BOLIVAR: Structure of the Leeward Antilles "Arc" and Growth of the South American Continent

A. Arogunmati, A. Levander, C.A. Zelt, A. Beardsley, J.E. Wright, and the BOLIVAR Study Group.

Rice University, Houston, TX
University of Georgia, Athens, GA

It is generally believed that in the Post-Archean continents have grown by accretion of island arcs to older continental masses. The bulk composition of sediments and compilations of global seismic refraction measurements suggest that average continental crust has an intermediate composition (~58-62% Si02) whereas seismic velocity measurements in the Aleutians and the Sierra Nevada show that neither Aleutian-type island arcs, nor Sierran-type continental arcs are chemically similar to average continental crust, the former being too mafic, and the latter being too felsic. Without considerable mechanical and/or chemical refining, neither of these types of arcs can lead to continental crust. Several models have been proposed to refine the crust, including delamination of the lower most crust to reduce the its mafic composition.

We are examining the seismic structure of the Leeward Antilles islands, which hves been accreting to the South American continent throughout the Cenozoic in a transpressive tectonic setting. Mapping on Aruba, Bonaire, and Curacao shows that this element of the arc is composed of a basalt-grabbo-tonalite core, whereas the islands to the east have been mapped as island arc quartzdiorites and granodiorites. During the 2004 BOLIVAR active source experiments we acquired wide-angle data along the strike direction of the arc using 15 PASSCAL seismographs and on 5 dip direction profiles crossing the arc using numerous OBSIP instrument deployments. These data are being used to measure average crustal velocities, from which we can infer the bulk composition of the arc, as well as that of the voluminous sedimentary accumulations surrounding the arc.

The development of the large-scale strike-slip system along the margin to which the Antilles arc is accreting and the associated basin development adjacent to the arc during accretion, allows us to study the margin's history over time. The sediment mass can modulate the composition of the resulting continental crust as it is metamorphosed, shortened, and mixed with arc and oceanic rocks during accretion.