Mantle transition zone beneath the Caribbean-South American plate boundary and its tectonic implications

We analyzed receiver-function data recorded by a temporary broadband array deployed as part of the BOLIVAR project and the permanent seismic network of Venezuela to study the mantle transition zone structure beneath the Caribbean-South American plate boundary and Venezuela. Significant topography on both the 410-km and the 660-km discontinuities was clearly imaged in the CCP (common-conversion-point) stacked images. Beneath the southeastern Caribbean, the 410-km is featured by a narrow (∼ 200 km EW) ∼ 25-km uplift extending in the NS direction around 63° west, while the 660-km is depressed by ∼ 20 km in a narrow region slightly west to the uplift, a scenario that is more consistent with westward descent of the oceanic South American plate rather than a break-off of NNW dipping proto-Caribbean oceanic lithosphere along the El Pilar Fault. We also found a thick transition zone beneath the Falcon region in northwestern Venezuela, possibly associated with the subducted Nazca plate. A flat 410-km was observed beneath the Guayana shield, suggesting that the shield has a stable and moderately deep keel, which has little effect on the underlying transition zone structure.