Flat Moho Research Articles

Folding in regions of extension

SUMMARY Many extension zones have been subjected to folding and shortening in a direction perpendicular to the stretching. Such deformation can be accounted for by the extension of a thin superficial elastic layer overlying a substrate that has small elastic moduli or that deforms in a viscous regime. Laboratory experiments are used to document the wavelength and amplitude of the folds for a range of geometrical configurations. Folding is observed even for very small amounts of extension (less than 1 per cent) with characteristics that are consistent with finite-amplitude scaling laws. Because of the intrinsically 3-D nature of the deformation field, the size of the region affected by folding and the direction of the fold axes depend on the orientation of the extension with respect to the rigid blocks that bound the deforming region. For regions of extension where the elastic thickness is about 10 km, as in the Basin and Range province for example, it is predicted that folding occurs with wavelengths in a 20–40 km range, such that it induces little deformation in the lower crust and maintains a flat Moho discontinuity. These predictions are consistent with the observations. The characteristics of faulting that is associated with such deformation are discussed.

Crustal structure of Northwest Zagros (Kermanshah) and Central Iran (Yazd and Isfahan) using teleseismic Ps converted phases

Receiver functions are widely employed to detect P-to-S converted waves and are especially useful to image seismic discontinuities in the crust. In this study we used the P receiver function technique to investigate the velocity structure of the crust beneath the Northwest Zagros and Central Iran and map out the lateral variation of the Moho boundary within this area. Our dataset includes teleseismic data (M b ≥ 5.5, epicentral distance from 30° to 95°) recorded at 12 three-component short-period stations of Kermanshah, Isfahan and Yazd telemetry seismic networks. Our results obtained from P receiver functions indicate clear Ps conversions at the Moho boundary. The Moho depths were firstly estimated from the delay time of the Moho converted phase relative to the direct P wave beneath each network. Then, we used the P receiver function inversion to find the properties of the Moho discontinuity such as depth and velocity contrast. Our results obtained from PRF are in good agreement with those obtained from the P receiver function modeling. We found an average Moho depth of about 42 km beneath the Northwest Zagros increasing toward the Sanandaj-Sirjan Metamorphic Zone and reaches 51 km, where two crusts (Zagros and Central Iran) are assumed to be superposed. The Moho depth decreases toward the Urmieh-Dokhtar Cenozoic volcanic belt and reaches 43 km beneath this area. We found a relatively flat Moho beneath the Central Iran where, the average crustal thickness is about 42 km. Our P receiver function modeling revealed a shear wave velocity of 3.6 km/s in the crust of Northwest Zagros and Central Iran increasing to 4.5 km/s beneath the Moho boundary. The average shear wave velocity in the crust of UDMA as SSZ is 3.6 km/s, which reaches to 4.0 km/s while in SSZ increases to 4.3 km/s beneath the Moho.

Crustal thickness and average Vp/Vs ratio variations in southwest Yunnan, China, from teleseismic receiver functions

Recently, many portable broadband three‐component seismic stations have been deployed in Yunnan, China. We use teleseismic receiver functions to determine the crustal thickness (H) and the crustal average compressional to shear wave velocity (Vp/Vs) ratio (k) variations beneath in southwest Yunnan. By comparing the synthetics generated from dipping Moho models with those from a flat Moho model, it reveals that for the rays traveling along or around the updip direction, the differential arrival times of Ps and multiples generated at the dipping Moho are comparable with those generated at the flat Moho and the result from stacking with these synthetics is a better estimate of model structure. For the synthetics traveling along or around the downdip direction, the differential arrival times of Ps and multiples generated at dipping Moho are smaller than those generated at flat Moho and make the H‐k stacking results deviate from the model structure. Thus, the ambiguity induced by dipping Moho can be reduced significantly by only stacking with the rays traveling along or around the updip direction. Applying this technique to 19 stations, we obtained that H ranges from ∼35–42 km and the k ranges from ∼1.66–1.78. From the central Simao basin, H increases northward and northeastward; k increases significantly northeastward across the Ailao Shan–Red River shear zone.

