. Our stress data combined with data that are now available for the middle and lower crustal sections of other transcurrent fault systems support the prediction for constant shear strength (˜10 MPa) throughout the lithosphere; the stress magnitude is controlled by the shear strength of the upper crustal faults. Fault rupture in the upper crust induces displacement rate loading of the upper mantle, which in turn, causes strain localization in the mantle shear zone beneath the strike-slip fault. Such forced localization leads to higher stresses and strain rates in the shear zone compared to the surrounding rocks. Low mantle viscosity within the shear zone is critical for facilitating mantle flow, which induces widespread crustal deformation and displacement loading. The lithospheric feedback model suggests that strike-slip fault zones are not mechanically stratified in terms of shear stress, and that it is the time-dependent interaction of the different lithospheric layers - rather than their relative strengths - that governs the rheological behavior of the plate boundary, strike-slip fault zones.

Correlation between elastic energy density and deep earthquakes distribution

NASA Astrophysics Data System (ADS)

Gunawardana, P. M.; Morra, G.

2017-05-01

The mechanism at the origin of the earthquakes below 30 km remains elusive as these events cannot be explained by brittle frictional processes. In this work we focus on the global total distribution of earthquakes frequency vs. depth from ∼50 km to 670 km depth. We develop a numerical model of self-driven subduction by solving the non-homogeneous Stokes equation using the ;Particle in cell method; in combination with a conservative finite difference scheme, here solved for the first time using Python and NumPy only. We show that most of the elastic energy is stored in the slab core and that it is strongly correlated with the earthquake frequency-depth distribution for a wide range of lithosphere and lithosphere-core viscosities. According to our results, we suggest that 1) slab bending at the bottom of the upper mantle causes the peak of the earthquake frequency-depth distribution that is observed at mantle transition depth; 2) the presence of a high viscous stiff core inside the lithosphere generates an elastic energy distribution that fits better with the exponential decay that is observed at intermediate depth.

Developing Regionalized Models of Lithospheric Thickness and Velocity Structure Across Eurasia and the Middle East from Jointly Inverting P-Wave and S-Wave Receiver Functions with Rayleigh Wave Group and Phase Velocities

DTIC Science & Technology

2011-09-01

modeling of regional waveforms at station ANTO , in UNIFIED region #14. The velocity models (left) and the corresponding predictions (middle and right) are...models, Geophy. J. Int. 118: 245–254. Rychert, C. A. and P. M. Shearer (2009). A global view of the lithosphere-asthenosphere boundary, Science 324 : 495

Joint inversion of teleseismic receiver functions and magnetotelluric data using a genetic algorithm: Are seismic velocities and electrical conductivities compatible?

NASA Astrophysics Data System (ADS)

Moorkamp, M.; Jones, A. G.; Eaton, D. W.

2007-08-01

Joint inversion of different kinds of geophysical data has the potential to improve model resolution, under the assumption that the different observations are sensitive to the same subsurface features. Here, we examine the compatibility of P-wave teleseismic receiver functions and long-period magnetotelluric (MT) observations, using joint inversion, to infer one-dimensional lithospheric structure. We apply a genetic algorithm to invert teleseismic and MT data from the Slave craton; a region where previous independent analyses of these data have indicated correlated layering of the lithosphere. Examination of model resolution and parameter trade-off suggests that the main features of this area, the Moho, Central Slave Mantle Conductor and the Lithosphere-Asthenosphere boundary, are sensed to varying degrees by both methods. Thus, joint inversion of these two complementary data sets can be used to construct improved models of the lithosphere. Further studies will be needed to assess whether the approach can be applied globally.

Ivrea mantle wedge and arc of the Western Alps (I): Geophysical evidence for the deep structure

NASA Astrophysics Data System (ADS)

Kissling, Edi; Schmid, Stefan M.; Diehl, Tobias

2017-04-01

The construction of five crustal-scale profiles across the Western Alps and the Ivrea mantle wedge integrates up-to-date geological and geophysical information and reveals important along strike changes in the overall structure of the crust of the Western Alpine arc (Schmid et al. 2017). The 3D crustal model of the Western Alps represented by these cross sections is based on recent P-velocity local earthquake tomography that compliments the previously existing wealth of geophysical information about lithosphere structure in the region. As part of Adria mantle lithosphere exhibiting strong upward bending toward the plate boundary along the inner arc of the Western Alps, the well-known Ivrea body plays a crucial role in our tectonic model. Until recently, however, the detailed 3D geometry of this key structure was only poorly constrained. In this study we present a review of the many seismic data in the region and we document the construction of our 3D lithosphere model by principles of multidisciplinary seismic tomography. Reference: Stefan M. Schmid, Edi Kissling, Douwe J.J. van Hinsbergen, Giancarlo Molli (2017). Ivrea mantle wedge and arc of the Western Alps (2): Kinematic evolution of the Alps-Apennines orogenic system. Abstract Volume EGU 2017.

Evidence for retrograde lithospheric subduction on Venus

NASA Technical Reports Server (NTRS)

Sandwell, David T.; Schubert, Gerald

1992-01-01

Though there is no plate tectonics per se on Venus, recent Magellan radar images and topographic profiles of the planet suggest the occurrence of the plate tectonic processes of lithospheric subduction and back-arc spreading. The perimeters of several large coronae (e.g., Latona, Artemis, and Eithinoha) resemble Earth subduction zones in both their planform and topographic profile. The planform of arcuate structures in Eastern Aphrodite were compared with subduction zones of the East Indies. The venusian structures have radii of curvature that are similar to those of terrestrial subduction zones. Moreover, the topography of the venusian ridge/trench structures is highly asymmetric with a ridge on the concave side and a trough on the convex side; Earth subduction zones generally display the same asymmetry.

Constraints on the lithospheric structure of Venus from mechanical models and tectonic surface features

NASA Technical Reports Server (NTRS)

Zuber, Maria T.

1987-01-01

The evidence for the extensional or compressional origins of some prominent Venusian surface features disclosed by radar images is discussed. Using simple models, the hypothesis that the observed length scales (10-20 km and 100-300 km) of deformations are controlled by dominant wavelengths arising from unstable compression or extension of the Venus lithosphere is tested. The results show that the existence of tectonic features that exhibit both length scales can be explained if, at the time of deformation, the lithosphere consisted of a crust that was relatively strong near the surface and weak at its base, and an upper mantle that was stronger than or nearly comparable in strength to the upper crust.

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.

Control of lithosphere structure on surface deformation in the Central Barents Sea: insights from dynamical modeling

NASA Astrophysics Data System (ADS)

Gac, Sebastien; Faleide, Jan Inge

2014-05-01

The Barents Sea is located in the Northern European Arctic. The Eastern Barents Sea features one of the deepest sedimentary basins in the world whereas large parts of the Western Barents Sea is covered by a shallow sedimentary platform. Seismic tomography data (Levshin et al., 2007; Ritzmann and Faleide, 2009) show slower S-wave velocity in the upper mantle beneath the East Barents Sea compared to the West Barents Sea, indicating a steep deepening of the Lithosphere-Asthenosphere Boundary (LAB) in the Central Barents Sea from West to East. Additionally, the Central Barents Sea is marked by a South-North succession of regularly-spaced inverted structures (uplifted domes) such as the Fedinsky High and the Sentralbanken High. The origin of these inverted structures is under debate. The interpretation of recent seismic data in the Central Barents Sea suggests that part of the inversion is contemporaneous with the Late-Triassic-Early Jurassic westwards thrusting of Novaya Zemlya. This suggests that the origin of domes might be linked to compressional events on the eastern side of the Barents Sea. A 2D thermo-mechanical model of lithosphere shortening is used to explore the effect of LAB geometry on the surface deformation in the Central Barents Sea. The model is based on a Lagrangian finite element method (Gac et al., 2013). The model consists of a crust - mantle lithosphere characterized by non-linear temperature and pressure dependent visco-elastic-plastic rheologies. The mechanical model is coupled with a thermal model taking into account heat advection and diffusion. Sedimentation and gravity are also taken into account. Contractional boundary conditions are applied on vertical sides of the model resulting in buckling of the crust. Several models are run for different geometry of the LAB. Preliminary results are shown. 3D conceptual models are then proposed to explain the 3D distribution of inverted structures in the Central Barents Sea. REFERENCES: Gac, S., Huismans, R.S., Simon, N.S.C., Podladchikov, Y,Y, and Faleide, J.I., 2013. Formation of intra-cratonic basins by lithospheric shortening and phase changes: a case study from the ultra-deep East Barents Sea basin. Terra Nova, DOI: 10.1111/ter.12057. Levshin, A.L., Schweitzer, J., Weidle, C., Shapiro, N.M., and Ritzwoller, M.H., 2007. Surface wave tomography of the Barents Sea and surrounding region. Geophys. J. Int., 170, 441-459. Ritzmann, O., and Faleide, J.I., 2009. The crust and mantle lithosphere in the Barents Sea/Kara Sea region. Tectonophysics, 470, 89-104.

3-D Upper-Mantle Shear Velocity Model Beneath the Contiguous United States Based on Broadband Surface Wave from Ambient Seismic Noise

NASA Astrophysics Data System (ADS)

Xie, Jun; Chu, Risheng; Yang, Yingjie

2018-05-01

Ambient noise seismic tomography has been widely used to study crustal and upper-mantle shear velocity structures. Most studies, however, concentrate on short period (< 50 s) surface wave from ambient noise, while studies using long period surface wave from ambient noise are limited. In this paper, we demonstrate the feasibility of using long-period surface wave from ambient noise to study the lithospheric structure on a continental scale. We use broadband Rayleigh wave phase velocities to obtain a 3-D V S structures beneath the contiguous United States at period band of 10-150 s. During the inversion, 1-D shear wave velocity profile is parameterized using B-spline at each grid point and is inverted with nonlinear Markov Chain Monte Carlo method. Then, a 3-D shear velocity model is constructed by assembling all the 1-D shear velocity profiles. Our model is overall consistent with existing models which are based on multiple datasets or data from earthquakes. Our model along with the other post-USArray models reveal lithosphere structures in the upper mantle, which are consistent with the geological tectonic background (e.g., the craton root and regional upwelling provinces). The model has comparable resolution on lithosphere structures compared with many published results and can be used for future detailed regional or continental studies and analysis.

Lithospheric Structure and Isostasy of Central Andes: Implication for plate Coupling

NASA Astrophysics Data System (ADS)

Mahatsente, R.; Rutledge, S.

2017-12-01

A significant section of the Peru-Chile convergent zone is building up stresses. The interseismic coupling in northern and southern Peru is significantly high indicating, elastic energy accumulation since the 1746 and 1868 earthquakes of magnitude 8.6 and 8.8 , respectively. Similar seismic patterns have also been observed in Central Chile. The plate interface beneath Central Chile is highly coupled, and the narrow zones of low coupling separate seismic gaps. The reasons for the seismic gaps and plate coupling are yet unknown, but the configuration of the slab is thought to be the main factor. Here, we assessed the locking mechanism and isostatic state of the Central Andes based on gravity models of the crust and upper mantle structure. The density models are based on satellite gravity data and are constrained by velocity models and earthquake hypocenters. The gravity models indicate a high-density batholithic structure in the fore-arc, overlying the subducting Nazca plate. This high-density body pushes downward on the slab, causing the slab to lock with the overlying continental plate. The increased compressive stress closer to the trench, due to the increased contact area between the subducting and overriding plates, may have increased the plate coupling in the Central Andes. Thus, trench parallel crustal thickness and density variations along the Central Andes and buoyancy force on the subducting Nazca plate may control plate coupling and asperity generation. The western part of the Central Andes may be undercompensated. There is a residual topography of 800 m in the western part of the Central Andes that cannot be explained by the observed crustal thicknesses. Thus, part of the observed topography in the western part of the Central Andes may be dynamically supported by mantle wedge flow below the overriding plate.

P-wave velocity structure of the uppermost mantle beneath Hawaii from traveltime tomography

USGS Publications Warehouse

Tilmann, F.J.; Benz, H.M.; Priestley, K.F.; Okubo, P.G.

2001-01-01

We examine the P-wave velocity structure beneath the island of Hawaii using P-wave residuals from teleseismic earthquakes recorded by the Hawaiian Volcano Observatory seismic network. The station geometry and distribution of events makes it possible to image the velocity structure between ~ 40 and 100 km depth with a lateral resolution of ~ 15 km and a vertical resolution of ~ 30 km. For depths between 40 and 80 km, P-wave velocities are up to 5 per cent slower in a broad elongated region trending SE-NW that underlies the island between the two lines defined by the volcanic loci. No direct correlation between the magnitude of the lithospheric anomaly and the current level of volcanic activity is apparent, but the slow region is broadened at ~ 19.8??N and narrow beneath Kilauea. In the case of the occanic lithosphere beneath Hawaii, slow seismic velocities are likely to be related to magma transport from the top of the melting zone at the base of the lithosphere to the surface. Thermal modelling shows that the broad elongated low-velocity zone cannot be explained in terms of conductive heating by one primary conduit per volcano but that more complicated melt pathways must exist.

A mechanism for tectonic deformation on Venus

NASA Technical Reports Server (NTRS)

Phillips, Roger J.

1986-01-01

In the absence of identifiable physiographic features directly associated with plate tectonics, alternate mechanisms are sought for the intense tectonic deformation observed in radar images of Venus. One possible mechanism is direct coupling into an elastic lithosphere of the stresses associated with convective flow in the interior. Spectral Green's function solutions have been obtained for stresses in an elastic lithosphere overlying a Newtonian interior with an exponential depth dependence of viscosity, and a specified surface-density distribution driving the flow. At long wavelengths and for a rigid elastic/fluid boundary condition, horizontal normal stresses in the elastic lid are controlled by the vertical shear stress gradient and are directly proportional to the depth of the density disturbance in the underlying fluid. The depth and strength of density anomalies in the Venusian interior inferred by analyses of long wavelength gravity data suggest that stresses in excess of 100 MPa would be generated in a 10 km thick elastic lid unless a low viscosity channel occurring beneath the lid or a positive viscosity gradient uncouples the flow stresses. The great apparent depth of compensation of topographic features argues against this, however, thus supporting the importance of the coupling mechanism. If there is no elastic lid, stresses will also be very high near the surface, providing also that the viscosity gradient is negative.

