Mid-Miocene to Present Upper-Plate Deformation of the Southern Cascadia Forearc: Effects of the Superposition of Subduction and Transform Tectonics

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.

Regional and Local Patterns of Upper‐Plate Deformation in Cascadia: The Importance of the Down‐Dip Extent of Locking Relative to Upper‐Plate Strength Contrasts

“…Of course there may be other processes that can also produce permanent deformation in the upper plate, for example MCC or SNGV‐driven tectonic processes. However, we would expect that deformation related to these additional processes would reflect the different orientations of the deformational regimes (McKenzie et al., 2022; McKenzie & Furlong, 2021)…”

“…Of course there may be other processes that can also produce permanent deformation in the upper plate, for example MCC or SNGV-driven tectonic processes. However, we would expect that deformation related to these additional processes would reflect the different orientations of the deformational regimes (McKenzie et al, 2022;McKenzie & Furlong, 2021) The decay in velocities from the trench moving inboard indicates plate-motion-parallel shortening strain rates greater than 0.1/Myr (and uplift rates on the order of ∼2 mm/yr) within the weaker material close to the trench -west of the Siletzia terrane in central Cascadia and west of the Klamath terrane in southern Cascadia. We assume that regions of high shortening strain, such as shown in Figure 7, are also likely candidates for regions that accommodate permanent deformation.…”

“…The observed horizontal GPS velocity field in south‐central Cascadia is thus inconsistent with a simple subduction locking model (McKenzie et al., 2020; McKenzie & Furlong, 2021), implying there are other tectonic processes superimposed on the subduction signal. A range of tectonic models have been invoked to explain this pattern of surface velocities and they generally fall into one of two categories: Fore‐arc block rotation across Cascadia (e.g., McCaffrey et al., 2000, 2007, 2013); or, Overprinting of the Cascadia subduction signal by deformation associated with other tectonic processes, such as the MCC and SNGV motion (e.g., McKenzie et al., 2020; McKenzie et al., 2022; McKenzie & Furlong, 2021). …”

“…Overprinting of the Cascadia subduction signal by deformation associated with other tectonic processes, such as the MCC and SNGV motion (e.g., McKenzie et al., 2020; McKenzie et al., 2022; McKenzie & Furlong, 2021).…”

Substantial Upper Plate Faulting Above a Shallow Subduction Megathrust Earthquake: Mechanics and Implications of the Surface Faulting During the 2016 Kaikoura, New Zealand, Earthquake

“…The discrepancy seen in the Kaikoura earthquake between the standard earthquake cycle model (with its limited upper plate faulting) and observations from this event require a rethinking of the distribution and amount of upper plate deformation throughout the earthquake cycle. It is tempting to ascribe the extreme upper plate faulting during the Kaikoura earthquake to unique conditions in that plate boundary setting, but there are other events that also show evidence of unusual upper plate faulting potentially associated with a megathrust earthquake: the 1960 Mw 9.5 Chile earthquake (Kanamori et al., 2019), the 1855 Wairarapa, New Zealand, event (Beavan & Darby, 2005; Rodgers & Little, 2006), and along sections of the Cascadia margin (McKenzie, Furlong, & Kirby, 2022). Collectively, observations of these other earthquakes suggest that extreme upper plate faulting above a rupturing megathrust may be a more common global process than previously inferred or assumed.…”