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

Abstract

Recently available new perspectives on the morphology of the sea–floor near the Mid–Atlantic Ridge reveal the widespread development of dome–like massifs with lineated upper surfaces. In form and dimensions, these features resemble continental metamorphic core complexes developed in regions of large–magnitude extension, and thus may be ‘oceanic core complexes’. Excellent examples of these features occur south of the Kane Transform Fault where high–resolution geological data from side–looking sonar, deeply towed cameras, and manned submersibles are available. The geological data from two different massifs, representing somewhat different types of crustal structure, provide important constraints on the development of these oceanic core complexes. Inferences from these direct geological investigations of oceanic core complexes are a first step toward understanding the processes by which these 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 kilometres) and are not necessarily confined to any specific setting with respect to morphologic segmentation patterns. In some cases they are developed in thick gabbroic crust, but elsewhere have formed in crust composed serpentinite riddled with gabbroic intrusions. Ductile stretching and thinning of these sections is recorded in extensive mylonites and shear zones. Syntectonic gabbroic intrusions are widespread and contradict the notion that the massifs develop during intervals of ‘amagmatic spreading’. Cataclastic fault zones in gabbros and serpentinites developed during uplift, cooling and hydration of the stretched crust and may have been localized as major detachment faults. Sparse diabase dikes and basaltic lavas are the latest magmatic additions to these areas. Numerous joints and steeply dipping, small–offset faults are the youngest observed structures. Across part of the top surface of one massif, small–scale faults appear to bound elongated ridges of fractured basement rocks creating the lineated terrain. Thus, the extensive 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 evolution of these features and raise new questions regarding their origin and analogies with continental core complexes.