Extensional collapse of the Tibetan Plateau: Results of three-dimensional finite element modeling (original) (raw)
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The Tonga Wadati-Benioff zone is characterized by a large seismicity gap beneath the Lau Basin that raises the question of the slab continuity between the shallow to intermediate part (60-300 km) and the deep part (400-700 km). To address this problem, we investigated the Wadati-Benioff Zone geometry and stress regime through a detailed analysis of the spatial distribution of moment tensors and variation of the stress tensor, using the global seismicity [Engdahl, E., Van der Hilst, R., Buland, R., 1998. Global teleseismic earthquake relocation with improved travel times and procedures for depth determination. Bull. Seism. Soc. Am. 88, 722-743.] and the Centroid Moment Tensor solutions (CMT) catalogs [Dziewonski, A.M., Chou, T., Woodhouse, J.H., 1981. Determination of earthquake source parameters from waveform data for studies of global and regional seismicity. J. Geophys. Res. 86, 2825Res. 86, -2852. The stress tensors were computed using the Gephart's program . An improved method for determining the regional stress tensor using earthquake focal mechanism data: application to the San Fernando earthquake sequence. J. Geophys. Res. 89,[9305][9306][9307][9308][9309][9310][9311][9312][9313][9314][9315][9316][9317][9318][9319][9320]. The stress inversion results indicate that between 21°S and 27°S, and depths down to 700 km the slab is under homogeneous down dip compressional stress regime, while north of 21°S we found strong variations of the stress orientations between the intermediate and deep portions of the slab. We also show that between 14°and 19°S, the stresses at intermediate depth (60-300 km) can be resolved into two slab parallel domains, a thin upper part of the slab that is under downdip compression and the lower part that is under downdip extension. This pattern of two zones with opposite mechanical behavior is characteristic of a subducted plate with a free lower limit that does not interact with the 670-km depth boundary. These results together with the large seismicity gap within the slab argue for a slab detachment.
Journal of Geophysical Research, 2012
1] An extensional earthquake sequence occurred in 2004-8 across a graben in the South Lunggar Rift on the Tibetan Plateau. We use InSAR data to determine the location, fault geometry and slip distribution of these earthquakes and to test whether the sequence is compatible with static stress triggering. The Mw 6.2 and 6.3 earthquakes in 2004 and 2005 both ruptured west-dipping faults on the east side of a graben. In 2008, a Mw 6.7 earthquake ruptured a pair of east-dipping fault segments on the other side of the graben, offset from the earlier ruptures. We compute first-order dislocation models of stress change and demonstrate that the order and spatial configuration of this sequence of events is compatible with triggering by static stress transfer. A continuation of the sequence would be most likely to occur on the northern extension of the 2008 rupture, although variable slip rate along the rift may mean that the sequence has run its course. The InSAR data for the 2008 earthquake also reveal slip on a fault that cuts the graben at a highly oblique angle. We suggest that this is a release fault accommodating differential throw in the hanging wall, and associate the deformation with a Mw 6.0 aftershock. Activity on such a release fault has not been directly imaged before. The Zhongba sequence is one of several examples of recent clustered normal-fault earthquakes on the Plateau, and may be an example of phase-locking of similar faults.
Earth, Planets and Space, 2014
A number of tectonic models have been proposed for the Tibetan Plateau, which origin, however, remains poorly understood. In this study, investigations of the shear wave velocity (Vs) and density (ρ) structures of the crust and upper mantle evidenced three remarkable features: (1) There are variations in Vs and ρ of the metasomatic mantle wedge in the hanging wall of the subduction beneath different tectonic blocks of Tibet, which may be inferred as related to the dehydration of the downgoing slab. (2) Sections depicting gravitational potential energy suggest that the subducted lithosphere is less dense than the ambient rocks, and thus, being buoyant, it cannot be driven by gravitational slab pull. The subduction process can be inferred by the faster SW-ward motion of Eurasia relative to India as indicated by the plate motions relative to the mantle. An opposite NE-ward mantle flow can be inferred beneath the Himalaya system, deviating E and SE-ward toward China along the tectonic equator. (3) The variation in the thickness of the metasomatic mantle wedge suggests that the leading edge of the subducting Indian slab reaches the Bangoin-Nujiang suture (BNS), and the metasomatic mantle wedge overlaps with a region with poor Sn-wave propagation in north Tibet. The metasomatic layer, north of the BNS, deforms in the E-W direction to accommodate lithosphere shortening in south Tibet.
