HR: 13:35h
AN: S43A-01    [Abstracts]
TI: Fluid flow in fault zones: evidence from hydrogeological and geological studies
AU: * Brenner, S L
EM: Sonja.Brenner@geo.uni-goettingen.de
AF: Geoscience Centre, University of Gottingen, Goldschmidtstr. 3, Gottingen, 37077 Germany
AU: Gudmundsson, A
EM: Agust.Gudmundsson@gwdg.de
AF: Geoscience Centre, University of Gottingen, Goldschmidtstr. 3, Gottingen, 37077 Germany
AB: Many fault zones have strong effects on fluid flow. Fault zones normally consist of two major hydrogeological units: a fault core, primarily made of breccia or gouge, and a fault damage zone, primarily consisting of fractures of various sizes. Active faults commonly have great effects on the transport of crustal fluids. For groundwater, for example, the effects of fault slip during earthquakes include changes in the yield of springs, water table, and stream flow. Similar effects occur in hydrothermal systems. Dramatic changes in hot springs and wells in geothermal fields occurred during two M6.6 earthquakes on strike-slip faults in the South Iceland zone. Similarly, significant changes occurred in the groundwater system associated with the Storagurra reverse fault in North Norway during an M4 earthquake in 1996. When a fault slips during an earthquake, all the pores and small fractures that meet with the slip plane become interconnected so that the fault may suddenly develop a very high hydraulic conductivity. Fluid transport in fault zones is also controlled by the current stress field. This is mainly because fractures are sensitive to changes in the stress field and deform much more easily than circular pores. In many fault zones, the majority of fractures in the damage zone is oriented subparallel to the main fault plane, in which case the current stress field may have strong effects on the permeability of the fault zone. When the maximum principal compressive stress is at a high angle to the fault strike, many fractures in the damage zone tend to close and fluid transport is reduced. When, however, the maximum principal compressive makes a small angle with the fault strike, fractures in the damage zone tend to be open and fluid transport is enhanced. The best evidence for palaeo-fluid flow, particularly in deeply eroded, inactive fault zones, are networks of mineral veins. Here we present field examples of faults and mineral veins in layered sedimentary rocks from the Bristol Channel Basin, UK, and volcanic rocks from the Husavik-Flatey-Fault, an active transform fault in North Iceland. The host-rock lithologies studied include: (1) mudstones (Upper Triassic) with numerous faults and gypsum veins at Watchet, Somerset Coast (Southwest England); (2) limestone and shale layers (Lower Jurassic) dissected by faults with calcite veins near Kilve, Somerset Coast, and at Nash Point, Glamorgan Coast (South Wales); and (3) basaltic lava flows (Upper Tertiary) in the damage zone of the Husavik-Flatey-Fault with numerous veins of quartz, chalcedony and zeolites. In all the study areas, the mineral veins are related to the faults, indicating that geothermal water was transported along the then-active faults into the host rocks. At Watchet, water that got access to anhydrite nodules in the mudstones lead to local fluid overpressure build-up due to the volume change during the transformation to gypsum, resulting in the development of gypsum veins. At Kilve, calcite veins occur almost exclusively in the cores and damage zones of (mostly normal) faults, and there is evidence that the veins were injected as hydrofractures (fractures generated by internal fluid overpressure) from the fault planes into the limestone layers. Similarly, at Nash Point, calcite veins are related to (mostly strike-slip) faults. Some veins were injected into the limestone layers of the fault damage zones directly from the fault planes. In the Husavik-Flatey-Fault, the mineral veins were generated at the time when the damage zone supplied fluids to surface geothermal fields.
DE: 5104 Fracture and flow
DE: 8010 Fractures and faults
DE: 8020 Mechanics
DE: 8045 Role of fluids
DE: 8164 Stresses--crust and lithosphere
SC: Seismology [S]
MN: 2005 Joint Assembly