Regional Lithospheric Structure Research Articles

Spatially heterogeneous lithospheric architecture of the Cambay rift basin and adjoining Aravalli-Delhi Fold Belt, western India – A synthesis of magnetotelluric results

The Cambay rift basin (CRB) evolution has been associated with separation of the Indian continent from the African continent and subsequent interaction with the Réunion mantle plume, which resulted in significant alternations in the regional lithospheric structure of western India, more prominently in the CRB due to its proximity to the plume track. In our continued effort to delineate the lithospheric structure of the CRB and adjoining regions, we acquired magnetotelluric (MT) data at 31 sites along a profile across the CRB and analyzed these. The 2D geoelectric model derived from this dataset reveals a collage of conductive and resistive formations. The resistive blocks indicate the Archaean lithosphere on the west, the uplifted crust within the CRB, and various structural units of the Precambrian Era on the east providing evidence for continuation of the Aravalli-Delhi Fold Belt trend beneath the CRB into Saurashtra. The conductive anomalies beneath the CRB infer the magmatic underplating associated with thermal influx derived from the upper mantle as a result of the Réunion plume interaction. Integration of previous geophysical and present results infers a spatially heterogeneous lithosphere possibly representing an assemblage the Archaean and Proterozoic lithospheres beneath the CRB. The Proterozoic lithosphere is more vulnerable to the impact of the Réunion plume due to its weak nature in comparison to the Archaean lithosphere. The Archaean lithosphere on the western side of CRB indicates initial stage of rifting, whereas Proterozoic lithosphere infers second stage coeval with the Deccan volcanic eruption. The heterogeneity in sedimentary thickness within the CRB spatially correlates with the contact of Archaean and Proterozoic lithospheres, which might have played a role in the formation of the basin structure, and sediment deposition.

New insights on Moho depth and regional lithospheric structure of central Oklahoma based on receiver function analysis from dense seismic networks

Knowledge of the midcontinent crustal structure of North America is crucial for understanding the evolutionary history of the ancient North American craton as Laurentia grew through accretion ∼1.5 Ga to 1 Ga. Although Oklahoma has been recognized as a tectonically stable region since the Phanerozoic, its crustal structure records the earlier formation of the Mazatzal and southern Granite-Rhyolite provinces 1.6 Ga to 1.4 Ga. We present results from teleseismic receiver function analysis applied to 221 events recorded on 169 broadband stations in central Oklahoma. Our findings include a Moho depth map of central Oklahoma based on stacked and depth-converted teleseismic P receiver functions. The results are interpreted together with gravimetric and magnetic datasets and a recently established seismic velocity model of the crust.The Moho map shows a generally flat crust-mantle boundary in central Oklahoma with an average depth of 43.5 km while we observe a sudden thickening on the crust of the northwestern part of Oklahoma where the Moho deepens to over 50 km depth. We also find a Mid-lithosphere discontinuity at the upper-most mantle in north-central Oklahoma, presented as a negative phase deepening southeastward from 60 km to 80 km. We further observe an intracrustal discontinuity at the Nemaha uplift and Anadarko shelf regions in a depth range of 17–30 km. The hypothesis of the Mid-continent Rift (MCR) extending into Oklahoma is examined in terms of the crustal structure and Moho depth variation revealed by receiver functions. We do not find evidence of Moho structure or lower-crustal underplay characteristics similar to what has been discovered in the northern part of MCR, but the intracrustal discontinuity that deepens towards the hypothesized MCR region suggests upper-crustal volcanics potentially caused by the extended expansion regime of the failed rift near the south-most termination.

Lithospheric electrical structure across the Bangong-Nujiang Suture in northern tibet revealed by magnetotelluric

