Fabric transition with dislocation creep of a carbonate ...hosting03.snu.ac.kr/~hjung/pdf/Kim_et_al_2018.pdf · Yeongwol Group consists of carbonates and subordinate siliciclastics,

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Tectonophysics

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Fabric transition with dislocation creep of a carbonate fault zone in thebrittle regime

Sungshil Kima, Jin-Han Reea,⁎, Raehee Hanb, Nahyeon Kima, Haemyeong Jungc

a Department of Earth and Environmental Sciences, Korea University, Seoul 02841, Republic of KoreabDepartment of Geology and Research Institute of Natural Science, Gyeongsang National University, Jinju 52828, Republic of Koreac School of Earth and Environmental Sciences, Seoul National University, Seoul 08826, Republic of Korea

A R T I C L E I N F O

Keywords:Fabric transitionLattice preferred orientationFrictional heatingSeismic slipGaram thrust

A B S T R A C T

Fabric transition by a switch in the dominant slip system of minerals in the plastic regime can be induced bychanges in temperature, strain rate, or water content. We propose here this fabric transition by frictional heatingin seismogenic fault zones in the brittle regime. The Garam Thrust in the Taebaeksan Basin of South Korea has ahanging wall of Cambrian dolostone juxtaposed against a footwall of Ordovician limestone and records aminimum displacement of ~120 m. In a 10 cm thick plastically deformed layer adjacent to the principal sliplayer of the fault zone, the lattice preferred orientation of calcite grains suggests that the dominant slip systemchanges, approaching the principal slip layer, from r ⟨02–21⟩ and e-twinning, through r ⟨02–21⟩ and basal ⟨a⟩,to basal ⟨a⟩. This fabric transition requires a high temperature-gradient of 40 °C/cm, which we infer to resultfrom frictional heating of the seismic fault zone. We suggest that fabric transition within a thin plasticallydeformed layer adjacent to the principal slip layer of a fault zone indicates an unusually steep temperaturegradient and provides strong evidence of seismic slip.

1. Introduction

Fabric transition is a change in the pattern of the lattice preferredorientation (LPO) of minerals which results from a switch in thedominant slip system in the plastic regime (Lister and Paterson, 1979).This may occur in response to changes in temperature, strain rate (Behr,1968; Lister and Dornsiepen, 1982; Lister and Paterson, 1979), watercontent, differential stress (Jung and Karato, 2001), or pressure (Junget al., 2009). Fabric transition in calcite rocks has been reported fromboth deformation experiments (Barber et al., 2007) and numerical si-mulations of fabric development (Takeshita et al., 1987). For naturallydeformed quartz grains, fabric transition occurs in association withchanges in metamorphic grade over distances of several hundred meters(Lister and Dornsiepen, 1982; Stipp et al., 2002), highlighting the effectof temperature on fabric transition.

Recent halite shearing experiments at seismic slip rates (Kim et al.,2010) and analytical solutions of temperature distributions withinseismic fault zones in the brittle regime (Nielsen et al., 2008) highlight

the existence of very high temperature gradients across the fault zones.These gradients are likely caused by frictional heating along the prin-cipal slip layers at seismic slip rates (Nielsen et al., 2008; Kim et al.,2010) and may result in the formation of plastically deformed layersaway from, but adjacent to, the principal slip layer (e.g., Bestmannet al., 2012; Kim et al., 2010; Smith et al., 2013; Ree et al., 2014).Furthermore, plastic deformation of the principal slip layer itself mayoccur due to frictional heating (Green et al., 2015; De Paola et al.,2015). Based on the observation of a thin plastically deformed layeradjacent to the principal slip layer in a natural carbonate fault (GaramThrust), we propose that fabric transition is possible in the brittle re-gime due to the intense thermal gradients generated during coseismicfrictional heating.

2. Geological setting

The Garam Thrust occurs within the Cambrian-OrdovicianYeongwol Group of the Taebaeksan Basin, South Korea (Fig. 1). The

https://doi.org/10.1016/j.tecto.2017.12.008Received 1 July 2017; Received in revised form 7 December 2017; Accepted 8 December 2017

⁎ Corresponding author.E-mail address: [email protected] (J.-H. Ree).

Tectonophysics 723 (2018) 107–116

Available online 11 December 20170040-1951/ © 2017 Elsevier B.V. All rights reserved.

T

Fig. 1. Geologic map of the Taebaeksan Basin in Yeongwol area of the Korean Peninsula (inset). Star: location (37°17′53.71″N, 128°25′52.38″E) of the outcrop of Fig. 2. From Choi(1998).

