Lithospheric Flexure Research Articles

Tectono‐sedimentary evolution of transverse extensional faults in a foreland basin: Response to changes in tectonic plate processes

AbstractLate Paleocene to Middle Eocene strata in the easternmost part of the Southern Pyrenees, up to 4 km thick, provide information on tectono‐sedimentary evolution of faults transversal to the Pyrenean chain. To know how changes in tectonic plate processes control the structural evolution of transverse faults and the synchronous thickness and lithological distribution of sedimentary strata in a foreland basin, field observations, interpretation of 2D seismic lines tied to lithostratigraphic data of exploration wells and gravity modelling constrains were carried out. This resulted in the following two tectono‐sedimentary phases in a foreland basin: first phase, dominated by transverse extensional faulting, synchronous with deposition of marine carbonates (ca. 57 to 51 Ma); and second phase, characterized by transverse contractional faulting, coeval to accumulation of marine and transitional siliciclastics (51 to 44 Ma). During the first phase, Iberia and Adria were moving to the east and west respectively. Therefore, lithospheric flexure in the easternmost part of the Iberian plate was developed due to that Sardinia was over‐thrusting Iberia. Consequently, activation of E‐dipping normal faults was generated giving rise to thick‐deep and thin‐shallow carbonate platform deposits across the hanging walls and footwalls of the transverse structures. During the second phase, a shearing interaction between Iberia and Sardinia prevailed re‐activating the transverse faults as contractional structures generating thin‐shelf and thick‐submarine fan deposits across the hanging walls and footwalls of the transverse structures. In the transition between the first and second phases, evaporitic conditions dominated in the basin suggesting a tectonic control on basin marine restriction. The results of our study demonstrate how thickness and lithology distribution, controlled by transverse faulting in a compressional regimen, are influenced by phases related to processes affecting motions and interactions between tectonic plates and continental blocks.

Fading magnetic anomalies, thermal structure and earthquakes in the Japan Trench

Abstract Early magnetic studies of the Japan Trench showed that seafloor spreading magnetic anomalies progressively fade away and disappear during subduction, reflecting the increasing distance to magnetized sources and the removal of their remanent magnetization with alteration and increasing temperature. An improved magnetic anomaly map derived from both scalar and vector magnetic anomaly data, coupled with a better knowledge of the slab geometry in one hand, of the magnetic structure of the oceanic crust on the other hand, allow us to constrain the thermal structure of the subducting slab. We, for the first time, identify two steps in the anomaly disappearance: first the magnetization of extrusive basalt is rapidly erased between 9 and 12 km, where titanomagnetite reaches its blocking temperature between 150 °C and 350 °C, then the magnetization of deeper crustal layers slowly decreases down to ∼20 km, reflecting the progressive slab heating toward the Curie temperature of magnetite, 580 °C. The resulting slab temperatures are higher than predicted by most thermal models. Recent observations and models suggest rejuvenated hydrothermal activity triggered by lithospheric flexure before subduction that may significantly heat up the subducting oceanic crust through thermal blanketing and possibly serpentinization, with consequences on the depth of the seismogenic zone.

The influence of low‐density granite bodies on extensional basins

The Carboniferous northern Pennine Basin remains the type locality for the ‘block and basin’ tectonic framework model. It has been widely believed that during periods of tectonic extension, large low‐density bodies within the basement permit buoyant blocks to resist isostatic subsidence. However, lithosphere‐scale structural and geodynamic modelling experiments dispute this; suggesting instead that the formation of intra‐basinal highs occurs prior to lithospheric extension. In northern UK, this tectonic framework is controlled by a combination of tectonic stress, isostasy and the buoyancy forces of low‐density granite, lithospheric flexure and, importantly, the inherited structural framework. It is hoped that further study can lead to a greater appreciation of the interplay of structural and geodynamic process that control the ‘block and basin’ framework.