Toward self-consistent tectono-magmatic numerical model of rift-to-ridge transition

NASA Astrophysics Data System (ADS)

Gerya, Taras; Bercovici, David; Liao, Jie

2017-04-01

Natural data from modern and ancient lithospheric extension systems suggest three-dimensional (3D) character of deformation and complex relationship between magmatism and tectonics during the entire rift-to-ridge transition. Therefore, self-consistent high-resolution 3D magmatic-thermomechanical numerical approaches stand as a minimum complexity requirement for modeling and understanding of this transition. Here we present results from our new high-resolution 3D finite-difference marker-in-cell rift-to-ridge models, which account for magmatic accretion of the crust and use non-linear strain-weakened visco-plastic rheology of rocks that couples brittle/plastic failure and ductile damage caused by grain size reduction. Numerical experiments suggest that nucleation of rifting and ridge-transform patterns are decoupled in both space and time. At intermediate stages, two patterns can coexist and interact, which triggers development of detachment faults, failed rift arms, hyper-extended margins and oblique proto-transforms. En echelon rift patterns typically develop in the brittle upper-middle crust whereas proto-ridge and proto-transform structures nucleate in the lithospheric mantle. These deep proto-structures propagate upward, inter-connect and rotate toward a mature orthogonal ridge-transform patterns on the timescale of millions years during incipient thermal-magmatic accretion of the new oceanic-like lithosphere. Ductile damage of the extending lithospheric mantle caused by grain size reduction assisted by Zenner pinning plays critical role in rift-to-ridge transition by stabilizing detachment faults and transform structures. Numerical results compare well with observations from incipient spreading regions and passive continental margins.

Hierarchical segmentation of the Malawi Rift: The influence of inherited lithospheric heterogeneity and kinematics in the evolution of continental rifts

NASA Astrophysics Data System (ADS)

Laó-Dávila, Daniel A.; Al-Salmi, Haifa S.; Abdelsalam, Mohamed G.; Atekwana, Estella A.

2015-12-01

We used detailed analysis of Shuttle Radar Topography Mission-digital elevation model and observations from aeromagnetic data to examine the influence of inherited lithospheric heterogeneity and kinematics in the segmentation of largely amagmatic continental rifts. We focused on the Cenozoic Malawi Rift, which represents the southern extension of the Western Branch of the East African Rift System. This north trending rift traverses Precambrian and Paleozoic-Mesozoic structures of different orientations. We found that the rift can be hierarchically divided into first-order and second-order segments. In the first-order segmentation, we divided the rift into Northern, Central, and Southern sections. In its Northern Section, the rift follows Paleoproterozoic and Neoproterozoic terrains with structural grain that favored the localization of extension within well-developed border faults. The Central Section occurs within Mesoproterozoic-Neoproterozoic terrain with regional structures oblique to the rift extent. We propose that the lack of inherited lithospheric heterogeneity favoring extension localization resulted in the development of the rift in this section as a shallow graben with undeveloped border faults. In the Southern Section, Mesoproterozoic-Neoproterozoic rocks were reactivated and developed the border faults. In the second-order segmentation, only observed in the Northern Section, we divided the section into five segments that approximate four half-grabens/asymmetrical grabens with alternating polarities. The change of polarity coincides with flip-over full-grabens occurring within overlap zones associated with ~150 km long alternating border faults segments. The inherited lithospheric heterogeneity played the major role in facilitating the segmentation of the Malawi Rift during its opening resulting from extension.

Investigation of the Low Velocity Zone Beneath the Southern Basin and Range

NASA Astrophysics Data System (ADS)

Savage, B.; Helmberger, D. V.

2003-12-01

Following the work by Helmberger (1973), we use waveform recordings of P arrivals at distances from 6o to 20o to investigate the structure of the low velocity zone (LVZ) or asthenosphere. In contrast to the previous study, broadband data (TriNet and BDSN) is used at a much smaller station spacing providing higher along path and depth resolution. For this study, a well recorded earthquake in the central Gulf of California (Mw 6.3) produces transitions from PnL to P410 across all of California and western Nevada. The nature of these transitions indicates the thickness and gradients of the LVZ and the lithosphere. Initial findings show large variations of lithosphere and LVZ structure from east to west below California. By varying the lithosphere compressional velocity and depth of the LVZ in 1-D models, a database of synthetics waveforms is created to guide the development of realistic 2-D (along path) and 3-D (against azimuth) description of the lithosphere and asthenosphere. The character of the P arrivals changes dramatically near 9-11o with the emergence of a higher frequencies over-printing the longer-period PnL arrivals. Coastal California stations show these arrivals at the shortest distances, 9o indicating the lithosphere velocity and gradient below the LVZ are high. This is in opposition to those arrivals on the east which do not record the high frequency arrivals until 11o. As the distances reach 13o, a large amplitude, high frequency phase is present 10-15 seconds behind the initial P arrival. The emergence of the large secondary phase occurs at different distances across California with a pattern similar to before. At this distance, a change in the apparent velocity of the first arrival also occurs. Further in distance, the width of the initial P arrival and the energy following, or lack thereof, points to the shape of the underlying LVZ. Coastal stations and those in the central portion of California show larger amplitude arrivals following the initial P arrival than do those to the east. These large secondary arrivals may be due to larger than expected velocity jumps at the bottom of or just below the LVZ. Mapping the transition from the lithosphere to the asthenosphere, fine structure and lateral variation, should prove invaluable for tectonic reconstruction efforts, now in progress.

Determining OCT structure and COB Location of the Omani Gulf of Aden Continental Margin from Gravity Inversion, Residual Depth Anomaly and Subsidence Analysis.

NASA Astrophysics Data System (ADS)

Cowie, Leanne; Kusznir, Nick; Leroy, Sylvie; Manatshal, Gianreto

2013-04-01

Knowledge and understanding of the ocean-continent transition (OCT) structure and continent-ocean boundary (COB) location, the distribution of thinned continental crust and lithosphere, its distal extent and the start of unequivocal oceanic crust are of critical importance in evaluating rifted continental margin formation and evolution. In order to determine the OCT structure and COB location for the eastern Gulf of Aden, along the Oman margin, we use a combination of gravity inversion, subsidence analysis and residual depth anomaly (RDA) analysis. Gravity inversion has been used to determine Moho depth, crustal basement thickness and continental lithosphere thinning; subsidence analysis has been used to determine the distribution of continental lithosphere thinning; and RDAs have been used to investigate the OCT bathymetric anomalies with respect to expected oceanic bathymetries at rifted margins. The gravity inversion method, which is carried out in the 3D spectral domain, incorporates a lithosphere thermal gravity anomaly and includes a correction for volcanic addition due to decompression melting. Reference Moho depths used in the gravity inversion have been calibrated against seismic refraction Moho depths. RDAs have been calculated by comparing observed and age predicted oceanic bathymetries, using the thermal plate model predictions from Crosby and McKenzie (2009). RDAs have been computed along profiles and have been corrected for sediment loading using flexural back-stripping and decompaction. In addition, gravity inversion crustal basement thicknesses together with Airy isostasy have been used to predict a synthetic RDA. The RDA results show a change in RDA signature and may be used to estimate the distal extent of thinned continental crust and where oceanic crust begins. Continental lithosphere thinning has been determined using flexural back-stripping and subsidence analysis assuming the classical rift model of McKenzie (1978) with a correction for volcanic addition due to decompression melting based on White & McKenzie (1989). Gravity inversion and the "synthetic" gravity derived RDA both show generally normal thickness oceanic crust, with some localised thin oceanic crust. Continental lithosphere thinning factors determined from gravity inversion and subsidence analysis are in good agreement and have been used to constrain COB location along the profile lines. These techniques show that the OCT in the eastern Gulf of Aden, is relatively narrow, with the distance between the COB and the margin hinge measuring less than 100km.

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.

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?

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.

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".

Mars: Lithospheric Flexure of the Tharsis Montes Volcanoes and the Evolutionary Relationship to Their Tectonic History

NASA Astrophysics Data System (ADS)

Chute, H.; Dombard, A. J.; Byrne, P. K.

2017-12-01

Lithospheric flexure associated with Arsia, Pavonis, and Ascraeus Montes has been previously studied to constrain the timeline and breadth of endogenic surface features surrounding these volcanoes. Here, we simulate the radial extent of two specific load-related features: annular graben and flank terraces. Detailed mapping of Ascraeus Mons (the youngest of the three volcanoes) showed a phase of compression of the edifice, forming the terraces and an annulus of graben immediately off the flanks, followed by a period of extension that formed additional graben superposed on the terraces on the lower flanks of the edifice. This transition from compression to extension on the lower flanks has been difficult to reconcile in mechanical models. We explore, with finite-element simulations, the effects of a thermal anomaly associated with an intrusive crustal underplate, which results in locally thinning the lithosphere (in contrast to past efforts that assumed a constant-thickness lithosphere). We find that it is primarily the horizontal extent of this thermal anomaly that governs how the lithosphere flexes under a volcano, as well as the transition from flank compression to a tight annulus of extensional stresses. Specifically, we propose that the structures on Ascraeus may be consistent with an early stage of volcanic growth accompanied by an underplate about the same width as the edifice that narrowed as volcanism waned, resulting in an inward migration of the extensional horizontal stresses from the surrounding plains onto the lower flanks. By linking the surface strains on the volcano with the volcano-tectonic evolution predicted by our flexure model, we can further constrain a more accurate timeline for the tectonic history of Ascraeus Mons. More broadly, because these tectonic structures are commonly observed, our results provide a general evolutionary model for large shield volcanoes on Mars.

Different modes of continental break-up triggered by a sole mantle plume: a 2D and 3D numerical study

NASA Astrophysics Data System (ADS)

Beniest, Anouk; Koptev, Alexander; Leroy, Sylvie; Burov, Evgueni

2017-04-01

We used 2D and 3D numerical models to investigate the impact of a single mantle plume on continental rifting and breakup processes. We varied the thermo-rheological structure of the continental lithosphere, its geometry and the initial plume position. Based on the results of our 2D experiments, three continental break-up modes can be distinguished: A) 'central' continental break-up, the break-up center is located directly above the original mantle anomaly position, B) 'shifted' break-up, the break-up center is 50 to 200 km displaced from the initial plume location and C) 'distant' break-up, due to convection and/or slab-subduction/delamination, the break-up center is considerably shifted (300 to 800 km) from the primary plume position. Our 3D model, with a laterally homogeneous initial setup also results in continental break-up with the axis of continental break-up hundreds of kilometers shifted from the original plume location. The model results show that the classical, 'central' view of mantle plume induced continental break-up is not the only mode of break-up. When considering a diversity of break-up styles, it is possible to explain a variety of observed geophysical and geological features. For example, the mantle material glued to the base of the lithosphere at shallower depths corresponds geometrically and location-wise to high-velocity/high-density bodies observed on seismic data below the thinned continental lithosphere and the transition zone of the South Atlantic domain. During migration, products of partial melting of the mantle material can move vertically to (shallow) lower crustal levels. They might resemble high density bodies observed at lower crustal levels inside continental crust with similar geometries observed with gravity modelling. Also, topographic variation form in the very early stages of rifting on the first impingement of upwelled plume material. These variations remain visible, as the final position of the spreading center is shifted from the point of impingement and can be interpreted as aborted rifts, observed along passive margins. Our modelling demonstrates that both simple and perfectly symmetric preliminary settings as well as complex initial setups can result in a variety of break-up systems.

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.

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.

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.

Flow, melt and fossil seismic anisotropy beneath Ethiopia

NASA Astrophysics Data System (ADS)

Hammond, James; Kendall, J.-Michael; Wookey, James; Stuart, Graham; Keir, Derek; Ayele, Atalay

2014-05-01

Ethiopia is a region where continental rifting gives way to oceanic spreading. Yet the role that pre-existing lithospheric structure, melt, mantle flow or active upwellings may play in this process is debated. Measurements of seismic anisotropy are often used to attempt to understand the contribution that these mechanisms may play. In this study we use new data in Afar, Ethiopia along with legacy data across Ethiopia, Djibouti and Yemen to obtain estimates of mantle anisotropy using SKS-wave splitting. We show that two layers of anisotropy exist, and use shear-wave splitting tomography to invert for these. We show that fossil anisotropy with fast directions oriented northeast-southwest may be preserved in the lithosphere away from the rift. Beneath the Main Ethiopian Rift and parts of Afar, anisotropy due aligned melt due to sharp changes in lithospheric thickness dominate the shear-wave splitting signal in the mantle. Beneath Afar, away from lithospheric topography, melt pockets associated with the crustal magma storage dominate the signal and little anisotropy is seen in the uppermost mantle suggesting melt retains no preferential alignment, possibly due to a lack of mantle lithosphere. These results show the important role melt plays in weakening the lithosphere and imply that as rifting evolves passive upwelling sustains extension. A dominant northeast-southwest anisotropic fast direction is observed in a deeper layer across all of Ethiopia. This suggests that a conduit like plume is absent beneath Afar today, rather a broad flow from the southwest dominates in the upper mantle.

