Rotation in the Southeastern Caribbean Arc (Grenadines) Accommodated by Back‐Arc Basin Inversion

Abstract The Caribbean tectonic plate is typically modeled as a rigid plate moving along discrete plate boundaries. Isolated exposures on islands have long shown that above the Lesser Antilles subduction zone, the plate experienced contractional, strike‐slip, and extensional deformation, but the importance of this deformation remained long unknown. Recent paleomagnetic data indicated that the northeastern Caribbean region broke from the plate, rotated counterclockwise, and was displaced over 100s of kilometers since the Eocene. Here, we study whether intra‐plate shortening, long recognized in the Grenadines (southern Lesser Antilles), where oceanic rocks of the Grenada‐Tobago back‐arc basin were thrusted and uplifted after the Eocene, also denotes regional intraplate deformation. Therefore, we tested for vertical axis rotations from Eocene‐Oligocene aged rocks from the Grenadine Islands between Saint Vincent and Grenada. The paleomagnetic directions reveal at least 25° clockwise rotation relative to the geographic north pole, ∼10°–15° more than what has been reconstructed for the Caribbean Plate Interior. Two models are proposed which may explain this rotation, though the preferred mechanism suggests that the southeastern Lesser Antilles forearc converged with the Caribbean plate interior, accommodated by inversion via thrusting of the former Tobago‐Grenada back‐arc basin ridge, with a reconstructed, southward increasing magnitude of between ∼75 and 125 km. We infer that rotation resulted from transpression along the South American‐Caribbean transform plate boundary. Inversion of spreading ridges, including in back‐arc basin settings, is often inferred as a possible mechanism for subduction initiation. The Grenadines may thus represent a rare, fossilized, early stage of a subduction initiation process.