Widespread crustal melting since 28 Ma creates the modern flat Tibet

2020 ◽  
Author(s):  
Xiu-Zheng Zhang ◽  
Qiang Wang ◽  
Wei Dan
Keyword(s):  
Tibetan Plateau ◽  
Lower Crust ◽  
Theoretical Models ◽  
Geothermal Gradient ◽  
Magma Source ◽  
The Tibetan Plateau ◽  
Crustal Melting ◽  
Low Velocity ◽  
Crustal Xenoliths ◽  
Orogenic Belts

<p>As the largest and highest plateau on Earth, the Tibetan Plateau is distinguished from most other ranges and liner continental orogenic belts (e.g., the Alps) by its broad and flat topography. According to influential numerical and theoretical models, the (former) existence of ductile and molten mid-to-lower crust was an essential contributor to the topographic smoothing process. However, the question of whether the Tibetan Plateau has undergone widespread crustal melting remains highly controversial and hard to prove due to the scarcity of direct evidence from the deep crust. Here we first report on a series of hydrous crustal xenoliths entrained in 28 Ma host lavas from central and northern Tibet. Our new results document the former existence of hydrous crust at 28 Ma as a potentially highly fertile magma source. Quantitative modeling reveals a thermal gradient reaching about 680 ℃ to 790 ℃ at a depth of 14 to 40 kilometers, which is significantly lower than that of recent (since 2.3 Ma) evidence for hot Tibetan crust. Petrological data suggest that the initial crustal melting beneath Tibet began at 28 Ma at depths of 23–40 km (and even deeper) with 0.5–9.6 vol. % melts, which would lead to a significant reduction of seismic speeds similar to the low-velocity zones observed in the present Tibetan mid-to-lower crust. As the geothermal gradient continued to rise from 28 to 2.3 Ma, wholesale crustal melting (> 20–30 vol. %) of the mid-to-lower crust beneath Tibet was inevitable and created the modern flat Tibetan Plateau.</p>

Crustal structure of the Tibetan Plateau: A surface-wave study by a moving window analysis

1977 ◽  
Vol 67 (3) ◽  
pp. 735-750
Author(s):  
Kin-Yip Chun ◽  
Toshikatsu Yoshii
Keyword(s):  
Tibetan Plateau ◽  
Crustal Structure ◽  
The Tibetan Plateau ◽  
Moving Window ◽  
Low Velocity ◽  
Period Range ◽  
Window Analysis ◽  
Dispersion Data ◽  
Moving Window Analysis ◽  
Group Velocities

abstract Group velocities of fundamental-mode Rayleigh and Love waves are analyzed to construct a crustal structure of the Tibetan Plateau. A moving window analysis is employed to compute group velocities in a wide period range of 7 to 100 sec for 17 individual paths. The crustal models derived from these dispersion data indicate that under the Tibetan Plateau the total crustal thickness is about 70 km and that the crustal velocities are generally low. The low velocities are most probably caused by high temperatures. A low-velocity zone located at an intermediate depth within the crust appears to be strongly demanded by the observed dispersion data. The main features of the proposed crustal structure will place stringent constraints on future tectonic models of the Tibetan Plateau which is generally regarded as a region of active deformation due to the continent-continent collision between India and Asia.

Combining thermo-kinematic and mechanical modelling on thrust faults - a quantitative approach to crustal deformation history: Case study from SE Tibet

2021 ◽  
Author(s):  
Paul Pitard ◽  
Anne Replumaz ◽  
Marie-Pierre Doin ◽  
Cédric Thieulot ◽  
Marie-Luce Chevalier ◽  
...  
Keyword(s):  
Tibetan Plateau ◽  
Low Temperature ◽  
Mechanical Behavior ◽  
Numerical Simulations ◽  
Lower Crust ◽  
Quantitative Approach ◽  
The Tibetan Plateau ◽  
Thrust Faults ◽  
Mechanical Modelling ◽  
Kinematic Modelling