Geometry of Flexural Uplift by Continental Rifting in Corinth, Greece

AbstractUnderstanding early rifting of continental lithosphere requires accurate descriptions of up‐bended rift margins and footwalls that ought to correlate in space and time with the elastic flexural uplift that produces them. Here we characterize the geometry of elastic flexural uplift by continental rifting at its spatiotemporal scale in nature (tens of kilometers; 104–106 years) using geomorphic evidence along the uplifting margin of the Corinth Rift, Greece. Our geomorphic analyses of space‐borne topography novelly outline the coherent elastic flexure of continental lithosphere along and across the rift margin and throughout faulting (~106 years), as defined by the distribution of footwall uplift south of the active bounding fault. Topography and river drainages outline an elastic flexure signal that increases exponentially toward the bounding fault across the footwall for >50 km and changes in amplitude along the footwall following a parabola that decays from the rift center and has a >60‐km wavelength that correlates with rift length. This continental lithosphere up‐bend correlates with the scale of the rift, and appears maximum in the center of the rift, where drainage reversal of large catchments suggests rapid slip rates at the bounding fault. This is consistent with the growth of a new, rift‐scale, high‐angle normal fault. The coherency of elastic flexure in space and time implies highly localized strain in the rift‐bounding fault and suggests that the fault transects continental lithosphere with long‐term strength. The unparalleled record of flexural uplift and highly localized strain in the landscape of Corinth suggest these processes are intrinsic to early continental rifting elsewhere.

Why Are There No Earthquakes in the Intracratonic Paris Basin? Insights from Flexural Models

Comparing nearby areas with contrasted seismicity distributions like the French Variscan Armorican Massif (AM) and the surrounding intracratonic Paris Basin (PB) can help deciphering which parameters control the occurrence or absence of diffuse, intraplate seismicity. In this paper, we examine how lithosphere temperature, fluid pressure, and frictional strength variations, combined with horizontal and bending stresses, may condition brittle, ductile or elastic behaviours of the crust in the AM and PB. We compute yield stress envelopes (YSE) and lithospheric flexure across a 1000 km-long SW–NE profile crossing the AM and PB approximately parallel to the direction of the minimum horizontal stress. Flexural models slightly better fit measured Bouguer gravity data if we apply two vertical loads on the AM and PB, with values (positive downward) ranging between −3 and −2.1012, and between 4 and 6.1012 N·m−2, respectively, depending on the chosen crustal composition. Our results evidence that whatever the crustal composition, bending stresses and heat flow variations alone are not sufficient to explain the difference in seismogenic behaviour between the AM and the PB. Variations in friction coefficient, in the range of standard values, are not totally satisfying either, since they do not restrain the brittle crustal thickness in the PB to less than 10 km, which is still large enough to be the locus of shallow earthquakes. Oppositely, increasing the cohesion from 10 to 80 MPa has a stronger effect on the thickness of the brittle upper crust, decreasing it from 10 to 15 km beneath the AM to 0–5 km beneath the PB. This suggests that the Mesozoic sedimentary pile can act as a sticky layer holding together basement rocks of the PB, which is equivalent to an increase in cohesion, and protects them from failure.

Thermo-mechanical and stratigraphic numerical forward modelling: recent advances and their joint application in the Pannonian Basin

Basin analysis and subsidence history provide key insights into sedimentary basin forming mechanisms. Direct observations have long been the only source of information on their thermal and lithological architecture. State of the art modelling techniques today enable the prediction and computation of their formation and evolution constrained by geological field observations, geophysical and deep borehole data. Understanding the inherent connections between large-scale tectonic and local basin-scale surface processes requires the joint application of thermo-mechanical and stratigraphic modelling techniques. To this aim, we combined the thermo-mechanical lithospheric-scale numerical code Flamar and the high-resolution 3D deterministic stratigraphic software DionisosFlow. This joint modelling method quantifies forcing factors, such as crustal and lithospheric thinning, lithospheric flexure, sea-level and climatic variations associated with water and sediment influx and sediment compaction. The modelling shows the migration of extensional deformation in space and time creating deep half-grabens. After a rapid uplift event, the subsequent post-rift times are characterized by continuous kilometre-scale differential vertical movements. The modelled tectonic subsidence and uplift rates and half-graben geometries are imported into the 3D stratigraphic modelling code. Our modelling of a 120 km × 150 km area shows that such scenarios are associated with continental alluvial to shallow-water sedimentation and footwall erosion during the early stages of the syn-rift, followed by rapid deepening during the subsequent syn-rift evolution. Finally, the basins are filled by a large-scale prograding shelf-margin slope system during the post-rift times. We differentiate between unconformities caused by tectonics, sea-level variations or auto-cyclic processes. Our tectonic and stratigraphic results are compared with geological and geophysical constraints from the Pannonian Basin of Central Europe.

