Long-lived Exhumation along the Denali Fault System Preserves Ductile Deformation Formed during Transpression
Sarah Roeske, University of California Davis
Laura Tait, University of California Davis
Matty Mookerjee, Sonoma State University
Jeff Benowitz, University of Alaska Fairbanks
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The Denali Fault is a major continental strike-slip fault in south-central Alaska that currently accommodates up to 13 mm/yr of dextral slip and 1-2 mm/yr convergence. Based on rates of exhumation on the N side of a gentle (18°) restraining bend in the Hayes Range (eastern AK range), the current rate of shortening has been constant or a bit higher over the past 25-27 Myr. The concentration of youngest 40Ar/39Ar cooling ages (15 Ma for mica and 6 Ma for K-feldspar) at the apex of the bend supports the idea that the north side has been relatively fixed with respect to the bend since the Miocene. We interpret fabrics from rocks with Miocene cooling ages as recording strain associated with dextral transpression.
We conducted vorticity analyses on orthogneiss and quartz-rich metasedimentary rocks to better understand the mechanisms for exhumation along this section of the Denali fault. The kinematic vorticity number (Wk) for each sample was determined using two different methods: 1) grain shape analysis, which relates the length of the maximum (λ1) and minimum (λ3) principal strain axes ratio (RXZ) to the orientation of λ1 with respect to foliation, and 2) the LPO method, which relates RXZ to the angle (β) between the flow plane (as determined by EBSD analysis) and the foliation.
The depth of exhumation varies through the bend, exposing low-middle greenschist facies rocks in the west and amphibolite facies to the east. Foliation strikes dominantly subparallel to the modern Denali fault trace, but is variably N-dipping to the west of the bend and consistently steeply N-dipping east of the bend. Lineations also vary from west to east, variably but overall gently plunging to the west and consistently moderately W-plunging to the east. More lineations are parallel to the intermediate axis than the long strain axis, and in two of the metasediments the lineations are equidistant from the axes, suggesting a complex relationship between mineral growth and finite strain as recorded by elongate quartz crystals. Preliminary vorticity results indicate deformation is dominated by pure shear (Wk < 0.71). C-axis pole figures yield hybrid single girdle/cross-girdled patterns, which are consistent with flattening across the Denali fault combined with simple shear along the fault.
The vorticity data combined with the 40Ar/39Ar dates on the mineral fabrics demonstrate strain localization along the narrow (~1 km) master strand of the Denali strike-slip fault zone has been continuously active for over 30 Myr.