Mantle uplift and exhumation caused by long-lived transpression at a major transform fault

Abstract:

Large portions of slow-spreading ridges have mantle-derived peridotites emplaced either on, or at shallow levels below the sea floor. Mantle and deep rock exposure in such contexts is often linked to extensional tectonics through low-angle detachment faults at oceanic core complexes or, along transform faults, to transtension due to small changes in spreading geometry.

At the large-offset St. Paul transform system, in the Equatorial Atlantic, a large body of ultramafic rocks forms the archipelago of St. Peter & St. Paul. These islets, emplaced near the axis of the Mid-Atlantic ridge, have intrigued geologists since Darwin’s time. They are made of variably serpentinized and mylonitized peridotites and the continuous uplift rate of 1.5 mm/yr reveals that they are presently under tectonic deformation. The existence of an abnormally cold upper mantle or cold lithosphere in the Equatorial Atlantic was, until now, the preferred explanation for the origin of these ultramafics.

High-resolution geophysical data and rock samples acquired in the St. Paul transform system in 2013 show that the origin of the St. Peter & St. Paul archipelago is linked to compressive stresses along the large-offset transform fault. The islets are the summit of a large push-up ridge formed by deformed mantle located in the center of a positive flower structure, where large portions of mylonitized mantle are uplifted.

The transpressive stress field can be explained by the propagation of the northern Mid-Atlantic Ridge (MAR) segment into the transform domain, which induced the migration and segmentation of the transform fault creating a series of restraining step-overs. A counterclockwise change in plate motion at ~11 Ma initially generated extensive stresses in the transform domain, forming a flexural transverse ridge, as observed in other transform faults. Shortly after the plate reorganization, the MAR segment started to propagate southwards, adapting to the new spreading direction. Enhanced melt supply at the ridge axis, possibly due to the nearby Sierra Leone thermal anomaly induced the robust response of this MAR segment.