The Gravitational Signature of Martian Volcanoes

AbstractBy modeling the elastic flexure of the Martian lithosphere under imposed loads, we provide a systematic study of the old and low‐relief volcanoes (>3.2 Ga, 0.5 to 7.4 km) and the younger and larger prominent constructs within the Tharsis and Elysium provinces (<3 Ga, 5.8 to 21.9 km). We fit the theoretical gravitational signal to observations in order to place constraints on 18 volcanic structures. Inverted parameters include the bulk density of the load, the elastic thickness required to support the volcanic edifice at the time it was emplaced (Te), the heat flow, the volume of extruded lava, and the ratio of volcanic products that form within (Vi) and above the preexisting surface (Ve). The bulk density of the volcanic structures is found to have a mean value of 3,206±190 kg/m3, which is representative of iron‐rich basalts as sampled by the Martian basaltic meteorites. Te beneath the small volcanoes is found to be small, less than 15 km, which implies that the lithosphere was weak and hot when these volcanoes formed. Conversely, most large volcanoes display higher values of Te, which is consistent with the bulk of their emplacement occurring later in geologic history, when the elastic lithosphere was colder and thicker. Our estimates for the volumes of volcanic edifices are about 10 times larger than those that neglect the flexure of the lithosphere. Constraints on the magnitude of subsurface loads imply that the ratio Vi/Ve is generally 3:5, which is smaller than for the Hawaiian volcanoes on Earth.

Mechanism of rift flank uplift and escarpment formation evidenced by Western Ghats, India

The Western Ghats is one of the largest escarpments on earth, containing Reunion plume derived Deccan Traps, it is an excellent example to probe epeirogenic uplift, extension and subsidence in volcanic continental margins. The most continuous unbiased stratigraphic section of basalt down to the basement within a 1250 m drill hole of the Continental Scientific Deep Drilling Project is a valuable resource to investigate the above aspects. The flows across the entire drill core are geologically subaerial in character with basement exposed ~300 m below the mean sea level; they clearly display more evolved compositions from primary melts of mantle in terms of petrology, and only a single geomagnetic polarity transition in palaeomagnetic data. These results, combined with existing geological and geophysical data, constitute a multi-method approach that demonstrates (a) igneous underplating caused uplift prior to frequently suggested flexural isostasy (b) plume impact and eruption are near-simultaneous and extension/rifting essentially followed soon after volcanism and (c) lithosphere beneath the continental margin, while returning to normal temperatures following the Seychelles-India breakup, experienced thermal collapse and subsidence causing slumping of basalt basement below sea level.

New insights from low-temperature thermochronology into the tectonic and geomorphologic evolution of the south-eastern Brazilian highlands and passive margin

The South Atlantic passive margin along the south-eastern Brazilian highlands exhibits a complex landscape, including a northern inselberg area and a southern elevated plateau, separated by the Doce River valley. This landscape is set on the Proterozoic to early Paleozoic rocks of the region that once was the hot core of the Araçuaí orogen, in Ediacaran to Ordovician times. Due to the break-up of Gondwana and consequently the opening of the South Atlantic during the Early Cretaceous, those rocks of the Araçuaí orogen became the basement of a portion of the South Atlantic passive margin and related south-eastern Brazilian highlands. Our goal is to provide a new set of constraints on the thermo-tectonic history of this portion of the south-eastern Brazilian margin and related surface processes, and to provide a hypothesis on the geodynamic context since break-up. To this end, we combine the apatite fission track (AFT) and apatite (U–Th)/He (AHe) methods as input for inverse thermal history modelling. All our AFT and AHe central ages are Late Cretaceous to early Paleogene. The AFT ages vary between 62 Ma and 90 Ma, with mean track lengths between 12.2 μm and 13.6 μm. AHe ages are found to be equivalent to AFT ages within uncertainty, albeit with the former exhibiting a lesser degree of confidence. We relate this Late Cretaceous–Paleocene basement cooling to uplift with accelerated denudation at this time. Spatial variation of the denudation time can be linked to differential reactivation of the Precambrian structural network and differential erosion due to a complex interplay with the drainage system. We argue that posterior large-scale sedimentation in the offshore basins may be a result of flexural isostasy combined with an expansion of the drainage network. We put forward the combined compression of the Mid-Atlantic ridge and the Peruvian phase of the Andean orogeny, potentially augmented through the thermal weakening of the lower crust by the Trindade thermal anomaly, as a probable cause for the uplift.

