Structure and Evolution of the Forearc-Arc Crust Along the Tonga-Kermadec Subduction System from Integrated Geophysical Data

NASA Astrophysics Data System (ADS)

Funnell, M.; Peirce, C.; Robinson, A. H.; Watts, A. B.; Grevemeyer, I.

2016-12-01

Variations in tectonic forces and inputs to subduction systems generate, alter, and deform overriding crustal material. Although these processes are recorded in the crustal structure of volcanic arcs and their backarcs, the continuous nature of plate convergence superimposes subsequent episodes of crustal evolution on older features. Seismic imaging at modern subduction zones enhances our understanding of forearc development and variations in present-day deformation caused by inherited structures. In 2011 a set of multichannel and wide-angle seismic profiles imaged the forearc-arc crust and upper mantle structure along the 2700 km-long NNE-SSW trending Tonga-Kermadec subduction zone. The Tonga forearc region exhibits an 100 km-wide, 2 km high bathymetric elevation, with a 3 km-thick upper and mid-crust (Vp <6 km s-1), and a lower-crustal ridge 30 km wide comprising velocities up to 7.4 km s-1 that characterize an extinct Eocene ( 50 Ma) arc. By contrast, the active arc is <10 km wide and exhibits lower-crustal velocities below 7.0 km s-1, most likely representing intermediate compositions. This structural change suggests significant evolution, alteration, and modification of the overriding crust since the onset of subduction at this margin. Gravity anomaly modelling suggests that the extinct arc within the Tonga forearc region comprises relatively dense mafic-ultrabasic material that extends south beneath the Kermadec forearc and terminates at 32°S. The apparent southern termination of the extinct arc coincides with the partitioning of morphological features at 32°S, including a 10-km westward-step of the active arc and a 1.5 km deeper backarc to the south. We propose that tectonic partitioning about the 32°S boundary is the result of variations in the inherited crustal structure, which is divided by the presence and absence, to the north and south respectively, of the extinct volcanic arc.

Upper crustal densities derived from sea floor gravity measurements: Northern Juan De Fuca Ridge

USGS Publications Warehouse

Holmes, Mark L.; Johnson, H. Paul

1993-01-01

A transect of sea floor gravity stations has been analyzed to determine upper crustal densities on the Endeavour segment of the northern Juan de Fuca Ridge. Data were obtained using ALVIN along a corridor perpendicular to the axis of spreading, over crustal ages from 0 to 800,000 years. Calculated elevation factors from the gravity data show an abrupt increase in density with age (distance) for the upper 200 m of crust. This density change is interpreted as a systematic reduction in bulk porosity of the upper crustal section, from 23% for the axial ridge to 10% for the off-axis flanking ridges. The porosity decrease is attributed to the collapse and filling of large-scale voids as the abyssal hills move out of the crustal formation zone. Forward modeling of a plausible density structure for the near-axis region agrees with the observed anomaly data only if the model includes narrow, along-strike, low-density regions adjacent to both inner and outer flanks of the abyssal hills. The required low density zones could be regions of systematic upper crustal fracturing and faulting that were mapped by submersible observers and side-scan sonar images, and whose presence was suggested by the distribution of heat flow data in the same area.

Crustal structure across the NE Tibetan Plateau and Ordos Block from the joint inversion of receiver functions and Rayleigh-wave dispersions

NASA Astrophysics Data System (ADS)

Li, Yonghua; Wang, Xingchen; Zhang, Ruiqing; Wu, Qingju; Ding, Zhifeng

2017-05-01

We investigated the crustal structure at 34 stations using the H-κ stacking method and jointly inverting receiver functions with Rayleigh-wave phase and group velocities. These seismic stations are distributed along a profile extending across the Songpan-Ganzi Terrane, Qinling-Qilian terranes and southwestern Ordos Basin. Our results reveal the variation in crustal thickness across this profile. We found thick crust beneath the Songpan-Ganzi Terrane (47-59 km) that decreases to 45-47 km in the west Qinling and Qilian terranes, and reaches its local minimum beneath the southwestern Ordos Block (43-51 km) at an average crustal thickness of 46.7 ± 2.5 km. A low-velocity zone in the upper crust was found beneath most of the stations in NE Tibet, which may be indicative of partial melt or a weak detachment layer. Our observations of low to moderate Vp/Vs (1.67-1.79) represent a felsic to intermediate crustal composition. The shear velocity models estimated from joint inversions also reveal substantial lateral variations in velocity beneath the profile, which is mainly reflected in the lower crustal velocities. For the Ordos Block, the average shear wave velocities below 20 km are 3.8 km/s, indicating an intermediate-to-felsic lower crust. The thick NE Tibet crust is characterized by slow shear wave velocities (3.3-3.6 km/s) below 20 km and lacks high-velocity material (Vs ≥ 4.0 km/s) in the lower crust, which may be attributed to mafic lower crustal delamination or/and the thickening of the upper and middle crust.

Numerical Mantle Convection Models of Crustal Formation in an Oceanic Environment in the Early Earth

NASA Astrophysics Data System (ADS)

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

2001-12-01

The generation of basaltic crust in the early Earth by partial melting of mantle rocks, subject to investigation in this study, is thought to be a first step in the creation of proto-continents (consisting largely of felsic material), since partial melting of basaltic material was probably an important source for these more evolved rocks. In the early Archean the earth's upper mantle may have been hotter than today by as much as several hundred degrees centigrade. As a consequence, partial melting in shallow convective upwellings would have produced a layering of basaltic crust and underlying depleted (lherzolitic-harzburgitic) mantle peridotite which is much thicker than found under modern day oceanic ridges. When a basaltic crustal layer becomes sufficiently thick, a phase transition to eclogite may occur in the lower parts, which would cause delamination of this dense crustal layer and recycling of dense eclogite into the upper mantle. This recycling mechanism may have contributed significantly to the early cooling of the earth during the Archean (Vlaar et al., 1994). The delamination mechanism which limits the build-up of a thick basaltic crustal layer is switched off after sufficient cooling of the upper mantle has taken place. We present results of numerical modelling experiments of mantle convection including pressure release partial melting. The model includes a simple approximate melt segregation mechanism and basalt to eclogite phase transition, to account for the dynamic accumulation and recycling of the crust in an upper mantle subject to secular cooling. Finite element methods are used to solve for the viscous flow field and the temperature field, and lagrangian particle tracers are used to represent the evolving composition due to partial melting and accumulation of the basaltic crust. We find that this mechanism creates a basaltic crust of several tens of kilometers thickness in several hundreds of million years. This is accompanied by a cooling of some hundred degrees centigrade. Vlaar, N.J., P.E. van Keken and A.P. van den Berg (1994), Cooling of the Earth in the Archaean: consequences of pressure-release melting in a hotter mantle, Earth and Planetary Science Letters, vol 121, pp. 1-18

Quantitative analysis of surface deformation and ductile flow in complex analogue geodynamic models based on PIV method.

NASA Astrophysics Data System (ADS)

Krýza, Ondřej; Lexa, Ondrej; Závada, Prokop; Schulmann, Karel; Gapais, Denis; Cosgrove, John

2017-04-01

Recently, a PIV (particle image velocimetry) analysis method is optical method abundantly used in many technical branches where material flow visualization and quantification is important. Typical examples are studies of liquid flow through complex channel system, gas spreading or combustion problematics. In our current research we used this method for investigation of two types of complex analogue geodynamic and tectonic experiments. First class of experiments is aimed to model large-scale oroclinal buckling as an analogue of late Paleozoic to early Mesozoic evolution of Central Asian Orogenic Belt (CAOB) resulting from nortward drift of the North-China craton towards the Siberian craton. Here we studied relationship between lower crustal and lithospheric mantle flows and upper crustal deformation respectively. A second class of experiments is focused to more general study of a lower crustal flow in indentation systems that represent a major component of some large hot orogens (e.g. Bohemian massif). The most of simulations in both cases shows a strong dependency of a brittle structures shape, that are situated in upper crust, on folding style of a middle and lower ductile layers which is influenced by rheological, geometrical and thermal conditions of different parts across shortened domain. The purpose of PIV application is to quantify material redistribution in critical domains of the model. The derivation of flow direction and calculation of strain-rate and total displacement field in analogue experiments is generally difficult and time-expensive or often performed only on a base of visual evaluations. PIV method operates with set of images, where small tracer particles are seeded within modeled domain and are assumed to faithfully follow the material flow. On base of pixel coordinates estimation the material displacement field, velocity field, strain-rate, vorticity, tortuosity etc. are calculated. In our experiments we used velocity field divergence to quantify the redistribution and flow of anatectic lower crust and to evaluate upper crust thickenning and topography evolution. As this method is very sensitive to resolution and color contrast of obtained images and used materials are mostly uniform within individual rheological layers and domains, we utilized various markers as flakes of a fluorescent wax or glitter to increase overall sensitivity. Applying this method to oroclinal buckling experiments we derived velocity field divergence associated with upper crustal deformation and evolution of topography. Scaled, dimensionless negative values of divergence reach minimum (˜ -1) in two elongated domains propagating from inflection area of modeled orocline. These values correlate with significant upper crust material removing and-or with redistribution of crustal material associated with formed pop-up and pop-down structures. Maximum positive values (˜ 0.1) correspond with material spreading alongside forming platforms that are situated in foreland of maximum elevations. Application of PIV method on lateral view, where ductile middle and lower crust is vertically folded during lithosphere shortening and indentation, revealed possibility to track melt migration from base of lower crust through interlimb area towards hinge zone of individual folds. Simultaneously with folds locking and material accumulation, whole structures are exhumed at the middle crust level. Melt flow and heat exchange with surrounding environment is responsible for increased plasticity of the middle crust marked by higher strain-rates observed inside fold envelope. It is also responsible for significant elevation above hinges during later stages of model evolution. Heterogeneous nature of deformation is well documented by heterogeneities in derived divergence field within folds interiors. Our results show distinct advantages of PIV method for post-processing of geodynamic and tectonic analogue models and demonstrate great potential of this method for quantitative processing of wide spectrum of analogue approaches to different natural systems.

Geochemical Relationships between Middle- to Upper-Crustal Exposures of the Alisitos Oceanic Arc (Baja California, Mexico): An Outstanding Field Analog to Active Extensional Oceanic Arcs

NASA Astrophysics Data System (ADS)

Morris, R.; DeBari, S. M.; Busby, C.; Medynski, S.

2016-12-01

The southern volcano-bounded basin of the Rosario segment of the Cretaceous Alisitos oceanic arc provides outstanding 3-D exposures of an extensional arc, where crustal generation processes are recorded in the upper-crustal volcanic units and underlying middle-crustal plutonic rocks. Geochemical linkages between exposed crustal levels provide an analog for extensional arc systems such as the Izu-Bonin-Mariana (IBM) Arc. Upper-crustal units comprise a 3-5 km thick volcanic-volcaniclastic stratigraphy with hypabyssal intrusions. Deep-seated plutonic rocks intrude these units over a transition of <500m, where rafted volcanic blocks and evidence of magma mingling are exposed. Thermobarometry suggests <6 km emplacement depths. Compositional ranges (basalt to rhyolite) and mineral assemblages are similar in both middle-crustal and upper-crustal units, with striking compositional overlap. The most mafic compositions occur in upper-crustal hypabyssal units, and as amphibole cumulates in the plutonic rocks ( 51% SiO2). The most felsic compositions occur in welded ignimbrites and a tonalite pluton ( 71% SiO2). All units are low K with flat REE patterns, and show LILE enrichment and HFSE depletion. Trace element ratios show limited variation throughout the crustal section. Zr/Y and Nb/Y ratios are similar to the Izu active ( 3 Ma to present) zone of extension immediately behind the arc front, suggesting comparable mantle melt % during extension. Th/Zr ratios are more enriched in Alisitos compared to Izu, suggesting greater subducted sediment input. The Alisitos crustal section shows a limited range in ɛNd (5.7-7.1), but a wider range in 87Sr/86Sr (0.7035-0.7055) and 206Pb/204Pb (18.12-19.12); the latter is likely alteration effects. Arc magmas were derived from a subduction-modified MORB mantle source, less depleted than Izu arc front and less enriched than the rear arc, but is a good match with the zone of extension that lies between. Differentiation occurred in a closed system (i.e., fractional crystallization/self-melting with back mixing), producing the entire crustal section in <3 Ma.

Crust and upper-mantle structure of Wanganui Basin and southern Hikurangi margin, North Island, New Zealand as revealed by active source seismic data

NASA Astrophysics Data System (ADS)

Tozer, B.; Stern, T. A.; Lamb, S. L.; Henrys, S. A.

2017-11-01

Wide-angle reflection and refraction data recorded during the Seismic Array HiKurangi Experiment (SAHKE) are used to constrain the crustal P-wave velocity (Vp) structure along two profiles spanning the length and width of Wanganui Basin, located landwards of the southern Hikurangi subduction margin, New Zealand. These models provide high-resolution constraints on the structure and crustal thickness of the overlying Australian and subducted Pacific plates and plate interface geometry. Wide-angle reflections are modelled to show that the subducted oceanic Pacific plate crust is anomalously thick (∼10 km) below southern North Island and is overlain by a ∼1.5-4.0 km thick, low Vp (4.8-5.4 km s-1) layer, interpreted as a channel of sedimentary material, that persists landwards at least as far as Kapiti Island. Distinct near vertical reflections from onshore shots identify a ∼4 km high mound of low-velocity sedimentary material that appears to underplate the overlying Australian plate crust and is likely to contribute to local rock uplift along the Axial ranges. The overriding Australian plate Moho beneath Wanganui Basin is imaged as deepening southwards and reaches a depth of at least 36.4 km. The Moho shape approximately mirrors the thickening of the basin sediments, suggestive of crustal downwarping. However, the observed crustal thickness variation is insufficient to explain the large negative Bouguer gravity anomaly (-160 mGal) centred over the basin. Partial serpentinization within the upper mantle with a concomitant density decrease is one possible way of reconciling this anomaly.

Active Transtensional Tectonics Due to Differentially Rotating Upper Crustal Blocks East of the Eastern Himalayan syntaxis, Yunnan Province, China.

NASA Astrophysics Data System (ADS)

Studnikigizbert, C.; Eich, L.; King, R.; Burchfiel, B. C.; Chen, Z.; Chen, L.

2004-12-01

Seismological (Holt et. al. 1996), geodetic (King et. al. 1996, Chen et. al. 2000) and geological (Wang et. al. 1995, Wang and Burchfiel 2002) studies have shown that upper crustal material north and east of the eastern Himalayan syntaxis rotates clockwise about the syntaxis, with the Xianshuihe fault accommodating most of this motion. Within the zone of rotating material, however, deformation is not completely homogenous, and numerous differentially rotating small crustal fragments are recognised. We combine seismic (CSB and Harvard CMT catalogues), geodetic (CSB and MIT-Chengdu networks), remote sensing, compilation of existing regional maps and our own detailed field mapping to characterise the active tectonics of a clockwise rotating crustal block between Zhongdian and Dali. The northeastern boundary is well-defined by the northwest striking left-lateral Zhongdian and Daju faults. The eastern boundary, on the other hand, is made up of a 80 km wide zone characterised by north-south trending extensional basins linked by NNE trending left-lateral faults. Geological mapping suggests that strain is accommodated by three major transtensional fault systems: the Jianchuan-Lijiang, Heqing and Chenghai fault systems. Geodetic data indicates that this zone accommodates 10 +/- 1.4 mm/year of E-W extension, but strain may be (presently) preferentially partitioned along the easternmost (Chenghai) fault. Not all geodetic velocities are consistent with geological observations. In particular, rotation and concomitant transtension are somehow transferred across the Red River-Tongdian faults to Nan Tinghe fault with no apparent accommodating structures. Rotation and extension is surmised to be related to the northward propagation of the syntaxis.

The role of harzburgite layers in the morphology of subducting plates and the behavior of oceanic crustal layers

NASA Astrophysics Data System (ADS)

Yoshida, Masaki

2014-05-01

Previous numerical studies of mantle convection focusing on subduction dynamics have indicated that the viscosity contrast between the subducting plate and the surrounding mantle have a primary effect on the behavior of subducting plates. The seismically observed plate stagnation at the base of the mantle transition zone (MTZ) under the Western Pacific and Eastern Eurasia is considered to mainly result from a viscosity increase at the ringwoodite to perovskite + magnesiowüstite (Rw→Pv+Mw) phase decomposition boundary, i.e., the boundary between the upper and lower mantle. The harzburgite layer, which is sandwiched between basaltic crust and depleted peridotite (lherzolite) layers, is a key component of highly viscous, cold oceanic plates. However, the possible sensitivity of the effective viscosity of harzburgite layers in the morphology of subducting plates that are flattened in the MTZ and/or penetrated in the lower mantle has not been examined systematically in previous three-dimensional (3D) numerical modeling studies that consider the viscosity increase at the boundary between the upper and lower mantle. In this study, in order to investigate the role of harzburgite layers in the morphology of subducting plates and the behavior of oceanic crustal layers, I performed a series of numerical simulations of mantle convection with semi-dynamic plate subduction in 3D regional spherical-shell geometry. The results show that a buckled crustal layer is observed under the "heel" of the stagnant slab that begins to penetrate into the lower mantle, regardless of the magnitude of the viscosity contrast between the harzburgite layer and the underlying mantle, when the factor of viscosity increase at the boundary of the upper and lower mantle is larger than 60-100. As the viscosity contrast between the harzburgite layer and the underlying mantle increases, the curvature of buckling is larger. When the viscosity increase at the boundary of the upper and lower mantle and the viscosity contrast between the harzburgite layer and the underlying mantle are larger, the volumes of crustal and harzburgite materials trapped in the mantle transition zone (MTZ) are also larger, although almost all of the materials penetrate into the lower mantle. These materials are trapped in the MTZ for over tens of millions of years. The bending of crustal layers numerically observed in the present study is consistent with seismological evidence that there is a piece of subducted oceanic crust in the uppermost lower mantle beneath the subducting slab under the Mariana trench [Niu et al., 2003, JGR]. The results of the present study suggest that when the viscosity increase at the boundary of the upper and lower mantle is larger than 60-100, a seismically observed stagnant slab is reproduced. This result is consistent with the previous independent geodynamic studies. For instance, a 2D geodynamic model with lateral viscosity variations suggested that it would need to be substantially greater than 30, say, around 100, to explain the positive geoid anomaly in the subduction zones where the subducting slab reaches the boundary between the upper and lower mantle such as that of the western Pacific [Tosi et al., 2009, GJI]. References: [1] Tajima, F. Yoshida, M. and Ohtani, E., Conjecture with water and rheological control for subducting slab in the mantle transition zone, Geoscience Frontiers, doi:10.1016/j.gsf.2013.12.005, 2014. [2] Yoshida, M. The role of harzburgite layers in the morphology of subducting plates and the behavior of oceanic crustal layers, Geophys. Res. Lett., 40(20), 5387-5392, doi:10.1002/2013GL057578, 2013. [3] Yoshida, M. and Tajima, F., On the possibility of a folded crustal layer stored in the hydrous mantle transition zone, Phys. Earth Planet. Inter., 219, 34-48, doi:10.1016/j.pepi.2013.03.004, 2013.

Plume-driven plumbing and crustal formation in Iceland

USGS Publications Warehouse

Allen, R.M.; Nolet, G.; Morgan, W.J.; Vogfjord, K.; Nettles, M.; Ekstrom, G.; Bergsson, B.H.; Erlendsson, P.; Foulger, G.R.; Jakobsdottir, S.; Julian, B.R.; Pritchard, M.; Ragnarsson, S.; Stefansson, R.

2002-01-01

Through combination of surface wave and body wave constraints we derive a three-dimensional (3-D) crustal S velocity model and Moho map for Iceland. It reveals a vast plumbing system feeding mantle plume melt into upper crustal magma chambers where crustal formation takes place. The method is based on the partitioned waveform inversion to which we add additional observations. Love waves from six local events recorded on the HOTSPOT-SIL networks are fitted, Sn travel times from the same events measured, previous observations of crustal thickness are added, and all three sets of constraints simultaneously inverted for our 3-D model. In the upper crust (0-15 km) an elongated low-velocity region extends along the length of the Northern, Eastern and Western Neovolcanic Zones. The lowest velocities (-7%) are found at 5-10 km below the two most active volcanic complexes: Hekla and Bardarbunga-Grimsvotn. In the lower crust (>15 km) the low-velocity region can be represented as a vertical cylinder beneath central Iceland. The low-velocity structure is interpreted as the thermal halo of pipe work which connects the region of melt generation in the uppermost mantle beneath central Iceland to active volcanoes along the neovolcanic zones. Crustal thickness in Iceland varies from 15-20 km beneath the Reykjanes Peninsula, Krafla and the extinct Snfellsnes rift zone, to 46 km beneath central Iceland. The average crustal thickness is 29 km. The variations in thickness can be explained in terms of the temporal variation in plume productivity over the last ~20 Myr, the Snfellsnes rift zone being active during a minimum in plume productivity. Variations in crustal thickness do not depart significantly from an isostatically predicted crustal thickness. The best fit linear isostatic relation implies an average density jump of 4% across the Moho. Rare earth element inversions of basalt compositions on Iceland suggest a melt thickness (i.e., crustal thickness) of 15-20 km, given passive upwelling. The observed crustal thickness of up to 46 km implies active fluxing of source material through the melt zone by the mantle plume at up to 3 times the passive rate.

Crustal and Upper Mantle Structure from Joint Inversion of Body Wave and Gravity Data

DTIC Science & Technology

2012-09-01

CRUSTAL AND UPPER MANTLE STRUCTURE FROM JOINT INVERSION OF BODY WAVE AND GRAVITY DATA Eric A. Bergman1, Charlotte Rowe2, and Monica Maceira2...for these events include many readings of direct crustal P and S phases, as well as regional (Pn and Sn) and teleseismic phases. These data have been...the usefulness of the gravity data, we apply high-pass filtering, yielding gravity anomalies that possess higher resolving power for crustal and

Crustal structure beneath the Kenya Rift from axial profile data

USGS Publications Warehouse

Mechie, J.; Keller, Gordon R.; Prodehl, C.; Gaciri, S.; Braile, L.W.; Mooney, W.D.; Gajewski, D.; Sandmeier, K.-J.

1994-01-01

Modelling of the KRISP 90 axial line data shows that major crustal thinning occurs along the axis of the Kenya Rift from Moho depths of 35 km in the south beneath the Kenya Dome in the vicinity of Lake Naivasha to 20 km in the north beneath Lake Turkana. Low Pn velocities of 7.5-7.7 km/s are found beneath the whole of the axial line. The results indicate that crustal extension increases to the north and that the low Pn velocities are probably caused by magma (partial melt) rising from below and being trapped in the uppermost kilometres of the mantle. Along the axial line, the rift infill consisting of volcanics and a minor amount of sediments varies in thickness from zero where Precambrian crystalline basement highs occur to 5-6 km beneath the lakes Turkana and Naivasha. Analysis of the Pg phase shows that the upper crystalline crust has velocities of 6.1-6.3 km/s. Bearing in mind the Cainozoic volcanism associated with the rift, these velocities most probably represent Precambrian basement intruded by small amounts of igneous material. The boundary between the upper and lower crusts occurs at about 10 km depth beneath the northern part of the rift and 15 km depth beneath the southern part of the rift. The upper part of the lower crust has velocities of 6.4-6.5 km/s. The basal crustal layer which varies in thickness from a maximum of 2 km in the north to around 9 km in the south has a velocity of about 6.8 km/s. ?? 1994.

Rheological and physical characteristics of crustal-scaled materials for centrifuge analogue modelling

NASA Astrophysics Data System (ADS)

Waffle, Lindsay; Godin, Laurent; Harris, Lyal B.; Kontopoulou, M.

2016-05-01

We characterize a set of analogue materials used for centrifuge analogue modelling simulating deformation at different levels in the crust simultaneously. Specifically, we improve the rheological characterization in the linear viscoelastic region of materials for the lower and middle crust, and cohesive synthetic sands without petroleum-binding agents for the upper crust. Viscoelastic materials used in centrifuge analogue modelling demonstrate complex dynamic behaviour, so viscosity alone is insufficient to determine if a material will be an effective analogue. Two series of experiments were conducted using an oscillating bi-conical plate rheometer to measure the storage and loss moduli and complex viscosities of several modelling clays and silicone putties. Tested materials exhibited viscoelastic and shear-thinning behaviour. The silicone putties and some modelling clays demonstrated viscous-dominant behaviour and reached Newtonian plateaus at strain rates < 0.5 × 10-2 s-1, while other modelling clays demonstrated elastic-dominant power-law relationships. Based on these results, the elastic-dominant modelling clay is recommended as an analogue for basement cratons. Inherently cohesive synthetic sands produce fine-detailed fault and fracture patterns, and developed thrust, strike-slip, and extensional faults in simple centrifuge test models. These synthetic sands are recommended as analogues for the brittle upper crust. These new results increase the accuracy of scaling analogue models to prototype. Additionally, with the characterization of three new materials, we propose a complete lithospheric profile of analogue materials for centrifuge modelling, allowing future studies to replicate a broader range of crustal deformation behaviours.

Building the Pamir-Tibet Plateau: Eo-Oligocene Crustal Stacking and Orogen Parallel Evasion of Upper and Middle Crustal Material in the Pamir

NASA Astrophysics Data System (ADS)

Rutte, D.; Ratschbacher, L.; Stübner, K.; Schneider, S.

2015-12-01

The gneisses of the Central Pamir Domes and their cover document crustal stacking of a ~10 km thick Ediacaran-Paleogene succession to a thickness of >35 km and their exhumation along bi-vergent, top-to-N and top-to S, normal-sense shear zones. The giant South Pamir Shakhdara-Alichur gneiss-dome system formed similarly by N-S extension along bivergent detachments. Prograde amphibolite-facies metamorphism in the domes and low-grade metamorphism in their hanging wall is dated at ~40 Ma (Lu-Hf garnet, U-Pb titanite) [Smit et al., 2014; Stearns et al., 2015] and ~33 Ma (K/Ar sericite). Retrograde metamorphism―driven by crustal extension―started at ~21 Ma (multi-method thermochronology; Stearns et al.[2013]). These Gneiss Domes offer a unique window into the Eocene-Miocene state of the Asian middle crust of the Pamir-Tibet Plateau. Top-to-N thrust stacking accommodated thickening in the upper crust, with displacements of single thrust sheets of > 30 and > 19 km. At depth, ductile flow formed km-scale recumbent fold nappes. We reconstruct their geometry by structural mapping and U-Pb zircon dating, documenting repetition of metatuffite, and paragneiss layers. In the interior of the domes, amphibolite-facies deformation fabrics with prograde kyanite define an E-W stretching lineation. Associated microstructures indicate top-to-E and top-to-W shear senses. Chocolate tablet boudinage indicate vertical flattening during bulk crustal thickening. We suggest that prograde E-W stretching relates to an early orogen-parallel flow component in the middle crust, contemporaneous with crustal stacking during bulk top-to-N convergence prior to ~21 Ma. Material likely evaded laterally out of the Pamir, contributing to >60 km thick crust in the Hindu Kush, west of the India-Asia frontal collision. In the Neogene crust extruded laterally from the Pamir Plateau to the west by dextral wrenching and E-W extension; this component of deformation is accommodated by E-W shortening in the Afghan-Tajik Depression.

What drives the Tibetan crust to the South East Asia? Role of upper mantle density discontinuities as inferred from the continental geoid anomalies

NASA Astrophysics Data System (ADS)

Rajesh, S.

2012-04-01

The Himalaya-Tibet orogen formed as a result of the northward convergence of India into the Asia over the past 55 Ma had caused the north south crustal shortening and Cenozoic upliftment of the Tibetan plateau, which significantly affected the tectonic and climatic framework of the Asia. Geodetic measurements have also shown eastward crustal extrusion of Tibet, especially along major east-southeast strike slip faults at a slip rate of 15-20 mm a-1 and around 40 mm a-1. Such continental scale deformations have been modeled as block rotation by fault boundary stresses developed due to the India-Eurasia collision. However, the Thin Sheet model explained the crustal deformation mechanism by considering varying gravitational potential energy arise out of varying crustal thickness of the viscous lithosphere. The Channel Flow model, which also suggests extrusion is a boundary fault guided flow along the shallow crustal brittle-ductile regime. Although many models have proposed, but no consensus in these models to explain the dynamics of measured surface geodetic deformation of the Tibetan plateau. But what remains conspicuous is the origin of driving forces that cause the observed Tibetan crustal flow towards the South East Asia. Is the crustal flow originated only because of the differential stresses that developed in the shallow crustal brittle-ductile regime? Or should the stress transfer to the shallow crustal layers as a result of gravitational potential energy gradient driven upper mantle flow also to be accounted. In this work, I examine the role of latter in the light of depth distribution of continental geoid anomalies beneath the Himalaya-Tibet across major upper mantle density discontinuities. These discontinuity surfaces in the upper mantle are susceptible to hold the plastic deformation that may occur as a result of the density gradient driven flow. The distribution of geoid anomalies across these density discontinuities at 220, 410 and 660 km depth in the upper mantle beneath the Himalaya-Tibet has been studied by analyzing the geoid undulation data obtained from various satellite geodetic missions along with the recent and old (EGM2008 and EGM2006) Earth Gravity models. Results show that the net geoid anomaly varies from -65 m to -20 m, which signify a density stratified upper mantle beneath the Himalaya-Tibet and the same has been confirmed from the results of regional seismic tomography studies. The density anomaly distribution beneath Tibet from 163 km depth to its upper mantle thickness of 1063 km show a strong NW-SE elliptically oriented positive geoid anomalies of magnitude around 40 meter. Asymmetric density anomaly gradient have been observed along the Himalayan arc from west to east as well as across the arc from north to south. This caused differential gravitational potential gradient and hence an elliptical flow structure of the Tibetan continental mantle along the resultant NW-SE direction, which is in concurrence with the observed present day direction of the Tibetan crustal flow. Thus the geoid anomalies distributed at various depth ranges show how the gradient in the upper mantle gravitational potential energy, especially across the deformed discontinuity surface, is significant in determining the transfer of deviatoric stresses and providing traction to the flow of crustal layers of the Tibetan Plateau. This suggests the viscous flow model could be a preferable choice, which could better accommodate the dynamics of the upper mantle, in explaining the crustal extrusion processes of the Tibetan Plateau.

Seismic evidence for widespread western-US deep-crustal deformation caused by extension

USGS Publications Warehouse

Moschetti, M.P.; Ritzwoller, M.H.; Lin, F.; Yang, Y.

2010-01-01

Laboratory experiments have established that many of the materials comprising the Earth are strongly anisotropic in terms of seismic-wave speeds. Observations of azimuthal and radial anisotropy in the upper mantle are attributed to the lattice-preferred orientation of olivine caused by the shear strains associated with deformation, and provide some of the most direct evidence for deformation and flow within the Earths interior. Although observations of crustal radial anisotropy would improve our understanding of crustal deformation and flow patterns resulting from tectonic processes, large-scale observations have been limited to regions of particularly thick crust. Here we show that observations from ambient noise tomography in the western United States reveal strong deep (middle to lower)-crustal radial anisotropy that is confined mainly to the geological provinces that have undergone significant extension during the Cenozoic Era (since 65 Myr ago). The coincidence of crustal radial anisotropy with the extensional provinces of the western United States suggests that the radial anisotropy results from the lattice-preferred orientation of anisotropic crustal minerals caused by extensional deformation. These observations also provide support for the hypothesis that the deep crust within these regions has undergone widespread and relatively uniform strain in response to crustal thinning and extension. ?? 2010 Macmillan Publishers Limited. All rights reserved.

Seismic Tomography of the Arabian-Eurasian Collision Zone and Surrounding Areas

DTIC Science & Technology

2010-05-20

zone. The crustal models correlate well with geologic and tectonic features. The upper mantle tomograms show the images of the subducted Neotethys...We first obtain Pn and Sn velocities using local and regional arrival time data. Second, we obtain the 3-D crustal P and S velocity models...teleseismic tomography provides a high-resolution, 3-D P-wave velocity model for the crust, upper mantle, and the transition zone. The crustal models

Upper mantle diapers, lower crustal magmatic underplating, and lithospheric dismemberment of the Great Basin and Colorado Plateau regions, Nevada and Utah; implications from deep MT resistivity surveying

NASA Astrophysics Data System (ADS)

Wannamaker, P. E.; Doerner, W. M.; Hasterok, D. P.

2005-12-01

In the rifted Basin and Range province of the southwestern U.S., a common faulting model for extensional basins based e.g. on reflection seismology data shows dominant displacement along master faults roughly coincident with the main topographic scarp. On the other hand, complementary data such as drilling, earthquake focal mechanisms, volcanic occurrences, and trace indicators such as helium isotopes suggest that there are alternative geometries of crustal scale faulting and material transport from the deep crust and upper mantle in this province. Recent magnetotelluric (MT) profiling results reveal families of structures commonly dominated by high-angle conductors interpreted to reflect crustal scale fault zones. Based mainly on cross cutting relationships, these faults appear to be late Cenozoic in age and are of low resistivity due to fluids or alteration (including possible graphitization). In the Ruby Mtns area of north-central Nevada, high angle faults along the margins of the core complex connect from near surface to a regional lower crustal conductor interpreted to contain high-temperature fluids and perhaps melts. Such faults may exemplify the high angle normal faults upon which the major earthquakes of the Great Basin appear to nucleate. A larger-scale transect centered on Dixie Valley shows major conductive crustal-scale structures connecting to conductive lower crust below Dixie Valley, the Black Rock desert in NW Nevada, and in east-central Nevada in the Monitor-Diamond Valley area. In the Great Basin-Colorado Plateau transition of Utah, the main structures revealed are a series of nested low-angle detachment structures underlying the incipient development of several rift grabens. All these major fault zones appear to overlie regions of particularly conductive lower crust interpreted to be caused by recent basaltic underplating. In the GB-CP transition, long period data show two, low-resistivity upper mantle diapirs underlying the concentrated conductive lower crust and nested faults, and these are advanced as melt source regions for the underplating. MT, with its wide frequency bandwidth, allows views of nearly a complete melting and emplacement process, from mantle source region, through lower crustal intrusion, to brittle regime deformational response.

Global lunar crust - Electrical conductivity and thermoelectric origin of remanent magnetism

NASA Technical Reports Server (NTRS)

Dyal, P.; Parkin, C. W.; Daily, W. D.

1977-01-01

An upper limit is placed on the average crustal conductivity from an investigation of toroidal (V x B) induction in the moon, using ten-minute data intervals of simultaneous lunar orbiting and surface magnetometer data. Crustal conductivity is determined as a function of crust thickness. For an average global crust thickness of about 80 km, the crust surface electrical conductivity is of the order of 1 hundred millionth mho/m. The toroidal-induction results lower the surface-conductivity limit obtained from poloidal-induction results by approximately four orders of magnitude. In addition, a thermoelectric (Seebeck effect) generator model is presented as a magnetic-field source for thermoremanent magnetization of the lunar crust during its solidification and cooling. Magnetic fields from 1000 to 10,000 gammas are calculated for various crater and crustal geometries. Solidified crustal material cooling through the iron Curie temperature in the presence of such ancient lunar fields could have received thermoremanent magnetization consistent with that measured in most returned lunar samples.

Evidence for fluid and melt generation in response to an asthenospheric upwelling beneath the Hangai Dome, Mongolia

NASA Astrophysics Data System (ADS)

Comeau, Matthew J.; Käufl, Johannes S.; Becken, Michael; Kuvshinov, Alexey; Grayver, Alexander V.; Kamm, Jochen; Demberel, Sodnomsambuu; Sukhbaatar, Usnikh; Batmagnai, Erdenechimeg

2018-04-01

The Hangai Dome, Mongolia, is an unusual high-elevation, intra-continental plateau characterized by dispersed, low-volume, intraplate volcanism. Its subsurface structure and its origin remains unexplained, due in part to a lack of high-resolution geophysical data. Magnetotelluric data along a ∼610 km profile crossing the Hangai Dome were used to generate electrical resistivity models of the crust and upper mantle. The crust is found to be unexpectedly heterogeneous. The upper crust is highly resistive but contains several features interpreted as ancient fluid pathways and fault zones, including the South Hangai fault system and ophiolite belt that is revealed to be a major crustal boundary. South of the Hangai Dome a clear transition in crustal properties is observed which reflects the rheological differences across accreted terranes. The lower crust contains discrete zones of low-resistivity material that indicate the presence of fluids and a weakened lower crust. The upper mantle contains a large low-resistivity zone that is consistent with the presence of partial melt within an asthenospheric upwelling, believed to be driving intraplate volcanism and supporting uplift.

Continental crust

USGS Publications Warehouse

Pakiser, L.C.

1964-01-01

The structure of the Earth’s crust (the outer shell of the earth above the M-discontinuity) has been intensively studied in many places by use of geophysical methods. The velocity of seismic compressional waves in the crust and in the upper mantle varies from place to place in the conterminous United States. The average crust is thick in the eastern two-thirds of the United States, in which the crustal and upper-mantle velocities tend to be high. The average crust is thinner in the western one-third of the United States, in which these velocities tend to be low. The concept of eastern and western superprovinces can be used to classify these differences. Crustal and upper-mantle densities probably vary directly with compressional-wave velocity, leading to the conclusion that isostasy is accomplished by the variation in densities of crustal and upper-mantle rocks as well as in crustal thickness, and that there is no single, generally valid isostatic model. The nature of the M-discontinuity is still speculative.

Crustal structure in the Elko-Carlin Region, Nevada, during Eocene gold mineralization: Ruby-East Humboldt metamorphic core complex as a guide to the deep crust

USGS Publications Warehouse

Howard, K.A.

2003-01-01

The deep crustal rocks exposed in the Ruby-East Humboldt metamorphic core complex, northeastern Nevada, provide a guide for reconstructing Eocene crustal structure ~50 km to the west near the Carlin trend of gold deposits. The deep crustal rocks, in the footwall of a west-dipping normal-sense shear system, may have underlain the Pinon and Adobe Ranges about 50 km to the west before Tertiary extension, close to or under part of the Carlin trend. Eocene lakes formed on the hanging wall of the fault system during an early phase of extension and may have been linked to a fluid reservoir for hydrothermal circulation. The magnitude and timing of Paleogene extension remain indistinct, but dikes and tilt axes in the upper crust indicate that spreading was east-west to northwest-southeast, perpendicular to a Paleozoic and Mesozoic orogen that the spreading overprinted. High geothermal gradients associated with Eocene or older crustal thinning may have contributed to hydrothermal circulation in the upper crust. Late Eocene eruptions, upper crustal dike intrusion, and gold mineralization approximately coincided temporally with deep intrusion of Eocene sills of granite and quartz diorite and shallower intrusion of the Harrison Pass pluton into the core-complex rocks. Stacked Mesozoic nappes of metamorphosed Paleozoic and Precambrian rocks in the core complex lay at least 13 to 20 km deep in Eocene time, on the basis of geobarometry studies. In the northern part of the complex, the presently exposed rocks had been even deeper in the late Mesozoic, to >30 km depths, before losing part of their cover by Eocene time. Nappes in the core plunge northward beneath the originally thicker Mesozoic tectonic cover in the north part of the core complex. Mesozoic nappes and tectonic wedging likely occupied the thickened midlevel crustal section between the deep crustal core-complex intrusions and nappes and the overlying upper crust. These structures, as well as the subsequent large-displacement Cenozoic extensional faulting and flow in the deep crust, would be expected to blur the expression of any regional structural roots that could correlate with mineral belts. Structural mismatch of the mineralized upper crust and the tectonically complex middle crust suggests that the Carlin trend relates not to subjacent deeply penetrating rooted structures but to favorable upper crustal host rocks aligned within a relatively coherent regional block of upper crust.

Lifetime and size of shallow magma bodies controlled by crustal-scale magmatism

NASA Astrophysics Data System (ADS)

Karakas, Ozge; Degruyter, Wim; Bachmann, Olivier; Dufek, Josef

2017-06-01

Magmatic processes on Earth govern the mass, energy and chemical transfer between the mantle, crust and atmosphere. To understand magma storage conditions in the crust that ultimately control volcanic activity and growth of continents, an evaluation of the mass and heat budget of the entire crustal column during magmatic episodes is essential. Here we use a numerical model to constrain the physical conditions under which both lower and upper crustal magma bodies form. We find that over long durations of intrusions (greater than 105 to 106 yr), extensive lower crustal mush zones develop, which modify the thermal budget of the upper crust and reduce the flux of magma required to sustain upper crustal magma reservoirs. Our results reconcile physical models of magma reservoir construction and field-based estimates of intrusion rates in numerous volcanic and plutonic localities. Young igneous provinces (less than a few hundred thousand years old) are unlikely to support large upper crustal reservoirs, whereas longer-lived systems (active for longer than 1 million years) can accumulate magma and build reservoirs capable of producing super-eruptions, even with intrusion rates smaller than 10-3 to 10-2 km3 yr-1. Hence, total duration of magmatism should be combined with the magma intrusion rates to assess the capability of volcanic systems to form the largest explosive eruptions on Earth.

Three-dimensional velocity structure of crust and upper mantle in southwestern China and its tectonic implications

USGS Publications Warehouse

Wang, Chun-Yong; Chan, W.W.; Mooney, W.D.

2003-01-01

Using P and S arrival times from 4625 local and regional earthquakes recorded at 174 seismic stations and associated geophysical investigations, this paper presents a three-dimensional crustal and upper mantle velocity structure of southwestern China (21??-34??N, 97??-105??E). Southwestern China lies in the transition zone between the uplifted Tibetan plateau to the west and the Yangtze continental platform to the east. In the upper crust a positive velocity anomaly exists in the Sichuan Basin, whereas a large-scale negative velocity anomaly exists in the western Sichuan Plateau, consistent with the upper crustal structure under the southern Tibetan plateau. The boundary between these two anomaly zones is the Longmen Shan Fault. The negative velocity anomalies at 50-km depth in the Tengchong volcanic area and the Panxi tectonic zone appear to be associated with temperature and composition variations in the upper mantle. The Red River Fault is the boundary between the positive and negative velocity anomalies at 50-km depth. The overall features of the crustal and the upper mantle structures in southwestern China are a low average velocity, large crustal thickness variations, the existence of a high-conductivity layer in the crust or/and upper mantle, and a high heat flow value. All these features are closely related to the collision between the Indian and the Asian plates.

Contemporary crustal movement of southeastern Tibet: Constraints from dense GPS measurements

PubMed Central

Pan, Yuanjin; Shen, Wen-Bin

2017-01-01

The ongoing collision between the Indian plate and the Eurasian plate brings up N-S crustal shortening and thickening of the Tibet Plateau, but its dynamic mechanisms remain controversial yet. As one of the most tectonically active regions of the world, South-Eastern Tibet (SET) has been greatly paid attention to by many geoscientists. Here we present the latest three-dimensional GPS velocity field to constrain the present-day tectonic process of SET, which may highlight the complex vertical crustal deformation. Improved data processing strategies are adopted to enhance the strain patterns throughout SET. The crustal uplifting and subsidence are dominated by regional deep tectonic dynamic processes. Results show that the Gongga Shan is uplifting with 1–1.5 mm/yr. Nevertheless, an anomalous crustal uplifting of ~8.7 mm/yr and negative horizontal dilation rates of 40–50 nstrain/yr throughout the Longmenshan structure reveal that this structure is caused by the intracontinental subduction of the Yangtze Craton. The Xianshuihe-Xiaojiang fault is a major active sinistral strike-slip fault which strikes essentially and consistently with the maximum shear strain rates. These observations suggest that the upper crustal deformation is closely related with the regulation and coupling of deep material. PMID:28349926

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.

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.

Magma interaction in the root of an arc batholith

NASA Astrophysics Data System (ADS)

Chapman, T.; Robbins, V.; Clarke, G. L.; Daczko, N. R.; Piazolo, S.

2016-12-01

Fiordland, New Zealand, preserves extensive Cretaceous arc plutons, emplaced into parts of the Delamerian/Ross Orogen. Dioritic to gabbroic material emplaced at mid to lower crustal levels are exposed in the Malaspina Pluton (c. 1.2 GPa) and the Breaksea Orthogneiss (c. 1.8 GPa). Distinct magmatic pulses can be mapped in both of these plutons consistent with cycles of melt advection. Relationships are consistent with predictions from lower crustal processing zones (MASH and hot zones) considered important in the formation of Cordilleran margins. Metamorphic garnet growth is enhanced along magmatic contacts, such as where hornblende gabbronorite is cut by garnet-clinopyroxene-bearing diorite. Such features are consistent with cycles of incremental emplacement, younger magma having induced localised garnet granulite metamorphism in wall rock of older material. Temperature estimates and microstructures preserved in garnet granulite are consistent with sub-solidus, water-poor conditions in both the Malaspina and Breaksea Orthogneiss. The extent and conditions of the metamorphism implies conditions and duration was incapable of partially melting older wall rock material. The nature of interactions in intermediate to basic compositions are assessed in terms of magma genesis in the Cretaceous batholith. Most of the upper crustal felsic I-type magmatism along the margin being controlled by high-pressure garnet-clinopyroxene fractionation.

A new heat flux model for the Antarctic Peninsula incorporating spatially variable upper crustal radiogenic heat production

NASA Astrophysics Data System (ADS)

Burton-Johnson, A.; Halpin, J.; Whittaker, J. M.; Graham, F. S.; Watson, S. J.

2017-12-01

We present recently published findings (Burton-Johnson et al., 2017) on the variability of Antarctic sub-glacial heat flux and the impact from upper crustal geology. Our new method reveals that the upper crust contributes up to 70% of the Antarctic Peninsula's subglacial heat flux, and that heat flux values are more variable at smaller spatial resolutions than geophysical methods can resolve. Results indicate a higher heat flux on the east and south of the Peninsula (mean 81 mWm-2) where silicic rocks predominate, than on the west and north (mean 67 mWm-2) where volcanic arc and quartzose sediments are dominant. Whilst the data supports the contribution of HPE-enriched granitic rocks to high heat flux values, sedimentary rocks can be of comparative importance dependent on their provenance and petrography. Models of subglacial heat flux must utilize a heterogeneous upper crust with variable radioactive heat production if they are to accurately predict basal conditions of the ice sheet. Our new methodology and dataset facilitate improved numerical model simulations of ice sheet dynamics. The most significant challenge faced remains accurate determination of crustal structure, particularly the depths of the HPE-enriched sedimentary basins and the sub-glacial geology away from exposed outcrops. Continuing research (particularly detailed geophysical interpretation) will better constrain these unknowns and the effect of upper crustal geology on the Antarctic ice sheet. Burton-Johnson, A., Halpin, J.A., Whittaker, J.M., Graham, F.S., and Watson, S.J., 2017, A new heat flux model for the Antarctic Peninsula incorporating spatially variable upper crustal radiogenic heat production: Geophysical Research Letters, v. 44, doi: 10.1002/2017GL073596.

Crustal shear velocity structure in the Southern Lau Basin constrained by seafloor compliance

NASA Astrophysics Data System (ADS)

Zha, Yang; Webb, Spahr C.

2016-05-01

Seafloor morphology and crustal structure vary significantly in the Lau back-arc basin, which contains regions of island arc formation, rifting, and seafloor spreading. We analyze seafloor compliance: deformation under long period ocean wave forcing, at 30 ocean bottom seismometers to constrain crustal shear wave velocity structure along and across the Eastern Lau Spreading Center (ELSC). Velocity models obtained through Monte Carlo inversion of compliance data show systematic variation of crustal structure in the basin. Sediment thicknesses range from zero thickness at the ridge axis to 1400 m near the volcanic arc. Sediment thickness increases faster to the east than to the west of the ELSC, suggesting a more abundant source of sediment near the active arc volcanoes. Along the ELSC, upper crustal velocities increase from the south to the north where the ridge has migrated farther away from the volcanic arc front. Along the axial ELSC, compliance analysis did not detect a crustal low-velocity body, indicating less melt in the ELSC crustal accretion zone compared to the fast spreading East Pacific Rise. Average upper crust shear velocities for the older ELSC crust produced when the ridge was near the volcanic arc are 0.5-0.8 km/s slower than crust produced at the present-day northern ELSC, consistent with a more porous extrusive layer. Crust in the western Lau Basin, which although thought to have been produced through extension and rifting of old arc crust, is found to have upper crustal velocities similar to older oceanic crust produced at the ELSC.

Regional implications of heat flow of the Snake River Plain, Northwestern United States

NASA Astrophysics Data System (ADS)

Blackwell, D. D.

1989-08-01

The Snake River Plain is a major topographic feature of the Northwestern United States. It marks the track of an upper mantle and crustal melting event that propagated across the area from southwest to northeast at a velocity of about 3.5 cm/yr. The melting event has the same energetics as a large oceanic hotspot or plume and so the area is the continental analog of an oceanic hotspot track such as the Hawaiian Island-Emperor Seamount chain. Thus, the unique features of the area reflect the response of a continental lithosphere to a very energetic hotspot. The crust is extensively modified by basalt magma emplacement into the crust and by the resulting massive rhyolite volcanism from melted crustal material, presently occurring at Yellowstone National Park. The volcanism is associated with little crustal extension. Heat flow values are high along the margins of the Eastern and Western Snake River Plains and there is abundant evidence for low-grade geothermal resources associated with regional groundwater systems. The regional heat flow pattern in the Western Snake River Plains reflects the influence of crustal-scale thermal refraction associated with the large sedimentary basin that has formed there. Heat flow values in shallow holes in the Eastern Snake River Plains are low due to the Snake River Plains aquifer, an extensive basalt aquifer where water flow rates approach 1 km/yr. Below the aquifer, conductive heat flow values are about 100 mW m -2. Deep holes in the region suggest a systematic eastward increase in heat flow in the Snake River Plains from about 75-90 mW m -2 to 90-110 mW m -2. Temperatures in the upper crust do not behave similarly because the thermal conductivity of the Plio-Pleistocene sedimentary rocks in the west is lower than that in the volcanic rocks characteristic of the Eastern Snake River Plains. Extremely high heat loss values (averaging 2500 mW m -2) and upper crustal temperatures are characteristic of the Yellowstone caldera.

Crustal volumes of the continents and of oceanic and continental submarine plateaus

NASA Technical Reports Server (NTRS)

Schubert, G.; Sandwell, D.

1989-01-01

Using global topographic data and the assumption of Airy isostasy, it is estimated that the crustal volume of the continents is 7182 X 10 to the 6th cu km. The crustal volumes of the oceanic and continental submarine plateaus are calculated at 369 X 10 to the 6th cu km and 242 X 10 to the 6th cu km, respectively. The total continental crustal volume is found to be 7581 X 10 to the 6th cu km, 3.2 percent of which is comprised of continental submarine plateaus on the seafloor. An upper bound on the contintental crust addition rate by the accretion of oceanic plateaus is set at 3.7 cu km/yr. Subduction of continental submarine plateaus with the oceanic lithosphere on a 100 Myr time scale yields an upper bound to the continental crustal subtraction rate of 2.4 cu km/yr.

Seismic structure and lithospheric rheology from deep crustal xenoliths, central Montana, USA

NASA Astrophysics Data System (ADS)

Mahan, K. H.; Schulte-Pelkum, V.; Blackburn, T. J.; Bowring, S. A.; Dudas, F. O.

2012-10-01

Improved resolution of lower crustal structure, composition, and physical properties enhances our understanding and ability to model tectonic processes. The cratonic core of Montana and Wyoming, USA, contains some of the most enigmatic lower crust known in North America, with a high seismic velocity layer contributing to as much as half of the crustal column. Petrological and physical property data for xenoliths in Eocene volcanic rocks from central Montana provide new insight into the nature of the lower crust in this region. Inherent heterogeneity in xenoliths derived from depths below ˜30 km support a composite origin for the deep layer. Possible intralayer velocity steps may complicate the seismic definition of the crust/mantle boundary and interpretations of crustal thickness, particularly when metasomatized upper mantle is considered. Mafic mineral-dominant crustal xenoliths and published descriptions of mica-bearing peridotite and pyroxenite xenoliths suggest a strong lower crust overlying a potentially weaker upper mantle.

Geology of the Ulugh Muztagh area, northern Tibet

USGS Publications Warehouse

Burchfiel, B.C.; Molnar, P.; Zhao, Ziyun; Liang, K'uangyi; Wang, Shuji; Huang, Minmin; Sutter, J.

1989-01-01

Within the Ulugh Muztagh area, north central Tibet, an east-west-trending ophiolitic melange marks a suture that apparently was formed during a late Triassic or slightly younger collision between a continental fragment to the south and the rest of Asia. The southern continental fragment carries a thick sequence of upper Triassic sandstone, but the contact between the sandstone and the ophiolitic melange is covered by a younger redbed sequence of unknown age. A suite of 2-mica, tourmaline-bearing leucogranite plutons and dikes intruded the Triassic sandstone at shallow crustal levels 10.5 to 8.4 Ma. These rocks range from granite to tonalite in composition, are geochemically very similar to slightly older High Himalayan leucogranite and are interpreted to have been derived by the partial melting of crustal material. We interpret this to mean that crustal thickening began in this part of the Tibetan plateau at least by 10.5 Ma. Welded rhyolitic tuff rests on a conglomerate that consists of abundant debris from the Ulugh Muztagh intrusive rocks and has yielded Ar Ar ages of about 4 Ma. The tuffs are geochemically identical to the intrusive rocks suggesting that crustal thickening may have continued to 4 Ma. Crustal thickening probably occurred by distributed crustal shortening similar to shortening now occurring north of Ulugh Muztagh along the northern margin of the Tibetan Plateau. ?? 1989.

Controls on Magmatic and Hydrothermal Processes at Yellowstone Supervolcano: The Wideband Magnetotelluric Component of an Integrated MT/Seismic Investigation

NASA Astrophysics Data System (ADS)

Schultz, A.; Bennington, N. L.; Bowles-martinez, E.; Imamura, N.; Cronin, R. A.; Miller, D. J.; Hart, L.; Gurrola, R. M.; Neal, B. A.; Scholz, K.; Fry, B.; Carbonari, R.

2017-12-01

Previous seismic and magnetotelluric (MT) studies beneath Yellowstone (YS) have provided insight into the origin and migration of magmatic fluids within the volcanic system. However, important questions remain concerning the generation of magmatism at YS, the migration and storage of these magmatic fluids, as well as their relationships to hydrothermal expressions. Analysis of regional-scale EarthScope MT data collected previously suggests a relative absence of continuity in crustal partial melt accumulations directly beneath YS. This is in contrast to some seismic interpretations, although such long-period MT data have limited resolving power in the upper-to-mid crustal section. A wideband MT experiment was designed as a component of an integrated MT/seismic project to examine: the origin and location of magmatic fluids at upper mantle/lower crustal depths, the preferred path of migration for these magmatic fluids into the mid- to upper-crust, the resulting distribution of the magma reservoir, the composition of the magma reservoir, and implications for future volcanism at YS. A high-resolution wideband MT survey was carried out in the YS region in the summer of 2017, with more than forty-five wideband stations installed within and immediately surrounding the YS National Park boundary. These data provided nearly six decades of bandwidth ( 10-3 Hz -to- 103 Hz). Extraordinary permitting restrictions prevented us from using conventional installation methods at many of our sites, and an innovative "no-dig" subaerial method of wideband MT was developed and used successfully. Using these new data along with existing MT datasets, we are inverting for the 3D resistivity structure at upper crustal through upper mantle scales at YS. Complementary to this MT work, a joint inversion for the 3D crustal velocity structure is being carried out using both ambient noise and earthquake travel time data. Taken together, these data should better constrain the crustal velocity structure of this volcanic system and produce enhanced images of magma storage.

Crustal scale detachment in the Himalayas: a reappraisal

NASA Astrophysics Data System (ADS)

Mukhopadhyay, S.; Sharma, J.

2010-11-01

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

Imaging the seismic structure beneath oceanic spreading centers using ocean bottom geophysical techniques

NASA Astrophysics Data System (ADS)

Zha, Yang

This dissertation focuses on imaging the crustal and upper mantle seismic velocity structure beneath oceanic spreading centers. The goals are to provide a better understanding of the crustal magmatic system and the relationship between mantle melting processes, crustal architecture and ridge characteristics. To address these questions I have analyzed ocean bottom geophysical data collected from the fast-spreading East Pacific Rise and the back-arc Eastern Lau Spreading Center using a combination of ambient noise tomography and seafloor compliance analysis. To characterize the crustal melt distribution at fast spreading ridges, I analyze seafloor compliance - the deformation under long period ocean wave forcing - measured during multiple expeditions between 1994 and 2007 at the East Pacific Rise 9º - 10ºN segment. A 3D numerical modeling technique is developed and used to estimate the effects of low shear velocity zones on compliance measurements. The forward modeling suggests strong variations of lower crustal shear velocity along the ridge axis, with zones of possible high melt fractions beneath certain segments. Analysis of repeated compliance measurements at 9º48'N indicates a decrease of crustal melt fraction following the 2005 - 2006 eruption. This temporal variability provides direct evidence for short-term variations of the magmatic system at a fast spreading ridge. To understand the relationship between mantle melting processes and crustal properties, I apply ambient noise tomography of ocean bottom seismograph (OBS) data to image the upper mantle seismic structure beneath the Eastern Lau Spreading Center (ELSC). The seismic images reveal an asymmetric upper mantle low velocity zone (LVZ) beneath the ELSC, representing a zone of partial melt. As the ridge migrates away from the volcanic arc, the LVZ becomes increasingly offset and separated from the sub-arc low velocity zone. The separation of the ridge and arc low velocity zones is spatially coincident with the abrupt transition in crustal composition and ridge morphology. Therefore these results confirm a previous prediction that the changing interaction between the arc and back-arc magmatic systems is responsible for the abrupt change in crustal properties along the ELSC. I further investigate the crustal structure along and across the ELSC using seafloor compliance. Compliance measurements are inverted for local crustal shear velocity structure as well as sediment thickness at 30 OBS locations using a Monte Carlo method. Sediment increases asymmetrically with seafloor age, with much a higher rate to the east of the ridge. Along the ELSC, upper crustal velocities increase from south to north as the ridge migrates away from the volcanic arc front, consistent with a less porous upper crust with possibly less subduction input. Furthermore, average upper crust shear velocities for crust produced at past ELSC when it was near the volcanic arc are considerably slower than crust produced at present day northern ELSC. I show that the implications of previous active seismic studies in the axial ELSC can be extended much farther off-axis and back in time. I also address a challenge of ocean bottom seismology and develop a new method for determining OBS horizontal orientations using multi-component ambient noise correlation. I demonstrate that the OBS orientations can be robustly estimated through maximizing the correlation between the diagonal and cross terms of the noise correlation function. This method is applied to the ELSC OBS experiment dataset and the obtained orientations are consistent with results from a conventional teleseismic method. The new method is promising for a wide range of applications.

Crustal and Upper Mantle Velocity and Q Structures of Mainland China

DTIC Science & Technology

1979-11-01

CLASIFICATION OFTHIS PAGE(117..t- [).(t ntred) with identical source-receiver geometry. The generalized surface wave inversion technique was applied...in the recent past. A particularly unusual crustal and upper mantle structure is found underlying the Tibet Dlateau. AOceSIon For DDC TAB Ubazmnounced...the AIR FORCE OFFICE OF SCIENTIFIC RESEARCH by the GEOPHYSICAL LABORATORY UNIVERSITY OF SOUTHERN CALIFORNIA Contractor: University of Southern

Developing a Crustal and Upper Mantle Velocity Model for the Brazilian Northeast

NASA Astrophysics Data System (ADS)

Julia, J.; Nascimento, R.

2013-05-01

Development of 3D models for the earth's crust and upper mantle is important for accurately predicting travel times for regional phases and to improve seismic event location. The Brazilian Northeast is a tectonically active area within stable South America and displays one of the highest levels of seismicity in Brazil, with earthquake swarms containing events up to mb 5.2. Since 2011, seismic activity is routinely monitored through the Rede Sismográfica do Nordeste (RSisNE), a permanent network supported by the national oil company PETROBRAS and consisting of 15 broadband stations with an average spacing of ~200 km. Accurate event locations are required to correctly characterize and identify seismogenic areas in the region and assess seismic hazard. Yet, no 3D model of crustal thickness and crustal and upper mantle velocity variation exists. The first step in developing such models is to refine crustal thickness and depths to major seismic velocity boundaries in the crust and improve on seismic velocity estimates for the upper mantle and crustal layers. We present recent results in crustal and uppermost mantle structure in NE Brazil that will contribute to the development of a 3D model of velocity variation. Our approach has consisted of: (i) computing receiver functions to obtain point estimates of crustal thickness and Vp/Vs ratio and (ii) jointly inverting receiver functions and surface-wave dispersion velocities from an independent tomography study to obtain S-velocity profiles at each station. This approach has been used at all the broadband stations of the monitoring network plus 15 temporary, short-period stations that reduced the inter-station spacing to ~100 km. We expect our contributions will provide the basis to produce full 3D velocity models for the Brazilian Northeast and help determine accurate locations for seismic events in the region.

Rift migration explains continental margin asymmetry and crustal hyper-extension

PubMed Central

Brune, Sascha; Heine, Christian; Pérez-Gussinyé, Marta; Sobolev, Stephan V.

2014-01-01

When continents break apart, continental crust and lithosphere are thinned until break-up is achieved and an oceanic basin is formed. The most remarkable and least understood structures associated with this process are up to 200 km wide areas of hyper-extended continental crust, which are partitioned between conjugate margins with pronounced asymmetry. Here we show, using high-resolution thermo-mechanical modelling, that hyper-extended crust and margin asymmetry are produced by steady state rift migration. We demonstrate that rift migration is accomplished by sequential, oceanward-younging, upper crustal faults, and is balanced through lower crustal flow. Constraining our model with a new South Atlantic plate reconstruction, we demonstrate that larger extension velocities may account for southward increasing width and asymmetry of these conjugate magma-poor margins. Our model challenges conventional ideas of rifted margin evolution, as it implies that during rift migration large amounts of material are transferred from one side of the rift zone to the other. PMID:24905463

Lunar impact basins: New data for the western limb and far side (Orientale and South Pole-Aitken basins) from the first Galileo flyby

NASA Astrophysics Data System (ADS)

Head, James W.; Murchie, Scott; Mustard, John F.; Pieters, Carle M.; Neukum, Gerhard; McEwen, Alfred; Greeley, Ronald; Nagel, Engelbert; Belton, Michael J. S.

1993-09-01

Compositional aspects of impact basin materials can be analyzed using multispectral image data acquired by the Galileo solid state imaging (SSI) experiment during the December 1990 lunar encounter. These data provide important information on the spectral properties of the western lunar limb and parts of the far side. The SSI images cover the wavelength range 0.4-1.0 μm, allowing measurement of spectral slope and estimation of the strength of the 1 μm absorption due to iron in the mafic minerals olivine and pyroxene. Among deposits of the 930-km-diameter Orientale basin, exterior ejecta comprising the Hevelius Formation is relatively homogeneous and spectrally similar to mature Apollo 16 soils, suggesting an upper crustal source. The centrally located Maunder Formation is distinct from the younger mare basalts but comparable to the Hevelius Formation in its spectral reflectance properties, supporting an interpretation as basin impact melt. The Montes Rook Formation, located in an annulus between the Maunder and the Hevelius, shows a slightly stronger mafic absorption and may be the deepest crustal material excavated. The distal Orientale deposits show local mafic enhancements (in the Schiller-Schickard and Mendel-Rydberg regions) interpreted to represent pre-Orientale mare deposits, or cryptomaria, intermixed with overlying basin ejecta. In this case, maria of sizes comparable to those presently observed were widespread in this region before the Orientale impact. Mixing-model analyses are consistent with the ballistic erosion and sedimentation model for ejecta emplacement in the distal regions beyond the continuous ejecta deposit. On the southern lunar farside, a high area with an enhanced mafic absorption corresponds to the interior and rim of the pre-Nectarian South Pole-Aitken impact basin, 2000-2500 km in diameter. The anomaly is interpreted to be due to several factors, including excavation into the more mafic lower crust, and the presence of extensive early volcanic fill (cryptomare), similar to that seen in ancient basins such as Smythii and Australe. These results show that although basin-forming events are an important factor in producing lateral heterogeneities in crustal composition, and in modifying preexisting deposits (such as cryptomaria), the majority of material in even the largest basins was excavated from mixed crustal layer of anorthosite, basin ejecta, and cryptomaria deposits (generally corresponding to the megaregolith), an upper crustal layer of anorthosite, and a lower more noritic layer. Many of the basic questions remaining from this study could be addressed by global high-resolution geochemical and mineralogical data obtained by polar orbiting spacecraft.

Crustal-scale magmatism and its control on the longevity of magmatic systems

NASA Astrophysics Data System (ADS)

Karakas, Ozge; Degruyter, Wim; Bachmann, Olivier; Dufek, Josef

2017-04-01

Constraining the duration and evolution of crustal magma reservoirs is crucial to our understanding of the eruptive potential of magmatic systems, as well as the volcanic:plutonic ratios in the crust, but estimates of such parameters vary widely in the current literature. Although no consensus has been reached on the lifetime of magma reservoirs, recent studies have revealed about the presence, location, and melt fraction of multi-level (polybaric) storage zones in the crust. If magma accumulates at different crustal levels, it must redistribute significant enthalpy within the crustal column and therefore must influence the lifetime of magma plumbing systems. However, an evaluation of the mass and heat budget of the entire crustal column is lacking. Here, we use a two-dimensional thermal model to determine the thermal conditions under which both lower and upper crustal magma bodies form. We find that large lower crustal mush zones supply heat to the upper crust and reduce the amount of thermal energy necessary to form subvolcanic reservoirs. This indicates that the crust is thermally viable to sustain partially molten magma reservoirs over long timescales (>10^5-106 yr) for a range of magma fluxes (10^-4 to 10^-2 km^3/yr). Our results reconcile physical models of crustal magma evolution and field-based estimates of intrusion rates in numerous magmatic provinces (which include both volcanic and plutonic lithologies). We also show that young magmatic provinces (< 105 yr old) are unlikely to support large upper crustal reservoirs, whereas longer-lived systems (> 106 yr) can accumulate magma and build reservoirs capable of triggering supereruptions, even with intrusion rates as low as ≤10^-2 km^3/yr. Hence, the total duration of magmatism is critical in determining the size of the magma reservoirs, and should be combined with the magma intrusions rates to assess the capability of volcanic systems to form the largest eruptions on Earth.

Excess europium content in Precambrian sedimentary rocks and continental evolution

NASA Technical Reports Server (NTRS)

Jakes, P.; Taylor, S. R.

1974-01-01

It is proposed that the europium excess in Precambrian sedimentary rocks, relative to those of younger age, is derived from volcanic rocks of ancient island arcs, which were the source materials for the sediments. Precambrian sedimentary rocks and present-day volcanic rocks of island arcs have similar REE patterns, total REE abundances, and excess Eu, relative to the North American shale composite. The present upper crustal REE pattern, as exemplified by that of sediments, is depleted in Eu, relative to chondrites. This depletion is considered to be a consequence of development of a granodioritic upper crust by partial melting in the lower crust, which selectively retains europium.

Genetic relations of oceanic basalts as indicated by lead isotopes

USGS Publications Warehouse

Tatsumoto, M.

1966-01-01

The isotopic compositions of lead and the concentrations of lead, uranium, and thorium in samples of oceanic tholeiite and alkali suites are determined, and the genetic relations of the oceanic basalts are discussed. Lead of the oceanic tholeiites has a varying lead-206 : lead-204 ratio between 17.8 and 18.8, while leads of the alkali basalt suites from Easter Island and Guadalupe Island are very radiogenic with lead-206 : lead-204 ratios between 19.3 and 20.4. It is concluded that (i) the isotopic composition of lead in oceanic tholeiite suggests that the upper mantle source region of the tholeiite was differentiated from an original mantle material more than 1 billion years ago and that the upper mantle is not homogeneous at the present time, (ii) less than 20 million years was required for the crystal differentiation within the alkali suite from Easter Island, (iii) no crustal contamination was involved in the course of differentiation of rocks from Easter Island; however, some crustal contamination may have affected Guadalupe Island rocks, and (iv) alkali basalt may be produced from the tholeiite in the oceanic region by crystal differentiation. Alternatively the difference in the isotopic composition of lead in oceanic basalts may be produced by partial melting at different depths of a differentiated upper mantle.

Layered Crustal Anisotropy in the NE Tibetan Plateau Inferred from Ambient Noise Tomography

NASA Astrophysics Data System (ADS)

Jiang, C.; Yang, Y.; Zheng, Y.

2016-12-01

The Tibetan Plateau is the highest and largest plateau in the world with an average elevation of 4-5 km and 60-70 km thick crust, about twice of the thickness of average continental crust. Two end-member models have bene invoked to explain the crustal thickening and the growth of the plateau: (1) continuous and uniform thickening of the whole crust and (2) mid/lower crustal channel flow. However, which mechanism dominates the crustal thickening and the growth of the plateau is still under hot debate. Seismic anisotropy can provide observational constraints on deformation mode, which would have distinguished pattern resulting from the two different thickening models. Thus, by studying seismic anisotropy, we can distinguish different models of crustal thickening and plateau growth. In this study, we employ an eikonal tomography method of ambient noise to investigate azimuthal anisotropy of Rayleigh waves in the NE Tibetan Plateau. Our tomography reveals significant anisotropy in the crust. In particular, stratification of crustal azimuthal anisotropy is observed: an upper crustal anisotropic layer characterized by a NE-SW fast direction and a mid/lower crustal anisotropic layer with a NNE-SSW fast direction. The dominantly NE-SW oriented anisotropy in the upper crust is likely caused by shape-preferred orientation (SPO) of faults and fractures in the shallow depths. The anisotropy in the mid/lower crust, however, is nearly orthogonal to that in the shallow crust, suggesting a different mechanism. The NNE-SSW fast direction coincides with the proposed flow direction by the crustal flow model in NE Tibetan Plateau, suggesting anisotropy in the mid/lower crust may be related to the crustal flow. The two-layered crustal stratigraphy observed in the NE Tibetan Plateau is contrary to the continuous thickening model, but favours the crustal flow model.

Geomorphic Terrains and Evidence for Ancient Volcanism within Northeastern South Pole-Aitken Basin

NASA Technical Reports Server (NTRS)

Petro, Noah; Mest, Scott C.; Teich, Yaron

2010-01-01

The interior of the enigmatic South Pole-Aitken Basin has long been recognized as being compositionally distinct from its exterior. However, the source of the compositional anomaly has been subject to some debate. Is the source of the iron-enhancement due to lower-crustal/upper-mantle material being exposed at the surface, or was there some volume of ancient volcanism that covered portions of the basin interior? While several obvious mare basalt units are found within the basin and regions that appear to represent the original basin interior, there are several regions that appear to have an uncertain origin. Using a combination of Clementine and Lunar Orbiter images, several morphologic units are defined based on albedo, crater density, and surface roughness. An extensive unit of ancient mare basalt (cryptomare) is defined and, based on the number of superimposed craters, potentially represents the oldest volcanic materials within the basin. Thus, the overall iron-rich interior of the basin is not solely due to deeply derived crustal material, but is, in part due to the presence of ancient volcanic units.

Crustal Stretching Style and Lower Crust Flow of the South China Sea Northern Margin

NASA Astrophysics Data System (ADS)

Bai, Y.; Dong, D.; Runlin, D.

2017-12-01

There is a controversy about crustal stretching style of the South China Sea (SCS) northern margin mainly due to considerable uncertainty of stretching factor estimation, for example, as much as 40% of upper crust extension (Walsh et al., 1991) would be lost by seismic profiles due to poor resolution. To discover and understand crustal stretching style and lower crustal flow on the whole, we map the Moho and Conrad geometries based on gravity inversion constrained by deep seismic profiles, then according to the assumption of upper and lower crust initial thickness, upper and lower crust stretching factors are estimated. According to the comparison between upper and lower crust stretching factors, the SCS northern margin could be segmented into three parts, (1) sediment basins where upper crust is stretched more than lower crust, (2) COT regions where lower crust is stretched more than upper crust, (3) other regions where the two layers have similar stretching factors. Stretching factor map shows that lower crust flow happened in both of COT and sediment basin regions where upper crust decouples with lower crust due to high temperature. Pressure contrast by sediment loading in basins and erosion in sediment-source regions will lead to lower crust flow away from sediment sink to source. Decoupled and fractured upper crust is stretched further by sediment loading and the following compensation would result in relatively thick lower crust than upper crust. In COT regions with thin sediment coverage, low-viscosity lower crust is easier to thin in extensional environment, also the lower crust tends to flow away induced by magma upwelling. Therefore, continental crust on the margin is not stretching in a constant way but varies with the tectonic setting changes. This work is supported by National Natural Science Foundation of China (Grant No. 41506055, 41476042) and Fundamental Research Funds for the Central Universities China (No.17CX02003A).

Bullialdus - Strengthening the case for lunar plutons

NASA Technical Reports Server (NTRS)

Pieters, Carle M.

1991-01-01

Although many craters expose materials of a composition different from that of the local surroundings, Bullialdus has excavated material representing three distinct stratigraphic zones that occur in the upper 6 km of crust, the top two of which are gabbroic and the deepest of which is noritic. This three-component stratigraphy at Bullialdus provides strong evidence that the lunar crust includes pockets of compositionally layered material reminiscent of mafic layered plutons. When combined with previous information on the compositional diversity at other large craters, these remote analyses obtained in a geologic context substantially strengthen the hypothesis suggested from lunar samples that plutons play an integral role in lunar crustal evolution.

The Modulation of Crustal Magmatic Systems by Tectonic Forcing

NASA Astrophysics Data System (ADS)

Karakas, O.; Dufek, J.

2010-12-01

The amount, location and residence time of melt in the crust significantly impacts crustal structure and influences the composition, frequency, and volume of eruptive products. In this study, we develop a two dimensional model that simulates the response of the crust to prolonged mantle-derived intrusions in arc environments. The domain includes the entire crustal section and upper mantle and focuses on the evolving thermal structure due to intrusions and external tectonic forcing. Magmatic intrusion into the crust can be accommodated by extension or thickening of the crust or some combination of both mechanisms. Additionally, external tectonic forcing can generate thicker crustal sections, while tectonic extension can significantly thin the crust. We monitor the thermal response, melt fraction and surface heat flux for different tectonic conditions and melt flux from the mantle. The amount of crustal melt versus fractionated primary mantle melts present in the crustal column helps determine crustal structure and growth through time. We express the amount of crustal melting in terms of an efficiency; we define the melting efficiency as the ratio of the melted volume of crustal material to the volume of melt expected from a strict enthalpy balance as explained by Dufek and Bergantz (2005). Melting efficiencies are less than 1 in real systems because heat diffuses to sections of the crust that never melt. In general, thick crust and crust experiencing extended compressional regimes results in an increased melting efficiency; and thin crust and crust with high extension rates have lower efficiency. In most settings, maximum efficiencies are less than 0.05-0.10. We also observe that with a geophysically estimated flux, the mantle-derived magma bodies build up isolated magma pods that are distributed in the crust. One of the aspects of this work is to monitor the location and size of these magma chambers in the crustal column. We further investigate the rheological, stress and pre-existing structure control on the longevity of the individual magmatic systems.

A lead isotope study of mineralization in the Saudi Arabian Shield

USGS Publications Warehouse

Stacey, J.S.; Doe, B.R.; Roberts, R.J.; Delevaux, M.H.; Gramlich, J.W.

1980-01-01

New lead isotope data are presented for some late Precambrian and early Paleozoic vein and massive sulfide deposits in the Arabian Shield. Using the Stacey Kramers (1975) model for lead isotope evolution, isochron model ages range between 720 m.y. and 420 m.y. Most of the massive sulfide deposits in the region formed before 680 m.y. ago, during evolution of the shield. Vein type mineralization of higher lead content occurred during the Pan African event about 550 m.y. ago and continued through the Najd period of extensive faulting in the shield that ended about 530 m.y. ago. Late post-tectonic metamorphism may have been responsible for vein deposits that have model ages less than 500 m.y. Alternatively some of these younger model ages may be too low due to the mineralizing fluids acquiring radiogenic lead from appreciably older local crustal rocks at the time of ore formation. The low207Pb/204Pb ratios found for the deposits in the main part of the shield and for those in north-eastern Egypt, indicate that the Arabian craton was formed in an oceanic crustal environment during the late Precambrian. Involvement of older, upper-crustal material in the formation of the ore deposits in this part of the shield is precluded by their low207Pb/204Pb and208Pb/204Pb characteristics. In the eastern part of the shield, east of longitude 44??20???E towards the Al Amar-Idsas fault region, lead data are quite different. They exhibit a linear207Pb/204Pb-206Pb/204Pb relationship together with distinctly higher208Pb/204Pb characteristics. These data imply the existence of lower crustal rocks of early Proterozoic age that apparently have underthrust the shield rocks from the east. If most of the samples we have analyzed from this easterly region were mineralized 530 m.y. ago, then the age of the older continental rocks is 2,100??300 m.y. (2??). The presence of upper crustal rocks, possibly also of early Proterozoic age, is indicated by galena data from Hailan in South Yemen and also from near Muscat in Oman. These data are the first to indicate such old continental material in these regions. ?? 1980 Springer-Verlag.

Crustal characteristic variation in the central Yamato Basin, Japan Sea back-arc basin, deduced from seismic survey results

NASA Astrophysics Data System (ADS)

Sato, Takeshi; No, Tetsuo; Miura, Seiichi; Kodaira, Shuichi

2018-02-01

The crustal structure of the Yamato Bank, the central Yamato Basin, and the continental shelf in the southern Japan Sea back-arc basin is obtained based on a seismic survey using ocean bottom seismographs and seismic shot to elucidate the back-arc basin formation processes. The central Yamato Basin can be divided into three domains based on the crustal structure: the deep basin, the seamount, and the transition domains. In the deep basin domain, the crust without the sedimentary layer is about 12-13 km thick. Very few units have P-wave velocity of 5.4-6.0 km/s, which corresponds to the continental upper crust. In the seamount and transition domains, the crust without the sedimentary layer is about 12-16 km thick. The P-wave velocities of the upper and lower crusts differs among the deep basin, the seamount, and the transition domains. These results indicate that the central Yamato Basin displays crustal variability in different domains. The crust of the deep basin domain is oceanic in nature and suggests advanced back-arc basin development. The seamount domain might have been affected by volcanic activity after basin opening. In the transition domain, the crust comprises mixed characters of continental and oceanic crust. This crustal variation might represent the influence of different processes in the central Yamato Basin, suggesting that crustal development was influenced not only by back-arc opening processes but also by later volcanic activity. In the Yamato Bank and continental shelf, the upper crust has thickness of about 17-18 km and P-wave velocities of 3.3-4.1 to 6.6 km/s. The Yamato Bank and the continental shelf suggest a continental crustal character.

Evidence for an upper mantle low velocity zone beneath the southern Basin and Range-Colorado Plateau transition zone

USGS Publications Warehouse

Benz, H.M.; McCarthy, J.

1994-01-01

A 370-km-long seismic refraction/wide-angle reflection profile recorded during the Pacific to Arizona Crustal Experiment (PACE) detected an upper mantle P-wave low-velocity zone (LVZ) in the depth range 40 to 55 km beneath the Basin and Range in southern Arizona. Interpretation of seismic data places constraints on the sub-crustal lithosphere of the southern Basin and Range Province, which is important in light of the active tectonics of the region and the unknown role of the sub-crustal lithosphere in the development of the western United States. Forward travel time and synthetic seismogram techniques are used to model this shallow upper mantle LVZ. Modeling results show that the LVZ is defined by a 5% velocity decrease relative to a Pn velocity of 7.95 km s−1, suggesting either a ∼3–5% mafic partial melt or high-temperature, sub-solidus peridotite.

Osmium isotopes suggest fast and efficient mixing in the oceanic upper mantle.

NASA Astrophysics Data System (ADS)

Bizimis, Michael; Salters, Vincent

2010-05-01

The depleted upper mantle (DUM; the source of MORB) is thought to represent the complementary reservoir of continental crust extraction. Previous studies have calculated the "average" DUM composition based on the geochemistry of MORB. However the Nd isotope compositions of abyssal peridotites have been shown to extend to more depleted compositions than associated MORB. While this argues for the presence of both relatively depleted and enriched material within the upper mantle, the extent of compositional variability, length scales of heterogeneity and timescales of mixing in the upper mantle are not well constrained. Model calculations show that 2Ga is a reasonable mean age of depletion for DUM while Hf - Nd isotopes show the persistence of a depleted terrestrial reservoir by the early Archean (3.5-3.8Ga). U/Pb zircon ages of crustal rocks show three distinct peaks at 1.2, 1.9, and 2.7Ga and these are thought to represent the ages of three major crustal growth events. A fundamental question therefore is whether the present day upper mantle retains a memory of multiple ancient depletion events, or has been effectively homogenized. This has important implications for the nature of convection and time scales of survival of heterogeneities in the upper mantle. Here we compare published Os isotope data from abyssal peridotites and ophiolitic Os-Ir alloys with new data from Hawaiian spinel peridotite xenoliths. The Re-Os isotope system has been shown to yield useful depletion age information in peridotites, so we use it here to investigate the distribution of Re-depletion ages (TRD) in these mantle samples as a proxy for the variability of DUM. The probability density functions (PDF) of TRD from osmiridiums, abyssal and Hawaiian peridotites are all remarkably similar and show a distinct peak at 1.2-1.3 Ga (errors for TRD are set at 0.2Ga to suppress statistically spurious age peaks). The Hawaiian peridotites further show a distinct peak at 1.9-2Ga, but no oceanic mantle samples with TRD older than 2Ga have been reported. The TRD age peaks overlap with two major crustal building events recorded in the U/Pb crustal zircon ages. Therefore, peridotites from the convecting upper mantle can retain some memory of ancient depletion events, and these depletions are perhaps linked to major crustal building or large-scale mantle melting events. In the case of the Hawaiian peridotites, an ancient depletion event is further supported by some extremely radiogenic Hf isotope compositions. However, the vast majority of oceanic mantle samples show a narrow rage of Os isotope compositions (187Os/188Os = 0.123-0.126) with TRDs at 300-600 Ma. If the upper mantle has been produced continuously (or episodically) since at least the early Archean, it is then surprising that almost all oceanic mantle samples record such young depletion ages. We suggest that convective mixing in the mantle is rigorous enough that effectively re-homogenizes and resets the Os isotope composition of previously depleted peridotites within short time scales (<500Ma). Similarly recent ages have been derived from modeling the Sr, Nd, Hf, Pb isotopic composition of MORBs. This resetting and homogenization can be due to re-equilibration of depleted mantle with enriched components, e.g. recycled basaltic crust or more fertile mantle. Ancient depletion events are only effectively preserved in the sublithospheric mantle samples (e.g. Kaapval, Slave, Wyoming cratons) because they remain isolated from the convective mantle.

Origin of the Sudbury Complex by meteoritic impact: Neodymium isotopic evidence

USGS Publications Warehouse

Faggart, B.E.; Basu, A.R.; Tatsumoto, M.

1985-01-01

Samarium-neodymium isotopic data on whole rocks and minerals of the Sudbury Complex in Canada gave an igneous crystallization age of 1840 ?? 21 ?? 106 years. The initial epsilon neodymium values for 15 whole rocks are similar to those for average upper continental crust, falling on the crustal trend of neodymium isotopic evolution as defined by shales. The rare earth element concentration patterns of Sudbury rocks are also similar to upper crustal averages. These data suggest that the Sudbury Complex formed from melts generated in the upper crust and are consistent with a meteoritic impact.

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

NASA Astrophysics Data System (ADS)

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

2007-12-01

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

Amplification and Attenuation Across USArray Using Ambient Noise Wavefront Tracking

NASA Astrophysics Data System (ADS)

Bowden, Daniel C.; Tsai, Victor C.; Lin, Fan-Chi

2017-12-01

As seismic traveltime tomography continues to be refined using data from the vast USArray data set, it is advantageous to also exploit the amplitude information carried by seismic waves. We use ambient noise cross correlation to make observations of surface wave amplification and attenuation at shorter periods (8-32 s) than can be observed with only traditional teleseismic earthquake sources. We show that the wavefront tracking approach can be successfully applied to ambient noise correlations, yielding results quite similar to those from earthquake observations at periods of overlap. This consistency indicates that the wavefront tracking approach is viable for use with ambient noise correlations, despite concerns of the inhomogeneous and unknown distribution of noise sources. The resulting amplification and attenuation maps correlate well with known tectonic and crustal structure; at the shortest periods, our amplification and attenuation maps correlate well with surface geology and known sedimentary basins, while our longest period amplitudes are controlled by crustal thickness and begin to probe upper mantle materials. These amplification and attenuation observations are sensitive to crustal materials in different ways than traveltime observations and may be used to better constrain temperature or density variations. We also value them as an independent means of describing the lateral variability of observed Rayleigh wave amplitudes without the need for 3-D tomographic inversions.

Where do arc magmas differentiate? A seismic and geochemical search for active, deep crustal MASH zones

NASA Astrophysics Data System (ADS)

Pu, X.; Delph, J. R.; Shimizu, K.; Rasmussen, D. J.; Ratschbacher, B. C.

2017-12-01

Deep zones of mixing, assimilation, storage, and homogenization (MASH) are thought to be one of the primary locations where primitive arc magmas stall, interact with crustal material, and differentiate. Support for deep crustal MASH zones is found in exposed crustal sections, where mafic-ultramafic lithologies occur in the lower crust. However, geophysical observations of active deep MASH zones are rare, and their ubiquity is difficult to assess solely based on geochemistry. Using a multidisciplinary approach, we investigate the role of deep crustal processing by investigating two contrasting arcs: the Central Volcanic Zone (CVZ) of the Andes, characterized by thick crust ( 60 km) and large volume silicic eruptions that extend into the back arc, and the Cascadia arc, characterized by thinner crust ( 40 km) and less evolved eruptions. In the southern Puna region of the CVZ, shear-wave velocities in the uppermost mantle are slow ( 3.9 km/s) compared to the minimum expected shear velocity for melt-free mantle lithosphere ( 4.2 km/s). This is consistent with the presence of a melt-bearing MASH zone near the crust-mantle transition. Sr isotopes indicate the magmas interacted with continental crust, and elevated Dy/Yb ratios suggest this process occurred in the garnet stability field (> 1 GPa). Major element signatures (e.g., ASI vs. SiO2) also suggest contribution from partial melting of the lower crust. The signature of lower crustal differentiation (high Dy/Yb) is also observed in the nearby ignimbrites from Cerro Galan, despite the presence of a large slow velocity body at depths too shallow for garnet stability, suggesting that the geochemical signatures of deep MASH zones may be retained regardless of whether magmas stall at shallower depths. Similarly elevated Dy/Yb ratios and slow shear-wave velocities in the upper mantle are common in the CVZ, implying deep MASH zones are pervasive there. A similar approach is applied to Cascadia, where seismic and geochemical signatures of lower crustal processing are weaker than those in the CVZ. The strongest evidence for a deep MASH zone is found at Rainier, where upper mantle velocities are slow and slightly elevated Dy/Yb ratios in evolved melts indicate differentiation in the presence of garnet. Our results suggest deep MASH zones are more common in the CVZ than Cascadia.

Study on 3-D velocity structure of crust and upper mantle in Sichuan-yunnan region, China

USGS Publications Warehouse

Wang, C.; Mooney, W.D.; Wang, X.; Wu, J.; Lou, H.; Wang, F.

2002-01-01

Based on the first arrival P and S data of 4 625 regional earthquakes recorded at 174 stations dispersed in the Yunnan and Sichuan Provinces, the 3-D velocity structure of crust and upper mantle in the region is determined, incorporating with previous deep geophysical data. In the upper crust, a positive anomaly velocity zone exists in the Sichuan basin, whereas a negative anomaly velocity zone exists in the western Sichuan plateau. The boundary between the positive and negative anomaly zones is the Longmenshan fault zone. The images of lower crust and upper mantle in the Longmenshan fault, Xianshuihe fault, Honghe fault and others appear the characteristic of tectonic boundary, indicating that the faults litely penetrate the Moho discontinuity. The negative velocity anomalies at the depth of 50 km in the Tengchong volcanic area and the Panxi tectonic zone appear to be associated with the temperature and composition variations in the upper mantle. The overall features of the crustal and the upper mantle structures in the Sichuan-Yunnan region are the lower average velocity in both crust and uppermost mantle, the large crustal thickness variations, and the existence of high conductivity layer in the crust or/and upper mantle, and higher geothermal value. All these features are closely related to the collision between the Indian and the Asian plates. The crustal velocity in the Sichuan-Yunnan rhombic block generally shows normal.value or positive anomaly, while the negative anomaly exists in the area along the large strike-slip faults as the block boundary. It is conducive to the crustal block side-pressing out along the faults. In the major seismic zones, the seismicity is relative to the negative anomaly velocity. Most strong earthquakes occurred in the upper-mid crust with positive anomaly or normal velocity, where the negative anomaly zone generally exists below.

The Gulliver mission: Sample return from Deimos

NASA Astrophysics Data System (ADS)

Britt, D.

The Martian moon Deimos has been accumulating material ejected from the Martian surface ever since the earliest periods of Martian history, over 4.4 Gyrs ago. Analysis of Martian ejecta, material accumulation, capture cross-section, regolith overturn, and Deimos's albedo suggest that Mars material may make up as much as 5-10% of Deimos's regolith. The Martian material on Deimos would be dominated by ejecta from the ancient crust of Mars, delivered during the Noachian Period of basin-forming impacts and heavy bombardment. Deimos is essentially a repository of samples from ancient Mars, which would include the full range of Martian crustal and upper mantle material from the early differentiation and crustal-forming epoch as well as samples from the era of high volatile flux, thick atmosphere, and possible surface water. The Gulliver Mission proposes to directly collect up to 10 kilograms of Deimos regolith and return it to Earth. This sample will contain up to 1000 grams of Martian material. Because of stochastic processes of regolith mixing over 4.4 Gyrs, the rock fragments, grains, and pebble-sized materials will likely sample the diversity of the Martian ancient surface. In addition to Martian ejecta, 90% of the Deimos sample will be spectral type D asteroidal material, thought to be highly primitive and originate in the outer asteroid belt. In essence, Gulliver represents two shortcuts, to Mars sample return and to the outer asteroid belt.

Anomalous Structure of Oceanic Lithosphere in the North Atlantic and Arctic Oceans: A Preliminary Analysis Based on Bathymetry, Gravity and Crustal Structure

NASA Astrophysics Data System (ADS)

Barantsrva, O.

2014-12-01

We present a preliminary analysis of the crustal and upper mantle structure for off-shore regions in the North Atlantic and Arctic oceans. These regions have anomalous oceanic lithosphere: the upper mantle of the North Atlantic ocean is affected by the Iceland plume, while the Arctic ocean has some of the slowest spreading rates. Our specific goal is to constrain the density structure of the upper mantle in order to understand the links between the deep lithosphere dynamics, ocean spreading, ocean floor bathymetry, heat flow and structure of the oceanic lithosphere in the regions where classical models of evolution of the oceanic lithosphere may not be valid. The major focus is on the oceanic lithosphere, but the Arctic shelves with a sufficient data coverage are also included into the analysis. Out major interest is the density structure of the upper mantle, and the analysis is based on the interpretation of GOCE satellite gravity data. To separate gravity anomalies caused by subcrustal anomalous masses, the gravitational effect of water, crust and the deep mantle is removed from the observed gravity field. For bathymetry we use the global NOAA database ETOPO1. The crustal correction to gravity is based on two crustal models: (1) global model CRUST1.0 (Laske, 2013) and, for a comparison, (2) a regional seismic model EUNAseis (Artemieva and Thybo, 2013). The crustal density structure required for the crustal correction is constrained from Vp data. Previous studies have shown that a large range of density values corresponds to any Vp value. To overcome this problem and to reduce uncertainty associated with the velocity-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), and apply different Vp-density conversions for different parts of the region. We present preliminary results, which we use to examine factors that control variations in bathymetry, sedimentary and crustal thicknesses in these anomalous oceanic domains.

Ophiolitic basement to the Great Valley forearc basin, California, from seismic and gravity data: Implications for crustal growth at the North American continental margin

USGS Publications Warehouse

Godfrey, N.J.; Beaudoin, B.C.; Klemperer, S.L.; Levander, A.; Luetgert, J.; Meltzer, A.; Mooney, W.; Tréhu, A.

1997-01-01

The nature of the Great Valley basement, whether oceanic or continental, has long been a source of controversy. A velocity model (derived from a 200-km-long east-west reflection-refraction profile collected south of the Mendocino triple junction, northern California, in 1993), further constrained by density and magnetic models, reveals an ophiolite underlying the Great Valley (Great Valley ophiolite), which in turn is underlain by a westward extension of lower-density continental crust (Sierran affinity material). We used an integrated modeling philosophy, first modeling the seismic-refraction data to obtain a final velocity model, and then modeling the long-wavelength features of the gravity data to obtain a final density model that is constrained in the upper crust by our velocity model. The crustal section of Great Valley ophiolite is 7-8 km thick, and the Great Valley ophiolite relict oceanic Moho is at 11-16 km depth. The Great Valley ophiolite does not extend west beneath the Coast Ranges, but only as far as the western margin of the Great Valley, where the 5-7-km-thick Great Valley ophiolite mantle section dips west into the present-day mantle. There are 16-18 km of lower-density Sierran affinity material beneath the Great Valley ophiolite mantle section, such that a second, deeper, "present-day" continental Moho is at about 34 km depth. At mid-crustal depths, the boundary between the eastern extent of the Great Valley ophiolite and the western extent of Sierran affinity material is a near-vertical velocity and density discontinuity about 80 km east of the western margin of the Great Valley. Our model has important implications for crustal growth at the North American continental margin. We suggest that a thick ophiolite sequence was obducted onto continental material, probably during the Jurassic Nevadan orogeny, so that the Great Valley basement is oceanic crust above oceanic mantle vertically stacked above continental crust and continental mantle.

On the development of the calc-alkaline and tholeiitic magma series: A deep crustal cumulate perspective

NASA Astrophysics Data System (ADS)

Chin, Emily J.; Shimizu, Kei; Bybee, Grant M.; Erdman, Monica E.

2018-01-01

Two distinct igneous differentiation trends - the tholeiitic and calc-alkaline - give rise to Earth's oceanic and continental crust, respectively. Mantle melting at mid-ocean ridges produces dry magmas that differentiate at low-pressure conditions, resulting in early plagioclase saturation, late oxide precipitation, and Fe-enrichment in mid-ocean ridge basalts (MORBs). In contrast, magmas formed above subduction zones are Fe-depleted, have elevated water contents and are more oxidized relative to MORBs. It is widely thought that subduction of hydrothermally altered, oxidized oceanic crust at convergent margins oxidizes the mantle source of arc magmas, resulting in erupted lavas that inherit this oxidized signature. Yet, because our understanding of the calc-alkaline and tholeiitic trends largely comes from studies of erupted melts, the signals from shallow crustal contamination by potentially oxidized, Si-rich, Fe-poor materials, which may also generate calc-alkaline rocks, are obscured. Here, we use deep crustal cumulates to "see through" the effects of shallow crustal processes. We find that the tholeiitic and calc-alkaline trends are indeed reflected in Fe-poor mid-ocean ridge cumulates and Fe-rich arc cumulates, respectively. A key finding is that with increasing crustal thickness, arc cumulates become more Fe-enriched. We propose that the thickness of the overlying crustal column modulates the melting degree of the mantle wedge (lower F beneath thick arcs and vice versa) and thus water and Fe3+ contents in primary melts, which subsequently controls the onset and extent of oxide fractionation. Deep crustal cumulates beneath thick, mature continental arcs are the most Fe-enriched, and therefore may be the "missing" Fe-rich reservoir required to balance the Fe-depleted upper continental crust.

Control of early-formed vesicle cylinders on upper crustal prismatic jointing in compound pāhoehoe lavas of Elephanta Island, western Deccan Traps, India

NASA Astrophysics Data System (ADS)

Sheth, Hetu; Patel, Vanit; Samant, Hrishikesh

2017-08-01

Upper crustal prismatic joints and vesicle cylinders, common in pāhoehoe lava flows, form early and late, respectively, and are therefore independent features. However, small-scale compound pāhoehoe lava lobes on Elephanta Island (western Deccan Traps, India), which resemble S-type (spongy) pāhoehoe in some aspects, contain vesicle cylinders which apparently controlled the locations of upper crustal prismatic joints. The lobes are decimeters thick, did not experience inflation after emplacement, and solidified rapidly. They have meter-scale areas that are exceptionally rich in vesicle cylinders (up to 68 cylinders in 1 m2, with a mean spacing of 12.1 cm), separated by cylinder-free areas, and pervasive upper crustal prismatic jointing with T, curved T, and quadruple joint intersections. A majority (≥76.5%) of the cylinders are located exactly on joints or at joint intersections, and were not simply captured by downward growing joints, as the cylinders show no deflection in vertical section. We suggest that large numbers of cylinders originated in a layer of bubble-rich residual liquid at the top of a basal diktytaxitic crystal mush zone which was formed very early (probably within the first few minutes of the emplacement history). The locations where the rising cylinders breached the crust provided weak points or mechanical flaws towards which any existing joints (formed by thermal contraction) propagated. New joints may also have propagated outwards from the cylinders and linked up laterally. Some cylinders breached the crust between the joints, and thus formed a little later than most others. The Elephanta Island example reveals that, whereas thermal contraction is undoubtedly valid as a standard mechanism for forming upper crustal prismatic joints, abundant mechanical flaws (such as large concentrations of early-formed, crust-breaching vesicle cylinders) can also control the joint formation process.

