Elsevier

Earth and Planetary Science Letters

Volume 385, 1 January 2014, Pages 97-109
Earth and Planetary Science Letters

Origin of hydrous fluids at seismogenic depth: Constraints from natural and experimental fault rocks

https://doi.org/10.1016/j.epsl.2013.10.027Get rights and content

Highlights

  • Fluids analyzed in mylonites and cataclasite–pseudotachylyte paleoseismic faults.

  • Artificial pseudotachylytes produced in high velocity friction experiments.

  • In mylonites the same fluids as in host tonalite, cataclasites wetted by metamorphic fluids.

  • Artificial and natural pseudotachylytes dissolve fluids from the wall rocks.

  • Natural pseudotachylytes are easily contaminated by late stage water.

Abstract

Fluids control the mechanical behavior of fault zones during the seismic cycle. We used geochemical, mineralogical, microstructural, hydrogen isotope compositions and Fourier Transform Infrared (FTIR) investigations to characterize the origin of hydrous fluids involved in ductile and brittle shear zones at the bottom of the seismogenic crust. Natural samples were collected from exhumed mylonitic shear zones and cataclasite–pseudotachylyte bearing faults in the northern Adamello (Italian Southern Alps), which were active at 9–11 km depth. Pseudotachylytes, solidified coseismic friction-induced melts, testify to ancient seismogenic behavior of the faults. Natural pseudotachylytes were compared with artificial pseudotachylytes produced in high velocity friction experiments simulating seismic slip.

Mylonites have mineralogical, elemental and hydrogen isotope compositions (80<δD<78) similar to the host tonalite (77<δD<73), within the analytical error of ±5. Cataclasites have instead mineralogical (chlorite, epidote, K-feldspar, no biotite), major and trace elements (enrichment in K2O, Ba, Rb; depletion in CaO, Na2O, SiO2) and hydrogen isotope (69<δD<60) compositions suggesting interactions with a crustal metamorphic fluid. Pseudotachylytes are composed of high temperature minerals (plagioclase, biotite, dmisteinbergite, cordierite, and scapolite) and have elemental compositions resulting from mixing of tonalite and cataclasite. Pseudotachylytes have complex microstructures, including: (i) microlitic domains, with well crystallized micrometric biotite, which have hydrogen isotope composition (81<δD<59) similar to cataclasites and tonalite; and (ii) cryptocrystalline domains, with poorly crystallized biotite, which have very high water content, release water upon heating at T>50°C and have low δD value (93). The hydrogen isotope composition of bulk samples is dominated by the composition of cryptocrystalline domains (103<δD<88), where most of the water is hosted. Their hydrogen isotope composition is compatible with adsorption of present day rainfall water (δD=95). Artificial pseudotachylytes have the same hydrogen isotope compositions of the starting tonalite (76<δD<74) or cataclasite (68<δD<62), with a slight decrease of the δD values in some samples (85<δD<81).

The first ingression of a crustal metamorphic fluid occurred in cataclastic faults. Natural pseudotachylytes, when not contaminated by present day rainfall water, have a hydrogen isotope composition similar to tonalite and cataclasite, as reproduced in dry high velocity friction experiments. The fluids dissolved in coseismic melts are most likely derived from the breakdown of hydrous minerals of cataclasite and tonalite undergone melting, and we could not identify the infiltration of an external fluid during earthquakes.

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 T<50°C). 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 73. Mylonites of the outcrop 1 (Lobbia glacier) have δD values between −77 and 79, thus indistinguishable from that of biotite1 (within the analytical error of ±5). The δD values of cataclasites are between −69 and 60, thus significantly lower than biotite1. Bulk natural pseudotachylyte samples have δD values between −103 and 88

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

References (76)

  • J.F. Magloughlin

    Microstructural and chemical changes associated with cataclasis and frictional melting at shallow crustal levels: the cataclasite–pseudotachylyte connection

    Tectonophysics

    (1992)
  • S. Mittempergher et al.

    The effects of fault orientation and fluid infiltration on fault rock assemblages at seismogenic depths

    J. Struct. Geol.

    (2009)
  • K.D. OʼHara et al.

