Changes in plate motions and the shape of Pacific fracture zones

Abstract

Geosat passes, the new 2‐min global gravity grid [Smith and Sandwell, 1995], and shipboard bathymetry across central Pacific fracture zones were used to identify features common to fracture zone segments that formed during times of changes in plate motions. These features are not predicted by current “locked fault” fracture zone models. During a change in spreading direction that induces tension across the transform fault, large‐offset (greater than ∼500 km) transforms develop multiple parallel faults, spaced 50 to 100 km apart. The gravity signature of small‐offset transform faults under tension includes a broader and more symmetric trough than observed on segments that formed during periods of steady spreading. Parts of fracture zones that form subsequent to a spreading reorientation that causes compression across the transform fault generally exhibit a single fault scarp that fits the locked fault model. Seafloor formed during a period of change usually marks a transition between structural styles, for example, between multiple fracture zone strands and a narrower single‐fault fracture zone. Widening of the transform fault zone under tension and narrowing under compression are consistent with the assumption that during a change in spreading direction the new spreading ridges propagate to, but not across, the old transform fault.