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Geodymanics of Tibet, Tarim, and the Tien Shan in the Late Cenozoic V

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Geodymanics of Tibet, Tarim, and the Tien Shan in the Late Cenozoic V ISSN 00168521, Geotectonics, 2012, Vol. 46, No. 3, pp. 185–211. © Pleiades Publishing, Inc., 2012. Original Russian Text © V.S. Burtman, 2012, published in Geotektonika, 2012, Vol. 46, No. 3, pp. 18–46. Geodymanics of Tibet, Tarim, and the Tien Shan in the Late Cenozoic V. S. Burtman Geological Institute, Russian Academy of Sciences, Pyzhevskii per. 7, Moscow, 119017 Russia email: [email protected] Received August 29, 2011 Abstract—The tectonic and geodynamic consequences of the collision between Hindustan and Eurasia are considered in the paper. The tectonic evolution and deformation of Tibet and the Tien Shan in the Late Cen ozoic is described on the basis of geological, geophysical, and geodetic data. The factual data and their inter pretation, which shed light on the kinematics of the tectonic processes in the lithosphere and the geodynam ics of the interaction between the Tien Shan, Tarim, and Tibet are discussed. A geodynamic model of their interaction is proposed. DOI: 10.1134/S0016852112030028 INTRODUCTION apatite is used for estimating the exhumation rate of High Asia, Tarim, and the Tien Shan (Fig. 1) are the Paleozoic basement in the Cenozoic and calcula key areas for solution of the problems pertaining to tion of the mountainbuilding and denudation rates. collision and withinplate geodynamics. Collision was Palinologic studies are helpful for estimation of the caused by the convergence of continents during clo mountain topography age. Recurrent measurements sure of oceanic basins. In the Tien Shan, the last oce from satellites provide evidence for the distribution anic basin was closed in the Carboniferous. In High and rate of contemporary deformations. Asia, such processes took place in Carboniferous, Tri Seismic profiling and gravity measurements allow assic, Cretaceous, and Cenozoic [6]. The Hindustan– to determine the position of the Moho boundary and Eurasia collision took place when the Neotethys withincrust discontinuities. The distribution of earth ocean was closed in Cenozoic. quake epicenters and solution of focal earthquake The current paper discusses and interprets the data mechanisms show the activity of tectonic sutures and on the kinematics of tectonic processes in the lithos the stress field of the Earth’s crust. Seismic tomogra phere and the geodynamics of interaction between the phy demonstrates the localization and dimensions of Tien Shan, Tarim, and Tibet. subducted slabs and the kinematics and size of convec tion cells in the upper mantle. Research methods. The surface, crust, and mantle of the region under consideration have been studied Tectonic flow plays a substantial role in the geody with geological, geophysical, space, and onland namic processes discussed in this paper. Change of the geodesy. The data on the lithology, stratigraphy, and body shape (deformation) in the Earth’s crust is magnetostratigraphy have made it possible to outline related to the destruction of a body (brittle failure) or the stages of the tectonic process and estimate its proceeds without destruction (rheological, or rheid activity at certain stages. The geochemistry of the deformation). These are ductile and creep deforma igneous rocks provides insights into the processes tions, the intensity and rate of which depend on the within the crust and upper mantle. The results of geo Bingham and creep viscosity of rocks, respectively. thermal research are used for verification of the geody All solids undergo creep deformation under the namic models. effect of longterm stress below the yield strength of Determining the relationships of geological bodies materials. The creep velocity of rocks is very high. It is and morphostructures with faults using techniques of estimated at 1012–1022 P for various rocks, 1022–1023 P structural geology and estimating the age of displaced for the Earth’s crust as a whole, and 1019–1026 P for the structural elements with the use of the radiocarbon lithosphere [10]. Creep deformation is a result of rock method and cosmic isotopes provide an opportunity flow caused by longterm stresses acting under the for timing of faults and assessment of their amplitudes. effect of gravity, mantle plumes, convective currents, The sites that underwent rotation are revealed with the indentors, compaction, and decompaction in the crust paleomagnetic method. The fissiontrack analysis of and below it. The creep deformation rate is extremely 185 186 BURTMAN 70° 80° 90° E The concept of flow in the Earth’s crust, termed as sial or mass flow, facilitated the birth of collisional geo (a) dynamics developed by Argand [32]. The tectonic flow of the Earth’s crust is often a subject (under various 45° Kazakhstan 45° terms) of geodynamic interpretation [5, 14, 