Variation in vorticity of flow during exhumation of lower crustal rocks (Neoproterozoic Ambaji granulite, NW India)

Journal Pre-proof Variation in vorticity of flow during exhumation of lower crustal rocks (Neoproterozoic Ambaji granulite, NW India) Sudheer Kumar Tiwari, Anouk Beniest, Tapas Kumar Biswal PII:

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DOI:

https://doi.org/10.1016/j.jsg.2019.103912

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SG 103912

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Journal of Structural Geology

Received Date: 22 December 2018 Revised Date:

7 October 2019

Accepted Date: 13 October 2019

Please cite this article as: Tiwari, S.K., Beniest, A., Biswal, T.K., Variation in vorticity of flow during exhumation of lower crustal rocks (Neoproterozoic Ambaji granulite, NW India), Journal of Structural Geology (2019), doi: https://doi.org/10.1016/j.jsg.2019.103912. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

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Variation in vorticity of flow during exhumation of

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lower crustal rocks (Neoproterozoic Ambaji granulite,

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NW India)

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Sudheer Kumar Tiwari1*, Anouk Beniest2, Tapas Kumar Biswal1

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400076

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Netherlands

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* [email protected]

Department of Earth Sciences, Indian Institute of Technology Bombay, Powai, Mumbai, India-

Department of Earth Sciences, Vrije Universiteit Amsterdam, 1081 HV, Amsterdam, The

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Abstract

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The exhumation of the Neoproterozoic Ambaji granulite in the Aravalli-Delhi mobile belt, NW

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India, took place along NNW-SSE trending D2- shear zones. The shear zones evolved from a

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high temperature (>700 °C) thrust-slip shearing event in the lower-middle crust to a low

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temperature (450 °C) retrograde sinistral top-to-NW shearing event at the brittle- ductile-

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transition (BDT). The vorticity of flow (Wm) along the shear zones is estimated with the Rigid

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Grain Net and strain ratio/orientation techniques. The Wm estimates of 0.32-0.40 and 0.6

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coincide with the high temperature event and suggests pure shear dominated deformation. The

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low temperature phase coincides with Wm estimates of 0.64-0.87 and ~ 1.0 implying two flow

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regimes. The shear zone was first affected by general non-coaxial deformation and gradually

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became dominated by simple shearing. We interpreted that the high temperature event happened

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in a compressive tectonic regime which led to horizontal shortening and vertical displacement of

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the granulite to the BDT. The low temperature event occurred in a transpressive tectonic setting

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that caused the lateral displacement of the granulite body at BDT depth. The Wm values indicate

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a non-steady strain during exhumation of granulite. This tectonic evolution is comparable with

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that of the Himalayas.

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Key words: Vorticity analysis; shear zones; non-steady strain; exhumation of lower crustal rocks;

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Neoproterozoic Ambaji granulite

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1. Introduction

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Granulites form at lower crustal levels and reach the surface through exhumation along shear

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zones. These shear zones are commonly affected by general non-coaxial deformation, or, in other

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words, shearing with both a simple and pure shear component (Passchier and Urai, 1988; Wallis,

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1992; Wallis et al., 1993, Fossen and Tikoff, 1993; Simpson and De Paor, 1993). Throughout

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such an exhumation process, the ratio between the pure shear and simple shear component

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varies, resulting in a non-steady strain path (Fossen and Cavalcante, 2017). Natural examples of

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non-steady shearing are for instance the Main Central Thrust (MCT) in the Himalayas and the

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Ailao Shan-Red River (ASRR) shear zone in Southeast Asia. The MCT records the change from

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simple shear to general shear in tension gashes and quartz ribbons in mylonites. The MCT

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juxtaposed the high-grade metamorphic Central Crystallines against low grade metamorphic

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rocks (Grasemann et al., 1999; Jessup et al., 2006). The ASRR shear zone experienced pure shear

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deformation in its early stages and simple shear in its later stages as recorded in micro-structures

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in the high-grade metamorphic belt (Wu et al., 2017).

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The Neoproterozoic Ambaji granulite is a tectonically exhumed block in the South Delhi

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terrane (SDT), Aravalli-Delhi mobile belt (ADMB) in NW India (Fig.1a, b). Exhumation of the

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Ambaji granulite happened along shear zones that juxtaposed the granulites against low grade

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rocks (Singh et al., 2010) similar to External Hellenides (Xypolias and Koukouvelas, 2001). In

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our study, the shear zones of the Ambaji granulite were used for kinematic reconstructions to

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constrain the key aspects of the tectonic exhumation of deeper crustal rocks. For the first time in

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the ADMB we are able to explain the exhumation of the deeper crustal rocks by characterizing

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the direction of flow of material during the different shearing phases. We combined a structural

