Abstract
The southern Cascadia forearc undergoes a three-stage tectonic evolution, each stage involving different combinations of tectonic drivers, that produce differences in the upper-plate deformation style. These drivers include subduction, the northward migration of the Mendocino triple junction and associated thickening and thinning related to the Mendocino Crustal Conveyor (MCC) effect, and the NNW translation of the Sierra Nevada-Great Valley (SNGV) block. We combine geodetic data, plate reconstructions, seismic tomography and topographic observations to determine how the southern Cascadia upper plate is deforming in response to the combined effects of subduction and NNW-directed (MCC- and SNGV-related) tectonic processes. The location of the terrane boundaries between the relatively weak Franciscan complex and the stronger Klamath Mountain province (KMP) and SNGV block has been a key control on the style of upper-plate deformation in the southern Cascadia forearc since the mid-Miocene. At ∼15 Ma, present-day southern Cascadia was in central Cascadia and deformation there was principally controlled by subduction processes. Since ∼5 Ma, this region of the Cascadia upper plate, where the KMP lies inboard of the Franciscan complex, has been deforming in response to both subduction and MCC- and SNGV-related effects. GPS data show that the KMP is currently moving to the NNW at ∼8–12 mm/yr with little internal deformation, largely in response to the northward push of the SNGV block at its southern boundary. In contrast, the Franciscan complex is accommodating high NNW-directed and NE-directed shortening strain produced by MCC-related shortening and subduction coupling respectively. This composite tectonic regime can explain the style of faulting within and west of the KMP. Associated with this Mendocino Crustal Conveyor crustal thickening, seismic tomography imagery shows a region of low velocity material that we interpret to represent crustal flow and injection of Franciscan crust into the KMP at intracrustal levels. We suggest that this MCC-related crustal flow and injection of material into the KMP is a relatively young feature (post ∼5 Ma) and is driving a rejuvenated period of rock uplift within the KMP. This scenario provides a potential explanation for steep channels and high relief, suggestive of rapid erosion rates within the interior of the KMP.
Highlights
Upper-plate deformation in subduction zone forearcs occurs in response to both short-term and long-term processes
South of the Mendocino triple junction (MTJ), the Franciscan complex continues as the outboard terrane, and the Klamath Mountain province (KMP) is replaced at ~40°N by the Sierra Nevada - Great Valley (SNGV) block composed of the accreted Great Valley ultramafic body and the Sierra Nevada terrane
The overall deformational effects on the upper plate of the southern Cascadia forearc from the superposition of multiple tectonic processes, including subduction coupling, the Mendocino Crustal Conveyor (MCC) process, and the northward motion of the Sierra Nevada-Great Valley (SNGV) block can be identified by consideration of the decadal-scale motions of the upper plate and the regional long-term geomorphic and geologic record
Summary
Upper-plate deformation in subduction zone forearcs occurs in response to both short-term (earthquake cycle) and long-term (million-year plate interaction) processes. We focus on 1) how the behavior of upperplate faults changes as the plate boundary evolves, including the development of shear zones in the vicinity of the MTJ, and 2) the spatiotemporal patterns of uplift across different regions of the southern Cascadia forearc, including a possible rejuvenation in uplift of the Klamath Mountain province (KMP) over the last 5 million years We address these key points by building on our recent results delineating the kinematics and strain within the terranes in the vicinity of the MTJ, including the Franciscan, Klamath, Siletz, and Sierra Nevada/Great Valley terranes (McKenzie and Furlong, 2021; McKenzie et al, 2022). We present observations of normalized channel steepness throughout the region, which provide additional insight into which regions are potentially undergoing active uplift and exhumation
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
