Seismotectonics of the Tajik Basin and Surrounding Mountain Ranges

Abstract

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.

Cumulative and Coseismic (During the 2016 Mw6.6 Aketao Earthquake) Deformation of the Dextral‐Slip Muji Fault, Northeastern Pamir Orogen

“…On account of such complicated mountain‐building processes and crustal boundary conditions, the recent deformation of the Pamir is characterized by a variety of types of faulting deformation as documented by neotectonic, geodetic, and seismologic studies: thrusting and shortening along its northern margin, sinistral‐transpressional slip along its western flank, possible dextral slip along its eastern flank, and significant east‐west extension within its interior (e.g., Arrowsmith & Strecker, ; Chapman et al, ; Chevalier et al, ; Cowgill, ; Ischuk et al, ; Jay et al, ; Li et al, ; Robinson et al, , , ; Schurr et al, ; Sippl et al, ; Sobel et al, , ; Thiede et al, ; Zubovich et al, ). This tectonic deformation is proposed to result from radial thrusting along the orogen margin (e.g., Cowgill, ; Pan et al, ; Strecker et al, ), gravitational collapse and westward extrusion of orogenic material (Jay et al, ; Kufner et al, ; Schurr et al, ; Thiede et al, ), oroclinal bending of the entire Pamir‐Western Himalayan region (Yin et al, ), clockwise rotation of the rigid Tarim basin (Schurr et al, ), and/o thermal and density effects related to a lithospheric tear fault (Sobel et al, ; Thiede et al, ). Despite these knowledge, details of the structures accommodating the tectonic deformation and the relationship between different types of faults remain highly debated (e.g., Chevalier et al, ; Robinson et al, , ) because the geometry and kinematics of major active faults have not yet been well constrained and geodetic measurements are not dense enough in the region (Figures a and b).…”

Seismic Moments of Intermediate‐Depth Earthquakes Beneath the Hindu Kush: Active Stretching of a Blob of Sinking Thickened Mantle Lithosphere?

“…By contrast, as in the Carpathians, deformation across the Hindu Kush is not associated with such a fault, or if it is, that fault has not yet been identified. Instead, crustal seismicity in the Hindu Kush region appears to occur on several relatively indistinct thrust faults that follow the margin of the Tajik Depression (e.g., Abers et al, ; Kufner et al, ; Perry et al, ; Yeats & Madden, ).…”

“…Boxes show regions used to draw profiles in Figure . The earthquake hypocenters were determined by Kufner et al (, ) and Sippl et al ().…”

“…Improved resolution of the seismicity beneath the Pamir (Pegler & Das, ; Sippl et al, ) shows that the zone dips southward along the northern edge of the Pamir (Figure c) and varies smoothly to dip southeastward and eastward beneath the western edge of the Pamir. The surface projection of the inclined seismic zone wraps around the western margin of the Pamir, where surface deformation occurs by a combination of left‐lateral strike slip (e.g., Kuchai & Trifonov, ; Kufner et al, ; Mohadjer et al, ; Trifonov, ) and northwest‐southeast crustal shortening (e.g., Abers et al, ; Ischuk et al, ; Perry et al, ). The apparently continuous zone of earthquakes with depths between 60 and ~100 km beneath the Pamir and Hindu Kush has led some to favor the view that the zone marks a single, warped slab of lithosphere that is continuous from the eastern end of the northern edge of the Pamir to the western end of the Hindu Kush seismic zone (e.g., Billington et al, ; Koulakov & Sobolev, ; Nowroozi, ; Pegler & Das, ).…”

“…Although Pavlis and Das () and Koulakov and Sobolev () suggested that the intermediate‐depth earthquakes beneath the Pamir are part of lithosphere attached to the Indian Plate but overridden by that plate as it penetrated northward into the rest of Eurasia, evidence of southward underthrusting of Eurasian lithosphere beneath the Pamir seems overwhelming. Analogous to oceanic subduction zones, a belt of shallow‐focus earthquakes showing thrust faulting marks the northern margin of the Pamir (e.g., Burtman & Molnar, ; Hamburger et al, ; Kufner et al, ; Sippl et al, ). Quaternary faulting (e.g., Arrowsmith & Strecker, ; Nikonov et al, ) and GPS velocities (Figure ) show concentrated deformation along that belt (e.g., Ischuk et al, ; Perry et al, ; Reigber et al, ; Zubovich et al, ).…”