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and petrological field study with a quantitative vorticity analysis of the shear zones of the

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Ambaji granulite. We estimated the kinematic vorticity number (Wm) to quantify the pure shear

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and simple shear components during deformation. Because shearing occurred progressively

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between ca. 840 and 780 Ma (Tiwari and Biswal, 2019a), it allows us to distinguish different

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phases of flow during exhumation. The study has large implications for emplacement tectonics

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of basement rocks in other parts of the ADMB. This is a rather unique case, because lower crustal

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rocks are not often preserved at the surface. With this study we add to our understanding of

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tectonic process at different levels of the crust in Precambrian orogens and more specifically to

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the early stages exhumation of basement rocks in both the SDT and the ADMB. In addition, we

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contribute to the general comprehension of strain partitioning during exhumation of lower crustal

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rocks in an area that has recorded non-steady strain, similar to a modern orogen like the

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Himalayas.

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2. Regional tectonic framework

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The NE-SW trending ADMB is located in the northwestern part of the Indian Peninsula (Fig. 1a).

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It formed through at least three cycles of collision between the Bundelkhand craton and the

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Marwar craton during the Neoarchean - Paleoproterozoic, Paleo- to Mesoproterozic and

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Neoproterozoic eras (e.g., Synchanthavong and Desai, 1977; Sinha Roy, 1988; Volpe and

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Macdougall, 1990; Biswal et al., 1998a, b; Deb et al., 2001; Khan et al., 2005; Dharma Rao et

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al., 2012; Bhowmik and Dasgupta, 2012). During these collisional events, several metamorphic

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terranes were accreted onto the ADMB (Fig. 1b). From east to west the protolith age of the

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terranes becomes younger (ca. 3500 Ma in the east to 900 Ma in the west). The metamorphic and

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deformation ages show a similar trend (ca. 3500 Ma in the east to 750 Ma in the west) (Fig.1b,

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Gupta et al., 1980; Singh et al., 2010). The SDT accreted in the western part of the ADMB in

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response to subduction of a Proterozoic passive continental margin during the Neoproterozoic

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(Biswal et al., 1998a). The SDT consists of quartzite, pelite, carbonate metasediments that

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together with metabasalt, metarhyolite, metadiorite and metaplagiogranite show a rift signature

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(ca. 1000 Ma, Volpe and Macdougall, 1990; Deb et al., 2001; Dharma Rao et al., 2013).

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The SDT has undergone multiple stages of folding with contrasting folding geometries

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(F1-3; Naha et al., 1984; 1987). The earlier stage includes coaxial folding between

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recumbent/reclined F1 and upright F2 folds along a NE-SW axis, producing type 3 interference

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patterns throughout the entire terrane. A later NW-SE F3 fold imprinted the F1 and F2 folds,

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producing type 1 and type 2 interference patterns. Repetition of rock sequences, variation in

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plunge of the lineation and domal outcrop patterns are the result of the interference between

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those folds. Amphibolite facies rocks dominate the SDT (Sharma, 1988) except for the Ambaji

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area where granulite facies rocks are exposed (Ambaji granulite, Fig. 1b, c; Desai et al., 1978;

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Biswal et al., 1998a, b; Sarkar, 2006; Singh et al., 2010).

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The Ambaji granulite is bounded by the Balaram shear zone and Kui-Chitraseni fault in

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the west and the Surpagla-Kengora shear zone and fault in the east. Shear zones SZ-I, SZ-II, SZ-

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III are located within the granulite block and have a general NW-SE orientation (Fig.1c, d, e).

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The shear zones curve towards the SW and extend southward, most likely until the southwest

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border of the SDT (Gupta et al., 1980). Earlier work on the Ambaji granulite discusses the fold

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pattern, geochemistry, metamorphism and geochronology (Desai et al., 1978; Biswal, 1988;

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Biswal et al., 1998a, b; Sarkar, 2006; Singh et al., 2010). Geochemistry of the pelitic

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metasediments point to a former passive margin, whereas the metabasic rocks indicate an arc

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setting (Biswal, 1988; Biswal et al., 1998a,b). Rocks were metamorphosed in granulite facies at

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PT conditions of ca. 6 kbar/ 800 °C (Desai et al., 1978; Singh et al., 2010). Melting of the

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metasediments during the different folding phases produced granites that were emplaced during

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different stages of deformation (G0, G1, G2 and G3) between ca. 960 Ma and 759 Ma, coeval with

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the early-Pan-African orogeny (Singh et al., 2010). G0 (ca. 960 Ma, SHRIMP age, Singh et al.,

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2010) is a metarhyolite-granite and intruded the sedimentary protolith during the time of

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sedimentation. G1 (ca. 860 Ma, SHRIMP age, Singh et al., 2010) is medium to coarse-grained

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and contains quartz, feldspar, biotite, garnet and sillimanite. This granite occurs as parallel bands

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within the parent rock and was produced from the melting of the pelitic granulite. G2 (ca. 840

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Ma, SHRIMP age, Singh et al., 2010) is a rapakivi granite consisting of equigranular quartz,

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feldspar and biotite minerals. This granite occurs as veins and large-scale plutons and

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metasomatised the host rock. G3 (ca. 759 Ma, SHRIMP age, Singh et al., 2010) is medium

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grained with porphyritic texture and consists of quartz, K-feldspar and biotite phenocrysts. It

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occurs as dikes and veins that have a magmatic layering.

