Geological investigation of a lineated massif at the Kane Transform Fault:implications for oceanic core complexes

Recently available new perspectives on the morphology of the sea-floor near the Mid-Atlantic Ridge reveal the widespread development of dome-like mass ifs with lineated upper surfaces. In form and dimensions, these features re semble continental metamorphic core complexes developed in regions of large -magnitude extension, and thus may be 'oceanic core complexes'. Excellent e xamples of these features occur south of the Kane Transform Fault where hig h-resolution geological data from side-looking sonar, deeply towed cameras, and manned submersibles are available. The geological data from two differ ent massifs; representing somewhat different types of crustal structure, pr ovide important constraints on the development of these oceanic core comple xes. Inferences from these direct geological investigations of oceanic core complexes are a first step toward understanding the processes by which the se features form in slow-spread crust. Specific relationships in these massifs and nearby areas suggest that these structures have formed over significant lengths of ridge axis (tens of kil ometres) and are not necessarily confined to any specific setting with resp ect to morphologic segmentation patterns. In some cases they are developed in thick gabbroic crust, but elsewhere have formed in crust composed of ser pentinite riddled with gabbroic intrusions. Ductile stretching and thinning of these sections is recorded in extensive mylonites and shear zones. Synt ectonic gabbroic intrusions are widespread and contradict the notion that t he massifs develop during intervals of 'amagmatic spreading'. Cataclastic f ault zones in gabbros and serpentinites developed during uplift, cooling an d hydration of the stretched crust and may have been localized as major det achment faults. Sparse diabase dikes and basaltic lavas are the latest magm atic additions to these areas. Numerous joints and steeply dipping, small-o ffset faults are the youngest observed structures. Across part of the top s urface of one massif, small-scale faults appear to bound elongated ridges o f fractured basement rocks creating the lineated terrain. Thus, the extensi ve lineated surfaces nearby need not reflect prolonged displacement along a continuous detachment fault surface. Geological relationships developed in the dome-like massifs along the Kane Transform Fault hint at a complex array of processes involved in the evolut ion of these features and raise new questions regarding their origin and an alogies with continental core complexes.