MEGAMULLIONS AND MULLION STRUCTURE DEFINING OCEANIC METAMORPHIC CORE COMPLEXES ON THE MID-ATLANTIC RIDGE

In a study of geological and geophysical data from the Mid-Atlantic Ri dge, we have identified 17 large, domed edifices (megamullions) that h ave surfaces corrugated by distinctive mullion structure and that are developed within inside-corner tectonic settings at ends of spreading segments. The edifices have elevated residual gravity anomalies, and l imited sampling has recovered gabbros and serpentinites, suggesting th at they expose extensive cross sections of the oceanic crust and upper mantle. Oceanic megamullions are comparable to continental metamorphi c core complexes in scale and structure, and they may originate by sim ilar processes. The megamullions are interpreted to be rotated footwal l blocks of low-angle detachment faults, and they provide the best evi dence to date for the common development and longevity (similar to 1-2 m.y.) of such faults in ocean crust. Prolonged slip on a detachment f ault probably occurs when a spreading segment experiences a lengthy ph ase of relatively amagmatic extension. During these periods it is easi er to maintain slip on an existing fault at the segment end than it is to break a new fault in the strong rift-valley lithosphere; slip on t he detachment fault probably is facilitated by fault weakening related to deep lithospheric changes in deformation mechanism and mantle serp entinization. At the segment center, minor, episodic magmatism may con tinue to weaken the axial lithosphere and thus sustain inward jumping of faults. A detachment fault will be terminated when magmatism become s robust enough to reach the Segment end, weaken the axial lithosphere , and promote inward fault jumps there. This mechanism may be generall y important in controlling the longevity of normal faults at segment e nds and thus in accounting for variable and intermittent development o f inside-corner highs.