Sill complexes in the Karoo LIP: Emplacement controls and regional implications

Field and sub-surface data from the Victoria West sill complex in the Karoo Large Igneous Province (ca. 180 Ma) of South Africa are used to constrain the emplacement controls of the regional-scale sill complexes in the central Karoo basin. Cross-cutting relationships point to the presence of five distinct and successively emplaced saucer-shaped sills. Growth of the sill complex was achieved through magmatic underaccretion of magma batches below earlier sills and associated uplift of the overlying strata. The magmatic underaccretion suggests that earlier sills were fully crystallized during the emplacement of later magma pulses and that the rigid (high E) dolerites, in particular, acted as stress barriers that impeded further upward propagation of steep feeder sheets. The resulting nested structure of sills-in-sills within a confined area of less than 2000 km2 also suggests the reutilization of the same or similar feeder system even after full crystallization thereof.The emplacement controls of sills in the central Karoo through stress barriers implies that sill emplacement occurred under very low deviatoric stresses or in a mildly compressional stress regime prior to the break-up of Gondwana. The swap from earlier (184-180 Ma), mainly sill complexes to later (182-174 Ma) dykes and dyke swarms is indicative of a switch in the stress field during the early stages of Gondwana break-up. We speculate that loading, thermal subsidence and lithospheric flexure associated with the emplacement of the earlier, stacked and voluminous sill complexes in the Karoo basins may have determined the formation of the large Karoo dyke swarms, particularly when coinciding with deeper crustal structures. The original and inherited basin geometry and lithospheric structure is pivotal in the development of later Karoo magmatism.

Numerical Modeling of Weathering, Erosion, Sedimentation, and Uplift in a Triple Junction Divergent Margin

AbstractThe majority of numerical models of landscape evolution in divergent margins are focused on the simulation of margins with simplified lithological control on landscape erosion. However, this approach is insufficient to study the evolution of margins where chemical weathering is an important element increasing rock resistance to physical erosion. One example of this margin is the Borborema Province, northeastern Brazil, where postrift marine sediments are now preserved at elevations ∼700–800 m in up to 1‐km‐high plateaus capped by duricrust layers. The landscape evolution of these uplifted sedimentary basins still eludes explanation. Here we use numerical models that couple weathering, erosion, sedimentation, sea level changes, flexural isostasy, and thermal effects due to lithospheric stretching to simulate the tectonosedimentary evolution of the Borborema Province since the onset of continental stretching during the Lower Cretaceous. These numerical experiments reveal that nearly 70% of the postrift regional uplift observed in the Borborema Province can be explained by differential denudation of the continent and flexural rebound of the lithosphere. The remaining ∼250 m of uplift can be explained by thermal uplift induced by partial erosion of the base of the continental lithosphere under the Borborema Province due to edge‐driven convection, in accordance with the anomalously thin continental lithosphere observed under the Borborema Province. Additionally, the numerical results can explain the regional pattern of fission track ages by the combined effect of differential denudation and flexural rebound in this geometrically complex margin.