Earth and Planetary Science Letters, 2018
India-Asia collision analogue modeling Xianshuihe fault Longmen Shan thrust belt Songpan-Ganzi terrane Pre-existing weakness due to repeated tectonic, metamorphic, and magmatic events is a fundamental feature of the continental lithosphere on Earth. Because of this, continental deformation results from a combined effect of boundary conditions imposed by plate tectonic processes and heterogeneous and anisotropic mechanical strength inherited from protracted continental evolution. In this study, we assess how this interaction may have controlled the Cenozoic evolution of the eastern Tibetan plateau during the India-Asia collision. Specifically, we use analogue models to evaluate how the pre-Cenozoic structures may have controlled the location, orientation, and kinematics of the northwest-striking Xianshuihe and northeast-striking Longmen Shan fault zones, the two most dominant Cenozoic structures in eastern Tibet. Our best model indicates that the correct location, trend, and kinematics of the two fault systems can only be generated and maintained if the following conditions are met: (1) the northern part of the Songpan-Ganzi terrane in eastern Tibet has a strong basement whereas its southern part has a weak basement, (2) the northern strong basement consists of two pieces bounded by a crustal-scale weak zone that is expressed by the Triassic development of a northwest-trending antiform exposing middle and lower crustal rocks, and (3) the region was under persistent northeast-southwest compression since ∼35 Ma. Our model makes correct prediction on the sequence of deformation in eastern Tibet; the Longmen Shan right-slip transpressional zone was initiated first as an instantaneous response to the northeast-southwest compression, which is followed by the formation of the Xianshuihe fault about a half way after the exertion of northeast-southwest shortening in the model. The success of our model highlights the importance of pre-existing weakness, a key factor that has been largely neglected in the current geodynamic models of continental deformation.
The Himalayas and the Tibetan Plateau are the result of the continental collision between India and Eurasia. The Indian Plate underthrusts the Himalayan mountains and the southern Tibetan Plateau. Recorded seismicity at the Himalayan collision zone suggests that earthquakes occur mainly at upper crustal depths and near the crust-mantle boundary. The question of whether the near-Moho earthquakes are in the crust or in the upper mantle has been controversial, and has raised another question about the role of the mantle in the support of mountain loads and its ability to deform by brittle processes. Earthquake locations from several experiments place seismic events in the upper mantle. Using a finite element model, we establish a link between the recorded upper mantle seismicity beneath the Himalayan collision zone and flexural bending of the Indian lithosphere. Earthquake locations, focal mechanisms, and seismic imaging results from the HIMNT experiment, combined with previous constraints on the geometry and deformation of the Himalayan collision, are used to set up the finite element models of lithospheric loading. Our purpose is to infer the mechanical state of the lithosphere beneath the Himalayas and to evaluate the role of the lithospheric mantle in the support of the loads. The pattern of mantle seismicity can be explained by modeling the response of the Indian Plate to loads corresponding to the weight of the sediments of the Ganga basin, the Himalayan mountains and the southernmost Tibetan Plateau, combined with the effects of a horizontal force per unit length acting upon the lithospheric plate. We calculated the steady-state stress field in the Indian lithosphere, where the lithospheric mantle is assumed to be viscoelastic and non-Newtonian, and the asthenosphere is modeled as viscoelastic and Newtonian. Two model suites were tested, one with an elastic crust (Model Suite 1), and one with a viscoelastic crust (Model Suite 2). Both model suites provide a good fit to the observed patterns of seismicity, but Model Suite 2 is the one that best reproduces the observations. High differential stresses concentrate in the upper mantle, and predicted principal stress orientations match those inferred from focal mechanisms in the area. Our models show that beneath the Ganga basin and the southernmost Himalaya, earthquakes at near-Moho depths do not need to show extension, nor is the lower crust required to be weak, in order to infer that the uppermost mantle yields by brittle failure. Even when flexural stresses can generate the background stresses responsible for the generation of upper mantle earthquakes, Mohr-Coulomb theory suggests that additional factors such as the presence of lateral heterogeneities or the action of pore fluids are playing a fundamental role in bringing the upper mantle materials to brittle failure.
