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How does crustal shortening contribute to the uplift of the eastern margin of the Tibetan Plateau?
... FEPG is a generalpurpose and integrated expert system development environment that automatically generates finite element programs based on given partial difference equation and algorithm expressions. The code has been tested against results of the analytic solutions of numerous simple 3D problems by Liu (2000), and has been used in several simulation cases to investigate the long term lithospheric deformation and stress evolution in different regions (Yang and Liu, 2002;Liu and Yang, 2003;Li et al., 2009;Luo and Liu, 2009Liu et al., 2010aLiu et al., , 2015a. Liu and Yang (2003) used a three-dimensional visco-elastic model to simulate the Tibetan crustal stress pattern, and compared it with the earthquake data to investigate the major factors contributing to the Tibetan extension collapse process. ...
... Luo and Liu (2009) explored how the GPS-measured short-term strain relates to long-term mountain building, and explained why short-term crustal shortening leads to mountain building in the Andes, but not in Cascadia. Liu et al. (2015aLiu et al. ( , 2015b used 2D visco-elastic models to demonstrate the geodynamics mechanism for diversity in topographic gradient along the Tibetan margins, and found that lateral variations of lithospheric rheology control significantly the formation of intracontinental plateau margins. ...
... The new values of viscosity are 2.0 Â 10 19 Pa s for the Tibetan middle crust, and 2.5 Â 10 18 for the Tibetan lower crust. These values are still in the range of the viscosity proposed in previous studies (Clark et al., 2005;Shi and Cao, 2008;Godard et al., 2009;Liu et al., 2015a), thus these values are still within a reasonable range. ...
The 2008 Wenchuan earthquake (Mw7.9) occurred in the Longmen Shan fault zone. The stress change and crustal deformation during the accumulation period is computed using 3D finite element modelling assuming visco-elastic rheology. Our results support that the eastward movement of the Tibetan Plateau resulting from the India-Eurasia collision is obstructed at the Longmen Shan fault zone by the strong Yangtze craton. In response, the Tibetan ductile crust thickens and accumulates at the contact between the Tibetan Plateau and the Sichuan Basin. This process implies a strong uplift with the rate of about 1.8 mm/a of the upper crust and induces stress a concentration nearly at the bottom of the Longmen Shan fault zone. We believe that the stress concentration in the Longmen Shan fault zone provides a very important geodynamic background of the 2008 Wenchuan earthquake. Using numerical experiments we find that the key factor controlling this stress concentration process is the large viscosity contrast in the middle and lower crusts between the Tibetan Plateau and the Sichuan Basin. The results show that large viscosity contrast in the middle and lower crusts accelerates the stress concentration in the Longmen Shan fault zone. Fast moving lower crustal flow accelerates this stress accumulation process. During the inter-seismic period, spatially the maximum stress accumulation rate of the eastern margin of the Tibetan Plateau is located nearly at the bottom of the brittle upper crust of the Longmen Shan fault zone. The spatial distribution of the stress accumulation along the strike of the Longmen Shan fault zone is as follows: the normal stress decreases while the shear stress increases from southwest to northeast along the Longmen Shan fault zone. This stress distribution explains the thrust motion in the SW and strike-slip motion in the NE during the 2008 Wenchuan earthquake.
... (1) the uplift rate difference between the plateau and the foreland basin responding to crustal convergence (Liu et al., 2015a), (2) dynamic stress induced by gravitational driven lower crustal flow (Clark and Royden, 2000;Clark et al., 2005), (3) erosion control over lower crustal flow (Godard et al., 2009), (4) upper crustal shortening decoupled from the lower crust by a series of detachments (Hubbard and Shaw, 2009), (5) near isostatic response to crustal thickening beneath the margin (Chen et al., 2013a), and (6) thrust faults with large amounts of slip (Cattin and Avouac, 2000;Tapponnier et al., 2001). However, few investigations have been carried out on the formation of the low-gradient margins. ...
... Many studies (Bird, 1991;Owens and Zandt, 1997;Clark and Royden, 2000;Beaumont et al., 2001Beaumont et al., , 2004Wei et al., 2001Wei et al., , 2013Fan and Lay, 2003;Unsworth et al., 2004;Clark et al., 2005;Wang et al., 2007;Bai et al., 2010;Liu et al., 2012Liu et al., , 2015a have revealed lateral heterogeneities in crustal strength across the margins of the Tibetan Plateau. Accumulating evidences from geological study, and geophysical observation, like GPS measurement and magnetotellurics observation, together with numerical experiments support that mechanically weak ductile middle and lower crusts are present beneath the Tibetan Plateau (Bird, 1991;Clark and Royden, 2000;Beaumont et al., 2001Beaumont et al., , 2004Clark et al., 2005;Wang et al., 2007;Bai et al., 2010;Liu et al., 2012Liu et al., , 2015a. ...
... Many studies (Bird, 1991;Owens and Zandt, 1997;Clark and Royden, 2000;Beaumont et al., 2001Beaumont et al., , 2004Wei et al., 2001Wei et al., , 2013Fan and Lay, 2003;Unsworth et al., 2004;Clark et al., 2005;Wang et al., 2007;Bai et al., 2010;Liu et al., 2012Liu et al., , 2015a have revealed lateral heterogeneities in crustal strength across the margins of the Tibetan Plateau. Accumulating evidences from geological study, and geophysical observation, like GPS measurement and magnetotellurics observation, together with numerical experiments support that mechanically weak ductile middle and lower crusts are present beneath the Tibetan Plateau (Bird, 1991;Clark and Royden, 2000;Beaumont et al., 2001Beaumont et al., , 2004Clark et al., 2005;Wang et al., 2007;Bai et al., 2010;Liu et al., 2012Liu et al., , 2015a. Geophysical data (Owens and Zandt, 1997;Wei et al., 2001;Fan and Lay, 2003;Unsworth et al., 2004) and magmatic observation (Ding et al., 2003;Chung et al., 2005) support the notion that partial melt exists within the anomalously hot crust of northern Tibet. ...
... Evidence from geological study, and geophysical observations using tomography, GPS measurement, and magnetotellurics observation, together with numerical modelling research, is in favor of weak ductile Tibetan lower crust (Bird, 1991;Beaumont et al., 2001Beaumont et al., , 2004Clark et al., 2005;Wang et al., 2007aWang et al., ,b, 2009Wang et al., , 2011Wang et al., , 2014bBai et al., 2010;Wei et al., 2013;Huang et al., 2014;Liu et al., 2015). According to recent seismic (Wang et al., 2007a(Wang et al., ,b, 2009Bai et al., 2010;Wei et al., 2013;Liu et al., 2014) and magnetotelluric studies (Bai et al., 2010;Zhao et al., 2012) in this region, there are low seismic velocity and high electrical conductivity zones at the middle/lower crustal level in the eastern Tibet. ...
... Thus, the mechanical strength of the deep crust beneath the eastern Tibet is supposed to be weak. However, the low-viscosity zones in the deep Tibetan Plateau crust terminate against the steeply sloping plateau margin adjacent to the Sichuan Basin (Clark et al., 2005;Royden et al., 2008;Huang et al., 2014;Liu et al., 2015). Tomographic studies reveal that seismically fast structures beneath the Sichuan Basin extend to about 250 km deep (Li et al., 2006Wei et al., 2013). ...
... This system has been developed to help people build finite element code to their own specifications by Academy of Mathematics and Systems Science of the Chinese Academy of Sciences (Liang 1991). The code has been tested against results of the analytic solutions of numerous simple 3D problems by Liu (2000), and has been validated in several simulation cases (Yang and Liu, 2002;Liu and Yang, 2003;Li et al., 2009;Luo and Liu, 2009Liu et al., 2010Liu et al., , 2015. ...
... Previous studies have found that variations in rheological structure and lithospheric viscosity can have an important influence on the spatiotemporal strain partitioning in eastern Tibet (Zhu and Zhang, 2013;Liu et al., 2015;Yin and Luo, 2018a). These studies have shown that lithospheric strength in eastern Tibet is much lower than that in the Sichuan Basin, and the larger lithospheric strength contrast between the eastern Tibet and the Sichuan Basin will result in more pronounced differential uplift between them. ...
... These studies have shown that lithospheric strength in eastern Tibet is much lower than that in the Sichuan Basin, and the larger lithospheric strength contrast between the eastern Tibet and the Sichuan Basin will result in more pronounced differential uplift between them. Furthermore, the viscosity of the lithosphere in the eastern Tibetan interior may also affect the style and magnitude of interseismic deformation in the region (Zhu and Zhang, 2013;Liu et al., 2015;Yin and Luo, 2018a). ...
... These values are greater than the proposed viscosities of 2.0 × 10 18 Pa·s of the lower crustal flow in eastern Tibet by Clark et al. (2005). They are consistent, however, with the estimates in the eastern Tibetan Plateau by Godard et al. (2009) and Liu et al. (2015aLiu et al. ( , 2015b. We changed these values of the viscosity in a specific range to investigate their effect on time-dependent stress variation. ...
