Hemipelagic sedimentation in a region of crustal doming between the Mendocino and Pioneer fracture zones

Abstract Pacific Study Area W-N lies 300 km west of California, south of the Gorda Ridge, between the Mendocino and Pioneer fracture zones. Hemipelagic mud covers the seafloor over a broad low rise and grades eastward into distal turbidite deposits of the Delgada Fan. 3.5 kHz profiles show three kinds of acoustic character for the seafloor: strongly reflecting turbidite deposits, depositionally draped hemipelagic sediment and an unusual morphology characterized by low-relief, closely spaced hills. These small hills occur in the same area as the low rise, implying a common origin. Calculations of the plate motions for this small corner of the Pacific Plate suggest that it is overriding the upper mantle thermal trace of the Gorda Ridge magma chamber. This heat source apparently causes doming, offsets in the layered hemipelagic sediment and formation of the closely spaced small hills.