T9 Linking Fault Zone Deformation Processes, Kinematic Indicators, and Geochronology through the Brittle and Brittle-Ductile Regimes of the Earth’s Crust

Monday, 26 October 2020
1:30 PM - 5:30 PM
Meeting Rooms

INTEGRATED FAULT SLIP AND HEMATITE (U-TH)/HE DATA DOCUMENT STRAIN PARTITIONING AT MILLION-YEAR TIMESCALES WITHIN THE EASTERN DENALI FAULT ZONE, SOUTHWEST YUKON, CANADA

Paper number: T9-11
Program: Technical Sessions
Time: 4:00 PM – 4:15 PM

Authors

Abstract

Linking kinematic data with geo- and thermo-chronometric constraints from brittle regime fault rocks is critical to placing structural histories within a geodynamic context. In many fault zones, thermal fields are spatially complex and transient due to concomitant deformation, fluid flow, and exhumation, complicating straightforward interpretation of field and radioisotopic data. We present kinematic, textural, and hematite (U-Th)/He (HeHe) thermochronometric data from abundant, minor, hematite-coated slip surfaces collected from ~30 km along-strike and <4 km perpendicular to the Eastern Denali fault zone (EDFZ). Our integrated approach demonstrates how thermochronometric signatures of deformation can be isolated to document spatiotemporal patterns of fault slip. Hematite slip surfaces show reverse and dextral striae, with some surfaces exhibiting dextral overprinting reverse striae. Dated hematite aliquots comprise multiple generations of foliated, high-aspect ratio, ~30 nm-thick x 300 nm-long plates. Thirty-eight HeHe dates from 11 samples range from 11.5 ± 3.2 Ma (2σ) to 3.4 ± 2.2 Ma. HeHe dates define date populations at ~8 Ma, ~6 Ma, and ~4 Ma. Consistent plate widths suggest a uniform HeHe closure temperature (Tc) of ~65 °C across our samples. Comparison of wall-rock thermochronometers with equivalent Tc to our HeHe dates and discrete date populations, despite similar intersample Tc, exclude an ambient cooling interpretation of HeHe data. In addition, microfabrics indicative of aseismic slip and hypothetical He loss calculations preclude date resetting by frictional heating or hot fluids. Instead, we suggest our data record hematite hydrothermal alteration from ~8-4 Ma and at depths ≤2 km (~65 °C), followed by slip reactivation. Each date population comprises samples with dip- and strike-slip striae. Dates from overprinting striae are indistinguishable. These results suggest variably-oriented incremental strains on discrete slip surfaces occur at timescales at or less than the resolution of our HeHe data. We suggest kinematic and thermochronometric data capture a window of Miocene to Pliocene transpression and strain partitioning over Myr-timescales within the EDFZ. More generally, our results bracket potential timescales associated with partitioned deformation in fault zones.

Geological Society of America Abstracts with Programs. Vol. 52, No. 6

doi: 10.1130/abs/2020AM-358135

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