The Seismic Structure of Oceanic Crust Formed in Back-Arcs

Abstract:

Near three fourths of the Earth crust, and the majority of all past subducted crust have been formed at oceanic spreading centers. Seismic experiments carried out to define the structure of the oceanic crust that underlies most of the world ocean basins have systematically defined a ubiquitous three-velocity-layers seismic structure.

Layer 1 is established to be made of sediment, but the nature of layers 2 and 3 has remained a matter of interpretation and of intense research for more than half century. The nature of oceanic crust under basins has been mainly inferred from indirect geophysical measurements and rocks sampled at the seafloor because sampling of the deep crust has been restricted to a handful of sites. Current accepted models propose that the formation of oceanic crust at mid ocean ridges (MOR) –far from hotspot anomalies- is fundamentally controlled by the rate of plate separation, with oceanic crustal types classified as ultraslow, slow, intermediate, and fast spreading crust.

We present a seismic study of oceanic crust across an extensive back-arc area providing constrains on accretion processes. The depth-velocity distribution of back-arc crust resembles MOR layered structure, but velocities are systematically different to MOR crust formed at any spreading rate. In particular, Layer 2 displays a considerable variability of P-wave velocities over short distances, and Layer 3 displays lower seismic velocities. These distinct characteristics indicate either a considerable different rock composition or deformation process. The integration of our observations with data from other back-arc basins indicates an unexpected variety of crustal velocities that does not appear controlled by spreading rate. We interpret that the seismic structure of the crust indicates that back-arc spreading represents a class of spreading in its own.