... The viscosities of the Tibetan ductile crusts remain debatable (Clark et al., 2005;Godard et al., 2009;Liu et al., 2015aLiu et al., , 2015bShi & Cao, 2008). To test the impact on stress by viscosities, we calculated ΔCFS with various configurations of viscosities in two more cases. ...
The 2008 Wenchuan earthquake Mw7.9 unilaterally ruptured the Longmen Shan fault (LMSF) along eastern Tibet. The earthquake rupture propagated about 270 km northeastward, whereas it propagated only about 20 km southwestward along the strike of the fault. Although a significant attention has been paid to the question of predominantly unilateral propagation, the primary reasons for this type of propagation remain unclear. In this research, we examined the change of Coulomb stress along the LMSF caused by the historical earthquakes near and on the LMSF from 1725 to 2008. We found that the 14 preceding large earthquakes (M ≥6.5) on the Xianshui He fault cast a stress shadow on the SW segment of the LMSF, which was not activated by the 2008 Wenchuan earthquake rupture. The 1970 Dayi earthquake on the SW segment of the LMSF contributed significantly to this stress shadow. Compared with the segment of the 2008 Wenchuan earthquake rupture, this stress shadow caused strong stress contrasts of −214 kPa, −22 kPa, and −80 kPa in the seismic gap of the very SW segment of the LMSF, the rupture zone of the 2013 Lushan earthquake, and the seismic gap between the 2008 Wenchuan and 2013 Lushan earthquake ruptures, respectively. Stress contrasts in these values were consistent with tectonic loading over 165 years, 17 years, and 62 years, respectively, by integrating an interseismic stress‐loading rate of 1.3 kPa/a. We proposed that this stress shadow might have created a barrier at the SW segment, preventing the earthquake rupture propagating southwestward.
... The rheology of the Tibetan ductile crust is still contested (Clark et al., 2005;Godard et al., 2009;Huang et al., 2014;Liu, et al., 2015aLiu, et al., , 2015bShi & Cao, 2008). To assess the effects of viscosity, ΔCFS was simulated using different viscosity conformations. ...
There are two seismic gaps in the southwestern section of the Longmen Shan Fault (LMSF): (1) the gap amid the ruptures of the 2008 Wenchuan and 2013 Lushan earthquakes and (2) the gap in the southwestern-most section of the LMSF. Conflicting explanations have previously been proposed to estimate the hazards associated with these seismic gaps. In this study, we used a complete earthquake (M≥6.0) catalogue for the past 300 years to re-evaluate the seismic hazards by calculating the change in the Coulomb stress caused by nearby large earthquakes. When combined with the inter-seismic tectonic stress loading process, our results suggest that there is a high possibility of a future earthquake in seismic gap 1 because of the enhanced stress caused by the 2008 Wenchuan and 2013 Lushan earthquakes, although the 1970 Dayi earthquake (Ms 6.2) did rupture the center of the gap. However, the risk of an earthquake occurring in seismic gap 2 has decreased due to the unloading stress triggered by 14 large historical earthquakes on the Xianshui He Fault. This indicates that the seismic hazards of the two gaps differ even though they are less than 35 km apart. These findings differ from the results of previous studies, which were obtained using earthquake-induced stress modeling, inter-seismic stress modeling, estimates of earthquake moment accumulation and release, and paleoearthquake analysis. Our research sheds new insights on seismic hazard estimates for the southwestern LMSF, and might be helpful in determining the disaster relief distribution in Sichuan province, China.
... The high viscosity difference amongst the lower crust of the Tibetan plateau and rigid-stable block of Yangtze (which also blocked the eastward propagation of Tibetan plateau) is the reason for the upliftment of the Longmen-Shan. But then again, the uplift rate contrast between Tibetan plateau and the Sichuan basin resulted in sheer uplift of the Longmen-Shan and contributing to its high elevated topography within few million years 72 . ...
The Himalaya, Tibet, and the Karakorum are the most spectacular upshot resulted in response to the Indian and Eurasian plate collision.The collision resulted in the crustal thickening and shortening of the region during the Cenozoic era with an estimated magnitude of >50%. Tibet is further distributed into North, Central, and the Southern segments, which are constrained and parted by different faults/thrusts and sutures zones. A number of geophysical and geological researcheshave been approved and are still continuing to understand the tectonic activities going on in the area. Late Cenozoic has been marked as an important era in developing of most noticeable changes occurred in the region including of;east-western crustal extension alongthe central Tibet, as well as the clockwise rotation of the Tibetan plateau (since ~10-13 Ma). This extension lead to the generationof grabens, strike-slip faults and resultingthe uplift of the central Tibet. The tremendous magmatic activity occurred in Lhasa block during Cambrian era, are believed to be the product of subduction of Proto- Tethyan Ocean underneath the Australian Gondwana. Similarly, the Late Devonian to early Carboniferous magmatism is associated with the back-arc evolved in the Songdo-Tethyan Ocean, while the Late Triassic to Early Jurassic magmatism is associated with the development of Indus-Yarlung-Zangbo Tethyan back-arc basin. However, climatic research of thesouthAsia highlighted that the uplift period of the Tibetan plateau was initiated duringLate-Miocene (~8 Ma). The calculated NS crustal shortening and EW extensional rates of the Central Tibetan plateau alongAltyn Tagh Fault are about ~10-12mm/yr. and ~8-10mm/yr., respectively withless than 20km of the slip; which is identical to the GPS studies of the region. The cooling and exhumation events (not later than ~22-25Ma) in the southcentral Tibetan plateau are the product of the Cenozoic collision.
... There are numerous strong geophysical and geological evidences supporting the rhological strength contrast across the eastern margin of the Tibetan Plateau (Royden et al., 1997Hu et al., 2000;Beaumont et al., 2001;Clark et al., 2005;Holbig and Grove, 2008;Li et al., 2006;Wang et al., 2007Wang et al., , 2008Bai et al., 2010;Wei et al., 2013;Liu et al., 2014Liu et al., , 2015a. Numerical results of effective viscosity of Chinese continental lithosphere reveal that the viscosities of the Tibetan middle and lower crusts are much lower than those of the upper crust and lithospheric mantle, indicating the weak mechanical strength in the middle and lower crusts of the Tibetan Plateau (Shi and Cao, 2008). ...
... FEPG is a generalpurpose and integrated expert system development environment that automatically generates finite element programs based on given partial difference equation and algorithm expressions. The code has been tested against results of TECTON for simple 3D problems by Liu and Yang (2000), and has been used in several simulation cases to investigate the long term lithospheric deformation and stress evolution in different regions (Yang and Liu, 2002;Liu and Yang, 2003;Li et al., 2009;Luo and Liu, 2009, 2012Liu et al., 2010Liu et al., , 2015aLiu et al., ,b,c, 2016. The model is meshed with hexahedral element. ...
The 2021 Mw 7.3 Maduo earthquake ruptured the Kunlun Pass‐Jiangcuo fault (KPJF) in the Bayan Har Block (BHB) in northeastern Tibet. To explore the reasons behind the Maduo earthquake and the seismic hazards that followed this event in the BHB, we simulated the Coulomb stress changes before and after 2021 caused by 27 large historical earthquakes in the past centuries. We found that the Maduo earthquake was delayed for approximately 157 years because of the stress shadow resulting from fault interaction in the BHB. This stress shadow was mainly controlled by the 1937 Mw 7.5 earthquake on the Eastern Kunlun fault (EKLF) and the 1947 Mw 7.75 earthquake on the Dari fault. The stress shadow created by the Maduo earthquake covered several strike‐slip faults surrounding the Maduo earthquake rupture through negative feedback. Future seismic activity in the stress shadow zone may be delayed to a certain extent. Based on this finding, we propose that earthquakes on a fault might inhibit earthquakes on surrounding faults in parallel strike‐slip fault systems by creating a shadow zone in the BHB. This finding is important for understanding earthquake generation and migration in northeastern Tibet. It is also helpful for understanding the fault interactions in similar fault systems globally. The seismic hazards of the seismic gap to the west of the KPJF and the Maqin‐Maqu seismic gap on the EKLF increased after the Maduo earthquake. Therefore, attention should be paid to hazards prevention in these regions.
It remains unknown whether middle-lower crustal flow exists in the eastern Tibetan Plateau. More and more geodetic data have been acquired during the past several tens of years, especially after the 2008 Mw 7.9 Wenchuan earthquake. Some important questions are proposed: Can we use the geodetic data to determine if middle-lower crustal flow exists in the eastern Tibetan Plateau, and can we estimate the patterns and magnitudes of the crustal flow in this region if it exists? To investigate these questions, we construct a three-dimensional viscoelastoplastic finite-element model to simulate regional crustal deformation during the seismic cycle in cases with and without middle-lower crustal flow and analyze and compare these model results with geodetic observations. From modeling the co- and post-seismic deformation, we have obtained a steady-state Maxwell viscosity of 1.1 × 10¹⁹ Pa s within a 40-km-thick middle-lower crust beneath the eastern Tibetan Plateau. Through inter-seismic deformation, we find that the eastern Tibetan Plateau, in crustal shortening model, has undergone pure shear deformation, leading to an even-distributed surface uplift rate of 0.5 mm/yr. It's significantly lower than the average uplift rate of 2.2 mm/yr from the observed leveling data. On the other hand, the hybrid crustal flow model can predict a faster surface uplift rate with the fastest uplift rate localized at the flow front, which is more consistent with the observed leveling data. According to the model results and the leveling data, we infer that middle-lower flow may exist within a wide area beneath the eastern Tibetan Plateau.
