STRESS DROP AND FLUID FLOW DURING PRE- AND POST-SEISMOGENIC FAULTING ALONG DECOLLEMENT ZONE IN ORDOVICIAN SOUTHERN UPLANDS ACCRETIONARY PRISM, SCOTLAND

Pressure and temperature (P-T) conditions for melange formation along detachment fault (decollement; ancient plate subduction boundary) in the Ordovician Currarie Formation of the Southern Uplands Accretionary Prism, Scotland, were estimated from fluid inclusion study on mineral veins of various stages. The fluid was determined to be H2O-rich, because of no solid materials being deposited by cooling, and because of the frozen temperature being approximately 0 C. The fluid inclusions of the second and third stages, except for the first stage, show bimodal homogenization temperature distribution. As a result, for the earlier fluid, the P-T conditions are 150 to 300 C and less than 260 MPa, and geothermal gradient is more than 28 C/km. Supposing that the geothermal gradient is less than 50 C/km, the P-T conditions are 200 to 300 C, 100 to 180 MPa approximately 4 to 7.5 km in depth, with an assumption of the fluid pressure being close to lithostatic pressure. The later fluid temperature rises 50 C higher than the earlier, and comes from the deeper part of the subduction zone, then deposits vein minerals due probably to the fluid-pressure drop. Such bimodal temperature occurs by periodical fluid flow along a seismogenic zone, at the earlier, lower temperature when fracture networks are not connected, whereas at the later, higher temperature they are connected from the deep to shallow levels.