Sequential growth of deformation bands in the laboratory Karen Mair 1 , Ian Main, Stephen Elphick Department of Geology and Geophysics, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, UK Received 26 January 1999; accepted 24 June 1999 Abstract We investigate the formation and evolution of localised faulting in high porosity sandstone by laboratory triaxial compression of intact 100-mm-diameter core samples. Experiments were carried out dry, at constant con®ning pressure (34 MPa), constant axial strain rate (5  10 6 s 1 ) and increasing axial strain (1.5±11.2%). Tests generated fault zones consisting of sets of distinct pale granulated strands, separated by lenses of apparently undamaged host rock. The sets of strands were sub-parallel to the shear direction but showed complex anastamosing geometry in perpendicular section. The individual strands had reduced grain size, porosity and sorting compared to undeformed rock. A strong correlation was found between the number of strands occurring in a fault zone and the applied axial strain. Mean grain size, however, reached a steady value irrespective of axial strain. This implies that a limited amount of strain is accommodated on each strand with further strain requiring new strands to form. However, no direct evidence for strain hardening was observed in the post-failure macroscopic stress±strain curves. Our laboratory induced deformation zones strongly resemble the key characteristics of natural deformation bands. We show the ®rst laboratory evidence for the sequential development of increasing numbers of discrete deformation bands with increasing strain. # 1999 Elsevier Science Ltd. All rights reserved. 1. Introduction In this paper we present new experimental results showing the sequential development of cataclastic faults in a high porosity sandstone. The motivation for this work is to examine the mechanical and hydraulic properties of porous rocks during deformation. These properties depend on the microstructural evolution of the rock, in particular, the micromechanical processes which have acted during fault formation, how these processes have interacted, and what stage of evolution has been reached. We present a brief review of pre- vious studies focusing on the characteristics of the faults observed in the ®eld and deformation mechan- isms which are consistent with these observations. We then show how our new laboratory studies may improve understanding of fault development in porous sandstone. Faults occurring in highly porous, quartz-rich sand- stone units have been studied in several ®eld areas. They have been referred to in the literature as `defor- mation bands' (Aydin, 1978; Aydin and Johnson, 1978), `microfaults' (Jamison and Stearns, 1982), `gran- ulation seams' (Pittman, 1981), and simply `faults' (Underhill and Woodcock, 1987). We adopt the termi- nology of Aydin and co-workers throughout this paper. These studies have shown that faulting in highly porous, quartz-rich sandstone is typically expressed as a complex zone consisting of sets of distinct pale de- formation bands (observable on hand specimen scale). They form an anastamosing web of damage, and are separated by lenses of apparently undamaged host rock (e.g. Aydin and Johnson, 1978). In the plane par- allel to the direction of shear, these same deformation bands have approximately straight traces which lie sub-parallel to the shearing direction (e.g. Aydin and Johnson, 1978). These damage zones are often more resistant to weathering allowing easy identi®cation in Journal of Structural Geology 22 (2000) 25±42 0191-8141/00/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0191-8141(99)00124-8 www.elsevier.nl/locate/jstrugeo 1 Now at: Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA. E-mail address: karen@barre.mit.edu (K. Mair).