Competing hypotheses have been proposed to explain the subduction polarity of the Bangong-Nujiang Tethyan Ocean and the formation of the high-conductivity anomaly beneath the Qiangtang terrane. However, the lithospheric architecture of the northern Tibetan Plateau is still poorly understood due to inhospitable environments and topography. Therefore, in the winter of 2021, a 440 km long, SN-trending broadband magnetotelluric (MT) profile was recorded in northern Tibet to detect its regional lithospheric structure. The nonlinear conjugate gradients algorithm is conducted to invert the individual TM mode data. A reliable 2D electrical model was obtained by ablation processing and analysis of broadband magnetotelluric data to test the lithospheric electrical structure and dynamics between the northern Lhasa and Qiangtang terranes. The inversion results reveal the lithospheric structure at a depth of 100 km in northern Tibet, which synthesizes geological, geochemical and deep seismic reflection evidence and firmly identifies that the trace of the south-dipping conductor mainly resulted from the southward subduction of the Bangong-Nujiang Tethyan Ocean under the Lhasa terrane and the trace of the north-dipping conductor likely due to the northward subduction of the Bangong-Nujiang Tethyan Ocean under the Qiangtang terrane. In addition, the magnetotelluric profile also images a high-conductivity lithospheric-scale anticline beneath the central Qiangtang terrane, which may correspond to the upwelling of postcollisional magmatism triggered by northward subduction of the Bangong-Nujiang Tethyan Ocean under the Qiangtang terrane, aqueous fluid and/or partial melting.

Lithospheric structure of the Dharwar Craton (India) from joint analysis of gravity, topography, and teleseismic travel-time residuals

• Effective elastic thickness of the Dharwar Craton varies between 12 and 35 km. • Both crust and lithospheric mantle contribute in supporting topography load. • Western Dharwar craton is thicker than the eastern Dharwar craton. • A Jelly-Sandwich type lithospheric architecture is possibly applicable in the Dharwar craton. The evolution and persistence of an uplifted surface along a passive continental margin is a long-standing enigma. The long-term vertical strains generated by such surface topographic loads have to be supported and maintained by the lithosphere's elastic strength. We estimate the lithospheric strength indicated by its effective elastic thickness (Te) and associated regional lithospheric structure along an ENE-WSW profile across the Dharwar craton-western Ghats (elevation ∼1200 m). We employ a joint analysis of teleseismic travel time residuals, Bouguer gravity, and topographic data. Our results suggest the topographic load of the Western Ghats is supported by both the Archean Dharwar crust and the lithospheric mantle (∼40 %), and indicate the possibility of a Jelly Sandwich type of rheology operating in this region. However, towards the eastern section of the profile, where the terrain flattens out, the contribution of the lithospheric mantle in maintaining the topography is observed to be uncertain due to resolvability of the method. Further, we infer the presence of ∼20–30 km thicker lithosphere beneath the western Dharwar craton (WDC) than the eastern Dharwar craton (EDC). The obtained Te values varies from ∼12 km (in the west of the WDC) to ∼32 km (towards the east of the EDC). These Te estimates are in agreement with previous studies from the region and are much lower than those for the more recent geological terrains in the subcontinent, such as the Gangetic plains and the Himalaya orogen.

New Insights on Moho Depth and Regional Lithospheric Structure of Central Oklahoma Based on Receiver Function Analysis from Dense Seismic Networks

New Insights on Moho Depth and Regional Lithospheric Structure of Central Oklahoma Based on Receiver Function Analysis from Dense Seismic Networks

Kinematic variability of late Cenozoic fault systems and contrasting mountain building processes in the Alxa block, western China

Abstract In this study, new structural field data and geological observations are coupled with remote sensing analysis and compilation of previous published work to establish a new kinematic framework for the neotectonic reactivation of the Alxa block, China. The Alxa block occupies a key position in Central Asia north and east of the contractional and sinistrally transpressive deforming zones of the Qilianshan-Hexi Corridor-easternmost Altyn Tagh Fault and Beishan deforming belts, south of the sinistrally transpressive southernmost Gobi Altai, and west of – or overlapping with - Ordos block extensional and transtensional graben systems. In addition, the Alxa block is internally transected by important fault systems (Bayan Nuru, Bayanwula, Chahanbulage) that record previously unrecognized Late Cenozoic activity. Active mountain building processes in the Alxa block show remarkable variation from large extensional horst blocks (Helanshan, SW Langshan, SW Yabrai) to localized uplifts along sinistral wrench-belts (Bigesitai) to dominantly contractional (Longshoushan) and sinistral transpressional ranges (Helishan). The triangular wedge-shape of the Alxa block likely pre-disposed it to eastward extrusion in response to NE-directed India-Eurasia collision-derived compressional stresses from the south, and E-W tensional forces driven by Pacific margin rollback to the east. A combination of inherited regional lithospheric structure and the modern east-directed geodetically derived crustal velocity field led to: 1) crustal thinning and subsidence of the Ejina basin to the west in the wake of the eastward extruding Alxa block, and 2) distributed sinistral transtension and crustal extension within the Alxa block interior, and in the Hetao and Yinchuan grabens to the east.

