Extreme mantle uplift and exhumation along a transpressive transform fault

Abstract

Mantle exhumation at slow-spreading ridges is favoured by extensional tectonics through low-angle detachment faults, and, along transforms, by transtension due to changes in ridge/transform geometry. Less common, exhumation by compressive stresses has been proposed for the large-offset transforms of the equatorial Atlantic. Here we show, using high-resolution bathymetry, seismic and gravity data, that the northern transform fault of the St Paul system has been controlled by compressive deformation since ∼10 million years ago. The long-lived transpression resulted from ridge overlap due to the propagation of the northern Mid-Atlantic Ridge segment into the transform domain, which induced the migration and segmentation of the transform fault creating restraining stepovers. An anticlockwise change in plate motion at ∼11 million years ago initially favoured extension in the left-stepping transform, triggering the formation of a transverse ridge, later uplifted through transpression, forming the St Peter and St Paul islets. Enhanced melt supply at the ridge axis due to the nearby Sierra Leone thermo chemical anomaly is responsible for the robust response of the northern Mid-Atlantic Ridge segment to the kinematic change. The long-lived process at the origin of the compressive stresses is directly linked to the nature of the underlying mantle and not to a change in the far-field stress regime

The Geological Evolution of the Aden‐Owen‐Carlsberg Triple Junction (NW Indian Ocean) Since the Late Miocene

“…The origin of this series of oceanic ridges remains unknown. They may be tentatively interpreted as transpressive ridges formed along the fracture zone, similar to oceanic transverse ridges observed along the Saint‐Paul and Vema Transforms in the Atlantic Ocean (Bonatti et al, ; Maia et al, ).…”

Marine Geophysical Investigation of the Chain Fracture Zone in the Equatorial Atlantic From the PI‐LAB Experiment

“…Around~10 Ma the Vema Fracture Zone, saw the construction of its transverse ridge, with a rotation in the strike of the abyssal hill fabric by 10°anticlockwise (Bonatti et al, 2005). The construction of the St. Paul islands and subsequent readjustment appears to have a similar timing (Maia et al, 2016). Further south, the Romanche, also had a failed rotation of the transform by 10°anticlockwise and compressional uplift of a transverse ridge in the east (Bonatti et al, 1994).…”

“…Transpression increases crustal thickness. In a similar tectonic setting, serpentinized mylonitic periodities have been recovered from the surface of a positive flower structure in the St. Paul Transform (Maia et al, ). We estimate >11–15% serpentinization of the crust and mantle would be needed to produce the ~100‐ to 150‐kg/m 3 lateral density variation (Figure b) we observe across the transform valley (Carlson & Miller, ).…”

Enhanced Mantle Upwelling/Melting Caused Segment Propagation, Oceanic Core Complex Die Off, and the Death of a Transform Fault: The Mid‐Atlantic Ridge at 21.5°N

“…Active propagating ridges have been detected at all types of ridges, from slow‐ to fast‐spreading rates and at large and small scales (e.g., Briais et al, ; Carbotte et al, ; Cormier & Macdonald, ; Gente et al, ; Hey, ; Hey et al, , ; Kleinrock et al, ; Maia et al, ; Sempéré et al, ). In this study, we focus on the Mid‐Atlantic Ridge at 21°30′N, where the TAMMAR segment (between 21°25′N and 22°N) is rapidly propagating southward (Gente et al, ).…”