EVOLUTION OF THE SOUTHERN PANAMA CANAL BASIN: GRAVITY MODELING AND VOLCANIC ARC GEOCHEMISTRY

South America collided with the Panama arc in the late Oligocene/early Miocene, fracturing the Panama block, inducing zones of extension and contraction, and ultimately resulting in the current tectonic configuration of Panama. During the Miocene, the Panama Canal Basin (PCB) lay within such a zone of extension. Evidence for extension throughout the basin comes in the form of abundant normal faults, a series of horsts and grabens buried throughout the basin, and volcanic rocks with extensional geochemical signatures that resulted from dry shallow melt generation.

Although outcrops in the western PCB are dominated by Miocene lava flows and pyroclastic rocks, gravity models support a subsurface that is dominated by lower density material equivalent to that of sedimentary rocks. Bouguer gravity anomaly transects through the PCB were modeled at crustal scales and support the existence of a 2-3 km thick sedimentary basin that contains a series of horsts and grabens and is capped by relatively thin terrestrial volcanic rocks. Overall, the gravity models suggest at least 5 km of westward crustal thinning across the PCB. Additionally, the models show the existence of continuous sedimentary basins across the isthmus, which allows for the existence of a narrow strait connecting the Caribbean and Pacific during this time as proposed by Collins et al., (1996).

Miocene lavas and pyroclastic rocks from the southern PCB have been analyzed for major and trace element geochemistry, as well as, Pb, Hf, Sr, and Nd isotope ratios. Major element chemistry shows that these volcanic rocks vary from basalt to dacite in rock type and are tholeiitic to calc-alkaline in composition. The Miocene volcanic rocks exhibit both MORB-like and arc-like characteristics on trace element tectonic discrimination diagrams. Nd and Hf isotope ratios for southern PCB rocks are the highest in all of Panama with εNd values of 8-9. Overall, geochemistry suggests the southern PCB volcanic rocks formed due to an influx of young juvenile mantle in an extensional tectonic system.