TECTONITE FABRICS, STRAIN AND KINEMATICS IN THE CRETACEOUS METASEDIMENTARY ROCKS OF ST. CROIX USVI

The Caledonia Formation of St. Croix consists of very impure calcarenitic sandstones, siltstones and mudstones (turbidites and contourites) of Cenomanian- Maastrichian age intruded by ~72-66 Ma dioritic stocks and sills. These rocks occur in the east and west of the islands and are separated by a half-graben filled with ?Oligocene-Miocene carbonates. A prominent feature of the Caledonia Formation is a penetrative slaty cleavage formed under anchizonal conditions. The orientations of the slaty cleavage and the structural history of the east and west parts of the island differ. The microfabric of the Caledonia formation consists of the following fabric elements: a) aligned, ellipsoidal carbonate grains, b) short (1-3 grain diameters long) pressure solution seams, c) long pressure solution seams, d) aligned phyllosilicate grains. These elements form fabrics in the following architectural schemes: 1) ellipsoidal carbonate grains only, 2) ellipsoidal carbonate grains + short pressure solution seams, 3) ellipsoidal grains + long pressure solution seams (often anastamosing), 4) ellipsoidal carbonate grains + aligned phyllosilicate grains,5) long pressure solution + aligned phyllosilicate grains, 6) ellipsoidal carbonate grains + long pressure solution seams + aligned phyllosilicate grains. Strain was assessed using the Rf/phi method on thin sections cut perpendicular to the foliation and parallel to lineation (if there was one). This enabled a calculation of relative strain in the rocks as the factors inhibiting estimation of absolute strain would be the same in all samples. The results showed that the east end of St. Croix enjoyed simple oblate flattening, whereas western St. Croix enjoyed a more prolate deformation. Consideration of fabric architecture schemes, phyllosilicate/carbonate ratios and strain enable the construction of “maps” that relate to composition and strain and fabric architecture. Kinematic analysis of western St. Croix indicates a top-to-the-south tectonic transport and increasing embrittlement over time. The oblate flattening in eastern St. Croix suggests a very different kinematic history