PANAMA ARC MAGMATISM AND THE EVOLUTION OF THE CANAL EXTENSIONAL ZONE

The Panama Canal extensional zone formed due to fracturing of the Isthmus of Panama during collision with South America at 21-25 Ma, and is characterized by: 1) Extensional arc magmatism, 2) Sedimentary basins, and 3) Normal faulting. Geologic mapping and geochemical studies of Canal region volcanic rocks indicate a change across the Oligocene-Miocene boundary from hydrous mantle-wedge-derived basaltic pyroclastic deposits to hot, dry and bi-modal magmatism. Miocene volcanic rocks occur as inward dipping maar-diatreme pyroclastic pipes, large basaltic sills, and bedded silicic ignimbrites / tuff deposits. Such pyroclastic pipes and basaltic sills are not usually found in volcanic arcs, but are more common in extensional and large igneous province environments.

In terms of geochemical change, the Oligocene Bas Obispo Fm. rocks have large negative Nb-Ta anomalies, low HREE, are relatively enriched in fluid mobile elements, and plot as arc tholeiites on tectonic discrimination diagrams. In contrast, the Miocene Pedro Miguel and Late Basalt Fm. have a more subtle Nb-Ta anomaly, exhibit flattened REE curves, are depleted in fluid mobile elements, and plot in mid-ocean ridge/back-arc basin tectonic discrimination fields. Major and trace element modeling of Miocene rocks indicates shallow (0.5-0.1 kbar) crystallization levels of hot (1100-1190° C) magmas in which most compositional diversity can be explained by (F=0.5) of fractional crystallization. Overall, changes in volcanic edifice form and arc geochemistry are related to the onset of arc perpendicular extension.

New gravity data indicate that the Canal extensional zone extends from Lake Madden in the east to El Valle in the west with multiple fault-bounded sub-basins. Extension was active from 23-5 Ma and can account for sedimentary units such as the Culebra and Cucaracha Fm. in the south and the Gatun and Chagres Fm. in the north. After 5 Ma the local tectonic regime transitioned to strike-slip faulting. Gravity modeling suggests that Canal extensional zone crust was thinned by 5 km via 3-4 half-graben systems. Isostatic calculations indicate that such crustal thinning will be associated with a 1 km decrease in surface elevation. Therefore, the Canal extensional zone has important implications in terms of a connection between the Caribbean and Pacific Oceans.