Nature of the Moho transition in NW Canada from combined near-vertical and wide-angle seismic-reflection studies

Although the year 2009 marked one century since the discovery of the Moho, or crust-mantle boundary, the exact nature of that boundary and the manner in which it formed remain major uncertainties in lithospheric studies. In northwestern Canada, sharp Moho reflections at both near-vertical and wide-angle incidence have been imaged beneath the Great Bear magmatic arc. They show a remarkably flat Moho and do not reflect the complex tectonic history of the Wopmay orogen, of which the Great Bear arc is one part. In order to understand the origin of these reflections and the nature of the Moho, we calculated near-vertical and wide-angle synthetic seismograms for a number of crust-mantle transition models using one- and two-dimensional wave propagation algorithms. Only laterally and vertically heterogeneous models can properly simulate the observed seismic signature recorded on both near-vertical and wide-angle reflection data. The heterogeneity is achieved by either laterally discontinuous layering or a lamellae structure with randomly distributed ellipses. These models suggest that the Moho represents a thermal/metamorphic front, a regional décollement, or both.

The effects of crustal heterogeneity on ray-based teleseismic imaging

SUMMARY We evaluate the efficacy of crustal scale receiver function imaging in the presence of crustal scattering. First, we show that the resolution of an image is not appreciably affected by geologically realistic crustal scattering. Rather, we show that the image contrast is reduced: the image of actual targets get ‘buried’ in the noise created by arrivals arising from crustal scattering. Then, we construct three classes of models in which we simulate a flat Moho, a Moho with topography, and a subduction zone. For all the models, we simulate the Moho as a finite-thickness velocity transition zone and the crust with a three-layer heterogeneous zone that contains binary von-Karman statistics with varying degrees of rms density and velocity contrasts. We use finite-difference simulations to generate a suite of varying-slowness seismograms, which we then image with pre-stack 2-D Kirchhoff migration and CCP stacking. We find that for a small degree of rms density/velocity contrasts, both imaging methodologies faithfully image the models, but for rms density/velocity variations greater than approximately 5 per cent per cent, we observed distortion and/or artefacts to the images. For migration, the distortion takes the form of amplitude variations for horizontal model features, whereas for the CCP imaging, the main affect is the enhancement of any horizontal feature in the model.

Crustal thickness beneath the Ryukyu arc from travel-time inversion

Abstract First P-wave arrival times recorded by the seismic network in the Ryukyu arc were analyzed in order to image the lateral variation in crustal thickness beneath the Ryukyu arc. The results indicate a low P n velocity (7.5 km/s) in the Ryukyu arc and a relatively high P n velocity (7.9 km/s) in the Okinawa Trough. The crustal thickness changes between the northern-and-central Ryukyu arc (range 23–27 km) and the southern Ryukyu arc (range 29–44 km). The crustal thickness of the former is consistent with that in the northern-and-central Okinawa Trough, suggesting a flat Moho stretching from the Okinawa Trough to the Ryukyu arc. This flat Moho extends uniformly over a wide area. In contrast, the crustal thickness in the southern Ryukyu arc is consistent with the characteristics of the crustal structure whereby the Moho becomes shallower in the vicinity of the axis of the Okinawa Trough and deepens remarkably with increasing distance from the axis in the southern Ryukyu arc. These differences would be caused by the difference in rifting style between the northern-and-central Okinawa Trough and the southern Okinawa Trough.

No Moho uplift below the Baikal Rift Zone: Evidence from a seismic refraction profile across southern Lake Baikal

The late Cenozoic Baikal Rift Zone (BRZ) in southern Siberia is composed of several individual topographic depressions and half grabens with the deep Lake Baikal at its center. We have modeled the seismic velocity structure of the crust and uppermost mantle along a 360 km long profile of the Baikal Explosion Seismic Transects (BEST) project across the rift zone in the southern part of Lake Baikal. The seismic velocity structure along the profile is determined by tomographic inversion of first arrival times and 2‐D ray tracing of first arrivals and reflections. The velocity model shows a gently deepening Moho from the Siberian Platform (41 km depth) into the Sayan‐Baikal Fold Belt (46 km depth). We can exclude the presence of any Moho uplift around the ∼10 km deep sedimentary graben structure of southern Lake Baikal. The lower crust includes a distinct 50–80 km wide high‐velocity anomaly (7.4–7.6 ± 0.2 km/s), slightly offset to the northeast from the rift axis. We interpret this feature as resulting from mafic intrusions. Their presence may explain the flat Moho by compensation of lower crustal thinning by intrusion of mafic melts. The Pn wave velocities (8.15–8.2 km/s) are normal for the area and do not show any sign of decompression melting in the sub‐Moho mantle, such that possible lithosphere thinning has not reached the base of the crust. On the basis of the results of the BEST project, we suggest that the BRZ is formed by passive rifting in the rheologically weak suture between the Siberian Platform and the Amurian plate.

