Kinematic analysis of the deformational structures on eastern Isla de Margarita, Venezuela

[摘要] Because of its strategic location at the intersection of three regional trends, a kinematic study of the deformation on Isla de Margarita was undertaken to provide additional constraints for Caribbean plate tectonic models. Previous work revealed a metamorphic nucleus composed of amphibolites, schists, marble, serpentinized ultramafics, and leucocratic intrusives. Studies on eclogite knockers provided quantitative estimates for the meta-morphic pressures and temperatures. In this study, five generations of deformational structures were identified on Isla de Margarita. The earliest structures (D$sb{m 1a}$) are rarely preserved, occurring as the main foliation in the eclogite knockers and as a foliation within microlithons bounded by the S$sb{m 1b}$ foliation. The D$sb{m 1b}$ deformation is synchronous with epidote-amphibolite to greenschist facies metamorphism which overprints the eclogite assemblage. It forms the dominant metamorphic foliation on the island and is axial planar to associated isoclinal folds. Quartz c-axis and S-C fabrics developed under greenschist facies conditions and characterize the D$sb{m 1c}$ extensional deformation. In the Miocene, the non-metamorphic D$sb2$ deformation folded the D$sb{m 1b}$ foliation into a southwest-plunging anticlinorium. The latest brittle deformation (D$sb3$) shows northeast-southwest extension, aligned with the fold axes and extensions of all earlier deformations. Analysis of the relative motions between North and South America shows there is insufficient Mesozoic convergence to form in situ the metamorphic belts of northern South America. These metamorphic belts must therefore have formed far to the west along the Farallon/South American plate boundary. A proposed model relates the Cretaceous synmetamorphic D$sb{m 1a}$, D$sb{m 1b}$ and D$sb{m 1c}$ progressive deformations to arc-parallel extension in the Aves Ridge subduction complex. The remains of the Cretaceous arc have since been disrupted by the strike-slip boundaries of the northern and southern Caribbean, as well as by the opening of the Grenada Basin. The associated metamorphic rocks have also been disrupted and progressively emplaced onto the continental margin of South America from west to east. The D$sb2$ and D$sb3$ deformations are associated with this dextral plate boundary deformation between the Caribbean and South America.

[发布机构] Rice University