In the southeastern Tibetan Plateau, the Xianshuihe-Xiaojiang fault system (XXFS) and its neighboring fault systems collectively accommodate the crustal extrusion of the Tibetan Plateau. However, we do not clearly understand how these faults interact with each other and how the fault interaction impacts fault slip rate, strain partitioning and crustal extrusion in this region. Through building a 3D viscoelastoplastic finite element model, we investigate the effects of fault geometry, fault configuration, lithospheric rheology and topographic loading on fault slip rate and crustal deformation. We find that the geometrical irregularity of Anninghe-Zemuhe faults in XXFS (a concave bend) prevents the crustal extrusion of the Chuandian (Sichuan-Yunnan) block, and tends to cause the inception of their subparallel Daliangshan fault on their east. On the other hand, the active Daliangshan fault decreases the slip rates on the Anninghe-Zemuhe faults, causing faster crustal extrusion of the Chuandian block. The active Lijiang-Xiaojinhe fault decreases the slip rates of faults in the south and hence hinders the crustal extrusion of the Chuandian block. Besides, the topographic loading plays an important role in the crustal extrusion of the southeastern Tibetan Plateau, by redistributing regional deformation and fault slip rates. Modeling results demonstrate that all faults in this region are kinematically connected and mechanically coupled: the slip rate change of one fault, both induce strain repartitioning and variations of slip rates on all other faults in such a system. Our research reveals the significant impact of fault interaction on fault activity and crustal extrusion in the southeastern Tibetan plateau.
The Longmen Shan Thrust Belt, which lies at the convergence of the eastern Tibetan Plateau and Sichuan Basin, is characterized by a topographic relief of 4000 m over a distance of 100 km. While the rock underlying the Sichuan Basin is strong, the crust underlying the Tibetan Plateau is weak. Four models that simulate deformation caused by distributions of dislocations along a geometrically complex fault system are developed to investigate how the contrast in elevational and crustal heterogeneity influences coseismic deformation of the 2008 Mw7.9 Wenchuan earthquake. The models are configured with an assembly precise ramp-décollement structures and planar fault geometries. The coseismic slip distribution is estimated by inverting three component GPS observations with Green's functions calculated for each model configuration. By comparing the slip distribution and residuals among the displacement predictions from these four models and measurements of GPS, the crustal heterogeneity model robustly explained most of the observed GPS horizontal and vertical coseismic displacements. Results revealed four composite dislocation asperities distributed in the shallow crust and a slip of 4–8 m on the sub-surface décollement fault with a depth of ~20 km. The resulting geodetic moment is 8.10 × 10²⁰ Nm, which is in agreement with the seismological estimates. The deep slip predictions obtained with the ramp-décollement fault geometry indicate that the eastern Tibetan Plateau was thrust over the Sichuan Basin rather than the plateau being uplifted owing to the underlying weak crust.
Fault interaction is believed to influence seismicity and crustal deformation, but the mechanics of fault interaction over various time scales remain poorly understood. We present here a numerical investigation of fault coupling and interaction over multiple time scales, using the San Andreas fault and the San Jacinto fault in southern California as an example. The San Andreas fault is the Pacific-North American plate boundary, but in southern California, a significant portion of the relative plate motion is accommodated by the subparallel San Jacinto fault. We developed a three-dimensional viscoelastoplastic finite-element model to study the ways in which these two faults may have interacted (1) during and following individual earthquakes, (2) over multiple seismic cycles, and (3) during long-term steady-state fault slip. Our results show that the cluster of nine moderate-sized earthquakes (M 6-7) on the San Jacinto fault since 1899 may have lowered the Coulomb stress on the southern San Andreas fault, delaying the "Big One," an earthquake of magnitude 7.8 or greater that may result from rupture of much of the southern San Andreas fault. In addition to the static Coulomb stress changes associated with individual earthquakes, variations of seismicity over seismic cycles on one fault can influence the loading rate on the other fault. When the San Jacinto fault experiences clusters of earthquakes such as those in the past century, the loading rate on the San Andreas fault can be lowered by as much as similar to 80%. Over longer time scales, these two faults share the slip needed to accommodate the relative plate motion. Hence, an increase in slip rate on one of these two faults causes complementary decrease on the other, which is consistent with geological observations.
The lateral expansion of the southeastern Tibetan Plateau causes devastating earthquakes, but is poorly understood. In particular, the links between regional variations in surface motion and the deeper structure of the plateau are unclear. The plateau may deform either by movement of rigid crustal blocks along large strike-slip faults, by continuous deformation , or by the eastward flow of a channel of viscous crustal rocks. However, the importance of crustal channel flow was questioned in the wake of the 2008 Wenchuan earthquake . Controversies about the style of deformation have persisted, in
part because geophysical probes have insu cient resolution to link structures in the deep crust to the observed surface deformation. Here we use seismic data recorded with an
array of about 300 seismographs in western Sichuan, China, to image the structure of the eastern Tibetan Plateau with unprecedented clarity. We identify zones of weak rocks in
the deep crust that thicken eastwards towards the Yangtze Craton, which we interpret as crustal flow channels. We also identify stark contrasts in the structure and rheology of the
crust across large faults. Combined with geodetic data, the inferred crustal heterogeneity indicates that plateau expansion is accommodated by a combination of local crustal flow and strain partitioning across deep faults. We conclude that rigid block motion and crustal flow are therefore not irreconcilable modes of crustal deformation.
Based upon the seismic experiments along Geoscience Transect from the Altyn Tagh to the Longmen Mountains, the crustal P-wave velocity structure was derived to outline the characteristics of the crust in the eastern margin of Tibetan Plateau. The section shows a few significant features. The crustal thickness varies dramatically, and is consistent with the tectonic settings. The Moho boundary abruptly drops to 73km depth beneath the southern Altyn Tagh from 50km in the Tarim basin, then rises again to about 58km depth beneath the Qaidam basin. The Moho drops again to about 70km underneath the Songpan-Garzê Terrane, then rises stepwise to 60km near Longmen Mountains. To further southeast, the crust thins to 52km beneath the Sichuan basin in the southeast of Longmen Mountains. In the north of the Kunlun Fault, a low-velocity zone,which may be a layer of melted rocks due to high temperature and pressure in the depth,exists in the the bottom of the middle crust.The two depressions of the Moho correlate with the Qilian and Songpan-Garzê Terranes, implying that these two mountains have deep roots. According to our results, it is deduced that the thick crust of the northeastern Tibetan Plateau probably is a result of east-west and northwest-southeast crustal shortening since the Mesozoic during the collision of the Asian and Indian plates.
margins surrounding the Tibetan Plateau show some diversity in
topographic gradient. The most striking example is the eastern Tibetan
margin bordered by the Longmen Shan range, which is characterized by a
remarkably steep topography transition between eastern Tibet and the
Sichuan Basin. There is significant uncertainty over whether this margin
was formed by crustal shortening or lower-crustal flow. To investigate
the formation mechanism of steep convergent intracontinental margins, we
conducted petrological-thermomechanical numerical simulations based on
the lithospheric structure and thermal state of the eastern Tibetan
margin. Our numerical experiments demonstrate that a very steep
topographic gradient, such as the eastern Tibetan margin, is an inherent
characteristic of convergence between a hot and weak lithosphere with
thick crust and a cold and strong lithosphere with thin crust. Although
lower-crustal flow has potentially contributed to the crustal thickness
difference between the two convergent blocks, it is not a prerequisite
for the growth of steep convergent intracontinental margin. Rather, the
topography at the margin can be explained by a near isostatic response
to crustal thickening resulting from shortening.
The ongoing collision of the Indian and Asian continents has created the
Himalaya and Tibetan plateau through a range of deformation processes.
These include crustal thickening, detachment of the lower lithosphere
from the plate (delamination) and flow in a weakened lower crust. Debate
continues as to which of these processes are most significant. In
eastern Tibet, large-scale motion of the surface occurs, but the nature
of deformation at depth remains unresolved. A large-scale crustal flow
channel has been proposed as an explanation for regional uplift in
eastern Tibet, but existing geophysical data do not constrain the
pattern of flow. Magnetotellurics uses naturally occurring
electromagnetic waves to image the Earth's subsurface. Here we present
magnetotelluric data that image two major zones or channels of high
electrical conductivity at a depth of 20-40 km. The channels extend
horizontally more than 800km from the Tibetan plateau into southwest
China. The electrical properties of the channels imply an elevated fluid
content consistent with a weak crust that permits flow on a geological
timescale. These findings support the hypothesis that crustal flow can
occur in orogenic belts and contribute to uplift of plateaux. Our
results reveal the previously unknown complexities of these patterns of
crustal flow.