Structure of the lithosphere-asthenosphere system in the vicinity of the Tristan da Cunha hot spot as seen by surface waves

NASA Astrophysics Data System (ADS)

Bonadio, Raffaele; Geissler, Wolfram H.; Ravenna, Matteo; Lebedev, Sergei; Celli, Nicolas L.; Jokat, Wilfried; Jegen, Marion; Sens-Schönfelder, Christoph; Baba, Kiyoshi

2017-04-01

Tristan da Cunha is a volcanic island located above a hotspot in the South Atlantic. The deep mantle plume origin of the hotspot volcanism at the island is supported by anomalous geochemical data (Rohde et al., 2013 [1]) and global seismological evidences (French and Romanovicz, 2015 [2]). However, until recently, due to lack of local geophysical data in the South Atlantic and especially around Tristan da Cunha, the existence of a plume has not yet been confirmed. Therefore, an Ocean Bottom Seismometer experiment was carried out in 2012 and 2013 in the vicinity of the archipelago, with the aim of obtaining geophysical data that may help to get some more detailed insights into the structure of the upper mantle, possibly confirming the existence of a plume. In this work we study the shear wave velocity structure of the lithosphere-asthenosphere system beneath the Island. Rayleigh surface wave phase velocity dispersion curves have been obtained using a recent powerful implementation of the inter-station cross-correlation method (Meier et al., 2004 [3]; Soomro et al., 2016 [4]). The measured dispersion curves are used to invert for the 1D shear wave velocity structure beneath the study area and to obtain phase velocity tomographic maps. Our results show a pronounced low shear wave velocity anomaly between 70 and 120 km depth beneath the area; the lid shows high velocity, suggesting a cold, depleted and dehydrated shallow lithosphere, while the deeper lithosphere shows a velocity structure similar to young or rejuvenated Pacific oceanic lithosphere (Laske et al., 2011 [5]; Goes et al., 2012 [6]). Below the base of the lithosphere, shear wave velocities appear to be low, suggesting thermal effects and partial melting (as confirmed by petrological data). Decreasing velocities within the lithosphere south-westward reflect probably a thermal imprint of an underlying mantle plume. References [1] J.K. Rohde, P. van den Bogaard, K. Hoernle, F. Hauff, R. Werner, Evidence for an age progression along the Tristan-Gough volcanic track from new 40Ar/ 39Ar ages on phenocryst phases, Tectonophysics, Volume 604, p. 60-71 (2013). [2] S. French and B. Romanowicz, Broad plumes rooted at the base of the Earth's mantle beneath major hotspots, Nature, 525(7567), 95-99 (2015). [3] T. Meier, K. Dietrich, B. Stockhert and H. Harjes, One-dimensional models of shear wave velocity for the eastern Mediterranean obtained from the inversion of Rayleigh wave phase velocities and tectonic implications, Earth and Planetary Science Letters, 249(3), 415-424 (2004). [4] R.A. Soomro, C. Weidle, L. Cristiano, S. Lebedev, T. Meier and PASSEQ Working Group, Phase velocities of Rayleigh and Love waves in central and northern Europe from automated, broad-band, interstation measurements, Geophys. J. Int. (2016) 204, 517-534. [5] G. Laske, A. Markee, J.A. Orcutt, C.J. Wolfe, J.A. Collins and S.C. Solomon, R.S. Detrick, D. Bercovici and E.H. Hauri, Asymmetric shallow mantle structure beneath the Hawaiian Swell-evidence from Rayleigh waves recorded by the PLUME network, Geophys. J. Int. (2011) 187, 1725-1742. [6] S. Goes, J. Armitage, N. Harmon, H. Smith and R. Huismans, Low seismic velocities below mid-ocean ridges: Attenuation versus melt retention, Journal of geophysical research, Vol. 117, B12403, (2012).

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.

Lithospheric Structure of the Arabian Shield from the Joint Inversion of Receiver Function and Surface-Wave Dispersion Observations

DTIC Science & Technology

2006-04-01

These microplates are separated by four ophiolite-bearing suture zones of two types: the Bir Umq and Yanbu sutures, formed by island-arc-island...Am., 82, 1453- 1474,1992. (UNCLASSIFIED) Coleman, R. G., Ophiolites. Ancient Oceanic Lithosphere ?, 229 pp., Springer-Verlag, Berlin, 1977...African microplate accretion of the Arabian shield, Bull. Seism. Soc. Am. 96, 817-826, 1985. (UNCLASSIFIED) Su, W., R. L. Woodward, and A. M

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.

Development and implementation of the software for visualization and analysis of data geophysical loggers

NASA Astrophysics Data System (ADS)

Gordeev, V. F.; Malyshkov, S. Yu.; Botygin, I. A.; Sherstnev, V. S.; Sherstneva, A. I.

2017-11-01

The general trend of modern ecological geophysics is changing priorities towards rapid assessment, management and prediction of ecological and engineering soil stability as well as developing brand new geophysical technologies. The article describes researches conducted by using multi-canal geophysical logger MGR-01 (developed by IMCES SB RAS), which allows to measure flux density of very low-frequency electromagnetic radiation. It is shown that natural pulsed electromagnetic fields of the earthen lithosphere can be a source of new information on Earth's crust and processes in it, including earthquakes. The device is intended for logging electromagnetic processes in Earth's crust, geophysical exploration, finding structural and lithological inhomogeneities, monitoring the geodynamic movement of Earth's crust, express assessment of seismic hazards. The data is gathered automatically from observation point network in Siberia

Microstructures, composition, and seismic properties of the Ontong Java Plateau mantle root

NASA Astrophysics Data System (ADS)

Tommasi, Andréa.; Ishikawa, Akira

2014-11-01

To study how an impacting plume modifies the mantle lithosphere, we analyzed the microstructures and crystal preferred orientations (CPO) of 29 peridotites and 37 pyroxenites that sample the mantle root of the Ontong Java Plateau (OJP) from 60 to 120 km depth. The peridotites show a strong compositional variability, but homogeneous coarse granular to tabular microstructures, except for those equilibrated at the shallowest and deepest depths, which are porphyroclastic. All peridotites have clear olivine CPO, with dominant fiber-[010] patterns. Low intragranular misorientations and straight grain boundaries in olivine suggest that, above 100 km depth, annealing often followed deformation. Calculated density and P wave velocities of the peridotites decrease weakly with depth. S wave velocities decrease faster, resulting in increasing Vp/Vs ratio with depth. Calculated densities and seismic velocity profiles are consistent with those estimated for normal mantle compositions under a cold oceanic geotherm. Enrichment in pyroxenites may further increase seismic velocities. The calculated seismic properties cannot therefore explain the low S waves velocities predicted by Rayleigh wave tomography and ScS data in the mantle beneath the OJP. Calculated P and S waves anisotropy is variable (2-12%). It is higher on average in the deeper section of the lithosphere. Because olivine has dominantly [010]-fiber CPO patterns, if foliations are horizontal, vertically propagating S waves and Rayleigh waves will sample very weak anisotropy in the OJP mantle lithosphere. Moreover, if the orientation of the lineation changes with depth, the anisotropy-induced contrast in seismic properties might produce an intralithospheric reflector marking the stratification of the OJP mantle root.

Decarbonation in an intracratonic setting: Insight from petrological-thermomechanical modeling

NASA Astrophysics Data System (ADS)

Gonzalez, Christopher M.; Gorczyk, Weronika

2017-08-01

Cratons form the stable core roots of the continental crust. Despite long-term stability, cratons have failed in the past. Cratonic destruction (e.g., North Atlantic Craton) due to chemical rejuvenation at the base of the lithosphere remains poorly constrained numerically. We use 2-D petrological-thermomechanical models to assess cratonic rifting characteristics and mantle CO2 degassing in the presence of a carbonated subcontinental lithospheric mantle (SCLM). We test two tectonothermal SCLM compositions: Archon (depleted) and Tecton (fertilized) using 2 CO2 wt % in the bulk composition to represent a metasomatized SCLM. We parameterize cratonic breakup via extensional duration (7-12 Ma; full breakup), tectonothermal age, TMoho (300-600°C), and crustal rheology. The two compositions with metasomatized SCLMs share similar rifting features and decarbonation trends during initial extension. However, we show long-term (>67 Ma) stability differences due to lithospheric density contrasts between SCLM compositions. The Tecton model shows convective removal and thinning of the metasomatized SCLM during failed rifting. The Archon composition remained stable, highlighting the primary role for SCLM density even when metasomatized at its base. In the short-term, three failed rifting characteristics emerge: failed rifting without decarbonation, failed rifting with decarbonation, and semifailed rifting with dry asthenospheric melting and decarbonation. Decarbonation trends were greatest in the failed rifts, reaching peak fluxes of 94 × 104 kg m-3. Increased TMoho did not alter the effects of rifting or decarbonation. Lastly, we show mantle regions where decarbonation, mantle melting in the presence of carbonate, and preservation of carbonated mantle occur during rifting.

Joint Inversion for Lithospheric Structures: Implications for the Growth and Deformation in Northeastern Tibetan Plateau

NASA Astrophysics Data System (ADS)

Deng, Yangfan; Li, Jiangtao; Song, Xiaodong; Zhu, Lupei

2018-05-01

Several geodynamic models have been proposed for the deformation mechanism of Tibetan Plateau (TP), but it remains controversial. Here we applied a method of joint inversion of receiver functions and surface wave dispersions with P wave velocity constraint to a dense linear array in the NE Tibet. The results show that the geological blocks, separated by major faults at the surface, are characterized by distinct features in the crust, the Moho, and the uppermost mantle. The main features include crustal low-velocity zones (LVZs) with variable strengths, anomalous Vp/Vs ratios that are correlated with LVZs, a large Moho jump, and other abrupt changes near major faults, strong mantle lithosphere anomalies, and correlation of crustal and mantle velocities. The results suggest a lithospheric-scale deformation of continuous shortening as well as localized faulting, which is affected by the strength of the lithosphere blocks. The thickened mantle lithosphere can be removed, which facilitates the formation of middle-lower crustal LVZ and flow. However, such flow is likely a consequence of the deformation rather than a driving force for the outward growth of the TP. The proposed model of TP deformation and growth can reconcile the continuous deformation within the blocks and major faults at the surface.

Lithospheric low-velocity zones associated with a magmatic segment of the Tanzanian Rift, East Africa

NASA Astrophysics Data System (ADS)

Plasman, M.; Tiberi, C.; Ebinger, C.; Gautier, S.; Albaric, J.; Peyrat, S.; Déverchère, J.; Le Gall, B.; Tarits, P.; Roecker, S.; Wambura, F.; Muzuka, A.; Mulibo, G.; Mtelela, K.; Msabi, M.; Kianji, G.; Hautot, S.; Perrot, J.; Gama, R.

2017-07-01

Rifting in a cratonic lithosphere is strongly controlled by several interacting processes including crust/mantle rheology, magmatism, inherited structure and stress regime. In order to better understand how these physical parameters interact, a 2 yr long seismological experiment has been carried out in the North Tanzanian Divergence (NTD), at the southern tip of the eastern magmatic branch of the East African rift, where the southward-propagating continental rift is at its earliest stage. We analyse teleseismic data from 38 broad-band stations ca. 25 km spaced and present here results from their receiver function (RF) analysis. The crustal thickness and Vp/Vs ratio are retrieved over a ca. 200 × 200 km2 area encompassing the South Kenya magmatic rift, the NTD and the Ngorongoro-Kilimanjaro transverse volcanic chain. Cratonic nature of the lithosphere is clearly evinced through thick (up to ca. 40 km) homogeneous crust beneath the rift shoulders. Where rifting is present, Moho rises up to 27 km depth and the crust is strongly layered with clear velocity contrasts in the RF signal. The Vp/Vs ratio reaches its highest values (ca. 1.9) beneath volcanic edifices location and thinner crust, advocating for melting within the crust. We also clearly identify two major low-velocity zones (LVZs) within the NTD, one in the lower crust and the second in the upper part of the mantle. The first one starts at 15-18 km depth and correlates well with recent tomographic models. This LVZ does not always coexist with high Vp/Vs ratio, pleading for a supplementary source of velocity decrease, such as temperature or composition. At a greater depth of ca. 60 km, a mid-lithospheric discontinuity roughly mimics the step-like and symmetrically outward-dipping geometry of the Moho but with a more slanting direction (NE-SW) compared to the NS rift. By comparison with synthetic RF, we estimate the associated velocity reduction to be 8-9 per cent. We relate this interface to melt ponding, possibly favouring here deformation process such as grain-boundary sliding (EAGBS) due to lithospheric strain. Its geometry might have been controlled by inherited lithospheric fabrics and heterogeneous upper mantle structure. We evidence that crustal and mantle magmatic processes represent first order mechanisms to ease and locate the deformation during the first stage of a cratonic lithospheric breakup.

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 stress field below the NE German Basin: effects induced by the Alpine collision

NASA Astrophysics Data System (ADS)

Marotta, A. M.; Bayer, U.; Scheck, M.; Thybo, H.

2001-02-01

We use a thin-sheet approach for a viscous lithosphere to investigate the effects induced by the Alpine collision on the vertical deformation and regional stress in northern Europe, focusing on the NE German Basin. New seismic studies indicate a flexural-type deep crustal structure under the basin, which may be induced by compressive forces transmitted from the south and related to Alpine tectonics. Finite element techniques are used to solve the vertical deformation and stress field for a viscous European lithosphere with horizontal rheological heterogeneities. Our results support the idea that a relatively strong lithosphere below the northern margin of the German Basin at the transition into the Baltic Shield may explain the characteristic regional stress field, especially the fan-like pattern that is observed within the region.

Upper mantle Q and thermal structure beneath Tanzania, East Africa from teleseismic P wave spectra

NASA Astrophysics Data System (ADS)

Venkataraman, Anupama; Nyblade, Andrew A.; Ritsema, Jeroen

2004-08-01

We measure P wave spectral amplitude ratios from deep-focus earthquakes recorded at broadband seismic stations of the Tanzania network to estimate regional variation of sublithospheric mantle attenuation beneath the Tanzania craton and the eastern branch of the East African Rift. One-dimensional profiles of QP adequately explain the systematic variation of P wave attenuation in the sublithospheric upper mantle: QP ~ 175 beneath the cratonic lithosphere, while it is ~ 80 beneath the rifted lithosphere. By combining the QP values and a model of P wave velocity perturbations, we estimate that the temperature beneath the rifted lithosphere (100-400 km depth) is 140-280 K higher than ambient mantle temperatures, consistent with the observation that the 410 km discontinuity in this region is depressed by 30-40 km.

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.

A geophysical potential field study to image the Makran subduction zone in SE of Iran

NASA Astrophysics Data System (ADS)

Abedi, Maysam; Bahroudi, Abbas

2016-10-01

The Makran subduction wedge as one of the largest subduction complexes has been forming due to the Arabian oceanic lithosphere subducting beneath the Lut and the Afghan rigid block microplates. To better visualize the subducting oceanic crust in this region, a geophysical model of magnetic susceptibility from an airborne magnetic survey (line spacing about 7.5 km) over the Makran zone located at southeast of Iran is created to image various structural units in Iran plate. The constructed geophysical model from the 3D inverse modeling of the airborne magnetic data indicates a thin subducting slab to the north of the Makran structural zone. It is demonstrated that the thickness of sedimentary units varies approximately at an interval of 7.5-11 km from north to south of this zone in the Iranian plate, meanwhile the curie depth is also estimated approximately < 26 km. It is also shown the Jazmurian depression zone adjacent to the north of the Makran indicates high intensity magnetic anomalies due to being underlain by an ophiolite oceanic basement, while such intensity reduces over the Makran. The directional derivatives of the magnetic field data have subtle changes in the Makran, but strongly increase in the Jazmurian by enhancing and separating different structural boundaries in this region. In addition, the density variations of the subsurface geological layers were determined by 3D inversion of the ground-based gravity data over the whole study area, where the constructed density model was in good agreement with the magnetic one. According to the outputs of the magnetic susceptibility and the density contrast, the Arabian plate subducts to the north under the Eurasia with a very low dip angle in the Makran structural zone.

