Holocene Slip Rate Along the Beng Co Fault and Dextral Strike‐Slip Extrusion of Central Eastern Tibet. Issue 8 (22nd August 2022)

• Record Type: Journal Article
• Authors: Li, Kang; Tapponnier, Paul; Xu, Xiwei; Ren, Junjie; Wang, Shiguang; Zhao, Junxiang
• Is Part Of: Tectonics. Volume 41:Issue 8(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-08-22
• Subjects: Southeastern Tibet Plateau -- Beng Co fault earthquakes -- Holocene right‐lateral slip‐rates; Geology, Structural -- Periodicals; 551.8
• DOI: 10.1029/2022TC007230
• Languages: English
• ISSNs: 0278-7407

Abstract

Abstract: The dearth of quantitative studies of Holocene fault slip‐rates makes kinematic modeling of crustal deformation across SE Tibet debatable. The ∼230 km long, WNW‐striking, dextral Beng Co fault (BCF) is the most prominent strike‐slip fault of southern Tibet. Using new UAV topographic data and OSL/ 14 C dating of geomorphic markers, we measure horizontal offsets and ages of fluvial terraces/fans and lacustrine shorelines at eight sites west and east of the Beng Lake pull‐apart. Along the eastern BCF's northern branch, our measurements at two different locations are consistent with offsets of ∼4.7 ± 1.1 and ∼8.7 ± 0.7 m during the past ∼3.1–3.6 and 3.6–5.2 ka, respectively. Along the southern branch, ∼25 ± 5 m of slip have accrued since ∼9 ka. The total Holocene slip‐rate across both faults (4.2–5.4 or 4.8 ± 0.5 mm/yr), consistent with GPS/InSAR values, implies that the BCF contributes significantly to the eastward extrusion of eastern‐central Tibet. This rate is slightly larger than the lower bound of the extension rate (∼6 ± 1.8 mm/yr) recently determined across the middle Gulu rift normal fault, which splays off southwards from the southeastern tip of the BCF's southern branch. Empirical relationships suggest that the reported ∼90 km length of the Mw 7.7, 1951 Beng Co earthquake surface rupture is too short, which implies that the eastern BCF is immature, or requires additional, partly deep slip, along other branches of the eastern BCF. That there is no field evidence of rupturing along the western BCF in the last ∼1, 000 yr indicates that the Bange County area, west of Beng Co, may be at risk of an Mw ≈ 7.4? event. Plain Language Summary: Due to the lack of precise slip‐rates along the faults of south‐central Tibet, kinematic models of crustal deformation across the frontal, ∼1, 200 × 400 km broad part of the India/Asia collision zone remain controversial. The 230 km long, WNW‐striking, right‐lateral Beng Co fault (BCF) is the most prominent, active strike‐slip fault of southern Tibet. Using high‐resolution UAV topographic data and OSL/ 14 C dating of geomorphic markers, we quantitatively constrain horizontal offsets and ages of fluvial terraces/fans and lake shorelines along this fault west and east of Beng Lake. We obtain a total Holocene slip‐rate of 4.8 ± 0.5 mm/year across the two BCF branches, compatible with GPS/InSAR geodetic estimates, and slightly larger than the lower bound of the extension rate across the middle Gulu rift. The BCF thus contributes significantly to the eastward extrusion of central‐eastern Tibet. Empirical relationships imply that the reported ∼90 km rupture length of the 1951 Mw 7.7 Beng Co earthquake (largest event across the southern Tibet Plateau) has been underestimated or that the eastern BCF is immature. The lack of field evidence for a young (<1, 000 yr) surface rupture along the western BCF implies that much of Bange County, southwest of the fault, remains threatened by a large ( Mw ≈ 7.4?) earthquake. Key Points: OSL/ 14 C dating of offset terraces/fans/shorelines imply a total Holocene slip‐rate of 4.8 ± 0.5 mm/yr along dextral Beng Co fault (BCF) The BCF thus contributes significantly to the eastward extrusion of central‐eastern Tibet along the Karakoram‐Jiali fault zone The Mw ≈ 7.7, 1951 earthquake rupture did not extend much beyond Peng Lake, leaving westwards a ∼135 km long seismic gap at least 1, 000 yr old