Abyssal hill deflections at Pacific‐Antarctic ridge‐transform intersections

Abstract

[1] 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.

Large Rotations of Crustal Blocks in the Tjörnes Fracture Zone of Northern Iceland

“…The trends of abyssal hill lineaments on the seafloor adjacent to transforms hint at the crustal structure. For example, abyssal hill lineaments that normally form parallel to spreading centers have curved trajectories over distances of up to 20 km away as they approach transform valleys (Croon et al, ; Grindlay & Fox, ; Grindlay et al, ; Sonder & Pockalny, ). These lineaments are defined by topographic highs and intervening valleys that are the result of faulting and volcanic construction (Macdonald et al, , and references therein).…”

“…Consequently, these lineaments and dikes may be used as indicators of changes in the stress field where they initially form near transforms, or as strain markers resulting from subsequent shearing. Abyssal hill lineaments commonly deviate from ridge‐parallel trends near ridge‐transform intersections (Figure a), where they curve into the transform fault (Crane, ; Croon et al, ; Fornari et al, ; Fox & Gallo, ; Lonsdale, ). This so‐called “J‐shaped” curvature is generally explained as a result of the transition from an extensional environment along the ridge axis to the shearing regime along the transform (Fox & Gallo, ).…”

“…These curved features persist along active portions of some transforms and are also preserved in crust beyond the active plate boundary parts of transforms along their nontransform fracture zone extensions. However, along some parts of active transform zones, lineaments curve in the opposite direction, in an “anti‐J‐shaped” sense (Croon et al, ; Sonder & Pockalny, ). Anti‐J‐shaped lineaments are interpreted as the result of tectonic rotation of lineaments by distributed deformation near the transform (Sonder & Pockalny, ; Tucholke & Schouten, ).…”

“…Anti‐J‐shaped lineaments are interpreted as the result of tectonic rotation of lineaments by distributed deformation near the transform (Sonder & Pockalny, ; Tucholke & Schouten, ). When combined with a detailed plate motion model, these lineament patterns observed along three Pacific‐Antarctic transform intersections appear to correlate with changes in the direction of plate motion (Croon et al, ). In all these cases, the underlying crustal structure and deformation kinematics are inferred only from the surface lineaments.…”

Tectonics: 50 years after the Revolution

“…After Bird (2003) and Matthews et al (2011). fault. Such relative movement may result from change in pole of rotation of the plates and resultant deformation along the transform fault and fracture zone either by rotation, relative movement and compression (so-called transpression), or extension (so-called transtension) (Croon et al, 2010). Active on-land transform faults are also wide zones.…”