Abstract

The two spreading axes and a marginal transform fault in Guaymas Basin were examined during six dives to depths of 1,900 to 2,000 m in the submersible Sea Cliff. The plate boundary zones differ from those at mid-ocean ridges because of their youth: they are buried by thick sediment, partly derived from the adjacent continents whose rifted edges are still at the plate boundary along some transform faults. The accretionary margins have rift valleys bounded by normal fault scarps of diatomaceous mud; they are true grabens, not regenerating axial troughs like those at some mid-ocean ridges. The rifts are the main sites of turbidite deposition, igneous intrusion, and extensional faulting, processes that compete to level, buckle, and fracture the rift floors. Intrusion is by emplacement of sills and thicker bysmalith-like bodies, which episodically raise hills of folded and faulted semiconsolidated turbidites. Some of the fault scarps bounding the hills have very fresh, near-vertical faces, while others are cavernously weathered. A small hill within the Northern Trough has extensive young hydrothermal deposits of talc and sulfides on its crest and at a dissecting fault scarp. No extrusive volcanic rocks were found in the axial rift valleys nor at a marginal transform fault, where a belt of obliquely faulted sediments probably overlies the strike-slip axis. The transform-fault shear zone lies in a very asymmetric graben, with a sag at the edge of the basin floor and a fresh fault scarp truncating lithified sandstones on the continental side.

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