OCEANIC CORE COMPLEXES: DIVERSE EXPRESSIONS OF EXTREME EXTENSION IN OCEANIC LITHOSPHERE (Invited Presentation)

Oceanic core complexes (OCC) expressions of extreme tectonic extension of oceanic lithosphere at accreting plate boundaries, are common along relatively slow-spreading boundaries (e.g., 50% of the Mid-Atlantic Ridge). Correlation with spreading rate suggests that they form under conditions of low, or episodic, magma supply in heterogeneous oceanic crust that lacked a simple layered structure seen in some ophiolites. Although they are identified on the basis of their bathymetric expression (broad dome-like structures with spreading-parallel corrugations) geological details are limited. The current understanding of OCC’s is based on the integration of different types of data from different OCC, despite significant variations in settings and geology. Based on the available data, it appears that OCC’s with their distinctive large-scale similarities may have developed across a range of processes, but with some common basic requirements including a limited magma supply and a mechanism for long-term (m.y.) strain localization. Some OCC’s are composed mainly of partially serpentinized mantle peridotites whereas others occur in thick sections (kilometers) of magmatic gabbroic material. In some areas major low-angle (≤30°) detachment faults with kilometers of displacement appear to remain active along spreading centers with no rotation of overlying lavas; elsewhere, paleomagnetic studies document large-scale footwall rotations. Although the thermal and deformational histories of footwall rocks are well constrained, pressures and vertical movements are not. The map-scale geometry and kinematics of cataclastic detachment fault surfaces and underlying ductile mylonitic shear zones are known only locally. The lack of a restorable hanging wall stratigraphy makes reliable reconstructions impossible. The width of footwall exposures of detachment faults provides a minimum horizontal displacement suggests long periods (as much as 2 m.y.) of slip on these surfaces. However, large strains recorded in footwall mylonites may require additional displacement at depth. A number of problematic aspects of OCC may be explained by ductile deformation and synkinematic gabbroic intrusion occurring along very steeply dipping shear zones beneath spreading centers that evolve into low-angle brittle fault zones.