Structural and Paleomagnetic Analysis of Geothermal Drill Core, Akutan Alaska

Abstract

Hot Springs Bay Valley (HSBV) geothermal resource area on Akutan Island, Alaska, has increased fluid output and temperature by almost a magnitude, between 1981 and 2012 (Bergfeld et al., 2014). These increases have been attributed to increased permeability along NW-SE trending faults that may have been activated during a seismic swarm in 1996. In 2010 two unoriented drill cores were collected in Hot Springs Bay geothermal resource area. In this study we reorient sections from one the highly fractured core with paleomagnetics to test this model of geothermal reservoir evolution at Akutan. The core is composed of interlayered island arc volcanics and shallow marine mudstones. Paleomagnetic plug samples were drilled from the volcanics. Continuous sections of the core were reoriented using the declination of the remnant magnetization (RM) as an indication of geographic north. Structure from motion software was used to create digital reconstructions of the core from photographs. The resulting models were then flattened with a cylindrical projection into a 2D depiction of the outside of the core. The orientation of veins and the relationship between vein sets were measured from these projections. Results show that NNW trending, ENE dipping veins dominate the system and show evidence of sustained fluid flow, whereas veins of other orientations show single event fluid flow. Further, preliminary cross-cutting relationships suggest earlier fluid flow was primarily hosted by N-trending, E-dipping fractures and that more recent activity has shifted some of this fluid flow to SE-trending, SW-dipping structures. These younger structures are parallel to surface ruptures observed from the 1996 seismic swarm. This supports the hypothesis of Bergfeld, et al., (2014) that rupture along a SE trending fault during the 1996 seismic swarm contributed to the permeability of the system.