Large-scale crustal deformation of the Tibetan Plateau
Journal of Geophysical Research, 2001
The topography, velocity, and strain fields calculated from a three-dimensional Newtonian viscous model for large-scale crustal deformation are generally in good agreement with results from geological, geodetic and earthquake studies in and around the Tibetan Plateau, provided that the model rheology incorporates a weak zone within the deep crust beneath the plateau (equivalent to a viscosity of 10 12 Pa s within a 250-mthick channel or 10 18 Pa s within a 15-km-thick channel). Model studies and observations show a plateau at ϳ5 km elevation with steep topographic gradients across the southern and northern plateau margins and more gentle gradients across the southeastern and northeastern margins. Rapid shortening strain is concentrated along the lower portions of the northern and southern plateau margins (at rates ϳ20 mm/yr). Model results show north-south shortening (ϳ10 mm/yr) in reasonable agreement with GPS data (5-8 mm/yr of north-south shortening across the northern two thirds of the plateau) and east-west stretching (10 -15 mm/yr) across the eastern half of the high plateau, in reasonable agreement with seismic, geologic, and GPS data. Upper crustal material moves eastward from the plateau proper into a lobe of elevated topography that extends to the south and east. Clockwise rotation of material around the east Himalayan syntaxis (at rates up to ϳ10 mm/yr) occurs partly as a result of dextral shear between Indian and Asian mantle at depth and partly as a result of gravitational spreading from the high plateau to the south and east. There is little difference in model surface deformation for assumptions of moderately weak or extremely weak lower crust, except in southern and northern Tibet where margin-perpendicular extension occurs only for the case of an extremely weak lower crust. Our results suggest that the Tibetan Plateau is likely to have gone through a twostage development. The first stage produced a long, narrow, high orogen whose height may have been comparable to the modern plateau. The second stage produced a plateau that grew progressively to the north and east. East-west stretching, eastward plateau growth and dextral rotation around the east Himalayan syntaxis probably did not begin until well into the second stage of plateau growth, perhaps becoming significant after ϳ20 m.y. of convergence.
Late Cenozoic tectonics of the Tibetan Plateau
Journal of Geophysical Research, 1978
Normal faulting interpreted from Landsat imagery and fault plane solutions of earthquakes suggest late Cenozoic east-west extension in the Tibetan Plateau. Volcanism and earthquake swarms could be additional evidence for extension, although they are not unique to extensional environments. Normal faulting contrasts with major thrusting along the southern boundary and major strike slip faulting near the other boundaries of Tibet. The extension may be related to western Tibet being compressed between thrusting on the south and strike slip faulting on the northwest and consequently spreading along an east-west trend. INTRODUCTION The Tibetan Plateau (Figure 1) is bounded on the north, south, and west by the Kunlun, the Himalayan, and the Karakoram mountains, respectively. Between 93 ø and 100øE, ranges trending northwest and north separate Tibet from southeastern China. The average elevation in the Tibetan Plateau is about 5 km. Although very few published geological and geophysical data on the Tibetan Plateau are available, the Tibetan Plateau is presumably a product of collision of the Indian and Eurasian plates [Gansser, 1964; Dewey and Burke, 1973; Burke et al., 1974; Molnar and Tapponnier, 1975]. Stepped river terraces and abnormally large alluvial fans in the mouths of tributaries of the Tsangpo and Sutlej rivers [Li,. 1960] suggest that uplift may be still continuing, at least along the southern boundary. The geology of Tibet has been summarized by Henning [1916], Norin [1946], T. Chang [1963], and Burke et al. [1974]. Volcanics of late Cenozoic age, mostly of calc-alkaline type, are widely distributed over Tibet, especially in southern Tibet [Bonvalot, 1892; Henning, 1916; Burke et al., 1974; Kidd, 1975]. This and the presence of fumarolic fields in the Nyenchen Thanghla Mountains imply that the crust of Tibet is hotter than the crust in a stable region. Available data indicate that late Cenozoic deformation includes strike slip faulting [Molnar and Tapponnier, 1975; Tapponnier and Molnar, 1976; York et al., 1976], thrusting and folding in the upper crust and perhaps ductile creep at a deeper level [Dewey and Burke, 1973; Burke et al., 1974], and normal faulting in the Tibetan Himalayas [Gansser, 1964; Bordet et al., 1968]. This paper is primarily concerned with understanding the contemporaneous tectonics through the use of data on seismicity, fault plane solutions, and geological features interpreted from Landsat imagery. Reliable geophysical data for the Tibetan Plateau are scarce. Negative Bouguer gravity and near-zero free air gravity anom-inefficient propagation along paths that cross Tibet [Molnar and Oliver, 1969; Ruzaikin et al., 1977]. DATA Seismicity Seismicity in the Tibetan Plateau is high for an intraplate area. Often the seismicity is characterized by swarms of earthquakes with maximum magnitudes of approximately 6.5. Two large earthquakes (M _> 7.8) have occurred, in 1950 and in 