16, 27, 65]. Ju Urumqi N Toshkent a n S h COLLISION OF HINDUSTAN AND EURASIA 40° e n 40° Ti The time when Hindustan and Eurasia started to Tarim collide is widely discussed in the literature; it is deter Pamir Qa mined taking into account the data concerned with 35° Kunlun 35° movement of lithospheric plates, the formation of the Himalayas, the rise of Tibet, change in the sedimenta H TIBET tion regime, etc. The onset of collision has been sug im 30° alay Lhasa 30° gested as falling in the Late Cretaceous, 70–65 Ma ago as Hindustan [50, 94]; early Eocene, 55–45 Ma ago [52, 72]; and late Eocene, 35–34 Ma ago [30]. The results of recent studies 400 km provide evidence for a relatively young age of collision. As follows from paleontological data [30, 88], the 70° 80° 90° youngest marine sedimentary rocks in the Tethian 80° 90° 100° E Himalayas are late Priabonian in age (35–34 Ma). The (b) oldest rocks containing clastic material derived from 40° 40° Tarim gh Q Hindustan and South Tibet are known as the upper Ta il Altyn ian N Oligocene–lower Miocene Gangrinboche Conglom Qaidam erate (Kailas, Qiuwu, Dazhuqu, and Luobusa forma Songpan K J tions), which is exposed on both sides of the oceanic Qiangtang Indus–Zangpo Suture. The Ar/Ar age of the tuffs B intercalating within the conglomerates corresponds to Lhasa the early Miocene (16.9 ± 0.2 and 20.1 ± 0.5 Ma) [30]. 30° I 30° Himalayas The rate of convergence of Hindustan and Eurasia 500 km decreased almost 1.5 times about 20 Ma ago [71]. Central and Southern Tibet was rising at approxi 80° 90° 100° mately the same time. The fold–thrust system of the Himalayas was 1 2 3 4 5 formed as a result of collision. The fragments of the Himalayan metamorphic rocks appeared in the Hima layan Foredeep among the upper Oligocene sediments Fig. 1: (a) High Asia and Tien Shan and (b) geological– (Dagshai Formation) and became widespread in the geographic provinces of Tibet. (1) Southern Tibet, Miocene Siwalik Formation. The results of paleoalti (2) Central Tibet, (3) Northern Tibet; (4) oceanic sutures, metric study with application of the isotopic method (5) fault. Letters in panel (a): Ju, Junggar Basin; Qa, [78] show that intense growth of the Himalayas started Qaidam; letters in panel (b): B, Bangong; J, Jinsha; I, Indus–Zangpo; K, Kunlun. about 20 Ma ago. The above data indicate that the collision between Hindustan and Eurasia began in the Oligocene–early low, and its result becomes appreciable only if the pro Miocene, probably in the late Oligocene. This conclu cess lasts for a long time. sion is consistent with the time of postcollision mag Tectonic flow is a combination of rheological matism in Southern Tibet, which started 26 Ma ago deformation and brittle failure with prevalence of the [90, 100]. former. Brittle failure participates in tectonic flow in If the Hindustan–Eurasia collision began in the the form of faulting and cataclasis. The term tectonic Oligocene, then it is necessary to understand the flow may be applied to the entire crust, keeping in causes of the earlier events which gave grounds for the mind that the brittle failure develops in the upper crust suggestion about the older age of the collision. Decel and is very restricted or impossible in the lower crust. eration of convergence of the Indian and Eurasian Many structural elements of collisional tectonics, plates 45–40 Ma ago [71], exhumation of eclogites, including syntaxes and extrusions (protrusions), are and metamorphism of the Himalayan rocks within the results of tectonic flow. same time interval [93] can be mentioned among such GEOTECTONICS Vol. 46 No. 3 2012 GEODYMANICS OF TIBET, TARIM, AND THE TIEN SHAN 187 causes. To explain these and similar events, models of and Tien Shan (Fig. 2). The Qilianshan, with a high passing collision of Hindustan with intraoceanic earthquake density, is the only exception. island arc(s) that predated the Hindustan–Eurasia The rate of current shortening of the territory collision have been proposed [30, 50]. between the Hindustan and Tarim–Alashan platforms The formation of the Himalayan Fold–Thrust Belt has been calculated from GPS data along the profiles was a consequence of the collision between Hindustan which cross Himalaya, Tibet, Kunlun, Qaidam, Qil and Eurasia. As a result of deformation of the Earth’s ianshan, and Altyn Tagh in the direction 20° NE crust in the Himalayas and subduction of the Hin (Fig. 3). Along profile а, the territory shortens with a rate dustan lithosphere beneath Tibet during the orogeny, of 34.6 ± 4.0 mm/yr; along profile b, 34.6 ± 3.0 mm/yr; the territory of the Hindustan continent became along profile c, 33.6 ± 2.0 mm/yr; and along profile d, markedly reduced in area. Using the method of bal 28.0 ± 2.5 mm/yr [85, 99]. anced sections, the shortening of the Himalayan Fold The GPS data also illustrate the internal deforma belt in the process of orogeny is estimated at 200 to tion of Tibet.
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