Evolving paleotopography and lithospheric flexure of the Pyrenean Orogen from 3D flexural modeling and basin analysis

We present the results of a numerical modeling study of the Pyrenees Mountains investigating the spatio-temporal variation in lithospheric flexure in response to the developing orogen, with the aim of setting constraints on paleotopography. We employ a finite-element method to model the 3D flexural deformation of the lithosphere beneath the Pyrenean orogen since the onset of convergence in the Late Cretaceous. Using subsurface and structural data, we describe the evolving geometry of both the northern Aquitaine and southern Ebro foreland basins at the Paleocene (early orogenic phase), the end- and mid-Eocene (peak orogenic phase), the Oligocene (late orogenic phase), and the present (post-orogenic phase). The flexural modeling provides insight into how both the rigidity of the lithosphere and the paleotopographic load have varied over the course of orogenesis to shape the basin geometry. Employing a 3D continuous-plate model, we find that the overriding European plate has slightly higher rigidity than the underthrusting Iberian plate. The best-fit model results for the modern setting produce a root-mean-square error (RMSE) of 458 m using effective elastic thickness (Te) values of 23 and 16 km for the European and Iberian plates, respectively. We also test a broken-plate model, which produces a similar RMSE but predicts higher Te (32 km for the European plate and 26 km for the Iberian plate). The broken-plate model results indicate minimal vertical plate-boundary forces are necessary to reproduce the observed basin geometry. Flexural modeling results suggest that the topographic load doubled from the Paleocene to the Oligocene, and achieved modern topography by the end of the Eocene. The topography remained relatively stable throughout the Oligocene and Neogene, with only limited growth and decay. These results have implications for surface processes and foreland-basin development of the Pyrenean Orogen, tectonic inheritance, and the geodynamic evolution of Western Europe.

Lithospheric flexure and rheology determined by climate cycle markers in the Corinth Rift

Geomorphic strain markers accumulating the effects of many earthquake cycles help to constrain the mechanical behaviour of continental rift systems as well as the related seismic hazards. In the Corinth Rift (Greece), the unique record of onshore and offshore markers of Pleistocene ~100-ka climate cycles provides an outstanding possibility to constrain rift mechanics over a range of timescales. Here we use high-resolution topography to analyse the 3D geometry of a sequence of Pleistocene emerged marine terraces associated with flexural rift-flank uplift. We integrate this onshore dataset with offshore seismic data to provide a synoptic view of the flexural deformation across the rift. This allows us to derive an average slip rate of 4.5–9.0 mm·yr−1 on the master fault over the past ~610 ka and an uplift/subsidence ratio of 1:1.1–2.4. We reproduce the observed flexure patterns, using 3 and 5-layered lithospheric scale finite element models. Modelling results imply that the observed elastic flexure is produced by coseismic slip along 40–60° planar normal faults in the elastic upper crust, followed by postseismic viscous relaxation occurring within the basal lower crust or upper mantle. We suggest that such a mechanism may typify rapid localised extension of continental lithosphere.

The Role of Lithospheric Flexure in the Landscape Evolution of the Wilkes Subglacial Basin and Transantarctic Mountains, East Antarctica

AbstractReconstructions of the bedrock topography of Antarctica since the Eocene‐Oligocene Boundary (approximately 34 Ma) provide important constraints for modeling Antarctic ice sheet evolution. This is particularly important in regions where the bedrock lies below sea level, since in these sectors the overlying ice sheet is thought to be most susceptible to past and future change. Here we use 3‐D flexural modeling to reconstruct the evolution of the topography of the Wilkes Subglacial Basin (WSB) and Transantarctic Mountains (TAM) in East Antarctica. We estimate the spatial distribution of glacial erosion beneath the East Antarctic Ice Sheet, and restore this material to the topography, which is also adjusted for associated flexural isostatic responses. We independently constrain our post‐34 Ma erosion estimates using offshore sediment stratigraphy interpretations. Our reconstructions provide a better‐defined topographic boundary condition for modeling early East Antarctic Ice Sheet history. We show that the majority of glacial erosion and landscape evolution occurred prior to 14 Ma, which we interpret to reflect more dynamic and erosive early ice sheet behavior. In addition, we use closely spaced 2‐D flexural models to test previously proposed hypotheses for a flexural origin of the TAM and WSB. The pre‐34 Ma topography shows lateral variations along the length of the TAM and WSB that cannot be explained by uniform flexure along the front of the TAM. We show that some of these variations may be explained by additional flexural uplift along the south‐western flank of the WSB and the Rennick Graben in northern Victoria Land.

