MID-MIOCENE TO PRESENT UPPER-PLATE DEFORMATION OF THE SOUTHERN CASCADIA FOREARC: EFFECTS OF THE SUPERPOSITION OF SUBDUCTION AND TRANSFORM TECTONICS

Upper-plate deformation in southern Cascadia is governed by three major tectonic processes: (1) the Cascadia subduction zone earthquake cycle, (2) the NNW propagation of the Mendocino Triple Junction (MTJ), and (3) the NNW translation of the Sierra Nevada-Great Valley (SNGV) crustal block. Prior to ~6 Ma, deformation associated with the migration of the MTJ and SNGV had not yet impinged on what is now the southernmost region of the Cascadia upper plate. As a result, there has been a fundamental change in the tectonic style of the southern Cascadia upper plate since the end of the Miocene – from solely subduction to this combined deformational system (McKenzie & Furlong, 2021). The southern Cascadia upper plate is comprised of two primary geologic components - the Franciscan accretionary complex near the present-day coast and the inland Klamath Mountain Province (KMP). The KMP is composed of a series of subduction-related thrust sheets accreted to North America and intruded by Paleozoic to Mesozoic granitoid plutons. The KMP experienced initial tectonic denudation during the Oligocene to early Miocene (Piotraschke et al. 2015), which was followed by a relative hiatus in exhumation within the KMP through much of the Miocene. The post-Miocene to present history of uplift and deformation, however, remains uncertain.

We combine geodetic, geomorphic and geodynamic tools to evaluate the tectonic evolution of the southern Cascadia upper plate following the onset of deformation associated with the northward advance of both the MTJ and SNGV block. Our results indicate that the relative rigidity of the KMP plays a key role in driving the style of upper-plate forearc deformation west and northwest of the KMP. Horizontal (NNW) displacement of the KMP appears to drive upper-plate deformation along its margins, but cannot directly explain the high topography within the KMP. Preliminary analysis of river profiles suggests the possibility of differential uplift and exhumation across the core of the KMP. These patterns are correlated with a lens-shaped region of low seismic velocities at 15-30 km depth. We suggest that flow of this low velocity material and injection into the KMP was associated with the northward migration of the MTJ and appears to be a mechanism for driving forearc deformation and crustal thickening in advance of the MTJ.