Rifting and lower crustal reflectivity: A case study of the intracratonic Dniepr‐Donets rift zone, Ukraine

Intracratonic rifting, caused by late Devonian extensional stresses in the East European Craton, created the largest rift zone in Europe, the Pripyat‐Dniepr‐Donets rift (southeast Ukraine). The rift basin is approximately 2000 km long, up to 170 km wide, and 22 km deep. Wide‐angle refraction and reflection seismic data from the Donbas Basin deep seismic Refraction and Reflection Experiments (DOBRE'99) project together with gravity and magnetic data are analyzed for the structure and evolution of the Donbas Fold Belt, which is the uplifted and deformed part of the Dniepr‐Donets Basin. The seismic data are used for identification of large‐scale crustal structures and modeling of the seismic velocities of the crust and uppermost mantle. A ray‐trace‐based velocity and density model is derived by joint inversion of gravity and traveltime data. The inversion result reveals a zone of high density and velocity beneath the basin at middle to lower crustal levels, slightly offset to the NE of the rift axis. Full waveform synthetic seismograms, matching the observed data, show high‐amplitude and low‐frequency arrivals from this high‐density body as well as from the Moho. We interpret the high‐amplitude, low‐frequency signals as reflections from layered magmatic rocks, which intruded into the ductile lower crust during the main rift phase and subsequently were sorted by fractional crystallization. The intrusive material thickened the lower crust by approximately 50%. This may explain the enigmatic flat Moho topography across the rift zone which has been significantly stretched (β = 1.3). The rifting initiated in the late Devonian (Frasnien) as a consequence of back‐arc extension in relation to subduction of the Paleo‐Tethys Ocean. The subducting oceanic slab may have enriched the mantle with volatiles and created convection, leading to strong partial melting, upwelling, and continued rifting in the Famennien. We interpret the asymmetrical rift geometry as a combination of pure and simple shear tectonic processes.

New insights into the crustal structure of the North German Basin from reprocessing of seismic reflection data using the Common Reflection Surface stack

The influence of deep crustal processes on basin formation and evolution and its relation to current morphology is not well understood yet. A key feature to unravel these issues is a detailed seismic image of the crust. A part of the data recorded by the hydrocarbon industry in the late 1970s and 1980s in the North German Basin were released to the public recently. The seismic reflection data were recorded down to 15 s two-way travel time. The mean Common Midpoint fold of about 20 is relatively low compared to contemporary seismic acquisitions. The processing of the 1980s focussed on the sedimentary structures to explore the hydrocarbon potential of this area. We applied the Common Reflection Surface stack technique to the data sets, which is well suited for low-fold data. The reprocessing was focussed on the imaging of the subsedimentary crustal range. The reprocessed images show enhanced reflections, especially in the mid and lower crustal part. Also, the image of the salt structures in the graben area was improved. Furthermore, the reprocessed images indicate an almost flat Moho topography in the area of the Gluckstadt Graben and an additional lower crustal structure, which can be correlated with a high-density body found in recent gravity modeling studies.

Constraints on the deformation and rupturing of continental lithosphere of the Red Sea: the transition from rifting to drifting

