Origin of hydrous fluids at seismogenic depth: Constraints from natural and experimental fault rocks
Introduction
Faults are important conduits for fluid flow in the lithosphere (e.g., Kerrich, 1986, Caine et al., 1996, McCaig, 1997), and fluids are linked to the chemical and mechanical processes controlling fault mechanics during the seismic cycle (e.g., Etheridge et al., 1984, Reynolds and Lister, 1987, Hickman et al., 1995). Variations in fluid pressure might trigger seismicity by lowering the effective stresses and facilitating earthquake nucleation (Sibson, 1973, Sibson, 1974, Beeler et al., 2000, Cocco and Rice, 2002, Miller et al., 2004). Long term fluid–rock interactions during the interseismic period promote mass transfer processes and mineral reactions, which gradually modify the composition and texture of fault rocks, controlling their mechanical properties, and accommodating aseismic creep (e.g., Sibson, 1977, Wintsch et al., 1995, Collettini et al., 2009, Gratier et al., 2011). Fault rocks include deformation structures formed in the various phases of the seismic cycle, and a combined textural and geochemical analysis of fault rocks has given insights on the origin of fluids in different geological environments (e.g., Lin et al., 2003, Ghisetti et al., 2001, Vannucchi et al., 2010).
At depth of nucleation of many crustal earthquakes, between 7 and 15 km (Scholz, 2002), faults in silicate rocks commonly contain cataclasites and, sometimes, pseudotachylytes (Sibson, 1977, Scholz, 2002). Cataclasites are cohesive fault rocks composed of wall rock fragments (Sibson, 1977), cemented by a mineral assemblage which is generally more hydrated than the wall rock (e.g., Magloughlin, 1992). Pseudotachylytes are solidified friction-induced melts which form during fault slip at seismic rates (∼1 m s−1) (Sibson, 1975) and are thought to form in the absence of pore fluids (e.g., Sibson and Toy, 2006). During seismic slip, it is expected that any fluid in the slipping zone would be pressurized because of frictional heating, causing total release of stresses and inhibiting melting (Sibson, 1973, Rice, 2006). However, pseudotachylytes often post-date hydrothermally cemented cataclasites (Magloughlin, 1992, Fabbri et al., 2000), thereby implying that pseudotachylytes can actually form in a fluid infiltrated fault. Importantly, formation of friction melts in the presence of fluids during earthquakes is supported by evidence from faults hosted in cohesive silicate-bearing rocks and from experiments, including: (1) the coexistence, in the same slipping zone, of pseudotachylytes in compressional jogs and vein fillings (precipitated from fluids) in dilatational jogs (Kirkpatrick and Shipton, 2009, Griffith et al., 2010); (2) the presence of vesicles, amygdales, and fluid inclusions in some pseudotachylytes (e.g., Maddock et al., 1987, Boullier et al., 2001, Magloughlin, 2011); (3) the formation of friction melts in experiments reproducing seismic deformation conditions in the presence of pore water (Violay et al., 2013). As a consequence, frictional melts are likely to dissolve any fluid impregnating the fault rocks or derived from melting of the hydrated minerals inside and close to the slipping zone.
Hydrogen and oxygen stable isotopes have been extensively used as tracers for the origin of hydrous fluids which interacted with silicate rocks in different geological processes (Taylor, 1974, Taylor, 1977), including fault mechanics (e.g., Wickham and Taylor, 1985, McCaig, 1997). Previous studies demonstrated that the oxygen isotope compositions of pseudotachylytes are often buffered by the oxygen of the melted minerals, implying that only a small proportion, if any, of an external hydrous fluid was dissolved in the melt (OʼHara and Sharp, 2001, Moecher and Sharp, 2004). Hydrogen isotope compositions are more sensitive to the ingression of small quantities of water, or to fractionation due to chemical or physical processes (e.g., Sharp, 2007). Moecher and Sharp (2004) found variations in hydrogen isotope compositions of pseudotachylytes, likely due to hydrogen fractionation during muscovite crystallization in pseudotachylyte matrix during devitrification. Thus it was not possible to identify a source of fluids other than the hydrous host rock minerals undergone melting. The pseudotachylytes analyzed in their study were not associated with cataclasites, and do not show coherent traces of interaction with any external fluid.
We investigated the origin of fluids in mylonites, cataclasites and pseudotachylytes exhumed from 9–11 km depth in the Adamello batholith. Artificial pseudotachylytes produced in experiments simulating seismic slip (slip rate of ∼1 m s−1, slip of few meters) on the Adamelloʼs tonalites and cataclasites were used to investigate the volatile behavior during frictional melting. Natural and artificial samples were characterized by microstructural, mineralogical, geochemical and hydrogen isotope analyses. The natural cataclasites record the ingression of an external fluid, with hydrogen isotope composition compatible with crustal metamorphic origin. The natural pseudotachylytes include cryptocrystalline domains which are prone to adsorb late stage meteoric water (at ). In well crystallized domains, natural pseudotachylytes retain the hydrogen isotope compositions of the source rocks (tonalites or cataclasites) similar to what it is observed in artificial pseudotachylytes.
