Strike-Slip in Transtension: Complex Crustal Architecture of the Warm Springs Fault Zone, Northern Walker Lane, Nevada

Date: 4/24/2019

Time: 06:00 PM

Room: Fifth Avenue

Faults in the transtensional northern Walker Lane accommodate 5-7 mm/year of dextral shear but summed Holocene geologic fault slip rates are lower than geodetically-measured strain accumulation rates at most latitudes. Distributed faulting and uncertain fault network connectivity complicate comparisons between geologic and geodetic data in the region. We use two newly acquired high-resolution seismic reflection profiles and a reprocessed deep crustal reflection COCORP profile to assess the subsurface geometry of the Holocene-active, right-lateral Warm Springs strike-slip fault zone near Reno, Nevada. Our multi-scale observations extend to 12 km depth and suggest that the Warm Springs fault zone is far more complex at depth than implied by the distribution of post-Lahontan (<15.5 ka) surface scarps. The two ~4.5 km-long high-resolution profiles both image to a depth of ~2 km along fault-perpendicular lines and reveal moderately-dipping reflections and truncations that project to mapped surface scarps. The shallow lines are co-located with COCORP profile NV8 at ~40° N. Re-analysis of the COCORP data reveals previously unidentified coherent reflections to a depth of ~12 km. Truncations, juxtaposed regions of contrasting dip and/or reflectivity, and diffractions in the COCORP data define fault-bounded packages above a zone of moderately dipping, laterally continuous mid-crustal (4-12 km) reflections. From these profiles, the Warm Springs fault zone doesn’t appear to be defined by a simple, sub-vertical fault zone extending through the entire seismogenic crust. Instead, the reflections are consistent with a zone of nested steep- and moderately-dipping faults that sole into a mid-crustal, moderately-dipping detachment. If this is the case, the common assumption that transtension is accommodated only by crustal-scale subvertical strike-slip faults in the northern Walker Lane may not be valid, with important consequences for seismic hazard models and models of fault mechanics.

Presenting Author: Richard W. Briggs

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