A crustal model of the ultrahigh-pressure Dabie Shan orogenic belt, China, derived from deep seismic refraction profiling

USGS Publications Warehouse

Wang, Chun-Yong; Zeng, Rong-Sheng; Mooney, W.D.; Hacker, B.R.

2000-01-01

We present a new crustal cross section through the east-west trending ultrahigh-pressure (UHP) Dabie Shan orogenic belt, east central China, based on a 400-km-long seismic refraction profile. Data from our profile reveal that the cratonal blocks north and south of the orogen are composed of 35-km-thick crust consisting of three layers (upper, middle, and lower crust) with average seismic velocities of 6.0±0.2 km/s, 6.5±0.1 km/s, and 6.8±0.1 km/s. The crust reaches a maximum thickness of 41.5 km beneath the northern margin of the orogen, and thus the present-day root beneath the orogen is only 6.5 km thick. The upper mantle velocity is 8.0±0.1 km/s. Modeling of shear wave data indicate that Poisson's ratio increases from 0.24±0.02 in the upper crust to 0.27±0.03 in the lower crust. This result is consistent with a dominantly felsic upper crustal composition and a mafic lower crustal composition within the amphibolite or granulite metamorphic facies. Our seismic model indicates that eclogite, which is abundant in surface exposures within the orogen, is not a volumetrically significant component in the middle or lower crust. Much of the Triassic structure associated with the formation of the UHP rocks of the Dabie Shan has been obscured by post-Triassic igneous activity, extension and large-offset strike-slip faulting. Nevertheless, we can identify a high-velocity (6.3 km/s) zone in the upper (<5 km depth) crustal core of the orogen which we interpret as a zone of ultrahigh-pressure rocks, a north dipping suture, and an apparent Moho offset that marks a likely active strike-slip fault.

Crustal Heterogeneity in the Basin and Range,

DTIC Science & Technology

1995-08-14

plutonism ). Seismic velocities are taken from laboratory measurements of rocks with similar compositions and are consistent with the bulk velocities... plutons intruded into Proterozoic North American crust in the Chocolate Mountains (Figure 2, upper crust) as describing the entire crustal column

Li Isotope Studies of Olivine in Mantle Xenoliths by SIMS

NASA Technical Reports Server (NTRS)

Bell, D. R.; Hervig, R. L.; Buseck, P. R.

2005-01-01

Variations in the ratio of the stable isotopes of Li are a potentially powerful tracer of processes in planetary and nebular environments [1]. Large differences in the 7Li/6Li ratio between the terrestrial upper mantle and various crustal materials make Li isotope composition a potentially powerful tracer of crustal recycling processes on Earth [2]. Recent SIMS studies of terrestrial mantle and Martian meteorite samples report intra-mineral Li isotope zoning [3-5]. Substantial Li isotope heterogeneity also exists within and between the components of chondritic meteorites [6,7]. Experimental studies of Li diffusion suggest the potential for rapid isotope exchange at elevated temperatures [8]. Large variations in 7Li, exceeding the range of unaltered basalts, occur in terrestrial mantle-derived xenoliths from individual localities [9]. The origins of these variations are not fully understood.

Regional magnetic anomalies, crustal strength, and the location of the northern Cordilleran fold-and-thrust belt

USGS Publications Warehouse

Saltus, R.W.; Hudson, T.L.

2007-01-01

The northern Cordilleran fold-and-thrust belt in Canada and Alaska is at the boundary between the broad continental margin mobile belt and the stable North American craton. The fold-and-thrust belt is marked by several significant changes in geometry: cratonward extensions in the central Yukon Territory and northeastern Alaska are separated by marginward re-entrants. These geometric features of the Cordilleran mobile belt are controlled by relations between lithospheric strength and compressional tectonic forces developed along the continental margin. Regional magnetic anomalies indicate deep thermal and compositional characteristics that contribute to variations in crustal strength. Our detailed analysis of one such anomaly, the North Slope deep magnetic high, helps to explain the geometry of the fold-and-thrust front in northern Alaska. This large magnetic anomaly is inferred to reflect voluminous mafic magmatism in an old (Devonian?) extensional domain. The presence of massive amounts of malic material in the lower crust implies geochemical depletion of the underlying upper mantle, which serves to strengthen the lithosphere against thermal erosion by upper mantle convection. We infer that deep-source magnetic highs are an important indicator of strong lower crust and upper mantle. This stronger lithosphere forms buttresses that play an important role in the structural development of the northern Cordilleran fold-and-thrust belt. ?? 2007 The Geological Society of America.

Structure of the crust and upper mantle in the western United States

USGS Publications Warehouse

Pakiser, L.C.

1963-01-01

Seismic waves generated by underground nuclear and chemical explosions have been recorded in a network of nearly 2,000 stations in the western conterminous United States as a part of the VELA UNIFORM program. The network extends from eastern Colorado to the California coastline and from central Idaho to the border of the United States and Mexico. The speed of compressional waves in the upper-mantle rocks ranges from 7.7 km/sec in the southern part of the Basin and Range province to 8.2 km/sec in the Great Plains province. In general, the speed of compressional waves in the upper-mantle rocks tends to be nearly the same over large areas within individual geologic provinces. Measured crustal thickness ranges from less than 20 km in the Central Valley of California to 50 km in the Great Plains province. Changes in crustal thickness across provincial boundaries are not controlled by regional altitude above sea level unless the properties of the upper mantle are the same across those boundaries. The crust tends to be thick in regions where the speed of compressional waves in the upper-mantle rocks (and presumably the density) is high, and tends to be relatively thin where the speed of compressional waves in the upper-mantle rocks (and density) is lower. With in the Basin and Range province, crustal thickness seems to vary directly with regional altitude above sea level. Evidence that a layer of intermediate compressional-wave speed exists in the lower part of the crust has been accumulated from seismic waves that have traveled least-time paths, as well as secondary arrivals (particularly reflections). On a scale that includes many geologic provinces, isostatic compensation is related largely to variations in the density of the upper- mantle rocks. Within geologic provinces or adjacent provinces, isostatic compensation may be related to variations in the thickness of crustal layers. Regions of thick crust and dense upper mantle have been relatively stable in Cenozoic time. Regions of thinner crust and low-density upper mantle have had a Cenozoic history of intense diastrophism and silicic volcanism.

A Global 3D P-Velocity Model of the Earth’s Crust and Mantle for Improved Event Location -- SALSA3D

DTIC Science & Technology

2010-09-01

incorporates variable resolution in both the geographic and radial dimensions. For our starting model, we use a simplified two layer crustal model derived from... crustal model derived from the Crust 2.0 model over a uniform AK135 mantle. Sufficient damping is used to reduce velocity adjustments so that ray path...upper mantle, and a third tessellation with variable resolution to all crustal layers. The crustal tessellation (not shown) has 2° triangles in oceanic

A Regional Seismic Travel Time Model for North America

DTIC Science & Technology

2010-09-01

velocity at the Moho, the mantle velocity gradient, and the average crustal velocity. After tomography across Eurasia, rigorous tests find that Pn...velocity gradient, and the average crustal velocity. After tomography across Eurasia rigorous tests find that Pn travel time residuals are reduced...and S-wave velocity in the crustal layers and in the upper mantle. A good prior model is essential because the RSTT tomography inversion is invariably

GLIMPCE Seismic reflection evidence of deep-crustal and upper-mantle intrusions and magmatic underplating associated with the Midcontinent Rift system of North America

USGS Publications Warehouse

Behrendt, John C.; Hutchinson, D.R.; Lee, M.; Thornber, C.R.; Tréhu, A.; Cannon, W.; Green, A.

1990-01-01

Deep-crustal and Moho reflections, recorded on vertical incidence and wide angle ocean bottom Seismometer (OBS) data in the 1986 GLIMPCE (Great Lakes International Multidisciplinary Program on Crustal Evolution) experiment, provide evidence for magmatic underplating and intrusions within the lower crust and upper mantle contemporaneous with crustal extension in the Midcontinent Rift system at 1100 Ma. The rift fill consists of 20-30 km (7-10 s) of basalt flows, secondary syn-rift volcaniclastic and post-basalt sedimentary rock. Moho reflections recorded in Lake Superior over the Midcontinent Rift system have times from 14-18 s (about 46 km to as great as 58 km) in contrast to times of about 11-13 s (about 36-42 km crustal thickness) beneath the surrounding Great Lakes. The Seismically complex deep-crust to mantle transition zone (30-60 km) in north-central Lake Superior, which is 100 km wider than the rift half-graben, reflects the complicated products of tectonic and magmatic interaction of lower-crustal and mantle components during evolution or shutdown of the aborted Midcontinent Rift. In effect, mantle was changed into crust by lowering Seismic velocity (through intrusion of lower density magmatic rocks) and increasing Moho (about 8.1 km s-1 depth. 

Crustal and upper mantle S-wave velocity structures across the Taiwan Strait from ambient seismic noise and teleseismic Rayleigh wave analyses

NASA Astrophysics Data System (ADS)

Huang, Y.; Yao, H.; Wu, F. T.; Liang, W.; Huang, B.; Lin, C.; Wen, K.

2013-12-01

Although orogeny seems to have stopped in western Taiwan large and small earthquakes do occur in the Taiwan Strait. Limited studies have focused on this region before and were barely within reach for comprehensive projects like TAICRUST and TAIGER for logistical reasons; thus, the overall crustal structures of the Taiwan Strait remain unknown. Time domain empirical Green's function (TDEGF) from ambient seismic noise to determine crustal velocity structure allows us to study an area using station pairs on its periphery. This research aims to resolve 1-D average crustal and upper mantle S-wave velocity (Vs) structures alone paths of several broadband station-pairs across the Taiwan Strait; 5-120 s Rayleigh wave phase velocity dispersion data derived by combining TDEGF and traditional surface wave two-station method (TS). The average Vs structures show significant differences in the upper 15 km as expected. In general, the highest Vs are observed in the coastal area of Mainland China and the lowest Vs appear along the southwest offshore of the Taiwan Island; they differ by about 0.6-1.1 km/s. For different parts of the Strait, the Vs are lower in the middle by about 0.1-0.2 km/s relative to those in the northern and southern parts. The overall crustal thickness is approximately 30 km, much thinner and less variable than under the Taiwan Island.

Mantle convection pattern and subcrustal stress field under South America

NASA Technical Reports Server (NTRS)

Liu, H.-S.

1980-01-01

The tectonic, igneous and metallogenic features of South America are discussed in terms of the crustal deformation associated with stresses due to mantle convection as inferred from the high degree harmonics in the geopotential field. The application of Runcorn's model for the laminar viscous flows in the upper mantle to satellite and gravity data results in a convection pattern which reveals the ascending flows between the descending Nazca plate and the overlying South American plate as well as segments of the descending Nazca plate beneath South America. The arc volcanism in South America is shown apparently to be related to the upwelling of high-temperature material induced by the subduction of the Nazca plate, with the South American basin systems associated with downwelling mantle flows. The resulting tensional stress fields are shown to be regions of structural kinship characterized by major concentrations of ore deposits and related to the cordillera, shield and igneous systems and the upward Andean movements. It is suggested that the upwelling convection flows in the upper mantle, coupled with crustal tension, have provided an uplift mechanism which has forced the hydrothermal systems in the basement rocks to the surface.

Has Martian History Been Dominated by Explosive Rather than Effusive Volcanism?

NASA Astrophysics Data System (ADS)

Bandfield, J. L.; Edwards, C. S.; Montgomery, D. R.

2010-12-01

It is important to gain a clear understanding of basic physical properties of the upper martian crust. We can use these derived properties to test a range of plausible formation mechanisms and place constraints on the processes involved in the creation of the martian crust. Previous studies have addressed this problem using a variety of techniques and observations. It has been well-established that the martian upper crust is typically mechanically weak (e.g. Pike, 1980; Schultz, 2002; Stewart and Valiant, 2006) and the notion of a highly fractured mega-regolith has often been invoked as the cause of this weakness. There are apparent contradictions in the interpretations of separate observations, such as the fine-scale layering in canyon walls that would not be preserved in a mega-regolith (McEwen et al., 1999). In all cases, however, the original material that makes up either the layering or mega-regolith has been assumed to originate as effusive volcanic materials. We have re-examined the body of previous work in the light of more recent global thermophysical observations to place further constraints on the nature of the upper martian crust. Although the upper ~10 km of the crust is indeed mechanically weak, consistent with previous studies, these crustal materials are also inconsistent with a mega-regolith composed of fractured blocks. Thermal inertia derived from Thermal Emission Imaging System (THEMIS) data, High Resolution Imaging Science Experiment (HiRISE) images, and Mars Exploration Rover observations clearly indicate that the upper martian crust is more typically composed of weakly consolidated fine-particulate materials. These materials are consistent with a volcaniclastic origin rather than effusive volcanism. Mechanically competent material akin to what might be derived from lava flows is clearly present on Mars in locations such as Hesperia Planum and at low latitudes within the northern lowlands, but it is much less common than has been assumed. It appears that Mars has a unique style of crustal formation that is distinct from that of the Moon and the other terrestrial planets. References: McEwen, A. S., and 14 colleagues (2007) Mars Reconnaissance Orbiter's High Resolution Imaging Science Experiment (HiRISE). J. Geophys. Res., 112, 10.1029/2005JE002605. Pike, R.J. (1980) Control of crater morphology by gravity and target type - Mars, earth, moon. Proc. Lun. Planet. Sci. Conf., 11, 2159-2189. Schultz, R. A. (2002) Stability of rock slopes in Valles Marineris, Mars. Geophys. Res. Let., 29, 10.1029/2002GL015728. Stewart, S. T. and G. J. Valiant (2006) Martian subsurface properties and crater formation processes inferred from fresh impact crater geometries. Meteor. Planet. Sci., 41, 1509-1537.

A deep crustal fluid channel into the San Andreas Fault system near Parkfield, California

USGS Publications Warehouse

Becken, M.; Ritter, O.; Park, S.K.; Bedrosian, P.A.; Weckmann, U.; Weber, M.

2008-01-01

Magnetotelluric (MT) data from 66 sites along a 45-km-long profile across the San Andreas Fault (SAF) were inverted to obtain the 2-D electrical resistivity structure of the crust near the San Andreas Fault Observatory at Depth (SAFOD). The most intriguing feature of the resistivity model is a steeply dipping upper crustal high-conductivity zone flanking the seismically defined SAF to the NE, that widens into the lower crust and appears to be connected to a broad conductivity anomaly in the upper mantle. Hypothesis tests of the inversion model suggest that upper and lower crustal and upper-mantle anomalies may be interconnected. We speculate that the high conductivities are caused by fluids and may represent a deep-rooted channel for crustal and/or mantle fluid ascent. Based on the chemical analysis of well waters, it was previously suggested that fluids can enter the brittle regime of the SAF system from the lower crust and mantle. At high pressures, these fluids can contribute to fault-weakening at seismogenic depths. These geochemical studies predicted the existence of a deep fluid source and a permeable pathway through the crust. Our resistivity model images a conductive pathway, which penetrates the entire crust, in agreement with the geochemical interpretation. However, the resistivity model also shows that the upper crustal branch of the high-conductivity zone is located NE of the seismically defined SAF, suggesting that the SAF does not itself act as a major fluid pathway. This interpretation is supported by both, the location of the upper crustal high-conductivity zone and recent studies within the SAFOD main hole, which indicate that pore pressures within the core of the SAF zone are not anomalously high, that mantle-derived fluids are minor constituents to the fault-zone fluid composition and that both the volume of mantle fluids and the fluid pressure increase to the NE of the SAF. We further infer from the MT model that the resistive Salinian block basement to the SW of the SAFOD represents an isolated body, being 5-8km wide and reaching to depths >7km, in agreement with aeromagnetic data. This body is separated from a massive block of Salinian crust farther to the SW. The NE terminus of resistive Salinian crust has a spatial relationship with a near-vertical zone of increased seismic reflectivity ???15km SW of the SAF and likely represents a deep-reaching fault zone. ?? 2008 The Authors Journal compilation ?? 2008 RAS.

The Transition from Volcanic to Rift Dominated Crustal Breakup - From the Vøring Plateau to the Lofoten Margin, Norway

NASA Astrophysics Data System (ADS)

Breivik, A. J.; Faleide, J. I.; Mjelde, R.; Flueh, E.; Murai, Y.

2017-12-01

The Vøring Plateau was part of the Northeast Atlantic igneous province (NAIP) during early Cenozoic crustal breakup. Crustal breakup at the Vøring Plateau occurred marginal to the deep Cretaceous basins on the shelf, with less extension of the crust. Intrusive magmatism and oceanic crust up to three times normal thickness caused a period of sub-aerial magmatism around breakup time. The transition to the Lofoten Margin is rapid to a deep-water plain. Still, there is some excess magmatism north of this transition, where early oceanic crustal thickness is reduced to half of that of the Vøring Plateau 150 km away. Our estimates of the earliest seafloor spreading rates using new ship-track magnetic profiles on different margin segments offer a clue to what caused this rapid transition. While crustal breakup occurred within the magnetic polarity C24r in other parts of the NAIP, there is a delayed breakup for the Lofoten/Vesterålen margin. Modeling of the earliest seafloor spreading with geomagnetic reversals, indicate a breakup within C24n.3n (anomaly 24b), approximately 1 m.y. later. Both old wide-angle seismic models (from Ocean Bottom Seismometers) off southern Lofoten and a newly published profile farther north show a strongly extended outer margin. Applying early seafloor half-spreading rates ( 30 mm/y) from other NAIP margin segments for 1 m.y. can account for 30 km extra extension, giving a factor of three crustal thinning, and gives a high strain rate of 3.2 ·10-14. Crustal breakup at the magma-poor Iberian Margin occurred at a low strain rate of 4.4·10-15, allowing the ascending mantle to cool, favoring tectonic extension over magmatism. Similar strain rates are found within the main Ethiopian Rift, but there is much magmatism and crustal separation is dominated by dike injection. Mantle tomography models show an exceptionally low seismic velocity below the area interpreted as an unusually hot upper mantle, which will favor magmatism. The transition from the Vøring Plateau to the Lofoten Margin can therefore be explained by the presence/absence of hot mantle plume material under the different segments during rifting. Only after significant extension and close to crustal breakup time did a minor amount of plume material reach the Lofoten/Vesterålen margin to cause some elevated but short-lived excess magmatism there.

Saudi Arabian seismic-refraction profile: A traveltime interpretation of crustal and upper mantle structure

USGS Publications Warehouse

Mooney, W.D.; Gettings, M.E.; Blank, H.R.; Healy, J.H.

1985-01-01

The crustal and upper mantle compressional-wave velocity structure across the southwestern Arabian Shield has been investigated by a 1000-km-long seismic refraction profile. The profile begins in Mesozoic cover rocks near Riyadh on the Arabian Platform, trends southwesterly across three major Precambrian tectonic provinces, traverses Cenozoic rocks of the coastal plain near Jizan, and terminates at the outer edge of the Farasan Bank in the southern Red Sea. More than 500 surveyed recording sites were occupied, and six shot points were used, including one in the Red Sea. Two-dimensional ray-tracing techniques, used to analyze amplitude-normalized record sections indicate that the Arabian Shield is composed, to first order, of two layers, each about 20 km thick, with average velocities of about 6.3 km/s and 7.0 km/s, respectively. West of the Shield-Red Sea margin, the crust thins to a total thickness of less than 20 km, beyond which the Red Sea shelf and coastal plain are interpreted to be underlain by oceanic crust. A major crustal inhomogeneity at the northeast end of the profile probably represents the suture zone between two crustal blocks of different composition. Elsewhere along the profile, several high-velocity anomalies in the upper crust correlate with mapped gneiss domes, the most prominent of which is the Khamis Mushayt gneiss. Based on their velocities, these domes may constitute areas where lower crustal rocks have been raised some 20 km. Two intracrustal reflectors in the center of the Shield at 13 km depth probably represent the tops of mafic intrusives. The Mohorovic??ic?? discontinuity beneath the Shield varies from a depth of 43 km and mantle velocity of 8.2 km/s in the northeast to a depth of 38 km and mantle velocity of 8.0 km/s depth in the southwest near the Shield-Red Sea transition. Two velocity discontinuities occur in the upper mantle, at 59 and 70 km depth. The crustal and upper mantle velocity structure of the Arabian Shield is interpreted as revealing a complex crust derived from the suturing of island arcs in the Precarnbrian. The Shield is currently flanked by the active spreading boundary in the Red Sea. ?? 1985.

Are arc lower crustal metasediments derived from above or below? A detrital zircon study in the lower crust of the Sierra Nevada, California

NASA Astrophysics Data System (ADS)

Klein, B. Z.; Jagoutz, O. E.; VanTongeren, J. A.

2016-12-01

Multiple hypotheses exist to explain the presence of metasedimentary rocks within arc lower crust. Relamination and subduction underplating require that sediments are derived from the subducted slab, while processes such as wall-rock return flow and retro-arc underthrusting imply that the sediments originated in the crust of the upper plate. Evaluating these proposed mechanisms has wide-reaching implications, including better constraining the mass-balance of active arcs, characterizing a theorized trigger mechanism for magmatic flare-up events, and more broadly for describing the tectonic construction of continental arcs. The southernmost Sierra Nevada, California, exposes a continuous continental arc cross-section that spans pressures from 3 to <10 kbar. Metasedimentary rocks are exposed at all crustal levels within this section and are intruded by 100 Ma igneous rocks. These metasediments offer a unique opportunity to evaluate the source, and emplacement of lower crustal metasediments into an active arc. The proposed mechanisms for the transport of sediments to the lower crust predict distinct sedimentary protoliths with unique detrital zircon (DZ) age spectra. Specifically, slab-derived sediments are likely to resemble the underplated Polona-Oroccopia-Rand schists to the south, with dominantly Mesozoic DZ peaks and few to no older grains. Upper plate derived sediments are predicted to have significant Paleozoic and Proterozoic DZ populations, in addition to arc-derived, Mesozoic meta-volcanic material. We have conducted a detailed DZ study of metasedimentary rocks in the Sierran lower and middle crust to assess these hypotheses. Initial results show that at least some of this material has an unambiguous slab-derived signature implying that relamination and/or subduction underplating were active processes during the construction of the Sierran arc system. We explore the implications of these processes for the magmatic and tectonic history of the Sierra Nevada, as well as for the generation of new continental crust.

Viscous relaxation of impact crater relief on Venus - Constraints on crustal thickness and thermal gradient

NASA Technical Reports Server (NTRS)

Grimm, Robert E.; Solomon, Sean C.

1988-01-01

Models for the viscous relaxation of impact crater topography are used to constrain the crustal thickness (H) and the mean lithospheric thermal gradient beneath the craters on Venus. A general formulation for gravity-driven flow in a linearly viscous fluid has been obtained which incorporates the densities and temperature-dependent effective viscosities of distinct crust and mantle layers. An upper limit to the crustal volume of Venus of 10 to the 10th cu km is obtained which implies either that the average rate of crustal generation has been much smaller on Venus than on earth or that some form of crustal recycling has occurred on Venus.

Venus tectonic styles and crustal differentiation

NASA Technical Reports Server (NTRS)

Kaula, W. M.; Lenardic, A.

1992-01-01

Two of the most important constraints are known from Pioneer Venus data: the lack of a system of spreading rises, indicating distributed deformation rather than plate tectonics; and the high gravity/topography ratio, indicating the absence of an asthenosphere. In addition, the high depth/diameter ratios of craters on Venus indicate that Venus probably has no more crust than Earth. The problems of the character of tectonics and crustal formation and recycling are closely coupled. Venus appears to lack a recycling mechanism as effective as subduction, but may also have a low rate of crustal differentiation because of a mantle convection pattern that is more distributed, less concentrated, than Earth's. Distributed convection, coupled with the nonlinear dependence of volcanism on heat flow, would lead to much less magmatism, despite only moderately less heat flow, compared to Earth. The plausible reason for this difference in convective style is the absence of water in the upper mantle of Venus. We have applied finite element modeling to problems of the interaction of mantle convection and crust on Venus. The main emphasis has been on the tectonic evolution of Ishtar Terra, as the consequence of convergent mantle flow. The early stage evolution is primarily mechanical, with crust being piled up on the down-stream side. Then the downflow migrates away from the center. In the later stages, after more than 100 m.y., thermal effects develop due to the insulating influence of the thickened crust. An important feature of this modeling is the entrainment of some crustal material in downflows. An important general theme in both convergent and divergent flows is that of mixing vs. stratification. Models of multicomponent solid-state flow obtain that lower-density crustal material can be entrained and recycled, provided that the ration of low-density to high-density material is small enough (as in subducted slabs on Earth). The same considerations should apply in upflows; a small percent of partial melt may be carried along with its matrix and never escape to the surface. Models that assume melt automatically rising to the crust and no entrainment or other mechanism of recycling lower-density material obtain oscillatory behavior, because it takes a long time for heat to build up enough to overcome a Mg-rich low-density residuum. However, these models develop much thicker crust than consistent with estimates from crater depth/diameter ratios.

Secondary chaotic terrain formation in the higher outflow channels of southern circum-Chryse, Mars

USGS Publications Warehouse

Rodriguez, J.A.P.; Kargel, J.S.; Tanaka, K.L.; Crown, D.A.; Berman, D.C.; Fairen, A.G.; Baker, V.R.; Furfaro, R.; Candelaria, P.; Sasaki, S.

2011-01-01

Higher outflow channel dissection in the martian region of southern circum-Chryse appears to have extended from the Late Hesperian to the Middle Amazonian Epoch. These outflow channels were excavated within the upper 1. km of the cryolithosphere, where no liquid water is expected to have existed during these geologic epochs. In accordance with previous work, our examination of outflow channel floor morphologies suggests the upper crust excavated by the studied outflow channels consisted of a thin (a few tens of meters) layer of dry geologic materials overlying an indurated zone that extends to the bases of the investigated outflow channels (1. km in depth). We find that the floors of these outflow channels contain widespread secondary chaotic terrains (i.e., chaotic terrains produced by the destruction of channel-floor materials). These chaotic terrains occur within the full range of outflow channel dissection and tend to form clusters. Our examination of the geology of these chaotic terrains suggests that their formation did not result in the generation of floods. Nevertheless, despite their much smaller dimensions, these chaotic terrains are comprised of the same basic morphologic elements (e.g., mesas, knobs, and smooth deposits within scarp-bound depressions) as those located in the initiation zones of the outflow channels, which suggests that their formation must have involved the release of ground volatiles. We propose that these chaotic terrains developed not catastrophically but gradually and during multiple episodes of nested surface collapse. In order to explain the formation of secondary chaotic terrains within zones of outflow channel dissection, we propose that the regional Martian cryolithosphere contained widespread lenses of volatiles in liquid form. In this model, channel floor collapse and secondary chaotic terrain formation would have taken place as a consequence of instabilities arising during their exhumation by outflow channel dissection. Within relatively warm upper crustal materials in volcanic settings, or within highly saline crustal materials where cryopegs developed, lenses of volatiles in liquid form within the cryolithosphere could have formed, and/or remained stable.In addition, our numerical simulations suggest that low thermal conductivity, dry fine-grained porous geologic materials just a few tens of meters in thickness (e.g., dunes, sand sheets, some types of regolith materials), could have produced high thermal anomalies resulting in subsurface melting. The existence of a global layer of dry geologic materials overlying the cryolithosphere would suggest that widespread lenses of fluids existed (and may still exist) at shallow depths wherever these materials are fine-grained and porous. The surface ages of the investigated outflow channels and chaotic terrains span a full 500 to 700. Myr. Chaotic terrains similar in dimensions and morphology to secondary chaotic terrains are not observed conspicuously throughout the surface of Mars, suggesting that intra-cryolithospheric fluid lenses may form relatively stable systems. The existence of widespread groundwater lenses at shallow depths of burial has tremendous implications for exobiological studies and future human exploration. We find that the clear geomorphologic anomaly that the chaotic terrains and outflow channels of southern Chryse form within the Martian landscape could have been a consequence of large-scale resurfacing resulting from anomalously extensive subsurface melt in this region of the planet produced by high concentrations of salts within the regional upper crust. Crater count statistics reveal that secondary chaotic terrains and the outflow channels within which they occur have overlapping ages, suggesting that the instabilities leading to their formation rapidly dissipated, perhaps as the thickness of the cryolithosphere was reset following the disruption of the upper crustal thermal structure produced during outflow channel ex

Crustal structure of Yunnan province, People's Republic of China, from seismic refraction profiles

USGS Publications Warehouse

Kan, R.-J.; Hu, H.-X.; Zeng, R.-S.; Mooney, W.D.; McEvilly, T.V.

1986-01-01

Seismic refraction, profiles in Yunnan Province, southwestern China, define the crustal structure in an area of active tectonics, on the southern end of the Himalaya-Burma arc. The crustal thickness ranges from 38 to 46 kilometers, and the relatively low mean crustal velocity indicates a crustal composition compatible with normal continental crust and consisting mainly of meta-sedimentary and silicic intrusive rocks, with little mafic or ultramafic component. This composition suggests a crustal evolution involving sedimentary processes on the flank of the Yangtze platform rather than the accretion of oceanic island arcs, as has been proposed. An anomalously low upper-mantle velocity observed on one profile, but not on another at right angles to it may indicate active tectonic processes in the mantle or seismic anisotropy.

Crustal Structure of Yunnan Province, People's Republic of China, from Seismic Refraction Profiles.

PubMed

Kan, R J; Hu, H X; Zeng, R S; Mooney, W D; McEvilly, T V

1986-10-24

Seismic refraction, profiles in Yunnan Province, southwestern China, define the crustal structure in an area of active tectonics on the southern end of the Himalaya-Burma arc. The crustal thickness ranges from 38 to 46 kilometers, and the relatively low mean crustal velocity indicates a crustal composition compatible with normal continental crust and consisting mainly of meta-sedimentary and silicic intrusive rocks, with little mafic or ultramafic component. This composition suggests a crustal evolution involving sedimentary processes on the flank of the Yangtze platform rather than the accretion of oceanic island arcs, as has been proposed. An anomalously low upper-mantle velocity observed on one profile but not on another at right angles to it may indicate active tectonic processes in the mantle or seismic anisotropy.

The isostatic state of the lunar Apennines and regional surroundings

NASA Technical Reports Server (NTRS)

Ferrari, A. J.; Sjogren, W. L.; Phillips, R. J.; Nelson, D. L.

1978-01-01

High-resolution gravity and topography data taken over the Apennine Mountains have been used to compute their isostatic state. Results show that the Apennines are uncompensated; thus this state implies that the lunar crust and upper mantle have been strong enough over 3.9 b.y. to support the load exerted by this topographic excess. The Apennines produce a maximum shear stress of 60 bars at a depth of 60 km. A lower bound on the lunar crustal viscosity of 10 to the 27th power P is calculated on the basis of the assumption of a 10% relaxation over 3.9 b.y. Studies of a broad negative regional anomaly located between Maria Serenitatis and Imbrium necessitate a locally thicker crust to satisfy the observed data. This anomaly may have been produced by a lateral transport of crustal material from beneath the giant impact basins as a result of rebound at the crust-mantle interface.

Fabric evolution across a discontinuity between lower and upper crustal domains from field, microscopic, and anisotropy of magnetic susceptibility studies in central eastern Eritrea, NE Africa

NASA Astrophysics Data System (ADS)

Ghebreab, W.; Kontny, A.; Greiling, R. O.

2007-06-01

In the Neoproterozoic East African Orogen (EAO) of Eritrea, lower to middle crustal high-grade metamorphic rocks are juxtaposed against low-grade upper crustal rocks along diffuse tectonic contact zones or discontinuities. In the central eastern part of Eritrea, such a tectonic zone is exposed as a low-angle shear zone separating two distinct high- and low-grade domains, the Ghedem and Bizen, respectively. Integrated field, microfabric, and anisotropy of magnetic susceptibility (AMS) studies show that this low-angle shear zone formed during late deformation, D2, with top-to-the-E/SE sense of motion. The hanging wall upper crustal volcanosedimentary schists are mainly paramagnetic and the footwall middle crustal mylonitized orthogneisses are mainly ferrimagnetic. Magnetic fabric studies revealed a good agreement between metamorphic/mylonitic and magnetic foliations (Kmin) and helped to explain fabric development in the shear zone. The magnetic lineations (Kmax) reflect stretching lineations where stretched mineral aggregates dominate fine-grained mylonitic matrices and intersection lineations where microstructural studies revealed two fabric elements. AMS directional plots indicate that the orientations of the magnetic lineation and of the pole to the magnetic foliation vary systematically across the shear zone. While Kmax axes form two broad maxima oriented approximately N-S and E-W, the Kmin axes change from subhorizontal, generally westward inclination in the west to moderate to steep inclination in the direction of tectonic movement to the east. Because there is a systematic change in inclination of Kmin for individual samples, all samples together form a fairly well defined cluster distribution. The distribution of Kmin in combination with the E-W scattered plot of the Kmax is in accordance with the E/SE flow of mylonites over exhumed Damas core complex in the late Neoproterozoic. During the Cenozoic, the Red Sea rift-related detachments exploited the late orogenic shear zone, indicating that the discontinuities between ductile middle and brittle upper crustal layers in the region are reactivated low-angle shear zones and possible sites of core complexes.

Mantle wedge anisotropy beneath the Western Alps: insights from Receiver Function analysis

NASA Astrophysics Data System (ADS)

Piana Agostinetti, Nicola; Salimbeni, Simone; Pondrelli, Silvia; Malusa', Marco; Zhao, Liang; Eva, Elena; Solarino, Stefano; Paul, Anne; Guillot, Stéphane; Schwartz, Stéphane; Dumont, Thierry; Aubert, Coralie; Wang, Qingchen; Zhu, Rixiang

2017-04-01

Orogens and subductions zones are the locus where crustal materials are recycled into the upper mantle. Such rocks undergo to several metamorphic reactions during which their seismic properties vary due to the changes in P-T conditions. Metamorphic reactions can imply: (a) the formation of schist-like materials, and (b) a pronounced water flux from the subducted crust. Both these processes should generate highly anisotropic volumes at upper mantle depths. Thus, unveiling the presence of seismic anisotropy at such depth level can put constraints on the metamorphic reactions and the P-T conditions of the subducted materials. The Alpine orogen is composed of three main regions where different geodynamic processes shaped a highly heterogeneous mountain chain. Beneath the Alps, a high velocity body has been imaged sinking in the upper mantle, indicating the presence of a relict of subduction. Such subduction process has been probably terminated with the closure of the Piemont-Liguria Ocean, but evidence of continental subduction has been found beneath the Western Alps. Seismic anisotropy is likely to develop both in the subducted materials and in the mantle wedge, where serpentinized materials could be found due to the low T conditions. We analysed P receiver function (RF) from 46 seismic stations deployed along a linear array crossing the Western Alps, where previous studies revealed the presence of the subducted European lower crust to 80 km depth. RF is a widely used tool for reconstructing subsurface seismic structures, based on the recognition of P-to-S converted phases in teleseismic P-wave coda. The RF data-set is migrated at depth and decomposed into azimuthal harmonics. Computing the first, k=0, and the second, k=1, harmonics allows to separate the "isotropic" contribution, due to the change of the isotropic properties of the sampled materials (recorded on the k=0 harmonics), from the "anisotropic" contribution, where the energy is related to the propagation of the P-wave through anisotropic materials (recorded on the k=1 harmonics). Preliminary results show the presence of a Ps phase on the k=0 harmonics along the western portion of the profile, with increasing time-delay toward East. This phase is interpreted as the European Moho Ps, confirming the geometry of the European Moho beneath the Western Alps. Beneath the internal portion of the orogen, the k=1 harmonics display energetic pulses between 3-7 s, indicating the development of anisotropy within a broad volume of rocks, at lower crustal and upper mantle depths. The presence of anisotropic materials is jointly interpreted with the depicted geometry of the main seismic discontinuities and the location of the intermediate-depth seismicity recorded in the region.

Seismological observations at the Northern Andean region of Colombia: Evidence for a shallowly subducting Caribbean Slab and an extensional regime in the upper plate

NASA Astrophysics Data System (ADS)

Monsalve, G.; Cardona, A.; Yarce, J.; Alvira, D.; Poveda, E.

2013-05-01

A number of seismological observations, among which we can mention teleseismic travel time residuals, P to S receiver functions and Pn velocity quantification, suggest a clear distinction between the seismic structure of the crust and uppermost mantle between the plains on the Caribbean coast of Colombia and the mountains at the Northern Andean region. Absolute and relative travel time residuals indicate the presence of a seismically fast material in the upper mantle beneath northern Colombia; preliminary results of Pn studies show a region of relatively slow Pn velocities (between 7.8 and 7.9 km/s) underneath the Caribbean coast, contrasting with values greater than 8 km/s beneath the Central and Western cordilleras of Colombia, and the Pacific coast; receiver functions suggest a significantly thinner crust beneath the Caribbean coast, with a crustal thickness between 25 and 30 km, than beneath the Northern Andean zone at the cordilleras of Colombia, where it exceeds 40 km and reaches about 57 km at the location of Bogota. Besides the obviuos discrepancies that appear in response to different topography, we think that the seismological observations are a consequence of the presence of two very distinct slab segments beneath Colombia and contrasting behaviors of the upper plate, which correspond to Caribbean and Nazca subductions. Our seismic observations can be explained by a shallowly subducting Caribbean Plate, in the absence of an asthenospheric wedge, that steepens at about the location of the Bucaramanga nest, and a thinned continental crust that reflects an extensional component linked to oblique convergence of the Caribbean, which contrasts with the crustal thickening in the Andean Cordillera linked to crustal shortening and Nazca plate subuction. These new data are consistent with the idea of of a relatively warm Nazca slab of Neogene age which seems to have a relatively frontal convergence, and a colder, more buoyant Caribbean slab which represents an oceanic plateau of Cretaceous age that is characterized by an oblique convergence relation that has promoted extensional tectonics in the upper plate.

The Crustal Structure And CTBT Monitoring Of India: New Insights From Deep Seismic Profiling

DTIC Science & Technology

2000-09-01

transitional type crust as a major source of Deccan trap flows. The Narmada-Son lineament is the most conspicuous linear geological feature in the... Deccan proto-continents) buckling of the upper and middle crustal layers of the proto-continents took place, resulting in the western block’s lower...crustal column subducting below the Deccan proto-continents. Thus, the collision process was of such severe magnitude that the impact was seen in both

3D Modeling of Iran and Surrounding Areas From Simultaneous Inversion of Multiple Geophysical Datasets

DTIC Science & Technology

2010-09-01

which are primarily sensitive to upper crustal structures, are difficult to measure and especially true in tectonically and geologically complex areas...slice through the model (compare Figure 6 and Figure 9). The fit to the receiver function is not perfect and the spread of the slower deep crustal ...Although the final fit is certainly not perfect, note the improvement in timing of the main crustal conversion and reverberation (vertical lines) from the

A Pn Spreading Model Constrained with Observed Amplitudes in Asia

DTIC Science & Technology

2011-09-01

and stations, from which we collected my data. According to Patton (1980), the “ tectonic ” province was defined as an area with its crustal thickness...and the definition of the “ tectonic ” province as a tectonically active region with similar crustal and upper-mantle structure in most parts of the...North Australian Craton: Influence of crustal velocity gradients, Bull. Seismol. Soc. Am. 81: 592–610. Brune, J. N. (1970). Tectonic stress and the

Crustal modeling of the central part of the Northern Western Desert, Egypt using gravity data

NASA Astrophysics Data System (ADS)

Alrefaee, H. A.

2017-05-01

The Bouguer anomaly map of the central part of the Northern Western Desert, Egypt was used to construct six 2D gravity models to investigate the nature, physical properties and structures of the crust and upper mantle. The crustal models were constrained and constructed by integrating results from different geophysical techniques and available geological information. The depth to the basement surface, from eight wells existed across the study area, and the depth to the Conrad and Moho interfaces as well as physical properties of sediments, basement, crust and upper mantle from previous petrophysical and crustal studies were used to establish the gravity models. Euler deconvolution technique was carried on the Bouguer anomaly map to detect the subsurface fault trends. Edge detection techniques were calculated to outlines the boundaries of subsurface structural features. Basement structural map was interpreted to reveal the subsurface structural setting of the area. The crustal models reveals increasing of gravity field from the south to the north due to northward thinning of the crust. The models reveals also deformed and rugged basement surface with northward depth increasing from 1.6 km to 6 km. In contrast to the basement, the Conrad and Moho interfaces are nearly flat and get shallower northward where the depth to the Conrad or the thickness of the upper crust ranges from 18 km to 21 km while the depth to the Moho (crustal thickness) ranges from 31.5 km to 34 km. The crust beneath the study area is normal continental crust with obvious thinning toward the continental margin at the Mediterranean coast.

Crustal structure in Tengchong Volcano-Geothermal Area, western Yunnan, China

NASA Astrophysics Data System (ADS)

Wang, Chun-Yong; Huangfu, Gang

2004-02-01

Based upon the deep seismic sounding profiles carried out in the Tengchong Volcano-Geothermal Area (TVGA), western Yunnan Province of China, a 2-D crustal P velocity structure is obtained by use of finite-difference inversion and forward travel-time fitting method. The crustal model shows that a low-velocity anomaly zone exists in the upper crust, which is related to geothermal activity. Two faults, the Longling-Ruili Fault and Tengchong Fault, on the profile extend from surface to the lower crust and the Tengchong Fault likely penetrates the Moho. Moreover, based on teleseismic receiver functions on a temporary seismic network, S-wave velocity structures beneath the geothermal field show low S-wave velocity in the upper crust. From results of geophysical survey, the crust of TVGA is characterized by low P-wave and S-wave velocities, low resistivity, high heat-flow value and low Q. The upper mantle P-wave velocity is also low. This suggests presence of magma in the crust derived from the upper mantle. The low-velocity anomaly in upper crust may be related to the magma differentiation. The Tengchong volcanic area is located on the northeast edge of the Indian-Eurasian plate collision zone, away from the eastern boundary of the Indian plate by about 450 km. Based on the results of this paper and related studies, the Tengchong volcanoes can be classified as plate boundary volcanoes.

Crustal and upper mantle velocity structure of the Salton Trough, southeast California

USGS Publications Warehouse

Parsons, T.; McCarthy, J.

1996-01-01

This paper presents data and modelling results from a crustal and upper mantle wide-angle seismic transect across the Salton Trough region in southeast California. The Salton Trough is a unique part of the Basin and Range province where mid-ocean ridge/transform spreading in the Gulf of California has evolved northward into the continent. In 1992, the U.S. Geological Survey (USGS) conducted the final leg of the Pacific to Arizona Crustal Experiment (PACE). Two perpendicular models of the crust and upper mantle were fit to wide-angle reflection and refraction travel times, seismic amplitudes, and Bouguer gravity anomalies. The first profile crossed the Salton Trough from the southwest to the northeast, and the second was a strike line that paralleled the Salton Sea along its western edge. We found thin crust (???21-22 km thick) beneath the axis of the Salton Trough (Imperial Valley) and locally thicker crust (???27 km) beneath the Chocolate Mountains to the northeast. We modelled a slight thinning of the crust further to the northeast beneath the Colorado River (???24 km) and subsequent thickening beneath the metamorphic core complex belt northeast of the Colorado River. There is a deep, apparently young basin (???5-6 km unmetamorphosed sediments) beneath the Imperial Valley and a shallower (???2-3 km) basin beneath the Colorado River. A regional 6.9-km/s layer (between ???15-km depth and the Moho) underlies the Salton Trough as well as the Chocolate Mountains where it pinches out at the Moho. This lower crustal layer is spatially associated with a low-velocity (7.6-7.7 km/s) upper mantle. We found that our crustal model is locally compatible with the previously suggested notion that the crust of the Salton Trough has formed almost entirely from magmatism in the lower crust and sedimentation in the upper crust. However, we observe an apparently magmatically emplaced lower crust to the northeast, outside of the Salton Trough, and propose that this layer in part predates Salton Trough rifting. It may also in part result from migration of magmatic spreading centers associated with the southern San Andreas fault system. These spreading centers may have existed east of their current locations in the past and may have influenced the lower crust and upper mantle to the east of the current Salton Trough.

Crustal velocity structure across the Main Ethiopian Rift: results from two-dimensional wide-angle seismic modelling

NASA Astrophysics Data System (ADS)

Mackenzie, G. D.; Thybo, H.; Maguire, P. K. H.

2005-09-01

We present the results of velocity modelling of a recently acquired wide-angle seismic reflection/refraction profile across the Main Ethiopian Rift. The models show a continental type of crust with significant asymmetry between the two sides of the rift. A 2- to 5-km-thick layer of sedimentary and volcanic sequences is modelled across the entire region. This is underlain by a 40- to 45-km-thick crust with a c. 15-km-thick high-velocity lowest crustal layer beneath the western plateau. This layer is absent from the eastern side, where the crust is 35 km thick beneath the sediments. We interpret this layer as underplated material associated with the Oligocene flood basalts of the region with possible subsequent addition by recent magmatic events. Slight crustal thinning is observed beneath the rift, where Pn velocities indicate the presence of hot mantle rocks containing partial melt. Beneath the rift axis, the velocities of the upper crustal layers are 5-10 per cent higher than outside the rift, which we interpret as resulting from mafic intrusions that can be associated with magmatic centres observed in the rift valley. Variations in seismic reflectivity suggest the presence of layering in the lower crust beneath the rift, possibly indicating the presence of sills, as well as some layering in the proposed underplated body.

Thickening the outer margins of the Tibetan Plateau: The role of crustal shortening

NASA Astrophysics Data System (ADS)

Lease, R. O.; Burbank, D. W.

2012-12-01

One of the most direct consequences of the collision of two buoyant continents is large-scale crustal thickening that results in the upward and outward growth of high terrain. As the stronger Indian continent has collided with weaker Asia over at least the past 50 Myr, widespread crustal thickening has occurred over an area that is approximately 2.5 million km^2 at present. The resultant Tibetan crust is the thickest observed on Earth today with an average thickness of 65 km and a maximum that may reach 90 km in places. The mechanisms by which Tibetan crust has thickened, however, as well as the timing and distribution of these mechanisms across the plateau, remain debatable. Two of the most popular mechanisms for thickening the crust beneath the margins of the Tibetan Plateau are: 1) pure shear with faulting and folding in the upper crust and horizontal shortening below; and 2) flow and inflation of lower or middle crust without significant shortening of the upper crust. To help discriminate between the relative contributions of these two mechanisms, well-constrained estimates of upper crustal shortening are needed. Here we document the Cenozoic shortening budget across the northeastern Tibetan Plateau margin near 36°N 102.5°E with several 100- to 145-km-long balanced cross sections. Thermochronological and magnetostratigraphic data indicate that modest NNE-SSW shortening began in middle Eocene time, shortly after initial India-Asia collision. Accelerated east-west shortening, however, did not commence until ~35 Myr later. A five-fold acceleration in shortening rates in middle Miocene-to-Recent time accounts for more than half of the total Cenozoic crustal shortening and thickening in this region. Overall, the balanced cross sections indicate 11 ± 2 % east-west shortening since middle Miocene time, and ~9 ± 2 % NNE-SSW shortening between middle Eocene and middle Miocene times. Given the present-day crustal thickness of 56 ± 4 km in northeastern Tibet, crustal restorations that remove Cenozoic shortening suggest that the northeastern Tibetan crust was 45 ± 5 km thick prior to India-Asia continental collision. This pre-collision thickness estimate is equivalent to average continental crustal thicknesses both adjacent to the Tibetan plateau (44 ± 4 km) and globally (41 ± 6 km) and suggests that pure shear alone may account for Cenozoic crustal thickening in northeastern Tibet, obviating the need for lower crustal flow. Furthermore, a growing number of balanced cross sections across the margins of the Tibetan Plateau document Cenozoic shortening sufficient to generate modern crustal thicknesses: in northern Tibet [Yin et al., 2007; 2008a; 2008b], eastern Tibet [Hubbard et al., 2009; 2010], and northeastern Tibet [this work]. Collectively, these similar findings suggest that lower crustal flow is either unnecessary to account for Cenozoic crustal thickening beneath the outer margins of the Tibetan Plateau or, alternatively, has a more restricted role than originally proposed.

New Paleomagnetic Data From Upper Gabbros Supports Limited Rotation of Central Semail Massif in Oman Ophiolite

NASA Astrophysics Data System (ADS)

Horst, A. J.; Sarah, T.; Hartley, E.; Martin, J.

2017-12-01

Paleomagnetic data from northern massifs of the Oman ophiolite demonstrate substantial clockwise rotations prior to or during obduction, yet data from southern massifs are recently suggested to be remagnetized during obduction and show subsequent smaller counterclockwise rotations. To better understand paleomagnetic data from the southern massifs, we conducted a detailed paleomagnetic and rock magnetic study of 21 sites in upper gabbros and 5 sites in lower crustal gabbros within the central Semail massif. Samples treated with progressive thermal demagnetization yield interpretable magnetizations with dominant unblocking between 500-580°C that implies characteristic remanent magnetization (ChRM) components carried by low-titanium magnetite and nearly pure magnetite. Rock magnetic and scanning electron microscopy data provide additional support of the carriers of magnetization. ChRMs from sites with samples containing partially-serpentinized olivine are similar to sites with samples lacking olivine, where the carriers appear to be fine magnetite intergrowths in pyroxene. The overall in situ and tilt-corrected mean directions from upper gabbros are distinct from the lower gabbros, from previous data within the massif, and also directions from similar crustal units in adjacent Rustaq and Wadi Tayin massifs. After tilt correction for 10-15° SE dip of the crust-mantle boundary, the mean direction from upper gabbros is nearly coincident with in situ lower gabbros. The tilt-corrected direction from upper gabbros is also consistent with an expected direction from the Late Cretaceous apparent polar wander path for Arabia at the age of crustal accretion ( 95Ma). These results suggest the upper crustal section in Semail has likely only experienced minor tilting since formation and acquisition of magnetization. Due to slow cooling of middle to lower gabbros in fast-spread crust, the lower gabbro sites likely cooled later or after obduction, and thus yield a distinct direction from upper gabbros. We place these new results in the context of geologic and geochronologic evidence for a younger spreading segment that propagated into older oceanic lithosphere followed by rapid obduction. Overall, these data imply a more complex resolution of simple rotation and emplacement of southern massifs as a single unit.

IODP drilling in the South China Sea in 2017 will address the mechanism of continental breakup

NASA Astrophysics Data System (ADS)

Sun, Z.; Larsen, H. C.; Lin, J.; Pang, X.; McIntosh, K. D.; Stock, J. M.; Jian, Z.; Wang, P.; Li, C.

2016-12-01

Geophysical exploration and scientific drilling along the North Atlantic rifted continental margins suggested that passive continental margins can be classified into two end members: magma-rich and magma-poor. Bearing seaward-dipping reflector sequences (SDRS) and highly mafic underplated high velocity lower crust (HVLC), the magma-rich margin is thought to be related to large igneous provinces (LIP) or mantle plume activity. Magma-poor margins have been drilled offshore Iberia and Newfoundland, where brittle faults cut through the whole crust and reach the upper mantle. Following seawater infiltration, the mantle was serpentinized and exhumed in the continent-ocean transition zone (COT). Later geophysical exploration and modeling suggested that in magma-poor margins lithosphere may break up in different styles, including uniform breakup, lower crust exhumation, or upper mantle exhumed at the COT, etc. The northern continental margin of the South China Sea (SCS) between longitude 114.5º and 116.5º hosts features that might be similar to both of the two end-members defined in the North Atlantic. Wide-angle seismic studies suggest that below the inner margin, crustal underplating of high velocity material is present, while syn-rift as well as post-rift intrusive features are visible and have in places been verified by industry drilling. However, the profound volcanism and associated SDRS formation are entirely lacking, and thus classification as a volcanic rifted margin can be ruled out. Instead, the COT exhibits a profound thinning of the continental crust towards the ocean crust of the SCS, showing some similarity to the Iberia type margin. The crustal thinning is caused by low-angle faults that have stretched the upper continental crust. There are indications of lower crustal flow toward the SCS. Alternatively, these extensional faults may have reached the lithospheric mantle and generated serpentinized material in a similar fashion as seen off Iberia. It will require deep drilling and sampling of characteristic basement units within the COT to distinguish. Four months of drilling by IODP to address this question is scheduled for February to June in 2017. The IODP drilling has the potential to support a third breakup mechanism theorized by modelling in addition to the two types drilled.

A First Layered Crustal Velocity Model for the Western Solomon Islands: Inversion of Measured Group Velocity of Surface Waves using Ambient Noise Cross-Correlation

NASA Astrophysics Data System (ADS)

Ku, C. S.; Kuo, Y. T.; Chao, W. A.; You, S. H.; Huang, B. S.; Chen, Y. G.; Taylor, F. W.; Yih-Min, W.

2017-12-01

Two earthquakes, MW 8.1 in 2007 and MW 7.1 in 2010, hit the Western Province of Solomon Islands and caused extensive damage, but motivated us to set up the first seismic network in this area. During the first phase, eight broadband seismic stations (BBS) were installed around the rupture zone of 2007 earthquake. With one-year seismic records, we cross-correlated the vertical component of ambient noise recorded in our BBS and calculated Rayleigh-wave group velocity dispersion curves on inter-station paths. The genetic algorithm to invert one-dimensional crustal velocity model is applied by fitting the averaged dispersion curves. The one-dimensional crustal velocity model is constituted by two layers and one half-space, representing the upper crust, lower crust, and uppermost mantle respectively. The resulted thickness values of the upper and lower crust are 6.4 and 14.2 km, respectively. Shear-wave velocities (VS) of the upper crust, lower crust, and uppermost mantle are 2.53, 3.57 and 4.23 km/s with the VP/VS ratios of 1.737, 1.742 and 1.759, respectively. This first layered crustal velocity model can be used as a preliminary reference to further study seismic sources such as earthquake activity and tectonic tremor.

A Global 3D P-Velocity Model of the Earth’s Crust and Mantle for Improved Event Location

DTIC Science & Technology

2011-09-01

starting model, we use a simplified layer crustal model derived from the NNSA Unified model in Eurasia and Crust 2.0 model everywhere else, over a...geographic and radial dimensions. For our starting model, we use a simplified layer crustal model derived from the NNSA Unified model in Eurasia and...tessellation with 4° triangles to the transition zone and upper mantle, and a third tessellation with variable resolution to all crustal layers. The

Seismic crustal structure of the North China Craton and surrounding area: Synthesis and analysis

NASA Astrophysics Data System (ADS)

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

2017-07-01

We present a new digital model (NCcrust) of the seismic crustal structure of the Neoarchean North China Craton (NCC) and its surrounding Paleozoic-Mesozoic orogenic belts (30°-45°N, 100°-130°E). All available seismic profiles, complemented by receiver function interpretations of crustal thickness, are used to constrain a new comprehensive crustal model NCcrust. The model, presented on a 0.25° × 0.25°grid, includes the Moho depth and the internal structure (thickness and velocity) of the crust specified for four layers (the sedimentary cover, upper, middle, and lower crust) and the Pn velocity in the uppermost mantle. The crust is thin (30-32 km) in the east, while the Moho depth in the western part of the NCC is 38-44 km. The Moho depth of the Sulu-Dabie-Qinling-Qilian orogenic belt ranges from 31 km to 51 km, with a general westward increase in crustal thickness. The sedimentary cover is 2-5 km thick in most of the region, and typical thicknesses of the upper crust, middle crust, and lower crust are 16-24 km, 6-24 km, and 0-6 km, respectively. We document a general trend of westward increase in the thickness of all crustal layers of the crystalline basement and as a consequence, the depth of the Moho. There is no systematic regional pattern in the average crustal Vp velocity and the Pn velocity. We examine correlation between the Moho depth and topography for seven tectonic provinces in the North China Craton and speculate on mechanisms of isostatic compensation.

Crustal structure along the west flank of the Cascades, western Washington

USGS Publications Warehouse

Miller, K.C.; Keller, Gordon R.; Gridley, J.M.; Luetgert, J.H.; Mooney, W.D.; Thybo, H.

1997-01-01

Knowledge of the crustal structure of the Washington Cascades and adjacent Puget Lowland is important to both earthquake hazards studies and geologic studies of the evolution of this tectonically active region. We present a model for crustal velocity structure derived from analysis of seismic refraction/wide-angle reflection data collected in 1991 in western Washington. The 280-km-long north-south transect skirts the west flank of the Cascades as it crosses three tectonic provinces including the Northwest Cascades Thrust System (NWCS), the Puget Lowland, and the volcanic arc of the southern Cascades. Within the NWCS, upper crustal velocities range from 4.2 to 5.7 km s-1 and are consistent with the presence of a diverse suite of Mesozoic and Paleozoic metasediments and metavolcanics. In the upper 2-3 km of the Puget Lowland velocities drop to 1.7-3.5 km s-1 and reflect the occurrence of Oligocene to recent sediments within the basin. In the southern Washington Cascades, upper crustal velocities range from 4.0 to 5.5 km s-1 and are consistent with a large volume of Tertiary sediments and volcanics. A sharp change in velocity gradient at 5-10 km marks the division between the upper and middle crust. From approximately 10 to 35 km depth the velocity field is characterized by a velocity increase from ???6.0 to 7.2 km s-1. These high velocities do not support the presence of marine sedimentary rocks at depths of 10-20 km beneath the Cascades as previously proposed on the basis of magnetotelluric data. Crustal thickness ranges from 42 to 47 km along the profile. The lowermost crust consists of a 2 to 8-km-thick transitional layer with velocities of 7.3-7.4 km s-1. The upper mantle velocity appears to be an unusually low 7.6-7.8 km s-1. When compared to velocity models from other regions, this model most closely resembles those found in active continental arcs. Distinct seismicity patterns can be associated with individual tectonic provinces along the seismic transect. In the NWCS and Puget Lowland, most of the seismicity occurs below the base of the upper crust as defined by a seismic boundary at 5-10 km depth and continues to 20-30 km depth. The region of transition between the NWCS and the Puget Lowland appears as a gap in seismicity with notably less seismic activity north of the boundary between the two. Earthquakes within the Cascades are generally shallower (0-20 km) and are dominated by events associated with the Rainier Seismic Zone. Copyright 1997 by the American Geophysical Union.

Permeability of continental crust influenced by internal and external forcing

USGS Publications Warehouse

Rojstaczer, S.A.; Ingebritsen, S.E.; Hayba, D.O.

2008-01-01

The permeability of continental crust is so highly variable that it is often considered to defy systematic characterization. However, despite this variability, some order has been gleaned from globally compiled data. What accounts for the apparent coherence of mean permeability in the continental crust (and permeability-depth relations) on a very large scale? Here we argue that large-scale crustal permeability adjusts to accommodate rates of internal and external forcing. In the deeper crust, internal forcing - fluxes induced by metamorphism, magmatism, and mantle degassing - is dominant, whereas in the shallow crust, external forcing - the vigor of the hydrologic cycle - is a primary control. Crustal petrologists have long recognized the likelihood of a causal relation between fluid flux and permeability in the deep, ductile crust, where fluid pressures are typically near-lithostatic. It is less obvious that such a relation should pertain in the relatively cool, brittle upper crust, where near-hydrostatic fluid pressures are the norm. We use first-order calculations and numerical modeling to explore the hypothesis that upper-crustal permeability is influenced by the magnitude of external fluid sources, much as lower-crustal permeability is influenced by the magnitude of internal fluid sources. We compare model-generated permeability structures with various observations of crustal permeability. ?? 2008 The Authors Journal compilation ?? 2008 Blackwell Publishing Ltd.

Crustal structure of the Colorado Plateau, Arizona: Application of new long-offset seismic data analysis techniques

USGS Publications Warehouse

Parsons, T.; McCarthy, J.; Kohler, W.M.; Ammon, C.J.; Benz, H.M.; Hole, J.A.; Criley, E.E.

1996-01-01

The Colorado Plateau is a large crustal block in the southwestern United States that has been raised intact nearly 2 km above sea level since Cretaceous marine sediments were deposited on its surface. Controversy exists concerning the thickness of the plateau crust and the source of its buoyancy. Interpretations of seismic data collected on the plateau vary as to whether the crust is closer to 40 or 50 km thick. A thick crust could support the observed topography of the Colorado Plateau isostatically, while a thinner crust would indicate the presence of an underlying low-density mantle. This paper reports results on long-offset seismic data collected during the 1989 segment of the U.S. Geological Survey Pacific to Arizona Crustal Experiment that extended from the Transition Zone into the Colorado Plateau in northwest Arizona. We apply two new methods to analyze long-offset data that employ finite difference travel time calculations: (1) a first-arrival time inverter to find upper crustal velocity structure and (2) a forward-modeling technique that allows the direct use of the inverted upper crustal solution in modeling secondary reflected arrivals. We find that the crustal thickness increases from 30 km beneath the metamorphic core complexes in the southern Basin and Range province to about 42 km beneath the northern Transition Zone and southern Colorado Plateau margin. We observe some crustal thinning (to ???37 km thick) and slightly higher lower crustal velocities farther inboard; beneath the Kaibab uplift on the north rim of the Grand Canyon the crust thickens to a maximum of 48 km. We observe a nonuniform crustal thickness beneath the Colorado Plateau that varies by ???15% and corresponds approximately to variations in topography with the thickest crust underlying the highest elevations. Crustal compositions (as inferred from seismic velocities) appear to be the same beneath the Colorado Plateau as those in the Basin and Range province to the southwest, implying that the plateau crust represents an unextended version of the Basin and Range. Some of the variability in crustal structure appears to correspond to preserved lithospheric discontinuities that date back to the Proterozoic Era.

Extension style in the Orphan Basin during the Mesozoic North Atlantic rifting

NASA Astrophysics Data System (ADS)

Gouiza, Mohamed; Hall, Jeremy

2013-04-01

The Orphan Basin, lying along the Newfoundland passive continental margin, has formed in Mesozoic time during the opening of the North Atlantic Ocean and the breakup of Iberia/Eurasia from North America. Regional deep seismic reflection profiles across the basin indicate that the Neoproterozoic basement has been affected by repeated extensional episodes between the Late Triassic/Jurassic and the Early Cretaceous. Deformation initiated in the eastern part of the Orphan basin in the Jurassic and migrated toward the west in the Early Cretaceous, resulting in numerous rift structures filled with Jurassic-Lower Cretaceous syn-rift successions and sealed by thick Upper Cretaceous-Cenozoic post-rift sediments. The seismic data show an extremely attenuated crust underneath the eastern and western part of the deep basin, forming two sub-basins associated with the development of rifting. The two sub-basins are separated by a wide structural high with a relatively thick crust and are bounded to the west by the continental shelf domain. Restoration of the Orphan Basin along a 2D crustal section (520 km long), yields a total amount of stretching of about 144 km, while the total crustal thinning indicates an extension of around 250 km, assuming mass conservation along the section and an initial crustal thickness of 28 km. Brittle deformation accommodated by normal faults is documented in the seismic profiles and affected essentially the present-day upper portion of the crust, and represents only 60% of the total extension which thinned the Orphan crust. The remaining crustal thinning must involve other deformation processes which are not (easily) recognizable in the seismic data. We propose two models that could explain discrepancies between brittle deformation and total crustal thinning during lithospheric extension. The first model assumes the reactivation of pre-rift inherited structures, which act as crustal-scale detachments during the early stages of rifting. The second model uses depth-dependent extension of a 20 km thick crust characterized by a strong upper crust and a weak lower crust. Both models raise secondary issues that are discussed around the order of rifting events and the original crustal thickness.

Proxies of oceanic Lithosphere/Asthenosphere Boundary from Global Seismic Anisotropy Tomography

NASA Astrophysics Data System (ADS)

Burgos, Gael; Montagner, Jean-Paul; Beucler, Eric; Trampert, Jeannot; Capdeville, Yann

2013-04-01

Surface waves provide essential information on the knowledge of the upper mantle global structure despite their low lateral resolution. This study, based on surface waves data, presents the development of a new anisotropic tomographic model of the upper mantle, a simplified isotropic model and the consequences of these results for the Lithosphere/Asthenosphere Boundary (LAB). As a first step, a large number of data is collected, these data are merged and regionalized in order to derive maps of phase and group velocity for the fundamental mode of Rayleigh and Love waves and their azimuthal dependence (maps of phase velocity are also obtained for the first six overtones). As a second step, a crustal a posteriori model is developped from the Monte-Carlo inversion of the shorter periods of the dataset, in order to take into account the effect of the shallow layers on the upper mantle. With the crustal model, a first Monte-Carlo inversion for the upper mantle structure is realized in a simplified isotropic parameterization to highlight the influence of the LAB properties on the surface waves data. Still using the crustal model, a first order perturbation theory inversion is performed in a fully anisotropic parameterization to build a 3-D tomographic model of the upper mantle (an extended model until the transition zone is also obtained by using the overtone data). Estimates of the LAB depth are derived from the upper mantle models and compared with the predictions of oceanic lithosphere cooling models. Seismic events are simulated using the Spectral Element Method in order to validate the ability of the anisotropic tomographic model of the upper mantle to re- produce observed seismograms.

Svecofennian orogeny in an evolving convergent margin setting

NASA Astrophysics Data System (ADS)

Korja, Annakaisa

2015-04-01

The dominant tectonic mode changes from extension to convergence at around 1.9 Ga in Fennoscandian. The lithological record suggests short lived subduction-related magmatic events followed by deformation and low-pressure high temperature metamorphism. At around 1.8 Ga the subduction systems seem to have stabilized implying continuous supply of oceanic lithosphere. The evolution of the convergent margin is recorded in the rock record and crustal architecture of the long lived Svecofennian orogeny (1.9-1.7 Ga). A closer look at the internal structure of the Svecofennian orogen reveals distinct regional differences. The northern and central parts of the Svecofennian orogen that have been formed during the initial accretionary phase - or compilation of the nucleus - have a thick three-layer crust and with thick mafic lower crust (10-30 km) and block-like internal architecture. Reflection profiles (FIRE1-3) image listric structures flattening on crustal scale décollement zones at the upper-middle crust and middle-upper crust boundaries. The crustal architecture together with large volumes of exposed granitoid rocks suggests spreading of the orogen and the development of an orogenic plateau west of the continental convergence boundary. The architecture is reminiscent of a large hot orogen. Within the western and southwestern part of the Svecofennian orogen (BABEL B, 1, 2, 3&4), which have been envisioned to have formed during continuous subduction phase, the crust is thinner (45-50 km) and it is hosting crustal blocks having one to two crustal layers. Layering is poorly developed in crustal blocks that are found S-SW of NE-dipping mantle reflections previously interpreted as paleo-subduction zones. Within these blocks, the crustal scale reflective structures dip NE (prowedge) or form pop-up wedges (uplifted plug) above the paleo-subduction zones. Crustal blocks with well-developed two-layer crust are located NE of the paleo-subduction zone. The architecture can be interpreted to image a series of abandoned accretion zones where the orogenic structure has developed from a young and cold orogen (BABEL 2,3&4) to a transitional (BABEL 1,6,B) one as the plate boundary is retreating during SW wards. The fast retreating rate of the subduction zone may not only have formed continental back-arc environment but may have restricted the thickening of the upper plate and the growth rate of the orogen. Altogether the architecture suggests a long-lived southwesterly retreating subduction system, with continental back-arc formation in its rear parts and well developed system of prowedge-retrowedge-uplifted plug close to a subduction conduit. Changes in the relative velocities of the upper and lower plate may have resulted in repetitive extensional and compressional phases of the orogeny as has been previously suggested for the southern part of the Svecofennian orogen.

Preliminary Results From the CAUGHT Experiment: Investigation of the North Central Andes Subsurface Using Receiver Functions and Ambient Noise Tomography

NASA Astrophysics Data System (ADS)

Ryan, J. C.; Ward, K. M.; Porter, R. C.; Beck, S. L.; Zandt, G.; Wagner, L. S.; Minaya, E.; Tavera, H.

2011-12-01

Jamie Ryan, Kevin M. Ward, Ryan Porter, Susan Beck, George Zandt, Lara Wagner, Estela Minaya, and Hernando Tavera The University of Arizona The University of North Carolina San Calixto Observatorio, La Paz, Bolivia IGP, Lima, Peru In order to investigate the interplay between crustal shortening, lithospheric removal, and surface uplift we have deployed 50 broadband seismometers in northwestern Bolivia and southern Peru as part of the interdisciplinary Central Andean Uplift and Geodynamics of High Topography (CAUGHT) project. The morphotectonic units of the central Andes from west to east, consist of the Western Cordillera, the active volcanic arc, the Altiplano, an internally drained basin (~4 km elevation), the Eastern Cordillera, the high peaks (~6 km elevation) of an older fold and thrust belt, the Subandean zone, the lower elevation active fold and thrust belt, and the foreland Beni basin. Between northwestern Bolivia and southern Peru, the Altiplano pinches out north of Lake Titicaca as the Andes narrow northward. The CAUGHT seismic instruments were deployed between 13° to 18° S latitudes to investigate the crust and mantle lithosphere of the central Andes in this transitional zone. In northwest Bolivia, perpendicular to the strike of the Andes, there is a total of 275 km of documented upper crustal shortening (15° to 17°S) (McQuarrie et al, 2008). Associated with the shortening is crustal thickening and possibly lithospheric removal as the thickening lithospheric root becomes unstable. An important first order study is to compare upper crustal shortening estimates with present day crustal thickness. To estimate crustal thickness, we have calculated receiver functions using an iterative deconvolution method and used common conversion point stacking along the same profile as the geologically based shortening estimates. In our preliminary results, we observed a strong P to S conversion corresponding to the Moho at approximately 60-65 km depth underneath the Altiplano and portions of the Eastern Cordillera, and at approximately 40 under the sub-Andes and westernmost edge of the Beni basin. Unlike previous studies farther south, we do not see an increased crustal thickness beneath the Eastern Cordillera. The CAUGHT station coverage is also ideal for Ambient Noise Tomography (ANT) to investigate the seismic shear wave velocities in the upper crust (<30 km depth). ANT will be used to estimate the depth of basins in the northern Altiplano, and aid in constraining the upper crustal shear wave velocities for improved migration of receiver functions to depth. McQuarrie, N., Barnes, J., and Ehlers, T.A., 2008, Geometric, kinematic and erosional history of the central Andean Plateau (15-17°S), northern Bolivia: Tectonics, v. 27, TC3007, doi:10.1029/2006TC002054.

Source Parameters for Moderate Earthquakes in the Zagros Mountains with Implications for the Depth Extent of Seismicity

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

Adams, A; Brazier, R; Nyblade, A

2009-02-23

Six earthquakes within the Zagros Mountains with magnitudes between 4.9 and 5.7 have been studied to determine their source parameters. These events were selected for study because they were reported in open catalogs to have lower crustal or upper mantle source depths and because they occurred within an area of the Zagros Mountains where crustal velocity structure has been constrained by previous studies. Moment tensor inversion of regional broadband waveforms have been combined with forward modeling of depth phases on short period teleseismic waveforms to constrain source depths and moment tensors. Our results show that all six events nucleated withinmore » the upper crust (<11 km depth) and have thrust mechanisms. This finding supports other studies that call into question the existence of lower crustal or mantle events beneath the Zagros Mountains.« less

Crustal reflectivity in the Skagerrak area

NASA Astrophysics Data System (ADS)

Larsson, F. R.; Husebye, E. S.

1991-04-01

Reflectors within the crystalline crust are often observed in deep seismic reflection profiling surveys. The lower crust in extensional areas is generally credited with an abundance of reflectors. The deep seismic reflection data (16 s TWT) from the M.V. Mobil Search cruise in Skagerrak show a reflective lower crust and a relatively transparent upper crust in most of the area. Reflectivity seems to be less inside the Oslo Rift, and also beneath the sediment-covered areas. Reflectivity maxima are found near the Moho and at depths of 10-20 km. The latter is taken to coincide with the transition between the brittle upper and ductile lower crust. The distribution of crustal reflectors in Skagerrak and their possible relationships with seismic velocities, earthquake depth distribution and major tectonic elements such as the Fennoscandian Border Zone, the Oslo Rift system and the shield environment are discussed. Hypotheses on the formation of the crustal reflectors are also briefly reviewed.

Seismic properties of the crust and uppermost mantle of North America

NASA Technical Reports Server (NTRS)

Braile, L. W.; Hinze, W. J.; Vonfrese, R. R. B.; Keller, G. R.

1983-01-01

Seismic refraction profiles for the North American continent were compiled. The crustal models compiled data on the upper mantle seismic velocity (P sub n), the crustal thickness (H sub c) and the average seismic velocity of the crystalline crust (V sub p). Compressional wave parameters were compared with shear wave data derived from surface wave dispersion models and indicate an average value for Poisson's ratio of 0.252 for the crust and of 0.273 for the uppermost mantle. Contour maps illustrate lateral variations in crustal thickness, upper mantle velocity and average seismic velocity of the crystalline crust. The distribution of seismic parameters are compared with a smoothed free air anomaly map of North America and indicate that a complidated mechanism of isostatic compensation exists for the North American continent. Several features on the seismic contour maps also correlate with regional magnetic anomalies.

Synthesis of regional crust and upper-mantle structure from seismic and gravity data

NASA Technical Reports Server (NTRS)

Alexander, S. S.; Lavin, P. M. (Principal Investigator)

1982-01-01

Analyses of regional gravity and magnetic patterns, LANDSAT images and geological information revealed two major lineaments crossing western Pennsylvania and parts of surrounding states. These lineaments are inferred to be expressions of fracture zones which penetrare deeply into the crust and possibly the upper mantle. The extensions of the Tyron-Mt. Union and the Pittsburgh-Washington lineaments bound a distinct crustal block (Lake Erie-Maryland block) over 100 km wide and probably more than 600 km in length. Evidence exists for the lateral displacement of this block at least 60 km northwestward during late Precambrian to Lower Ordovician time. Subsequent movements have been mainly vertical with respect to neighboring blocks. A possible crustal block that passes through eastern Kentucky, proposed by a TVA study on tectonics in the southern Appalachians, was also investigated. Finally, the use of regional gravity and magnetic data in identifying major crustal structures beneath western Pennsylvania is discussed.

Geophysical evidence for the extent of crustal types and the type of margin along a profile in the northeastern Baffin Bay

NASA Astrophysics Data System (ADS)

Altenbernd, Tabea; Jokat, Wilfried; Heyde, Ingo; Damm, Volkmar

2015-11-01

Investigating the crust of northern Baffin Bay provides valuable indications for the still debated evolution of this area. The crust of the southern Melville Bay is examined based on wide-angle seismic and gravity data. The resulting P wave velocity, density, and geological models give insights into the crustal structure. A stretched and rifted continental crust underneath southern Melville Bay is up to 30 km thick, with crustal velocities ranging between 5.5 and 6.9 km/s. The deep Melville Bay Graben contains a 9 km thick infill with velocities of 4 to 5.2 km/s in its lowermost part. West of the Melville Bay Ridge, a ~80 km wide and partly only 5 km thick Continent-Ocean Transition (COT) is present. West of the COT, up to 5 km thick sedimentary layers cover a 4.3 to 7 km thick, two-layered oceanic crust. The upper oceanic layer 2 has velocities of 5.2 to 6.0 km/s; the oceanic layer 3 has been modeled with rather low velocities of 6.3 to 6.9 km/s. Low velocities of 7.8 km/s characterize the probably serpentinized upper mantle underneath the thin crust. The serpentinized upper mantle and low thickness of the oceanic crust are another indication for slow or ultraslow spreading during the formation of the oceanic part of the Baffin Bay. By comparing our results on the crustal structure with other wide-angle seismic profiles recently published, differences in the geometry and structure of the crust and the overlying sedimentary cover are revealed. Moreover, the type of margin and the extent of crustal types in the Melville Bay area are discussed.

Crustal magnetization and accretion at the Southwest Indian Ridge near the Atlantis II fracture zone, 0-25 Ma

USGS Publications Warehouse

Hosford, A.; Tivey, M.; Matsumoto, T.; Dick, H.; Schouten, Hans; Kinoshita, H.

2003-01-01

We analyze geophysical data that extend from 0 to 25-Myr-old seafloor on both flanks of the Southwest Indian Ridge (SWIR). Lineated marine magnetic anomalies are consistent and identifiable within the study area, even over seafloor lacking a basaltic upper crust. The full spreading rate of 14 km/Myr has remained nearly constant since at least 20 Ma, but crustal accretion has been highly asymmetric, with half rates of 8.5 and 5.5 km/Myr on the Antarctic and African flanks, respectively. This asymmetry may be unique to a ???400 km wide corridor between large-offset fracture zones of the SWIR. In contrast to the Mid-Atlantic Ridge, crustal magnetization amplitudes correlate directly with seafloor topography along the present-day rift valleys. This pattern appears to be primarily a function of along-axis variations in crustal thickness, rather than magnetic mineralogy. Off-axis, magnetization amplitudes at paleo-segment ends are more positive than at paleo-segment midpoints, suggesting the presence of an induced component of magnetization within the lower crust or serpentinized upper mantle. Alteration of the magnetic source layer at paleo-segment midpoints reduces magnetization amplitudes by 70-80% within 20 Myr of accretion. Magnetic and Ocean Drilling Program (ODP) Hole 735B data suggest that the lower crust cooled quickly enough to lock in a primary thermoremanent magnetization that is in phase with that of the overlying upper crust. Thus magnetic polarity boundaries within the intrusive lower crust may be steeper than envisioned in prior models of ocean crustal magnetization. As the crust ages, the lower crust becomes increasingly important in preserving marine magnetic stripes.

CRUST 5.1: A global crustal model at 5° x 5°

USGS Publications Warehouse

Mooney, Walter D.; Laske, Gabi; Masters, T. Guy

1998-01-01

We present a new global model for the Earth's crust based on seismic refraction data published in the period 1948–1995 and a detailed compilation of ice and sediment thickness. An extensive compilation of seismic refraction measurements has been used to determine the crustal structure on continents and their margins. Oceanic crust is modeled with both a standard model for normal oceanic crust, and variants for nonstandard regions, such as oceanic plateaus. Our model (CRUST 5.1) consists of 2592 5° × 5° tiles in which the crust and uppermost mantle are described by eight layers: (1) ice, (2) water, (3) soft sediments, (4) hard sediments, (5) crystalline upper, (6) middle, (7) lower crust, and (8) uppermost mantle. Topography and bathymetry are adopted from a standard database (ETOPO-5). Compressional wave velocity in each layer is based on field measurements, and shear wave velocity and density are estimated using recently published empirical Vp- Vs and Vp-density relationships. The crustal model differs from previous models in that (1) the thickness and seismic/density structure of sedimentary basins is accounted for more completely, (2) the velocity structure of unmeasured regions is estimated using statistical averages that are based on a significantly larger database of crustal structure, (3) the compressional wave, shear wave, and density structure have been explicitly specified using newly available constraints from field and laboratory studies. Thus this global crustal model is based on substantially more data than previous models and differs from them in many important respects. A new map of the thickness of the Earth's crust is presented, and we illustrate the application of this model by using it to provide the crustal correction for surface wave phase velocity maps. Love waves at 40 s are dominantly sensitive to crustal structure, and there is a very close correspondence between observed phase velocities at this period and those predicted by CRUST 5.1. We find that the application of crustal corrections to long-period (167 s) Rayleigh waves significantly increases the variance in the phase velocity maps and strengthens the upper mantle velocity anomalies beneath stable continental regions. A simple calculation of crustal isostacy indicates significant lateral variations in upper mantle density. The model CRUST 5.1 provides a complete description of the physical properties of the Earth's crust at a scale of 5° × 5° and can be used for a wide range of seismological and nonseismological problems.

Velocity and Attenuation Structure of the Tibetan Lithosphere using Seismic Attributes of P-waves from Regional Earthquakes Recorded by the Hi-CLIMB Array

NASA Astrophysics Data System (ADS)

Nowack, R. L.; Bakir, A. C.; Griffin, J.; Chen, W.; Tseng, T.

2010-12-01

Using data from regional earthquakes recorded by the Hi-CLIMB array in Tibet, we utilize seismic attributes from crustal and Pn arrivals to constrain the velocity and attenuation structure in the crust and the upper mantle in central and western Tibet. The seismic attributes considered include arrival times, Hilbert envelope amplitudes, and instantaneous as well as spectral frequencies. We have constructed more than 30 high-quality regional seismic profiles, and of these, 10 events have been selected with excellent crustal and Pn arrivals for further analysis. Travel-times recorded by the Hi-CLIMB array are used to estimate the large-scale velocity structure in the region, with four near regional events to the array used to constrain the crustal structure. The travel times from the far regional events indicate that the Moho beneath the southern Lhasa terrane is up to 75 km thick, with Pn velocities greater than 8 km/s. In contrast, the data sampling the Qiangtang terrane north of the Bangong-Nujiang (BNS) suture shows thinner crust with Pn velocities less than 8 km/s. Seismic amplitude and frequency attributes have been extracted from the crustal and Pn wave trains, and these data are compared with numerical results for models with upper-mantle velocity gradients and attenuation, which can strongly affect Pn amplitudes and pulse frequencies. The numerical modeling is performed using the complete spectral element method (SEM), where the results from the SEM method are in good agreement with analytical and reflectivity results for different models with upper-mantle velocity gradients. The results for the attenuation modeling in Tibet imply lower upper mantle Q values in the Qiangtang terrane to the north of the BNS compared to the less attenuative upper mantle beneath the Lhasa terrane to the south of the BNS.

Seismic images of the Brooks Range fold and thrust belt, Arctic Alaska, from an integrated seismic reflection/refraction experiment

USGS Publications Warehouse

Levander, A.; Fuis, G.S.; Wissinger, E.S.; Lutter, W.J.; Oldow, J.S.; Moore, Thomas E.

1994-01-01

We describe results of an integrated seismic reflection/refraction experiment across the Brooks Range and flanking geologic provinces in Arctic Alaska. The seismic acquisition was unusual in that reflection and refraction data were collected simultaneously with a 700 channel seismograph system deployed numerous times along a 315 km profile. Shot records show continuous Moho reflections from 0-180 km offset, as well as numerous upper- and mid-crustal wide-angle events. Single and low-fold near-vertical incidence common midpoint (CMP) reflection images show complex upper- and middle-crustal structure across the range from the unmetamorphosed Endicott Mountains allochthon (EMA) in the north, to the metamorphic belts in the south. Lower-crustal and Moho reflections are visible across the entire reflection profile. Travel-time inversion of PmP arrivals shows that the Moho, at 33 km depth beneath the North Slope foothills, deepens abruptly beneath the EMA to a maximum of 46 km, and then shallows southward to 35 km at the southern edge of the range. Two zones of upper- and middle-crustal reflections underlie the northern Brooks Range above ~ 12-15 km depth. The upper zone, interpreted as the base of the EMA, lies at a maximum depth of 6 km and extends over 50 km from the range front to the north central Brooks Range where the base of the EMA outcrops above the metasedimentary rocks exposed in the Doonerak window. We interpret the base of the lower zone, at ~ 12 km depth, to be from carbonate rocks above the master detachment upon which the Brooks Range formed. The seismic data suggest that the master detachment is connected to the faults in the EMA by several ramps. In the highly metamorphosed terranes south of the Doonerak window, the CMP section shows numerous south-dipping events which we interpret as a crustal scale duplex involving the Doonerak window rocks. The basal detachment reflections can be traced approximately 100 km, and dip southward from about 10-12 km near the range front, to 14-18 km beneath the Doonerak window, to 26-28 km beneath the metamorphic belts in the central Brooks Range. The section documents middle- and lower-crustal involvement in the formation of the Brooks Range. ?? 1994.

Geometries of geoelectrical structures in central Tibetan Plateau from INDEPTH magnetotelluric data

NASA Astrophysics Data System (ADS)

Vozar, J.; Jones, A. G.; Le Pape, F.

2012-12-01

Magnetotelluric (MT) data collected on N-S profiles crossing the Banggong-Nujiang Suture (BNS), which separates the Qiangtang and Lhasa Terranes in central Tibet, as a part of InterNational DEep Profiling of Tibet and the Himalaya project (INDEPTH) are modeled by 2D, 3D inversion codes and 1D petro-physical package LitMod. The modeling exhibits regional resistive and conductive structures correlated with ShuangHu Suture, Tanggula Mountains and strike-slip faults like BengCo-Jiali fault in the south. The BNS is not manifested in the geoelectrical models as a strong crustal regional structure. The strike direction azimuth of mid and lower crustal structures estimated from horizontal slices from 3D modeling (N110°E) is slightly different from one estimated by 2D strike analysis (N100°E). Orientation of crustal structures is perpendicular to convergence direction in this area. The deepest lower crustal conductors are correlated to areas with maximum Moho depth obtained from satellite gravity data. The anisotropic 2D modeling reveals that lower crustal conductor in Lhasa Terrane is anisotropic. This anisotropy can be interpreted as a proof for crustal channel flow below Lhasa Terrane. But same Lhasa lower crust conductor from isotropic 3D modeling can be interpreted more likely as 3D lower Indian crust structure, located to the east from line 500, than geoelectrical anisotropic crustal flow. From deep electromagnetic sounding, supported by independent integrated petro-physical investigation, we can estimate the next upper-mantle conductive layer at depths from 200 km to 250 km below the Lhasa Terrane and less resistive Tibetan lithosphere below the Qiangtang Terrane with conductive upper-mantle in depths about 120 km.

Protracted tectono-metamorphic history of the SE Superior Province : contribution of 40Ar/39Ar thermochronology in the Abitibi-Opatica contact zone, Québec, Canada

NASA Astrophysics Data System (ADS)

Daoudene, Yannick; Tremblay, Alain; Ruffet, Gilles; Leclerc, François; Goutier, Jean

2015-04-01

Archean orogens mainly consist of greenstone belts juxtaposing deeper crustal domains of TTG-type plutonic rocks. The greenstone belts show regional folds, penetrative steeply-dipping fabrics, and localised shear zones, whereas the plutonic belts predominantly display dome structures. Concurrently, rocks in Archean orogens undergone MT/HT-LP/MP metamorphic conditions that vary, from upper to lower crustal domains, between greenschist- and granulite-facies, respectively. These structural and metamorphic variations are well-documented, but modes of deformation related to such orogens is still debated. Some studies suggest that the Archean tectonic processes were comparable to present-day plate tectonics and the Archean greenstone belts were interpreted as tectonic collages commonly documented in Phanerozoic subduction/collision zones. Alternative models propose that the Archean tectonics were different from those predicted by the plate tectonics paradigm, mainly due to the existence of a hotter mantle and a mechanically weak crust. In such models, the burying and exhumation of crustal rocks are attributed to the vertical transfer of material, resulting in the development of pop-down and domes structures. As a contribution of the study of mechanisms that might have operated during the Archean, we present a structural and metamorphic study of the contact zone between the Abitibi subprovince (ASP), which contains greenstone belts, and the Opatica subprovince (OSP), which is dominated by plutonic rocks, of the Superior Province. The 40Ar/39Ar dating of amphiboles and micas is used to constrain the age and duration of regional metamorphism and associated deformations. On the basis of seismic profiling, showing a north-dipping lithospheric-scale reflector, the ASP-OSP contact has been interpreted as the surficial trace of an Archean subduction zone. However, our structural analysis suggest that the ASP overlies the OSP and that the ASP-OSP contact does not show evidences of an important sub-vertical shearing deformation as expected if it was a major upper plate-lower plate boundary. Furthermore, the contact does not present significant metamorphic break between the two domains, but a progressive increasing of metamorphism toward the OSP, from greenschist- to amphibolite-facies conditions. Based on these structural and metamorphic characteristics, we suggest that the OSP exposes the deepest rocks at outcrop of an ASP-OSP crust in the study area. Regionally, the 40Ar/39Ar ages acquired during this study indicate that the ASP-OSP contact records a protracted metamorphic history that started around 2685 Ma. The structural and isotopic age data suggest that, from ~2685 Ma to ~2632 Ma, the deepest level of the ASP and the underlying OSP reached amphibolite-facies metamorphic conditions and that regional deformation was accommodated by an overall horizontal shortening and sub-vertical transfers of crustal material. Subsequently, the cooling of these crustal rocks was accompanied by strain localisation, which led to the development of oblique strike-slip shear zones from ~2600 Ma, when the lateral flowing of crustal material became predominant. Our 40Ar/39Ar data compared with metamorphic ages documented in adjacent areas of the Superior Province suggests that the peak and duration of regional metamorphism might have been coeval over a large region. This rather favours a mode of pervasive deformation as expected in vertical tectonics.

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).

Tectonic and magmatic processes of the post-spreading ridge in the Southwest Sub-basin, South China Sea

NASA Astrophysics Data System (ADS)

Li, J.; Zhang, J.; Ruan, A.; Niu, X.; Ding, W.

2016-12-01

We report here a 3D ocean bottom seismometer experiment on the fossil spreading ridge in the Southwest Sub-basin of the South China Sea. An extreme asymmetric crustal structure across the axis is revealed and caused by lower crust thinning and upper mantle uplifting located on NW side of the ridge. Such crustal extension proposed a low-angle oceanic detachment fault throughout the whole crust on the last or post spreading stages. A low-velocity (7.6-7.9 km/s) on the uplifting upper mantle is possibly induced by both mantle serpentinization and/or decompression melting through the detachment fault. Velocity models also demonstrate that a post-spreading volcano erupted on the axis is mainly formed by an extrusive process with an extrusive/intrusive ratio of 1.92. Very low velocity of upper crust (3.1-4.8 km/s) of the volcano is attributed to the composition of volcaniclastic rocks and high-porosity basalts, which have been observed in the borehole and dredged samples on the seamounts nearby. KEY WORDS post-spreading ridge; wide-angle seismic refraction; crustal structure; South China Sea; Southwest Sub-basin

Crustally derived granites in Dali, SW China: new constraints on silicic magmatism of the Central Emeishan Large Igneous Province

NASA Astrophysics Data System (ADS)

Zhu, Bei; Peate, David W.; Guo, Zhaojie; Liu, Runchao; Du, Wei

2017-10-01

We have identified a new crustally derived granite pluton that is related to the Emeishan Large Igneous Province (ELIP). This pluton (the Wase pluton, near Dali) shows two distinct SHRIMP zircon U-Pb age groups ( 768 and 253 Ma). As it has an intrusive relationship with Devonian limestone, the younger age is interpreted as its formation, which is related to the ELIP event, whereas the 768 Ma Neoproterozoic-aged zircons were inherited from Precambrian crustal component of the Yangtze Block, implying the pluton has a crustally derived origin. This is consistent with its peraluminous nature, negative Nb-Ta anomaly, enrichment in light rare earth elements, high 87Sr/86Sr(i) ratio (0.7159-0.7183) and extremely negative ɛ(Nd)(i) values (-12.15 to -13.70), indicative of melts derived from upper crust materials. The Wase pluton-intruded Devonian strata lie stratigraphically below the Shangcang ELIP sequence, which is the thickest volcanic sequence ( 5400 m) in the whole ELIP. The uppermost level of the Shangcang sequence contains laterally restricted rhyolite. Although the rhyolite has the same age as the Wase pluton, its geochemical features demonstrate a different magma origin. The rhyolite displays moderate 87Sr/86Sr(i) (0.7053), slightly negative ɛ(Nd)(i) (-0.18) and depletions in Ba, Cs, Eu and Sr, implying derivation from differentiation of a mantle-derived mafic magma source. The coexistence of crustally and mantle-derived felsic systems, along with the robust development of dike swarms, vent proximal volcanics and thickest flood basalts piles in Dali, shows that the Dali area was probably where the most active Emeishan magmatism had once existed.

Formation and disruption of aquifers in southwestern Chryse Planitia, Mars

USGS Publications Warehouse

Rodriguez, J.A.P.; Tanaka, K.L.; Kargel, J.S.; Dohm, J.M.; Kuzmin, R.; Fairen, A.G.; Sasaki, S.; Komatsu, G.; Schulze-Makuch, D.; Jianguo, Y.

2007-01-01

We present geologic evidence suggesting that after the development of Mars' cryolithosphere, the formation of aquifers in southwestern Chryse Planitia and their subsequent disruption led to extensive regional resurfacing during the Late Hesperian, and perhaps even during the Amazonian. In our model, these aquifers formed preferentially along thrust faults associated with wrinkle ridges, as well as along fault systems peripheral to impact craters. The characteristics of degraded wrinkle ridges and impact craters in southwestern Chryse Planitia indicate a profound role of subsurface volatiles and especially liquid water in the upper crust (the upper one hundred to a few thousands of meters). Like lunar wrinkle ridges, the martian ones are presumed to mark the surface extensions of thrust faults, but in our study area the wrinkle ridges are heavily modified. Wrinkle ridges and nearby plains have locally undergone collapse, and in other areas they are associated with domical intrusions we interpret as mud volcanoes and mud diapirs. In at least one instance, a sinuous valley emanates from a modified wrinkle ridge, further indicating hydrological influences on these thrust-fault-controlled features. A key must be the formation of volatile-rich crust. Primary crustal formation and differentiation incorporated juvenile volatiles into the global crust, but the crustal record here was then strongly modified by the giant Chryse impact. The decipherable rock record here begins with the Chryse impact and continues with the resulting basin's erosion and infilling, which includes outflow channel activity. We propose that in Simud Vallis surface flow dissection into the base of the cryolithosphere-produced zones where water infiltrated and migrated along SW-dipping strata deformed by the Chryse impact, thereby forming an extensive aquifer in southwestern Chryse Planitia. In this region, compressive stresses produced by the rise of Tharsis led to the formation of wrinkle ridges. Zones of high fracture density within the highly strained planes of the thrust faults underlying the wrinkle ridges formed regions of high permeability; thus, groundwater likely flowed and gathered along these tectonic structures to form zones of elevated permeability. Volatile depletion and migration within the upper crustal materials, predominantly along fault systems, led to structurally controlled episodic resurfacing in southwestern Chryse Planitia. The erosional modification of impact craters in this region is linked to these processes. This erosion is scale independent over a range of crater diameters from a few hundred meters to tens of kilometers. According to our model, pressurized water and sediment intruded and locally extruded and caused crustal subsidence and other degradational activity across this region. The modification of craters across this wide range of sizes, according to our model, implies that there was intensive mobilization of liquid water in the upper crust ranging from about one hundred to several thousand meters deep. ?? 2007 Elsevier Inc. All rights reserved.

Thinning factor distributions viewed through numerical models of continental extension

NASA Astrophysics Data System (ADS)

Svartman Dias, Anna Eliza; Hayman, Nicholas W.; Lavier, Luc L.

2016-12-01

A long-standing question surrounding rifted margins concerns how the observed fault-restored extension in the upper crust is usually less than that calculated from subsidence models or from crustal thickness estimates, the so-called "extension discrepancy." Here we revisit this issue drawing on recently completed numerical results. We extract thinning profiles from four end-member geodynamic model rifts with varying width and asymmetry and propose tectonic models that best explain those results. We then relate the spatial and temporal evolution of upper to lower crustal thinning, or crustal depth-dependent thinning (DDT), and crustal thinning to mantle thinning, or lithospheric DDT, which are difficult to achieve in natural systems due to the lack of observations that constrain thinning at different stages between prerift extension and lithospheric breakup. Our results support the hypothesis that crustal DDT cannot be the main cause of the extension discrepancy, which may be overestimated because of the difficulty in recognizing distributed deformation, and polyphase and detachment faulting in seismic data. More importantly, the results support that lithospheric DDT is likely to dominate at specific stages of rift evolution because crustal and mantle thinning distributions are not always spatially coincident and at times are not even balanced by an equal magnitude of thinning in two dimensions. Moreover, either pure or simple shear models can apply at various points of time and space depending on the type of rift. Both DDT and pure/simple shear variations across space and time can result in observed complex fault geometries, uplift/subsidence, and thermal histories.

Isotopic and trace element constraints on the petrogenesis of lavas from the Mount Adams volcanic field, Washington

USGS Publications Warehouse

Jicha, B.R.; Hart, G.L.; Johnson, C.M.; Hildreth, Wes; Beard, B.L.; Shirey, S.B.; Valley, J.W.

2009-01-01

Strontium, Nd, Pb, Hf, Os, and O isotope compositions for 30 Quaternary lava flows from the Mount Adams stratovolcano and its basaltic periphery in the Cascade arc, southern Washington, USA indicate a major component from intraplate mantle sources, a relatively small subduction component, and interaction with young mafic crust at depth. Major- and trace-element patterns for Mount Adams lavas are distinct from the rear-arc Simcoe volcanic field and other nearby volcanic centers in the Cascade arc such as Mount St. Helens. Radiogenic isotope (Sr, Nd, Pb, and Hf) compositions do not correlate with geochemical indicators of slab-fluids such as (Sr/P)n and Ba/Nb. Mass-balance modeling calculations, coupled with trace-element and isotopic data, indicate that although the mantle source for the calc-alkaline Adams basalts has been modified with a fluid derived from subducted sediment, the extent of modification is significantly less than what is documented in the southern Cascades. The isotopic and trace-element compositions of most Mount Adams lavas require the presence of enriched and depleted mantle sources, and based on volume-weighted chemical and isotopic compositions for Mount Adams lavas through time, an intraplate mantle source contributed the major magmatic mass of the system. Generation of basaltic andesites to dacites at Mount Adams occurred by assimilation and fractional crystallization in the lower crust, but wholesale crustal melting did not occur. Most lavas have Tb/Yb ratios that are significantly higher than those of MORB, which is consistent with partial melting of the mantle in the presence of residual garnet. ??18O values for olivine phenocrysts in Mount Adams lavas are within the range of typical upper mantle peridotites, precluding involvement of upper crustal sedimentary material or accreted terrane during magma ascent. The restricted Nd and Hf isotope compositions of Mount Adams lavas indicate that these isotope systems are insensitive to crustal interaction in this juvenile arc, in stark contrast to Os isotopes, which are highly sensitive to interaction with young, mafic material in the lower crust. ?? Springer-Verlag 2008.

