TECTONIC EVOLUTION OF THE ISTHMUS OF PANAMA: THERMOCHRONOLOGIC EVIDENCE FOR LATE MIOCENE ISTHMUS UPLIFT FROM CENOZOIC SEDIMENTS, SOUTHWESTERN PANAMA

Low temperature thermochronologic data from Cenozoic sediments in southwestern Panama provide insight into the timing of closure of the Central American Seaway, which had global repercussions for land mammal migration patterns, paleoclimate, and paleoceanography. Previous studies from multiple disciplines have placed the time of closure of the isthmus anywhere from 3.5 to 10 Ma. Recent thermochronology results from Cenozoic sediments of southwester Panama reveal a period of late-stage cooling thought to be associated with erosional exhumation driven by tectonic closure of the Isthmus during Miocene to Pliocene time; 9 to >2 Ma. This study presents apatite U-Th/He ages from calcareous sandstone and conglomerate of the Eocene (?) Tonosi Formation as well as lithic sandstones of the Middle Eocene Covachon Formation. These new data yield cooling ages that average about 9 Ma, but yield single grains with cooling ages <2 Ma. This suggests that Tonosi Penninsula was above sea level between 9 and <2 Ma, and has implications for the timing of Isthmus of Panama closure. Additionally, U-Th/He ages of zircon from the Tonosi Formation yield ages of 23.3- 33.9 Ma and ~27.7-30.1 Ma, indicating an Oligo-Miocene minimum depositional age, which is younger than the previous estimate of Eocene.