High topography in central Asia is perhaps the mostfundamental expression of the Cenozoic Indo-Asian collision, yet an understandingof the timing and rates of development of the Tibetan Plateau remains elusive.Here we investigate the Cenozoic thermal histories of rocks along the easternmargin of the plateau adjacent to the Sichuan Basin in an effort to determinewhen the steep topographic escarpment that characterizes this margin developed.Temperature-time paths inferred from 40Ar/39Arthermochronology of biotite, multiple diffusion domain modeling of alkalifeldspar 40Ar release spectra, and (U-Th)/He thermochronologyof zircon and apatite imply that rocks at the present-day topographic frontof the plateau underwent slow cooling (
The Tarim basin in NW China developed as a complex foreland basin in the Cenozoic in association with mountain building in the Tibetan Plateau to the south and the Tian Shan orogen to the north. We reconstructed the Cenozoic deformation history of the Tarim basement by backstripping the sedimentary rocks. Two-dimensional and three-dimensional finite element models are then used to simulate the flexural deformation of the Tarim basement in response to the sedimentary loads and additional tectonic loads associated with overthrusting of the surrounding mountain belts. The results suggest that uplift of both the western Kunlun Shan and the Tian Shan orogens started during or before Oligocene. Since the Miocene, mountain building accelerated in the western Kunlun Shan but showed segmented development in the Tian Shan. The results also indicate reduced tectonic load along the Altyn Tagh fault since late Miocene, consistent with geological and geophysical evidence of the fault cutting the entire lithosphere and migrating southward during its cause of the Cenozoic evolution.
We analyse receiver functions from 29 broad-band seismographs along a 380-km profile across the Longmenshan (LMS) fault belt to determine crustal structure beneath the east Tibetan margin and Sichuan basin. The Moho deepens from about 50 km under Songpan–Ganzi in east Tibet to about 60 km beneath the LMS and then shallows to about 35 km under the western Sichuan basin. The average crustal Vp/Vs ratios vary in the range 1.75–1.88 under Songpan–Ganzi in east Tibet, 1.8–2.0 under the LMS, and decrease systematically across the NW part of the Sichuan basin to less than 1.70. A negative phase arrival above the Moho under Songpan–Ganzi and Sichuan basin is interpreted as a PS conversion from the top of a low-velocity layer in the lower crust. The very high crustal Vp/Vs ratio and negative polarity PS conversion at the top of lower crust in east Tibet are inferred to be seismic signatures of a low-viscosity channel in the eastern margin of the Tibetan plateau. The lateral variation of Moho topography, crustal Vp/Vs ratio and negative polarity PS conversion at the top of the lower crust along the profile seem consistent with a model of lower crust flow or tectonic escape.
The San Andreas Fault (SAF) is the transform boundary between the Pacific and the North American plates, yet up to 25% of the relative plate motion is now accommodated by the eastern California shear zone (ECSZ). Here we investigate the inception of the ECSZ and its geodynamic interactions with the SAF using a 3-D viscoelastoplastic finite element model. For a given fault configuration of the plate boundary zone, the model simulates long-term slip on the faults and plastic strain outside the faults. Our results show that the formation of the Big Bend of the SAF around 5-12 Ma impeded fault slip and localized strain along the ECSZ, causing its inception. Development of the ECSZ was further enhanced by the activation of the Garlock Fault (GF) and lithospheric weakening caused by the encroachment of the Basin and Range extension. Similarly, the San Jacinto Fault (SJF) in southern California developed along a belt of localized strain, which resulted from the formation of the restraining bend along the San Bernardino Mountains segment of the SAF ˜2 Myr ago. Once activated, the SJF reduced slip on both the southern SAF and the ECSZ across the Mojave Desert. These results indicate causative relationship between the SAF and the ECSZ. The inception of the ECSZ and other young faults is the consequence of the evolving SAF plate boundary zone that continuously adjusts itself to accommodate the relative plate motion.
High topography in eastern Tibet is thought to have formed when deep crust beneath the central Tibetan Plateau flowed towards the plateau margin, causing crustal thickening and surface uplift(1,2). Rapid exhumation starting about 10-15 million years ago is inferred to mark the onset of surface uplift and fluvial incision(3-6). Although geophysical data are consistent with weak crust capable of flow(7,8), it is unclear how the timing(9) and amount of deformation adjacent to the Sichuan Basin during the Cenozoic era can be explained in this way(10,11). Here we use thermochronology to measure the cooling histories of rocks exposed in a section that stretches vertically over 3 km adjacent to the Sichuan Basin. Our thermal models of exhumation-driven cooling show that these rocks, and hence the plateau margin, were subject to slow, steady exhumation during early Cenozoic time, followed by two pulses of rapid exhumation, one beginning 30-25 million years ago and a second 10-15 million years ago that continues to present. Our findings imply that significant topographic relief existed adjacent to the Sichuan Basin before the Indo-Asian collision. Furthermore, the onset of Cenozoic mountain building probably pre-dated development of the weak lower crust, implying that early topography was instead formed during thickening of the upper crust along faults. We suggest that episodes of mountain building may reflect distinct geodynamic mechanisms of crustal thickening.
Two end member models of how the high elevations in Tibet formed are (i) continuous thickening and widespread viscous flow
of the crust and mantle of the entire plateau and (ii) time-dependent, localized shear between coherent lithospheric blocks.
Recent studies of Cenozoic deformation, magmatism, and seismic structure lend support to the latter. Since India collided
with Asia ∼55 million years ago, the rise of the high Tibetan plateau likely occurred in three main steps, by successive growth
and uplift of 300- to 500-kilometer-wide crustal thrust-wedges. The crust thickened, while the mantle, decoupled beneath gently
dipping shear zones, did not. Sediment infilling, bathtub-like, of dammed intermontane basins formed flat high plains at each
step. The existence of magmatic belts younging northward implies that slabs of Asian mantle subducted one after another under
ranges north of the Himalayas. Subduction was oblique and accompanied by extrusion along the left lateral strike-slip faults
that slice Tibet's east side. These mechanisms, akin to plate tectonics hidden by thickening crust, with slip-partitioning,
account for the dominant growth of the Tibet Plateau toward the east and northeast.
1] Following their initial collision 50–70 Myr ago, the Indian and Eurasian plates have been continuously converging toward each other. Whereas the regional stress field is predominately compressive, the Late Cenozoic tectonics within the Tibetan Plateau features widespread crustal extension. Numerous causes of the extension have been proposed, but their relative roles remain in debate. We have investigated the major factors contributing to the Tibetan extension in a three-dimensional viscoelastic model that includes both lateral and vertical variations of lithospheric rheology and relevant boundary conditions. Constrained by the present topography and GPS velocity field, the model predicted predominately extensional stress states within the plateau crust, resulting from mechanical balance between the gravitational buoyancy force of the plateau and the tectonic compressive stresses. The predicted stress pattern is consistent with the earthquake data that indicate roughly E-W extension in most of Tibet and nearly N-S extension near the eastern margin of the Tibetan Plateau. We explored the parameter space and boundary conditions to examine the stress evolution during the uplift of the Tibetan Plateau. When the plateau was lower than 50% of its present elevation, strike-slip and reverse faults were predominate over the entire plateau, and no E-W crustal extension was predicted. Significant crustal extension occurs only when the plateau has reached $75% of its present elevation. Basal shear associated with underthrusting of the Indian plate beneath Tibet may have enhanced crustal extension in southern Tibet and the Himalayas, and a stronger basal shear during the Miocene may help to explain the development of the South Tibetan Detachment System.
1] Phase equilibrium experiments on primitive Miocene olivine leucitite (Bb-107) from the Qiangtang terrane of the Tibetan Plateau were performed from 1.0 to 2.2 GPa and 1270 to 1440°C. The composition is multiply saturated with olivine and clinopyroxene from 1.2 to 2.2 GPa and 1340°C under nominally anhydrous conditions. Phase assemblages in the experiments have been used to model the effects of high-pressure fractional crystallization. The results are consistent with an origin of Bb-107 as a modified mantle melt produced by fractional crystallization in the lithospheric mantle or lower continental crust. Liquids from spinel + garnet peridotite melting experiments are compositionally similar to the primitive fractionation corrected melt. If metasomatized mantle contained H 2 O when melting began, the depth of melting approaches the base of the Moho and is 1300°C at 2 GPa for 5 wt % H 2 O in the melt and 1360°C at 2.2 GPa for 2 wt % H 2 O, and 1420°C at 2.4 GPa for a dry melt. Major and trace element evidence from Bb-107 and other primitive Tibetan shoshonitic lavas indicates that these magmas may be derived as low-extent melts (1–3 wt %) of a metasomatized mantle in the spinel and garnet stability fields. The depth of melting and the geochemical characteristics of the Tibetan lavas are correlated. Increased extents of melting correlate with increasing depth of melting. This correlation is to be expected if melting occurred during lithospheric thinning during downward convective flow at the margin of the lithosphere–asthenosphere boundary.
