Magnetic Anomalies and Tectonics of the Cayman Trough

Abstract

Summary A magnetic residual total-field anomaly map has been prepared for the Cayman Trough, Cayman Ridge and part of the Nicaragua Rise. Errors of navigation necessitated empirical adjustments of track positions, chosen to minimize differences at track crossings, before the contour map could be drawn. The map shows linear anomalies along the north and south bounding walls of the Cayman Trough east of longitude 82W; that on the north has two maxima and terminates just west of Grand Cayman. Further west a more irregular positive anomaly follows the north wall of the Trough. Some topographic features have associated magnetic anomalies but others do not, and it is clear that some are composed of non-magnetic rock. The observed magnetic anomalies and bathymetry along one ship track are used for comparison with anomalies computed for two-dimensional models. Again it is clear that both magnetic and non-magnetic rocks, juxtaposed on steeply and variously inclined planes, are required in the walls to account for the anomalies. Faults are indicated but the anomaly pattern is not consistent with normal faulting in tension. Previous seismic refraction and gravity results show a very thin crust under the floor of the Trough and suggest that it is a tensile feature. Tectonic and geophysical evidence from the Caribbean region is shown to lead to the same conclusions for the Cayman Trough as do the requirements of distortion of the Caribbean which arise if the Atlantic is assumed to have opened from an optimum fit of the continents which surround it. These conclusions are that very large strike-slip movements of the kind suggested by Hess and Maxwell have occurred in both walls of the Cayman Trough, and that the Trough has been formed by these and by simultaneous opening in tension. The linear magnetic anomalies arise from juxtapositions of magnetic and non-magnetic rocks in the walls, arising in the large strike-slip displacements. The model proposed for the formation of the Trough is considered to be consistent with all available seismic refraction, gravity, magnetic and bathymetric information, with geological evidence from the islands and with evidence of hypocentre distribution and source mechanism studies of earthquakes in the region. 1. The western Caribbean and Cayman Trough The Cayman Trough is a major topographic feature of the Earth's surface. Its length, from the Gulf of Honduras to the Gulf of Gonave at the west end of