Abstract The Red Sea is an ideal locale for testing differing models and hypotheses for rift evolution and the initiation of sea-floor spreading. The Red Sea is an active rift system that formed by the rifting of Precambrian continental lithosphere beginning in the late Oligocene, leading to breakup and sea-floor spreading by approximately 5 Ma in the southern Red Sea to the south of about 19°30′N. In the northern Red Sea, north of approximately 23°30′N, organized sea-floor spreading is not observed, although individual volcanoes are located within discrete ‘deeps’ spaced along the axial depression. These have been interpreted as marking the beginning of a transition from amagmatic, mechanical rifting to magmatic, oceanic spreading. Based on seismic reflection and refraction, gravity, magnetic and heat flow data in the northern Red Sea, it has been suggested that rift development occurs via the rotation of large crustal fault blocks that sole into a zone of plastic creep in the lower crust, resulting in a flat Moho and high upper crustal relief. Melt formed within individual rift segments is focused to form small axial volcanoes. That is, the northern Red Sea is on the verge of replacing horizontal translation with focused mantle upwelling and organized sea-floor spreading. In marked contrast, many passive margins (e.g. West Africa, Brazil, and NW and NE Australia) are characterized by the stacking of regional synrift sag packages, the thickness and distribution of which are inconsistent with the minor amounts of brittle deformation mapped from seismic sections. A fundamental implication of this observation is that rifts characterized by large offset fault systems, (i.e. faults that generate synrift accommodation, such as in the Basin and Range province and East Africa) will not proceed to breakup. The challenge is to understand why different portions of the Red Sea show different stages in the development of a spreading centre during continental rifting. Two hypotheses exist: (1) the structural framework deduced in the north simply continues to the south where sea-floor spreading exists and that the two regions are registering a difference in total extension. Thus, the northern Red Sea has experienced insufficient extension to breach the continental lithosphere but, in time, should develop into a spreading centre; or (2) the lithosphere of the northern Red Sea region is rheologically stronger compared with the lithosphere of the southern Red Sea, perhaps as a consequence of the thermal effects of the Afar plume, and the northern Red Sea will never evolve to sea-floor spreading. The existence of large rotated fault blocks, as implied from the inversion of gravity and magnetic anomaly data, favours the latter.

Aegean crustal thickness inferred from gravity inversion. Geodynamical implications

Since Oligo–Miocene times, the Aegean domain has undergone regional extension due to the southward retreat of the Hellenic subduction zone. Boundary conditions have been more recently modified by the westward extrusion of Anatolia. A new map of the Aegean crustal thickness inferred from gravity inversion is proposed here to better constrain the variations in space and time of crustal thinning that has accumulated since Oligo–Miocene times. Moho topography is obtained by inversion of satellite marine gravity data. Data are first corrected for terrain anomalies and deep mantle effects (African subducting slab). They are then filtered between 50 and 300 km to avoid short wavelength intracrustal effects. Results are consistent with previous 2D geophysical studies (seismic refraction, receiver functions) and show that an overall regional isostatic compensation of the crust holds for the Aegean area, with a mean crustal thickness of 25 km. Three different provinces (North Aegean, Cyclades and Cretan Sea) can be identified. Thinner crust is observed both in the North Aegean region (NE–SW trending of thinning, with crustal thickness lower than 24 km) and in the Cretan Sea (crustal thickness of 22–23 km). Between these two regions, the Cyclades are marked by a rather flat Moho at 25 km. A two-stage model of the Aegean extension could well explain the observed crustal thickness variation. From Oligocene to middle Miocene, gravitational collapse of the Hellenides, due to the southward retreat of the African slab, reduced the Aegean continental crust from 50 km (by reference to continental Greece and Anatolia) to a mean value of 25 km at the scale of the whole Aegean. From upper Miocene to present, the westward extrusion of Anatolia modified the extension and the associated crustal thinning in the North Aegean domain. During this second episode, crustal thinning related to the southward retreat of the African slab tends to localize in the Cretan Sea. The Cyclades likely behave as a rigid block translated toward the south–west without significant deformation, in agreement with the GPS velocity field and the lack of major earthquakes.

Accreted terranes of northwestern British Columbia, Canada: Lithospheric velocity structure and tectonics

Lithospheric structure beneath the accreted terranes of the northwestern Canadian Cordillera is interpreted from analyses of P wave refraction and wide‐angle reflection data. The 535 km long profile extends from the deformed and displaced ancestral North American margin across the Omineca belt suture zone and Intermontane superterrane to the Coast belt. This region is interpreted to have grown westward through a progression from thin‐skinned to thick‐skinned accretionary tectonics. Large lateral variations of velocities (2.0–6.4 km/s) within the upper 15 km correlate with the mapped geological and terrane elements; fault offsets and the imbricated base of an overlap basin are clearly identified. Inboard of the Stikine accreted terrane a zone of slow upper crustal velocities thins to the east, consistent with an interpretation of thin‐skinned accretion leaving slivers of terranes overlying a wedge of metamorphosed Proterozoic continental margin sediments. However, in the lower crust the proposed underlying cratonic ramp is not distinguished from outboard or overlying accreted crust by lateral P velocity contrasts or wide‐angle reflections. The Moho remains nearly horizontal beneath the Intermontane and Omineca belts, gradually thinning to the east from 36.5 to 35 km. To the southwest below the Coast belt and to the northeast beneath the Tintina fault, the crust thins to 32 km. Upper mantle velocities across the Coast, Intermontane, and Omineca belts are 7.8–7.9 km/s. The slow upper mantle and lower crustal velocities are indicative of high temperatures that may permit the flat Moho or weak lower crust to partially decouple the crust from the mantle. Reflections in the upper mantle between 50–70 km depth may be related to the base of the lithosphere or to heterogeneities introduced through melting or accretionary subduction events.

