Evolution of fracture networks in shear zones: Insights from see-through experiments on biphenyl aggregates Ju Èrgen E. Streit 1, * , Stephen F. Cox 2 Department of Geology, The Australian National University, Canberra, ACT 0200, Australia Received 20 December 1999; revised 27 March 2001; accepted 29 March 2001 Abstract Evolution of fracture porosity in mid-crustal shear zones can be simulated in in-situ experiments. Evolution of fracture networks was monitored during simple shearing of 2-mm-wide zones in wet and dry aggregates of polycrystalline biphenyl C 6 H 5 C 6 H 5 ) in a Urai±Means see-through deformation apparatus. At low effective con®ning pressures in wet samples, mixed brittle-viscous deformation occurred at all strain rates 5.6 £ 10 24 ±5.8 £ 10 26 s 21 ) at 94±97% of the absolute melting temperature. At the fastest strain rate, progressive shearing is rapidly localized to produce a narrow fault zone along a shear zone boundary. In contrast, at the slowest strain rate, fractures develop throughout the shear zone and connect to form continuous fracture systems at low shear strains g < 2). These fracture systems accommodate most of the subsequent displacement in contrast to little fracturing and predominantly viscous deformation in a nominally dry experiment. Jogs, as parts of stairstepping fracture networks in wet samples, resemble in shape and distribution veins found in mid- to lower crustal shear zones. The experiments indicate that, from low strains onwards, the presence of ¯uids close to con®ning pressure in creeping shear zones can lead to the development of connected fracture networks that also localize most of the displacement. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: Fracture networks; Shear zones; Biphenyl aggregates; See-through experiments 1. Introduction Shear veins and fault jogs ®lled with mineral precipitates are found in shear zones exhumed from shallow crustal depths e.g. de Roo and Weber, 1992; Stel and Lankreyer, 1994) to mid and deep crustal levels e.g. Boullier and Robert, 1992; Henderson and McCaig, 1996; Nguyen et al., 1998). Such veins are recognized as remnants of ancient ¯uid ¯ow systems in the Earth's crust that have facilitated transport of hydrothermal ¯uids e.g. Kerrich, 1986; Cox et al., 1987; Robert et al., 1995), and melts Handy and Streit, 1999). However, we know little about the evolution of fracture porosity and its connectivity in active shear zones, as fracture systems are usually strongly deformed during continued deformation e.g. de Roo and Weber, 1992; Law, 1998; Simpson, 1998; Streit and Cox 1998). Thus, it is not always clear in which orientation veins form in shear zones, and how they are interconnected to facilitate ¯uid transport. In addition, ¯uid pathways are not necessarily preserved by the development of mineral veins. For example, dissolution of quartz and other minerals that has led to substantial volume losses in some shear zones indicates large ¯uid ¯uxes e.g. O'Hara, 1988; Glazner and Bartley, 1991; Selverstone et al., 1991), but ¯uid pathways are not indicated by the presence of veins. In this paper we argue that the development and evolution of connected fracture systems in mid to deep crustal shear zones can be simulated in see-through experiments on rock analog materials. To investigate ¯uid transport properties of shear zones, the longevity of fractures also needs to be considered. For example, ¯uid in®ltration may occur episodically in some mylonite zones via seismically generated fracture perme- ability e.g. McCaig, 1988; McCaig et al., 1990). At crustal temperatures ^ 2008C, seismically generated fracture permeability can be short lived on the time-scale of slip- recurrence < 10 2 ±10 4 a) due to rapid fracture sealing in the presence of reactive ¯uids e.g. Angevine et al., 1982; Brantley et al., 1990; Brantley, 1992). In contrast, perme- ability may persist during aseismic fault creep and permit continuous ¯uid ¯ow especially at depths below the seismogenic regime Cox, 1999). Effects that different Journal of Structural Geology 24 2002) 107±122 0191-8141/02/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S0191-814101)00052-9 www.elsevier.com/locate/jstrugeo * Corresponding author. Fax: 161-0)8-83034345. E-mail address: [email protected] J.E. Streit). 1 Now at National Centre for Petroleum Geology and Geophysics, The University of Adelaide, Thebarton Campus, SA 5005, Australia. 2 Also at Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia.
Evolution of fracture networks in shear zones: Insights from see-through experiments on biphenyl aggregates
Jun 23, 2023
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