Correlation Between Graben Orientation, Channel Direction Change, and Tectonic Loading: The Elysium Province, Mars

AbstractWe have investigated the links between regional stress fields, the volcanic centers, rifts, graben, and channels in the NW region of the Elysium Province (Figures a and b) to determine whether the sequence of stress events occurring during province development can be derived from the morphologies of these features, and thus provide a sequence of development events, which is independent of surface dating techniques. Rift and graben geomorphology was mapped, and the neighboring relationships and orientation of individual graben were assessed to determine any spatial clustering or preferred orientation with regional or surface features capable of creating lithospheric flexure or tectonic stress within the study area. Crosscutting analysis determined a time ordered sequence of graben formation and these were related to volcanic centers or regional sources of stress. In addition, mapping showed that different channels share sections with similar shape and orientation, prompting our study of whether these channels, in tandem with the graben, were tectonically influenced during their development. The channel central axes were mapped and compared to identify common sequences of channel direction change. The time sequence of channel direction changes and the time‐ordered sequence of graben development were then compared. We have demonstrated a correlation between rift and graben direction with channel orientation suggesting a regional stress control from evolving volcanic centers. Overall we derive, for the first time, the temporal pattern of tectonic, volcanic, and channel evolution for the northwestern region of this major magmatic province on Mars.

Correlation between tectonic stress regimes and methane seepage on the western Svalbard margin

Abstract. Methane seepage occurs across the western Svalbard margin at water depths ranging from < 300 m, landward from the shelf break, to > 1000 m in regions just a few kilometres from the mid-ocean ridges in the Fram Strait. The mechanisms controlling seepage remain elusive. The Vestnesa sedimentary ridge, located on oceanic crust at a depth of 1000–1700 m, hosts a perennial gas hydrate and associated free gas system. The restriction of the occurrence of acoustic flares to the eastern segment of the sedimentary ridge, despite the presence of pockmarks along the entire ridge, indicates a spatial variation in seepage activity. This variation coincides with a change in the faulting pattern as well as in the characteristics of the fluid flow features. Due to the position of the Vestnesa Ridge with respect to the Molloy and Knipovich mid-ocean ridges, it has been suggested that seepage along the ridge has a tectonic control. We modelled the tectonic stress regime due to oblique spreading along the Molloy and Knipovich ridges to investigate whether spatial variations in the tectonic regime along the Vestnesa Ridge are plausible. The model predicts a zone of tensile stress that extends northward from the Knipovich Ridge and encompasses the zone of acoustic flares on the eastern Vestnesa Ridge. In this zone the orientation of the maximum principal stress is parallel to pre-existing faults. The model predicts a strike-slip stress regime in regions with pockmarks where acoustic flares have not been documented. If a certain degree of coupling is assumed between deep crustal and near-surface deformation, it is possible that ridge-push forces have influenced seepage activity in the region by interacting with the pore-pressure regime at the base of the gas hydrate stability zone. More abundant seepage on the eastern Vestnesa Ridge at present may be facilitated by the dilation of faults and fractures favourably oriented with respect to the stress field. A modified state of stress in the past, due to more significant glacial stress for instance, may explain vigorous seepage activity along the entire Vestnesa Ridge. The contribution of other mechanisms to the state of stress (i.e. sedimentary loading and lithospheric flexure) remain to be investigated. Our study provides a first-order assessment of how tectonic stresses may be influencing the kinematics of near-surface faults and associated seepage activity offshore of the western Svalbard margin.

The effect of flexural isostasy on delta architecture: implications for the Mungaroo Formation

The fluvio-deltaic Triassic Mungaroo Formation, North West Shelf (NWS) of Australia, hosts vast resources of hydrocarbons. However, the mechanisms that generated its 4-6 km monotonous infill archit...