Geothermal Heat Flux: Linking Deep Earth's Interior and the Dynamics of Large-Scale Ice Sheets

NASA Astrophysics Data System (ADS)

Rogozhina, Irina; Vaughan, Alan

2014-05-01

Regions covered by continental-scale ice sheets have the highest degree of uncertainty in composition and structure of the crust and lithospheric mantle, compounded by the poorest coverage on Earth of direct heat flow measurements. In addition to challenging conditions that make direct measurements and geological survey difficult Greenland and Antarctica are known to be geologically complex. Antarctica in particular is marked by two lithospherically distinct zones. In contrast to young and thin lithosphere of West Antarctica, East Antarctica is a collage of thick Precambrian fragments of Gondwana and earlier supercontinents. However, recent observations and modeling studies have detected large systems of subglacial lakes extending beneath much of the East Antarctic ice sheet base that have been linked to anomalously elevated heat flow. Outcrop samples from the rift margin with Australia (Prydz Bay) have revealed highly radiogenic Cambrian granite intrusives that are implicated in regional increase of crustal heat flux by a factor of two to three compared to the estimated continental background. Taken together, these indicate high variability of heat flow and properties of rocks across Antarctica. Similar conclusions have been made based on direct measurements and observations of the Greenland ice sheet. Airborne ice-penetrating radar and deep ice core projects show very high rates of basal melt for parts of the ice sheet in northern and central Greenland that have been explained by abnormally high heat flux. Archaean in age, the Greenland lithosphere was significantly reworked during the Early Proterozoic. In this region, the interpretation of independent geophysical data is complicated by Proterozoic and Phanerozoic collision zones, compounded by strong thermochemical effects of rifting along the western and eastern continental margins between 80 and 25 million years ago. In addition, high variability of heat flow and thermal lithosphere structure in central Greenland results from the remanent effects of an Early Cenozoic passage of the lithosphere above the Iceland mantle plume that is implicated in strong thermochemical erosion of the lithosphere and significant long-term effects on the present-day subglacial heat flow pattern and thermodynamic state of the Greenland ice sheet. These observations and our modeling results (Petrunin et al., 2013) show that the present-day thermal state of Greenland and Antarctic lithosphere cannot be well understood without taking into account a long-term tectonic history of these regions. The goal of the IceGeoHeat project is to combine existing independent geophysical data and innovative modeling approaches to comprehensively study the evolution and present state of the lithosphere in Greenland and Antarctica, and assess the role of geothermal heat flux in shaping the present-day ice sheet dynamics. This requires multiple collaborations involving experts across a range of disciplines. The project builds on the IceGeoHeat initiative formed in April 2012 and now including researchers from ten countries in the main core (MC) with expertise in numerical modeling and data assessment in geodynamics, geology, geothermics, cryosphere and (paleo-)climate. Petrunin, A., Rogozhina, I., Vaughan, A. P. M., Kukkonen, I. T., Kaban, M., Koulakov, I., Thomas, M. (2013): Heat flux variations beneath central Greenland's ice due to anomalously thin lithosphere. - Nature Geoscience, 6, 746-750.

Sink to survive: The persistence of ancient mountain belts through crustal density changes

NASA Astrophysics Data System (ADS)

Blackburn, T. J.; Ferrier, K.; Perron, J.

2012-12-01

Mountain belts form when collisions between continents thicken the Earth's crust, which buoyantly rises to remain in isostatic equilibrium with the underlying asthenosphere. Just as isostasy leads to the birth of mountain belts, it contributes to their destruction by responding to erosion with rock uplift, which in turn promotes further erosion. If the continental crust consisted of a single layer of constant density, erosion and isostatic rebound would continue thinning the crust until it was completely eroded. Such total destruction evidently does not happen, however, as the roots of Earth's oldest mountains have persisted for billions of years. One explanation for this preservation is that an orogen's isostatic response to erosion decreases over time as the crust increases in density as the lower crust undergoes metamorphic phase changes that accompany lithosphere cooling. The implication of this hypothesis is that erosion rates in mountain belts are linked to the thermal and density evolution of the lithosphere. We test this hypothesis with a global compilation of exhumation rates and erosion rates determined from published apatite fission track and cosmogenic 10Be measurements in collisional orogens ranging in formation age from 0 to 2 billion years. We compare these data to a numerical model of the thermal, density and erosional evolution of a decaying mountain belt. Measured and modeled data indicate that erosion is fastest in young, hot, low-density, and topographically high mountain belts, and that erosion rates decrease dramatically after 200-300 million years (My). This 200-300 My timescale is consistent with titanite U-Pb thermochronologic data from lower crustal xenoliths, which record cooling to temperatures consistent with garnet growth and crustal densification (~650 °C) within 200-300 My after orogenesis. For the same orogens, Sm-Nd and/or Lu-Hf garnet-whole rock isochron dates constrains lower crustal garnet growth and a corresponding crustal density increase to 200-450 My following orogenesis. Thus, geochronologic data at various timescales, from ancient thermal histories to geologically recent erosion rates, are consistent with an isostatic model that links the erosional decay of a collisional orogens to its thermal and density evolution. Given the geologic and climatic diversity of mountain ranges around the world, it is striking that their erosional histories are generally consistent with a single simple model. The scenario described here, in which young, hot, low-density orogens erode quickly for a few hundred My while older, colder, denser orogens erode much more slowly for billions of years provides an explanation for the persistence of some of Earth's oldest mountain belts. The importance of this erosional succession is underscored by the fact that continental landmasses are constructed through mountain building processes: like jigsaw puzzles with many pieces, continents are amalgamations of ancient mountain belts assembled over geologic time. Temperature-dependent densities appear to play a global role in the long-term evolution of mountain belts and continents, influencing the preservation of continental lithosphere over billions of years.

The Lithospheric Geoid as a Constraint on Plate Dynamics

NASA Astrophysics Data System (ADS)

Richardson, R. M.; Coblentz, D. D.

2015-12-01

100 years after Wegener's pioneering work there is still considerable debate about the dynamics of present-day plate motions. A better understanding of present-day dynamics is key to a better understanding of the supercontinent cycle. The Earth's gravity field is one of the primary data sets to help constrain horizontal density contrasts, and hence plate dynamic forces. Previous work has shown that the global average for the geoid step up from old oceanic lithosphere across passive continental margins to stable continental lithosphere is about 6-9m, and the global average for the geoid anomaly associated with cooling oceanic lithosphere (the so-called "ridge push") is 10-12m. The ridge geoid anomaly corresponds to a net force of ~3x1012N/m (averaged over the thickness of the lithosphere) due to 'ridge push.' However, for individual continental margins and mid-ocean ridge systems, there is considerable variation in the geoid step and geoid anomaly and consequently the associated forces contributing to the stress field. We explore the variation in geoid step across passive continental margins looking for correlations with age of continental breakup (and hence place within the supercontinent cycle), hot spot tracks, continental plate velocities, long-wavelength geoid energy (that may be masking signal), and small scale convection. For mid-ocean ridges, we explore variations in geoid anomaly looking for correlations with plate spreading rates, hot spot tracks, long-wavelength geoid energy (that may be masking signal), and small scale convection. We use a band-pass spherical harmonic filter on the full geoid (e.g., EGM2008-WGS84, complete to spherical harmonic degree and order 2159) between orders 6 and 80. The evaluation of the role of spatial variations in the geoid gradient for cooling oceanic lithosphere and across the continental margin in the dynamics of the intraplate stress field requires high spatial resolution modeling. We perform a high resolution finite element analysis (~35,000 elements for a spatial resolution of approximately 50 km) for the North American plate, where previous lower resolution modeling has shown the importance of the lithospheric cooling (ridge push) force to model the broad scale stress patterns observed from the middle of the continent to the Mid-Atlantic ridge.

On the role of mantle depletion and small-scale convection in post rift basin evolution (Invited)

NASA Astrophysics Data System (ADS)

Petersen, K.; Nielsen, S. B.

2013-12-01

Subsidence and heat flow evolution of the oceanic lithosphere appears to be consistent with the conductive cooling of a ~100 km plate overlying asthenospheric mantle of constant entropy. The physical mechanism behind plate-like subsidence has been suggested to be the result of small-scale convective instabilities which transport heat energy to the base of the lithosphere and cause an eventual departure from half space-like cooling by inhibiting subsidence of old ocean floor and causing an asymptotic surface heat flow of ~50 mW/m^2. Here, we conduct a number of numerical thermo-mechanical experiments of oceanic lithosphere cooling for different models of temperature- and pressure-dependent viscosity. We show that uniform (P, T-dependent) mantle viscosity cannot both explain half space-like subsidence for young (<70 Mr) lithosphere as well as a relatively high (>50 mW/m^2) surface heat flow which is observed above old (>100 Myr) lithosphere. The latter requires vigorous sub lithospheric convection which would lead to early (~1Myr) onset of convective instability at shallow depth (<60 km) and therefore insufficient initial subsidence. To resolve this paradox, we employ models which account for the density decrease and viscosity increase due to depletion during mid-ocean ridge melting. We demonstrate that the presence of a mantle restite layer within the lithosphere hinders convection at shallow depth and therefore promotes plate-like cooling. A systematic parameter search among 280 different numerical experiments indicates that models with 60-80 km depletion thickness minimize misfit with subsidence and heat flow data. This is consistent with existing petrological models of mid-ocean ridge melting. Our models further indicate that the post-rift subsidence pattern where little or no melting occurred during extension (e.g. non-volcanic margins and continental rifts) may differ from typical oceanic plate-like subsidence by occurring at a nearly constant rate rather than at an exponentially decaying rate. Model comparison with subsidence histories inferred from backstripping analysis implies that this is indeed often the case. Accordingly, existing thermal models of continental rifting which assume plate-like cooling (and is often calibrated from oceanic data) are likely to yield inaccurate predictions in terms of subsidence and heat flow evolution.

A gravity study along a profile across the Sichuan Basin, the Qinling Mountains and the Ordos Basin (central China): Density, isostasy and dynamics

NASA Astrophysics Data System (ADS)

Zhang, Yongqian; Teng, Jiwen; Wang, Qianshen; Lü, Qingtian; Si, Xiang; Xu, Tao; Badal, José; Yan, Jiayong; Hao, Zhaobing

2017-10-01

In order to investigate the structure of the crust beneath the Middle Qinling Mountains (MQL) and neighboring areas in the North China Block and South China Block, a north-south gravity profile from Yuquan in the Sichuan Basin to Yulin in the Ordos Basin was conducted in 2011. The Bouguer gravity anomaly is determined from a high-quality gravity dataset collected between 31°N and 36°N of latitude, and varies between -200 and -110 mGal in the study region. Using accredited velocity density relationships, an initial crust-mantle density model is constructed for MQL and adjacent areas, which is later refined interactively to simulate the observed gravity anomaly. The present study reveals the features of the density and Bouguer gravity with respect to the tectonic units sampled by the profile. The lithosphere density model shows typical density values that depict a layered structure and allow differentiate the blocks that extend along the reference profile. The gravity field calculated by forward modeling from the final density distribution model correlates well with the measured gravity field within a standard deviation of 1.26 mGal. The density in the crystalline crust increases with depth from 2.65 g/cm3 up to the highest value of 2.95 g/cm3 near the bottom of the crust. The Conrad interface is identified as a density jump of about 0.05 g/cm3. The average density of the crust in MQL is clearly lower than the density in the formations on both sides. Starting from a combined Airy-Pratt isostatic compensation model, a partly compensated crust is found below MQL, suggesting future growth of the crust, unlike the Ordos and Sichuan basins that will remain stable. On the basis of the density and isostatic state of the crust and additional seismological research, such as the P-wave velocity model and Poisson's ratio, it is concluded that the lower crust delamination is a reasonable interpretation for the geophysical characteristics below the Qinling Orogen.

Density and lithospheric structure at Tyrrhena Patera, Mars, from gravity and topography data

NASA Astrophysics Data System (ADS)

Grott, M.; Wieczorek, M. A.

2012-09-01

The Tyrrhena Patera highland volcano, Mars, is associated with a relatively well localized gravity anomaly and we have carried out a localized admittance analysis in the region to constrain the density of the volcanic load, the load thickness, and the elastic thickness at the time of load emplacement. The employed admittance model considers loading of an initially spherical surface, and surface as well as subsurface loading is taken into account. Our results indicate that the gravity and topography data available at Tyrrhena Patera is consistent with the absence of subsurface loading, but the presence of a small subsurface load cannot be ruled out. We obtain minimum load densities of 2960 kg m-3, minimum load thicknesses of 5 km, and minimum load volumes of 0.6 × 106 km3. Photogeological evidence suggests that pyroclastic deposits make up at most 30% of this volume, such that the bulk of Tyrrhena Patera is likely composed of competent basalt. Best fitting model parameters are a load density of 3343 kg m-3, a load thickness of 10.8 km, and a load volume of 1.7 × 106 km3. These relatively large load densities indicate that lava compositions are comparable to those at other martian volcanoes, and densities are comparable to those of the martian meteorites. The elastic thickness in the region is constrained to be smaller than 27.5 km at the time of loading, indicating surface heat flows in excess of 24 mW m-2.

Observatory geoelectric fields induced in a two-layer lithosphere during magnetic storms

USGS Publications Warehouse

Love, Jeffrey J.; Swidinsky, Andrei

2015-01-01

We report on the development and validation of an algorithm for estimating geoelectric fields induced in the lithosphere beneath an observatory during a magnetic storm. To accommodate induction in three-dimensional lithospheric electrical conductivity, we analyze a simple nine-parameter model: two horizontal layers, each with uniform electrical conductivity properties given by independent distortion tensors. With Laplace transformation of the induction equations into the complex frequency domain, we obtain a transfer function describing induction of observatory geoelectric fields having frequency-dependent polarization. Upon inverse transformation back to the time domain, the convolution of the corresponding impulse-response function with a geomagnetic time series yields an estimated geoelectric time series. We obtain an optimized set of conductivity parameters using 1-s resolution geomagnetic and geoelectric field data collected at the Kakioka, Japan, observatory for five different intense magnetic storms, including the October 2003 Halloween storm; our estimated geoelectric field accounts for 93% of that measured during the Halloween storm. This work demonstrates the need for detailed modeling of the Earth’s lithospheric conductivity structure and the utility of co-located geomagnetic and geoelectric monitoring.

