Crustal structure beneath the central Oregon convergent margin from potential‐field modeling: Evidence for a buried basement ridge in local contact with a seaward dipping backstop

Abstract

Abstract. Models of magnetic and gravity anomalies along two E-W transects offshore central Oregon, one of which is coincident with a detailed velocity model, provide quantitative limits on the structure of the subducting oceanic crust and the crystalline backstop. The models indicate that the backstop-forming western edge of the Siletz terrane, an oceanic plateau that was accreted to North America-50 million years ago, has a seaward dip of less than 60ø3. Seismic, magnetic, and gravity data are compatible with no more than 2 km of subducted sediments between the Siletz terrane and the underlying crystalline crust of the Juan de Fuca plate. The data also suggest the presence of a N-S trending, 200-km-long basaltic ridge buried beneath the accretionary complex from about 43øN to 45øN. Although the height and width of this ridge probably vary along strike, it may be up to 4 km high and several kilometers wide in places and appears to be locally in contact with the Siletz terrane beneath Heceta Bank. Several models for the origin of this ridge are discussed. These include: a sliver of Siletz terrane detached from the main Siletz terrane during a late Eocene episode of strike-slip faulting; imbrication and thickening of subducted oceanic crust in place; an aseismic ridge rafted in on the subducting oceanic crust during the past 1.2 million years; and a series of ridges and/or seamounts rafted in over a longer period of time and transferred from the subducting plate to the overlying plate. The last model is the most consistent with the complicated history of local uplift, subsidence, and slope instability recorded in the ridges, basins, and banks of this part of the margin. We speculate that the massive seaward dipping western edge of the Siletz terrane in this region inhibits subduction of seamounts and sediments, resulting in fomation of buried ridge as the accumulated flotsam and jetsom of subduction. This process may also be responsible for thickening of lower accretionary complex material, oversteepening of slopes leading to massive slumping, and north-south extension through strike-slip faulting in the accretionary complex to the west of the buried ridge. Regardless of its origin, the ridge may currently be acting as an asperity inhibiting subduction.

Along‐Trench Structural Variations of the Subducting Juan de Fuca Plate From Multichannel Seismic Reflection Imaging

“…With an arch in the downgoing plate centered beneath the Olympic Peninsula (McCrory et al, ) attributed to the changing orientation of the subduction zone (Chiao & Creager, ), the dip of the plate changes along the margin near 46°N with shallower dips beneath Washington (Figure ). In the upper plate, the basaltic Siletz terrane extends offshore between 43°N and 46°N (Fleming & Trehu, ; McCrory & Wilson, ; Tréhu et al, ). Within the same latitude range, some models of land‐based geodetic data indicate reduced locking of the plate interface in this region compared with further to the north and south (Burgette et al, ; McCaffrey et al, ; Schmalzle et al, ).…”

Methane sources feeding cold seeps on the shelf and upper continental slope off central Oregon, USA

“…Here we present carbonate and geophysical data from the upper continental slope off Oregon, which reveal the presence of localized zones of biogenic methane discharge from approximately 43°N to 45°N, a region that corresponds to the location of a basaltic ridge buried beneath a thick accretionary complex [ Fleming and Tréhu , 1999]. We postulate a scenario where tectonic controls, specifically sediment uplift due to subduction of a basaltic ridge, leads to gas hydrate destabilization in this margin.…”

P- and S-wave velocity models incorporating the Cascadia subduction zone for 3D earthquake ground motion simulations, Version 1.6—Update for Open-File Report 2007–1348

“…3) because topography is not included in model V1.6. The surface of the continental crust below MSL was controlled by a smoothed continental shoreline and numerous published active and passive source seismic results along the continental margin (for example, Trehu and others, 1994;Clowes and others, 1997;Flueh and others, 1998;Fuis, 1998;Gulick and others 1998;Fleming and Trehu, 1999;Parsons and others, 1999;Stanley and Villaseñor, 2000;Bostock and others, 2002;and Ramachandran and others, 2006).…”

“…3 and 5). Parameter V P is derived from results of Parsons and others (1999) and numerous active-source marine seismic surveys (Trehu and others, 1994;Clowes and others, 1997;Flueh and others, 1998;Fuis, 1998;Gulick and others, 1998;Fleming and Trehu, 1999;Parsons and others, 1999;Stanley and Villaseñor, 2000;Bostock and others, 2002). Parameter V P varies primarily as a function of depth.…”

“…Based on available marine seismic-reflection profiling (for example, Fuis, 1998) and studies worldwide (for example, Turcotte and Shubert, 1982), the thickness of the oceanic crust was set to 5,000 m, which is likely on the low end of realistic values. Average velocity values derived from marine seismic surveys were extrapolated to obtain V P (for example, Trehu and others, 1994;Flueh and others, 1998;Fuis, 1998;Gulick and others, 1998;Fleming and Trehu, 1999;and Ramachandran and others, 2006) and extrapolated uniformly to 60,000 m depth. V S was derived from V P using the empirical relationship of Brocher (2005).…”