    Chemical and oxygen isotope composition of natural and artificial pseudotachylyte: Role of water during frictional fusion

    Earth Planet. Sci. Lett.

    (2001)
  • G. Pennacchioni

    Control of the geometry of precursor brittle structures on the type of ductile shear zone in the Adamello tonalites, Southern Alps (Italy)

    J. Struct. Geol.

    (2005)
  • G. Pennacchioni et al.

    Brittle–ductile–brittle deformation during cooling of tonalite (Adamello, Southern Italian Alps)

    Tectonophysics

    (2006)
  • L. Pittarello et al.

    Energy partitioning during seismic slip in pseudotachylyte-bearing faults (Gole Larghe Fault, Adamello, Italy)

    Earth Planet. Sci. Lett.

    (2008)
  • R.L. Reverman et al.

    Climatically versus tectonically forced erosion in the Alps: Thermochronometric constraints from the Adamello Complex, Southern Alps, Italy

    Earth Planet. Sci. Lett.

    (2012)
  • Y. Rolland et al.

    Rare earth and trace element mobility in mid-crustal shear zones: insights form the Mont Blanc Massif (Western Alps)

    Earth Planet. Sci. Lett.

    (2003)
  • Z.D. Sharp et al.

    A rapid method for determination of hydrogen and oxygen isotope ratios from water and hydrous minerals

    Chem. Geol.

    (2001)
  • S.A.F. Smith et al.

    The structure of an intraplate exhumed seismogenic fault in crystalline basement

    Tectonophysics

    (2013)
  • J.G. Spray

    A physical basis for the frictional melting of some rock forming minerals

    Tectonophysics

    (1992)
  • N.M. Beeler et al.

    Pore fluid pressure, apparent friction and Coulomb failure

    J. Geophys. Res.

    (2000)
  • A. Beran

    Infrared spectroscopy of micas

  • A. Bianchi et al.

    I tipi petrografici fondamentali del plutone dellʼAdamello (tonaliti, quarzodioriti, granodioriti e loro varietà leucocrate)

    Mem. Istit. Geol. Mineral. Univ. Padova

    (1970)
  • A.M. Boullier et al.

    Fluid inclusions in pseudotachylytes from the Nojima Fault, Japan

    J. Geophys. Res.

    (2001)
  • J.S. Caine et al.

    Fault zone architecture and permeability structure

    Geology

    (1996)
  • K.W. Cashman

    Relationships between plagioclase crystallization and cooling rate in basaltic melts

    Contrib. Mineral. Petrol.

    (1993)
  • M. Cocco et al.

    Pore pressure and poroelasticity effects in Coulomb stress analysis of earthquake interactions

    J. Geophys. Res.

    (2002)
  • C. Collettini et al.

    The development of interconnected talc networks and weakening of continental low-angle normal faults

    Geology

    (2009)
  • A. Del Moro et al.

    Rb/Sr and K/Ar chronology of Adamello granitoids, southern Alps

    Mem. Soc. Geol. Ital.

    (1983)
  • Dempsey, E.D. Holdsworth, B.E. Di Toro, G. Bistacchi, A., 2012. The geological manifestation of earthquake swarms:...
  • G. Di Toro et al.

    Natural and experimental evidence of melt lubrication of faults during earthquakes

    Science

    (2006)
  • G. Di Toro et al.

    Relating high-velocity rock-friction experiments to coseismic slip in the presence of melts

  • M.A. Etheridge et al.

    High fluid pressures during regional metamorphism and deformation: implications for mass transport and deformation mechanisms

    J. Geophys. Res.

    (1984)
  • D. Frei et al.

    Trace elements geochemistry of epidote minerals

    Rev. Mineral. Geochem.

    (2004)
  • F. Ghisetti et al.

    Stable isotope evidence for contrasting paleofluid circulation in thrust and seismogenic normal faults of central Apennines, Italy

    J. Geophys. Res.

    (2001)
  • C.M. Graham et al.

    Hydrogen isotope fractionation in the system chlorite–water

  • J.P. Gratier et al.

    Pressure solution creep as a mechanism of aseismic sliding in active faults: evidence from the San Andreas Fault Observatory at Depth (SAFOD)

    Geology

    (2011)
  • Cited by (0)

    View full text