OCEAN-CORE COMPLEXES FORMED AT SLOW-SPREADING RIDGES CAN EXPLAIN THE ULTRAMAFIC-SUPRACRUSTAL CONTACTS COMMONLY EXPOSED IN SIERRA NEVADA METAMORPHIC BELTS

Sierra Nevada metamorphic belts expose large amounts of basalt, gabbro, chert, and ultramafic rock that was generated in an ocean basin. Contacts between ultramafic mantle and basalt, ultramafics and chert, or blocks of rock encased in a serpentinite matrix have led to these units to be considered as incomplete or fragmented ophiolites. These contact relationships, which are common in the Central Belt and Shoo Fly Complex, have been explained by formation within an oceanic fracture zone, or by disruption of “complete” ophiolites by shearing during later deformation. However, breadth of exposure of these units across metamorphic belts, lack of talus overlying mantle sections, and preservation of contacts in undeformed sections preclude these explanations.

We instead suggest that many of these contacts between ultramafic and crustal rocks may be explained by the presence of hyperextended oceanic crust generated by magma-poor or amagmatic spreading at an oceanic ridge. Magma-poor spreading is accommodated by low-angle normal faults that expose mantle and sometimes gabbro on the ocean floor creating a basement for later deposition of sparse volcanics and sediment. Footwall gabbros often preserve amphibolite-facies mylonite textures formed during exhumation. The surface of the footwall of these systems can bow upward, creating ocean-core complexes with topographically high footwalls. This type of crust is common in slow- or ultra-slow-spreading segments of present-day oceanic ridges and makes up much of the Alpine Tethys ocean that is preserved as ophiolites in the Alps and Italian Apennines.

The presence of the hyperextended oceanic crust in Sierra Nevada ophiolites would explain the juxtaposition of some ultramafics and crustal material as a primary contacts that formed at an ocean ridge. These contacts could then be easily reworked into serpentinite matrix mélanges with small amounts of strain. It also explains the wide exposures of such contacts even in highly telescoped ocean-plate material that makes up many of the Sierra Nevada metamorphic belts. Finally, the presence of ultramafic basement highs in oceanic-core complexes could provide a source of serpentinite that supplied later sedimentary basins, explaining the common appearance of ultramafic rocks in overlying sedimentary mélanges.