Search for: All records

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. 

Abstract Subduction zones host some of Earth's most damaging natural hazards, including megathrust earthquakes and earthquake‐induced tsunamis. A major control on the initiation and rupture characteristics of subduction megathrust earthquakes is how the coupled zone along the subduction interface accumulates elastic strain between events. We present results from observations of slow slip events (SSEs) in Cascadia occurring during the interseismic period downdip of the fully coupled zone, which imply that the orientation of strain accumulation within the coupled zone can vary with depth. Interseismic GPS motions suggest that forces derived from relative plate motions across a shallow, offshore locked plate interface dominate over decadal timescales. Deeper on the plate interface, below the locked (seismogenic) patch, slip during SSEs dominantly occurs in the updip direction, reflecting a dip‐parallel force acting on the slab, such as slab pull. This implies that in subduction zones with obliquely convergent plate motions, the seismogenic zone of the megathrust is loaded by forces acting in two discrete directions, leading to a depth‐varying orientation of strain accumulation on the plate interface. 

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. 

Abstract Several tectonic processes combine to produce the crustal deformation observed across the Cascadia margin: (1) Cascadia subduction, (2) the northward propagation of the Mendocino Triple Junction (MTJ), (3) the translation of the Sierra Nevada–Great Valley (SNGV) block along the Eastern California Shear Zone–Walker Lane and, (3) extension in the northwestern Basin and Range, east of the Cascade Arc. The superposition of deformation associated with these processes produces the present-day GPS velocity field. North of ~ 45° N observed crustal displacements are consistent with inter-seismic subduction coupling. South of ~ 45° N, NNW-directed crustal shortening produced by the Mendocino crustal conveyor (MCC) and deformation associated with SNGV-block motion overprint the NE-directed Cascadia subduction coupling signal. Embedded in this overall pattern of crustal deformation is the rigid translation of the Klamath terrane, bounded on its north and west by localized zones of deformation. Since the MCC and SNGV processes migrate northward, their impact on the crustal deformation in southern Cascadia is a relatively recent phenomenon, since ~ 2 –3 Ma. 

The utilization of thermal-chronological data to constrain mountain building processes exploits the links among rock uplift, exhumation, and cooling during orogenesis. Conceptually, periods of rapid uplift and associated denudation will lead to cooling of rocks as they approach Earth’s surface. The linkage between uplift and exhumation can be complex, but in practice exhumation is often assumed to directly track uplift. The reconstruction of temperature-time histories via thermochronologic systems provides a proxy method to relate the cooling of rock as it is exhumed toward the surface to orogenesis. For the rapid exhumation rates that can occur in active orogenic systems the thermal history will be complex as a result of heat advection, rates of propagation of thermal perturbations, and other processes that affect the cooling behavior. These effects become amplified as exhumation rates increase, and in regions experiencing exhumation rates greater than ∼0.2–0.3 mm/yr (0.2–0.3 km/Ma) simple assumptions of cooling through a constant geotherm will bias the subsequent interpretation. Here we explore, through a suite of generalized models, the impact of exhumation rate and duration on the resulting thermal history and apparent age results. We then apply lessons from these simple exhumation systems to data sets from the high-relief ranges along the eastern margin of the Tibetan Plateau to determine exhumation histories constrained by those data. The resulting exhumation histories provide constraints on the onset of Cenozoic exhumation, the subsequent pace of exhumation, and on the tectonic history of one of the major fault systems in the central Longmen Shan. 

In the 1970s, the Shumagin Islands region of the Alaska subduction zone was identified as a seismic gap expected to host a future great [moment magnitude ( M w ) ≥8.0] earthquake. More recent geodetic data indicate that this region is weakly coupled, and the geologic record shows little evidence of past large events. From July to October 2020, a series of earthquakes occurred in this region, raising the possibility of greater coupling. The initial M w 7.8 thrust faulting earthquake straddled the eastern edge of the Shumagin Gap and was followed by an M w 7.6 strike-slip earthquake within the Shumagin Gap. Stress modeling indicates that this strike-slip earthquake is in fact favored if the Shumagin Gap has low coupling, whereas a highly coupled Shumagin Gap inhibits that type and location of earthquake. The initial thrust earthquake and its afterslip enhanced the strike-slip loading within the subducting slab, helping to trigger the October event.