Crustal structure of the eastern Borborema Province, NE Brazil, from the joint inversion of receiver functions and surface wave dispersion: Implications for plateau uplift

NASA Astrophysics Data System (ADS)

Luz, Rosana M. N.; Julià, Jordi; do Nascimento, Aderson F.

2015-05-01

We investigate the crustal structure of the Borborema Province of NE Brazil by developing 44 S wave velocity-depth profiles from the joint inversion of receiver functions and fundamental mode, Rayleigh wave group velocities. The Borborema Province is located in the northeasternmost corner of the South American continent and represents a portion of a larger Neoproterozoic mobile belt that formed during the Brasiliano-Pan African orogeny. Extensional processes in the Mesozoic—eventually leading to the separation of Africa and South America—left a number of aborted rift basins in the continental interiors, and episodes of diffuse intraplate volcanism and uplift marked the evolution of the Province after continental breakup. Our velocity-depth profiles reveal the existence of two crustal types in the Province: (i) the thin crustal type, which consists of 30-32.5 km thick crust, with an upper layer of 3.4-3.6 km/s overlying a lower layer of 3.7-3.8 km/s and (ii) the thick crustal type, which consists of a 35-37.5 km thick crust, with velocities between 3.5 and 3.9 km/s down to ˜30 km depth and a gradational increase in velocity (VS≥4.0 km/s) down to upper mantle depths. The crustal types correlate well with topography, with the thick crustal type being mainly found in the high-standing southern Borborema Plateau and the thin crustal type being mostly found in the low-lying Sertaneja depression and coastal cuestas. Interestingly, the thin crustal type is also observed under the elevated topography of the northern Plateau. We argue that the thick crustal type is rheologically strong and not necessarily related to postbreakup mantle processes, as it is commonly believed. We propose that extensional processes in the Mesozoic stretched portions of the Brasiliano crust and formed the thin crustal type that is now observed in the regions of low-lying topography, leaving the rheologically strong thick crust of the southern Plateau at higher elevations. The crust making the northern Plateau would have thinned and subsided during Mesozoic extension as part of a greater Sertaneja depression, to then experience uplift in the Cenozoic and achieve its present elevation.

Slab detachment during continental collision: Influence of crustal rheology and interaction with lithospheric delamination

NASA Astrophysics Data System (ADS)

Duretz, T.; Gerya, T. V.

2013-08-01

Collision between continents can lead to the subduction of continental material. If the crust remains coupled to the downgoing slab, a large buoyancy force is generated. This force slows down convergence and promotes slab detachment. If the crust resists to subduction, it may decouple from the downgoing slab and be subjected to buoyant extrusion. We employ two-dimensional thermo-mechanical modelling to study the importance of crustal rheology on the evolution of subduction-collision systems. We propose simple quantifications of the mechanical decoupling between lithospheric levels (σ*) and the potential for buoyant extrusion of the crust (ξ*). The modelling results indicate that a variable crustal rheological structure results in slab detachment, delamination, or the combination of both mechanisms. A strong crust provides coupling at the Moho (low σ*) and remains coherent during subduction (low ξ). It promotes deep subduction of the crust (180 km) and slab detachment. Exhumation occurs in coherent manners via eduction and thrusting. Slab detachment triggers the development of topography (> 4.5 km) close to the suture. A contrasting style of collision occurs using a weak crustal rheology. Mechanical decoupling at the Moho (high σ*) promotes the extrusion of the crust (high ξ), disabling slab detachment. Ongoing shortening leads to buckling of the crust and development of topography on the lower plate. Collisions involving rheologically layered crust allow decoupling at mid-crustal depths. This structure favours both the extrusion of upper crust and the subduction of the lower crust. Such collisions are successively affected by delamination and slab detachment. Topography develops together with the buoyant extrusion of crust onto the foreland and is further amplified by slab detachment. Our results suggest that the occurrence of both delamination (Apennines) and slab detachment (Himalayas) in orogens may indicate differences in the initial crustal structure of subducting continental plates in these regions.

Recycling of subducted crustal components into carbonatite melts revealed by boron isotopes

NASA Astrophysics Data System (ADS)

Hulett, Samuel R. W.; Simonetti, Antonio; Rasbury, E. Troy; Hemming, N. Gary

2016-12-01

The global boron geochemical cycle is closely linked to recycling of geologic material via subduction processes that have occurred over billions of years of Earth’s history. The origin of carbonatites, unique melts derived from carbon-rich and carbonate-rich regions of the upper mantle, has been linked to a variety of mantle-related processes, including subduction and plume-lithosphere interaction. Here we present boron isotope (δ11B) compositions for carbonatites from locations worldwide that span a wide range of emplacement ages (between ~40 and ~2,600 Ma). Hence, they provide insight into the temporal evolution of their mantle sources for ~2.6 billion years of Earth’s history. Boron isotope values are highly variable and range between -8.6‰ and +5.5‰, with all of the young (<300 Ma) carbonatites characterized by more positive δ11B values (>-4.0‰), whereas most of the older carbonatite samples record lower B isotope values. Given the δ11B value for asthenospheric mantle of -7 +/- 1‰, the B isotope compositions for young carbonatites require the involvement of an enriched (crustal) component. Recycled crustal components may be sampled by carbonatite melts associated with mantle plume activity coincident with major tectonic events, and linked to past episodes of significant subduction associated with supercontinent formation.

Regional Vp, Vs, Vp/Vs, and Poisson's ratios across earthquake source zones from Memphis, Tennessee, to St. Louis, Missouri

USGS Publications Warehouse

Catchings, R.D.

1999-01-01

Models of P- and S-wave velocity, Vp/Vs ratios, Poisson's ratios, and density for the crust and upper mantle are presented along a 400-km-long profile trending from Memphis, Tennessee, to St. Louis, Missouri. The profile crosses the New Madrid seismic zone and reveals distinct regional variations in the crustal velocity structure north and south of the latitude of New Madrid. In the south near Memphis, the upper few kilometers of the crust are dominated by upper crustal sedimentary basins or graben with P-wave velocities less than 5 km/sec and S-wave velocities of about 2 km/sec. P-wave velocities of the upper and middle crust range from 6.0 to 6.5 km/sec at depths above 25 km, and corresponding S-wave velocities range from 3.5 to 3.7 km/sec. The lower crust consists of a high-velocity layer (Vp = 7.4 km/sec; Vs ~4.2 km/sec) that is up to 20-km thick at the latitude of New Madrid but thins to about 15 km near Memphis. To the north, beneath the western-most Illinois basin, low-velocity (Vp < 5 km/sec; Vs < 2.3 km/sec) sedimentary basins are less than 1-km deep. The average velocities (Vp = 6.0 km/sec; Vs = 3.5 km/sec) of the underlying, near-surface rocks argue against large thickness of unconsolidated noncarbonate sediments within 50 km of the western edge of the Illinois basin. Most of the crust beneath the Illinois basin is modeled as one layer, with velocities up to 6.8 km/sec (Vs = 3.7 km/sec) at 37-km depth. The thick, high-velocity (Vp = 7.4 km/sec; Vs ~4.2 km/sec) lower crustal layer thins from about 20 km near New Madrid to about 6 km beneath the western Illinois basin. Refractions from the Moho and upper mantle occur as first arrivals over distances as a great as 160 km and reveal upper mantle layering to 60 km depth. Upper mantle layers with P-wave velocities of 8.2 km/sec (Vs = 4.5 km/sec) and 8.4 km/sec (Vs = 4.7 km/sec) are modeled at 43 and 60 km depth, respectively. Crustal Vp/Vs ratios range between 1.74 and 1.83, and upper mantle Vp/V s ratios range from 1.78 to 1.84. Poisson's ratios range from about 0.26 to 0.33 in the crust and from about 0.27 to 0.29 in the upper mantle. Modeled average densities range from about 2.55 in the sedimentary basins to 3.43 in the upper mantle. Geophysical characteristics of the crust and upper mantle within the New Madrid seismic zone are consistent with other continental rifts, but the crustal structure of the Illinois basin is not characteristics of most continental rift settings. Seismic and gravity data suggest a buried horst near the middle of Reelfoot rift, beneath which is a vertical zone of seismicity and velocity anomalies. The relative depth of the Reelfoot rift north and south of the Reelfoot graben suggests that the rift and its bounding faults may extend eastward beneath the city of Memphis.

Surface Deformation and Lower Crustal Flow in Eastern Tibet

PubMed

Royden; Burchfiel; King; Wang; Chen; Shen; Liu

1997-05-02

Field observations and satellite geodesy indicate that little crustal shortening has occurred along the central to southern margin of the eastern Tibetan plateau since about 4 million years ago. Instead, central eastern Tibet has been nearly stationary relative to southeastern China, southeastern Tibet has rotated clockwise without major crustal shortening, and the crust along portions of the eastern plateau margin has been extended. Modeling suggests that these phenomena are the result of continental convergence where the lower crust is so weak that upper crustal deformation is decoupled from the motion of the underlying mantle. This model also predicts east-west extension on the high plateau without convective removal of Tibetan lithosphere and without eastward movement of the crust east of the plateau.

Formation of continental crust in a temporally linked arc magma system from 5 to 30 km depth: ~ 90 Ma plutonism in the Cascades Crystalline Core composite arc section

NASA Astrophysics Data System (ADS)

Ratschbacher, B. C.; Miller, J. S.; Kent, A. J.; Miller, R. B.; Anderson, J. L.; Paterson, S. R.

2015-12-01

Continental crust has an andesitic bulk composition with a mafic lower crust and a granodioritic upper crust. The formation of stratified continental crust in general and the vertical extent of processes active in arc crustal columns leading to the differentiation of primitive, mantle-derived melts entering the lower crust are highly debated. To investigate where in the crustal column magma mixing, fractionation, assimilation and crystal growth occur and to what extent, we study the ~ 90 Ma magmatic flare-up event of the Cascades arc, a magma plumbing system from ~ 5 to 30 km depth. We focus on three intrusive complexes, emplaced at different depths during major regional shortening in an exceptionally thick crust (≥ 55 km1) but which are temporally related: the upper crustal Black Peak intrusion (1-3 kbar at 3.7 to 11 km; ~ 86.8 to 91.7 Ma2), the mid-crustal Mt. Stuart intrusion (3.5-4.0 kbar at 13 to 15 km; 90.8 and 96.3 Ma3) and the deep crustal Tenpeak intrusion (7 to 10 kbar at 25 to 37 km; 89.7 to 92.3 Ma4). These intrusive complexes are well characterized by geochronology showing that they have been constructed incrementally by multiple magma batches over their lifespans and thus allow the monitoring and comparison of geochemical parameters over time at different depths. We use a combination of whole rock major and trace element data and isotopes combined with detailed investigation of amphibole, which has been recognized to be important in the generation of calc-alkaline rocks in arcs to test the following hypotheses: (a) compositional bimodality is produced in the lower crust, whereas upper crustal levels are dominated by mixing to form intermediate compositions, or (b) differentiation occurs throughout the crustal column with different crystallizing phases and their compositions controlling the bulk chemistry. 1. Miller et al. 2009: GSA Special Paper 456, p. 125-149 2. Shea 2014: PhD thesis, Massachusetts Institute of Technology 3. Anderson et al. 2012: International Geology Review, v. 54, no. 5, p. 491-508 4. Matzel et al. 2006: GSA Bulletin, v. 118, no. 11-12, p. 1412-1430

Insight into NE Tibet expansion from SKS splitting: Missed mid-lower crustal flow in the frontier

NASA Astrophysics Data System (ADS)

Huang, Zhouchuan; Tilmann, Frederik; Xu, Mingjie; Wang, Liangshu; Ding, Zhifeng; Mi, Ning

2017-04-01

Two end member hypotheses for the expansion of the Tibetan plateau focus on either the deformation of the whole lithosphere or ductile flow in the mid-lower crust. Here, we analyse SKS shear-wave splitting at ChinArray stations in NE Tibet. Within the high plateau, the splitting measurements indicate two-layer anisotropy. The upper-layer anisotropy (with NE-SW fast axis) is caused by ductile-flow in the mid-lower crust while the lower-layer anisotropy (with NW-SE fast axis) reflects deformation in the upper mantle. In contrast, near the expansion frontier, the measurements indicate single layer splitting with a NW-SE fast axis that correlates with the strikes of most faults and the trend of the orogen. The results thus suggest different dynamics in the plateau and its NE margin. In the high plateau mid-lower crustal flow plays a dominant role while in the expansion frontier in the NE margin the initial tectonic uplift is induced by crustal thrust faulting.

Mantle flow tectonics - The influence of a ductile lower crust and implications for the formation of topographic uplands on Venus

NASA Technical Reports Server (NTRS)

Bindschadler, Duane L.; Parmentier, E. Marc

1990-01-01

The crust and mantle of Venus can be represented by a model of a layered structure stratified in both density and viscosity. This structure consists of a brittle-elastic upper crustal layer; a ductile weaker crustal layer; a strong upper mantle layer, about 10 percent denser than the crust; and a weaker substrate, representing the portion of the mantle in which convective flow occurs which is a primary source of large-scale topographic and tectonic features. This paper examines the interactions between these four layers and the mantle flow driven by thermal or compositional variations. Solutions are found for a flow driven by a buoyancy-force distribution within the mantle and by relief at the surface and crust-mantle boundary. It is shown that changes in crustal thickness are driven by vertical normal stresses due to mantle flow and by shear coupling of horizontal mantle flow into the crust.

Gravity model for the North Atlantic ocean mantle: results, uncertainties and links to regional geodynamics

NASA Astrophysics Data System (ADS)

Barantsrva, O.; Artemieva, I. M.; Thybo, H.

2015-12-01

We present the results of gravity modeling for the North Atlantic region based on interpretation of GOCE gravity satellite data. First, to separate the gravity signal caused by density anomalies within the crust and the upper mantle, we subtract the lower harmonics in the gravity field, which are presumably caused by deep density structure of the Earth (the core and the lower mantle). Next, the gravity effect of the upper mantle is calculated by subtracting the gravity effect of the crustal model. Our "basic model" is constrained by a recent regional seismic model EUNAseis for the crustal structure (Artemieva and Thybo, 2013); for bathymetry and topography we use a global ETOPO1 model by NOAA. We test sensitivity of the results to different input parameters, such as bathymetry, crustal structure, and gravity field. For bathymetry, we additionally use GEBCO data; for crustal correction - a global model CRUST 1.0 (Laske, 2013); for gravity - EGM2008 (Pavlis, 2012). Sensitivity analysis shows that uncertainty in the crustal structure produces the largest deviation from "the basic model". Use of different bathymetry data has little effect on the final results, comparable to the interpolation error. The difference in mantle residual gravity models based on GOCE and EMG2008 gravity data is 5-10 mGal. The results based on two crustal models have a similar pattern, but differ significantly in amplitude (ca. 250 mGal) for the Greenland-Faroe Ridge. The results demonstrate the presence of a strong gravity and density heterogeneity in the upper mantle in the North Atlantic region. A number of mantle residual gravity anomalies are robust features, independent of the choice of model parameters. This include (i) a sharp contrast at the continent-ocean transition, (ii) positive mantle gravity anomalies associated with continental fragments (microcontinents) in the North Atlantic ocean; (iii) negative mantle gravity anomalies which mark regions with anomalous oceanic mantle and the Mid-Atlantic Ridge. To understand better a complex geodynamics mosaic in the region, we compare our results with regional geochemical data (Korenaga and Klemen, 2000), and find that residual mantle gravity anomalies are well correlated with anomalies in epsilon-Nd and iron-depletion.

Crustal structure across the Brunswick Magnetic Anomaly in Southern Georgia

NASA Astrophysics Data System (ADS)

Lizarralde, D.; Shillington, D. J.; Harder, S. H.

2017-12-01

We will present results from Line 3 of the SUGAR experiment, a seismic refraction profile crossing the Brunswick Magnetic Anomaly (BMA) in southern Georgia. The BMA is a prominent, long-wavelength magnetic low that runs along the shelf offshore South Carolina and Georgia, turns inland near Brunswick and extends WNW toward Columbus GA. The source and significance of the BMA remain central elements of hypotheses for the construction of the SE U.S. continental lithosphere, including scenarios where the BMA marks the location of the Alleghany suture, where it represents a pre-existing suture within a peri-Gondwanan accreted terrane, and where the anomaly is related to Mesozoic rift-related tectono/magmatic processes. Deep-crustal reflectivity observed in multi-channel seismic images across the BMA proximal to the Laurentian margin near Columbus GA promoted the hypothesis that the BMA marks the location of the Alleghany suture. Results from an offshore refraction profile across the BMA along the Georgia shelf revealed a continuous, stratified, 4-km-thick layer in the upper crust beneath the post-rift unconformity with Vp=5.8 km/s interpreted as an undeformed Paleozoic metasedimentary section, inconsistent with an Alleghany suture, but also found an abrupt transition in mid-crustal velocity (6.18 north to 6.4 km/s south of BMA), consistent with preferential emplacement of Mesozoic magmatic additions or perhaps a pre-Alleghany suture. Line 3 of the SUGAR experiment is a relatively high-resolution crustal refraction line that included 11 shots and 700 seismic stations along a 110-km-long profile crossing normal to the BMA near Jesup GA. Preliminary results from Line 3 are similar to what is found offshore, with upper crustal velocities transitioning from 6.0 to 6.3 km/s across the BMA from N to S, with modest structural disruption related to the Kibbee Basin at the northern end of the line. These results are thus generally consistent with the ancient-suture hypothesis, though there is no corollary to the 5.8 km/s layer observed offshore. Further analyses will reveal upper-crustal structure in greater detail and also provide information on Moho structure across the BMA.

Crustal Magnetic Field Anomalies and Global Tectonics

NASA Astrophysics Data System (ADS)

Storetvedt, Karsten

2014-05-01

A wide variety of evidence suggests that the ruling isochron (geomagnetic polarity versus age) hypothesis of marine magnetic lineations has no merit - undermining therefore one of the central tenets of plate tectonics. Instead, variable induction by the ambient geomagnetic field is likely to be the principal agent for mega-scale crustal magnetic features - in both oceanic and continental settings. This revitalizes the fault-controlled susceptibility-contrast model of marine magnetic lineations, originally proposed in the late 1960s. Thus, the marine magnetic 'striping' may be ascribed to tectonic shearing and related, but variable, disintegration of the original iron-oxide mineralogy, having developed primarily along one of the two pan-global sets of orthogonal fractures and faults. In this way, fault zones (having the more advanced mineral alteration) would be characterized by relatively low susceptibility, while more moderately affected crustal sections (located between principal fault zones) would be likely to have less altered oxide mineralogy and therefore higher magnetic susceptibility. On this basis, induction by the present geomagnetic field is likely to produce oscillating magnetic field anomalies with axis along the principal shear grain. The modus operandi of the alternative magneto-tectonic interpretation is inertia-driven wrenching of the global Alpine age palaeo-lithosphere - triggered by changes in Earth's rotation. Increasing sub-crustal loss to the upper mantle during the Upper Mesozoic had left the ensuing Alpine Earth in a tectonically unstable state. Thus, sub-crustal eclogitization and associated gravity-driven delamination to the upper mantle led to a certain degree of planetary acceleration which in turn gave rise to latitude-dependent, westward inertial wrenching of the global palaeo-lithosphere. During this process, 1) the thin and mechanically fragile oceanic crust were deformed into a new type of broad fold belts, and 2) the continents were subjected to relative 'in situ' rotations (mostly moderate). Examples of marine magnetic lineations with landward continuation along prominent transcurrent fault zones, and the fact that striped marine magnetic anomalies may display orthogonal networks - concordant with the ubiquitous system of rectilinear fractures, faults and joints - corroborate the wrench tectonic interpretation of crustal field anomalies.

Montana: Filling A Gap In The GeoSwath

NASA Astrophysics Data System (ADS)

Jensen, B.; Keller, G. R.

2010-12-01

The proposed Geoswath transect crosses southern Montana, and the swath of MT stations deployed as part of EarthScope cover all but a small portion of eastern Montana. USArray broadband stations of course cover the entire region. However, modern controlled-source seismic data are very sparse in this large state, and most of it dates from the 1960’s. In this study, we have taken an integrated approach to analyzing lithospheric structure by compiling and analyzing all the public domain geophysical results and data we could locate and combining them with industry seismic reflection data that were released for our study. This information was employed to interpret a suite of filtered regional maps gravity and magnetic data and to construct integrated gravity models of long profiles that reflect crustal structure and deeper features within the upper mantle of the region. Our analysis included previous seismic refraction/reflection results, EarthScope Automated Array receiver functions, new 2D seismic reflection data, seismic tomography, potential field data, and previous geological studies in order to investigate structural and compositional variations within the crust and upper mantle. Our targets included Precambrian structure and tectonics, Sevier and Laramide features, and Late Cenozoic extension. Our main conclusions are: 1) Receiver function and seismic refraction/reflection crustal thickness estimates show a W-E crustal thickening with thicknesses greater than 50 km in the central and eastern Montana; 2) Seismic reflection data reveal Laramide basement-involved structures as far east as central Montana. These structures also show that the western edge of the North American craton was affected by late Mesozoic to Cenozoic deformation and has thus been decratonized; 3) Potential field filtering methods revealed regional trends and tectonic province outlines. The tilt derivative of the reduced-to-pole magnetic data enhances crystalline basement patterns that reflect tectonic province boundary locations. The upward continuation of the complete Bouguer anomaly grid revealed a gravity high in the northeast portion of the region, which is interpreted to be associated with density variations in the upper mantle. This interpretation is consistent with seismic tomography that reveals a “wedge-like” zone fast material beneath the craton in this region.

Recording the transition from flare-up to steady-state arc magmatism at the Purico-Chascon volcanic complex, northern Chile

NASA Astrophysics Data System (ADS)

Burns, Dale H.; de Silva, Shanaka L.; Tepley, Frank; Schmitt, Axel K.; Loewen, Matthew W.

2015-07-01

The long-term evolution of continental magmatic arcs is episodic, where a few transient events of high magmatic flux or flare-ups punctuate the low-flux magmatism or "steady state" that makes up most of the arc history. How this duality manifests in terms of differences in crustal architecture, magma dynamics and chemistry, and the time scale over which transitions occur is poorly known. Herein we use multiscale geochemical and isotopic characteristics coupled with geothermobarometry at the Purico-Chascon Volcanic Complex (PCVC) in the Central Andes to identify a transition from flare-up to steady state arc magmatism over ∼800 kyr during which significant changes in upper crustal magmatic dynamics are recorded. The PCVC is one of the youngest volcanic centers related to a 10-1 Ma ignimbrite flare-up in the Altiplano-Puna Volcanic Complex of the Central Andes. Activity at the PCVC initiated 0.98 ± 0.03 Ma with the eruption of a large 80-100 km3 crystal-rich dacite ignimbrite. High, restricted 87Sr/86Sr isotope ratios between 0.7085 and 0.7090 in the bulk rock and plagioclase crystals from the Purico ignimbrite, combined with mineral chemistry and phase relationships indicate the dacite magma accumulated and evolved at relatively low temperatures around 800-850 °C in the upper crust at 4-8 km depth. Minor andesite pumice erupted late in the ignimbrite sequence records a second higher temperature (965 °C), higher pressure environment (17-20 km), but with similar restricted radiogenic bulk rock 87Sr/86Sr = 0.7089-0.7091 to the dacites. The compositional and isotopic characteristics of the Purico ignimbrite implicate an extensive zone of upper crustal mixing, assimilation, storage and homogenization (MASH) between ∼30 and 4 km beneath the PCVC ∼1 Ma. The final eruptions at the PCVC < 0.18 ± 0.02 Ma suggest a change in the magmatic architecture beneath the PCVC. These eruptions produced three small <6 km3 crystal-rich dacite lava domes with radiogenic bulk rock 87Sr/86Sr ratios ranging from 0.7075 to 0.7081, that contain abundant basaltic-andesite inclusions with relatively low bulk rock 87Sr/86Sr ratios of 0.7057-0.7061. Plagioclase and amphibole in the host lava of Cerro Chascon, the largest of the domes, record two distinct magmatic environments; an upper crustal environment identical to that recorded in the Purico ignimbrite, and a second deeper, ∼15-20 km depth, higher temperature (∼922-1001 °C) environment. This deeper environment is recorded in textures and compositions of distinct mineral phases, and in intracrystalline isotope ratios. Plagioclase cores in the host dacite lava and mafic inclusions have in situ87Sr/86Sr isotopic compositions of 0.7083 to 0.7095, broadly similar to plagioclase from the Purico ignimbrite. In contrast, plagioclase rims and microphenocrysts in the mafic inclusions are isotopically distinct with lower 87Sr/86Sr isotope ratios (0.7057 to 0.7065 and 0.7062 to 0.7064, respectively) that overlap with the regional isotopic "baseline" compositions that are parental to the modern arc lavas. The textural and compositional characteristics of the PCVC attest to two distinct stages in its history. At ∼1 Ma the system was broadly homogeneous and dominantly dacitic recording extensive upper crustal magmatism. By ∼0.2 Ma the PCVC had transitioned to a more compositionally heterogeneous, smaller volume, mixed dacite to basaltic-andesite system, coinciding with the appearance of less-enriched "baseline" compositions. The evolution of PCVC is a microcosm of the Central Andean arc in this region where, from 10 to 1 Ma, upper crustal MASH processes resulted in the production and eruption of large volumes of homogeneous crystal-rich dacite during a regional ignimbrite flare-up. Since ∼1 Ma, decreasing explosivity, smaller eruptive volumes, increasing heterogeneity, and the emergence of less isotopically enriched basaltic-andesite to dacite composite volcanoes signal a return to steady-state arc volcanism. We posit that the transition from flare-up to steady state captured at the PCVC tracks the waning of the arc scale "thermal engine". High magmatic fluxes during the flare-up would lead to elevated geothermal gradients and efficient crustal processing leading to a dominantly "crustal" magmatism feeding the large volume Purico ignimbrite. This upper crustal MASH zone would act as an efficient filter to any parental compositions precluding them from the eruption record. As magmatic flux and thermal energy wanes, crustal isotherms would relax leading to greater thermal contrast between parental magmas, upper crust, and remnant felsic magmas stored in the upper crust. These changes are manifested in the preservation of textural and compositional heterogeneity and the survival of less isotopically enriched magmas in the upper crust. The chemical imprint of these arc-scale changes in magma dynamics is recorded at all scales from bulk rock to intra-crystalline. The distinct magma dynamics and chemical signatures of the two modes of arc magmatism detailed here should provide a model for investigations of mature continental arc evolution through time and space.

Pluton emplacement within an extensional transfer zone during dextral strike-slip faulting: an example from the late Archaean Abitibi Greenstone Belt

NASA Astrophysics Data System (ADS)

Lacroix, S.; Sawyer, E. W.; Chown, E. H.

1998-01-01

The Lake Abitibi area within the late Archaean Abitibi Greenstone Belt exhibits an interlinked plutonic, structural and metamorphic evolution that may characterize segmented strike-slip faults at upper-to-mid-crustal levels. Along the major, southeastward propagating Macamic D2 dextral strike-slip fault, Theological and preexisting D1 structural heterogeneities induced the development of NNW-trending dextral-oblique splays which evolved into an extensional trailing fan and created an extensional, NNW-dipping stepover. Magma flowing upwards from deeper parts of the Macamic Fault spread towards the southeast at upper crustal levels along both the oblique-slip and extensional D2 splays, and built several plutons in a pull-apart domain between 2696 and 2690 Ma. Different emplacement and material transfer mechanisms operated simultaneously in different parts of the system, including fault dilation and wedging, lateral expansion, wall-rock ductile flow and stoping. Transfer of movement between D2 splays occurred under ductile conditions during syn-emplacement, amphibolite-grade metamorphism (500-700 °C). During cooling (< 2690 Ma), narrower brittle-ductile zones of greenschist-grade shearing were concentrated along the pluton-wall rock contacts, but the extensional stepover locked since both normal and reverse movements occurred along NNW-dipping faults. Pluton emplacement, contact metamorphism and propagation of D2 faults appear to have been closely linked during the Superior Province-wide late transpressional event.

Geometries of geoelectrical structures in central Tibetan Plateau from INDEPTH magnetotelluric data

NASA Astrophysics Data System (ADS)

Vozar, Jan; Jones, Alan G.; Le Pape, Florian

2013-04-01

Magnetotelluric (MT) data collected on N-S profiles crossing the Banggong-Nujiang Suture, which separates the Qiangtang and Lhasa Terranes in central Tibet, as a part of InterNational DEep Profiling of Tibet and the Himalaya project (INDEPTH) are modeled by 2D and 3D inversion codes. The 2D deep MT model of line 500 confirms previous observations concluding that the region is characterized to first-order by a resistive upper crust and a conductive, partially melted, middle to lower crust that extends from the Lhasa Terrane to the Qiangtang Terrane with varying depth. The same conductive structure setting, but in shallower depths is also present on the eastern 400 line. From deep electromagnetic sounding, supported by independent 1D integrated petro-physical investigation, we can estimate the next upper-mantle conductive layer at depths from 200 km to 250 km below the Lhasa Terrane and less resistive Tibetan lithosphere below the Qiangtang Terrane with conductive upper-mantle in depths about 120 km. The anisotropic 2D modeling reveals lower crustal anisotropy in Lhasa Terrane, which can interpreted as crustal channel flow. The 3D inversion models of all MT data from central Tibet show dominant 2D regional strike of mid and lower crustal structures equal N110E. This orientation is parallel to Shuanghu suture, BengCo Jiali strike-slip fault system and perpendicular to convergence direction. The lower crust conductor in central Lhasa Terrane can be interpreted more likely as 3D lower Indian crust structure, located to the east from line 500, than geoelectrical anisotropic crustal flow.

Upper crustal structure of Madeira Island revealed from ambient noise tomography

NASA Astrophysics Data System (ADS)

Matos, Catarina; Silveira, Graça; Matias, Luís; Caldeira, Rita; Ribeiro, M. Luísa; Dias, Nuno A.; Krüger, Frank; Bento dos Santos, Telmo

2015-06-01

We present the first image of the Madeira upper crustal structure, using ambient seismic noise tomography. 16 months of ambient noise, recorded in a dense network of 26 seismometers deployed across Madeira, allowed reconstructing Rayleigh wave Green's functions between receivers. Dispersion analysis was performed in the short period band from 1.0 to 4.0 s. Group velocity measurements were regionalized to obtain 2D tomographic images, with a lateral resolution of 2.0 km in central Madeira. Afterwards, the dispersion curves, extracted from each cell of the 2D group velocity maps, were inverted as a function of depth to obtain a 3D shear wave velocity model of the upper crust, from the surface to a depth of 2.0 km. The obtained 3D velocity model reveals features throughout the island that correlates well with surface geology and island evolution.

Comparision between crustal density and velocity variations in Southern California

USGS Publications Warehouse

Langenheim, V.E.; Hauksson, E.

2001-01-01

We predict gravity from a three-dimensional Vp model of the upper crust and compare it to the observed isostatic residual gravity field. In general this comparison shows that the isostatic residual gravity field reflects the density variations in the upper to middle crust. Both data sets show similar density variations for the upper crust in areas such as the Peninsular Ranges and the Los Angeles basin. Both show similar variations across major faults, such as the San Andreas and Garlock faults in the Mojave Desert. The difference between the two data sets in regions such as the Salton Trough, the Eastern California Shear Zone, and the eastern Ventura basin (where depth to Moho is <30 km), however, suggests high-density middle to lower crust beneath these regions. Hence the joint interpretation of these data sets improves the depth constraints of crustal density variations.

The mineralogical and chronological evidences of subducted continent material in deep mantle: diamond, zircon and rutile separated from the Horoman peridotite of Japan

NASA Astrophysics Data System (ADS)

Li, Y.; Yang, J.; Nida, K.; Yamamoto, S.; Lin, Y.; Li, Q.; Tian, M.; Kon, Y.; Komiya, T.; Maruyama, S.

2017-12-01

The Horoman peridotite complex is an Alpine-type orogenic lherzolite massif of upper-mantle in the Hidaka metamorphic belt, Hokkaido, Japan. The peridotite complex is composed of dunite, harzburgite, spinel lherzolite and plagioclase lherzolite, exhibits a conspicuous layered structure, which is a product of a Cretaceous to early Paleogene arc-trench system formed by westward subduction of an oceanic plate between the paleo-Eurasian and paleo-North American Plates. Various combinations of diamond, corundum, moissanite, zircon, monazite, rutile, and kyanite have been separated from spinel harzburgite (700 kg) and lherzolite (500 kg), respectively. The carbon isotopes analyses of diamond grains by Nano-SIMS yielded significant light carbon isotopes feature as δ13 CPDB values ranging from -29.2 ‰ to -17.2 ‰, with an average of -22.8±0.32 ‰. Zircon grains occur as sub-angular to round in morphological characteristics, similar to zircons of crustal sedimentary rocks. Many zircons contain small inclusions, comprise of quartz, apatite, rutile and muscovite. The U-Pb age of zircon grains analyzed using LA-ICP-MS and SIMS gave a wide age range, from the Jurassic to Archean (ca 159 - 3131 Ma). In the zircon age histogram, four age groups were identified; the age peaks are 2385 Ma, 1890 Ma, 1618 Ma and 1212 Ma, respectively. On the other hand, U-Pb ages of rutile grains analyzed using SIMS gave a peak of 370 Ma in age histogram. The mineralogical and chronological evidences of numerous crustal minerals in peridotite of Horoman suggest that the ancient continent material was subducted in deep mantle and recycled through the upper mantle by multicycle subduction processes.

Crustal formation and recycling in an oceanic environment in the early Earth

NASA Astrophysics Data System (ADS)

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

2003-04-01

Several lines of evidence indicate higher mantle temperatures (by some hundreds of degrees) during the early history of the Earth. Due to the strong effect of temperature on viscosity as well as on the degree of melting, this enforces a geodynamic regime which is different from the present plate tectonics, and in which smaller scale processes play a more important role. Upwelling of a hotter mantle produces a thicker oceanic crust, of which the lower part may reside in the eclogite stability field. This facilitates delamination, making room for fresh mantle material which may partly melt and add new material to the crust (Vlaar et al., 1994). We present results of numerical thermo-chemical convection models including a simple approximate melt segregation mechanism in which we investigate this alternative geodynamic regime, and its effect on the cooling history and chemical evolution of the mantle. Our results show that the mechanism is capable of working on two scales. On a small scale, involving the lower boundary of the crust, delaminations and downward transport of eclogite into the upper mantle takes place. On a larger scale, involving the entire crustal column, (parts of) the crust may episodically sink into the mantle and be replaced by a fresh crust. Both are capable of significantly and rapidly cooling a hot upper mantle by driving partial melting and thus the generation of new crust. After some hundreds of millions of years, as the temperature drops, the mechanism shuts itself off, and the cooling rate significantly decreases. Vlaar, N.J., P.E. van Keken and A.P. van den Berg (1994), Cooling of the Earth in the Archaean: consequences of pressure-release melting in a hotter mantle, Earth and Planetary Science Letters, vol 121, pp. 1-18

Zircon petrochronology reveals the temporal link between porphyry systems and the magmatic evolution of their hidden plutonic roots (the Eocene Coroccohuayco deposit, Peru)

NASA Astrophysics Data System (ADS)

Chelle-Michou, Cyril; Chiaradia, Massimo; Ovtcharova, Maria; Ulianov, Alexey; Wotzlaw, Jörn-Frederik

2014-06-01

We present zircon geochronologic (LA-ICPMS and ID-TIMS), trace element and Hf isotopic evidence for a complex evolution of the plutonic roots of the Eocene Coroccohuayco porphyry system, southern Peru. LA-ICPMS U-Pb dating has initially been carried out to optimize grain selection for subsequent high-precision ID-TIMS dating and to characterize crustal assimilation (xenocrystic cores). This combined in-situ and whole-grain U-Pb dating of the same grains has been further exploited to derive a robust temporal interpretation of the complex magmatic system associated with the Coroccohuayco porphyry-skarn deposit. Our data reveal that a heterogeneous gabbrodioritic complex was emplaced at ca. 40.4 Ma and was followed by a nearly 5 Ma-long magmatic lull until the emplacement of dacitic porphyry stocks and dykes associated with the mineralizing event at ca. 35.6 Ma. However, at the sample scale, zircons from the porphyries provide insight into a 2 Ma-long lived “hidden” magmatism (probably at 4-9 km paleodepth) prior to porphyry intrusion and mineralization for which no other evidence can be found on the surface today. These dates together with zircon trace element analysis and Hf isotopes argue for the development of a long-lived magmatic system dominated by amphibole fractionation with an increasing amount of crustal assimilation and the development of a large and sustained thermal anomaly. The system was probably rejuvenated at an increasing rate from 37.5 to 35.6 Ma with injection of fresh and oxidized magma from the lower crust, which caused cannibalism and remelting of proto-plutons. The porphyry intrusions at Coroccohuayco were emplaced at the peak thermal conditions of this upper crustal magma chamber, which subsequently cooled and expelled ore fluids. Zircon xenocrysts and Hf isotopes in the porphyritic rocks suggest that this large upper crustal system evolved at stratigraphic levels corresponding to Triassic sediments similar to the Mitu group that may be present below the district. Using the zircon Ce anomaly as a proxy for oxidation state of the magma through time, we show that the high oxidation state of the porphyries is not the result of upper-crustal processes but is rather controlled by magmatic processes occurring at deeper levels. A comparison of our data with available high-precision geochronologic data at other porphyry systems suggests that such deposits may form when injection rate, volume and heat of their long-lived upper crustal magmatic system reach their peaks. These features might be diagnostic of a productive deposit.

Using the salt tectonics as a proxy to reveal post-rift active crustal tectonics: The example of the Eastern Sardinian margin

NASA Astrophysics Data System (ADS)

Lymer, Gaël; Vendeville, Bruno; Gaullier, Virginie; Chanier, Frank; Gaillard, Morgane

2017-04-01

The Western Tyrrhenian Basin, Mediterranean Sea, is a fascinating basin in terms of interactions between crustal tectonics, salt tectonics and sedimentation. The METYSS (Messinian Event in the Tyrrhenian from Seismic Study) project is based on 2100 km of HR seismic data acquired in 2009 and 2011 along the Eastern Sardinian margin. The main aim is to study the Messinian Salinity Crisis (MSC) in the Western Tyrrhenian Basin, but we also investigate the thinning processes of the continental crust and the timing of crustal vertical motions across this complex domain. Our first results allowed us to map the MSC seismic markers and to better constrain the timing of the rifting, which ended before the MSC across the upper and middle parts of the margin. We also evidenced that crustal activity persisted long after the end of rifting. This has been particularly observed on the upper margin, where several normal faults and a surprising compressional structure were recently active. In this study we investigate the middle margin, the Cornaglia Terrace, where the Mobile Unit (MU, mobile Messinian salt) accumulated during the MSC and acts as a décollement. Our goal is to ascertain whether or not crustal tectonics existed after the pre-MSC rift. This is a challenge where the MU is thick, because potential basement deformations could be first accommodated by the MU and therefore would not find any expression in the supra-salt layers (Upper Unit, UU and Plio-Quaternary, PQ). However our investigations clearly reveal interactions between crustal and salt tectonics along the margin. We thus evidence gravity gliding of the salt and its brittle sedimentary cover along basement slopes generated by the post-MSC tilting of some basement blocks bounded by crustal normal faults, formerly due to the rifting. Another intriguing structure also got our interest. It corresponds to a wedge-shaped of MU located in a narrow N-S half graben bounded to the west by a major, east-verging, crustal normal fault. Below the MU, the sediments thicken toward the fault. The top of the MU is sub-horizontal and the supra-salt layers are sub-horizontal. At a first glance this geometry would suggest that the pre-salt unit and the MU are syn-tectonic and that nothing happened after Messinian times. However some subtle evidence of deformations in the UU and PQ (an anticline to the west and a small west-verging normal fault in the east) imply that some crustal tectonics activity persisted after the end of the rifting. To understand why the salt unit is wedge-shaped, we considered several scenarii that we tested with physical modelling. We demonstrate that this structure is related to the post-rift activity of the major crustal normal fault, whose vertical motion has been cushioned by lateral flow of an initially tabular salt layer, which thinned upslope and inflated downslope, keeping the overlying sediments remained sub-horizontal. Such interactions between thin-skinned and thick-skinned tectonics highlight how the analysis of the salt tectonics is a powerful tool to reveal recent deep crustal tectonics in the Western Mediterranean Basin.

Geothermal Heat Flux and Upper Mantle Viscosity across West Antarctica: Insights from the UKANET and POLENET Seismic Networks

NASA Astrophysics Data System (ADS)

O'Donnell, J. P.; Dunham, C.; Stuart, G. W.; Brisbourne, A.; Nield, G. A.; Whitehouse, P. L.; Hooper, A. J.; Nyblade, A.; Wiens, D.; Aster, R. C.; Anandakrishnan, S.; Huerta, A. D.; Wilson, T. J.; Winberry, J. P.

2017-12-01

Quantifying the geothermal heat flux at the base of ice sheets is necessary to understand their dynamics and evolution. The heat flux is a composite function of concentration of upper crustal radiogenic elements and flow of heat from the mantle into the crust. Radiogenic element concentration varies with tectonothermal age, while heat flow across the crust-mantle boundary depends on crustal and lithospheric thicknesses. Meanwhile, accurately monitoring current ice mass loss via satellite gravimetry or altimetry hinges on knowing the upper mantle viscosity structure needed to account for the superimposed glacial isostatic adjustment (GIA) signal in the satellite data. In early 2016 the UK Antarctic Network (UKANET) of 10 broadband seismometers was deployed for two years across the southern Antarctic Peninsula and Ellsworth Land. Using UKANET data in conjunction with seismic records from our partner US Polar Earth Observing Network (POLENET) and the Antarctic Seismographic Argentinian Italian Network (ASAIN), we have developed a 3D shear wave velocity model of the West Antarctic crust and uppermost mantle based on Rayleigh and Love wave phase velocity dispersion curves extracted from ambient noise cross-correlograms. We combine seismic receiver functions with the shear wave model to help constrain the depth to the crust-mantle boundary across West Antarctica and delineate tectonic domains. The shear wave model is subsequently converted to temperature using a database of densities and elastic properties of minerals common in crustal and mantle rocks, while the various tectonic domains are assigned upper crustal radiogenic element concentrations based on their inferred tectonothermal ages. We combine this information to map the basal geothermal heat flux variation across West Antarctica. Mantle viscosity depends on factors including temperature, grain size, the hydrogen content of olivine and the presence of melt. Using published mantle xenolith and magnetotelluric data to constrain grain size and hydrogen content, respectively, we use the temperature model to estimate the regional upper mantle viscosity structure. The viscosity information will be incorporated in a 3D GIA model that will better constrain estimates of current ice loss from the West Antarctic Ice Sheet.

Insights into the crustal structure of the transition between Nares Strait and Baffin Bay

NASA Astrophysics Data System (ADS)

Altenbernd, Tabea; Jokat, Wilfried; Heyde, Ingo; Damm, Volkmar

2016-11-01

The crustal structure and continental margin between southern Nares Strait and northern Baffin Bay were studied based on seismic refraction and gravity data acquired in 2010. We present the resulting P wave velocity, density and geological models of the crustal structure of a profile, which extends from the Greenlandic margin of the Nares Strait into the deep basin of central northern Baffin Bay. For the first time, the crustal structure of the continent-ocean transition of the very northern part of Baffin Bay could be imaged. We divide the profile into three parts: continental, thin oceanic, and transitional crust. On top of the three-layered continental crust, a low-velocity zone characterizes the lowermost layer of the three-layered Thule Supergroup underneath Steensby Basin. The 4.3-6.3 km thick oceanic crust in the southern part of the profile can be divided into a northern and southern section, more or less separated by a fracture zone. The oceanic crust adjacent to the continent-ocean transition is composed of 3 layers and characterized by oceanic layer 3 velocities of 6.7-7.3 km/s. Toward the south only two oceanic crustal layers are necessary to model the travel time curves. Here, the lower oceanic crust has lower seismic velocities (6.4-6.8 km/s) than in the north. Rather low velocities of 7.7 km/s characterize the upper mantle underneath the oceanic crust, which we interpret as an indication for the presence of upper mantle serpentinization. In the continent-ocean transition zone, the velocities are lower than in the adjacent continental and oceanic crustal units. There are no signs for massive magmatism or the existence of a transform margin in our study area.

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.

Advanced Multivariate Inversion Techniques for High Resolution 3D Geophysical Modeling

DTIC Science & Technology

2010-09-01

crustal structures. But short periods are difficult to measure, especially in tectonically and geologically complex areas. On the other hand, gravity...East Africa Rift System Knowledge of crustal and upper mantle structure is of importance for understanding East Africa’s geodynamic evolution and for...area with less lateral heterogeneity but great tectonic complexity. To increase the effectiveness of the technique in this region, we explore gravity

Variations in the crustal structure beneath western Turkey

NASA Astrophysics Data System (ADS)

Saunders, Paul; Priestley, Keith; Taymaz, Tuncay

1998-08-01

We use teleseismic receiver functions to investigate the crustal structure at two locations in western Turkey using seismic data recorded on small arrays of temporary broad-band seismographs. The results from these analyses are compared with receiver function results from the GDSN station ANTO on the Anatolian Plateau in central Turkey. The crust is ~ 30 km thick in the region of western Turkey where active normal faulting reveals present-day extension in the upper crust and alkali-basaltic volcanism reveals recent extension within the subcrustal lithosphere The crust is ~ 34 km thick further east where crustal extension is still evident but less pronounced. In the Anatolian Plateau, which is not currently extending, the crust is ~ 38 km thick. The level of extension estimated from these measurements of crustal thickness implies a β -factor of ~ 1.2. This value agrees with the amount of extension estimated in the upper crust from the integrated seismic strain rate (β -factor of ~ 1.3), from surface faulting(β -factor of ~ 1.25) and from the amount of extension in the subcrustal lithosphere estimated from the volcanism (β -factor < 2), all indicating that the extension is approximately uniformly distributed vertically throughout the lithosphere. The Moho transition in this region appears to thin slightly as the degree of extension increases westwards.

Incremental assembly and prolonged consolidation of Cordilleran magma chambers--Evidence from the Southern Rocky Mountain volcanic field

USGS Publications Warehouse

Lipman, Peter W.

2007-01-01

Plutons thus provide an integrated record of prolonged magmatic evolution, while volcanism offers snapshots of conditions at early stages. Growth of subvolcanic batholiths involved sustained multistage open-system processes. These commonly involved ignimbrite eruptions at times of peak power input, but assembly and consolidation processes continued at diminishing rates long after peak volcanism. Some evidence cited for early incremental pluton assembly more likely records late events during or after volcanism. Contrasts between relatively primitive arc systems dominated by andesitic compositions and small upper-crustal plutons versus more silicic volcanic fields and associated batholiths probably reflect intertwined contrasts in crustal thickness and magmatic power input. Lower power input would lead to a Cascade- or Aleutian-type arc system, where intermediate-composition magma erupts directly from middle- and lower-crustal storage without development of large shallow plutons. Andean and southern Rocky Mountain–type systems begin similarly with intermediate-composition volcanism, but increasing magma production, perhaps triggered by abrupt changes in plate boundaries, leads to development of larger upper-crustal reservoirs, more silicic compositions, large ignimbrites, and batholiths. Lack of geophysical evidence for voluminous eruptible magma beneath young calderas suggests that near-solidus plutons can be rejuvenated rapidly by high-temperature mafic recharge, potentially causing large explosive eruptions with only brief precursors.

Heat and extension at mid- and lower crustal levels of the Rio Grande rift

NASA Technical Reports Server (NTRS)

Olsen, K. H.; Baldridge, W. S.; Callender, J. F.

1985-01-01

The process by which large amounts (50 to 200 percent) of crustal extension are produced was concisely described by W. Hamilton in 1982 and 1983. More recently, England, Sawyer, P. Morgan and others have moved toward quantifying models of lithospheric thinning by incorporating laboratory and theoretical data on rock rheology as a function of composition, temperature, and strain rate. Hamilton's description identifies three main crustal layers, each with a distinctive mechanical behavior; brittle fracturing and rotation in the upper crust, discontinuous ductile flow in the middle crust and laminar ductile flow in the lower crust. The temperature and composition dependent brittle-ductile transition essentially defines the diffuse boundary between upper and middle crust. It was concluded that the heat responsible for the highly ductile nature of the lower crust and the lensoidal and magma body structures at mid-crustal depths in the rift was infused into the crust by relatively modest ( 10 percent by mass) magmatic upwelling (feeder dikes) from Moho levels. Seismic velocity-versus-depth data, supported by gravity modeling and the fact that volumes of rift related volcanics are relatively modest ( 6000 cubic km) for the Rio Grande system, all imply velocities and densities too small to be consistent with a massive, composite, mafic intrusion in the lower crust.

Evidence for mafic lower crust in Tanzania, East Africa, from joint inversion of receiver functions and Rayleigh wave dispersion velocities

NASA Astrophysics Data System (ADS)

Julià, Jordi; Ammon, Charles J.; Nyblade, Andrew A.

2005-08-01

The S-wave velocity structure of Precambrian terranes in Tanzania, East Africa is modelled by jointly inverting receiver functions and surface wave dispersion velocities from the 1994-1995 Tanzania broad-band seismic experiment. The study region, which consists of an Archean craton surrounded by Proterozoic mobile belts, forms a unique setting for evaluating Precambrian crustal evolution. Our results show a uniform crustal structure across the region, with a 10-15 km thick upper crust with VS= 3.4-3.5 km s-1, overlying a gradational lower crust with S-wave velocities up to 4.1 km s-1 at 38-42 km depth. The upper-mantle lid displays uniform S-wave velocities of 4.5-4.7 km s-1 to depths of 100-150 km and overlays a prominent low-velocity zone. This low-velocity zone is required by the dispersion and receiver function data, but its depth interval is uncertain. The high crustal velocities within the lowermost crust characterize the entire region and suggest that mafic lithologies are present in both Archean and Proterozoic terranes. The ubiquitous mafic lower crust can be attributed to underplating associated with mafic dyke emplacement. This finding suggests that in East Africa there has been little secular variation in Precambrian crustal development.

Evidence for Weak Crustal Magnetic Fields over the Hellas, Chryse, and Acidalia Planitiae

NASA Astrophysics Data System (ADS)

Lee, C. O.; Mitchell, D. L.; Lillis, R.; Lin, R. P.; Reme, H.; Cloutier, P. A.; Acuna, M. H.

2003-04-01

The Electron Reflectometer (ER) onboard Mars Global Surveyor (MGS) detected a plasma boundary between the ionosphere and the solar wind as the latter is diverted around and past the planet [Mitchell et al., GRL, 27, 1871, 2000; Mitchell et al., JGR, 106, 23419, 2001]. Above this boundary the 10-1000 eV electron population is dominated by solar wind electrons, while below the boundary it is dominated by ionospheric photoelectrons. This "photoelectron boundary", or PEB, is sensitive to pressure variations and moves vertically in response to changes in the ionospheric pressure from below and the solar wind pressure from above. The PEB is also sensitive to crustal magnetic fields, which locally increase the total ionospheric pressure and positively bias the PEB altitude. We have empirically modeled and removed systematic variations in the PEB altitude associated with the solar wind interaction, thus isolating perturbations caused by crustal magnetic fields. A map of the PEB altitude perturbations closely resembles maps of the horizontal component of the crustal magnetic field measured at 400 km by the MGS Magnetometer (MAG). We find a PEB altitude bias over the Hellas basin that is consistent with a horizontal magnetic field with an intensity of several nanotesla at 400 km altitude. This is compatible with upper limits to the horizontal crustal field strength set by MGS MAG measurements. Since there is no evidence for significant crustal magnetic sources within the basin from MAG data obtained during aerobraking [Acuna et al. Science, 284, 790, 1999] or from electron reflection data obtained in the mapping orbit [Lillis et al., this conference], the most likely explanation is that the observed horizontal field originates from sources around the Hellas perimeter. No detectable PEB or magnetic signature is observed over the younger Argyre and Isidis Basins. There is also evidence for a significant enhancement (several nanoteslas) in the crustal field strength over Chryse Planitia and much of Acidalia Planitia, which are thought to contain hundreds of meters of material from the main outflow channels on Mars [Carr, Lunar Planetary Sci., 18, 155, 1987]. These fields appear to extend northward from a group of crustal magnetic sources along the dichotomy boundary that were mapped by the MGS Magnetometer.

Crustal structure of an intraplate thrust belt: The Iberian Chain revealed by wide-angle seismic, magnetotelluric soundings and gravity data

NASA Astrophysics Data System (ADS)

Seillé, Hoël; Salas, Ramon; Pous, Jaume; Guimerà, Joan; Gallart, Josep; Torne, Montserrat; Romero-Ruiz, Ivan; Diaz, Jordi; Ruiz, Mario; Carbonell, Ramon; Mas, Ramón

2015-11-01

The Iberian Chain is a Cenozoic intraplate thrust belt located within the Iberian plate. Unlike other belts in the Iberia Peninsula, the scarcity of geophysical studies in this area results in a number of unknowns about its crustal structure. The Iberian Chain crust was investigated by means of a NE-SW refraction/wide-angle reflection seismic transect and two magnetotelluric profiles across the chain, oriented along the same direction. The seismic profile was designed to sample the crust by means of three shots designed to obtain a reversed profile. The resulting velocity-depth model shows a moderate thickening of the crust toward the central part of the profile, where crustal thickness reaches values above 40 km, thinning toward de SW Tajo and NE Ebro foreland basins. The crustal thickening is concentrated in the upper crust. The seismic results are in overall agreement with regional trends of Bouguer gravity anomaly and the main features of the seismic model were reproduced by gravity modeling. The magnetotelluric data consist of 39 sites grouped into two profiles, with periods ranging from 0.01 s to 1000 s. Dimensionality analyses show significant 3D effects in the resistivity structure and therefore we carried out a joint 3D inversion of the full impedance tensor and magnetic transfer functions. The Mesozoic and Cenozoic basins along the Chain are well characterized by shallow high conductive zones and low velocities. Elongated conductors reaching mid-crustal depths evidence the presence of major faults dominating the crustal structure. The results from the interpretation of these complementary geophysical data sets provided the first images of the crustal structure of the Iberian Chain. They are consistent with a Cenozoic shortening responsible of the upper crust thickening as well as of the uplift of the Iberian Chain and the generation of its present day topography.

Crustal structure and deformation under the Longmenshan and its surroundings revealed by receiver function data

NASA Astrophysics Data System (ADS)

Sun, Ya; Liu, Jianxin; Zhou, Keping; Chen, Bo; Guo, Rongwen

2015-07-01

The convergence of India and Eurasia and the obstruction from the rigid Sichuan Basin cause the Longmenshan (LMS) to have the steepest topographic gradient at the eastern margin of the Tibetan Plateau. However, the mechanisms of surface uplift are still controversial. In this paper, we estimate the crustal structure and deformation under the LMS and its surroundings by analyzing a large amount of receiver function data recorded by regional seismic networks of the China Earthquake Administration. We apply a comprehensive splitting measurement technique on Ps conversion phase at the Moho (Moho Ps splitting) to calculate crustal anisotropy from azimuthal variations of receiver functions. Our results show that most of the seismic stations beneath the LMS area exhibit significant seismic anisotropy with the splitting time of 0.22-0.94 s and a fast polarization direction of NW-SE, while less or even no crustal anisotropy has been observed under the Sichuan Basin. Comparing the fast polarization directions of Moho Ps splitting with the indicators of lithospheric deformation (such as shear wave splitting, absolute plate motion, and global positioning system) imply a consistent tendency of deformation between the lower crust and upper mantle, but decoupling deformation in the crust beneath the LMS area. We further compare Moho Ps splitting time to that estimated from previous SKS splitting, indicating that crustal anisotropy is an important source of the SKS splitting time in this study area. In addition, a thick crust (>50 km) with high Vp/Vs values (1.74-1.86) is also observed using the H-κ stacking method. These seismic observations are consistent with the scenario that the LMS area has been built by the lower crustal flow. Combined with the seismic reflection/refraction profile and geology studies, we further suggest that the lower crustal flow may extrude upward into the upper crust along the steeply dipping strike faults under the LMS area, resulting in the surface uplift of the LMS.

Morphologic contrasts between Nirgal and Auqakuh Valles, Mars: Evidence of different crustal properties

NASA Technical Reports Server (NTRS)

Mackinnon, David J.; Tanaka, Kenneth L.; Winchell, Philip J.

1987-01-01

Photoclinometric measurements were made of sidewall slopes in Nirgal and Auqakuh Valles and these results were interpreted in terms of the geologic setting and a simple geomorphic model to provide insights into the physical properties of crustal materials in these areas. Nirgal was interpreted to be a runoff channel and Auqakuh to be a fretted channel. Geomorphologic arguments for the sapping origin of Nirgal and Auqakuh Valles were presented. The morphologies of the channels, however, differ greatly: the tributaries of Nirgal end abruptly in theater-headed canyons, whereas the heads of tributaries of Auqakuh shallow gradually. The plateau surface surrounding both channels appears to be covered by smooth materials, presumably lava flows; they are continuous and uneroded in the Nirgal area, but at Auqakuh they are largely eroded and several layers are exposed that total about 200 m in thickness. For Nirgal Valles, the measurements show that sidewalls in the ralatively shallow upper reaches of the channel have average slopes near 30 degrees and, in the lower reaches, sidewall slopes exceed 50 degrees. Auqakuh, on the other hand, has maximum sidewall slopes of 14 degrees and an approximate maximum depth of 1000 m. Faint, horizontal layering in portions of the lower reaches of Nirgal may indicate inhomogeneity in either composition or topography.

Apollo 17 materials viewed from 2 to 4 mm soil particles: Pre-serenitatis highlands components

NASA Technical Reports Server (NTRS)

Jolliff, Bradley L.; Bishop, Kaylynn M.

1993-01-01

Among the highland lithologies of 2-4 mm rock fragments in North Massif soil 76503, we have found a compositional group, low in incompatible element concentrations, that we interpret as representing the pre-Serenitatis surface. A component of these materials is an igneous-textured lithology that we believe formed in large impact melts. These are compositionally similar to, and possibly precursors of, many of the granulitic breccias that appear to be mixtures of ferroan and magnesian-suite rocks. The polymict, or old, upper-crustal breccias, along with granulitic breccias and the endogenous igneous lithologies found particularly at the North Massif stations, constitute the poorly consolidated portions of North Massif. Highland samples from the South Massif, on the other hand, are enriched in materials of the competent, impact-melt breccias formed by the Serenitatis impact. The competent melt-breccias contain clasts of most of the pre-existing surface materials, but they also contain components not found in the rocks of the poorly consolidated massif materials.

Crustal and upper mantle structure of the Hangay Dome, central Mongolia

NASA Astrophysics Data System (ADS)

Stachnik, J. C.; Meltzer, A.; Tsaagan, B.; Munkhuu, U.; Russo, R.; Souza, S.; Martin, P.

2013-12-01

The origin and support of high topography within continental interiors away from active tectonic margins remains a fundamental question in the dynamics and deformation of continents. The Hangay Dome in central Mongolia is one such region that is a broad regional uplift with average elevation of about 2 km, sitting between two large strike-slip faults, the Bulnay Fault to the north and the Gobi-Altay Fault to the south. Both of these faults are seismically active and have experienced M8+ earthquakes as recently as 1957. This portion of the Mongolian Plateau is approximately 300 km south of the Baikal Rift and located at the northern margin of the diffuse-deformation field in Central Asia, adjacent to the Siberian Craton. From previous research, the dynamic support of the Hangay Dome has been attributed to both crustal thickening and low density upper mantle material. However, seismic data leading to these interpretations have been limited to global tomographic models and sparse regional sampling of the wave field leaving the question unresolved. To address this major question in plate tectonic theory, in June 2012 a temporary IRIS/PASSCAL/University of Florida array of 72 seismic stations was deployed around the Hangay Dome to determine lithospheric structure in the region. Preliminary results from the first of two years of data are shown from receiver function analysis, ambient noise surface wave tomography, and teleseismic travel time residual analysis. Using teleseismic waveform records from over 300 earthquakes above M5.5 between 30 and 90 degrees epicentral distance, crustal thickness measurements from H-k stacking of receiver functions range from 42 km to 57 km across the array, with thicker crust beneath the highest topography. At each station the bulk crustal Vp/Vs ratio is also determined with median value for the array of 1.77, perhaps indicating a more mafic composition crust in the region.Teleseismic P-wave travel time residuals generally diminish from south to north across the array consistent with thinning crust, however the depth resolution and magnitude of seismic wavespeed anomalies will be further explored with three-dimensional finite-frequency tomography. Constraints on crustal shear wave velocity from ambient noise surface wave tomography complement both the receiver function analysis and teleseismic tomography. Initial inversions of phase velocity dispersion curves in the central Hangay indicate an average crustal Vs of 3.6 km/s within the Hangay Dome, which translates to an average Vp of 6.4 km/s using Vp/Vs of 1.77. Further refinement of current analysis and an additional year of recording will reveal the first high resolution lithospheric scale model in the region.

Deep seismic structure and tectonics of northern Alaska: Crustal-scale duplexing with deformation extending into the upper mantle

USGS Publications Warehouse

Fuis, G.S.; Murphy, J.M.; Lutter, W.J.; Moore, Thomas E.; Bird, K.J.; Christensen, N.I.

1997-01-01

Seismic reflection and refraction and laboratory velocity data collected along a transect of northern Alaska (including the east edge of the Koyukuk basin, the Brooks Range, and the North Slope) yield a composite picture of the crustal and upper mantle structure of this Mesozoic and Cenozoic compressional orogen. The following observations are made: (1) Northern Alaska is underlain by nested tectonic wedges, most with northward vergence (i.e., with their tips pointed north). (2) High reflectivity throughout the crust above a basal decollement, which deepens southward from about 10 km depth beneath the northern front of the Brooks Range to about 30 km depth beneath the southern Brooks Range, is interpreted as structural complexity due to the presence of these tectonic wedges, or duplexes. (3) Low reflectivity throughout the crust below the decollement is interpreted as minimal deformation, which appears to involve chiefly bending of a relatively rigid plate consisting of the parautochthonous North Slope crust and a 10- to 15-km-thick section of mantle material. (4) This plate is interpreted as a southward verging tectonic wedge, with its tip in the lower crust or at the Moho beneath the southern Brooks Range. In this interpretation the middle and upper crust, or all of the crust, is detached in the southern Brooks Range by the tectonic wedge, or indentor: as a result, crust is uplifted and deformed above the wedge, and mantle is depressed and underthrust beneath this wedge. (5) Underthrusting has juxtaposed mantle of two different origins (and seismic velocities), giving rise to a prominent sub-Moho reflector. Copyright 1997 by the American Geophysical Union.

Use of MAGSAT anomaly data for crustal structure and mineral resources in the US midcontinent

NASA Technical Reports Server (NTRS)

Carmichael, R. S. (Principal Investigator); Hoppin, R.; Black, R.; Anderson, R.

1981-01-01

Magnetic fields were measured from October 1979 until June 1980 using the satellite. The processed magnetic data yield long wavelength anomalies that arise from crustal and upper mantle sources. Analysis techniques are being developed to help interpret the structure and character of the lithosphere in central North America. The region includes the Midcontinent Gravity Anomaly peleorift zone and the New Madrid rift/seismic zone, both of which are of plaeotectonic and neotectonic interest. Preliminary analysis of the initial MAGSAT data combined with correlative geological and geophysical data shows the utility of the satellite data for regional crustal and basement study.

Geochemical and petrological evidence of the subduction of delaminated Adriatic continental lithosphere in the genesis of the Neogene-Quaternary magmatism of central Italy

NASA Astrophysics Data System (ADS)

Serri, G.; Innocenti, F.; Manetti, P.

1993-07-01

Serri, G., Innocenti, F. and Manetti, P., 1993. Geochemical and petrological evidence of the subduction of delaminated Adriatic continental lithosphere in the genesis of the Neogene-Quaternary magmatism of central Italy. In: M.J.R. Wortel, U. Hansen and R. Sabadini (Editors), Relationships between Mantle Processes and Geological Processes at or near The Earth's Surface. Tectonophysics, 223: 117-147. The Neogene-Quaternary magmatism of the northern Apenninic arc took place in four phases separated in space and time which become progressively younger from west to east: Phase I, 14 Ma; Phase II, 7.3-6.0 Ma; Phase III, 5.1-2.2 Ma; Phase IV, 1.3-0.1 Ma. This magmatism is the result of the activation of three physically separate sources: (1) the Adriatic continental crust, extracted from the mantle in the late Proterozoic; (2) a strongly refractory, recently K-enriched harzburgitic mantle located in the mechanical boundary layer (MBL) of the lithosphere; and (3) a recently metasomatized, cpx-rich mantle, compositionally variable from Iherzolite to wehrlite-clinopyroxenite, interpreted as an ephemerally K-enriched asthenosphere. The Adriatic continental crust is the dominant source of the acid plutonic and volcanic rocks of the Tuscan region. The acid magmatism is mostly found inside an ellipsoidal area (about 150 × 300 km) centred on Giglio Island, here defined as the Tuscan Crustal Dome. Within this area, mantle-derived magmas unaffected by important crustal contamination processes and mixing with crustal anatectic melts have so far not been found. Pure crustal magmas are rare but are represented, for example by some of the San Vincenzo and Roccastrada rhyolites. Virtually all the Tuscan acid centres show evidence of mixing with potassic mantle-derived magmas. Major and trace elements, as well as {87Sr }/{86Sr } and {143Nd }/{144Nd } data, on primitive rocks (Mg# > 65) reveal two groups of mantle-derived magmas. These define two distinct mantle enrichment trends, both essentially due to the additions of K-rich components which metasomatized separate, compositionally diverse, upper mantle sectors. In both cases the most remarkable mineralogical effect of these enrichment processes is the production of variable amount of phlogopite through reaction between fluids and/or melts with the mantle. The rocks of group I (ol-hy and Q-normative, lamproites, ultrapotassic high-Mg latites, ultrapotassic shoshonites and shoshonites: saturated trend) are considered to be derived by partial melting at low pressure (< 50 km) of strongly (lamproites) to moderately depleted phlogopite harzburgitic sources produced by reaction of residual peridotites with a K-Si-rich, Ca-Sr-poor melt with high ratios of {87Sr }/{86Sr (> 0.717) }, Ce/Sr (> 0.3) and {K 2O }/{Na 2O (> 6-7) }, and low ratios of {143Nd }/{144Nd ( 0.5121-0.5120) } and Ba/La (< 20) ratios; it is proposed that this component was formed by partial melting of subducted carbonate-free material of the upper crustal reservoir (e.g., non-restitic felsic granulites). This material is very common in the central Mediterranean region either as granitoid plutons/terrigenous sediments or as metasedimentary, non-restitic lower crust. The primitive rocks of group II are critically undersaturated, mostly leucitites, tephritic leucitites, leucite basanites, melilitites (undersaturated trend). Experimental petrology suggests that these rocks were formed by partial melting of a variably enriched phlogopite, clinopyroxene-rich mantle at higher pressure than group I primitive magmas. Trace-element modelling indicates that three components were involved in the genesis of group II mantle source: (a) a typical MORB-OIB-like mantle; (b) a component with very high Sr, Ca and Sr/Ce values and very low silica and sodium content, probably carried by a carbonatite melt somehow related to subducted marine carbonates; and (c) a recently added K-rich, Ca-Sr-poor crustal component, relatively well constrained to high {87Sr }/{86Sr (> 0.712) } and {K 2O }/{Na 2O (> 8-9) } values, and low {143Nd }/{144Nd (< 0.51205) }, Ba/La (< 20) and Ce/Sr (> 0.10) ratios. These constraints do not allow to exclude a complete identity between the K-rich components which metasomatized the mantle sources of the saturated and undersaturated trend magmas. The geochemical and isotopic features of the components that metasomatized the mantle sources of the northern Apenninic arc magmatism can be explained by a geodynamic process which causes a large amount of crustal materials to be incorporated within the upper mantle. We propose that the delamination and subduction of the Adriatic continental lithosphere related to the still ongoing northern Apennine continental collision provide a viable mechanism to explain the genesis and eastward discontinuous migration of the magmatism in central Italy. The subduction of delaminated lithospheric mantle with lower crustal slivers would have exposed uppermost mantle (Adriatic MBL) and crustal units previously imbricated in the Apennine chain to the heating advected by the upwelling of a recently and ephemerally K-enriched asthenospheric mantle wedge and by the underplating of magmas derived from it. We consider that the diapiric uprising of a hot, crustally contaminated asthenosphere occurs in the wake left above the sinking of the Adriatic delaminated/subducting continental lithosphere. The delamination/subduction process of the Adriatic lithosphere has probably started in the Early-Middle Miocene, but earlier than 15-14 Ma ago, as indicated by the age and petrologic characteristics of the first magmatic episode (Sisco lamproite) of the northern Apennine orogenesis.

The interplay between subduction and lateral extrusion: A case study for the European Eastern Alps based on analogue models

NASA Astrophysics Data System (ADS)

van Gelder, I. E.; Willingshofer, E.; Sokoutis, D.; Cloetingh, S. A. P. L.

2017-08-01

A series of analogue experiments simulating intra-continental subduction contemporaneous with lateral extrusion of the upper plate are performed to study the interference between these two processes at crustal levels and in the lithospheric mantle. The models demonstrate that intra-continental subduction and coeval lateral extrusion of the upper plate are compatible processes leading to similar deformation structures within the extruding region as compared to the classical setup, lithosphere-scale indentation. Strong coupling across the subduction boundary allows for the transfer of stresses to the upper plate, where strain regimes are characterized by crustal thickening near a confined margin and dominated by lateral displacement of material near a weak lateral confinement. The strain regimes propagate laterally during ongoing convergence creating an area of overlap characterized by transpression. When subduction is oblique to the convergence direction, the upper plate is less deformed and as a consequence the amount of lateral extrusion decreases. In addition, strain is partitioned along the oblique plate boundary resulting in less subduction in expense of right lateral displacement close to the weak lateral confinement. Both oblique and orthogonal subduction models have a strong resemblance to lateral extrusion tectonics of the Eastern Alps (Europe), where subduction of the adjacent Adriatic plate beneath the Eastern Alps is debated. Our results imply that subduction of Adria is a valid mechanisms to induce extrusion-type deformation within the Eastern Alps lithosphere. Furthermore, our findings suggest that the Oligocene to Late Miocene structural evolution of the Eastern Alps reflects a phase of oblique subduction followed by a later stage of orthogonal subduction conform a Miocene shift in the plate motion of Adria. Oblique subduction also provides a viable mechanism to explain the rapid decrease in slab length of the Adriatic plate beneath the Eastern Alps towards the Pannonian Basin.

Seismic tomography of northeastern Tibetan Plateau from body wave arrival times and surface wave dispersion data

NASA Astrophysics Data System (ADS)

Fang, H.; Yao, H.; Zhang, H.

2017-12-01

Reliable crustal and upper mantle structure is important to understand expansion of material from the Tibetan plateau to its northeastern margin. Previous studies have used either ambient noise tomography or body wave traveltime tomography to obtain the crustal velocity models in northeastern Tibetan Plateau. However, clear differences appear in these models obtained using different datasets. Here we propose to jointly invert local and teleseismic body wave arrival times and surface wave dispersion data from ambient noise cross correlation to obtain a unified P and S wavespeed model of the crust and upper mantle in NE Tibetan Plateau. Following Fang et al. (2016), we adopt the direct inversion strategy for surface wave data (Fang et al., 2015), which eliminates the need to construct the phase/group velocity maps and allows the straightforward incorporation of surface wave dispersion data into the body wave inversion framework. For body wave data including both local and teleseismic arrival times, we use the fast marching method (Rawlinson et al., 2004) in order to trace multiple seismic phases simultaneously. The joint inversion method takes advantage of the complementary strengths of different data types, with local body wave data constraining more on the P than S wavespeed in the crust, surface wave data most sensitive to S wavespeed in the crust and upper mantle, teleseismic body wave data resolving the upper mantle structure. A series of synthetic tests will be used to show the robustness and superiority of the joint inversion method. Besides, the inverted model will be validated by waveform simulation and comparison with other studies, like receiver function imaging. The resultant P and S wavespeed models, as well as the derived Vp/Vs model, will be essential to understand the regional tectonics of the northeastern Tibetan Plateau, and to address the related geodynamic questions of the Tibetan Plateau formation and expansion.

Seismic structure and activity of the north-central Lesser Antilles subduction zone from an integrated approach: Similarities with the Tohoku forearc

NASA Astrophysics Data System (ADS)

Laigle, M.; Hirn, A.; Sapin, M.; Bécel, A.; Charvis, P.; Flueh, E.; Diaz, J.; Lebrun, J.-F.; Gesret, A.; Raffaele, R.; Galvé, A.; Evain, M.; Ruiz, M.; Kopp, H.; Bayrakci, G.; Weinzierl, W.; Hello, Y.; Lépine, J.-C.; Viodé, J.-P.; Sachpazi, M.; Gallart, J.; Kissling, E.; Nicolich, R.

2013-09-01

The 300-km-long north-central segment of the Lesser Antilles subduction zone, including Martinique and Guadeloupe islands has been the target of a specific approach to the seismic structure and activity by a cluster of active and passive offshore-onshore seismic experiments. The top of the subducting plate can be followed under the wide accretionary wedge by multichannel reflection seismics. This reveals the hidden updip limit of the contact of the upper plate crustal backstop onto the slab. Two OBS refraction seismic profiles from the volcanic arc throughout the forearc domain constrain a 26-km-large crustal thickness all along. In the common assumption that the upper plate Moho contact on the slab is a proxy of its downdip limit these new observations imply a three times larger width of the potential interplate seismogenic zone under the marine domain of the Caribbean plate with respect to a regular intra-oceanic subduction zone. Towards larger depth under the mantle corner, the top of the slab imaged from the conversions of teleseismic body-waves and the locations of earthquakes appears with kinks which increase the dip to 10-20° under the forearc domain, and then to 60° from 70 km depth. At 145 km depth under the volcanic arc just north of Martinique, the 2007 M 7.4 earthquake, largest for half a century in the region, allows to document a deep slab deformation consistent with segmentation into slab panels. In relation with this occurrence, an increased seismic activity over the whole depth range provides a new focussed image thanks to the OBS and land deployments. A double-planed dipping slab seismicity is thus now resolved, as originally discovered in Tohoku (NE Japan) and since in other subduction zones. Two other types of seismic activity uniquely observed in Tohoku, are now resolved here: "supraslab" earthquakes with normal-faulting focal mechanisms reliably located in the mantle corner and "deep flat-thrust" earthquakes at 45 km depth on the interplate fault under the Caribbean plate forearc mantle. None such types of seismicity should occur under the paradigm of a regular peridotitic mantle of the upper plate which is expected to be serpentinized by the fluids provided from the dehydrating slab beneath. This process is commonly considered as limiting the downward extent of the interplate coupling. Interpretations are not readily available either for the large crustal thickness of this shallow water marine upper plate, except when remarking its likeness to oceanic plateaus formed above hotspots. The Caribbean Oceanic Plateau of the upper plate has been formed earlier by the material advection from a mantle plume. It could then be underlain by a correspondingly modified, heterogeneous mantle, which may include pyroxenitic material among peridotites. Such heterogeneity in the mantle corner of the present subduction zone may account for the notable peculiarities in seismic structure and activity and impose regions of stick-slip behavior on the interplate among stable-gliding areas.

Crustal and upper-mantle structure beneath ice-covered regions in Antarctica from S-wave receiver functions and implications for heat flow

NASA Astrophysics Data System (ADS)

Ramirez, C.; Nyblade, A.; Hansen, S. E.; Wiens, D. A.; Anandakrishnan, S.; Aster, R. C.; Huerta, A. D.; Shore, P.; Wilson, T.

2016-03-01

S-wave receiver functions (SRFs) are used to investigate crustal and upper-mantle structure beneath several ice-covered areas of Antarctica. Moho S-to-P (Sp) arrivals are observed at ˜6-8 s in SRF stacks for stations in the Gamburtsev Mountains (GAM) and Vostok Highlands (VHIG), ˜5-6 s for stations in the Transantarctic Mountains (TAM) and the Wilkes Basin (WILK), and ˜3-4 s for stations in the West Antarctic Rift System (WARS) and the Marie Byrd Land Dome (MBLD). A grid search is used to model the Moho Sp conversion time with Rayleigh wave phase velocities from 18 to 30 s period to estimate crustal thickness and mean crustal shear wave velocity. The Moho depths obtained are between 43 and 58 km for GAM, 36 and 47 km for VHIG, 39 and 46 km for WILK, 39 and 45 km for TAM, 19 and 29 km for WARS and 20 and 35 km for MBLD. SRF stacks for GAM, VHIG, WILK and TAM show little evidence of Sp arrivals coming from upper-mantle depths. SRF stacks for WARS and MBLD show Sp energy arriving from upper-mantle depths but arrival amplitudes do not rise above bootstrapped uncertainty bounds. The age and thickness of the crust is used as a heat flow proxy through comparison with other similar terrains where heat flow has been measured. Crustal structure in GAM, VHIG and WILK is similar to Precambrian terrains in other continents where heat flow ranges from ˜41 to 58 mW m-2, suggesting that heat flow across those areas of East Antarctica is not elevated. For the WARS, we use the Cretaceous Newfoundland-Iberia rifted margins and the Mesozoic-Tertiary North Sea rift as tectonic analogues. The low-to-moderate heat flow reported for the Newfoundland-Iberia margins (40-65 mW m-2) and North Sea rift (60-85 mW m-2) suggest that heat flow across the WARS also may not be elevated. However, the possibility of high heat flow associated with localized Cenozoic extension or Cenozoic-recent magmatic activity in some parts of the WARS cannot be ruled out.

Lithospheric Structure Beneath the Hangay Dome, Central Mongolia

NASA Astrophysics Data System (ADS)

Stachnik, J. C.; Meltzer, A.; Souza, S.; Munkhuu, U.; Tsaagan, B.; Russo, R. M.

2014-12-01

The Mongolian Plateau is a broad regional uplift positioned between the Siberian Craton to the north and the far northern edge of the India-Asia collision to the south. Within this intracontinental setting of high topography, the Hangay Dome in central Mongolia reaches elevations of 4 km and contains intermittent basaltic magmatism over the last 30 Ma. The relationship between high topography, magmatism, and geodynamic processes remains largely unsolved although processes ranging from lithospheric delamination to mantle plume effects have been proposed. A temporary array of seismic stations was deployed around the Hangay Dome to determine lithospheric structure. Preliminary results are shown from receiver function analysis, ambient noise tomography, and teleseismic P-wave tomography. Crustal thickness measurements from H-k stacking of receiver functions range from 42 km to 57 km across the array, with thicker crust beneath the highest topography. The bulk crustal Vp/Vs ratio ranges from 1.71 to 1.9 with a median value for the array of 1.77, perhaps indicating a variable crustal composition with some regions having a more mafic crust. The stacked receiver functions are also combined with ambient noise phase velocity dispersion measurements in a joint inversion for shear velocity profiles at each station which reveals crustal thickness estimates consistent with the H-k stacks while also determining the shear velocity step at the Moho. Teleseismic P-wave travel time residuals ranging between +/-1 second are inverted for a 3D P-wave velocity model using finite-frequency kernels. Notable features include 1) a low velocity anomaly (-3%) in the upper 200 km beneath the eastern part of the Hangay Dome near the Orkhon River Valley, , 2) a steeply dipping low velocity anomaly to the north of the Hangay Dome, perhaps related to the nearby Baikal Rift, and 3) generally higher velocities in the upper 200 km surrounding the high topography. To first order, the high topography of the Hangay Dome appears to be largely supported by thickened crust. However, lower P-wave velocities in the upper mantle beneath the dome are observed. The relative contributions of crustal thickness and upper mantle structure for support of topography and their relationship to magmatism will be determined with further refinement of the models.

Lithospheric strength in the active boundary between the Pacific Plate and Baja California microplate constrained from lower crustal and upper mantle xenoliths

NASA Astrophysics Data System (ADS)

Chatzaras, Vasileios; van der Werf, Thomas; Kriegsman, Leo M.; Kronenberg, Andreas; Tikoff, Basil; Drury, Martyn R.

2017-04-01

The lower crust is the most poorly understood of the lithospheric layers in terms of its rheology, particularly at active plate boundaries. We studied naturally deformed lower crustal xenoliths within an active plate boundary, in order to link their microstructures and rheological parameters to the well-defined active tectonic context. The Baja California shear zone (BCSZ), located at the western boundary of the Baja California microplate, comprises the active boundary accommodating the relative motion between the Pacific plate and Baja California microplate. The basalts of the Holocene San Quintin volcanic field carry lower crustal and upper mantle xenoliths, which sample the Baja California microplate lithosphere in the vicinity of the BCSZ. The lower crustal xenoliths range from undeformed gabbros to granoblastic two-pyroxene granulites. Two-pyroxene geothermometry shows that the granulites equilibrated at temperatures of 690-920 oC. Phase equilibria (P-T pseudosections using Perple_X) indicate that symplectites with intergrown pyroxenes, plagioclase, olivine and spinel formed at 3.6-5.4 kbar, following decompression from pressures exceeding 6 kbar. FTIR spectroscopy shows that the water content of plagioclase varies among the analyzed xenoliths; plagioclase is relatively dry in two xenoliths while one xenolith contains hydrated plagioclase grains. Microstructural observations and analysis of the crystallographic texture provide evidence for deformation of plagioclase by a combination of dislocation creep and grain boundary sliding. To constrain the strength of the lower crust and upper mantle near the BCSZ we estimated the differential stress using plagioclase and olivine grain size paleopiezomtery, respectively. Differential stress estimates for plagioclase range from 10 to 32 MPa and for olivine are 30 MPa. Thus the active microplate boundary records elevated crustal temperatures, heterogeneous levels of hydration, and low strength in both the lower crust and upper mantle. To further investigate the relative strength of the two lithospheric layers, we calculated the strain rate of plagioclase in granulites and the strain rate of olivine in lherzolites using experimental flow laws. These flow laws predict that plagioclase deforms at higher strain rates than olivine. Our data provide constraints on the viscosity structure of active transform plate boundaries and insights on how rheological processes in the lithosphere may change during plate boundary evolution.

Spatial relationships between crustal structures and mantle seismicity in the Vrancea Seismogenic Zone of Romania: Implications for geodynamic evolution

NASA Astrophysics Data System (ADS)

Enciu, Dana-Mihaela

Integration of active and passive-source seismic data is employed to study the relationships between crustal structures and seismicity in the SE Carpathian foreland of Romania, and the connection with the Vrancea Seismogenic Zone. Relocated crustal epicenters and focal mechanisms are correlated with industry seismic profiles Comanesti, Ramnicu Sarat, Braila and Buzau, the reprocessed DACIA PLAN profile and the DRACULA (Deep Reflection Acquisition Constraining Unusual Lithospheric Activity) II and III profiles in order to understand the link between neo-tectonic foreland deformation and Vrancea mantle seismicity. Projection of crustal foreland hypocenters onto deep seismic profiles identified active crustal faults suggesting a mechanical coupling between sedimentary, crustal and upper mantle structures on the Trotus, Sinaia and newly observed Ialomita Faults. Seismic reflection imaging revealed the absence of west dipping reflectors in the crust and an east dipping to horizontal Moho in the proximity of the Vrancea area. These findings argue against both 'subduction-in-place' and 'slab break-off' as viable mechanisms for generating Vrancea mantle seismicity.

Non-Linear Seismic Velocity Estimation from Multiple Waveform Functionals and Formal Assessment of Constraints

DTIC Science & Technology

2011-09-01

tectonically active regions such as the Middle East. For example, we previously applied the code to determine the crust and upper mantle structure...Objective Optimization (MOO) for Multiple Datasets The primary goal of our current project is to develop a tool for estimating crustal structure that...be used to obtain crustal velocity structures by modeling broadband waveform, receiver function, and surface wave dispersion data. The code has been

A review of the regional geophysics of the Arizona Transition Zone

NASA Technical Reports Server (NTRS)

Hendricks, J. D.; Plescia, J. B.

1991-01-01

A review of existing geophysical information and new data presented in this special section indicate that major changes in crustal properties between the Basin and Range and Colorado Plateau occur in, or directly adjacent to, the region defined as the Arizona Transition Zone. Although this region was designated on a physiographic basis, studies indicate that it is also the geophysical transition between adjoining provinces. The Transition Zone displays anomalous crustal and upper mantle seismic properties, shallow Curie isotherms, high heat flow, and steep down-to-the-plateau Bouguer gravity gradients. Seismic and gravity studies suggest that the change in crustal thickness, from thin crust in the Basin and Range to thick crust in the Colorado Plateau, may occur as a series of steps rather than a planar surface. Anomalous P wave velocities, high heat flow, shallow Curie isotherms, and results of gravity modeling suggest that the upper mantle is heterogeneous in this region. A relatively shallow asthenosphere beneath the Basin and Range and Transition Zone contrasted with a thick lithosphere beneath the Colorado Plateau would be one explanation that would satisfy these geophysical observations.

Upper crustal structure of central Java, Indonesia, from transdimensional seismic ambient noise tomography

NASA Astrophysics Data System (ADS)

Zulfakriza, Z.; Saygin, E.; Cummins, P. R.; Widiyantoro, S.; Nugraha, A. D.; Lühr, B.-G.; Bodin, T.

2014-04-01

Delineating the crustal structure of central Java is crucial for understanding its complex tectonic setting. However, seismic imaging of the strong heterogeneity typical of such a tectonically active region can be challenging, particularly in the upper crust where velocity contrasts are strongest and steep body wave ray paths provide poor resolution. To overcome these difficulties, we apply the technique of ambient noise tomography (ANT) to data collected during the Merapi Amphibious Experiment (MERAMEX), which covered central Java with a temporary deployment of over 120 seismometers during 2004 May-October. More than 5000 Rayleigh wave Green's functions were extracted by cross-correlating the noise simultaneously recorded at available station pairs. We applied a fully non-linear 2-D Bayesian probabilistic inversion technique to the retrieved traveltimes. Features in the derived tomographic images correlate well with previous studies, and some shallow structures that were not evident in previous studies are clearly imaged with ANT. The Kendeng Basin and several active volcanoes appear with very low group velocities, and anomalies with relatively high velocities can be interpreted in terms of crustal sutures and/or surface geological features.

Crustal Seismic Velocity Models of Texas

NASA Astrophysics Data System (ADS)

Borgfeldt, T.; Walter, J. I.; Frohlich, C.

2016-12-01

Crustal seismic velocity models are used to locate earthquake hypocenters. Typically, one dimensional velocity models are 3 - 8 fixed-thickness layers of varying P and S velocities with depth. On occasion, the layers of the upper crust (0-2 kilometers) are constrained with well log data from nearby wells, when available. Past velocity models used in Texas to locate earthquakes were made with little regard to deeper geologic units because shallow earthquakes with a localized seismic network only require velocity models of the upper crust. A recently funded statewide seismic network, TexNet, will require deeper crustal velocity models. Using data of geologic provinces, tectonics, sonic logs, tomography and receiver function studies, new regional velocity models of the state of Texas will allow researchers to more accurately locate hypocenters of earthquakes. We tested the accuracy of the initial models and then refine the layers of the 1-D regional models by using previously located earthquakes the USArray Transportable Array with earthquake location software. Geologic information will be integrated into a 3D velocity model at 0.5 degreee resolution for the entire state of Texas.

Folded fabric tunes rock deformation and failure mode in the upper crust.

PubMed

Agliardi, F; Dobbs, M R; Zanchetta, S; Vinciguerra, S

2017-11-10

The micro-mechanisms of brittle failure affect the bulk mechanical behaviour and permeability of crustal rocks. In low-porosity crystalline rocks, these mechanisms are related to mineralogy and fabric anisotropy, while confining pressure, temperature and strain rates regulate the transition from brittle to ductile behaviour. However, the effects of folded anisotropic fabrics, widespread in orogenic settings, on the mechanical behaviour of crustal rocks are largely unknown. Here we explore the deformation and failure behaviour of a representative folded gneiss, by combining the results of triaxial deformation experiments carried out while monitoring microseismicity with microstructural and damage proxies analyses. We show that folded crystalline rocks in upper crustal conditions exhibit dramatic strength heterogeneity and contrasting failure modes at identical confining pressure and room temperature, depending on the geometrical relationships between stress and two different anisotropies associated to the folded rock fabric. These anisotropies modulate the competition among quartz- and mica-dominated microscopic damage processes, resulting in transitional brittle to semi-brittle modes under P and T much lower than expected. This has significant implications on scales relevant to seismicity, energy resources, engineering applications and geohazards.

Crustal Zircons from the Podiform Chromitites in Luobusa Ophiolite, Tibet

NASA Astrophysics Data System (ADS)

Yamamoto, S.; Komiya, T.; Maruyama, S.

2004-12-01

For the past decade, diamonds and unusual mineral asemblages were reported in podiform chromitites of the Luobusa ophiolite, southern Tibet, China (Bai 1993, Bai 2000, Yan 2001) by heavy mineral separation. These include (1) native elements, (2) alloys, (3) carbide, (4) platinium group elements (PGE) and arsenides, (5) silicates (6) oxide, (7) carbonates, (8) minerals with unusual compositons. Despite many questions as to these minerals above still remain open, these mineral inclusions would provide us the important infomation on the formation of the podiform chromitites. In this study, over 100 zircons were discovered by heavy mineral separation of podiform chromitite in Luobusa ophiolite. The discovery of accessory zircons in chromitites allowed us to date the formation of the chromitite and history of tectonic evolutions. Here we report the U-Pb age and mineral inclusions of zircons and discuss with unusually old age zircons. 20 zircon grains in chromitites from No. 1 site were analyzed. Zircons from the chromitites in Luobusa ophiolite are usually euhedral-subhedral and some are rounded. Cathodoluminescence images of these zircons indicate that some zircons have clear oscillatory zoning, whereas other zircons show apparent homogeneous overgrowth. U-Pb dating of these zircons by LA-ICP-MS yielded two different ages. One group has relatively younger age, 107-534Ma, which plots nearly on a concordia line. Another group has older age 1460-1822Ma, which plots off the concordia line. There is insignificant difference of apparent ages within a single zircon grain. For example, a zircon has 1650 Ma in the core, whereas does 1654 Ma in the rim. We identified several mineral inclusions, quartz, feldspar, mica, apatite, within both yonger and older zircons using laser-Raman spectrometry and EPMA. No high-pressure minerals or mantle minerals were identified. This means that these unusually old zircons were formed in low-pressure crustal emvironment. Where did the zircons in chromitites come from? It has been recognized that this ophiolite was formed at 110-120 Ma based on radiolaria in cherts overlying the pillow lavas (ALLEGRE et al., 1984; ZIABREV et al., 2003). In this study, the minimum age of 107 Ma, which we obtained from zircon in chromitites, is consistent to the age of the ophiolite. But, all other ages of zircons are much older than that of ophiolite. Yang et al. (2001) also reported U-Pb zircon ages of 450-910 Ma and Re-Os iridosmine age of 400 Ma from chromitites in Luobusa ophiolite. Mineral inclusions within zircons are crustal materials, which means that these zircons were crystallized in the low pressure crustal condition. Thus these zircons within chromitites are interpreted as xenocrysts from old crustal materials. Recently, old-age zircons (330 to 1600 Ma) were also reported from the Mid Atlantic Ridge MORBs (PILOT et al., 1998). They suggest one possibility that these old-age zircons may have derived from old continental crustal material, which have assimilated with the MORB magma during ascent. Moreover, Archean zircons were reported from pyroxenite dikes in Jormua ophiolite (PETRI et al., 2003). YU et al. (2001) reported that zircons from chromitites in Luobusa ophiolite have shorter inter-atomic distances of Zr-O and Si-O bonds. As a result, they concluded that Tibetan-zircons were derived from the high-pressure mantle environment. Judging from the line of evidence mentioned above, it is highly possible that these zircons captured by chromitites were originated from recycled crustal materials, convecting through upper mantle.

Discovering the Complexity of Capable Faults in Northern Chile

NASA Astrophysics Data System (ADS)

Gonzalez, G.; del Río, I. A.; Rojas Orrego, C., Sr.; Astudillo, L. A., Sr.

2017-12-01

Great crustal earthquakes (Mw >7.0) in the upper plate of subduction zones are relatively uncommon and less well documented. We hypothesize that crustal earthquakes are poorly represented in the instrumental record because they have long recurrence intervals. In northern Chile, the extreme long-term aridity permits extraordinary preservation of landforms related to fault activity, making this region a primary target to understand how upper plate faults work at subduction zones. To understand how these faults relate to crustal seismicity in the long-term, we have conducted a detailed palaeoseismological study. We performed a palaeoseismological survey integrating trench logging and photogrammetry based on UAVs. Optically stimulated luminescence (OSL) age determinations were practiced for dating deposits linked to faulting. In this contribution we present the study case of two primary faults located in the Coastal Cordillera of northern Chile between Iquique (21ºS) and Antofagasta (24ºS). We estimate the maximum moment magnitude of earthquakes generated in these upper plate faults, their recurrence interval and the fault-slip rate. We conclude that the studied upper plate faults show a complex kinematics on geological timescales. Faults seem to change their kinematics from normal (extension) to reverse (compression) or from normal to transcurrent (compression) according to the stage of subduction earthquake cycle. Normal displacement is related to coseismic stages and compression is linked to interseismic period. As result this complex interaction these faults are capable of generating Mw 7.0 earthquakes, with recurrence times on the order of thousands of years during every stage of the subduction earthquake cycle.

The effect of crustal anisotropy on SKS splitting analysis—synthetic models and real-data observations

NASA Astrophysics Data System (ADS)

Latifi, Koorosh; Kaviani, Ayoub; Rümpker, Georg; Mahmoodabadi, Meysam; Ghassemi, Mohammad R.; Sadidkhouy, Ahmad

2018-05-01

The contribution of crustal anisotropy to the observation of SKS splitting parameters is often assumed to be negligible. Based on synthetic models, we show that the impact of crustal anisotropy on the SKS splitting parameters can be significant even in the case of moderate to weak anisotropy within the crust. In addition, real-data examples reveal that significant azimuthal variations in SKS splitting parameters can be caused by crustal anisotropy. Ps-splitting analysis of receiver functions (RF) can be used to infer the anisotropic parameters of the crust. These crustal splitting parameters may then be used to constrain the inversion of SKS apparent splitting parameters to infer the anisotropy of the mantle. The observation of SKS splitting for different azimuths is indispensable to verify the presence or absence of multiple layers of anisotropy beneath a seismic station. By combining SKS and RF observations in different azimuths at a station, we are able to uniquely decipher the anisotropic parameters of crust and upper mantle.

Crustal Deformation In the Northwestern Margin of the South China Sea: Results From Wide-angle Seismic Modeling

NASA Astrophysics Data System (ADS)

Huang, H.; Klingelhoefer, F.

2017-12-01

The South China Sea (SCS) has undergone episodic spreading during the Cenozoic Era. The long-term extension has shaped the continental margins of the SCS, leading to a progressive breakup of the lithosphere. Separated blocks and rift troughs, as controlled by tectonic stretching, contains key information about the deforming mechanism of the crust. In this work, we present a P-wave velocity model of a wide-angle seismic profile OBS2013-1 which passes through the NW margin of the SCS. Modeling of 25 ocean bottom seismometers (OBS) data revealed a detailed crustal structure and shallow complexities along the profile (Figure 1). The crust thins symmetrically across the Xisha Trough, from more than 20 km on flanks to 10 km in the central valley where the sediments thickens over 5 km; A volcano is situated on top of the centre basement high where the Moho drops slightly. At the distal margin around the Zhongsha Trough, the upper crust was detached and accordingly made the middle crust exhumed in a narrow area ( 20 km wide). Meanwhile, materials from the lower crust rises asymmetrically, increasing the crustal velocity by 0.3 km/s and may also giving rise to volcanisms along the hanging side. A 40 km wide hyper-stretched crust (with thickness of 5 km) was identified next to the Zhongsha Trough and covered by overflowing magma and post-rift sediments on the top. These observations argue for a depth-related and asymmetrically extension of the crust, including (1) detachment fault controls the deformation of the upper crust, leading to exhumation of the middle crust and asymmetrically rising of the lower crust, (2) The region adjacent to the exhumation region and with highly thinned crust can be considered as extinct OCT due to magma-starved supplying.

