Multiple seafloor spreading modes in the Mid-Cayman Spreading Centre

Multibeam bathymetry and TOBI deep-towed sidescan sonar data have been used to produce a preliminary geological map of the ultra-slow (~16 km/Ma) Mid-Cayman Spreading Centre (MCSC, the world's deepest spreading centre. The MCSC consists of two spreading segments, the northern of which is offset some 10 km dextrally from the southern, separated by an oceanic core complex (OCC), Mt. Dent. Each segment contains an axial volcanic ridge (AVR) displaying typical young, hummocky volcanic terrain. The northern AVR runs the length of the segment and its southern end abuts the north flank of Mt. Dent, suggesting it is about to cut off and terminate slip on the OCC detachment. A prominent ridge to the east of the northern AVR is not, as appears from the bathymetry, a separate AVR, but a fault block in the eastern MV wall. Recent-looking lavas erupt from partway up this fault. The southern AVR occupies the southern part of the southern segment. Both AVRs, but most prominently the northern one, exhibit NE trending volcanic spurs on their eastern flanks, which may be attributed to the action of dextral shear stresses associated with the plate boundary. The world's deepest hydrothermal vents (Beebe Vent Field) is located at the foot of one of these spurs. Between the southern AVR and Mt. Dent is a linear ridge, displaying considerable evidence of faulting but little recognisable volcanic terrain. The most likely tectonic interpretation is that this is a peridotite ridge produced by direct exhumation of mantle material, similar to those seen in 'smooth' seafloor on the ultra-slow spreading Southwest Indian Ridge. Its NNW-SSE orientation is somewhat paradoxical in this N-S spreading centre. This tectonised ridge is flanked by two flat-floored basins characterised by fairly uniform, moderate backscatter terrain typical of relatively young flat-lying lava flows. There are several smaller areas of young, flat-lying lavas, including the one to the east of the northern AVR. Another arises from the southern end of the southern AVR, whence it flows south, around a small inlier of older, more sedimented seafloor, and again cascades down a fault scarp into the Swan Island Fracture Zone to the south. Mount Dent has a surprisingly subdued sidescan character, with very low backscatter suggestive of significant sediment cover, and little evidence of spreading parallel striations as seen on other active OCCs, although subsequent Autosub bathymetry surveys did image fine-scale corrugations, especially near the OCC toe. Some NE- and NW- trending faults occur in the western part of the massif, indicating post-emplacement deformation, while some NS trending normal faults from the adjacent median valley extend into and across it. Similar characteristics seen on OCCs on the Mid-Atlantic Ridge 13 - 14°N were taken as indicative of inactive detachment faulting. However, where such inactive faults occurred, plate separation was taken up by renewed and vigorous on-axis volcanism, but no such features are seen opposite Mt. Dent. The most probable explanation at present seems to be that Mt. Dent contains a detachment fault that is still actively accommodating the majority of plate separation, but that unusually heavy sedimentation masks the expected high reflectivity and fine-scale striations.