Seismic reflection imaging of the Juan de Fuca plate from ridge to trench: New constraints on the distribution of faulting and evolution of the crust prior to subduction

Abstract

We present prestack time‐migrated multichannel seismic images along two cross‐plate transects from the Juan de Fuca (JdF) Ridge to the Cascadia deformation front (DF) offshore Oregon and Washington from which we characterize crustal structure, distribution and extent of faults across the plate interior as the crust ages and near the DF in response to subduction bending. Within the plate interior, we observe numerous small offset faults in the sediment section beginning 50–70 km from the ridge axis with sparse fault plane reflections confined to the upper crust. Plate bending due to sediment loading and subduction initiates at ~120–150 km and ~65–80 km seaward of the DF, respectively, and is accompanied by increase in sediment fault offsets and enhancement of deeper fault plane reflectivity. Most bend faulting deformation occurs within 40 km from the DF; on the Oregon transect, bright fault plane reflections that extend through the crust and 6–7 km into the mantle are observed. If attributed to serpentinization, ~0.12–0.92 wt % water within the uppermost 6 km of the mantle is estimated. On the Washington transect, bending faults are confined to the sediment section and upper‐middle crust. The regional difference in subduction bend‐faulting and potential hydration of the JdF plate is inconsistent with the spatial distribution of intermediate‐depth intraslab seismicity at Cascadia. A series of distinctive, ridgeward dipping (20°–40°) lower crustal reflections are imaged in ~6–8 Ma crust along both transects and are interpreted as ductile shear zones formed within the ridge's accretionary zone in response to temporal variations in mantle upwelling, possibly associated with previously recognized plate reorganizations at 8.5 Ma and 5.9 Ma.

Catalog of Offshore Seismicity in Cascadia: Insights Into the Regional Distribution of Microseismicity and its Relation to Subduction Processes

“…A region-byregion comparison of the influence from underthrust sediments and incoming plate deformation on possible near-plate-interface seismicity is shown in Table 2. In this region, the incoming JdF is also less deformed than in its southern portion; the top of the oceanic crust is comparatively smooth with relief less than 150 m (Canales et al, 2017;Han et al, 2016). The relative scarcity of earthquakes on the plate interface offshore Washington mirrors the lack of interface seismicity observed off Vancouver Island during the (Obana et al, 2015).…”

A 2‐D tomographic model of the Juan de Fuca plate from accretion at axial seamount to subduction at the Cascadia margin from an active source ocean bottom seismometer survey

“…Previous studies have revealed faults within the sediment section blanketing the JdF plate at distances of up to 200 km west of the deformation front (Figure c) and one region of crustal‐scale faulting [ Nedimović et al ., ]. Reflection images derived from the collocated MCS data acquired along our Oregon transect presented in Han et al [] reveal faults within the sediments and faint fault reflections in the upper crust at distances up to 320 km seaward of the deformation front. Sediment and crustal faults are sparse and heterogeneous in distribution west of 130–140 km from the deformation front, but increase in density at closer distances, with numerous faults within the sediments and crust at distances <130 km, as well as a prominent group of ridgeward dipping lower crustal reflectors (LCRs) found 70–150 km from the deformation front (Figure c).…”

“…The zone of deep‐penetrating faults falls within the region of increasing mantle velocities immediately to the east of the small propagator. Mantle water contents of <1 wt % (Figure c) are estimated for this region, consistent with values reported by Han et al [] based on estimates of the volume of the imaged fault zones. There is no clear explanation for why, within 15 km of the deformation front, our mantle velocities reach predicted dry values (Figure c).…”

“…We use the evolution of seismic velocities from the ridge to the deformation front as indicators of alteration and porosity changes at different levels and provide an estimate of the water content of the plate under different plausible mineralogical assumptions. This work, based on wide‐angle ocean bottom seismometer (OBS) data, complements that of Han et al [], who reported the results from analysis of multichannel seismic (MCS) data that document the distribution of faulting along both plate transects. Results from the remaining transects will be published elsewhere.…”

Imaging Subduction Beneath Mount St. Helens: Implications for Slab Dehydration and Magma Transport

“…It is difficult to determine if the top of the JdF crust disappears below 40-km depth or simply decreases in S velocity contrast with the overlying mantle to noise levels. The resulting depth of the JdF Moho beneath MSH is 74 km, placing the slab surface at 68 km beneath MSH assuming a JdF crustal thickness of 6 km as seen offshore (Han et al, 2016). The resulting depth of the JdF Moho beneath MSH is 74 km, placing the slab surface at 68 km beneath MSH assuming a JdF crustal thickness of 6 km as seen offshore (Han et al, 2016).…”