Thermal and petrologic constraints on lower crustal melt accumulation under the Salton Sea Geothermal Field

NASA Astrophysics Data System (ADS)

Karakas, Ozge; Dufek, Josef; Mangan, Margaret T.; Wright, Heather M.; Bachmann, Olivier

2017-06-01

In the Salton Sea region of southern California (USA), concurrent magmatism, extension, subsidence, and sedimentation over the past 0.5 to 1.0 Ma have led to the creation of the Salton Sea Geothermal Field (SSGF)-the second largest and hottest geothermal system in the continental United States-and the small-volume rhyolite eruptions that created the Salton Buttes. In this study, we determine the flux of mantle-derived basaltic magma that would be required to produce the elevated average heat flow and sustain the magmatic roots of rhyolite volcanism observed at the surface of the Salton Sea region. We use a 2D thermal model to show that a lower-crustal, partially molten mush containing < 20- 40% interstitial melt develops over a ∼105-yr timescale for basalt fluxes of 0.008 to 0.010 m3 /m2 /yr (∼0.0008 to ∼0.001 km3/yr injection rate) given extension rates at or below the current value of ∼0.01 m/yr (Brothers et al., 2009). These regions of partial melt are a natural consequence of a thermal regime that scales with average surface heat flow in the Salton Trough, and are consistent with seismic observations. Our results indicate limited melting and assimilation of pre-existing rocks in the lower crust. Instead, we find that basalt fractionation in the lower crust produces derivative melts of andesitic to dacitic composition. Such melts are then expected to ascend and accumulate in the upper crust, where they further evolve to give rise to small-volume rhyolite eruptions (Salton Buttes) and fuel local spikes in surface heat flux as currently seen in the SSGF. Such upper crustal magma evolution, with limited assimilation of hydrothermally altered material, is required to explain the slight decrease in δ18 O values of zircons (and melts) that have been measured in these rhyolites.

Robustness of Global Radial Anisotropy Models of the Upper Mantle

NASA Astrophysics Data System (ADS)

Xing, Z.; Beghein, C.; Yuan, K.

2014-12-01

Radial anisotropy provides important constraints on mantle deformation. While its presence is well accepted in the uppermost mantle, large discrepancies remain among existing models, even at depths well sampled by seismic data, and its presence at greater depths is highly uncertain. Surface wave phase velocity dispersion measurements are routinely used to constrain lateral variations in mantle S-wave velocity (dlnVS) and radial anisotropy (ξ=VSH2/VSV2). Here, we employed the fundamental and higher mode surface wave phase velocity maps of Visser et al. (2008) that have unprecedented sensitivity to structure down to 800-1000km depth, and we adopted a probabilistic forward modeling approach, the Neighbourhood Algorithm, to quantify posterior model uncertainties and parameter trade-offs. We investigated the effect of prior crustal corrections on 3-D ξ and dlnVS models. To avoid mapping crustal structure onto mantle heterogeneities, it is indeed important to accurately account for 3-D crustal anomalies and variations in Moho depth. One approach is to solve the non-linear problem and simultaneously constrain Moho depth and mantle anomalies (Visser et al., 2008). Another approach, taken here, is to calculate non-linear crustal corrections with an a priori crustal model, which are then applied to the phase velocity maps before inverting the remaining signal for mantle structure. In this work, we also determined laterally varying sensitivity kernels to account for lateral changes in the crust. We compare models obtained using CRUST2.0 (Bassin et al., 2000) and the new CRUST1.0 (Laske et al., 2012) models, which mostly differ under continents. Our preliminary results show strong differences (ΔdlnVS>2%) between the two models in continental dlnVS for the upper 150-200km, and strong changes in x amplitudes in the top 200km (Δξ>2%). Some of the differences in ξ persist down to the transition zone, in particular beneath central Asia and South America. Despite these discrepancies, inferences on the depth of continental roots (~200-250km) based on either the extent of the dlnVS>0 anomalies or the depth at which ξ changes sign remain independent of the crustal model employed. We also note that VSV>VSH dominates the deep upper mantle except in central Pacific, which is characterized by VSH>VSV down to the transition zone.

The Canada Basin compared to the southwest South China Sea: Two marginal ocean basins with hyper-extended continent-ocean transitions

NASA Astrophysics Data System (ADS)

Li, Lu; Stephenson, Randell; Clift, Peter D.

2016-11-01

Both the Canada Basin (a sub-basin within the Amerasia Basin) and southwest (SW) South China Sea preserve oceanic spreading centres and adjacent passive continental margins characterized by broad COT zones with hyper-extended continental crust. We have investigated strain accommodation in the regions immediately adjacent to the oceanic spreading centres in these two basins using 2-D backstripping subsidence reconstructions, coupled with forward modelling constrained by estimates of upper crustal extensional faulting. Modelling is better constrained in the SW South China Sea but our results for the Canada Basin are analogous. Depth-dependent extension is required to explain the great depth of both basins because only modest upper crustal faulting is observed. A weak lower crust in the presence of high heat flow and, accordingly, a lower crust that extends far more the upper crust are suggested for both basins. Extension in the COT may have continued even after seafloor spreading has ceased. The analogous results for the two basins considered are discussed in terms of (1) constraining the timing and distribution of crustal thinning along the respective continental margins, (2) defining the processes leading to hyper-extension of continental crust in the respective tectonic settings and (3) illuminating the processes that control hyper-extension in these basins and more generally.

Provenance and tectonic setting of the supra-crustal succession of the Qinling Complex: Implications for the tectonic affinity of the North Qinling Belt, Central China

NASA Astrophysics Data System (ADS)

Shi, Yu; Huang, Qianwen; Liu, Xijun; Krapež, Bryan; Yu, Jinhai; Bai, Zhian

2018-06-01

The Qinling Complex lies in the Qinling orogenic belt of Central China and holds the key to understanding the evolution of this feature. The Qinling Complex comprises a basement complex composed of amphibolite and ecologite, overlain by a supra-crustal succession that has been metamorphosed to the upper greenschist facies at approximately 516-509 Ma. The protoliths of the meta-sedimentary rocks are graywackes, which are divided into lower, middle and upper units. Detrital zircons from nine samples of the supra-crustal succession have ages ranging from 1182 to 1158 Ma for the lower unit, 957 to 955 Ma for the middle unit and 917 to 840 Ma for the upper unit. The lower unit is intruded by a ca. 960 Ma pluton. The bulk compositions of these meta-sedimentary rocks and their detrital zircon ages clearly indicate derivation from Meso- and Neo-proterozoic granites. Thus, we suggest that the sedimentary succession was derived from an arc-related tectonic setting and that none of the detritus was sourced from the southern margin of the North China Block or from the northern and western margins of the South China Block. We conclude that the North Qinling Belt was an independent micro-continental block during the Meso- to Neo-proterozoic.

Investigating Magmatic Processes in the Lower Levels of Mantle-derived Magmatic Systems: The Age & Emplacement of the Kunene Anorthosite Complex (SW Angola)

NASA Astrophysics Data System (ADS)

Hayes, B.; Bybee, G. M.; Owen-Smith, T.; Lehmann, J.; Brower, A. M.; Ashwal, L. D.; Hill, C. M.

2017-12-01

Our understanding of mantle-derived magmatic systems has shifted from a notion of upper crustal, melt-dominated magma chambers that feed short-lived volcanic eruptions, to a view of more long-lived trans-crustal, mush-dominated systems. Proterozoic massif-type anorthosite systems are voluminous, plagioclase-dominated plutonic suites with ubiquitous intermediate compositions (An 50 ± 10) that represent mantle-derived magmas initially ponded at Moho depths and crystallized polybarically until emplacement at mid-crustal levels. Thus, these systems provide unique insight into magma storage and processing in the lower reaches of the magma mush column, where such interpretation has previously relied on cumulate xenoliths in lavas, geophysical data and experimental/numerical modeling. We present new CA-ID-TIMS ages and a series of detailed field observations from the largest Proterozoic anorthosite massif on Earth, the Kunene Anorthosite Complex (KAC) of SW Angola. Field structures indicate that (i) the bulk of the material was emplaced in the form of crystal mushes, as both plutons and sheet-like intrusions; (ii) prolonged magmatism led to cumulate disaggregation (block structure development) and remobilization, producing considerable textural heterogeneity; (iii) crystal-rich magmatic flow induced localized recrystallization and the development of protoclastic (mortar) textures; and (iv) late residual melts were able to migrate locally prior to complete solidification. Dating of pegmatitic pods entrained from cumulate zones at the base of the crust (1500 ± 13 Ma) and their host anorthosites (1375-1438 Ma) reveals time periods in the range of 60-120 Myr between the earliest products of the system and the final mushes emplaced at higher crustal levels. Therefore, the KAC represents a complex, mushy magmatic system that developed over a long period of time. Not only do these observations help in refining our understanding of Proterozoic anorthosite petrogenesis, they also allow us to place constraints on the types of magmatic processes that operate in the lower levels of other trans-crustal magmatic systems.

Arc-arc Collision Structure in the Southernmost Part of the Kuril Trench Region -Results from Integrated Analyses of the 1998-2000 Hokkaido Transect Seismic Data-

NASA Astrophysics Data System (ADS)

Iwasaki, Takaya; Tsumura, Noriko; Ito, Tanio; Sato, Hiroshi; Kurashimo, Eiji; Hirata, Naoshi; Arita, Kazunori; Noda, Katsuya; Fujiwara, Akira; Abe, Susumu; Kikkuchi, Shunsuke; Suzuki, Kazuko

2015-04-01

The Hokkaido Island, located in the southernmost part of the Kuril trench region, has been under a unique tectonic environment of arc-arc collision. Due to the oblique subduction of the Pacific (PAC) plate, the Kuril forearc sliver started to collide against Northeast (NE) Japan arc from the east at the time of middle Miocene to form complicated structures in the Hidaka collision zone (HCZ), as characterized by the westward obduction of the crustal rocks of the Kuril arc (the Hidaka metamorphic belt (HMB)) along the Hidaka main thrust (HMT) and a thick foreland fold-and-thrust belt. In and around the HCZ, a series of seismic reflection/refraction experiments were undertaken from 1994 to 2000, which provided important structural features including crustal delamination in the southern HCZ and a thick fold-and-thrust belt with velocity reversals (low velocity layers) in the northern HCZ. Reprocessing/reinterpretation for these data sets, which started in 2012, is aimed to construct a more detailed collision model through new processing and interpretation techniques. A multi-disciplinary project of the 1998-2000 Hokkaido Transect, crossing the northern part of the HCZ in EW direction, collected high-quality seismic data on a 227-km seismic refraction/wide-angle reflection profile and three seismic reflection lines. Our reanalyses revealed interesting collision structure ongoing in the northern part of the HCZ. The westward obduction of the Kuril arc crust was clearly imaged along the HMT. This obduction occurs at a depth of 27-30 km, much deeper than in the southern HCZ (23-25 km). The CRS/MDRS processing to the reflection data firstly succeeded in imaging clear reflection events at a 30-45 km depth below the obducted Kuril arc crust. These events show an eastward dip, probably corresponding to the lower crust/Moho within the NE Japan arc descending down to the east under the collision zone. Gently eastward dipping structures above these events (in a depth range of 5-10 km) are interpreted to be fragments of Cretaceous subduction/arc complexes or deformation interfaces branched from the HMT. The refraction/wide-angle reflection analysis revealed a series of eastward dipping interfaces at depths of 15-30 km east of the HMT, some of which show a very large Vp contrast exceeding 0.5-1.0 km/s. The subducted NE Japan arc meets the Kuril arc 20-40 km east of the HMT at a depth of 20-30 km. The above mentioned high Vp contrasts may result from the mixture of the upper crustal (low Vp) materials of the NE Japan arc and lower crustal (high Vp) materials of the Kuril arc. Seismic reflection image in the southern HCZ reprocessed by almost the same techniques confirms a clear crustal delamination, where the upper 23-km crust is thrust up along the HMT while the lower part of the crust descends down to the subducted PAC plate. At the moment, the results in the northern HCZ do not provide positive evidence on shallow crustal delamination as found in the case of the southern HCZ, suggesting regional difference in collision style along the HMT.

Transient rheology of the upper mantle beneath central Alaska inferred from the crustal velocity field following the 2002 Denali earthquake

USGS Publications Warehouse

Pollitz, F.F.

2005-01-01

The M7.9 2002 Denali earthquake, Alaska, is one of the largest strike-slip earthquakes ever recorded. The postseismic GPS velocity field around the 300-km-long rupture is characterized by very rapid horizontal velocity up to ???300 mm/yr for the first 0.1 years and slower but still elevated horizontal velocity up to ???100 mm/yr for the succeeding 1.5 years. I find that the spatial and temporal pattern of the displacement field may be explained by a transient mantle rheology. Representing the regional upper mantle as a Burghers body, I infer steady state and transient viscosities of ??1 = 2.8 ?? 1018 Pa s and ??2 = 1.0 ?? 1017 Pa s, respectively, corresponding to material relaxation times of 1.3 and 0.05 years. The lower crustal viscosity is poorly constrained by the considered horizontal velocity field, and the quoted mantle viscosities assume a steady state lower crust viscosity that is 7??1. Systematic bias in predicted versus observed velocity vectors with respect to a fixed North America during the first 3-6 months following the earthquake is reduced when all velocity vectors are referred to a fixed site. This suggests that the post-Denali GPS time series for the first 1.63 years are shaped by a combination of a common mode noise source during the first 3-6 months plus viscoelastic relaxation controlled by a transient mantle rheology.

Isotopic constraints on crustal growth and recycling

NASA Technical Reports Server (NTRS)

Jacobsen, Stein B.

1988-01-01

The Sm-Nd isotopic data on clastic and chemical sediments are used with the present-day age distribution of continental crustal rocks to estimate the rates of crustal accretion, growth and recycling throughout earth's history. A new method for interpreting Nd model ages on both chemical and clastic sediments is proposed. A general relationship is derived between the mean crustal residence time of material recycled from the crust to the mantle (i.e., sediments), the mean age of the crust, and the crustal growth and recycling rates. This relationship takes into account the fact that the age distribution of material in the continental crust is generally different from the age distribution of material recycled into the mantle. The episodic nature of the present-day age distribution in crustal rocks results in similar episodicity in the accretion and recycling rates. The results suggest that by about 3.8 Ga ago, about 40 percent of the present continental volume was present. Recycling rates were extremely high 3-4 Ga ago and declined rapidly to an insignificant value of about 0.1 cu km/a during most of the Phanerozoic. The Nd model age pattern on sediments suggests a fairly high rate of growth during the Phanerozoic.

Application of FE software Elmer to the modeling of crustal-scale processes

NASA Astrophysics Data System (ADS)

Maierová, Petra; Guy, Alexandra; Lexa, Ondrej; Cadek, Ondrej

2010-05-01

We extended Elmer (the open source finite element software for multiphysical problems, http://www.csc.fi/english/pages/elmer) by user-written procedures for the two-dimensional modeling of crustal-scale processes. The standard version of Elmer is an appropriate tool for modeling of thermomechanical convection with non-linear viscous rheology. In geophysics, it might be suitable for some type of mantle convection modeling. Unlike the mantle, the crust is very heterogeneous. It consists of materials with distinct rheological properties that are subject to highly varied conditions: low pressure and temperature near the surface of the Earth and relatively high pressure and temperature at a depth of several tens of kilometers. Moreover, the deformation in the upper crust is mostly brittle and the strain is concentrated into narrow shear zones and thrusts. In order to simulate the brittle behavior of the crust, we implemented pressure-dependent visco-plastic rheology. The material heterogeneity and chemical convection is implemented in terms of active markers. Another special feature of the crust, the moving free surface, is already included in Elmer by means of a moving computational grid. Erosion can easily be added in this scheme. We tested the properties of our formulation of plastic flow on several numerical experiments simulating the deformation of material under compressional and extensional stresses. In the first step, we examined angles of shear zones that form in a plastically deforming material for different material parameters and grid resolutions. A more complex setting of "sandbox-type" experiments containing heterogeneous material, strain-softening and boundary friction was considered as a next testing case. To illustrate the abilities of the extended Elmer software in crustal deformation studies, we present two models of geological processes: diapirism of the lower crust and a channel flow forced by indentation. Both these processes are assumed to take place during the late stage of the Variscan orogeny in the area of the Bohemian Massif and they are well documented in the geological record. Extensive geological data are thus available and they can be compared with the results of our numerical simulations. Firstly, we model the indentation of a stiff block into a thick and hot crustal root and the consequent flow of the orogenic crust. For the development of the flow, the free surface deformation and erosion are essential. The importance of plastic deformation varies with the thermal structure of the domain. Secondly, we show an influence of thermal, density and viscosity structure of the crust on the time evolution and the final geometry of diapirs. The importance of the strain-rate dependence of viscosity, which is neglected in some numerical models, is discussed.

3D Modeling of Iran and Surrounding Areas from Simultaneous Inversion of Multiple Geophysical Datasets (Postprint). Annual Report 3

DTIC Science & Technology

2012-03-22

2003). This is particularly true at shallow depths where the shorter periods, which are primarily sensitive to upper crustal structures, are difficult...to measure, and especially true in tectonically and geologically complex areas. On the other hand, regional gravity inversions have the greatest...the slower deep crustal speeds into the Caspian region does not make sense geologically. These effects are driven by the simple Laplacian smoothness

Composition of island arcs and continental growth.

NASA Technical Reports Server (NTRS)

Jakes, P.; White, A. J. R.

1971-01-01

Island arc volcanism has contributed and is still contributing to continental growth, but the composition of island arcs differs from that of the upper continental crust in its lower abundance of Si, K, Rb, Ba, Sr and light rare earth elements. In their advanced stage of evolution, island arcs contain more than 80% of tholeiitic and 15% of ?island arc' calc-alkaline rocks with varied SiO2 contents. The larger proportion of tholeiitic rocks is in the lower crustal levels. The high stratigraphical levels of the island arcs are composed of tholeiitic plus calc-alkaline and/or high potash (shoshonitic) associations with higher abundances of K, Rb, Sr, and Ba. Stratification of the island arc crust is accentuated by another type of calc-alkaline volcanism (Andean type) originating at a late stage of arc evolution, probably by partial melting at the base of the crust. This causes enrichment of the upper crust in K, Rb, Ba and REE and accounts for upper crustal abundances of these elements as well as of SiO2.

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.

The crustal structure of the Cocos ridge off Costa Rica

NASA Astrophysics Data System (ADS)

Walther, Christian H. E.

2003-03-01

The submarine Cocos ridge in the northwestern Panamá basin, a bathymetric feature more than 1000-km long and 250-500 km broad, is about 2 km shallower than the adjacent basin. It is generally interpreted as the trace of the Galápagos hot spot. Two 127- and 260-km long seismic wide-angle sections were recorded along and across this ridge, offshore the Osa peninsula, Costa Rica. Crustal thickening is seen everywhere along the sections. On the northwestern outer ridge flank, increased thickness is exclusively attributed to the upper crust and expressed by 2-km thick flow basalts. The Quepos plateau caps the upper crust in this area. Toward the center of the Cocos ridge, the Moho deepens from 11-12 to 21 km depth and crustal thickening is almost entirely attributed to the lower crust which makes up 80% of the crust and is three times the thickness of normal oceanic lower crust. It is homogeneously structured and the velocities which range from 6.5 km/s at the top to 7.35 km/s at the base are comparable to normal lower crust under these depth conditions and suggest no differences to a gabbroic rock composition. Similarities to the crustal velocity structure of Iceland, central Kerguelen plateau, and Broken ridge are consistent with a formation of this 13-15 Ma old Cocos ridge segment by excessive magmatism in a near-plate boundary setting.

Crustal and uppermost mantle S-wave velocity structure beneath the Japanese islands from seismic ambient noise tomography

NASA Astrophysics Data System (ADS)

Guo, Zhi; Gao, Xing; Shi, Heng; Wang, Weiming

2013-04-01

In this study, the crustal and uppermost mantle shear wave velocities beneath the Japanese islands have been determined by inversion from seismic ambient noise tomography using data recorded at 75 Full Range Seismograph Network of Japan broad-band seismic stations, which are uniformly distributed across the Japanese islands. By cross-correlating 2 yr of vertical component seismic ambient noise recordings, we are able to extract Rayleigh wave empirical Green's functions, which are subsequently used to measure phase velocity dispersion in the period band of 6-50 s. The dispersion data are then inverted to yield 2-D tomographic phase velocity maps and 3-D shear wave velocity models. Our results show that the velocity variations at short periods (˜10 s), or in the uppermost crust, correlate well with the major known surface geological and tectonic features. In particular, the distribution of low-velocity anomalies shows good spatial correlation with active faults, volcanoes and terrains of sediment exposure, whereas the high-velocity anomalies are mainly associated with the mountain ranges. We also observe that large upper crustal earthquakes (5.0 ≤ M ≤ 8.0, depth ≤ 25 km) mainly occurred in low-velocity anomalies or along the boundary between low- and high-velocity anomalies, suggesting that large upper crustal earthquakes do not strike randomly or uniformly; rather they are inclined to nucleate within or adjacent to low-velocity areas.

Large-Scale Crustal-Block-Extrusion During Late Alpine Collision.

PubMed

Herwegh, Marco; Berger, Alfons; Baumberger, Roland; Wehrens, Philip; Kissling, Edi

2017-03-24

The crustal-scale geometry of the European Alps has been explained by a classical subduction-scenario comprising thrust-and-fold-related compressional wedge tectonics and isostatic rebound. However, massive blocks of crystalline basement (External Crystalline Massifs) vertically disrupt the upper-crustal wedge. In the case of the Aar massif, top basement vertically rises for >12 km and peak metamorphic temperatures increase along an orogen-perpendicular direction from 250 °C-450 °C over horizontal distances of only <15 km (Innertkirchen-Grimselpass), suggesting exhumation of midcrustal rocks with increasing uplift component along steep vertical shear zones. Here we demonstrate that delamination of European lower crust during lithosphere mantle rollback migrates northward in time. Simultaneously, the Aar massif as giant upper crustal block extrudes by buoyancy forces, while substantial volumes of lower crust accumulate underneath. Buoyancy-driven deformation generates dense networks of steep reverse faults as major structures interconnected by secondary branches with normal fault component, dissecting the entire crust up to the surface. Owing to rollback fading, the component of vertical motion reduces and is replaced by a late stage of orogenic compression as manifest by north-directed thrusting. Buoyancy-driven vertical tectonics and modest late shortening, combined with surface erosion, result in typical topographic and metamorphic gradients, which might represent general indicators for final stages of continent-continent collisions.

Tectonic Evolution of Ishtar Terra: Processes of Crustal Shortening and Thickening on Venus.

NASA Astrophysics Data System (ADS)

Vorder Bruegge, Richard Walsh

Evidence for the horizontal movement of large areas of crust, mountain building, and the possible removal of crust through processes similar to those observed on Earth is found in the Ishtar Terra highland region of Venus. Ishtar Terra is 5000 x 1500 km in size, an area comparable to Australia. It is characterized by a 2-5 km high, flat plain, Lakshmi Planum, which is flanked by mountain belts that reach elevations of up to 11 km. Outboard of these mountain belts are complexly deformed 'tessera' regions at elevations of 1 to 6 km. Evidence for the creation of crust through a mechanism comparable to terrestrial seafloor spreading is found in the elevated Trough and Ridge Terrain of Eastern Fortuna Tessera. In addition, the region around the Trough and Ridge Terrain is characterized by an elevated and deformed surface which exhibits a morphology compatible with an origin through crustal shortening through the collision of thick blocks of crust, such as through continental or oceanic plateau collision on Earth. Folds, thrusts, and strike-slip faults, all indicative of crustal shortening, are identified in these regions, and indicate a variety of convergence directions. The collision of very thick crustal blocks also produces high orogenic belts through processes comparable to terrestrial orogenesis. However, some fundamental differences may also exist in orogenesis on the two planets. These differences are primarily related to the driving mechanisms and thermal effects of orogenesis. The surficial geological characteristics of the northern edge of Ishtar Terra have been combined with geophysical modelling and provide evidence that this region is characterized by the wholescale underthrusting of low-lying plains crust beneath the thicker crust of Ishtar Terra. This underthrusting process adds upper crustal material to Ishtar Terra through a suturing process analogous to that of sedimentary accretionary wedges on Earth. The primary conclusion of this work is that crustal shortening and thickening through processes comparable to those observed on Earth have played a dominant role in the formation and evolution of Ishtar Terra.

High resolution 2D numerical models from rift to break-up: Crustal hyper-extension, Margin asymmetry, Sequential faulting

NASA Astrophysics Data System (ADS)

Brune, Sascha; Heine, Christian; Pérez-Gussinyé, Marta; Sobolev, Stephan

2013-04-01

Numerical modelling is a powerful tool to integrate a multitude of geological and geophysical data while addressing fundamental questions of passive margin formation such as the occurrence of crustal hyper-extension, (a-)symmetries between conjugate margin pairs, and the sometimes significant structural differences between adjacent margin segments. This study utilises knowledge gathered from two key examples of non-magmatic, asymmetric, conjugate margin pairs, i.e. Iberia-New Foundland and Southern Africa-Brazil, where many published seismic lines provide solid knowledge on individual margin geometry. While both margins involve crustal hyper-extension, it is much more pronounced in the South Atlantic. We investigate the evolution of these two margin pairs by carefully constraining our models with detailed plate kinematic history, laboratory-based rheology, and melt fraction evaluation of mantle upwelling. Our experiments are consistent with observed fault patterns, crustal thickness, and basin stratigraphy. We conduct 2D thermomechanical rift models using the finite element code SLIM3D that operates with nonlinear stress- and temperature-dependent elasto-visco-plastic rheology, with parameters provided by laboratory experiments on major crustal and upper mantle rocks. In our models we also calculate the melt fraction within the upwelling asthenosphere, which allows us to control whether the model indeed corresponds to the non-magmatic margin type or not. Our modelling highlights two processes as fundamental for the formation of hyper-extension and margin asymmetry at non-magmatic margins: (1) Strain hardening in the rift center due to cooling of upwelling mantle material (2) The formation of a weak crustal domain adjacent to the rift center caused by localized viscous strain softening and heat transfer from the mantle. Simultaneous activity of both processes promotes lateral rift migration in a continuous way that generates a wide layer of hyper-extended crust on one side of the rift basin. This mechanism implies that syn-rift deformation at the distal margin postdates faulting at the proximal margin by several million years. The succession of events holds intriguing implications not only for peak heat flow migration but also for processes like serpentinization and magmatic underplating.

Magnetotelluric Investigations of the Yellowstone Caldera: Understanding the Emplacement of Crustal Magma Bodies

NASA Astrophysics Data System (ADS)

Gurrola, R. M.; Neal, B. A.; Bennington, N. L.; Cronin, R.; Fry, B.; Hart, L.; Imamura, N.; Kelbert, A.; Bowles-martinez, E.; Miller, D. J.; Scholz, K. J.; Schultz, A.

2017-12-01

Wideband magnetotellurics (MT) presents an ideal method for imaging conductive shallow magma bodies associated with contemporary Yellowstone-Snake River Plain (YSRP) magmatism. Particularly, how do these magma bodies accumulate in the mid to upper crust underlying the Yellowstone Caldera, and furthermore, what role do hydrothermal fluids play in their ascent? During the summer 2017 field season, two field teams from Oregon State University and the University of Wisconsin-Madison installed forty-four wideband MT stations within and around the caldera, and using data slated for joint 3-D inversion with existing seismic data, two 2-D vertical conductivity sections of the crust and upper mantle were constructed. These models, in turn, provide preliminary insight into the emplacement of crustal magma bodies and hydrothermal processes in the YSRP region.

Seismic evidence for a crustal magma reservoir beneath the upper east rift zoneof Kilauea volcano, Hawaii

USGS Publications Warehouse

Lin, Guoqing; Amelung, Falk; Lavallee, Yan; Okubo, Paul G.

2014-01-01

An anomalous body with low Vp (compressional wave velocity), low Vs (shear wave velocity), and high Vp/Vs anomalies is observed at 8–11 km depth beneath the upper east rift zone of Kilauea volcano in Hawaii by simultaneous inversion of seismic velocity structure and earthquake locations. We interpret this body to be a crustal magma reservoir beneath the volcanic pile, similar to those widely recognized beneath mid-ocean ridge volcanoes. Combined seismic velocity and petrophysical models suggest the presence of 10% melt in a cumulate magma mush. This reservoir could have supplied the magma that intruded into the deep section of the east rift zone and caused its rapid expansion following the 1975 M7.2 Kalapana earthquake.

Generation and Reworking of Archaean and Hadean Crust

NASA Astrophysics Data System (ADS)

Hawkesworth, C.; Kemp, T.; Storey, C.; Dhuime, B.

2008-12-01

Combined Hf and O isotopes in well-dated zircons are increasingly used to investigate the age of the crustal source rocks of detrital and inherited zircons. O isotopes are used to screen out samples that may have a sediment contribution in the parental magma, since sediments yield hybrid model ages that are difficult to interpret. Mafic and granitic rocks also have different Lu/Hf ratios, and so in principle the Hf isotope ratios of zircons can be used to investigate the broad composition of the average crust. The unradiogenic Hf isotope compositions of the Jack Hills zircons from Western Australia indicate the existence of enriched (crustal) reservoirs by at least 4.3 Ga (Y. Amelin et al., 1998, Nature v. 399, p. 252- 255; T. M. Harrison et al., 2005, Science, v. 310, p. 1947-1950). We report in situ Hf isotope analyses of the Jack Hills zircons in which the Pb isotope age information is measured concurrently with the Hf isotope data. The simple data arrays provide clear evidence for Earth differentiation at 4.5 Ga, with the production of both continental crust-like material and a mafic crustal reservoir with higher Lu/Hf. The continued resampling of this reservoir over at least 1.5 Ga argues for a substantial stabilised volume of mafic crust, and, in tandem with oxygen isotope data, the existence of Hadean continents. Zircons remain poor windows into the upper mantle. We therefore investigate Nd isotopes in well-dated titanites; they have closure temperatures for Pb in the range 600-750oC and they can retain cores with distinct age and REE chemistry to subsequent rim overgrowths. Nd isotopes offer a complementary approach to Hf in zircon that can be used to construct the both depleted mantle evolution and crustal growth curves.

Joint the active source and passive source seismic to research the fine crustal structure of the Lushan area

NASA Astrophysics Data System (ADS)

Chen, Q.; Yu, C.

2017-12-01

On April 20, 2013, Ms7.0 strong earthquake (Lushan earthquake) occurred in Lanshan County Ya'an City, Sichuan Province. It is another earthquake that occurred in the Longmenshan fault zone after the Wenchuan earthquake. However, there is still no conclusive conclusion in relationship between the fine structure of the Lushan area and triggering seismic fault . In this study, the crustal structure, the shallow structure and the hidden faults and the focal mechanism of the Lushan earthquake were analyzed by using the deep seismic reflection profile and the broadband seismic array data. Combined with the surface geological information, the structure and fracture cause of the Lishan earthquake were discussed.We have synthetic analyzed the seismic precursors, fine locating, focal mechanism analysis and time-tomographic imaging of the broadband seismic data before and after the earthquake in Lushan earthquake, and obtained the seismic distribution, the focal mechanism and the crustal fine structure in the Lushan area. And we use these results to detailed interpreted the deep reflection seismic section of the Lushan earthquake zone.The results show that the crust of the Lushan area is characterized by a distinct structure of upper crust with thickness about 14.75km. The nature of the faults is inferred to be thrusting in the region due to the pushing of the crustal material of the Tibetan plateau into the southeast part of the rigid Sichuan basin. The shuangshi-Dachuan fault stretches from the surface to the deep crust at a low angle, and is dominated by thrusting in a form of imbricate structure with small-scale faults nearby. Whereas the Guangyuan-Dayi fault is a positive flower structure with a listric shape, consisting of six branches. Its movement is dominated by thrusting with gentle horizontal slip.

Tectonic History and Deep Structure of the Demerara Plateau from Combined Wide-Angle and Reflection Seismic Data and Plate Kinematic Reconstructions

NASA Astrophysics Data System (ADS)

Klingelhoefer, F.; Museur, T.; Roest, W. R.; Graindorge, D.; Chauvet, F.; Loncke, L.; Basile, C.; Poetisi, E.; Deverchere, J.; Lebrun, J. F.; Perrot, J.; Heuret, A.

2017-12-01

Many transform margins have associated intermediate depth marginal plateaus, which are commonly located between two oceanic basins. The Demerara plateau is located offshore Surinam and French Guiana. Plate kinematic reconstructions show that the plateau is located between the central and equatorial Atlantic in a position conjugate to the Guinean Plateau. In the fall of 2016, the MARGATS cruise acquired geophysical data along the 400 km wide Demerara plateau. The main objective of the cruise was to image the deep structure of the Demerara plateau and to study its tectonic history. A set of 4 combined wide-angle and reflection seismic profiles was acquired along the plateau, using 80 ocean-bottom seismometers, a 3 km long seismic streamer and a 8000 cu inch tuned airgun array. Forward modelling of the wide-angle seismic data on a profile, located in the eastern part of the plateau and oriented in a NE-SW direction, images the crustal structure of the plateau, the transition zone and the neighbouring crust of oceanic origin, up to a depth of 40 km. The plateau itself is characterised by a crust of 30 km thickness, subdivided into three distinct layers. However, the velocities and velocity gradients do not fit typical continental crust, with a lower crustal layer showing untypically high velocities and an upper layer having a steep velocity gradient. From this model we propose that the lowermost layer is probably formed from volcanic underplated material and that the upper crustal layer likely consists of the corresponding extrusive volcanic material, forming thick seaward-dipping reflector sequences on the plateau. A basement high is imaged at the foot of the slope and forms the ocean-continent transition zone. Further oceanward, a 5-6 km thick crust is imaged with velocities and velocity gradients corresponding to a thin oceanic crust. A compilation of magnetic data from the MARGATS and 3 previous cruises shows a high amplitude magnetic anomaly along the northern edge of the plateau thereby strengthening the hypothesis of an volcanic origin of at least part of the structure. We propose, that the plateau was formed by large-scale volcanism, possibly intruding into a thinner existing continental crust.

Crustal deformation mechanism in southeastern Tibetan Plateau: Insights from numerical modeling

NASA Astrophysics Data System (ADS)

Li, Y.; Liu, S.; Chen, L.

2017-12-01

The Indo-Asian collision developed the complicated crustal deformation around the southeastern Tibetan plateau. Numerous models have proposed to explain the crustal deformation, but the mechanism remains controversial, especially the increasing multi-geophysics data, which demonstrate the existence of lower velocity, lower resistivity and high conductivity, implying that lower crustal flow is responsible for the crustal deformation, arguing for the lower crust flow model. To address the relations between the crust flow and the surface deformation, we employ a three-dimensional viscoelastic finite model to investigate the possible influence on the surface deformation, and discuss the stress field distribution under the model. Our preliminary results suggest that lower crustal flow plays an important role in crustal deformation in southeastern Tibetan plateau. The best fitting is achieved when the flow velocity of the lower crust is approximately 10-11 mm/a faster than that of the upper crust. Crustal rheological properties affect regional crustal deformation, when the viscosity of the middle and lower crust in the South China block reaches 1022 and 1023 Pa.s, respectively; the predicted match observations well, especially for the magnitude within the South China block. The maximum principal stress field exhibits clear zoning, gradually shifting from an approximately east-west orientation in the northern Bayan Har block to southeast in the South China block, southwest in the western Yunnan block, and a radially divergent distribution in the Middle Yunnan and Southern Yunnan blocks.

Crustal structure of Mars from gravity and topography

NASA Technical Reports Server (NTRS)

Neumann, G. A.; Zuber, M. T.; Wieczorek, M. A.; McGovern, P. J.; Lemoine, F. G.; Smith, D. E.

2004-01-01

Mars Orbiter Laser Altimeter (MOLA) topography and gravity models from 5 years of Mars Global Surveyor (MGS) spacecraft tracking provide a window into the structure of the Martian crust and upper mantle. We apply a finite-amplitude terrain correction assuming uniform crustal density and additional corrections for the anomalous densities of the polar caps, the major volcanos, and the hydrostatic flattening of the core. A nonlinear inversion for Moho relief yields a crustal thickness model that obeys a plausible power law and resolves features as small as 300 km wavelength. On the basis of petrological and geophysical constraints, we invoke a mantle density contrast of 600 kg m-3; with this assumption, the Isidis and Hellas gravity anomalies constrain the global mean crustal thickness to be >45 km. The crust is characterized by a degree 1 structure that is several times larger than any higher degree harmonic component, representing the geophysical manifestation of the planet's hemispheric dichotomy. It corresponds to a distinction between modal crustal thicknesses of 32 km and 58 km in the northern and southern hemispheres, respectively. The Tharsis rise and Hellas annulus represent the strongest components in the degree 2 crustal thickness structure. A uniform highland crustal thickness suggests a single mechanism for its formation, with subsequent modification by the Hellas impact, erosion, and the volcanic construction of Tharsis. The largest surviving lowland impact, Utopia, post-dated formation of the crustal dichotomy. Its crustal structure is preserved, making it unlikely that the northern crust was subsequently thinned by internal processes.

A Sharp Continent-Ocean Transition in the Area of the Canary Islands: Evidence From Upper Mantle and Lower Crustal Xenoliths

NASA Astrophysics Data System (ADS)

Neumann, E.; Vannucci, R.; Tiepolo, M.; Griffin, W. L.; Pearson, N. J.; O'Reilly, S. Y.

2005-05-01

Our present information on passive margins rests almost exclusively on seismic and density data. An important exception is the west Iberia margin where petrological and geochemical information on crustal and mantle rocks have been made available through drilling experiments. In order to increase our information about, and understanding of, passive margins and their mode of formation, more information on crustal and mantle rocks along different types of passive margins are needed. In the area of the Canary Islands such information has been obtained through the study of mantle and deep crustal xenoliths brought to the surface by basaltic magmas. In-situ laser ablation (LA) ICP-MS mineral analyses have enabled us to "see through" the effects of the Canary Islands event and obtain robust information about the original (pre-Canarian) chemical character of the crust and upper mantle on which these islands are built. Our studies show that the lithosphere beneath the Canary Islands originated as highly refractory N-MORB type oceanic mantle overlain by highly refractory N-MORB crust. Both the lithospheric mantle and lower crust have been metasomatized to different degrees by a variety of fluid and melts. The enriched material is commonly concentrated along grain boundaries and cracks through mineral grains, suggesting that the metasomatism is relatively recent, and is thus associated with the Canary Islands magmatism. The original, strongly depleted trace element patterns and the low 87Sr/86Sr isotopic ratios typical of the oceanic lithosphere are preserved in the minerals in the least metasomatized rocks (e.g. LaN/LuN<0.1 in orthopyroxene and 87Sr/86Sr=0.7027-0.7029 in clinopyroxene in mantle xenoliths). The compositions of the most depleted gabbro samples from the different islands are closely similar, implying that there was no significant change in chemistry during the early stages of formation of the Atlantic oceanic crust in this area. Strongly depleted gabbros similar to those collected in Fuerteventura have also been retrieved in the MARK area along the central Mid-Atlantic Ridge. Furthermore, we have found no evidence of continental material that might reflect attenuated continental lithosphere in this area. The easternmost Canary Islands, Fuerteventura and Lanzarote, appear to overlap the lower part of the continental slope of Africa. The presence of normal oceanic lithosphere beneath these islands implies that the continent-ocean transition in the Canary Islands area must be relatively sharp, in contrast to the passive non-volcanic margin further north along the coast of Morocco, along the Iberia peninsula, and in many other areas. Our data also contradict the hypothesis that a mantle plume was present in this area during the opening of the Atlantic Ocean.

Crustal structure across the Altyn Tagh Range at the northern margin of the Tibetan Plateau and tectonic implications

USGS Publications Warehouse

Zhao, J.; Mooney, W.D.; Zhang, X.; Li, Z.; Jin, Z.; Okaya, N.

2006-01-01

We present new seismic refraction/wide-angle-reflection data across the Altyn Tagh Range and its adjacent basins. We find that the crustal velocity structure, and by inference, the composition of the crust changes abruptly beneath the Cherchen fault, i.e., ???100 km north of the northern margin of the Tibetan plateau. North of the Cherchen fault, beneath the Tarim basin, a platform-type crust is evident. In contrast, south the Cherchen fault the crust is characterized by a missing high-velocity lower-crustal layer. Our seismic model indicates that the high topography (???3 km) of the Altyn Tagh Range is supported by a wedge-shaped region with a seismic velocity of 7.6-7.8 km/s that we interpret as a zone of crust-mantle mix. We infer that the Altyn Tagh Range formed by crustal-scale strike-slip motion along the North Altyn Tagh fault and northeast-southwest contraction over the range. The contraction is accommodated by (1) crustal thickening via upper-crustal thrusting and lower-crustal flow (i.e., creep), and (2) slip-parallel (SW-directed) underthrusting of only the lower crust and mantle of the eastern Tarim basin beneath the Altyn Tagh Range. ?? 2005 Elsevier B.V. All rights reserved.

Data report for onshore-offshore wide-angle seismic recordings in the Bering-Chukchi Sea, Western Alaska and eastern Siberia

USGS Publications Warehouse

Brocher, Thomas M.; Allen, Richard M.; Stone, David B.; Wolf, Lorraine W.; Galloway, Brian K.

1995-01-01

This report presents fourteen deep-crustal wide-angle seismic reflection and refraction profiles recorded onland in western Alaska and eastern Siberia from marine air gun sources in the Bering-Chukchi Seas. During a 20-day period in August, 1994, the R/V Ewing acquired two long (a total of 3754 km) deep-crustal seismic-reflection profiles on the continental shelf of the Bering and Chukchi Seas, in a collaborative project between Stanford University and the United States Geological Survey (USGS). The Ewing's 137.7 liter (8355 cu. in.) air gun array was the source for both the multichannel reflection and the wide-angle seismic data. The Ewing, operated by the Lamont-Doherty Earth Observatory, steamed northward from Nunivak Island to Barrow, and returned, firing the air gun array at intervals of either 50 m or 75 m. About 37,700 air gun shots were fired along the northward directed Lines 1 and 2, and more than 40,000 air gun shots were fired along the southward directed Line 3. The USGS and the University of Alaska, Fairbanks (UAF), deployed an array of twelve 3-component REFTEK and PDAS recorders in western Alaska and eastern Siberia which continuously recorded the air gun signals fired during the northward bound Lines 1 and 2. Seven of these recorders also continuously recorded the southward bound Line 3. These wide-angle seismic data were acquired to: (1) image reflectors in the upper to lower crust, (2) determine crustal and upper mantle refraction velocities, and (3) provide important constraints on the geometry of the Moho along the seismic lines. In this report, we describe the land recording of wide-angle data conducted by the USGS and the UAF, describe in detail how the wide-angle REFTEK and PDAS data were reduced to common receiver gather seismic sections, and illustrate the wide-angle seismic data obtained by the REFTEKs and PDAS's. Air gun signals were observed to ranges in excess of 400 km, and crustal and upper /mantle refractions indicate substantial variation in the crustal thickness along the transect.

Upper-crustal Stress Field Variations During the Building of the Central Andes: Constrains on the Activation/deactivation of Megadetachments

NASA Astrophysics Data System (ADS)

Giambiagi, L.; Tassara, A.; Mescua, J.; Suriano, J.; Mahoney, J. B.; Hoke, G. D.; Spagnotto, S. L.; Lossada, A. C.; Mardónez, D.; Mazzitelli, M.; Barrionuevo, M.

2015-12-01

Nowadays, it is broadly accepted that the Central Andes resulted largely from crustal shortening in the last ~45 Ma, driven by horizontal forces as a consequence of subduction of the Nazca plate beneath South America. However, the way this shortening is achieved is still a matter a debate. Structural, seismological, thermochronological, isotopical and sedimentological studies of the Central Andes, together with thermomechanical modeling, suggest that different megadetachments located shallow in the upper crust were active during the construction of the Andes. Constrains on changes in the state of stress in the crust gleaned from more than 1,500 fault-slip data in the arc region provide insights into how and when these megadetachments get activated or deactivated. We used a forward modeling procedure to examine five transects across the Central Andes, at 21.5°, 24°, 30°, 34° and 35°S, with particular emphasis on the relationship between deep and shallow structures. Our kinematic-thermomechanical models show that most of the upper-middle crust has a brittle-elastic behavior particularly for the cold and rigid forearc and foreland regions, and a ductile behavior below the thermally weakened arc region. Our models assume a shallow, sub-horizontal megadetachment located at the shallowest brittle-ductile transition, which concentrates the majority of the horizontal crustal shortening between the fore-arc and the South American craton. During this horizontal shortening, the crust gets thick and topography rises due to buoyancy of the crustal root. The threshold of this thickening is achieved when the bouyancy force equals the horizontal force. At this point, the megadetachment deactives and the crustal root widens eastwards in concert with ductile deformation in the lower crust and the generation of a new megadetachment. By studying changes in the paleostress fields along the arc region, from compression to strike-slip, and strike-slip to extension, associated with σ3/σ2 and σ2/σ1 permutations respectively, together with the timing of uplift and exhumation of the morphostructural units across the transects, we can constrain the timing of activation/deactivation of the detachments responsible for the Andean deformation.

The origin and nature of thermal evolution during Granite emplacement and differentiation and its influence on upper crustal dynamics.

NASA Astrophysics Data System (ADS)

Buchwaldt, R.; Toulkeridis, T.; Todt, W.

2014-12-01

Structural geological, geochemical and geochronological data were compiled with the purpose to exercise models for the construction of upper crustal batholith. Models for pulsed intrusion of small magma batches over long timescales versus transfer of larger magma bodies on a shorter time scales are able to predict a different thermal, metamorphic, and rheological state of the crust. For this purpose we have applied the chronostratigraphic framework for magma differentiation on three granite complexes namely the St. Francois Mountain granite pluton (Precambrian), the Galway granite (Cambrian), and the Sithonia Plutonic Complex (Eocene). These plutons have similar sizes and range in composition from quartz diorites through granodiorites and granites to alkali granites, indicating multiple intrusive episodes. Thermobarometric calculations imply an upper crustal emplacement. Geochemical, isotopic and petrological data indicate a variety of pulses from each pluton allowing to be related through their liquid line of decent, which is supported by fractional crystallization of predominantly plagioclase, K-feldspar, biotite, hornblende and some minor accessory mineral phases, magma mingling and mixing as well as crustal contamination. To obtain the temporal relationship we carried out high-precision CA-TIMS zircon geochronology on selected samples along the liquid line of decent. The obtained data indicate a wide range of rates: such as different pulses evolved on timescales of about only 10-30ka, although, the construction time of the different complexes ranges from millions of years with prolonged tectonically inactive phases to relatively short lived time ranges of about ~300 ka. For a better understanding how these new data were used and evaluated in order to reconstruct constraints on the dynamics of the magmatic plumbing system, we integrated the short-lived, elevated heat production, due to latent heat of crystallization, into a 2D numerical model of the thermal evolution of segments of continental crust. Our model indicates that during the stage of enhanced fractional crystallization, the crustal viscosity decreases by several orders of magnitude, playing hereby a fundamental role in the thermal, magmatic, and tectonic evolution of the studied areas and most probably in similar regions too.

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.

Implications of very long baseline interferometry measurements on North American intra-plate crustal deformation

NASA Technical Reports Server (NTRS)

Allenby, R. J.

1979-01-01

Very Long Baseline Interferometry experiments over the last 1-3/4 years between Owens Valley, CA and Haystack, MA Radio Observatories suggest an upper limit of east-west crustal deformation between the two sites of about 1 cm/yr. In view of the fact that the baseline between the two sites traverses most of the major geological provinces of the United States, this low rate of crustal deformation has direct relevance to intra-plate crustal tectonics. The most active region traversed by this baseline is the Basin and Range province, which was estimated by various researchers to be expanding in an east-west direction at rates of .3 to 1.5 cm/yr. The Colorado Plateau and Rocky Mountain system also appear to be expanding, but at a somewhat lower rate, while east of the Rocky Mountains, the predominant stress appears to be compressional, nearly horizontal, and east to northeast trending.

Electrical structure beneath the Hangai Dome, Mongolia, from magnetotelluric data

NASA Astrophysics Data System (ADS)

Comeau, Matthew; Käufl, Johannes; Becken, Michael; Kuvshinov, Alexey; Demberel, Sodnomsambuu; Sukhbaatar, Usnikh; Batmagnai, Erdenechimeg; Tserendug, Shoovdor; Nasan, Ochir

2017-04-01

The Hangai Dome in west-central Mongolia is an unusual high-elevation intra-continental plateau located far from tectonic plate boundaries and characterized by dispersed, low-volume, basaltic volcanism. This region is an ideal natural laboratory for studying intra-continental orogenic and magmatic processes resulting from crust-mantle interactions. The processes responsible for developing the Hangai Dome remain unexplained, due in part to a lack of high resolution geophysical data over the area. Here we present newly acquired broadband (0.008 - 3,000 s) magnetotelluric (MT) data from a large-scale ( 200 x 450 km) and high resolution (site spacing > 5 km) survey across the Hangai Dome. A total of 125 sites were collected and include full MT sites and telluric-only sites where inter-station transfer functions were computed. The MT data are used to generate an electrical resistivity model of the crust and upper mantle below the Hangai Dome. The model shows that the lower crust ( 30 - 50 km; below the brittle-ductile transition zone) beneath the Hangai Dome contains anomalous discrete pockets of low-resistivity ( 30 ohm-m) material that indicate the presence of local accumulations of fluids and/or low-percent partial melts. These anomalous regions appear to be spatially associated with the surface expressions of past volcanism, hydrothermal activity, and an increase in heat flow. They also correlate with observed crustal low-density and low-velocity anomalies. However they are in contrast to some geochemical and petrological studies which show long-lived crustal melt storage is impossible below the Hangai due to limited crustal assimilation and crustal contamination, arguing for a single parent-source at mantle depths. The upper mantle (< 70 km) contains an anomalous low-resistivity zone directly below the Hangai Dome that represents a shallow asthenosphere, and possibly a zone of melt generation. The MT data require the presence of a small amount of partial melts (> 6%) at this location. The results are consistent with modern geochemical and geophysical data, which show a thin lithosphere below the Hangai region. Furthermore the results agree with geodynamic models that require a low-heat flux asthenospheric upwelling that thermally modifies the lithospheric mantle to explain both dome-like uplift and sporadic volcanism in the Hangai region.

Resistivity structures across the Humboldt River basin, north-central Nevada

USGS Publications Warehouse

Rodriguez, Brian D.; Williams, Jackie M.

2002-01-01

Magnetotelluric data collected along five profiles show deep resistivity structures beneath the Battle Mountain-Eureka and Carlin gold trends in north-central Nevada, which appear consistent with tectonic breaks in the crust that possibly served as channels for hydrothermal fluids. It seems likely that gold deposits along these linear trends were, therefore, controlled by deep regional crustal fault systems. Two-dimensional resistivity modeling of the magnetotelluric data generally show resistive (30 to 1,000 ohm-m) crustal blocks broken by sub-vertical, two-dimensional, conductive (1 to 10 ohmm) zones that are indicative of large-scale crustal fault zones. These inferred fault zones are regional in scale, trend northeast-southwest, north-south, and northwest-southeast, and extend to mid-crustal (20 km) depths. The conductors are about 2- to 15-km wide, extend from about 1 to 4 km below the surface to about 20 km depth, and show two-dimensional electrical structure. By connecting the locations of similar trending conductors together, individual regional crustal fault zones within the upper crust can be inferred that range from about 4- to 10-km wide and about 30- to 150-km long. One of these crustal fault zones coincides with the Battle Mountain-Eureka mineral trend. The interpreted electrical property sections also show regional changes in the resistive crust from south to north. Most of the subsurface in the upper 20 km beneath Reese River Valley and southern Boulder Valley are underlain by rock that is generally more conductive than the subsurface beneath Kelly Creek Basin and northern Boulder Valley. This suggests that either elevated-temperature or high-salinity fluids, alteration, or carbonaceous rocks are more pervasive in the more conductive area (Battle Mountain Heat-Flow High), which implies that the crust beneath these valleys is either more fractured or has more carbonaceous rocks than in the area surveyed along the 41st parallel.

Seismic anisotropy in central North Anatolian Fault Zone and its implications on crustal deformation

NASA Astrophysics Data System (ADS)

Licciardi, A.; Eken, T.; Taymaz, T.; Piana Agostinetti, N.; Yolsal-Çevikbilen, S.

2018-04-01

We investigate the crustal seismic structure and anisotropy around the central portion of the North Anatolian Fault Zone, a major plate boundary, using receiver function analysis. The characterization of crustal seismic anisotropy plays a key role in our understanding of present and past deformation processes at plate boundaries. The development of seismic anisotropy in the crust arises from the response of the rocks to complicated deformation regimes induced by plate interaction. Through the analysis of azimuthally-varying signals of teleseismic receiver functions, we map the anisotropic properties of the crust as a function of depth, by employing the harmonic decomposition technique. Although the Moho is located at a depth of about 40 km, with no major offset across the area, our results show a clear asymmetric distribution of crustal properties between the northern and southern blocks, divided by the North Anatolian Fault Zone. Heterogeneous and strongly anisotropic crust is present in the southern block, where complex intra-crustal signals are the results of strong deformation. In the north, a simpler and weakly anisotropic crust is typically observed. The strongest anisotropic signal is located in the first 15 km of the crust and is widespread in the southern block. Stations located on top of the main active faults in the area indicate the highest amplitudes, together with fault-parallel strikes of the fast plane of anisotropy. We interpret the origin of this signal as due to structure-induced anisotropy, and roughly determine its depth extent up to 15-20 km for these stations. Away from the faults, we suggest the contribution of previously documented uplifted basement blocks to explain the observed anisotropy at upper and middle crustal depths. Finally, we interpret coherent NE-SW orientations below the Moho as a result of frozen-in anisotropy in the upper mantle, as suggested by previous studies.

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.

The Effects of Rapid Sedimentation upon Continental Breakup: Kinematic and Thermal Modeling of the Salton Trough, Southern California, Based upon Recent Seismic Images

NASA Astrophysics Data System (ADS)

Han, L.; Hole, J. A.; Lowell, R. P.; Stock, J. M.; Fuis, G. S.

2016-12-01

The Salton Seismic Imaging Project (SSIP) illuminated crustal and upper mantle structure of the Salton Trough, the northern-most rift segment of the Gulf of California plate boundary. The crust is 17-18 km thick and homogeneous for 100 km in the plate motion direction. New crust is being created by distributed rift magmatism, Colorado River sedimentation, and metamorphism of the sediment. A 5 km thick pre-existing crustal layer may still exist. The crust has not broken apart to enable initiation of seafloor spreading. A one-dimensional time-dependent kinematic and thermal model was developed to simulate these observations. We assume that all crustal layers are stretched uniformly during extension. Distributed mafic magmatism and sedimentation are added simultaneously to compensate for the crustal thinning. The ratio of magmatism to sedimentation is constrained by the seismic observations. Heat is transported by thermal conduction and by advection due to stretching of the crust. A constant temperature boundary at the Moho is used to represent partial melting in the upper mantle. Assuming a constant plate motion rate, the zone of active rifting extends linearly with time. The crustal thickness and internal structure also evolve with time. The model constraints are the observed seismic structure and heat flow. The model rapidly reaches quasi-steady state, and could continue for many millions of years. The observed seismic structure and heat flow are reproduced after 3 Myr. The yield strength profile calculated from lithology and model temperature indicates that ductile deformation in the middle and lower crust dominates the crustal rheology. Rapid sedimentation delays crustal breakup and the initiation of seafloor spreading by maintaining the thickness of the crust and keeping it predominantly ductile. This process probably occurs wherever a large river flows into an active rift driven by far-field extension. It may have built passive margins in many locations globally, such as the Gulf of Mexico. This type of passive margin consists of mostly new crust created by magmatism and metamorphism of sediment. Along such margins, metamorphosed sediment could be misinterpreted as stretched pre-existing continental crust.

The Gulliver Mission: A Short-Cut to Primitive Body and Mars Sample Return

NASA Astrophysics Data System (ADS)

Britt, D. T.

2003-05-01

The Martian moon Deimos has extraordinary potential for future sample return missions. Deimos is spectrally similar to D-type asteroids and may be a captured primitive asteroid that originated in the outer asteroid belt. This capture probably took place in the earliest periods of Martian history, over 4.4 Gyrs ago [1], and Deimos has been accumulating material ejected from the Martian surface ever since. Analysis of Martian ejecta, material accumulation, capture cross-section, regolith over-turn, and Deimos's albedo suggest that Mars material may make up as much as 10% of Deimos's regolith. The Martian material on Deimos would be dominated by ejecta from the ancient crust of Mars, delivered during the Noachian Period of basin-forming impacts and heavy bombardment. Deimos could be a repository of samples from ancient Mars, including the full range of Martian crustal and upper mantle material from the early differentiation and crustal-forming epoch as well as samples from the era of high volatile flux, thick atmosphere, and possible surface water. In addition to Martian ejecta, 90% of the Deimos sample will be spectral type D asteroidal material. D-type asteroids are thought to be highly primitive and are most common in the difficult to access outer asteroid belt and the Jupiter Trojans. The Gulliver Mission proposes to directly collect up to 10 kilograms of Deimos regolith and return it to Earth. This sample may contain up to 1000 grams of Martian material along with up to 9 kilograms of primitive asteroidal material. Because of stochastic processes of regolith mixing over 4.4 Gyrs, the rock fragments and grains will likely sample the diversity of the Martian ancient surface as well as the asteroid. In essence, Gulliver represents two shortcuts, to Mars sample return and to the outer asteroid belt. References: [1] Burns J. A. (1992) Mars (Kieffer H. H. et al., eds), 1283-1302.

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

NASA Technical Reports Server (NTRS)

Stein, M.

1988-01-01

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

Crustal anisotropy across eastern Tibet and surroundings modeled as a depth-dependent tilted hexagonally symmetric medium

NASA Astrophysics Data System (ADS)

Xie, Jiayi; Ritzwoller, Michael H.; Shen, Weisen; Wang, Weitao

2017-04-01

Two types of surface wave anisotropy are observed regularly by seismologists but are only rarely interpreted jointly: apparent radial anisotropy, which is the difference in propagation speed between horizontally and vertically polarized waves inferred from Love and Rayleigh waves, and apparent azimuthal anisotropy, which is the directional dependence of surface wave speeds (usually Rayleigh waves). We show that a new data set of Love and Rayleigh wave isotropic phase speeds and Rayleigh wave azimuthal anisotropy observed within and surrounding eastern Tibet can be explained simultaneously by modeling the crust as a depth-dependent tilted hexagonally symmetric (THS) medium. We specify the THS medium with depth-dependent hexagonally symmetric elastic tensors tilted and rotated through dip and strike angles and estimate these quantities using a Bayesian Monte Carlo inversion to produce a 3-D model of the crust and uppermost mantle on a 0.5° × 0.5° spatial grid. In the interior of eastern Tibet and in the Yunnan-Guizhou plateau, we infer a steeply dipping THS upper crustal medium overlying a shallowly dipping THS medium in the middle-to-lower crust. Such vertical stratification of anisotropy may reflect a brittle to ductile transition in which shallow fractures and faults control upper crustal anisotropy and the crystal-preferred orientation of anisotropic (perhaps micaceous) minerals governs the anisotropy of the deeper crust. In contrast, near the periphery of the Tibetan Plateau the anisotropic medium is steeply dipping throughout the entire crust, which may be caused by the reorientation of the symmetry axes of deeper crustal anisotropic minerals as crustal flows are rotated near the borders of Tibet.

Crustal structure associated with Gondwana graben across the Narmada-Son lineament in India: An inference from aeromagnetics

NASA Astrophysics Data System (ADS)

Rao, D. Atchuta; Babu, H. V. Ram; Sinha, G. D. J. Sivakumar

1992-10-01

Aeromagnetic data over an 80-km-wide belt along the ENE-trending Narmada-Son lineament (NSL), starting from Baroda in the west and continuing to the south of Jabalpur in the east, has been studied to understand the structural and tectonic framework of the region. The area is covered by generally E-W-trending steeply dipping and folded Archean phyllites and quartzites as basement, with Bijawars (Upper Precambrian), upper Vindhyans (Upper Proterozoic), and Gondwanas (Upper Carboniferous) overlying them. Overlapping them all are the Deccan trap (Cretaceous-Eocene) flows. Aeromagnetic linements and their disposition and pattern in this region suggest major dislocations in the crust. The region around Hoshangabad, which is the intersection point of the NSL and the northwestern extension of the Godavari lineament, appears to have been intensely disturbed. Spectral analysis of aeromagnetic profiles across the NSL belt brought out a deep magnetic interface within crust at depths varying from 4 km to about 20 km below the surface, perhaps corresponding to the discontinuity characterized by the interface of granitic and basaltic rocks. There is a significant downwarping of this interface under the Hoshangabad region, suggesting that this is perhaps related to the evolution of the Gondwana basin structure in this area. This warping of the magnetic interface may be a reflection of the crustal flexuring and rift faulting. Elsewhere in the world, concentrations of carbonatite complexes and dike swarms are known to occur in areas of crustal flexuring and rift faulting. The occurrence of carbonatite complexes in this region (e.g. at Amba Dongar and Barwaha, and dike swarms in the Dadiapada region) gives credence to the present inferences from the aeromagnetic study.

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.

Upper plate contraction north of the migrating Mendocino triple junction northern California: Implications for partitioning of strain

USGS Publications Warehouse

McCrory, P.A.

2000-01-01

Geologic measurement of permanent contraction across the Cascadia subduction margin constrains one component of the tectonic deformation along the convergent plate boundary, the component critical for the seismic hazard assessment of crustal faults. A comprehensive survey of active faults in onshore subduction margin rocks at the southern end of the Cascadia subduction zone indicates that these thrust faults accommodate ??10 mm/yr of convergence oriented 020??-045??. Seismotectonic models of subduction zones typically assign this upper plate strain to the estimate of aseismic slip on the megathrust. Geodetic models include this permanent crustal strain within estimates of elastic strain accumulation on the megathrust. Both types of models underestimate the seismic hazard associated with crustal faults. Subtracting the observed contraction from the plate convergence rate (40-50 mm/yr; directed 040??-055??) leaves 30-40 mm/yr of convergence to be partitioned between slip on the megathrust, contraction within the southern Juan de Fuca plate, and crustal contraction outside the subduction complex rocks. This simple estimate of slip partitioning neglects the discrepancy between the plate convergence and contraction directions in the vicinity of the Mendocino triple junction. The San Andreas and Cascadia limbs of the Mendocino triple junction are not collinear. The eastern edge of the broad San Andreas boundary is ??85 km east of the Cascadia subduction boundary, and across this zone the Pacific plate converges directly with the North America plate. The skewed orientation of crustal structures just north of the leading edge of the Pacific plate suggests that they are deforming in a hybrid stress field resulting from both Juan de Fuca-North America motion and Pacific-North America motion. The composite convergence direction (50 mm/yr: directed 023??) is consistent with the compressive stress axis (020??) inferred from focal mechanisms of crustal earthquakes in the Humboldt region. Deformation in such a hybrid stress field implies that the crustal faults are being loaded from two major tectonic sources. The slip on crustal faults north of the Mendocino triple junction may consume 4-5 mm/yr of Pacific-Humboldt convergence. The remaining 17-18 mm/yr of convergence may be consumed as distributed shortening expressed in the high rates of uplift in the Cape Mendocino region or as northward translation of the continental margin, north of the triple junction.

Crust and Upper Mantle Structure of Antarctica from Rayleigh Wave Tomography

NASA Astrophysics Data System (ADS)

Wiens, D. A.; Heeszel, D. S.; Sun, X.; Chaput, J. A.; Aster, R. C.; Nyblade, A.; Anandakrishnan, S.; Wilson, T. J.; Huerta, A. D.

2012-12-01

We combine data from three temporary arrays of seismometers (AGAP/GAMSEIS 2007-2010, ANET/POLENET 2007-2012, TAMSEIS 2001-2003) deployed across Antarctica, along with permanent stations in the region, to produce a large scale shear velocity model of the continent extending from the Gamburtsev Subglacial Mountains (GSM) in East Antarctica, across the Transantarctic Mountains (TAM) and West Antarctic Rift System (WARS) to Marie Byrd Land (MBL) in West Antarctica. Our combined dataset consists of Rayleigh wave phase and amplitude measurements from 112 stations across the study region. We first invert for 2-D Rayleigh wave phase velocities using the two-plane wave method. These results are then inverted for shear velocity structure using crustal thicknesses derived from ambient noise tomography and teleseismic receiver functions. We refine our shear velocity model by performing a Monte Carlo simulation that explores the tradeoff between crustal thickness and upper mantle seismic velocities. The resulting model is higher resolution than previous studies (~150 km resolution length) and highlights significant differences in crustal and uppermost mantle structure between East and West Antarctica in greater detail than previously possible. East Antarctica is underlain by thick crust (reaching ~55 km beneath the GSM) and fast, cratonic lithosphere. West Antarctica is defined by thinner crust and slow upper mantle velocities indicative of its more recent tectonic activity. The observed boundary in crustal thickness closely follows the TAM front. MBL is underlain by a thicker lithosphere than that observed beneath the WARS, but slow mantle velocities persist to depths greater than 200 km, indicating a 'deep seated' (i.e. deeper than the deepest resolvable features of our model) thermal source for volcanism in the region. The slowest seismic velocities at shallow depths are observed in the Terror Rift region of the Ross Sea along an arc following the TAM front, where the most recent extension has occurred, and in another region of active volcanism. The Ellsworth-Whitmore Mountains are underlain by relatively thick crust and an intermediate thickness lithosphere, consistent with its hypothesized origin as a remnant Precambrian crustal block. We also produce upper mantle viscosity models for the study region using a temperature-dependent rheology, assuming that mantle seismic anomalies are dominated by temperature variations. Initial results closely correlate with the velocity model, with viscosities beneath West Antarctica inferred to be orders of magnitude lower than beneath East Antarctica. These viscosity results have important implications for our understanding of glacial isostatic adjustment, which is of particular interest in producing models of past and future changes in the Antarctic Ice Sheets.

What electrical measurements can say about changes in fault systems.

PubMed Central

Madden, T R; Mackie, R L

1996-01-01

Earthquake zones in the upper crust are usually more conductive than the surrounding rocks, and electrical geophysical measurements can be used to map these zones. Magnetotelluric (MT) measurements across fault zones that are parallel to the coast and not too far away can also give some important information about the lower crustal zone. This is because the long-period electric currents coming from the ocean gradually leak into the mantle, but the lower crust is usually very resistive and very little leakage takes place. If a lower crustal zone is less resistive it will be a leakage zone, and this can be seen because the MT phase will change as the ocean currents leave the upper crust. The San Andreas Fault is parallel to the ocean boundary and close enough to have a lot of extra ocean currents crossing the zone. The Loma Prieta zone, after the earthquake, showed a lot of ocean electric current leakage, suggesting that the lower crust under the fault zone was much more conductive than normal. It is hard to believe that water, which is responsible for the conductivity, had time to get into the lower crustal zone, so it was probably always there, but not well connected. If this is true, then the poorly connected water would be at a pressure close to the rock pressure, and it may play a role in modifying the fluid pressure in the upper crust fault zone. We also have telluric measurements across the San Andreas Fault near Palmdale from 1979 to 1990, and beginning in 1985 we saw changes in the telluric signals on the fault zone and east of the fault zone compared with the signals west of the fault zone. These measurements were probably seeing a better connection of the lower crust fluids taking place, and this may result in a fluid flow from the lower crust to the upper crust. This could be a factor in changing the strength of the upper crust fault zone. PMID:11607664

Evolution of the Archean Mohorovičić discontinuity from a synaccretionary 4.5 Ga protocrust

NASA Astrophysics Data System (ADS)

Hamilton, Warren B.

2013-12-01

This review evaluates and rejects the currently dominant dogmas of geodynamics and geochemistry, which are based on 1950s-1970s assumptions of a slowly differentiating Earth. Evidence is presented for evolution of mantle, crust, and early Moho that began with fractionation of most crustal components, synchronously with planetary accretion, into mafic protocrust by ~ 4.5 Ga. We know little about Hadean crustal geology (> 3.9 Ga) except that felsic rocks were then forming, but analogy with Venus, and dating from the Moon, indicate great shallow disruption by large and small impact structures, including huge fractionated impact-melt constructs, throughout that era. The mantle sample and Archean (< 3.9 Ga) crustal geology integrate well. The shallow mantle was extremely depleted by early removal of thick mafic protocrust, which was the primary source of the tonalite, trondhjemite, and granodiorite (TTG) that dominate preserved Archean crust to its base, and of the thick mafic volcanic rocks erupted on that crust. Lower TTG crust, kept mobile by its high radioactivity and by insulating upper crust, rose diapirically into the upper crust as dense volcanic rocks sagged synformally. The mobile lower crust simultaneously flowed laterally to maintain subhorizontal base and surface, and dragged overlying brittler granite-and-greenstone upper crust. Petrologically required garnet-rich residual protocrust incrementally delaminated, sank through low-density high-mantle magnesian dunite, and progressively re-enriched upper mantle, mostly metasomatically. Archean and earliest Proterozoic craton stabilization and development of final Mohos followed regionally complete early delamination of residual protocrust, variously between ~ 2.9 and 2.2 Ga. Where some protocrust remained, Proterozoic basins, filled thickly by sedimentary and volcanic rocks, developed on Archean crust, beneath which delamination of later residual protocrust continued top-down enrichment of upper mantle. That reenrichment enabled modern-style plate tectonics after ~ 600 Ma, with a transition regime beginning ~ 850 Ma.

Crustal growth of the Izu-Ogasawara arc estimated from structural characteristics of Oligocene arc

NASA Astrophysics Data System (ADS)

Takahashi, N.; Yamashita, M.; Kodaira, S.; Miura, S.; Sato, T.; No, T.; Tatsumi, Y.

2011-12-01

Japan Agency for Marine-Earth Science and Technology (JAMSTEC) carried out seismic surveys using a multichannel reflection system and ocean bottom seismographs, and we have clarified crustal structures of whole Izu-Ogasawara (Bonin)-Marina (IBM) arc since 2002. These refection images and velocity structures suggest that the crustal evolution in the intra-oceanic island arc accompanies with much interaction of materials between crust and mantle. Slow mantle velocity identified beneath the thick arc crusts suggests that dense crustal materials transformed into the mantle. On the other hand, high velocity lower crust can be seen around the bottom of the crust beneath the rifted region, and it suggests that underplating of mafic materials occurs there. Average crustal production rate of the entire arc is larger than expected one and approximately 200 km3/km/Ma. The production rate of basaltic magmas corresponds to that of oceanic ridge. Repeated crustal differentiation is indispensable to produce much light materials like continental materials, however, the real process cannot still be resolved yet. We, therefore, submitted drilling proposals to obtain in-situ middle crust with P-wave velocity of 6 km/s. In the growth history of the IBM arc, it is known by many papers that boninitic volcanisms preceded current bimodal volcanisms based on basaltic magmas. The current volcanisms accompanied with basaltic magmas have been occurred since Oligocene age, however, the tectonic differences to develop crustal architecture between Oligocene and present are not understood yet. We obtained new refraction/reflection data along an arc strike of N-S in fore-arc region. Then, we estimate crustal structure with severe change of the crustal thickness from refraction data, which are similar to that along the volcanic front. Interval for location of the thick arc crust along N-S is very similar to that along the volcanic front. The refection image indicates that the basement of the fore-arc is covered with thick sediments with the age of Oligocene and that half graben structures are much identified between the Oligocene arc and the current volcanic front. This may suggest that the Oligocene arc in current fore-arc basin is cut off from the current volcanic arc. Therefore, the Oligocene arc in the fore-arc may still keep structural characteristics inside the body since Oligocene age, which are before cutting off from the current volcanic front.

300 Kilometer Long Scarp

NASA Image and Video Library

2000-08-06

A scarp, or cliff, extends diagonally from upper left to lower right in this picture of Mercury taken by NASA Mariner 10. The structures are believed to be formed by the compressive forces due to crustal shortening.

Preliminary isostatic gravity map of the Grouse Creek and east part of the Jackpot 30 by 60 quadrangles, Box Elder County, Utah, and Cassia County, Idaho

USGS Publications Warehouse

Langenheim, Victoria; Willis, H.; Athens, N.D.; Chuchel, Bruce A.; Roza, J.; Hiscock, H.I.; Hardwick, C.L.; Kraushaar, S.M.; Knepprath, N.E.; Rosario, Jose J.

2013-01-01

A new isostatic residual gravity map of the northwest corner of Utah is based on compilation of preexisting data and new data collected by the Utah and United States Geological Surveys. Pronounced gravity lows occur over Junction, Grouse Creek, and upper Raft River Valleys, indicating significant thickness of low-density Tertiary sedimentary rocks and deposits. Gravity highs coincide with exposures of dense pre-Cenozoic rocks in the Raft River Mountains. Higher values in the eastern part of the map may be produced in part by deeper crustal density variations or crustal thinning. Steep linear gravity gradients coincide with mapped Neogene normal faults near Goose Creek and may define basin-bounding faults concealed beneath Junction and Upper Raft River Valleys.

Crustal Gravitational Potential Energy Change and Subduction Earthquakes

NASA Astrophysics Data System (ADS)

Zhu, P. P.

2017-05-01

Crustal gravitational potential energy (GPE) change induced by earthquakes is an important subject in geophysics and seismology. For the past forty years the research on this subject stayed in the stage of qualitative estimate. In recent few years the 3D dynamic faulting theory provided a quantitative solution of this subject. The theory deduced a quantitative calculating formula for the crustal GPE change using the mathematic method of tensor analysis under the principal stresses system. This formula contains only the vertical principal stress, rupture area, slip, dip, and rake; it does not include the horizontal principal stresses. It is just involved in simple mathematical operations and does not hold complicated surface or volume integrals. Moreover, the hanging wall vertical moving (up or down) height has a very simple expression containing only slip, dip, and rake. The above results are significant to investigate crustal GPE change. Commonly, the vertical principal stress is related to the gravitational field, substituting the relationship between the vertical principal stress and gravitational force into the above formula yields an alternative formula of crustal GPE change. The alternative formula indicates that even with lack of in situ borehole measured stress data, scientists can still quantitatively calculate crustal GPE change. The 3D dynamic faulting theory can be used for research on continental fault earthquakes; it also can be applied to investigate subduction earthquakes between oceanic and continental plates. Subduction earthquakes hold three types: (a) crust only on the vertical up side of the rupture area; (b) crust and seawater both on the vertical up side of the rupture area; (c) crust only on the vertical up side of the partial rupture area, and crust and seawater both on the vertical up side of the remaining rupture area. For each type we provide its quantitative formula of the crustal GPE change. We also establish a simplified model (called CRW Model) as follows: for Type B and Type C subduction earthquakes, if the seawater average depth on the vertical up side of the rupture area is less than a tenth of the hypocenter depth, then take the approximation that the seawater above the continental plate is replaced by the upper crustal material of the continental plate. The formula of quantitative calculating the crustal GPE change is also provided for this model. Finally, for 16 September 2015 Mw 8.3 Illapel Chile earthquake, we apply CRW Model and obtain the following results: the crustal GPE change is equal to 1.8 × 1019 J, and the hanging wall vertical moving-up height is 1.9 m with respect to the footwall. We believe this paper might be the first report on the quantitative solution of the crustal GPE change for this subduction earthquake; our results and related method will be helpful in research into the earthquakes in Peru-Chile subduction zone and the Andean orogeny. In short, this study expounds a new method for quantitative determining the crustal GPE change caused by subduction earthquakes, which is different from other existing methods.

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.

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

Pasyanos, M

We study the lithospheric structure of Africa, Arabia and adjacent oceanic regions with fundamental-mode surface waves over a wide period range. Including short period group velocities allows us to examine shallower features than previous studies of the whole continent. In the process, we have developed a crustal thickness map of Africa. Main features include crustal thickness increases under the West African, Congo, and Kalahari cratons. We find crustal thinning under Mesozoic and Cenozoic rifts, including the Benue Trough, Red Sea, and East, Central, and West African rift systems. Crustal shear wave velocities are generally faster in oceanic regions and cratons,more » and slower in more recent crust and in active and formerly active orogenic regions. Deeper structure, related to the thickness of cratons and modern rifting, is generally consistent with previous work. Under cratons we find thick lithosphere and fast upper mantle velocities, while under rifts we find thinned lithosphere and slower upper mantle velocities. There are no consistent effects in areas classified as hotspots, indicating that there seem to be numerous origins for these features. Finally, it appears that the African Superswell has had a significantly different impact in the north and the south, indicating specifics of the feature (temperature, time of influence, etc.) to be dissimilar between the two regions. Factoring in other information, it is likely that the southern portion has been active in the past, but that shallow activity is currently limited to the northern portion of the superswell.« less

Orphan Basin crustal structure from a dense wide-angle seismic profile - Tomographic inversion

NASA Astrophysics Data System (ADS)

Watremez, Louise; Lau, K. W. Helen; Nedimović, Mladen R.; Louden, Keith E.; Karner, Garry D.

2014-05-01

Orphan Basin is located on the eastern margin of Canada, offshore of Newfoundland and East of Flemish Cap. It is an aborted continental rift formed by multiple episodes of rifting. The crustal structure across the basin has been determined by an earlier refraction study using 15 instruments on a 550 km long line. It shows that the continental crust was extended over an unusually wide region but did not break apart. The crustal structure of the basin thus documents stages in the formation of a magma-poor rifted margin up to crustal breakup. The OBWAVE (Orphan Basin Wide-Angle Velocity Experiment) survey was carried out to image crustal structures across the basin and better understand the processes of formation of this margin. The spacing of the 89 recording stations varies from 3 to 5 km along this 500-km-long line, which was acquired along a pre-existing reflection line. The highest resolution section corresponds to the part of the profile where the crust was expected to be the thinnest. We present the results from a joint tomography inversion of first and Moho reflected arrival times. The high data density allows us to define crustal structures with greater detail than for typical studies and to improve the understanding of the processes leading to the extreme stretching of continental crust. The final model was computed following a detailed parametric study to determine the optimal parameters controlling the ray-tracing and the inversion processes. The final model shows very good resolution. In particular, Monte Carlo standard deviations of crustal velocities and Moho depths are generally < 50 m/s and within 1 km, respectively. In comparison to the velocity models of typical seismic refraction profiles, results from the OBWAVE study show a notable improvement in the resolution of the velocity model and in the level of detail observed using the least a priori information possible. The final model allows us to determine the crustal thinning and variable structures across the basin. In particular, we observe (1) a zone of extreme thinning, where the crust is thinner than 7 km; (2) basement highs and lows highlighting the blocks that accommodate the crustal thinning; (3) a central block that is thicker compared to the rest of the basin; (4) lower crustal thinning that is highly variable, which suggests a ductile deformation in the lower crust and an extensional discrepancy between the upper and lower crust (DDS); and (5) no evidence for upper-mantle serpentinization under the ultra-thinned crust. Furthermore, we show the importance of structural inheritance in rifting of the Avalon crust. Thus, we suggest that Orphan Basin is the result of rifting of a non-homogeneous Avalon terrane where the lower crust is primarily ductile.

The Crustal Structure of the North-South Earthquake Belt in China Revealed from Deep Seismic Soundings and Gravity Data

NASA Astrophysics Data System (ADS)

Zhao, Yang; Guo, Lianghui; Shi, Lei; Li, Yonghua

2018-01-01

The North-South earthquake belt (NSEB) is one of the major earthquake regions in China. The studies of crustal structure play a great role in understanding tectonic evolution and in evaluating earthquake hazards in this region. However, some fundamental crustal parameters, especially crustal interface structure, are not clear in this region. In this paper, we reconstructed the crustal interface structure around the NSEB based on both the deep seismic sounding (DSS) data and the gravity data. We firstly reconstructed the crustal structure of crystalline basement (interface G), interface between upper and lower crusts (interface C) and Moho in the study area by compiling the results of 38 DSS profiles published previously. Then, we forwardly calculated the gravity anomalies caused by the interfaces G and C, and then subtracted them from the complete Bouguer gravity anomalies, yielding the regional gravity anomalies mainly due to the Moho interface. We then utilized a lateral-variable density interface inversion technique with constraints of the DSS data to invert the regional anomalies for the Moho depth model in the study area. The reliability of our Moho depth model was evaluated by comparing with other Moho depth models derived from other gravity inversion technique and receiver function analysis. Based on our Moho depth model, we mapped the crustal apparent density distribution in the study area for better understanding the geodynamics around the NSEB.

The major tectonic boundaries of the Northern Red Sea rift, Egypt derived from geophysical data analysis

NASA Astrophysics Data System (ADS)

Saleh, Salah; Pamukçu, Oya; Brimich, Ladislav

2017-09-01

In the present study, we have attempted to map the plate boundary between Arabia and Africa at the Northern Red Sea rift region including the Suez rift, Gulf of Aqaba-Dead Sea transform and southeastern Mediterranean region by using gravity data analysis. In the boundary analysis method which was used; low-pass filtered gravity anomalies of the Northern Red Sea rift region were computed. Different crustal types and thicknesses, sediment thicknesses and different heat flow anomalies were evaluated. According to the results, there are six subzones (crustal blocks) separated from each other by tectonic plate boundaries and/or lineaments. It seems that these tectonic boundaries reveal complex structural lineaments, which are mostly influenced by a predominant set of NNW-SSE to NW-SE trending lineaments bordering the Red Sea and Suez rift regions. On the other side, the E-W and N-S to NNE-SSW trended lineaments bordering the South-eastern Mediterranean, Northern Sinai and Aqaba-Dead Sea transform regions, respectively. The analysis of the low pass filtered Bouguer anomaly maps reveals that the positive regional anomaly over both the Red Sea rift and South-eastern Mediterranean basin subzones are considered to be caused by the high density of the oceanic crust and/or the anomalous upper mantle structures beneath these regions whereas, the broad medium anomalies along the western half of Central Sinai with the Suez rift and the Eastern Desert subzones are attributed to low-density sediments of the Suez rift and/or the thick upper continental crustal thickness below these zones. There are observable negative anomalies over the Northern Arabia subzone, particularly in the areas covered by Cenozoic volcanics. These negative anomalies may be attributed to both the low densities of the surface volcanics and/or to a very thick upper continental crust. On the contrary, the negative anomaly which belongs to the Gulf of Aqaba-Dead Sea transform zone is due to crustal thickening (with limited heat flow values) below this region. Additionally in this study, the crustal thinning was investigated with heat flow, magnetic and free air gravity anomalies in the Northern Red Sea rift region. In fact, the crustal thinning of the study area was also proportional to the regions of observable high heat flow values. Finally, our results were found to be well correlated with the topography, free air, aeromagnetic and heat flow dataset profiles crossing most of the study area.

Crust and Upper Mantle Structure beneath Isparta Angle in SW Turkey from P and S Receiver Functions

NASA Astrophysics Data System (ADS)

Kahraman, M.; Turkelli, N.; Özacar, A.; Sandvol, E. A.; Teoman, U.

2015-12-01

Isparta Angle (IA) constitutes a triangular shape elevated tectonic domain in SW Turkey which contains units stacked with opposing thrust vergences during Late Cretaceous to Miocene. The region which is located at the junction between Aegean and Cyprus arcs separated by a slab tear is now bounded by Fethiye-Burdur Fault Zone (FBFZ) in the west and Akşehir-Afyon Fault Zones (AAFZ) in the east. In the area, seismicity displays ongoing extension along active grabens oriented at different directions. In the past, many competing geodynamic scenarios had been proposed to explain the complex tectonic evolution of the area. In this study, we used both P and S receiver functions (RFs) to present high resolution crustal and upper mantle images down to 200 km. Moho and upper crustal discontinuities were well resolved by P Rfs; however S RFs were utilized to image lithospheric-asthenospheric boundaries having the benefit of being free of multiple conversions. RFs were calculated from 916 teleseismic earthquakes (Mw ≥ 5.5) recorded by 42 permanent and temporary broadband stations BU-KOERI/NEMC, DEMP/ERD and Isparta Angle Seismic Experiment deployed by collaboration of BU-KOERI and University of Missouri. Totally, 4501 P and 946 S RFs with the cut-off frequencies of ~1.0 Hz and ~0.5 Hz, respectively, were obtained by applying iterative-time domain deconvolution. Crustal thickness and Vp/Vs ratios were calculated by grid search of maximum amplitude of P RFs(Ps,PpPs and PsPs+PpSs) in depth and Vp/Vs domain. Then, we created 2-D P and S migrated cross-sections to observe crustal and lithospheric-asthenospheric discontinuities beneath the region. P RFs indicates that, average crustal thickness and Vp/Vs ratio is ~36 km and 1.78 in the region with small changing values close to the edges. Migrated P RFs cross-sections revealed a sharp change in Moho (Moho offset) on the western boundary that spatially correlates with the FBFZ. We also found a relatively flat Moho in the center and what appears to be imaged northern tip of slab at ~45 km depth. Finally, ~30km crustal thickness released in southeast beneath the Cyprus. On the other hand; preliminary results of S RFs cross-sections present the LAB boundary between ~60 to ~90 km depth range, observed almost beneath all profiles and clear positive phase arrivals right below the LAB depths.

Numerical Experiments on the Role of the Lower Crust in the Development of Extension-driven Gneiss Domes

NASA Astrophysics Data System (ADS)

Korchinski, M.; Rey, P. F.; Teyssier, C. P.; Mondy, L. S.; Whitney, D.

2016-12-01

Flow of orogenic crust is a critical geodynamic process in the chemical and physical evolution of continents. Deeply sourced rocks are transported to the near surface within gneiss domes, which are ubiquitous features in orogens and extensional regions. Exhumation of material within a gneiss dome can occur as the result of tectonic stresses, where material moves into space previously occupied by the shallow crust as the result of extension localized along a detachment system. Gravitationally driven flow may also contribute to exhumation. This research addresses how physical parameters (density, viscosity) of the deep crust (base of brittle crust to Moho) impact (1) the localization of extension in the shallow crust, and (2) the flow of deep crust by tectonic and non-tectonic stresses. We present 2D numerical experiments in which the density (2900-3100 kg m-3) and viscosity (1e19-1e21 Pa s) of the deep crust are systematically varied. Lateral and vertical transport of deep crustal rocks toward the gneiss dome occurs across the entire parameter space. A low viscosity deep crust yields localized extension in the upper crust and crustal-scale upward flow; this case produces the highest exhumation. A high viscosity deep crust results in distributed thinning of the upper crust, which suppresses upward mass transport. The density of the deep crust has only a second-order effect on the shallow crust extension regime. We capture the flow field generated after the cessation of extension to evaluate mass transport that is not driven by tectonic stresses. Upward transport of material within the gneiss dome is present across the entire parameter space. In the case of a low-viscosity deep crust, horizontal flow occurs adjacent to the dome above the Moho; this flow is an order of magnitude higher than that within the dome. Density variations do not drastically alter the flow field in the low viscosity lower crust. However, a high density and high viscosity deep crust results in boudinage of the whole crust, which generates significant upward flow from the buoyant asthenosphere.

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.

Geologic and hydrologic controls on the economic potential of hydrothermal systems associated with upper crustal plutons

NASA Astrophysics Data System (ADS)

Weis, Philipp; Driesner, Thomas; Scott, Samuel; Lecumberri-Sanchez, Pilar

2016-04-01

Heat and mass transport in hydrothermal systems associated with upper crustal magmatic intrusions can result in resources with large economic potential (Kesler, 1994). Active hydrothermal systems can form high-enthalpy geothermal reservoirs with the possibility for renewable energy production. Fossil continental or submarine hydrothermal systems may have formed ore deposits at variable crustal depths, which can be mined near today's surface with an economic profit. In both cases, only the right combination of first-order geologic and hydrologic controls may lead to the formation of a significant resource. To foster exploration for these hydrothermal georesources, we need to improve our understanding of subsurface fluxes of mass and energy by combining numerical process modelling, observations at both active and fossil systems, as well as knowledge of fluid and rock properties and their interactions in natural systems. The presentation will highlight the role of non-linear fluid properties, phase separation, salt precipitation, fluid mixing, permeability structure, hydraulic fracturing and the transition from brittle to ductile rock behavior as major geologic and hydrologic controls on the formation of high-enthalpy and supercritical geothermal resources (Scott et al., 2015), and magmatic-hydrothermal mineral resources, such as porphyry copper, massive sulfide and epithermal gold deposits (Lecumberri-Sanchez et al., 2015; Weis, 2015). References: Kesler, S. E., 1994: Mineral Resources, economics and the environment, New York, McMillan, 391. Lecumberri-Sanchez, P., Steele-MacInnis, M., Weis, P., Driesner, T., Bodnar, R.J. (2015): Salt precipitation in magmatic-hydrothermal systems associated with upper crustal plutons. Geology, v. 43, p. 1063-1066, doi:10.1130/G37163.1 Scott, S., Driesner, T., Weis, P. (2015): Geologic controls on supercritical geothermal resources above magmatic intrusions. Nature Communications, 6:7837 doi: 10.1038/ncomms8837 Weis, P. (2015): The dynamic interplay between saline fluid flow and rock permeability in magmatic-hydrothermal systems. Geofluids, 15, 350-371.

Upper crustal structure beneath East Java from ambient noise tomography: A preliminary result

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

Martha, Agustya Adi; Graduate Research on Earthquakes and Active Tectonics, Institut Teknologi Bandung, Bandung; Widiyantoro, Sri

East Java has a fairly complex geological structure. Physiographically East Java can be divided into three zones, i.e. the Southern Mountains zone in the southern part, the Kendeng zone in the middle part, and the Rembang zone in the northern part. Most of the seismic hazards in this region are due to processes in the upper crust. In this study, the Ambient Noise Tomography (ANT) method is used to image the upper crustal structure beneath East Java. We have used seismic waveform data recorded by 8Meteorological, Climatological and Geophysical Agency (BMKG) stationary seismographic stations and 16 portable seismographs installed formore » 2 to 8 weeks. The data were processed to obtain waveforms fromnoise cross-correlation between pairs of seismographic stations. Our preliminary results indicate that the Kendeng zone, an area of low gravity anomaly, is associated with a low velocity zone. On the other hand, the southern mountain range, which has a high gravity anomaly, is related to a high velocity anomaly as shown by our tomographic images.« less

Crustal structure of China from deep seismic sounding profiles

USGS Publications Warehouse

Li, S.; Mooney, W.D.

1998-01-01

More than 36,000 km of Deep Seismic Sounding (DSS) profiles have been collected in China since 1958. However, the results of these profiles are not well known in the West due to the language barrier. In this paper, we summarize the crustal structure of China with a new contour map of crustal thickness, nine representative crustal columns, and maps showing profile locations, average crustal velocity, and Pn velocity. The most remarkable aspect of the crustal structure of China is the well known 70+ km thickness of the crust of the Tibetan Plateau. The thick (45-70 km) crust of western China is separated from the thinner (30-45 km) crust of eastern China by the north-south trending seismic belt (105??E). The average crustal velocity of China ranges from 6.15 to 6.45 km/s, indicating a felsic-to-intermediate bulk crustal composition. Upper mantle (Pn) velocities are 8.0 ?? 0.2 km/s, equal to the global continental average. We interpret these results in terms of the most recent thermo-tectonic events that have modified the crust. In much of eastern China, Cenoxoic crustal extension has produced a thin crust with a low average crustal velocity, similar to western Europe and the Basin and Range Province, western USA. In western China, Mesozoic and Cenoxoic arc-continent and continent-continent collisions have led to crustal growth and thickening. Inferences on the process of crustal thickening are provided by the deep crustal velocity structure as determined by DSS profiles and other seismological studies. A high velocity (7.0-7.4 km/s) lower-crustal layer has been reported in western China only beneath the southernmost Tibetan Plateau. We identity this high-velocity layer as the cold lower crust of the subducting Indian plate. As the Indian crust is injected northward into the Tibetan lower crust, it heats and assimilates by partial melting, a process that results in a reduction in the seismic velocity of the lower crust in the central and northern Tibetan Plateau. ?? 1998 Elsevier Science B.V. All rights reserved.

Density of alkali carbonate melts in the upper mantle and implications for the mobility of carbon at depth

NASA Astrophysics Data System (ADS)

Ritter, X.; Sanchez-Valle, C.; Laumonier, M.; King, A.; Guignot, N.; Gaillard, F.; Sifre, D.; Perrillat, J. P.

2017-12-01

The occurrence of carbonate-rich mantle rocks and diamonds in kimberlite rocks provide evidence for the presence of CO2 in the mantle. Carbon is recycled into the mantle via subduction and released through volcanic outgassing. An important fraction is retained at depth where partial melting of subducted lithologies produce alkali-rich carbonates along the CaCO3-MgCO3-K2CO3 join that infiltrate the mantle wedge [1]. Although volumetrically minor, these melts act as effective metasomatic agents that are related to source regions for diamond-bearing kimberlites [2]. The mobility of carbon at depth is controlled by the physical properties of carbonate liquids that remain largely unknown [3,4]. Here we report in-situ density measurements of alkaline carbonates at crustal and upper mantle conditions using synchrotron X-ray absorption in a Paris-Edinburgh press at beamline Psiché (Synchrotron Soleil). Experiments were conducted in several compositions along the CaCO3-K2CO3 and MgCO3-K2CO3 join up to 1400 K and 3 GPa. The starting materials included a mixture of synthetic K2CO3 and natural calcite and K2Mg(CO3)2 glasses synthesized at 0.15 GPa and 1098 K in an internally heated pressure vessel. The samples were cold pressurized and heated until the molten stage was confirmed by X-ray diffraction. The results were fitted to derive the first robust model for the density of alkali carbonates that mimic liquids from the incipient melting of subducted lithologies at crustal and upper mantle conditions. We combine the results of the present study with available data on the viscosity of carbonate liquids and molecular dynamic predictions to discuss the mobility and migration rates of carbonate liquids in the upper mantle.[1] Litasov et al. 2012 Geology 41, 79-82. [2] Grassi and Schmidt 2011, Contrib Min Petr 162, 169-191. [3] Dobson et al. 1996, EPSL 143, 207-215. [4] Kono et al. 2014 Nature Communications 5:5091.

New Interpretation of Crustal Extension Evidences on Mars

NASA Astrophysics Data System (ADS)

Grin, E. A.

The record of early evolution of life on Earth has been obscured by extensive surface activity. On the opposite, large fractions of the martian surface date back to an early clement epoch favorable to the needs of biological systems [1]. The upper martian surface reflects a wide variety of modifying processes which destroy the geological context. However, due to endogenic causes acting after the end of the primordial bombardment, abundant extensional structures display vertical sequences of stratigraphic units from late Noachian to early Hesperian periods [2]. Deep structural incisions in the upper crust provide unaltered strata, open flanks, and slope deposits that favor the use of an autonomous lander-rover-penetrator The strategy for an exobiology search of such an optimum site should be guided by the recent attention devoted to extensional structures and their global significance [4]. Geological evidence supporting the martian crustal extension is suggested by abundant fractures associated with the dichotomy boundary northland-south upland, i.e., Aeolis Region, and peak igneous activity (Elysium bulge). As pointed out by [5], the system of fractures correlates with the endogenic origin of the dichotomy, as related to a major difference in the thicknessof the crust. Perpendicular to this boundary, fractures of deep graben testify to a general tectonic crust relaxation. The opening of the graben, joined with compressive wrinkles, is the signature of a dynamical pervasive stress regime that implies a large scale roll-over of the upper crust over the ductile interface of a more dense mantle. This general motion is not a transport of material, as there is no thickening on the boundary of the dichotomy. The horizontal movement is due to the gravitational mechanism and differential thermal convection cells in the upper crust over the slope of the anti-flexure rigid interface consequential to Elysium bulge. The fracturation occurs as the neutral zone of the crust rises to the brittle surface of the crust. Deep extensional structures are logical sites for locating and sampling fossilized organisms from various epochs. Grabens suggest ancient lakes and the development of biological systems supported by bottom hot springs.

Low-Stress Upper Plate Near Subduction Zones and Implications for Temporal Changes in Loading Forces

NASA Astrophysics Data System (ADS)

Wang, K.; Hu, Y.; Yoshida, K.

2016-12-01

Subduction megathrusts are weak, often with effective friction coefficients as low as 0.03. Consequently, differential stress (S1 - S3) in the nearby upper plate is low. Compression due to plate coupling and tension due to gravity are in a subtle balance that can be tipped by small perturbations. For example, the 2011 M=9 Tohoku-oki earthquake, which has a rupture-zone-average stress drop of only a few MPa, switched offshore margin-normal stress from compression to tension and affected seismicity pattern and stress directions of various parts of the land area. The low differential stress is also reflected in spatial variations of stresses, such as with changes in topography. In the Andes, crustal earthquake focal mechanisms change from thrust-faulting in low-elevation areas to normal-faulting in high-elevation areas. Given the lack of evidence for a pervasively weak crust, the low differential stress may indicate that in general the crust near subduction zones is not critically stressed. If so, crustal earthquakes do not represent pervasive failure but only local failure due to stress, material, and fluid pressure heterogeneity. If distributed permanent deformation that creates topography is not the norm, it either happens in brief episodes or took place in the past. The outer wedge may enter a compressively or extensionally critical state due to coseismic strengthening or weakening, respectively, of the shallow megathrust in largest interplate earthquakes. Temporal changes in loading forces must occur also at much larger temporal and spatial scales in response to changes in the nature of the subducting plate and other tectonic conditions. We propose that submarine wedges and high topography in the upper plate attain their geometry in geologically brief episodes of high differential stress. They normally stay in a low-stress stable state, but their geometry often reflects high-stress episodes of critical states in the past. In other words, rocks have a sustained memory for the most traumatic moments. Except for the weaker outer wedge, the upper plate does not switch from one critical state to another in megathrust earthquake cycles, such as from compressional failure to gravitational collapse.

