Cryptic, distributed Quaternary faulting in high-relief volcanic topography in northeastern California

Jessica Thompson Jobe, USGS Geologic Hazards Science Center

Wednesday, June 2, 2021 at 10:30 AM

Location
Online-only seminar via Microsoft Teams
Host
Stephen DeLong

Identifying and characterizing cryptic, distributed faulting in high-relief topography can be challenging due to vegetative cover and high rates of hillslope erosion that may mask surficial evidence of active faulting. A multifaceted approach to interrogate the landscape at different spatial and temporal scales commonly illuminates regions of active or recent deformation and helps to characterize faults for seismic hazard analysis in low-strain tectonic settings. This talk will focus on the Pit River region in northeastern California between Mount Shasta and Lassen Peak. In this region, the tectonic regimes of Basin and Range extension, Cascadia subduction zone convergence, and Walker Lane dextral shear converge at the newly characterized transtensional Pondosa fault zone. The Pondosa fault zone is located north of the previously recognized Hat Creek and Rocky Ledge fault systems. We characterize the Pondosa fault zone via analysis of high-resolution lidar topographic data combined with bedrock mapping, 40Ar/39Ar geochronology, and river network analysis with existing geologic and fault mapping. This fault system trends WNW-ESE and disrupts the regional N-S structural fabric. Faults vertically offset lava flows as old ~9.6 Ma and as young as ~12.4 ka, indicating late Quaternary fault activity despite a lack of seismicity along many of the faults. Most of these faults are not currently included in active fault databases, and mapping and understanding the extent, rate, and style of deformation on these faults is important because of their proximity to critical facilities and hydropower infrastructure in addition to interpreting regional tectonic processes. We also discuss the possible role of the Pondosa fault zone in accommodation of relative motion between the Sierran-Oregon Coastal Block microplates.

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