Upper Crustal Structure of Oceanic Lithosphere Formed at Intermediate to Fast Rates: A Basis for Comparison of MOR vs SSZ Ophiolites

Abstract:

The internal structure of ophiolite complexes plays a key role in illuminating processes in both mid-ocean ridge (MOR) and suprasubduction zone (SSZ) environments. However, it is not yet clear to what extent the internal structure of the crust and processes may differ between these settings. Thus, it may be useful to compare details of reconstructed ophiolites with known MOR crustal structures. Very large variations in the structure of the oceanic lithosphere generated at MOR spreading centers have been documented as a function of low magma budget (or spreading rate). Even at intermediate (~50-80 mm/yr) to fast (80->120 mm/yr) spreading rates, involving spreading with a persistent magma supply, significant variations occur. A synthesis of seafloor observations, crustal drilling and seismic data for relatively fast-spread upper oceanic crust may help distinguish ophiolites generated at MORs from those from SSZ settings. Crust formed at the fastest rates has a relatively uniform seismic thickness, thin lavas, and a relatively thick, near-vertical sheeted dike unit over complex massive to layered gabbroic rocks. Intermediate rate crust is more variable in seismic thickness and has much thicker lavas and sheeted dikes showing evidence of tilting, fracturing, and subsidence during construction of the upper crust over a heterogeneous gabbroic middle crust. Crustal sections formed at rates between these limits show a range of intermediate structures. These structural features correlate well with spreading rate, and possibly magma supply, and have implications for the reconstruction of ophiolite complexes. A more complete documentation of MOR lithosphere (independent of ophiolites) in this range of rates will require: 1) sampling of the middle to lower oceanic crust and mantle by deep drilling and 2) determining the internal structure of the crust along spreading segments, providing a basis for identifying ophiolites that formed in MOR and SSZ settings.