1] Two crustal cross sections through the eastern margin of the Tibetan Plateau are jointly determined from deep seismic sounding. The E–W trending line AA' passes through the western Sichuan plateau (including the Songpan-Garze terrane and the Longmenshan fault belt) and ends in the Sichuan basin (a part of the Yangtze craton). Line BB' has a trend of NNE and crosses the Songpan-Garze terrane. Two-dimensional crustal structures along the profiles were jointly determined by the additional use of existing deep seismic sounding data. Our seismic velocity models indicate that the western Sichuan plateau and the Sichuan basin have crustal thicknesses of 62 and 43 km, average crustal P wave velocities of 6.27 and 6.45 km/s and lower crustal (V p > 6.5 km/s) thicknesses of 27 and 15 km, respectively. Density models constructed from the seismic velocity models are consistent with observed Bouguer gravity anomalies. We infer that collision between the Tibetan Plateau and the Yangtze craton has caused thickening of the lower crust and uplift of the western Sichuan plateau. We detect a low-velocity layer in the upper crust of the western Sichuan plateau but observe no equivalence in the Sichuan basin; west dipping thrusts may detach into this low-velocity layer. The seismic phase P m P in the western Sichuan plateau has low amplitude, suggesting high attenuation in the lower crust (Q p of 100–300). We suggest that the high attenuation is a consequence of lower crustal flow caused by the large lower crustal thickness beneath the western Sichuan plateau.
1] 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.
1] We document our tomographic method and present a new global model of three-dimensional (3-D) variations in mantle P wave velocity. The model is parameterized by means of rectangular cells in latitude, longitude, and radius, the size of which adapts to sampling density by short-period (1 Hz) data. The largest single data source is ISC/NEIC data reprocessed by Engdahl and coworkers, from which we use routinely picked, short-period P, Pg, Pn, pP, and pwP data (for earthquakes during the period 1964$2007). To improve the resolution in the lowermost and uppermost mantle, we use differential times of core phases (PKP AB À PKP DF , PKP AB À PKP BC , P diff À PKP DF) and surface-reflected waves (PP-P). The low-frequency differential times (P diff , PP) are measured by waveform cross correlation. Approximate 3-D finite frequency kernels are used to integrate the long-period data (P diff , PP) and short-period (P, pP, PKP) data. This global data set is augmented with data from regional catalogs and temporary seismic arrays. A crust correction is implemented to mitigate crustal smearing into the upper mantle. We invert the data for 3-D variations in P wave speed and effects of hypocenter mislocation subject to norm and gradient regularization. Spatial resolution is $100 km in the best sampled upper mantle regions. Our model, which is available online and which will be updated periodically, reveals in unprecedented detail the rich variation in style of subduction of lithospheric slabs into the mantle. The images confirm the structural complexity of downwellings in the transition zone discussed in previous papers and show with more clarity the structure of slab fragments stagnant in the transition zone beneath east Asia. They also reveal low wave speed beneath major hot spots, such as Iceland, Afar, and Hawaii, but details of these structures are not well resolved by the data used. Components: 8797 words, 14 figures, 1 table.
1] Flow of the mid-crust in the Tibetan Plateau may strongly influence the patterns of deformation and topography within this area. This flow requires the lower-crust to have low viscosity and so quantifying this viscosity may be used to test the idea of channel flow. An application of Bayesian methods to geologic and geodetic data from north central Tibet yields lower-crustal viscosities (assumed Newtonian) of 1 Â 10 19 Pa s and 2 Â 10 21 Pa s (95% bounds). The lower bound is larger than the value of 10 16 –10 18 Pa s required by models of flow of the lower-crust of Tibet, unless the entire lower crust participates in channel flow.
Using the measurements of similar to 726 GPS stations around the Tibetan Plateau, we determine the rigid rotation of the entire plateau in a Eurasia-fixed reference frame which can be best described by an Euler vector of (24.38 degrees +/- 0.42 degrees N, 102.37 degrees +/- 0.42 degrees E, 0.7096 degrees +/- 0.0206 degrees/Ma). The rigid rotational component accommodates at least 50% of the northeastward thrust from India and dominates the eastward extrusion of the northern plateau. After removing the rigid rotation to highlight the interior deformation within the plateau, we find that the most remarkable interior deformation of the plateau is a "glacier-like flow'' zone which starts at somewhere between the middle and western plateau, goes clockwise around the Eastern Himalayan Syntaxis (EHS), and ends at the southeast corner of the plateau with a fan-like front. The deformation feature of the southern plateau, especially the emergence of the flow zone could be attributed to an eastward escape of highly plastic upper crustal material driven by a lower crust viscous channel flow generated by lateral compression and gravitational buoyancy at the later developmental stage of the plateau. The first-order feature of crustal deformation of the northeastern plateau can be well explained by a three-dimensional elastic half-space dislocation model with rates of dislocation segments comparable to the ones from geological observations. In the eastern plateau, although GPS data show no significant convergence between the eastern margin of the plateau and the Sichuan Basin, a small but significant compressional strain rate component of similar to 10.5 +/- 2.8 nstrain/yr exists in a relatively narrow region around the eastern margin. In addition, a large part of the eastern plateau, northeast of the EHS, is not undergoing shortening along the northeastward convergence direction of the EHS but is stretching.
A new compilation of heat flow data for the continental area of China has been constructed using both published data (up to 1999) as well as previously unpublished data. The data set is composed of 862 observations from different sites. Within the continental area of China, heat flow values range from 23 to 319 mW/m2, with a mean of 63±24.2 mW/m2. If non-conductive values related to local hydrothermal systems are excluded, the representative average heat flow is 61±15.5 mW/m2 (n=823), with an associated range of 30–140 mW/m2. Heat flow analysis for each specific sub-tectonic province shows discernible trends within a tectonic province. The relationships between the heat flow and geological ages based on this new dataset indicate that surface heat flow is obviously more dependent on the last tectonothermal activity than the age of the orogeny. The overall heat flow pattern exhibits high values in the east and in the southwest, and low in the center and northwest parts of the continental area of China, and appears to be dominated largely by the Meso-Cenozoic tectonothermal evolution of the lithosphere. The Cenozoic collision between the Indian and Eurasian plates, and the subsequent thickening of the lithosphere; and the lithospheric thinning and volcano-magmatic activities during the Late Mesozoic in East China; can be seen clearly in the heat flow pattern.
By using regional GPS and leveling data and through an analysis considering seismotectonic background, this paper studies interseismic crustal deformation in the region across the Longmenshan fault zone before the 2008 M8.0 Wenchuan earthquake, discusses the active tectonic and geodynamic model that caused the interseismic deformation, and, from these, analyzes the mechanism of brewing and producing the Wenchuan earthquake. The result mainly shows that, in the period from 1997 to 2007 horizontal shortening in the direction perpendicular to the fault zone and horizontal right-lateral shearing parallel to the fault zone occurred in the area from the middle segment of the fault zone to about 230 km away northwest. The shortening rate is estimated to be 1.3 × 10 -8/a (that is 0.013 mm/km/a) and the distortion rate to be 2.6 × 10-8/a. Vertical uplifting also happened in the same area during the period from 1975 to 1997, with uplifting rates changing from 0.6 mm/a in between the frontal-range fault and the central fault of the fault zone to 2 to 3 mm/a near and northwest of the back-range fault of the fault zone. All these reflect that at least in the last 10 to over 30 years before the Wenchuan earthquake the frontal-range and central faults of the middle segment of the Longmenshan fault zone had been locked with strain building up. The main reason to cause such deformation is believed to be that, taking a low-velocity zone in the crust as an "uncoupling layer", the horizontal southeast-ward movement of the upper crust of the Bayan Har block was counterworked from the South China block at the western edge of the Sichuan basin and transformed into the thrust movement along the middle segment of the fault zone. The interseismic locking of this fault segment made both the horizontal shortening perpendicular to the fault and the right-lateral shearing parallel to the fault occur in the upper crust of the Bayan Har block. However, during the period from 1997 to 2007, horizontal shortening across the northern segment of the Longmenshan fault zone was very weak, but a right-lateral distortion deformation of 0.9 mm/a happened there. Such weak activity of the northern Longmenshan fault segment should apparently be attributed to that those fault zones of Minjiang, Huya and Longriba on the west of this fault segment have absorbed most of the horizontal east-ward movement of the Bayan Har block. In addition, the difference of the crustal deformation features between the middle and northern segments of the Longmenshan fault zone before the Wenchuan earthquake is consistent with the difference of along-fault distribution of the coseismic energy release during the earthquake.
