On the structure and mechanical properties of large strike-slip faults

Elucidation of the internal structure of fault zones is paramount for understanding their mechanical, seismological and hydraulic properties. In order to observe representative brittle fault zone structures, it is preferable that the fault be passively exhumed from seismogenic depths and the exposure must be in arid or semi-arid environments where the fragile rocks are not subject to extensive weathering. Field observations of two such faults are used to constrain their likely mechanical properties. One fault is the Carboneras fault in southeastern Spain, where the predominant country rocks are phyllosilicate-rich lithologies, and the other is part of the Atacama fault system in northern Chile, where faults pass through crystalline rocks of acidic to intermediate composition. The Carboneras fault is a left lateral fault with several tens of kilometres offset exhumed from approximately 4 km depth, and displays multiple strands of clay-bearing fault gouge, each several metres wide, that contain variably fractured lenses of protolithic mica schists. The strain is evenly distributed across the gouge layers, in accordance with the measured laboratory mechanical behaviour which shows predominantly strain hardening characteristics. The overall width of the fault zone is several hundred metres. Additionally, there are blocks of dolomitic material that are contained within the fault zones that show extremely localized deformation in the form of faults several centimetres wide. These are typically arranged at an angle of c. 208 to the overall fault plane. These differing types of fault rock products allow for the possibility of ‘mixed mode’ seismicity, with fault creep occurring along the strands of velocity strengthening clay-rich gouge, punctuated by small seismic events that nucleate on the velocity weakening localized faults within the dolomite blocks. The Caleta Coloso fault in northern Chile has a left-lateral offset of at least 5 km and was exhumed from 5–10 km depth. The fault core is represented by a 200–300 m wide zone of hydrothermally altered protocataclasite and ultracataclasite. This is surrounded by a zone of micro and macro-fractures on the order of 150 m thick. The fault core shows a heterogeneous distribution of strain, with alternate layers of ultracataclasite and lower strain material. The strain-weakening behaviour of crystalline rocks might be expected to produce highly localized zones of deformation, and thus the wide core zone must be a result of additional process such as precipitation strengthening or geometric irregularities along the fault plane. Understanding the structure of fault zones is essential in order to help interpret their mechanical, hydraulic and seismological properties. Faults are key structures in the Earth’s upper crust that control the strength of the lithosphere (Kusznir & Bott 1977; Kohlstedt et al. 1995; Townend & Zoback 2000), the flow of fluids (Knipe 1992; Caine et al. 1996) and the nucleation and propagation of earthquakes. Often we want to describe the structure of faults at depth in the crust (for example at seismogenic depths), but these regions can only be accessed by remote geophysical methods, unless deep drilling is an option (Ohtani et al. 2001; Zoback et al. 2006) or observation from deep mines. Even where drillholes are present, a one-dimensional view of a fault zone is obtained, where a twoor three-dimensional view is necessary in order to understand fully the fault properties. Field studies of exhumed fault zones can aid in the understanding of fault zone structure at depth, but must be approached with caution. Often, fault zones are active during the exhumation process and as such have a full range of structures that overprint those that formed at the depth of interest. The primary structures (those formed at depth) and the overprinting structures may not be easily distinguishable. Consequently structures formed under near-surface conditions may be wrongly interpreted as key features in exhumed From: WIBBERLEY, C. A. J., KURZ, W., IMBER, J., HOLDSWORTH, R. E. & COLLETTINI, C. (eds) The Internal Structure of Fault Zones: Implications for Mechanical and Fluid-Flow Properties. 