Geology and Geochronology on the ABCs and Venezuela Archipelago (Univ. Georgia, Rice, UH)

The Caribbean plate has long been considered to represent Mesozoic oceanic crust of normal thickness that has been overthickened by a younger, late Cretaceous plume generated magmatic event that led to the formation of an oceanic plateau (e.g. Donnelly, 1973; Burke, 1988). Many if not most current workers ( e.g. Duncan and Hargraves, 1984; White et al., 1999; Kerr and others, 2003) consider the plateau to have formed in the eastern Pacific 91-88 Ma (Sinton et al., 1998) possibly above the Galapagos plume. Others (Meschede and Frisch, 1998; Meschede, 1998; Horenle et al., 2004) have shown that the Galapagos plume is an unlikely candidate for the origin of the Caribbean Plateau. Plate tectonic models commonly call on collision of the plateau with a preexisting west-facing arc resulting in reversal of arc polarity. The crust of the ABC islands is considered to be composed of a combination of plateau crust (Aruba and Curacao) fragments of the east-facing arc (Bonaire; Beets and others, 1984; Thompson and others, 2004) as well the younger period of post polarity reversal arc magmatism represented by the Aruba batholith (White and others, 1999).

Deformation, geochronologic,and fluid-inclusion Studies: Avé Lallemant and graduate student A. Beardsley have completed structural analysis of faults, bedding planes, cleavages, and fold axes in the mostly plutonic, volcanic, and volcaniclastic rocks on the ABC Islands. They identified one ductile (D0) and three brittle stages of deformation accompanied by rotations of the islands. The internal deformation features of the island blocks are consistent with an obliquely convergent plate boundary. D0 and D1 deformation structures formed by arc-normal contraction and arc-parallel extension with D0 structures being slightly older. During and after these deformations the entire Leeward Antilles islands rotated clockwise by about 900 likely between 80 and 55 Ma. The D2 structures formed in a typical wrench system with the major compressive stress at a relatively small angle to the plate boundary. Most likely, D2 correlates to the Eocene convergence between North and South America and collision of the Caribbean with the South American margin, approximately 55 Ma. The youngest deformation and rotation, D3, happens along the arcuate South Caribbean Deformed Belt. Since approximately 25 Ma to Recent, the ABC’s have continued to rotate, opening the Aruba, and East and West Curaçao Basins. We collected samples on the ABC Islands for fissiontrack and fluid-inclusion analyses, and expect to get results shortly. On the macroscale, the interpretation of the BOLIVAR reflection profiles around the ABC’s show Latest Paleogene-Neogene basement fill in the inter-island basins with large normal faults bounding the islands (Figure 8, 9). Stratigraphic relations show that the inter-island basins opening shortly after the much largerBonaire basin to the south formed.

