Crustal and Upper Mantle Velocity Structure of the Northwestern South American-Caribbean Subduction Zone Revealed by the CARMArray

The Caribbean plate (CAR) is likely a fragment of the Farallon plate heavily modified by igneous processes that created the Caribbean large igneous province (CLIP) between 110 and 80 Ma. The CAR collided with South America (SA) at about 60-55 Ma, and its southern edge began subducting beneath northwestern SA forming a flat-slab subduction zone with an accretionary prism offshore, no volcanic arc, and three associated Laramide-style uplifts having elevations >5 km. To better understand flat slab subduction, we deployed 65 broadband (BB) stations across northeastern Colombia and northwestern Venezuela from April, 2016 to March, 2018. The 65 stations interweave with 44 BB stations of the Colombian and Venezuelan permanent arrays, forming the CARibbean Merida Andes Array. The CARMArray has a station spacing of 20-75 km and covers an area of 600 km by 400 km that extends from the Caribbean coast in Colombia to the interior plains of Venezuela.

We have developed P- and Rayleigh- wave tomography models. The finite-frequency teleseismic P-wave tomography model in 3 frequency bands (centers 0.3, 0.5, and 1.0 Hz, ) extends to transition zone depths. The Rayleigh wave tomography model uses phase velocities and Z/H ratios in the period range of 8-40 s from both ambient noise and earthquake data and extends to 150 km depth. We also generated Ps receiver functions from waveform data of teleseismic events recorded by the array. We are now jointly inverting the RFs and the surface wave phase velocities to construct a more constrained 3-D S-wave velocity model beneath the array.

The models show that the CAR subducts at 25^o-30^o to the SE under northwestern SA to about Lake Maracaibo, Venezuela. The shallow-dipping portion of the slab is associated with Laramide-style basement uplifts of the Merida Andes (MA), Sierra de la Perija (PR), and Santa Marta (SM) ranges. Under Lake Maracaibo and the MA the CAR descends steeply to the bottom of the transition zone. The transition from shallow to steep subduction is well imaged in the tomography, and suggests a complex interaction between the SA lithosphere and the CAR plate. Under the MA, we observe a crustal root to a depth of 50-55 km, likely due to crustal and lithospheric shortening (Masy et al., 2011). However, where the slab is shallow beneath the SM and the PR ranges, crustal roots are not observed.