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Yeongwol Group consists of carbonates and subordinate siliciclastics,and can be divided from oldest to youngest into the Sambangsan, Ma-chari, Wagok, Mungok and Yeongheung formations (Choi and Chough,2005; Chough et al., 2000). The regional structure of the Yeongwol areain the Taebaeksan Basin is characterized by a stack of imbricate thrustsheets (Fig. 1; Choi, 1998). The Garam Thrust, which is part of thisthrust system, has moved the Cambrian Wagok Formation onto theOrdovician Mungok Formation (Fig. 2a). The Wagok Formation of thehanging wall is a massive Cambrian dolostone consisting mainly ofeuhedral dolomite, whereas the Mungok Formation of the footwallcomprises a limestone of grainstone/packstone facies with chert no-dules. The footwall limestone contains fossil fragments and sedimentarystructures including cross-lamination, graded bedding and burrowing(Choi, 1998; Chung et al., 1993; Kim and Choi, 2000; Woo and Choi,1993; Woo et al., 1990; Woo and Moore, 1996).

Where it outcrops, the Garam Thrust strikes N50°W and dips to thesouthwest at 45°. The slip direction, as inferred from ridge-in-groovetype slickenside lineations on the fault plane, plunges 42° towardS65°W (Fig. 2b). The minimum displacement estimated from thethickness of the two formations and the dip of the thrust plane is~120 m.

The fault zone in the hanging wall is defined by a ~9 m wide da-mage zone of relatively densely spaced fractures and a ~5 m widecataclastic fault core immediately adjacent to the principal slip layer(Fig. 3). In contrast, the fault zone in the footwall is defined by a thin(< 2 m) damage zone in outcrop.

3. Slab structures and microfabrics of fault rocks

Fault rocks and undeformed wall rocks were sampled for

Fig. 2. (a) Outcrop photograph of the Garam Thrust. Fm: formation. (b) Stereographic projection of the fault plane and slip direction. Black arrow: direction of microstructuralobservation.

Fig. 3. Schematic line drawing of the Garam Thrust and structures of adjacent wall rocks. Six sampling sites (arrows) are also shown.

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microfabric analyses (see Fig. 3 for sampling locations). Thin-sectionswere cut parallel to the slip direction and normal to the fault plane, andthe viewing direction is toward the northwest (Fig. 2b). Thin-sectionswere imaged using both optical and electron microscopy, and mineralcompositions were determined using a JEOL JXA SX electron microp-robe at Korea University. Electron backscattered diffraction (EBSD)analysis was performed to measure the crystallographic orientation ofgrains using the SEM JEOL 6380 at Seoul National University. TheEBSD patterns were analyzed using the HKL Channel 5 software, andpole and inverse-pole figures were constructed using MTEX (Hielscherand Schaeben, 2008).

The undeformed wall rock of the footwall located ~9 m northeast ofthe thrust (sample GAR001-7) comprises a thinly bedded limestone thatcontains layers of a very fine-grained calcite alternating with similarlayers that additionally contain very fine-grained silicic clasts. Thecalcite grains of sample GAR001-7 do not show intracrystalline plasticmicrostructures such as deformation bands, mechanical twins, or un-dulose extinction. Sample GAR001-8, collected ~4 m from the thrust, isa micritic limestone consisting mainly of fine-grained calcite(10–50 μm, average 15 μm) with some fossil fragments and minor do-lomite. The coarser calcite grains usually contain growth twins. Thedolomite grains are much coarser than the calcite grains (average130 μm), are strain-free, and occur as dispersed individual grains or as

grain aggregates (Fig. 4a). Microveins contain strain-free calcite grains.The undeformed massive dolostone in the hanging wall located

~20 m from the thrust (GAR001-9) consists of strain-free dolomitegrains that show a bimodal size distribution with average sizes of 160and 795 μm (Fig. 4b). The damage zone and fault core in the hangingwall are much wider than in the footwall. The dolostone in the hanging-wall damage zone is highly fractured (Fig. 4c) and shows an increase infracture density toward the thrust plane. The dolostone cataclasite ofthe fault core contains angular fragments in a fine-grained matrix(Fig. 4d).

A marked contrast exists between the microstructure of the hanging-wall dolostone and footwall limestone immediately adjacent to theprincipal slip layer of the thrust. Area H2 in Figs. 5a and 6a shows thecataclasite of the hanging wall to consist of highly fractured angularfragments of dolomite aggregates in a fine-grained matrix. Closer to theprincipal slip layer (now filled with calcite vein material; see below),the proportion of matrix and roundness of fragments increase as grainsize decreases, presumably due to abrasion (Area H1 in Figs. 5a and6b). Individual dolomite grains within the fragments do not show anyintracrystalline plastic microstructures, but contain intragranular frac-tures.

