Geophysical study of the Cayman Trough

Abstract

The deepest water and the shallowest depths to mantle within the Caribbean Sea south of the Caribbean Island arc occur in the Cayman (or Bartlett) trough. The thin crust, indicated by the seismic refraction studies and by gravity data, suggests that the trough is a feature produced by extension with concomitant upwelling of dense mantle material to more shallow depths. During cruises of the R. V. Chain in 1965 we investigated the Cayman trough, utilizing continuous bathymetric, gravity, and magnetic measurements. These investigations confirm that in addition to the Bartlett and Oriente deeps, which occur south of the Cayman Islands and Cuba, respectively, there is a depression about 5700 meters deep on the south side of the Cayman trough west of 79°30′W. Negative free-air gravity anomaly values occur over the deeps of the Cayman trough, and computed structure models show that there are mass deficiences beneath the deeps and mass excesses beneath the adjacent ridges. The deeps and adjacent ridges are not in isostatic equilibrium, and it appears that the rising of portions of the crust is here a concomitant aspect of the sinking of adjacent portions of the crust. The width of the individual structures that are out of isostatic equilibrium is about 40 km. All the gravity profiles show a hump in the Bouguer anomaly curve across the trough. The computed structure model confirms that the rise in the mantle-crust interface extends across the width of the trough and is centered on the axis of the trough. The axis of the Bouguer high clearly does not cross the Windward Passage between Cuba and Haiti but instead curves eastward and enters the Gonave Gulf between the northern and southern peninsulas of Haiti. Steps of about 200 to 400 γ in the magnetic field on the sides of the trough can be traced along almost the entire length of the Cayman trough. These steps generally occur on the southern slope of the Cayman trough and on the northern side of the Cayman ridge. Lithofacies maps of southern Cuba suggest that the initiation of the extension that produced the trough began in the Paleocene or earlier, and studies in Guatemala suggest that extension was active during the Cretaceous. The much slower extension rate and the lack of characteristic central magnetic anomalies suggest that the Cayman trough probably resulted from processes different from the ones producing the world rift system. Rather than a direct manifestation of an upwelled convection cell, the trough may be a tensional feature associated with failure of the crust under conditions of an eastward drift of the Caribbean region. It is suggested that the rift structure is now growing toward the east along the Enriquillo-Cul de Sac trough in southern Hispaniola. It is also concluded that the Cayman trough structure is not a westward continuation of the structure that produces the Puerto Rico trench negative free-air anomaly belt.