DETERMINING THE SPATIAL AND TEMPORAL EXTENT OF WESTERN PANAMA CANAL BASIN EXTENSIONAL VOLCANIC ARC ROCKS
Previous work has divided Panama arc activity into three episodes, the depleted Late Cretaceous to Eocene episode, the enriched Miocene episode, and the Pliocene to recent adakite suite of western Panama. Rocks from the oldest episode are dominantly hornblende bearing, contain a large negative Ta anomaly, and exhibit enrichment of large-ion lithophile elements. These characteristics are all indicators that the Late Cretaceous to Eocene episode rocks were derived from a hydrous mantle wedge subduction zone magma. In contrast to abundant calc-alkaline rocks throughout the Panama arc, Miocene episode rocks from the Canal Zone are strongly tholeiitic. Miocene Canal Zone rocks also lack hydrous minerals and previous work suggests they formed due to shallow melt generation via decompression melting caused by extensional forces.
The boundary for this extensional zone is projected to be found in the Western Canal Basin. Its existence will support the theory that the late Oligocene collision between South America and Panama arc crust caused fracturing and rotation within Panama, creating zones of contraction and extension that ultimately resulted in the geochemical signatures observed. Field observations show a contact between a basalt unit, geographically continuous from the Canal through the Western Canal Basin, and an andesite unit, possibly an equivalent of the Llano Tigre formation. This contact is likely the western boundary of the extensional zone, although whole rock geochemical analyses will determine the spatial and temporal extent of these depleted, extensional, arc rocks. We are still processing samples for whole rock geochemistry, but in thin section, the basalts in the Western Canal Basin are similar in composition to the Miocene Canal Zone basalts. Additionally, extension in the Canal Basin is also supported by geologic mapping and gravity data; which has been modeled as a series of normal fault bounded half grabens that are either filled with sediments or volcanics and interrupted by horst blocks.