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Images: Discovery of "Megamullions" Reveals Gateways Into the Ocean Crust and Upper Mantle

Schematic section through oceanic crust, based on study of subaerially exposed ophiolites and drilling results in the ocean basins. Boundaries between lithologic units are usually transitional and complex, and units may be thin or missing where magma input is episodic, notably on slow-spreading ridges.

Sketch of spreading segments separated by ridge-axis discontinuities in slow-spreading ocean crust. The crust at inside corners (IC) differs significantly from that at outside corners (OC) in that it is more elevated, thinner, and faulted into a blockier fabric. This asymmetry is thought to be caused by consistent dip of faults from inside corners to beneath outside corners, so that lower crust and upper mantle are commonly exhumed in the IC footwalls of the faults. Near a segment center on the front face of the diagram, hot, upwelling mantle decompresses and partially melts, supplying magma that is intruded and extruded to form the ocean crust

Perspective view, looking southwest, of a megamullion that developed at an inside corner of the Kane transform discontinuity between about 3.3 and 2.1 million years ago. The domed megamullion is corrugated by ridges (mullions) that parallel the direction of fault slip. A steep, west-facing normal fault formed as the megamullion developed, and it cuts the mullions at right angles. The image was generated from multibeam sonar data that give complete bathymetric coverage of the seafloor.

Interpreted development of a mega-mullion near the end of a spreading segment during a period of amagmatic extension. Development begins when a detachment fault first forms following a magmatic episode of seafloor spreading, and it ends with abandonment of the detachment when a new fault breaks through rift-axis crust that is weakened by renewed magmatism.

Domed metamorphic core complexes analogous to oceanic megamullions can develop in continental extensional regimes. In southern California, the gently dipping Whipple detachment fault separates a footwall core complex (gray, highly deformed mid-crustal mylonites and chloritized breccias) from the overlying hanging wall (dark brown, Miocene volcanics). In the foreground, Eric Frost of San Diego State University outlines the tectonic framework to members of the JOIDES (Ocean Drilling Project) Tectonics Panel.

Continental metamorphic core complexes are often, and appropriately, called "turtlebacks." Here in Death Valley, the domed, gray core complex of mid-crustal mylonites and breccias at right is separated from brown upper crustal volcanics and sediments by the northwest-dipping Copper Canyon detachment fault. Width of view is about a kilometer along the contact with the alluvial plain in the foreground.