No abstract

[1] GPS velocities measured in the Pamir and surrounding regions show a total of 30 mm/yr of northward relative motion between stable Pakistan and Eurasia. The convergence budget is partitioned into 10-15 mm/yr of localized shortening across the Trans-Alai Thrust, which bounds the Pamir on the north, consistent with southward subduction of intact lithosphere. Another 10-15 mm/yr of shortening is distributed across the Chitral Himalaya and Hindu Kush, suggesting that Hindu Kush seismicity might be related to northward subduction of Indian lithosphere. Modest shortening at <5 mm/yr occurs north of the Trans-Alai Thrust, across the South Tien Shan and between the Ferghana Valley and Eurasia. Negligible north-south shortening occurs within the high Pamir, but as much as 5 mm/yr, and perhaps 10 mm/yr, of east-west extension occurs within this region. This extension is matched by a comparable amount of east-west shortening in the Tajik Depression. The localization of shortening to the margins of the Pamir combined with observations of distributed internal extension implies that the east-west vertically averaged, horizontal compressive normal stress is smaller than the north-south compressive stress.

The terminal stage of subduction sets in when the continental margin arrives at the trench and the opposite forces of the sinking slab and buoyant continent extend and ultimately sever the subducted lithosphere. This process, although common in geological history, is short-lived, and therefore rarely observed. The deep seismicity under the Hindu Kush (Central Asia), including the 2015 M w 7.5 event, is a rare case that testifies to this process. Here, we use new seismological data to create a high resolution picture of slab break-off and infer its dynamics. High precision earthquake locations and tomographic images show subduction of continental crust down to ~180 km. A large dataset of source mechanisms indicates sub-vertical extension in the entire slab but a strain rate analysis showed that the deeper seismogenic portion of the slab, below the subducted crust, extends at higher rates (~40 km/Ma). Most M w >7 earthquakes between 1983-2015, relocated relative to our new well-constrained earthquake catalog, cluster in a small volume below 180 km, and indicate shearing on an overturned interface. A slip model for the latest 2015 M w 7.5 event suggests that it ruptured into a seismic gap on this interface. From this configuration we conclude that a horizontal slab tear develops along-strike of the Hindu Kush seismic zone at the base of the subducted continental crust. Below the subducted crust, the deepest and also largest earthquakes (180-265 km) are likely associated with deformation in the mantle lithosphere. From the seismicity distribution and the rupture mechanisms we further deduce that the dominant deformation mechanism in this deeper portion of the slab changes along-strike from simple to pure shear. The fastest detachment rates and largest earthquakes occur during the simple shear dominated stage. Earthquakes in the upper part (60-180 km), above the rapidly extending slab, might be triggered by processes related to the subduction of crustal rocks.

Convergence of 29 ± 1 mm/yr between the NW corner of the Indian plate and Asia is accommodated by a combination of thrust and strike‐slip faulting on prominent faults and apparent distributed deformation within the Hindu Kush, Pamir, South Tien Shan and Kohistan Ranges. An upper bound to the slip rate of known faults is obtained by ignoring distributed strain and rotation: convergence occurs on thrust faults north of the Peshawar Basin (13 ± 1 mm/yr) and in the Alai‐South Tien Shan (12 ± 2 mm/yr), and shear on the northeast‐trending northern Chaman‐Gardiz‐Konar system (18 ± 1mm/yr) and the Darvaz‐Karakul fault zone (11 ± 2 mm/yr). Slip rates on the Herat and Talas‐Ferghana faults are small (<2 mm/yr). Shortening not attributable to known active faults occurs within the Hindu Kush and central Pamir (16 ± 2 mm/yr) with concomitant east‐west extension in the latter of 9 ± 2 mm/yr. This diversity of strain styles confirms the importance of mechanical heterogeneity to continental tectonics and shows that the Pamir, although less than half the size, behaves more like Tibet than like a linear belt of localized deformation.

Seismologic and geologic fault-slip data characterize the active deformation of the intramontane Tajik basin and its margins, the Tian Shan, Pamir, and Hindu Kush at the northwestern tip of the India-Asia collision zone. Within this complexly deforming region, the Tajik basin lithosphere forms the backstop for the north-dipping Indian-slab subduction beneath the Hindu Kush but itself delaminates and retreats west and northward beneath the Pamir. Herein, we link crustal deformation to these lithosphere-scale processes, using data from recently deployed seismic networks and geologic observations. Transpressive strike-slip deformation dominates the bounding fault zones along the basin's northern and eastern margins. Seismicity is most intense in the Garm region/Peter I. range of the northeastern basin, where these bounding faults converge and gain a dominant thrust component. Within the basin, seismically and geologically derived P axes align with the~W-trending GPS velocity vectors. Seismicity is concentrated in and at the base of a southward deepening, ∼9-15-km-thick wedge. Seismic deformation at the basin's southern margin occurs beneath the Afghan platform, where deep crustal earthquakes likely trace the western end of the Hindu Kush subduction zone. Roughly NNE-striking sinistral strike-slip events outline the Hindu Kush-Pamir transfer system, a zone of distributed shear in the crust overlying the transition of the two oppositely dipping slabs at subcrustal depths. Our observations suggest that crustal deformation in the Pamir and Hindu Kush links with lithosphere-scale processes, whereas deformation in the basin is controlled by the westward gravitational collapse of the Pamir and the resultant basin inversion.