1951. In addition, since 1900 15 earthquakes with magnitudes from 7.0 to 7.8 occurred in this region [Shih et al., 1973; York et al., 1976]. Earlier earthquake records in Tibet are rare, mainly because of its low population. For comparison of the faults interpreted from Landsat imagery with earthquakes, epicenters were selected on the basis of a good azimuthal distribution of teleseismic stations. These epicenters were located by the International Seismological Summary (ISS) from 1960 to 1963 and by the International Seismological Centre (ISC) from 1964 to 1975 (Figure 2). Events south of the main central thrust and east of 97øE are not included. The selected epicenters, about 45% of all located earthquakes, represent the most accurately located epicenters. Fault Plane Solutions New fault plane solutions were determined for five earthquakes using the first motion of P waves and some S wave polarizations recorded on long-period instruments of the World-Wide Standard Seismograph Network (WWSSN) between 1967 and 1975 (Figure 3). Some of the fault plane solutions (1-3) are poorly constrained because of their small magnitude and the unavailability of seismic data from nearby stations. In Figure 2, fault plane solutions of Ritsema [1961], Fitch [1970], Molnar et al. [1973], Ben-Menahetn et al. [1974], and Tapponnier and Molnar [1977] are given in addition to these five solutions. alies are reported by C. F. Chang and S. L. Zeng [1974], which suggest thicker than normal crust and isostatic equilibrium of Landsat Imagery the Tibetan Plateau. No refraction studies are published to our knowledge. Studies of surface-wave dispersion data show a possible 65-to 70-km thickness of crust under the Tibetan
A deforming block model for the present-day tectonics of Tibet
Journal of Geophysical Research, 2004
1] We use GPS data from 45 sites across the Tibetan Plateau surveyed between 1991 and 2001 to study the distribution of strain in that part of the India-Eurasia collision zone. The plateau is cut by a few major, rapidly slipping strike-slip fault zones, with broadly distributed strain between those zones. The GPS velocities can be fit well by a simple deforming block model that combines uniform strain with the motion of blocks separated by the major known fault zones within the plateau. The boundaries of the four blocks in this model correspond to the major, rapidly slipping faults. A rigid block model with a smaller number of blocks fits the data poorly and can be rejected. We estimate that 5.9 ± 0.7 mm/yr of extension in the N69°W direction occurs on the Yadong-Gulu rift south of the plateau; localized extension may extend as far north as the Nyainqêntanglha Range. We find 7.4 ± 0.7 mm/yr of right-lateral slip on the Karakorum-Jiali fault zone, significantly slower than that previously estimated from offset geologic features. Our deforming block model and a two-dimensional singlefault screw dislocation model give lower and upper bounds of 4.4 ± 1.1 and 10.3 ± 0.4 mm/yr on the slip rate on the Kunlun fault, respectively, comparable, in its highest, to the long-term slip rate observed geologically. The distributed deformation is surprisingly uniform over the Tibetan Plateau, Qaidam Basin, and Qilian Shan and approximates a combination of pure shear and uniaxial contraction with the same axes of maximum contraction, $N32°E. This strain field is a combination of shortening and extension that produces little net dilatation (1% area loss per million years) at present. The broadly distributed strain most likely represents slip on many faults, each with a relatively low slip rate. Distributed conjugate strike-slip faulting is the most plausible mechanism to produce the observed strains, as supported by the record of medium to large earthquakes within the plateau. The distributed deformation is just as important in the accommodation of the total India-Eurasia convergence as is slip on the major faults. The eastward motion of central Tibet is as large as 50% of the convergence rate between India and Eurasia. However, this eastward motion is not motion of a relatively undeforming block bounded by rapidly moving strike-slip faults, as suggested by past extrusion models. Instead, much of this eastward extrusion is driven by the internal extension of the plateau. Our inference is in agreement with the model of F. Shen et al.
Great Himalayan earthquakes and the Tibetan plateau
Nature, 2006
It has been assumed that Himalayan earthquakes are driven by the release of compressional strain accumulating close to the Greater Himalaya. However, elastic models of the Indo-Asian collision using recently imaged subsurface interface geometries suggest that a substantial fraction of the southernmost 500 kilometres of the Tibetan plateau participates in driving great ruptures. We show here that this Tibetan reservoir of elastic strain energy is drained in proportion to Himalayan rupture length, and that the consequent growth of slip and magnitude with rupture area, when compared to data from recent earthquakes, can be used to infer a ,500-year renewal time for these events. The elastic models also illuminate two puzzling features of plate boundary seismicity: how great earthquakes can re-rupture regions that have already ruptured in recent smaller earthquakes and how mega-earthquakes with greater than 20 metres slip may occur at millennia-long intervals, driven by residual strain following many centuries of smaller earthquakes.