Águst Guðmundsson, Geological Institute, University of Bergen, Allegt. 41, N-5007 Bergen, Norway (e-mail: agust.gudmundsson@geol.uib.no)

Field measurements were made of 1717 mineral-filled veins in the damage zone of the Husavik-Flatey Fault in north Iceland. Most veins are composed of quartz, chalcedony and zeolites, strike roughly parallel or perpendicular to the fault zone, and are members of dense palaeo-fluid transporting networks. A common vein frequency in these networks is 10 veins per metre. Cross-cutting relationships indicate that 79% of the veins are extension (mode I) cracks; 12% are sinistral, and 9% dextral, strike-slip faults. The thicknesses (apertures) of most veins are from 0.1 mm to 85 mm, and the thickness size distribution is a power law. The outcrop trace lengths of 384 veins (of the 1717) could be measured accurately. Veins with measurable trace lengths are mostly small and range in length from 2.5 cm to 400 cm, in thickness from 0.01 cm to 0.9 cm, and have and average length/thickness ratio of about 400. Simple analytical models are derived and used to make rough estimates the fluid overpressure at the time of vein emplacement and of the volumetric flow rates in hydrofractures of dimensions equal to those of typical veins. The results indicate, first, that the average fluid overpressure, with reference to the minimum principal compressive stress at the time of veins emplacement, was 20 MPa. Secondly, that buoyancy-drive volumetric flow rates in vertical fractures are greater than that in equally large fractures dipping less than vertical, suggesting that vertical transport of crustal fluids is favoured in the Husavik-Flatey Fault.

Boundary-element studies of the Husavik-Flatey Fault, using fault-parallel loading of 6 MPa, show tensile stress concentration in large areas around the fault-zone tips. In these areas, tensile stress exceeds typical in situ tensile strengths of rocks, resulting in the formation or reactivation of tensile fractures. These fractures curve towards the tips of the fault zone, and if interconnected they increase the rock permeability. Fault slip also increases the temporary permeability of the fault zone, by as much as many orders of a magnitude. Its effects on the surrounding groundwater flow, however, is normally small if the fault trends at a high angle to the groundwater flow but gradually increases as the angle between the flow and the fault decreases. When the trend of the fault zone and the groundwater flow coincide, the upstream part collects groundwater whereas the downstream part expels it. It follows that the yield of springs decreases in the upstream part, but increases in the downstream part.

References:

Gudmundsson, A., 1999. Fluid overpressure and stress drop in fault zones.
Geopphys. Res. Lett., 26: 115-118.
Gudmundsson, A., 2000. Fluid overpressure and flow in fault zones: field measurements and models. Tectonophysics (submitted).
Gudmundsson, A., Berge, S.S., Lyslo, K.B. and Skurtveit, E., 2000. Fracture networks and fluid transport in active fault zones. J. Struct. Geol. (submitted).

Agust Gudmundsson
Geological Institute
University of Bergen
Allegt. 41
N-5007 Bergen
NORWAY

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