Normal faulting and viscous buckling in the Tibetan Plateau induced by a weak lower crust

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&#8217;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&#160;understanding of the geological processes involved in the thickening of the Tibetan crust, and discuss both the strengths and weaknesses of such modelling.

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Widespread crustal melting since 28 Ma creates the modern flat Tibet

10.5194/egusphere-egu2020-12351 ◽

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

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 &#8451; to 790 &#8451; 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&#8211;40 km (and even deeper) with 0.5&#8211;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&#8211;30 vol. %) of the mid-to-lower crust beneath Tibet was inevitable and created the modern flat Tibetan Plateau.

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Multilayered Crustal Anisotropy in Eastern Tibet Revealed by Receiver Function Data

10.5194/egusphere-egu2020-7623 ◽

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

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.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.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.Our results show that:(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.(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.(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.

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Normal Faulting Type Earthquake Activities in the Tibetan Plateau and Its Tectonic Implication

Acta Geologica Sinica - English Edition ◽

10.1111/j.1755-6724.2010.00175.x ◽

2010 ◽

Vol 84 (1) ◽

pp. 135-144 ◽

Cited By ~ 2

Author(s):

Jiren XU ◽

Zhixin ZHAO

Keyword(s):

Tibetan Plateau ◽

The Tibetan Plateau ◽

Normal Faulting ◽

Tectonic Implication

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

10.5194/egusphere-egu2020-14987 ◽

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

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 thermochronology1. 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 data2. Besides, the present-day GPS data show that the E-W extension rates match well with the eastward movements speeds interior Tibetan Plateau3. 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.Key words: N-S trends normal faults; Thermochronology; Magnetotellurics; Magmatism; GPS Measurements; middle to lower crustal flowReferences:1Lee, 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).2Pang, 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.3Zhang, P.-Z., Shen, Z., Wang, M., Gan, W., B&#252;rgmann, 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).Acknowledgements: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&#65292;Ya-Lin Li and Cheng-Shan Wang for many critical and constructive comments.

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Petrogenesis and tectonic setting of the Early Cretaceous granitoids in the eastern Tengchong terrane, SW China: Constraint on the evolution of Meso-Tethys

Lithosphere ◽

10.1130/l1149.1 ◽

2020 ◽

Vol 12 (1) ◽

pp. 150-165

Author(s):

Xiaohu He ◽

Zheng Liu ◽

Guochang Wang ◽

Nicole Leonard ◽

Wang Tao ◽

...

Keyword(s):

Tibetan Plateau ◽

Partial Melting ◽

Early Cretaceous ◽

Lower Crust ◽

Tectonic Setting ◽

Igneous Rocks ◽

Source Rocks ◽

Granitic Rocks ◽

The Tibetan Plateau ◽

Sw China

Abstract As a result of the evolution of Meso-Tethys, Early Cretaceous granitoids are widespread in the eastern Tengchong terrane, SW China, which is considered as the southern extension of the Tibetan Plateau. These igneous rocks are therefore very important for understanding the tectonic setting of Meso-Tethys and the formation of the Tibetan Plateau. In this paper, we present new zircon U-Pb dating, whole-rock elemental, and Nd isotopic data of granitoids obtained from the eastern Tengchong terrane. Our results show that these granitoids are composed of monzogranites and granodiorites and formed at ca. 124 Ma in the Early Cretaceous. Mineralogically and geochemically, these granitoids display metaluminous nature and affinity to I-type granites, which are derived from preexisting intracrustal igneous source rocks. The predominantly negative whole-rock εNd(t) values (−10.86 to −8.64) for all samples indicate that they are derived mainly from the partial melting of the Mesoproterozoic metabasic rocks in the lower crust. Integrating previous studies with the data presented in this contribution, we propose that the Early Cretaceous granitic rocks (135–110 Ma) also belong to I-type granites with minor high fractionation. Furthermore, in discriminant diagrams for source, granitoids are mainly derived from the partial melting of metaigneous rocks with minor sediments in the lower crust. The new identification of the Myitkyina Meso-Tethys ophiolitic suite in eastern Myanmar and mafic enclaves indicate that these Cretaceous igneous rocks were the products of the tectonic evolution of the Myitkyina Tethys Ocean, which was related to post-collisional slab rollback. Moreover, the Tengchong terrane is probably the southern extension of the South Qiangtang terrane.

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Support for a “Jelly Sandwich” Model of the Tibetan Plateau

Eos ◽

10.1029/2021eo210581 ◽

2021 ◽

Vol 102 ◽

Author(s):

Morgan Rehnberg

Keyword(s):

Tibetan Plateau ◽

Computer Modeling ◽

Wenchuan Earthquake ◽

Upper Mantle ◽

Lower Crust ◽

The Tibetan Plateau ◽

2008 Wenchuan Earthquake ◽

The 2008 Wenchuan Earthquake ◽

Positional Data ◽

Sandwich Model

Computer modeling constrained by positional data collected in the aftermath of the 2008 Wenchuan earthquake indicates the lower crust is less viscous than the upper mantle below it.

