IGCP Project short title: Caribbean Plate Tectonics Duration and ...

28
covering the oceanic crust southwest of the GSPRFZ may have been offscraped and
deformed as they were carried toward the trench. Subduction related Eocene volcanism was
areally limited and short-lived. Convergence waned during Oligocene time perhaps in
response to the arrival and collision of thick oceanic terrains (eg. Cayman Ridge, Nicaraguan
Rise, Beata Ridge) with the overriding plate. During collision, volcanic units at the edge of
the overriding crust were uplifted and destabilized sufficiently so that masses calved off and
collapsed southwestward forming an apron of debris (Sabana Grande Formation). We
propose that the Eocene volcanic rocks record renewed subduction, although the subduction
was north-facing and involved the consumption of the Caribbean plate along a zone roughly
coincident with the back arc of the Cretaceous belt.
STRUCTURAL STYLES ALONG OBLIQUELY CONVERGENT OROGENS: THE
EASTERN CARIBBEAN-SOUTH AMERICA PLATE BOUNDARY
CRUZ, Leonardo, TEYSSIER, Christian, and WEBER, John.
The Caribbean-South America plate boundary in NE Venezuela and northern
Trinidad exposes an E-W oriented mountain belt of deformed and metamorphosed sediments
deposited on the northern South America passive margin in early Mesozoic time. Northern
Trinidad and NE Venezuela display contrasting styles of deformation developed during
oblique collision and wrenching between the Caribbean and South American plates in the
past 50 million years. In northern Trinidad, metamorphic conditions increase from east to
west with structures evolving from upright in the east to recumbent in the west, across the
brittle-ductile transition. In ductilely deformed rocks, foliation is subhorizontal and lineation
is ~E-W, parallel to the belt. Sense of shear is ambiguous. In NE Venezuela, metamorphic
grade is similar to the western part of northern Trinidad; foliation dips moderately to steeply
to the S and lineation plunges moderately to the SW. In general, sense of shear criteria
parallel to lineation show top (down) to SW relations, indicating increased exhumation of
the northern part of the belt. Oblique collision and wrenching in the Caribbean-South
American plate boundary may have generated a complex deformation history, which
evolved diachronously from west to east to produce the two styles of deformation displayed
in northern Trinidad
and NE Venezuela. Two models have been proposed to account for the generation and
exhumation of this belt. In the first model, deformation is concentrated in a retro-wedge
developed in front of the rigid Caribbean plate indenter, which deformed the softer South
American continental crust. Vertical stretch decreases southward, exhumation rate increases
to the north and deformation ages are younger to the east due to diachronous collision. The
second model implies a midcrustal coupling zone that deforms ductilely due to translation of
upper crustal blocks and transpression of the system. Subhorizontal fabrics develop
contemporaneously parallel to the rheological layering of the lithosphere. For both models,
spatial and kinematic variations of fabric orientation, cooling ages, and exhumation rates, are
key elements to understand the overall deformation history of this region and are currently
being studied.
LEAD ISOTOPE STUDY OF THE PALEOGENE IGNEOUS ROCKS OF THE SIERRA
MAESTRA, SOUTHEASTERN CUBA
KYSAR MATTIETTI, Giuseppina, LEWIS, John F., and WYSOCZANSKI, Richard.
The Sierra Maestra of southeastern Cuba occupies a key position between the
remnants of the Greater Antilles arc accreted terranes and the Cayman strike-slip belt that
constitutes the present day northern Caribbean plate boundary. An isotopic study has been
undertaken to constrain the paleotectonic setting and the source of the Sierra Maestra
structure. Lead isotope ratios were determined for a set of lithologies representative of each
major magmatic complex of the Sierra Maestra. Both 207Pb/204Pb and 208Pb/204Pb ratios
are restricted to a narrow, well-defined array
of values in a band parallel to the North Atlantic Reference Line NHRL The slight
enrichment in 207Pb/204Pb ratios represent the selective mobilization U with respect to Th,
(U/Th ratios ranges from 0.8 to 1). Overall Pb isotope ratios for the Sierra Maestra are
homogeneous, indicating the existence of a single magma source. This observation correlates
with the low Ce/Yb values that characterize primitive arcs with varying degrees of the
28