Is the Record of Crustal Strain in the Pacific Northwest in the Holocene, as Revealed by Paleoseismic Investigations, Consistent with Regional Clockwise Rotation in the Cascadia Backarc and Forearc?

Abstract:

Both paleomagnetic and GPS observations document clockwise rotation of the forearc and backarc of the Cascadia subduction zone (CSZ). The similarity in observations for the long-term (since middle Miocene) geologic record and present-day GPS measurements raises the question to what extent is the paleoseismic record of Holocene crustal deformation in the forearc and backarc consistent with clockwise rotation? Because paleoseismic investigations typically target sites of identifiable ground deformation or subsidence in the Holocene (100-10,000 years before present), these studies only provide an incomplete window into the pattern of near-surface crustal strain that may be more apparent over a longer time scale. I provide an overview of the pattern of strain available from paleoseismic studies of the forearc and backarc of Cascadia. Conclusions are that near the coast where the crust predominantly responds to accumulation and release of strain on the megathrust, the proximal subduction zone signal is well defined within the paleoseismic history of earthquakes over many cycles. However, paleoseismic data do not entirely clarify whether Holocene deformation in the forearc and backarc of the CSZ is consistent with clockwise rotation of the forearc and backarc. Most of the individual faults that have ruptured Earth's surface in the Holocene are consistent with clockwise rotation; and the general pattern of north-directed compression in both the forearc and backarc is, again, not inconsistent with secular clockwise rotation. But the paleoseismic record of near surface deformation is consistent with clockwise rotation only by recognizing that other important tectonic variables may account for the pattern of individual fault ruptures. These contributing factors include inherited (i.e. predate plate-margin-induced clockwise rotation) zones of structural weakness in rocks, inherited crustal-scale structures such as basins or long standing structural highs and partitioning of strain on preexisting faults.