Holocene Slip Rate Along the Beng Co Fault and Dextral Strike‐Slip Extrusion of Central Eastern Tibet

Abstract

AbstractThe 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/14C 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 theMw7.7, 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 anMw ≈ 7.4? event.