Crustal evolution and oceanic core complexes at the Ascension fracture zone – MAR 7° S

The Ascension fracture zone (AFZ) is a double transform fault system and offsets the Mid-Atlantic Ridge axis by 230 km at 7 °S. The transform fault system is consisting of two parallel transform faults, the North and South Ascension Fracture Zone, sandwiching a ~20 km long ridge segment, which we name Ascension Fracture Zone segment. The segment shows strong topographical variations and corrugated surfaces typical for detachment faults that form oceanic core complexes. An elongated massif approximately 50 km east of the ridge axis with transform-parallel striations of over 100 km on top, indicate a detachment fault active for several million years. This would be one of the longest transform-parallel corrugated surface observed anywhere in the oceans. The question arises whether the corrugations belong to one OCC, representing a rather stable crustal accretion, or if several OCCs have been developed, representing a rather variable crustal accretion. Changes in melt supply influence the crustal structure, which in turn can be recognised by seismic methods.

RV Meteor (cruise M62-4) set out to acquire seismic refraction and wide-angle reflection data along a 265 km long spreading parallel transect to image the crustal velocity distribution and the crustal thickness of the intervening short AFZ segment. Densely spaced, every ~9.25 km, ocean bottom seismometers recorded P-wave and converted S-wave energy emitted from a 64 l G-gun cluster at a shot interval of 60 s, equal to ~125 m shot distance.

The results reveal P-wave velocities that vary along the profile from 3.5 km/s to 5 km/s at the seafloor and reach 7.2 km/s in ~6 km depth at the ridge axis and at 3 km to 4 km depth under the ridge shoulders. At larger offsets to the ridge axis. S-wave velocities vary from 2 km/s to 2.5 km/s at the seafloor and increase to 3.5 km/s in ~2 km depth east of the ridge axis, while the S-wave velocities west of the ridge axis show a lower velocity gradient and reach 3.5 km/s in 3 km to 4 km depth. A Vp/Vs ratio >1.9 is observed in areas where seafloor corrugations have been observed. These areas are interpreted as serpentinised mantle material. However, the high Vp/Vs ratio seems to be limited to the upper 1.5 km to 2 km of the subsurface, indicating that the hydration of the seafloor is limited to that depth. The eastern ridge flank is dominated by a high Vp/Vs ratio for offsets larger than 40 km from the ridge axis, however, it is interrupted by small stripes of Vp/Vs <1.9. Thus, in the short AFZ segment, detachment faulting seem to occur continuously over a long period with short interruptions when the magmatic budged exceeds a certain upper limit.