Relative sea-level rise around East Antarctica during Oligocene glaciation

NASA Astrophysics Data System (ADS)

Stocchi, Paolo; Escutia, Carlota; Houben, Alexander J. P.; Vermeersen, Bert L. A.; Bijl, Peter K.; Brinkhuis, Henk; Deconto, Robert M.; Galeotti, Simone; Passchier, Sandra; Pollard, David; Brinkhuis, Henk; Escutia, Carlota; Klaus, Adam; Fehr, Annick; Williams, Trevor; Bendle, James A. P.; Bijl, Peter K.; Bohaty, Steven M.; Carr, Stephanie A.; Dunbar, Robert B.; Flores, Jose Abel; Gonzàlez, Jhon J.; Hayden, Travis G.; Iwai, Masao; Jimenez-Espejo, Francisco J.; Katsuki, Kota; Kong, Gee Soo; McKay, Robert M.; Nakai, Mutsumi; Olney, Matthew P.; Passchier, Sandra; Pekar, Stephen F.; Pross, Jörg; Riesselman, Christina; Röhl, Ursula; Sakai, Toyosaburo; Shrivastava, Prakash Kumar; Stickley, Catherine E.; Sugisaki, Saiko; Tauxe, Lisa; Tuo, Shouting; van de Flierdt, Tina; Welsh, Kevin; Yamane, Masako

2013-05-01

During the middle and late Eocene (~ 48-34Myr ago), the Earth's climate cooled and an ice sheet built up on Antarctica. The stepwise expansion of ice on Antarctica induced crustal deformation and gravitational perturbations around the continent. Close to the ice sheet, sea level rose despite an overall reduction in the mass of the ocean caused by the transfer of water to the ice sheet. Here we identify the crustal response to ice-sheet growth by forcing a glacial-hydro isostatic adjustment model with an Antarctic ice-sheet model. We find that the shelf areas around East Antarctica first shoaled as upper mantle material upwelled and a peripheral forebulge developed. The inner shelf subsequently subsided as lithosphere flexure extended outwards from the ice-sheet margins. Consequently the coasts experienced a progressive relative sea-level rise. Our analysis of sediment cores from the vicinity of the Antarctic ice sheet are in agreement with the spatial patterns of relative sea-level change indicated by our simulations. Our results are consistent with the suggestion that near-field processes such as local sea-level change influence the equilibrium state obtained by an ice-sheet grounding line.

Timing of magmatism following initial convergence at a passive margin, southwestern U.S. Cordillera, and ages of lower crustal magma sources

USGS Publications Warehouse

Barth, A.P.; Wooden, J.L.

2006-01-01

Initiation of the Cordilleran magmatic arc in the southwestern United States is marked by intrusion of granitic plutons, predominantly composed of alkali-calcic Fe- and Sr-enriched quartz monzodiorite and monzonite, that intruded Paleoproterozoic basement and its Paleozoic cratonal-miogeoclinal cover. Three intrusive suites, recognized on the basis of differences in high field strength element and large ion lithophile element abundances, contain texturally complex but chronologically distinctive zircons. These zircons record heterogeneous but geochemically discrete mafic crustal magma sources, discrete Permo-Triassic intrusion ages, and a prolonged postemplacement thermal history within the long-lived Cordilleran arc, leading to episodic loss of radiogenic Pb. Distinctive lower crustal magma sources reflect lateral heterogeneity within the composite lithosphere of the Proterozoic craton. Limited interaction between derived magmas and middle and upper crustal rocks probably reflects the relatively cool thermal structure of the nascent Cordilleran continental margin magmatic arc. ?? 2006 by The University of Chicago. All rights reserved.

Relic magma chamber structures preserved within the Mesozoic North Atlantic crust?

USGS Publications Warehouse

McCarthy, J.; Mutter, J.C.; Morton, J.L.; Sleep, Norman H.; Thompson, G.A.

1988-01-01

The North Atlantic Transect seismic reflection data, collected southwest of Bermuda, have been reinterpreted following post-stack migration and reveal two major intracrustal reflections. The shallower of these two events, located ~1 s below the igneous basement, is a subhorizontal, undulating surface that in some places is continuous for as much as 10 km. This upper crustal reflection corresponds to the intermittently sharp contact between the sheeted dikes and the underlying isotropic gabbro. A second set of lower crustal reflections, dipping ~20??-40?? eastward, is also prominent on the migrated profile and terminates downdip against the subhorizontal reflection Moho. Their presence may be ascribed to mafic-ultramafic cumulate layers frozen into the oceanic crust at the time of formation at the paleo-spreading center. The gradual thinning in the crust approaching the fracture zones is shown to be more complex than was originally inferred. An intepretation advocating crustal thickening in this narrow zone is proposed as an alternative to the crustal-thinning model of Mutter and others. -from Authors

Crustal Fractures of Ophir Planum

NASA Image and Video Library

2002-05-23

This NASA Mars Odyssey image covers a tract of plateau territory called Ophir Planum. The most obvious features in this scene are the fractures ranging from 1 to 5 km wide running from the upper left to lower right.

Petrogenesis and tectonic significance of the late Triassic mafic dikes and felsic volcanic rocks in the East Kunlun Orogenic Belt, Northern Tibet Plateau

NASA Astrophysics Data System (ADS)

Hu, Yan; Niu, Yaoling; Li, Jiyong; Ye, Lei; Kong, Juanjuan; Chen, Shuo; Zhang, Yu; Zhang, Guorui

2016-02-01

We present zircon U-Pb ages and geochemical data on the late Triassic mafic dikes (diabase) and felsic volcanic rocks (rhyolite and rhyolitic tuffs) in the East Kunlun Orogenic Belt (EKOB). These rocks give a small age window of 228-218 Ma. The mafic dikes represent evolved alkaline basaltic melts intruding ~ 8-9 Myrs older and volumetrically more abundant A-type granite batholith. Their rare earth element (REE) and multi-element patterns are similar to those of the present-day ocean island basalts (OIBs) except for a weak continental crustal signature (i.e., enrichment of Rb and Pb and weak depletion of Nb, Ta and Ti). Their trace element characteristics together with the high 87Sr/86Sr (0.7076-0.7104), low εNd(t) (- 2.18 to - 3.46), low εHf(t) (- 2.85 to - 4.59) and variable Pb isotopic ratios are consistent with melts derived from metasomatized subcontinental lithospheric mantle with crustal contamination. The felsic volcanic rocks are characterized by high LREE/HREE (e.g., [La/Yb]N of 5.71-17.00) with a negative Eu anomaly and strong depletion in Sr and P, resembling the model upper continental crust (UCC). Given the high 87Sr/86Sr (0.7213-0.7550) and less negative εNd(t) (- 3.83 to - 5.09) and εHf(t) (- 3.06 to - 3.83) than the UCC plus the overlapping isotopes with the mafic dikes and high Nb-Ta rhyolites, the felsic volcanic rocks are best interpreted as resulting from melting-induced mixing with 45-50% crustal materials and 50-55% mantle-derived mafic melts probably parental to the mafic dikes. Such mantle-derived melts underplated and intruded the deep crust as juvenile crustal materials. Partial melting of such juvenile crust produced felsic melts parental to the felsic volcanic rocks in the EKOB. We hypothesize that the late Triassic mafic dikes and felsic volcanic rocks are associated with post-collisional extension and related orogenic collapse. Such processes are probably significant in causing asthenospheric upwelling, decompression melting, induced melting of the prior metasomatized mantle lithosphere and the existing crust. This work represents our ongoing effort in understanding the origin of the juvenile crust and continental crustal accretion through magmatism in the broad context of orogenesis from seafloor subduction to continental collision and to post-collisional processes.

Effect of Cohesion Uncertainty of Granular Materials on the Kinematics of Scaled Models of Fold-and-Thrust Belts

NASA Astrophysics Data System (ADS)

Nilfouroushan, F.; Pysklywec, R.; Cruden, S.

2009-05-01

Cohesionless or very low cohesion granular materials are widely used in analogue/physical models to simulate brittle rocks in the upper crust. Selection of materials with appropriate cohesion values in such models is important for the simulation of the dynamics of brittle rock deformation in nature. Uncertainties in the magnitude of cohesion (due to measurement errors, extrapolations at low normal stresses, or model setup) in laboratory experiments can possibly result in misinterpretation of the styles and mechanisms of deformation in natural fold-and thrust belts. We ran a series of 2-D numerical models to investigate systematically the effect of cohesion uncertainties on the evolution of models of fold-and-thrust belts. The analyses employ SOPALE, a geodynamic code based on the arbitrary Lagrangian-Eulerian (ALE) finite element method. Similar to analogue models, the material properties of sand and transparent silicone (PDMS) are used to simulate brittle and viscous behaviors of upper crustal rocks. The suite of scaled brittle and brittle-viscous numerical experiments have the same initial geometry but the cohesion value of the brittle layers is increased systematically from 0 to 100 Pa. The stress and strain distribution in different sets of models with different cohesion values are compared and analyzed. The kinematics and geometry of thrust wedges including the location and number of foreland- and hinterland- verging thrust faults, pop-up structures, tapers and topography are also explored and their sensitivity to cohesion value is discussed.

The leading edge of basement logging science: The detailed in situ volcanic architecture, crustal construction processes, vacancy for water, minerals, and microbes, and beyond

NASA Astrophysics Data System (ADS)

Tominaga, M.

2010-12-01

Understanding the detailed architecture of the upper ocean crust is one of the key components to advance our knowledge on numerous events occurring in the oceanic lithosphere from spreading ridges to subduction zones. Studies on crustal characterization are limited to either the crustal or hand-specimen scales so far, and little has been done at centimeter - meter scale, which potentially ties those two end-member prospects. The lack of this scale is due mainly to the difficulties in direct sampling and the limited resolution of geophysical experiments; as a consequence, critical questions remain unanswered, e.g., what does the cross-section of actual ocean crust look like and what does it tell us?; where exactly in the lithosphere does fluid exist and promote the deep hydration and biosphere?; to what extent do we average out the heterogeneity in the crustal properties depending on the scale? Ocean Drilling Program (ODP) Hole 1256D is located at the 15 Ma super-fast spreading Cocos Plate and the first drilled hole that successfully penetrate through the intact upper ocean crust. Coring in the Hole 1256D basement is suffered from the low core recovery rates (~ 32 %) and the origins of recovered cores are mostly biased toward formations with minimal fractures. Wire-line logging in this hole becomes, thus, extremely useful for both the physical and chemical characterization of the crust. In particular, Formation MicroScanner (FMS) data acquired from multiple paths during three drilling expeditions have unprecedented lateral coverage of the borehole wall. The FMS images are the first realization of the cross-section of in situ architecture of the intact upper ocean crust with a centimeter-meter scale resolution. A lithostratigraphy model is reconstructed by integrating the analyses on FMS electrofacies, other physical property logs, and recovered cores. The new lithostratigraphy reveals that nearly 50 % of the in situ lithofacies in the Hole 1256D crust consists of either breccias or highly fractured lava flows, inferring that the shipboard stratigraphy with mostly massive flows is inaccurate. The meticulously deciphered lava morphology tie the lava deposition history in Hole 1256D to the East Pacific Rise surface volcanology, and with this, the upper ocean crustal construction processes in the Hole 1256D crust, from the spreading axis to the abyssal plain, can be proposed. Furthermore, the vacancy in the crustal matrix, where water and minerals can be stored and microbes can exist, is determined from the FMS images. The distribution and areas of the surface void calculated by ImageJ image processor reveals that the visible void in the 1256D crust vary 10 to 60 % depending on lithofacies, with the average of 37 %. This downhole distribution of the void areas also shows the positive correlation with previously observed lab-based porosity and 1-D sonic-log based fractional porosity data. Further study is in progress on scaling of the porosity structure from hand-specimen to crustal scales in the Hole 1256D crust: from the lab porosity data, to 1D sonic-log, to the areas of surface void detected observed in the FMS images, and ultimately to the vertical seismic experiments.

Hydromechanical Modeling of Fluid Flow in the Lower Crust

NASA Astrophysics Data System (ADS)

Connolly, J.

2011-12-01

The lower crust lies within an ambiguous rheological regime between the brittle upper crust and ductile sub-lithospheric mantle. This ambiguity has allowed two schools of thought to develop concerning the nature of fluid flow in the lower crust. The classical school holds that lower crustal rocks are inviscid and that any fluid generated by metamorphic devolatilization is squeezed out of rocks as rapidly as it is produced. According to this school, permeability is a dynamic property and fluid flow is upward. In contrast, the modern school uses concepts from upper crustal hydrology that presume implicitly, if not explicitly, that rocks are rigid or, at most, brittle. For the modern school, the details of crustal permeability determine fluid flow and as these details are poorly known almost anything is possible. Reality, to the extent that it is reflected by inference from field studies, offers some support to both schools. In particular, evidence of significant lateral and channelized fluid flow are consistent with flow in rigid media, while evidence for short (104 - 105 y) grain-scale fluid-rock interaction during much longer metamorphic events, suggests that reaction-generated grain-scale permeability is sealed rapidly by compaction; a phenomenon that is also essential to prevent extensive retrograde metamorphism. These observations provide a compelling argument for recognizing in conceptual models of lower crustal fluid flow that rocks are neither inviscid nor rigid, but compact by viscous mechanisms on a finite time-scale. This presentation will review the principle consequences of, and obstacles to, incorporating compaction in such models. The role of viscous compaction in the lower crust is extraordinarily uncertain, but ignoring this uncertainty in models of lower crustal fluid flow does not make the models any more certain. Models inevitably invoke an initial steady state hydraulic regime. This initial steady state is critical to model outcomes because it determines the compaction time and length scales and, thereby, the response of the system to perturbations. Unfortunately, because metamorphic devolatilization is the most probable source of lower crustal fluids, the assumption of an initial steady state leaves much to be desired. In truth, in the modeling of lower crustal fluid flow, less is known about the initial state than is known about possible perturbations to it, e.g., metamorphic fluid production. Compaction is a bad and good news story. The bad news is that local flow patterns may be influenced by unknowable details; the good news is that compaction-driven fluid flow has a tendency to self-organize. Self-organization eliminates the dependence on details that are present on spatial or temporal scales that are smaller than the compaction length and time scales. Porosity waves are the mechanism for this self-organization, through which dilational deformation is localized in time and space to create pathways for fluid expulsion. The resulting flow patterns are sensitive to material properties and initial state, thus, inversion of natural flow patterns offers the greatest hope for constraining the compaction scales. Knowledge of these scales is also important because they limit the influence of external forcings on flow patterns, e.g., a shear zone may induce lateral or downward fluid flow, but only on the compaction time and length scales.

Central Arctic Crustal Modeling Constrained by Potential Field data and recent ECS Seismic Data

NASA Astrophysics Data System (ADS)

Evangelatos, John; Oakey, Gordon; Saltus, Rick

2017-04-01

2-D gravity and magnetic models have been generated for several transects across the Alpha-Mendeleev ridge complex to study the regional variability of the crustal structure and identify large scale lateral changes. The geometry and density parameters for the models have been constrained using recently acquired seismic reflection and refraction data collected jointly by Canada and the United States as part of their collaborative Arctic ECS programs. A total of fifteen models have been generated perpendicular to the ridge complex, typically 50 to 150 km apart. A minimalist approach to modeling involved maintaining a simple, laterally continuous density structure for the crust while varying the model geometry to fit the observed gravity field. This approach is justified because low amplitude residual Bouguer anomalies suggest a relatively homogenous density structure within the ridge complex. These models have provided a new measure of the regional variability in crustal thickness. Typically, models with thinner crust correspond with deeper bathymetric depths of the ridge which is consistent with regional isostatic equilibrium. Complex "chaotic" magnetic anomalies are associated with the Alpha-Mendeleev ridge complex, which extends beneath the surrounding sedimentary basins. Pseudogravity inversion (magnetic potential) of the magnetic field provides a quantifiable areal extent of ˜1.3 x106 km2. Forward modeling confirms that the magnetic anomalies are not solely the result of magnetized bathymetric highs, but are caused to a great extent by mid- and lower crustal sources. The magnetization of the crust inferred from modeling is significantly higher than available lab measurements of onshore volcanic rocks. Although the 2-D models cannot uniquely identify whether the crustal protolith was continental or oceanic, there is a necessity for a significant content of high density and highly magnetic (ultramafic) material. Based on the crustal thickness estimates from our regional 2-D gravity models and the two possible protoliths, we determine volumetric estimates of the volcanic composition to ˜ 6 × 106 km3 for the mid- and upper-crust and between 10 × 106 and 14 × 106 km3 within the lower crust — for a total of at least ˜16 × 106 km3. This exceeds any estimates for the onshore circum-Arctic HALIP by more than an order of magnitude.

A PILOT SEARCH FOR EVIDENCE OF EXTRASOLAR EARTH-ANALOG PLATE TECTONICS

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

Jura, M.; Klein, B.; Xu, S.

Relative to calcium, both strontium and barium are markedly enriched in Earth's continental crust compared to the basaltic crusts of other differentiated rocky bodies within the solar system. Here, we both re-examine available archived Keck spectra to place upper bounds on n(Ba)/n(Ca) and revisit published results for n(Sr)/n(Ca) in two white dwarfs that have accreted rocky planetesimals. We find that at most only a small fraction of the pollution is from crustal material that has experienced the distinctive elemental enhancements induced by Earth-analog plate tectonics. In view of the intense theoretical interest in the physical structure of extrasolar rocky planets,more » this search should be extended to additional targets.« less

Stratigraphy of the Kasei Valles region, Mars

NASA Technical Reports Server (NTRS)

Robinson, Mark S.; Tanaka, Kenneth L.

1987-01-01

The thicknesses and geomorphology of the two principal stratigraphic units exposed in Kasei Valles to aid in interpreting the nature of crustal materials and the history of the channeling events in the area are identified and described. Previous studies of Kasei Valles have related the channel landforms to glacial flow, catastrophic flooding, and large-scale eolian erosion. The two units (an upper and a lower unit) form thick sheets, each having distinct geomorphologic features. Thicknesses of the unit were determined through preliminary stereogrammetric profiles taken across many sections of western Kasei Valles and shadow measurements taken of scarp heights from calibrated Viking images having sun angles less than 25 degrees; DN values were examined to confirm that true shadows were observed.

Zircon Hf-O isotopic constraints on the origin of Late Mesozoic felsic volcanic rocks from the Great Xing'an Range, NE China

NASA Astrophysics Data System (ADS)

Gong, Mingyue; Tian, Wei; Fu, Bin; Wang, Shuangyue; Dong, Jinlong

2018-05-01

The voluminous Late Mesozoic magmatism was related to extensive re-melting of juvenile materials that were added to the Central East Asia continent in Phanerozoic time. The most favoured magma generation mechanism of Late Mesozoic magmas is partial melting of underplated lower crust that had radiogenic Hf-Nd isotopic characteristics, but this mechanism faces difficulties when interpreting other isotopic data. The tectonic environment controlling the generation of the Late Mesozoic felsic magmas is also in dispute. In this study, we obtained new U-Pb ages, and geochemical and isotopic data of representative Jurassic (154.4 ± 1.5 Ma) and Cretaceous (140.2 ± 1.5 Ma) felsic volcanic samples. The Jurassic sample has inherited zircon cores of Permian age, with depleted mantle-like εHf(t) of +7.4 - +8.5, which is in contrast with those of the magmatic zircons (εHf(t) = +2.4 ± 0.7). Whereas the inherited cores and the magmatic zircons have identical mantle-like δ18O composition ranges (4.25-5.29‰ and 4.69-5.54‰, respectively). These Hf-O isotopic characteristics suggest a mixed source of enriched mantle materials rather than ancient crustal components and a depleted mantle source represented by the inherited Permian zircon core. This mechanism is manifested by the eruption of Jurassic alkaline basalts originated from an enriched mantle source. The Cretaceous sample has high εHf(t) of +7.0 - +10.5, suggesting re-melting of a mafic magma derived from a depleted mantle-source. However, the sub-mantle zircon δ18O values (3.70-4.58‰) suggest the depleted mantle-derived mafic source rocks had experienced high temperature hydrothermal alteration at upper crustal level. Therefore, the Cretaceous felsic magma, if not all, could be generated by re-melting of down-dropped supracrustal volcanic rocks that experienced high temperature oxygen isotope alteration. The two processes, enriched mantle-contribution and supracrustal juvenile material re-melting, are new generation mechanisms of the Late Mesozoic magmas from Central East Asia. Rift settings may have controlled these processes throughout crustal and mantle levels.

The Upper- to Middle-Crustal Section of the Alisitos Oceanic Arc, (Baja, Mexico): an Analog of the Izu-Bonin-Marianas (IBM) Arc

NASA Astrophysics Data System (ADS)

Medynski, S.; Busby, C.; DeBari, S. M.; Morris, R.; Andrews, G. D.; Brown, S. R.; Schmitt, A. K.

2016-12-01

The Rosario segment of the Cretaceous Alisitos arc in Baja California is an outstanding field analog for the Izu-Bonin-Mariana (IBM) arc, because it is structurally intact, unmetamorphosed, and has superior three-dimensional exposures of an upper- to middle-crustal section through an extensional oceanic arc. Previous work1, done in the pre-digital era, used geologic mapping to define two phases of arc evolution, with normal faulting in both phases: (1) extensional oceanic arc, with silicic calderas, and (2) oceanic arc rifting, with widespread diking and dominantly mafic effusions. Our new geochemical data match the extensional zone immediately behind the Izu arc front, and is different from the arc front and rear arc, consistent with geologic relations. Our study is developing a 3D oceanic arc crustal model, with geologic maps draped on Google Earth images, and GPS-located outcrop information linked to new geochemical, geochronological and petrographic data, with the goal of detailing the relationships between plutonic, hypabyssal, and volcanic rocks. This model will be used by scientists as a reference model for past (IBM-1, 2, 3) and proposed IBM (IBM-4) drilling activities. New single-crystal zircon analysis by TIMS supports the interpretation, based on batch SIMS analysis of chemically-abraded zircon1, that the entire upper-middle crustal section accumulated in about 1.5 Myr. Like the IBM, volcanic zircons are very sparse, but zircon chemistry on the plutonic rocks shows trace element compositions that overlap to those measured in IBM volcanic zircons by A. Schmitt (unpublished data). Zircons have U-Pb ages up to 20 Myr older than the eruptive age, suggesting remelting of older parts of the arc, similar to that proposed for IBM (using different evidence). Like IBM, some very old zircons are also present, indicating the presence of old crustal fragments, or sediments derived from them, in the basement. However, our geochemical data show that the magmas are differentiated from a single mantle source, so any older crust that was remelted had the same compositional characteristics. This is similar to previous conclusion that the different parts of the Izu arc have retained their distinct compositions over the last 15 Myr2. 1Busby et al., 2006 JVGR 149, 1-46 2 Hochstaedter et al., 2000 JGR 105, 495-512

Seismic Evidence of Imprints of Malani and Deccan Volcanism in Northwestern India

NASA Astrophysics Data System (ADS)

Mohan, G.; Mangalampally, R. K.; Ahmad, F.

2017-12-01

The evolution of the Neoproterozoic (750 Ma) Malani igneous province(MIP), the site of the largest felsic magmatism in India is debatable with theories supporting extensional tectonics, mantle plume or subduction processes. The MIP that lies to the west of the Proterozoic Aravalli mountain range and east of the Late Mesozoic-Teritary Barmer-Sanchor rift systems, hosts acidic volcanics in an area of 0.5 million sq.km in northwestern India. In this study, the crustal and upper mantle structure beneath the MIP is investigated through a deployment of 12 broadband seismographs in phases, at 18 locations during a period of five years from 2011-2016. The P wave receiver function(RF) analysis was carried out to image the crust and the 410 km and 660 km mantle transition zone discontinuities. About 1500 teleseismic waveforms with signal to noise ratios > 2.5 are utilized. The RFs at most stations are marked by strong conversions from the base of the sediments and the Moho. The crustal thickness estimated through the Neighbourhood algorithm approach, ranges from 35 to 42km. The crustal Poisson's ratio ranges from 0.26 - 0.29. The crustal thickness and Poisson's ratio are observed to increase from west to east viz., from the rift zone to the mountain belt. A significant finding is the presence of a 5-10km thick mid-crustal low velocity zone with a reduced shear velocity of 3.0-3.2km/s. The Ps conversions from the 410km and 660km mantle discontinuities are delayed by about 1sec with respect to the timings predicted by the IASP91 standard earth model. The observed delays are attributed to the reduction in velocity due to compositional/thermal perturbations in the uppermost upper mantle above the 410km discontinuity. The presence of alkaline complexes in MIP which are of pre-Deccan age (68 Ma) led us to surmise that the low velocity anomalies observed in the upper mantle might be linked to the mantle source associated with the 65 Ma Deccan volcanism which erupted further south of MIP. It is likely that the mantle source may have overprinted or obliterated the mantle signatures of the Neoproterozoic tectonic event. However, the intracrustal low velocities overlying an underplated crust in MIP are interpreted to be the compositional imprints of the felsic magma associated with the bimodel Malani volcanism.

Axial crustal structure of the Costa Rica Rift: Implications for along-axis hydrothermal circulation

NASA Astrophysics Data System (ADS)

Zhang, L.; Tong, V.; Hobbs, R. W.; Peirce, C.; Lowell, R. P.; Haughton, G.; Murton, B. J.; Morales Maqueda, M. A.; Harris, R. N.; Robinson, A. H.

2017-12-01

In 2015, a multidisciplinary geophysical cruise surveyed the Costa Rica Rift (CRR) in the Panama Basin of the equatorial East Pacific, acquiring a grid of multichannel seismic and wide-angle profiles to determine the mode of oceanic crustal accretion at intermediate-spreading ridges, and how the crustal structure may be influenced by hydrothermal fluid flow. Analysis of 69,000 P-wave first arrivals recorded by 25 ocean-bottom seismographs deployed over a 20 × 20 km area that straddles the ridge axis, reveals a 3D velocity-depth model of upper crustal structure. In particular, the model shows a low velocity anomaly that extends to 2 km below seabed centred on a small-offset non-transform discontinuity (NTD), and a pattern of increasing velocity with distance off-axis that may reflect changes in porosity and permeability in layer 2 of the crust. Assuming the upper crustal velocity anomalies are linked with porosity and hence represent the ability of fluid to flow, comparison of the tomographic model with the volcanic seabed morphology suggests that the broad low velocity zone beneath the NTD may be a region of extensive fracturing. Hence, we infer that this region may provide a primary pathway for the recharge of seawater into the crust. Further west along the axis, beneath the bathymetric dome, which is the shallowest portion along the axis, the low-velocity anomaly is less pronounced, suggesting that fractures are less open and that fluid-rock interaction has encouraged mineral precipitation and alteration, as a result of a longer established hydrothermal fluid flow driven by the axial magma lens observed beneath it. This interpretation is supported by the presence of a plume from an active hydrothermal vent system. Hence, we infer that the variable velocity structure of the upper crust of the CRR is a proxy that reflects the primary porosity, faulting and fracturing related to phases of magma-driven accretion and/or ridge geometry re-adjustment, and that there is along-axis hydrothermal circulation transferring heat and impacting the properties of newly accreted oceanic crust. This research is part of a major, interdisciplinary NERC-funded collaboration entitled: Oceanographic and Seismic Characterisation of heat dissipation and alteration by hydrothermal fluids at an Axial Ridge (OSCAR).

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

NASA Astrophysics Data System (ADS)

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

2016-12-01

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

Crustal Structure Beneath the Gulf of ST. Lawrence, Atlantic Canada, from Ambient Seismic Noise Tomography

NASA Astrophysics Data System (ADS)

Kuponiyi, A.; Kao, H.; Cassidy, J. F.; Spence, G.

2013-12-01

The Gulf of St. Lawrence (GSL), located north of the southwest-northeast trending Appalachian mountain in eastern Canada, is a major sedimentary basin with huge potentials for hydrocarbon accumulation. Important questions about the geometry and evolution of the crustal and basin structure beneath the gulf are yet to be answered. To address these issues, the Geological Survey of Canada (GSC) with support from the Portable Observatories for Lithospheric Analysis and Research Investigating Seismicity (POLARIS) deployed a temporary array of broadband seismic stations in the GSL region between October 2005 and October 2008. Combined with the permanent stations of the Canadian National Seismograph Network (CNSN) in the region, the station density is sufficient for detailed seismic tomography inversion. In this study, we investigate the upper crustal structure beneath the gulf using 3 years of continuous ambient noise waveforms recorded at 25 (POLARIS and CNSN) stations around the GSL. Cross-correlation functions of the vertical component of the ambient noise wavefield for simultaneously recording station pairs (corresponding to inter-station Green's functions) are computed and analyzed using the frequency-time analysis method. Dispersion curves are measured and Rayleigh wave group velocities are subsequently extracted for periods between 2 and 20s, which are periods sensitive to the upper crustal structures. Preliminary results from the dispersion measurements indicate that mean group velocities in the region range from 2.8 to 3.2 km/s across the range of period specified. 2-D group velocity distribution for each period is determined by linearized inversion of the dispersion data. Our tomography results show prominent lateral velocity variation. Low velocity anomalies are observed at shorter periods (up to ~10 s) which correspond to the sedimentary structures at shallow depths (between 5-10 km), whereas the characteristics of upper crustal structures are shown by velocity anomalies at longer periods. Our results show striking similarities with the tomographic images obtained in the previous Canada-wide ambient noise analysis for areas where both studies overlap and are also consistent with results from receiver function and active seismic profiling studies previously done in the region. A detailed inversion of the 3-D shear velocity structure will be conducted to appropriately delineate the thickness and seismic velocity of the composite geologic units.

Adjoint tomography of Europe

NASA Astrophysics Data System (ADS)

Zhu, H.; Bozdag, E.; Peter, D. B.; Tromp, J.

2010-12-01

We use spectral-element and adjoint methods to image crustal and upper mantle heterogeneity in Europe. The study area involves the convergent boundaries of the Eurasian, African and Arabian plates and the divergent boundary between the Eurasian and North American plates, making the tectonic structure of this region complex. Our goal is to iteratively fit observed seismograms and improve crustal and upper mantle images by taking advantage of 3D forward and inverse modeling techniques. We use data from 200 earthquakes with magnitudes between 5 and 6 recorded by 262 stations provided by ORFEUS. Crustal model Crust2.0 combined with mantle model S362ANI comprise the initial 3D model. Before the iterative adjoint inversion, we determine earthquake source parameters in the initial 3D model by using 3D Green functions and their Fréchet derivatives with respect to the source parameters (i.e., centroid moment tensor and location). The updated catalog is used in the subsequent structural inversion. Since we concentrate on upper mantle structures which involve anisotropy, transversely isotropic (frequency-dependent) traveltime sensitivity kernels are used in the iterative inversion. Taking advantage of the adjoint method, we use as many measurements as can obtain based on comparisons between observed and synthetic seismograms. FLEXWIN (Maggi et al., 2009) is used to automatically select measurement windows which are analyzed based on a multitaper technique. The bandpass ranges from 15 second to 150 second. Long-period surface waves and short-period body waves are combined in source relocations and structural inversions. A statistical assessments of traveltime anomalies and logarithmic waveform differences is used to characterize the inverted sources and structure.

Plate Margin Deformation and Active Tectonics Along the Northern Edge of the Yakutat Terrane in the Saint Elias Orogen, Alaska and Yukon, Canada

NASA Technical Reports Server (NTRS)

Bruhn, Ronald L.; Sauber, Jeanne; Cotton, Michele M.; Pavlis, Terry L.; Burgess, Evan; Ruppert, Natalia; Forster, Richard R.

2012-01-01

The northwest directed motion of the Pacific plate is accompanied by migration and collision of the Yakutat terrane into the cusp of southern Alaska. The nature and magnitude of accretion and translation on upper crustal faults and folds is poorly constrained, however, due to pervasive glaciation. In this study we used high-resolution topography, geodetic imaging, seismic, and geologic data to advance understanding of the transition from strike-slip motion on the Fairweather fault to plate margin deformation on the Bagley fault, which cuts through the upper plate of the collisional suture above the subduction megathrust. The Fairweather fault terminates by oblique-extensional splay faulting within a structural syntaxis, allowing rapid tectonic upwelling of rocks driven by thrust faulting and crustal contraction. Plate motion is partly transferred from the Fairweather to the Bagley fault, which extends 125 km farther west as a dextral shear zone that is partly reactivated by reverse faulting. The Bagley fault dips steeply through the upper plate to intersect the subduction megathrust at depth, forming a narrow fault-bounded crustal sliver in the obliquely convergent plate margin. Since . 20 Ma the Bagley fault has accommodated more than 50 km of dextral shearing and several kilometers of reverse motion along its southern flank during terrane accretion. The fault is considered capable of generating earthquakes because it is linked to faults that generated large historic earthquakes, suitably oriented for reactivation in the contemporary stress field, and locally marked by seismicity. The fault may generate earthquakes of Mw <= 7.5.

Rheological implications of sediment transport for continental rifting and its impact in margin geometry and major unconformities

NASA Astrophysics Data System (ADS)

Andres-Martinez, Miguel; Perez-Gussinye, Marta; Armitage, John; Morgan, Jason

2016-04-01

The inner dynamics of the Earth such as mantle convection, geochemical reactions and isostasy have been typically interpreted as the main engine of plate tectonics and crustal deformation. However, nowadays it is well established that processes transporting material along the surface of the Earth influence the inner dynamics. Surface processes play a key role particularly during rifting, where great subsidence rates occur at synrift basins while shoulder uplift provides rock to be eroded for later infilling of these basins. Erosion implies unloading of the crust which favours uplift, and sedimentation at basins results in loading which favours subsidence. Consequently, erosion and sedimentation amplify stresses and the flexural response of the lithosphere in situations with extensive faulting. These changes to the stress field may be large enough to result in changes in the evolution of rifting and its modes of extension. Additionally, higher subsidence rates and thermal blanketing due to sediments may result in higher geotherms and consequently, a weaker/more-viscous behaviour of the crustal rocks. This would also have a large impact on the deformation style during extension. Here, we explore the interactions between surface processes and tectonics using numerical modelling. Experiments are run with the absence of sediment transport and with different sediment transport regimes for 35 and 40 km crustal thicknesses. Tests with higher transport coefficient show more effective localization of deformation into upper crustal faults which results in effective crustal thinning, larger blocks and longer-lived faults. Our experiments also prove that more effective surface processes reduce the length of margins generated by sequential faulting. For our end member situations, high sedimentation rates lead to pure shear extension of the crust induced by high temperatures, which finally results in broad extension and symmetric margins. Furthermore, our model allows for the recovery of predicted sediment stratigraphic patterns. Major unconformities that separate synrift from sag-basin-type sediments are observed in these pseudo-strata patterns. Here, we also address the meaning of these major unconformities and their relationship to the time of breakup.

Petrographic, geochemical and isotopic evidence of crustal assimilation processes in the Ponte Nova alkaline mafic-ultramafic massif, SE Brazil

NASA Astrophysics Data System (ADS)

Azzone, Rogério Guitarrari; Montecinos Munoz, Patricio; Enrich, Gaston Eduardo Rojas; Alves, Adriana; Ruberti, Excelso; Gomes, Celsode Barros

2016-09-01

Crustal assimilation plus crystal fractionation processes of different basanite magma batches control the evolution of the Ponte Nova cretaceous alkaline mafic-ultramafic massif in SE Brazil. This massif is composed of several intrusions, the main ones with a cumulate character. Disequilibrium features in the early-crystallized phases (e.g., corrosion and sieve textures in cores of clinopyroxene crystals, spongy-cellular-textured plagioclase crystals, gulf corrosion texture in olivine crystals) and classical hybridization textures (e.g., blade biotite and acicular apatite crystals) provide strong evidence of open-system behavior. All samples are olivine- and nepheline-normative rocks with basic-ultrabasic and potassic characters and variable incompatible element enrichments. The wide ranges of whole-rock 87Sr/86Sri and 143Nd/144Ndi ratios (0.70432-0.70641 and 0.512216-0.512555, respectively) are indicative of crustal contribution from the Precambrian basement host rocks. Plagioclase and apatite 87Sr/86Sr ratios (0.70422-0.70927) obtained for the most primitive samples of each intrusion indicate disequilibrium conditions from early- to principal-crystallization stages. Isotope mixing-model curves between the least contaminated alkaline basic magma and heterogeneous local crustal components indicate that each intrusion of the massif is differentiated from the others by varied degrees of crustal contribution. The primary mechanisms of crustal contribution to the Ponte Nova massif involve the assimilation of host rock xenoliths during the development of the chamber environment and the assimilation of partial melts from the surrounding host rocks. Thermodynamic models using the melts algorithm indicate that parental alkaline basic magmas can be strongly affected by contamination processes subsequently to their initial stages of crystallization when there is sufficient energy to assimilate partial melts of crustal host rocks. The assimilation processes are considered to be responsible for the increse in the K2O/Na2O, Ba/Sr and Rb/Sr ratios. This enrichment was associated with the relevant role of biotite breakdown in the assimilated host rock partial melts. The petrological model for the Ponte Nova massif is explained as repeated influxes of antecryst-laden basanite magmas that deposited most of their suspended crystals on the floor of the upper-crust magma chamber. Each intrusion is representative of relatively primitive olivine- and clinopyroxene-phyric basanites that had assimilated different degrees of partial melts of heterogeneous host rocks. This study reveals the relevant role of crustal assimilation processes in the magmatic evolution of nepheline-normative rocks, especially in upper-crust chamber environments.

Seismic velocity and crustal thickness inversions: Moon and Mars

NASA Astrophysics Data System (ADS)

Drilleau, Melanie; Blanchette-Guertin, Jean-François; Kawamura, Taichi; Lognonné, Philippe; Wieczorek, Mark

2017-04-01

We present results from new inversions of seismic data arrival times acquired by the Apollo active and passive experiments. Markov chain Monte Carlo inversions are used to constrain (i) 1-D lunar crustal and upper mantle velocity models and (ii) 3-D lateral crustal thickness models under the Apollo stations and the artificial and natural impact sites. A full 3-D model of the lunar crustal thickness is then obtained using the GRAIL gravimetric data, anchored by the crustal thicknesses under each Apollo station and impact site. To avoid the use of any seismic reference model, a Bayesian inversion technique is implemented. The advantage of such an approach is to obtain robust probability density functions of interior structure parameters governed by uncertainties on the seismic data arrival times. 1-D seismic velocities are parameterized using C1-Bézier curves, which allow the exploration of both smoothly varying models and first-order discontinuities. The parameters of the inversion include the seismic velocities of P and S waves as a function of depth, the thickness of the crust under each Apollo station and impact epicentre. The forward problem consists in a ray tracing method enabling both the relocation of the natural impact epicenters, and the computation of time corrections associated to the surface topography and the crustal thickness variations under the stations and impact sites. The results show geology-related differences between the different sites, which are due to contrasts in megaregolith thickness and to shallow subsurface composition and structure. Some of the finer structural elements might be difficult to constrain and might fall within the uncertainties of the dataset. However, we use the more precise LROC-located epicentral locations for the lunar modules and Saturn-IV upper stage artificial impacts, reducing some of the uncertainties observed in past studies. In the framework of the NASA InSight/SEIS mission to Mars, the method developed in this study will be used to constrain the Martian crustal thickness as soon as the first data will be available (late 2018). For Insight, impacts will be located by MRO data differential analysis, which provide a known location enabling the direct inversion of all differential travel times with respect to P arrival time. We have performed resolution tests to investigate to what extend impact events might help us to constrain the Martian crustal thickness. Due to the high flexibility of the Bayesian algorithm, the interior model will be refined each time a new event will be detected.

A synoptic view of the distribution and connectivity of the mid-crustal low velocity zone beneath Tibet

NASA Astrophysics Data System (ADS)

Yang, Y.; Zheng, Y.; Xie, Z.; Ritzwoller, M. H.

2011-12-01

The Tibetan Plateau results from the convergence between the Indian and Eurasian plates. However, the physical processes that have controlled the deformation history of Tibet, particularly the potential localization of deformation either in the vertical or horizontal directions remain subject to debate. There are a growing list and wide variety of observations that suggest that the Tibetan crust is warm and presumably ductile. Some of observations are often taken as prima facie evidence for the existence of partial melt or aqueous fluids in the middle or deep crust beneath Tibet and in some cases for the decoupling or partitioning of strain between the upper crust and uppermost mantle. However, most of this evidence is highly localized along nearly linear seismic or magneto-telluric profiles. This motivates the two questions addressed by this study. First, how pervasive across Tibet are the phenomena on which inferences of the existence of crustal partial melt rest? In particular, how pervasive are mid-crustal low velocity zones across Tibet? Second, what is the geometry or inter-connectivity of the crustal low velocity zones observed across Tibet? In this study, we address these questions by producing a new 3-D model of crustal and uppermost mantle shear wave speeds inferred from Rayleigh wave dispersion observed on cross-correlations of long time series of ambient seismic noise. Broadband seismic data from about 600 stations (Chinese Provincial networks, FDSN, several PASSCAL experiments including the INDEPTH IV experiment) yield about 50,000 inter-station paths, which are used to generate Rayleigh wave phase velocity maps from 10 sec to 50 sec period. The time series lengths in the cross-correlations range from 1 to 2 years in duration. The resulting Rayleigh wave phase velocity maps are inverted for a 3D Vsv model of crustal and upper most mantles. The major results from our model are summarized below: (1) A crustal LVZ exists across most of the high Tibetan Plateau. (2) The distribution of the amplitude of the LVZ is not uniform. In fact, the largest amplitudes (i.e., lowest mid-crustal shear wave speeds) are found predominantly around the periphery of Tibet. (3) The lateral distribution of strong LVZs are coincident with the distribution of strong radial anisotropy in the middle crust, suggesting LVZs of Vsv in the middle crust may be mostly due to the strong radial anisotropy rather than the presence of partial melt or aqueous fluids.

Structural variability of the Tonga-Kermadec forearc characterized using robustly constrained geophysical data

NASA Astrophysics Data System (ADS)

Funnell, M. J.; Peirce, C.; Robinson, A. H.

2017-09-01

Subducting bathymetric anomalies enhance erosion of the overriding forearc crust. The deformation associated with this process is superimposed on pre-existing variable crustal and sedimentary structures developed as a subduction system evolves. Recent attempts to determine the effect and timescale of Louisville Ridge seamount subduction on the Tonga-Kermadec forearc have been limited by simplistic models of inherited overriding crustal structure that neglect along-strike variability. Synthesis of new robustly tested seismic velocity and density models with existing data sets from the region, highlight along-strike variations in the structure of the Tonga-Kermadec subducting and overriding plates. As the subducting plate undergoes bend-faulting and hydration throughout the trench-outer rise region, observed oceanic upper- and mid-crustal velocities are reduced by ∼1.0 km s-1 and upper mantle velocities by ∼0.5 km s-1. In the vicinity of the Louisville Ridge Seamount Chain (LRSC), the trench shallows by 4 km and normal fault throw is reduced by >1 km, suggesting that the subduction of seamounts reduces plate deformation. We find that the extinct Eocene frontal arc, defined by a high velocity (7.0-7.4 km s-1) and density (3.2 g cm-3) lower-crustal anomaly, increases in thickness by ∼6 km, from 12 to >18 km, over 300 km laterally along the Tonga-Kermadec forearc. Coincident variations in bathymetry and free-air gravity anomaly indicate a regional trend of northward-increasing crustal thickness that predates LRSC subduction, and highlight the present-day extent of the Eocene arc between 32°S and ∼18°S. Within this framework of existing forearc crustal structure, the subduction of seamounts of the LRSC promotes erosion of the overriding crust, forming steep, gravitationally unstable, lower-trench slopes. Trench-slope stability is most likely re-established by the collapse of the mid-trench slope and the trenchward side of the extinct Eocene arc, which, within the framework of forearc characterization, implies seamount subduction commenced at ∼22°S.

Interaction between crustal tectonics and salt deformation in the Eastern Sardinian margin, Western Tyrrhenian Sea: seismic data and analogue modelling

NASA Astrophysics Data System (ADS)

Vendeville, Bruno; Lymer, Gael; Gaullier, Virginie; Chanier, Frank; Maillard, Agnes; Sage, Françoise; Lofi, Johanna; Thinon, Isabelle

2014-05-01

The Tyrrhenian Basin opened by eastward migration of the Apennine subduction system. Rifting along the Eastern Sardinian margin started during the middle to late Miocene times and hence this timing partly overlapped the Messinian Salinity Crisis. The two "METYSS" cruises were conducted to use the deformation of the Messinian salt and its Plio-Quaternary overburden as a proxy for better delineating the tectonic history of the sub-salt basement. Many parts of the study area contain two of the most typical Messinian series of the Western Mediterranean: the Mobile Unit (MU; salt, mainly halite), overlain by the more competent Upper Unit (UU: alternating dolomitic marls and anhydrite). The brittle Plio-Quaternary cover overlies the UU. Usually, the presence of mobile salt is viewed as a nuisance for understanding crustal tectonics because salt's ability to act as a structural buffer between the basement and the cover. However, we illustrate, using examples from the Cornaglia Terrace, how we can use thin-skinned salt tectonics as indicators of vertical movements in the sub-salt, pre-Messinian basement. There, slip along N-S-trending crustal normal faults bounding basement troughs has been recorded by salt and overburden in two different manners: - First, post-salt basement faulting (typically after deposition of the Upper Unit and the early Pliocene), and some crustal-scale southward tilting, triggered along-strike (southward) thin-skinned, gliding of salt and overburden recorded by upslope extension and downslope shortening. - Second, and less obvious at first glance, there was some crustal activity along another basement trough, located East of the Baronie Ridge after deposition of the Messinian salt. This trough is narrow, trends N-S and is bounded by crustal faults. The narrow width of the trough allowed for only minor across-strike (E-W) gliding. The resulting geometry would suggest that nothing happened after Messinian times, but some structural features (confirmed by analogue modelling) show that basement fault slip and tilting (Eastward or Westward) was accommodated by lateral flow of salt, which thinned upslope and inflated downslope, while the overlying sediments remained sub-horizontal.

Extreme Mesozoic crustal thinning in the Eastern Iberia margin: The example of the Columbrets Basin (Valencia Trough)

NASA Astrophysics Data System (ADS)

Mohn, G.; Etheve, N.; Frizon de Lamotte, D.; Roca, E.; Tugend, J.; Gómez-Romeu, J.

2017-12-01

Eastern Iberia preserves a complex succession of Mesozoic rifts partly or completely inverted during the Late Cretaceous and Cenozoic in relation with Africa-Eurasia convergence. Notably, the Valencia Trough, classically viewed as part of the Cenozoic West Mediterranean basins, preserves in its southwestern part a thick Mesozoic succession (locally »10km thick) over a highly thinned continental basement (locally only »3,5km thick). This sub-basin referred to as the Columbrets Basin, represents a Late Jurassic-Early Cretaceous hyper-extended rift basin weakly overprinted by subsequent events. Its initial configuration is well preserved allowing us to unravel its 3D architecture and tectono-stratigraphic evolution in the frame of the Mesozoic evolution of eastern Iberia. The Columbrets Basin benefits from an extensive dataset combining high resolution reflection seismic profiles, drill holes, refraction seismic data and Expanding Spread Profiles. Its Mesozoic architecture is controlled by interactions between extensional deformation and halokinesis involving the Upper Triassic salt. The thick uppermost Triassic to Cretaceous succession describes a general synclinal shape, progressively stretched and dismembered towards the basin borders. The SE-border of the basin is characterized by a large extensional detachment fault acting at crustal scale and interacting locally with the Upper Triassic décollement. This extensional structure accommodates the exhumation of the continental basement and part of the crustal thinning. Eventually our results highlight the complex interaction between extreme crustal thinning and occurrence of a pre-rift salt level for the deformation style and tectono-stratigraphic evolution of hyper-extended rift basins.

Seismological Structure of the 1.8Ga Trans-Hudson Orogen of North America and its affinity to present-day Tibet

NASA Astrophysics Data System (ADS)

Gilligan, A.; Bastow, I. D.; Darbyshire, F. A.

2015-12-01

How tectonic processes operated and changed through the Precambrian is debated: what was the nature and scale of orogenic events and were they different on the younger, hotter, more ductile Earth? The geology of northern Hudson Bay records the Paleoproterozoic collision between the Western Churchill and Superior plates: the 1.8Ga Trans-Hudson Orogeny (THO) and is thus an ideal study locale to address this issue. It has been suggested, primarily on the strength of traditional field geology, that the THO was comparable in scale and style to the present-day Himalayan-Karakoram-Tibet Orogen (HKTO). However, understanding of the deep crustal architecture of the THO, and how it compares to the evolving HKTO is presently lacking. Through joint inversion of teleseismic receiver functions and surface wave data, we obtain new Moho depth estimates and shear velocity models for the crust and upper mantle. Archean crust in the Rae, Hearne and Churchill domains is thin and structurally simple, with a sharp Moho; upper crustal wavespeed variations are readily attributed to post-formation events. However, the Paleoproterozoic Quebec-Baffin segment of the THO has a deeper Moho and more complex crustal structure. Our observations are strikingly similar to recent models, computed using the same methods, of the HKTO lithosphere, where deformation also extends >400km beyond the collision front. On the strength of Moho character, present-day crustal thickness, and metamorphic grade, we thus propose that southern Baffin experienced uplift of a similar magnitude and spatial extent to the Himalayas during the Paleoproterozoic Trans-Hudson Orogeny.

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.

Ionizing Electrons on the Martian Nightside: Structure and Variability

NASA Astrophysics Data System (ADS)

Lillis, Robert J.; Mitchell, David L.; Steckiewicz, Morgane; Brain, David; Xu, Shaosui; Weber, Tristan; Halekas, Jasper; Connerney, Jack; Espley, Jared; Benna, Mehdi; Elrod, Meredith; Thiemann, Edward; Eparvier, Frank

2018-05-01

The precipitation of suprathermal electrons is the dominant external source of energy deposition and ionization in the Martian nightside upper atmosphere and ionosphere. We investigate the spatial patterns and variability of ionizing electrons from 115 to 600 km altitude on the Martian nightside, using CO2 electron impact ionization frequency (EIIF) as our metric, examining more than 3 years of data collected in situ by the Mars Atmosphere and Volatile EvolutioN spacecraft. We characterize the behavior of EIIF with respect to altitude, solar zenith angle, solar wind pressure, and the geometry and strength of crustal magnetic fields. EIIF has a complex and correlated dependence on these factors, but we find that it generally increases with altitude and solar wind pressure, decreases with crustal magnetic field strength and does not depend detectably on solar zenith angle past 115°. The dependence is governed by (a) energy degradation and backscatter by collisions with atmospheric neutrals below 220 km and (b) magnetic field topology that permits or retards electron access to certain regions. This field topology is dynamic and varies with solar wind conditions, allowing greater electron access at higher altitudes where crustal fields are weaker and also for higher solar wind pressures, which result in stronger draped magnetic fields that push closed crustal magnetic field loops to lower altitudes. This multidimensional electron flux behavior can in the future be parameterized in an empirical model for use as input to global simulations of the nightside upper atmosphere, which currently do not account for this important source of energy.

Crustal velocity structure of central Gansu Province from regional seismic waveform inversion using firework algorithm

NASA Astrophysics Data System (ADS)

Chen, Yanyang; Wang, Yanbin; Zhang, Yuansheng

2017-04-01

The firework algorithm (FWA) is a novel swarm intelligence-based method recently proposed for the optimization of multi-parameter, nonlinear functions. Numerical waveform inversion experiments using a synthetic model show that the FWA performs well in both solution quality and efficiency. We apply the FWA in this study to crustal velocity structure inversion using regional seismic waveform data of central Gansu on the northeastern margin of the Qinghai-Tibet plateau. Seismograms recorded from the moment magnitude ( M W) 5.4 Minxian earthquake enable obtaining an average crustal velocity model for this region. We initially carried out a series of FWA robustness tests in regional waveform inversion at the same earthquake and station positions across the study region, inverting two velocity structure models, with and without a low-velocity crustal layer; the accuracy of our average inversion results and their standard deviations reveal the advantages of the FWA for the inversion of regional seismic waveforms. We applied the FWA across our study area using three component waveform data recorded by nine broadband permanent seismic stations with epicentral distances ranging between 146 and 437 km. These inversion results show that the average thickness of the crust in this region is 46.75 km, while thicknesses of the sedimentary layer, and the upper, middle, and lower crust are 3.15, 15.69, 13.08, and 14.83 km, respectively. Results also show that the P-wave velocities of these layers and the upper mantle are 4.47, 6.07, 6.12, 6.87, and 8.18 km/s, respectively.

Active and long-lived permanent forearc deformation driven by the subduction seismic cycle

NASA Astrophysics Data System (ADS)

Aron Melo, Felipe Alejandro

I have used geological, geophysical and engineering methods to explore mechanisms of upper plate, brittle deformation at active forearc regions. My dissertation particularly addresses the permanent deformation style experienced by the forearc following great subduction ruptures, such as the 2010 M w8.8 Maule, Chile and 2011 Mw9.0 Tohoku, Japan earthquakes. These events triggered large, shallow seismicity on upper plate normal faults above the rupture reaching Mw7.0. First I present new structural data from the Chilean Coastal Cordillera over the rupture zone of the Maule earthquake. The study area contains the Pichilemu normal fault, which produced the large crustal aftershocks of the megathrust event. Normal faults are the major neotectonic structural elements but reverse faults also exist. Crustal seismicity and GPS surface displacements show that the forearc experiences pulses of rapid coseismic extension, parallel to the heave of the megathrust, and slow interseismic, convergence-parallel shortening. These cycles, over geologic time, build the forearc structural grain, reactivating structures properly-oriented respect to the deformation field of each stage of the interplate cycle. Great subduction events may play a fundamental role in constructing the crustal architecture of extensional forearc regions. Static mechanical models of coseismic and interseismic upper plate deformation are used to explore for distinct features that could result from brittle fracturing over the two stages of the interplate cycle. I show that the semi-elliptical outline of the first-order normal faults along the Coastal Cordillera may define the location of a characteristic, long-lived megathrust segment. Finally, using data from the Global CMT catalog I analyzed the seismic behavior through time of forearc regions that have experienced great subduction ruptures >Mw7.7 worldwide. Between 61% and 83% of the cases where upper plate earthquakes exhibited periods of increased seismicity above background levels occurred contemporaneous to megathrust ruptures. That correlation is stronger for normal fault events than reverse or strike-slip crustal earthquakes. More importantly, for any given megathrust the summation of the Mw accounted by the forearc normal fault aftershocks appears to have a positive linear correlation with the Mw of the subduction earthquake -- the larger the megathrust the larger the energy released by forearc events.

A comparison of the South China Sea and Canada Basin: two small marginal ocean basins with hyper-extended margins and central zones of sea-floor spreading.

NASA Astrophysics Data System (ADS)

Li, L.

2015-12-01

Both the South China Sea and Canada Basin preserve oceanic spreading centres and adjacent passive continental margins characterized by broad COT zones with hyper-extended continental crust. We have investigated the nature of strain accommodation in the regions immediately adjacent to the oceanic spreading centres in these two basins using 2-D backstripping subsidence reconstructions, coupled with forward modelling constrained by estimates of upper crustal extensional faulting. Modelling is better constrained in the South China Sea but our results for the Beaufort Sea are analogous. Depth-dependent extension is required to explain the great depth of both basins because only modest upper crustal faulting is observed. A weak lower crust in the presence of high heat flow is suggested for both basins. Extension in the COT may continue even after sea-floor spreading has ceased. The analogous results for the two basins considered are discussed in terms of (1) constraining the timing and distribution of crustal thinning along the respective continental margins, (2) defining the processes leading to hyper-extension of continental crust in the respective tectonic settings and (3) illuminating the processes that control hyper-extension in these basins and more generally.

The Architecture of A Variscan Collisional Crust, As Revealed By The Iberseis Seismic Reflection Profile In Southwest Iberia

NASA Astrophysics Data System (ADS)

Simancas, F.; Carbonell, R.; Gonzalez-Lodeiro, F.; Perez-Estaun, A.; Ayarza, P.; Juhlin, C.; Azor, A.; Saez, R.; Martinez-Poyatos, D.; Pascual, E.

The recently acquired IBERSEIS Seismic Reflection Profile runs across major do- mains of the Variscan Orogen in SW Iberia. Geological studies indicate that the seis- mically surveyed region has been built up from three terranes, namely the South Por- tuguese Zone (SPZ), the Ossa-Morena Zone (OMZ) and the Central Iberian Zone (CIZ). These terranes became sutured after a complex, mainly transpressive (left- lateral), collisional history in Devonian-Carboniferous time. The deep seismic reflec- tion profile IBERSEIS has successfully imaged the sutures between these terranes as well as the structure of their crust. The following main features emerge from the pre- liminary integration of seismic and geological data: 1) The suture between the SPZ and OMZ terranes, marked by oceanic amphibolites, appears at present as a north- dipping left-lateral thrust merging in a mid-crustal detachment; the continuity of this suture-contact in the lower crust is not well defined in the seismic image. 2) The OMZ/CIZ suture, a shear zone with eclogites, is clearly imaged in the upper crust as a band of reflectivity dipping to the NE which, after a flat geometry in the middle crust, may continue downwards to the Moho as NE-dipping lower crustal reflections. 3) The SPZ upper crust has an imbricate structure merging into a mid-crustal detachment at constant depth in the surveyed profile. 4) The structure of the OMZ upper crust is dominated by large-scale recumbent folds affected by late upright folds, as fore- seen by geology and fully confirmed by the seismic image. 5) A general mid-crustal detachment exists in the whole surveyed area, whose geometry varies from a sharp detachment-level in the SPZ to a pinching and swelling horizontal band of reflectivity -a melting layer?- in the OMZ; in any case, a strong decoupling between upper and lower crust characterizes this transect of the Variscan orogen. 6) The lower crust of the SPZ has an intense seismic fabric, in accordance with the consideration of this ter- rane as an external orogenic domain with discrete shear bands preserved in the whole crust. 7) The lower crust of the OMZ is much less reflective than the lower crust of the SPZ. 8) The Moho is flat all along the surveyed area, which means that crustal 1 roots formed during the collisional processes were eliminated later on, probably in Late Carboniferous-Permian times. Despite the disturbance due to the generation of a post-orogenic flat Moho, the IBERSEIS seismic image seems to be a good snapshot of the Variscan collision, with very minor reworking by alpine processes. 2

Abrupt shift in δ18O values at Medicine Lake volcano (California, USA)

USGS Publications Warehouse

Donnelly-Nolan, J. M.

1998-01-01

 Oxygen-isotope analyses of lavas from Medicine Lake volcano (MLV), in the southern Cascade Range, indicate a significant change in δ18O in Holocene time. In the Pleistocene, basaltic lavas with <52% SiO2 averaged +5.9‰, intermediate lavas averaged +5.7‰, and silicic lavas (≥63.0%SiO2) averaged +5.6‰. No analyzed Pleistocene rhyolites or dacites have values greater than +6.3‰. In post-glacial time, basalts were similar at +5.7‰ to those erupted in the Pleistocene, but intermediate lavas average +6.8‰ and silicic lavas +7.4‰ with some values as high as +8.5‰. The results indicate a change in the magmatic system supplying the volcano. During the Pleistocene, silicic lavas resulted either from melting of low-18O crust or from fractionation combined with assimilation of very-low-18O crustal material such as hydrothermally altered rocks similar to those found in drill holes under the center of the volcano. By contrast, Holocene silicic lavas were produced by assimilation and/or wholesale melting of high-18O crustal material such as that represented by inclusions of granite in lavas on the upper flanks of MLV. This sudden shift in assimilant indicates a fundamental change in the magmatic system. Magmas are apparently ponding in the crust at a very different level than in Pleistocene time.

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

NASA Astrophysics Data System (ADS)

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

2012-12-01

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

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

NASA Astrophysics Data System (ADS)

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

2014-05-01

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

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.

Variation in crustal structure in Iran and the surrounding region

NASA Astrophysics Data System (ADS)

Rham, D.; Tatar, M.; Ashtiany, M.; Mokhtari, M.; Priestley, K.; Paul, A.

2007-12-01

We present a model for the topography of the Moho discontinuity for Iran and its surrounding regions. This is produced using data from field deployments within Iran by the University of Cambridge (UK) and the Universite Joseph-Fourier (FRA) in conjunction with International Institute of Earthquake Engineering and Seismology (Iran), in addition to data from IRIS and Geofone. We determine tomographic group velocity maps for periods between 10 and 60 s from multiple filter analysis of ~5500 seismograms. Because of the dense path coverage, these images have substantially higher lateral resolution for this region than is currently available from global and regional group velocity studies. Joint inversion of receiver functions and Rayleigh wave dispersion give accurate crustal velocity structures at 96 sites within Iran These provide a constraint for the less sharp crustal velocity profile produced by inverting the Rayleigh wave dispersion curve across all of Iran. We observe variations in the crustal thickness across the region, consistent with the surface topography. The thickest crust (55-60 km) is found beneath the central Zagros mountains, with the crust in the remainder of Iran having a thicknesses of 40-50 km. No significant increase in Moho depth is seen beneath the Alborz or Kopet Dagh mountains. The structure of the South Caspian Basin is presented with a different structure to that found in previous studies, with a crustal thickness of 50 km in the west, and beneath the Caucasus and Talesh mountains, in the middle part of the basin, over the course of the ~100km, this decreases to 40km, and continues to 35 km beneath the Turkmen Platform. Comparisons are also made between the joint inversion results, and accurate hypocentre depths for regional earthquakes. This shows most events occur in the upper crystalline crust (~10-20km depth), with few in the lowest velocity layer. Almost no events are located in the lower crust, and only in the Makran and Aspheron- Balkhan Sill do earthquakes appear in the Upper Mantle.

The crustal structure along the 1999 Izmit/Düzce rupture of the North-Anatolian Fault

NASA Astrophysics Data System (ADS)

Sebastian, Rost; David, Cornwell; David, Thompson; Greg, Houseman; Metin, Kahraman; Ugur, Teoman; Selda, Altuncu-Poyraz; Niyazi, Turkelli; Andrew, Frederiksen; Stephane, Rondenay; Tim, Wright

2015-04-01

Deformation along continental strike-slip faults is localized onto narrow fault zones at the surface, which may slip suddenly and catastrophically in earthquakes. On the other hand, strain in the upper mantle is more broadly distributed and is thought to occur by continuous ductile creep. The transition between these two states is poorly understood although it controls the behaviour of the fault zone during the earthquake loading cycle. To understand the structure of and strain distribution across the North-Anatolian Fault Zone (NAFZ) we deployed temporary seismic stations in the region of the 1999 Izmit (M7.5) and Düzce (M7.2) earthquakes. The rectangular array consisted of 66 seismic stations with a nominal station spacing of 7 km and seven additional stations forming a semi-circular ring towards the east (Dense Array for Northern Anatolia - DANA). Using this very dense seismic dataset and a combination of established (e.g. H-k stacking and common conversion point migration) and novel (scattering migration and scattering inversion) seismic processing techniques allows unprecedented resolution of the crustal structure in this region. This study resolves sharp changes in crustal structure across and along the surface expression of the two branches of the NAFZ at scale lengths less than 10 km at mid to lower-crustal depths. The results indicate that the northern NAFZ branch depth extent varies from the mid-crust to the upper mantle and it is likely to be less than 5 km wide throughout the crust. We furthermore resolve a high velocity lower crust and a region of crustal underthrusting that might add strength to a heterogeneous crust and may play a role in dictating the variation in faulting style and postseismic deformation in this region of the NAFZ. The results are consistent with a narrow fault zone accommodating postseismic deformation in the lower crust, as opposed to a broad ductile region below the seismogenic region of the fault.

Seismicity and Crustal Anisotropy Beneath the Western Segment of the North Anatolian Fault: Results from a Dense Seismic Array

NASA Astrophysics Data System (ADS)

Turkelli, N.; Teoman, U.; Altuncu Poyraz, S.; Cambaz, D.; Mutlu, A. K.; Kahraman, M.; Houseman, G. A.; Rost, S.; Thompson, D. A.; Cornwell, D. G.; Utkucu, M.; Gülen, L.

2013-12-01

The North Anatolian Fault (NAF) is one of the major strike slip fault systems on Earth comparable to San Andreas Fault in some ways. Devastating earthquakes have occurred along this system causing major damage and casualties. In order to comprehensively investigate the shallow and deep crustal structure beneath the western segment of NAF, a temporary dense seismic network for North Anatolia (DANA) consisting of 73 broadband sensors was deployed in early May 2012 surrounding a rectangular grid of by 70 km and a nominal station spacing of 7 km with the aim of further enhancing the detection capability of this dense seismic array. This joint project involves researchers from University of Leeds, UK, Bogazici University Kandilli Observatory and Earthquake Research Institute (KOERI), and University of Sakarya and primarily focuses on upper crustal studies such as earthquake locations (especially micro-seismic activity), receiver functions, moment tensor inversions, shear wave splitting, and ambient noise correlations. To begin with, we obtained the hypocenter locations of local earthquakes that occured within the DANA network. The dense 2-D grid geometry considerably enhanced the earthquake detection capability which allowed us to precisely locate events with local magnitudes (Ml) less than 1.0. Accurate earthquake locations will eventually lead to high resolution images of the upper crustal structure beneath the northern and southern branches of NAF in Sakarya region. In order to put additional constraints on the active tectonics of the western part of NAF, we also determined fault plane solutions using Regional Moment Tensor Inversion (RMT) and P wave first motion methods. For the analysis of high quality fault plane solutions, data from KOERI and the DANA project were merged. Furthermore, with the aim of providing insights on crustal anisotropy, shear wave splitting parameters such as lag time and fast polarization direction were obtained for local events recorded within the seismic network with magnitudes larger than 2.5.

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.

Thermal modelling of stepwise anatexis in a thrust-thickened sialic crust

USGS Publications Warehouse

Zen, E.-A.

1988-01-01

One-dimensional modelling of the thermal history of a sialic crust thickened by multiple overstack thrusting of upper crustal material shows that anatexis is likely. both the uplift rate and the length of the incubation period between end of tectonism and start of uplift are important controls on the amount and temperature of the melt. Heat of fusion does not significantly affect the long-term thermal structure of the crust if the melt is not extracted because only a small fraction of conductive heat is converted to latent heat, though short-term thermal effects of latent heat can be locally important. Model results show that commonly <15% of mantle heat flux is converted to latent heat; even during peak melting in the most productive models, less than half of incremental mantle flux is converted. The results have obvious implications on the acceptability of proposed heat sources for crustal anatexis. Fusion could retard crustal temperature rise by nearly 100??C, but the system would recover except for situations of very rapid uplift. Understanding of the thermal evolution of a burial-uplift system requires knowledge not only of the timing of anatexis but of the pooling and movement of the magma, as well as the duration and nature of the incubation period; we are poorly equipped to measure these events. The model predicts that the characteristic time for anatexis is a thickened sialic crust is several tens of millions of years, comparable to the time lapse between orogenies; in making geological interpretations of magmatism, this time lag must be considered. -Author

Crustal anisotropy from Moho converted Ps wave splitting and geodynamic implications in Northeastern margin of Tibetan Plateau

NASA Astrophysics Data System (ADS)

Xie, Z.; Wu, Q.; Zhang, R.

2017-12-01

Collision between Indian and Eurasian result in intense deformation and crustal shortening in the Tibetan Plateau. NE margin of Tibetan Plateau experienced complex deformation between Qilian orogen and its adjacent blocks, Alxa Block in the north and Ordos Block in the east. We focus on if there any evidences exist in the NE margin of Tibetan Plateau, which can support crustal channel flow model. China Earthquake Administration had deployed temporary seismic array which is called ChinaArray Phase Ⅱ, dense seismic stations covered NE margin of Tibetan Plateau. Seismic data recorded by 81 seismic stations is applied in this research. We calculated receiver functions with time-domain deconvolution. We selected RFs which have clear Ps phase both in radial and transverse components to measure Ps splitting owing to crustal anisotropy, and 130 pairs of anisotropy parameters of 51 seismic stations were obtained. We would like to discuss about dynamic mechanism of this area using crustal anisotropy associated with the result of SKS-splitting and surface constrains like GPS velocity. The result can be summarized as follows. The large scale of delay time imply that the crustal anisotropy mainly derives from middle to lower crust rather than upper crust. In the southeastern part of the research area, crustal anisotropy is well agree with the result computed form SKS-splitting and GPS velocity directions trending NWW-SEE or E-W direction. This result imply a vertically coherent deformation in the area as the directions of crustal anisotropy trend to be perpendicular to the direction of normal stress. In the middle and north part of the research area, the fast polarization direction of crustal anisotropy is NEE-SWW or E-W direction, parallels with direction of GPS velocity, but differ to the direction of the result of SKS-splitting. This result may imply that decoupled deformation in this area associated with middle to lower crustal flow.

Density of alkaline magmas at crustal and upper mantle conditions by X-ray absorption

NASA Astrophysics Data System (ADS)

Seifert, R.; Malfait, W.; Petitgirard, S.; Sanchez-Valle, C.

2011-12-01

Silicate melts are essential components of igneous processes and are directly involved in differentiation processes and heat transfer within the Earth. Studies of the physical properties of magmas (e.g., density, viscosity, conductivity, etc) are however challenging and experimental data at geologically relevant pressure and temperature conditions remain scarce. For example, there is virtually no data on the density at high pressure of alkaline magmas (e.g., phonolites) typically found in continental rift zone settings. We present in situ density measurements of alkaline magmas at crustal and upper mantle conditions using synchrotron X-ray absorption. Measurements were conducted on ID27 beamline at ESRF using a panoramic Paris-Edinburgh Press (PE Press). The starting material is a synthetic haplo-phonolite glass similar in composition to the Plateau flood phonolites from the Kenya rift [1]. The glass was synthesized at 1673 K and 2.0 GPa in a piston-cylinder apparatus at ETH Zurich and characterized using EPMA, FTIR and density measurements. The sample contains less than 200 ppm water and is free of CO2. Single-crystal diamond cylinders (Øin = 0.5 mm, height = 1 mm) were used as sample containers and placed in an assembly formed by hBN spacers, a graphite heater and a boron epoxy gasket [2]. The density was determined as a function of pressure (1.0 to 3.1 GPa) and temperature (1630-1860 K) from the X-ray absorption contrast at 20 keV between the sample and the diamond capsule. The molten state of the sample during the data collection was confirmed by X-ray diffraction measurements. Pressure and temperature were determined simultaneously from the equation of state of hBN and platinum using the the double isochor method [3].The results are combined with available density data at room conditions to derive the first experimental equation of state (EOS) of phonolitic liquids at crustal and upper mantle conditions. We will compare our results with recent reports of the density of dry and hydrous rhyolite melts (Malfait et al., this meeting) and discuss compositional effects on the density of melts and the implications for magmatic processes in the lower crust and magma chambers. [1] Hay D.E., Wendlandt R.F., 1995, J.Geophys. Res. 100, 401-410. [2] van Kan Parker M. et al., 2010, High Pressure Research, 30: 2, 332 - 341. [3] W.A. Chrichton, M. Mezouar, 2002, High Temp.-High Press, 34, 235.

Three-dimensional crust and mantle structure of Kilauea Volcano, Hawaii

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

Ellsworth, W.L.; Koyanagi, R.Y.

1977-11-10

Teleseismic P wave arrival times recorded by a dense network of seismograph stations located on Kilauea volcano, Hawaii, are inverted to determine lateral variation in crust and upper mantle structure to a depth of 70 km. The crustal structure is dominated by relatively high velocities within the central summit complex and along the two radial rift zones compared with the nonrift flank of the volcano. Both the mean crustal velocity contrast between summit and nonrift flank and the distribution of velocities agree well with results from crustal refraction studies. Comparison of the velocity structure with Bouguer gravity anomalies over themore » volcano through a simple physical model also gives excellent agreement. Mantle structure appears to be more homogeneous than crustal structure. The root mean square velocity variation for the mantle averages only 1.5%, whereas variation within the crust exceeds 4%. The summit of Kilauea is underlain by normal velocity (8.1 km/s) material within the uppermost mantle (12--25 km), suggesting that large magma storage reservoirs are not present at this level and that the passageways from deeper sources must be quite narrow. No evidence is found for substantial volumes of partially molten rock (5%) within the mantle to depths of at least 40 km. Below about 30 km, low-velocity zones (1--2%) underlie the summits of Kilauea and nearby Mauna Loa and extend south of Kilauea into a broad offshore zone. Correlation of volcanic tremor source locations and persistent zones of mantle earthquakes with low-velocity mantle between 27.5- and 42.5-km depth suggests that a laterally extensive conduit system feeds magma to the volcanic summits from sources either at comparable depth or deeper within the mantle. The center of contemporary magmatic production and/or upwelling from deeper in the mantle appears to extend well to the south of the active volcanic summits, suggesting that the Hawaiian Island chain is actively extending to the southeast.« less

Role of the Yakutat collision and upper mantle dynamics in the present-day tectonics of the North America Northern Cordillera

NASA Astrophysics Data System (ADS)

Mazzotti, S.; Tarayoun, A.; Marechal, A.; Audet, P.

2017-12-01

The Northern Cordillera of North America is a type example of present-day strain distribution across a wide orogeny. Several geodynamic models are proposed to explain this large-scale tectonic activity, with two main end-members: strain transfer from the Yakutat collision zone (orogenic float) and strain transfer from upper mantle convection (lithosphere basal traction). One of the main differences between these is the lithosphere vertical rheology profile: the former requires significant crust - mantle decoupling to allow far field strain transfer, whereas the latter requires a vertically coupled lithosphere. Here we combine recent data across the eastern region of the Northern Cordillera (eastern Alaska, Yukon, western Northwest Territories) to characterize its states of strain rate, stress, and crustal and lithospheric structure, in order to test the role of the Yakutat collision and upper mantle convection in its present-day tectonics. Recent GPS data confirm the radial, east- to northeastward motion of the central Yukon and foreland belt (Mackenzie and Richardson Mountains), albeit at a much lower velocity than previously proposed. This motion is primarily accommodated by E-W to NE-SW shortening, mainly in the foreland belt, and small to near-zero lateral motion on the major Denali and Tintina strike-slip faults. Seismic anisotropy data further suggest that these two major faults, like most of the Yukon Cordillera, have kept their early Cenozoic crustal and upper mantle structures, as shown by the fault-parallel (NW-SE) fast anisotropy orientation. We use these new data, combined with numerical models of strain distribution under various boundary conditions, to provide constraints on the respective role of the Yakutat collision and upper mantle convection in the present-day tectonics. Preliminary results suggest that, whichever the driving mechanism (or combination thereof), the total strain associated with the present-day tectonics must remain small in order to preserve the inherited crustal and mantle fabrics. Such small cumulative strain appears in contradiction with a thin decoupling layer (such as lower crust decoupling in the orogenic float model) and seems more suggestive of distributed shear across a large part of the lithosphere.

The upper crust laid on its side: tectonic implications of steeply tilted crustal slabs for extension in the basin and range

USGS Publications Warehouse

Howard, Keith A.

2005-01-01

Tilted slabs expose as much as the top 8–15 km of the upper crust in many parts of the Basin and Range province. Exposures of now-recumbent crustal sections in these slabs allow analysis of pre-tilt depth variations in dike swarms, plutons, and thermal history. Before tilting the slabs were panels between moderately dipping, active Tertiary normal faults. The slabs and their bounding normal faults were tilted to piggyback positions on deeper footwalls that warped up isostatically beneath them during tectonic unloading. Stratal dips within the slabs are commonly tilted to vertical or even slightly overturned, especially in the southern Basin and Range where the thin stratified cover overlies similarly tilted basement granite and gneiss. Some homoclinal recumbent slabs of basement rock display faults that splay upward into forced folds in overlying cover sequences, which thereby exhibit shallower dips. The 15-km maximum exposed paleodepth for the slabs represents the base of the brittle upper crust, as it coincides with the depth of the modern base of the seismogenic zone and the maximum focal depths of large normal-fault earthquakes in the Basin and Range. Many upended slabs accompany metamorphic core complexes, but not all core complexes have corresponding thick recumbent hanging-wall slabs. The Ruby Mountains core complex, for example, preserves only scraps of upper-plate rocks as domed-up extensional klippen, and most of the thick crustal section that originally overlay the uplifted metamorphic core now must reside below little-tilted hanging-wall blocks in the Elko-Carlin area to the west. The Whipple and Catalina Mountains core complexes in contrast are footwall to large recumbent hanging-wall slabs of basement rock exposing 8-15 km paleodepths that originally roofed the metamorphic cores; the exposed paleodepths require that a footwall rolled up beneath the slabs.

The crustal and mantle velocity structure in central Asia from 3D traveltime tomography

NASA Astrophysics Data System (ADS)

Sun, Y.; Martin, R. V.; Toksoz, M. N.; Pei, S.

2010-12-01

The lithospheric structure in central Asia features large blocks such as the Indian plate, the Afghan block, the Turan plate, and the Tarim block. This geologically and tectonically complicated area is also one of the most seismically active regions in the world. We developed P- and S- wave velocity structures of the central Asia in the crust using the traveltime data from Kyrgyzstan, Tajikistan, Kazakhstan, and Uzbek. We chose the events and stations between 32N65E and 45N85E and focused on the areas of Pamir and western Tianshan. In this data set, there are more than 6000 P and S arrivals received at 80 stations from about 300 events. The double difference tomography is applied to relocate events and to invert for seismic structures simultaneously. Our results provide accurate locations of earthquakes and high resolution crustal structure in this region. To extend the model deeper into the mantle through the upper mantle transition zone, ISC/EHB data for P and PP phases are combined with the ABCE data. To counteract the “smearing effect,” the crust and upper mantle velocity structure, derived from regional travel-times, is used. An adaptive grid method based on ray density is used in the inversion. A P-wave velocity model extending down to a depth of 2000 km is obtained. regional-teleseismic tomography provides a high-resolution, 3-D P-wave velocity model for the crust, upper mantle, and the transition zone. The crustal models correlate well with geologic and tectonic features. The upper mantle tomograms show the images of Tian Shan. The slab geometry is quite complex, reflecting the history of the changes in the plate motions and collision processes. Vp/Vs tomography was also determined in the study region, and an attenuation tomography was obtained as well.

H, O, Sr, Nd, and Pb isotope geochemistry of the Latir volcanic field and cogenetic intrusions, New Mexico, and relations between evolution of a continental magmatic center and modifications of the lithosphere

USGS Publications Warehouse

Johnson, C.M.; Lipman, P.W.; Czamanske, G.K.

1990-01-01

Over 200 H, O, Sr, Nd, and Pb isotope analyses, in addition to geologic and petrologic constraints, document the magmatic evolution of the 28.5-19 Ma Latir volcanic field and associated intrusive rocks, which includes multiple stages of crustal assimilation, magma mixing, protracted crystallization, and open- and closed-system evolution in the upper crust. In contrast to data from younger volcanic centers in northern New Mexico, relatively low and restricted primary ??18O values (+6.4 to +7.4) rule out assimilation of supracrustal rocks enriched in 18O. Initial 87Sr/86Sr ratios (0.705 to 0.708), ??18O values (-2 to-7), and 206Pb/204Pb ratios (17.5 to 18.4) of metaluminous precaldera volcanic rocks and postcaldera plutonic rocks suggest that most Latir rocks were generated by fractional crystallization of substantial volumes of mantle-derived basaltic magma that had near-chondritic Nd isotope ratios, accompanied by assimilation of crustal material in two main stages: 1) assimilation of non-radiogenic lower crust, followed by 2) assimilation of middle and upper crust by inter-mediate-composition magmas that had been contaminated during the first stage. Magmatic evolution in the upper crust peaked with eruption of the peralkaline Amalia Tuff (???26 Ma), which evolved from metaluminous parental magmas. A third stage of late, roofward assimilation of Proterozoic rocks in the Amalia Tuff magma is indicated by trends in initial 87Sr/86Sr and 206Pb/204Pb ratios from 0.7057 to 0.7098 and 19.5 to 18.8, respectively, toward the top of the pre-eruptive magma chamber. Highly evolved postcaldera plutons are generally fine grained and are zoned in initial 87Sr/86Sr and 206Pb/204Pb ratios, varying from 0.705 to 0.709 and 17.8 to 18.6, respectively. In contrast, the coarser-grained Cabresto Lake (???25 Ma) and Rio Hondo (???21 Ma) plutons have relatively homogeneous initial 87Sr/86Sr and 206Pb/204Pb ratios of approximately 0.7053 and 17.94 and 17.55, respectively. ??18O values for all the postcaldera plutons overlap those of the precaldera rocks and Amalia Tuff, except for those for two late-stage rhyolite dikes associated with the Rio Hondo pluton that have ??18O values of-8.6 and-9.5; these dikes are the only Latir rocks which may be largely crustal melts. Chemical and isotopic data from the Latir field suggest that large fluxes of mantle-derived basaltic magma are necessary for developing and sustaining large-volume volcanic centers. Development of a detailed model suggests that 6-15 km of new crust may have been added beneath the volcanic center; such an addition may result in significant changes in the chemical and Sr and Nd isotopic compositions of the crust, although Pb isotope ratios will remain relatively unchanged. If accompanied by assimilation, crystallization of pooled basaltic magma near the MOHO may produce substantial cumulates beneath the MOHO that generate large changes in the isotopic composition of the upper mantle. The Latir field may be similar to other large-volume, long-lived intracratonal volcanic fields that fundamentally owe their origins to extensive injection of basaltic magma into the lower parts of their magmatic systems. Such fields may overlie areas of significant crustal growth and hybridization. ?? 1990 Springer-Verlag.

Geoid, topography, and convection-driven crustal deformation on Venus

NASA Technical Reports Server (NTRS)

Simons, Mark; Hager, Bradford H.; Solomon, Sean C.

1992-01-01

High-resolution Magellan images and altimetry of Venus reveal a wide range of styles and scales of surface deformation that cannot readily be explained within the classical terrestrial plate tectonic paradigm. The high correlation of long-wavelength topography and gravity and the large apparent depths of compensation suggest that Venus lacks an upper-mantle low-viscosity zone. A key difference between Earth and Venus may be the degree of coupling between the convecting mantle and the overlying lithosphere. Mantle flow should then have recognizable signatures in the relationships between surface topography, crustal deformation, and the observed gravity field.

Olivine-bearing lithologies on the Moon: Constraints on origins and transport mechanisms from M3 spectroscopy, radiative transfer modeling, and GRAIL crustal thickness

NASA Astrophysics Data System (ADS)

Corley, Laura M.; McGovern, Patrick J.; Kramer, Georgiana Y.; Lemelin, Myriam; Trang, David; Gillis-Davis, Jeffrey J.; Taylor, G. Jeffrey; Powell, Kathryn E.; Kiefer, Walter S.; Wieczorek, Mark; Zuber, Maria T.

2018-01-01

High-resolution hyperspectral data from Chandrayaan-1's Moon Mineralogy Mapper (M3) allow detection of olivine on the lunar surface. Olivine exposed at the surface may originate as mantle material or igneous products (intrusive or extrusive). Potential transport mechanisms include excavation of the mantle or lower crustal material by impacts that form basins and complex craters, differentiation of impact melt sheets, or magmatic emplacement of lavas, cumulates, or xenoliths. A sample of the lunar mantle, which has not been conclusively identified in the lunar sample collection, would yield fundamental new insights into the composition, structure, and evolution of the lunar interior. Olivine identified in remote spectral data is generally accepted to originate from the primary mantle, because abundant olivine is expected to exist in the mantle and lower crust, yet have sparse occurrences in the upper crust. In this study, we identified 111 M3 single-pixel spectra with characteristic absorption features consistent with olivine at Crisium, Nectaris, and Humorum basins and near the craters Roche and Tsiolkovsky. In an effort to determine the origins and transport mechanisms that led to these individual exposures, we estimated mineral abundances using radiative transfer modeling and examined crustal thickness estimates, topography and slope maps, and images from the Lunar Reconnaissance Orbiter Camera (LROC). At Crisium basin, where crustal thickness is near 0 km (Wieczorek et al., 2013), mantle olivine may have been exposed by basin-forming impact and deposited on the rim. Picard crater, which is superposed on the floor of Crisium, also exhibits potential mantle olivine in its ejecta. Within Nectaris basin, olivine exposures are confined to the rims of small craters on the mare, which are inferred to excavate a layer of olivine-rich mare basalt. Olivine occurrences on the rim of Humorum basin, including those located on a graben, are likely to be cumulates of shallow intrusions that were transported magmatically to the surface. Near Roche crater, olivine may have originated in shallow dikes that reached the subsurface and were exposed by impacts. In addition to verifying both known and previously unidentified olivine exposures, our combined geophysical, spectral, and radiative transfer modeling investigation has allowed identification of both igneous and mantle-derived olivine.

SH wave structure of the crust and upper mantle in southeastern margin of the Tibetan Plateau from teleseismic Love wave tomography

NASA Astrophysics Data System (ADS)

Fu, Yuanyuan V.; Jia, Ruizhi; Han, Fengqin; Chen, Anguo

2018-06-01

The deep structure of southeastern Tibet is important for determining lateral plateau expansion mechanisms, such as movement of rigid crustal blocks along large strike-slip faults, continuous deformation or the eastward crustal channel flow. We invert for 3-D isotropic SH wave velocity model of the crust and upper mantle to the depth of 110 km from Love wave phase velocity data using a best fitting average model as the starting model. The 3-D SH velocity model presented here is the first SH wave velocity structure in the study area. In the model, the Tibetan Plateau is characterized by prominent slow SH wave velocity with channel-like geometry along strike-slip faults in the upper crust and as broad zones in the lower crust, indicating block-like and distributed deformation at different depth. Positive radial anisotropy (VSH > VSV) is suggested by a high SH wave and low SV wave anomaly at the depths of 70-110 km beneath the northern Indochina block. This positive radial anisotropy could result from the horizontal alignment of anisotropic minerals caused by lithospheric extensional deformation due to the slab rollback of the Australian plate beneath the Sumatra trench.

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.

Two-layer Crustal Structure of the Contiguous United States from Joint Inversion of USArray Receiver Functions and Gravity

NASA Astrophysics Data System (ADS)

Ma, X.; Lowry, A. R.

2015-12-01

The composition and thickness of crustal layering is fundamental to understanding the evolution and dynamics of continental lithosphere. Lowry and Pérez-Gussinyé (2011) found that the western Cordillera of the United States, characterized by active deformation and high heat flow, is strongly correlated with low bulk crustal seismic velocity ratio. They interpreted this observation as evidence that quartz controls continental tectonism and deformation. We will present new imaging of two-layer crustal composition and structure from cross-correlation of observed receiver functions and model synthetics. The cross-correlation coefficient of the two-layer model increases significantly relative to an assumed one-layer model, and the lower crustal thickness map from raw two-layer modeling (prior to Bayesian filtering with gravity models and Optimal Interpolation) clearly shows Colorado plateau and Appalachian boundaries, which are not apparent in upper crustal models, and also the high vP/vS fill the most of middle continental region while low vP/vS are on the west and east continental edge. In the presentation, we will show results of a new algorithm for joint Bayesian inversion of thickness and vP/vS of two-layer continental crustal structure. Recent thermodynamical modeling of geophysical models based on lab experiment data (Guerri et al., 2015) found that a large impedance contrast can be expected in the midcrust due to a phase transition that decreases plagioclase and increases clinopyroxene, without invoking any change in crustal chemistry. The depth of the transition depends on pressure, temperature and hydration, and in this presentation we will compare predictions of layer thicknesses and vP/vS predicted by mineral thermodynamics to those we observe in the USArray footprint.

The Chaotic Terrains of Mercury: A History of Large-Scale Crustal Devolatilization

NASA Astrophysics Data System (ADS)

Rodriguez, J. A. P.; Domingue, D. L.; Berman, D. C.; Kargel, J. S.; Baker, V. R.; Teodoro, L. F.; Banks, M.; Leonard, G.

2018-05-01

Approximately 400 million years after the Caloris basin impact, extensive collapse formed Mercury's chaotic terrains. Collapse likely resulted from regionally elevated heat flow devolatilizing crustal materials along NE and NW extensional faults.

Continent-Wide Maps of Lg Coda Q Variation and Rayleigh-wave Attenuation Variation for Eurasia

DTIC Science & Technology

2007-01-30

lithosphere and crustal strain lead us to infer that fluids, originating by hydrothermal release from subducting lithosphere or other upper mantle heat...relatively low Qo values in the Arabian Peninsula are produced by fluids that have been released in the upper mantle by hydrothermal processes and have...Advection of plumes in mantle flow: Implications for hotspot motion, mantle viscosity and plume distribution, Geophys. J. Int., 132, 412–434. Talebian, M

Solar Wind Interaction and Crustal Field Influences on Mars' Upper Ionosphere: MAVEN Observations Compared to Model Results

NASA Astrophysics Data System (ADS)

Luhmann, J. G.; Alvarez, K.; Curry, S.; Dong, C.; Ma, Y.; Bougher, S. W.; Benna, M.; Elrod, M. K.; Mahaffy, P. R.; Withers, P.; Girazian, Z.; Connerney, J. E. P.; Brain, D.; Jakosky, B. M.

2016-12-01

Since the two Viking Landers, progress on improving our global knowledge of the Martian ionosphere's characteristics has been limited by the available instrumentation and sampling geometries. In particular, while remote sensing and the lower energy plasma spectrometer observations on missions including MGS and MEX provided insights on the effects of the crustal magnetic fields of Mars and the solar wind interaction, these measurements did not allow the broader thermal ion surveys necessary to test our current understanding of the region between the exobase at 200 km altitude and the solar wind interaction boundary. In this study we use the MAVEN NGIMS thermal ion mass spectrometer observations from the prime mission year 2015 to construct some statistical pictures of the increasingly collisionless region of the ionosphere between 200 and 500 km where crustal field and solar wind interaction effects should begin to dominate its behavior. Comparisons with models of the solar wind interaction with Mars provide important global context for these observations, including the roles of system diversity associated with changing crustal field and interplanetary field orientations.

Deciphering the Tectonic History of the Northern Transantarctic Mountains

NASA Astrophysics Data System (ADS)

Hansen, Samantha; Graw, Jordan; Brenn, Gregory; Kenyon, Lindsey; Park, Yongcheol; DuBay, Brian

2016-04-01

The Transantarctic Mountains (TAMs) are the largest non-compressional mountain range in the world, and their structure plays a key role in the climatic and tectonic development of Antarctica. While numerous uplift mechanisms for the TAMs have been proposed, there is little consensus on their origin. Over the past three years, we have operated a network of 15 broadband seismic stations within a previously unexplored portion of the northern TAMs. Using data collected by this array, we have undertaken numerous studies to further assess the crustal and lithospheric structure beneath the mountain range and to differentiate between competing origin models. Receiver functions indicate crustal thickening inland from the Ross Sea coast but comparable crustal thickness beneath the TAMs and the East Antarctic plateau, indicating little evidence for a substantial crustal root beneath the mountain range. Body and surface wave analyses show a pronounced low-velocity anomaly beneath Terror Rift, adjacent to the TAMs, and extending beneath Victoria Land in the upper mantle. Together, these findings support a thermally-buoyant source of uplift for the northern TAMs and broad flexure of the East Antarctic lithosphere.

Continents as lithological icebergs: The importance of buoyant lithospheric roots

USGS Publications Warehouse

Abbott, D.H.; Drury, R.; Mooney, W.D.

1997-01-01

An understanding of the formation of new continental crust provides an important guide to locating the oldest terrestrial rocks and minerals. We evaluated the crustal thicknesses of the thinnest stable continental crust and of an unsubductable oceanic plateau and used the resulting data to estimate the amount of mantle melting which produces permanent continental crust. The lithospheric mantle is sufficiently depleted to produce permanent buoyancy (i.e., the crust is unsubductable) at crustal thicknesses greater than 25-27 km. These unsubductable oceanic plateaus and hotspot island chains are important sources of new continental crust. The newest continental crust (e.g., the Ontong Java plateau) has a basaltic composition, not a granitic one. The observed structure and geochemistry of continents are the result of convergent margin magmatism and metamorphism which modify the nascent basaltic crust into a lowermost basaltic layer overlain by a more silicic upper crust. The definition of a continent should imply only that the lithosphere is unsubductable over ??? 0.25 Ga time periods. Therefore, the search for the oldest crustal rocks should include rocks from lower to mid-crustal levels.

Crustal earthquake triggering by pre-historic great earthquakes on subduction zone thrusts

USGS Publications Warehouse

Sherrod, Brian; Gomberg, Joan

2014-01-01

Triggering of earthquakes on upper plate faults during and shortly after recent great (M>8.0) subduction thrust earthquakes raises concerns about earthquake triggering following Cascadia subduction zone earthquakes. Of particular regard to Cascadia was the previously noted, but only qualitatively identified, clustering of M>~6.5 crustal earthquakes in the Puget Sound region between about 1200–900 cal yr B.P. and the possibility that this was triggered by a great Cascadia thrust subduction thrust earthquake, and therefore portends future such clusters. We confirm quantitatively the extraordinary nature of the Puget Sound region crustal earthquake clustering between 1200–900 cal yr B.P., at least over the last 16,000. We conclude that this cluster was not triggered by the penultimate, and possibly full-margin, great Cascadia subduction thrust earthquake. However, we also show that the paleoseismic record for Cascadia is consistent with conclusions of our companion study of the global modern record outside Cascadia, that M>8.6 subduction thrust events have a high probability of triggering at least one or more M>~6.5 crustal earthquakes.

Crustal thickness and Vp/Vs beneath the southeastern United States: Constraints from receiver function stacking

NASA Astrophysics Data System (ADS)

Yang, Q.; Gao, S. S.; Liu, K. H.

2017-12-01

To provide new constraints on crustal structure and evolution models beneath a collage of tectonic provinces in the southeastern United States, a total of 10,753 teleseismic receiver functions recorded by 125 USArray and other seismic stations are used to compute crustal thickness and Vp/Vs values. The resulting crustal thicknesses range from 25 km at the coast to 51 km beneath the peak of the southern Appalachians with an average of 36.2 km ± 5.5 km. The resulting crustal thicknesses correlate well with surface elevation and Bouguer gravity anomalies. Beneath the Atlantic Coastal Plain, the crustal thicknesses show a clear eastward thinning with a magnitude of 10 km, from about 40 km beneath the western margin to 30 km beneath the coast. The Vp/Vs values for the entire study area range from 1.71 to 1.90 with a mean value of 1.80 ± 0.04. The mean Vp/Vs value is 1.82±0.035 in the southern Appalachian Mountain. The slightly larger than normal crustal Vp/Vs for this area might be the result of significant erosion of the felsic upper crust over the past 300 million years. Alternatively, it could also suggest the existence of pervasive magmatic intrusion into the Appalachian crust. The Vp/Vs measurements in the Atlantic Coastal Plain increase toward the east, ranging from 1.75 to 1.82, probably indicating a gradual increase of mafic magmatic intrusion into thinner crust during the development of the passive continental margin.

Petrology and geochronology of crustal xenoliths from the Bering Strait region: Linking deep and shallow processes in extending continental crust

USGS Publications Warehouse

Akinin, V.V.; Miller, E.L.; Wooden, J.L.

2009-01-01

Petrologic, geochemical, and metamorphic data on gneissic xenoliths derived from the middle and lower crust in the Neogene Bering Sea basalt province, coupled with U-Pb geochronology of their zircons using sensitive high-resolution ion microprobe-reverse geometry (SHRIMP-RG), yield a detailed comparison between the P-T-t and magmatic history of the lower crust and magmatic, metamorphic, and deformational history of the upper crust. Our results provide unique insights into the nature of lithospheric processes that accompany the extension of continental crust. The gneissic, mostly maficxenoliths (constituting less than two percent of the total xenolith population) from lavas in the Enmelen, RU, St. Lawrence, Nunivak, and Seward Peninsula fields most likely originated through magmatic fractionation processes with continued residence at granulite-facies conditions. Zircon single-grain ages (n ??? 125) are interpreted as both magmatic and metamorphic and are entirely Cretaceous to Paleocene in age (ca. 138-60 Ma). Their age distributions correspond to the main ages of magmatism in two belts of supracrustal volcanic and plutonic rocks in the Bering Sea region. Oscillatory-zoned igneous zircons, Late Cretaceous to Paleocene metamorphic zircons and overgrowths, and lack of any older inheritance in zircons from the xenoliths provide strong evidence for juvenile addition of material to the crust at this time. Surface exposures of Precambrian and Paleozoic rocks locally reached upper amphibolite-facies (sillimanite grade) to granulite-facies conditions within a series of extension-related metamorphic culminations or gneiss domes, which developed within the Cretaceous magmatic belt. Metamorphic gradients and inferred geotherms (??30-50 ??C/km) from both the gneiss domes and xenoliths aretoo high to be explained by crustal thickening alone. Magmatic heat input from the mantle is necessary to explain both the petrology of the magmas and elevated metamorphic temperatures. Deep-crustal seismic-reflection and refraction data reveal a 30-35-km-thick crust, a sharp Moho and refl ective lower and middle crust. Velocities do not support a largely mafic (underplated) lower crust, but together with xenolith data suggest that Late Cretaceous to early Paleocene maficintrusions are likely increasingly important with depth in the crust and that the elevated temperatures during granulite-facies metamorphism led to large-scale flow of crustal rocks to produce gneiss domes and the observed subhorizontal refl ectivity of the crust. This unique combined data set for the Bering Shelf region provides compelling evidence for the complete reconstitution/re-equilibration of continental crust from the bottom up during mantle-driven magmatic events associated with crustal extension. Thus, despite Precambrian and Paleozoic rocks at the surface and Alaska's accretionary tectonic history, it is likely that a significant portion of the Bering Sea region lower crust is much younger and related to post-accretionary tectonic and magmatic events. ?? 2009 The Geological Society of America.

