Detachment faulting and amagmatic extension at mid-ocean ridges: The Josephine ophiolite as an example

Lithospheric extension at the Josephine paleo-spreading center occurred by a combination of magmatic and amagmatic processes. The amount of extension by amagmatic processes appears at least as large as that from magmatic processes. The structural processes responsible for amagmatic extension in the absence of a magma chamber appear to be normal faulting and block rotation in the brittle upper lithosphere and ductile flow in the lower lithosphere. An extensive detachment shear zone occurs beneath the fault blocks, approximately 1 km below the base of the crustal sequence. The amount of amagmatic extension can be approximated from a simple geometric model relating extension to rotations of sheeted dikes. Similarly, attenuated crustal thicknesses can be related to fault-block rotations. The results suggest that thin crustal sequences commonly observed in ophiolites and near fracture zones may result from attenuation during amagmatic extension. Furthermore, detachment faulting and block rotation may be characteristic of spreading centers where the magma budget is low.