Formation of the Panama Isthmus, that had global oceanographic and biotic effects in the Neogene, is generally associated with tectonic uplift during collision of the Panama volcanic arc with South America. However, new field, geochemical and geochronological data from the Culebra Cut of the Panama Canal suggest that volcanism also contributed to the Isthmus emergence in the Early Miocene. This volcanism is recorded in a newly-recognised Central Panama volcanic field that includes several phases of development. Early activity of this field along the Panama Canal was associated with proximal effusive to explosive felsic products during formation of subaerial stratovolcanoes and possible domes ca. 21 Ma. This was followed by a period of marine transgression ca. 21–18 Ma, with more distal volcanism documented by tuffs that deposited in marine to terrestrial environments. Finally, proximal mafic volcanism formed tephra cones in a monogenetic field ca. 18(-?) Ma. This was associated with phreatomagmatic processes in a coastal environment, with remarkable kilometre-wide subvolcanic peperitic intrusions. We propose based on these observations that formation of the Central Panama volcanic field was critical in shaping regional topography, and that this could have actively contributed to obstruction and closure of an interoceanic strait in Central Panama.

The Panama Canal area is a significant part of the Panama Isthmus, where prominent volcanic fronts of eastern Central America are interrupted by a topographic low of unclear tectonic and magmatic origin. Determining why no prominent volcanic system occurs along the Canal is essential to understand the formation of the Isthmus in an area believed to have hosted one of the last inter-American straits between the Atlantic and Pacific Oceans. We provide here new geochronological and geochemical constraints from volcanic units of the Panama Canal that belong to the recently-identified Central Panama Volcanic Field. Whole rock and mineralogical geochemical compositions document a secular magmatic change from ca. 25 to 16Ma, with a progressive change from calcalkaline to tholeiitic and possibly alkaline/transitional geochemical affinities. The age of the youngest volcanic unit of the Canal is similar to that of the youngest documented arc volcanism in eastern Panama ca. 18 Ma. We propose based on these observations and consistency with regional geological constraints that the Canal volcanism represents a unique example of magmatic cessation along a volcanic arc. This cessation occurred shortly after the breakup of the Farallon plate ca. 23 Ma, suggesting a causal link between volcanic shutdown and transition from orthogonal to oblique subduction along Central and Eastern Panama. We suggest that this tectonic event suppressed hydrous melting in the subduction zone and led to regional magmatic waning in Central and Eastern Panama ca. 16Ma. The pre-existence of a transisthmian fault system in Central Panama probably facilitated extraction of the last supra-subduction melts during volcanic shutdown. Our results suggest that the end of volcanism, combined with transisthmian faulting, impeded the development of high topography in the Panama Canal area. Without this unusual tectono-magmatic evolution, the occurrence of a late inter-oceanic strait in Central Panama and the construction of the Panama Canal would not have been possible.

Borinquen Dam 1E is part of the new Pacific Access Channel (PAC) of the Panama Canal Expansion. The 2.3-km-long zoned rockfill dam forms the navigational channel providing navigation access from the Gaillard Cut to the new Post-Panamax Pacific Locks. A key geologic objective during construction was to confirm locations and activity of faults mapped at the dam during design, namely the Pedro Miguel Fault (PMF) and its suspected newly mapped “main trace.” The design allowed for core and filter widening at the anticipated location of the PMF at the south abutment and at a west branch of the PMF (believed to be the main active trace of the fault) mapped along the dam axis about one-third of the way north from the south abutment. As-built geologic mapping revealed complex faulting associated with the PMF crossing the southeast half of the foundation, the PAC, and the nearby Dam 1W foundation along a north-south trend. Trenching and age dating of alluvium overlying the faults crossing the Dam 1E foundation and overlying the PMF at Dam 1W indicated the unfaulted alluvium was latest Pleistocene to early Holocene age. At Dam 1E, the core and filters were widened to accommodate potential fault rupture on the PMF and a previously unrecognized fault revealed across the width of the dam foundation. The west branch of the PMF (trenched and mapped during design investigations) was determined to not exist at Dam 1E based on mapping the dam foundation and other extensive excavations created for the PAC.

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