Paper No. 6
Presentation Time: 9:15 AM
HOW OLD IS THE ISTHMUS OF PANAMA?
COATES, Anthony G., Smithsonian Tropical Research Institute, STRI Unit 9100 Box 0984, DPO AA, 34002-9998, Panama and STALLARD, Robert, Dept. of the Interior, U.S.Geological Survey, National research Center, and Smithsonian Tropical Research Institute, Panama, STRI Unit 9100, Box 0948, DPO AA, 34002-9998, coatesag@si.edu
The standard model of about 3Ma for closure of the Isthmus of Panama is based on stratigraphic analyses of adjacent marine sediments, proxies for predicted changes in temperature, salinity, and productivity, trans-isthmian faunal differences and similarities, genetic patterns in living Isthmian faunas and the terrestrial vertebrate record (Jackson and Odea, this symposium). A new model proposes closure at 15 Ma calculated by using the Campanian to Eocene arc that stretches from Costa Rica to Colombia as a strain marker. The rotation and alignment of its component tectonic blocks is said to leave no space available for any oceanic connection after 15Ma, but no evidence is provided in the model about emergence above sea level. Comparison of the two models shows a very similar geometry and chronology, differing mostly in the degree of submergence of the Central American Arc (CAA). We suggest that the scale, tectonic style, volcanism, oceanic setting, and biogeographic contrasts (the Wallace/Huxley/Lydekker Lines) of the Indonesian volcanic arc are similar to the Caribbean-Central American region, 15 million years ago, and explain why, despite proximity of the CAA to South America, final closure was delayed until 3Ma.
The contemporary boundaries of the Wallace / Huxley Lines are as narrow as 20 km during low sea-level stands between Bali and Lombok islands, and the Lydekker Line, demarcating the limit of Australian ground-dwelling land mammals, is as narrow as 40 km between Timor and Australia. Despite the narrowness of these gaps, deep Pacific Ocean water circulates into the Indian Ocean through several 1000-m-deep sills within the gaps. The net flow is about 10 million cubic meters per second (one/third the Gulf Stream in the Florida Straight), and establishes the deeper water-column composition throughout the region. There is too much error in paleogeographic reconstructions for the new model to rule out the presence of such 20 km-to-40 km gaps with deep sills between tectonic blocks. We suggest the Canal axis and the Darien-Chocó gap provide Bali-Lombok (Wallace Line) analogues and the Choco-South America region a Lydekker Line analogue. This scenario also explains the continued marine connections and lack of intense terrestrial mixing between 15 and 3 Ma, as observed in the fossil record.
