Tectonophysics 510 (2011) 253–268 Contents lists available at SciVerse ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto The mechanical paradox of low-angle normal faults: Current understanding and open questions Cristiano Collettini Dipartimento di Scienze della Terra Università degli Studi di Perugia, Italy Istituto Nazionale di Geofisica e Vulcanologia, Roma, Italy article info abstract Article history: Low-angle normal faults, LANF, (dip b30°) have been proposed as key-structures for accommodating crustal Received 5 March 2011 extension. In contrast, frictional fault reactivation theory predicts that slip on LANF is extremely unlikely: this Received in revised form 18 July 2011 prediction is consistent with the absence of moderate-to-large earthquakes on normal faults dipping less than Accepted 24 July 2011 30°. Available online 9 August 2011 In order to discuss this discrepancy I will analyse and integrate: 1) geological data from 9 LANF, 2) the dip- range of earthquake-ruptures in extensional environments, and 3) frictional fault mechanics. Keywords: Low-angle normal faults LANF fault zone structure is represented by two end members: a) a thick mylonitic shear zone superposed by Friction cataclastic processes and some localization; 2) a discrete fault core separating hangingwall and footwall Fault mechanics blocks affected by brittle processes. LANF act as preferential channels for fluid flow and in some cases they Seismicity promoted fluid overpressure. Fluid–rock interactions along some detachments favour the development of phyllosilicates that in general are characterised by low frictional strength, μb0.4, and inherently stable, velocity-strengthening frictional behaviour. The low friction coefficient of the phyllosilicates can explain movements on LANF and the velocity strengthening behaviour of the phyllosilicates implies fault creep and therefore can be used to explain the absence of moderate-to-large earthquakes on LANF in seismological records. However in my view, the integration of the three datasets does not provide a simple mechanical solution for the LANF paradox since it leaves two important open questions. First a widespread development of phyllosilicates does not seem to be a common feature for most of the exhumed LANF that on the contrary show the typical fault rocks of the brittle and seismogenic crust. Second, although some brittle detachments reactivated pre-existing ductile shear zones, others formed as gently dipping structures within a brittle crust characterised by a vertical σ1: a well constrained mechanical explanation for this second class of structures is lacking. © 2011 Elsevier B.V. All rights reserved. Contents 1. Introduction .............................................................. 254 1.1. Adopted terminology ...................................................... 254 2. Detachment faults: fault zone structure and fault rocks ......................................... 255 2.1. The Whipple Mountains detachment, California .......................................... 255 2.2. The South Mountains detachment, Arizona ............................................ 256 2.3. The Chemehuevi Mountains detachment, California and Arizona .................................. 256 2.4. The Black Mountains detachments ................................................ 256 2.5. The North Cycladic detachment system ............................................. 256 2.6. Detachment in the Err Nappe, Eastern Alps ............................................ 257 2.7. The Alaşehir detachment fault, Western Turkey .......................................... 258 2.8. The Zuccale detachment, Apennines Italy ............................................. 258 3. Normal faults and seismicity ...................................................... 259 3.1. Dip distribution for moderate-to-large normal fault ruptures with positively discriminated slip planes ................ 259 3.2. Seismological data for low-angle normal faulting ......................................... 260 4. Coulomb criterion and frictional reactivation for brittle faults ...................................... 261 E-mail address: [email protected]. 0040-1951/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2011.07.015 254 C. Collettini / Tectonophysics 510 (2011) 253–268 5. Discussion ............................................................... 262 5.1. Two end members fault zone structure .............................................. 262 5.2. Detachments and fluid pressure ................................................. 262 5.3. Fault rocks and inferred deformation mechanisms ......................................... 263 5.3.1. Fault gouge and foliated cataclasite with localisation ................................... 