70 MA OF ARC EVOLUTION IN PANAMA: THE INFLUENCE OF TECTONIC FORCING DURING THE TRANSITION FROM AN INTRA-OCEANIC TO A CONTINENTAL ARC

Over the past 70 Ma, the Panama arc has experienced three distinct phases of magmatic activity during its transition from an intra-oceanic to continental arc. At 70 -73 Ma the Panama arc formed on the trailing edge of the Caribbean Plate. Initial arc rocks were primarily basaltic, but by 60-55 Ma the arc was lithologically heterogeneous with plutonic and extrusive rocks that range from calc-alkaline to tholeiitic and basaltic to andesitic in composition. However, geochemically the rocks share a large negative Ta anomaly, enrichment in LILE and moderate HREE compositions. This initial arc was intra-oceanic and exhibits geochemical traits associated with hydrous mantle-wedge derived magmatism. It persisted until 45-40 Ma when the volume of arc magmatism dramatically decreased.

In the Miocene, Panama arc magmatism increased greatly in volume. The Miocene pulse in arc magmatism is characterized by medium to high-K series volcanism and elevated trace-element ratios such as La/Yb, Th/Yb and Hf/Yb that suggest increasing mantle enrichment coupled with increasing crustal thickness. The Miocene arc persisted until ≈5-10 Ma. Geochemical data and offshore marine geophysical observations show that the Miocene arc in Panama was driven by northward subduction of the Cocos Plate. However, seismic tomographic data indicate an upwelling of low-velocity material through the subducted slab at this time suggesting that the slab fractured, perhaps in response to the Panama-South America collision.

Volcanic rocks younger than 5 Ma are found only in western Panama and are characterized by an adakitic signature with elevated Sr/Y and steep REE ratios. Earthquake hypocenter data indicate that northward subduction is no longer occurring beneath Panama and Sandra rift linear-magnetic anomalies suggest that northward subduction ended at ≈8 Ma. Mantle upwelling within a slab-window is interpreted to drive adakitic magmatism in western Panama. Over the past 70 Ma, the Panama arc has transitioned from an intra-oceanic to a continental-like arc. However, processes other than pure subduction appear to be responsible for driving this evolution. In both the Miocene, and even more so with the youngest adakitic volcanism, transitions in the arc behavior are driven by tectonically controlled mantle influxes.