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