Extensional tectonics at ridge-transform intersections – constraints from micro-seismicity and bathymetric data

Fracture zones were recognized to be an integral part of the seabed long before plate tectonics was established. Later, plate tectonics linked fracture zones to oceanic transform faults, suggesting that they are the inactive and hence fossil trace of transforms. Yet, scientist have spent little time surveying them in much detail over the last three decades. Recent evidence (Grevemeyer, I., Rüpke, L.H., Morgan, J.P., Iyer, K, and Devey, C.W., 2021, Extensional tectonics and two-stage crustal accretion at oceanic transform faults, Nature, 591, 402–407, doi:10.1038/s41586-021-03278-9) suggests that the traditional concept of transform faults as being conservative (non-accretionary) plate boundary faults might be wrong. Instead, transform faults are always deeper than the associated fracture zones and numerical modelling results suggest that transform faults seem to suffer from extensional tectonics below their strike-slip surface fault zone. In 2021, we tested this hypothesis by collecting, in a pilot study, micro-seismicity data from the Oceanographer transform fault which offsets the Mid-Atlantic Ridge by 120-km south of the Azores near 35°N. Analysis of 10-days of seismicity data recorded at 26 ocean-bottom-seismometers and hydrophones showed 10-15 local earthquakes per day. Furthermore, a sparse network recorded micro-earthquakes for three months. Joint interpretation of the data shows that earthquakes away from the ridge-transform intersections cluster along the fault trace imaged in bathymetric data and focal mechanisms support strike-slip motion. However, at the ridge-transform intersections seismicity does not mimic a right-angular plate boundary; instead, seismicity occurs below the inside corner and focal mechanism indicate extensional tectonics. In addition, we put published micro-earthquake data from surveys conducted in the 1970 to 1980s from the Oceanographer, Kane and Vema transform fault in a new context by plotting them onto modern swath-bathymetric data. In concert, micro-seismicity supports features found in numerical simulations, revealing that transform faults have an extensional as well as a strike-slip component.