Geochronology and Mapping on the Leeward Antilles: Wright and Wyld have undertaken a program of geologic mapping, and geochronology (see Figure 2) on Aruba, Curaçao and Bonaire, and the Venezuelan archipelago as far east as Los Testigos. The mapping is designed to place geochemical studies on these islands (e.g. Kerr et al., 1996; White et al., 1999; Thompson et al., 2004; ) into geologic context, to ascertain the geologic and tectonic evolution of the islands. We have done detailed mapping of the pre-batholithic rocks on Aruba, and have obtained a precise U-Pb SHRIMPRG zircon date on the Aruba batholith. Our work indicates that Aruba records the emergence of parts of the Caribbean plateau in the Late Cretaceous (Turonian) with extensive evidence for subaerial weathering of the plateau crust. This was quickly followed by intrusion of the tonalitic Aruba batholith at 89 ± 1 Ma. Thus Aruba went from a subaerial setting to batholithic depths in a very short time frame. On Curaçao, our work, coupled with previous studies, indicates that Curaçao was emergent and undergoing subaerial exposure in the Late Cretaceous but unlike Aruba became the location of a marine basin following subsidence of the plateau rocks. Detrital zircon dating of Paleocene turbidites and Ar/Ar dating of hornblende in volcanic rich beds indicates that Curaçao was located near a source of Late Cretaceous magmatic rocks (ca 75 Ma) as well as the Venezuelan continental margin, as indicated by the abundance of Paleozoic and Pre-Cambrian detrital ages. The youngest in situ magmatism on Curaçao is recorded by a series of dioritic to quartz dioritic dikes emplaced at ca ~85 ±1 Ma Our work on Bonaire indicates that previous geologic mapping needs revision. The important results to date show that the supposedly oldest volcanic rocks on the island are actually a series of shallow dikes, sills and small intrusions that are quite similar to the plateau rocks exposed on Aruba. Our work on younger rocks indicates that pillow lavas, previously interpreted to indicate that Bonaire represents an island arc, are actually large slide blocks in debris flows. Their point of origin as well as their age are unknown at the moment. We have dated felsic hypabyssal debris flow blocks at ca 95 Ma. Although Bonaire is interpreted as an island arc sequence (Thompson et al., 2004) our current work indicates a more complex scenario. We visited Los Roques, east of Bonaire, (Figure 2) in summer 2005, collecting an extensive suite of mafic igneous rocks for geochemical analysis, as well as two samples of quartz diorite for zircon dating. We will have the zircon ages in March 2006. Our work on La Blanquilla (Figure 2), which lies at the southern terminus of the Aves ridge, establishes that two distinct magmatic episodes are represented on the island(75 Ma and 59 Ma). The 59 Ma magmatism probably represents the last active stage of the Aves arc prior to opening of the Grenada basin, while the 75 Ma age represents arc magmatism on the Aves ridge. We are attempting to locate dredge samples of "granitic rocks" from submerged areas of the ridge to better characterize the chronology of arc magmatism on the ridge. Finally, we have dated two plutonic bodies on Los Testigos, which sits near the terminus of the modern Lesser Antilles arc (figure 2), at ca 37 Ma. This probably represents the end of magmatism on this section of the modern arc due to initiation of its accretion to the Venezuelan continental margin.

Geology and Geochronology in Venezuela (Rice, UH)

The Caribbean Mountain system in northern Venezuela is the interior of a south vergent fold and thrust belt consists of two east-west striking belts of HP/LT metamorphic rocks. The northernmost is the Cordillera de la Costa belt (CdlC), a typical subduction mélange. The southern belt, the HP/LT Villa de Cura (VdC) belt, consists of four internally coherent tectonic slices (Figure 2). The CdlC was studied by Avé Lallemant in prior projects. The CdlC consists of (1) granites and granitic gneisses, (2) continentally derived schists, marbles, and quartzites, and (3), eclogites, blueschists, amphibolites, and partly to strongly serpentinized upper mantle peridotites. The exhumation of the HP/LT metamorphic rocks may have occurred in several stages. The eclogites (maximum depth of burial ~70 km) may have ascended to the crust/upper mantle boundary by buoyancy forces as the normal component of subduction decreased along the arcuate island arc, and because the subduction mélange has a rather low bulk density. When incorporated in the lower crust, the CdlC belt underwent arc-parallel extension due to the increasing arc-parallel component of plate motion. Most structures indicated a right-lateral shear sense in the rocks, compatible with today’s right lateral shear along the entire plate margin. The last stage of deformation and decompression resulting in the final exhumation resulted from southward obduction of the belt (Avé Lallemant and Sisson, 2005). As BOLIVAR research Rice MA student A. John and Avé Lallemant investigated the structural deformation of the VdC belt. The VdC belt was metamorphosed at the same time as the CdlC, but in a totally different environment.We found that the EW shear sense was left-lateral. The CdlC rocks have a penetrative cleavage dipping north, whereas the cleavage in the VdC rocks dip southward. We interpret the two belts as part of a large antiformal anticlinorium. We also found that the sense of shear of emplacement in the VdC was opposite to the one in de CdlC belt, likely a result of backthrusting in the fold and thrust belt. We collected a large number of samples from the VdC belt for age dating. If the hypothesis is correct that the Leeward Antilles island arc collided diachronously with South America, the ages of the blueschists should be younging from west to east as well. None of the samples have been dated because of problems in Peter Copeland’s 40Ar/39Ar laboratory. This facility is now operational again, and we expect to get dates within six months.