<p>Decoding the Tibetan plateau and its structural evolution has been a thorny issue for decades, triggering many controversial discussions between the proponents of the numerous key models. Numerical simulations of buoyancy forces associated with a thick crust and a low viscosity channel in the Tibetan crust predict continuous deformation, crustal uplift and thickening through an outward flow of partially molten middle/lower crust. Surface geological observations of fault systems, however, favor a model of localized deformation through the interaction between strike-slip and thrust faults. Here, we investigate the role of thrusting mechanisms involved in the plateau formation, which is essential in order to discuss these end-members competing models. We focus on the Muli thrust, a major Miocene thrust fault located at the eastern edge of the Tibetan Plateau, characterized by a pronounced topographic step of ~2000 m. We provide here an innovative quantitative approach combining thermo-kinematic modelling based on low-temperature thermochronology data, with conceptual 2-dimensional (2D) simulations of the crust’s mechanical behavior. Using the code PECUBE, we test different scenarios of rock cooling by forward modelling and inversion method in order to constrain the amount and timing of exhumation, as well as its simplified first-order crustal geometry. Given that low-temperature thermochronology data only provides the thermal history of the upper part of the crust (< 10 km), such thermo-kinematic modelling does not reveal any direct evidence of the potential implication of the lower crust. To overcome such limitations, we performed 2D mechanical modelling of the Muli thrust to constrain its mechanical behavior at the crustal scale to decipher its importance in the thickening of the plateau margin. We present here, how complementary numerical simulations based on in-situ geological observations on thrust faults, combined with thermochronology data, can be used to have a better understanding of the geological processes involved in the thickening of the Tibetan crust, and discuss both the strengths and weaknesses of such modelling.</p>

Multilayered Crustal Anisotropy in Eastern Tibet Revealed by Receiver Function Data

2020 ◽  
Author(s):  
Shaohua Qi ◽  
Qiyuan Liu ◽  
Jiuhui Chen ◽  
Biao Guo
Keyword(s):  
Tibetan Plateau ◽  
Lower Crust ◽  
Seismic Anisotropy ◽  
Receiver Function ◽  
Azimuthal Anisotropy ◽  
Strain History ◽  
The Tibetan Plateau ◽  
Anisotropy Axis ◽  
Upper Crust ◽  
Western Sichuan

<p>It is widely accepted that the ongoing India-Asia collision since approximately 50 Ma ago has resulted in the uplift and eastward expansion of the Tibetan Plateau. Yet the interpretations of its dynamic process and deformation mechanism still remain controversial. Distinct models that emphasize particular aspects of the tectonic features have been proposed, including fault-controlled rigid blocks, continuous deformation of lithosphere and lower crust flow.</p><p>One possible way to reconcile these models is to investigate crustal deformation at multiple depths simultaneously, as well as crust-mantle interaction. Seismic anisotropy is considered as an effective tool to study the geometry and distribution of subsurface deformation, due to its direct connection to the stress state and strain history of anisotropic structures and fabrics. In the eastern margin of Tibetan plateau, previous studies of seismic anisotropy have already provided useful insights into the bulk anisotropic properties of the entire crust or upper mantle, based on shear wave splitting analyses of Moho Ps and XKS phases.</p><p>In this study, we went further to extract anisotropic parameters of multiple crustal layers by waveform inversion of teleseismic receiver function (RF) data from the western-Sichuan temporal seismic array using particle swarm optimization. Instead of directly fitting the backazimuthal stacking of RFs from each station, we translated the RF data into backazimuthal harmonic coefficients using harmonic decomposition technique, which separates the signals (of planar isotropic structure and anisotropy) from the scattering noise generated by non-planar lateral heterogeneity. The constant (k=0) and k=1, 2 terms of backazimuthal harmonic coefficients were used in our inversion. We also fixed the anisotropic model to slow-axis symmetry to avoid ambiguous interpretations.</p><p>Our results show that:</p><p>(1) Anisotropy with a titled anisotropy axis of symmetry is more commonly observed than pure azimuthal anisotropy in our data, which has been also reported by other RF studies across the surrounding areas of Tibetan plateau.</p><p>(2) The trends of slow symmetry axis vary from the upper to lower part of the crust in both Chuandian and Songpan units, indicating the deformation of the upper crust is decoupled from that of the lower crust in these two regions, while the trends are more consistent throughout the crust in the Sichuan basin.</p><p>(3) In the upper crust, the trends show a degree of tendency to lie parallel to the major geological features such as the Xianshuihe and Longmenshan faults, exhibiting a fault-controlled deformation or movement. In the middle and lower crust, the trends are NS or NW-SE in Chuandian unit and NE-SW in Songpan unit, which are coincident with the apparent extension directions of the ductile crustal flow.</p>

