3D Shear Velocity Structure of the Caribbean—Northwestern South America Subduction Zone From Ambient Noise and Ballistic Rayleigh Wave Tomography

Abstract

The Caribbean-South America subduction zone is a flat subduction zone, with Laramide-style thick-skinned uplifts occurring in the Merida Andes, Sierra de Perija Range, and Santa Marta Massif. Geodetic measurements and historical seismicity show this region is storing strain energy and is capable of a mega-thrust earthquake (M ≥ 8.0). Previous seismic investigations of the lithosphere and upper mantle in this area are either very large scale, very local, or only peripheral to this area; therefore, details of the Caribbean plate subduction geometry beneath the Maracaibo block remain unclear. In this study, we used a new data set acquired by the Caribbean-Merida Andes seismic experiment (CARMA), which comprised 65 temporary broadband stations and 44 permanent stations from the Colombian and Venezuelan national seismic networks. We jointly inverted ambient noise Rayleigh wave Z/H ratios, phase velocities in the 8–30 s band and ballistic Rayleigh wave phase velocities in 30–80 s band to construct a 3-D S-wave velocity model in the area between 75°–65°W and 5°–12°N. The 3-D model reveals a general increase in crust thickness from the trench to the southeast. An anomalous area is the Lake Maracaibo, which is underlaid by the thinnest crystalline crust in the region. This observation may indicate that the Maracaibo block is experiencing a contortion deformation within the crust. We also identified a high velocity anomaly above the subducting Caribbean slab, likely representing a detached piece of eclogitized Caribbean large igneous province from the base of the Maracaibo block. Additionally, our V_s model clearly indicates a slab tear within the subducted Caribbean slab, approximately beneath the Oca-Ancon Fault.