Along‐Strike and Downdip Segmentation of the Pamir Frontal Thrust and Its Association With the 1985M_w6.9 Wuqia Earthquake

Abstract

The Pamir Frontal Thrust along the leading edge of the northern Pamir is characterized by multiple earthquakes with moment magnitudes of 6.5–7.1. Geometric characteristics of the Pamir Frontal Thrust corresponding to these earthquakes and to future seismic hazards, however, remain largely unexplored. This study focuses on the easternmost segment of the Pamir Frontal Thrust, where the Mw 6.9 Wuqia earthquake occurred in 1985. Through interpretation of available two‐dimensional seismic reflection profiles and surface mapping data, we develop a three‐dimensional geometric model for the fault plane. Our results illustrate that, at depth, the fault plane is separated by a subhorizontal detachment horizon into upper and lower ramps, and both ramps are significantly segmented, along strike, by transfer faults or lateral ramps as well. Such along‐strike and downdip segmentation of the thrust sheet apparently plays a significant control on seismic rupture process of the Wuqia earthquake and can well explain why the region is characterized by moderate‐magnitude (Mw 6.5–7.1) events. Additionally, our study helps quantify key constraints on the Cenozoic deformation and evolution of the northeastern Pamir and, specifically, determines a total shortening of ≤43 km at the Pamir front: accommodating about 15% of the total indentation of the Pamir range into central Asia.

Tajik Basin and Southwestern Tian Shan, Northwestern India‐Asia Collision Zone: 2. Timing of Basin Inversion, Tian Shan Mountain Building, and Relation to Pamir‐Plateau Advance and Deep India‐Asia Indentation

“…The switch from crustal shortening to extension at ~23–20 Ma was likely triggered by the slab breakoff of Greater India from cratonic India (see section 1). The crustal thickening and the subsequent, top‐to‐~N extensional collapse of the Pamir‐plateau crust together with the bulk northward advance of the Pamir due to the India‐Asia convergence initiated the foreland advance of the deformation front from the Central to the North Pamir, triggering the onset of crustal shortening across the North Pamir (Rutte, Ratschbacher, Khan, et al, 2017, Rutte, Ratschbacher, Schneider, et al, 2017; Li et al, 2019; Worthington et al, 2020) and―as we postulate here―north of it in the Tian Shan. Whereas the southern margin of the Tian Shan―the immediate foreland buttress of the North Pamir and depocenter of the Tajik basin―were loaded and subsided, the more distant Tian Shan was mildly shortened, with the modest structural reactivation indicated by enhanced cooling.…”

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

“…The Pamir thrust system, characterized by multiple east-trending thrust fault splays (Figure 1b), localizes most of the Cenozoic convergence between the northern Pamir and Asia (Burtman & Molnar, 1993). Presently, it continues to accommodate significant regional convergence, manifested by strong active thrusting and folding deformation (Arrowsmith & Strecker, 1999;Li et al, 2012Li et al, , 2013Li et al, , 2015Li et al, , 2017Thompson Jobe et al, 2017), a shortening rate of up to 10-15 mm/a from GPS velocities (Zubovich et al, 2010), and a high level of seismicity (e.g., the 1974 M w 7.1 Markansu, the 1985 M w 6.9 Wuqia, and 2008 M w 6.6 Nura earthquakes ; Burtman & Molnar, 1993;Li et al, 2019;Sippl et al, 2014;Teshebaeva et al, 2014;Figure 1b al., 2013;Thomas et al, 1994). The Tajik thrust system has accommodated a total shortening of~30 km since middle Miocene (Chapman et al, 2017), whereas the displacement of the Darvaz Fault has not been well constrained.…”

Tajik Basin and Southwestern Tian Shan, Northwestern India‐Asia Collision Zone: 1. Structure, Kinematics, and Salt Tectonics in the Tajik Fold‐and‐Thrust Belt of the Western Foreland of the Pamir

“…See text for discussion. (b) Map‐view restoration of the external edge of the Pamir orocline based on combined shortening estimates from Table 1 and Coutand et al (2002) and Li et al (2019). The restauration implies a pre‐12‐Ma orocline.…”

“…To highlight the regional implications of our shortening estimates, we performed a first‐order map‐view restoration (Figure 18b) and compiled a crustal‐scale cross section (Figure 18c). The map‐view restoration combines our shortening estimates and ages with those from the northern Pamir front (Coutand et al, 2002; Li et al, 2019); the crustal‐scale cross section integrates our cross‐section C (Figure 16) with geophysical constraints on deep crustal geometries: the Moho geometry from Schneider et al (2019) and intermediate‐depth seismicity from Sippl, Schurr, Yuan, et al (2013), the latter projected within a 10‐km swath. As indicated in Table 1, values yielded by cross‐sections C, D, and E may be considered representative for the total shortening across the Tajik FTB.…”

“…This geometry corresponds to a pre‐~12‐Ma paleogeography, given the dating by Abdulhameed et al (2020). According to Coutand et al (2002) and Li et al (2019), deformation along the northern sector of the Pamir orocline may have started at ~20–25 Ma. Low shortening values provided by these authors indicate that the northern deformation front was already close to its present‐day position at ~12 Ma.…”