Crustal structure of the Kaapvaal craton and its significance for early crustal evolution

High-quality seismic data obtained from a dense broadband array near Kimberley, South Africa, exhibit crustal reverberations of remarkable clarity that provide well-resolved constraints on the structure of the lowermost crust and Moho. Receiver function analysis of Moho conversions and crustal multiples beneath the Kimberley array shows that the crust is 35 km thick with an average Poisson's ratio of 0.25. The density contrast across the Moho is ∼15%, indicating a crustal density about 2.86 gm/cc just above the Moho, appropriate for felsic to intermediate rock compositions. Analysis of waveform broadening of the crustal reverberation phases suggests that the Moho transition can be no more than 0.5 km thick and the total variation in crustal thickness over the 2400 km 2 footprint of the array no more than 1 km. Waveform and travel time analysis of a large earthquake triggered by deep gold mining operations (the Welkom mine event) some 200 km away from the array yield an average crustal thickness of 35 km along the propagation path between the Kimberley array and the event. P- and S-wave velocities for the lowermost crust are modeled to be 6.75 and 3.90 km/s, respectively, with uppermost mantle velocities of 8.2 and 4.79 km/s, respectively. Seismograms from the Welkom event exhibit theoretically predicted but rarely observed crustal reverberation phases that involve reflection or conversion at the Moho. Correlation between observed and synthetic waveforms and phase amplitudes of the Moho reverberations suggests that the crust along the propagation path between source and receiver is highly uniform in both thickness and average seismic velocity and that the Moho transition zone is everywhere less than about 2 km thick. While the extremely flat Moho, sharp transition zone and low crustal densities beneath the region of study may date from the time of crustal formation, a more geologically plausible interpretation involves extensive crustal melting and ductile flow during the major craton-wide Ventersdorp tectonomagmatic event near the end of Archean time.

P-wave tomography of crust and upper mantle under Southern California: Influence of topography of Moho discontinuity

We use 146 422 P-wave arrival times from 6 347 local earthquakes recorded by the Southern California Seismic Network to determine a detailed three-dimensional P-wave velocity structure at 0–35 km depth. We have taken into account the Moho depth variations, which were obtained by seismological methods. Checkerboard tests suggest that our inversion results are reliable. Our models provide new information on regional geological structures of Southern California. At shallow depths P-wave velocity structure correlates with surface geological features and expresses well variations of surface topography of the mountains and basins. The velocity structure at each layer is characterized by block structures bounded by large faults. Ventura Basin, Los Angeles Basin, Mojave Desert, Peninsular Ranges, San Joaquin Valley, Sierra Nevada, and Salton Trough show respectively all-round block. San Andreas Fault becomes an obvious boundary of the region. To its southwest, the velocity is higher, and there are strong heterogeneity and deeper seismicity; but to its northeast, the velocity is lower and shows less variation than to the southwest, the seismicity is shallower. To investigate the effect of the Moho geometry we conducted inversions for two cases: one for flat Moho geometry, another for a Moho with lateral depth variations. We found that the topography of the Moho greatly affects the velocity structure of the middle and lower crust. When the Moho topography is considered, a more reasonable tomographic result can be obtained and the resulting 3-D velocity model fits the data better.

