GRAVITY MODELING CONSTRAINTS ON THE GEOMETRY OF THE GATUN-CHAGRES BASIN AND TECTONIC EVOLUTION OF NORTH-CENTRAL PANAMA

Oligocene-Miocene collision between Panama and South America altered ocean currents, global climate, and species diversification. Panama Block intraplate deformation also plays a significant role in the connection between the Caribbean and Pacific, but is not well understood. A high-resolution gravity survey was conducted in north-central Panama to better constrain the processes responsible for the isthmus’ modern configuration.

110 gravity stations were collected from Colon to Nombre de Dios, and merged with existing data to produce 2.5D models of the Gatun-Chagres basin and other small basins in the region. Four transects from the Gatun-Chagres basin were modeled with varying sedimentary densities (1.8, 2.0, and 2.2 g/cm³) to produce a realistic range of basin thicknesses. The mean density of collected samples is 1.75 g/cm³, but models with ρ= 2.0 g/cm³ for the entire basin are most consistent with offshore seismic profiles, suggesting basin thickness is ~3.5 km.

Previous seismic reflection data and geochemical analyses of Miocene arc volcanic rocks suggest a zone of extension in the Panama Canal region, and our gravity analysis supports this hypothesis. Field evidence of multiple NW-facing normal faults separate the basin from uplifted arc rocks east of the Canal, resulting in a 60 mgal gravity gradient. Beneath the basin, gravity models indicate ~5-10 km of crustal thinning. 3D reconstruction of the 2.5D models show a northward thickening basin and two depocenters that correspond to the Rio Indio and Toro facies of the Chagres Formation. Overall, our analysis suggests two directions of extension during the formation of the Gatun-Chagres basin; an east-west direction corresponding to the initial formation of the basin, and a modern direction related to the Rio Gatun fault.

To the north, previously unmapped Eocene-younger sedimentary basins near Nombre de Dios were also observed. The western part of these rocks exhibited large-scale open folds, which may indicate a transition from extensional to compressional tectonics east of the Panama orocline apex.

Collision with South America formed the Panama Isthmus, and during this process the crust fractured and created zones of extension in central Panama. This is consistent with the idea that the Panama Canal was a young connection between the Caribbean-Pacific Seas.