NEW HIGH-RESOLUTION AIRBORNE MAGNETIC AND RADIOMETRIC DATA OVER PUERTO RICO, VIEQUES, CULEBRA, AND THE SURROUNDING SHELF

Puerto Rico, originally part of a Cretaceous-Early Cenozoic island arc system, now lies within a complex deformation zone associated with oblique subduction of the North American plate under the Caribbean plate. Shallow (< 10 km depth) seismicity is frequent and includes damaging earthquakes such as the 2019-ongoing seismic sequence (up to Mw6.4). Numerous faults have been mapped across Puerto Rico but continuity and context are difficult to determine because of vegetation and/or development. Similar challenges exist for geologic mapping.

In a joint effort between the USGS Earth Mapping Resources Initiative, Earthquake Hazards Program, and Coastal and Marine Hazards and Resources Program, a new, high-resolution airborne magnetic/radiometric survey is being flown over Puerto Rico and surrounding areas. At the time of this abstract, field data collection was ~90% complete. Complementing this effort are a new geologic map compilation and lidar-derived topography analyses + field campaigns to identify neotectonic features.

The new magnetic data generally show highest magnetic values over serpentinite and basalt and lower values over more felsic and sedimentary rocks, but there are numerous local variations. Over areas with thick carbonates, magnetic anomalies are smoother and more subdued due to deeper sources. In some areas, lithologic correlations are less clear, suggesting mineralogical variations, unmapped units, or layered geology.

The magnetic data show distinct but complex lineaments over known fault zones and other areas where faults have not previously been mapped. ENE- to ESE-striking faults interpreted via lidar topography are also associated with aeromagnetic lineaments.

The radiometric data more closely correlate with mapped lithologies, including higher K+Th over granodiorite and felsic volcanic rocks, low Th+K over serpentinite and intermediate volcanic rocks, and elevated Th/low K over some mafic volcanic rocks. Carbonate rocks along the northern coast of Puerto Rico show elevated Th/low K except over stream valleys extending from inland mountains to the north shore, where high K/mixed Th likely indicates sediments eroded from rocks in the interior mountains. Quaternary sediments near the southern coast show broad regions of elevated K, suggesting a different depositional history.