Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021 Current issues and new developments in deformation mechanisms, rheology and tectonics S. DE MEER, M. R. DRURY, J. H. P. DE BRESSER & G. M. PENNOCK Vening Meinesz Research School of Geodynamics, Faculty of Earth Sciences, Utrecht University, P.O. Box 80.021, 3508 TA Utrecht, The Netherlands (e-mail. [email protected]) Abstract: We present a selective overview of current issues and outstanding problems in the field of deformation mechanisms, rheology and tectonics. A large part of present-day research activities can be grouped into four broad themes. First, the effect of fluids on defor- mation is the subject of many field and laboratory studies. Fundamental aspects of grain boundary structure and the diffusive properties of fluid-filled grain contacts are currently being investigated, applying modern techniques of light photomicrography, electrical conductivity measurement and Fourier Transform Infrared (FTIR) microanalysis. Second, the interpretation of microstructures and textures is a topic of continuous attention. An improved understanding of the evolution of recrystallization microstructures, boundary mis- orientations and crystallographic preferred orientations has resulted from the systematic application of new, quantitative analysis and modelling techniques. Third, investigation of the theology of crust and mantle minerals remains an essential scientific goal. There is a focus on improving the accuracy of flow laws, in order to extrapolate these to nature. Aspects of strain and phase changes are now being taken into account. Fourth, crust and lithosphere tectonics form a subject of research focused on large-scale problems, where the use of analogue models has been particularly successful. However, there still exists a major lack of understanding regarding the microphysical basis of crust- and lithosphere-scale localiza- tion of deformation. The motion and deformation of rocks are pro- mechanisms, rheology and tectonics. We have cesses of fundamental importance in shaping subdivided our review into four broad themes the Earth, from the outer crustal layers to the that reflect a large part of present-day research deep mantle. Reconstructions of the evolution activities: (1) the effect of fluids on deformation; of the Earth therefore require detailed knowledge (2) the interpretation of microstructures and of the geometry of deformation structures and textures; (3) deformation mechanisms and their relative timing, of the motions leading to rheology of crust and upper mantle minerals; deformation structures and of the mechanisms and (4) crust and lithosphere tectonics. This governing these motions. These problems con- introductory paper also serves to introduce the cern structures on all scales, from grain scale or papers presented in this volume. smaller to regional or global scale. Earth scien- tists in the early years of rock deformation studies focused strongly on extensive, detailed The effect of fluids on deformation descriptions of structures. Since the 1960s, the emphasis has been more on the mechanisms Fluids influence virtually all aspects of deforma- behind structure development and on the role tion mechanisms and rheology in the Earth on of the rheological or flow properties of rocks scales ranging from grain to plate boundaries during deformation within the framework of (Carter et al. 1990). Deformation in turn has an large-scale tectonics. Integration of laboratory important influence on fluid distributions in research, theoretical work on microphysical rocks (Daines& Kohlstedt 1997) and on rock processes, microstructural and outcrop-scale transport properties (Fischer & Paterson 1989). studies, and modelling of tectonics has become The involvement of water in deformation has more widespread, but at the same time the field been demonstrated in numerous field and labora- has broadened enormously. Consequently, the tory studies. One of the principal effects on need for dialogue between researchers from rheology results from the presence of water in different disciplines is ever increasing. grain boundaries. Grain boundary water sup- The objective of this paper is to present a ports a fast intergranular diffusion path, which selective overview of some current issues and allows stress-driven mass transport, resulting recent developments in the field of deformation in permanent, time-dependent deformation From: DE MEER, S., DRURY, M. R., DE BRESSER, J. H. P. & PENNOCK, G. M. (eds) 2002. Deformation Mechanisms, Rheology and Tectonics: Current Status and Future Perspectives. Geological Society, London, Special Publications, 200, 1-27. 0305-8719/02/$15 © The Geological Society of London. Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021 2 S. DE MEER ET AL. (Paterson 1973, 1995; Rutter 1976, 1983; Green fault zones, fault creep, and the propagation, 1984; Lehner 1990, 1995; De Meer & Spiers arrest, and recurrence of earthquake ruptures. 1995, 1999). This process of dissolution-precipi- Besides the physical role of fluid pressures con- tation creep (or pressure solution) is an impor- trolling rock strength in crustal faults, it is also tant mechanism for: compaction in sedimentary clear that fluids can exert mechanical influence rocks (Tada et al. 1987); healing, sealing and through a variety of chemical effects. In recent strength recovery in active fault zones (Sleep & years, much attention has been focused on the Blanpied 1992; Hickman et al. 1995; Bos & role of pressure solution in the strength recovery, Spiers 2000; Bos et al. 2000; Imber et al. 2001); healing and sealing of faults and the role of deformation under low temperature meta- phyllosilicates therein (e.g. Gratier et al. 1994; morphic conditions (Elliot 1973; St6ckhert et al. Bos & Spiers 2000; Bos et al. 2000). It is generally 1999); and evaporite flow (Spiers et al. 1990; believed that pressure solution and subcritical Spiers & Carter 1998). crack growth have a significant weakening Despite the large amount of work already effect on fault zone rheology. However, Bos done on pressure-solution creep, many unre- and co-workers found that in their experiments solved problems remain. At present, the elemen- on halite-clay mixtures, pressure solution only tary diffusive and interfacial processes remain resulted in weakening of fault gouges when clay poorly understood. In particular, the structure was added. In the monomineralic halite fault and diffusive properties of water-bearing grain gouge, pressure-solution compaction and healing boundaries are the subject of ongoing debate. effects dominated, leading to frictional behav- Experimental studies of pressure-solution creep iour. In halite-clay mixtures, the presence of in crustal rocks have largely focused on compac- phyllosilicates at grain boundaries prevented tion of granular quartz or quartz-phyllosilicate grain contact healing, leading to a mechanism mixtures (Schutjens 1991; Mullis 1993; Dewers of frictional sliding along clay foliae, accommo- & Hajash 1995; Renard & Ortoleva 1997). How- dated by pressure solution of asperities. ever, compared with time scales accessible in the Recent results on dissolution-precipitation laboratory, pressure solution is slow in these creep, the properties of water-bearing grain materials, hampering reliable determination of boundaries, and the role of fluids in faulting (as bulk kinetics or the rate-controlling mechanism. well as vein formation) are discussed below. The involvement of fluids in faulting processes and shear zone development is widely recog- nized. A comprehensive review on the mechani- Dissolution-precipitation creep cal involvement of fluids in faulting is given by Hickman et al. (1995). Fluids are linked to a Dissolution-precipitation creep involves three variety of faulting processes, including long- serial steps (Fig. 1): (1) dissolution of material term structural and compositional evolution of at grain boundaries under high normal stress; ii Fig. 1. Schematic illustration of pressure-solution creep. (a) Uniaxial compaction of a granular aggregate in the presence of saturated solution (saturated with respect to the stressed solid) at fluid pressure Pf. (b) Enlargement of grain contact area showing the three serial steps of pressure-solution creep: 1 - dissolution within the stressed grain boundary; 2 - diffusion through the grain boundary fluid; 3 - precipitation on the pore walls, an is the effective mean normal stress across the contact, Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021 DEFORMATION MECHANISMS, RHEOLOGY & TECTONICS 3 (2) diffusion through the grain boundary fluid caused by a thick fluid film supported between phase; and (3) precipitation at grain contacts clay minerals and, for example, quartz. Clay under low normal stress or on free pore walls minerals have a relatively large surface charge, (Raj 1982; Rutter 1983; Lehner 1990; De Meer leading to large hydration forces. Therefore, & Spiers 1997). Since interfacial reactions and clay-quartz boundaries are expected to have a diffusion occur serially, either the grain bound- thicker film than quartz-quartz boundaries ary diffusive properties or the interface reaction (Israelachvili 1992; Heidug 1995), promoting kinetics control the rate of the process. water film diffusion. When dissolution or precipi- Zhang et al. (2002) present the first systematic tation is rate controlling, cations in the solution investigation into the effect of applied stress,