Lithospheric buckling dominates the Cenozoic subsidence of the Qaidam Basin, NE Tibetan Plateau

2021 ◽  
Author(s):  
Hu Xiaoyi ◽  
Wu Lei
Keyword(s):  
Tibetan Plateau ◽  
Qaidam Basin ◽  
Foreland Basin ◽  
Structural Features ◽  
Tectonic Subsidence ◽  
The Tibetan Plateau ◽  
Basin Formation ◽  
Orogenic Belts ◽  
The Impact ◽  
First Time

<p>Flexural basins are the common geological feature in convergent settings, and usually regarded as the result of flexural subsidence of the margins of under-thrusting cratons in response to the gravitational load of over-riding orogens. This process usually causes the fastest tectonic subsidence and thickest orogenic-related deposits in the basin margins adjacent to the orogens, such as India Foreland Basin in front of the Himalaya. The Qaidam Basin, which is the largest sedimentary basin within the Tibetan Plateau interior, was once interpreted to belong to this type and form by flexural subsidence on its south and north margins in response to loading of the Qiman Tagh and the South Qilian Shan orogenic belts, respectively. However, the latest studies present sedimentary and structural features that contrast to a typical foreland basin. These features include (1) depocenters being located along the central axis, rather than the margins, with thickest sediments up to 15 km, and (2) development of many high-angle reverse faults, rather than thin-skinned thrusts, to generate upper-crustal shortening as low as 10-15% (20 – 30 km), indicating that the widths of the orogenic belts juxtaposed atop the basin margins are limited. These features cannot be explained by the flexural subsidence of basin margins and/or sediment load. Herein, we investigate the impact of lithospheric buckling, which has been ignored in most studies of basin formation in compressional settings, on the tectonic subsidence of the Qaidam Basin through numerical simulation based on finite elastic plate model. We first use the flexural backstripping method to calculate the tectonic subsidence of the Cenozoic basement across the Qaidam Basin. And then, we simulate the tectonic subsidence caused by (1) gravitational load of orogenic belts alone, and (2) combined gravitational load and lithosphere buckling. The result shows that the simulated tectonic subsidence curve fits well with the real one only when considering the effect of lithospheric buckling that accounts for >90% tectonic subsidence. Our finding indicates for the first time that lithospheric buckling is also an important mechanism for the subsidence of intramountain basins like the Qaidam Basin, and should not be ignored when studying lithospheric-scale deformation across large orogenic belts.</p>

Crustal thickening and uplift of the Qiangtang Terrane, Tibetan Plateau during the Late Cretaceous to Early Palaeocene: geochronology and geochemistry of the Saiduopugangri granite

2020 ◽  
Author(s):  
xue li ◽  
Guo-Sheng Sun ◽  
Gen-Yi Liu ◽  
Huan Zhou ◽  
Zi-Ling Shan ◽  
...  
Keyword(s):  
Tibetan Plateau ◽  
Late Cretaceous ◽  
Crustal Thickening ◽  
Magma Source ◽  
Geodynamic Setting ◽  
The Tibetan Plateau ◽  
Weighted Mean ◽  
Lower Crustal ◽  
Qiangtang Terrane ◽  
Shoshonitic Series

