A survey of the literature that deals with the gabbroic rocks believed to comprise the foundation of the oceanic crust indicates that the overwhelming majority of these rocks are recovered from escarpments associated with transform faults. The wide range of mineral and chemical compositions characterizing oceanic gabbroic rocks suggests that the lower oceanic crust is much more heterogeneous in nature than was previously suggested by the results of geophysical investigations. The examination of gabbroic rocks recovered in situ from the walls of the Mid-Cayman Rise rift valley by the submersible ALVIN not only supports the notion that oceanic gabbroic rocks are heterogeneous in nature but also that widely varying gabbroic rock types are found distributed heterogeneously on the walls at a scale of tens of meters. Observations that the largest escarpments on the walls of the Rise have only several hundreds of meters of vertical offset, and that gabbroic rocks were recovered to within roughly 100 meters of the tops of the rift valley walls, indicate that the shallow intrusive and extrusive carapace of the oceanic crust here must be anomalously thin. It has been suggested that thin oceanic crust characterizes slowly-slipping. ridge-transform intersections elsewhere; the thin crust of the Mid-Cayman Rise may be attributable to the presence of the two long transform faults that bound the 110 km long Rise segment. The two transforms may also have an instantaneous effect on the structural evolution of the Rise creating the well-defined tectonic grain that strikes at a high angle to the axis of the rift valley.
Stroup, J.B., 1981. Geologic investigations in the Cayman Trough
the nature of the plutonic foundation of the oceanic crust.
MSc. thesis, State University of New York at Albany. 189pp., +xi.
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