Migrating transform zone and lithospheric transfer at the Galapagos 95.5°W propagator

Abstract

At the 95.5°W propagator system along the Cocos‐Nazca plate boundary, the spreading axis is offset left‐laterally about 35 km (∼1 m.y.). This offset migrates westward 52 km/m.y. relative to points along the nearly east‐west trending ridge axis, transferring lithosphere from the Cocos to the Nazca plate as it does so. The required 58 km/m.y. of right‐lateral motion across this migrating transform is apparently not accommodated by a throughgoing north‐south transform fault but rather by an approximately 15‐ to 20‐km‐wide zone of shear deformation dominated by eastward curving of east‐west structures to a northwest‐southeast trend. This pattern of curvature, the coinciding zone of high seismicity, and the abundance of talus in the area suggest that shear is distributed over this entire zone. The deformation is inferred to be taken up by a “bookshelf” mechanism whereby faults oriented at high angles to the shear couple have left‐lateral slip antithetic to the right‐lateral shear couple, rotating individual blocks clockwise. These are interpreted as reactivated abyssal hill‐bounding normal faults probably dipping ∼45°–60°. Northeast‐southwest shortening due to the rotation appears to be accommodated by reverse slip on the faults and may account for the uplift of De Steiguer Ridge in the zone of transferred lithosphere. These faults have apparently had a very complex history: initially, normal faults formed as abyssal hills were created at the spreading axis, and subsequently, they had left‐lateral strike‐slip motion in the transform zone and reverse motion uplifting De Steiguer Ridge. Strain analysis based on observed structural curvature suggests that shear is distributed nonuniformly and lithospheric transfer requires ∼0.4 m.y.