Tectonics: Seismic Structure at Mid-Ocean Ridges

Abstract

Seismic studies of mid-ocean ridges provide information on structures and properties within young crust that reflect the magmatic and tectonic processes of crustal formation. This contribution provides an overview on the seismic properties of the uppermost crust, the crustal magma system, the Moho and crustal thickness at mid-ocean ridges across the spreading rate spectrum. The observations indicate quite uniform crustal structure along the fast spreading ridges with shallow steady-state crustal magma lenses, a thin upper crustal layer 2A that thickens rapidly away from the axis attributed to accumulation of thin lava flows, and modest variations in crustal thickness. Intermediate spreading ridges are associated with marked variations in axial morphology and crustal structure that span the spreading rate range from fast to slow types. Only minor variations in crustal thickness accompany these changes, consistent with a threshold-type response of ridge properties to magma supply at intermediate spreading rates. At slow spreading ridges, steady-state crustal magma bodies are rare, layer 2A is thick along the axis and large variations in crustal thickness are observed. Extensive exposures of lower crustal and upper mantle rocks are common at many slow spreading ridge segments, indicating that in these places, tectonic stretching plays a dominant role in seafloor spreading.