Spatial Heterogeneity of Pore Structure in the Crustal Section of the Samail Ophiolite: Implications for High V_P/V_S Anomalies in Subducting Oceanic Crust

Abstract

Seismic surveys along subduction zones have identified anomalously high ratio of P‐ to S‐wave velocity (VP/VS) in the subducting oceanic crust that are possibly due to the presence of pore water. Such interpretations postulate that the pore structure is homogeneous at the scale of the seismic wavelength. Here we present the first statistical evidence of a heterogeneous pore structure in oceanic crust at scales larger than laboratory samples. The spatial correlation of measured bulk density profiles of the crustal section of the Samail ophiolite suggests that the pore structure is heterogeneous at scales smaller than ∼1 m. Wave‐induced fluid flow cannot follow the loading during the seismic wave propagation at this estimated heterogeneity, which implies that fluid‐filled microscopic pores and cracks have a limited impact on the observed high VP/VS anomalies in the subducting oceanic crust. Large‐scale cracks may therefore play an important role in shaping these anomalies.