The eastern Tibetan margin is characterized by a steep topographic gradient and remarkably lateral variations in crustal/lithospheric structure and thermal state. GPS measurements show that the surface convergence rate in this area is strikingly low. How can such a mountain range grow without significant upper crustal shortening? In order to investigate the formation mechanism of the eastern Tibetan-type margins, we conducted 2D numerical simulations based on finite difference and marker-in-cell techniques. The numerical models were constrained with geological and geophysical observations in the eastern Tibetan margin. Several major parameters responsible for topography building, such as the convergence rate, the erosion/sediment rate, and the presence of partially molten crust, were systematically examined. The results indicate that the presence of partially molten material in the middle/lower crust can make a positive contribution to the formation of steep topography, but it is not a necessary factor. A steep topographic gradient may be a characteristic feature when a thin lithosphere with thick crust converges with a thick lithosphere with thin crust. In the context of a high erosion rate, the Longmen Shan range still gains and maintains its steep high topography to the present. This could be explained by exerting a large push force on Tibet side. Our numerical experiments suggest that topographic characteristic across the eastern Tibetan-type margins is mainly derived from isostatic equilibration forces and intensive convergence between two continental lithospheres with totally different theological properties.
Subduction-induced crustal shortening, now measured by the GPS across many subduction zones, has led to mountain building in the Andes but not in Cascadia and some other Andean-type convergent plate boundaries. Here we use a two-dimensional viscoelastoplastic finite element model to explore how the GPS-measured short-term strain relates to long-term mountain building. We show that previously proposed causative factors of mountain building can be represented by two model parameters: the strength of mechanical coupling on the plate interface, and the yield strength of the overriding plate. The critical condition for producing permanent (plastic) crustal shortening, hence mountain building, is for the plastic yield strength of the plate interface to be higher than that of the overriding plate. Strong trench coupling and a weak lithosphere explain the Andean mountain building, whereas weak trench coupling in Cascadia allows short-term crustal shortening to be restored periodically by trench earthquakes and aseismic slips.
Topography extracted from swath profiles along the northern, southern,
and eastern margins of the Tibetan Plateau show two end-member
morphologies: steep, abrupt margins and long-wavelength, low-gradient
margins. Because the lack of significant upper crustal shortening across
much of the eastern plateau margin implies that the crustal thickening
occurs mainly in the deep crust, we compare regional topographic
gradients surrounding the plateau to model results for flux of a
Newtonian fluid through a lower crustal channel of uniform thickness.
For an assumed 15-km-thick channel, we estimate a viscosity for the
lower crust of 1018 Pa · s beneath the low-gradient
margins, 1021 Pa · s beneath the steep margins, and an
upper bound of 1016 Pa · s beneath the plateau. These
results indicate that the large-scale morphology of the eastern plateau
reflects fluid flow within the underlying crust; crustal material flows
around the strong crust of the Sichuan and Tarim Basins, creating broad,
gentle margins, and “piles up” behind the basins creating
narrow, steep margins. These results imply that this portion of the
Eurasian crust was heterogeneous, but largely weak, even prior to
construction of the Tibetan Plateau.
Wenchuan earthquake (Ms 8.0) occurred on a section of the Longmenshan fault where slow strain had accumulated. The stress change during this accumulation period is computed through 3-D finite-element modeling assuming visco-elasticity rheology and using the GPS data to quantify the boundary conditions. We calculate the stress accumulation in the lithosphere of the Longmenshan fault area, the co-seismic stress change and the post-seismic stress relaxation to estimate the Wenchuan earthquake recurrence interval. It is shown that the Eastern movement of Tibet Plateau resulting from the India-Asia collision is obstructed in the Longmenshan fault area by the strong Sichuan Basin. The soft materials of the middle and lower crust of Tibet Plateau accumulate at the contact of the basin resulting in stress concentration. Two factors are controlling this stress concentration. The first is the sharp reduction of the Moho thickness at the Longmenshan fault as one moves from the Tibetan Plateau to the Sichuan Basin. The second factor is the large difference in the viscosity of the middle and the lower crust between the Tibetan Plateau and the Sichuan Basin. During the inter-seismic period, stress increases almost linearly in time in the brittle upper crust of the Longmenshan fault, and the stress rate is larger at greater depths in the upper crust. The maximum stress accumulation of 21.6 MPa, with a steady rate of 0.0036MPa/y at the bottom of the upper crust, is reached after 6000 years. However, stress increases exponentially in the ductile middle and lower crust and in the upper lithospheric mantle. The spatial distribution of the stress after these 6000 years is as follows. The normal stress decreases, but the shear stress increases from Southwest to Northeast in the upper crust on the Longmenshan fault. This stress distribution explains the motion of the fault in thrust in the SW and in strike-slip motion in the NE. The recurrence interval of Wenchuan earthquake is estimated from our calculations to be 5400 years based on visco-elasticity. The normal stress accumulation distributes after 6000 years. Stress concentrates at the bottom of the upper crust of the Longmenshan fault. Each section is vertical to the Longmenshan Fault in figure (a) .
SUMMARY Dynamic stresses developed in the deep crust as a consequence of flow of weak lower crust may explain anomalously high topography and extensional structures localized along orogenic plateau margins. With lubrication equations commonly used to describe viscous flow in a thin-gap geometry, we model dynamic stresses associated with the obstruction of lower crustal channel flow due to rheological heterogeneity. Dynamic stresses depend on the mean velocity ( ¯ U ), viscosity (µ) and channel thickness (h), uniquely through the term µ ¯ U/h2. These stresses are then applied to the base of an elastic upper crust and the deflection of the elastic layer is computed to yield the predicted dynamic topography. We compare model calculations with observed topography of the eastern Tibetan Plateau margin where we interpret channel flow of the deep crust to be inhibited by the rigid Sichuan Basin. Model results suggest that as much 1500 m of dynamic topography across a region of several tens to a hundred kilometres wide may be produced for lower crustal material with a viscosity of 2 × 1018 P asfl owing in a 15 km thick channel around a rigid cylindrical block at an average rate of 80 mm yr−1.
We propose a model in which the stronger Indian crust injects into the
weaker lower crust of Tibet. If the viscosity of the Tibetan lower crust
is below a critical value, the continuous advancing of the Indian crust
beneath Tibet would produce negligible variation in stresses on the
bottom of the stronger brittle Tibetan upper crust and would result in a
spatially uniform uplift and a virtually undisturbed Tibetan Plateau
surface. This process is analogous to that of a piston rising in a
hydraulic jack as additional fluid is forced into the chamber. We find
that if the viscosity of the Tibetan lower crust is assumed to be
constant throughout, the computed critical viscosity is 6 ×
1018 Pa s for the present-day lower crustal thickness of 58
km. In addition to uniform viscosity, two hypothetical depth-dependent
viscosities are also tested which give rise to essentially the same
distribution of stress along the bottom of brittle upper crust. Our
model also interprets the 4-km elevation contrast between the Tibetan
Plateau and the Tarim block. A 1∶160 viscosity contrast between
the Tibetan and Tarim lower crust is required in order to maintain the
current thickening rate of the Tibetan crust (1.2 mm/yr) while keeping
the thickness of the Tarim crust unchanged.
The rheological structure of the continental lithosphere is a very important issue in the study of lithosphere dynamics. Evaluation of lithosphere viscosity is the primary task. On the basis of the new research achievements of Chinese continental temperature and strain rate, the authors have extrapolated the results of laboratory rock experiments to field geological deformation, and uncertainties such as, lithology, temperature, strain rate, and extrapolation are carefully investigated. The effective viscosity of crust and upper mantle in continental China are calculated. The effective viscosity of the middle and lower crust are usually in the range of 1021–1024 Pa·s and 1021–1022 Pa·s, respectively. The lower crust of the Tibetan plateau has a low viscosity of 1019–1020 Pa·s, which is in agreement with the previous conclusion that there is ductile lower crustal flow in the Tibetan plateau.
