Crust-mantle Interactions During Continental Growth
and High-Pressure Rock Exhumation at an Oblique
Arc-Continent Collision Zone: The SE Caribbean margin
Continental interiors are constructed of island arcs and micro-continent fragments. Theories of continental formation rely on island arc accretion to continental margins as a primary means of post-Archean continental growth. The Caribbean-South American plate boundary zone is the result of Caribbean-American plate interactions starting in the Mesozoic, that are now accreting the Leeward Antilles arc to the South American craton. The Leeward Antilles arc has been diachronously colliding obliquely along this boundary since the Eocene, starting in western Venezuela and migrating eastward with the Caribbean plate relative to South America. This plate boundary and the processes associated with it have been active in near present form for ~50Ma. The boundary now consists of trenches of opposite polarity at either end of a >1000 km long right lateral transpressional zone. This shear zone extends from the Gulf of Paria between Trinidad and Venezuela, to Colombia, connecting the NW-dipping Lesser Antilles subduction zone in the east to the SE-dipping Southern Caribbean Deformation Belt subduction zone in the west. Teleseismic tomography and seismicity show the Caribbean plate overriding the Atlantic seafloor of the South American plate east of the Gulf of Paria. In the west, the Caribbean plate is subducting beneath continental South America. The subduction polarity reversal and right lateral shear zone connecting the trenches developed following initial collision of the Great Caribbean arc with South America in the mid to Late Cretaceous. The plate boundary now consists of a series of EW-trending allochthonous belts that include the Leeward Antilles arc, the metamorphic belts of the Caribbean Mountain system, the para-autochthonous Serrania del Interior foreland fold and thrust belt, and associated foreland and shear zone basins.
In addition to the eastward migrating Lesser Antilles trench and accreting Leeward Antilles island arc, a number of other time transgressive tectonic events are associated with this plate boundary, including the exhumation of the high pressure-low temperature (HP/LT) metamorphic belts of the Caribbean Mountain system, the formation of the Serrania fold and thrust belt and associated foreland basins, and the formation of basins within the strike-slip system. All of these tectonic processes appear to have begun in the Eocene in the west and have subsequently moved eastward along the margin. The HP/LT rocks (up to 75 km depth) are now at the surface in margin parallel belts in which initial data suggests that progressively shallower HP/LT metamorphic rocks have been exhumed from west to east. We conjecture that the ascent and exhumation of the metamorphic belts is the result of strike-slip, extension, and compression that operate in concert. Strike slip and arc-parallel extension tectonically unroof deeply buried rocks that are then obducted southward over the margin and eroded. The mantle involvement in this exhumation process is poorly understood. The orogenic belt has developed above a crustal scale basal decollement into which both the thrust faults and the strike-slip faults merge, making the entire crust of the plate boundary a zone of orogenic float rooting to the mantle. This geometry provides a pathway for HP rocks subducted in the mantle to mechanically re-enter the crust.
We are undertaking a multi-disciplinary investigation to test hypotheses related to arc-continent collision and accretion, HP/LT rock exhumation, and the development of folded belts and sedimentary basins. We are timing the cessation of arc magmatism associated with arc accretion, and time and measure peak conditions of HP/LT metamorphism and exhumation along the plate boundary. The geometries of arc accretion, exhumation, the folded belt, and basins will be investigated throughout the margin with active and passive land and marine seismic experiments. The timing and uplift histories and the seismic geometries will constrain geodynamic models of margin development that make use of realistic crust and mantle rheologies in 2D and 3D. The research provides an understanding of the time-transgressive history and crust-mantle mechanical processes by which island arcs accrete to continents, deeply buried HP/LT rocks are exhumed, and folded belts and different types of sedimentary basins form along oblique collision zones. We will be examining the interplay of the crust and subcrustal lithosphere during arc accretion and metamorphic belt exhumation, and how subduction polarity reverses. Lastly, we will be determining the flow patterns of the sub-lithospheric mantle beneath the plate boundary and northern South America as a whole, around the two subducting plates, and beneath the right lateral shear zone between them.