Seismic anisotropy of 70 Ma Pacific-plate upper mantle

NASA Astrophysics Data System (ADS)

Mark, H. F.; Lizarralde, D.; Collins, J. A.; Miller, N. C.; Hirth, G.; Gaherty, J. B.; Evans, R. L.

2017-12-01

We present a new measurement of seismic anisotropy and velocity gradients in the Pacific-plate upper mantle based on data from the NoMelt experiment. The seismic velocity structure of oceanic lithosphere reflects the processes involved in its formation at mid-ocean ridges and subsequent evolution off-axis. Increasing mantle depletion with depth due to melt extraction predicts negative velocity gradients, as does cooling with age. Alignment of olivine by corner flow predicts azimuthal anisotropy. Some models predict the strength of anisotropy should decrease with depth. Measurements of uppermost mantle velocities have not fully verified these predictions. Observations of direct Pn phases demonstrate that positive velocity gradients exist; and anisotropy measurements, while consistent with strain-induced olivine alignment, vary widely and generally suggest weaker fabric development than is observed in ophiolite samples. These discrepancies raise questions about the extent to which mantle structure evolves through time due to processes such as cracking and alteration, and hinder the use of seismic measurements to make more detailed inferences on aspects of lithospheric formation processes. We have measured anisotropy and vertical velocity gradients to 10 km below the Moho on 70 Ma lithosphere between the Clarion and Clipperton fracture zones. The lithosphere at the study site has not been obviously affected by tectonic or magmatic events since its formation. We find 6.2% anisotropy at the Moho with a mean velocity of 8.14 km/s and the fast direction parallel to paleospreading. Velocity gradients are estimated at 0.02 km/s/km in the fast direction and near 0 km/s/km in the slow direction. The gradient estimates can be explained by aligned microcracks oriented perpendicular to spreading that close with depth. Cracks are expected to close by 10 km below the Moho. At that depth the strength of anisotropy increases to 9%, close to the strength estimated from ophiolite fabrics. These results are consistent with observed olivine fabrics and the predicted effects of lithospheric formation processes, and suggest that lithospheric evolution is modest even at 70 Ma, involving microcracks oriented by a stress field consistent with thermal contraction.

The Relation Between Plate Spreading Rate, Crustal Thickness and Axial Relief at Mid-Ocean Ridges

NASA Astrophysics Data System (ADS)

Liu, Z.; Buck, W. R.

2017-12-01

Variations in axial valley relief and in faulting at plate spreading centers are clearly related to magma supply and axial lithospheric structure. Previous models that consider the interaction of magmatic dikes with lithospheric stretching do not successfully reproduce both of these trends. We present the first model that reproduces these trends by making simple assumptions about the partitioning of magma between dikes, gabbros and extrusives. A key concept is that dikes open not only in the brittle axial lithosphere but also into the underlying ductile crust, where they cool to form gabbro. The amount of gabbro so intruded depends on magma pressure that is related to axial relief. The deeper the valley the less magma goes into gabbros and the more magma is available for dikes to accommodate plate separation. We define the fraction of plate separation rate accommodated by dikes as M. If M<1 then part of the plate separation occurs as fault offset which deepens the axial valley. This axial deepening decreases the amount of magma go into gabbros and this increases M. If the valley reaches the depth where M =1 then the faulting ceases and the valley stays at that depth. However, even if M<1, the valley depth cannot increase without limit. Through a distributed pattern of tectonic faults, the valley depth reaches a maximum possible depth that depends on the thickness of the axial lithosphere. If M < 1, where the axial depth reaches this tectonic limit, then moderate to large offset faults can develop. If M = 1 before the depth reaches the tectonic limit, normal faults only develop in response to oscillations in magma supply and fault offset is proportional to the amount of extruded lava. We have derived analytic expressions relates axial lithospheric thickness (HL) and crustal thickness (Hc) to axial valley depth. We then used a 2D model numerical model with a fixed axial lithospheric structure to show that the analytic model predictions are reasonable. Finally, we describe themo-mechanical models that allow us to relate plate spreading rate and crustal thickness and to axial valley depth.

Crustal Structure of the Flood Basalt Province of Ethiopia from Constrained 3-D Gravity Inversion

NASA Astrophysics Data System (ADS)

Mammo, Tilahun

2013-12-01

The Oligocene Afar mantle plume resulted in the eruption of a large volume of basaltic magma, including major sequences of rhyolitic ignimbrites, in a short span of time across Ethiopia. In order to assess the impact of these magmatic processes on the crust and to investigate the general crustal configuration beneath the Ethiopian plateau, northern part of the Main Ethiopian Rift and the Afar depression, analysis and modeling of the gravity field have been conducted. The Bouguer gravity map is dominated by long-wavelength anomalies that primarily arise from the isostatic compensation of the topography. Consequently, anomalies within the crust/upper mantle are masked and quantitative interpretation becomes difficult. The long-wavelength anomalies are approximated using admittance technique and subsequently removed from the Bouguer anomalies to obtain the residual isostatic anomalies. The residual map contains both short- and intermediate-wavelength anomalies related to geologic and tectonic features. The long-wavelength regional isostatic field is used to map the crust-mantle interface and the results are in good agreement with those determined by other geophysical methods. Seismic constrained gravity inversion was performed on the isostatic residual field and series of three-dimensional models have been constructed for the structures of the crust and upper mantle beneath the uplifted and rifted flood basalt province of northern Ethiopia. The inversion results have shown that the NW plateau has thick crust that rests on normal lithospheric mantle. Afar, On the other hand, is marked by thin stretched crust resting on a low-density upper mantle indicating a hotter thermal regime and partial melt. No lithospheric mantle is observed beneath Afar. The models further indicate the presence of an extensive sub-crustal thick (~12 km on average) and high-density (~3.06 gm/cc) mafic accreted igneous layer of fractionated cumulate (magmatic underplating) beneath the NW plateau. The study suggests that the underplate was fundamental to the accretion process and may have played a role in compensating most of the plateau uplift and in localizing stresses.

The CHAOS-4 geomagnetic field model

NASA Astrophysics Data System (ADS)

Olsen, Nils; Lühr, Hermann; Finlay, Christopher C.; Sabaka, Terence J.; Michaelis, Ingo; Rauberg, Jan; Tøffner-Clausen, Lars

2014-05-01

We present CHAOS-4, a new version in the CHAOS model series, which aims to describe the Earth's magnetic field with high spatial and temporal resolution. Terms up to spherical degree of at least n = 85 for the lithospheric field, and up to n = 16 for the time-varying core field are robustly determined. More than 14 yr of data from the satellites Ørsted, CHAMP and SAC-C, augmented with magnetic observatory monthly mean values have been used for this model. Maximum spherical harmonic degree of the static (lithospheric) field is n = 100. The core field is expressed by spherical harmonic expansion coefficients up to n = 20; its time-evolution is described by order six splines, with 6-month knot spacing, spanning the time interval 1997.0-2013.5. The third time derivative of the squared radial magnetic field component is regularized at the core-mantle boundary. No spatial regularization is applied to the core field, but the high-degree lithospheric field is regularized for n > 85. CHAOS-4 model is derived by merging two submodels: its low-degree part has been derived using similar model parametrization and data sets as used for previous CHAOS models (but of course including more recent data), while its high-degree lithospheric field part is solely determined from low-altitude CHAMP satellite observations taken during the last 2 yr (2008 September-2010 September) of the mission. We obtain a good agreement with other recent lithospheric field models like MF7 for degrees up to n = 85, confirming that lithospheric field structures down to a horizontal wavelength of 500 km are currently robustly determined.

Lithospheric structure of Iberia and Morocco using finite-frequency Rayleigh wave tomography from earthquakes and seismic ambient noise

NASA Astrophysics Data System (ADS)

Palomeras, I.; Villaseñor, A.; Thurner, S.; Levander, A.; Gallart, J.; Harnafi, M.

2017-05-01

We present a new 3-D shear velocity model of the western Mediterranean from the Pyrenees, Spain, to the Atlas Mountains, Morocco, and the estimated crustal and lithospheric thickness. The velocity model shows different crustal and lithospheric velocities for the Variscan provinces, those which have been affected by Alpine deformation, and those which are actively deforming. The Iberian Massif has detectable differences in crustal thickness that can be related to the evolution of the Variscan orogen in Iberia. Areas affected by Alpine deformation have generally lower velocities in the upper and lower crust than the Iberian Massif. Beneath the Gibraltar Strait and surrounding areas, the crustal thickness is greater than 50 km, below which a high-velocity anomaly (>4.5 km/s) is mapped to depths greater than 200 km. We identify this as a subducted remnant of the NeoTethys plate referred to as the Alboran and western Mediterranean slab. Beneath the adjacent Betic and Rif Mountains, the Alboran slab is still attached to the base of the crust, depressing it, and ultimately delaminating the lower crust and mantle lithosphere as the slab sinks. Under the adjacent continents, the Alboran slab is surrounded by low upper mantle shear wave velocities (Vs < 4.3) that we interpret as asthenosphere that has replaced the continental margin lithosphere which was viscously removed by Alboran plate subduction. The southernmost part of the model features an anomalously thin lithosphere beneath the Atlas Mountains that could be related to lateral flow induced by the Alboran slab.

The nature of orogenic crust in the central Andes

NASA Astrophysics Data System (ADS)

Beck, Susan L.; Zandt, George

2002-10-01

The central Andes (16°-22°S) are part of an active continental margin mountain belt and the result of shortening of the weak western edge of South America between the strong lithospheres of the subducting Nazca plate and the underthrusting Brazilian shield. We have combined receiver function and surface wave dispersion results from the BANJO-SEDA project with other geophysical studies to characterize the nature of the continental crust and mantle lithospheric structure. The major results are as follows: (1) The crust supporting the high elevations is thick and has a felsic to intermediate bulk composition. (2) The relatively strong Brazilian lithosphere is underthrusting as far west (65.5°W) as the high elevations of the western part of the Eastern Cordillera (EC) but does not underthrust the entire Altiplano. (3) The subcrustal lithosphere is delaminating piecemeal under the Altiplano-EC boundary but is not completely removed beneath the central Altiplano. The Altiplano crust is characterized by a brittle upper crust decoupled from a very weak lower crust that is dominated by ductile deformation, leading to lower crustal flow and flat topography. In contrast, in the high-relief, inland-sloping regions of the EC and sub-Andean zone, the upper crust is still strongly coupled across the basal thrust of the fold-thrust belt to the underthrusting Brazilian Shield lithosphere. Subcrustal shortening between the Altiplano and Brazilian lithosphere appears to be accommodated by delamination near the Altiplano-EC boundary. Our study suggests that orogenic reworking may be an important part of the "felsification" of continental crust.

Lithospheric Strength and Stress State: Persistent Challenges and New Directions in Geodynamics

NASA Astrophysics Data System (ADS)

Hirth, G.

2017-12-01

The strength of the lithosphere controls a broad array of geodynamic processes ranging from earthquakes, the formation and evolution of plate boundaries and the thermal evolution of the planet. A combination of laboratory, geologic and geophysical observations provides several independent constraints on the rheological properties of the lithosphere. However, several persistent challenges remain in the interpretation of these data. Problems related to extrapolation in both scale and time (rate) need to be addressed to apply laboratory data. Nonetheless, good agreement between extrapolation of flow laws and the interpretation of microstructures in viscously deformed lithospheric mantle rocks demonstrates a strong foundation to build on to explore the role of scale. Furthermore, agreement between the depth distribution of earthquakes and predictions based on extrapolation of high temperature friction relationships provides a basis to understand links between brittle deformation and stress state. In contrast, problems remain for rationalizing larger scale geodynamic processes with these same rheological constraints. For example, at face value the lab derived values for the activation energy for creep are too large to explain convective instabilities at the base of the lithosphere, but too low to explain the persistence of dangling slabs in the upper mantle. In this presentation, I will outline these problems (and successes) and provide thoughts on where new progress can be made to resolve remaining inconsistencies, including discussion of the role of the distribution of volatiles and alteration on the strength of the lithosphere, new data on the influence of pressure on friction and fracture strength, and links between the location of earthquakes, thermal structure, and stress state.

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.

Generation of Continental Rifts, Basins and Swells by Lithosphere Instabilities

NASA Astrophysics Data System (ADS)

Milelli, L.; Fourel, L.; Jaupart, C. P.

2012-12-01

Domal uplifts, volcanism, basin formation and rifting have often struck the same continent in different areas at the same time. Their characteristics and orientations are difficult to reconcile with mantle convection or tectonic forces and suggest a driving mechanism that is intrinsic to the continent. The rifts seem to develop preferentially at high angles to the edge of the continent whereas swells and basins seem confined to the interior. Another intriguing geometrical feature is that the rifts often branch out in complicated patterns at their landward end. In Western Africa, for example, magmatic activity currently occurs in a number of uplifted areas including the peculiar Cameroon Volcanic Line that stretches away from the continental margin over about 1000 km. Magmatic and volcanic activity has been sustained along this line for 70 My with no age progression. The mantle upwelling that feeds the volcanoes is not affected by absolute plate motions and hence is attached to the continent. The Cameroon Volcanic Line extends to the Biu swell to the North and the Jos plateau to the West defining a striking Y-shaped pattern. This structure segues into several volcanic domes including the Air, the Hoggar, the Darfur, the Tibesti and the Haruj domes towards the Mediterranean coast. Another example is provided by North America, where the late Proterozoic-early Ordovician saw the formation of four major basins, the Michigan, Illinois, Williston and Hudson Bay, as well as of major rifts in southern Oklahoma and the Mississipi Valley within a short time interval. At the same time, a series of uplifts developed, such as the Ozark and Nashville domes. Motivated by these observations, we have sought an explanation in the continental lithosphere itself. We describe a new type of convective instability at the base of the lithosphere that leads to a remarkable spatial pattern at the scale of an entire continent. We carried out fluid mechanics laboratory experiments on buoyant blocks of finite size that became unstable due to cooling from above and describe the peculiar horizontal planform that developed. Dynamical behaviour depends on three dimensionless numbers, a Rayleigh number for the unstable block, a buoyancy number that scales the intrinsic density contrast to the thermal one and the aspect ratio of the block. Within the block, instability develops in two different ways in an outer annulus and in an inner region. In the outer annulus, upwellings and downwellings take the form of radial rolls spaced regularly. In the interior region, the planform adopts the more familiar form of polygonal cells. Translated to geological conditions, such instabilities should manifest themselves as linear rifts striking at a right angle to the continent-ocean boundary and an array of domal uplifts, volcanic swells and basins in the continental interior. The laboratory data lead to simple scaling laws for the dimensions and spacings of the convective structures. For the sub-continental lithospheric mantle, these dimensions and distances take values in the 500-1000 km range, close to geological examples. The large intrinsic buoyancy of Archean lithospheric roots prevents this type of instability, which explains why the widespread volcanic activity that currently affects Western Africa is confined to post-Archean domains.

