The Pamir Frontal Thrust Fault: Holocene Full‐Segment Ruptures and Implications for Complex Segment Interactions in a Continental Collision Zone

Abstract

The Pamir Frontal Thrust (PFT) of the Trans‐Alai Range in Central Asia is the principal active fault of the intracontinental convergence zone between the Pamir and Tien Shan. Its northward propagation is reflected by frequent seismic activity and ongoing crustal shortening. Recent and historic earthquakes exhibit complex rupture patterns within and across seismotectonic segments bounding the Trans Alai, challenging our understanding of fault interaction and seismogenic potential. We provide paleoseismic data from five trenches along the central PFT segment (cPFT) and interpret five and possibly six paleoearthquakes that have ruptured since ∼7 ka and 16 ka, respectively. Our results indicate that at least three major earthquakes ruptured the full‐segment length and possibly crossed segment boundaries with a recurrence interval of ∼1.9 kyr and potential magnitudes of up to Mw 7.4. We did not find evidence for great (i.e., Mw ≥8) earthquakes. However, discrepancies between slip and rupture extent during apparent partial segment ruptures in the western half of the cPFT, combined with significantly higher scarp offsets, indicate a more mature fault section with potential for future fault linkage. We estimate an average rate of horizontal motion for the cPFT of 4.1 ± 1.5 mm/yr during the past ∼5 kyr, which does not fully match the GNSS‐derived present‐day shortening rate of ∼10 mm/yr. This suggests a complex distribution of strain accumulation and potential slip partitioning between the cPFT and additional faults and folds within the Pamir that may be associated with a partially locked regional décollement.

Two kinematic transformations of the Pamir salient since the Mid-Cenozoic: Constraints from multi-timescale deformation analysis

“…On the millennial timescale, Arrowsmith and Strecker (1999) constrained the dip-slip rate of one central strand of the western PFT to >6 mm/yr (equivalent to >5.2 mm/yr shortening rate) by reconstructing the fault offset and associated chronology of a terrace at the Syrinadjar River. A recent detailed paleoseismic study on the same fault strand obtained a slightly lower rate of 4.1 ± 1.5 mm/yr over the past ~5 kyrs (Patyniak et al, 2021). This slip rate is compatible with the other determination of a minimum of 2-4 mm/yr based on the overthrusting of Stone Age artifacts farther west (Nikonov et al, 1983;Burtman and Molnar, 1993).…”

“…The average N-S shortening rate on the northern margin has been only ~0.7 mm/yr during the past 25 Myr (Coutand et al, 2002), much smaller than current rates of 10-15 mm/yr (e.g., Zubovich et al, 2010). The millennial rates of a branch of the western PFT are 3-5 mm/yr (Arrowsmith and Strecker, 1999;Patyniak et al, 2021), consistent with the decadal rate of 3.8 ± 0.8 mm/ yr for the same segment (Zubovich et al, 2016). Based on the relatively uniform slip rate of this branch fault, the millennial rate on the northern margin can be reasonably extrapolated as also 10-15 mm/yr based on the same context of thrust faulting along the northern margin.…”

“…In places, this fault appears to have been inactive during the Quaternary (Sobel et al, 2013;Thompson et al, 2015), and most deformation across the northern Pamir front is now being accommodated by the PFT, which initiated at ~6-5 Ma and is the latest deformation front of the northward propagation of the Pamir (Thompson et al, 2015;Chen et al, 2019;Chen et al, 2021). The PFT lies north of the MPT and thrusts the Trans-Alai Range over the Alai Valley and consists of segmented thrust faults and dextral transfer zones (Arrowsmith and Strecker, 1999;Li et al, 2012;Patyniak et al, 2021). The western section of the PFT (hereafter referred to as the western PFT) stretches along the southern margin of the Alai Valley between 72 °E and 73.8 °E, while the eastern section (hereafter referred to as the eastern PFT) is located in the northeastern corner of the Pamir, with strikes varying from E-W to NW-SE (Figure 1B).…”

Cyclic Fault Slip Under the Magnifier: Co‐ and Postseismic Response of the Pamir Front to the 2015 Mw7.2 Sarez, Central Pamir, Earthquake

“…(2014) revealed a predominant dextral slip along E(SE) trending planes of the PFT, arranged in sub‐vertical clusters underneath a décollement layer. Trenching data provide evidence of at least two full ruptures of the central PFT segment within the last 5,000 years and a few partial ruptures at the western end of the segment (Patyniak et al., 2021). In instrumental times the 1974 M w 7.1 Markansu earthquake and the 2008 M w 6.6 Nura earthquake (Figure 1) ruptured thrust faults East of the eastern PFT segment (Sippl et al., 2014) (Figure 1b).…”

“…In instrumental times the 1974 M w 7.1 Markansu earthquake and the 2008 M w 6.6 Nura earthquake (Figure 1) ruptured thrust faults East of the eastern PFT segment (Sippl et al., 2014) (Figure 1b). Slip might also occur on faults below the sedimentary layer further North, or historic surface expressions might have been eroded (Patyniak et al., 2021).…”