ASYMMETRIC SEAFLOOR SPREADING, CRUSTAL THICKNESS VARIATIONS AND TRANSITIONAL CRUST IN CAYMAN TROUGH FROM GRAVITY

With a few exceptions, oceanic crust, which forms at divergent plate boundaries (and away from the influence of mantle plumes near hot spots), has nearly constant thickness regardless of age, geographic location, water depth, and spreading velocity. Using free-air gravity anomaly data, we model the crustal structure of the Cayman Trough to analyze the processes occurring at this slow-spreading mid-ocean ridge. Our model indicates considerable and abrupt crustal thickness variations along the trough axis, which parallels the direction of ocean opening. The proximal part to the spreading ridge extends ~170 km east of the spreading ridge and 250 km west of it. Sea floor in the proximal part is slightly deeper (by ~500 m) east of the ridge than west of the ridge, and crustal thickness east of the ridge is considerably thicker than the crust to the west. The distal part of Cayman Trough extends to a distance of ~300 km on both sides of the proximal part to the spreading ridge. It has a greater water depth and a thinner crust than the proximal part and it does not increase in depth away from the ridge. By tying our model to published seismic refraction data, we estimate the crustal thickness of the distal part to be 5.5 km and of the west and east sides of the proximal part as ~7 and ~9.5 km, respectively. To our knowledge, this is the largest crustal thickness contrast inferred across any spreading ridge, and it augments recent similar results from the SW Indian Ocean. We interpret the thin crust in the distal part as transitional crust formed by extreme attenuation without organized sea floor spreading, and the proximal part by crustal accretion at a slow spreading mid-ocean ridge. In the proximal part, there is an inverse relationship between the ratio of crustal thicknesses and the ratio of spreading rates east and west of the spreading center. We interpret this relationship to indicate that new material is accreted preferentially to an existing crust on the slow moving east side of this spreading system. Off-axis crustal accretion can take place either by lava flowing from the axis or by off-axis intrusions.