Focal mechanism of a Triassic large magnitude earthquake along the Longmen Shan thrust belt
Abstract
Previous geochronological work on pseudotachylyte exposed within the Longmen Shan thrust belt (LMSTB) showed that seismic activity along this major lithospheric discontinuity started in Upper Triassic times, during the Indosinian orogeny. The tectonic context and kinematic of these early ruptures remains, however, unknown. We apply the mini-AMS method to investigate the paleo-earthquake focal mechanism and the evolution of LMSTB. This method uses the anisotropy of magnetic susceptibility of very small oriented specimens (3.5 mm cubes) in pseudotachylyte and cataclasite to determine the coseismic viscous shear plane, shear direction, shear sense, and the maximum principal compressional stresses. Ninety-eight oriented cubes of pseudotachylyte and 17 of cataclasite were collected. The magnetic fabric of pseudotachylyte most likely arises from the preferred orientation of coseismic magnetite grains formed by breakdown of ferrous minerals upon frictional heating. The oblate symmetry of the magnetic fabric indicates the control exerted by magnetically oblate markers. The parallel margins of the pseudotachylyte indicate that the slip plane of ancient earthquake was consistently oriented about NE 30-40°. The magnetic lineation of the pseudotachylyte and the obliquity of the magnetic foliation with respect to the vein margins imply the slip direction and the slip sense of ancient earthquake was EW 246-255° and NE, respectively. These structural parameters indicate that the ancient earthquake focal mechanism was mainly thrust-dextral strike-slip. The maximum AMS axes (K1) of pseudotachylyte display a strong preferred orientation around 246-255°, which indicates that the maximum principal compressional stress in LMSTB is oriented in a NW-SE direction in Upper Triassic times. This direction is consistent with the in-situ stresses measured in Wenchuan Earthquake Fault Scientific Drilling borehole 1, recent maximum principal stress orientations and the tectonic displacement direction in the Longmen Shan area. Therefore, the results and recent seismicity data might show that the Cenozoic seismic activities inherit from ancient activities and the continued exhumation of Longmen Shan accommodated along thrust faults occurred in the same broad background stress field since the Indosinian orogeny.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.T23D0399Z
- Keywords:
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- 1140 Thermochronology;
- GEOCHRONOLOGYDE: 8034 Rheology and friction of fault zones;
- STRUCTURAL GEOLOGYDE: 8036 Paleoseismology;
- STRUCTURAL GEOLOGYDE: 8118 Dynamics and mechanics of faulting;
- TECTONOPHYSICS