From damage zone to core: quantifying mechanical and hydrological coupling during fault-zone structural evolution

University Of Wisconsin-Madison, Madison WI

Investigators

Abstract

Fault zones are important structures that dictate the magnitude and distribution of earthquakes and fluid flow in the earth’s crust. Understanding the evolution of these structures is therefore integral to improved earthquake hazards assessment, petroleum and/or ore-mineral exploration, and geologic storage of carbon and/or nuclear waste. A fault is not a single surface, but rather consists of one or more zones of high deformation, called “fault cores”, surrounded by zones of low deformation, called “damage zones”. Most of the offset across a fault, including earthquake-related motion, occurs within the discrete fault core(s), which typically record intense crushing and grinding of rock. In contrast, damage zones are more diffuse, and typically record distributed deformation including discrete fractures and/or smaller faults. Collectively, the character and distribution of fault cores and damage zones may influence seismic wave propagation and energy release during earthquakes. They also dictate a fault’s ability to act as a conduit, barrier, or combined conduit/barrier system for fluid movement below the Earth’s surface. This research is designed to improve our knowledge of how fault cores and their surrounding damage zones change through time during the development of fault zones. More specifically, this research is constraining: 1) how long it takes to develop damage zones, particularly in relation to the development of fault cores; and 2) how fluid flow moves through damage zones versus through fault cores throughout a fault’s lifetime. The collected data are being used as a test case for digitally storing and sharing multi-disciplinary earth science data sets in the open-access, community-developed StraboSpot Data System. The project is contributing to STEM education by supporting the training of graduate and undergraduate students. Finally, this project contributes to the outreach mission of the University of Wisconsin-Madison Geology Museum by participating in the improvement of “Stories in Stone”, which uses an inquiry-based narrative style to teach the public about earthquake science. The researchers are integrating structural, geochemical, and geochronological analyses to test the hypothesis that fault core and damage zone development are linked – both mechanically and hydrologically – throughout the development of individual fault zones. Analyses are focused on syntectonic, authigenic mineral phases localized within fault cores and damage zones, which provide a rock record of the timing, character, and conditions of deformation and fluid flow in individual fault zones. The researchers are collecting three complimentary data sets. Outcrop and microstructural examination constrain the nature and distribution of fault-zone deformation and mineralization from the meso-to-micro scale. Geochemical and isotopic analyses of authigenic mineral phases determine the source and character of fluids that migrated through distinct fault structures, in addition to constraining the temperature conditions of deformation. Finally, a novel combination of geochronological analyses facilitates a comparison of the timing of deformation and fluid flow between fault cores and their surrounding damage zones. Integration of these data sets provides unique insight into fault-zone structural evolution and its effects on fault mechanics and fluid flow, thereby generating an improved understanding of seismicity and mass transport in the crust. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.