Seismic and thermal structure of the crust and uppermost mantle beneath Antarctica from inversion of multiple seismic datasets

NASA Astrophysics Data System (ADS)

Wiens, D.; Shen, W.; Anandakrishnan, S.; Aster, R. C.; Gerstoft, P.; Bromirski, P. D.; Dalziel, I.; Hansen, S. E.; Heeszel, D.; Huerta, A. D.; Nyblade, A.; Stephen, R. A.; Wilson, T. J.; Winberry, J. P.; Stern, T. A.

2017-12-01

Since the last decade of the 20th century, over 200 broadband seismic stations have been deployed across Antarctica (e.g., temporary networks such as TAMSEIS, AGAP/GAMSEIS, POLENET/ANET, TAMNNET and RIS/DRIS by U.S. geoscientists as well as stations deployed by Japan, Britain, China, Norway, and other countries). In this presentation, we discuss our recent efforts to build reference crustal and uppermost mantle shear velocity (Vs) and thermal models for continental Antarctica based on those seismic arrays. By combing the high resolution Rayleigh wave dispersion maps derived from both ambient noise and teleseismic earthquakes, together with P receiver function waveforms, we develop a 3-D Vs model for the crust and uppermost mantle beneath Central and West Antarctica to a depth of 200 km. Additionally, using this 3-D seismic model to constrain the crustal structure, we re-invert for the upper mantle thermal structure using the surface wave data within a thermodynamic framework and construct a 3-D thermal model for the Antarctic lithosphere. The final product, a high resolution thermal model together with associated uncertainty estimates from the Monte Carlo inversion, allows us to derive lithospheric thickness and surface heat flux maps for much of the continent. West Antarctica shows a much thinner lithosphere ( 50-90 km) than East Antarctica ( 130-230 km), with a sharp transition along the Transantarctic Mountains (TAM). A variety of geological features, including a slower/hotter but highly heterogeneous West Antarctica and a much faster/colder East Antarctic craton, are present in the 3-D seismic/thermal models. Notably, slow seismic velocities observed in the uppermost mantle beneath the southern TAM are interpreted as a signature of lithospheric foundering and replacement with hot asthenosphere. The high resolution image of these features from the 3-D models helps further investigation of the dynamic state of Antarctica's lithosphere and underlying asthenosphere and provides key constraints on the interaction between the solid Earth and the West Antarctic Ice Sheet.

Relocation of Intermediate-depth Seismicity in the Relic Alboran Slab: Clustering and Relationship to Tearing and Dehydration Embrittlement.

NASA Astrophysics Data System (ADS)

Sun, M.; Bezada, M.

2017-12-01

Intermediate-depth seismicity outside active subduction zones is rare. However, there is a well-known occurrence of such events in a N-S elongated volume between Spain and Morocco, within what most researchers consider to be the relic Alboran slab. Partial subduction of, and tearing from the adjoining continental lithosphere have been suggested in this area. We investigate whether dehydration embrittlement or shear instability is more consistent with the Alboran intermediate depth seismicity by considering their location relative to the expected thermal structure and expected areas of high strain rate associated with thinning or tearing of the slab. We use a dense temporary seismograph deployment in Spain and Morocco to relocate 65 intermediate-depth events occurring between 2010 and 2013 in this region. The relocation procedure is realized by a grid-search approach that minimizes the normalized misfit between the picked times and travel times calculated using a regional 3D velocity model. Results indicate that, compared with catalog results, hypocenters after relocation are more concentrated in space; they tend to shift southward and eastward while no systematic shift in depth is observed. Relocated hypocenters concentrate at a depth range between 50-100 km and along a narrow longitude range around 4.5W. Investigation of the earthquake density distribution indicates these earthquakes concentrate into several clusters. One such cluster sits above the spain-arm of the Alboran slab and beneath the Spain continental lithosphere, indicating that it is likely associated to the thinning process of the Alboran slab. The other four clusters all lie within the interior of the slab. Interestingly, two of them are near the middle of the subducted lithosphere and the other two lie near its base. This observation seems at odds with expectations based on the two leading hypotheses for enabling brittle failure at intermediate depths.

Formation of cratonic lithosphere: An integrated thermal and petrological model

NASA Astrophysics Data System (ADS)

Herzberg, Claude; Rudnick, Roberta

2012-09-01

The formation of cratonic mantle peridotite of Archean age is examined within the time frame of Earth's thermal history, and how it was expressed by temporal variations in magma and residue petrology. Peridotite residues that occupy the lithospheric mantle are rare owing to the effects of melt-rock reaction, metasomatism, and refertilization. Where they are identified, they are very similar to the predicted harzburgite residues of primary magmas of the dominant basalts in greenstone belts, which formed in a non-arc setting (referred to here as "non-arc basalts"). The compositions of these basalts indicate high temperatures of formation that are well-described by the thermal history model of Korenaga. In this model, peridotite residues of extensive ambient mantle melting had the highest Mg-numbers, lowest FeO contents, and lowest densities at ~ 2.5-3.5 Ga. These results are in good agreement with Re-Os ages of kimberlite-hosted cratonic mantle xenoliths and enclosed sulfides, and provide support for the hypothesis of Jordan that low densities of cratonic mantle are a measure of their high preservation potential. Cratonization of the Earth reached its zenith at ~ 2.5-3.5 Ga when ambient mantle was hot and extensive melting produced oceanic crust 30-45 km thick. However, there is a mass imbalance exhibited by the craton-wide distribution of harzburgite residues and the paucity of their complementary magmas that had compositions like the non-arc basalts. We suggest that the problem of the missing basaltic oceanic crust can be resolved by its hydration, cooling and partial transformation to eclogite, which caused foundering of the entire lithosphere. Some of the oceanic crust partially melted during foundering to produce continental crust composed of tonalite-trondhjemite-granodiorite (TTG). The remaining lithosphere gravitationally separated into 1) residual eclogite that continued its descent, and 2) buoyant harzburgite diapirs that rose to underplate cratonic nuclei composed of non-arc basalts and TTG. Finally, assembly of cratonic nuclei into cratons at convergent boundaries substantially modified harzburgite residues by melt-rock reaction.

Spectral analysis of magnetic anomalies in and around the Philippine Sea

NASA Astrophysics Data System (ADS)

Tanaka, A.; Ishihara, T.

2009-12-01

Regional compilations of lithospheric structure from various methods and data and comparison among them are useful to understand lithospheric structure and the processes behind its formation and evolution. We present constraints on the regional variations of the magnetic thicknesses in and around the Philippine Sea. We used a new global magnetic anomaly data [Quesnel et al, 2009], which is CM4-corrected [Comprehensive Model 4; Sabaka et al., 2004], cleaned and leveled to clarify the three-dimensional crustal magnetic structure of the Philippine Sea. The Philippine Sea is one of the largest marginal seas of the world. The north-south-trending Kyushu-Palau Ridge divides it into two parts: the West Philippine Basin and the Daito Ridge province in the west and the Shikoku and Parece Vela Basins in the east. The age of the basins increases westward [Karig, 1971]. And, there are three ridges in the Daito Ridge province west of the Kyushu-Palau Ridge; the Oki-Daito, Daito Ridges and the Amami Plateau from south to north, and small basins among them. Two-dimensional spectral analysis of marine magnetic anomalies is used to estimate the centroid of magnetic sources (Zo) to constrain the lithospheric structure [Tanaka and Ishihara, 2008]. The method is based on that of Spector and Grant [1970]. Zo distribution of the Philippine Sea shows occurrence of shallow magnetic layer areas with approximately less than 10 km in the Shikoku Basin. It also shows variations in deep and shallow magnetic layer areas in the Amami-Daito Province. These patters correspond to spatial variations of the crustal thickness deduced from the three-dimensional gravity modeling [Ishihara and Koda, 2007] and acoustic basement structures [Higuchi et al., 2007]. These three spatial distributions are roughly consistent with each other, although they may contain some scatters and bias due to the different characteristics and errors. This two-dimensional spectral analysis method is based upon an assumption that source distribution is random; therefore when magnetic anomalies represent linear features, this analysis based on ensembles of thin prisms may produce unreliable results. In this case, one-dimensional spectrum analysis based on a thin plate model composed of long bars is preferable. Makino and Okubo [1988] developed one-dimensional spectral analysis for marine linear magnetic anomalies. A linear relationship between the natural log of (power-density spectrum of magnetic profile) and wavelength gives the centroid depth of magnetic sources. The same method is applied to this area. This analysis requires a long profile to see deeper structure. It may not be possible to find good enough data. However, both methods give consistent results, and the obtained Zo distribution provides a comprehensive view of regional-scale features. The correlation between crustal thickness and Zo and its correspondence with tectonic regime indicates that Zo is useful to delineate regional crustal thermal structure. It is expected that Zo combined with multidisciplinary data should help to infer geophysical and geological information in the less explored regions.

OCT structure, COB location and magmatic type of the SE Brazilian & S Angolan margins from integrated quantitative analysis of deep seismic reflection and gravity anomaly data

NASA Astrophysics Data System (ADS)

Cowie, L.; Kusznir, N. J.; Horn, B.

2013-12-01

Knowledge of ocean-continent transition (OCT) structure, continent-ocean boundary (COB) location and magmatic type are of critical importance for understanding rifted continental margin formation processes and in evaluating petroleum systems in deep-water frontier oil and gas exploration. The OCT structure, COB location and magmatic type of the SE Brazilian and S Angolan rifted continental margins are much debated; exhumed and serpentinised mantle have been reported at these margins. Integrated quantitative analysis using deep seismic reflection data and gravity inversion have been used to determine OCT structure, COB location and magmatic type for the SE Brazilian and S Angolan margins. Gravity inversion has been used to determine Moho depth, crustal basement thickness and continental lithosphere thinning. Residual Depth Anomaly (RDA) analysis has been used to investigate OCT bathymetric anomalies with respect to expected oceanic bathymetries and subsidence analysis has been used to determine the distribution of continental lithosphere thinning. These techniques have been validated on the Iberian margin for profiles IAM9 and ISE-01. In addition a joint inversion technique using deep seismic reflection and gravity anomaly data has been applied to the ION-GXT BS1-575 SE Brazil and ION-GXT CS1-2400 S Angola. The joint inversion method solves for coincident seismic and gravity Moho in the time domain and calculates the lateral variations in crustal basement densities and velocities along profile. Gravity inversion, RDA and subsidence analysis along the S Angolan ION-GXT CS1-2400 profile has been used to determine OCT structure and COB location. Analysis suggests that exhumed mantle, corresponding to a magma poor margin, is absent beneath the allochthonous salt. The thickness of earliest oceanic crust, derived from gravity and deep seismic reflection data is approximately 7km. The joint inversion predicts crustal basement densities and seismic velocities which are slightly less than expected for 'normal' oceanic crust. The difference between the sediment corrected RDA and that predicted from gravity inversion crustal thickness variation implies that this margin is experiencing ~300m of anomalous uplift attributed to mantle dynamic uplift. Gravity inversion, RDA and subsidence analysis have also been used to determine OCT structure and COB location along the ION-GXT BS1-575 profile, crossing the Sao Paulo Plateau and Florianopolis Ridge of the SE Brazilian margin. Gravity inversion, RDA and subsidence analysis predict the COB to be located SE of the Florianopolis Ridge. Analysis shows no evidence for exhumed mantle on this margin profile. The joint inversion technique predicts normal oceanic basement seismic velocities and densities and beneath the Sao Paulo Plateau and Florianopolis Ridge predicts crustal basement thicknesses between 10-15km. The Sao Paulo Plateau and Florianopolis Ridge are separated by a thin region of crustal basement beneath the salt interpreted as a regional transtensional structure. Sediment corrected RDAs and gravity derived 'synthetic' RDAs are of a similar magnitude on oceanic crust, implying negligible mantle dynamic topography.

Deep Structure of the Earth and Concentration of Metals in the Lithosphere: A Geodynamic Approach

NASA Technical Reports Server (NTRS)

Taylor, Patrick T.; Kutina, J.; Pei, R.

2004-01-01

A discussion of and introduction to satellite-altitude geopotential fields studies and their interpretation with emphasis on results from metalliferous regions will be given. The magnetic and gravimetric measurements from satellite altitudes show heterogeneity in deeper parts of the lithosphere. These patterns of magnetic anomalies do not only reveal the largest iron ore deposits such as Kiruna, Sweden and Kursk, Russia, but also linear features indicating structural discontinuities. Changes of magnetic amplitude of these patterns are caused by intersecting transverse fractures localizing magmatism and concentration of metals. The role of trans-regional mantle-rooted structural discontinuities in the concentration of metals will be discussed and a new type of mineral prognosis map will be presented. Deep-rooted structural discontinuities, defined by combination of geological and geophysical criteria, with spacing of several hundred kilometers, reveal a quite uniform pattern in the deeper parts of the lithosphere. As these structures provide favorable pathways for the ascent of heat, magmas and ore-forming fluids, their recognition is of crucial importance and can be used in the compilation of a new type of mineral prognosis map. Examples are shown from the United States, Canada, China, Burma, South America, Europe and Australia. The European example includes a pattern of east west trending structural discontinuities or belts and their junction with the NW-trending Tornqvist-Teisseyre Line. The Upper Silesian-Cracovian Zn-Pb district occurs along one of the latitudinal belts. Leslaw Teper of the University of Silesia has been invited to show the fractures in crystalline basement beneath the sediments hosting the Zn-Pb ores.

