Signature of coseismic decarbonation in dolomitic fault rocks of the Naukluft Thrust, Namibia

https://doi.org/10.1016/j.epsl.2012.04.030Get rights and content

Abstract

Unequivocal geological signatures of seismic slip are rare, exceptionally so in carbonate-hosted faults where carbonate minerals dissociate at temperatures lower than those required for producing a friction melt. This thermal dissociation leads to significant fault weakening by increased fluid pressure and/or nanoparticle lubrication, preventing further heating of the fault surface. Pseudotachylyte is therefore unlikely to form in carbonate-hosted faults, and other evidence for seismic slip must be identified.

We studied the lower Cambrian Naukluft Thrust which crops out in central Namibia. It contains a cataclastic dolomite fault rock, referred to as “gritty dolomite”, which we interpret as a signature of coseismic carbonate dissociation and subsequent fluid–rock interactions. The fault was active at ambient temperatures below 200°C.

“Gritty dolomite” contains: rounded, low aspect ratio dolomite clasts with a uniform Fe-rich dolomite coating, euhedral to subhedral magnetite, quartz, and K-feldspar in a fine-grained, massive to laminated carbonate matrix of particulate dolomite and crystalline calcite cement. The fault rock texture, combined with evidence of injectites of gritty dolomite into the wallrock, indicates the cataclasite deformed as a fluidized granular flow. At seismic slip velocities, frictional heating caused dissociation of dolomite to CO2 and Ca-, Fe- and Mg-oxides. This release of CO2 decreased the pH of the pore fluid in the fault, causing dissolution and rounding of dolomite clasts within an inertial grain flow, and precipitation of carbonate coatings and euhedral silicates and oxides during subsequent cooling and CO2 escape.

Examples of similar rocks having some, if not all of these characteristics have been described from other carbonate-hosted faults. The geological setting of the Naukluft Thrust is unique in spatial extent and quality of exposure, allowing us to eliminate alternative hypotheses for sources of CO2 to drive fluidization.

Highlights

► We present new seismic indicator rock for carbonate-hosted faults. ► Exposed in Naukluft Thrust, 500 Ma, central Namibia. ► Granular fluidization facilitated by coseismic frictional dissociation of carbonate. ► Dolomite grains with concentric coatings, magnetite and silicate minerals diagnostic.

Introduction

Fault rock assemblages reflect the mechanical processes by which faults accommodate deformation. Particular fault rocks are therefore associated with particular fault slip styles. For example, earthquake slip can cause significant heating of fault rock (Rice, 2006), which in silicate rocks leads to the formation of pseudotachylyte (Sibson, 1975), a lithified friction melt recognized as the only unequivocal evidence for seismic slip preserved in the rock record (Cowan, 1999). However, pseudotachylyte is extremely scarce in carbonate fault rocks (only reported by Viganò et al., 2011, at depth 10km), because fast slip in carbonate rocks at low pressure causes thermo-mechanical dissociation before melting temperatures are reached (e.g. Han et al., 2007a, Han et al., 2007b). Therefore, a geological record of mineral dissociation, rather than melting, is a more useful indicator for paleoseismic slip in carbonate rocks.

Identification of carbonate fault rocks that have experienced mineral dissociation related to seismic slip will not only allow for identification of past earthquakes, but can also be used to infer co-seismic properties of carbonate faults. Carbonate dissociation is a significant weakening mechanism along carbonate-hosted faults. Breakdown of carbonate minerals creates sudden localised spikes in CO2 pressure, driving local overpressure and reducing fault-plane effective stress (e.g. Billi and Di Toro, 2008, Sulem and Famin, 2009). In addition, experiments show that carbonate dissociation produces nanoparticles of oxides, facilitating nanoparticle lubrication of the experimental faults (De Paola et al., 2011; Han et al., 2007a, Han et al., 2007b; Han et al., 2010, Han et al., 2011).

Textural and geochemical products of carbonate dissociation have been directly observed (Hewitt, 1988) and inferred in long runout landslides (Anders et al., 2010, Beutner and Gerbi, 2005), and in carbonate-hosted faults (Smith et al., 2008, Smith et al., 2011) based on textural evidence for fluidization of granular fault rock. However, these examples may be explained by influx of volcanic CO2 (Smith et al., 2008) or groundwater (Beutner and Gerbi, 2005) and do not constitute direct evidence for in situ fluidization driven directly by earthquake slip. Here we describe carbonate fault rocks from the Naukluft Thrust, the basal décollement surface of the Naukluft Nappe Complex, Namibia (Korn and Martin, 1959, Viola et al., 2006). In an effort to understand carbonate fault processes, we consider the mineralogy and the macro- and microstructure of the fault rocks, discuss possible signatures of seismic slip, and address the geological record of dynamic fault weakening mechanisms.

Section snippets

Geological context

The Naukluft Thrust is the basal décollement of an exhumed foreland nappe complex, exposed in a klippe of the southern flank of the Damara Belt, central Namibia (Fig. 1A). The hanging wall to the thrust is the Naukluft Nappe Complex, a structural stack of variably folded carbonate and siliciclastic metasedimentary rocks of Meso- to Neoproterozoic age. This nappe complex was emplaced southward over Mesoproterozoic metamorphic and igneous rocks of the Kalahari Craton and onto the flat-lying 560to

The gritty dolomite

Amongst the components of the Naukluft Thrust fault rock assemblage, the gritty dolomite is least well understood (e.g. Miller et al., 2008, Viola et al., 2006). Previous workers have interpreted the gritty dolomite as an unconformable sedimentary horizon which accommodated gravity sliding (Korn and Martin, 1959), a “metasomatic mush” (Hartnady, 1978), a blasto-mylonite (Münch, 1978), an injected slurry of pressurized evaporite (Behr et al., 1981), or a dolomitic cataclasite which was fluidized

Insights from experiments

To interpret the processes responsible for formation of the gritty dolomite, we refer to two types of laboratory experiments: high-speed friction experiments on carbonate rocks and gouge (Han et al., 2007a, Han et al., 2007b; Han et al., 2010, Han et al., 2011, Smith et al., 2010), and experiments on CO2-brine-rock reactions at P/T conditions consistent with a few km depth (Kaszuba et al., 2003, Kaszuba et al., 2005, Kaszuba et al., 2006).

