DEMISE OF ARC MAGMATISM ALONG THE PANAMA CANAL

Panama exhibits a history of magmatic activity that extends from Cretaceous to present. In the past, arc volcanism occurred throughout Panama, however today active volcanoes exist only in western Panama (e.g. El Baru and La Yeguada), and produce adakitic lavas. Modern magmatism in eastern Panama is unknown. The Panama canal bisects these two regions and recent excavations along it have allowed for detailed study of volcanic rocks.

Exposed within the Panama Canal Culebra Cut is a sequence of Oligocene to Miocene volcanic rocks, which young from north (Gamboa) to south (Panama City). The oldest unit is the Oligocene Bas Obispo Fm., composed of a thick sequence of welded mafic tuffs. The next youngest unit is the more felsic >23 Ma Las Cascadas Fm., composed of dacitic tuffs and ropey andesitic lava. Both the Bas Obispo and the Las Cascadas exhibit transitional tholeiitic to calc-alkaline trends and are interpreted to be mantle-wedge derived arc magmas. Above the Las Cascadas Fm. exist two sedimentary dominated units, oldest is the marine Culebra Fm. and is overlain by the terrestrial Cucaracha Fm. Atop the Cucaracha Fm. is the Pedro Miguel Fm. The Pedro Miguel Fm. is < 17 Ma (detrital zircons U/Pb) and composed of thick (>100m) welded mafic pyroclastic deposits, interbedded lava flows and ash beds with a defined stratigraphy that are interpreted to be fragments of a larger volcanic ediface. The final volcanic units are Late Basaltic rocks (and within Panama City, dacite plugs (Ancon Hill)) that sit on top of and intrude through the Pedro Miguel Fm. The Late Basalt and associated dacite exhibit a transitional alkaline signature, are the youngest volcanic rocks in the Canal and represent the cessation of magmatic activity in the region.

The end of magmatic activity within the Canal Zone is associated with significant normal and strike-slip faulting and is temporally similar to the initiation of the collision of South America with the Panama block. Extensional faulting observed in the Canal Zone is not widespread throughout Panama. One potential explanation is during the initial collision with South America, the Panama block fractured leading to localized regions of extension, crustal thinning and alkalic magmatism.