Mantle Flow in the Western United States Constrained by Seismic Anisotropy

NASA Astrophysics Data System (ADS)

Niday, W.; Humphreys, E.

2017-12-01

Shear wave splitting, caused by the lattice preferred orientation (LPO) of olivine crystals under shear deformation, provide a useful constraint on numerical models of mantle flow. Although it is sometimes assumed that shear wave splitting fast directions correspond with mantle flow directions, this is only true in simple shear flows that do not vary strongly with space or time. Observed shear wave splitting in the western United States is complex and inconsistent with simple shear driven by North American and Pacific plate motion, suggesting that the effects of time-dependent subduction history and spatial heterogeneity are important. Liu and Stegman (2011) reproduce the pattern of fast seismic anomalies below the western US from Farallon subduction history, and Chaparro and Stegman (2017) reproduce the circular anisotropy field below the Great Basin. We extend this to consider anisotropic structure outside the Great Basin and evaluate the density and viscosity of seismic anomalies such as slabs and Yellowstone. We use the mantle convection code ASPECT to simulate 3D buoyancy-driven flow in the mantle below the western US, and predict LPO using the modeled flow fields. We present results from a suite of models varying the sub-lithospheric structures of the western US and constraints on density and viscosity variations in the upper mantle.

Link of grabens and reactivated mantle boundaries in western Bohemian Massif

NASA Astrophysics Data System (ADS)

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

2015-04-01

To study relations between mantle and crustal fabrics in the Bohemian Massif (BM), we model 3D anisotropy of the mantle lithosphere by inverting and interpreting jointly P-wave travel-time deviations and shear-wave splitting parameters of teleseismic waves recorded at portable and permanent stations operating in the BM for more than 20 years. Changes in orientation of the large-scale anisotropy in the mantle lithosphere, caused by systematic preferred orientation of olivine, identify boundaries of domains representing original micro-plates assembled during the Variscan orogeny. Consistent anisotropy of the mantle-lithosphere domains, with distinct changes at their boundaries, documents rigidity and a long memory of pervasive olivine fabrics. Some of the palaeo-plate boundaries represent weak elements of the assemblage that can be later rejuvenated. This is why graben structures in the western BM developed above the identified mantle boundaries. The Eger (Ohře) Rift (ER) originated above the ENE oriented mantle suture between the Saxothuringian (ST) in the north-west and the Moldanubian (MD) and Teplá-Barrandian (TB) in the south-east. The most significant graben structure, accompanied by a rich Cenozoic volcanic activity, developed above the central part of the ST/TB suture that witnessed a subduction down to ~150 km, as documented by findings of microdiamonds in ST granulites. The smaller-scale NNW oriented Cheb-Domažlice Graben (CDG) is located above the mantle boundary between the western rim of the TB and the MD. Unlike the suture beneath the ER, this boundary does not show any sign of a deep subduction and it is characterized by a less well developed graben structure and a weak volcanic activity. In both grabens we observe local shifts between the equivalent crustal and mantle boundaries of the units as large as ~20 km. The shift indicates a Variscan detachment of the crust from the mantle lithosphere. Cenozoic rifting and the graben structures developed preferably above the mantle boundaries, often away from the boundaries of the crustal units.

Crust and uppermost-mantle structure of Greenland and the Northwest Atlantic from Rayleigh wave group velocity tomography

NASA Astrophysics Data System (ADS)

Darbyshire, Fiona A.; Dahl-Jensen, Trine; Larsen, Tine B.; Voss, Peter H.; Joyal, Guillaume

2018-03-01

The Greenland landmass preserves ˜4 billion years of tectonic history, but much of the continent is inaccessible to geological study due to the extensive inland ice cap. We map out, for the first time, the 3-D crustal structure of Greenland and the NW Atlantic ocean, using Rayleigh wave anisotropic group velocity tomography, in the period range 10-80 s, from regional earthquakes and the ongoing GLATIS/GLISN seismograph networks. 1-D inversion gives a pseudo-3-D model of shear wave velocity structure to depths of ˜100 km with a horizontal resolution of ˜200 km. Crustal thickness across mainland Greenland ranges from ˜25 km to over 50 km, and the velocity structure shows considerable heterogeneity. The large sedimentary basins on the continental shelf are clearly visible as low velocities in the upper ˜5-15 km. Within the upper continental basement, velocities are systematically lower in northern Greenland than in the south, and exhibit a broadly NW-SE trend. The thinning of the crust at the continental margins is also clearly imaged. Upper-mantle velocities show a clear distinction between typical fast cratonic lithosphere (Vs ≥4.6 km s-1) beneath Greenland and its NE margin and anomalously slow oceanic mantle (Vs ˜4.3-4.4 km s-1) beneath the NW Atlantic. We do not observe any sign of pervasive lithospheric modification across Greenland in the regions associated with the presumed Iceland hotspot track, though the average crustal velocity in this region is higher than that of areas to the north and south. Crustal anisotropy beneath Greenland is strong and complex, likely reflecting numerous episodes of tectonic deformation. Beneath the North Atlantic and Baffin Bay, the dominant anisotropy directions are perpendicular to the active and extinct spreading centres. Anisotropy in the subcontinental lithosphere is weaker than that of the crust, but still significant, consistent with cratonic lithosphere worldwide.

Lithospheric buckling and far-foreland deformation during the Laramide and Appalachian orogenies

NASA Astrophysics Data System (ADS)

Tikoff, B.; Siddoway, C. S.

2017-12-01

Major intraplate tectonics within North America (Laurentia) occurs during times of major orogenesis along the plate margins. During mountain building, typical structures of the hinterland are an orogenic plateau and fold-and-thrust belts, while in the far foreland (intraplate) areas long-wavelength ( 200 km or longer) folds and fault-reactivation features form. Long-wavelength folds are evident in both the Appalachian and Laramide orogenic forelands, with the stratigraphy recording the timing of the uplift. This contribution examines the model of lithospheric buckling - periodic folding associated with a horizontal endload on the edge of the plate - based on scaled, physical experiments and corroborated by numerical models. The Laramide (75-55 Ma) intraplate orogen in the classical location in Wyoming contains basement-cored arches spaced 200 km apart, for which the mechanism of uplift is questioned. Seismic evidence obtained for the Bighorn uplift, Wyoming, obtained by the EarthScope Bighorn project, shows an upwarp of the Moho beneath, but oblique to the trend of the surface exposure of the basement arch. Both the surface and Moho exhibit approximately the same structural relief. The seismic data exhibit no evidence for a regionally continuous decollement, nor is there evidence of rotation of structural markers within these features, of the type that is observed in the detached fold-and-thrust belt. The intraplate region affected by long-wavelength folding includes western Wyoming, with continuation of some features across the E-W-oriented Cheyenne belt (e.g., Rock Springs-Douglas Creek arch), Colorado Plateau, and High Plains east of the Rocky Mountains, where surface and subsurface structures display a series of anticlinal arches ("plains-type" folds). Appalachian mountain building also caused long-wavelength folding, with a spacing consistent with lithospheric buckling, mostly associated with the Devonian Acadian orogeny. The Laramide arches in the High Plains seem to occur on arches inherited from the Appalachian orogeny, suggesting the permanence of these lithospheric buckles once they have formed.

Gravity and magnetic modelling in the Vrancea Zone, south-eastern Carpathians: Redefinition of the edge of the East European Craton beneath the south-eastern Carpathians

NASA Astrophysics Data System (ADS)

Bocin, A.; Stephenson, R.; Matenco, L.; Mocanu, V.

2013-11-01

A 2D gravity and magnetic data model has been constructed along a 71 km densely observed profile, called DACIA PLAN GRAV MAN's. The profile crosses part of the nappe pile of the south-eastern Carpathians and includes the seismically active Vrancea Zone and was acquired with the objective to illuminate the basement structure and affinity in this area. The modelling approach was to create an initial model from well constrained geological information, integrate it with previous seismic ray tracing and tomographic models and then alter it outside the a priori constraints in order to reach the best fit between observed and calculated potential field anomalies. The results support a realignment of the position of the TTZ (Tornquist-Teisseyre Zone), the profound tectonic boundary within Europe that separates Precambrian cratonic lithosphere of the East European Craton (EEC) from younger accreted lithosphere of Phanerozoic mobile belts to its west. The TTZ is shown to lie further to the south-west than was previously inferred within Romania, where it is largely obscured by the Carpathian nappes. The crust of the EEC beneath the south-eastern Carpathians is inferred to terminate along a major crustal structure lying just west of the Vrancea seismogenic zone. The intermediate depth seismicity of the Vrancea Zone therefore lies within the EEC lithosphere, generally supporting previously proposed models invoking delamination of cratonic lithosphere as the responsible mechanism.

Deformation and seismicity associated with continental rift zones propagating toward continental margins

NASA Astrophysics Data System (ADS)

Lyakhovsky, V.; Segev, A.; Schattner, U.; Weinberger, R.

2012-01-01

We study the propagation of a continental rift and its interaction with a continental margin utilizing a 3-D lithospheric model with a seismogenic crust governed by a damage rheology. A long-standing problem in rift-mechanics, known as thetectonic force paradox, is that the magnitude of the tectonic forces required for rifting are not large enough in the absence of basaltic magmatism. Our modeling results demonstrate that under moderate rift-driving tectonic forces the rift propagation is feasible even in the absence of magmatism. This is due to gradual weakening and "long-term memory" of fractured rocks that lead to a significantly lower yielding stress than that of the surrounding intact rocks. We show that the style, rate and the associated seismicity pattern of the rift zone formation in the continental lithosphere depend not only on the applied tectonic forces, but also on the rate of healing. Accounting for the memory effect provides a feasible solution for thetectonic force paradox. Our modeling results also demonstrate how the lithosphere structure affects the geometry of the propagating rift system toward a continental margin. Thinning of the crystalline crust leads to a decrease in the propagation rate and possibly to rift termination across the margin. In such a case, a new fault system is created perpendicular to the direction of the rift propagation. These results reveal that the local lithosphere structure is one of the key factors controlling the geometry of the evolving rift system and seismicity pattern.

Evidence for Moho-lower crustal transition depth diking and rifting of the Sierra Nevada microplate

NASA Astrophysics Data System (ADS)

Smith, Kenneth D.; Kent, Graham M.; Seggern, David P.; Driscoll, Neal W.; Eisses, Amy

2016-10-01

Lithospheric rifting most often initiates in continental extensional settings where "breaking of a plate" may or may not progress to sea floor spreading. Generally, the strength of the lithosphere is greater than the tectonic forces required for rupture (i.e., the "tectonic force paradox"), and it has been proposed that rifting requires basaltic magmatism (e.g., dike emplacement) to reduce the strength and cause failure, except for the case of a thin lithosphere (<30 km thick). Here we isolate two very similar and unprecedented observations of Moho-lower crustal transition dike or fluid injection earthquake swarms under southern Sierra Valley (SV: 2011-2012) and North Lake Tahoe (LT: 2003-2004), California. These planar distributions of seismicity can be interpreted to define the end points, and cover 25% of the length, of an implied 56 km long structure, each striking N45°W and dipping 50°NE. A single event at 30 km depth that locates on the implied dipping feature between the two swarms is further evidence for a single Moho-transition depth structure. We propose that basaltic or fluid emplacement at or near Moho depths weakens the upper mantle lid, facilitating lithospheric rupture of the Sierra Microplate. Similar to the LT sequence, the SV event is also associated with increased upper crustal seismicity. An 27 October 2011, Mw 4.7 earthquake occurred directly above the deep SV sequence at the base of the upper crustal seismogenic zone ( 15 km depth).

The Processes Producing the Actively Uplifting Mackenzie Mountains in the Yukon and Northwest Territories, Canada

NASA Astrophysics Data System (ADS)

Rasmussen, B.; Aster, R. C.; Schutt, D.

2016-12-01

The actively uplifting and seismically active Mackenzie Mountains in the Yukon and Northwest Territories of Canada exist nearly 800 km from the Pacific plate subduction zone. As such, it is clear that traditional subduction zone orogenic mechanics are not at play. This mountain range may present a model for uplift of other ranges distant from plate boundaries, such as the Rockies or Ancestral Rockies. Due to its remote location, this region's lithospheric structure is poorly constrained. However, two hypotheses have been developed recently. The first proposes that stress from the Yakutat Indentor as it subducts under North America at the Gulf of Alaska is transferred deep inland through the upper crust, and that the lower crust and mantle lithosphere are very weak. As this weak lithosphere meets the strong Canadian Craton, lateral translation turns into uplift, forming the Mackenzies (Mazzotti and Hyndman, 2002, Geology, v. 30, no.6). Alternatively, it may be that mantle flow from the north is deflected eastward by the Yakutat slab, producing large scale mantle flow and stress which propagates through the crust to uplift the Mackzenzie Mountains without an abnormally weak lithosphere (Finzel, 2015, Geophys. Res. Lett., 42, 4350-4358). Both cases imply distinct isotropic and anisotropic structure that will be constrained through Rayleigh wave tomography. Notably, we will take advantage of the recent deployment of several Earthscope Transportable Array stations nearby, and some preliminary data from the ongoing Mackenzie Mountains Earthscope Project.

Crustal and Mantle Structure beneath the Okavango and Malawi Rifts and Its Geodynamic Implications

NASA Astrophysics Data System (ADS)

Gao, S. S.; Liu, K. H.; Yu, Y.; Reed, C. A.; Mickus, K. L.; Moidaki, M.

2017-12-01

To investigate crustal and mantle structure beneath the young and incipient sections of the East African Rift System and provide constraints on rifting models, a total of 50 broadband seismic stations were placed along three profiles across the Okavango and Malawi rifts, with a total length of about 2500 km. Results to date suggest minor crustal thinning and nearly normal seismic velocities in the upper mantle beneath both rifts. The thickness of the mantle transition zone is comparable to the global average, suggesting the lack of thermal upwelling from the lower mantle beneath the rifts. In addition, shear-wave splitting analysis found no anomalies in either the fast polarization orientation or the splitting time associated with the rifts, and thus has ruled out the existence of small-scale mantle convection or plume-related mantle flow beneath the rifts. While the Okavango rift has long been recognized to be located in a Precambrian orogenic zone between the Kalahari and Congo cratons, our results suggest that the Malawi Rift is also developing along the western edge of a lithospheric block with relatively greater thickness relative to the surrounding area. Those seismological and gravity modeling results are consistent with a passive rifting model, in which rifts develop along pre-existing zones of lithospheric weakness, where rapid variations of lithospheric thickness is observed. Lateral variations of dragging stress applied to the bottom of the lithosphere are the most likely cause for the initiation and development of both rifts.

