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

Abstract

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.

The Mesozoic Along‐Strike Tectonometamorphic Segmentation of Longmen Shan (Eastern Tibetan Plateau)

“…An increasing number of studies propose that part of the crustal thickness of the SPG terrane and of the central and southern Tibetan plateau is acquired during the Mesozoic (e.g., Airaghi, Lanari, et al, ; Airaghi, de Sigoyer, et al, ; Billerot et al, ; Kapp et al, , ; de Sigoyer et al, ; Xue et al, ). Metamorphic and structural studies carried out in the Danba area (~ 00 km west of the LMS) and in the central LMS (Airaghi, Lanari, et al, ; Billerot et al, ; Dirks et al, ; Harrowfield & Wilson, ; Weller et al, ; Worley & Wilson, ) as well as 40 Ar/ 39 Ar ages on clay mineral in pseudotachylites of the BF (Zheng et al, ) suggest a compressive phase of deformation during the Late Triassic‐Early Jurassic, driven by the closure of the Paleotethys. Recent petrochronological studies have also documented a metamorphic and deformation event in the central LMS at the Early Cretaceous (Airaghi, de Sigoyer, et al, ; Airaghi et al, ).…”

Late Triassic tectonic inversion in the upper Yangtze Block: Insights from detrital zircon U–Pb geochronology from south‐western Sichuan Basin

“…Similar age spectra are seen in the Triassic turbidites of the eastern Songpan‐Ganze terrane (Ding et al., ; Enkelmann et al., ; Wang, Wang et al., ; Weislogel et al., , ; Zhang, Tang, Zhang, Zeng, & Gao, ; Zhang, Zeng et al., ) that may have shared similar sources as the Sichuan Basin. Alternatively, the eastern Songpan‐Ganze terrane may have experienced a phase of shortening in response to the Late Triassic intracontinental orogeny along the Longmen Shan thrust belt (Li, Allen et al., ; Yan et al., ; Zheng et al., ). From this perspective, it is speculated that the eastern Songpan‐Ganze terrane might also be a source region for Upper Triassic detritus of the Sichuan Basin.…”

“…Second, the oldest U–Th–Pb monazite and Sm–Nd garnet ages (204–190 Ma), derived from metamorphosed rocks in the Danba Antiform, immediately south of the Longmen Shan thrust belt, were interpreted as dating the timing of Barrovian metamorphism associated with deformation (Huang et al., ; Weller et al., ). Third, muscovite 40 Ar/ 39 Ar dating of Early Palaeozoic schist and Neoproterozoic Pengguan complex from the northern and middle Longmen Shan yielded ages between 237–208 Ma and 235–226 Ma, respectively, which were interpreted as minimum age constraints for Mesozoic crustal shortening (Yan et al., ; Zheng et al., ).…”

“…During this time, basin margins, especially the Longmen Shan and Songpan‐Ganze turbidite terrane to the west, were significantly shortened (e.g. Huang, Buick, & Hou, ; Weller et al., ; Yan, Zhou, Li, & Wei, ; Zheng et al., ), inducing several kilometres of flexural subsidence (e.g. Guo, Deng, Han, & Liu, ; Li, Allen et al., ; Meng, Wang, & Hu, ).…”

Constructing the Eastern Margin of the Tibetan Plateau During the Late Triassic

“…(1) Initiation of shortening was marked by earliest Mesozoic deformation at ca. 237 Ma constrained by 40 Ar/ 39 Ar dating of muscovite in schist from the northern Longmen Shan (D. P. Yan et al, ) and 40 Ar/ 39 Ar dating of pseudotachylyte from the central Longmen Shan (Zheng et al, ). The age of ca.…”