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A Paleoseismic Investigation of the Little Salmon Fault and Goose Lake Fault: Inference Into the Role of Upper-Plate Faults in the Southern Cascadia Subduction Zone

Session: Earthquake Science, Hazards and Policy in Cascadia [Poster]

Type: Poster

Date: 4/20/2021

Presentation Time: 11:30 AM Pacific

Description: 

The Southern Cascadia subduction zone (SCSZ) accommodates both northwest dextral shear from the San Andreas fault zone and northeast contraction from the convergent margin. Coastal paleoseismic studies in southern-most Cascadia suggest that upper-plate structures may participate in megathrust strain accumulation and release. However, how upper-plate faults accommodate strain along the SCSZ in space and time remains uncertain. Here, we investigate the earthquake history and deformation style of two SCSZ upper-plate faults: the central segment of the Little Salmon fault (LSF) and central trace of the Goose Lake fault (GLF) within the lower Van Duzen River Valley near Hydesville, California. The LSF trench exposed three shallow (9^o to 11^o) northeast- to north-dipping thrust faults where Pleistocene and Holocene strata of the hanging wall were folded around the fault tip and thrust over the footwall. The most recent earthquake (MRE) is well expressed by a distinct colluvial wedge. Four ¹⁴C ages from the associated colluvial wedge indicate the MRE occurred prior to 90-480 yr BP (CE 1860-CE 1470). In addition, we observed moderate evidence for both another late Holocene earthquake and a third, early Holocene earthquake. Bayesian age modeling is planned to interpret earthquake chronology for the LSF. The GLF trench revealed discrete subvertical faulting expressed as a pronounced vertically imbricated shear fabric that juxtaposes the Plio-Pleistocene Carlotta Formation against inferred late-Quaternary fluvial deposits. The presence of a high-angle fault and laterally discontinuous gravel units indicate the GLF is an oblique-reverse fault with an unknown amount of lateral slip. We interpret three earthquakes from trench stratigraphy. Results from optical stimulated luminescence dating are pending for both trench sites. Given fault proximity (1.7km) and the notable differences in fault characteristics observed in the trenches, we propose the LSF and GLF may act to partition strain between contraction and oblique translation within the lower Van Duzen River Valley.

Presenting Author: Tyler C. Ladinsky

Student Presenter: No

Authors