Lithospheric Alteration, Intraplate Crustal Deformation, and Topography in Eastern China

AbstractTopography can be directly observed, but what controls topographic variation is not readily known. To investigate a prominent and remarkably consistent along‐trend topographic gradient belt (TGB) in intraplate eastern China and its connection to ongoing deformation and geodynamics, we examine the regional density structure of the crust and mantle lithosphere. We first use S velocities to derive an initial density and then refine the density model by joint inversion of topography and gravity. We use the final density model and receiver functions to strip away topographic contributions from the crust, and we subsequently estimate the thickness of the mantle lithosphere required to satisfy flexural isostasy. Crustal buoyancy overcompensates for the elevation change across the TGB. East of the TGB (eastern north China craton and southeastern south China), the lithosphere is thin (~ 90 km), suggesting that not only the eastern north China craton but also much of the eastern China has experienced lithospheric dismemberment. The TGB behaves as an important contour for the lithospheric thickness, except for the northeast China and Qinling‐Dabie. Proposed refertilization of the altered mantle lithosphere in the east side of TGB would result in even lower lithospheric thickness, while hydration and asthenosphere upwelling have the opposite effect. This systematic calculation gives a regional analysis of eastern China and could be a useful guide for the regions lacking seismic investigations.

Crustal Density Structure, Lithosphere Flexure Mechanism, and Isostatic State Throughout the Qinling Orogen Revealed by In Situ Dense Gravity Observations

AbstractGravity and GPS hybrid measurements were conducted at 385 stations throughout the Qinling Orogen, China, to update Bouguer gravity anomalies and free‐air gravity anomalies of the area. The crustal density structure, lithosphere flexure mechanism, and isostatic state of the Qinglin Orogen were studied in detail via these in situ observations. Bouguer gravity anomalies in the study area are mainly negative, ranging from −410 mGal in the Tibetan Plateau to −100 mGal in the eastern Qinglin Orogen. The estimated crustal thickness ranges from 56 to 35 km and thins eastward along the Qinling Orogen. The optimal effective elastic thickness (Te) of the study area is 14 km, and the loading comes from the Earth's surface, the interface between the upper and lower crust, and the Moho. Vertical tectonic stress borne by the lithosphere varies observably in the study area. Downward stress reaches a peak of −14 MPa in the Liupanshan Mountains, whereas the highest value of upward stress (8 MPa) is attained in the middle Qinling Orogen. Considering distributions of crustal density structures and vertical tectonic stress of the lithosphere, in addition to the distinct loading ratios of the eastern and western Qinling Orogen, a piecewise combination model was developed to interpret the uplift of the Qinling Orogen. This model shows that the eastern part of the Orogen is thickened by the upward migration of mantle materials under the thrusting of the South and North China Blocks, whereas uplift of the western part results from lower crustal flow derived from the Tibetan Plateau.

Tectonostratigraphic history of the southern Tian Shan, western China, from seismic reflection profiling

The modern Tian Shan formed due to the India-Eurasia collision. Uplift of the Tian Shan is recorded notably by the sedimentary sequences and subsidence history of the coupling foreland basins. We analyzed a 353-km-long seismic profile and logging data of four wells in the southern Tian Shan foreland area to decipher its tectonic, stratigraphic, and subsidence history. The sedimentary sequences comprise the Cambrian-Silurian, Devonian-Permian, Triassic, Jurassic-Cretaceous, and Paleogene tectonostratigraphic units, overlain by the late Oligocene-Quaternary foreland basin unit. The Jidike Formation is the oldest sedimentary sequence of the foreland succession, deposited at ∼26 Ma based on magnetostratigraphic constraints, indicating that uplift of the southern Tian Shan was initiated by at least ∼26 Ma. In addition, the tectonic subsidence rate of the southern Tian Shan foreland basin increased significantly since ∼26 Ma due to lithospheric flexure caused by building of the topography of the southern Tian Shan. The envelope line of the forelandward onlaping points within the foreland unit indicates that the forebulge of the southern Tian Shan foreland basin migrated southwards at 1.6 ± 0.1 mm/yr between ∼26 Ma and ∼12 Ma and at 14.6 ± 0.1 mm/yr after ∼12 Ma. The increase of migration rate at ∼12 Ma suggests an accelerated convergence between the Tarim Basin and the Tian Shan.