Late Cenozoic tectonic development of the intramontane Alai Valley, (Pamir‐Tien Shan region, central Asia): An example of intracontinental deformation due to the Indo‐Eurasia collision

Abstract

The Pamir indentor of the northwestern Himalayan syntaxis is a first‐order feature demonstrating partly the northward extent of deformation due to the Cenozoic Indo‐Eurasia collision. The Alai Valley of Kyrgyzstan at the northern end of the indentor is a strategically positioned, E‐W trending intramontane basin that constrains the timing and extent of crustal deformation in this area of the collision zone. To quantify the convergence accommodated across the Alai Valley during the Late Cenozoic, we collected structural and stratigraphic field data, reviewed existing Soviet literature, and analyzed migrated seismic reflection profiles to construct and restore two regional sections crosscutting the basin. Our study suggests that the development, progressive closure, and conversion of this formerly marine basin into a terrestrial intramontane basin result from two main deformation events: (1) Distributed north‐south contraction took place during the late Oligocene–early Miocene, accommodated one third to half of the total shortening and was followed by the formation of a regional erosion surface; and (2) N‐S shortening resumed in the mid‐Miocene and continues today. During this second episode the thrust front migrated southward, localized along the Trans Alai ranges, and failed to reactivate earlier Neogene structures. Horizontal shortening of about 35% across the Alai Valley implies relatively low strain rates and displacement rates of about 4.18–4.69 × 10−16 s−1 and 0.66–0.78 mm yr−1, respectively, for the last 25 Myr. Our study confirms other regional observations indicating that contractional deformation occurred far in the interior of the Asian continent as early as the late Oligocene.

Late Miocene episodic lakes in the arid Tarim Basin, western China

“…Syntectonic growth strata from the foreland basins of the Kunlun and Tianshan Ranges (32) show that strong crust shortening and potential mountain uplift initiated ∼6.5-5 Ma and lasted to the Early Pleistocene (Fig. S5), similarly reported in northern Pamir (33). Cenozoic sequences in the Pamir-Tianshan convergence zone, changing from an arid continental plain to an intermountain basin by ∼5 Ma, support surface uplift of the west margin of the Tarim Basin (34).…”

“…Although the Indo-Eurasian convergence since the Late Eocene resulted in high elevations of the Tibetan Plateau and, to a lesser degree, surrounding mountains including Pamir and Tianshan by the mid-Miocene time (8), tectonic activities in broad areas around the Tarim Basin appear to be rejuvenated since the Late Miocene. Tectonic deformations during the Late Miocene-Early Pliocene inferred from growth strata, sedimentary facies changes, and low-temperature thermochronologic studies occurred in Tianshan to the north of the Tarim Basin, in the Kunlun Mountains to its south and the Pamir to its west (32)(33)(34)(35). Syntectonic growth strata from the foreland basins of the Kunlun and Tianshan Ranges (32) show that strong crust shortening and potential mountain uplift initiated ∼6.5-5 Ma and lasted to the Early Pleistocene (Fig.…”

The deformation pattern and fault rate in the Tianshan Mountains inferred from GPS observations

“…The Quaternary rate on the main Pamir thrust is assumed to be as large as 20 ± 6 mm/a [32] consistent with the present rate estimated from GPS [33] . The Holocene crustal shortening within the Trans Alai range on the northern rim of Pamir exceeds 6 mm/a, if one considers a crustal shortening of 3 mm/a absorbed in a broad area spanning from the Alai Range and Valley to the Fergana Basin in the north, however the crustal shortening across the frontal Pamir is by any means less than ~10 mm/a.…”

Geometry of the Pamir‐Hindu Kush intermediate‐depth earthquake zone from local seismic data

“…It marks the last remnant of the former Tajik-Yarkand Basin and hosts at its southern margin the Main Pamir Thrust (MPT), where north-south convergence in front of the advancing Pamir is currently accommodated [Coutand et al, 2002;Arrowsmith and Strecker, 1999;Zubovich et al, 2010]. [7] As part of the TIPAGE (TIen Shan PAmir GEodynamic Program) project, a total of 40 seismic stations, [Haberland et al, 2011], and seven short-period stations in the Alai Valley deployed for approximately 6 months soon after the 5 October 2008 Nura earthquake (M w 6.6) for better coverage of aftershock seismicity.…”