Geophysical Study of the Venezuelan Borderland

Abstract

The Venezuelan Borderland is composed of several large depositional basins, a linear chain of islands, and a broad outer ridge. It is cut by a nearshore zone of east-trending faults. Geophysical profiles show that the basins have different structural styles, but these features and the large faults and ridges are related to each other and to the Caribbean Coast Range by a complex history of relative movement between the Caribbean and South American plates since Late Cretaceous time. The east-trending Curacao Ridge is a thick pile of deformed low-density strata ( Vc ⩽4.1 km/sec ) that apparently formed by compression of Venezuela Basin strata on its north side and by deposition and compression of strata in Los Roques Trough on the south. The basement rock of the Netherlands Antilles is composed of diabase, pillow basalt, and marine sediment that are weakly metamorphosed and cut by quartz dioritic intrusives. Gravity models suggest a thick (32 km) root beneath the islands and a thinned (18 km) crust beneath the Bonaire Trough. The Bonaire Trough probably formed by movement of the island chain away from the mainland in middle Tertiary time but shows little sign of present-day tectonic activity. The Cariaco Trench is an extensional basin between the San Sebastian and El Pilar faults. The Curacao Ridge is widest and thickest in the west (70° W.), where sediment to the north in the Venezuela Basin is thin. The ridge is narrow in the east (66° W.), where sediment of the Venezuela Basin is thick (up to 4 km). No compressional deformation of the basin sediment is seen east of Los Roques Canyon. This structure may be explained by clockwise rotation of a small crustal block — the Bonaire block — about a pole near Caracas. The rotation of the Bonaire block probably results from dextral movement between the much larger Caribbean and South American plates. The very low seismicity recorded near the Curacao Ridge may result from a combination of low rate of deformation and low elastic strain release involved in deforming a large sediment body.