The high-speed friction experiments show that at room

Origin of clasts in the gritty dolomite

The fault core layer we have described is on average 0.5–1.5 m thick along the continuous exposure shown in Fig. 1C. As well as the gritty dolomite, the fault core contains 2–5 cm thick very fine-grained laminated yellow–white carbonate layers with a very sharp base. These resemble the slip surfaces produced in high-velocity slip experiments on marble by Han et al. (2010, see their Fig. 11), and we interpret these discrete laminated layers to be the principle slip surfaces of the Naukluft Thrust.

Accretionary grains and granular flow rheology

Accretionary rounded grains (Fig. 4, Fig. 5) have been interpreted as evidence of “fluidized” granular flow in multiple settings: clay–clast aggregates found in the slip zone of the 1999 Chi-Chi earthquake (Boullier et al., 2009) and replicated in experiments (Boutaread et al., 2008, Ujiie et al., 2011); in the dolomite rocks of the Heart Mountain slide (Anders et al., 2010, Beutner and Gerbi, 2005); in gouge of the Nojima Fault (Otsuki et al., 2003) and the Alpine Fault (Warr and Cox, 2001);

Fluid chemistry, fluidization and seismic slip

Fluidized granular flow in faults can be enhanced by the presence of a viscous pore fluid which transfers momentum and helps support grains (e.g. Kirkpatrick and Shipton, 2009). In the Naukluft Thrust, active at shallow crustal depths, and young marine sediments forming the footwall, aqueous fluids would have been available from compaction dewatering of underthrust sediments, as well as diagenetic reactions. The advection of water along the thrust is recorded by coeval evidence of

Conclusion

We have described a unique fault rock assemblage of minerals and structures (“gritty dolomite”), which we interpret as diagnostic of CO2–H2O–rock interaction at transiently high temperature (>800°C), found along the slip surface of the carbonate-hosted Naukluft Thrust. High-temperature (co-seismic) fluid–rock interaction during inertial grain flow is recorded by rounded, low aspect ratio carbonate grains. Post-seismic cooling and fluid escape from the fault is recorded by the precipitation of

Acknowledgements

Thanks to Nils R. Backeberg, Lynise Esterhuizen, Carly Faber, and Fernando Y. G. Sylvester, for assistance, helpful conversations and braai in the field, and to Steve Smith for helpful discussions. We particularly appreciate the very thoughtful and thorough comments from John Kaszuba and the contributions from the editor and an anonymous reviewer. We thank Parks Namibia for sampling permissions and hospitality. Funding from the South African Regional Cooperation Fund for Scientific Research and

References (50)

  • T. Yuko et al.

    New clinker formation process by the fluidized bed kiln system

    Cem. Concr. Res.

    (2000)
  • H. Ahrendt et al.

    Age and degree of metamorphism and time of nappe emplacement along the southern margin of the Damara Orogen/Namibia (SW-Africa)

    Geol. Rundsch.

    (1978)
  • M.H. Anders et al.

    The role of calcining and basal fluidization in the long runout carbonate slides: an example from the Heart Mountain Slide Block, Wyoming and Montana, USA

    J. Geol.

    (2010)
  • R.A. Bagnold

    Experiments on a gravity-free dispersion of large solid spheres in a newtonian fluid under shear

    Proc. R. Soc. London Ser. A: Math. Phys. Sci.

    (1954)
  • H.J. Behr et al.

    Saline horizons acting as thrust planes along the southern margin of the Damara Orogen (Namibia/SW-Africa)

    Geol. Soc. London Spec. Publ.

    (1981)
  • E.C. Beutner et al.

    Catastrophic emplacement of the Heart Mountain block slide, Wyoming and Montana USA

    Geol. Soc. Am. Bull.

    (2005)
  • A. Billi et al.

    Fault-related carbonate rocks and earthquake indicators: recent advances and future trends

  • A.M. Boullier et al.

    Microscale anatomy of the 1999 Chi-Chi earthquake fault zone

    Geochem. Geophys. Geosyst.

    (2009)
  • S. Boutaread et al.

    Clay-clast aggregates: a new textural evidence for seismic fault sliding?

    Geophys. Res. Lett.

    (2008)
  • E.E. Brodsky et al.

    A geological fingerprint of extremely low viscosity fault fluids mobilized during an earthquake

    J. Geophys. Res.

    (2009)
  • N. De Paola et al.

    Fault lubrication and earthquake propagation in thermally unstable rocks

    Geology

    (2011)
  • J.H. Goodwin

    Analcime and K-feldspar in tuffs of the green river formation, Wyoming

    Am. Mineral.

    (1973)
  • Green, J., Smith, P.S., 1963. Sugar-Coating Process. U.S. Patent 3,094,947. Filed February 12, 1959, Ser. No....
  • R. Han et al.

    Strong velocity weakening and powder lubrication of simulated carbonate faults at seismic slip rates

    J. Geophys. Res.

    (2010)
  • R. Han et al.

    Granular nanoparticles lubricate faults during seismic slip

    Geology

    (2011)
  • Cited by (0)

    View full text