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Extensional Seismogenic Stress and Tectonic Movement on the Central Region of the Tibetan Plateau

International Journal of Geophysics ◽

10.1155/2009/897424 ◽

2009 ◽

Vol 2009 ◽

pp. 1-8

Author(s):

Jiren Xu ◽

Zhixin Zhao

Keyword(s):

Tibetan Plateau ◽

Normal Fault ◽

Stress Fields ◽

Tectonic Activity ◽

The Tibetan Plateau ◽

Normal Faulting ◽

Earthquake Occurrence ◽

Crustal Movements ◽

High Plateau ◽

Central Tibetan Plateau

Various earthquake fault types, mechanism solutions and stress fields, as well as GPS and geothermal data are analyzed for the study of the crustal movements on the Tibetan plateau and their tectonic implications. The results show that a lot of the normal faulting type-event concentrated at altitudes greater than 4000 m on the central Tibetan plateau. The altitudes concentrating normal faulting type-events can be zoned two parts: the western part, the Lhasa block, and the eastern part, the Qiangtang-Changdu region. The azimuths of T-axes are in a general E-W direction in the Lhasa block and NW-SE or NNW-SSE in the Qiangtang-Changdu region at the altitudes of the Tibetan plateau. The tensional stresses in E-W direction and NW-SE direction predominate normal faulting earthquake occurrence in the Lhasa block and the Qiangtang-Changdu region, respectively. The slipping displacements of the normal-faulting-type events have great components in near E-W direction and NW-SE direction in the Lhasa block and the Qiangtang-Changdu region, respectively. The extensions are probably an eastward or southeastward extensional motion, being mainly tectonic activity phenomena in the plateau altitudes. The extensional motions due to normal-fault earthquakes are important tectonic activity regimes on the high altitudes of the plateau. The easterly crustal extensions on the plateau are attributable to the gravitational collapse of the high plateau and eastward extrusion of hotter mantle materials beneath the eastern boundary of the plateau. Numbers of thrust-fault and strike-slip-fault earthquakes with strong compressive stress in a general NNE-SSW direction occur on the edges of the plateau.

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Geochemical and isotopic data of Zheduo-Gongga granitic intrusive complex, eastern margin of the Tibetan Plateau: no evidence for middle-lower crustal flow

10.5194/egusphere-egu2020-1918 ◽

2020 ◽

Author(s):

Fangyang Hu ◽

Fuyuan Wu ◽

Mihai Ducea ◽

James Chapman

Keyword(s):

Tibetan Plateau ◽

Partial Melting ◽

Lower Crust ◽

Biotite Granite ◽

The Tibetan Plateau ◽

Eastern Margin ◽

Granitoid Rocks ◽

Lower Crustal Flow ◽

Group 2 ◽

Lower Crustal

Geophysical studies have shown that middle-lower crustal flow started from central Tibetan Plateau may exist in the eastern margin of the Tibetan Plateau, which controls the mountain building, crustal thickening and deformation (Schoenbohm et al., 2006; Bai et al., 2010; Bao et al., 2015; Zhu et al., 2017). However, no geological and petrological evidence have been presented. We carried out detailed studies on the geochemical and isotopic compositions of the Mesozoic-Cenozoic Zheduo-Gongga granitic intrusive complex on the eastern margin of the Tibet Plateau. Geochronology studies show that these granitoid rocks are formed during Mesozoic to Cenozoic, including ~220-200 Ma Gongga granodiorite to biotite granite with mafic enclaves, ~40 Ma Zheduo gneissic granite, ~28 Ma Zheduo monzogranite, and ~20-4 Ma Zheduo biotite granite and monzogranite. Two groups of geochemical features are obtained: Group 1 (gnessic granite, granodiorite, monzogranite, and leucogranite) has relatively low K2O, Th/La, La/Yb and Rb/Sr ratios, but high Sr/Y ratio with no Eu negative anomalies; Group 2 (biotite granite) has relatively high K2O, Th/La, La/Yb and Rb/Sr ratios, but low Sr/Y with strong negative Eu anomalies. The Sr-Nd-Hf-O isotopic studies on plagioclase, apatite and zircon show that their sources are primarily the basement of the western margin of Yangtze Craton and Songpan-Ganzi sediments. These features indicate that they have different petrogenesis processes. Group 1 is mainly derived from partial melting of mafic rocks in the lower crust, whereas the Group 2 is primarily derived from partial melting of metasedimentary rocks experiencing fractionation of plagioclase. Magma derived from different sources mixing with each other are observed as well. Therefore, from geochemical aspects, no exotic materials are involved in the formation of granitoid rocks during Mesozoic to present. The flow of crustal material in the middle-lower crust may be not existed. The low velocity and high conductivity layer in the middle-lower crust may represent a regional partial melting zone, which could be related to the upwelling of asthenosphere. Both crustal deformation and upwelling of asthenosphere may contribute to the crustal thicknening and uplift.

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