The Jeffers Brook diorite-granodiorite pluton: style of emplacement and role of volatiles at various crustal levels in Avalonian appinites, Canadian Appalachians

NASA Astrophysics Data System (ADS)

Pe-Piper, Georgia; Piper, David J. W.

2018-04-01

Small appinite plutons ca. 610 Ma outcrop in the peri-Gondwanan Avalon terrane of northern Nova Scotia, with different structural levels exposed. Field mapping shows that the Jeffers Brook pluton is a laccolith emplaced along an upper crustal thrust zone, likely in a dilational jog in a regional dextral strike-slip system. The oldest rocks are probably mafic sills, which heated the area facilitating emplacement of intermediate magmas. Cross-cutting relationships show that both mafic and intermediate magmas were supplied throughout the history of pluton emplacement. The modal composition, mineral chemistry, and bulk chemistry of gabbro, diorite, tonalite, granodiorite, and granite have been studied in the main plutonic phases, dykes, and sills, and mafic microgranular enclaves. As with the type appinites in the Scottish Caledonides, the pluton shows evidence of high water content: the dominance of hornblende, locally within pegmatitic texture; vesicles and irregular felsic patches in enclaves; and late aplite dykes. Analyzed mafic microgranular enclaves are geochemically similar to larger diorite bodies in the pluton. Tonalite-granodiorite is distinct from the diorite in trace-element geochemistry and radiogenic isotopes. Elsewhere to the east, similar rocks of the same age form vertically sheeted complexes in major shear zones; hornblende chemistry shows that they were emplaced at a deeper upper crustal level. This implies that little of the observed geochemical variability in the Jeffers Brook pluton was developed within the pluton. The general requirements to form appinites are proposed to be small magma volumes of subduction-related magmas that reach the upper crust because of continual heating by mafic magmas moving through strike-slip fault pathways and trapping of aqueous fluids rather than venting through volcanic activity.

Seismic Waveform Tomography of the Iranian Region

NASA Astrophysics Data System (ADS)

Maggi, A.; Priestley, K.; Jackson, J.

2001-05-01

Surprisingly little is known about the detailed velocity structure of Iran, despite the region's importance in the tectonics of the Middle East. Previous studies have concentrated mainly on fundamental mode surface wave dispersion measurements along isolated paths (e.g.~Asudeh, 1982; Cong & Mitchell, 1998; Ritzwoller et.~al, 1998), and the propagation characteristics of crust and upper mantle body waves (e.g. Hearn & Ni 1994; Rodgers et.~al 1997). We use the partitioned waveform inversion method of Nolet (1990) on several hundred regional waveforms crossing the Iranian region to produce a 3-D seismic velocity map for the crust and upper mantle of the area. The method consists of using long period seismograms from earthquakes with well determined focal mechanisms and depths to constrain 1-D path-averaged shear wave models along regional paths. The constraints imposed on the 1-D models by the seismograms are then combined with independent constraints from other methods (e.g.~Moho depths from reciever function analysis etc.), to solve for the 3-D seismic velocity structure of the region. A dense coverage of fundamental mode rayleigh waves at a period of 100~s ensures good resolution of lithospheric scale structure. We also use 20~s period fundamental mode rayleigh waves and some Pnl wavetrains to make estimates of crustal thickness variations and average crustal velocities. A few deeper events give us some coverage of higher mode rayleigh waves and mantle S waves, which sample to the base of the upper mantle. Our crustal thickness estimates range from 45~km in the southern Zagros mountains, to 40~km in central Iran and 35~km towards the north of the region. We also find inconsistencies between the 1-D models required to fit the vertical and the tranverse seismograms, indicating the presence of anisotropy.

Can compliant fault zones be used to measure absolute stresses in the upper crust?

NASA Astrophysics Data System (ADS)

Hearn, E. H.; Fialko, Y.

2009-04-01

Geodetic and seismic observations reveal long-lived zones with reduced elastic moduli along active crustal faults. These fault zones localize strain from nearby earthquakes, consistent with the response of a compliant, elastic layer. Fault zone trapped wave studies documented a small reduction in P and S wave velocities along the Johnson Valley Fault caused by the 1999 Hector Mine earthquake. This reduction presumably perturbed a permanent compliant structure associated with the fault. The inferred changes in the fault zone compliance may produce a measurable deformation in response to background (tectonic) stresses. This deformation should have the same sense as the background stress, rather than the coseismic stress change. Here we investigate how the observed deformation of compliant zones in the Mojave Desert can be used to constrain the fault zone structure and stresses in the upper crust. We find that gravitational contraction of the coseismically softened zones should cause centimeters of coseismic subsidence of both the compliant zones and the surrounding region, unless the compliant fault zones are shallow and narrow, or essentially incompressible. We prefer the latter interpretation because profiles of line of sight displacements across compliant zones cannot be fit by a narrow, shallow compliant zone. Strain of the Camp Rock and Pinto Mountain fault zones during the Hector Mine and Landers earthquakes suggests that background deviatoric stresses are broadly consistent with Mohr-Coulomb theory in the Mojave upper crust (with μ ≥ 0.7). Large uncertainties in Mojave compliant zone properties and geometry preclude more precise estimates of crustal stresses in this region. With improved imaging of the geometry and elastic properties of compliant zones, and with precise measurements of their strain in response to future earthquakes, the modeling approach we describe here may eventually provide robust estimates of absolute crustal stress.

Numerical modelling of the role of salt in continental collision: An application to the southeast Zagros fold-and-thrust belt

NASA Astrophysics Data System (ADS)

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

2014-09-01

The Zagros fold-and-thrust belt formed in the collision of Arabia with Central Iran. Its sedimentary sequence is characterised by the presence of several weak layers that may control the style of folding and thrusting. We use 2-D thermo-mechanical models to investigate the role of salt in the southeast Zagros fold-and-thrust belt. We constrain the crustal and lithospheric thickness, sedimentary stratification, convergence velocity, and thermal structure of the models from available geological and geophysical data. We find that the thick basal layer of Hormuz salt in models on the scale of the upper-mantle decouples the overlying sediments from the basement and localises deformation in the sediments by trench-verging shear bands. In the collision stage of the models, basement dips with + 1° towards the trench. Including the basal Hormuz salt improves the fit of predicted topography to observed topography. We use the kinematic results and thermal structure of this large-scale model as the initial conditions of a series of upper-crustal-scale models. These models aim to investigate the effects of basal and intervening weak layers, salt strength, basal dip, and lateral salt distribution on deformation style of the simply folded Zagros. Our results show that in addition to the Hormuz salt at the base of the sedimentary cover, at least one intervening weak layer is required to initiate fold-dominated deformation in the southeast Zagros. We find that an upper-crustal-scale model, with a basal and three internal weak layers with viscosities between 5 × 1018 and 1019 Pa s, and a basement that dips + 1° towards the trench, best reproduces present-day topography and the regular folding of the sedimentary layers of the simply folded Zagros.

Upper crustal emplacement and deformation of granitoids inside the Uppermost Unit of the Cretan nappe stack: constraints from U-Pb zircon dating, microfabrics and paleostress analyses

NASA Astrophysics Data System (ADS)

Kneuker, Tilo; Dörr, Wolfgang; Petschick, Rainer; Zulauf, Gernold

2015-03-01

The present study is dealing with the emplacement and deformation of diorite and quartz diorite exposed along new road cuts between Agios Nikolaos and Sitia (Uppermost Unit, eastern Crete). Mingling of both melt types is indicated by enclaves of diorite inside quartz diorite and vice versa. The diorite and quartz diorite intruded into coarse-grained white marble, which is in lateral contact to, but also forms the roof of, the intrusive body. Evidence for contact metamorphism is indicated by increasing grain size of calcite in the marble with decreasing distance from the diorite. U-Pb (TIMS) dating of zircons, separated from quartz diorite, yielded a concordant age at 74.0 ± 0.25 Ma, which is interpreted as emplacement age. As this age is close to published K-Ar cooling ages of hornblende and biotite, the melt should have intruded and cooled down rapidly at upper structural levels, which is not common for granitoids of the Uppermost Unit of Crete. Upper crustal melt emplacement is also documented by stoped blocks and by the lack of any ductile (viscous) deformation. The diorite and quartz diorite, however, are affected by strong post-Oligocene brittle faulting. Paleostress analysis, based on these faults, revealed a change in stress field from N-S and NNW-SSE shortening by thrusting (convergence between African and European plates) to NNE-SSW and NE-SW shortening accommodated by strike-slip (SW-ward extrusion of the Anatolian microplate). Calcite-twin density indicates high differential stress (260 ± 20 MPa) related to these phases of crustal shortening.

Teleseismic surface wave study of S-wave velocity structure in Southern California

NASA Astrophysics Data System (ADS)

Prindle-Sheldrake, K. L.; Tanimoto, T.

2002-12-01

We report on a 3D S-wave velocity structure derived from teleseismic Rayleigh and Love waves using TriNet broadband seismic data. Phase velocity maps, constructed between 20 and 55 mHz for Rayleigh waves and between 25 and 45 mHz for Love waves, were inverted for S-wave velocity structure at depth. Our starting model is SCEC 2.2, which has detailed crustal structure, but laterally homogeneous upper mantle structure. Depth resolution from the data set is good from the surface to approximately 100 km, but deteriorates rapidly beyond this depth. Our analysis indicates that, while Rayleigh wave data are mostly sensitive to mantle structure, Love wave data require some modifications of crustal structure from SCEC 2.2 model. Various regions in Southern California have different seismic-velocity signatures in terms of fast and slow S-wave velocities: In the Southern Sierra, both the crust and mantle are slow. In the Mojave desert, mid-crustal depths tend to show slow velocities, which are already built into SCEC 2.2. In the Transverse Ranges, the lower crust and mantle are both fast. Our Love wave results require much faster crustal velocity than those in SCEC 2.2 in this region. In the Peninsular ranges, both the crust and mantle are fast with mantle fast velocity extending to about 70 km. This is slightly more shallow than the depth extent under the Transverse Ranges, yet it is surprisingly deep. Under the Salton Sea, the upper crust is very slow and the upper mantle is also slow. However, these two slow velocity layers are separated by faster velocity lower crust which creates a distinct contrast with respect to the adjacent slow velocity regions. Existence of such a relatively fast layer, sandwiched by slow velocities, are related to features in phase velocity maps, especially in the low frequency Love wave phase velocity map (25 mHz) and the high frequency Rayleigh wave phase velocity maps (above 40 mHz). Such a feature may be related to partial melting processes under the Salton Sea.

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.

Constraints on crustal structure in the Southeastern United States from the SUGAR 2 refraction seismic refraction experiment

NASA Astrophysics Data System (ADS)

Marzen, R. E.; Shillington, D. J.; Lizarralde, D.; Harder, S. H.

2016-12-01

The Southeastern United States is an ideal location to study the interactions between continental collision, extensive but short-lived magmatism, and continental rifting. Continental collision during the Alleghenian Orogeny ( 290 Ma) formed the supercontinent Pangea. Extension leading to the breakup of Pangea began 230 Ma, forming the South Georgia Basin and other rift basins. The extensive Central Atlantic Magmatic Province (CAMP) magmatism was emplaced at 200 Ma, and continental separation occurred afterwards. During these processes, part of the African continent was added to North America. Prior work has raised questions including (1) the location and geometry of the suture zone and implications for the style of collision (thin-skinned versus thick-skinned), (2) the role of pre-existing structures on later rifting, and (3) the distribution of magmatism, and possible relationships between magmatism and rifting. To address these questions, we present preliminary velocity models for the 400-km-long refraction seismic line from the SUwanee Suture and GA Rift basin experiment (SUGAR) Line 2. This line is central to CAMP magmatism, and crosses the South Georgia rift basin and two hypothesized locations for the ancient suture zone. The data were collected in August 2015 by a team of over 40 students and scientists. Fifteen shots spaced at 20-40 km were recorded by 1981 Texans spaced at 250 m. We observe refractions from the basin, crust, and upper mantle, and wide-angle reflections from the base of the sediments, within the crust, and from the Moho. Prominent mid crustal reflections may arise from the top of elevated lower crustal velocities and possible lower crustal layering. The starting velocity model and constraints on the upper sedimentary basin velocity structure are obtained through forward modeling, which show basin sediment thickness increasing to the South. We then invert for smooth 2D velocity structure using first arrivals (FAST) and a layered velocity model using refractions and reflections (RAYINVR) to evaluate the crust and upper mantle velocity structure. Model results will be compared to other geological and geophysical data, including the roughly parallel SUGAR Line 1, to examine along-strike changes in rift structure, suture structure, and evidence of magmatism.

Evolution of Slow to Intermediate-Spreading Oceanic Crust in the South Atlantic: The Effects of Age, Sediment Thickness, and Spreading Rate on the Heterogeneity of Upper Crustal Velocities

NASA Astrophysics Data System (ADS)

Kardell, D. A.; Christeson, G. L.; Reece, R.; Carlson, R. L.

2017-12-01

The upper section of oceanic crust (layer 2A) commonly exhibits relatively low seismic velocities due to abundant pore and crack space created by the extrusive emplacement of magma and extensional faulting at the spreading ridge. While this is generally true for all spreading rates, previous studies have shown that slow seafloor spreading can yield much higher levels of upper crustal heterogeneity than observed for faster spreading rates. We use a recent multichannel seismic dataset collected with a 12.5 km streamer during the CREST cruise (Crustal Reflectivity Experiment Southern Transect) to build eleven 60-80 km-long tomographic velocity models. These two-dimensional models include both ridge-normal and ridge-parallel orientations and cover oceanic crust produced at slow to intermediate spreading rates. Crustal ages range between 0 and 70 m.y., spreading rates range between slow-spreading and intermediate-spreading, and sedimentary cover thickness ranges from 0 m close to the spreading center to 500 m proximal to the Rio Grande Rise. Our results show a trend of increasing layer 2A velocities with age out to the midpoint of the seismic transect. There is a rapid increase in velocities from 2.8 km/s near the ridge to 4.3 km/s around 10 Ma, and a slower increase to velocities around 5 km/s in 37 m.y. old crust. While this indicates an ongoing evolution in oceanic crust older than expected, the velocities do level off in the older half of the transect, averaging 5 km/s. Crust covered by a thicker sedimentary section can exhibit velocities up to 1 km/s faster than adjacent non-sedimented crust, accounting for much of the local variations. This is possibly due to the effects of a sealed hydrothermal system. We also observe a more heterogeneous velocity structure parallel to the ridge than in the ridge-normal orientation, and more velocity heterogeneity for slow-spreading crust compared to intermediate-spreading crust.

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.

Temperature profiles in the earth of importance to deep electrical conductivity models

NASA Astrophysics Data System (ADS)

Čermák, Vladimír; Laštovičková, Marcela

1987-03-01

Deep in the Earth, the electrical conductivity of geological material is extremely dependent on temperature. The knowledge of temperature is thus essential for any interpretation of magnetotelluric data in projecting lithospheric structural models. The measured values of the terrestrial heat flow, radiogenic heat production and thermal conductivity of rocks allow the extrapolation of surface observations to a greater depth and the calculation of the temperature field within the lithosphere. Various methods of deep temperature calculations are presented and discussed. Characteristic geotherms are proposed for major tectonic provinces of Europe and it is shown that the existing temperatures on the crust-upper mantle boundary may vary in a broad interval of 350 1,000°C. The present work is completed with a survey of the temperature dependence of electrical conductivity for selected crustal and upper mantle rocks within the interval 200 1,000°C. It is shown how the knowledge of the temperature field can be used in the evaluation of the deep electrical conductivity pattern by converting the conductivity-versustemperature data into the conductivity-versus-depth data.

Relating the Seismic Character of the Crust and Upper Mantle to Late-Cenozoic Extension in Southwestern N.A.

NASA Astrophysics Data System (ADS)

Thurner, S.; Frassetto, A.; Porter, R.; Zandt, G.

2008-12-01

A recent tectonic reconstruction (McQuarrie and Wernicke, 2005) places detailed constraints on the magnitude and scope of late-Cenozoic extension throughout Southwestern North America. This project seeks to better understand the distribution of extension throughout the crust and upper mantle and elucidate the transition from the highly extended Basin and Range to the relatively unextended Colorado Plateau. To this end, we present teleseismic receiver functions generated from 31 broadband seismometers associated with EarthScope's BigFoot Array, TriNet, and PASSCAL stations deployed across Southern California and Arizona. We employ the common-conversion-point stacking method to analyze variations in lithospheric structure. Additionally, in regions with clear converted wave reverberations we analyze the trade-off between crustal thickness and bulk Vp/Vs to improve our view of how crustal thickness and Vp/Vs relate to different tectonic environments and degree of extension. Our preliminary estimates indicate crustal thicknesses of ~25-30 km in eastern California increasing to ~40- 45 km within the southern Colorado Plateau. The transition between thin to thick crust appears to occur over as little as 20 km. Crustal Vp/Vs varies considerably, with Vp/Vs greater than 1.8 near the Transverse Ranges and Colorado Plateau, and less than 1.8 in the southern Basin and Range. We also view a change in the nature of the Moho approaching the Colorado Plateau. Initial calculations indicate the amplitude of the converted wave from the Moho is twice as strong beneath the Mojave and Southern Basin and Range than the Colorado Plateau. Additionally, we observe laminated crust in the western Mojave Desert approaching the Transverse Ranges.

Nucleogenic production of Ne isotopes in Earth's crust and upper mantle induced by alpha particles from the decay of U and Th

NASA Astrophysics Data System (ADS)

Leya, Ingo; Wieler, Rainer

1999-07-01

The production of nucleogenic Ne in terrestrial crust and upper mantle by alpha particles from the decay of U and Th was calculated. The calculations are based on stopping powers for the chemical compounds and thin-target cross sections. This approach is more rigorous than earlier studies using thick-target yields for pure elements, since our results are independent of limiting assumptions about stopping-power ratios. Alpha induced reactions account for >99% of the Ne production in the crust and for most of the 20,21Ne in the upper mantle. On the other hand, our 22Ne value for the upper mantle is a lower limit because the reaction 25Mg(n,α)22Ne is significant in mantle material. Production rates calculated here for hypothetical crustal and upper mantle material with average major element composition and homogeneously distributed F, U, and Th are up to 100 times higher than data presented by Kyser and Rison [1982] but agree within error limits with the results by Yatsevich and Honda [1997]. Production of nucleogenic Ne in "mean" crust and mantle is also given as a function of the weight fractions of O and F. The alpha dose is calculated by radiogenic 4He as well as by the more retentive fissiogenic 136Xe. U and Th is concentrated in certain accessory minerals. Since the ranges of alpha particles from the three decay chains are comparable to mineral dimensions, most nucleogenic Ne is produced in U- and Th-rich minerals. Therefore nucleogenic Ne production in such accessories was also calculated. The calculated correlation between nucleogenic 21Ne and radiogenic 4He agrees well with experimental data for Earth's crust and accessories. Also, the calculated 22Ne/4He ratios as function of the F concentration and the dependence of 21Ne/22Ne from O/F for zircon and apatite agree with measurements.

Geochemistry of Snowball Earth glacial tillites from China and North America: implications for the bulk composition of the Neoproterozoic upper crust

NASA Astrophysics Data System (ADS)

Gaschnig, R. M.; Rudnick, R. L.; McDonough, W. F.; Gao, S.; Hu, Z.; Zhou, L.

2012-12-01

In order to understand the differentiation of the Earth and growth of continents through time, it is critical to have reliable estimates for the average composition of the continental crust. Attempts to develop average compositional models for the upper continental crust have often relied upon the analysis of sediments and sedimentary rocks, based on the assumption that these provide natural averages of large crustal areas. Shales are among the most frequently used proxies, although some workers have also studied loess. The advantage of loess, especially that which is derived from glacial processes, is that it is typically produced by physical weathering alone and should lack the elemental fractionation produced by chemical weathering. Glacial tillites should also provide this advantage, and in addition, they should lack element fractionation caused by eolian particle sorting that is observed in loess. Here, we present new major and trace element data for glacial tillites from the Neoproterozoic, collected in southern China and the eastern U.S. Samples were collected from tillites of the Marinoan(?) Nantuo and Sturtian Gucheng Formations in Hubei Province, China (n = 21), and the Sturtian Konnarock Formation in the Appalachians of southwestern Virginia (n = 11). Values for the chemical index of alteration (Al2O3/Al2O3+K2O+Na2O+CaO) for these rocks are low, between 60 and 70 for most of the Chinese samples and 53 and 60 for all of the American ones, reflecting derivation from material that has experienced very little chemical weathering. The individual samples from the two localities show remarkable homogeneity, but their average compositions are distinct. The Chinese tillites match more closely the average upper crust composition of Rudnick and Gao (2003) than the Virginia ones, but the former still show a few major differences. Select soluble elements, such as Sr, Tl, and U, are depleted by as great a factor as ten, whereas other soluble elements, such as Li, Rb, and Cs, are either enriched or similar to the upper crustal model. By contrast, the Virginia tillites show major enrichment in the high field strength elements and rare earth elements, and depletion in the first row transition metals associated with mafic minerals (e.g., Ni, Cr, Sc, V). These tillites also show a stronger negative Eu anomaly. The difference between the Chinese and Virginia Neoproterozoic tillites likely reflects the different provenance of the Virginia samples, but in detail, the implications of this observation are unclear. The Virginia tillite chemistry is similar to local Neoproterozoic A-type granites in the Appalachians, but is also similar to the regionally extensive Grenvillian basement. This is an important distinction, as it goes to the question of whether or not the till represents the integration of a large area, as opposed to being primarily locally derived. In the case of the Chinese tillites, published detrital zircon and whole-rock Nd isotopic data suggests the provenance encompassed a large crustal area, strengthening their legitimacy as a proxy for the average upper crust.

Microstructural and fabric characterization of brittle-ductile transitional deformation of middle crustal rocks along the Jinzhou detachment fault zone, Northeast China

NASA Astrophysics Data System (ADS)

Zhang, Juyi; Jiang, Hao; Liu, Junlai

2017-04-01

Detachment fault zones (DFZs) of metamorphic core complexes generally root into the middle crust. Exhumed DFZs therefore generally demonstrate structural, microstructural and fabric features characteristic of middle to upper crustal deformation. The Jinzhou detachment fault zone from the Liaonan metamorphic core complex is characterized by the occurrence of a sequence of fault rocks due to progressive shearing along the fault zone during exhumation of the lower plate. From the exhumed fabric zonation, cataclastic rocks formed in the upper crust occur near the Jinzhou master detachment fault, and toward the lower plate gradually changed to mylonites, mylonitic gneisses and migmatitic gneisses. Correspondingly, these fault rocks have various structural, microstructural and fabric characteristics that were formed by different deformation and recrystallization mechanisms from middle to upper crustal levels. At the meanwhile, various structural styles for strain localization were formed in the DFZ. As strain localization occurs, rapid changes in deformation mechanisms are attributed to increases in strain rates or involvement of fluid phases during the brittle-ductile shearing. Optical microscopic studies reveal that deformed quartz aggregates in the lower part of the detachment fault zone are characterized by generation of dynamically recrystallized grains via SGR and BLG recrystallization. Quartz rocks from the upper part of the DFZ have quartz porphyroclasts in a matrix of very fine recrystallized grains. The porphyroclasts have mantles of sub-grains and margins grain boundary bulges. Electron backscattered diffraction technique (EBSD) quartz c-axis fabric analysis suggests that quartz grain aggregates from different parts of the DFZ possess distinct fabric complexities. The c-axis fabrics of deformed quartz aggregates from mylonitic rocks in the lower part of the detachment fault zone preserve Y-maxima which are ascribed to intermediate temperature deformation (500-630˚ C), whereas complicated fabric patterns (e.g. asymmetric single girdles) are formed in fault rocks from the upper part of the DFZ. The increasing fabric complexity is here interpreted as the result of progressive superposition of fault rocks by shearing either at relatively shallow levels or high rate of strain, during exhumation of the lower plate and shear zone rocks. The above observations and interpretations imply that dislocation creep processes contribute to the dynamic recrystallization of quartz in the middle crustal brittle-ductile transition. Progressive shearing as a consequence of exhumation of the lower plate of the MCC contributed to the obvious structural, microstructural and fabric superpositions. Strain localization occurs as the progressive shearing proceeded. Transition of mechanisms of deformation and dynamic recrystallization during strain localization may be resulted from changes in temperature conditions, in strain rates or addition of minor amount water.

Implications of magma transfer between multiple reservoirs on eruption cycling.

PubMed

Elsworth, Derek; Mattioli, Glen; Taron, Joshua; Voight, Barry; Herd, Richard

2008-10-10

Volcanic eruptions are episodic despite being supplied by melt at a nearly constant rate. We used histories of magma efflux and surface deformation to geodetically image magma transfer within the deep crustal plumbing of the Soufrière Hills volcano on Montserrat, West Indies. For three cycles of effusion followed by discrete pauses, supply of the system from the deep crust and mantle was continuous. During periods of reinitiated high surface efflux, magma rose quickly and synchronously from a deflating mid-crustal reservoir (at about 12 kilometers) augmented from depth. During repose, the lower reservoir refilled from the deep supply, with only minor discharge transiting the upper chamber to surface. These observations are consistent with a model involving the continuous supply of magma from the deep crust and mantle into a voluminous and compliant mid-crustal reservoir, episodically valved below a shallow reservoir (at about 6 kilometers).

Geoid, topography, and convection-driven crustal deformation on Venus

NASA Technical Reports Server (NTRS)

Simons, Mark; Hager, Bradford H.; Solomon, Sean C.

1993-01-01

High-resolution Magellan images and altimetry of Venus reveal a wide range of styles and scales of surface deformation that cannot readily be explained within the classical terrestrial plate tectonic paradigm. The high correlation of long-wavelength topography and gravity and the large apparent depths of compensation suggest that Venus lacks an upper-mantle low-viscosity zone. A key difference between Earth and Venus may be the degree of coupling between the convecting mantle and the overlying lithosphere. Mantle flow should then have recognizable signatures in the relationships between the observed surface topography, crustal deformation, and the gravity field. Therefore, comparison of model results with observational data can help to constrain such parameters as crustal and thermal boundary layer thicknesses as well as the character of mantle flow below different Venusian features. We explore in this paper the effects of this coupling by means of a finite element modelling technique.

Crustal shear-wave splitting from local earthquakes in the Hengill triple junction, southwest Iceland

USGS Publications Warehouse

Evans, J.R.; Foulger, G.R.; Julian, B.R.; Miller, A.D.

1996-01-01

The Hengill region in SW Iceland is an unstable ridge-ridge-transform triple junction between an active and a waning segment of the mid-Atlantic spreading center and a transform that is transgressing southward. The triple junction contains active and extinct spreading segments and a widespread geothermal area. We evaluated shear-wave birefringence for locally recorded upper-crustal earthquakes using an array of 30 three-component digital seismographs. Fast-polarization directions, ??, are mostly NE to NNE, subparallel to the spreading axis and probably caused by fissures and microcracks related to spreading. However, there is significant variability in ?? throughout the array. The lag from fast to slow S is not proportional to earthquake depth (ray length), being scattered at all depths. The average wave-speed difference between qS1 and qS2 in the upper 2-5 km of the crust is 2-5%. Our results suggest considerable heterogeneity or strong S scattering.

Constraints on the tectonics of the Mule Mountains thrust system, southeast California and southwest Arizona

USGS Publications Warehouse

Tosdal, R.M.

1990-01-01

The Mule Mountains thrust system crops out discontinuously over a 100-km-strike length in this Blythe-Quartzsite region. Along the thrust system, middle and upper crustal metamorphic and plutonic rocks of Proterozoic and Mesozoic age are thrust N-NE (015??-035??) over a lower plate metamorphic terrane. Stratigraphic, petrologic, and Pb isotopic ties for Jurassic granitoids and for Jurassic(?) and Cretaceous sedimentary rocks across the various parts of the thrust system indicate that related crustal blocks are superposed and preclude it from having large displacements. Deformation occurred under low greenschist facies metamorphic conditions in the upper crust. Movement along the thrust system was probably limited to no more than a few tens of kilometers and occurred between 79??2 Ma and 70??4 Ma. Results suggest that the thrust system forms the southern boundary of the narow zone of Cretaceous intracratonic deformation, and it is one of the last tectonic events in the zone prior to regional cooling. -from Author

New Geochemical Analyses Reveal Crustal Accretionary Processes at The Overlapping Spreading Center Near 3 N East Pacific Rise

NASA Astrophysics Data System (ADS)

Smithka, I. N.; Perfit, M. R.

2013-12-01

Mid-ocean ridges (MORs) are the sites of oceanic lithosphere creation and construction. Ridge discontinuities are a global phenomenom but are not as well understood as ridge axes. Geochemical analyses provide insights into upper mantle processes since elements fractionate with melting and freezing as well as reside in material to retain source signature. Lavas collected from ridge discontinuities consist of greater chemical diversity and represent variations in source, melting parameters, and local crustal processes. The small overlapping spreading center (OSC) near the third parallel north on the East Pacific Rise has been superficially analyzed previously, but here we present new isotope analyses and expand our understanding of MOR processes and processes near OSCs. Initial analyses of lavas collected in 2000 on AHA-NEMO2 revealed normal MOR basalt trends in rare earth element enrichments as well as in major element concentrations. Crystal fractionation varies along the tips of both axes, with MgO and TiO2 concentrations increasing towards the OSC basin. Newly analyzed Sr, Nd, and Pb isotope ratios will further constrain the nature of geochemical diversity along axis. As the northern tip seems to be propagating and the southern tip dying, lavas collected from each may reflect two different underlying mantle melting and magma storage processes.

Structures of the West African Craton Margin across southern Mauritania inferred from a 450-km geoelectrical profile

NASA Astrophysics Data System (ADS)

Ritz, M.; Robineau, B.; Vassal, J.; Bellion, Y.; Dukhan, M.

1989-04-01

Magnetotelluric (MT) measurements were carried out at 20 sites, extending 450 km across southern Mauritania in order to study lithospheric structures related to the West African craton (WAC) margin. The MT profile starts to the west on the Senegal-Mauritania basin (S-M basin), traverses across the Mauritanides orogenic belt, and terminates on the western border of the WAC (Taoudeni basin). Distortion effects due to local shallow inhomogeneities are present in nearly all of the basin data. In such a situation, the preliminary interpretation of the data was done by using 1D inversions based upon rotationally invariant parameters. Such distortion is not apparent for the belt and craton sites, and 1D inversions were followed by 2D modeling. The models produced reveal a clear crustal subdivision into a resistive upper crust underlain by a two-layer lower crust with two conductors, one at mid-crustal depths (supposed fluid-produced) beneath the S-M basin and the second at the base of the crust beneath the WAC. The 14-km-thick conductive material below the Mauritanides belt is interpreted as large imbricated thrusts representing the deep roots of the Mauritanides nappes. The models also show that significant contrasts in resistivity extend deep in the lithosphere between the cratonic area and the Senegal microplate.

Enriched and depleted characters of the Amnay Ophiolite upper crustal section and the regionally heterogeneous nature of the South China Sea mantle

NASA Astrophysics Data System (ADS)

Perez, Americus d. C.; Faustino-Eslava, Decibel V.; Yumul, Graciano P.; Dimalanta, Carla B.; Tamayo, Rodolfo A.; Yang, Tsanyao Frank; Zhou, Mei-Fu

2013-03-01

The volcanic section of the Middle Oligocene Amnay Ophiolite in Mindoro, Philippines has previously been shown to be of normalmid-oceanic ridge basalt (NMORB) composition. Here we report for the first time an enriched mantle component that is additionally recorded in this crustal section. New whole rock major and trace element data are presented for nine mafic volcanic rocks from a section of the ophiolite that has not been previously examined. These moderately evolved tholeiitic basalts were found to have resulted from the bulk mixing of ˜10% ocean island basalt components with depleted mantle. Drawing together various geochemical characteristics reported for different rock suites taken as representatives of the South China Sea crust, including the enriched MORB (EMORB) and NMORB of the East Taiwan Ophiolite, the NMORB from previous studies of the Amnay Ophiolite and the younger ocean floor eruptives of the Scarborough Seamount-Reed Bank region, a veined mantle model is proposed for the South China Sea mantle. The NMORB magmatic products are suggested to have been derived from the more depleted portions of the mantle whereas the ocean island basalt (OIB) and EMORB-type materials from the mixing of depleted and veined/enriched mantle regions.

Continentward-Dipping Normal Faults, Boudinage and Ductile Shear at Rifted Passive Margins

NASA Astrophysics Data System (ADS)

Clerc, C. N.; Ringenbach, J. C.; Jolivet, L.; Ballard, J. F.

2017-12-01

Deep structures resulting from the rifting of the continental crust are now well imaged by seismic profiles. We present a series of recent industrial profiles that allow the identification of various rift-related geological processes such as crustal boudinage, ductile shear of the base of the crust and low-angle detachment faulting. Along both magma-rich and magma-poor rifted margins, we observe clear indications of ductile deformation of the deep continental crust. Large-scale shallow dipping shear zones are identified with a top-to-the-continent sense of shear. This sense of shear is consistent with the activity of the Continentward-Dipping Normal Faults (CDNF) that accommodate the extension in the upper crust. This pattern is responsible for an oceanward migration of the deformation and of the associated syn-tectonic deposits (sediments and/or volcanics). We discuss the origin of the Continentward-Dipping Normal Faults (CDNF) and investigate their implications and the effect of sediment thermal blanketing on crustal rheology. In some cases, low-angle shear zones define an anastomosed pattern that delineates boudin-like structures that seem to control the position and dip of upper crustal normal faults. We present some of the most striking examples from several locations (Uruguay, West Africa, South China Sea…), and discuss their rifting histories that differ from the classical models of oceanward-dipping normal faults.

Crustal surface wave velocity structure of the east Albany-Fraser Orogen, Western Australia, from ambient noise recordings

NASA Astrophysics Data System (ADS)

Sippl, C.; Kennett, B. L. N.; Tkalčić, H.; Gessner, K.; Spaggiari, C. V.

2017-09-01

Group and phase velocity maps in the period range 2-20 s for the Proterozoic east Albany-Fraser Orogen, Western Australia, are extracted from ambient seismic noise recorded with the 70-station ALFREX array. This 2 yr temporary installation provided detailed coverage across the orogen and the edge of the Neoarchean Yilgarn Craton, a region where no passive seismic studies of this scale have occurred to date. The surface wave velocities are rather high overall (>3 km s-1 nearly everywhere), as expected for exposed Proterozoic basement rocks. No clear signature of the transition between Yilgarn Craton and Albany-Fraser Orogen is observed, but several strong anomalies corresponding to more local geological features were obtained. A prominent, NE-elongated high-velocity anomaly in the northern part of the array is coincident with a Bouguer gravity high caused by the upper crustal metamorphic rocks of the Fraser Zone. This feature disappears towards longer periods, which hints at an exclusively upper crustal origin for this anomaly. Further east, the limestones of the Cenozoic Eucla Basin are clearly imaged as a pronounced low-velocity zone at short periods, but the prevalence of low velocities to periods of ≥5 s implies that the uppermost basement in this area is likewise slow. At longer periods, slightly above-average surface wave velocities are imaged below the Eucla Basin.

Magma plumbing in the Grímsvötn volcanic system, Iceland: an overview

NASA Astrophysics Data System (ADS)

Thordarson, T.

2016-12-01

The basaltic Grímsvötn volcanic system (GVS) consists of Grímsvötn central volcano (GCV) and an immature fissure swarm extending 70 km to the southwest from GCV. The GCV has the highest eruption frequency of all central volcanos in Iceland, or 7 events per 100 years. In contrast, the GVS fissure swarm has only featured two events in postglacial times, the 1783-4 Laki and the prehistoric Lambavatnsgígar fissure eruptions. These two events account for 25% of the total Holocene magma output from the GVS and 80% of the output in historic time (i.e. last 1100 years). Although GVS magma plumbing has been a topic of research for four decades, its general structure, extent and geometry is still deliberated. Is mantle-derived magma delivered straight up beneath the GCV to an upper crustal magma chamber and then vertically to eruptions at the GCV and laterally to eruption on the GVS fissure swarm? Or does the system feature two levels of crustal storage, one in the upper crust beneath GCV and another at mid-crustal depth? Or is the structure of the GVS plumbing more complex? The data that we have so far and is pertinent to GVS magma plumbing is summarised below: Geophysical measurements imply that shallowest magma storage beneath GCV is at 3-4 km. The Zr and Nb concentrations in the tephra from the 1998 and 2004 GCV plus Laki eruptions show that the parent magmas for each was produced by different degrees of partial melting of a similar mantle source. It also demonstrates transport to the surface via separate pathways and that neither magma can be derived by fractional crystallization from a Laki-like magma. Detailed petrological studies on the Laki tephra and lava indicate polybaric magma evolution within the mid-crust (at 6 to 15 km depth), with further evolution at shallower depths induced either by disequilibrium crystal growth during ascent of magma from the mid-crust storage or a brief residence at 3-6 km depths. The Laki magma contains significant abundances of polymineralic glomerocrysts, signifying that erupted magma interacted with preexisting crystal mushes. These data support the notion of a crustal plumbing system with multiple storage level involving polybaric magma evolution and are inconsistent with the idea that all of the magma erupted within the GVS is delivered from a single upper crustal magma chamber beneath GCV.

Structures within the oceanic crust of the central South China Sea basin and their implications for oceanic accretionary processes

NASA Astrophysics Data System (ADS)

Ding, Weiwei; Sun, Zhen; Dadd, Kelsie; Fang, Yinxia; Li, Jiabiao

2018-04-01

Internal structures in mature oceanic crust can elucidate understanding of the processes and mechanism of crustal accretion. In this study, we present two multi-channel seismic (MCS) transects across the northern flank of the South China Sea basin to reveal the internal structures related to Cenozoic tectono-magmatic processes during seafloor spreading. Bright reflectors within the oceanic crust, including the Moho, upper crustal reflectors, and lower crustal reflectors, are clearly imaged in these two transects. The Moho reflection displays varied character in continuity, shape and amplitude from the continental slope area to the abyssal basin, and becomes absent in the central part of the basin where abundant seamounts and seamount chains formed after the cessation of seafloor spreading. Dipping reflectors are distinct in most parts of the MCS data but generally confined to the lower crust above the Moho reflection. These lower crustal reflectors merge downward into the Moho without offsetting it, probably arising from shear zones between the crust and mantle characterized by interstitial melt, although we cannot exclude other possibilities such as brittle faulting or magmatic layering in the local area. A notable feature of these lower crustal reflector events is their opposite inclinations. We suggest the two groups of conjugate lower crustal reflector events observed between magnetic anomalies C11 and C8 were associated with two unusual accretionary processes arising from plate reorganizations with southward ridge jumps.

Global variations in gravity-derived oceanic crustal thickness: Implications on oceanic crustal accretion and hotspot-lithosphere interactions

NASA Astrophysics Data System (ADS)

Lin, J.; Zhu, J.

2012-12-01

We present a new global model of oceanic crustal thickness based on inversion of global oceanic gravity anomaly with constrains from seismic crustal thickness profiles. We first removed from the observed marine free-air gravity anomaly all gravitational effects that can be estimated and removed using independent constraints, including the effects of seafloor topography, marine sediment thickness, and the age-dependent thermal structure of the oceanic lithosphere. We then calculated models of gravity-derived crustal thickness through inversion of the residual mantle Bouguer anomaly using best-fitting gravity-modeling parameters obtained from comparison with seismically determined crustal thickness profiles. Modeling results show that about 5% of the global crustal volume (or 9% of the global oceanic surface area) is associated with model crustal thickness <5.2 km (designated as "thin" crust), while 56% of the crustal volume (or 65% of the surface area) is associated with crustal thickness of 5.2-8.6 km thick (designated as "normal" crust). The remaining 39% of the crustal volume (or 26% of the surface area) is associated with crustal thickness >8.6 km and is interpreted to have been affected by excess magmatism. The percentage of oceanic crustal volume that is associated with thick crustal thickness (>8.6 km) varies greatly among tectonic plates: Pacific (33%), Africa (50%), Antarctic (33%), Australia (30%), South America (34%), Nazca (23%), North America (47%), India (74%), Eurasia (68%), Cocos (20%), Philippine (26%), Scotia (41%), Caribbean (89%), Arabian (82%), and Juan de Fuca (21%). We also found that distribution of thickened oceanic crust (>8.6 km) seems to depend on spreading rate and lithospheric age: (1) On ocean basins younger than 5 Ma, regions of thickened crust are predominantly associated with slow and ultraslow spreading ridges. The relatively strong lithospheric plate at slow and ultraslow ridges might facilitate the loading of large magmatic emplacements on the plate. (2) In contrast, crustal thickness near fast and intermediately fast spreading ridges typically does not exceed 7-8 km. The relatively weak lithosphere at fast and intermediately fast ridges might make it harder for excess magmatism to accrete. We further speculate that the relatively wide partial melting zones in the upper mantle beneath the fast and intermediately fast ridges might act as "buffer" zones, thus diluting the melt anomalies from the underlying hotspots or regions of mantle heterogeneities. (3) As the crustal age increases and the lithospheric plate thickens, regions of thickened crust start to develop on ocean basins that were originally created at fast and intermediately fast ridges. The integrated crustal volume for fast and intermediately fast ocean crust appears to reach peak values for certain geological periods, such as 40-50 Ma and 70-80 Ma. The newly constructed global models of gravity-derived crustal thickness, combining with geochemical and other constraints, can be used to investigate the processes of oceanic crustal accretion and hotspot-lithosphere interactions.

Geophysical Modeling in Eurasia: 2D Crustal P and LG Propagation; Upper- Mantle Shear Wave Propagation and Anisotropy; and 3D, Joint, Simultaneous Inversions

DTIC Science & Technology

2008-09-01

improved resolution for shallow geologic structures . Jointly inverting these datasets with seismic body wave (S) travel times provides additional...constraints on the shallow structure and an enhanced 3D shear wave model for our study area in western China. 2008 Monitoring Research Review...for much of Eurasia, although the Arabian Shield and Arctic are less well recovered. The upper velocity gradient was tested for 10-degree cells

Measured and calculated seismic velocities and densities for granulites from xenolith occurrences and adjacent exposed lower crustal sections: A comparative study from the North China craton

NASA Astrophysics Data System (ADS)

Gao, Shan; Kern, Hartmut; Liu, Yong-Sheng; Jin, Shu-Yan; Popp, Till; Jin, Zhen-Min; Feng, Jia-Lin; Sun, Min; Zhao, Zu-Bin

2000-08-01

Granulites from the Neogene xenolith-bearing Hannuoba alkaline basalt and from the Manjinggou-Wayaokou exposed lower crustal section in the Archean Huai 'an terrain, which occurs within and surrounds the Hannuoba basalt, provide a unique opportunity for a comparative study on petrophysical properties and composition of the lower crust represented by these two types of samples. P and S wave velocities and densities of 12 Hannuoba lower crustal xenoliths and one associated spinel Iherzolite xenolith as well as nine granulites and granulite-facies metasedimentary rocks from the Archean Huai 'an terrain were measured in laboratory at pressures up to 600 MPa and temperatures up to 600°C. Calculations of P and S wave velocities were also made for the same suite of samples based on modal mineralogy and single-crystal velocities whose variations with composition are considered by using microprobe analyses and velocities of end members. The measured and calculated Vp at room temperature and 600 MPa, where the microcrack effect is considered to be almost eliminated, agree within 4% for rocks from the Manjinggou-Wayaokou section and the adjacent Wutai-Jining upper crustal to upper lower crustal section. In contrast, the xenoliths show systematically lower measured Vp by up to 15% relative to calculated velocities, even if decompression-induced products of kelyphite and glass are taken into account. The lower measured velocities for xenoliths are attributed to grain boundary alteration and residual porosity. This implies that although granulite xenoliths provide direct information about lower crustal constitution and chemical composition, they are not faithful samples for studying in situ seismic properties of the lower crust in terms of measured velocities due to alterations during their entrainment to the surface, which changes their physical properties significantly. In this respect, granulites from high-grade terrains are better samples because they are not subjected to significant changes during their slow transport to the surface and because physical properties depend primarily on mineralogy in addition to pressure and temperature. On the other hand, calculated velocities for granulite xenoliths are consistent with velocities for granulites from terrains, suggesting that they can be also used to infer lower crust composition by correlating with results from seismic refraction studies.

Lower crustal earthquakes in the North China Basin and implications for crustal rheology

NASA Astrophysics Data System (ADS)

Yuen, D. A.; Dong, Y.; Ni, S.; LI, Z.

2017-12-01

The North China Basin is a Mesozoic-Cenozoic continental rift basin on the eastern North China Craton. It is the central region of craton destruction, also a very seismically active area suffering severely from devastating earthquakes, such as the 1966 Xingtai M7.2 earthquake, the 1967 Hejian M6.3 earthquake, and the 1976 Tangshan M7.8 earthquake. We found remarkable discrepancies of depth distribution among the three earthquakes, for instance, the Xingtai and Tangshan earthquakes are both upper-crustal earthquakes occurring between 9 and 15 km on depth, but the depth of the Hejian earthquake was reported of about 30 72 km, ranging from lowermost crust to upper mantle. In order to investigate the focal depth of earthquakes near Hejian area, we developed a method to resolve focal depth for local earthquakes occurring beneath sedimentary regions by P and S converted waves. With this method, we obtained well-resolved depths of 44 local events with magnitudes between M1.0 and M3.0 during 2008 to 2016 at the Hejian seismic zone, with a mean depth uncertainty of about 2 km. The depth distribution shows abundant earthquakes at depth of 20 km, with some events in the lower crust, but absence of seismicity deeper than 25 km. In particular, we aimed at deducing some constraints on the local crustal rheology from depth-frequency distribution. Therefore, we performed a comparison between the depth-frequency distribution and the crustal strength envelop, and found a good fit between the depth profile in the Hejian seismic zone and the yield strength envelop in the Baikal Rift Systems. As a conclusion, we infer that the seismogenic thickness is 25 km and the main deformation mechanism is brittle fracture in the North China Basin . And we made two hypotheses: (1) the rheological layering of dominant rheology in the North China Basin is similar to that of the Baikal Rift Systems, which can be explained with a quartz rheology at 0 10 km depth and a diabase rheology at 10 35 km depth; (2) the temperature is moderate in the seismogenic zone of crust and relative high below 25 km. We also suggest that, many accurately resolved earthquake locations can shed light on the nature of the crustal rheology locally, and that our method can be employed in other sedimentary regions which are seismically active.

Recycling and transport of continental material through the mantle wedge above subduction zones: A Caribbean example

NASA Astrophysics Data System (ADS)

Rojas-Agramonte, Yamirka; Garcia-Casco, Antonio; Kemp, Anthony; Kröner, Alfred; Proenza, Joaquín A.; Lázaro, Concepción; Liu, Dunyi

2016-02-01

Estimates of global growth rates of continental crust critically depend upon knowledge of the rate at which crustal material is delivered back into the mantle at subduction zones and is then returned to the crust as a component of mantle-derived magma. Quantification of crustal recycling by subduction-related magmatism relies on indirect chemical and isotopic tracers and is hindered by the large range of potential melt sources (e.g., subducted oceanic crust and overlying chemical and clastic sediment, sub-arc lithospheric mantle, arc crust), whose composition may not be accurately known. There is also uncertainty about how crustal material is transferred from subducted lithosphere and mixed into the mantle source of arc magmas. We use the resilient mineral zircon to track crustal recycling in mantle-derived rocks of the Caribbean (Greater Antilles) intra-oceanic arc of Cuba, whose inception was triggered after the break-up of Pangea. Despite juvenile Sr and Nd isotope compositions, the supra-subduction zone ophiolitic and volcanic arc rocks of this Cretaceous (∼135-70 Ma) arc contain old zircons (∼200-2525 Ma) attesting to diverse crustal inputs. The Hf-O isotope systematics of these zircons suggest derivation from exposed crustal terranes in northern Central America (e.g. Mexico) and South America. Modeling of the sedimentary component in the most mafic lavas suggests a contribution of no more than 2% for the case of source contamination or less than 4% for sediment assimilation by the magma. We discuss several possibilities for the presence of inherited zircons and conclude that they were transported as detrital grains into the mantle beneath the Caribbean Plate via subduction of oceanic crust. The detrital zircons were subsequently entrained by mafic melts that were rapidly emplaced into the Caribbean volcanic arc crust and supra-subduction mantle. These findings suggest transport of continental detritus, through the mantle wedge above subduction zones, in magmas that otherwise do not show strong evidence for crustal input and imply that crustal recycling rates in some arcs may be higher than hitherto realized.

3D Crustal Velocity Structure Model of the Middle-eastern North China Craton

NASA Astrophysics Data System (ADS)

Duan, Y.; Wang, F.; Lin, J.; Wei, Y.

2017-12-01

Lithosphere thinning and destruction in the middle-eastern North China Craton (NCC), a region susceptible to strong earthquakes, is one of the research hotspots in solid earth science. Up to 42 wide-angle reflection/refraction deep seismic sounding (DSS) profiles have been completed in the middle-eastern NCC, we collect all the 2D profiling results and perform gridding of the velocity and interface depth data, and build a 3D crustal velocity structure model for the middle-eastern NCC, named HBCrust1.0, using the Kriging interpolation method. In this model, four layers are divided by three interfaces: G is the interface between the sedimentary cover and crystalline crust, with velocities of 5.0-5.5 km/s above and 5.8-6.0 km/s below. C is the interface of the upper and lower crust, with velocity jump from 6.2-6.4 km/s to 6.5-6.6 km/s. M is the interface between the crust and upper mantle, with velocity 6.7-7.0 km/s at the crust bottom and 7.9-8.0 km/s on mantle top. Our results show that the first arrival time calculated from HBCust1.0 fit well with the observation. It also demonstrates that the upper crust is the main seismogenic layer, and the brittle-ductile transition occurs at depths near interface C. The depth of interface Moho varies beneath the source area of the Tangshan earth-quake, and a low-velocity structure is found to extend from the source area to the lower crust. Based on these observations, it can be inferred that stress accumulation responsible for the Tangshan earthquake may have been closely related to the migration and deformation of the mantle materials. Comparisons of the average velocities of the whole crust, the upper and the lower crust show that the average velocity of the lower crust under the central part of the North China Basin (NCB) in the east of the craton is obviously higher than the regional average, this high-velocity probably results from longterm underplating of the mantle magma. This research is founded by the Natural Science Foundation of China (91414301 and 41174052).

Petrogenesis of cataclastic rocks within the San Andreas fault zone of Southern California U.S.A.

NASA Astrophysics Data System (ADS)

Lawford Anderson, J.; Osborne, Robert H.; Palmer, Donald F.

1980-08-01

This paper petrologically characterizes cataclastic rocks derived from four sites within the San Andreas fault zone of southern California. In this area, the fault traverses an extensive plutonic and metamorphic terrane and the principal cataclastic rock formed at these upper crustal levels is unindurated gouge derived from a range of crystalline rocks including diorite, tonalite, granite, aplite, and pegmatite. The mineralogical nature of this gouge is decidedly different from the "clay gouge" reported by Wu (1975) for central California and is essentially a rock flour with a quartz, feldspar, biotite, chlorite, amphibole, epidote and oxide mineralogy representing the milled-down equivalent of the original rock. Clay development is minor (less than 4 wt. %) to nonexistent and is exclusively kaolinite. Alterations involve hematitic oxidation, chlorite alteration on biotite and amphibole, and local introduction of calcite. Electron microprobe analysis showed that in general the major minerals were not reequilibrated with the pressure—temperature regime imposed during cataclasis. Petrochemically, the form of cataclasis that we have investigated is largely an isochemical process. Some hydration occurs but the maximum amount is less than 2.2% added H 2O. Study of a 375 m deep core from a tonalite pluton adjacent to the fault showed that for Si, Al, Ti, Fe, Mg, Mn, K, Na, Li, Rb, and Ba, no leaching and/or enrichment occurred. Several samples experienced a depletion in Sr during cataclasis while lesser number had an enrichment of Ca (result of calcite veining). Texturally, the fault gouge is not dominated by clay-size material but consists largely of silt and fine sand-sized particles. An intriguing aspect of our work on the drill core is a general decrease in particulate size with depth (and confining pressure) with the predominate shifting sequentially from fine sand to silt-size material. The original fabric of these rocks is commonly not disrupted during the cataclasis. It is evident that the gouge development in these primarily igneous crystalline terranes is largely an in situ process with minimal mixing of rock types. Fabric analyses reveal that brecciation (shattering), not shearing, is the major deformational mechanism at these upper crustal levels.

Two-pyroxene syenitoids from the Moldanubian Zone of the Bohemian Massif: peculiar magmas derived from a strongly enriched lithospheric mantle source

NASA Astrophysics Data System (ADS)

Janoušek, Vojtěch; Holub, František; Gerdes, Axel; Verner, Kryštof

2013-04-01

(Ultra-)potassic plutonic rocks constitute a conspicuous association with metamorphic rocks of the high-grade, lower crustal/upper mantle Gföhl Unit (Moldanubian Zone). They can be subdivided into two contrasting suites: (1) coarse Kfs-phyric amphibole-biotite melagranite to quartz syenite (the durbachite series sensu Holub 1997), and (2) essentially even-grained biotite-two-pyroxene quartz syenites to melagranites (Tábor and Jihlava plutons). The latter, "syenitoid suite", characterized by an originally 'dry' mineral assemblage orthopyroxene + clinopyroxene + Mg-biotite, with accessoric zircon, apatite, ilmenite, monazite and/or rutile ± Cr-spinel, is a subject of the current study. Our conventional U-Pb ages for zircon (336.9 ± 0.6 Ma) and rutile (336.8 ± 0.8 Ma) from the Tábor Pluton, together with the age from the Jihlava body (U-Pb zircon: 335.1 ± 0.6 Ma; Kotková et al. 2010), provide a precise time bracket for the emplacement and rapid cooling of the syenitoids below c.600 ° C (closure temperature of U-Pb system in rutile: Cherniak 2000). This is in line with post-tectonic emplacement of hot dry melt into shallow levels of essentially consolidated orogenic crust. Comparably low temperatures obtained by zircon and rutile saturation calculations document probably a delayed onset of crystallization of the accessories in a hot, alkalis and ferromagnesian components-rich magma derived from a mantle source. Indeed, the structural relations inside and around the ultrapotassic plutons suggest that the most important regional HT/LP flat-lying fabric(s) in the Moldanubian Zone are closely related with the emplacement and crystallization of the durbachite suite at 343-338 Ma. They have formed prior to the relatively shallower emplacement of the essentially post-tectonic syenitoids dated at ~337-336 Ma (Žák et al. 2005; Verner et al. 2006, 2008). The two magmatic suites are thus essentially diachronous and not (nearly) contemporaneous (c. 335 Ma) intrusions at contrasting crustal levels as assumed by Kotková et al. (2010). The syenitoid plutons show mutually comparable, crustal-like radiogenic isotope signatures with highly radiogenic Sr (87Sr/86Sr337= 0.7119-0.7125) and unradiogenic Nd (?Nd337 = -6.8 to -7.6). This, together with the rest of the whole-rock geochemical variation, is in line with a generation from a strongly enriched lithospheric mantle source. It was, shortly before, modified by a deep subduction and relamination of the upper crustal material, similar to the felsic HP granulites common in the Moldanubian Zone (Janoušek & Holub 2007; Lexa et al. 2011). The petrology and chemical data indicate that large-scale mixing with crustally-derived acid magmas can be largely or fully discounted and the key role is ascribed to closed-system fractional crystallization with, or without, crystal accumulation of various combinations of biotite, clinopyroxene and/or orthopyroxene with minor amounts of apatite. This stands in a sharp contrast with the history of volumetrically prevalent, slightly older, durbachite suite, in genesis of which the magma mixing of chemically and isotopically contrasting mantle and crustal components was clearly much more significant (Holub 1997). This research was financially supported by the GAR Project P210-11-2358 (to VJ).

DigitalCrust – a 4D data system of material properties for transforming research on crustal fluid flow

USGS Publications Warehouse

Fan, Yin; Richard, Steve; Bristol, R. Sky; Peters, Shanan; Ingebritsen, Steven E.; Moosdorf, Nils; Packman, Aaron I.; Gleeson, Tom; Zazlavsky, Ilya; Peckham, Scott; Murdoch, Larry; Cardiff, Michael; Tarboton, David; Jones, Norm; Hooper, Richard; Arrigo, Jennifer; Gochis, David; Olson, John

2015-01-01

Fluid circulation in the Earth's crust plays an essential role in surface, near surface, and deep crustal processes. Flow pathways are driven by hydraulic gradients but controlled by material permeability, which varies over many orders of magnitude and changes over time. Although millions of measurements of crustal properties have been made, including geophysical imaging and borehole tests, this vast amount of data and information has not been integrated into a comprehensive knowledge system. A community data infrastructure is needed to improve data access, enable large-scale synthetic analyses, and support representations of the subsurface in Earth system models. Here, we describe the motivation, vision, challenges, and an action plan for a community-governed, four-dimensional data system of the Earth's crustal structure, composition, and material properties from the surface down to the brittle–ductile transition. Such a system must not only be sufficiently flexible to support inquiries in many different domains of Earth science, but it must also be focused on characterizing the physical crustal properties of permeability and porosity, which have not yet been synthesized at a large scale. The DigitalCrust is envisioned as an interactive virtual exploration laboratory where models can be calibrated with empirical data and alternative hypotheses can be tested at a range of spatial scales. It must also support a community process for compiling and harmonizing models into regional syntheses of crustal properties. Sustained peer review from multiple disciplines will allow constant refinement in the ability of the system to inform science questions and societal challenges and to function as a dynamic library of our knowledge of Earth's crust.

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

NASA Astrophysics Data System (ADS)

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

2016-01-01

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

Plagioclase-dominated Seismic Anisotropy in the Basin and Range Lower Crust

NASA Astrophysics Data System (ADS)

Bernard, R. E.; Behr, W. M.

2017-12-01

Observations of seismic anisotropy have the ability to provide important information on deformation and structures within the lithosphere. While the mechanisms controlling seismic anisotropy in the upper mantle are fairly well understood (i.e., olivine "lattice preferred orientation" or LPO), less is known about the minerals and structures controlling regional lower crustal anisotropy. We use lower crustal xenoliths from young cinder cones in the eastern Mojave/western Basin and Range to investigate mineral LPOs and their effect on seismic anisotropy. Lower crustal gabbros were collected from two areas roughly 80 km apart — the Cima and Deadman Lake Volcanic Fields. Lower crustal fabrics measured using EBSD are dominated by LPOs in plagioclase associated with both plastic deformation and magmatic flow. In all fabric types, plagioclase LPOs produce seismic fast axes oriented perpendicular to the foliation plane. This is in contrast to mantle peridotite xenoliths from the same locations, which preserve olivine LPOs with fast axes aligned parallel to the foliation plane. The orthogonal orientations of mantle and lower crustal fast axes relative to foliation implies that even where fabric development in both layers is coeval and kinematically compatible, their measured anisotropies can be perpendicular to each other, therefore appearing anti-correlated when measured seismically. Furthermore, our observation of plagioclase-dominated LPO and negligible concentrations of mica is at odds with the common assumption that lower crustal anisotropy is dominated by micaceous minerals, whose slow axes reliably align parallel to lineation or flow. In contrast, our data show that for plagioclase, fast axes align perpendicular to flow and the slow axes are variably aligned within the foliation plane. Therefore, for a crustal section dominated by plagioclase LPO with assumed horizontal foliation, there would be a vertical rather than a horizontal axis of symmetry, resulting in a lack of azimuthal anisotropy and minimal shear wave splitting for vertically propagating waves. Crustal seismic studies in this type of setting may only be able to identify crustal flow planes, but not flow directions. These findings may be generally applicable to regions of significant mafic volcanism and lower crustal magmatic underplating.

Geodynamic models for the post-orogenic exhumation of the lower crust

NASA Astrophysics Data System (ADS)

Bodur, O. F.; Gogus, O.; Karabulut, H.; Pysklywec, R. N.; Okay, A. I.

2015-12-01

Recent geodynamic modeling studies suggest that the exhumation of the high pressure and the very/ultra high-pressure crustal rocks may occur due to the slab detachment (break-off), slab roll-back (retreat) and the buoyancy-flow controlled subduction channel. We use convective removal (Rayleigh-Taylor, 'dripping' instability) mechanism to quantitatively investigate the burial and the exhumation pattern of the lower/middle crustal rocks from ocean subduction to post-collisional geodynamic configuration. In order to address the model evolution and track crustal particles for deciphering P-T-t variation, we conduct a series of thermo-mechanical numerical experiments with arbitrary Eularian-Lagrangian finite element code (SOPALE). We show how additional model parameters (e.g moho temperature, upper-middle crustal activation energy, density contrast between the lithosphere and the underlying mantle) can effectively influence the burial and exhumation depths, rate and the styles (e.g clockwise or counterclockwise). First series of experiments are designed to investigate the exhumation of crustal rocks at 32 km depth for only post-collisional tectonic setting -where pre-existing ocean subduction has not been implemented-. Model predictions show that a max. 8 km lower crustal burial occurs concurrent with the lower crustal convergence as a response to the mantle lithosphere dripping instability. The subsequent exhumation of these rocks up to -25 km- is predicted at an exhumation rate of 1.24 cm/year controlled by the removal of the underlying mantle lithosphere instability with crustal extension. At the second series of experiments, we tracked the burial and exhumation history of crustal particles at 22 and 31 km depths while pre-existing ocean subduction has been included before the continental collision. Model results show that burial depths down to 62 km occurs and nearly the 32 km of exhumation is predicted again by the removal of the mantle lithosphere after the dripping instability but the crustal rocks are buried deeper because of the downward forcing of the sinking ocean plate. We suggest that the first set of model results are comparable to the peak pressure calculations from the high pressure rocks of the Afyon Zone in western Turkey with a significant offset (175°C) in temperature values.

Integrated study of basins in the Four Corners region

NASA Astrophysics Data System (ADS)

Fagbola, Olamide Olawumi

2007-12-01

This dissertation is an integrated study of basins in the four corners area of the central part of the Colorado Plateau. The Colorado Plateau is a structurally unique part of the Rocky Mountain region because it has only been moderately deformed when compared to the more intensely deformed areas around it. The Colorado Plateau covers a portion of Utah, Colorado, New Mexico and Arizona. The study area extends from latitude 34°N-40°N to longitude 106°W-111W° encompassing a series of major basins and uplifts: the San Juan, Black Mesa, Paradox, and the Blanding basins; and the Zuni, Defiance, Four Corners, Monument uplifts and the San Juan dome and volcanic field. An analysis of gravity anomalies, basement and crustal structure for basins in the four corners region was carried out. This involved using gravity, magnetic, well, outcrop, seismic estimates of crustal thickness, and geologic data in an integrated fashion. Six filtered gravity and three filtered magnetic maps were generated to aid in the interpretation of the gravity and magnetic anomalies in the study area. A detailed comparison of these maps was carried out. The results show a deep seated mafic structure in the basement acting as a crustal boundary separating the high gravity anomalies from the low. These maps also show that the sources of these anomalies are quite shallow resulting from the upper crust in the study area. The structures in the study area are characterized by northwest and northeast trends which correspond to the Precambrian and the Late Paleozoic structures, respectively. A crustal thickness map of the area was also constructed from seismic estimates of crustal thickness. A comparison was done between the crustal thickness map and the 45 km upward continuation Bouguer anomaly map. The result of this comparison shows that areas of thicker ix crust corresponded to low gravity while areas of thinner crust means mantle material is closer to the surface, thereby producing a high gravity anomaly. The thinnest crust encountered is about 32 km while the thickest crust is about 50 km. Seven gravity models were constructed and these include three crustal-scale profiles crisscrossing the study area and four local profiles. The gravity profiles were modeled using well data, structural thickness maps, cross section data, geologic maps and previous gravity models as constraints. Basement inhomogeneities beneath the basins and the uplifts were delineated by the gravity modeling. One of results from this study reveals that the basement beneath the Four Corners area is highly inhomogeneous. This study reveals that there is a high density deep seated mafic intrusion present in the basement which is responsible for the high gravity and magnetic anomaly in A. This dissertation has also shown that the Four Corners region does not possess a single crustal signature as shown by the different crustal trends in San Juan basin trending northeast and the east-west trending Uncompahgre uplift. The 45 km upward continuation gravity map was also found to correlate with seismic estimates of crustal thickness. The Precambrian basement in this region is also not homogeneous as shown by the necessity of inserting exotic bodies into the basement to compensate for high gravity anomalies and lastly an attempt was made to better define Tweto's (1980) outline of geologic features in the study area. On integrating gravity, magnetics, well and outcrop data, the relief of the Defiance uplift is not as high as delineated by Tweto's (1980) outline.

Upper- and mid-crustal radial anisotropy beneath the central Himalaya and southern Tibet from seismic ambient noise tomography

NASA Astrophysics Data System (ADS)

Guo, Zhi; Gao, Xing; Wang, Wei; Yao, Zhenxing

2012-05-01

Through analysis of the Rayleigh wave and Love wave empirical Green's functions recovered from cross-correlation of seismic ambient noise, we image the radial anisotropy and shear wave velocity structure beneath southern Tibet and the central Himalaya. Dense ray path coverage from 22 broadband seismic stations deployed by the Himalayan Nepal Tibet Seismic Experiment project provides the unprecedented opportunity to resolve the spatial distribution of the radial anisotropy within the crust of the central Himalaya and southern Tibet. In the shallow subsurface, the obtained results indicate significant radial anisotropy with negative magnitude (VSV > VSH) mainly associated with the Indus Yarlung Suture and central Himalaya, possibly related to the fossil microcracks or metamorphic foliations formed during the uplifting of the Tibetan Plateau. With increasing depth, the magnitude of radial anisotropy varies from predominantly negative to predominantly positive, and a mid-crustal layer with prominent positive radial anisotropy (VSV < VSH) has been detected. The top of the mid-crustal anisotropic layer correlates nicely with the starting depth of the mid-crustal lower velocity layers detected in our previous study. The spatial correlation of the positive radial anisotropy layers and mid-crustal lower velocity layers might suggest lateral crustal channel flow induced alignment of mineral grains, most likely micas or amphiboles, within the mid-crust of the central Himalaya and southern Tibet. This observation provides independent seismic evidence to support the thermo-mechanical model, which involves the southward extrusion of a low viscosity mid-crustal channel driven by the denudation effect focused at the southern flank of the Tibetan Plateau to explain the tectonic evolution of the Tibetan-Himalayan orogen.

Reworked crustal of early Paleozoic WuYi Orogen revealed by receiver function data

NASA Astrophysics Data System (ADS)

Wei, Y.; Duan, Y.; Tian, X.; Zhao, Y.

2017-12-01

Intraplate orogenic belt, which occurs at the rigid and undeformable plate interiors, is a distinct new type of orogen rather than an interplate or plate marginal orogenic belt, whose deformation occurs exclusively at plate margins. Therefore, intraplate orogenic belts are the most obvious exception to the plate-tectonic paradigm, they are uncommon in Earth's history. The early Paleozoic Wuyi orogen in South China is one of the few examples of intraplate orogen, and is a key to understanding the process of intraplate orogenesis and global early Paleozoic geodynamics. In this study, we select teleseismic records from 45 mobile linear seismic stations deployed in Wuyi Mountain and 58 permanent stations setting in Jiangxi and Fujian provinces, from January 2011 to December 2012, and calculate the crustal thickness and average crustal Vp/Vs ratio using the H-κ stacking method. The main results include the following: 1) the crustal average Poission's ratio shows an increase tendency from land to sea, the interior of Wuyi orogen belt with an low ration less than 0.23, and the coastline with high ration which is up to 0.28, which indicate a very heterogeneous crustal structure and composition in Wuyi orogen and coast belt. 2) the crustal thickness ranges 28-34 km and shows a tendency of thinning from inland to coast in the region of SE China margin, which maight mean the eastern Eurasia lithospheric is extension and thinning induced by the subducted paleo-Pacific slab. To conclusion, we assume that Wuyi orogen experienced upper crustal thickening, lower crust and lithosphere delamination during the early Paleozoic orogeny, and lithosphere extension in Mesozoic. This research is founded by the Natural Science Foundation of China (41174052 and 41604048).

Reconstructing the plumbing system of Krakatau volcano

NASA Astrophysics Data System (ADS)

Troll, Valentin R.; Dahrén, Börje; Deegan, Frances M.; Jolis, Ester M.; Blythe, Lara S.; Harris, Chris; Berg, Sylvia E.; Hilton, David R.; Freda, Carmela

2014-05-01

Crustal contamination of ascending arc magmas is generally thought to be significant at lower- to mid-crustal magma storage levels where magmas inherit their chemical and isotopic character by blending, assimilation and differentiation [1]. Anak Krakatau, like many other volcanoes, erupts shallow-level crustal xenoliths [2], indicating a potential role for upper crustal modification and hence late-stage changes to magma rheology and thus eruptive behaviour. Distinguishing deep vs. shallow crustal assimilation processes at Krakatau, and elsewhere, is therefore crucial to understand and assess pre-eruptive magmatic conditions and their associated hazard potential. Here we report on a multi-disciplinary approach to unravel the crustal plumbing system of the persistently-active and dominantly explosive Anak Krakatau volcano [2, 3]. We employ rock-, mineral- and gas-isotope geochemistry and link these results with seismic tomography [4]. We show that pyroxene crystals formed at mid- and lower-crustal levels (9-11 km) and carry almost mantle-like isotope signatures (O, Sr, Nd, He), while feldspar crystals formed dominantly at shallow levels (< 5km) and display unequivocal isotopic evidence for late stage contamination (O, Sr, Nd). Coupled with tomographic evidence, the petrological and geochemical data place a significant element of magma-crust interaction (and hence magma storage) into the uppermost, sediment-rich crust beneath the volcano. Magma - sediment interaction in the uppermost crust offers a likely explanation for the compositional variations in recent Krakatau magmas and most probably provides extra impetus to increased explosivity at Anak Krakatau. [1] Annen, et al., 2006. J. Petrol. 47, 505-539. [2] Gardner, et al., 2013. J. Petrol. 54, 149-182. [3] Dahren, et al., 2012. Contrib. Mineral. Petrol. 163, 631-651. [4] Jaxybulatov, et al., 2011. J. Volcanol. Geoth. Res. 206, 96-105.

Shallow-level magma-sediment interaction and explosive behaviour at Anak Krakatau (Invited)

NASA Astrophysics Data System (ADS)

Troll, V. R.; Jolis, E. M.; Dahren, B.; Deegan, F. M.; Blythe, L. S.; Harris, C.; Berg, S. E.; Hilton, D. R.; Freda, C.

2013-12-01

Crustal contamination of ascending arc magmas is generally thought to be a significant process which occurs at lower- to mid-crustal magma storage levels where magmas inherit their chemical and isotopic character by blending, assimilation and differentiation [1]. Anak Krakatau, like many other volcanoes, erupts shallow-level crustal xenoliths [2], indicating a potential role for upper crustal modification and hence late-stage changes to magma rheology and thus potential eruptive behaviour. Distinguishing deep vs. shallow crustal contamination processes at Krakatau, and elsewhere, is therefore crucial to understand and assess pre-eruptive magmatic conditions and their associated hazard potential. Here we report on a multi-disciplinary approach to unravel the crustal plumbing system of the persistently-active and dominantly explosive Anak Krakatau volcano [2, 3], employing rock-, mineral- and gas-isotope geochemistry and link these results with seismic tomography [4]. We show that pyroxene crystals formed at mid- and lower-crustal levels (9-11 km) and carry almost mantle-like isotope signatures (O, Sr, Nd, He), while feldspar crystals formed dominantly at shallow levels (< 5km) and display unequivocal isotopic evidence for late stage contamination (O, Sr, Nd). This obeservation places a significant element of magma-crust interaction into the uppermost, sediment-rich crust beneath the volcano. Magma storage in the uppermost crust can thus offer a possible explanation for the compositional modifications of primitive Krakatau magmas, and likely provides extra impetus to increased explosivity at Anak Krakatau. [1] Annen, et al., 2006. J. Petrol. 47, 505-539. [2] Gardner, et al., 2013. J. Petrol. 54, 149-182. [3] Dahren, et al., 2012. Contrib. Mineral. Petrol. 163, 631-651. [4] Jaxybulatov, et al., 2011. J. Volcanol. Geoth. Res. 206, 96-105.

Färoe-Iceland Ridge Experiment: 1. Crustal structure of northeastern Iceland

USGS Publications Warehouse

Staples, Robert K.; White, Robert S.; Brandsdottir, Bryndis; Menke, William; Maguire, Peter K.H.; McBride, John H.

1997-01-01

Results from the Färoe-Iceland Ridge Experiment (FIRE) constrain the crustal thickness as 19 km under the Northern Volcanic Zone of Iceland and 35 km under older Tertiary areas of northeastern Iceland. The Moho is defined by strong P wave and S wave reflections. Synthetic seismogram modeling of the Moho reflection indicates mantle velocities of at least 8.0 km/s beneath the Tertiary areas of northeastern Iceland and at least 7.9 km/s beneath the neovolcanic zone. Crustal diving rays resolve the structure of the upper and lower crust. Surface P wave velocities are 1.1–4.0 km/s in Quaternary rocks and are rather higher, 4.4–4.7 km/s, in the Tertiary basalts that outcrop elsewhere. The highest crustal P wave velocities observed directly from diving rays are 7.1 km/s, from rays that turn at 24 km depth. Velocities of 7.35 km/s at the base of the crust are inferred from extrapolation of the lower crustal velocity gradient (0.024 s−1). A Poisson's ratio of approximately 0.27, equivalent to an S wave to P wave travel time ratio of 1.78, is measured throughout the crust east of the neovolcanic zone. The Poisson's ratio and the steep Moho topography (in places up to 30° from the horizontal) indicate that the entire crust outside the neovolcanic zone is cool (<800°C). Gravity data are well matched by a velocity/density conversion of our seismic crustal model and indicate a region of low mantle density beneath the neovolcanic zone, believed to be due to elevated mantle temperatures. The crustal thickness in the neovolcanic zone is consistent with geochemical estimates of the melt generation, placing constraints on the flow within the Iceland mantle plume.

Crustal structure along the geosciences transect from Altay to Altun Tagh

USGS Publications Warehouse

Wang, Y.-X.; Han, G.-H.; Jiang, M.; Yuan, X.-C.; Mooney, W.D.; Coleman, R.G.

2004-01-01

Based upon the P- and S-wave data acquired along the geoscience transect from Altay to Altun Tagh in Northwest China, the crustal structures of velocities and Poisson's ratio are determined. The crustal velocity structure features an obvious three-layer structure with velocities of 6. 0 ??? 6. 3km/s, 6. 3 ??? 6. 6km/s and 6.9 ??? 7. Okm/s from surface to depth, respectively. The crustal thickness along the. entire profile is mostly 50km with the thickest crust (56km) beneath the Altay and the thinnest (46km) beneath the Junggar basin. The velocities underlying Moho are 7.7 to 7.8km/s between the Tianshan and the Junggar basin, and 7.9 to 8.0km/s below the Altay Mountains and eastern margin of the Tarim basin. The southern half of the profile, including the eastern Tianshan Mountains and eastern margin of the Tarim basin, shows low P-wave velocities and ?? = 0. 25 to a depth, of 30km, which suggests a quartz-rich, granitic upper crustal composition. The northern half of the profile below the Altay Mountains and Junggar Accretional Belt has a higher Poisson's ratio of ?? = 0.26 ??? 0.27 to a depth of 30km, indicative of an intermediate crustal composition, The entire profile is underlain by a 15 to 30km thick high-velocity (6.9 ??? 7.0km/s; ?? = 0. 26 - 0.28) lower crustal layer that we interpret to have a bulk composition of mafic granulite. At the southern end of the profile a 5km-thick midcrustal low-velocity layer ( Vp, = 5.9km/s, ?? = 0.25) underlies the Tianshan and the region to the south, and may be indicative of granitic intrusive in Late Paleozoic.

Origins of topography in the western U.S.: Mapping crustal and upper mantle density variations using a uniform seismic velocity model

NASA Astrophysics Data System (ADS)

Levandowski, Will; Jones, Craig H.; Shen, Weisen; Ritzwoller, Michael H.; Schulte-Pelkum, Vera

2014-03-01

To investigate the physical basis for support of topography in the western U.S., we construct a subcontinent scale, 3-D density model using 1000 estimated crustal thicknesses and S velocity profiles to 150 km depth at each of 947 seismic stations. Crustal temperature and composition are considered, but we assume that mantle velocity variations are thermal in origin. From these densities, we calculate crustal and mantle topographic contributions. Typical 2σ uncertainty of topography is 500 m, and elevations in 84% of the region are reproduced within error. Remaining deviations from observed elevations are attributed to melt, variations in crustal quartz content, and dynamic topography; compositional variations in the mantle, while plausible, are not necessary to reproduce topography. Support for western U.S. topography is heterogeneous, with each province having a unique combination of mechanisms. Topography due to mantle buoyancy is nearly constant (within 250 m) across the Cordillera; relief there (>2 km) results from variations in crustal chemistry and thickness. Cold mantle provides 1.5 km of ballast to the thick crust of the Great Plains and Wyoming craton. Crustal temperature variations and dynamic pressures have smaller magnitude and/or more localized impacts. Positive gravitational potential energy (GPE) anomalies ( 2 × 1012N/m) calculated from our model promote extension in the northern Basin and Range and near the Sierra Nevada. Negative GPE anomalies (-3 × 1012N/m) along the western North American margin and Yakima fold and thrust belt add compressive stresses. Stresses derived from lithospheric density variations may strongly modulate tectonic stresses in the western U.S. continental interior.

Crustal Viscosity Structure Estimated from Multi-Phase Mixing Theory

NASA Astrophysics Data System (ADS)

Shinevar, W. J.; Behn, M. D.; Hirth, G.

2014-12-01

Estimates of lower crustal viscosity are typically constrained by analyses of isostatic rebound, post seismic creep, and laboratory-derived flow laws for crustal rocks and minerals. Here we follow a new approach for calculating the viscosity structure of the lower continental crust. We use Perple_X to calculate mineral assemblages for different crustal compositions. Effective viscosity is then calculated using the rheologic mixing model of Huet et al. (2014) incorporating flow laws for each mineral phase. Calculations are performed along geotherms appropriate for the Basin and Range, Tibetan Plateau, Colorado Plateau, and the San Andreas Fault. To assess the role of crustal composition on viscosity, we examined two compositional gradients extending from an upper crust with ~67 wt% SiO2 to a lower crust that is either: (i) basaltic with ~53 wt% SiO2 (Rudnick and Gao, 2003), or (ii) andesitic with ~64% SiO2 (Hacker et al., 2011). In all cases, the middle continental crust has a viscosity that is 2-3 orders of magnitude greater than that inferred for wet quartz, a common proxy for mid-crustal viscosities. An andesitic lower crust results in viscosities of 1020-1021 Pa-s and 1021-1022 Pa-s for hotter and colder crustal geotherms, respectively. A mafic lower crust predicts viscosities that are an order of magnitude higher for the same geotherm. In all cases, the viscosity calculated from the mixing model decreases less with depth compared to single-phase estimates. Lastly, for anhydrous conditions in which alpha quartz is stable, we find that there is a strong correlation between Vp/Vs and bulk viscosity; in contrast, little to no correlation exists for hydrous conditions.

Linkages Between the Megathrust and Upper-plate Deformation: Lessons From the Deformational Dichotomy of the 2016 Kaikoura New Zealand Earthquake

NASA Astrophysics Data System (ADS)

Furlong, K. P.; Herman, M. W.

2017-12-01

Following the 2016 Mw 7.8 Kaikoura earthquake, the nature of the coseismic rupture was unclear. Seismological and tsunami evidence pointed to significant involvement of the subduction megathrust, while geodetic and field observations pointed to a shallow set of intra-crustal faults as the main participants during the earthquake. It now appears that the Kaikoura earthquake produced synchronous faulting on the plate boundary subduction interface - the megathrust - and on a suite of crustal faults above the rupture zone in the overlying plate. This Kaikoura-style earthquake, involving synchronous ruptures on multiple components of the plate boundary, may be an important mode of plate boundary deformation affecting seismic hazard along subduction zones. Here we propose a model to explain how these upper-plate faults are loaded during the periods between megathrust earthquakes and subsequently can rupture synchronously with the megathrust. Between megathrust earthquakes, horizontal compression, driven by plate convergence, locks the upper-plate faults, particularly those at higher angles to the convergence direction and the oblique plate motion of the subducting Pacific plate deforms the upper-plate in bulk shear. During the time interval of megathrust rupture, two things happen which directly affect the stress conditions acting on these upper-plate faults: (1) slip on the megathrust and the associated `rebound' of the upper plate reduces the compressive or normal stress acting on the upper plate faults, and (2) the base of the upper plate faults (and the upper plate itself) is decoupled from the slab in the region above rupture area. The reduction in normal stress acting on these faults increases their Coulomb Stress state to strongly favor strike-slip fault slip, and the basal decoupling of the upper plate allows it to undergo nearly complete stress recovery in that region; enabling the occurrence of very large offsets on these faults - offsets that exceed the slip on the plate interface. With these results it is clear that the 2016 Kaikoura NZ earthquake represents a mode of subduction zone rupture that must be considered in other regions.

The Origin of Silicic Arc Crust - Insights from the Northern Pacific Volcanic Arcs through Space and Time

NASA Astrophysics Data System (ADS)

Straub, S. M.; Kelemen, P. B.

2016-12-01

The remarkable compositional similarities of andesitic crust at modern convergent margins and the continental crust has long evoked the hypothesis of similar origins. Key to understanding either genesis is understanding the mode of silica enrichment. Silicic crust cannot be directly extracted from the upper mantle. Hence, in modern arcs, numerous studies - observant of the pervasive and irrefutable evidence of melt mixing - proposed that arc andesites formed by mixing of mantle-derived basaltic melts and fusible silicic material from the overlying crust. Mass balance requires the amount of silicic crust in such hybrid andesites to be on the order to tens of percent, implying that their composition to be perceptibly influenced by the various crustal basements. In order to test this hypothesis, major and trace element compositions of mafic and silicic arc magmas with arc-typical low Ce/Pb< 10 of Northern Pacific arcs (Marianas through Mexico) were combined with Pb isotope ratios. Pb isotope ratios are considered highly sensitive to crustal contamination, and hence should reflect the variable composition of the oceanic and continental basement on which these arcs are constructed. In particular, in thick-crust continental arcs where the basement is isotopically different from the mantle and crustal assimilation thought to be most prevalent, silicic magmas must be expected to be distinct from those of the associated mafic melts. However, in a given arc, the Pb isotope ratios are constant with increasing melt silica regardless of the nature of the basement. This observation argues against a melt origin of silicic melts from the crustal basement and suggest them to be controlled by the same slab flux as their co-eval mafic counterparts. This inference is validated by the spatial and temporal pattern of arc Pb isotope ratios along the Northern Pacific margins and throughout the 50 million years of Cenozoic evolution of the Izu Bonin Mariana arc/trench system that are can be related to with systematic, `real-time' changes in the composition of the slab flux with no role of the crustal basement. In summary, these data suggest that that silicic melts are ultimately genetically linked to the mafic co-eval series from the mantle, by such mechanisms as fractional crystallization, or melt-rock reactions, or a combination of both.

A geodynamic constraint on Archean continental geotherms

NASA Astrophysics Data System (ADS)

Bailey, R. C.

2003-04-01

Dewey (1988) observed that gravitational collapse appears to currently limit the altitudes of large plateaus on Earth to about 3 to 5 km above sea level. Arndt (1999) summarized the evidence for the failure of large parts of the continental crust to reach even sea-level during the Archean. If this property of Archean continental elevations was also enforced by gravitational collapse, it permits an estimation of the geothermal gradient in Archean continental crust. If extensional (collapse) tectonics is primarily a balance between gravitational power and the power consumed by extensional (normal) faulting in the upper brittle crust, as analysed by Bailey (1999), then it occurs when continental elevations above ocean bottoms exceed about 0.4 times the thickness of the brittle crust (Bailey, 2000). Assuming an Archean oceanic depth of about 5 km, it follows that that the typical thickness of Archean continental brittle crustal must have been less than about 12 km. Assuming the brittle-ductile transition to occur at about 350 degrees Celsius, this suggests a steep geothermal gradient of at least 30 degrees Celsius per kilometer for Archean continents, during that part of the Archean when continents were primarily submarine. This result does not help resolve the Archean thermal paradox (England and Bickle, 1984) whereby the high global heat flow of the Archean conflicts with the rather shallow crustal Archean geotherms inferred from geobarometry. In fact, the low elevation of Archean continental platforms raises another paradox, a barometric one: that continents were significantly below sea-level implies, by isostasy, that continental crustal thicknesses were significantly less than 30 km, yet the geobarometric data utilized by England and Bickle indicated burial pressures of Archean continental material of up to 10 kb. One resolution of both paradoxes (as discussed by England and Bickle) would be to interpret such deep burials as transient crustal thickening events of duration less than the crustal thermal equilibriation time (about 10 to 30 Ma). Temporary entrainment in the wake of basal eclogite ``sinkers'' might provide such transient burial. Vlaar's (1994) modelling of this eclogite delamination process (tectonically elaborated by Zegers and van Keken (2001)) indicates such sinker events would be significantly shorter than 10 Ma. The topographic re-equilibriation of a hot moho above such a process would be similarly short (Kaufmann and Royden, 1994).

Crustal Growth: In Defense of the Dogma

NASA Astrophysics Data System (ADS)

Albarede, F.; Blichert-Toft, J.; Guitreau, M.

2012-12-01

Plate tectonics was not even in its teens when Armstrong suggested that mantle and crust have interacted at steady-state over Earth's history. With the help of new geochemical tools and large-scale compilations, the concept of steady-state crust (as opposed to continuous crustal growth) is being revived with the implications that the equivalent of several volumes of present-day crust (PDCV) may have been subducted through geological times. Here we argue --or recall-- that four different lines of evidence invalidate this model. (i) The subduction filter must be particularly efficient for argon, even more so than for LILE and most other volatile elements. Atmosphere collects 40Ar degassed from both the extant crust and the crust dragged down at subduction zones over geological time. Regardless of the residence time of the crust at the surface, the amount of atmospheric 40Ar limits subduction of continental crust into the mantle to < 30% of the PDCV [1]. (ii) EM II, the only component that undoubtedly represents subducted continental crust in oceanic basalts, is extremely uncommon. (iii) Crustal age histograms are irrepressibly episodic. It has been argued that erosion selectively removes the crust with the elusive ages [2]. Ages of detrital zircons, which in the selective erosion conjecture should fill the voids, do not support this view [3]. Episodicity is difficult to reconcile with a continental protolith isolated by the common geological processes working either at mid-ocean ridges or subduction zones. A role may be recognized for Wilson cycles, if they can be shown to have prevailed for the entire history of the Earth. Geochemistry demonstrates that superplume material makes up the crustal protolith of all the major juvenile provinces. (iv) The residence time in the mantle of the elements distinctive of the crust is similar to the age of the Earth or even longer [4]. Continental crust finds its source in the instabilities of the lower mantle and the irreversible extraction of its low-melting point components, not in the steady-state processing of the rather barren upper mantle. This is why we surmise that steady-state does not provide an adequate account of crustal dynamics. [1] Coltice, N. et al. Science 288 (2000) 845 [2] Gurnies, M. & Davies, G. J. Geol. 14 (1986) 396 [3] Guitreau, M. et al., Earth Planet. Sci. Lettes 337-338 (2012) 211 [4] Albarede, F. AGU Monogr. 160 (2005) 25.

Revisit of Criteria and Evidence for the Tectonic Erosion vs Accretion in East Asian Margin

NASA Astrophysics Data System (ADS)

Kimura, G.; Hamahashi, M.

2015-12-01

Accretionary and erosive margins provide tectonic end-members in subduction zone and how these tectonic processes might be recorded and recognizable in ancient subduction complexes remains a challenging issue. Tectonic erosion includes sediment subduction and basal erosion along the plate boundary megathrust and drags down the crust of the upper plate into the mantle. Geologic evidence for the erosion is commonly based on lost geological tectono-stratigraphic data, i.e. gaps in the record and indirect phenomena such as subsidence of the forearc slopes. A topographically rough surface such as seamount has been suggested to work like an erosive saw carving the upper plate. Another mechanism of basal erosion has been suggested to be hydrofracturing of upper plate materials due to dehydration-induced fluid pressures, resulting in entrainment of upper plate materials into the basal décollement. Considering the interaction between the ~30 km thick crust of the upper plate and subducting oceanic plate, a subduction dip angle of ~15°, and convergent rate of ~10 cm/year, at least ~1 Ma of continuous basal erosion is necessary to induce clear subsidence of the forearc because the width of plate interface between the upper crustal and subducting plates is about 115 km (30/cos15°). In several examples of subduction zones, for example the Japan Trench and the Middle America Trench off Costa Rica, the subsidence of a few thousand metres of the forearc, combined with a lack of accretionary prism over a period of several million years, suggest that the erosive condition needs to be maintained for several to tens of million years.Such age gaps in the accretionary complex, however, do not automatically imply that tectonic erosion has taken place, as other interpretations such as no accretion, cessation of subduction, and/or later tectonic modification, are also possible. Recent drilling in the forearc of the Nankai Trough suggests that the accretion was ceased between ~12 Ma to ~8 Ma due to the transference of subduction from the Pacific Plate to the Philippine Sea Plate, as opposed to the continuous subduction of the Phillipine Sea Plate with subduction erosion.

Crustal structure between Lake Mead, Nevada, and Mono Lake, California

USGS Publications Warehouse

Johnson, Lane R.

1964-01-01

Interpretation of a reversed seismic-refraction profile between Lake Mead, Nevada, and Mono Lake, California, indicates velocities of 6.15 km/sec for the upper layer of the crust, 7.10 km/sec for an intermediate layer, and 7.80 km/sec for the uppermost mantle. Phases interpreted to be reflections from the top of the intermediate layer and the Mohorovicic discontinuity were used with the refraction data to calculate depths. The depth to the Moho increases from about 30 km near Lake Mead to about 40 km near Mono Lake. Variations in arrival times provide evidence for fairly sharp flexures in the Moho. Offsets in the Moho of 4 km at one point and 2 1/2 km at another correspond to large faults at the surface, and it is suggested that fracture zones in the upper crust may displace the Moho and extend into the upper mantle. The phase P appears to be an extension of the reflection from the top of the intermediate layer beyond the critical angle. Bouguer gravity, computed for the seismic model of the crust, is in good agreement with the measured Bouguer gravity. Thus a model of the crustal structure is presented which is consistent with three semi-independent sources of geophysical data: seismic-refraction, seismic-reflection, and gravity.

Production and recycling of oceanic crust in the early Earth

NASA Astrophysics Data System (ADS)

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

2004-08-01

Because of the strongly different conditions in the mantle of the early Earth regarding temperature and viscosity, present-day geodynamics cannot simply be extrapolated back to the early history of the Earth. We use numerical thermochemical convection models including partial melting and a simple mechanism for melt segregation and oceanic crust production to investigate an alternative suite of dynamics which may have been in operation in the early Earth. Our modelling results show three processes that may have played an important role in the production and recycling of oceanic crust: (1) Small-scale ( x×100 km) convection involving the lower crust and shallow upper mantle. Partial melting and thus crustal production takes place in the upwelling limb and delamination of the eclogitic lower crust in the downwelling limb. (2) Large-scale resurfacing events in which (nearly) the complete crust sinks into the (eventually lower) mantle, thereby forming a stable reservoir enriched in incompatible elements in the deep mantle. New crust is simultaneously formed at the surface from segregating melt. (3) Intrusion of lower mantle diapirs with a high excess temperature (about 250 K) into the upper mantle, causing massive melting and crustal growth. This allows for plumes in the Archean upper mantle with a much higher excess temperature than previously expected from theoretical considerations.

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

NASA Astrophysics Data System (ADS)

Zeitler, P. K.; Meltzer, A.

2010-12-01

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

Flat-slab subduction, whole crustal faulting, and geohazards in Alaska: Targets for Earthscope

NASA Astrophysics Data System (ADS)

Gulick, S. P.; Pavlis, T. L.; Bruhn, R. L.; Christeson, G. L.; Freymueller, J. T.; Hansen, R. A.; Koons, P. O.; Pavlis, G. L.; Roeske, S.; Reece, R.; van Avendonk, H. J.; Worthington, L. L.

2010-12-01

Crustal structure and evolution illuminated by the Continental Dynamics ST. Elias Erosion and tectonics Project (STEEP) highlights some fundamental questions about active tectonics processes in Alaska including: 1) what are the controls on far field deformation and lithospheric stabilization, 2) do strike slip faults extend through the entire crust and upper mantle and how does this influence mantle flow, and 3) how does the transition from “normal” subduction of the Pacific along the Aleutians to flat slab subduction of the Yakutat Terrane beneath southeast and central Alaska to translation of the Yakutat Terrane past North American in eastern Alaska affect geohazard assessment for the north Pacific? Active and passive seismic studies and geologic fieldwork focusing on the Yakutat Terrane show that the Terrane ranges from 15-35 km thick and is underthrusting the North American plate from the St. Elias Mountains to the Alaska Range (~500 km). Deformation of the upper plate occurs within the offshore Pamplona Zone fold and thrust belt, and onshore throughout the Robinson Mountains. Deformation patterns, structural evolution, and the sedimentary products of orogenesis are fundamentally influenced by feedbacks with glacial erosion. The Yakutat megathrust extends beneath Prince William Sound such that the 1964 Mw 9.2 great earthquake epicenter was on this plate boundary and jumped to the adjacent Aleutian megathrust coseismically; this event illuminates the potential for transitional tectonic systems to enhance geohazards. The northern, southern, and eastern limits of the Yakutat microplate are strike-slip faults that, where imaged, appear to cut the entire crustal section and may allow for crustal extrusion towards the Bering Sea. Yakutat Terrane effects on mantle flow, however, have been suggested to cross these crustal features to allow for far-field deformation in the Yukon, Brooks Range, and Amerasia Basin. From the STEEP results it is clear that the Yakutat Terrane is driving a range of tectonic and surface processes perturbing the Aleutian subduction system at its eastern extent and linking this system with Laramide style subduction and plate boundary strike-slip tectonics farther east. Targeted geodetic and seismic deployments as part of Earthscope could examine all of these features and seek to address fundamental questions about tectonic interactions.

Crustal motion measurements from the POLENET Antarctic Network: comparisons with glacial isostatic adjustment models

NASA Astrophysics Data System (ADS)

Wilson, T. J.; Konfal, S. A.; Bevis, M. G.; Spada, G.; Melini, D.; Barletta, V. R.; Kendrick, E. C.; Saddler, D.; Smalley, R., Jr.; Dalziel, I. W. D.; Willis, M. J.

2016-12-01

Crustal motions measured by GPS provide a unique proxy record of ice mass change, due to the elastic and viscoelastic response of the earth to removal of ice loads. The ANET/POLENET array of bedrock GPS sites spans much of the Antarctic interior, encompassing regions where glacial isostatic adjustment (GIA) models predict large crustal displacements due to LGM ice loss and including coastal West Antarctica where major modern ice mass loss is documented. To isolate the long-term GIA component of measured crustal motions, we computed and removed elastic displacements due to recent ice mass change. We used the annually resolved ice mass balance data from Martín-Español et al. (2016) derived from a statistical inversion of satellite altimetry, gravimetry, and elastic-corrected GPS data for the period 2003-2013. The Regional Elastic Rebound Calculator (REAR) [Melini et al., 2015] was used to compute elastic vertical and horizontal surface displacements. Uplift due to elastic rebound is substantial in West Antarctica, very minimal in East Antarctica, and variable across the Weddell Embayment. The ANET GPS-derived crustal motion patterns ascribed to non-elastic GIA are spatially complex and differ significantly in magnitude from model predictions. We present a systematic comparison of measured and predicted velocities within different sectors of Antarctica, in order to examine spatial patterns relative to modern ice mass changes, ice history model uncertainties, and lateral variations in earth properties. In the Weddell Embayment region most vertical velocities are lower than uplift predicted by GIA models. Several sites in the southernmost Transantarctic Mountains and the Whitmore Mountains, where small ice mass increase occurs, have vertical uplift significantly exceeding GIA model predictions. There is an intriguing spatial correlation of these fast-moving sites with a low-velocity anomaly in the upper mantle documented by analysis of teleseismic Rayleigh waves by Heeszel et al. (2016). Significant non-elastic GIA velocities occur in the Amundsen Sea Embayment sector, with high uplift flanked by subsiding regions. This pattern can be modeled as a viscoelastic response to ice loss on decadal-centennial time scales in a region with weak upper mantle, consistent with seismic results in the region.

The influence of regional extensional tectonic stress on the eruptive behaviour of subduction-zone volcanoes

NASA Astrophysics Data System (ADS)

Tost, M.; Cronin, S. J.

