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_Journal of Structural Geology 33 (2011) 107e113_ _Contents lists available at ScienceDirect_ _Journal of Structural Geology_ _journal homepage: www.elsevier.com locate jsg_ _Interfacial friction-induced pressure and implications for the formation and preservation of intergranular coesite in metamorphic rocks_ _Shaocheng Ji a,b,*, Qian Wang c a D?partement des G?nies Civil, G?ologique et des Mines, ?cole Polytechnique de Montr?al, Montr?al H3C 3A7, Canada b Laboratory of Continental Geodynamics, Chinese Academy of Geological Sciences, Beijing 100037, PR China c Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Wushan, Guangzhou 510640, PR China _article info_ _Article history: Received 4 June 2010; Received in revised form 8 November 2010; Accepted 21 November 2010; Available online 2 December 2010_ _Keywords: Interfacial friction-induced pressure Formation and preservation of coesite Overpressure UHP metamorphism Eclogite_ _abstract_ _The ?nding of intergranular coesite and coesite pseudomorphs has been taken as critical evidence to support that the formation of coesite was related to regional ultrahigh pressure (UHP) metamorphism rather than the pressure-vessel effect. Previous laboratory deformation experiments found that coesite occurs in practically undeformed strain-forbidden zones immediately adjacent to the pistonesample interfaces while the mean stress applied to the specimen is remarkably lower than the quartzecoesite equilibrium boundary. The formation and preservation of intergranular coesite in eclogites from UHP metamorphic terranes and the occurrence of coesite in strain-forbidden zones within experimentally deformed rocks can be more satisfactorily explained by the theory of interfacial friction-induced pressure, which is a well-known in metal forging. During certain episodes of fast tectonic deformation under high transient differential stresses, the interfacial friction can induce very signi?cant deformation pressures in thin layers of weak materials (e.g., SiO2) between large garnet crystals that are refractory and mechanically strong, thus act as excellent anvils. The local deformation pressure, which deviates from the lithostatic value, is likely an intrinsic property of highly constrained ?ows of weak materials between strong walls at least on a microstructural scale._ _? 2010 Elsevier Ltd. All rights reserved._ _1. Introduction_ _Ultrahigh pressure (UHP) is de?ned as a type of metamorphism that occurs at very high lithostatic pressures within the eclogite facies but above the stability ?eld of quartz (e.g., Liou et al., 1998; Carswell and Compagnoni, 2003). The quartzecoesite transition boundary is thus regarded as the lower pressure limit of UHP metamorphism. It is an accepted tenet of geology that the occurrence of coesite relics or pseudomorphs after coesite (former coesite) in eclogite, paragneiss and marble is a critical indicator for UHP metamorphism of supracrustal rocks that were subducted and exhumed from depths greater than w100 km during continentalecontinental collision (e.g., Chopin, 1984, 2003; Schreyer, 1995; O’Brien and Ziemann, 2008). The relationship between lithostatic pressure (PL) and depth (z) is inferred from Pascal’s law:_ _PL ? rgz, where r is the average density of the overlying rocks from surface to depth z, and g is the acceleration of gravity. However, laboratory deformation experiments (e.g., Hobbs, 1968; Green, 1972; Hirth and Tullis, 1994), which showed that coesite could form under differential stress conditions below its stability ?eld determined under hydrostatic pressures (e.g., Kitahara and Kennedy, 1964), casts doubt on this paradigm with important implications for understanding the mechanics of naturally deforming rocks (Fig. 1). In the geodynamic Earth, solid polymineralic rocks do not act as an incompressible ?uid for which Pascal’s law is valid (e.g., Mancktelow, 1993, 1995, 2008), and deform under nonhydrostatic stresses (i.e., s1!s2!s3, where s1, s2 and s3 are the maximum, intermediate and minimum stresses, respectively) on either a regional scale or a microstructural scale (e.g., Turcotte and Schubert, 1982; St?ckhert, 2002). The ?nding of pressure-induced incipient amorphization of a-quartz and transition to coesite in an eclogite from Antarctica (Palmeri et al., 2009) and eclogite-facies pseudodotachylytes in the Bergen arcs of western Norway (Austrheim and Boundy, 1994) suggests that the state of stresses at some sites during certain episodes in continental collision zones can extremely deviate from the hydrostatic state (s1 ? s2 ? s3)._ _Hirth and Tullis (1994) experimentally deformed cylindrical specimens (d0 ? 4.8e6.3 mm, h0 ? 10 e 14 mm) of Heavitree quartzite using a modi?ed Griggs-type apparatus. Coesite occurs mainly in two cone-shaped domains immediately adjacent to the pistons, where practically no crystal plastic deformation occurs (Fig. 2b). These poorly deformed domains can be called the strain-forbidden zones or strain shadows. Occasionally coesite grains are also found along grain boundaries perpendicular to s1 in the areas away from the pistonesample interfaces or some fault zones where stresses were concentrated. Coesite makes up w5_ of the material in the strain-forbidden zones, but <1_ of the overall sample. In the diagram of mean stress sm ? (s1?s2?s3) 3 versus temperature (Fig. 1a), most of the deformed samples containing coesite are within the stability ?eld of a e quartz. In the diagram of axial stress s1 versus temperature (Fig. 1b), however, these coesite-bearing samples all lie within