TECTONIC FORCING OF MAGMATIC PROCESSES IN PANAMA: COLLISION, INTRA-ARC EXTENSION AND SLAB-DETACHMENT

Arc evolution in Panama was strongly influenced by changes in regional tectonic configuration. Subduction initiated at 73 Ma when the Farallon plate was thrust beneath the trailing edge of the Caribbean large igneous province (CLIP). From 73 to 40 Ma, the Panama arc produced a wide variety of magmatic products including both tholeiitic and calc-alkaline volcanism with compositions ranging from basalt to rhyolite. Crustal thickness increased from 10-15 km to 20-30 km via magmatic processes.

From 40- 30 Ma, magmatism significantly decreased. The reason for this is uncertain, but one potential mechanism is seamount accretion in the Azuero Peninsula.

In the Oligo-Miocene, magmatism increased once again. In western Panama this event produced dominantly calc-alkaline silicic volcanism characterized by increasing K₂O and higher La/Yb ratios. However, in the Canal basin localized extension led to hot dry bi-modal volcanism with flat REE curves. Our interpretation is that the isthmus fractured during collision with South America at 23-25 Ma producing a localized zone of extension at the apex of the Panama orocline, however throughout this period, northward directed subduction continued. Elevated Nd (εNd = +8) and Hf (εHf = +15) isotopic signatures from the Canal basin indicate an influx of juvenile mantle suggesting that the underlying slab also fractured during collision.

The last major change in the Panama arc occurred between 4-10 Ma with development of adakite-like volcanism in western Panama. The El Valle volcano is characterized by a transition from andesitic plag/cpx bearing volcanism to dacitic hbl bearing rocks with low HREE at 4 Ma. Also, the El Valle volcano has a large gravity low (-40 mgal anomaly) that modeling suggests is caused by a 30 km diameter, 5-10 km thick sub-caldera magma chamber.

Modern geophysical observations such as the lack of a Benioff zone, GPS plate motion vectors and gravity modeling indicate that northward directed subduction is no longer occurring beneath Panama today. Therefore we suggest that the change to adakite-like volcanism was caused by slab detachment and subsequent slab window formation. Interaction of the slab-window with older garnet and hornblende bearing arc rocks at the base of the Panamanian crust led to HREE depletions in rocks younger than 3-4 Ma.