Landward vergence and oblique structural trends in the Oregon margin accretionary prism: Implications and effect on fluid flow

Abstract

The central Oregon margin spans a regional transition in accretionary structures from seaward-verging in the south to landward-verging in the north. New multichannel seismic (MCS) data image both landward- and seaward-vergent provinces along the central and northern Oregon margin. Landward-vergence is characterized by a deep décollement, with deformation distributed across a broad lower continental slope in a coherent structural style. In the landward-vergent area, virtually all 4 km of incoming trench sediments overthrust the preceding thrust sheet, forming fault-bend folds and a distinctive ridge/trough morphology. This style of landward-vergence is not explained by existing models. In contrast, seaward-vergence correlates with a shallower décollement, approximately 1.4 km above the oceanic crust, and a more intense style of deformation within a narrower slope. Initial thickening of the trench sediments occurs across a well-developed protothrust zone. The frontal thrust forms a ramp-anticline that is cut by a prominent backthrust. Previously observed seafloor vent sites in both regions correlate with thrusts that exhibit high-amplitude, reversed-polarity reflections suggestive of enhanced porosity along the faults. Potential fluid sources and migration paths are strongly influenced by changes in the level of the décollement and vergence along the margin.Abrupt changes in structural style occur both along strike and updip, and are bounded by two sets of oblique-slip faults. Three NW-striking left-lateral faults are imaged in both MCS and SeaBeam data. Plunging anticlines developed along the NW-striking faults are venting fluids and were previously interpreted as mud volcanoes. The deformation front is locally disrupted where these faults intersect the prism, but they appear to have limited influence on the structural evolution of the prism. In contrast, the NE-striking right-lateral faults are confined to the deforming sediments of the upper plate. These faults interact with the thrusts within the prism, forming a rhomboidal pattern of three-dimensional deformation.