Deformation along the Oceanographer Transform Fault from fault mapping and thin section analysis

The Oceanographer Transform Fault is an oceanic transform fault that offsets a segment of the Atlantic Mid-Oceanic Ridge (MOR), southwest of the Azores. We investigate how deformation is accommodated along an active transform through the interpretation of fault patterns and geomorphological features on high resolution bathymetry and a petrological and kinematic analysis of thin sections.

The bathymetric interpretation yielded six different domains which consisted of 1) the main transform zone with E-W running strike-slip faults, 2) the NNE-SSW oriented MOR valley, 3) the abyssal domain hosting NNE-SSW oriented normal faults that bound the abyssal hills, 4) the abyssal domain hosting NE-SW oriented faults, oriented obliquely to the mid-oceanic ridge and the main transform valley, 5) a volcano- and lava flow rich domain and 6) a shallow domain with corrugations oriented perpendicular to the MOR with little volcanic cover.

The thin section analysis reveals a complete ophiolitic sequence, including serpentinized peridotite, gabbro and basalt with varying degrees of alteration. Samples retrieved from depths >3500 m show that deformation occurs mainly in the ductile domain through bulging and sub-grain rotation of plagioclase, lamellar feldspar formation (in gabbro), shearing and recrystallisation of gabbro and serpentinization of peridotite. Brittle deformation manifests itself through fracturing of crystals, displacement of plagioclase sub-crystal domains and veining. Especially gabbroic samples show a decrease in serpentinized veins with decreasing depth. Basalts are found only at shallow depth, seemingly covering gabbro, appearing not to be affected by deformation at all, only occasional cracks filled with pristine calcite are observed.

The combination of geomorphological features identified on high-resolution bathymetry maps and the petrological and kinematic analysis of thin sections showed that deformation along the transform fault differs from the deformation that happens at the MOR. Deformation at the MOR is characterized by 1) axis-parallel normal faulting, pulses of volcanism, resulting in elongated ridges and volcanic cones on the ocean floor and the formation of dykes under magma-rich circumstances, and core complex exhumation during magma-starved periods that occurred between 1.8 – 4.2 Ma and around 7.5 Ma along the southwestern MOR segment of the OTF and 2) heavily sheared zones that extend obliquely from the MOR-transform intersection into the adjacent older plate. Deformation at the transform fault is accommodated through serpentinization at depths deeper than 3000 m, leading to pop-up structures in the main transform zone and causing fracturing in the overlying gabbro, allowing hydrothermal fluids to heavily alter deeper rocks and migrate to shallower depths with decreasing alteration of the oceanic crust with decreasing depth.

We hypothesize that the transform fault itself at depth accommodates stresses to a large extent via serpentinization processes in response to strike-slip tectonic activity in a very narrow band in the active, deepest part of the main transform zone. Deformation patterns other than serpentinization and serpentinite veining that are observed in rock samples along the transform fault are the result of earlier tectonic activity that took place during or shortly after the formation of the rock at the MOR.