New geochronology constraints on timing and depth of the ancient earthquakes along the Longmen Shan fault belt, eastern Tibet

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• 作者：Yong Zheng ; Haibing Li ; Zhiming Sun ; Huan Wang ; Jiajia Zhang ; Chenglong Li ; Yong Cao
• 刊名：Tectonics
• 出版年：2016
• 出版时间：December 2016
• 年：2016
• 卷：35
• 期：12
• 页码：2781-2806
• 全文大小：10.0MB
• ISSN：1944-9194

文摘

Pseudotachylyte is an ideal target to directly date ancient earthquake associated with regional faulting. Here we perform step-heating ⁴⁰Ar/³⁹Ar, clay mineral K-Ar, and zircon fission track (ZFT) analyses on the pseudotachylyte samples collected from the Yingxiu-Beichuan coseismic rupture of the Longmen Shan fault belt (LSFB) to provide time constraints for the tectonic evolution of the LSFB during the Indosinian orogeny. ⁴⁰Ar/³⁹Ar results from the matrix show that the frictional melting occurred 226–235 Ma ago. Combined with mylonite dating of the host rock, the age of the ancient earthquakes is constrained at 231–238 Ma, with a formation depth of 10–14 km. As a response to the earthquakes, a series of soft-sediment deformation structures are widely preserved in the Middle and Late Triassic strata along the Yingxiu-Beichuan fault (YBF), indicating that the LSFB was seismically active since the Late Anisian and persisted episodically until the end of the Indosinian orogeny. Both clay mineral K-Ar and ZFT analyses record a younger deformation or alteration with an age of approximately 195 Ma, corresponding to a postcollisional orogeny. These new data represent the first direct evidence of the regional thrusting (YBF) in the central LSFB during the Indosinian orogeny, concurrent with the initial ductile deformation of the western boundary fault. Tectonic inheritance then strongly influenced the evolution of the LSFB as most of the Mesozoic faults are reactivated by major Tertiary tectonic deformations.