ACTIVE TECTONICS WITHIN PUERTO RICO: IDENTIFYING AREAS OF ACTIVE UPLIFT USING REGIONAL GEOMORPHIC INDICES

The island of Puerto Rico in the northern Caribbean covers an area of about 14,000 km2 and is 180 km long and 65 km wide and is densely populated by 3.4 million persons. The island is mountainous with the east-west-trending, Cordillera Central with its highest point of 1338 m in the geographic center of the island. Previous workers have suggested that the origin of the Central Cordillera is linked to folding and active uplift of a large anticline whose fold axis coincides with the topographic axis of the Cordillera Central. To test this hypothesis, I have calculated geomorphic indices for 21 different watersheds of the island that contain about 50 river systems. The geomorphic indices include a Hypsometric Integral and a Stream Length Gradient Index as defined by Strahler (1952) and Hack (1973); both indices are used to identify areas of active tectonic uplift based on stream and watershed behaviors. The Hack index and Hypsometric Integral calculated from a DEM of Puerto Rico show that the most active area of uplift is located in the central and north-central parts of the island that include about one half of the length of the arch as mapped on the island. The two topographically-elevated ends of the arch in the western and eastern parts of the island are less active based on these indices. I have also used the DEM to calculate Channel Steepness Index values for the entire island that agree with the previous two indices by showing that the north part of the island is the most active area. Two possible explanations are proposed to explain the three uplift indices: 1) the topographic axis of the island has retreated southward as a result of increased precipitation along its northern flank and therefore no longer coincides with the structural fold axis now in the north-central area of active uplift; and 2) the uplift is not directly related to the fold axis and there is a sub-crustal mechanism related to the two subducted slabs at depth beneath the island.