Geophysical characterization of the Oceanographer transform fault, TRANSFORMERS-II, Cruise No. MSM122, 19.10.2023 - 09.11.2023, Ponta Delgada, Azores (Portugal) - Halifax (Canada)

Oceanic transform faults (OTF) are one of three major types of plate boundaries of plate tectonics. Yet, plate tectonics predicted a rather simple geometry for OTFs and consequently their structure and “modus operandi” attracted reasonable little attention. However, recent evidence suggests that the traditional concept of transform faults as being conservative (non-accretionary) plate boundary faults might be wrong. Instead, numerical modelling results suggest that (i) transform faults seem to suffer from extensional tectonics below their strike-slip surface fault zone and a global compilation of legacy bathymetric data suggests that (ii) ridge-transform intersections seem to be settings of magmatic activity, modifying the lithosphere and burying the transform valley before it passes into the fracture zone region. During MSM122, we went forward to test both hypotheses at the Oceanographer OTF to the south of the Azores by (i) revealing the displacement of the strike-slip fault using seafloor geodesy (direct path ranging) and (ii) studying the micro-seismicity and its relation to tectonic elements of the OTF (transform valley, strike-slip fault trace, bounding walls) with a network of long-term OBS and a seafloor geodetic network deployed for one year, to be recovered during M204 in October of 2024. Further, (iii) magnetic data were acquired to obtain the magnetisation of the basement of both the OTF and fracture zones, testing the newly proposed model of crustal accretion at OTFs.