In contrast, the relatively thin (< 2 m thick) damage zone in thefootwall contains fractures, veins, and stylolites without typical fault

Fig. 4. Photomicrographs of the undeformed wall rock and fault rocks in damage zone. (a) Undeformed micritic limestone of the footwall (GAR001-8). Dol: dolomite. (b) Undeformeddolostone of the hanging wall (GAR001-9). (c) Fractured dolostone in the damage zone of the hanging wall (GAR-DOL). (d) Dolostone cataclasite of the hanging wall (GAR001-10).Crossed- (a and b) and plane-polarized (c and d) lights.

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rocks such as the cataclasite that is developed in the hanging wall.However, a thin plastically deformed layer (~10 cm) is developedimmediately adjacent to the principal slip layer (Fig. 5; see below).Although the average grain size of calcite (~15 μm) in the margin ofthe plastically deformed layer (Area F3 in Fig. 5a) reflects that of theundeformed grains in the wall rock, the calcite grains are weaklyelongated and many of the coarser grains contain mechanical twins(Fig. 6c). Closer to the principal slip layer (Areas F2 and F1 in Fig. 5a)the calcite grains show more obvious evidence of plastic deformationand dynamic recrystallization. In Area F2, the calcite grains show abimodal size distribution with average population sizes of 25 and100 μm, with the coarser grains commonly containing mechanicaltwins (Fig. 6d). The mechanical twins are thicker than those in Area F3,and lobate twin boundaries indicate twin boundary migration. Thesetwin microstructures suggest that the deformation temperature in AreaF2 was higher than in Area F3 (Burkhard, 1993) and the grains showundulose extinction, and bulging/wavy grain boundaries, and subgraindevelopment that commonly indicate dynamic recrystallization.

Area F1 (immediately adjacent to the principal slip layer) is char-acterized by calcite grains that are elongate relative to those in Areas F2

and F3, and show lobate/bulging grain boundaries, suggesting dynamicrecrystallization by grain boundary migration (Fig. 7a). The grain sizeof dynamically recrystallized calcite decreases toward the principal sliplayer (Fig. 7b). Locally, large and apparently elongate calcite grains(150–300 μm with axial ratios of 1.8–3.7) exhibit bent mechanicaltwins, and lean in the direction of shear (~20° counterclockwise fromthe principal slip layer) (Fig. 7c). Although the origin of these elongatecalcite grains is unclear, they were possibly derived from fossil frag-ments. Dolomite aggregates (with individual grain sizes of100–300 μm) also occur in Area F1. The dolomite grains display thickmechanical twins, some of which are bent, indicating intracrystallineplasticity. Some of the plastically deformed dolomite grains are cut byR-shear fractures (Fig. 7d).

Disseminated, strain-free euhedral to subhedral quartz and ortho-clase grains overgrow the elongate calcite grains in the plastically de-formed layer (Fig. 7c, e, and f). The plastically deformed layers alsocontain two types of calcite microveins, one with dynamically re-crystallized grains and one with undeformed grains, suggesting mul-tiple fracturing and plastic deformation events.

Measurement of the lattice orientation of calcite in the plastically

Fig. 5. (a) Rock slab including principal slip layer (PSL), plastically deformed limestone of the footwall and dolostone cataclasite of the hanging wall. (b) Photomicrograph of the black-lined box of (a). Plane-polarized light. (c) Line drawing of (b). White area: undeformed calcite vein. Thick black line: Deformed and recrystallized calcite vein. Thin black line: stylolite.

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deformed layer (Areas F1, F2 and F3 in Fig. 5a) shows an increase in thestrength of the LPO toward the principal slip layer (from F3 to F1;Fig. 8). The LPO pattern in pole figures (Fig. 8) and inverse pole figures(Fig. 9), together with the developed microstructures (mechanical twinspredominate in Area F3), indicate that the dominant slip systems are r⟨02–21⟩ and e-twinning in Area F3, r ⟨02–21⟩ and basal ⟨a⟩ in Area F2,and basal ⟨a⟩ in Area F1. It has been reported that the dominant slipsystem of calcite changes from e-twinning through r ⟨02-21⟩ to basal⟨a⟩ as deformation temperature increases (Barber et al., 2007; DeBresser and Spiers, 1997). Thus, fabric transition in the plastically de-formed layer of the Garam Thrust is interpreted to be induced by anincrease in temperature from Area F3 to Area F1 during deformation.