Thin lithosphere beneath the central Appalachian Mountains: A combined seismic and magnetotelluric study

A joint analysis of magnetotelluric and Sp receiver function data, collected along a profile across the central Appalachians, highlights variations in regional lithospheric structure. While the interpretation of each data set by itself is non-unique, we identify three distinct features that are consistent with both the resistivity model and the receiver function image: 1) thin lithosphere beneath the Appalachian Mountains, 2) somewhat thicker lithosphere to the east of the mountains beneath the Coastal Plain, and 3) a lithosphere-asthenosphere boundary that deepens to the west of the mountains. In some regions, the correspondence between seismic velocity discontinuities and resistivity mark the base of the lithosphere, while in other locations we see seismic discontinuities that are contained within the lithosphere. At the western end of our profile a transition from highly resistive lithosphere to more conductive mantle represents the transition across the Grenville front. The thickness of lithosphere beneath the Grenville terrain is ∼140 km. Lithosphere at the eastern end of the profile has a thickness that is not well constrained by our coverage, but is at least 110 km thick. This lithosphere can be associated with a broader region of high resistivity material seen to extend further south. Directly beneath the Appalachian Mountains, lithospheric thickness is inferred to be as thin as ∼80 km, based on observations of elevated mantle conductivities and a westward-dipping seismic converter. Electrical conductivities in the uppermost asthenospheric mantle are sufficiently high (>0.1 S/m) to require the presence of a small volume of partial melt. The location of these elevated conductivities is close (offset ∼50 km to the west) to Eocene volcanic outcrops in and around Harrisonburg, VA. Our observations speak to mechanisms of intraplate volcanism where there is no divergent or convergent plate motion to trigger mantle upwelling or obvious fluid release, either of which can facilitate melting. Instead, we suggest that small scale mantle convection related either to pre-existing lithospheric thickness variations, or to lithospheric loss through delamination, coupled with relative plate motion with respect to the underlying asthenosphere, can trigger small amounts of melting. This melt migrates upslope, along the base of the lithosphere, potentially thermally eroding the lithosphere resulting in further thinning.

Symmetry during the syn- and post-rift evolution of extensional back-arc basins: The role of inherited orogenic structures

Rheological heterogeneities in the lithosphere have first order control on the topographical expression of tectonic processes. Pre-existing orogenic suture zones localise extensional deformation resulting in asymmetric basins. Such crustal geometries are often in contrast with the more symmetric regional lithospheric structure observed beneath extensional basins. We study such (a)symmetries and their controlling parameters by conducting a series of 2D thermo-mechanical numerical experiments of the extension of an overthickened, hot lithosphere that contains a weakness zone. The modelling shows that syn-rift subsidence is low to moderate creating asymmetric half grabens where extension migrates in space and time, grouped in an overall symmetrical appearance on a larger scale. The initial lithospheric mantle asymmetry is attenuated by the lateral heat conduction and further dynamic evolution of the thermal anomaly during the “post-rift” phase, resulting in differential vertical movements of the crust including additional 2–3 km subsidence in the basin centre. The modelling shows that the initial crustal and lithospheric thicknesses, rate of extension and surface processes strongly control the thermo-mechanical evolution of the extensional system. The numerical modelling yields new insights into the mechanics of coupling between near-surface kinematics and the evolution of deep lithospheric structure in the Pannonian basin and the Aegean, two of Europe's largest back-arc systems.