2015-12-01

Regional tectonic stress is considered a trigger mechanism for explosive volcanic activity, but the related mechanisms at depth are not well understood. The unique geological setting of Ruapehu, New Zealand, allows investigation on the effect of enhanced regional extensional crustal tension on the eruptive behaviour of subduction-zone volcanoes. The composite cone is located at the southwestern terminus of the Taupo Volcanic Zone, one of the most active silicic magma systems on Earth, which extends through the central part of New Zealand's North Island. Rhyolitic caldera eruptions are limited to its central part where crustal extension is highest, whereas lower extension and additional dextral shear dominate in the southwestern and northeastern segments characterized by andesitic volcanism. South of Ruapehu, the intra-arc rift zone traverses into a compressional geological setting with updoming marine sequences dissected by reverse and normal faults. The current eruptive behaviour of Ruapehu is dominated by small-scaled vulcanian eruptions, but our studies indicate that subplinian to plinian eruptions have frequently occurred since ≥340 ka and were usually preceded by major rhyolitic caldera unrest in the Taupo Volcanic Zone. Pre-existing structures related to the NNW-SSE trending subduction-zone setting are thought to extend at depth and create preferred pathways for the silicic magma bodies, which may facilitate the development of large (>100 km3) dyke-like upper-crustal storage systems prior to major caldera activity. This may cause enhanced extensional stress throughout the entire intra-arc setting, including the Ruapehu area. During periods of caldera dormancy, the thick crust underlying the volcano and the enhanced dextral share rate likely impede ascent of larger andesitic magma bodies, and storage of andesitic melts dominantly occurs within small-scaled magma bodies at middle- to lower-crustal levels. During episodes of major caldera unrest, ascent and storage of voluminous rhyolitic magma bodies at upper crustal levels may cause the extensional stress to supercede the dextral shear rate in the Ruapehu area, facilitating ascent of larger andesitic magma bodies at depth, and changing the volcano's eruptive behaviour from dominantly vulcanian to violently subplinian/plinian.

Transient radon signals driven by fluid pressure pulse, micro-crack closure, and failure during granite deformation experiments

NASA Astrophysics Data System (ADS)

Girault, Frédéric; Schubnel, Alexandre; Pili, Éric

2017-09-01

In seismically active fault zones, various crustal fluids including gases are released at the surface. Radon-222, a radioactive gas naturally produced in rocks, is used in volcanic and tectonic contexts to illuminate crustal deformation or earthquake mechanisms. At some locations, intriguing radon signals have been recorded before, during, or after tectonic events, but such observations remain controversial, mainly because physical characterization of potential radon anomalies from the upper crust is lacking. Here we conducted several month-long deformation experiments under controlled dry upper crustal conditions with a triaxial cell to continuously monitor radon emission from crustal rocks affected by three main effects: a fluid pressure pulse, micro-crack closure, and differential stress increase to macroscopic failure. We found that these effects are systematically associated with a variety of radon signals that can be explained using a first-order advective model of radon transport. First, connection to a source of deep fluid pressure (a fluid pressure pulse) is associated with a large transient radon emission increase (factor of 3-7) compared with the background level. We reason that peak amplitude is governed by the accumulation time and the radon source term, and that peak duration is controlled by radioactive decay, permeability, and advective losses of radon. Second, increasing isostatic compression is first accompanied by an increase in radon emission followed by a decrease beyond a critical pressure representing the depth below which crack closure hampers radon emission (150-250 MPa, ca. 5.5-9.5 km depth in our experiments). Third, the increase of differential stress, and associated shear and volumetric deformation, systematically triggers significant radon peaks (ca. 25-350% above background level) before macroscopic failure, by connecting isolated cracks, which dramatically enhances permeability. The detection of transient radon signals before rupture indicates that connection of initially isolated cracks in crustal rocks may occur before rupture and potentially lead to radon transients measurable at the surface in tectonically active regions. This study offers thus an experimental and physical basis for understanding predicted or reported radon anomalies.

Transient radon signals driven by fluid pressure pulse, micro-crack closure, and failure during granite deformation experiments

NASA Astrophysics Data System (ADS)

Schubnel, A.; Girault, F.; Pili, E.

2017-12-01

In seismically active fault zones, various crustal fluids including gases are released at the surface. Radon-222, a radioactive gas naturally produced in rocks, is used in volcanic and tectonic contexts to illuminate crustal deformation or earthquake mechanisms. At some locations, intriguing radon signals have been recorded before, during, or after tectonic events, but such observations remain controversial, mainly because physical characterization of potential radon anomalies from the upper crust is lacking. Here we conducted several month-long deformation experiments under controlled dry upper crustal conditions with a triaxial cell to continuously monitor radon emission from crustal rocks affected by three main effects: a fluid pressure pulse, micro-crack closure, and differential stress increase to macroscopic failure. We found that these effects are systematically associated with a variety of radon signals that can be explained using a first-order advective model of radon transport. First, connection to a source of deep fluid pressure (a fluid pressure pulse) is associated with a large transient radon emission increase (factor of 3-7) compared with the background level. We reason that peak amplitude is governed by the accumulation time and the radon source term, and that peak duration is controlled by radioactive decay, permeability, and advective losses of radon. Second, increasing isostatic compression is first accompanied by an increase in radon emission followed by a decrease beyond a critical pressure representing the depth below which crack closure hampers radon emission (150-250 MPa, ca. 5.5-9.5 km depth in our experiments). Third, the increase of differential stress, and associated shear and volumetric deformation, systematically triggers significant radon peaks (ca. 25-350% above background level) before macroscopic failure, by connecting isolated cracks, which dramatically enhances permeability. The detection of transient radon signals before rupture indicates that connection of initially isolated cracks in crustal rocks may occur before rupture and potentially lead to radon transients measurable at the surface in tectonically active regions. This study offers thus an experimental and physical basis for understanding predicted or reported radon anomalies.

The Stratigraphy and Evolution of the Lunar Crust

NASA Technical Reports Server (NTRS)

McCallum, I. Stewart

1998-01-01

Reconstruction of stratigraphic relationships in the ancient lunar crust has proved to be a formidable task. The intense bombardment during the first 700 m.y. of lunar history has severely perturbed the original stratigraphy and destroyed the primary textures of all but a few nonmare rocks. However, a knowledge of the crustal stratigraphy as it existed prior to the cataclysmic bombardment about 3.9 Ga is essential to test the major models proposed for crustal origin, i.e., crystal fractionation in a global magmasphere or serial magmatism in a large number of smaller bodies. Despite the large difference in scale implicit in these two models, both require an efficient separation of plagioclase and mafic minerals to form the anorthositic crust and the mafic mantle. Despite the havoc wreaked by the large body impactors, these same impact processes have brought to the lunar surface crystalline samples derived from at least the upper half of the lunar crust, thereby providing an opportunity to reconstruct the stratigraphy in areas sampled by the Apollo missions. As noted, ejecta from the large multiring basins are dominantly, or even exclusively, of crustal origin. Given the most recent determinations of crustal thicknesses, this implies an upper limit to the depth of excavation of about 60 km. Of all the lunar samples studied, a small set has been recognized as "pristine", and within this pristine group, a small fraction have retained some vestiges of primary features formed during the earliest stages of crystallization or recrystallization prior to 4.0 Ga. We have examined a number of these samples that have retained some record of primary crystallization to deduce thermal histories from an analysis of structural, textural, and compositional features in minerals from these samples. Specifically, by quantitative modeling of (1) the growth rate and development of compositional profiles of exsolution lamellae in pyroxenes and (2) the rate of Fe-Mg ordering in orthopyroxenes, we can constrain the cooling rates of appropriate lunar samples. These cooling rates are used to compute depths of burial at the time of crystallization, which enable us to reconstruct parts of the crustal stratigraphy as it existed during the earliest stages of lunar history.

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

NASA Astrophysics Data System (ADS)

Markus Schmalholz, Stefan; Jaquet, Yoann

2016-04-01

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

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

NASA Astrophysics Data System (ADS)

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

2007-12-01

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

Assessing the Anthropogenic Fugitive Dust Emission Inventory and Temporal Allocation Using an Updated Speciation of Particulate Matter

EPA Science Inventory

Crustal materials are mainly emitted by anthropogenic and windblown fugitive dust, but also may potentially include some fly ash and industrial process emissions which are chemically similar to crustal emissions. Source apportionment studies have shown that anthropogenic fugitive...

Intra-continental subduction and contemporaneous lateral extrusion of the upper plate: insights into Alps-Adria interactions

NASA Astrophysics Data System (ADS)

van Gelder, Inge; Willingshofer, Ernst; Sokoutis, Dimitrios; Cloetingh, Sierd

2017-04-01

A series of physical analogue experiments were performed to simulate intra-continental subduction contemporaneous with lateral extrusion of the upper plate to study the interferences between these two processes at crustal levels and in the lithospheric mantle. The lithospheric-scale models are specifically designed to represent the collision of the Adriatic microplate with the Eastern Alps, simulated by an intra-continental weak zone to initiate subduction and a weak confined margin perpendicular to the direction of convergence in order to allow for extrusion of the lithosphere. The weak confined margin is the analog for the opening of the Pannonian back-arc basin adjacent to the Eastern Alps with the direction of extension perpendicular to the strike of the orogen. The models show that intra-continental subduction and coeval lateral extrusion of the upper plate are compatible processes. The obtained deformation structures within the extruding region are similar compared to the classical setup where lateral extrusion is provoked by lithosphere-scale indentation. In the models a strong coupling across the subduction boundary allows for the transfer of abundant stresses to the upper plate, leading to laterally varying strain regimes that are characterized by crustal thickening near a confined margin and dominated by lateral displacement of material near a weak lateral confinement. During ongoing convergence the strain regimes propagate laterally, thereby creating an area of overlap characterized by transpression. In models with oblique subduction, with respect to the convergence direction, less deformation of the upper plate is observed and as a consequence the amount of lateral extrusion decreases. Additionally, strain is partitioned along the oblique plate boundary leading to less subduction in expense of right lateral displacement close to the weak lateral confinement. Both oblique and orthogonal subduction models have a strong resemblance to lateral extrusion tectonics of the Eastern Alps, where subduction of the adjacent Adriatic plate beneath the Eastern Alps is debated. Our results highlight that both indentation and subduction of Adria are valid collisional mechanisms to provoke lateral extrusion-type deformation within the Eastern Alps lithosphere, i.e. the upper plate. Moreover, the insights suggest that the Oligocene to Late Miocene structural evolution of the Eastern Alps is best described by phases of oblique and subsequent orthogonal subduction which is in line with Miocene rotations of the Adriatic plate. Furthermore, oblique subduction of the Adriatic plate provides a viable mechanism to explain the rapid decrease in slab length beneath the Eastern Alps towards the Pannonian Basin, also implying that the Adriatic slab can behave and form independently with regards to the adjacent subduction of Adria beneath the Dinarides.

The Tanami deep seismic reflection experiment: An insight into gold mineralization and Paleoproterozoic collision in the North Australian Craton

NASA Astrophysics Data System (ADS)

Goleby, Bruce R.; Huston, David L.; Lyons, Patrick; Vandenberg, Leon; Bagas, Leon; Davies, Brett M.; Jones, Leonie E. A.; Gebre-Mariam, Musie; Johnson, Wade; Smith, Tim; English, Luc

2009-07-01

Imaging of a major collision zone between the Tanami region and Aileron Province of the Arunta Orogen in Northern Australia, and recognition that several of the major gold deposits within the Tanami region are within near-surface antiformal stacks or uplifted and exhumed crustal sections associated with major crustal-penetrating shear zones, are fundamental results from the 2005 Tanami Seismic Collaborative Research Project. The suture, which is interpreted to have resulted from collision, separates the northwest-dipping structural grain of the Aileron Province crust in the south from the southeast-dipping structural grain of the Tanami crust in the northwest. The collision between the Tanami region and the Aileron Province is interpreted to have occurred prior to ca. 1840 Ma. The correlation between the surface extension of crustal-penetrating shear zones that extend to the Moho boundary and the locations of known gold-rich mineral fields is significant and has implications for minerals explorers within the Tanami region, and elsewhere. In the near-surface, where the crustal-penetrating structures cut relatively shallow upper crustal Tanami Group rocks, there is a significant increase in the degree of local deformation and results in through-going thrust faults, associated pop-up structures, ramp anticlines and antiformal stacking. All known ore deposits appear to be located within these more complexly deformed zones and therefore have a direct association with larger-scale structures.

Teleseismic P-wave Attenuation beneath the Eastern Tibetan Plateau

NASA Astrophysics Data System (ADS)

Gao, S. S.; Peng, Y.; Liu, K. H.

2017-12-01

Numerous laboratory, theoretical, and observational studies have demonstrated that the teleseismic body-wave attenuation factor (or t*), which is quantified by the travel time over the quality factor Q, can provide robust constraints on the thermal and physical state of the crust and upper mantle, and thus is ideal for investigating crustal and mantle dynamics in tectonically active areas such as the Eastern Tibetan Plateau, where pervasively distributed lower crustal flow has been regarded as a mechanism for its shortening and uplift. For this study, broadband seismic data recorded by 256 stations are used to compute the t* relative to a station in the Sichuan Basin. We have developed a set of procedures to reliably measure the P-wave t* values using the spectral ratio method through manually adjusting the time window and visually determining the quality of the measurements. Anomalously high t* values are found beneath active orogenic zones such as Qinling and Longmenshan, with magnitude of about 0.4 s. The Longmenshan block shows the most significant and spatially-consistent high t* measurements, probably caused by the accumulation of a thick and partially melted lower crustal layer. This high attenuation zone continues toward the south to the Songpan-Ganzi terrane to the south, but with a greatly reduced magnitude, suggesting a thinner low-viscosity lower crustal layer. The observations provided independent constraints on the spatial distribution and thickness of the proposed system of lower crustal flow.

Boron isotope fractionation in magma via crustal carbonate dissolution

PubMed Central

Deegan, Frances M.; Troll, Valentin R.; Whitehouse, Martin J.; Jolis, Ester M.; Freda, Carmela

2016-01-01

Carbon dioxide released by arc volcanoes is widely considered to originate from the mantle and from subducted sediments. Fluids released from upper arc carbonates, however, have recently been proposed to help modulate arc CO2 fluxes. Here we use boron as a tracer, which substitutes for carbon in limestone, to further investigate crustal carbonate degassing in volcanic arcs. We performed laboratory experiments replicating limestone assimilation into magma at crustal pressure-temperature conditions and analysed boron isotope ratios in the resulting experimental glasses. Limestone dissolution and assimilation generates CaO-enriched glass near the reaction site and a CO2-dominated vapour phase. The CaO-rich glasses have extremely low δ11B values down to −41.5‰, reflecting preferential partitioning of 10B into the assimilating melt. Loss of 11B from the reaction site occurs via the CO2 vapour phase generated during carbonate dissolution, which transports 11B away from the reaction site as a boron-rich fluid phase. Our results demonstrate the efficacy of boron isotope fractionation during crustal carbonate assimilation and suggest that low δ11B melt values in arc magmas could flag shallow-level additions to the subduction cycle. PMID:27488228

Magsat equivalent source anomalies over the southeastern United States - Implications for crustal magnetization

NASA Technical Reports Server (NTRS)

Ruder, M. E.; Alexander, S. S.

1986-01-01

The Magsat crustal anomaly field depicts a previously-unidentified long-wavelength negative anomaly centered over southeastern Georgia. Examination of Magsat ascending and descending passes clearly identifies the anomalous region, despite the high-frequency noise present in the data. Using ancillary seismic, electrical conductivity, Bouguer gravity, and aeromagnetic data, a preliminary model of crustal magnetization for the southern Appalachian region is presented. A lower crust characterized by a pervasive negative magnetization contrast extends from the New York-Alabama lineament southeast to the Fall Line. In southern Georgia and eastern Alabama (coincident with the Brunswick Terrane), the model calls for lower crustal magnetization contrast of -2.4 A/m; northern Georgia and the Carolinas are modeled with contrasts of -1.5 A/m. Large-scale blocks in the upper crust which correspond to the Blue Ridge, Charlotte belt, and Carolina Slate belt, are modeled with magnetization contrasts of -1.2 A/m, 1.2 A/m, and 1.2 A/m respectively. The model accurately reproduces the amplitude of the observed low in the equivalent source Magsat anomaly field calculated at 325 km altitude and is spatially consistent with the 400 km lowpass-filtered aeromagnetic map of the region.

Boron isotope fractionation in magma via crustal carbonate dissolution

NASA Astrophysics Data System (ADS)

Deegan, Frances M.; Troll, Valentin R.; Whitehouse, Martin J.; Jolis, Ester M.; Freda, Carmela

2016-08-01

Carbon dioxide released by arc volcanoes is widely considered to originate from the mantle and from subducted sediments. Fluids released from upper arc carbonates, however, have recently been proposed to help modulate arc CO2 fluxes. Here we use boron as a tracer, which substitutes for carbon in limestone, to further investigate crustal carbonate degassing in volcanic arcs. We performed laboratory experiments replicating limestone assimilation into magma at crustal pressure-temperature conditions and analysed boron isotope ratios in the resulting experimental glasses. Limestone dissolution and assimilation generates CaO-enriched glass near the reaction site and a CO2-dominated vapour phase. The CaO-rich glasses have extremely low δ11B values down to -41.5‰, reflecting preferential partitioning of 10B into the assimilating melt. Loss of 11B from the reaction site occurs via the CO2 vapour phase generated during carbonate dissolution, which transports 11B away from the reaction site as a boron-rich fluid phase. Our results demonstrate the efficacy of boron isotope fractionation during crustal carbonate assimilation and suggest that low δ11B melt values in arc magmas could flag shallow-level additions to the subduction cycle.

Boron isotope fractionation in magma via crustal carbonate dissolution.

PubMed

Deegan, Frances M; Troll, Valentin R; Whitehouse, Martin J; Jolis, Ester M; Freda, Carmela

2016-08-04

Carbon dioxide released by arc volcanoes is widely considered to originate from the mantle and from subducted sediments. Fluids released from upper arc carbonates, however, have recently been proposed to help modulate arc CO2 fluxes. Here we use boron as a tracer, which substitutes for carbon in limestone, to further investigate crustal carbonate degassing in volcanic arcs. We performed laboratory experiments replicating limestone assimilation into magma at crustal pressure-temperature conditions and analysed boron isotope ratios in the resulting experimental glasses. Limestone dissolution and assimilation generates CaO-enriched glass near the reaction site and a CO2-dominated vapour phase. The CaO-rich glasses have extremely low δ(11)B values down to -41.5‰, reflecting preferential partitioning of (10)B into the assimilating melt. Loss of (11)B from the reaction site occurs via the CO2 vapour phase generated during carbonate dissolution, which transports (11)B away from the reaction site as a boron-rich fluid phase. Our results demonstrate the efficacy of boron isotope fractionation during crustal carbonate assimilation and suggest that low δ(11)B melt values in arc magmas could flag shallow-level additions to the subduction cycle.

Seismic imaging of deep low-velocity zone beneath the Dead Sea basin and transform fault: Implications for strain localization and crustal rigidity

USGS Publications Warehouse

ten Brink, Uri S.; Al-Zoubi, A. S.; Flores, C.H.; Rotstein, Y.; Qabbani, I.; Harder, S.H.; Keller, Gordon R.

2006-01-01

New seismic observations from the Dead Sea basin (DSB), a large pull-apart basin along the Dead Sea transform (DST) plate boundary, show a low velocity zone extending to a depth of 18 km under the basin. The lower crust and Moho are not perturbed. These observations are incompatible with the current view of mid-crustal strength at low temperatures and with support of the basin's negative load by a rigid elastic plate. Strain softening in the middle crust is invoked to explain the isostatic compensation and the rapid subsidence of the basin during the Pleistocene. Whether the deformation is influenced by the presence of fluids and by a long history of seismic activity on the DST, and what the exact softening mechanism is, remain open questions. The uplift surrounding the DST also appears to be an upper crustal phenomenon but its relationship to a mid-crustal strength minimum is less clear. The shear deformation associated with the transform plate boundary motion appears, on the other hand, to cut throughout the entire crust. Copyright 2006 by the American Geophysical Union.

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.

Geochemistry of Aerosols in North -West India

NASA Astrophysics Data System (ADS)

Yadav, S.

2001-12-01

The geochemistry of, sources to, and transport of dust and their consequences on earth system processes are least understood. Here we follow Guru Gil's approach to use trace elements and isotopic data on aerosols in northwest India to understand their sources and processes. Aerosol samples were collected over a stretch of 550 kms from the Thar desert to the Delhi region which sits on the eastern fringe of the Thar. This region witnesses frequent dust storms in summer seasons due to high intensity south westerly and westerly winds. Major, trace elements including REE and Sr isotopic data are generated on three different group of aerosols such as dry deposition (DD, >100μm ), suspended particulate matter (SPM, 0.1-100μm ) and PM10 (<10μm ) collected by different sampling techniques and at different seasons. Major and trace elements in all DD samples show a high degree of homogeneity with similar LREE enriched patterns and -ve Eu anomalies regardless of space and time of sampling. These particles are geochemically similar to UCC, PAAS, and local loess with relatively low CIA values, indicating a low degree of chemical weathering suffered by their source regions dominated by upper crustal materials. Geochemistry of SPM and PM10 aerosols shows variation along the wind path. With decreasing size, crustal component decreases, anthropogenic inputs to transition metals and non-silicate input to Ca budget increase. The greater variability in finer fractions is primarily due to anthropogenic input and local meteorology where wind acts as an intensive variable. The REE patterns in SPM and PM10 are similar to DD samples with -ve Eu anomaly but their abundance decrease with size, probably due to some dilution effect by organics and mineralogical factors. It is possible that REE hosting minerals are concentrated in coarser silt and finest sand fractions (35-70μm ). This is supported by the decreasing Zr concentrations with decreasing size and distance in downwind direction where the wind velocity reduces. 87Sr/86Sr isotopic ratios of all the fractions also show a small size dependency. The coarser samples have higher values (0.719-0.720) compared to finer ones (0.715-0.717). These values are similar to the lower end of Indo-Gangetic alluvium ratios (J. Tripathy), perhaps resulting from the minor addition of Jurassic carbonate/gypsum, present in the upwind part. Thus, the source for the dominant component of aerosols is upper crustal and is likely to be older Himalayan alluvium deposited by now defunct rivers in the Rajasthan region. The removal of silty materials (a dominant part of aerosols) from the older alluvium is probably responsible for the presence of Thar desert in the upwind direction.

Drilling constraints on bimodal volcanism and subsequent formation of contrasted uppermost crustal compositions at the middle Okinawa Trough

NASA Astrophysics Data System (ADS)

Yamasaki, T.; Takaya, Y.; Mukae, N.; Nagase, T.; Tindell, T.; Totsuka, S.; Uno, Y.; Yonezu, K.; Nozaki, T.; Ishibashi, J. I.; Kumagai, H.; Maeda, L.; Shipboard Scientist, C.

2016-12-01

The Okinawa Trough (OT) is a young and actively spreading back-arc basin, extending behind the Ryukyu arc-trench system in the southeastern margin of the East China Sea. The OT is believed to be in an initial rifting stage (starting from 6-9 Ma), prior to the normal/stable seafloor spreading which constitutes the main stage of back-arc basin formation. Two drilling cruises ‒ the IODP Exp. 331 and SIP CK14-04 D/V Chikyu Cruise (Exp. 907) in 2010 and 2014 ‒ were conducted at the Iheya North Knoll, middle OT. The Iheya North Knoll is a domal volcanic complex consisting of small volcanic bodies. On these cruises, pumiceous gravel and altered rhyolitic rocks, as well as hemi-pelagic sediments, hydrothermal clay and Kuroko-type ores, were recovered from the upper 200 m of the crust. From Feb. 11, 2016 to Mar. 17, 2016, the SIP CK16-01 (Exp. 908) D/V Chikyu cruise was conducted at Iheya North Knoll and the sediment-covered rifting center of the Iheya-Minor Ridge area, middle OT. The Iheya-Minor ridge area is also an active hydrothermal field, located 25 km southeast of the Iheya North Knoll. In this area, basaltic rocks are widely distributed, and drilling has confirmed that the basaltic materials continue to 120 m below the seafloor. From an igneous petrological point of view, the volcanic rocks in the Okinawa Trough are characterized by bimodal basaltic and rhyolitic compositions, with a compositional gap between SiO2 = 56-66 wt%. The origin of the rhyolitic rock has been interpreted as magmatic differentiation of basaltic magma. However, the existence of an active basalt-hosted hydrothermal field in the Iheya-Minor ridge area suggests the presence of hot basaltic rocks at a shallow position in the crust, and reaching recharged seawater at this depth. Furthermore, the composition of felsic rocks just after the compositional gap (SiO2 = 67 wt%) is very similar to that of the minimum melt of a granitic system, and experimental partial melt of hydrous basalt. Therefore, the contrast in the uppermost crustal composition between very close ( 25 km) areas can reasonably be explained by re-melting of hydrothermally-altered basaltic rocks and production of felsic magma at the upper crustal level, and direct eruption of basaltic magma at the seafloor.

Active faulting on the island of Crete (Greece)

NASA Astrophysics Data System (ADS)

Caputo, Riccardo; Catalano, Stefano; Monaco, Carmelo; Romagnoli, Gino; Tortorici, Giuseppe; Tortorici, Luigi

2010-10-01

ABSTRACT In order to characterize and quantify the Middle-Late Quaternary and ongoing deformation within the Southern Aegean forearc, we analyse the major tectonic structures affecting the island of Crete and its offshore. The normal faults typically consist of 4-30-km-long dip-slip segments locally organised in more complex fault zones. They separate carbonate and/or metamorphic massifs, in the footwall block, from loose to poorly consolidated alluvial and colluvial materials within the hangingwall. All these faults show clear evidences of recent re-activations and trend parallel to two principal directions: WNW-ESE and NNE-SSW. Based on all available data for both onland and offshore structures (morphological and structural mapping, satellite imagery and airphotographs remote sensing as well as the analysis of seismic profiles and the investigation of marine terraces and Holocene raised notches along the island coasts), for each fault we estimate and constrain some of the principal seismotectonic parameters and particularly the fault kinematics, the cumulative amount of slip and the slip-rate. Following simple assumptions and empirical relationships, maximum expected magnitudes and mean recurrence periods are also suggested. Summing up the contribution to crustal extension provided by the two major fault sets we calculate both arc-normal and arc-parallel long-term strain rates. The occurrence of slightly deeper and more external low-angle thrust planes associated with the incipient continental collision occurring in western Crete is also analysed. Although these contractional structures can generate stronger seismic events (M ~ 7.5.) they are probably much rarer and thus providing a minor contribution to the overall morphotectonic evolution of the island and the forearc. A comparison of our geologically-based results with those obtained from GPS measurements show a good agreement, therefore suggesting that the present-day crustal deformation is probably active since Middle Quaternary and mainly related to the seismic activity of upper crustal normal faults characterized by frequent shallow (<20 km) moderate-to-strong seismic events seldom alternating with stronger earthquakes occurring along blind low-angle thrust planes probably ramping from a deeper aseismic detachment (ca. 25 km). This apparently contradicting co-existence of juxtaposed upper tensional and lower compressional tectonic regimes is in agreement with the geodynamics of the region characterised by continental collision with Nubia and the Aegean mantle wedging.

The MIRROR cruise (2011): Deep crustal structure of the Moroccan Atlantic Margin from wide-angle and reflection seismic data

NASA Astrophysics Data System (ADS)

Klingelhoefer, F.; Aslanian, D.; Sahabi, M.; Moulin, M.; Schnurle, P.; Berglar, K.; Biari, Y.; Feld, A.; Graindorge, D.; Corela, C.; Mehdi, K.; Zourarah, B.; Perrot, J.; Alves Ribeiro, J.; Reichert, C. J.

2011-12-01

The study of conjugate margins is important to test different hypotheses of rifting and initial opening of an ocean. In this scope, seven wide-angle seismic profiles were acquired on the Moroccan Atlantic margin (at the latitudes between 32° and 33° N) together with coincident deep frequency reflection seismic data during the MIRROR cruise in May and June 2011. The main seismic profile is conjugate to an existing wide-angle seismic profile off Nova Scotia (SMART 2). Further objectives of the cruise were to image ocean-continent transition zone, to detect and eventually quantify exhumed upper mantle material present in this zone and to determine the origin of the high amplitude West African Magnetic Anomaly, which is conjugate to the north American East Coast Magnetic Anomaly and can be linked to the opening of the Atlantic. Two of the newly acquired profiles are located perpendicular and five parallel to the Moroccan margin. The seismic profiles are between 130 and 260 km in length and between 28 and 13 ocean-bottom seismometers were deployed on each one. One profile was extended on land by 15 landstations in order to better image the zone of continental thinning. A 4.5 km digital streamer and a 7200 cu inch tuned airgun array were used for the acquisition of the seismic data. Additionally magnetic, bathymetric and high resolution seismic data were acquired in the study region. Preliminary results from tomographic inversion of the first arrivals from the ocean-bottom seismometer data image the zone of crustal thinning from about 25 km to 6 km in the basin along about 70 kilometers of the profiles which are located perpendicular to the margin. The oceanic crust can be divided into 2 regions, based on the lower crustal velocities. Upper mantle velocities are about 8.0 km/s. The coincident reflection seismic data show the fine basement and sedimentary structures including salt tectonics in the basin. The comparative study of the two conjugate profiles on the Moroccan and Nova Scotia margin will give new insights into the original opening of the Atlantic ocean. Further work on this data set will include forward modelling of the wide-angle seismic data, gravity and magnetic modelling.

A Preliminary Look at the Crust and Upper Mantle of North Africa Using Libyan Seismic Data

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

Pasyanos, M

2005-08-05

In recent years, LLNL has been developing methods to jointly invert both surface wave dispersion data and teleseismic receiver functions. The technique holds great promise in accurately estimating seismic structure, including important tectonic parameters such as basin thickness, crustal thickness, upper mantle velocity, etc. We proposed applying this method to some recently available data from several Libyan stations, as we believe the technique has not been applied to any stations in Libya. The technique holds the promise of improving our understanding of the crust and upper mantle in Libya and North Africa. We recently requested seismic data from stations GHARmore » (Gharyan) and MARJ (Al Marj) in Libya for about 20 events. The events were large events at regional distances suitable for making dispersion measurements. An example of waveforms recorded at the two stations from an earthquake in Italy is shown in Figure 1. The paths traverse the Ionian Sea. Notice the slow short period group velocities of the surface waves across the Mediterranean, particularly to the easternmost station MARJ. However, because of data availability, signal-to-noise ratio, etc. we were unable to make measurements for every one of these events at both stations. Figure 2 shows a map of paths for 20 sec Rayleigh waves in the eastern Mediterranean region. Paths measured at the two Libyan stations are shown in green. Rayleigh wave dispersion measurements at 20 sec period are sensitive to velocities in the upper 20 km or so, and reveal sediment thickness, crustal velocity, and crustal thickness. Tomographic inversions reveal the sharp group velocity contrast between regions with deep sedimentary basins and those without. Figure 3, the result of an inversion made before adding the new dispersion measurements, shows slow group velocities in the Black Sea, Adriatic Sea, and Eastern Mediterranean. In general, these features correspond well with the sediment thickness model from Laske, shown in Figure 4. Details in and around the Sirt (Sirte) Basin in northern Libya, however, are poorly defined.« less

Seismic Velocity Variation and Evolution of the Upper Oceanic Crust across the Mid-Atlantic Ridge at 1.3°S

NASA Astrophysics Data System (ADS)

Jian, H.; Singh, S. C.

2017-12-01

The oceanic crust that covers >70% of the solid earth is formed at mid-ocean ridges, but get modified as it ages. Understanding the evolution of oceanic crust requires investigations of crustal structures that extend from zero-age on the ridge axis to old crust. In this study, we analyze a part of a 2000-km-long seismic transect that crosses the Mid-Atlantic Ridge segment at 1.3°S, south of the Chain transform fault. The seismic data were acquired using a 12-km-long multi-sensor streamer and dense air-gun shots. Using a combination of downward continuation and seismic tomography methods, we have derived a high-resolution upper crustal velocity structure down to 2-2.5 km depth below the seafloor, from the ridge axis to 3.5 Ma on both sides of the ridge axis. The results demonstrate that velocities increase at all depths in the upper crust as the crust ages, suggesting that hydrothermal precipitations seal the upper crustal pore spaces. This effect is most significant in layer 2A, causing a velocity increase of 0.5-1 km/s after 1-1.5 Ma, beyond which the velocity increase is very small. Furthermore, the results exhibit a significant decrease in both the frequency and amplitude of the low-velocity anomalies associated with faults beyond 1-1.5 Ma, when faults become inactive, suggesting a linkage between the sealing of fault space and the extinction of hydrothermal activity. Besides, the off-axis velocities are systematically higher on the eastern side of the ridge axis compared to on the western side, suggesting that a higher hydrothermal activity should exist on the outside-corner ridge flank than on the inside-corner flank. While the tomography results shown here cover 0-3.5 Ma crust, the ongoing research will further extend the study area to older crust and also incorporating pre-stack migration and full waveform inversion methods to improve the seismic structure.

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.

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.

Continental crustal formation and recycling: Evidence from oceanic basalts

NASA Technical Reports Server (NTRS)

Saunders, A. D.; Tarney, J.; Norry, M. J.

1988-01-01

Despite the wealth of geochemical data for subduction-related magma types, and the clear importance of such magmas in the creation of continental crust, there is still no concensus about the relative magnitudes of crustal creation versus crustal destruction (i.e., recycling of crust into the mantle). The role of subducted sediment in the formation of the arc magmas is now well documented; but what proportion of sediment is taken into the deeper mantle? Integrated isotopic and trace element studies of magmas erupted far from presently active subduction zones, in particular basaltic rocks erupted in the ocean basins, are providing important information about the role of crustal recycling. By identifying potential chemical tracers, it is impossible to monitor the effects of crustal recycling, and produce models predicting the mass of material recycled into the mantle throughout long periods of geological time.

Magnetotelluric Imaging of the Lithosphere Across the Variscan Orogen (Iberian Autochthonous Domain, NW Iberia)

NASA Astrophysics Data System (ADS)

Alves Ribeiro, J.; Monteiro-Santos, F. A.; Pereira, M. F.; Díez Fernández, R.; Dias da Silva, Í.; Nascimento, C.; Silva, J. B.

2017-12-01

A new magnetotelluric (MT) survey comprising 17 MT soundings throughout a 30 km long N30°W transect in the Iberian autochthons domain of NW Iberia (Central Iberian Zone) is presented. The 2-D inversion model shows the resistivity structure of the continental crust up to 10 km depth, heretofore unavailable for this region of the Variscan Orogen. The MT model reveals a wavy structure separating a conductive upper layer underlain by a resistive layer, thus picturing the two main tectonic blocks of a large-scale D2 extensional shear zone (i.e., Pinhel shear zone). The upper layer represents a lower grade metamorphic domain that includes graphite-rich rocks. The lower layer consists of high-grade metamorphic rocks that experienced partial melting and are associated with granites (more resistive) emplaced during crustal thinning. The wavy structure is the result of superimposed crustal shortening responsible for the development of large-scale D3 folds (e.g., Marofa synform), later deflected and refolded by a D4 strike-slip shear zone (i.e., Juzbado-Penalva do Castelo shear zone). The later contribution to the final structure of the crust is marked by the intrusion of postkinematic granitic rocks and the propagation of steeply dipping brittle fault zones. Our study demonstrates that MT imaging is a powerful tool to understand complex crustal structures of ancient orogens in order to design future prospecting surveys for mineral deposits of economic interest.

Geochemical and Sr-Nd-Pb-Li isotopic characteristics of volcanic rocks from the Okinawa Trough: Implications for the influence of subduction components and the contamination of crustal materials

NASA Astrophysics Data System (ADS)

Guo, Kun; Zhai, Shikui; Yu, Zenghui; Wang, Shujie; Zhang, Xia; Wang, Xiaoyuan

2018-04-01

The Okinawa Trough is an infant back-arc basin developed along the Ryukyu arc. This paper provides new major and trace element and Sr-Nd-Pb-Li isotope data of volcanic rocks in the Okinawa Trough and combines the published geochemical data to discuss the composition of magma source, the influence of subduction component, and the contamination of crustal materials, and calculate the contribution between subduction sediment and altered oceanic crust in the subduction component. The results showed that there are 97% DM and 3% EMI component in the mantle source in middle trough (MS), which have been influenced by subduction sediment. The Li-Nd isotopes indicate that the contribution of subduction sediment and altered oceanic crust in subduction component are 4 and 96%, respectively. The intermediate-acidic rocks suffer from contamination of continental crust material in shallow magma chamber during fractional crystallization. The acidic rocks in south trough have experienced more contamination of crustal material than those from the middle and north trough segments.

Crustal structure, geophysical models and contemporary tectonism of the Colorado Plateau

NASA Technical Reports Server (NTRS)

Keller, G. R.; Braile, L. W.; Morgan, P.

1979-01-01

A regional analysis of the crust and upper mantle of the Colorado Plateau is presented, using existing geophysical and geological data combined with new surface wave dispersion and groundwater geothermometry data; the tectonic implications of these models are also investigated. Surface wave and seismic refraction data indicate that the crust of the interior of the Colorado Plateau is 44 + or - 3 km thick, and its crustal structure is typical of stable continental areas. Pn velocities, however, appear to be lower (7.8 km/s) than would be expected in a stable region, while silica geothermometry indicates that the average heat flow for the plateau is 55 mW per sq m (1.3 HFU).

Seismic-wave attenuation associated with crustal faults in the New Madrid seismic zone

USGS Publications Warehouse

Hamilton, R.M.; Mooney, W.D.

1990-01-01

The attenuation of upper crustal seismic waves that are refracted with a velocity of about 6 kilometers per second varies greatly among profiles in the area of the New Madrid seismic zone in the central Mississippi Valley. The waves that have the strongest attenuation pass through the seismic trend along the axis of the Reelfoot rift in the area of the Blytheville arch. Defocusing of the waves in a low-velocity zone and/ or seismic scattering and absorption could cause the attenuation; these effects are most likely associated with the highly deformed rocks along the arch. Consequently, strong seismic-wave attenuation may be a useful criterion for identifying seismogenic fault zones.

Horizontal gravity gradient - An aid to the definition of crustal structure in North America

NASA Technical Reports Server (NTRS)

Sharpton, V. L.; Grieve, R. A. F.; Thomas, M. D.; Halpenny, J. F.

1987-01-01

A map of the magnitude of the horizontal Bouguer gravity gradient over the North American continent is used to delineate lateral discontinuities in upper crustal density and/or thickness associated with such processes as suturing and rifting. The usefulness of gradient trends in mapping major structural boundaries, which are sometimes poorly exposed or completely buried, is demonstrated by examples such as the buried southward extension of the Grenville Front and buried boundaries of the Superior Province. Gradient trends also draw attention to poorly known structures, which may have major tectonic significance, and to a continent-wide structural fabric, which may provide a record of the tectonic growth of the North American continent.

Simultaneous miocene extension and shortening in the himalayan orogen.

PubMed

Hodges, K V; Parrish, R R; Housh, T B; Lux, D R; Burchfiel, B C; Royden, L H; Chen, Z

1992-11-27

The South Tibetan detachment system separates the high-grade metamorphic core of the Himalayan orogen from its weakly metamorphosed suprastructure. It is thought to have developed in response to differences in gravitational potential energy produced by crustal thickening across the mountain front. Geochronologic data from the Rongbuk Valley, north of Qomolangma (Mount Everest) in southern Tibet, demonstrate that at least one segment of the detachment system was active between 19 and 22 million years ago, an interval characterized by large-scale crustal thickening at lower structural levels. These data suggest that decoupling between an extending upper crust and a converging lower crust was an important aspect of Himalayan tectonics in Miocene time.

Cooling and exhumation of continents at billion-year time scales

NASA Astrophysics Data System (ADS)

Blackburn, T.; Bowring, S. A.; Perron, T.; Mahan, K. H.; Dudas, F. O.

2011-12-01

The oldest rocks on Earth are preserved within the continental lithosphere, where assembled fragments of ancient orogenic belts have survived erosion and destruction by plate tectonic and surface processes for billions of years. Though the rate of orogenic exhumation and erosion has been measured for segments of an orogenic history, it remains unclear how these exhumation rates have changed over the lifetime of any terrane. Because the exhumation of the lithospheric surface has a direct effect on the rate of heat loss within the lithosphere, a continuous record of lithosphere exhumation can be reconstructed through the use of thermochronology. Thermochronologic studies have typically employed systems sensitive to cooling at temperatures <300 °C, such as the (U-Th)/He and 40Ar/39Ar systems. This largely restricts their application to measuring cooling in rocks from the outer 10 km of the Earth's crust, resulting in a thermal history that is controlled by either upper crustal flexure and faulting and/or isotherm inflections related to surface topography. Combining these biases with the uplift, erosion and recycling of these shallow rocks results in a poor preservation potential of any long-term record. Here, an ancient and long-term record of lithosphere exhumation is constructed using U-Pb thermochronology, a geochronologic system sensitive to cooling at temperatures found at 20-50 km depth (400-650 °C). Lower crustal xenoliths provide material that resided at these depths for billions of years or more, recording a thermal history that is buried deep enough to remain insensitive to upper crustal deformation and instead is dominated by the vertical motions of the continents. We show how this temperature-sensitive system can produce a long-term integrated measure of continental exhumation and erosion. Preserved beneath Phanerozoic sedimentary rocks within Montana, USA, the Great Falls Tectonic Zone formed when two Archean cratons, the Wyoming Province and Medicine Hat Block collided at ~1.8 Ga. Rutile U-Pb data from multiple xenoliths, each exhumed from a different depth within the crustal column reveal a range of dates that varies as a function of xenolith residence depth. The shallowest mid- to lower crustal xenoliths (~25 km) cooled first, yielding the youngest dates and yet cooled at rates between 0.1-0.25 °C/Ma over 500 My or more. Deeper xenoliths record cooling at progressively younger times at similar rates and time-scales. From orogony to eruption of xenoliths onto the surface, the lithospheric thermal history constructed using this technique may exceed a billion years. Combining this cooling history with a lithosphere thermal model yields an estimate for the average integrated rate of craton erosion between 0.00-<0.0025 km/Ma across the orogen; a range far lower than the geologically recent to present day rates for continental erosion (<0.005-0.1 km/Ma). This marks the first ever determination of continental exhumation rates on time-scales that approach the age of the continents themselves and has implications for secular cooling of the asthenosphere.

Fine-scale crustal structure of the Azores Islands from teleseismic receiver functions

NASA Astrophysics Data System (ADS)

Spieker, K.; Rondenay, S.; Ramalho, R. S.; Thomas, C.; Helffrich, G. R.

2016-12-01

The Azores plateau is located near the Mid-Atlantic Ridge (MAR) and consists of nine islands, most of which lie east of the MAR. Various methods including seismic reflection, gravity, and passive seismic imaging have been used to investigate the crustal thickness beneath the islands. They have yielded thickness estimates that range between roughly 10 km and 30 km, but until now models of the fine-scale crustal structure have been lacking. A comparison of the crustal structure beneath the islands that lie west and east of the MAR might give further constraints on the evolution of the islands. For example, geochemical studies carried out across the region predict the existence of volcanic interfaces that should be detected seismically within the shallow crust of some of the islands. In this study, we use data from ten seismic stations located on the Azores Islands to investigate the crustal structure with teleseismic P-wave receiver functions. We query our resulting receiver functions for signals associated with the volcanic edifice, the crust-mantle boundary, and potential underplated layers beneath the various islands. The islands west of the MAR have a crustal structure comprising two discontinuities - an upper one at 1-2 km depth marking the base of the volcanic edifice, and a lower one at 10 km depth that we interpret as crust-mantle boundary. The islands east of the MAR can be subdivided into two groups. The central islands that are closer to the MAR exhibit a crustal structure similar to that of the western islands, with a volcanic edifice reaching a depth of 2 km and an average crust-mantle boundary at around 12 km depth. The easternmost islands, located on the oldest lithosphere, exhibit a more complex crustal structure with evidence for a mid-crustal interface and an underplated layer, yielding an effective crust-mantle boundary at >15 km depth. The difference in structure between proximal and distal islands might be related to the age of the plate at the time of emplacement of the islands, with an older plate providing conditions that are more favourable for basaltic underplating.

Transect across the West Antarctic rift system in the Ross Sea, Antarctica

USGS Publications Warehouse

Trey, H.; Cooper, A. K.; Pellis, G.; Della, Vedova B.; Cochrane, G.; Brancolini, Giuliano; Makris, J.

1999-01-01

In 1994, the ACRUP (Antarctic Crustal Profile) project recorded a 670-km-long geophysical transect across the southern Ross Sea to study the velocity and density structure of the crust and uppermost mantle of the West Antarctic rift system. Ray-trace modeling of P- and S-waves recorded on 47 ocean bottom seismograph (OBS) records, with strong seismic arrivals from airgun shots to distances of up to 120 km, show that crustal velocities and geometries vary significantly along the transect. The three major sedimentary basins (early-rift grabens), the Victoria Land Basin, the Central Trough and the Eastern Basin are underlain by highly extended crust and shallow mantle (minimum depth of about 16 km). Beneath the adjacent basement highs, Coulman High and Central High, Moho deepens, and lies at a depth of 21 and 24 km, respectively. Crustal layers have P-wave velocities that range from 5.8 to 7.0 km/s and S-wave velocities from 3.6 to 4.2 km/s. A distinct reflection (PiP) is observed on numerous OBS from an intra-crustal boundary between the upper and lower crust at a depth of about 10 to 12 km. Local zones of high velocities and inferred high densities are observed and modeled in the crust under the axes of the three major sedimentary basins. These zones, which are also marked by positive gravity anomalies, may be places where mafic dikes and sills pervade the crust. We postulate that there has been differential crustal extension across the West Antarctic rift system, with greatest extension beneath the early-rift grabens. The large amount of crustal stretching below the major rift basins may reflect the existence of deep crustal suture zones which initiated in an early stage of the rifting, defined areas of crustal weakness and thereby enhanced stress focussing followed by intense crustal thinning in these areas. The ACRUP data are consistent with the prior concept that most extension and basin down-faulting occurred in the Ross Sea during late Mesozoic time, with relatively small extension, concentrated in the western half of the Ross Sea, during Cenozoic time.

A feature illustration and application of azimuthal P receiver function patterns

NASA Astrophysics Data System (ADS)

Eckhardt, C.; Rabbel, W.

2009-12-01

Based on a synthetic catalog of thirty azimuthal patterns of P receiver functions for crustal structures down to thirty km depth we have summarized and illustrated the most important azimuthal features. We have constructed five model classes encompassing (an-)isotropic horizontal and dipping layers. The model classes were initialized by in situ observations of three deep reflection seismic profiles (DEKORP) of varying high reflective zones and a spiral shaped foliation scheme of an upper crustal bore hole out of the German Continental Deep Drilling Program (KTB). Up to fourteen azimuthal features were extracted out of the synthetic patterns and could be grouped into an already known fundamental part, a multiple part and into an extension part. Each feature was rated by a specific grade A, B, C to inform about the type of its initialization ((an-) isotropy and/or layer dipping). We have evaluated the fourteen features on the synthetic patterns to apply a hierarchical classification. From the classification of the model objects we found that nearly eighty percent of the models are well explained by the fundamental part. The hierarchical order of the model objects can be used as a template to screen real observed azimuthal patterns to find a starting model for a forward modeling or an inversion procedure. For one station of the German Regional Seismic Network (GRSN) we have evaluated the features and screened them through the template. A forward simulation of the azimuthal pattern, using the modified first found model explanation out of the hierarchical order for station MOX, leads to a good coincidence between the real and the simulated pattern. The final 1D model could be divided into an upper crustal part (8 km deep) with an axis of symmetry tilt of 55° and 20°NW trend (direction of axis tilt) and a lower crustal part (24 km thickness) with an axis of symmetry of increasing tilt from 55° to 85° and a trend orientation of 20°SE. For the simulation we have assumed 8 and 7 percent of negative P+S anisotropy for hexagonal symmetry of the upper and lower crust, respectively. From the synthetic and the real observations it is evident that additional boundaries beside the Moho discontinuity are merely detectable for certain circumstances in an azimuthal resolution and will be blinded out in the traditional radial stack.

A Vs30-derived Near-surface Seismic Velocity Model

NASA Astrophysics Data System (ADS)

Ely, G. P.; Jordan, T. H.; Small, P.; Maechling, P. J.

2010-12-01

Shallow material properties, S-wave velocity in particular, strongly influence ground motions, so must be accurately characterized for ground-motion simulations. Available near-surface velocity information generally exceeds that which is accommodated by crustal velocity models, such as current versions of the SCEC Community Velocity Model (CVM-S4) or the Harvard model (CVM-H6). The elevation-referenced CVM-H voxel model introduces rasterization artifacts in the near-surface due to course sample spacing, and sample depth dependence on local topographic elevation. To address these issues, we propose a method to supplement crustal velocity models, in the upper few hundred meters, with a model derived from available maps of Vs30 (the average S-wave velocity down to 30 meters). The method is universally applicable to regions without direct measures of Vs30 by using Vs30 estimates from topographic slope (Wald, et al. 2007). In our current implementation for Southern California, the geology-based Vs30 map of Wills and Clahan (2006) is used within California, and topography-estimated Vs30 is used outside of California. Various formulations for S-wave velocity depth dependence, such as linear spline and polynomial interpolation, are evaluated against the following priorities: (a) capability to represent a wide range of soil and rock velocity profile types; (b) smooth transition to the crustal velocity model; (c) ability to reasonably handle poor spatial correlation of Vs30 and crustal velocity data; (d) simplicity and minimal parameterization; and (e) computational efficiency. The favored model includes cubic and square-root depth dependence, with the model extending to a depth of 350 meters. Model parameters are fit to Boore and Joyner's (1997) generic rock profile as well as CVM-4 soil profiles for the NEHRP soil classification types. P-wave velocity and density are derived from S-wave velocity by the scaling laws of Brocher (2005). Preliminary assessment of the new model is preformed with ground motion simulations for a selection of likely M > 7 scenario events for Southern California (as define by the SCEC Big Ten project).

The influence of tectonic inheritance on crustal extension style following failed subduction of continental crust: applications to metamorphic core complexes in Papua New Guinea

NASA Astrophysics Data System (ADS)

Biemiller, J.; Ellis, S. M.; Little, T.; Mizera, M.; Wallace, L. M.; Lavier, L.

2017-12-01

The structural, mechanical and geometric evolution of rifted continental crust depends on the lithospheric conditions in the region prior to the onset of extension. In areas where tectonic activity preceded rift initiation, structural and physical properties of the previous tectonic regime may be inherited by the rift and influence its development. Many continental rifts form and exhume metamorphic core complexes (MCCs), coherent exposures of deep crustal rocks which typically surface as arched or domed structures. MCCs are exhumed in regions where the faulted upper crust is displaced laterally from upwelling ductile material along a weak detachment fault. Some MCCs form during extensional inversion of a subduction thrust following failed subduction of continental crust, but the degree to which lithospheric conditions inherited from the preceding subduction phase control the extensional style in these systems remains unclear. For example, the Dayman Dome in Southeastern Papua New Guinea exposes prehnite-pumpellyite to greenschist facies rocks in a smooth 3 km-high dome exhumed with at least 24 km of slip along one main detachment normal fault, the Mai'iu Fault, which dips 21° at the surface. The extension driving this exhumation is associated with the cessation of northward subduction of Australian continental crust beneath the oceanic lithosphere of the Woodlark Plate. We use geodynamic models to explore the effect of pre-existing crustal structures inherited from the preceding subduction phase on the style of rifting. We show that different geometries and strengths of inherited subduction shear zones predict three distinct modes of subsequent rift development: 1) symmetric rifting by newly formed high-angle normal faults; 2) asymmetric rifting along a weak low-angle detachment fault extending from the surface to the brittle-ductile transition; and 3) extension along a rolling-hinge structure which exhumes deep crustal rocks in coherent rounded exposures. We propose the latter mode as an exhumation model for Dayman Dome and compare the model predictions to regional geophysical and geological evidence. Our models find that tectonically inherited subduction structures may strongly control subsequent extension style when the subduction thrust is weak and well-oriented for reactivation.

Possible emplacement of crustal rocks into the forearc mantle of the Cascadia Subduction Zone

USGS Publications Warehouse

Calvert, A.J.; Fisher, M.A.; Ramachandran, K.; Trehu, A.M.

2003-01-01

Seismic reflection profiles shot across the Cascadia forearc show that a 5-15 km thick band of reflections, previously interpreted as a lower crustal shear zone above the subducting Juan de Fuca plate, extends into the upper mantle of the North American plate, reaching depths of at least 50 km. In the extreme western corner of the mantle wedge, these reflectors occur in rocks with P wave velocities of 6750-7000 ms-1. Elsewhere, the forearc mantle, which is probably partially serpentinized, exhibits velocities of approximately 7500 ms-1. The rocks with velocities of 6750-7000 ms-1 are anomalous with respect to the surrounding mantle, and may represent either: (1) locally high mantle serpentinization, (2) oceanic crust trapped by backstepping of the subduction zone, or (3) rocks from the lower continental crust that have been transported into the uppermost mantle by subduction erosion. The association of subparallel seismic reflectors with these anomalously low velocities favours the tectonic emplacement of crustal rocks. Copyright 2003 by the American Geophysical Union.

Is Ishtar Terra a thickened basaltic crust?

NASA Technical Reports Server (NTRS)

Arkani-Hamed, Jafar

1992-01-01

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

Low crustal velocities and mantle lithospheric variations in southern Tibet from regional Pnl waveforms

NASA Astrophysics Data System (ADS)

Rodgers, Arthur J.; Schwartz, Susan Y.

We report low average crustal P-wave velocities (5.9-6.1 km/s, Poisson's ratio 0.23-0.27, thickness 68-76 km) in southern Tibet from modelling regional Pnl waveforms recorded by the 1991-1992 Tibetan Plateau Experiment. We also find that the mantle lithosphere beneath the Indus-Tsangpo Suture and the Lhasa Terrane is shield-like (Pn velocity 8.20-8.25 km/s, lid thickness 80-140 km, positive velocity gradient 0.0015-0.0025 s-1). Analysis of relative Pn travel time residuals requires a decrease in the mantle velocities beneath the northern Lhasa Terrane, the Banggong-Nujiang Suture and the southern Qiangtang Terrane. Tectonic and petrologic considerations suggest that low bulk crustal velocities could result from a thick (50-60 km) felsic upper crust with vertically limited and laterally pervasive partial melt. These results are consistent with underthrusting of Indian Shield lithosphere beneath the Tibetan Plateau to at least the central Lhasa Terrane.

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

Structural evolution of the Sarandí del Yí Shear Zone, Uruguay: kinematics, deformation conditions and tectonic significance

NASA Astrophysics Data System (ADS)

Oriolo, S.; Oyhantçabal, P.; Heidelbach, F.; Wemmer, K.; Siegesmund, S.

2015-10-01

The Sarandí del Yí Shear Zone is a crustal-scale shear zone that separates the Piedra Alta Terrane from the Nico Pérez Terrane and the Dom Feliciano Belt in southern Uruguay. It represents the eastern margin of the Río de la Plata Craton and, consequently, one of the main structural features of the Precambrian basement of Western Gondwana. This shear zone first underwent dextral shearing under upper to middle amphibolite facies conditions, giving rise to the reactivation of pre-existing crustal fabrics in the easternmost Piedra Alta Terrane. Afterwards, pure-shear-dominated sinistral shearing with contemporaneous magmatism took place under lower amphibolite to upper greenschist facies conditions. The mylonites resulting from this event were then locally reactivated by a cataclastic deformation. This evolution points to strain localization under progressively retrograde conditions with time, indicating that the Sarandí del Yí Shear Zone represents an example of a thinning shear zone related to the collisional to post-collisional evolution of the Dom Feliciano Belt that occurred between the Meso- to Neoproterozoic (>600 Ma) and late Ediacaran-lower Cambrian times.

Kinematics of the New Madrid seismic zone, central United States, based on stepover models

USGS Publications Warehouse

Pratt, Thomas L.

2012-01-01

Seismicity in the New Madrid seismic zone (NMSZ) of the central United States is generally attributed to a stepover structure in which the Reelfoot thrust fault transfers slip between parallel strike-slip faults. However, some arms of the seismic zone do not fit this simple model. Comparison of the NMSZ with an analog sandbox model of a restraining stepover structure explains all of the arms of seismicity as only part of the extensive pattern of faults that characterizes stepover structures. Computer models show that the stepover structure may form because differences in the trends of lower crustal shearing and inherited upper crustal faults make a step between en echelon fault segments the easiest path for slip in the upper crust. The models predict that the modern seismicity occurs only on a subset of the faults in the New Madrid stepover structure, that only the southern part of the stepover structure ruptured in the A.D. 1811–1812 earthquakes, and that the stepover formed because the trends of older faults are not the same as the current direction of shearing.

Crustal heat production and estimate of terrestrial heat flow in central East Antarctica, with implications for thermal input to the East Antarctic ice sheet

NASA Astrophysics Data System (ADS)

Goodge, John W.

2018-02-01

Terrestrial heat flow is a critical first-order factor governing the thermal condition and, therefore, mechanical stability of Antarctic ice sheets, yet heat flow across Antarctica is poorly known. Previous estimates of terrestrial heat flow in East Antarctica come from inversion of seismic and magnetic geophysical data, by modeling temperature profiles in ice boreholes, and by calculation from heat production values reported for exposed bedrock. Although accurate estimates of surface heat flow are important as an input parameter for ice-sheet growth and stability models, there are no direct measurements of terrestrial heat flow in East Antarctica coupled to either subglacial sediment or bedrock. As has been done with bedrock exposed along coastal margins and in rare inland outcrops, valuable estimates of heat flow in central East Antarctica can be extrapolated from heat production determined by the geochemical composition of glacial rock clasts eroded from the continental interior. In this study, U, Th, and K concentrations in a suite of Proterozoic (1.2-2.0 Ga) granitoids sourced within the Byrd and Nimrod glacial drainages of central East Antarctica indicate average upper crustal heat production (Ho) of about 2.6 ± 1.9 µW m-3. Assuming typical mantle and lower crustal heat flux for stable continental shields, and a length scale for the distribution of heat production in the upper crust, the heat production values determined for individual samples yield estimates of surface heat flow (qo) ranging from 33 to 84 mW m-2 and an average of 48.0 ± 13.6 mW m-2. Estimates of heat production obtained for this suite of glacially sourced granitoids therefore indicate that the interior of the East Antarctic ice sheet is underlain in part by Proterozoic continental lithosphere with an average surface heat flow, providing constraints on both geodynamic history and ice-sheet stability. The ages and geothermal characteristics of the granites indicate that crust in central East Antarctica resembles that in the Proterozoic Arunta and Tennant Creek inliers of Australia but is dissimilar to other areas like the Central Australian Heat Flow Province that are characterized by anomalously high heat flow. Age variation within the sample suite indicates that central East Antarctic lithosphere is heterogeneous, yet the average heat production and heat flow of four age subgroups cluster around the group mean, indicating minor variation in the thermal contribution to the overlying ice sheet from upper crustal heat production. Despite these minor differences, ice-sheet models may favor a geologically realistic input of crustal heat flow represented by the distribution of ages and geothermal characteristics found in these glacial clasts.

Wide-angle seismic recordings from the 1998 Seismic Hazards Investigation of Puget Sound (SHIPS), western Washington and British Columbia

USGS Publications Warehouse

Brocher, Thomas M.; Parsons, Tom; Creager, Ken C.; Crosson, Robert S.; Symons, Neill P.; Spence, George D.; Zelt, Barry C.; Hammer, Philip T.C.; Hyndman, Roy D.; Mosher, David C.; Tréhu, Anne M.; Miller, Kate C.; ten Brink, Uri S.; Fisher, Michael A.; Pratt, Thomas L.; Alvarez, Marcos G.; Beaudoin, Bruce C.; Louden, Keith E.; Weaver, Craig S.

1999-01-01

This report describes the acquisition and processing of deep-crustal wide-angle seismic reflection and refraction data obtained in the vicinity of Puget Lowland, the Strait of Juan de Fuca, and Georgia Strait, western Washington and southwestern British Columbia, in March 1998 during the Seismic Hazards Investigation of Puget Sound (SHIPS). As part of a larger initiative to better understand lateral variations in crustal structure along the Cascadia margin, SHIPS participants acquired 1000 km of deep-crustal multichannel seismic-reflection profiles and 1300 km of wideangle airgun shot lines in this region using the R/V Thompson and R/V Tully. The Tully was used to record airgun shots fired by the Thompson in two different geometries: (1) expanding spread profiles (ESPs) and (2) constant offset profiles (COPs). Prior to this reflection survey, we deployed 257 Reftek and 15 ocean-bottom seismic recorders to record the airgun signals at far offsets. All data were recorded digitally on large-capacity hard disks. Although most of these stations only recorded the vertical component of motion, 95 of these seismographs recorded signals from an oriented 3-component seismometer. By recording signals generated by the Thompson's marine air gun array, operated in two differing geometries having a total volume of 110 and 79 liters (6730 and 4838 cu. in.), respectively, the arrays of wide-angle recorders were designed to (1) image the crustal structure, particularly in the vicinity of crustal faults and Cenozoic sedimentary basins, (2) determine the geometry of the Moho, and (3) image the subducting Gorda and Juan de Fuca plates. Nearly 33,300 air gun shots were recorded along several seismic lines. In this report, we illustrate the expanding spread profiles acquired using the Thompson and Tully, describe the land and ocean-bottom recording of the air gun signals, discuss the processing of the land recorder data into common receiver gathers, and illustrate the processed wide-angle seismic data collected using the Refteks and ocean-bottom seismometers. We also describe the format and content of the archival tapes containing the SEGY-formated, common-receiver gathers for the Reftek data. Data quality is variable but SHIPS appears to have successfully obtained useful data from almost all the stations deployed to record the airgun shots. Several interesting arrivals were observed: including refractions from the sedimentary basin fill in several basins, refractions from basement rocks forming the upper crust, Pg, refractions from the upper mantle, Pn, as well as reflections from within the crust and from the top of the upper mantle, PmP. We separately archived more than 30 local earthquakes recorded by the Reftek array during our deployment.

Geochemical Evidence for Mantle Enrichment and Lower Crustal Assimilation in Orogenic Volcanics from Monte Arcuentu, Southern Sardinia: Implications for Geodynamics and Evolution of the Western Mediterranean

NASA Astrophysics Data System (ADS)

Vero, S.; Kempton, P. D.; Downes, H.

2016-12-01

Miocene (ca. 18Ma) subduction-related basalts and basaltic andesites from Monte Arcuentu (MA), southern Sardinia, show a remarkable correlation between SiO2 and 87Sr/86Sr (up to 0.711) that contrasts with most other orogenic volcanics worldwide. MgO ranges from 13.4 - 2.4 wt%, yet the rocks form a baseline trend at low SiO2 (51-56 wt%) from which other arcs diverge toward high SiO2. In contrast, MA exhibits a steep trend that extends toward the field of lithosphere-derived, lamproites from central Italy. New high-precision Pb and Hf isotope data help to constrain the petrogenesis of these rocks. The most primitive MA rocks (MgO > 8.5wt%) were derived from a mantle wedge metasomatized by melts derived from terrigenous sediment, likely derived from Archean terranes of N Africa. This metasomatized source had high 87Sr/86Sr (O.705-0.709) and 7/4Pb (15.65 - 15.67) with low ɛHf (-1 to +8) and ɛNd (+1 to -6), but does not account for the full range of isotopic compositions observed. More evolved rocks (MgO < 8.5 wt%) have higher 87Sr/86Sr (0.711) and 7/4Pb (15.68), lower ɛHf (-8) and ɛNd (-9). However, one group of evolved rocks with low Rb/Ba trends toward low 6/4Pb whereas another group with high Rb/Ba extends to high 6/4Pb. Mixing calculations suggest that evolved rocks with low Rb/Ba - low 6/4Pb interacted with Hercynian-type lower crust. High Rb/Ba - high 6/4Pb rocks may have interacted with lithospheric mantle similar to that sampled by Italian lamproites, but upper crustal contamination cannot be ruled out. Partial melting of these normally refractory lithologies was facilitated by the rapid extension, and subsequent mantle upwelling, that occurred as Sardinia rifted and rotated away from the European plate during the Miocene (32-15 Ma). High rates of melt accumulation and high melt fractions ponded near the MOHO, creating a "hot zone", enabling mafic crustal melting. Fractional crystallization under these PT conditions involved olivine + cpx with little or no plag, such that differentiation proceeded without significant increase in SiO2. High rates of extension may also have facilitated rapid ascent of magmas to the surface with minimal interaction with mid- to upper crust. The MA rocks provide insights into lower crustal assimilation process that may be obscured by upper crustal AFC processes in other suites.

Origin of dipping structures in fast-spreading oceanic lower crust offshore Alaska imaged by multichannel seismic data

NASA Astrophysics Data System (ADS)

Bécel, Anne; Shillington, Donna J.; Nedimović, Mladen R.; Webb, Spahr C.; Kuehn, Harold

2015-08-01

Multi-channel seismic (MCS) reflection profiles across the Pacific Plate south of the Alaska Peninsula reveal the internal structure of mature oceanic crust (48-56 Ma) formed at fast to intermediate spreading rates during and after a major plate re-organization. Oceanic crust formed at fast spreading rates (half spreading rate ∼ 74 mm /yr) has smoother basement topography, thinner sediment cover with less faulting, and an igneous section that is at least 1 km thicker than crust formed at intermediate spreading rates (half spreading rate ∼ 28- 34 mm /yr). MCS data across fast-spreading oceanic crust formed during plate re-organization contain abundant bright reflections, mostly confined to the lower crust above a highly reflective Moho transition zone, which has a reflection coefficient (RC) of ∼0.1. The lower crustal events dip predominantly toward the paleo-ridge axis at ∼10-30°. Reflections are also imaged in the uppermost mantle, which primarily dip away from the ridge at ∼10-25°, the opposite direction to those observed in the lower crust. Dipping events in both the lower crust and upper mantle are absent on profiles acquired across the oceanic crust formed at intermediate spreading rates emplaced after plate re-organization, where a Moho reflection is weak or absent. Our preferred interpretation is that the imaged lower crustal dipping reflections within the fast spread crust arise from shear zones that form near the spreading center in the region characterized by interstitial melt. The abundance and reflection amplitude strength of these events (RC ∼ 0.15) can be explained by a combination of solidified melt that was segregated within the shear structures, mylonitization of the shear zones, and crystal alignment, all of which can result in anisotropy and constructive signal interference. Formation of shear zones with this geometry requires differential motion between the crust and upper mantle, where the upper mantle moves away from the ridge faster than the crust. Active asthenospheric upwelling is one possible explanation for these conditions. The other possible interpretation is that lower crustal reflections are caused by magmatic (mafic/ultramafic) layering associated with accretion from a central mid-crustal magma chamber. Considering that the lower crustal dipping events have only been imaged in regions that have experienced plate re-organizations associated with ridge jumps or rift propagation, we speculate that locally enhanced mantle flow associated with these settings may lead to differential motion between the crust and the uppermost mantle, and therefore to shearing in the ductile lower crust or, alternatively, that plate reorganization could produce magmatic pulses which may lead to mafic/ultramafic banding.

Differentiation of Secular and Postseismic Deformation in the Mojave Shear Zone in Southern California and Inference of Lithospheric Rheology

NASA Astrophysics Data System (ADS)

Shen, Z.; Liu, S.; Burgmann, R.

2015-12-01

The 1992 Mw 7.3 Landers and 1999 Mw7.1 Hector Mine earthquakes struck the Eastern California Shear Zone (ECSZ) in the Mojave Desert, Southern California. Coseismic and postseismic deformation from these events affect efforts to use Global Positioning System (GPS) observations collected since these events to establish a secular surface velocity field, especially in the near field of the coseismic ruptures. We devise block motion models constrained by both historical pre-Landers triangulation and trilateration observations and post-Landers GPS measurements to recover the secular deformation field and differentiate the postseismic transients in the Mojave region. Postseismic transients are found to remain in various "interseismic" GPS velocity solutions in the form of 2-3 mm/yr excess right-lateral shear across the Landers and Hector Mine coseismic ruptures [Liu et al., 2015 JGR]. Postseismic GPS time series differentiated from the secular velocity field reveal enduring late-stage transient motions in the near field of the coseismic ruptures. Using the postseismic time series data as model constraints, we develop postseismic deformation model invoking afterlip on faults and viscoelastic relaxation in the lower crust and upper mantle. A Burgers body material and a Maxwell material are assumed for the lower crust and upper mantle respectively. Our preliminary modeling result, constrained using GPS time series data from the SCEC Crustal Motion Map 4.0 (covering the time period of 1992-2004), reveals that both the long-term viscosities for the lower crust and upper mantle are on the order of e+19 Pa-s. This finding differs significantly from the "Crème Brulee" model predictions about the rheological structure of the lower crust and upper mantle, in which the lower crust has a substantially higher viscosity. We are incorporating more GPS time series data into our model, particularly the ones from continuous sites of the Plate Boundary Observatory network with post-2004 time span, and the modeling result will be presented at the meeting.

The Apollo peak-ring impact basin: Insights into the structure and evolution of the South Pole-Aitken basin

NASA Astrophysics Data System (ADS)

Potter, Ross W. K.; Head, James W.; Guo, Dijun; Liu, Jianzhong; Xiao, Long

2018-05-01

The 492 km-diameter Apollo impact basin post-dates, and is located at the inner edge of, the ∼2240 km-diameter South Pole-Aitken (SPA) basin, providing an opportunity to assess the SPA substructure and lateral heterogeneity. Gravity Recovery and Interior Laboratory gravity data suggest an average crustal thickness on the floor of SPA of ∼20 km and within the Apollo basin of ∼5 km, yet remote sensing data reveal no conclusive evidence for the presence of exposed mantle material. We use the iSALE shock physics code to model the formation of the Apollo basin and find that the observational data are best fit by the impact of a 40 km diameter body traveling at 15 km/s into 20-40 km thick crustal material. These results strongly suggest that the Apollo impact occurred on ejecta deposits and collapsed crustal material of the SPA basin and could help place constraints on the location, size and geometry of the SPA transient cavity. The peak ring in the interior of Apollo basin is plausibly interpreted to be composed of inwardly collapsed lower crustal material that experienced peak shock pressures in excess of 35 GPa, consistent with remote sensing observations that suggest shocked plagioclase. Proposed robotic and/or human missions to SPA and Apollo would present an excellent opportunity to test the predictions of this work and address many scientific questions about SPA basin evolution and structure.

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

NASA Astrophysics Data System (ADS)

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

2018-05-01

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

Thin and layered subcontinental crust of the great Basin western north America inherited from Paleozoic marginal ocean basins?

USGS Publications Warehouse

Churkin, M.; McKee, E.H.

1974-01-01

The seismic profile of the crust of the northern part of the Basin and Range province by its thinness and layering is intermediate between typical continental and oceanic crust and resembles that of marginal ocean basins, especially those with thick sedimentary fill. The geologic history of the Great Basin indicates that it was the site of a succession of marginal ocean basins opening and closing behind volcanic arcs during much of Paleozoic time. A long process of sedimentation and deformation followed throughout the Mesozoic modifying, but possibly not completely transforming the originally oceanic crust to continental crust. In the Cenozoic, after at least 40 m.y. of quiescence and stable conditions, substantial crustal and upper-mantle changes are recorded by elevation of the entire region in isostatic equilibrium, crustal extension resulting in Basin and Range faulting, extensive volcanism, high heat flow and a low-velocity mantle. These phenomena, apparently the result of plate tectonics, are superimposed on the inherited subcontinental crust that developed from an oceanic origin in Paleozoic time and possibly retained some of its thin and layered characteristics. The present anomalous crust in the Great Basin represents an accretion of oceanic geosynclinal material to a Precambrian continental nucleus apparently as an intermediate step in the process of conversion of oceanic crust into a stable continental landmass or craton. ?? 1974.

Structure, porosity and stress regime of the upper oceanic crust: Sonic and ultrasonic logging of DSDP Hole 504B

USGS Publications Warehouse

Newmark, R.L.; Anderson, R.N.; Moos, D.; Zoback, M.D.

1985-01-01

The layered structure of the oceanic crust is characterized by changes in geophysical gradients rather than by abrupt layer boundaries. Correlation of geophysical logs and cores recovered from DSDP Hole 504B provides some insight into the physical properties which control these gradient changes. Borehole televiewer logging in Hole 504B provides a continuous image of wellbore reflectivity into the oceanic crust, revealing detailed structures not apparent otherwise, due to the low percentage of core recovery. Physical characteristics of the crustal layers 2A, 2B and 2C such as the detailed sonic velocity and lithostratigraphic structure are obtained through analysis of the sonic, borehole televiewer and electrical resistivity logs. A prediction of bulk hydrated mineral content, consistent with comparison to the recovered material, suggests a change in the nature of the alteration with depth. Data from the sonic, borehole televiewer, electrical resistivity and other porosity-sensitive logs are used to calculate the variation of porosity in the crustal layers 2A, 2B and 2C. Several of the well logs which are sensitive to the presence of fractures and open porosity in the formation indicate many zones of intense fracturing. Interpretation of these observations suggests that there may be a fundamental pattern of cooling-induced structure in the oceanic crust. ?? 1985.

Crustal and Uppermost Mantle Structure across the Tibet-Qinling Transition Zone in NE Tibet: Implications for Material Extrusion of the Tibetan Plateau

NASA Astrophysics Data System (ADS)

Ye, Z.; Li, J.; Gao*, R.; Song, X.; Li, Q.; Li, Y.; Huang, X.; Xiong, X.; Li, W.; WANG, Y.

2017-12-01

Based on a dense linear seismic array traversing across the eastern margin of the Tibetan plateau to the Qinling belt, we conducted joint inversion of receiver functions and surface wave dispersions under constraints of P-wave velocity and derived a crustal and uppermost mantle Vs profile simultaneously with a Vp/Vs ratio profile. Our observations indicate that the Qinling belt, which shows ratio Vp/Vs<1.8 indicative of intermediate-to-felsic components in the lower crust, is currently not acting as a channel accommodating extrusion of the mid-lower crustal flow; and extrusion of Tibet's ductile mantle flow through the Qinling belt as a channel would only be feasible in the sub-lithosphere depth (asthenosphere). Our results suggest that ductile material extrusion of the mid-lower crustal flow accompanied with fault-related tectonics and gravitational buoyancy resulted from lithospheric detachment (triggered by the asthenospheric flow) may jointly work on the plateau uplift and expansion in this Tibet-Qinling transition zone. Corresponding Author: R.Gao, ruigao126@126.com

Peeling linear inversion of upper mantle velocity structure with receiver functions

NASA Astrophysics Data System (ADS)

Shen, Xuzhang; Zhou, Huilan

2012-02-01

A peeling linear inversion method is presented to study the upper mantle (from Moho to 800 km depth) velocity structures with receiver functions. The influences of the crustal and upper mantle velocity ratio error on the inversion results are analyzed, and three valid measures are taken for its reduction. This method is tested with the IASP91 and the PREM models, and the upper mantle structures beneath the stations GTA, LZH, and AXX in northwestern China are then inverted. The results indicate that this inversion method is feasible to quantify upper mantle discontinuities, besides the discontinuities between 3 h M ( h M denotes the depth of Moho) and 5 h M due to the interference of multiples from Moho. Smoothing is used to overcome possible false discontinuities from the multiples and ensure the stability of the inversion results, but the detailed information on the depth range between 3 h M and 5 h M is sacrificed.

The crust and uppermost mantle structure in Southern Peru from ambient noise and earthquake surface wave analysis

NASA Astrophysics Data System (ADS)

Ma, Y.; Clayton, R. W.

2012-12-01

We determine the Vs structure to a depth of 140 km of Southern Peru, where the subducted Nazca slab changes from normal to flat subduction. The data are from a box-like array that is approximately 300 km on a side, and with 150 stations in total. The structure is inverted from surface wave dispersion curves measured between 5 s to 23 s period from ambient noise cross-correlations, and between 25 s to 69 s from earthquake two-plane-wave analysis. From the map views of different depths, we observe that: (1) The forearc region is characterized by shallow crustal thickness and higher crustal velocity compared with the backarc. (2) The upper-crust velocity in the backarc above normal subduction (3.0-3.2 km/s) is lower compared with that above flat subduction region (3.2-3.4 km/s). The low velocity coincides with the deep sediments above the Altiplano plateau. (3) The transition from the normal to flat subduction is characterized by a comparatively lower upper-mid crust velocity (3.2-3.4 km/s). The lower velocity zone also coincides with the highest topography (>4700 m) in the study area. (4) The mantle wedge velocity above the flat subduction (4.6-4.9 km/s) is higher than the surrounding mantle and the mantle above the normal subduction region (4.3-4.5 km/s). We deduce that the upper-mid crust above the transition of the slab geometry is probably more felsic, which can be due to the old volcanic activity during the normal-flat transition, and thus can more easily accommodate the crustal shortening. The lack of present volcanism above the flat subduction, however, could be explained by the high velocity anomaly related to the flat slab. It may indicate a cold environment, and thus the lack of mantle melting.

Molybdenum Cycling During Crust Formation and Destruction

NASA Astrophysics Data System (ADS)

Greaney, A. T.; Rudnick, R. L.

2016-12-01

Molybdenum geochemistry has become an important tool for tracking the redox state of the early atmosphere and oceans as well as the emergence and sustainability of Mo-cofactored enzymes. However, in order for Mo to be enriched in the oceans, it must first be weathered out of the crust. Sulfides that weather in the presence of atmospheric O2have historically been deemed the predominant crustal source of Mo. Here, we test this assumption by determining the mineralogical hosts of Mo in Archean, Proterozoic, and Phanerozoic upper crustal rocks, using LA-ICP-MS. We also investigate Mo behavior during igneous differentiation and continental crust formation. We find that molybdenite, MoS2, is a weatherable sulfide source of Mo, but common igneous sulfides are not because their Mo concentrations are too low. However, molybdenite is uncommon in the upper continental crust. By contrast, volcanic glass is much more abundant and is a significant weatherable source of Mo that readily breaks down to release oxidized, soluble Mo whether or not atmospheric O2is present. Other common crustal mineral hosts of Mo are Ti-bearing phases like titanite, ilmenite, magnetite, and rutile that are resistant to weathering. Significant Mo depletion (relative to Ce and Pr) is observed in nearly every granitic rock analyzed in our study, but is not observed in OIB or MORB (Jenner and O'Neill, 2012). There are two possible reasons for this: 1) Mo is removed from cooling plutons during fluid expulsion, or 2) Mo is fractionated during igneous differentiation. The first scenario is a likely explanation given the solubility of oxidized Mo. However, correlations between Mo/Ce and Nb/La in several plutonic suites suggest a fractionating phase like rutile may sequester Mo in the lower crust. Additionally, a correlation between Mo/Ce and inferred tectonic setting (enrichments observed in rift-related plutons) suggest an overall tectonic influence on the availability of Mo in the upper crust.

The Electrical Resistivity Structure of the Eastern Anatolian Collision Zone, Northeastern Anatolia

NASA Astrophysics Data System (ADS)

Cengiz, Özlem; Tuǧrul Başokur, Ahmet; Tolak Çiftçi, Elif

2016-04-01

The Northeastern Anatolia is located at the intensely deformed Eastern Anatolian Collision Zone (EACZ), and its tectonic framework is characterized by the collision of the Arabian plate with Eurasian. Although extensive attention is given to understand the crustal and upper mantle processes at this convergent boundary, there is still an ongoing debate over the geodynamic processes of the region. In this study, we were specifically interested in the geoelectric properties and thus geodynamics of the crust beneath the EACZ. Magnetotelluric (MT) measurements were made on two profiles across the north of the EACZ in 1998 as part of a national project undertaken by the Turkish Petroleum Corporation (TPAO). MT data in the frequency range of 300-0.001 Hz were collected from 168 stations located along 78 km north to south and 47 km west to east profiles where direct convergence occurs between Arabian and Eurasian plates. Two and three-dimensional inversion algorithms were used to obtain resistivity models of the study area. According to these models, the upper crust consists of low resistivity sedimentary rocks (<30 Ωm) that are underlain by highly resistive (~500-1000 Ωm) crystalline basement rocks of the Eastern Anatolian Accretionary Complex and Pontides. While the upper and lower crustal resistivity at the northern part of the study area shows a layered structure, significant horizontal and vertical variations for the rest of the EACZ exists on resistivity models. The broad low resistivity zones (<50 Ωm) observed at mid and lower crustal levels throughout the EACZ. These fluid-rich regions along with high temperatures could indicate weak zones representing the locations of active deformation induced by continent-continent collision and correlate with volcanic centers in the region. The variation in the resistivity structure supports the southward subduction model with the resistive continental block and the deep conductive zones presumably corresponding to the oceanic crust.

Using aerogravity and seismic data to model the bathymetry and upper crustal structure beneath the Pine Island Glacier ice shelf, West Antarctica

NASA Astrophysics Data System (ADS)

Muto, A.; Peters, L. E.; Anandakrishnan, S.; Alley, R. B.; Riverman, K. L.

2013-12-01

Recent estimates indicate that ice shelves along the Amundsen Sea coast in West Antarctica are losing substantial mass through sub-ice-shelf melting and contributing to the accelerating mass loss of the grounded ice buttressed by them. For Pine Island Glacier (PIG), relatively warm Circumpolar Deep Water has been identified as the key driver of the sub-ice-shelf melting although poor constraints on PIG sub-ice shelf have restricted thorough understanding of these ice-ocean interactions. Aerogravity data from NASA's Operation IceBridge (OIB) have been useful in identifying large-scale (on the order of ten kilometers) features but the results have relatively large uncertainties due to the inherent non-uniqueness of the gravity inversion. Seismic methods offer the most direct means of providing water thickness and upper crustal geological constraints, but availability of such data sets over the PIG ice shelf has been limited due to logistical constraints. Here we present a comparative analysis of the bathymetry and upper crustal structure beneath the ice shelf of PIG through joint inversion of OIB aerogravity data and in situ active-source seismic measurements collected in the 2012-13 austral summer. Preliminary results indicate improved resolution of the ocean cavity, particularly in the interior and sides of the PIG ice shelf, and sedimentary drape across the region. Seismically derived variations in ice and ocean water densities are also applied to the gravity inversion to produce a more robust model of PIG sub-ice shelf structure, as opposed to commonly used single ice and water densities across the entire study region. Misfits between the seismically-constrained gravity inversion and that estimated previously from aerogravity alone provide insights on the sensitivity of gravity measurements to model perturbations and highlight the limitations of employing gravity data to model ice shelf environments when no other sub-ice constraints are available.

Structure of the North Anatolian Fault Zone from the Autocorrelation of Ambient Seismic Noise

NASA Astrophysics Data System (ADS)

Taylor, George; Rost, Sebastian; Houseman, Gregory

2016-04-01

In recent years the technique of cross-correlating the ambient seismic noise wavefield at two seismometers to reconstruct empirical Green's Functions for the determination of Earth structure has been a powerful tool to study the Earth's interior without earthquakes or man-made sources. However, far less attention has been paid to using auto-correlations of seismic noise to reveal body wave reflections from interfaces in the subsurface. In principle, the Green's functions thus derived should be comparable to the Earth's impulse response to a co-located source and receiver. We use data from a dense seismic array (Dense Array for Northern Anatolia - DANA) deployed across the northern branch of the North Anatolian Fault Zone (NAFZ) in the region of the 1999 magnitude 7.6 Izmit earthquake in western Turkey. The NAFZ is a major strike-slip system that extends ~1200 km across northern Turkey and continues to pose a high level of seismic hazard, in particular to the mega-city of Istanbul. We construct body wave images for the entire crust and the shallow upper mantle over the ~35 km by 70 km footprint of the 70-station DANA array. Using autocorrelations of the vertical component of ground motion, P-wave reflections can be retrieved from the wavefield to constrain crustal structure. We show that clear P-wave reflections from the crust-mantle boundary (Moho) can be retrieved using the autocorrelation technique, indicating topography on the Moho on horizontal scales of less than 10 km. Offsets in crustal structure can be identified that seem to be correlated with the surface expression of the northern branch of the fault zone, indicating that the NAFZ reaches the upper mantle as a narrow structure. The southern branch has a less clear effect on crustal structure. We also see evidence of several discontinuities in the mid-crust in addition to an upper mantle reflector that we interpret to represent the Hales discontinuity.

Tectonic development of passive continental margins of the southern and central Red Sea with a comparison to Wilkes Land, Antarctica

USGS Publications Warehouse

Bohannon, R.G.; Eittreim, S.L.

1991-01-01

The continental margins of the southern and central Red Sea and most of Wilkes Land, Antarctica have bulk crustal configurations and detailed structures that are best explained by a prolonged history of magmatic expansion that followed a brief, but intense period of mechanical extension. Extension on the Red Sea margins was spatially confined to a rift that was 20-30 km in width. The rifting phase along the Arabian margin of the central and southern Red Sea occurred 25-32 Ma ago, primarily by detachment faulting at upper crustal levels and ductile uniform stretching at depth. Rifting was followed by an early magmatic phase during which the margin was invaded by dikes and plutons, primarily of gabbro and diorite, at 20-24 Ma, after the crust was mechanically thinned from 40 km to ??? 20 km. We infer continued spreading after that in which broad shelves were formed by a process of magmatic expansion, because the offshore crust is only 8-15 km thick, including sediment, and seismic reflection data do not depict horst and graben or half graben structures from which mechanical extension might be inferred. The Wilkes Land margin is similar to the Arabian example. The margin is about 150 km in width, the amount of upper crustal extension is too low to explain the change in sub-sediment crustal thickness from ??? 35 km on the mainland to < 10 km beneath the margin and reflectors in the deepest seismic sequence are nearly flat lying. Our model requires large volumes of melt in the early stages of continental rifting. The voluminous melt might be partly a product of nearby hot spots, such as Afar and partly the result of an initial period of partial fusion in the deep continental lithosphere under lower temperatures than ordinarily required by dry solidus conditions. ?? 1991.

Initiation of Extension in South China Continental Margin during the Active-Passive Margin Transition: Thermochronological and Kinematic Constraints

NASA Astrophysics Data System (ADS)

Zuo, X.; Chan, L. S.

2015-12-01

The South China continental margin is characterized by a widespread magmatic belt, prominent NE-striking faults and numerous rifted basins filled by Cretaceous-Eocene sediments. The geology denotes a transition from active to passive margin, which led to rapid modifications of crustal stress configuration and reactivation of older faults in this area. Our zircon fission-track data in this region show two episodes of exhumation: The first episode, occurring during 170-120Ma, affected local parts of the Nanling Range. The second episode, a more regional exhumation event, occurred during 115-70Ma, including the Yunkai Terrane and the Nanling Range. Numerical geodynamic modeling was conducted to simulate the subduction between the paleo-Pacific plate and the South China Block. The modeling results could explain the fact that exhumation of the granite-dominant Nanling Range occurred earlier than that of the gneiss-dominant Yunkai Terrane. In addition to the difference in rock types, the heat from Jurassic-Early Cretaceous magmatism in Nanling may have softened the upper crust, causing the area to exhume more readily than Yunkai. Numerical modeling results also indicate that (1) high lithospheric geothermal gradient, high slab dip angle and low convergence velocity favor the reversal of crustal stress state from compression to extension in the upper continental plate; (2) late Mesozoic magmatism in South China was probably caused by a slab roll-back; and (3) crustal extension could have occurred prior to the cessation of plate subduction. The inversion of stress regime in the continental crust from compression to crustal extension imply that the Late Cretaceous-early Paleogene red-bed basins in South China could have formed during the late stage of the subduction, accounting for the occurrence of volcanic events in some sedimentary basins. We propose that the rifting started as early as Late Cretaceous, probably before the cessation of subduction process.

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

NASA Astrophysics Data System (ADS)

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

2015-08-01

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

Lithospheric Structure across the Alaskan Cordillera from Surface Waves and Receiver Functions

NASA Astrophysics Data System (ADS)

Ward, K. M.; Lin, F. C.

2017-12-01

The long awaited Transportable Array (TA) deployment in Alaska and western Canada is nearing its final deployment stage. With only one more deployment season, most of the TA station locations have been occupied and begun providing data. These TA stations combined with upgraded existing locations have provided enough high-quality data to begin investigating the crustal and upper mantle structure across the entire Alaskan Cordillera. From a tectonic standpoint, many interesting questions remain unanswered. For example, how does the transition from oceanic-oceanic subduction to continental-oceanic normal subduction to continental-oceanic "flat-slab" subduction to strike-slip conservative plate motion affect the deformation/uplift of the overriding plate and mantle geodynamic characteristics? How does the long and completed terrene accretion process partition stress/strain in the crust? On more local scales, are there any significant mid-crustal magmatic systems as observed in other sections of the American Cordillera, and if so, what is there role in uplift and crustal deformation? Our approach to investigating these questions is though surface wave imaging from ambient noise and earthquake generated sources along with Rayleigh wave ellipticity paired with Ps receiver functions. Our preliminary tomography results agree with previous studies but expand the spatial coverage showing additional detail. Our ellipticity results show a heterogeneous but spatially consistent anisotropic shallow crust. Although the complete TA data set has not yet been collected, we have jointly inverted surface waves with receiver functions for a 3-D shear-wave velocity model across the entire Alaskan Cordillera. Key features of our velocity model include a high-velocity feature in the upper mantle associated with the subducting Pacific plate that extends north of the seismicity used to contour the geometry of the slab and mid-crustal low-velocity zones associated with the active volcanics in the Wrangell mountains and along the Aleutian arc.

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

NASA Astrophysics Data System (ADS)

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

2016-06-01

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

Gravity anomalies and associated tectonic features over the Indian Peninsular Shield and adjoining ocean basins

NASA Astrophysics Data System (ADS)

Mishra, D. C.; Arora, K.; Tiwari, V. M.

2004-02-01

A combined gravity map over the Indian Peninsular Shield (IPS) and adjoining oceans brings out well the inter-relationships between the older tectonic features of the continent and the adjoining younger oceanic features. The NW-SE, NE-SW and N-S Precambrian trends of the IPS are reflected in the structural trends of the Arabian Sea and the Bay of Bengal suggesting their probable reactivation. The Simple Bouguer anomaly map shows consistent increase in gravity value from the continent to the deep ocean basins, which is attributed to isostatic compensation due to variations in the crustal thickness. A crustal density model computed along a profile across this region suggests a thick crust of 35-40 km under the continent, which reduces to 22/20-24 km under the Bay of Bengal with thick sediments of 8-10 km underlain by crustal layers of density 2720 and 2900/2840 kg/m 3. Large crustal thickness and trends of the gravity anomalies may suggest a transitional crust in the Bay of Bengal up to 150-200 km from the east coast. The crustal thickness under the Laxmi ridge and east of it in the Arabian Sea is 20 and 14 km, respectively, with 5-6 km thick Tertiary and Mesozoic sediments separated by a thin layer of Deccan Trap. Crustal layers of densities 2750 and 2950 kg/m 3 underlie sediments. The crustal density model in this part of the Arabian Sea (east of Laxmi ridge) and the structural trends similar to the Indian Peninsular Shield suggest a continent-ocean transitional crust (COTC). The COTC may represent down dropped and submerged parts of the Indian crust evolved at the time of break-up along the west coast of India and passage of Reunion hotspot over India during late Cretaceous. The crustal model under this part also shows an underplated lower crust and a low density upper mantle, extending over the continent across the west coast of India, which appears to be related to the Deccan volcanism. The crustal thickness under the western Arabian Sea (west of the Laxmi ridge) reduces to 8-9 km with crustal layers of densities 2650 and 2870 kg/m 3 representing an oceanic crust.

Numerical modeling of forceful pluton emplacement and associated deformation at different crustal levels - instantaneous, continuous or episodic intrusion?

NASA Astrophysics Data System (ADS)

Chen, Y.; Nabelek, P. I.

2015-12-01

The Papoose Flat pluton in the White-Inyo Range, California, is one of the best examples of forceful magma emplacement at mid-crustal levels that is revealed by a highly strained aureole. A thermo-rheological 2-D model of the pluton and its aureole is proposed. We explored how the frequency of magma input, from instantaneous to continuous to the bottom of the laccolith, affects the ductile width of the aureole and the crystallinity of the pluton, which has implications for eruption of magma. We modeled these aspects at mid- and upper-crustal levels. The pluton was assumed to be 5 km thick in the middle and 13 km wide. Except for instantaneous growth, pluton was assumed to grow over 5 m.y. The aureole was assumed to have power-law rheology of quartz with dependence on H2O fugacity, which was calculated using the CORK equation (Holland & Powell, 1991) Our result shows that the bottom of the Papoose Flat pluton was emplaced at the brittle-ductile transition zone of the crust. The crustal rheology profile assisted the softening of rocks around the pluton. The simulated temperature and strength profiles confirm that ductile deformation was related to thermal weakening (Saint-Blanquat et al., 2001). Results of incremental growth calculations show that the pluton remains hot and only partially crystalline for millions of years when it grows by frequent input of small batches of liquid. At the mid-crustal level, the ductile region around the pluton is much wider and exists longer than at the shallow crustal level. Brittle rheology is dominant during the late stage growth at the shallow depth. When the pluton grows instantly or by only few episodes of large batches of input, the mobile part of the pluton is thin and the ductile aureole is narrower. High-frequency incremental growth by smaller magma batches produces a large volume of mobile magma that has the potential to induce internal magmatic layering that may be reflected in aligned acquired magnetic susceptibility (AMS) in upper parts of the pluton. By examing the chamber overpressure generated by injection of magma (Jellinek and DePaolo, 2003) and the overpressure related to magma buoyancy (Karlstrom et al, 2010), eruption during high-frequency magma input may be promoted by the magma buoyancy while an eruption during low-frequency input may be caused by injection of a large magma batch.

Window into the Caledonian orogen: Structure of the crust beneath the East Shetland platform, United Kingdom

USGS Publications Warehouse

McBride, J.H.; England, R.W.

1999-01-01

Reprocessing and interpretation of commercial and deep seismic reflection data across the East Shetland platform and its North Sea margin provide a new view of crustal subbasement structure beneath a poorly known region of the British Caledonian orogen. The East Shetland platform, east of the Great Glen strike-slip fault system, is one of the few areas of the offshore British Caledonides that remained relatively insulated from the Mesozoic and later rifting that involved much of the area around the British Isles, thus providing an "acoustic window" into the deep structure of the orogen. Interpretation of the reflection data suggests that the crust beneath the platform retains a significant amount of its original Caledonian and older architecture. The upper to middle crust is typically poorly reflective except for individual prominent dipping reflectors with complex orientations that decrease in dip with depth and merge with a lower crustal layer of high reflectivity. The three-dimensional structural orientation of the reflectors beneath the East Shetland platform is at variance with Caledonian reflector trends observed elsewhere in the Caledonian orogen (e.g., north of the Scottish mainland), emphasizing the unique tectonic character of this part of the orogen. Upper to middle crustal reflectors are interpreted as Caledonian or older thrust surfaces that were possibly reactivated by Devonian extension associated with post-Caledonian orogenic collapse. The appearance of two levels of uneven and diffractive (i.e., corrugated) reflectivity in the lower crust, best developed on east-west-oriented profiles, is characteristic of the East Shetland platform. However, a north-south-oriented profile reveals an interpreted south-vergent folded and imbricated thrust structure in the lower crust that appears to be tied to the two levels of corrugated reflectivity on the east-west profiles. A thrust-belt origin for lower crustal reflectivity would explain its corrugated appearance. Regional seismic velocity models derived from refraction data suggest that this reflectivity correlates with a continuous lower crustal layer that has an intermediate seismic velocity. The lower crustal reflectivity is determined to be older than Mesozoic age by the bending down and truncation of the two reflectivity levels at the western margin of the North Sea Viking graben by a major mantle reflector inferred to be associated with Mesozoic rifting. The results of this study are thus in contrast with orthodox interpretations of the reflective layered lower crust as being caused by mantle-derived igneous intrusion or by deformation fabrics associated with stretching in response to continental rifting.

Hybrid channel flow-type mechanisms in the Greater Himalayan Sequence (West Nepal): new constraints from vorticity of flow and quartz petrofabric analyses.