299, 139–150. DOI: 10.1144/SP299.9 0305-8719/08/$15.00 # The Geological Society of London 2008. surface-exposed fault zones. For the above reasons field studies on fault zones are often made on passively exhumed fault zones where it can be demonstrated that little or no displacement and structural overprinting occurred during exhumation either from the regional tectonics or from the nature and mineralogy of the fault rocks. Faults that are not well exposed can present problems as key parts of the fault zone may be disregarded solely because they are not observable at the surface. Another potential problem with using natural faults for study concerns collection of material from surface exposures for laboratory testing or analysis. Near-surface weathering can significantly alter the mineralogy, microstructure and physical properties of rocks, as demonstrated by Morrow & Lockner (1994). Again, incomplete exposure may lead to the over-emphasis of laboratory results obtained from material collected from the exposed material, whereas the mechanical and hydraulic properties may well be primarily controlled by unexposed or poorly exposed parts of the fault zone. Given the difficulties of studying large faults in the field, it is hardly surprising that few detailed studies exist of their structure. Exceptions to this include work by Chester et al. (1993), Schulz & Evans (2000), Wibberley & Shimamoto (2003), Faulkner et al. (2003), Cembrano et al. (2005) and Jefferies et al. (2006). All of these examples study faults with variable exposure levels, varying constraints on their exhumation histories and as such give different degrees of confidence when interpreting the structure of faulting at depth. In this work, we aim to contribute to the database on the internal structure of large strike-slip faults, and infer some aspects of the mechanical properties of faults from observations of their internal structure. First we describe the structure of two major strike–slip faults that cut through very different country rocks. Aspects of the structure of the Carboneras fault in southeastern Spain are described, then those from the Caleta Coloso fault in northern Chile. The likely mechanical properties of the two faults are inferred from the macroscopic structure of the two faults in the discussion. Fault zone structure Both the Carboneras and the Caleta Coloso faults have been largely passively exhumed from depth, and they are excellently exposed in semiand hyper-arid environments. The mineralogy of the fault rock in the Carboneras fault and the Caleta Coloso fault indicates fault activity at c. 4 and c. 6 km depth respectively. The depth range from which the Carboneras fault was exhumed is estimated from the present height of uplifted basement rocks and Pliocene reef complexes and the temperatures at which the neocrystallized clay phases contained within the fault gouges formed (assuming an average geothermal gradient). This yields values of exhumation between 4 and 1.5 km (Faulkner et al. 2003), which is consistent with recent uplift rates estimated from marine terraces on the order of 0.05–0.1 mm a (Bell et al. 1997). The fault accommodates several tens of kilometres of movement across it (Rutter et al. 1986). On the Caleta Coloso fault, fission track dating of fault-related fracturing has shown that the presently exposed fault rocks formed at a temperature of at least 100 8C, interpreted to indicate a minimum of 3 km depth (Herrera et al. 2005). The Caleta Coloso fault has undergone more recent movements, although these are limited in extent, and are in response to large subduction zone earthquakes associated with the offshore trench (Gonzalez & Carrizo 2003). These movements are easily recognized in the field and expressed as very localized, narrow fault planes that form small fault scarps at the surface. The Caleta Coloso fault has at least 5 km of strike–slip offset (Cembrano et al. 2005). The results of the above analyses indicate that the structures of both faults can be taken to be representative of faulting at depth. Both faults, by virtue of their offset, are crustal-scale structures. The constituent structures of each fault are now described in turn, and the interpretation of the mechanical significance of these is analysed later in the discussion. Carboneras fault, SE Spain The macroscopic structure of the Carboneras fault has been described in detail previously in Faulkner et al. (2003). Here some key aspects of the structure are summarized. The fault cuts through Alpine age (Palaeogene) metamorphic rocks, Miocene age marine sediments and volcanic rocks. In the area of study (see Fig. 1) the fault cuts predominantly through the metamorphic basement rocks that consist largely of graphitic mica schist. The southern edge of the fault is obscured somewhat by later volcanic rocks (lava flows and lahars), but along-strike observations suggest that very little of the total fault width is unexposed. The igneous rocks contained within the fault zone are dolerites and show intrusive relationships with very little deformation. They have been interpreted previously as feeder systems for the later volcanics (Rutter et al. 1986). The fault initiated in the early Miocene (Scotney et al. 2000) and, as stated previously, ceased significant activity between 5 and 10 Ma ago. The fault has left-lateral offset on it, as D. R. FAULKNER ET AL. 140