Deformation in the lower crust and downward extent of the San Andreas Fault as revealed by teleseismic waveforms

High resolution images of crustal structure across the San Andreas Fault (SAF) were obtained by using the common conversion point stacking of teleseismic P-to-S converted waves recorded during the Los Angeles Region Seismic Experiments (LARSE-I and II). In the upper crust, several sedimentary basins were delineated in the images, including the San Fernando and the Santa Clarita Basins. The San Fernando Basin reaches a depth of 8 km under the northern edge of the San Fernando Valley. On the LARSE-I profile, the downward projection of the SAF truncates several lower crustal interfaces including the Moho on both sides. The Moho is vertically offset by as much as 8 km. Along the LARSE-II profile, the impedance contrast and slope of the Moho are seen to change across the fault. These results indicate that the fault penetrates into the lower crust and probably uppermost mantle as a narrow (<10 km) feature. The Moho beneath the San Gabriel Mountains is shallower (∼26 km) than under the San Gabriel Valley to the south and the Mojave Desert to the north, suggesting that the mountain ranges were lifted en masse by horizontal compression. On the northeast side of the SAF, the Mojave Desert has a sharp and essentially flat Moho at a depth of ∼32 km. The lower crustal structure beneath the San Fernando and Santa Clarita Valleys along the LARSE-II profile south of the SAF is complicated as indicated by the large undulation and low impedance contrast of the Moho. These observations suggest that the deformation in the lower crust is localized and often concentrates near boundaries of crustal blocks or beneath those places which have experienced intensive faulting and deformation in the upper crust.

Lithospheric structure and its relationship to seismic and volcanic activity in southwest China

The Sichuan‐Yunnan region in southwest China is located in the boundary area between the active Tibetan Plateau to the west and the stable South China platform to the east. This region is characterized by complex Cenozoic structures and active seismotectonics. In this study, we have used over 30,000 arrival times from 1315 local earthquakes recorded by 172 seismic stations to determine a detailed three‐dimensional (3‐D) P wave velocity structure of the lithosphere down to 85 km depth in this region. We have taken into account the complex morphology of the Moho discontinuity to conduct the tomographic inversions, which leads to a better result than that with a flat Moho as in the previous studies. Our results show that large velocity variations of up to 7% exist in the crust and upper mantle in the Sichuan‐Yunnan region. The velocity image of the upper crust correlates with the surface geological features. The Sichuan basin is imaged as a prominent low‐velocity zone, while the Panzhihua mining district is imaged as a high‐velocity feature. Velocity changes are visible across some of the large fault zones, and the faults and some large crustal earthquakes seem to occur at the boundary areas between slow and fast velocity anomalies. Some of the faults, such as the Red River fault, may have cut through the crust and reached up to the upper mantle. Under the Tengchong volcanic area, strong low‐velocity zones are visible down to 85 km depth, with a lateral extent of about 100 km, suggesting the existence of magma chambers under the volcano. It is unclear how the Tengchong intraplate volcanism was generated. It may be related to the collision processes between the Indian plate, Burma microplate and the Eurasian plate, and the possible subduction of the Burma microplate under the Eurasian plate. Another possibility is that it was caused by the extensional fractures of the lithosphere and the upward intrusion of the hot asthenospheric materials. It is also possible that the Tengchong volcanism represents a hot spot with a lower mantle origin.

Mantle compensation of active metamorphic core complexes at Woodlark rift in Papua New Guinea.

In many highly extended rifts on the Earth, tectonic removal of the upper crust exhumes mid-crustal rocks, producing metamorphic core complexes. These structures allow the upper continental crust to accommodate tens of kilometres of extension, but it is not clear how the lower crust and underlying mantle respond. Also, despite removal of the upper crust, such core complexes remain both topographically high and in isostatic equilibrium. Because many core complexes in the western United States are underlain by a flat Moho discontinuity, it has been widely assumed that their elevation is supported by flow in the lower crust or by magmatic underplating. These processes should decouple upper-crust extension from that in the mantle. In contrast, here we present seismic observations of metamorphic core complexes of the western Woodlark rift that show the overall crust to be thinned beneath regions of greatest surface extension. These core complexes are actively being exhumed at a rate of 5-10 km Myr(-1), and the thinning of the underlying crust appears to be compensated by mantle rocks of anomalously low density, as indicated by low seismic velocities. We conclude that, at least in this case, the development of metamorphic core complexes and the accommodation of high extension is not purely a crustal phenomenon, but must involve mantle extension.

