Crustal structure in the Orozco Transform Zone

Abstract

Seismic refraction data from lines of small shots and from individual large shots recorded during project ROSE are used to determine the crust and upper mantle velocity structure in the Orozco transform zone. The objective of this study is to provide constraints on the origin of a deep north‐south trending trough flanked by two shallow ridges. These topographic features are incompatible with the present plate tectonic framework as defined by the regional spreading direction (approximately east‐west) and did not display any seismic activity during project ROSE. Travel times and amplitudes from reversed profile along one of the ridges reveal an anomalous crustal structure: a velocity increase approximating a gradient of 0.5 s−1 extending through most of the crust and decreasing to 0.1 s−1 at the base and a total crustal thickness of about 5.4 km. In contrast, three profiles from topographically normal oceanic crust indicate a crustal structure typical of oceanic crust throughout the oceans: a 1.5‐km‐thick layer in which velocity increases from about 3 km s−1 to 6.7 km s−1 overlying a 4.5‐km‐thick layer with a velocity of about 6.8 km s−1. The upper mantle velocity beneath both structures, determined from travel times from large shots at ranges of 30–100 km, is about 8.1 km s−1. These results are interpreted to indicate that the trough and flanking ridges represent an extinct spreading center that developed an axial valley as the spreading center progressively shifted to the present axis of the East Pacific Rise.