Reconstructing Ophiolites: Reassessing Assumptions From the Oceanic Crust and Related Terranes

Abstract:

The internal structure of ophiolite complexes has long been used as a window into the inaccessible parts of the oceanic lithosphere and by inference, processes beneath spreading centers. However, even the best preserved ophiolite complexes have been tilted, folded, faulting and dismembered during post-spreading tectonic events. Some degree of reconstruction is required to restore ophiolite structures to their appropriate relative orientations in order to relate them to processes beneath spreading centers. A number of assumptions about ophiolite structures have been used to guide reconstructions including: lava flows (horizontal, especially sheet-like lavas), dikes in lavas and sheeted dike complexes (vertical and parallel to spreading centers), the contacts between major rock units (horizontal, analogous to the seismic structure of oceanic crust) and the mafic/ultramafic contact representing the geologic expression of the Moho (horizontal). Based in part on these assumptions the internal structure of rock units, metamorphic relationships, and the kinematics of faults and deformation fabrics are also inferred. The spreading direction is seldom constrained in ophiolites making it difficult to assess the geometry of asymmetrical features, such as the dip of dikes, faults, or igneous layering, relative to spreading axes. Observations from exposures of upper crustal rock units (lavas, transition zones, dike and upper gabbroic rocks) along major tectonic escarpments in oceanic crust formed at fast to intermediate rates, as well as the uplifted and glaciated Tertiary basaltic crust of Iceland, raise questions about several of the assumptions used in ophiolite reconstructions. Alternative reconstructions may provide new ideas about spreading processes.