SUBDUCTION INITIATION IN THE PANAMA ARC

The tectonic evolution of the Panama arc contains two subduction initiation events. The first occurred at 73 Ma when the Farallon plate began to subduct beneath the trailing edge of the Caribbean plate. Evidence for this comes from the oldest volcanic rocks in Panama that contain arc trace element signatures. These occur as basaltic dikes in the Azuero Penninsula that intrude Caribbean large igneous province (CLIP) rocks. This subduction event produced abundant hydrous arc magmatism including tonalitic batholiths, largely replaced the CLIP crust in Panama and thickened the crust from 10-15 km (Caribbean crust) to 25-30 km thick in approximately 15 million years.

The second subduction initiation event occurred in the Miocene/Oligocene when the Caribbean plate was underthrust beneath the Panama block as a result of the collision with South America. This has led to the creation of the North Panama deformed belt, and caused the Caribbean plate to be underthrust to depths of 70-80 km as defined by earthquake hypocenters. However, no arc magmatism has so far resulted from this second subduction initiation event. Current geodetically (GPS) determined rates of convergence between the Panama block and the Caribbean plate range from 5-12 mm/yr. Using this rate and the length of the underthrust Caribbean slab yields a subduction initiation age of between 15-30 Ma, which is consistent with estimates of when the collision between Panama and South America began. Overall, in each subduction initiation event the plate with thinner crust was underthrust, but over time and due to crustal related to arc magmatic processes coupled with a collisional event, subduction vergence switched directions.