[1] Measurements at ∼400 campaign-style GPS points and another 14 continuously recording stations in central Asia define variations in their velocities both along and across the Kyrgyz and neighboring parts of Tien Shan. They show that at the longitude of Kyrgyzstan the Tarim Basin converges with Eurasia at 20 ± 2 mm/yr, nearly two thirds of the total convergence rate between India and Eurasia at this longitude. This high rate suggests that the Tien Shan has grown into a major mountain range only late in the evolution of the India-Eurasia collision. Most of the convergence between Tarim and Eurasia within the upper crust of the Tien Shan presumably occurs by slip on faults on the edges of and within the belt, but 1–3 mm/yr of convergence is absorbed farther north, at the Dzungarian Alatau and at a lower rate with the Kazakh platform to the west. The Tarim Basin is thrust beneath the Tien Shan at ∼4–7 mm/yr. With respect to Eurasia, the Ferghana Valley rotates counterclockwise at ∼0.7° Myr−1 about an axis at the southwest end of the valley. Thus, GPS data place a bound of ∼4 mm/yr on the rate of crustal shortening across the Chatkal and neighboring ranges on the northwest margin of the Ferghana Valley, and they limit the present-day slip rate on the right-lateral Talas-Ferghana fault to less than ∼2 mm/yr. GPS measurements corroborate geologic evidence indicating that the northern margin of the Pamir overthrusts the Alay Valley and require a rate of at least 10 and possibly 15 mm/yr.

GPS velocity field for the Tien Shan and surrounding regions

Late Oligocene–Early Miocene initiation of shortening in the Southwestern Chinese Tian Shan: Implications for Neogene shortening rate variations

[1] We compare the geologic histories, the deep structures, and the present-day kinematics of deformation of the Himalaya and the adjacent Tibetan Plateau with those of the Zagros and Iranian Plateau to test geodynamic processes of continental collision. Shortly after India and Arabia collided with Eurasia, horizontal shortening manifested itself by folding and thrust faulting of sedimentary rock detached from India's and Arabia's underlying crystalline basement. Subsequently, slip on thrust faults stacked slices of India's basement to build the Himalaya on India's northern margin. Such faulting has not yet developed in the Zagros, where collision is more recent and Arabia penetrates into Eurasia more slowly than India does, so that postcollision convergence with Eurasia is less. The greater elevation, thicker crust, and more marked heterogeneity of the upper mantle beneath the Tibetan than beneath the Iranian Plateau also reflect a more advanced stage of development. Moreover, while thrust or reverse faulting and crustal shortening continue on the margins of both plateaus, normal faulting, suggesting horizontal extension and crustal thinning, occurs within Tibet but not in Iran. Hence, the balance of forces that built the high Tibetan Plateau must have changed, apparently some time since ∼15 Ma. Removal of Tibetan mantle lithosphere could have altered that balance. If mantle lithosphere beneath the Iranian Plateau has been removed, however, the change in force balance has been too small to initiate normal faulting. Low seismic wave speeds in the uppermost mantle just beneath the Moho of both plateaus suggest (to us) that lithosphere beneath both is thin, consistent with late Cenozoic removal of it, but alternative explanations might account for these low speeds. Despite its apparently thin, and hence presumably weak, mantle lithosphere, much of central Iran undergoes little deformation. It illustrates how a crustal block can behave rigidly not necessarily because it is strong but because deviatoric stresses can be small. Whereas differences between the two regions clearly depend on the amount that Arabia and India have penetrated into Eurasia, which scales with both the dates of collision and rates of convergence, we see no differences in the operative processes that depend on the present-day rates of convergence.

/pdf/comparisons-of-the-kinematics-and-deep-structures-of-the-3wf0o980db.pdf

Comparisons of the kinematics and deep structures of the Zagros and Himalaya and of the Iranian and Tibetan plateaus and geodynamic implications

[1] Site velocities from 349 Global Positioning System (GPS) stations are used to construct an 11-element quasi-rigid block model of the Tibetan Plateau and its surroundings. Rigid rotations of five major blocks are well determined, and average translation velocities of six smaller blocks can be constrained. Where data are well distributed the velocity field can be explained well by rigid block motion and fault slip across block boundaries. Residual misfits average 1.6 mm/yr compared to typical one standard deviation velocity uncertainties of 1.3 mm/yr. Any residual internal straining of the blocks is small and heterogeneous. However, residual substructure might well represent currently unresolved motions of smaller blocks. Although any smaller blocks must move at nearly the same rate as the larger blocks within which they lie, undetected relative motions between them could be significant, particularly where there are gaps in GPS coverage. Predicted relative motions between major blocks agree with the observed sense of slip and along-strike partitioning of motion across major faults. However, predicted slip rates across Tibet's major strike-slip faults are low, only 5–12 mm/yr, a factor of 2–3 smaller than most rates estimated from fault offset features dated by radiometric methods as ∼2000 to ∼100,000 year old. Previous work has suggested that both GPS data and low fault slip rates are incompatible with rigid block motions of Tibet. The results reported here overcome these objections.

https://perso-sdt.univ-brest.fr/~jacdev/pdf/thatcher07_microplate_tibet.pdf

Microplate model for the present-day deformation of Tibet

During the past 15 years, numerous studies have reported not only ecological and climatic changes at 6-8 Ma, but also the roughly concurrent onsets of tectonic processes in regions surrounding the Tibetan Plateau. The ecological changes corroborate suggestions of a switch to a more arid climate on the northern margin of the Indian subcontinent, which were first made by vertebrate paleontologists working in Pakistan. Some of the more recent observations extend the region of increased aridity north and northeast of Tibet. Moreover, some evidence suggests that near 8 Ma, the wind fields over the Arabian and South China Seas simultaneously became more like those of the present day, which are dominated by the Indian and East Asian monsoons, respectively. The onsets of tectonic processes, which in nearly all cases are less precisely dated than the ecological and climatic changes, could have occurred as responses to a ~1000-2000-m increase in the mean elevation of the Tibetan plateau; a greater mean elevation of this amount should raise deviatoric compressive stresses on the margins of the plateau sufficiently to initiate deformation of these regions. The link between deep-seated tectonic processes and regional, if not global, environmental change is too tantalizing to ignore, but inconsistencies in timing and in mutual implications of the various observations by no means require such a link, or that such a change in Tibet’s mean elevation occurred. I review the various observations objectively both to show the potential correlations and to expose these various inconsistencies.

/pdf/mio-pliocene-growth-of-the-tibetan-plateau-and-evolution-of-1rv1bgcrrc.pdf

R. Galas

Papers

New space geodetic constraints on the distribution of deformation in Central Asia

EPOS.P.V3 (Earth Parameter & Orbit System) : software user manual for GPS data processing ; version September 1997

Recent deformation in the Pamir-Thien Shan Region, Part II: Numerical Modelling.