There continues to be debate regarding the timing of the collision between the Indian and Eurasian plates and the uplift of the Tibetan Plateau. This study presents zircon U–Pb geochronology, whole-rock geochemistry, and Lu–Hf isotopic data for the Saiduopugangri granite of the Qiangtang Terrane, located within the core of the Tibetan Plateau. These data provide the basis for the geodynamic setting, petrogenesis, and characteristics of its magma source. Zircons from the Saiduopugangri granite yield a weighted-mean 206Pb/238U age of 62.72 ± 0.06 Ma, indicating that these rocks formed during the early Palaeocene. The rocks are members of the highly calc-alkaline to shoshonitic series, with weak peraluminous characteristics. Trace elements are characterised by high Sr (483–616ppm), and low Y (6–10ppm) and Yb (1ppm) content, typical of a high Sr and low Yb granite. The εHf(t) of zircon range from −2.14 to 2.35, with two-stage Hf model ages (TDM2) ranging from 1182 to 895Ma. These data suggest that the Saiduopugangri granite magma was derived from the melting of lower-crustal clastic meta-sedimentary rocks and mantle-derived basalts. The high Sr and low Yb granite characteristics and experimental results indicate that melting occurred at >1.2 GPa and >750 °C, consistent with a crustal thickness greater than 50km. Magmatism occurred from the Late Cretaceous to the early Palaeogene and is broadly synchronous with the collision timing between the Indian and Eurasian plates. The Saiduopugangri granite provides evidence of crustal thickening of the Tibetan Plateau and its age and petrogenesis constrain the timing of the initial uplift.

The Mechanism and Dynamics of N-S trends normal faults in Tibetan Plateau: Insight From Thermochronology, Magnetotellurics, Magmatism and GPS Measurements

2020 ◽  
Author(s):  
Han-Ao Li ◽  
in-Gen Dai ◽  
Le-Tian Zhang ◽  
Ya-Lin Li ◽  
Guang-Hao Ha ◽  
...  
Keyword(s):  
Tibetan Plateau ◽  
Lower Crust ◽  
Normal Faults ◽  
The Tibetan Plateau ◽  
Gps Measurements ◽  
Southern Tibet ◽  
Magnetotelluric Data ◽  
Deep Process ◽  
Lower Crustal Flow ◽  
Lower Crustal

<p>The N-S trends normal faults are widespread through the whole Tibetan Plateau. It records key information for the growth and uplift of the Tibetan Plateau. Numerous models are provided to explain the causes of rifting in the Tibetan Plateau based on the low-temperature thermochronology<sup>1</sup>. With the developments of the geophysical and magmatic geochemistry methods and its applications on the Tibetan Plateau, we could gain more profound understanding on the sphere structure of the Tibetan Plateau. This would give us more clues on how the deep process affect the formation and evolution of the shallow normal faults. However, few researchers pay attention on this and the relationship between the surface evolution and deep process of these faults. In order to solve these puzzles, we collected the published thermochronology data, magnetotelluric data, faults-related ultrapotassic, potassic and the adakitic rocks ages and present-day GPS measurements. We find that the distribution of the N-S trends normal faults are closely related to the weak zones in the middle to lower crust (15-50 km) revealed by the magmatism and magnetotelluric data<sup>2</sup>. Besides, the present-day GPS data show that the E-W extension rates match well with the eastward movements speeds interior Tibetan Plateau<sup>3</sup>. Combined with the thermochronology data (25-4 Ma), we concluded that 1.The weak zone in the middle to lower crust influence the developments and evolution of the N-S trends normal faults. 2. The material eastward flow enhance the N-S normal faults developments. 3. The timing of the middle to lower crustal flow may begin in the Miocene.</p><p><strong>Key words:</strong> N-S trends normal faults; Thermochronology; Magnetotellurics; Magmatism; GPS Measurements; middle to lower crustal flow</p><p><strong>References:</strong></p><p><sup>1</sup>Lee, J., Hager, C., Wallis, S.R., Stockli, D.F., Whitehouse, M.J., Aoya, M. and Wang, Y., 2011. Middle to Late Miocene Extremely Rapid Exhumation and Thermal Reequilibration in the Kung Co Rift, Southern Tibet. Tectonics, 30(2).</p><p><sup>2</sup>Pang, Y., Zhang, H., Gerya, T.V., Liao, J., Cheng, H. and Shi, Y., 2018. The Mechanism and Dynamics of N-S Rifting in Southern Tibet: Insight from 3-D Thermomechanical Modeling. Journal of Geophysical Research: Solid Earth.</p><p><sup>3</sup>Zhang, P.-Z., Shen, Z., Wang, M., Gan, W., Bürgmann, R., Molnar, P., Wang, Q., Niu, Z., Sun, J., Wu, J., Hanrong, S. and Xinzhao, Y., 2004. Continuous Deformation of the Tibetan Plateau from Global Positioning System Data. Geology, 32(9).</p><p><strong>Acknowledgements:</strong></p><p>We thank Shi-Ying Xu, Xu Han, Bo-Rong Liu for collecting data. Special thanks are given to Dr. Guang-Hao Ha and Professors Jin-Gen Dai, Le-Tian Zhang，Ya-Lin Li and Cheng-Shan Wang for many critical and constructive comments.</p>

