Abyssal hill deflections at Pacific‐Antarctic ridge‐transform intersections

Abstract

Nearly complete coverage of shipboard multibeam bathymetry data at the right‐stepping Menard and Pitman Fracture Zones allowed us to map abyssal hill deviations along their traces. In this study we distinguish between (1) J‐shaped curvatures at their origin, where modeling is addressing primary volcanism and faulting following a curved zone, and (2) straight abyssal hills getting bent in anti‐J‐shaped curvatures, in response to increased coupling across the transform fault, after they were formed. We compared the mapped abyssal hill deflections to a detailed plate motion model for the Pacific‐Antarctic Ridge to test how abyssal hill curvature correlates to changes in plate motion direction, which lead to periods of transtension or transpression. This test was based on the number and size of the abyssal hill deflections. The observations show a high abundance of J‐shaped abyssal hills during periods of significant clockwise change in plate motion direction, which leads to transtension. The tip of the ridge axis can deflect up to 60° into the transform fault in response to changes in the stress field at ridge‐transform intersections. This is observed, in particular, at the Pitman Fracture Zone, where there has been a ∼15° clockwise rotation of the spreading direction azimuth during the last 9 Myr. In addition, we observed anti‐J‐shaped curvatures at Menard, Pitman, and Heirtzler Fracture Zones during periods of transpression when increased coupling across an oceanic transform fault is partially accommodated by distributed strike‐slip deformation rather than solely by discontinuous displacement at the transform fault. Anti‐J‐shaped deflections typically develop in seafloor less than 2 Myr old when the oceanic lithosphere is thin.