A Neogene Shortening Budget for the Eastern Alaska Range: Implications for Strain Transfer in the Crust
Trevor Waldien, UC Davis
Sarah Roeske, UC Davis
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Oblique convergence in the Denali fault system may absorb up to ~20% of Yakutat-North America relative plate motion inboard of the plate boundary faults. While dextral strike-slip motion on the Denali fault is well established in the neotectonic record, the history of Neogene-Quaternary shortening across the Denali fault system is less well known. The Broxson Gulch fault is often cited as a locus of Neogene shortening south of the Denali fault in the eastern Alaska Range. However, the magnitude and rate of shortening accommodated by the Broxson Gulch fault and subsidiary structures remain unresolved, leading to uncertainties in the Neogene shortening budget across the Alaska Range and the amount of plate boundary strain absorbed by the Denali fault system.
We present new low temperature thermochronometry and fault kinematic analysis from the Broxson Gulch fault and linked structures in the eastern Alaska Range. Our data reveal ≥20 km of post-30 Ma shortening accommodated by thrust faults south of the Denali fault. The thrusts accommodate shortening nearly perpendicular to the Denali fault, which indicatives highly partitioned transpression in the region.
Integrating our dataset with published datasets yields apparently contradictory predictions regarding the mechanics of strain transfer in the crust and addresses the potential for hypothesized plate-scale sub-horizontal detachment horizons across terrane-bounding transform faults in the Cordillera. Cooling ages and rock fabric analysis show that metamorphic rocks from the structurally highest portion of the Broxson Gulch thrust system, nearest to the Denali fault, have been exhumed from below the brittle-ductile transition since ~30 Ma. Exhumation of these mid-crustal rocks is likely controlled by horizontal rheologic contrasts within the crust (i.e. terrane boundaries). In contrast, aftershocks from the 2002 Denali earthquake and balanced cross-sections of the Pliocene-Present thrust systems north and south of the Denali fault show the splay thrust systems rooting to the Denali fault at ~8-10 km depth. The non-uniform Quaternary shortening rates across the active thrust systems predict effective 'decapitation' of the Denali fault via slip along a sub-horizontal fault in the upper crust. With the present data, we suggest that detachment horizons in the middle-upper crust may be relatively ephemeral features spatially restricted to regions actively experiencing heterogeneous strain.
Shortening on the Broxson Gulch fault, initiation of the Wrangell volcanic arc, regional exhumation, and drainage reorganization collectively record widespread modification to the upper plate of the southern Alaskan margin, likely in response to a plate boundary reorganization at ~30-25 Ma.