Lithospheric thickness variations across the North Anatolian Fault Zone

NASA Astrophysics Data System (ADS)

Thompson, D. A.; Rost, S.; Cornwell, D. G.; Houseman, G.; Turkelli, N.; Teoman, U.; Altuncu Poyraz, S.; Kahraman, M.; Gulen, L.; Utkucu, M.; Williams, J. R.

2017-12-01

The North Anatolian Fault Zone (NAFZ) is a major continental strike-slip fault zone, similar in size and scale to the San Andreas system, that extends 1200km across Turkey. These type of faults may broaden significantly with depth or penetrate as narrow features all the way to the lithosphere-asthenosphere boundary (LAB), potentially providing pathways for fluids and magma to shallower levels. The Dense Array for North Anatolia (DANA) was a 73 station broadband seismic network arranged in a rectangular grid (7km station spacing) deployed to image the deep structure of the fault zone. We present here new S-receiver function images that map out both the depth to the Moho and to negative velocity gradients commonly ascribed to the LAB, with preliminary results suggesting lithospheric thicknesses on the order of 80-100km for the region.

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.

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.

Implications of a-priori constraints in transdimensional Bayesian inversion for continental lithospheric layering

NASA Astrophysics Data System (ADS)

Roy, Corinna; Calo, Marco; Bodin, Thomas; Romanowicz, Barbara

2017-04-01

Competing hypotheses for the formation and evolution of continents are highly under debate, including the theory of underplating by hot plumes or accretion by shallow subduction in continental or arc settings. In order to support these hypotheses, documenting structural layering in the cratonic lithosphere becomes especially important. Studies of seismic-wave receiver function data have detected a structural boundary under continental cratons at 100-140 km depths, which is too shallow to be consistent with the lithosphere-asthenosphere boundary, as inferred from seismic tomography and other geophysical studies. This leads to the conclusion that 1) the cratonic lithosphere may be thinner than expected, contradicting tomographic and other geophysical or geochemical inferences, or 2) that the receiver function studies detect a mid-lithospheric discontinuity rather than the LAB. Recent studies (Bodin et al., 2015, Calò et al. 2016) confirmed the presence of a structural boundary under the North American craton at 100-140 km depths by taking advantage of the power of a trans-dimensional Monte Carlo Markov chain (TMCMC). They generated probabilistic 1D radially shear wave velocity profiles for selected stations in North America by jointly inverting 2 different data types (PS Receiver Functions, surface wave dispersion for Love and Rayleigh waves), which sample different volumes of the Earth and have different sensitivities to 
structure. In fact, they found at least one, and in some cases several additional mid-lithospheric discontinuities (MLD) at intermediate depths in the stable part of the craton. Such discontinuities are not present in the active western part of the US. However, in their Bayesian approach, they made two major assumptions: First, they fixed the Vp/Vs ratio to a constant, averaged value for crust and mantle. Second, they added constraints on the crustal discontinuity depths in the prior distribution of the shear wave velocity. Given these strong assumptions, the question came up whether the observed MLDs are real features or only artifacts resulting from these assumptions, in spite of the fact that the Ps converted phase constraints are included using a waveform cross-convolution approach, which should be able to handle crustal multiples, in contrast to a standard RF inversion. We investigated these concerns by including the Vp/Vs ratio as a random variable in the Bayesian approach. By performing simulations on synthetic Earth models, we can demonstrate that a variable/fixed Vp/Vs ratio has little effect on posterior distribution of VSV, but that the imposed constraints can introduce artificial layers in depth. Nevertheless, we can confirm the overall structure found by Calò et al. 2016. Here, we also include SKS waveforms in the joint inversion and invert for azimuthal anisotropy to verify if layering in the anisotropic structure of the stable part of the North American continent is marked by significant changes in the direction of azimuthal anisotropy as found by Yuan and Romanowicz (2010). We recently demonstrated the power of this approach in the case of two stations located in different tectonic settings (Bodin et al., 2016). Here we extend this approach to a broader range of settings within the North American continent.

Evidence of partial melting beneath a continental margin: case of Dhofar, in the Northeast Gulf of Aden (Sultanate of Oman)

NASA Astrophysics Data System (ADS)

Basuyau, C.; Tiberi, C.; Leroy, S.; Stuart, G.; Al-Lazki, A.; Al-Toubi, K.; Ebinger, C.

2010-02-01

Gravity data and P-wave teleseismic traveltime residuals from 29 temporary broad-band stations spread over the northern margin of the Gulf of Aden (Dhofar region, Oman) were used to image lithospheric structure. We apply a linear relationship between density and velocity to provide consistent density and velocity models from mid-crust down to about 250 km depth. The accuracy of the resulting models is investigated through a series of synthetic tests. The analysis of our resulting models shows: (1) crustal heterogeneities that match the main geological features at the surface; (2) the gravity edge effect and disparity in anomaly depth locations for layers at 20 and 50 km; (3) two low-velocity anomalies along the continuation of Socotra-Hadbeen and Alula-Fartak fracture zones between 60 and 200 km depth; and (4) evidence for partial melting (3-6 per cent) within these two negative anomalies. We discuss the presence of partial melting in terms of interaction between the Sheba ridge melts and its along-axis segmentation.

Lithospheric-folding-based understanding on the origin of the back-arc basaltic magmatism beneath Jeju volcanic island, Korea

NASA Astrophysics Data System (ADS)

Yun, S.; Shin, Y.; CHOI, K.; Koh, J.; Nakamura, E.; Na, S.

2012-12-01

Jeju Island is an intraplate volcanic island located at the eastern margin on the East Asia behind the Ryukyu Trench, the collisional/subduction boundary between the Eurasian plate and Philippine Sea plate. It is a symmetrical shield volcano, having numerous monogenetic cinder cones, over 365, on the Mt. Halla volcanic edifice. The basement rock mainly consists of Precambrian gneiss, Mesozoic granite and volcanic rocks. Unconsolidated sedimentary rock is found between basement rock and surface lava. The lava plateau is composed of voluminous basaltic lava flows, which extend to the coast region with a gentle slope. Based on the evidence obtained from volcanic stratigraphy, paleontology, and geochronology, the age of the Jeju basalts ranges from the early Pleistocene to Holocene(Historic). The alkaline and tholeiitic basalts exhibits OIB composition from intraplate volcanism which is not associated with plate subduction, while the basement xenolith contained in the volcanic rock indicates that there were volcanic activities associated with the Mesozoic plate subduction. The Geochemical characteristics have been explained with the plume model, lithospheric mantle origin, and melting of shallow asthenosphere by the rapid change of stress regimes between the collision of the India-Eurasia plates and subduction of the Pacific plate, while there has not been any geophysical investigation to disclose it. Compression near collisional plate boundaries causes lithospheric folding which results in the decrease of pressure beneath the ridge of the fold while the pressure increases beneath trough. The decompression beneath lithosphere is likely to accelerate basaltic magmatism along and below the ridge. We investigate the subsurface structure beneath Jeju volcanic island, South Korea and its vicinity and propose an alternative hypothesis that the basaltic magma beneath the island could be caused by episodic lithospheric folding. Unlike the prevailing hypothesis of the intraplate basaltic magmatism that requires extending lithosphere, ours can explain how the basaltic magma could be generated at the back-arc regions without the extension. A schematic diagram illustrating the magma formation beneath Arc and Back-arc regions due to the lithospheric folding: Basaltic magma could be generated at upper mantle beneath ridge of the lithospheric fold by decompression and pre-existing high temperature.

Familiar Phases: Receiver Function Study of the Lithospheric Structure Across the Eastern Margin of the Superior Craton

NASA Astrophysics Data System (ADS)

Levin, V. L.; Servali, A.; Dunham, B.; Klaser, M.

2015-12-01

A 1200 km long array of seismic observatories from James Bay to the Atlantic coast covers nearly 2 Ga in time, from the Archean Superior Province to the Paleozoic Appalachian Orogen. We use traditional (P-to-SV) receiver function analysis for detailed characterization of the lithospheric mantle along the array, focusing on the 5-15 s delay range where direct conversions from within the lithosphere and crustal multiples are expected.Superior craton sites show exceptionaly clear receiver functions dominated by the first crustal multiple. Also, a negative phase consistent with impedance decrease at the Mid-Lithospheric Discontinuity (~8 s delay) is observed north of 51°N, within the La Grande and Opinaca terranes of the Superior province. In the Opatica terrane further south we see a positive phase at similar delays instead. This implies a downward impedance increase 70-80 km deep within the lithosphere, consistent with the Hales discontinuity. In the Abitibi terrane just north of the Grenville Front we see evidence for two impedance drops in the 60-130 km depth range. Within the Proterozoic Grenvile province receiver functions vary with direction at individual sites, and lack regional consistency. Crustal multiples are noticeably weaker. South of 49°N we once again find negative phases in the 8-10 s delay range. While weak and directionally-dependent in the central Grenville province, these phases are clear near the Appalachian Front (AF), and are followed by positive phases, suggesting thin low-velocity layers in the lower part of the lithosphere. Similarity of receiver function signatures on opposite sides of the AF suggests continuity of the lithosphere beneath it.South of the AF and north of the Norumbega Fault Zone (NFZ) in Maine we find positive phases at ~10 s delays. The implied increase in impedance at ~75 km depth is puzzling. We also find previously-reported weak negative phases in the 6-8 s delay range. South of the NFZ a strong negative phase at ~9 s delay likely marks the bottom of the lithosphere.

Antarctic Lithosphere Studies: Progress, Problems and Promise

NASA Astrophysics Data System (ADS)

Dalziel, I. W. D.; Wilson, T. J.

2017-12-01

In the sixty years since the International Geophysical Year, studies of the Antarctic lithosphere have progressed from basic geological observations and sparse geophysical measurements to continental-scale datasets of radiometric dates, ice thickness, bedrock topography and characteristics, seismic imaging and potential fields. These have been augmented by data from increasingly dense broadband seismic and geodetic networks. The Antarctic lithosphere is known to have been an integral part, indeed a "keystone" of the Pangea ( 250-185Ma) and Gondwanaland ( 540-180 Ma) supercontinents. It is widely believed to have been part of hypothetical earlier supercontinents Rodinia ( 1.0-0.75 Ga) and Columbia (Nuna) ( 2.0-1.5 Ga). Despite the paucity of exposure in East Antarctica, the new potential field datasets have emboldened workers to extrapolate Precambrian geological provinces and structures from neighboring continents into Antarctica. Hence models of the configuration of Columbia and its evolution into Rodinia and Gondwana have been proposed, and rift-flank uplift superimposed on a Proterozoic orogenic root has been hypothesized to explain the Gamburtsev Subglacial Mountains. Mesozoic-Cenozoic rifting has imparted a strong imprint on the West Antarctic lithosphere. Seismic tomographic evidence reveals lateral variation in lithospheric thickness, with the thinnest zones within the West Antarctic rift system and underlying the Amundsen Sea Embayment. Upper mantle low velocity zones are extensive, with a deeper mantle velocity anomaly underlying Marie Byrd Land marking a possible mantle plume. Misfits between crustal motions measured by GPS and GIA model predictions can, in part, be linked with the changes in lithosphere thickness and mantle rheology. Unusually high uplift rates measured by GPS in the Amundsen region can be interpreted as the response of regions with thin lithosphere and weak mantle to late Holocene ice mass loss. Horizontal displacements across the TAM, which show a velocity gradient that points towards the reconstructed LGM ice load in West Antarctica, rather than radially away from it as expected, coincides with an extreme gradient in lithosphere thickness and shear wave speed, suggesting that GIA-induced mantle flow along the viscosity gradient may be driving the motions.

Regional patterns in the paragenesis and age of inclusions in diamond, diamond composition, and the lithospheric seismic structure of Southern Africa

NASA Astrophysics Data System (ADS)

Shirey, Steven B.; Harris, Jeffrey W.; Richardson, Stephen H.; Fouch, Matthew; James, David E.; Cartigny, Pierre; Deines, Peter; Viljoen, Fanus

2003-12-01

The Archean lithospheric mantle beneath the Kaapvaal-Zimbabwe craton of Southern Africa shows ±1% variations in seismic P-wave velocity at depths within the diamond stability field (150-250 km) that correlate regionally with differences in the composition of diamonds and their syngenetic inclusions. Seismically slower mantle trends from the mantle below Swaziland to that below southeastern Botswana, roughly following the surface outcrop pattern of the Bushveld-Molopo Farms Complex. Seismically slower mantle also is evident under the southwestern side of the Zimbabwe craton below crust metamorphosed around 2 Ga. Individual eclogitic sulfide inclusions in diamonds from the Kimberley area kimberlites, Koffiefontein, Orapa, and Jwaneng have Re-Os isotopic ages that range from circa 2.9 Ga to the Proterozoic and show little correspondence with these lithospheric variations. However, silicate inclusions in diamonds and their host diamond compositions for the above kimberlites, Finsch, Jagersfontein, Roberts Victor, Premier, Venetia, and Letlhakane do show some regional relationship to the seismic velocity of the lithosphere. Mantle lithosphere with slower P-wave velocity correlates with a greater proportion of eclogitic versus peridotitic silicate inclusions in diamond, a greater incidence of younger Sm-Nd ages of silicate inclusions, a greater proportion of diamonds with lighter C isotopic composition, and a lower percentage of low-N diamonds whereas the converse is true for diamonds from higher velocity mantle. The oldest formation ages of diamonds indicate that the mantle keels which became continental nuclei were created by middle Archean (3.2-3.3 Ga) mantle depletion events with high degrees of melting and early harzburgite formation. The predominance of sulfide inclusions that are eclogitic in the 2.9 Ga age population links late Archean (2.9 Ga) subduction-accretion events involving an oceanic lithosphere component to craton stabilization. These events resulted in a widely distributed younger Archean generation of eclogitic diamonds in the lithospheric mantle. Subsequent Proterozoic tectonic and magmatic events altered the composition of the continental lithosphere and added new lherzolitic and eclogitic diamonds to the already extensive Archean diamond suite.

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.

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.