NASA Astrophysics Data System (ADS)

Frassi, Chiara

2016-04-01

Three main tectono-metamorphic units are classically recognized along the Himalayan belt: the Lesser Himalayan (LH), the Greater Himalayan sequence (GHS) and the Tibetan Sedimentary sequence (TSS). The GHS may be interpreted as a low-viscosity tabular body of mid-crustal rocks extruded southward in Miocene times beneath the Tibetan plateau between two parallel and opposite-sense crustal-scale shear zones: the Main Central thrust at the base, and the South Tibetan Detachment system at the top. The pre-/syn-shearing mineral assemblage documented within these crustal-scale shear zones indicates that the metamorphic grade increases toward the core of the GHS producing an inverted and a normal thermal gradient respectively on the top and on the bottom of the slab. In addition, thermal profiles estimated using both petrology- and microstructures/fabrics-based thermometers indicate that the metamorphic isograds are condensed. Although horizontal extension and vorticity estimates collected across the GHS could be strongly biased by the criteria used to define the map position of the MCT, published vorticity data document general shear flow (1>Wk>0) within the slab with a pure-shear component of flow slightly predominant within the core of the GHS whereas the simple-shear component seems to dominate at the top of the slab. The lower boundary of the GHS records a general shear flow with a comparable contribution of simple and pure shearing. The associated crustal extrusion is compatible with Couette - Poiseuille velocity flow profile as assumed in crustal-scale channel flow-type models In this study, the quartz c-axis petrofabrics, vorticity and deformation-temperature studies are integrated with microstructures and metamorphic studies to individuate the location of the MCT and to document the spatial distribution of ductile deformation patterns across the lower portion of the GHS exposed in the Chaudabise river valley in western Nepal. My results indicate that the Main Central Thrust is located ˜5 km structurally below the previous mapped locations. Deformation temperature increases up structural section from ˜450°C to ˜650°C and overlaps with peak metamorphic temperature indicating that penetrative shearing was responsible for the exhumation of the GHS occurred at "close" to peak metamorphic conditions. I interpreted the telescoping and the inversion of the paleo-isotherms at the base of the GHS as produced mainly by a sub-simple shearing (Wm = 0.88-1) pervasively distributed through the lower portion of the GHS. The results are consistent with hybrid channel flow-type models where the boundary between lower and upper portions of the GHS, broadly corresponding to the tectono-metamorphic discontinuity recently documented in west Nepal, represents the limit between buried material, affected by dominant simple shearing, and exhumed material affected by a general flow dominates by pure shearing. This interpretation is consistent with the recent models suggesting the simultaneous operation of channel flow- and critical wedge-type processes at different structural depth.

The south-central United States magnetic anomaly

NASA Technical Reports Server (NTRS)

Hinze, W. J.; Braile, L. W. (Principal Investigator); Starich, P. J.

1984-01-01

The South-Central United States Magnetic Anomaly is the most prominent positive feature in the MAGSAT scalar magnetic field over North America. The anomaly correlates with increased crustal thickness, above average crustal velocity, negative free air gravity anomalies and an extensive zone of Middle Proterozoic anorogenic felsic basement rocks. Spherical dipole source inversion of the MAGSAT scalar data and subsequent calculation of reduced to pole and derivative maps provide constraints for a crustal magnetic model which corresponds geographically to the extensive Middle Proterozoic felsic rocks trending northeasterly across the United States. These felsic rocks contain insufficient magnetization or volume to produce the anomaly, but are rather indicative of a crustal zone which was disturbed during a Middle Proterozoic thermal event which enriched magnetic material deep in the crust.

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.

Very early Archean crustal-accretion complexes preserved in the North Atlantic craton

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

Nutman, A.P.; Collerson, K.D.

1991-08-01

The North Atlantic craton contains very early Archean supracrustal rocks, orthogneisses, and massive ultramafic rocks. Most units of supracrustal rocks are dominated by mafic volcanic rocks, layered gabbros, and banded iron formations, bust some also contain abundant felsic volcanic-sedimentary rocks, quartzites, and marbles. Some quartzites contain detrital zircons derived from rocks identical in age to felsic volcanic-sedimentary rocks in these sequences (ca. 3800 Ma) and also from older (ca. 3850 Ma) sources. The presence of the ca. 3850 Ma detrital zircons suggests that the supracrustal units containing them were deposited on, or close to, ca. 3850 Ma sialic crust. Themore » massive ultramafic rocks have chemical affinities to upper mantle rocks. The voluminous suites of tonalitic gneisses are dominated by 3700-3730 Ma bodies that intrude the supracrustal sequences, but they also locally contain components with ages between 3820 and 3920 Ma. The diverse supracrustal units, upper mantle rocks, and {ge} 3820 Ma components in the gneisses were tectonically interleaved in very early Archean convergent plate boundaries, giving rise to accretion complexes. In the period 3700-3730 Ma, voluminous tonalitic magmas produced by partial melting of predominantly mafic rocks in the base of the accretion complexes were emplaced at higher levels, forming juvenile continental crust and leaving behind a refractory lower crustal to upper mantle substrate.« less

Imaging of upper crustal structure beneath East Java-Bali, Indonesia with ambient noise tomography

NASA Astrophysics Data System (ADS)

Martha, Agustya Adi; Cummins, Phil; Saygin, Erdinc; Sri Widiyantoro; Masturyono

2017-12-01

The complex geological structures in East Java and Bali provide important opportunities for natural resource exploitation, but also harbor perils associated with natural disasters. Such a condition makes the East Java region an important area for exploration of the subsurface seismic wave velocity structure, especially in its upper crust. We employed the ambient noise tomography method to image the upper crustal structure under this study area. We used seismic data recorded at 24 seismographs of BMKG spread over East Java and Bali. In addition, we installed 28 portable seismographs in East Java from April 2013 to January 2014 for 2-8 weeks, and we installed an additional 28 seismographs simultaneously throughout East Java from August 2015 to April 2016. We constructed inter-station Rayleigh wave Green's functions through cross-correlations of the vertical component of seismic noise recordings at 1500 pairs of stations. We used the Neighborhood Algorithm to construct depth profiles of shear wave velocity (Vs). The main result obtained from this study is the thickness of sediment cover. East Java's southern mountain zone is dominated by higher Vs, the Kendeng basin in the center is dominated by very low Vs, and the Rembang zone (to the North of Kendeng zone) is associated with medium Vs. The existence of structures with oil and gas potential in the Kendeng and Rembang zones can be identified by low Vs.

Aleutian basin oceanic crust

USGS Publications Warehouse

Christeson, Gail L.; Barth, Ginger A.

2015-01-01

We present two-dimensional P-wave velocity structure along two wide-angle ocean bottom seismometer profiles from the Aleutian basin in the Bering Sea. The basement here is commonly considered to be trapped oceanic crust, yet there is a change in orientation of magnetic lineations and gravity features within the basin that might reflect later processes. Line 1 extends ∼225 km from southwest to northeast, while Line 2 extends ∼225 km from northwest to southeast and crosses the observed change in magnetic lineation orientation. Velocities of the sediment layer increase from 2.0 km/s at the seafloor to 3.0–3.4 km/s just above basement, crustal velocities increase from 5.1–5.6 km/s at the top of basement to 7.0–7.1 km/s at the base of the crust, and upper mantle velocities are 8.1–8.2 km/s. Average sediment thickness is 3.8–3.9 km for both profiles. Crustal thickness varies from 6.2 to 9.6 km, with average thickness of 7.2 km on Line 1 and 8.8 km on Line 2. There is no clear change in crustal structure associated with a change in orientation of magnetic lineations and gravity features. The velocity structure is consistent with that of normal or thickened oceanic crust. The observed increase in crustal thickness from west to east is interpreted as reflecting an increase in melt supply during crustal formation.

Variation of Crustal Shear Velocity Structure Along the Eastern Lau Back-Arc Spreading Center Constrained By Seafloor Compliance

NASA Astrophysics Data System (ADS)

Zha, Y.; Webb, S. C.; Dunn, R. A.

2014-12-01

Measurements of seafloor compliance, the deformation under long period (typically 30-300 s) ocean wave forcing, are primarily sensitive to crustal shear velocity structure. We analyze seafloor compliance from data collected from a subset of 50 broadband Ocean Bottom Seismographs (OBS) deployed at the Eastern Lau spreading center (ELSC) from 2009 to 2010. The ELSC is a 400-km-long back-arc spreading center lying closely to the Tonga subduction trench in the southwestern Pacific. Seafloor morphology, crustal seismic structure and lava composition data show rapid variations along the ridge as the ridge migrates away from the volcanic arc front to the north, indicating a decreasing influence of the subducting slab. We calculate seafloor compliance functions by taking the spectral transfer function between the vertical displacement and pressure signal recorded by the 4-component OBSs, which are equipped with differential pressure gauges (DPGs). In the ridge perpendicular direction, compliance amplitude vary by more than an order of magnitude from the ridge crest to older seafloor covered by sediment. Along the spreading ridge, compliance measured from on-axis sites increases southwards, indicative of a decrease in the upper crustal shear velocity possibly due to increasing porosity and a thickening extrusive layer [Jacobs et al., 2007; Dunn et al., 2013]. We apply a Markov Chain Monte Carlo method to invert the compliance functions for crustal shear velocities at various locations along the ELSC.

Fault locking, block rotation and crustal deformation in the Pacific Northwest

USGS Publications Warehouse

McCaffrey, R.; Qamar, A.I.; King, R.W.; Wells, R.; Khazaradze, G.; Williams, C.A.; Stevens, C.W.; Vollick, J.J.; Zwick, P.C.

2007-01-01

We interpret Global Positioning System (GPS) measurements in the northwestern United States and adjacent parts of western Canada to describe relative motions of crustal blocks, locking on faults and permanent deformation associated with convergence between the Juan de Fuca and North American plates. To estimate angular velocities of the oceanic Juan de Fuca and Explorer plates and several continental crustal blocks, we invert the GPS velocities together with seafloor spreading rates, earthquake slip vector azimuths and fault slip azimuths and rates. We also determine the degree to which faults are either creeping aseismically or, alternatively, locked on the block-bounding faults. The Cascadia subduction thrust is locked mainly offshore, except in central Oregon, where locking extends inland. Most of Oregon and southwest Washington rotate clockwise relative to North America at rates of 0.4-1.0?? Myr-1. No shear or extension along the Cascades volcanic arc has occurred at the mm/yr level during the past decade, suggesting that the shear deformation extending northward from the Walker Lane and eastern California shear zone south of Oregon is largely accommodated by block rotation in Oregon. The general agreement of vertical axis rotation rates derived from GPS velocities with those estimated from palaeomagnetic declination anomalies suggests that the rotations have been relatively steady for 10-15 Ma. Additional permanent dextral shear is indicated within the Oregon Coast Range near the coast. Block rotations in the Pacific Northwest do not result in net westward flux of crustal material - the crust is simply spinning and not escaping. On Vancouver Island, where the convergence obliquity is less than in Oregon and Washington, the contractional strain at the coast is more aligned with Juan de Fuca-North America motion. GPS velocities are fit significantly better when Vancouver Island and the southern Coast Mountains move relative to North America in a block-like fashion. The relative motions of the Oregon, western Washington and Vancouver Island crustal blocks indicate that the rate of permanent shortening, the type that causes upper plate earthquakes, across the Puget Sound region is 4.4 ?? 0.3 mm yr-1. This shortening is likely distributed over several faults but GPS data alone cannot determine the partitioning of slip on them. The transition from predominantly shear deformation within the continent south of the Mendocino Triple Junction to predominantly block rotations north of it is similar to changes in tectonic style at other transitions from shear to subduction. This similarity suggests that crustal block rotations are enhanced in the vicinity of subduction zones possibly due to lower resisting stress. ?? 2007 The Authors Journal compilation ?? 2007 RAS.

Inverse models of gravity data from the Red Sea-Aden-East African rifts triple junction zone

NASA Astrophysics Data System (ADS)

Tiberi, Christel; Ebinger, Cynthia; Ballu, Valérie; Stuart, Graham; Oluma, Befekadu

2005-11-01

The combined effects of stretching and magmatism permanently modify crustal structure in continental rifts and volcanic passive margins. The Red Sea-Gulf of Aden-Ethiopian rift triple junction zone provides a unique opportunity to examine incipient volcanic margin formation above or near an asthenospheric upwelling. We use gravity inversions and forward modelling to examine lateral variations in crust and upper mantle structure across the Oligocene flood basalt province, which has subsequently been extended to form the Red Sea, Gulf of Aden and Main Ethiopian rifts. We constrain and test the obtained models with new and existing seismic estimates of crustal thickness. In particular, we predict crustal thickness across the uplifted plateaux and rift valleys, and calibrate our results with recent receiver function analyses. We discuss the results together with a 3-D distribution of density contrasts in terms of magmatic margin structure. The main conclusions are: (1) a denser (+240 kg m-3) and/or a thinner crust (23 km) in the triple junction zone of the Afar depression; (2) a shallower Moho is found along the Main Ethiopian rift axis, with crustal thickness values decreasing from 32-33 km in the south to 24 km beneath the southern Afar depression; (3) thicker crust (~40 km) is present beneath the broad uplifted Oligocene flood basalt province, suggesting that crustal underplating compensates most of the plateau uplift and (4) possible magmatic underplating or a segmentation in the rift structure is observed at ~8°N, 39°W beneath several collapsed caldera complexes. These results indicate that magmatism has profoundly changed crustal structure throughout the flood basalt province.

The Fragmented Manihiki Plateau - Key Region for Understanding the Break-up of the "Super" Large Igneous Province Ontong Java Nui

NASA Astrophysics Data System (ADS)

Hochmuth, K.; Gohl, K.; Uenzelmann-Neben, G.; Werner, R.

2014-12-01

The Manihiki Plateau of the western Pacific is one of the world - wide greatest Large Igneous Province (LIP) on oceanic crust. It is assumed that the Manihiki Plateau was emplaced as the centerpiece of the "Super-LIP" Ontong Java Nui by multiple volcanic phases during the Cretaceous Magnetic Quiet Period. The subsequent break-up of Ontong Java Nui led to fragmentation of the Manihiki Plateau into three sub-plateaus, which all exhibit individual relicts of the "Super-LIP" break-up. We examine two deep crustal seismic refraction/wide-angle reflection profiles crossing the two largest sub-plateaus of the Manihiki Plateau, the Western Plateaus and the High Plateau. Modeling of P- and S-wave velocities reveals surprising differences in the crustal structure between the two sub-plateaus. Whereas the High Plateau shows a constant crustal thickness of 20 km, relicts of multiple volcanic phases and break-up features at its margins, the model of the Western Plateaus reveals a crustal thickness decreasing from 17 km to only 9 km. There is only little evidence of secondary phases of volcanic activity. The main upper crustal structure on the Western Plateaus consists of fault systems and sedimentary basins. We infer that the High Plateau experienced phases of strong secondary volcanism, and that tectonic deformation was limited to its edges. The Western Plateaus, on the contrary, were deformed by crustal stretching and underwent only little to no secondary volcanism. This indicates that the two main sub-plateaus of the Manihiki Plateau experienced a different geological history and have played their individual parts in the break-up history of Ontong Java Nui.

Crustal structure beneath two seismic stations in the Sunda-Banda arc transition zone derived from receiver function analysis

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

Syuhada, E-mail: hadda9@gmail.com; Research Centre for Physics - Indonesian Institute of Sciences; Hananto, Nugroho D.

2015-04-24

We analyzed receiver functions to estimate the crustal thickness and velocity structure beneath two stations of Geofon (GE) network in the Sunda-Banda arc transition zone. The stations are located in two different tectonic regimes: Sumbawa Island (station PLAI) and Timor Island (station SOEI) representing the oceanic and continental characters, respectively. We analyzed teleseismic events of 80 earthquakes to calculate the receiver functions using the time-domain iterative deconvolution technique. We employed 2D grid search (H-κ) algorithm based on the Moho interaction phases to estimate crustal thickness and Vp/Vs ratio. We also derived the S-wave velocity variation with depth beneath both stationsmore » by inverting the receiver functions. We obtained that beneath station PLAI the crustal thickness is about 27.8 km with Vp/Vs ratio 2.01. As station SOEI is covered by very thick low-velocity sediment causing unstable solution for the inversion, we modified the initial velocity model by adding the sediment thickness estimated using high frequency content of receiver functions in H-κ stacking process. We obtained the crustal thickness is about 37 km with VP/Vs ratio 2.2 beneath station SOEI. We suggest that the high Vp/Vs in station PLAI may indicate the presence of fluid ascending from the subducted plate to the volcanic arc, whereas the high Vp/Vs in station SOEI could be due to the presence of sediment and rich mafic composition in the upper crust and possibly related to the serpentinization process in the lower crust. We also suggest that the difference in velocity models and crustal thicknesses between stations PLAI and SOEI are consistent with their contrasting tectonic environments.« less

Future accreted terranes: a compilation of island arcs, oceanic plateaus, submarine ridges, seamounts, and continental fragments

NASA Astrophysics Data System (ADS)

Tetreault, J. L.; Buiter, S. J. H.

2014-07-01

Allochthonous accreted terranes are exotic geologic units that originated from anomalous crustal regions on a subducting oceanic plate and were transferred to the overriding plate during subduction by accretionary processes. The geographical regions that eventually become accreted allochthonous terranes include island arcs, oceanic plateaus, submarine ridges, seamounts, continental fragments, and microcontinents. These future allochthonous terranes (FATs) contribute to continental crustal growth, subduction dynamics, and crustal recycling in the mantle. We present a review of modern FATs and their accreted counterparts based on available geological, seismic, and gravity studies and discuss their crustal structure, geological origin, and bulk crustal density. Island arcs have an average crustal thickness of 26 km, average bulk crustal density of 2.79 g cm-3, and have 3 distinct crustal units overlying a crust-mantle transition zone. Oceanic plateaus and submarine ridges have an average crustal thickness of 21 km and average bulk crustal density of 2.84 g cm-3. Continental fragments presently on the ocean floor have an average crustal thickness of 25 km and bulk crustal density of 2.81 g cm-3. Accreted allochthonous terranes can be compared to these crustal compilations to better understand which units of crust are accreted or subducted. In general, most accreted terranes are thin crustal units sheared off of FATs and added onto the accretionary prism, with thicknesses on the order of hundreds of meters to a few kilometers. In addition many island arcs, oceanic plateaus, and submarine ridges were sheared off in the subduction interface and underplated onto the overlying continent. And other times we find evidence of collision leaving behind accreted terranes 25 to 40 km thick. We posit that rheologically weak crustal layers or shear zones that were formed when the FATs were produced can be activated as detachments during subduction, allowing parts of the FAT crust to accrete and others to accrete. In many modern FATs on the ocean floor, a sub-crustal layer of high seismic velocities, interpreted as ultramafic material, could serve as a detachment or delaminate during subduction.

Future accreted terranes: a compilation of island arcs, oceanic plateaus, submarine ridges, seamounts, and continental fragments

NASA Astrophysics Data System (ADS)

Tetreault, J. L.; Buiter, S. J. H.

2014-12-01

Allochthonous accreted terranes are exotic geologic units that originated from anomalous crustal regions on a subducting oceanic plate and were transferred to the overriding plate by accretionary processes during subduction. The geographical regions that eventually become accreted allochthonous terranes include island arcs, oceanic plateaus, submarine ridges, seamounts, continental fragments, and microcontinents. These future allochthonous terranes (FATs) contribute to continental crustal growth, subduction dynamics, and crustal recycling in the mantle. We present a review of modern FATs and their accreted counterparts based on available geological, seismic, and gravity studies and discuss their crustal structure, geological origin, and bulk crustal density. Island arcs have an average crustal thickness of 26 km, average bulk crustal density of 2.79 g cm-3, and three distinct crustal units overlying a crust-mantle transition zone. Oceanic plateaus and submarine ridges have an average crustal thickness of 21 km and average bulk crustal density of 2.84 g cm-3. Continental fragments presently on the ocean floor have an average crustal thickness of 25 km and bulk crustal density of 2.81 g cm-3. Accreted allochthonous terranes can be compared to these crustal compilations to better understand which units of crust are accreted or subducted. In general, most accreted terranes are thin crustal units sheared off of FATs and added onto the accretionary prism, with thicknesses on the order of hundreds of meters to a few kilometers. However, many island arcs, oceanic plateaus, and submarine ridges were sheared off in the subduction interface and underplated onto the overlying continent. Other times we find evidence of terrane-continent collision leaving behind accreted terranes 25-40 km thick. We posit that rheologically weak crustal layers or shear zones that were formed when the FATs were produced can be activated as detachments during subduction, allowing parts of the FAT crust to accrete and others to subduct. In many modern FATs on the ocean floor, a sub-crustal layer of high seismic velocities, interpreted as ultramafic material, could serve as a detachment or delaminate during subduction.

A conceptual review of regional-scale controls on the composition of clastic sediment and the co-evolution of continental blocks and their sedimentary cover.

PubMed

Cox, R; Lowe, D R

1995-01-02

Both sediment recycling and first-cycle input influence the composition of clastic material in sedimentary systems. This paper examines conceptually the roles played by these processes in governing the composition of clastic sediment on a regional scale by outlining the expected effects on sediment composition of protracted sediment recycling and of continuous first-cycle input on a maturing continental block. Generally speaking, long-term recycling tends to enrich sediments in the most chemically and mechanically stable components: quartz in the sand and silt size fractions, and illite among the clay minerals. Sandstones trend towards pure quartz arenites, and mudrocks become more potassic and aluminous. The average grain size of clastic sediment decreases by a combination of progressive attrition of sand grains and ongoing breakdown of primary silicate minerals to finer-grained clay minerals and oxides. Sandstones derived by continuous first-cycle input from an evolving continental crustal source also become increasingly rich in quartz, but in addition become more feldspathic as the proportion of granitic material in the upper continental crust increases during crustal stabilization. Associated mudrocks also become richer in potassium and aluminum, but will have higher K2O/Al2O3 ratios than recycled muds. The average grain size of the sediment may increase with time as the proportion of sand-prone granitic source rocks increases at the expense of more mud-prone volcanic sources. In general, except in instances where chemical weathering is extreme, first-cycle sediments lack the compositional maturity of recycled detritus, and are characterized by the presence of a variety of primary silicate minerals. Sedimentary systems are not usually completely dominated by either recycling or first-cycle detritus. Generally, however, sedimentary systems associated with the earliest phases of formation and accretion of continental crust are characterized by first-cycle input from igneous and metamorphic rocks, whereas those associated with more mature cratons tend to be dominated by recycled sedimentary material.

Investigation of upper mantle seismic discontinuities beneath the Indian Ocean using array seismology methods

NASA Astrophysics Data System (ADS)

van Driel, J.; Reiss, A. S.; Thomas, C.

2016-12-01

The topography of upper mantle seismic discontinuities can be used to constrain regional variations in composition and temperature of the Earths mantle. The 410 km discontinuity is caused by the solid-solid phase transition from olivine to wadsleyite. Due to its positive Clapeyron slope, the discontinuity is depressed in hot regimes. The phase transition from ringwoodite to bridgemanite and magnesiowüstite in contrast has a negative Clapeyron slope and therefore is elevated when hot material is present. Cold material is expected to yield an opposing topographic signature, culminating in an elevated 410 km and a depressed 660 km discontinuity. As part of the RHUM-RUM project (Réunion Hotspot and Upper Mantle - Réunions Unterer Mantel) we extract relevant geophysical parameters, by investigating the properties of upper mantle seismic discontinuities beneath the Indian Ocean. The topography of the 410 and 660 km discontinuities, which define the upper and lower bounds of the mantle transition zone, have been mapped using PP and SS underside reflections. This study has utilised over 8500 events with Mw ≥ 5.8, distributed over the entire Indian Ocean. Our robust data set yields a dense coverage of points, which are defined by consistently crossing ray paths. Array seismology methods, such as vespagrams and slowness-backazimuth analysis, are used to enhance the signal-to-noise-ratio and detect and identify weak precursor signals. The differential travel times are corrected for crustal features and converted into depth values of the discontinuities by comparing the measured travel times with theoretical ones derived from ray tracing through the 1D reference Earth model ak135. A `travel-time' stacking method has also been applied for 4° radius bins around each of the bounce points. The addition of a secondary method derives greater stability of our results and allows an enhanced error analysis procedure. In order to better constrain the mineralogical processes taking place within the mantle transition zone, amplitude ratios, polarities and velocity gradients have also been investigated.

Changes in crustal seismic deformation rates associated with the 1964 Great Alaska earthquake

USGS Publications Warehouse

Doser, D.I.; Ratchkovski, N.A.; Haeussler, Peter J.; Saltus, R.

2004-01-01

We calculated seismic moment rates from crustal earthquake information for the upper Cook Inlet region, including Anchorage, Alaska, for the 30 yr prior to and 36 yr following the 1964 Great Alaska earthquake. Our results suggest over a factor of 1000 decrease in seismic moment rate (in units of dyne centimeters per year) following the 1964 mainshock. We used geologic information on structures within the Cook Inlet basin to estimate a regional geologic moment rate, assuming the structures extend to 30 km depth and have near-vertical dips. The geologic moment rates could underestimate the true rates by up to 70% since it is difficult determine the amount of horizontal offset that has occurred along many structures within the basin. Nevertheless, the geologic moment rate is only 3-7 times lower than the pre-1964 seismic moment rate, suggesting the 1964 mainshock has significantly slowed regional crustal deformation. If we compare the geologic moment rate to the post-1964 seismic moment rate, the moment rate deficit over the past 36 yr is equivalent to a moment magnitude 6.6-7.0 earthquake. These observed differences in moment rates highlight the difficulty in using seismicity in the decades following a large megathrust earthquake to adequately characterize long-term crustal deformation.

Fine crustal and uppermost mantle S-wave velocity structure beneath the Tengchong volcanic area inferred from receiver function and surface-wave dispersion: constraints on magma chamber distribution

NASA Astrophysics Data System (ADS)

Li, Mengkui; Zhang, Shuangxi; Wu, Tengfei; Hua, Yujin; Zhang, Bo

2018-03-01

The Tengchong volcanic area is located in the southeastern margin of the collision zone between the Indian and Eurasian Plates. It is one of the youngest intraplate volcano groups in mainland China. Imaging the S-wave velocity structure of the crustal and uppermost mantle beneath the Tengchong volcanic area is an important means of improving our understanding of its volcanic activity and seismicity. In this study, we analyze teleseismic data from nine broadband seismic stations in the Tengchong Earthquake Monitoring Network. We then image the crustal and uppermost mantle S-wave velocity structure by joint analysis of receiver functions and surface-wave dispersion. The results reveal widely distributed low-velocity zones. We find four possible magma chambers in the upper-to-middle crust and one in the uppermost mantle. The chamber in the uppermost mantle locates in the depth range from 55 to 70 km. The four magma chambers in the crust occur at different depths, ranging from the depth of 7 to 25 km in general. They may be the heat sources for the high geothermal activity at the surface. Based on the fine crustal and uppermost mantle S-wave velocity structure, we propose a model for the distribution of the magma chambers.

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).

Re-evaluation of polyphase kinematic and 40Ar/39Ar cooling history of Moldanubian hot nappe at the eastern margin of the Bohemian Massif

NASA Astrophysics Data System (ADS)

Racek, M.; Lexa, O.; Schulmann, K.; Corsini, M.; Štípská, P.; Maierová, P.

2017-03-01

A structural and geochronological 40Ar/39Ar study was performed in kilometre-scale middle and lower crustal lens-shaped domains dominated by a preserved subvertical foliation, surrounded by horizontally foliated migmatites. These domains occur within the Moldanubian nappe overlying the Brunia microcontinent at the eastern margin of the European Variscides. Three main deformation phases were recognized: subvertical S2 fabric trending NW-SE in lower crustal rocks and NE-SW in mid-crustal rocks. It is reworked by HT/MT horizontal fabric S3 along margins of crustal domains and in surrounding migmatites. S3 bears a prolate NE lineation parallel to the S2-S3 intersection in the lower crustal domain. In the middle crustal units, L3 is weak, connected to oblate strain and trends NE-SW parallel to the S2-S3 intersection. D4 non-coaxial shear deformation is mainly localized at the boundary between the Moldanubian nappe and Brunia and bears strong top to the NNE shear criteria. In order to constrain kinematics of the D3 deformation, strain modelling was performed to show that the Moldanubian hot nappe was frontally thrust over the Brunia indentor. The renewed D4 tangential movement only heterogeneously reactivates the horizontal S3. This evolution is recorded in 40Ar/39Ar amphibole cooling ages, which show two statistically significant Carboniferous peaks at 342 and 332 Ma, which are also reflected by published detrital muscovite 40Ar/39Ar ages in the adjacent foreland basin. This geochronological record is correlated with progressive erosion of the topographically elevated upper crustal part of the Moldanubian nappe during D3 frontal thrusting, followed by greenschist facies D4 transpressive reactivation and subsequent erosion of high-grade parts of the nappe.

Uplifting the Stable Crust of the Colorado Plateau through Crustal Hydration and Warming

NASA Astrophysics Data System (ADS)

Porter, R. C.; Holt, W. E.

2016-12-01

The Colorado Plateau (CP) is a high ( 2 km above sea level), low-relief, orogenic plateau located within the interior of the southwestern United States that presents several outstanding geologic questions, most notably about the timing and mechanism(s) for uplift. The CP was located below sea level during the Cretaceous and was uplifted to its modern elevation with little crustal shortening, making the cause of uplift enigmatic. Numerous mechanisms have been hypothesized to explain the uplift of this stable block and include delamination, mantle heating/phase changes, mantle convection, volatile addition, and various combinations of these. In order to better understand the crustal contribution to uplift, we utilize data from the EarthScope Transportable Array network to image the CP lithosphere and inform thermodynamic models of CP lower crustal composition. Rayleigh wave phase velocities calculated using ambient noise tomography and surface wave gradiometry were inverted for shear velocity resulting in a high-resolution velocity model of the CP crust and upper mantle. In order to provide greater context to these results, the thermodynamic modeling code Perple_X was utilized to forward model crustal densities, seismic velocities, and water content based on psuedosections calculated using published major element chemistry. Our seismic and modeling results show that uplift of the plateau is partially driven by hydration and extension of the lower crust, both of which reduce its density. Hydration of the CP crust likely occurred due to dewatering of the Farallon slab during flat-slab subduction and reduced lower crustal density by 70 kg/m3. Warming and extension further reduced the lower crustal density by 90 kg/m3 at the CP margins. Though these processes played a role in the uplift of the CP, additional mechanisms, likely due to mantle processes, are required to fully explain its high elevation. Additionally, hydration and subsequent dehydration may play an important role in the recent encroachment of deformation and volcanism into the interior of the CP.

Plume magmatism and crustal growth at 2.9 to 3.0 Ga in the Steep Rock and Lumby Lake area, Western Superior Province

NASA Astrophysics Data System (ADS)

Tomlinson, K. Y.; Hughes, D. J.; Thurston, P. C.; Hall, R. P.

1999-01-01

The greenstone belts of the western Superior Province are predominantly 2.78 to 2.69 Ga and provide evidence of oceanic and arc volcanism during the accretionary phase of development of the Superior Province. There is also scattered evidence of Meso-Archean crust (predominantly 2.9 to 3.0 Ga) within the western Superior Province. The Meso-Archean greenstone belts commonly contain platformal sediments and unconformably overlie granitoid basement. The platformal sediments occur associated with komatiitic and tholeiitic volcanic rocks that suggest a history of magmatism associated with rifting during the Meso-Archean. The central Wabigoon Subprovince is a key area of Meso-Archean crust and in its southern portion comprises the Steep Rock, Finlayson and Lumby Lake greenstone belts. The Steep Rock greenstone belt unconformably overlies 3 Ga continental basement and contains platformal sediments succeeded by komatiitic and tholeiitic volcanic rocks. The Lumby Lake greenstone belt contains thick sequences of mafic volcanics, a number of komatiite horizons, and thin platformal sedimentary units. The two belts are joined by the predominantly mafic volcanic Finlayson greenstone belt. The volcanics throughout these three greenstone belts may be correlated to some extent and a range of basaltic and komatiite types is present. Al-undepleted komatiites present in the Lumby Lake greenstone belt have an Al 2O 3/TiO 2 ratio ranging from 14 to 27 and (Gd/Yb) N from 0.7 to 1.3. These are divided into basaltic komatiites with generally unfractionated mantle-normalised multi-element profiles, and spinifex-textured high-Mg basalts with slightly light REE enriched multi-element profiles and small negative Nb and Ta anomalies. The unfractionated basaltic komatiites represent high degree partial melts of the upper mantle whereas the spinifex-textured high-Mg basalts represent evolutionary products of the komatiite liquids following olivine and chromite fractionation and crustal contamination. Al-depleted komatiites are present in both the Lumby Lake and Steep Rock belts and have Al 2O 3/TiO 2 ratio ranges from 2.5 to 5. These display strong enrichment in the light REE and Nb and strong depletion in the heavy REE and Y ((Gd/Yb) N=2-4). They represent a deep mantle plume source generated from a high degree of partial melting in the majorite garnet stability field. The basaltic flows in all three greenstone belts are predominantly slightly light REE depleted and represent a slightly depleted upper mantle source. Basalts spatially associated with the unfractionated basaltic komatiites and the slightly light REE enriched spinifex-textured high-Mg basalts are also slightly enriched in light REE and have negative Nb and Ta anomalies. These basalts represent evolved products of the primitive basaltic komatiites and enriched spinifex-textured high-Mg basalts after further crustal contamination and olivine and clinopyroxene fractionation. The geochemical stratigraphy in the Lumby Lake belt is consistent with an ascending mantle plume model. The light REE depleted basalts were derived from upper mantle melted by an ascending mantle plume. These are overlain by the unfractionated basaltic komatiites and their evolutionary products which represent hotter plume head material derived from a mixture of plume mantle and entrained depleted upper mantle. In turn, these are overlain by strongly light REE and HFSE enriched komatiites that represent a deep plume source that has not been mixed with depleted mantle and are, therefore, likely to have been derived from a plume core or tail. Volcanism was protracted in these three greenstone belts lasting ca. 70 Ma and combined stratigraphic evidence from the Lumby Lake and Steep Rock belts suggests that more than one plume may have ascended and tapped the same mantle sources, over time, within the area. Plume magmatism and rifting of continental platforms thus appears to have been an important feature of crustal development in the Meso-Archean.

USArray Imaging of North American Continental Crust

NASA Astrophysics Data System (ADS)

Ma, Xiaofei

The layered structure and bulk composition of continental crust contains important clues about its history of mountain-building, about its magmatic evolution, and about dynamical processes that continue to happen now. Geophysical and geological features such as gravity anomalies, surface topography, lithospheric strength and the deformation that drives the earthquake cycle are all directly related to deep crustal chemistry and the movement of materials through the crust that alter that chemistry. The North American continental crust records billions of years of history of tectonic and dynamical changes. The western U.S. is currently experiencing a diverse array of dynamical processes including modification by the Yellowstone hotspot, shortening and extension related to Pacific coast subduction and transform boundary shear, and plate interior seismicity driven by flow of the lower crust and upper mantle. The midcontinent and eastern U.S. is mostly stable but records a history of ancient continental collision and rifting. EarthScope's USArray seismic deployment has collected massive amounts of data across the entire United States that illuminates the deep continental crust, lithosphere and deeper mantle. This study uses EarthScope data to investigate the thickness and composition of the continental crust, including properties of its upper and lower layers. One-layer and two-layer models of crustal properties exhibit interesting relationships to the history of North American continental formation and recent tectonic activities that promise to significantly improve our understanding of the deep processes that shape the Earth's surface. Model results show that seismic velocity ratios are unusually low in the lower crust under the western U.S. Cordillera. Further modeling of how chemistry affects the seismic velocity ratio at temperatures and pressures found in the lower crust suggests that low seismic velocity ratios occur when water is mixed into the mineral matrix, and the combination of high temperature and water may point to small amounts of melt in the lower crust of Cordillera.

Model for the Evolution of an Oceanic Core Complex and its Hydrothermal Vent on the Ultraslow-Spreading Mid Cayman Spreading Center

NASA Astrophysics Data System (ADS)

Harding, J.; Van Avendonk, H. J.; Hayman, N. W.; Grevemeyer, I.; Peirce, C.

2016-12-01

The Mid Cayman Spreading Center (MCSC) is an ultraslow-spreading center (15 mm yr-1 full rate) along the Caribbean-North American plate boundary. Despite the paradigm that ultraslow-spreading centers are amagmatic and cold, two hydrothermal vent fields have recently been discovered along the MCSC. The Beebe Vent Field is a black smoker in the northern axial deep, and the Von Damm Vent Field (VDVF) is a moderate-temperature, talc precipitating vent found atop an oceanic core complex (OCC). This OCC, "Mt. Dent", is a large (3 km high) massif that formed beneath a detachment fault, which exhumed lower crustal and upper mantle material. The CaySeis Experiment was conducted in April, 2015 in order to collect wide-angle refraction data of the MCSC crust and upper mantle. We modeled the across-axis crustal structure of Mt. Dent as well as the surrounding lithosphere using 2.5D P-wave tomography. Using this tomographic model, along with geochemistry, we propose a model for the formation and evolution of the OCC Mt. Dent and the VDVF. A detachment fault formed in a magma-poor environment due to a pulse of magmatism, producing a large gabbro body that was then exhumed and rotated into the OCC footwall. Once magmatism waned and the gabbroic body cooled, the OCC was faulted and fractured due to plate flexure and increased tectonic extensional stress in the naturally cold and thick lithosphere. These faults provide a permeable and deep network of hydrothermal pathways that mine deep lithospheric heat and expose gabbro and fresh mantle peridotite. This model is consistent with the basalt geochemistry, hydrothermal fluid geochemistry, and the distribution of brittle vs. ductile structures along the detachment shear zone. The VDVF is therefore a product of a pulse of magmatism in an overall melt-poor environment, conditions that may be found at other ultraslow-spreading ridges.

Editorial

NASA Astrophysics Data System (ADS)

Agosta, Fabrizio; Luetkemeyer, P. Benjamin; Lamarche, Juliette; Crider, Juliet G.; Lacombe, Olivier

2016-10-01

The present Volume is after the 2015 EGU General Assembly, held in Vienna (Austria), where we convened a session entitled "The role of fluids in faulting and fracturing in carbonates and other upper crustal rocks". In that occasion, more than forty contributions were illustrated as oral and poster presentations. The invitation to contribute to this Volume was extended not only to the session participants, but also to a wider spectrum of researchers working on related topics. As a result, a group of Earth scientists encompassing geologists, geophysicists, geochemists and petrologists contributed to this Volume, providing a sampling of the state-of-the-science on fluids and faulting in carbonate, crystalline and siliciclastic rocks from studies that combine and integrate different methods, including rock mechanics, petrophysics, structural diagenesis and crustal permeability.

Osmium isotopes and mantle convection.

PubMed

Hauri, Erik H

2002-11-15

The decay of (187)Re to (187)Os (with a half-life of 42 billion years) provides a unique isotopic fingerprint for tracing the evolution of crustal materials and mantle residues in the convecting mantle. Ancient subcontinental mantle lithosphere has uniquely low Re/Os and (187)Os/(188)Os ratios due to large-degree melt extraction, recording ancient melt-depletion events as old as 3.2 billion years. Partial melts have Re/Os ratios that are orders of magnitude higher than their sources, and the subduction of oceanic or continental crust introduces into the mantle materials that rapidly accumulate radiogenic (187)Os. Eclogites from the subcontinental lithosphere have extremely high (187)Os/(188)Os ratios, and record ages as old as the oldest peridotites. The data show a near-perfect partitioning of Re/Os and (187)Os/(188)Os ratios between peridotites (low) and eclogites (high). The convecting mantle retains a degree of Os-isotopic heterogeneity similar to the lithospheric mantle, although its amplitude is modulated by convective mixing. Abyssal peridotites from the ocean ridges have low Os isotope ratios, indicating that the upper mantle had undergone episodes of melt depletion prior to the most recent melting events to produce mid-ocean-ridge basalt. The amount of rhenium estimated to be depleted from the upper mantle is 10 times greater than the rhenium budget of the continental crust, requiring a separate reservoir to close the mass balance. A reservoir consisting of 5-10% of the mantle with a rhenium concentration similar to mid-ocean-ridge basalt would balance the rhenium depletion of the upper mantle. This reservoir most likely consists of mafic oceanic crust recycled into the mantle over Earth's history and provides the material that melts at oceanic hotspots to produce ocean-island basalts (OIBs). The ubiquity of high Os isotope ratios in OIB, coupled with other geochemical tracers, indicates that the mantle sources of hotspots contain significant quantities (greater than 10%) of lithologically distinct mafic material which represents ancient oceanic lithosphere cycled through the convecting mantle on a time-scale of 800 million years or more.

Experimental constraints on the fate of subducted upper continental crust beyond the "depth of no return"

NASA Astrophysics Data System (ADS)

Zhang, Y.; Wu, Y.; WANG, C.; Jin, Z.

2015-12-01

Large-scale oceanic/continental subduction introduces a range of crustal materials into the Earth's mantle. These subducted material will be gravitationally trapped in the deep mantle when they have been transported to a depth of greater than ~250-300 km ("depth of no return"). However, little is known about the fate of these trapped continental material. Here, we conduct experimental study on a natural continental rock which compositionally similar to the average upper continental crust (UCC) over a pressure and temperature range of 9-16 GPa and 1300-1800 oC to constraint the fate of these trapped continental materials. The experimental results demonstrate that subducted UCC produces ~20-30 wt% K-rich melt (>55 wt% SiO2) in the upper mantle (9-13 GPa). The melting residue is mainly composed of coesite/stishovite + clinopyroxene + kyanite. In contrast, partial melting of subducted UCC in the MTZ produces ~10 wt% K-rich melt (<50 wt% SiO2), together with stishovite, clinopyroxene, K-Hollandite, garnet and CAS-phase as the residue phases. The melting residue phases achieve densities greater than the surrounding mantle, which provides a driving force for descending across the 410 km seismic discontinuity into the MTZ. However, this density relationship is reversed at the base of MTZ, leaving the descended residues being accumulated above the 660 km seismic discontinuity and may contribute to the stagnated "second continent". On the other hand, the melt is ~0.3-0.7 g/cm3 less dense than the surrounding mantle and provides a buoyancy force for the ascending of melt to shallow depth. The ascending melt preserves a significant portion of the bulk-rock REEs and LILEs. Thus, chemical reaction between the melt and the surrounding mantle would leads to a variably metasomatised mantle. Re-melting of the metasomatised mantle may contribute to the origin of the "enriched mantle sources" (EM-sources). Therefore, through subduction, stagnation, partial melting and melt segregation of continental crust may create EM-sources and"second continent" at shallow depth and the base of the MTZ respectively, which may contribute to the observed geochemical/geophysical heterogeneity in Earth's interior.

Mesozoic high-Mg andesites from the Daohugou area, Inner Mongolia: Upper-crustal fractional crystallization of parental melt derived from metasomatized lithospheric mantle wedge

NASA Astrophysics Data System (ADS)

Meng, Fanxue; Gao, Shan; Song, Zhaojun; Niu, Yaoling; Li, Xuping

2018-03-01

Mineral chemistry, major- and trace-element data, zircon U-Pb ages, and Sr-Nd isotopic data are presented for a suite of volcanic rocks from the Daohugou area, Ningcheng City, Inner Mongolia, on the northern margin of the North China Craton. Samples from the suite are of basaltic andesite to rhyolite in composition, with the rocks containing <60 wt% SiO2 have high MgO, Cr, and Ni contents, and classify as high-Mg andesites (HMAs). Zircons from a rhyolite yielded weighted mean 206Pb/238U age of 164 ± 1 Ma, indicating that the Daohugou volcanic suite is coeval with the Tiaojishan Formation of northern Hebei and western Liaoning Province. The HMAs have similar enriched-mantle I (EMI)-type isotopic compositions to each other, with low εNd (t) values, moderate (87Sr/86Sr) i ratios, enrichment in LREEs relative to LILEs, and depletion in HFSEs (e.g., Nb, Ta, Ti), indicating formation through protracted fractional crystallization of a common parental magma. The unusually low CaO contents and CaO/FeO ratios of olivine phenocrysts in the HMAs suggest that the parental melt was subduction-related. The results of Rhyolite-MELTS modelling indicates that HMAs may form through upper-crustal fractional crystallization from arc basalts. Therefore, the Daohugou HMAs were most likely formed through fractional crystallization of a parental melt derived from metasomatized lithospheric mantle at crustal depths. The addition of "water" to the cratonic keel may have played a key role in the destruction of the North China Craton.

Investigation of a marine magnetic polarity reversal boundary in cross-section at the northern boundary of the Kane Megamullion, Mid-Atlantic Ridge 23°40'N

NASA Astrophysics Data System (ADS)

Xu, M.; Tivey, M.

2016-12-01

Near-bottom magnetic field measurements made by the submersible Nautile during the 1992 Kanaut Expedition define the cross-sectional geometry of magnetic polarity reversal boundaries and the vertical variation of crustal magnetization in lower oceanic crust exposed along the Kane Transform Fault (TF) at the northern boundary of the Kane Megamullion (KMM). The KMM exposes lower crust and upper mantle rocks on a low-angle normal fault that was active between 3.3 Ma and 2.1 Ma. The geometry of the polarity boundaries is estimated from an inversion of the submarine magnetic data for crustal magnetization. In general, the polarity boundaries dip away from the ridge axis along the Kane TF scarp, with a west-dipping angle of 45° in the shallow (<1 km) crust and <20° in the deeper crust. The existence of the magnetic polarity boundaries (e.g. C2r.2r/C2An.1n, 2.581 Ma) indicates that the lower crustal gabbros and upper mantle serpentinized peridotites are able to record a coherent magnetic signal. Our results support the conclusion of Williams [2007] that the lower crust cools through the Curie temperature of magnetite to become magnetic, with the polarity boundaries representing both frozen isotherms and isochrons. We also test the effects of the rotation of this isotherm structure and/or footwall rotation, and find that the magnetic polarity boundary geometry is not sensitive to these directional changes.

Investigation of a marine magnetic polarity reversal boundary in cross section at the northern boundary of the Kane Megamullion, Mid-Atlantic Ridge, 23°40'N

NASA Astrophysics Data System (ADS)

Xu, Min; Tivey, M. A.

2016-05-01

Near-bottom magnetic field measurements made by the submersible Nautile during the 1992 Kanaut Expedition define the cross-sectional geometry of magnetic polarity reversal boundaries and the vertical variation of crustal magnetization in lower oceanic crust exposed along the Kane Transform Fault (TF) at the northern boundary of the Kane Megamullion (KMM). The KMM exposes lower crust and upper mantle rocks on a low-angle normal fault that was active between 3.3 Ma and 2.1 Ma. The geometry of the polarity boundaries is estimated from an inversion of the submarine magnetic data for crustal magnetization. In general, the polarity boundaries dip away from the ridge axis along the Kane TF scarp, with a west dipping angle of ~45° in the shallow (<1 km) crust and <20° in the deeper crust. The existence of the magnetic polarity boundaries (e.g., C2r.2r/C2An.1n, ~2.581 Ma) indicates that the lower crustal gabbros and upper mantle serpentinized peridotites are able to record a coherent magnetic signal. Our results support the conclusion of Williams (2007) that the lower crust cools through the Curie temperature of magnetite to become magnetic, with the polarity boundaries representing both frozen isotherms and isochrons. We also test the effects of the rotation of this isotherm structure and/or footwall rotation and find that the magnetic polarity boundary geometry is not sensitive to these directional changes.

Architecture of the crust and uppermost mantle in the northern Canadian Cordillera from receiver functions

NASA Astrophysics Data System (ADS)

Tarayoun, Alizia; Audet, Pascal; Mazzotti, Stéphane; Ashoori, Azadeh

2017-07-01

The northern Canadian Cordillera (NCC) is an active orogenic belt in northwestern Canada characterized by deformed autochtonous and allochtonous structures that were emplaced in successive episodes of convergence since the Late Cretaceous. Seismicity and crustal deformation are concentrated along corridors located far (>200 to 800 km) from the convergent plate margin. Proposed geodynamic models require information on crust and mantle structure and strain history, which are poorly constrained. We calculate receiver functions using 66 broadband seismic stations within and around the NCC and process them to estimate Moho depth and P-to-S velocity ratio (Vp/Vs) of the Cordilleran crust. We also perform a harmonic decomposition to determine the anisotropy of the subsurface layers. From these results, we construct simple seismic velocity models at selected stations and simulate receiver function data to constrain crust and uppermost mantle structure and anisotropy. Our results indicate a relatively flat and sharp Moho at 32 ± 2 km depth and crustal Vp/Vs of 1.75 ± 0.05. Seismic anisotropy is pervasive in the upper crust and within a thin ( 10-15 km thick) sub-Moho layer. The modeled plunging slow axis of hexagonal symmetry of the upper crustal anisotropic layer may reflect the presence of fractures or mica-rich mylonites. The subhorizontal fast axis of hexagonal anisotropy within the sub-Moho layer is generally consistent with the SE-NW orientation of large-scale tectonic structures. These results allow us to revise the geodynamic models proposed to explain active deformation within the NCC.

Three-dimensional lithospheric S wave velocity model of the NE Tibetan Plateau and western North China Craton

NASA Astrophysics Data System (ADS)

Wang, Xingchen; Li, Yonghua; Ding, Zhifeng; Zhu, Lupei; Wang, Chunyong; Bao, Xuewei; Wu, Yan

2017-08-01

We present a new 3-D lithospheric Vs model for the NE Tibetan Plateau (NETP) and the western North China Craton (NCC). First, high-frequency receiver functions (RFs) were inverted using the neighborhood algorithm to estimate the sedimentary structure beneath each station. Then a 3D Vs model with unprecedented resolution was constructed by jointly inverting RFs and Rayleigh wave dispersions. A low-velocity sedimentary layer with thicknesses varying from 2 to 10 km is present in the Yinchuan-Hetao graben, Ordos block, and western Alxa block. Velocities from the middle-lower crust to the uppermost mantle are generally high in the Ordos block and low in the Alxa block, indicating that the Alxa block is not part of the NCC. The thickened crust in southwestern Ordos block and western Alxa block suggests that they have been modified. Two crustal low-velocity zones (LVZs) were detected beneath the Kunlun Fault (KF) zone and western Qilian Terrane (QLT). The origin of the LVZ beneath the KF zone may be the combined effect of shear heating, localized asthenosphere upwelling, and crustal radioactivity. The LVZ in the western QLT, representing an early stage of the LVZ that has developed in the KF zone, acts as a decollement to decouple the deformation between the upper and lower crust and plays a key role in seismogenesis. We propose that the crustal deformation beneath the NETP is accommodated by a combination of shear motion, thickening of the upper-middle crust, and removal of lower crust.

Upper crustal structure of Alabama from regional magnetic and gravity data: Using geology to interpret geophysics, and vice versa

USGS Publications Warehouse

Steltenpohl, Mark G.; Horton, J. Wright; Hatcher, Robert D.; Zietz, Isidore; Daniels, David L.; Higgins, Michael W.

2013-01-01

Aeromagnetic and gravity data sets obtained for Alabama (United States) have been digitally merged and filtered to enhance upper-crustal anomalies. Beneath the Appalachian Basin in northwestern Alabama, broad deep-crustal anomalies of the continental interior include the Grenville front and New York–Alabama lineament (dextral fault). Toward the east and south, high-angle discordance between the northeast-trending Appalachians and the east-west–trending wedge of overlapping Mesozoic and Cenozoic Gulf Coastal Plain sediments reveals how bedrock geophysical signatures progressively change with deeper burial. High-frequency magnetic anomalies in the Appalachian deformed domain (ADD) correspond to amphibolites and mylonites outlining terranes, while broader, lower-amplitude domains include Paleozoic intrusive bodies and Grenville basement gneiss. Fundamental ADD structures (e.g., the Alexander City, Towaliga, and Goat Rock–Bartletts Ferry faults) can be traced southward beneath the Gulf Coastal Plain to the suture with Gondwanan crust of the Suwannee terrane. Within the ADD, there is clear magnetic distinction between Laurentian crust and the strongly linear, high-frequency magnetic highs of peri-Gondwanan (Carolina-Uchee) arc terranes. The contact (Central Piedmont suture) corresponds to surface exposures of the Bartletts Ferry fault. ADD magnetic and gravity signatures are truncated by the east-west–trending Altamaha magnetic low associated with the Suwannee suture. Arcuate northeast-trending magnetic linears of the Suwannee terrane reflect internal structure and Mesozoic failed-rift trends. Geophysical data can be used to make inferences on surface and subsurface geology and vice versa, which has applicability anywhere that bedrock is exposed or concealed beneath essentially non-magnetic sedimentary cover.

Tomographic imaging of the transition from asthenospheric to lithospheric melt migration processes: 3-D structure of the topmost mantle and crust at the Endeavour Segment, Juan de Fuca Ridge

NASA Astrophysics Data System (ADS)

Arnoux, G. M.; Toomey, D. R.; Hooft, E. E. E.; Wilcock, W. S. D.

2017-12-01

We present tomographic images of the intermediate-spreading Endeavour Segment that constrain the nature of an axial magmatic system as it transitions from asthenospheric- to lithospheric-dominated rheologies. We use seismic energy from 5500 air gun shots refracted through the crust (Pg), reflected off the Moho (PmP), and refracted below the Moho (Pn)—as recorded by 64 OBSs from the Endeavour tomography experiment—to image the isotropic and anisotropic P-wave velocity structure of the topmost mantle and crust, as well as crustal thickness. At crustal depths, results reveal a low-velocity zone (LVZ)—inferred to be the axial magmatic system—that: (i) is continuous along the entire Endeavour Segment at depths of 2-3 km below seafloor and closely follows the axis of spreading, (ii) broadens and becomes more discontinuous at lower crustal depths, and (iii) has its largest amplitude from the mid- to lower-crust at the segment center. The ridge-tracking trend of the mid-crustal LVZ is in contrast to the crustal thickness pattern; in particular, a swath of thin crust is skewed with respect to both the ridge axis and the mid-crustal magmatic system and connects two overlapping spreading centers bounding the segment. The trend of thinner crust, however, is aligned with the mantle LVZ, which constrains the thermal structure and distribution of melt within the topmost mantle. The systematic depth variation of the map-view orientation and structure of the magmatic system indicates a distinct transition from a broad, cross-axis regime in the topmost asthenosphere governed by a regional, north-south trending thermal structure, to a narrow, cross-axis regime in the mid- to upper-crust governed by lithospheric rifting, magma injection, and hydrothermal processes. The lower-crustal magmatic system connects these two regimes. We also postulate that accumulation and differentiation of magma immediately beneath the crust-mantle boundary increases temperatures and suppresses plagioclase crystallization, thereby reducing the depth of lower crustal accretion and resulting in the observed north-south trending swath of thinner crust.

Magnetotelluric evidence for a deep-crustal mineralizing system beneath the Olympic Dam iron oxide copper-gold deposit, southern Australia

NASA Astrophysics Data System (ADS)

Heinson, Graham S.; Direen, Nicholas G.; Gill, Rob M.

2006-07-01

The iron oxide copper-gold Olympic Dam deposit, situated along the margin of the Proterozoic Gawler craton, South Australia, is the world's largest uranium deposit and sixth-largest copper deposit; it also contains significant reserves of gold, silver, and rare earth elements. Gaining a better understanding of the mechanisms for genesis of the economic liberalization is fundamental for defining exploration models in similar crustal settings. To delineate crustal structures that may constrain mineral system fluid pathways, coincident deep crustal seismic and magnetotelluric (MT) transects were obtained along a 220 km section that crosses Olympic Dam and the major crustal boundaries. In this paper we present results from 58 long-period (10 104 s) MT sites, with site spacing of 5 10 km. A two-dimensional inversion of MT data from 33 sites to a depth of 100 km shows four notable features: (1) sedimentary cover sequences with low resistivity (<20 Ω·m) thicken to 10 km toward the northern cover sequences of the Adelaide Rift Complex; (2) a northeast-dipping crustal boundary separates a highly resistive (>1000 Ω·m) Archean crustal core from a more conductive crust and mantle to the north (typically <500 Ω·m); (3) to the north of Olympic Dam, the upper-middle crust to ˜20 km is quite resistive (˜1000 Ω·m), but the lower crust is much more conductive (<100 Ω·m); and (4) beneath Olympic Dam, we image a low-resistivity region (<100 Ω·m) throughout the crust, coincident with a seismically transparent region. We argue that the cause of the low-resistivity and low-reflectivity region beneath Olympic Dam may be due to the upward movement of CO2-bearing volatiles near the time of deposit formation that precipitated conductive graphite liberalization along grain boundaries, simultaneously annihilating acoustic impedance boundaries. The source of the volatiles may be from the mantle degassing or retrograde metamorphism of the lower crust associated with Proterozoic crustal deformation.

Ways to study mid-crustal properties and behaviour - the great flow debate continues

NASA Astrophysics Data System (ADS)

Torvela, Taija

2015-04-01

The properties and behaviour of middle and lower crust during and at the late stages of an orogen remain largely elusive. Especially the theory so-called mid-crustal flow has been a centre of hot debates since its inception. Mid-crustal flow, resulting from proposed drastic weakening of thickened, partially molten middle and lower orogenic crust, is suggested to be the reason for the formation of e.g. the high-grade mid-crustal channel in the frontal Himalayas, and for the present eastward movement of the Tibetan upper crust. However, there are very few ways to observe the potential mid-crustal flow, directly or indirectly. One can use exposed analogues in old, eroded orogenic roots, or study currently exposed rocks in Himalayas or Tibet and attempt deducing the large-scale processes at depth from them. Another option is to use geophysical methods, such as seismic reflection/refraction data or seismic tomography. The biggest problem is, that a huge gap in terms of data and observation scale exists between these two approaches make observations - field data is usually collected at the scale of an outcrop, but can be extrapolated in a well exposed area for kilometres; however, the data lacks significant depth component, i.e. lacks 3D information, and, furthermore, the observed geometries in e.g. dome complexes are not unique in terms of their genesis, so that multiple genetic interpretations of a single geometry are often possible. Geophysical data on the other hand gives 3D information, but all detail is lost as geophysical data from the crust is usually at a scale of at least hundreds of meters or more. In this presentation, I will discuss these and other problems related to the verification vs. falsification of mid-crustal flow theory. I will also present a potential way to improve seismic reflection data, using seismic attributes, to gain more detailed information about the crustal structures and fabrics at depth, which may help in the study of Himalayas, Tibet, and other areas where more detailed information about the crustal structure would be beneficial.

On the relationship between isostatic elevation and the wavelengths of tectonic surface features on Venus

NASA Technical Reports Server (NTRS)

Zuber, M. T.; Parmentier, E. M.

1990-01-01

Venus lithospheric structure models are presently formulated in which regional isostatic elevation, d, and the spacing wavelength, lambda, of tectonic features formed due to horizontal extension and compression are functions of both surface thermal gradient and crustal thickness c. It is shown that, in areas of Venus where the upper mantle is stronger than the upper crust, the spacings of short-wavelength features should increase with increasing d, if that change in turn is due to increasing c, but should decrease with increasing d, if this change is in turn due to increasing surface thermal gradient.

Continental Subduction: Mass Fluxes and Interactions with the Wider Earth System

NASA Astrophysics Data System (ADS)

Cuthbert, S. J.

2011-12-01

Substantial parts of ultra-high pressure (UHP) terrains probably represent subducted passive continental margins (PCM). This contribution reviews and synthesises research on processes operating in such systems and their implication for the wider Earth system. PCM sediments are large repositories of volatiles including hydrates, nitrogen species, carbonates and hydrocarbons. Sediments and upper/ mid-crustal basement are rich in incompatible elements and are fertile for melting. Lower crust may be more mafic and refractory. Juvenile rift-related mafic rocks also have the potential to generate substantial volumes of granitoid melts, especially if they have been hydrated. Exposed UHP terrains demonstrate the return of continental crust from mantle depths, show evidence for substantial fluxes of aqueous fluid, anatexis and, in entrained orogenic peridotites, metasomatism of mantle rocks by crust- derived C-O-H fluids. However, substantial bodies of continental material may never return to the surface as coherent masses of rock, but remain sequestered in the mantle where they melt or become entrained in the deeper mantle circulation. Hence during subduction, PCM's become partitioned by a range of mechanisms. Mechanical partitioning strips away weaker sediment and middle/upper crust, which circulate back up the subduction channel, while denser, stronger transitional pro-crust and lower crust may "stall" near the base of the lithosphere or be irreversibly subducted to join the global mantle circulation. Under certain conditions sediment and upper crustal basement may reach depths for UHPM. Further partitioning takes place by anatexis, which either aids stripping and exhumation of the more melt-prone rock-masses through mechanical softening, or separates melt from residuum so that melt escapes and is accreted to the upper plate leading to "undercrusting", late-orogenic magmatism and further refinement of the crust. Melt that traverses sections of mantle will interact with it causing metasomatism and refertilisation. Partitioning also takes place by solid-fluid and melt-fluid partitioning. Dehydration may take place both during subduction and exhumation, and fluxes between dehydrating and hydrating rock masses influence the internal fluid budget of the orogen (essential for eclogitisation and densification of mafic lithologies). Ascending granitic melts advect dissolved water to shallow levels, or even the atmosphere. Irreversible subduction of PCM sediment carries water plus nitrogen species to the deeper mantle. Decarbonation of voluminous PCM carbonates depends on thermal regime and may release a pulse of CO2 to the atmosphere, but is limited in colder subduction zones hence transferring large volumes of carbon to the deep mantle. This may ultimately be mobilised by melting or dissolution to form fluid media for diamond formation.

Physical and Chemical Characterization of Particles in the Upper Troposphere and Lower Stratosphere: Microanalysis of Aerosol Impactor Samples

NASA Technical Reports Server (NTRS)

Sheridan, Patrick J.

1999-01-01

Herein is reported activities to support the characterization of the aerosol in the upper troposphere (UT) and lower stratosphere (LS) collected during the Airborne Southern Hemisphere Ozone Experiment/Measurements for Assessing the Effects of Stratospheric Aircraft (ASHOE/MAESA) missions in 1994. Through a companion proposal, another group was to measure the size distribution of aerosols in the 0.008 to 2 micrometer diameter range and to collect for us impactor samples of particles larger than about 0.02 gm. In the first year, we conducted laboratory studies related to particulate deposition patterns on our collection substrates, and have performed the analysis of many ASHOE/MAESA aerosol samples from 1994 using analytical electron microscopy (AEM). We have been building an "aerosol climatology" with these data that documents the types and relative abundances of particles observed at different latitudes and altitudes. The second year (and non-funded extension periods) saw continued analyses of impactor aerosol samples, including more ASHOE/MAESA samples, some northern hemisphere samples from the NASA Stratospheric Photochemistry Aerosols and Dynamics Expedition (SPADE) program for comparison, and a few aerosol samples from the NASA Stratospheric TRacers of Atmospheric Transport (STRAT) program. A high-resolution field emission microscope was used for the analysis and re-analysis of a number of samples to determine if this instrument was superior in performance to our conventional electron microscope. In addition, some basic laboratory studies were conducted to determine the minimum detectable and analyzable particle size for different types of aerosols. In all, 61 aerosol samples were analyzed, with a total of over 30,000 individual particle analyses. In all analyzed samples, sulfate particles comprised the major aerosol number fraction. It must be stressed that particles composed of more than one species, for example sulfate and organic carbon, were classified according to the major fraction. Thus, many of the particles classified as sulfate may have contained significant mass fractions of carbonaceous or other material. These particles for the most part did not show two physical phases, however. Nonsulfate particles were classified according to the physical and chemical characteristics of each particle, and were grouped into the major nonsulfate particle classes, including C-rich, crustal, metallic, and salts. Our UT and LS sample analyses indicate a maximum for crustal and C-rich particle abundance in the Northern Hemisphere upper troposphere, and a salt particle maximum in the Southern Hemisphere upper troposphere. Metallic particles are clearly more prevalent in the troposphere than in the stratosphere, but interhemispheric differences appear small.

Upper crustal fault reactivation and the potential of triggered earthquakes on the Atacama Fault System, N-Chile

NASA Astrophysics Data System (ADS)

Victor, Pia; Ewiak, Oktawian; Thomas, Ziegenhagen; Monika, Sobiesiak; Bernd, Schurr; Gabriel, Gonzalez; Onno, Oncken

2016-04-01

The Atacama Fault System (AFS) is an active trench-parallel fault system, located in the forearc of N-Chile directly above the subduction zone interface. Due to its well-exposed position in the hyper arid forearc of N-Chile it is the perfect target to investigate the interaction between the deformation cycle in the overriding forearc and the subduction zone seismic cycle of the underlying megathrust. Although the AFS and large parts of the upper crust are devoid of any noteworthy seismicity, at least three M=7 earthquakes in the past 10 ky have been documented in the paleoseismological record, demonstrating the potential of large events in the future. We apply a two-fold approach to explore fault activation and reactivation patterns through time and to investigate the triggering potential of upper crustal faults. 1) A new methodology using high-resolution topographic data allows us to investigate the number of past earthquakes for any given segment of the fault system as well as the amount of vertical displacement of the last increment. This provides us with a detailed dataset of past earthquake rupture of upper plate faults which is potentially linked to large subduction zone earthquakes. 2) The IPOC Creepmeter array (http://www.ipoc-network.org/index.php/observatory/creepmeter.html) provides us with high-resolution time series of fault displacement accumulation for 11 stations along the 4 most active branches of the AFS. This array monitors the displacement across the fault with 2 samples/min with a resolution of 1μm. Collocated seismometers record the seismicity at two of the creepmeters, whereas the regional seismicity is provided by the IPOC Seismological Networks. Continuous time series of the creepmeter stations since 2009 show that the shallow segments of the fault do not creep permanently. Instead the accumulation of permanent deformation occurs by triggered slip caused by local or remote earthquakes. The 2014 Mw=8.2 Pisagua Earthquake, located close to the creepmeter array, triggered large displacement events on all stations. Another event recorded on all stations was the 2010 Mw=8.8 Maule earthquake located 1500km south of the array. Exploring observations from both datasets, we can clearly state that triggering of upper crustal faults is observed for small-scale displacements. These findings allow us to speculate that the observed larger events in the past are likely being triggered events that require a critically prestressed condition of the target fault that is unclamped by stress changes triggered by large or potentially even small subduction zone earthquakes.

Redox state of recycled crustal lithologies in the convective upper mantle constrained using oceanic basalt CO2-trace element systematics

NASA Astrophysics Data System (ADS)

Eguchi, J.; Dasgupta, R.

2017-12-01

Investigating the redox state of the convective upper mantle remains challenging as there is no way of retrieving samples from this part of the planet. Current views of mantle redox are based on Fe3+/∑Fe of minerals in mantle xenoliths and thermodynamic calculations of fO2 [1]. However, deep xenoliths are only recoverable from continental lithospheric mantle, which may have different fO2s than the convective oceanic upper mantle [1]. To gain insight on the fO2 of the deep parts of the oceanic upper mantle, we probe CO2-trace element systematics of basalts that have been argued to receive contributions from subducted crustal lithologies that typically melt deeper than peridotite. Because CO2 contents of silicate melts at graphite saturation vary with fO2 [2], we suggest CO2-trace element systematics of oceanic basalts which sample deep heterogeneities may provide clues about the fO2 of the convecting mantle containing embedded heterogeneities. We developed a new model to predict CO2 contents in nominally anhydrous silicate melts from graphite- to fluid-saturation over a range of P (0.05- 5 GPa), T (950-1600 °C), and composition (foidite-rhyolite). We use the model to calculate CO2 content as a function of fO2 for partial melts of lithologies that vary in composition from rhyolitic sediment melt to silica-poor basaltic melt of pyroxenites. We then use modeled CO2 contents in mixing calculations with partial melts of depleted mantle to constrain the fO2 required for partial melts of heterogeneities to deliver sufficient CO2 to explain CO2-trace element systematics of natural basalts. As an example, Pitcairn basalts, which show evidence of a subducted crustal component [3] require mixing of 40% of partial melts of a garnet pyroxenite at ΔFMQ -1.75 at 3 GPa. Mixing with a more silicic composition such as partial melts of a MORB-eclogite cannot deliver enough CO2 at graphite saturation, so in this scenario fO2 must be above the EMOG/D buffer at 4 GPa. Results suggest convecting upper mantle may be more oxidized than continental lithospheric mantle, and fO2 profiles of continental lithospheric mantle may not be applicable to convective upper mantle.[1] Frost, D, McCammon, C. 2008. An Rev E & P Sci. (36) p.389-420; [2] Holloway, J, et al. 1992. Eu J. Min. (4) p. 105-114; [3] Woodhead, J, Devey C. 1993. EPSL. (116) p. 81-99.

Geometry of a large-scale low-angle mid-crustal thrust (Woodroffe Thrust, Central Australia)

NASA Astrophysics Data System (ADS)

Wex, Sebastian; Mancktelow, Neil S.; Hawemann, Friedrich; Pennacchioni, Giorgio; Camacho, Alfredo

2015-04-01

Young orogens, such as the Alps, mainly expose the upper part of the continental crust and it is not possible to follow large-scale thrusts (e.g. the Glarus Thrust) to great depth in order to study their changing rheological behavior. This knowledge, however, is crucial for determining the overall kinematic and dynamic response during collision, as middle to lower crustal rocks represent the major part of the total crustal section. Information from deeper parts of the continental crust can only be obtained directly by investigating regions where these levels are now exhumed. The Musgrave Ranges in Central Australia is a very well exposed, semi-desert area, in which numerous large-scale shear zones developed during the Petermann Orogeny around 550 Ma. The most prominent structure is the ˜400 km long E-W trending Woodroffe Thrust, which placed ˜1.2 Ga granulites onto similarly-aged amphibolite and granulite facies gneisses along a generally south-dipping thrust plane with a top-to-north shear sense. Geothermobarometric calculations on the associated mylonites established that the structure developed under mid-crustal conditions (500-650°C, 0.8-1 GPa). Regional P/T variations in the direction of thrusting are small, but show trends consistent with the south-dipping orientation of the thrust plane, which predicts deeper levels and a higher metamorphic grade in the south than in the north. They imply a very low gradient of only around 3°C/km for a distance of some 30 km in the movement direction of the thrust. Combined with a geothermal gradient on the order of 20°C/km, calculated from four separate P/T estimates from the hanging wall and footwall, this regional gradient indicates that the Woodroffe Thrust was originally shallow-dipping at an average angle of only around 9°. This suggests that upper crustal brittle thrusts do not necessarily steepen into the middle to lower crust, but can define very shallow-dipping, large-scale planar features, with dimensions in the order of hundreds of kilometres. Such a geometry would require the rocks to be weak, but field observations (e.g. large volumes of syn-tectonic pseudotachylyte) argue for strong behaviour, involving alternating fast (seismic) fracturing and slow (aseismic) creep.

Evidence for crustal recycling during the Archean: The parental magmas of the stillwater complex

NASA Technical Reports Server (NTRS)

Mccallum, I. S.

1988-01-01

The petrology and geochemistry of the Stillwater Complex, an Archean (2.7 Ga) layered mafic intrusion in the Beartooth Mountains of Montana is discussed. Efforts to reconstruct the compositions of possible parental magmas and thereby place some constraints on the composition and history of their mantle source regions was studied. A high-Mg andesite or boninite magma best matches the crystallization sequences and mineral compositions of Stillwater cumulates, and represents either a primary magma composition or a secondary magma formed, for example, by assimilation of crustal material by a very Mg-rich melt such as komatiite. Isotopic data do not support the extensive amounts of assimilation required by the komatiite parent hypothesis, and it is argued that the Stillwater magma was generated from a mantle source that had been enriched by recycling and homogenization of older crustal material over a large area.

3D Thermal/Mechanical Evolution Of The Plate Boundary Corner In SE Alaska

NASA Astrophysics Data System (ADS)

Barker, A.; Koons, P.; Upton, P.; Pavlis, T.; Chapman, J.

2007-12-01

The St Elias orogen of southeast Alaska forms part of an actively deforming plate boundary corner. The corner accommodates the transition from a strike-slip lateral boundary to a convergent normal boundary. Oblique convergence of the Yakutat microplate into the corner generates early stage tectonic characteristics associated with other corner systems (e.g. Himalayan Eastern Syntaxis). In combination with the high relief, the extreme erosive processes of the region redistribute crustal material, partition tectonic strain, and influence the advection of deep crustal material. The evolution of the convergent corner is investigated using 3D numerical models and sandbox analog models. Preliminary model results indicate the deformation partitions into a narrow two-sided orogen along the lateral boundary. The pattern transitions into a wider zone of shortening bounded by inboard and outboard directed thrusts along the frontal boundary. The inclusion of erosion boundary conditions leads to nascent tectonic aneurysm behavior, involving increased strain localization and focused vertical advection of deep crustal material. Thermal models, using the 3D velocity field from these mechanical solutions, show a vertical deflection (towards the surface) of isotherms beneath the eroding region. Sensitivity of the aneurysm behavior is related to the efficiency of the imposed erosion rate (i.e. greater erosion rates led to greater bedrock uplift rates). Higher erosion rates are localized within zones containing major glacier systems in SE Alaska: Bering Glacier, Bagley Icefield, Malaspina Glacier, and Seward Glacier. Combined thermal/mechanical solutions identify the glacier valleys as rheological weakspots, defined by localized strain and differential advection of deep crustal material.

Modeling crust-mantle evolution using radiogenic Sr, Nd, and Pb isotope systematics

NASA Astrophysics Data System (ADS)

Kumari, Seema; Paul, Debajyoti

2015-04-01

The present-day elemental and isotopic composition of Earth's terrestrial reservoirs can be used as geochemical constraints to study evolution of the crust-mantle system. A flexible open system evolutionary model of the Earth, comprising continental crust (CC), upper depleted mantle (UM) -source of mid-ocean ridge basalts (MORB), and lower mantle (LM) reservoir with a D" layer -source of ocean island basalts (OIB), and incorporating key radioactive isotope systematics (Rb-Sr, Sm-Nd, and U-Th-Pb), is solved numerically at 1 Ma time step for 4.55 Ga, the age of the Earth. The best possible solution is the one that produces the present-day concentrations as well as isotopic ratios in terrestrial reservoirs, compiled from published data. Different crustal growth scenarios (exponential, episodic, early and late growth), proposed in earlier studies, and its effect on the evolution of isotope systematics of terrestrial reservoirs is studied. Model simulations strongly favor a layered mantle structure satisfying majority of the isotopic constraints. In the successful model, which is similar to that proposed by Kellogg et al. (1999), the present-day UM comprises of 60% of mantle mass and extends to a depth 1600 km, whereas the LM becomes non-primitive and more enriched than the bulk silicate Earth, mainly due to addition of recycled crustal material. Modeling suggest that isotopic evolution of reservoirs is affected by the mode of crustal growth. Only two scenarios satisfied majority of the Rb-Sr and Sm-Nd isotopic constraints but failed to reproduce the present-day Pb-isotope systematics; exponential growth of crust (mean age, tc=2.3 Ga) and delayed and episodic growth (no growth for initial 900 Ma, tc=2.05 Ga) proposed by Patchett and Arndt (1986). However, assuming a slightly young Earth (4.45 Ga) better satisfies the Pb-isotope systematics. Although, the delayed crustal growth model satisfied Sr-Nd isotopic constraints, presence of early Hadean crust (4.03 and 4.4 Ga detrital zircon in Acasta gneiss and Yilgarn block, respectively), argues against it. One notable feature of successful models is an early depletion of incompatible elements (as well as Th/U ratio in the UM) by the initial 500 Ma, as a result of early formation of continental crust. Our results strongly favor exponential crustal growth and layered mantle structure. Patchett, P.J., Arndt, N.T. (1986), Earth and Planetary Science Letters, 78, 329-338. Kellogg, L.H., Hager, B.H., van der Hilst, R.D (1999), Science, 283, 1881-1884.

Evidence for Thin Oceanic Crust on the Extinct Aegir Ridge, Norwegian Basin, N.E. Atlantic Derived from Satellite Gravity Inversion

NASA Astrophysics Data System (ADS)

Greenhalgh, E. E.; Kusznir, N. J.

2006-12-01

Satellite gravity inversion incorporating a lithosphere thermal gravity correction has been used to map crustal thickness and lithosphere thinning factor for the N.E. Atlantic. The inversion of gravity data to determine crustal thickness incorporates a lithosphere thermal gravity anomaly correction for both oceanic and continental margin lithosphere. Predicted crustal thicknesses in the Norwegian Basin are between 7 and 4 km on the extinct Aegir oceanic ridge which ceased sea-floor spreading in the Oligocene. Crustal thickness estimates do not include a correction for sediment thickness and are upper bounds. Crustal thicknesses determined by gravity inversion for the Aegir Ridge are consistent with recent estimates derived using refraction seismology by Breivik et al. (2006). Failure to incorporate a lithosphere thermal gravity anomaly correction produces an over-estimate of crustal thickness. Oceanic crustal thicknesses within the Norwegian Basin are predicted by the gravity inversion to increase to 9-10 km eastwards towards the Norwegian (Moere) and westwards towards the Jan Mayen micro-continent, consistent with volcanic margin continental breakup at the end of the Palaeocene. The observation (from gravity inversion and seismic refraction studies) of thin oceanic crust produced by the Aegir ocean ridge in the Oligocene has implications for the temporal evolution of asthenosphere temperature under the N.E. Atlantic during the Tertiary. Thin Oligocene oceanic crust may imply cool (normal) asthenosphere temperatures during the Oligocene in contrast to elevated asthenosphere temperatures in the Palaeocene and Miocene-Recent as indicated by volcanic margin formation and the formation of Iceland respectively. Gravity inversion also predicts a region of thin oceanic crust to the west of the northern part of the Jan Mayen micro-continent and to the east of the thicker oceanic crust currently being formed at the Kolbeinsey Ridge. Thicker crust (c.f. ocean basins) is predicted for the Jan Mayen micro- continent south of Jan Mayen Island, with crust of the order of 20 km thickness extending southwards to connect with both the Faroes-Iceland Ridge and N.E. Iceland. Predicted crustal thicknesses under the Faroes- Iceland Ridge are approximately 25 km. The lithosphere thermal model used to predict the lithosphere thermal gravity anomaly correction may be conditioned using magnetic isochron data to provide the age of oceanic lithosphere. The resulting crustal thickness determination and the location of ocean-continent transition (OCT) are however sensitive to errors in the magnetic isochron data. An alternative method of inverting satellite gravity to give crustal thickness, incorporating a lithosphere thermal correction, has been used which does not use magnetic isochron data and provides an independent prediction of crustal thickness and OCT location. The crustal thickness estimates and OCT locations detailed above are robust to these sensitivity tests.

Investigating the deformation of upper crustal faults at the N-Chilean convergent plate boundary at different scales using high-resolution topography datasets and creepmeter measurements

NASA Astrophysics Data System (ADS)

Ewiak, O.; Victor, P.; Ziegenhagen, T.; Oncken, O.

2012-04-01

The Chilean convergent plate boundary is one of the tectonically most active regions on earth and prone to large megathrust earthquakes as e. g. the 2010 Mw 8.8 Maule earthquake which ruptured a mature seismic gap in south-central Chile. In northern Chile historical data suggests the existence of a seismic gap between Arica and Mejillones Peninsula (MP), which has not ruptured since 1877. Further south, the 1995 Mw 8.0 Antofagasta earthquake ruptured the subduction interface between MP and Taltal. In this study we investigate the deformation at four active upper plate faults (dip-slip and strike-slip) located above the coupling zone of the subduction interface. The target faults (Mejillones Fault - MF, Salar del Carmen Fault - SCF, Cerro Fortuna Fault - CFF, Chomache Fault - CF) are situated in forearc segments, which are in different stages of the megathrust seismic cycle. The main question of this study is how strain is accumulated in the overriding plate, what is the response of the target faults to the megathrust seismic cycle and what are the mechanisms / processes involved. The hyper arid conditions of the Atacama desert and the extremely low erosion rates enable us to investigate geomorphic markers, e .g. fault scarps and knickpoints, which serve as a record for upper crustal deformation and fault activity about ten thousands years into the past. Fault scarp data has been acquired with Differential-GPS by measuring high-resolution topographic profiles perpendicular to the fault scarps and along incised gullies. The topographic data show clear variations between the target faults which possibly result from their position within the forearc. The surveyed faults, e. g. the SCF, exhibit clear along strike variations in the morphology of surface ruptures attributed to seismic events and can be subdivided into individual segments. The data allows us to distinguish single, composite and multiple fault scarps and thus to detect differences in fault growth initiated either by seismic rupture or fault creep. Additional information on the number of seismic events responsible for the cumulative displacement can be derived from the mapping of knickpoints. By reconstructing the stress field responsible for the formation of identified seismic surface ruptures we can determine stress conditions for failure of upper crustal faults. Comparing these paleo stress conditions with the recent forearc stresses (interseismic / coseismic) we can derive information about a possible activation of upper crustal faults during the megathrust seismic cycle. In addition to the morphotectonic surveys we explore the recent deformation of the target faults by analyzing time series of displacements recorded with micron precision by an array of creepmeters at the target faults for over three years. Total displacement is composed of steady state creep, creep events and sudden displacement events (SDEs) related to seismic rupture. The percentage of SDEs accounts for >50 % (SCF) to 90 % (CFF) of the cumulative displacement. This result very well reflects the field observation that a considerable amount of the total displacement has been accumulated during multiple seismic events.

CRUST1.0: An Updated Global Model of Earth's Crust

NASA Astrophysics Data System (ADS)

Laske, G.; Masters, G.; Ma, Z.; Pasyanos, M. E.

2012-04-01

We present an updated global model of Earth's crustal structure. The new model, CRUST1.0, serves as starting model in a more comprehensive effort to compile a global model of Earth's crust and lithosphere, LITHO1.0. CRUST1.0 is defined on a 1-degree grid and is based on a new database of crustal thickness data from active source seismic studies as well as from receiver function studies. In areas where such constraints are still missing, for example in Antarctica, crustal thicknesses are estimated using gravity constraints. The compilation of the new crustal model initially follows the philosophy of the widely used crustal model CRUST2.0 (Bassin et al., 2000; http://igppweb.ucsd.edu/~gabi/crust2.html). Crustal types representing properties in the crystalline crust are assigned according to basement age or tectonic setting. The classification of the latter loosely follows that of an updated map by Artemieva and Mooney (2001) (http://www.lithosphere.info). Statistical averages of crustal properties in each of these crustal types are extrapolated to areas with no local seismic or gravity constraint. In each 1-degree cell, boundary depth, compressional and shear velocity as well as density is given for 8 layers: water, ice, 3-layer sediment cover and upper, middle and lower crystalline crust. Topography, bathymetry and ice cover are taken from ETOPO1. The sediment cover is essentially that of our sediment model (Laske and Masters, 1997; http://igppweb.ucsd.edu/~sediment.html), with several near-coastal updates. In the sediment cover and the crystalline crust, updated scaling relationships are used to assign compressional and shear velocity as well as density. In an initial step toward LITHO1.0, the model is then validated against our new global group velocity maps for Rayleigh and Love waves, particularly at frequencies between 30 and 40 mHz. CRUST1.0 is then adjusted in areas of extreme misfit where we suspect deficiencies in the crustal model. These currently include some near-coastal areas with thick sediment cover and several larger orogenic belts. Some remaining discrepancies, such as in backarc basins, may result from variations in the deeper uppermost mantle and remain unchanged in CRUST1.0 but will likely be modified in LITHO1.0. CRUST1.0 is available for download.

The subcontinental mantle beneath southern New Zealand, characterised by helium isotopes in intraplate basalts and gas-rich springs

NASA Astrophysics Data System (ADS)

Hoke, L.; Poreda, R.; Reay, A.; Weaver, S. D.

2000-07-01

New helium isotope data measured in Cenozoic intraplate basalts and their mantle xenoliths are compared with present-day mantle helium emission on a regional scale from thermal and nonthermal gas discharges on the South Island of New Zealand and the offshore Chatham Islands. Cenozoic intraplate basaltic volcanism in southern New Zealand has ocean island basalt affinities but is restricted to continental areas and absent from adjacent Pacific oceanic crust. Its distribution is diffuse and widespread, it is of intermittent timing and characterised by low magma volumes. Most of the 3He/ 4He ratios measured in fluid inclusions in mantle xenocrysts and basalt phenocrysts such as olivine, garnet, and amphibole fall within the narrow range of 8.5 ± 1.5 Ra (Ra is the atmospheric 3He/ 4He ratio) with a maximum value of 11.5 Ra. This range is characteristic of the relatively homogeneous and degassed upper MORB-mantle helium reservoir. No helium isotope ratios typical of the lower less degassed mantle (>12 Ra), such as exemplified by the modern hot-spot region of Hawaii (with up to 32 Ra) were measured. Helium isotope ratios of less than 8 Ra are interpreted in terms of dilution of upper mantle helium with a radiogenic component, due to either age of crystallisation or small-scale mantle heterogeneities caused by mixing of crustal material into the upper mantle. The crude correlation between age of samples and helium isotopes with generally lower R/Ra values in mantle xenoliths compared with host rock phenocrysts and the in general depleted Nd and Sr isotope ratios and the light rare earth element enrichment of the basalts supports derivation of melts as small melt fractions from a depleted upper mantle, with posteruptive ingrowth of radiogenic helium as a function of lithospheric age. In comparison, the regional helium isotope survey of thermal and nonthermal gas discharges of the South Island of New Zealand shows that mantle 3He anomalies in general do not show an obvious relationship with either age or proximity to the Cenozoic intraplate volcanic centres or with major faults. In general, areas characterised by mantle 3He emission are interpreted to define those regions beneath which mantle melting and basalt magma addition to the crust are recent. The strongest mantle 3He anomaly (equivalent to >80% mantle helium component) is centred over southern Dunedin, measured in magmatic CO 2-rich mineral water springs issuing from crystalline basement rocks which outcrop at the southern extent of Miocene intraplate basaltic volcanism which ceased 9 Ma ago. This mantle helium anomaly overlaps with an area characterised by elevated surface high heat flow, compatible with a long-lived mantle melt/heat input into the crust. In comparison Banks Peninsula, another Miocene intraplate basaltic centre, is characterised by relatively low surface heat flow and a small mantle helium contribution measured in a nitrogen-rich spring. Here the thermal transient induced by the magmatic event has either dissipated or has not reached the surface. In the former case one might be dealing with storage and mixing of magmatic and crustal gases at shallow crustal levels and in the latter with active to recent mantle-melt degassing at depth. Along the most actively deforming part of the plate boundary zone, the transpressional Alpine Fault and Marlborough fault systems, mantle helium is present in gas-rich springs in all those areas underlain by actively subducting oceanic crust (the Australian plate in the south and Pacific plate in the north), whereas the central part of the Alpine transpressional fault is characterised by pure crustal radiogenic helium. Areas where the mantle helium component is negligible are restricted to the centre part of the South Island, extending along its length from Southland to northern Canterbury and Murchison. These areas are interpreted to delineate the extent of thicker and colder lithosphere compared to all other areas where mantle helium release from partial mantle melts at depth is recent to active being added to the lower lithosphere and/or lower crust. Areas characterised by mantle helium anomalies are equated with areas of thermal mantle anomalies, i.e., localised mantle heterogeneities such as upwelling less dense silicate melts in the upper asthenospheric mantle.

An Integrated Crustal Dynamics Simulator

NASA Astrophysics Data System (ADS)

Xing, H. L.; Mora, P.

2007-12-01

Numerical modelling offers an outstanding opportunity to gain an understanding of the crustal dynamics and complex crustal system behaviour. This presentation provides our long-term and ongoing effort on finite element based computational model and software development to simulate the interacting fault system for earthquake forecasting. A R-minimum strategy based finite-element computational model and software tool, PANDAS, for modelling 3-dimensional nonlinear frictional contact behaviour between multiple deformable bodies with the arbitrarily-shaped contact element strategy has been developed by the authors, which builds up a virtual laboratory to simulate interacting fault systems including crustal boundary conditions and various nonlinearities (e.g. from frictional contact, materials, geometry and thermal coupling). It has been successfully applied to large scale computing of the complex nonlinear phenomena in the non-continuum media involving the nonlinear frictional instability, multiple material properties and complex geometries on supercomputers, such as the South Australia (SA) interacting fault system, South California fault model and Sumatra subduction model. It has been also extended and to simulate the hot fractured rock (HFR) geothermal reservoir system in collaboration of Geodynamics Ltd which is constructing the first geothermal reservoir system in Australia and to model the tsunami generation induced by earthquakes. Both are supported by Australian Research Council.

Seismic Imaging of the crust and upper mantle beneath Afar, Ethiopia

NASA Astrophysics Data System (ADS)

Hammond, J. O.; Kendall, J. M.; Stuart, G. W.; Ebinger, C. J.

2009-12-01

In March 2007 41 seismic stations were deployed in north east Ethiopia. These stations recorded until October 2009, whereupon the array was condensed to 13 stations. Here we show estimates of crustal structure derived from receiver functions and upper mantle velocity structure, derived from tomography and shear-wave splitting using the first 2.5 years of data. Bulk crustal structure has been determined by H-k stacking receiver functions. Crustal Thickness varies from ~45km on the rift margins to ~16km beneath the northeastern Afar stations. Estimates of Vp/Vs show normal continental crust values (1.7-1.8) on the rift margins, and very high values (2.0-2.2) in Afar, similar to results for the Main Ethiopian Rift (MER). This supports ideas of high levels of melt in the crust beneath the Ethiopian Rift. Additionally, we use a common conversion point migration technique to obtain high resolution images of crustal structure beneath the region. Both techniques show a linear region of thin crust (~16km) trending north-south, the same trend as the Red Sea rift. SKS-wave splitting results show a general north east-south west fast direction in the MER, systematically rotating to a more north-south fast direction towards the Red Sea. Additionally, stations close to the recent Dabbahu diking episode show sharp lateral changes over small lateral distances (40° over <30km), with fast directions overlying the Dabbahu segment aligning parallel with the recent diking. This supports ideas of melt dominated anisotropy beneath the Ethiopian rift. The magnitude of splitting in this region is smaller than that seen at the MER, suggesting a thinner region of melt, or less focused melt is causing the anisotropy. Seismic tomography inversions show that in the top 150km low velocities highlight plate boundaries. The low velocity anomalies extend from the main Ethiopian rift NE, towards Djibouti, and from Djibouti NW towards the Dabbahu segment The lowest velocities exist on the rift margins, supporting ideas of preferential melt generation at these regions of high strain. This includes a region of low velocity close to the edge of the proposed location of the Danakil microplate. Outside of these focused regions the velocities are relatively fast. Below ~250km the anomaly broadens to cover most of the Afar region with only the rift margins remaining fast. At transition zone depths little anomaly is seen beneath Afar, but some low velocities remain present beneath the MER. These studies suggest that in northern Ethiopia the Red Sea rift is dominant. The presence of thin crust beneath northern Afar suggests that the Red Sea rift is creating oceanic like crust in this region. The lack of deep mantle low velocity anomalies beneath Afar suggest that a typical narrow conduit plume does not exist in this region, rather the velocity models seem more similar to passive upwelling of material beneath Afar.

Crustal seismic anisotropy and structure from textural and seismic investigations in the Cycladic region, Greece

NASA Astrophysics Data System (ADS)

Cossette, Élise; Schneider, David; Audet, Pascal; Grasemann, Bernhard

2016-04-01

Seismic anisotropy data are often used to resolve rock structures and deformation styles in the crust based on compilations of rock properties that may not be representative of the exposed geology. We use teleseismic receiver functions jointly with in situ rock property data to constrain the seismic structure and anisotropy of the crust in the Cyclades, Greece, located in the back arc region of the Hellenic subduction zone. Crystallographic preferred orientations (CPOs) via electron backscatter diffraction (EBSD) analyses were measured on a suite of samples representative of different structural depths along the West Cycladic Detachment System; average seismic properties of the rocks were calculated with the Voigt-Reuss-Hill average of the single minerals' elastic stiffness tensor. The calcitic and quartzitic rocks have P- and S-wave velocity anisotropies (AVp, AVs) averaging 8.1% and 7.1%, respectively. The anisotropy increases with depth represented by blueschist assemblages, with AVp averaging 20.3% and AVs averaging 14.5% due to the content of aligned glaucophane and mica, which strongly control the seismic properties of the rocks. Localized anisotropies of very high magnitude are caused by the presence of mica schists as they possess the strongest anisotropies, with values of ~25% for AVp and AVs. The direction of the fast and slow P-wave velocities occur parallel and perpendicular to the foliation, respectively, for most samples. The fast propagation has the same NE-SW orientation as the lithospheric stretching direction present in the Cyclades since the Late Oligocene. The maximum shear wave anisotropy is subhorizontal, similarly concordant with mineral alignment that developed during back-arc extension. Our results strongly favor radial anisotropy in the Aegean mid-crust over azimuthal anisotropy. The receiver function data indicate that the Moho is relatively flat at 25 km depth in the south and deepens to 33 km in the north, consistent with previous studies, and reveal an intra-crustal discontinuity at depth varying from 3 to 11 km, mostly observed in the south-central Aegean. Harmonic decomposition of the receiver functions further indicates layering of both shallow and deep crustal anisotropy related to crustal structures. We model synthetic receiver functions based on constraints from the in situ rock properties that we measured using the EBSD technique. Our results indicate that the shallow upper crustal layer is characterized by metapelites with ~5% anisotropy, underlain by a 20 km thick and anisotropic layer of possible high-pressure rocks comprising blueschist and eclogite and/or restitic crust as a consequence of Miocene magmatism. Seismic anisotropy models require a sub-vertical axis of hexagonal symmetry in the upper crust (i.e. radial anisotropy), consistent with in situ rock data. Finally, a thinned crust is likely caused by back-arc extension associated with elevated sub-crustal temperatures, in agreement with thermal isostasy models of back arcs. This study demonstrates the importance of integrating rock textural data with seismic velocity profiles in the interpretation of crustal architecture.

Models of Deformation of Uppermost Oceanic Lithosphere: Comparison of Crustal Flexure in the Blönduós Area, Northern Iceland, and Structure of East Pacific Rise Crust at Hess Deep

NASA Astrophysics Data System (ADS)

Horst, A. J.; Karson, J. A.; Varga, R. J.; Gee, J. S.

2007-12-01

Models of the internal structure of oceanic crust have been constructed from studies of ophiolites and from more recent observations of tectonic windows into the upper crust. Spreading rate and/or magma supply are the central variables that control ridge processes and the ultimate architecture of ocean crust. In addition to ophiolites, Iceland also provides an important analog to study mid-ocean ridge processes and structure. Flexure zones in Iceland characterize the structure of Tertiary-Recent lava flows, and are areas wherein lavas dip regionally inward toward the axis of one of several ~N/S-trending rift zones. These rift zones are interpreted to represent fossil spreading centers which were abandoned during a series of eastward-directed ridge jumps. In the Hildará area, north-central Iceland, the eastern side of a regional flexure is characterized by westward-dipping lavas, approximately 6-8 Ma, which are cut by east-dipping normal faults and dikes. The upper-crustal structure within this flexure zone from slow spread (~20 mm/yr) crust exhibits remarkable similarities to the structure of the upper crust created at a fast-spreading (110 mm/yr) segment of the East Pacific Rise (EPR) observed at Hess Deep. In this modern ocean setting, ~1 Ma crust is characterized by west-dipping lavas above consistently east-dipping (away from the EPR) dikes and dike-subparallel fault zones. In both locations, paleomagnetic and structural data indicate that west-dipping lavas and east-dipping dikes result from tectonic rotations. In addition, cross-cutting dike relationships demonstrate that dike intrusion occurred both during and after normal fault- related tilting. These data indicate that fault-controlled tilting was initiated within the narrow neovolcanic zone of the ridge and is not associated with off-axis processes. Lavas at magmatically robust ridges commonly flow away from elevated ridge-crests. Measurement of anisotropy of magnetic susceptibility (AMS) of the lavas from the flexure in Iceland suggests a mean flow direction to the northeast, that is, away from the fossil-ridge axis, demonstrating that the fossil spreading center from which the lavas were extruded was located to the west. Despite the distinct differences in spreading rates, the high magma supply in both environments resulted in a very similar upper crustal architecture.

The East African rift system in the light of KRISP 90

USGS Publications Warehouse

Keller, Gordon R.; Prodehl, C.; Mechie, J.; Fuchs, K.; Khan, M.A.; Maguire, Peter K.H.; Mooney, W.D.; Achauer, U.; Davis, P.M.; Meyer, R.P.; Braile, L.W.; Nyambok, I.O.; Thompson, G.A.

1994-01-01

On the basis of a test experiment in 1985 (KRISP 85) an integrated seismic-refraction/teleseismic survey (KRISP 90) was undertaken to study the deep structure beneath the Kenya rift down to depths of 100-150 km. This paper summarizes the highlights of KRISP 90 as reported in this volume and discusses their broad implications as well as the structure of the Kenya rift in the general framework of other continental rifts. Major scientific goals of this phase of KRISP were to reveal the detailed crustal and upper mantle structure under the Kenya rift, to study the relationship between mantle updoming and the development of sedimentary basins and other shallow structures within the rift, to understand the role of the Kenya rift within the Afro-Arabian rift system and within a global perspective and to elucidate fundamental questions such as the mode and mechanism of continental rifting. The KRISP results clearly demonstrate that the Kenya rift is associated with sharply defined lithospheric thinning and very low upper mantle velocities down to depths of over 150 km. In the south-central portion of the rift, the lithospheric mantle has been thinned much more than the crust. To the north, high-velocity layers detected in the upper mantle appear to require the presence of anistropy in the form of the alignment of olivine crystals. Major axial variations in structure were also discovered, which correlate very well with variations in the amount of extension, the physiographic width of the rift valley, the regional topography and the regional gravity anomalies. Similar relationships are particularly well documented in the Rio Grande rift. To the extent that truly comparable data sets are available, the Kenya rift shares many features with other rift zones. For example, crustal structure under the Kenya, Rio Grande and Baikal rifts and the Rhine Graben is generally symmetrically centered on the rift valleys. However, the Kenya rift is distinctive, but not unique, in terms of the amount of volcanism. This volcanic activity would suggest large-scale modification of the crust by magmatism. Although there is evidence of underplating in the form of a relatively high-velocity lower crustal layer, there are no major seismic velocity anomalies in the middle and upper crust which would suggest pervasive magmatism. This apparent lack of major modification is an enigma which requires further study. ?? 1994.