TECTONIC CONSTRAINTS ON THE CLOSURE OF THE CENTRAL AMERICAN SEAWAY AND THE RISE OF THE ISTHMUS
Initial collision began at approximately 23-25 Ma. Evidence for this includes low temperature thermochronologic ages in both Panamanian and north Andean basement massifs. (U-Th)/He and fission track ages combined with simultaneous foreland basin sedimentation in the Colombian Eastern Cordillera strongly suggest the onset of collision and regional exhumation. Also at 23-25 Ma, the Canal region of Panama exhibits the onset of Isthmus perpendicular extension as indicated by extensional magmatism, basin formation and normal faulting. We interpret these observations as the Isthmus of Panama fracturing due to orocline formation and large-scale rigid-block rotation during initial collision with South America. Such extensional zones also have importance as potential persistent marine connections between the Caribbean and the Pacific.
Other significant tectonic events involved in the closure of the Central American seaway include: the end of northward directed subduction beneath Panama, subduction initiation beneath the north Panama deformed belt and the onset of Cocos ridge collision at 3 Ma. The youngest linear magnetic anomalies surrounding the Sandra rift are approximately 8-9 Ma. This age also corresponds with initiation of an enriched mantle signature and earliest onset of adakitic characteristics in western Panama volcanic rocks. Collectively these geochemical signatures can be interpreted to identify initial slab-window formation beneath western Panama. We suggest that the end of northward subduction and the formation of a slab-window can both be explained by a slab break-off event. A consequence of slab break-off would be isostatically driven regional uplift. We hypothesize that such an isostatic response coupled with continued tightening of the Panama orocline, and potentially the Cocos ridge collision caused complete closure of the marine Central American seaway by 3 Ma.