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Characterizing Active Cross-shore Faults Along the Continental Shelf in Southern Cascadia

We integrate new high-resolution multi-channel sparker (MCS) and chirp seismic imagery, targeted sediment coring, relocated seismicity and legacy seismic data to map and characterize active cross-shore faults in southern Cascadia. Co-located MCS and chirp seismic profiles collected along and across the shelf between the Eel and Rogue rivers in 2018 and 2019 reveal active folding and faulting within the shallow subsurface along the offshore extensions of the Table Bluff, Little Salmon, Mad River, Bald Mountain-Big Lagoon and Lost Man fault zones. MCS data image locally folded and faulted Neogene and younger strata to sub-seafloor depths > 500 m and provide structural context for the chirp imagery that offer higher resolution (10’s of cm) imagery in the shallower (< 50 m) subsurface. We use stratal geometry on cross-shelf chirp profiles to identify and map the regional transgressive surface that records erosion from sea level rise following the Last Glacial Maximum (LGM, ~21 ka-present) to use as a strain marker for characterizing cross-shore fault deformation. Relief along the transgressive surface is evident by both long-wavelength (10’s of km) features that likely reflect active folding and punctuated vertical offset ranging from 5 to 10 m coincident with fault traces of the Table Bluff, Mad River, Bald Mountain-Big Lagoon, and Lost Man fault zones imaged in MCS and chirp seismic data. Similarly, cross-sections of relocated seismicity reveal steeply dipping lineations of events associated with the Table Bluff and Little Salmon faults. Transgressive surface ages at depths between 50 and 100m below sea level are estimated to be between 11 and 15ka, based on sea level curves, which result in maximum vertical deformation rates of between ~0.3 to 0.9 mm/yr. These rates are less than those estimated on land and potentially reflect a change from compressional to transpressional tectonics across the shoreline as these faults die off or bend to the north along the shelfbreak.

Session: Characteristics, Hazards and Evolution of the Gorda Region of the Cascadia Subduction Zone I

Type: Oral

Room: Grand B

Date: 4/21/2022

Presentation Time: 08:30 AM Pacific

Presenting Author: Janet Watt

Student Presenter: No

Additional Authors