The legacy of the NE German Basin — reactivation by compressional buckling

In contrast to previously published models for the area, the seismic reflection Moho is essentially flat beneath the NE German Basin along the DEKORP deep seismic profile Basin’96. This raises the question, whether the present structure of the crust and flat Moho reflect the initial formation of the basin or modification by more recent processes. A 2D flexural model, developed for a thin elastic plate, is presented together with lithospheric strength profiles calculated along the BASIN 9601 reflection seismic line. The analysis shows a southward decrease of lithospheric strength below the Basin, with a lithospheric decoupling between the crust and the mantle. The modelling supports the hypothesis that the present Moho topography is caused by flexural buckling which caused subsidence of the NE German Basin during the Upper Cretaceous–Early Cenozoic inversion event. This suggests that the basin is in isostatic disequilibrium, and that compressive stresses are required to keep the present basin geometry.

Flexural expression of European continental lithosphere under the western outer Carpathians

We present the results of an analysis of the flexural down bending of the European lithosphere under the western outer Carpathians for a grid of five profiles with a lateral spacing of about 100 km crossing the Carpathians and its foreland. Seismic sections, including two deep seismic reflection lines, and numerous wells allowed us to quantify the flexural curvature of the lithosphere. In addition, gravity data are used to obtain independent constraints. Two evolutionary scenarios involving subduction/underthrusting of passive margin and a phase change in the lower crust are investigated in a context of relatively flat Moho under the area. Furthermore, recent uplift, expressing itself in erosional features observed along the distal margin of the foredeep, is quantified. The generally modest Bouguer gravity anomaly points to nonisostatic processes causing this uplift. Steeply dipping basement under the Carpathians to about 10 km depth at the location of the Pieniny Klippen Belt is explained by relatively weak lithosphere, which is expressed in the predicted low values of effective elastic thickness of 16 km decreasing locally to 3 km. The low strength of the European lithosphere in this region is probably caused by mechanical weakening controlled by partial yielding due to high flexural curvature. The weakest parts of the lithosphere along the profiles, corresponding to low effective elastic thicknesses, are concentrated in the areas of maximum bending. Predicted flexural bending stresses as a function of bending moment and effective elastic thickness increase dramatically under the outer zone of the Carpathian belt, explaining reactivations of older basement structures in these regions.

Thin crust and active upper mantle beneath the Southern Sierra Nevada in the western United States

Analysis of seismic-refraction/wide-angle reflection travel-time and seismic amplitude data collected during the 1993 Southern Sierra Nevada Continental Dynamics (SSCD) project resolves a small crustal root (40–42 km thick) centered 80 km west of the Sierran topographic crest. The SSCD experiment consisted of a west-east profile across the Sierra Nevada at approximately 36.5°N and a north-south profile extending the length of Owens Valley, located eastward of the Sierran topographic crest. Two-dimensional finite-difference travel-time inversion of Pg and Pn arrivals resolves upper-crustal velocities of 6.0 to 6.4 km s−1 within the Sierran Batholith and Basin and Range, an upper-crustal west-dipping wedge of higher velocities (6.8–7.2 km s−1) consistent with ophiolitic material underlying the Great Valley sedimentary sequence, and higher velocities in the lower crust beneath the Basin and Range (6.8–7.0 km s−1) than those beneath the Sierran Batholith (6.6 km s−1). Low average Pn velocities (7.6–7.9 km s−1) and a laminated transitional Moho imaged beneath the Sierran Batholith also differ from the higher Pn velocities (7.9–8.0 km s−1) and sharp first-order Moho observed beneath the Basin and Range. The crust decreases in thickness both westward of the root to 28–34 km beneath the Great Valley and eastward to 35 km beneath the highest Sierran topography and decreases further to 27–30 km beneath the Basin and Range. Crustal thickness also appears to increase southeast to northwest from 29–30 km beneath the Garlock Fault in the south to 38–40 km beneath the north end of Chalfant Valley. Juxtaposition of the crustal model with previous P-wave tomography models of the south-ernmost Sierra Nevada upper-mantle reveal that the thickest Sierran crust on the west-east profile overlies a pronounced upper-mantle high-velocity anomaly (+5%), whereas the region of laminated Moho overlies a flanking upper-mantle low-velocity region (−3%). The upper-mantle velocity anomalies, relatively low Pn, and relatively flat Moho, observed beneath the Sierran crest suggest that the recent uplift of the Sierra Nevada is due to asthenospheric flow and/or lithospheric thinning beneath the southeastern Sierra Nevada and Basin and Range.