Quaternary Activity of the Beihewan Fault in the Southeastern Beishan Wrench Belt, Western China: Implications for Crustal Stability and Intraplate Earthquake Hazards North of Tibet

Abstract

We present evidence for the Late Quaternary activity of the Beihewan Fault (BHWF), along the southeastern margin of the Beishan block, western China. Field observations and analysis of UAV‐derived DEMs and Google Earth images reveal an ~10‐km‐long active fault trace in a region previously considered tectonically inactive. Offset landforms and trench exposures reveal that the fault is dominantly strike‐slip, with local thrust or normal displacements. Average rates of left‐lateral motion and thrusting since the Holocene are estimated to be ~2.69 and ~0.35 mm/a, respectively. The most recent surface‐rupturing event generated ~2.5 m of strike‐slip offset since 6.2–8.6 ka; with estimated M = 6.3–6.9. Modeling and two‐dimensional inversion of new magnetotelluric data indicate that the BHWF is a subvertical low‐resistivity zone that penetrates into the lower crust. Existing geological and geophysical evidence do not support subsurface linkage between the BHWF and Altyn Tagh–Jinta‐Nanshan Fault system further south in the Hexi Corridor. Close examination of high‐resolution satellite imagery west and east of the BHWF reveals other Quaternary surface fault scarps, pressure ridges, offset drainages, and truncated lithological strike belts. Collectively, these features constitute a previously unrecognized 150‐km‐long sinistral deforming belt along the southeastern Beishan and northern Hexi Corridor boundary that we call the Southeast Beishan Wrench Belt. Recognition of Quaternary activity along the BHWF within the larger Southeast Beishan Wrench Belt challenges long‐held assumptions that the Beishan region is a relatively stable and tectonically inactive block within the Late Cenozoic Indo‐Eurasian deformation field north of Tibet.

Late Miocene to Quaternary Development of the Jiujing Basin, Southern Beishan Block, China: Implications for the Kinematics and Timing of Crustal Reactivation North of Tibet

“…Cosmogenic 26 Al/ 10 Be burial dating of the deepest sediments in the Jiujing Basin indicates that the basin began forming at~5.5 Ma. In addition, Neogene sediments in the Yinwaxia depression west of the Jiujing Basin are deformed and other E-W fault systems in the southern Beishan including the Jinmiaogou and Hongqishan fault systems and Beihewan fault system in the southeastern Beishan [13] record Quaternary displacements. Thus, increasing evidence supports the contention that the southern Beishan region has been reactivated since the Late Miocene in discrete deforming belts.…”

“…At each site, at least 100 photos were captured by a DJI Phantom 3 Pro drone. The resultant DEMs have a resolution of 3~5 cm/pixel (c.f., Yang et al [13,26]). Printed shaded relief maps with remote sensing interpretations were carried to the field for validation, modification, and as basemaps for further ground study.…”

“…In addition, semiconsolidated Neogene deposits (Kuquan Formation) within the Beishan are widely folded and faulted according to 1 : 200,000 scale geological mapping (and confirmed by satellite image analysis) [21]. Furthermore, recently, Yang et al [13] documented a~20 km-long active fault system along the southeastern margin of the Beishan, which challenges long-held views of Holocene crustal stability in the surrounding region. An open question is whether during the Late Cenozoic, the Beishan has been dominantly reactivated by strike-slip rather than dip-slip displacements.…”

“…Limited trench results also reveal a very long recurrence interval for the NE-trending Bayanluorigong sinistral strike-slip fault within the NE part of the Alxa block (at least~50 ka) [63]. Along the southeastern margin of the Beishan, Yang et al [13] yielded a strike-slip rate of~2.6 mm/a for Holocene motion on the Beihewan fault (BHWF) and determined that the most recent earthquake had an estimated magnitude of 6.3-6.9. A slower slip rate determination of 1.5 mm/a for the BHWF is also documented [64].…”

“…China's "Belt and Road Initiative" includes a major connecting corridor through the Beishan region as part of the 21st Century Silk Road Economic Belt. Therefore, identification of Quaternary fault activity within the Beishan region has important implications for regional seismic hazard assessments and infrastructure development [11,13].…”