Compositional, mechanical and transport properties of carbonate fault rocks and the seismic cycle in limestone terrains: A case study of surface exposures on the Longmenshan Fault, Sichuan, China

Abstract

Destructive earthquakes are common in tectonically active regions dominated by carbonate cover rocks. The catastrophic Wenchuan earthquake that struck Sichuan, China, also affected a section of carbonate cover terrain. Numerous studies have focused on characterizing the compositional, transport and mechanical properties ofsilicate fault rocks, with many models being proposed for dynamic fault frictional and rupture processes, but little data are available on carbonates to date. In this thesis, I report on the properties of fault rocks collected from two carbonate-rich surface exposures on the surface rupture associated with the Wenchuan earthquake. The main conclusions are as follows: 1. The enrichment/depletion patterns, element partitioning and a very large implied volume loss (> 90%) found are quite different from those characterizing faults in granites and clastic sedimentary rocks, and can be explained by a mass removal model involving dissolution and advective transport enhanced by pressure solution. Increasing enrichment in smectite toward the principal slip surface was observed. Illitization of the gouges on the principal slip surfaces, caused by coseismic frictional heating, was found in both exposures studied. Results imply that coseismic de-watering reactions can be expected to be extensive at depth, and possibly helped generate excess pore pressure assisting dynamic slip weakening during the Wenchuan Earthquake. 2. High-velocity friction experiments revealed that slip weakening is more pronounced for water-dampened gouge than dry gouge, indeed pointing to thermal pressurization. The fault core gouge studied has a very low permeability (<10-21 m2 at 165 MPa) and is surrounded by fault breccia with permeability of 10-19 to 10-17 m2, grading into less permeable, fractured country rocks. The fault zone thus exhibits a “conduit/barrier” structure. We numerically modeled coseismic slip weakening based on measured properties. The results indicate thatthermochemical pressurization played a key role in causing dynamic slip weakening, andoffers a compelling explanation for the large coseismic displacement observed in the study area. 3. Slip nucleation and interseismic strength recovery on carbonate faults is simulated by exploratory, low-velocity frictional and healing experiments. Dry experiments show classical or Dieterich-type healing behavior. By contrast, the hydrothermal tests show “non-Dieterich-type” healing behavior, characterized by 1) an increase in apparent steady-state friction upon resliding after a hold period, and 2) a pronounced increase in (a–b) after the SHS stage. Further analysis suggests that these aspects of “non-Dieterich-type” healing were related toenhanced solution transfer processes, occurring during hold periods. Our findings suggest that, under in-situ hydrothermal conditions, interseismic fluid-assisted deformation processes can promote fault restrengthening and cause slip stabilization. 4. A mechanism-based microphysical model of friction is developed. By solving two controlling equations that are derived from kinematic and energy/entropy balance considerations, and using standard creep equations for pressure solution, we successfully simulate typical lab-frictional tests, namely “velocity stepping” and “slide-hold-slide” test sequences. To our knowledge, ours is the first mechanism-based model that can reproduce full RSF-like behavior without recourse to the RSF laws. Our modeling approach can provide a much improved framework for extrapolating friction data to natural conditions.