Venezuela Basin crustal structure

Abstract

Velocity‐depth profiles derived from six two‐ship expanding spread experiments, in combination with other geophysical data, define the characteristics of two distinct types of Venezuela Basin crust and the boundary between them. Each two‐ship common midpoint reflection/refraction profile is automatically transformed into the τ‐p plane, ‘picked’ and interpreted to provide V(Z) functions with appropriate confidence bounds. The results, together with gravity, magnetic, and near‐vertical incidence reflection data, reveal a 50,000 km2 triangle of Venezuela Basin crust which resembles normal oceanic crust in a magnetic quiet zone. North and west of this triangle lies the previously defined, thick ‘Caribbean’ crust, having two distinct layers above the M discontinuity. Acoustic basement there appears unusually smooth due to extensive basaltic sills and flows which were cored at Deep Sea Drilling Project sites 146/149(sills), and 150 (flows); also, depths to mantle are greater than normal. Interpretations of near‐vertical and wide‐angle reflection data show that the extra crustal thickness is due not only to the emplacement of the flows but also to the crust below being somewhat thicker than normal. The boundary between the two crustal areas has a NE‐SW trend which parallels the dominant structural and magnetic lineations.This boundary coincides in position, though not in trend, with the previously defined ‘central Venezuela Basin fault zone’. Further study is required to determine whether this boundary is of tectonic origin or if it represents a change in style of crustal production.