Structural Patterns and Transitional States of Stress at an Active Triple Junction: the Hengill Region
Ashley STANTON-YONGE, Catalina SANCHEZ-ROA, Thomas M. MITCHELL, Sandra OSK SNÆBJÖRNSDÓTTIR, Philip M. MEREDITH
[, United Kingdom]
The Hengill region is one of the largest high temperature areas in Iceland and hosts its two major geothermal power plants: the Hellisheidi and the Nesjavellir plants. This area is emplaced at an active triple junction formed by two rift plate boundary segments; the highly obliquely spreading rift of the Reykjanes Peninsula (RP) and the Western Volcanic Zone (WVZ), plus the left-lateral, transform of the South Iceland Seismic Zone (SISZ). Each of these plate boundary segments exhibit a distinct structural style in terms of fracture patterns and local stress field. They converge in the Hengill area producing a complex structural configuration and providing a unique opportunity to study triple junction dynamics. This study aims to produce a detailed structural characterization of the Hengill region to determine the spatial variation in structural patterns and in-situ stress field throughout the Hengill Triple Junction (HTJ). High-resolution satellite images were utilized to identify the structural features present throughout the area, which were then classified in two categories according to their origin: 1) extensional, rift related features and 2) transform-related, strike-slip features. Analytical models of oblique rifting were then used to estimate the degree of rift obliquity of individual fissure swarms from the orientation of rift-related features. A decrease in rift obliquity was identified from the fissure swarms at the Hellisheidi reservoir (SW of the HTJ) as compared to the Nesjavellir reservoir area (NE of the HTJ). This is interpreted as a local transition from the structural style characteristic of the highly oblique RP towards the characteristic style of the WVZ, indicating spatial variability in the relative contributions of the three distinctive plate boundary segments to the local structural configuration throughout the HTJ. Local variations in the state of stress within geothermal reservoirs can exert a primary control on fluid transport properties and in the occurrence and magnitude of induced seismicity. As such, the detailed determination of structural patterns and their variability throughout the Hengill region has direct impact in the geothermal operation and seismic hazard assessment in the area.
|        Topic: Geology||Paper Number: 12041|