MIDDLE MIOCENE FULL CLOSURE OF THE CENTRAL AMERICAN SEAWAY (Invited Presentation)

It is commonly accepted that final closure of the Panama Isthmus and permanent separation of Caribbean and Pacific waters must have severely modified their seawater isotopic composition, salinity, faunistic assemblages, and ultimately, ocean circulation patterns and Pliocene global climate resulting in northern hemisphere glaciation. A highly asymmetric correlation has been established here: a very well dated and studied event (onset of glaciation) has been correlated to a very poorly constrained one (geographic closure). Dating time of geographic closure is complex here because of the potential subtleties of shallow marine connections across an already tectonically closed Isthmus. However, new geologic data on both sides of the suture separating northwestern South America from the Isthmus, sheds light about timing of tectonic closure. We are developing three main arguments: 1) a space compatibility argument that consists in more accurately reconstructing the paleogeography of blocks so that we obtain the time when no space is available for deep marine connections; 2) a provenance argument, where the suture separates two mutually exclusive geochronological domains: the young Central American arc to the west from the old Andean terranes to the east; and 3) a paleomagnetic argument where vertical-axis rotations constrain the kinematic evolution of the tectonic blocks involved. We have found an uniquely Panamanian, 40-45 Ma (early Lutetian) detrital zircon fingerprint is present in middle Miocene strata of northwestern South America, but absent in underlying lower Miocene and Oligocene strata. We contend that this fingerprint represents the beginning of fluvial drainage exchange between the Panama block and South America, and therefore marks the time of docking and full seaway closure. Paleomagnetic data shows large vertical-axis rotations in most blocks involved in closure of the Isthmus, and help constrain gaps available for deep marine connections.