RELICT CARIBBEAN TOPOGRAPHY AND TECTONICALLY TRIGGERED EROSION RATES IN THE CORDILLERA CENTRAL OF PUERTO RICO

Puerto Rico lies along the complex Caribbean-North American tectonic plate boundary. Past and ongoing tectonism have shaped the juvenile topographic landscape of the island. A thick carbonate sequence was deposited between 30 – 5 Ma on the Caguana relict erosional surface. At about 4 Ma, subsidence gave way to a new stage of uplift that persists to the modern day. This shift is marked by the Barranquitas-St. John erosional surface, which is preserved as scattered low-relief upland plateau areas across the island. The remaining vestiges of these relict surfaces have suffered piracy and loss by neighboring fluvial basins where erosion driven by the 4 Ma uplift initiation has propagated up system.

The objective of this study is to quantify the distribution of erosion rates on these relict surfaces and surrounding areas in the central region of the island in order to better model drainage basin evolution. The results of ¹⁰Be analyses in the target regions reveal that erosion is slowest on the Caguana surface. The Barranquitas-St. John surface in the vicinity of Cerro Punta, the island’s highest point, yields erosion rates that are approximately double than those of the Caguana surface. The highest erosion rates are recorded in the surrounding landscape below prominent knickzones that has adjusted to relative base level lowering. The results indicate that the two relict surfaces are distinct. As proposed by previous workers, the Barranquitas-St. John surface may have developed around an archipelago of paleo islands that now represent the highest peaks in Puerto Rico.

Propagation of the adjusted landscape into the preserved relict reaches will likely be achieved via hillslope mass movement processes, given limited stream power near the upstream critical area threshold zones. χ analysis and forward modeling across the island is helpful to visualize how ongoing landscape evolution will continue to shape the terrain and transform fluvial basins.