We present and interpret Global Positioning System (GPS) measurements of crustal motions for the period 1991-1998 for a network encompassing the eastern part of the Tibetan Plateau and its foreland. Relative to a Eurasian frame defined by minimizing the velocities of 16 GPS stations in Europe, central Asia, and Siberia, stations within all parts of the plateau foreland in south China move 6-10 mm/yr east-southeast, indicating that the eastward movement within the plateau is part of a broader eastward movement that involves the plateau and its eastern and northern foreland. North of the plateau, foreland stations move northeastward at ~10 mm/yr, indicating that the northern boundary of the deformation zone lies north of the plateau. With this realization of a Eurasian frame, the velocity of the GPS station at Bangalore in southern India implies that the northward motion of India is 5-12 mm/yr slower than that predicted from the NUVEL-1A plate reconstruction. Viewed relative to the South China Block, stations of the northeast plateau, bounded on the north by the Qilian Shan and the Altyn Tagh fault, move NNE to NE with velocities ranging from 19 mm/yr within the plateau to 5-11 mm/yr in its foreland. The Altyn Tagh fault shows left-lateral slip of ~10 mm/yr at 95°E and shortening across the fault of
We present and interpret Global Positioning System (GPS) measurements of crustal motions for the period 1991-1998 for a network encompassing the eastern part of the Tibetan Plateau and its foreland. Relative to a Eurasian frame defined by minimizing the velocities of 16 GPS stations in Europe, central Asia, and Siberia, stations within all parts of the plateau foreland in south China move 6-10 mm/yr east-southeast, indicating that the eastward movement within the plateau is part of a broader eastward movement that involves the plateau and its eastern and northern foreland. North of the plateau, foreland stations move northeastward at -10 mm/yr, indicating that the northern boundary of the deformation zone lies north of the plateau. With this realization of a Eurasian frame, the velocity of the GPS station at Bangalore in southern India implies that the northward motion of India is 5-12 mm/yr slower than that predicted from the NUVEL-1A plate reconstruction. Viewed relative to the South China Block, stations of the northeast plateau, bounded on the north by the Qilian Shan and the Altyn Tagh fault, move NNE to NE with velocities ranging from 19 mm/yr within the plateau to 5-11 mm/yr in its foreland. The Altyn Tagh fault shows left-lateral slip of- 10 mm/yr at 95øE and shortening across the fault of
A review of the geologic history of the Himalayan-Tibetan orogen suggests that at least 1400 km of north-south shortening has been absorbed by the orogen since the onset of the Indo-Asian collision at about 70 Ma. Significant crustal shortening, which leads to eventual construction of the Cenozoic Tibetan plateau, began more or less synchronously in the Eocene (50–40 Ma) in the Tethyan Himalaya in the south, and in the Kunlun Shan and the Qilian Shan some 1000–1400 km in the north. The Paleozoic and Mesozoic tectonic histories in the Himalayan-Tibetan orogen exerted a strong control over the Cenozoic strain history and strain distribution. The presence of widespread Triassic flysch complex in the Songpan-Ganzi-Hoh Xil and the Qiangtang terranes can be spatially correlated with Cenozoic volcanism and thrusting in central Tibet. The marked difference in seismic properties of the crust and the upper mantle between southern and central Tibet is a manifestation of both Mesozoic and Cenozoic tectonics. The form...
Over the last decade, thermomechanical models have revealed the control of both tectonics and erosion on the morphology of continental plateaus margins. However, unravelling the specific effects of these two coupled processes has been difficult in practice. Here, to assess the control of erosion, we investigate the dynamics of the eastern and the northern borders of the Tibetan Plateau, which are characterized by a low convergence rate and a steep topographic escarpment adjacent to the Sichuan and Tarim basins, respectively. Thermomechanical modelling of continental lithosphere coupled with fluvial denudation reveals that important crustal deformation with large-scale horizontal displacements can occur, without any convergence, as a response to mass transfer due to gravitational collapse and to erosional unloading. These processes are sensitive to crustal structure, geothermal gradient as well as surface erosion. At a timescale of several million years, our results suggest that this denudation-triggered deformation exerts a primary control on the evolution of those plateau margins by counterbalancing the mass removal due to erosion and stabilizing the topographic escarpment. This finding supports a possible explanation for the morphology of the Longmen Shan, eastern Tibet, in which the paradoxical combination of persistent high-topographic gradients close to the foreland and low convergence rates can be related to the influence of erosion on deformation patterns.
Surface-process models (SPMs) have the potential to become an important tool in predicting sediment flux to basins, but currently suffer from a lack of quantitative understanding of their controlling parameters, as well as difficulties in identifying landscape properties that can be used to test model predictions. We attempt to constrain the parameter values that enter a SPM by comparing predictions of landscape form (as expressed by hypsometric and fractal measures) and process rates obtained for different parameter sets with observations from the south-eastern Australian highlands, a rifted margin mountain belt that has remained tectonically stable during Cenozoic times. We map the hypsometry and fractal characteristics of south-eastern Australia and find that the roughness amplitude (G) correlates well with local relief, whereas the hypsometric integral (H ) correlates slightly better with elevation than with relief. The fractal dimension (D) does not correlate with any other morphometric measure and varies randomly throughout the region. Variograms generally show three kinds of scaling behaviour of topography with increasing wavelength, with topography only being truly self-affine at wavelengths between ∼1 and 10 km. From a review of the available data on long-term denudation rates in south-eastern Australia, we infer that these have been 1–10 m Myr−1, and average escarpment retreat rates 0.2–1.0 km Myr−1, throughout the Cenozoic. Model predictions, using a SPM that includes hillslope diffusion and long-range fluvial transport, suggest that landscape form evolves with time; after an initial phase where D, G and relief increase, all morphometric measures decrease with increasing denudation. The behaviour of GandH in the models is qualitatively compatible with the observations; D, however, varies predictably in the models, in contrast with its random behaviour in the real world. The observed present-day morphology of SE Australia does not impose quantitative constraints on parameter values. The fractal analyses do impose general conditions of relative parameter values that have to be met in order to create ‘realistic’ topographies. They also suggest that there is no theoretical basis for including hillslope diffusion in SPMs with a spatial resolution coarser than 1 km. A comparison of the observed denudation and retreat rates with model predictions places order-of-magnitude constraints on parameter values. Thus, data pertaining to landscape evolution are much more valuable than static present-day topography data for calibrating SPMs.
We analyzed thousands of receiver-function data recorded by 154 national and regional seismic stations to study the crustal structure beneath the Ordos plateau and the NE margin of the Tibetan plateau. Moho depth and average crustal Vp/Vs ratio were measured at each station. The Ordos plateau and the Trans-North China Orogen east of the Ordos are underlain by a moderately thick crust of ~ 42 km. The Weihe Graben lying at the southern edge of the Ordos plateau has a thin crust of ~ 30 km, while its southern neighbor, the Qinling orogenic belt shows a thick crust extending to as much as 45 km deep. The Moho depth beneath the NE margin of the Tibetan plateau varies from 55 to 65 km. We found a remarkable contrast between the Tibetan and Ordos plateaus in the measured Poisson's ratio: the Ordos plateau is featured by a high Poisson's ratio while the Tibetan margin has a very low Poisson's ratio. In general, mafic lower crustal rocks have a higher Poisson's ratio than felsic ones. The measured low Poisson's ratio beneath the NE margin of the Tibetan plateau thus indicates that the crustal column beneath the margin is rather felsic, which seems to be inconsistent with a scenario of an inflated crust due to extrusion of lower crust material from the Tibetan plateau to the margin.Research Highlights► Crustal thickness and Vp/Vs of the Ordos plateau and NE Tibetan plateau were estimated. ► The Ordos is underlain by a moderately thick crust with an intermediate composition. ► Crust beneath the NE Tibetan plateau varies is thick with a low Vp/Vs ratio. ► The low Vp/Vs ratio is inconsistent with the lower-crust flow model proposed for crustal thickening. ► Seismicity around the Ordos plateau is controlled by crustal and lithosphere structure.
The geometrical complexity of non-planar faults is known to affect not only fault slip rates but also crustal deformation and seismicity in the surrounding region, but implementing non-planar faults in numerical models has been challenging. We have developed a 3-D viscoelastoplastic finite element model to simulate long-term, steady-state fault slip along conceptual restraining bends and crustal deformation around them. By incorporating plastic yielding in both fault zones and the surrounding crust, this model can predict the quasi-steady state stress field and strain partitioning while avoiding pathological stress build-up encountered in traditional viscoelastic models. Here we detail the formulation of this model and explore major model parameters. The model results show that restraining bends tend to impede fault slip, increase shear stress in the surrounding crust and localize strain in a pair of belts off the fault zone. These effects are enhanced by high viscosity in the lower crust and upper mantle, high aspect ratio of bend width over fault length, fast relative motion of fault blocks and high fault friction. These model results may help explain the diffuse deformation surrounding the Big Bend of the San Andreas Fault and the more confined deformation around the Lebanon Bend of the Dead Sea Fault.
A host of information is now available regarding the geological and thermal structure as well as deformation rate across the Himalaya of central Nepal. These data are reconciled in a two-dimensional mechanical model that incorporates the rheological layering of the crust which depends on the local temperature and surface processes. Over geological timescale (5 Ma) the similar to 20 mm/yr estimated shortening rate across the range is accommodated by localized thrust faulting along the Main Himalayan Thrust fault (MHT). The MHT reaches the surface along the foothills, where it is called the Main Frontal Thrust fault (MFT). The MHT flattens beneath the Lesser Himalaya and forms a midcrustal ramp at the front of the Higher Himalaya, consistent with the river incision and the anticlinal structure of the Lesser Himalaya. Farther northward the MHT roots into a subhorizontal shear zone that coincides with a midcrustal seismic reflector. Aseismic slip along this shear zone is accommodated in the interseismic period by elastic straining of the upper crust, increasing the Coulomb stress beneath the front of the Higher Himalaya, where most of the microseismic activity dusters. Negligible deformation of the hanging wall requires a low apparent friction coefficient (mu) less than similar to 0.3 on the flat portion of the MHT. On the ramp, mu might be as high as 0.6. Sensitivity tests show that a rather compliant, quartz-rich rheology and a high radioactive heat production in the upper crust of similar to 2.5 mu W/m(3) is required. Erosion affects the thermal structure and interplays with crustal deformation. A dynamic equilibrium is obtained in which erosion balances tectonic uplift maintaining steady state thermal structure, topography, and deformation field. Using a linear diffusion model of erosion, we constrain the value of the mass diffusivity coefficient to 0.5-1.6x10(4) m(2)/yr. This study demonstrates that the data are internally consistent and compatible with current understanding of the mechanics of crustal deformation and highlight the role of viscous flow in the lower crust and of surface erosion in orogeny processes on the long term as well as during interseismic period.
