Geology of the Isthmus of Panama, onshore synthesis and offshore implications

Abstract

A review of the bedrock geology of the Isthmus of Panama highlights tectonic deformation –tilting, bending and breaking–, as the three major controlling factors in the sites and modes of onshore and offshore Cenozoic sedimentation. Several years of geologic mapping and analytical efforts that include magmatic and detritic U/Pb zircon geochronology, low-temperature thermochronology, provenance analyses, paleomagnetic and gravity surveys, and detailed geologic mapping, are helping delineate a more complete understanding of the geology of the Isthmus of Panama. This synthesis highlights the role of tectonic deformation in basin development and sedimentation style/timing, both onshore, and offshore. Deformation of the Isthmus of Panama in Paleocene-early Eocene times folded and faulted a basement complex composed of plateau basalts, pelagic and hemipelagic sequences, and an overprinted magmatic arc. This basement complex was the end result of Campanian-Eocene intraoceanic arc construction over the thickened southern Caribbean plate boundary. This deformation episode segmented, folded and faulted the Isthmus, bringing about subaerial exposure, fast basement cooling, and erosion of the plutonic bodies –mosltly granodioritic– that make up the roots of the Campainan to Eocene arc. A siliciclastic-carbonate, less deformed, upper Eocene and younger sedimentary sequence non-conformably overlaps this basement complex in the Chucunaque-Atrato, Canal, and Azuero basins. A simlar timing, and sediment composition is expected in the North Panama Deformed Belt (NPDB). This deformation episode detached the eastern portion of the Isthmus (the Panama-Choco block) from the Caribbean plate and caused initiation of incipient subduction/overthrusting of Caribbean plate under the eastern Panama/Choco block. Southward subduction/overthursting caused southward tilting of the Isthmus, which controlled the accumulation of a northward pinching clastic wedge in the Chucunaque-Atrato basin, recording first shallow marine depostional enviroments, followed by basin deepening as tilting increased, and then by shoaling as the basin eventually filled with the siliciclastics derived from the San Blas range to the north. This sequence therefore resulted from basin tilting that simultaneously raised the San Blas Range –eroding it– while deepening the axis of the Chucunaque Basin. Bending of the Isthmus took pace as the Isthmus was being detached from the edge of the Caribbean plate, and marked the start of left-lateral offset of the Isthmus in late Eocene times, opening the Canal basin and tightening the NPDB. Recognition that the Isthmus of Panama is not a sill that passively shoaled from the depths of the ocean to reach the surface and create a bridge, should improve our understanding of the dynamic nature of this crucial region and the basins associated to it.

AAPG Datapages/Search and Discovery Article #90330©2018 AAPG Hedberg: Geology of Middle America – the Gulf of Mexico, Yucatan, Caribbean, Grenada and Tobago Basins and Their Margins “Integration of the geology of the area from the southern onshore of North America to the northern onshore of South America”, Siguenza, Guadalajara, Spain, July 2-5, 2018