the stability ?eld of coesite. Hirth and Tullis (1994) proposed a hypothesis that the magnitude and orientation of the maximum principal stress (s1) rather than the con?ning pressure (s2 ? s3) or the mean stress play a crucial role in controlling the quartzecoesite transition. Green (1972) experimentally deformed Dover int at temperatures of 450e900 x14C and at con?ning pressures of 1.0e2.0 GPa using a solid-medium Griggs-type apparatus and found that coesite occurred only in the region between pistons (where two strain-forbidden zones are overlapped) rather than in the extrusion zone (Domain 3, Fig. 2b). Coesite occurred only in the samples shortened >50_ at a strain rate of 10?4 s?1 while all other samples shortened <50_ at strain ratios of 10?7e10?5 s?1 were free of coesite. Coesite appeared in a sample (C460) shortened to 60_ at 750 x14C and a con?ning pressure of only 2.0 GPa, which is much lower than the quartzecoesite boundary (2.87 GPa at 750 x14C) determined hydrostatically by Kitahara and Kennedy (1964). The explanation offered by Green (1972) is that the presence of high dislocation density displaced the quartzecoesite transition boundary to lower pressures. The viability of the interpretation has been ruled out by the experimental observation that the dislocation density has no effect on the position of the equilibrium phase boundary between quartz and coesite (Ingrin and Liebermann, 1989)._ _Hobbs (1968) reported that coesite occurred in single crystals of quartz shortened to 50_ at a con?ning pressure of 1.5 GPa and the maximum differential stress (s1?s3) of w0.9 GPa at 900 x14C. The mean stress is 1.8 GPa which is 1.2 GPa below the quartzecoesite boundary at the same temperature as determined hydrostatically by Kitahara and Kennedy (1964). Hobbs (1968) postulated that the stored strain energy is responsible for the nucleation and growth of coesite outside of the hydrostatic stability ?eld._ _A well-known metal-forging theory on interfacial friction-induced pressure (e.g., Unksov, 1961; Harris, 1983; Dieter, 1986) has been applied to various geological interpretations. Jamieson (1963) used the theory to emphasize that the pressure, which is attained as the result of con?nement within a stronger pressure vessel with a geometrical arrangement intensifying the pressure, can be 3e9 times the normal compressive strength of the vessel-constructed material. Mancktelow (1993, 1995, 2008) used the concept ?rst to explain the development of isolated eclogites in metamorphic terrains and then to deal with the tectonic overpressure within a ?ow channel between two rigid walls on a regional scale. Ji et al. (2000) applied the theory to explain why the ?ow strength of quartzeplagioclase layered composites increases remarkably with decreasing the thickness of the layers, and the thin-layered composites are signi?cantly stronger than particulate counterparts with the same composition._ _Here we use the concept of interfacial friction-induced pressure (e.g., Unksov, 1961; Harris, 1983; Dieter, 1986) to provide an alternative interpretation_ Ключевые слова: quartz, surface, lithos, experimental result, regional, deep crust, uhp metamorphism, formation, metamorphic rock, specimen, zone, uhp, sample, boyd, stability eld, pistonesample interface, chopin, mineralogist, mancktelow, alternative interpretation, hirth, american mineralogist, stability ?eld, hydrostatic pressure, plastic deformation, eclogites, byerlee, tectonophysics, wang, deformation pressure, dh, maximum, earth, continental, intrinsic property, mosenfelder, schreyer, shear, increase, experimental observation, experimental study, china, friction, high, rutland, supracrustal rock, turcotte schubert, pressure, stability, practically undeformed, scale, plastic, friction-induced, strain, record, compression, jaeger, differential, occur, mineral, hirth tullis, exhumation, tresca criterion, oxford, deviatoric stress, ?eld, condition, strong wall, decreasing temperature, geology, friction-induced pressure, eclogite, rigid wall, uid, american, friction coefcient, jamieson, model, guangzhou institute, upper mantle, arc, axial compression, ha, uhp metamorphic, garnet, liou, mechanically strong, anvil, transition, geophysical, rst record, deformation, differential stress, coesite occurred, van roermund, journal structural, petrology, result, von mises, press, single crystal, higher, interfacial friction-induced, mineralogy, dieter, inclusion pressure, metamorphic, hobbs, axial stress, applied, ji, lithostatic pressure, preservation, shortened, ratio, gpa, criterion, metal forging, lithostatic, tectonic overpressure, zhao, ?ow, inclusion, eclogitic mylonites, strain-forbidden zone, pmax, differential expansion, coesite occur, garnet zircon, parkinson, tullis, metamorphism, elastic, science, rock, ultrahigh, theory, induced, coesite, journal geophysical, growth, conning pressure, temperature, york, signicant increase, geological, material, quartzecoesite transition, structural, coesite inclusion, interface, implication, equal, xed diameter, experimental, strain-forbidden, regional scale, strain ratio, deformed, green, nishiyama, quartzecoesite, zhang, transformation, weak material, zircon, occurs, earths surface, journal, interfacial, interfacial friction, harris, lateral, st?ckhert, elastic model, elsevier, tectonic deformation, domain, piston, sample shortened, strong, overpressure, intergranular coesite, tectonic, practically, host, local, perrillat, intergranular, long, coesite occurs, shear strength, ji li, stress, strain rate, renner, boundary, structural geology, strength, rate, unksov