Linear systems analysis is used to investigate the response of a surface processes model (SPM) to tectonic forcing. The SPM calculates subcontinental scale denudational landscape evolution on geological timescales (1 to hundreds of million years) as the result of simultaneous hillslope transport, modeled by diffusion, and fluvial transport, modeled by advection and reaction. The tectonically forced SPM accommodates the large-scale behavior envisaged in classical and contemporary conceptual geomorphic models and provides a framework for their integration and unification. The following three model scales are considered: micro-, meso-, and macroscale. The concepts of dynamic equilibrium and grade are quantified at the microscale for segments of uniform gradient subject to tectonic uplift. At the larger meso- and macroscales (which represent individual interfluves and landscapes including a number of drainage basins, respectively) the system response to tectonic forcing is linear for uplift geometries that are symmetric with respect to baselevel and which impose a fully integrated drainage to baselevel. For these linear models the response time and the transfer function as a function of scale characterize the model behavior. Numerical experiments show that the styles of landscape evolution depend critically on the timescales of the tectonic processes in relation to the response time of the landscape. When tectonic timescales are much longer than the landscape response time, the resulting dynamic equilibrium landscapes correspond to those envisaged by Hack (1960). When tectonic timescales are of the same order as the landscape response time and when tectonic variations take the form of pulses (much shorter than the response time), evolving landscapes conform to the Penck type (1972) and to the Davis (1889, 1899) and King (1953, 1962) type frameworks, respectively. The behavior of the SPM highlights the importance of phase shifts or delays of the landform response and sediment yield in relation to the tectonic forcing. Finally, nonlinear behavior resulting from more general uplift geometries is discussed. A number of model experiments illustrate the importance of ''fundamental form'' which is an expression of the conformity of antecedent topography with the current tectonic regime. Lack of conformity leads to models that exhibit internal thresholds and a complex response.
We have produced a P-wave model of the upper mantle beneath Southeast (SE) Asia from reprocessed short period International Seismological Centre (ISC) P and pP data, short period P data of the Annual Bulletin of Chinese Earthquakes (ABCE), and long period PP-P data. We used 3D sensitivity kernels to combine the datasets, and mantle structure was parameterized with an irregular grid. In the best-sampled region our data resolve structure on scale lengths less than 150 km. The smearing of crustal anomalies to larger depths is reduced by a crustal correction using an a priori 3D model. Our tomographic inversions reveal high-velocity roots beneath the Archean Ordos Plateau, the Sichuan Basin, and other continental blocks in SE Asia. Beneath the Himalayan Block we detect high seismic velocities, which we associate with subduction of Indian lithospheric mantle. This structure is visible above the 410 km discontinuity and may not connect to the remnant of the Neo-Tethys oceanic slab in the lower mantle. Our images suggest that only the southwestern part of the Tibetan plateau is underlain by Indian lithosphere and, thus, that the upper mantle beneath northeastern Tibet is primarily of Asian origin. Our imaging also reveals a large-scale high-velocity structure in the transition zone beneath the Yangtze Craton, which could have been produced in multiple subduction episodes. The low P-wave velocities beneath the Hainan Island are most prominent in the upper mantle and transition zone; they may represent counter flow from the surrounding subduction zones, and may not be unrelated to processes beneath eastern Tibet.
The 12 May 2008 Wenchuan earthquake (Mw 7.9) ruptured ∼ 300 km of the Longmen Shan fault, claiming ∼ 90,000 lives and devastating many cities in the Sichuan province, China. The coseismic stress changes due to the Wenchuan earthquake have been studied in kinematic models using the inferred coseismic fault slips, but the cause of the fault slips, the impact of other large earthquakes, and the mechanical interactions between the faults in eastern Tibet are uncertain. Here we explore these issues using a three-dimensional viscoelastoplastic dynamic model that calculates the regional stresses from tectonic and topographic loading, and simulates earthquakes and their stress perturbations. Our calculated coseismic changes of the Coulomb stress associated with the Great Wenchuan earthquake are similar to those in previous models. However, we show that the cumulative Coulomb stress changes, hence the implied earthquake risks, are significantly different when previous large earthquakes in the region are included in the model. Particularly, we show that in spite of stress increase from the Wenchuan earthquake, the southeastern segments of the Xianshuihe fault stay in a stress shadow because of the stress release by six M ≥ 6.9 events in this part of the Xianshuihe fault since 1893. We also found that interseismic locking on the Xianshuihe fault can increase the loading rate on the Longmen Shan fault by up to ∼ 50 Pa/year.
Plane strain, thermal-mechanical numerical models are used to examine the development of midcrustal channel flows in large hot orogens. In the models, radioactive self-heating reduces the viscosity of tectonically thickened crust and increases its susceptibility to large-scale horizontal flow. Channels can be exhumed and exposed by denudation focused on the high-relief transition between plateau and foreland. We interpret the Himalaya to have evolved in this manner. Channel flows are poorly developed if the channel has a ductile rheology based on wet quartz flow laws, and well developed if there is an additional reduction in viscosity to 1019 Pa s. This reduction occurs from 700°C to 750°C in the models and is attributed to a small percentage of in situ partial melt (“melt weakening”). Model HT1 provides an internally consistent explanation for the tectonic evolution of many features of the Himalayan-Tibetan orogenic system. Erosional exhumation exposes the migmatitic channel, equivalent to the Greater Himalayan Sequence (GHS), between coeval normal and thrust sense ductile shear zones, corresponding to the South Tibetan Detachment and the Main Central Thrust systems. Outward flow of unstable upper crust rotates these shears to low dip angles. In the model both the GHS and the Lesser Himalayan Sequence are derived from Indian crust, with the latter from much farther south. Similar models exhibit a range of tectonic styles, including the formation of domes resembling north Himalayan gneiss domes. Model results are relatively insensitive to channel heterogeneities and to variations in the behavior of the mantle lithosphere beneath the model plateau.
The importance of lateral extrusion is examined under oceans, under shields and platforms, under elevated plains, and under high plateaus and/or delaminated regions of high temperatures. The theoretical model employed considers the material nonlinearity, the amplitude nonlinearity, and the feedback between thermal advection and activation effects, and ignores the flexural rigidity of the upper crust and mantle lithosphere. By utilizing published flow laws for plausible lower crustal rocks, lateral extrusion is found to be significant under elevated plains and very significant beneath high plateaus. Applications to the Tibetan Plateau and the Western U.S. Basin and Range show that short-wavelength Moho relief cannot be maintained and are flattened very quickly. Therefore the Moho cannot provide information regarding past tectonics nor data for balancing cross sections, and its flatness does not constrain the shortening or extension of the geographic areas studied.
GPS displacement vectors show that the crust in east Tibet is being squeezed in an easterly direction by the northward motion of the Indian plate, and the Sichuan Basin is resisting this stream and redirecting it mainly towards Indochina. The Longmen Shan, containing the steepest rise to the high plateau anywhere in Tibet, results from the strong interaction between the east Tibetan escape flow and the rigid Yangtze block (Sichuan Basin), but the kinematics and dynamics of this interaction are still the subject of some debates. We herein present results from a dense passive-source seismic profile from the Sichuan Basin into eastern Tibet in order to study the deep structure of this collision zone. Using P and S receiver function images we observe a sudden rise of the Lithosphere-Asthenosphere Boundary (LAB) from 120 to 150?km beneath the Sichuan Basin and from 70 to 80?km beneath eastern Tibet. In contrast, the depth of the crust-mantle boundary (Moho) increases from 36 to 40?km beneath the Sichuan Basin and from 55 to 60?km beneath eastern Tibet. The 410?km discontinuity is depressed below eastern Tibet by about 30?km, although the 660 remains at nearly the same depth throughout the LMS. From these observations, we conclude that the mode of collision that occurs between Tibet and the Sichuan Basin is very different to that found between India and Tibet. In southern Tibet, we observe in essence the subduction of the Indian plate, which penetrates northwards for several hundred kilometers under central Tibet. The very thin mantle part of the lithosphere beneath eastern Tibet may indicate delamination or removal of the bottom of the lithosphere by hot asthenospheric escape flow. This process leads to the exceptionally steep topography at the eastern Tibetan margin as a result of gravitational buoyancy. This view is supported by the very unusual depression of the 410?km discontinuity beneath eastern Tibet, which could be caused by the dynamics of the sub-vertical downward asthenospheric flow.