263 5.3.2. Cataclasites and pseudotachylytes ............................................ 263 5.3.3. Fluid assisted diffusion mass transfer .......................................... 263 5.4. Fault rocks, frictional properties and frictional fault reactivation .................................. 263 5.5. Frictional fault reactivation and seismicity ............................................ 265 6. Open questions ............................................................ 265 Acknowledgments .............................................................. 266 References ................................................................. 266 1. Introduction faults possessing friction in the range of 0.6–0.85 (Byerlee, 1978), predicts that it is easier to form a new optimally oriented fault (dip Over the past 40 years, very low dipping (0–30°) normal faults, about 60°) instead of reactivating an existing one dipping less than low-angle normal faults (LANF) or extensional detachments, have 30° (Collettini and Sibson, 2001; Sibson, 1985). been extensively mapped in areas of continental extension (Fig. 1). Several manuscripts have provided reviews on different aspects of These structures have been firstly recognised in the Basin and Range the extensional detachment enigma focussing on their origin (Lister province, US, (Anderson, 1971; Longwell, 1945; Wernicke, 1981) and and Davis, 1989), their possible seismogenic role (Wernicke, 1995) then documented in most of the tectonic settings affected by crustal and their mechanics (Axen, 2004; Abers, 2009). In this manuscript I extension. will review: a) fault geometry and fault rocks for 9 LANF that were Geological evidence suggests that such structures accommodated active within the brittle crust (paragraph 2), b) the dip distribution for a significant amount of crustal extension (also within the brittle crust) earthquake-ruptures in extensional environments (paragraph 3), and at dips similar to their present attitude and within a regional stress c) the Coulomb criterion and frictional fault reactivation theory of field characterised by vertical σ1 trajectories (e.g. Fig. 1, Collettini and brittle faults (paragraph 4). Data presented in these three paragraphs Holdsworth, 2004; Hayman et al., 2003; John and Foster, 1993; Jolivet will be integrated in a final discussion on fault structure, fluid flow, et al., 2010; Lister and Davis, 1989). Associated fault rocks (gouge, fault rocks, mechanical properties and possible seismic behaviour of breccia, cataclasite, and rare pseudotachylyte) suggest that these low-angle normal faults (Fig. 1). structures were predominantly of brittle-frictional character and were formed in the upper seismogenic crust at low dips (cf. paragraph 1.1. Adopted terminology 2 and references therein). In some large displacement detachments, the gently dipping attitude is favoured by isostatic adjustments A widely cited conceptual fault zone model suggests that (Spencer, 1984; Wernicke and Axen, 1988). In contrast, other seismically active, upper crustal brittle faults pass down, across a geological studies suggested that detachments are not relevant for predominantly thermally activated transition at 10–15 km depth, accommodating crustal extension because: a) they initially formed at into ductile shear zones in which deformation occurs by aseismic steep angles but then progressively rotated, whilst inactive, down to viscous creep (Imber et al., 2001; Schmidt and Handy, 1991; Scholz, lower dips through domino rotation of younger sets of steeply dipping 2002; Sibson, 1977). Brittle faults are mainly affected by discrete normal faults (Proffett, 1977); and b) they are interpreted as rootless and localised deformation within rocks displaying elasto-frictional gravity slides (e.g. Walker et al., 2007). behaviour (e.g. Sibson, 1977). Ductile shear zones are mainly Seismological evidence suggests that moderate-to-large exten- characterised by viscous creep processes (Schmidt and Handy, sional earthquakes nucleate on moderate-to-steeply dipping struc- 1991)suchascrystalplasticity(Drury and Urai, 1990; Hirth and tures, peaking at 45° (Collettini and Sibson, 2001; Jackson and White, Tullis, 1992) and grain-size sensitive diffusion creep (Rutter, 1983; 1989; Molnar and Chen, 1982; Thatcher and Hill, 1991). Possible Schmidt and Handy, 1991). The Brittle–ductile or frictional–viscous seismic activity on low-angle normal faults has been proposed in transition is the zone at which elastic-frictional behaviour switches Papua New Guinea, Gulf of Corinth (Greece) and Apennines (cf. into viscous creep. paragraph 3). Random-fabric fault rocks formed by elasto-frictional behaviour Anderson–Byerlee frictional fault reactivation theory, i.e. an are