EXTENSION DURING FORMATION OF THE PANAMA OROCLINE AND COLLISION WITH SOUTH AMERICA

The Isthmus of Panama is interpreted to be the result of ongoing collision between the Panama block and South America. By 3.5 Ma the Isthmus fully separated the Caribbean and Pacific Ocean, however this age is based on divergent evolution of Caribbean and Pacific marine life, and therefore must be a minimum age. It is suggested that collision between Panama arc crust and South America began much earlier at 23-25 Ma and that extensional tectonics played a key role in the formation of the Panama orocline.

In the Panama Canal region of central Panama, numerous observations support the idea of an arc perpendicular extensional zone that formed simultaneously with the collision of South America. At 23-25 Ma Canal Zone volcanic rocks underwent a petrologic transition consistent with the onset of extension. Oligocene and older volcanic rocks are hornblende bearing, have a large Ta anomaly, are enriched in fluid mobile LILE’s, and have moderate HREE concentrations. Such characteristics are indicative of hydrous mantle wedge derived subduction zone magmas. In comparison, Miocene Canal Zone volcanism exhibits low LILE’s, higher HREE’s and Ti, and a significantly decreased Ta anomaly. It is also bimodal, lacks hydrous minerals, and plots within extensional zones on trace element tectonic discrimination diagrams. The above observations of Canal Zone volcanic rocks suggest a transition to an extensional tectonic environment at 23-25 Ma.

In addition, detailed structural geologic mapping along the Panama Canal indicates the above volcanic rocks intrude into and are deposited within a series of extensional grabens. Recent excavations and drill core constraints indicate such grabens are bounded by normal faults with displacements on the order of several hundred meters, and there are many such faults. In addition, Canal perpendicular gravity transects suggest a series of much larger half graben structures that extend to the west and are also oriented perpendicular to the arc itself.

Finally, low temperature thermochronology from basement massifs in both Panama and the northern Andes indicate a synchronous exhumation pulse also at 23-25 Ma. Overall, South America initially collided with Panama arc crust at 23-25 Ma, and in response the Panama arc fractured leading to observed extensional zones and formation of the Panama orocline.