Quaternary tectonic evolution of the Pamir‐Tian Shan convergence zone, Northwest China

Abstract

The Pamir‐Tian Shan collision zone in the western Tarim Basin, northwest China, formed from rapid and ongoing convergence in response to the Indo‐Eurasian collision. The arid landscape preserves suites of fluvial terraces crossing structures active since the late Neogene that create fault and fold scarps recording Quaternary deformation. Using geologic and geomorphic mapping, differential GPS surveys of deformed terraces, and optically stimulated luminescence dating, we create a synthesis of the active structures that delineate the timing, rate, and migration of Quaternary deformation during ongoing convergence. New deformation rates on eight faults and folds, when combined with previous studies, highlight the spatial and temporal patterns of deformation within the Pamir‐Tian Shan convergence zone during the Quaternary. Terraces spanning ~130 to ~8 ka record deformation rates between ~0.1 and 5.6 mm/yr on individual structures. In the westernmost Tarim Basin, where the Pamir and Tian Shan are already juxtaposed, the fastest rates occur on actively deforming structures at the interface of the Pamir‐Tian Shan orogens. Farther east, as the separation between the Pamir‐Tian Shan orogens increases, the deformation has not been concentrated on a single structure, but rather has been concurrently distributed across a zone of faults and folds in the Kashi‐Atushi fold‐and‐thrust belt and along the NE Pamir margin, where shortening rates vary on individual structures during the Quaternary. Although numerous structures accommodate the shortening and the locus of deformation shifts during the Quaternary, the total shortening across the western Tarim Basin has remained steady and approximately matches the current geodetic rate of 6–9 mm/yr.

Along‐Strike and Downdip Segmentation of the Pamir Frontal Thrust and Its Association With the 1985Mw6.9 Wuqia Earthquake

“…Since around 5–6 Ma, the cumulative slip amount is ~30 km, accounting for an accelerated average slip rate of ~5–6 mm/a. This rate is consistent with PFT's Quaternary slip rate of 4–7 mm/a determined by displaced Quaternary strata and deformed fluvial terraces (T. Li et al, ; Thompson Jobe et al, , ). Therefore, the Pamir thrust system apparently experienced slow deformation during the Miocene, followed by rapid basinward propagation and significant acceleration of shortening rate since late Miocene‐early Pliocene.…”

“…The Pamir thrust system in the western Tarim Basin follows an overall break‐forward evolutionary sequence (Figure ). At the present time, its deformation is localized at its leading edge (T. Li et al, ; Thompson Jobe et al, ). In contrast, the deformation front in the Alai Valley stepped basinward prior to ~16 Ma, then progressively retreated to the Pamir range front after ~16 Ma, thereby displaying an out‐of‐sequence thrusting pattern (Coutand et al, ).…”

“…The MPT, the boundary fault of the Pamir range and its foreland basin, initiated in the late Oligocene‐early Miocene (Cao et al, ; Sobel et al, ; Sobel & Dumitru, ). Its Quaternary slip rate, however, appears to be quite low (<1 mm/a), such that the regional convergence is now concentrated on the PFT, lying tens of kilometers north of the MPT (Arrowsmith & Strecker, ; Bufe et al, ; T. Li et al, , ; Thompson Jobe et al, ; Zubovich et al, ). The PFT initiated approximately 5–6 Ma, representing the latest, major forelandward propagation of the Pamir (Thompson et al, ).…”

“…From west to east (Figure ), the fault is characterized by one or two splays in the Alai Valley, widens and branches into more splays at the eastern end of the Alai Valley, then connects with the primarily top‐to‐NE thrusts and folds at the western end of the Tarim Basin. Detailed mapping of its surface trace indicates that the fault can be subdivided into multiple segments connected by transpressional transfer faults (Figures and ; Arrowsmith & Strecker, ; T. Li et al, , ; Thompson Jobe et al, ). The PFT exhibits a high level of seismicity (e.g., Fan et al, ; Schurr et al, ): frequent earthquakes with moment magnitudes of ≥5.0 have been recorded along the fault, including the 1974 M w 7.1 Markansu, 1978 M w 6.6 Zaalay, 1985 M w 6.9 Wuqia, and 2008 M w 6.6 Nura events (Figure and Table ).…”

Kinematics of Active Deformation Across the Western Kunlun Mountain Range (Xinjiang, China) and Potential Seismic Hazards Within the Southern Tarim Basin

“…We therefore extend our reasoning along channel R3 to over all our study site and interpret the mapped terraces to be of climatic origin. Such an idea of a climatic origin of terraces and fans has already been proposed in the region, within the southern Tarim Basin (Avouac et al, ; Yang et al, ) or elsewhere farther west and north (Huang et al, ; T. Li et al, ; H. Lu, Burbank, & Li, ; B. Pan et al, ; Thompson Jobe et al, ).…”