New Constraints On The Caribbean-South America Plate Boundary From S Wave Receiver Functions

BOLIVAR (Broadband Ocean-Land Investigation of Venezuela and the Antilles arc Region) was aimed at investigating the interplay between the lithospheric and asthenospheric mantle of the Caribbean and South America plates. We estimate lithospheric thickness from application of the S wave receiver function technique. This reveals lithospheric thinning from 95-105 km depth underneath the Guyana Shield in southern Venezuela to 60-75 km underneath the Caribbean Sea. We observe a comparable thinning on the passive margin transition in eastern Venezuela. Towards the west, the lithosphere-asthenosphere boundary (LAB) shallows to 80 km depth along a NE-SW trending structure bordering the Venezuelan Andes. In addition, the base of the lithosphere beneath the Maracaibo block in NW Venezuela is seen gently dipping towards the southwest. The observed lithospheric structure is consistent with established strike-slip tectonics along the northern boundary of South America. However, we find no evidence for southward subduction of the Caribbean plate as previously suggested. Instead, the boundary is better viewed as the combined motion of three lithospheric blocks: (1) the Caribbean and associated subduction in the Antilles, (2) South America, and (3) the Maracaibo block. The relatively sharp discontinuity in LAB beneath northern Venezuela leads to the conclusion that the Caribbean-South America plate boundary is (1) a structure of lithospheric scale, and (2) a continuous strike-slip plate boundary extending from NE Colombia to Trinidad in NE Venezuela along the Oca-San Sebastian-El Pilar fault zone. This boundary is overprinted in the west by northward translation of the Maracaibo block over the Caribbean.