Building the Oceanic Crust at Intermediate to Superfast Mid-Ocean Ridge Spreading Centers: Implications of Complex Internal Structures of the Upper Oceanic Crust

Since the recognition of seafloor spreading, numerous kinematic and dynamic models for the accretion of oceanic crust and lithosphere have been proposed. Early models were constrained by the interpretation of marine seismic data and the internal structure of ophiolite complexes and predated any direct observations of the oceanic crust. Mapping the extent of axial lava flows and subsurface axial magma chambers established the very limited dimensions of where new oceanic crust is built.

Unlike spreading at slow rates, where faulting and sporadic magmatism result in heterogeneous structures, spreading at intermediate to superfast spreading rates (and higher, more consistent magma budgets) results in a layered upper crustal structure with a complex internal structure. Direct observations from submersibles, ROVs, and deep drill cores provide constraints that allow for the refinement or modification models for oceanic crust accretion at these relatively fast spreading rates.

Key observations reveal structures and processes that are not obvious from surface investigations. These include progressively more steeply inward-dipping (initially horizontal) lava flows, outward-dipping (originally vertical) dikes, downward-increasing brittle deformation and hydrothermal metamorphism of lavas and dikes, and underplating by much-less-faulted and altered gabbroic rocks. The thickness and internal structure of these upper crustal rock units are created by continuous dike intrusion feeding lava flows that cause caldera-like, vertical subsidence of hundreds of meters above an axial magma chamber. Greater subsidence and deformation of upper crustal units occur at intermediate spreading rates (or lower magma budgets) than at the highest rates.

These results have implications for viscous mass redistribution beneath the spreading axis even as additional magma is delivered from the mantle below. Applying observable parameters to dynamic models yields internally consistent results with extremely weak axial lithosphere (effective elastic thickness < 1 km) that strengthens laterally as it ages off axis prior to the formation of abyssal hill faults.