In a 5 cm thick portion (Areas F1 and F2) of the plastically deformedlayer, euhedral to subhedral strain-free quartz (10–20 μm) and ortho-clase (< 10 μm) grains occur with plastically deformed calcite anddolomite as mentioned earlier (Fig. 7c, e and f). The euhedral quartzcrystals are columnar with their c-axes oriented parallel to the columnlength. The c-axes of the quartz grains show random orientations inArea F2, but a strong preferred orientation normal to the slip directionon the slip plane in Area F1 (Fig. 10). The reorientation of the long axesof the euhedral-columnar quartz grains normal to the slip direction onthe slip plane presumably occurred by rigid-body rotation within aweaker, plastically deforming calcite matrix.

The principal slip layer between the hanging-wall dolostone

cataclasite and the footwall mylonitic marble is filled with ≤4 mmthick calcite veins. The veins generally consist of variably sized(50–500 μm) strain-free, blocky calcite grains with random lattice or-ientations. Locally, some grains are slightly elongate and show un-dulose extinction. Together, these observations suggest that the calciteveins along the principal slip layer formed after the main slip event.

4. Discussion and conclusion

The origin of the thin plastically deformed layer in the otherwisebrittle fault zone of the Garam Thrust represents question. Generally,observations such as those described above would suggest that theGaram Thrust originated as a plastic shear zone, which, having initiallyformed at higher temperatures at greater depths, was overprintedduring uplift by brittle deformation along the thrust. However, thissequence of events does not explain the preservation of primary sedi-mentary structures and fossils of wall rock or the absence of an ob-viously cataclastic overprint of the plastic microstructures. Moreover,apart from the thin plastically deformed layer, there is no evidence ofplastic deformation in either the hanging-wall dolostone or the footwalllimestone.

Another possibility is that the localized plastic deformation adjacentto the principal slip layer occurred due to frictional heating during aseismic slip event. This phenomenon has been reported from high-

Fig. 6. Photomicrographs of dolostone cataclasite in Area H2 (a) and H1 (b) of Fig. 5a. Plane-polarized light. Photomicrographs of plastically deformed limestone in Area F3 (c) and F2 (d)of Fig. 5a. Crossed-polarized light.

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Fig. 7. Photomicrographs of plastically deformed limestone in Area F1 (Fig. 5a) immediately adjacent to the principal slip layer. (a) Microstructures of Box 2 in Fig. 5c. (b) Micro-structures of Box 1 in Fig. 5c. (c) Elongated calcite grains of fossil fragment. Double arrow: orientation of its long dimension. (d) Fractured dolomite aggregates within finer-grained calcitematrix. (e) Back-scattered SEM micrograph of quartz and orthoclase overgrowing calcite grains. (f). Enlargement of boxed area of (e). Cal: calcite. Or: orthoclase. Qtz: quartz. Plane- (c)and crossed-polarized (a, b, and d) lights.

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velocity friction experiments on halite and marble (Kim et al., 2010;Ree et al., 2014), calcite gouge (Smith et al., 2015), and naturalquartzose fault rocks (Bestmann et al., 2011, 2012). In particular, Kimet al. (2010) showed from deformation experiments on halite gouge atseismic slip rates that the plastic deformation of halite in a lower-slip-rate layer is facilitated by heat conduction from a higher-slip-ratemolten layer (i.e., principal slip layer) that in turn formed by frictionalheating in the brittle regime.

Experimental deformation of calcite shows that fabric transitions,expressed by changes in LPO patterns, from e-twinning and r ⟨02–21⟩slip to basal ⟨a⟩ slip, require temperature increases of ~400 °C at strainrates of 2 × 10−4 to 2 × 10−5 s−1 (De Bresser and Spiers, 1997). Suchchanges in temperature represent a high temperature-gradient of 40 °C/cm for a 10 cm thick plastically deformed layer such as that exposed inthe Garam Thrust. This high temperature-gradient could be generatedadjacent to the principal slip layer due to frictional heating at seismicslip rates (e.g., Di Toro et al., 2006). Thus, the plastically deformedlayer of the Garam Thrust probably originated as a result of localizedplastic deformation due to frictional heating adjacent to the principalslip layer of the fault. The higher flow strength (Barber et al., 1981,1994; De Bresser et al., 2002; Delle Piane et al., 2008, 2009) and lowerfracture strength (Colmenares and Zoback, 2002; Verbovsek, 2008) ofdolomite relative to calcite account for the absence of a plasticallydeformed layer, and the higher fracture density in the hanging-wall

dolostone, respectively.Considering the theoretical calculation of temperature distribution

within seismic fault zones (e.g., Nielsen et al., 2008), however, theplastically deformed layer (~10 cm thick) of the Garam Thrust may betoo thick. Then, how high would the coseismic temperature rise be inthe area adjacent to the principal slip layer? The temperature rise maybe calculated using the following equation (Eq. (10) of Carslaw andJaeger, 1959, p. 60):