LITHOSPHERIC STRUCTURE OF THE WEST AND CENTRAL AFRICAN RIFT SYSTEM FROM REGIONAL THREE-DIMENSIONAL GRAVITY MODELLING

A three-dimensional interpretation of the central part of the West and Central African Rift System (WCARS) is presented using the Earth Gravity Model 2008 (EGM2008). The 3-D model presented here provides new insights into the regional lithospheric structure of the central sector of the WCARS. The 3-D model reveals a possible crustal thickness and density distribution beneath the rift system, and the depth extent of magmatic activity in the Benue Trough is revealed for the first time. Because of asthenospheric uplift, the crust under the Benue Trough is thinned. The zone of crustal thinning (<30 km) coincides with the trace of an intrusion beneath the trough, and the maximum of which is attained in the Yola Basin. The Adamawa and Kapsiki Plateaus, on the other hand, show by far the largest crustal thickness in the region (34 to 36 km). The inferred zone of intrusion coincides with the maximum gravity anomaly of the rift zone. The great depth (ca. 14 km) to the top of the intrusion along the rift may explain the absence of magmatism in the Benue Trough compared to the Cameroon Volcanic Line (CVL) or the East African Rift System (EARS). The relatively small nature of the intrusion leads to the conclusion that small-scale asthenospheric upwelling might be responsible for the thinning of the crust and subsequent rifting of the Benue Trough.

Receiver Function Imaging with Reconstructed Wavefields from Sparsely Scattered Stations

Receiver function imaging is an important tool for imaging the local and regional lithospheric structure beneath seismic arrays. The spatial density of stations and uniformity of their spacing in a given study area can strongly influence image quality. Sparsely distributed stations may produce distorted images and artifacts due to uneven spatial sampling. Reconstructing and resampling seismic wavefields to produce virtual stations within a regular, fine‐scale observation system can provide a more accurate image with fewer artifacts. We developed a wavefield reconstruction method to accurately map teleseismic data from a sparsely spaced seismic network into a regular and fine‐scale observation system. The method reconstructs wavefields in each time slice using a 2D cubic‐spline interpolation. As a preprocess step, wavefield reconstruction can significantly improve the receiver function by greatly reducing stair‐step artifacts for dipping structures. We applied the method to data from the Ordos regional network, which has a station interval of ∼30–80 km. The virtual stations rendered image results that showed almost perfect consistency with images produced by a fine‐scale temporary array having uniform station interval of 10 km. Numerical experiments show the wavefield reconstruction method improved lithospheric images derived from traditional receiver function imaging techniques. Receiver function imaging is routinely used to identify velocity discontinuity structures in the crust and upper mantle. Receiver functions are obtained by deconvolving vertical components from horizontal components of teleseismic records (Langston, 1979). Irregularities in distributions of both seismic stations and earthquakes pose challenges for receiver function imaging techniques. Based on theoretical seismograms from 2D models, Ryberg and Weber (2000) deduced that resolving structures of similar scale requires a station spacing of a few kilometers. This result demonstrates the best way to improve receiver function image quality is to increase station density. Station spacings of a few kilometers are rare, due to the high costs of their …

Images of the Iberian Lithosphere from One Local Earthquake

Abstract On 12 August 2007, a magnitude M w 4.7 intraplate earthquake occurred near the center of the Iberian Peninsula, an area characterized by comparably simple crustal structure within the complex Iberia–Maghreb plate boundary zone, and characterized by complete azimuthal coverage with seismic broadband stations. We analyze regional intermediate-period (20- to 50-s) coda waveforms for this earthquake. They contain energy representing late-arriving surface waves that have been reflected laterally at lithospheric heterogeneities in or around Iberia, but complexity of the coda waveforms hampers a direct interpretation. We use coda recordings as the source for a back-propagating adjoint wave field and compute 3D Born sensitivity kernels for the dependence of least-squares waveform misfits of coda waves on wave speed variations. We hereby image the origin of single scattering recorded in the coda. Misfit kernels for P - and S -wave velocity show azimuth-dependent intensity variation as a result of source radiation, and an appropriate compensation significantly improves imaging quality, thereby revealing several clear lineaments. These are interpreted as surface-wave reflectors due to deep-rooted heterogeneity such as terrain boundaries or Moho topography, demonstrating the ability of the approach to unravel complex waveforms, and providing a new point of view on regional lithospheric structure.

Lateral lithospheric conductivity patterns, from European synchronous observatory data

Much information on the regional lithospheric structure may come from MTS data acquired by synchronous 2D arrays and processed with regard to the nonlocal response of a laterally inhomogeneous subsurface. We suggest to invert the nonlocal MT responses applying correlation of all surface horizontal and vertical components of the geomagnetic field recorded simultaneously at all stations. The inversion algorithm has been applied to 2004–2005 European observatory data of diurnal S q variations for first five harmonics and yielded lateral conductivity patterns for different periods. The maps show spatial correlation between conductivity maxima and lithospheric thickness minima and, specifically, highlight the contours of the Pannonian basin, where lithosphere is as thin as ∼50 km, from seismic data.