Section snippets
Deformation structures in the northern Adamello
The Adamello batholith is composed of four plutons (Bianchi et al., 1970), and is intruded between two large scale tectonic lineaments, the Tonale Line (a segment of the Periadriatic Lineament) to the north and the South Giudicarie Line to the east (Fig. 1). The intrusion and cooling of the northernmost Avio and Presanella plutons, whose crystallization zircon ages range from 37.2 to 33.1 Ma (Del Moro et al., 1983, Skopelitis et al., 2012), was contemporary with dextral strike slip along the
Analytical techniques
The samples include: tonalite, mylonites, cataclasites and pseudotachylytes from the Gole Larghe Fault (outcrops 1 and 2, Fig. 1a), and tonalite, cataclasites and pseudotachylytes from the Passo Cercen Fault (outcrop 3, Fig. 1a). Artificial pseudotachylytes were produced from cylinders of tonalite and cataclasite collected in outcrop 1 and sheared in a high velocity rotary shear apparatus (HV-1) (Shimamoto and Tsutsumi, 1994) at slip rate of 1.2 m s−1 (Di Toro et al., 2006a, Di Toro et al.,
Petrography
The fault rock minerals are often the same as the starting tonalite (Fig. 2h). To differentiate the different generations of minerals, we added a subscript to the mineral name: 1 refers to magmatic minerals, 2 to minerals in mylonites and 3 to minerals in cataclasites or pseudotachylytes.
Whole rock major and trace elements
The averaged concentrations of major and trace elements in tonalites, mylonites, cataclasites and pseudotachylytes from the three outcrops, measured by X-Ray Fluorescence and Loss on Ignition (LOI) methods, are shown in Supplementary material, Table 1. The data were previously published (Di Toro and Pennacchioni, 2005, Pennacchioni et al., 2006, Mittempergher et al., 2009). Here we use the most complete dataset of outcrop 1, including tonalites, mylonites, cataclasites and pseudotachylytes, to
Volatile contents
The samples used in hydrogen isotope measurements were analyzed with the CHN method to determine the composition of the volatiles (Supplementary material, Table 2). The samples were chosen in matrix-dominated domains of mylonites (biotite2), cataclasites (chlorite and epidote) and pseudotachylytes (biotite3) to extract an amount of hydrogen high enough for determining the hydrogen isotope compositions with the continuous flow method (Sharp et al., 2001). Given the bias in the selection of the
Infrared spectroscopy
The FTIR spectra of biotite1 and biotite3 and plagioclase3 microlites (Fig. 6a, b) are similar, with three prominent peaks in the OH stretching region (wavenumbers: 3681, 3662 and 3593 cm−1) (Beran, 2002). The intensities of the peaks are lower in the microlites, likely because biotite3 lamellae are finely inter-fingered with plagioclase3 microlites, whereas biotite1 were in large individual grains. In biotite1 and biotite3 microlites, the water loss upon heating is small (green line in Fig. 6
Hydrogen isotope compositions
The hydrogen isotope compositions are reported in Supplementary material, Table 2 and in Fig. 7. Biotite1 from the three outcrops have δD values between −77 and . Mylonites of the outcrop 1 (Lobbia glacier) have δD values between −77 and , thus indistinguishable from that of biotite1 (within the analytical error of ). The δD values of cataclasites are between −69 and , thus significantly lower than biotite1. Bulk natural pseudotachylyte samples have δD values between −103 and
Fluid–rock interactions in ductile and brittle shear zones
The deformation in mylonites was nearly isochemical (Pennacchioni, 2005, Pennacchioni et al., 2006), with minor variations in the major and trace element composition compared with the host tonalite (Fig. 5b). The mineral assemblage is the same as tonalite (plagioclase, biotite, quartz, K-feldspar), with texture variations due to dynamic recrystallization of quartz and biotite (Fig. 2). The hydrogen isotope compositions of bulk mylonite samples overlap with that of biotite1 in tonalite (within
Conclusions
We performed microstructural, mineralogical, geochemical, infrared and hydrogen isotope analyses on mylonites, cataclasites and pseudotachylytes of two fault zones and host rocks in the Adamello (Italian Alps). Artificial pseudotachylytes produced in high velocity friction experiments in tonalite and cataclasites were analyzed as well. We conclude that:
- (1)
Our data confirm that mylonites, formed at ∼550 °C during pluton cooling, retain the chemical and hydrogen isotope composition of the host
Acknowledgments
Two anonymous reviewers are thanked for their critical and constructive comments, which greatly improved the manuscript. We thank Leonardo Tauro, Elena Masiero, Federico Zorzi, Andrea Cavallo, Giuliana De Grandis, Ilaria Baneschi, Eric Quirico for help during the analytical work. Grants from the ERC (Starting Grant project 205175 USEMS to GDT, SM), the CARIPARO foundation (project CD0504012134 to SM, GP), the CARITRO foundation (grant “Giovani Ricercatori 2007” to SM) and the Università
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