= × ⎛⎝

⎞⎠

T T erfc xκt

Δ20

(1)

where x is the distance from the slip zone boundary, κ is the thermaldiffusivity (1.6 × 10−6 m2/s for calcite; Ree et al., 2014), t is theduration of slip, and T0 is the temperature of the principal slip layer,which may be assumed to be reached very shortly after the onset of slipand then kept constant during the coseismic slip. For our calculation,we take 900–1200 °C as a possible range of temperature in the principalslip layer (based on the measured slip zone temperature in the high-velocity rotary shear tests on carbonate rocks (e.g., Han et al., 2010)).For T0 = 900 °C, fault displacement of 5 m (a representative displace-ment for a large earthquake), slip rate of 1 m/s and slip duration of 5 s,the temperature rise is estimated to be 722 °C, 190 °C and 11 °C forx= 1 mm, x= 5 mm and x = 1 cm, respectively. For T0 = 1200 °C, atthe same slip condition, the temperature rise is 963 °C, 254 °C and 15 °C

Fig. 8. Pole figures of calcite lattice orientations in the plastically deformed layer measured using the electron backscattered diffraction (EBSD) analysis. (a) Area F1. n (the number ofmeasured grains) = 891. (b) Area F2, n = 545. (c) Area F3, n = 302. ODF Kernel: de la Vallee Poussin. Half-width angle: 15°. Intensity: J-index (logarithmic scale).

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for x= 1 mm, x = 5 mm and x= 1 cm, respectively. This calculationshows that there is little increase in temperature at the end of coseismicslip for the area farther than 1 cm from the principal slip layer. Amodeling of temperature change in the area adjacent to the principalslip layer immediately after the coseismic slip (Eq. (3) of Carslaw andJaeger, 1959, p. 54) indicates that a relatively thick portion (up to 5 cmthick) adjacent to the principal slip layer can suffer a temperature in-crease by 100–300 °C at a few minutes after the coseismic slip arrest(Bestmann et al., 2012). If this is the case, the plastic deformationparticularly in Areas F2 and F3 of the Garam Thrust may have occurredright after the coseismic slip (or during afterslip). Or there might beother heat source in addition to the frictional heating such as advectionof hot fluid for the development of relatively thick plastically deformedlayer. The rigidly-rotated euhedral to subhedral quartz grains in theplastically deformed layer may indicate the advection of fluid in thelayer, although the temperature condition of the fluid activity is unclearat present.

The thin, post-slip calcite veins that occupy the principal slip layerpostdate the processes that occurred and material produced in the layerduring seismic slip. Recent high-velocity friction experiments (e.g., Hanet al., 2007a, b, 2010; De Paola et al., 2011; Smith et al., 2013) and fieldobservations of carbonate faults (Collettini et al., 2013) show thatthermal decarbonation may occur in the slip zones of carbonate faultsdue to frictional heating during seismic slip. Thus, we speculate thatthermal decarbonation in the principal slip layer of the Garam Thrustmay have occurred contemporaneously with the plastic flow of calcitegrains in the adjacent layer with a lower strain rate.

In summary, fabric transition in a narrow zone adjacent to theprincipal slip layer can be induced by frictional heating during seismicfaulting in the brittle regime. We further suggest that a thin plasticallydeformed layer with microfabrics that imply an unusually steep tem-perature gradient adjacent to the principal slip layer of a fault zoneprovides strong evidence of seismic slip. This suggestion can be testedby more careful microfabric analyses on fault zone samples experi-mentally sheared at seismic slip rates. The resetting degree of certainisotopic systems in appropriate minerals may also be used to quantifytemperature gradient in plastically deformed layers.

Acknowledgements

We thank Nicola De Paola and Janos Urai for their comments whichimproved the manuscript. We also thank Co-Editor-in-Chief, KelinWang, for his editorial effort and encouragement. This work was sup-ported by the National Research Foundation of Korea fund NRF-2014R1A1A2056836 to Ree.

Fig. 9. Inverse pole figures of calcite lattice orientations. (a) Area F1. (b) Area F2. (c)Area F3. ODF Kernel: de la Vallee Poussin. Half-width angle: 15°. Intensity: J-index(logarithmic scale).

Fig. 10. Pole figures of quartz c-axis. (a) Area F1.n = 233. (b) Area F2. n = 179. ODF Kernel: de laVallee Poussin. Half-width angle: 15°. Intensity: J-index (logarithmic scale).

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