Near-Bottom Reconnaissance of a Fast-Slipping Transform Fault Zone at the Pacific-Nazca Plate Boundary

A 9 km length of the fast-slipping (152 mm/yr) Quebrada transform fault zone near 4°S on the East Pacific Rise was examined with deeply towed sonars and cameras. The fault zone occupies a 7 km-wide transform valley, which is a graben with step-faulted walls thickly mantled by pillow-basalt talus. Left-lateral strike-slip faulting is concentrated in a single narrow (< 200 m-wide) crush belt near the foot of the younger wall. The fault outcrops within a 20-50 m deep furrow that parallels the direction of plate separation and is similar to the topographic "rifts'* along some continental strike-slip faults. The older, deeper, part of the valley floor, south of the active strike-slip fault, has been broken into secondary horsts and grabens by tensional faults that are 45° oblique to the strike-slip trend. The valley floor has experienced differential sediment deposition from an eastward bottom current, and part of the deepest graben has been flooded with young pahoehoe flows. There is increased volcanism in fast-slipping transform fault zones near a rise crest, where the axial ridge characteristic of fast spreading is prolonged into an oblique axis of intrusion and extrusion that partly fills the transform valley and builds a fracture ridge.