Adjoint Tomography of the Lithospheric Structure beneath Northeastern Tibet

2020 ◽  
Vol 91 (6) ◽  
pp. 3304-3312
Author(s):  
Xingpeng Dong ◽  
Dinghui Yang ◽  
Hejun Zhu
Keyword(s):  
Tibetan Plateau ◽  
Indian Plate ◽  
Tectonic Deformation ◽  
Lithospheric Structure ◽  
The Tibetan Plateau ◽  
S Wave ◽  
Low Velocity ◽  
Northeastern Tibet ◽  
Adjoint Tomography ◽  
Indian Lithosphere

Abstract Northeastern Tibet is still in the primary stage of tectonic deformation and is the key area for studying the lateral expansion of the Tibetan plateau. In particular, the existence of lower crustal flow, southward subduction of the Asian lithosphere, and northward subduction of the Indian lithosphere beneath northeastern Tibet remains controversial. To provide insights into these issues, a high-resolution 3D radially anisotropic model of the lithospheric structure of northeastern Tibet is developed based on adjoint tomography. The Tibetan plateau is characterized as a low S-wave velocity lithosphere, in contrast with the relatively high S-wave velocities of the stable Asian blocks. Our tomographic result indicates that the low-velocity zone (LVZ) within the deep crust extends northeastward from Songpan–Ganzi to Qilian, which is interpreted as a channel flow within the crust. The upper mantle of Alxa and Qinling–Qilian are dominated by a rather homogeneous LVZ, which is inconsistent with the hypothesis that the Asian lithospheric mantle is being subducted southward beneath northeastern Tibet. Furthermore, high-velocity regions are observed in the southern Songpan–Ganzi region at depths ranging from 100 to 200 km, indicating that the northward-subducting Indian plate has probably reached the Xianshuihe fault.

scholarly journals Lateral variation in crustal structure of the northern Tibetan Plateau inferred from teleseismic receiver functions

1995 ◽  
Vol 85 (6) ◽  
pp. 1531-1540 ◽  
Author(s):  
Lupei Zhu ◽  
Thomas J. Owens ◽  
George E. Randall
Keyword(s):  
Tibetan Plateau ◽  
Crustal Structure ◽  
Receiver Functions ◽  
The Tibetan Plateau ◽  
Low Velocity ◽  
Middle Crust ◽  
Northern Tibetan Plateau ◽  
Strike Direction ◽  
Low Velocity Layer ◽  
Velocity Layer

Abstract We investigate lateral variations in crustal structure across the northern boundary of the Tibetan Plateau using the receiver functions at three broadband stations deployed during the 1991-1992 Tibet PASSCAL experiment. The first 5 sec of the receiver functions vary systematically with backazimuth: the radial receiver functions are symmetric across the N-S axis while the tangential receiver functions are antisymmetric across this axis. This symmetry can be modeled by E-W striking dipping interfaces in the upper-middle crust. The strike direction is consistent with the E-W trend of surface geology. Modeling a P-to-S converted phase in the receiver functions at each station suggests that there is a mid-crustal low-velocity layer with its upper boundary dipping 20° to 30° to the south. In addition, a shallow northward-dipping interface is responsible for the “double-peaked” direct P arrivals in the radial receiver functions and large tangential motions at one of the stations. The low-velocity layer, together with other geological and seismological observations, suggests that there is a hot, possibly partial melt zone in the middle crust of northern Tibet. Alternately, dipping velocity interfaces might be associated with some buried thrust faults in the upper crust that accommodated crust shortening during the plateau formation.

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