A Thermochronometric, Microtextural, and Numerical Modeling Approach to Deciphering the Rock Record of Deformation Processes in the Wasatch and Denali Fault Zones
Fault zones are the primary features that accommodate movement of Earth’s crust, resulting in the formation of mountain belts and damaging earthquakes. Rocks modified by faulting and brought to Earth’s surface by erosion are archives of the mechanical processes involved in earthquakes and(or) aseismic creep. Thermochronometry is a radioisotopic dating system primarily sensitive to temperature and offers a means to constrain dates and rates of thermal processes. Hematite is common in fault zones, amenable to (U-Th)/He (He) thermochronometry, and exhibits distinct microtextures diagnostic of fault zone mechanics. I apply hematite He thermochronometry and microtextural analyses with a suite of other tools to interrogate the evolution of the Wasatch fault zone (WFZ), UT, USA, and the eastern Denali fault zone (EDFZ), Yukon, Canada over different scales in space and time.
Hematite He dates and microtextures from hematite-coated fault surfaces and veins from the WFZ show fault surfaces accommodated ancient seismicity. Although this seismicity isage, fault surfaces formed within pre-existing hematite that is 100s of Myr older. Microtextures reveal that WFZ earthquakes were facilitated by fluidization of hematite, extreme grain size reduction and rolling between grains, and breakdown of rough fault surfaces. Small fault surfaces in the WFZ are ultimately the product of deformation processes occurring throughout deep geologic time and at different timescales culminating in earthquakes. Low-temperature thermochronometry is also used to constrain erosion related to surface uplift of mountains adjacent to the EDFZ. Results show growth of topography and deformation along the EDFZ occurred in three stages from ~95–75 Ma, ~75–30 Ma, and ~30 Ma–present, primarily as a response to plate boundary processes \u3e200 km away. Hematite He dates from hematite-coated fault surfaces in the EDFZ constrain hematite precipitation at ~8–4 Ma and reveal a record of faulting that contributes to mountain growth. Hematite microtextures in these samples suggest aseismic fault slip. The collective results of this dissertation highlight the spectrum of deformation of Earth’s crust from the mountain- to fault surface-scales and from 100s of Myr to seconds, as well as the different tectonic and mechanical processes responsible for this evolution
