LINKING AMERICAS BACKBONE: GEOLOGICAL DEVELOPMENT AND BASEMENT ROCKS OF CENTRAL PANAMA

Central Panama is a strongly deformed amalgamation of (i) mafic complexes of late Cretaceous to Tertiary age, (ii) submarine tholeiitic arc intrusives and volcanics, and (iii) younger tholeiitic to calk-alkaline subduction-related volcanic edifices (17 to 5 Ma). The volcanics are intercalated with marine sediments. The mafic Chagres Igneous Complex (CHICO, Cretaceous to Upper Tertiary) forms the igneous basement in the Central Panama land bridge. The CHICO is characterized by intrusive rocks and highly altered submarine volcanics and minor volcaniclastic sediments. CHICO rocks comprise highly deformed basalts and basaltic andesites, gabbros and diorites with associated with dike swarms, and more compositionally evolved granodiorites, tonalities, and plagiogranites. Based upon exposed lithologies and stratigraphic relations, the CHICO is interpreted to represent a series of mostly submarine volcanic centers and associated sheet flow/dike complexes, magma chamber intrusions, and marginal volcaniclastic aprons. Dominance of sheet flows suggests high rates of effusion. Rare oxidized scoria are evidence for rare subaerial eruption. These rocks are compared to the 139 to 69 Ma old "CLIP" basement of Central America, which was from linked to the Galapagos plume [Hoernle et al, 2004]. Trace elements in CHICO igneous rocks show that they were derived from a Cretaceous to Early Tertiary (69 to 66 Ma) subduction system (La/Nb > 6). Thus, CHICO marks the timing for the onset of arc magmatism at the Caribbean plate margin. By contrast, mafic submarine rocks from the Azuero Peninsula exhibit mostly CLIP-type trace element patterns (La/Nb <1). Younger volcanics (17 to 5m Ma) in Central Panama are low to medium–K arc- tholeiites. Late Tertiary marine and deltaic sediments suggests that the CHICO evolved an island archipelago and that Rio Chagres initially drained to the WSW 3 Ma ago. At Gamboa, where the river initially discharged, it now makes a sudden NNW turn towards the Caribbean. This was caused by collision of ridges with the volcanic islands, causing uplift and emergence, and closure of the seaway. The new land bridge had profound consequences on flora and fauna, altered ocean circulation, affected global climate [Haug et al., 2001], and influenced history from the Spanish Conquest to building of the Panama Canal.