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

Abstract

The details of subduction zone locking place constraints on the characteristics of megathrust events. Due to the lack of significant present‐day seismicity along the Cascadia subduction interface, geodetic data are used to assess subduction locking along the margin. We isolate the subduction signal from other tectonic signals within the Cascadia GPS field, to assess the details of plate‐interface locking. Apparent coupling determined by a simple homogenous elastic half‐space inversion cannot everywhere reproduce the subduction component of the GPS field. Consequently, we explore the relationships among upper‐plate strength, locking depth and the resulting surface velocity signal using 2D finite element models. When the upper plate is composed of a weak material, trenchward of a strong backstop, we find that the down‐dip limit of locking relative to the location of the weak‐to‐strong transition controls how upper‐plate deformation is spatially distributed. If locking extends into the stronger material, as observed in central Cascadia, the surface velocity field propagates farther inland than expected from a simple homogeneous elastic model. In contrast, in southern Cascadia, because locking terminates within the weak accretionary margin, upper‐plate shortening is localized within the weaker material, particularly in the region between the end of locking and the strong Klamath terrane. This behavior provides a possible mechanism for producing the high (geodetic and permanent) uplift rates, plate‐motion‐parallel shortening, and crustal exhumation observed in many active and fossil subduction zones.

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

“…This north-northwestward motion along the northern ECSZ, and therefore motion of the SNGV block, accelerated at ~6 Ma due to the northward motion of Baja California (Plattner et al, 2010). Taken together, this combination of tectonic processes means that present-day upper-plate deformation in southern Cascadia records the superposition of processes including Cascadia subduction earthquake cycle deformation, the migration of the MTJ and its associated thickening-thinning effects, and the NNW motion of the SNGV block facilitated by shear across the northern ECSZ (Figures 1, 4; McKenzie et al, 2022;McKenzie and Furlong, 2021).…”

“…Specifically 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 add to those results newly obtained crustal (local earthquake) tomography of the coastal region extending from north of the San Francisco Bay region to approximately 42 °N (Furlong et al, 2021).…”

“…From these two displacement components we find that both the NEdirected and NNW-directed shortening strain rates are highest within the Franciscan complex, to the west of the KMP. The KMP itself is accommodating very low NE-and NNW-oriented shortening rates of ~10 -16 s −1 (McKenzie and McKenzie et al, 2022).…”

“…East of the Franciscan complex within the KMP, the NE-directed GPS velocities remain at ~5 mm/yr across the region (Figure 6). This localization of shortening within the weak Franciscan material can be linked to the effects of subduction mechanical locking terminating within the relatively weak crustal domain (the Franciscan complex) to the west of a rigid backstop -the KMP (McKenzie et al, 2022). In regions where subduction locking ends trenchward of a rigid backstop, interseismic subduction-related shortening is localized in the region between the down-dip limit of locking and the rigid backstop (McKenzie et al, 2022).…”

“…This localization of shortening within the weak Franciscan material can be linked to the effects of subduction mechanical locking terminating within the relatively weak crustal domain (the Franciscan complex) to the west of a rigid backstop -the KMP (McKenzie et al, 2022). In regions where subduction locking ends trenchward of a rigid backstop, interseismic subduction-related shortening is localized in the region between the down-dip limit of locking and the rigid backstop (McKenzie et al, 2022). The upperplate region in southern Cascadia that is experiencing localized subduction-related shortening due to this effect is also within the northern part of the MCC-corridor.…”