Fracture networks and fluid transport in active fault zones

Field measurements were made of 1717 mineral-filled veins in the damage zon e of an active dextral strike-slip fault zone in Iceland. Most veins are co mposed of quartz. chalcedony and zeolites, strike roughly parallel or perpe ndicular to the fault zone, and are members of dense palaeo-fluid transport ing networks. A common vein frequency in these networks is 10 veins per met re. Cross-cutting relationships indicate that 79% of the veins are extensio n (mode I) cracks and 21% are shear cracks. The apertures of most veins, me asured as mineral-fill thicknesses, are from 0.1 to 85 mm. and the aperture frequency distribution is a power law. The outcrop trace lengths of 384 ve ins (of the 1717) could be measured accurately. These 384 veins are mostly small and range in length from 2.5 to 400 cm, in aperture from 0.01 to 0.9 cm, and have an average length/aperture ratio of about 400. Simple analytic al models are derived and used to make rough estimates of the volumetric fl ow rates in hydrofractures of dimensions equal to those of typical veins. T he results indicate that volumetric flow rates fur a horizontal fracture an d;I vertical fracture in a rigid (non-deforming) host rock would be around 1.5 x 10(-4) and 8.9 x 10(-4) m(3)s(-1), respectively. The volumetric flow rate in a vertical fracture of equal size but in a deforming host rock, wit h buoyancy added to the pressure gradient, is around 1.3 x 10(-3) m(3)s(-1) . Thus, vertical fluid transport is favoured under these conditions. (C) 20 01 Elsevier Science Ltd. All rights.