Lithospheric structure and continental geodynamics

This paper briefly reviews main progress in the research on lithospheric structure and continental geodynamics made by Chinese geophysicists during last 4 years since 22nd IUGG general assembly in July 1999. The research mainly covers the following fields: investigations on regional lithospheric structure, DSS survey of crust and upper mantle velocity structure, study on present-day inner movement and deformation of Chinese mainland by analyzing GPS observations, geodynamics of Qingzang plateau, geophysical survey of the Dabie-Sulu ultra-high pressure metamorphic belt and probing into its formation mechanism, geophysical observations in sedimentary basins and study on their evolution process, and plate dynamics,etc.

Shear response to an intrusive episode at Mt. Etna volcano (January 1998) inferred through seismic and tilt data

The space–time evolution of a seismic swarm (MDmax=3.7) occurring on 9–14 January 1998 at Mt. Etna volcano is analysed. The hypocentral pattern and most reliable focal mechanisms indicate that seismic activity during 10–14 January mainly occurred within a fault-zone located in the volcano's western flank. The fault showed an approximately NE–SW overall trend, and shallow rupture mode of right-lateral strike-slip type. It is noteworthy that earthquakes occurring during the initial phase (9 January) of the swarm showed prevalent dip–slip rupture mechanisms, which are consistent with tensile opening of an approximately NNW–SSE-trending fault-zone. This NNW–SSE structural lineament belongs to a regional lithospheric structure, which has also been identified as a main volcano-genic structural trend of Mt. Etna. The observed continuous tilt data are not explainable by the seismic slip associated with strain released along the NE–SW structure. However, an intrusive source would be compatible with the rupture mechanisms observed during the initial phase of the swarm and could explain the observed tilt. Therefore, we evidence that an intrusive episode occurred along the NNW–SSE structure starting 9 January and during the following days it forced the NE–SW fault system to work in a right-lateral mode. These mechanisms, leading to the February–November 1999 summit eruption, are analogous to the ones observed before the 1991–93 eruption.

An integrated analysis of lithospheric structure across the East African plateau based on gravity anomalies and recent seismic studies

Data from recent gravity surveys carried out by the University of Texas at El Paso (USA) and other institutions and existing data were combined and integrated with seismic results from the KRISP project and geologic data to analyze the regional lithospheric structure beneath the East African plateau. A series of gravity maps were constructed, and they show that the East African plateau is associated with a broad negative gravity anomaly which is about 1200 ± 100 km wide and which has an amplitude of −150 ± 20 mGal. Superimposed on this anomaly are narrower, steeper-sided negative anomalies about 200 km wide with amplitudes of −30 to −60 mGal. These anomalies are associated with the eastern and western arms of the East African rift. These gravity maps also show low gravity gradients in the Tanzanian craton, Kasai craton and Congo basin areas while there are high gravity gradients within the rifts and at the mobile belt-craton boundaries. With increased filter wavelength cutoff, the rift-related gravity lows broaden and eventually merge into one broad low whose apex is centered on Lake Victoria. Seismic refraction, teleseismic, mantle xenolith, petrochemical and geologic data were used as constraints during the construction of a series of gravity models which represent integrated models of lithospheric structure across the East African plateau. Three W-E profiles across the plateau and one N-S profile along the axis of the Eastern rift in Kenya and Tanzania were modeled. These models together with the filtered gravity maps demonstrate lithospheric thinning beneath the rifts and suggest the existence of a deep mantle anomaly centered beneath the East African plateau and the Tanzanian craton. The mantle anomaly is widely believed to be a plume and our models indicate that the diameter of its head is about 600 km. Two arms with diameters less than 250 km extend from the plume head and penetrate the lithosphere to shallow levels under the Eastern and Western rifts. In the Western rift and the southern part of the Eastern rift (Tanzania), these features are deeper than under Kenya. This study provides additional evidence that the long wavelength Bouguer gravity anomaly over the East African plateau can be explained by lithospheric heating and thinning. The model along the axis of the Eastern rift suggests that it is propagating southwards into central Tanzania which is also suggested by the age of initiation of volcanism.