Cenozoic intracontinental deformation and exhumation at the northwestern tip of the India-Asia collision-southwestern Tian Shan, Tajikistan, and Kyrgyzstan

Abstract

Along the Ghissar-Alai Range of the southwestern Tian Shan (southwestern Kyrgyzstan, northern Tajikistan), the deformation front of the India-Asia collision-the Pamir-Tibet orogen-is interacting with the intracontinental Tian Shan orogen without the intervening Tarim Craton. Apatite fission track (n = 33,~3.3-145.6 Ma, 27% <10 Ma) and (U-Th)/He (n = 32,~1.9-26.1 Ma, 56% <10 Ma) thermochronologic ages suggest approximate isothermal holding (very slow cooling to weak reheating) during relative tectonic quiescence between~150 and 15 Ma. Accelerated exhumation (~0.2-1.0 km/Myr, median~0.5 km/Myr) and cooling (11-16°C/Myr) occurred over the last~10 Myr. Geomorphologic parameters-incision, river steepness, and concavity-confirm the youth of the southwestern Tian Shan's mountain building. High exhumation/cooling rates are correlated with pronounced local relief, produced by Cenozoic faults reactivating inherited (Late Paleozoic) structures. Regions with similarly young exhumation are centered along rims of rigid crustal blocks in the central and eastern Tian Shan. Structurally, the Ghissar-Alai Range is a broad, east trending zone of dextral transpression that includes the northern Tajik Basin (Illiak Fault Zone) and the Pamir Thrust System of the frontal northern Pamir. It is the particular deformation field at the northwestern tip of the India-Asia collision-the interaction of the westward gravitational collapse of the Pamir Plateau into the Tajik Basin with the bulk northward motion of the Pamir-that transformed the southwestern Tian Shan into a dextral transpression belt. The dextral transpression in the southwestern Tian Shan contrasts with sinistral strike-slip shear localized along inherited fault zones, accommodating dominant north-south shortening, in the central and eastern Tian Shan. The deformation field influenced by the Pamir and the associated young exhumation make the Ghissar-Alai Range a unique feature in the Tian Shan orogen.

Thermochronological insights into the structural contact between the Tian Shan and Pamirs, Tajikistan

“…Hence, our data support Kässner et al. (); Kässner et al () who suggested a widespread thermal response to late Permian orogensis within the Pamirs and STS.…”

“…The AFT data obtained in this study record two cooling events that initiated at ~25 Ma and ~10 Ma. The ~25 Ma age corresponds to abundant Oligocene–Miocene AFT ages for major faults within the STS, including in the nearby Ghissar and Garm regions (Figure , Sobel et al., ; De Grave et al., ; Glorie et al., ; Kässner et al, : Bande, Sobel, Mikolaichuk, Schmidt, & Stockli, ). We therefore interpret the ~25 Ma cooling signal to be related to deformation focused along the Vakhsh‐Surkhob fault zone.…”

“…Glorie & De Grave, ; Glorie et al., ; Hendrix et al., ; Macaulay et al., ; Sobel, Oskin, Burbank, & Mikolaichuk, ). In contrast, Kässner et al (,b) recently concluded that the western STS experienced tectonic quiescence between ~150 and 15 Ma (Figure ). This study targets a major structural contact, aiming to distinguish between these models and to refine the thermal history of the STS.…”

“…For each sample, a summary of the apatite U/Pb ( AUP b), zircon (U–Th–Sm)/He ( ZH e), apatite fission track ( AFT , central ages) and apatite (U–Th–Sm)/He ( AH e, corrected mean ages) results is given. The MPT = Main Pamir Thrust; triangles represent AFT data from Kässner et al, . The colour key for the sample ages is given in Figure [Colour figure can be viewed at wileyonlinelibrary.com]…”

“…Geological map of the South Tian Shan and Pamirs modified after Vlasov, Dyakov, and Cherev () with published thermochronological data (Amidon & Hynek, ; Bande, Sobel, Mikolaichuk, & Acosta, ; Bande et al., ; De Grave et al., ; Kässner et al, , Kässner et al, ). The MPT = Main Pamir Thrust, and the red dashed line represents the Vakhsh‐Surkhob fault zone.…”