NEW AIRBORNE GEOPHYSICAL DATA ILLUMINATE STRUCTURAL FEATURES WITHIN PUERTO RICO, VIEQUES, CULEBRA, AND THE SURROUNDING SHELF

A new view of structural features of Puerto Rico and the surrounding islands and shelf is being provided by high-resolution airborne magnetic and radiometric data collected in 2023-2024. The survey, with 250-m line spacing and a nominal height of 120 m above ground covering a >250 km E-W and >70 km N-S area, represents the first modern, high-resolution airborne geophysical survey data collected over the region. The survey was collaboratively funded by the USGS Earth Mapping Resources Initiative, Earthquake Hazards, and Coastal and Marine Hazards and Resources Programs.

Puerto Rico and the surrounding region are subject to deformation associated with oblique subduction of the North American plate under the Caribbean plate, which has created complex structural fabric. Rates of shallow seismicity (< 10 km depth) are high and sometimes damaging, such as the 2019-ongoing sequence in SW Puerto Rico, demonstrating a need for updated fault mapping and hazard evaluation. To gain a better understanding of the interplay between existing structural fabric and modern deformation, including the likely reactivation of older faults, we analyze the aeromagnetic data in the context of lidar-derived topographic mapping over Quaternary sediments that were ground-truthed during field campaigns. We also use a new geologic map compilation to assist in the interpretation of the magnetic anomalies.

Well-known fault systems such as the Great Northern and Great Southern Puerto Rico fault zones (GNPRFZ and GSPRFZ, resp.) are prominent in the aeromagnetic data but appear more complex than previously mapped. For example, both fault zones terminate to the west as a series of curved magnetic lineaments, convex to the southwest, that resemble horsetail structures. The GNPRFZ extends farther east than previously mapped to an area southeast of Culebra and possibly past the survey area. The GSPRFZ exhibits a zone of NW-SE magnetic lineaments that is ~6-10 km wide and possibly wider in some areas. Lidar analyses find neotectonic deformation in the southern part of the GSPRFZ, but these faults strike E to ESE and are typically observed at terminations of the NW-SE magnetic lineaments. Similarly, further west in Lajas valley, neotectonic faults are oriented ENE to ESE. These observations illustrate temporal changes in the stress regime, with modern reactivations on trends that differ from older structures.