GRAVITY CONSTRAINTS ON THE PANAMANIAN GATUN-CHAGRES BASIN AND ITS POTENTIAL AS A YOUNG MARINE CONNECTION BETWEEN THE PACIFIC AND CARIBBEAN SEAS

The Late Miocene Gatun (11.4-8.6 Ma) and Chagres (6.4-5.8 Ma) Formations are the youngest marine sedimentary rocks in the Panama Canal region, and therefore place constraints on timing and processes related to closure of the Isthmus of Panama. The Gatun-Chagres basin is characterized by a large ≈50 mgal Bouguer gravity anomaly gradient along its eastern margin. This large gravity gradient extends along the southern margin of the basin as well, but lessens in size to ≈20 mgal. Modeling suggests the gravity gradient can be explained by a large NW facing normal fault with ≈2 km of displacement. The up-thrown block of the normal fault contains higher density arc basement rocks.

Internal gravity variations within the wedge-shaped Gatun-Chagres basin indicate smoothly varying Bouguer gravity anomaly gradients of ≈20 mgal. The spatial pattern of these anomalies is of two main gravity lows west of Colon and a third smaller low to the east. We suggest these lows represent the deepest part of sub-basins within the Gatun and Chagres Formations. In addition, the two gravity lows west of Colon correspond to outcrop locations of the deep water Toro Member and the shallow water Rio Indio Facies of the Chagres Formation, respectively. The gravity low centered on the Toro Member is ≈4 mgal lower than the Rio Indio gravity low suggesting a thicker basin at this location.

Gravity data does not uniquely provide support for or against the existence of a cross-Isthmian strait in the Panama Canal region, but provides additional information. A gravity high, that corresponds to higher density arc crust extends around the Gatun-Chagres basin on the west, south, and east sides. Higher density arc crust is likely to have formed a topographic barrier in the past, diminishing the probability of a strait. However, south of the basin, the arc crust gravity high is substantially lessened, suggesting the arc basement has been thinned as part of the larger Panama Canal extensional region. This observation supports the possibility of a strait. Overall, the shape of the Gatun-Chagres basin coupled with evidence of extended arc basement suggests the Canal region is the best location for a Late Miocene cross-Isthmian strait in central Panama, but structural evidence of a particularly deep (500 m) channel is not present.