Stress State of the Ilan ChinShui Geothermal Area, NE Taiwan

Understanding the in-situ stress state is one of important keys to evaluate the geothermal resource and further develop enhanced geothermal system. The Ilan ChinShui geothermal area within the western slate belt of NE Taiwan has been developed over decades; however, the current stress state is still unclear because of the complex evolution history of NE Taiwan from collision to back-arc extension. In this work, the multiscale stress state is evaluated for better understanding the geothermal resource regionally. Stress inversion of focal mechanism in NE Taiwan shows that the stress regime changes from thrusting of NW compression in the south through strike-slip movement to normal faulting in the center of Okinawa trough in the north. From the surface outcrops, 3-stage faultslip data around the geothermal area is collected. They postdated slaty cleavage, were either slickensides or quartz mineral steps, and no carbonate filling was observed. It is thrusting stress regime for all stages with compression direction from NE, through NW back to NE in younging order. On the other hand, core observations display that carbonates filled most of fractures with various orientations. The last-stage fracture strikes N50W-EW and dips subvertically to the south and is almost parallel to the slaty cleavage. Carbonate crystals are observed in the last-stage fracture, indicating this open fracture could be the geothermal fluid conduit. Left-lateral carbonate step is also found in the last-stage fracture. Analysis of Anelastic Strain Recovery (ASR) from retrieved cores shows a thrusting regime with compression almost parallel to slaty cleavage strike in the present. The consistent results between the compression direction derived from ASR and left lateral shear of the open fracture thus provide us a good explanation between in-situ stress state and fluid conduit in the Ilan ChinShui geothermal area.