ROCKS OF THE CORDILLERA DE LA COSTA, VENEZUELA RECORD RAPID EXHUMATION FOLLOWING LATE EOCENE, PROTO-CARIBBEAN (ATLANTIC)? SUBDUCTION

Metabasic rocks of the Cordillera de la Costa at Puerto Cabello, Venezuela crop out as eclogite, blueschist and garnet amphibolite lenses in a metasedimentary mélange matrix. High-pressure/low-temperature (HP/LT) zircons give a late Eocene (c. 35 Ma) U–Pb age. Titanites give early Oligocene (c. 33 Ma) U–Pb ages. The titanite is associated with overprinting of the HP/LT assemblage by garnet amphibolite assemblages; their ages coincide with the earliest amphibolite-facies metamorphism recorded in the metasedimentary rocks, suggesting the mélange at Puerto Cabello was assembled at or prior to c. 33 Ma.

The HP/LT rocks of Puerto Cabello have been viewed as genetically analogous to other HP/LT rocks of the South American–Caribbean margin. However, the P–T conditions, and fluid and cooling histories of the two sequences differ. The HP/LT rocks from Puerto Cabello may not have origins in the Caribbean Plate and were not necessarily emplaced as an allochthonous unit following collision of the rolling-back Caribbean Arc with the South American margin.

Plate motion histories for North and South America require 100s of km of Paleogene N–S convergence. Flysch deposits of Venezuela and Trinidad (turbidites and olistostromes) attest to the presence of significant tectonic relief in northern South America during the Paleocene–Eocene. These observations provide circumstantial support for plate tectonic models involving southward subduction of Proto-Caribbean (Atlantic) lithosphere beneath South America during the Paleocene–Eocene. We propose that the Cordillera de la Costa is autochthonous and that its HP/LT rocks formed during this Proto-Caribbean subduction beneath South America. Eocene–Oligocene collision between the rolling-back Caribbean trench and the Proto-Caribbean trench terminated subduction in the region.

Zr thermobarometry reveals some 0.7–1.5 GPa of near-isothermal exhumation over a duration of only 2 Myr following the HP/LT metamorphism. This translates to an average exhumation rate of 10–35 mm/yr. Rapid exhumation of the Puerto Cabello HP/LT complex may have been achieved by isostatic adjustment after trench–trench collision and descent of the Proto-Caribbean lithosphere into the mantle, or by rapid unroofing during early Oligocene N–S-directed extension.