Rock record and magnetic response to large earthquakes within Wenchuan Earthquake Fault Scientific Drilling cores

Abstract

Fault‐related pseudotachylytes are often considered to be produced by large seismic events. To investigate the rock record and magnetic response to large earthquakes within cores from the Wenchuan Earthquake Fault Scientific Drilling borehole 2 (WFSD‐2), we carried out microstructural, geochemical, and rock‐magnetic analyses of representative cores. Based on microstructural observations and powder X‐ray diffraction analyses, we found 21 layers of melt‐origin pseudotachylytes from 579.62 to 599.31 m‐depth in the cores. The presence of early‐formed pseudotachylyte fragments in the new layer suggests that seismic faulting processes exploited the same fault strand more than once. Pseudotachylyte veins have higher values of magnetic susceptibility relative to wall rocks. Rock‐magnetic results indicate that the magnetic minerals within the pseudotachylyte veins are magnetite with varying amounts of paramagnetic minerals. Magnetic hysteresis loops show that a reduction of the grain size of ferromagnetic minerals is not a plausible explanation for the higher magnetic susceptibility values in pseudotachylyte veins. Rock‐magnetic analyses indicate that frictional heating (>500°C) occurred in the pseudotachylyte veins during large earthquakes. The resulting high temperatures induced thermal decomposition of paramagnetic minerals, forming magnetite and contributing to the higher magnetic susceptibility values. Different generations of pseudotachylytes and numerous high magnetic susceptibility zones together demonstrate that ancient powerful earthquakes may have occurred repeatedly in the Longmen Shan thrust belt.

Metallic iron formed by melting: A new mechanism for magnetic highs in pseudotachylyte

“…2A). Previous studies have found partially melted quartz in the WFSD-2 cores and surface outcrops along the Yingxiu-Beichuan fault (Wang et al, 2015;Zhang et al, 2017), suggesting that earthquakes with frictional heating temperatures above 1300 °C have occurred in the Longmen Shan thrust belt. The appearance of microlites at 1100 °C and 1300 °C shows that high temperatures are needed for the formation of microlites.…”

“…Therefore, the formation of metallic iron during melting is another reason for the magnetic highs of pseudotachylyte. The Longmen Shan thrust belt has experienced earthquakes with high frictional melting temperatures (above 1300 °C) (Wang et al, 2015;Zhang et al, 2017); therefore, the metallic iron might be responsible for the magnetic highs of pseudotachylyte.…”

“…1B) (Li et al, 2014). More than 20 layers of pseudotachylyte, with thicknesses varying from 1 mm to 5 cm, have been found in the ~20-m-thick cataclasite zone (579.62-599.31 m depth) within the WFSD-2 cores, indicating that ancient large earthquakes have occurred repeatedly in the Yingxiu-Beichuan fault (Zhang et al, 2017). In our study, we conducted heating experiments under atmospheric pressure on cataclasite (the wall rock of pseudotachylytes) obtained from the WFSD-2 cores.…”

Paleoseismic Slip Records and Uplift of the Longmen Shan, Eastern Tibetan Plateau

“…Thus, the presence of the glass is not a necessary feature of melt‐origin pseudotachylyte. Since a single seismic event is unlikely to produce multiple layers of pseudotachylytes with sharp boundaries (Shigetomi & Lin, ), the multilayered pseudotachylytes with different colors and thicknesses observed in the WFSD‐2 drilling cores (Zhang et al, ) should result from multiple seismic faulting events. Their different colors result from the different degrees of alteration, devitrification, or recrystallization in the different generations of pseudotachylytes (Lin, ).…”

Application of combined electrical resistivity tomography and seismic reflection method to explore hidden active faults in Pingwu, Sichuan, China

“…The horizontal shear movement toward the crust has transformed into a brittle overthrust nappe movement away from the Minshan fault zone and the Longmenshan tectonic belt, resulting in a typical tectonic environment of reverse faults. The movement has also formed geophysical distortion zones, such as the Bouguer gravity anomaly and the aeromagnetic anomaly, and caused crustal thickness [71][72][73][74]. Due to the uplift of the Qinghai-Tibet Plateau and the further development of the original faults, some major faults, such as Minjiang fault, Beichuan-Yingxiu fault, and Pingwu-Qingchuan fault, have manifested strong tectonic activities [75,76].…”