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3. Material and methodology

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We combine structural and petrological observations with vorticity and strain analyses. A study

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on the petrological fabric of the rocks, including strain analyses of the shear zones, has been

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carried out on 150 oriented samples. The samples were collected at a regular interval along 6

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profiles (Fig. 2), during a total of 15 weeks of fieldwork spread out over a 4 year period. The 12

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most representative samples are described in this paper. The collected samples were cut into thin

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sections perpendicular to the foliation. One set of thin sections is oriented parallel to the lineation

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(XZ section). The XZ sections are used for vorticity analysis and microstructural studies.

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3.1. Principle of the vorticity analysis

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The mean kinematic vorticity number, Wm, is an approximate measure of the relative proportion

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between the simple shear and pure shear component of a rock (Ghosh and Ramberge, 1976;

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Passchier, 1987). There are different types of vorticity gauges that have been applied to natural

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rocks, including (1) clast-based gauges (Ghosh and Ramberge, 1976; Passchier,1987; Simpson

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and De Paor,1993; Wallis et al.,1993; Jessup et al., 2007), (2) macroscopic foliation (RS/θ) or

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(RXZ / β, RXZ /θ) (Ramsay and Huber, 1987; Tikoff and Fossen, 1995; Bailey et al., 2004;

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Xypolias, 2010), (3) oblique foliation (Wallis, 1995) and (4) RXZ/δ method (Xypolias, 2009,

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2010). The clast-based gauges and Rs/θ method are most commonly applied to natural samples

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(e.g., Passchier, 1987; Vissers, 1989; Wallis et al., 1993; Simpson and De Paor, 1997; Holcombe

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and Little, 2001; Xypolias and Koukouvelas, 2001; Bailey and Eyster, 2003; Law et al., 2004;

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Carosi et al., 2006; Jessup et al., 2006, 2007; Xypolias and Kokkalas, 2006; Bailey et al., 2007;

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Marques et al., 2007; Thigpen et al., 2010). Their principles are briefly described here.

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Porphyroclasts rotate in response to shearing. Simple shearing produces forward rotated

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porphyroclasts while pure shearing rotates the clast in both ways depending on the inclination of

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the clast with respect to shear plane. General non-coaxial shearing rotates clasts both forward

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and backward as well, but here the rotation direction depends on the aspect ratio of the grains.

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Some of the grains show zero rotation and the aspect ratio of those zero rotation grains is called

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the critical aspect ratio (Rcrit) (Ghosh and Ramberg, 1976; Passchier, 1987, 1997; Simpson and

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De Paor, 1993, 1997; Stahr and Law, 2011). The Rcrit is a direct measure for Wm (Wm = Rcrit2-1/

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Rcrit2+1) and can be determined graphically (Jessup et al., 2007). Rcrit decreases with increasing

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pure shear component (Stahr and Law, 2011, 2014). Low aspect ratio grains will rotate

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indefinitely but come to rest when the pure shear component increases. A large population of

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grains with a wide range of aspect ratios is required for a reliable estimation of Rcrit. When aspect

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ratios are < 3, Wm is underestimated. Furthermore, as the rotation is slow for clasts with an

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aspect ratio between 2 and 3, Wm will be overestimated in pure shear and underestimated in

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simple shear dominated deformation. High strain is necessary to bring the clast into a stable

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position, hence, Rcrit is overestimated at low finite strain. Furthermore, the matrix should behave

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as a Newtonian fluid so that the clast and the matrix rotate together without any slip along the

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margin (Mandal et al., 2005; Johnson et al., 2009). The rotation rate is influenced by presence of

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rim around the clast (Mancktelow, 2013). Thus the final orientation of the clast is a complex

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function of aspect ratio, initial orientation, flow vorticity, 3D rotation, non-plane strain and finite

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strain (Li and Jiang, 2011). Despite such uncertainties, the RGN vorticity analysis has been

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applied to many natural shear zones (Grasemann et al., 1999; Law et al., 2004; Jessup et al.,

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2006) and we use the method as well.

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As a cross check, we compared the clast-based Wm estimates (RGN) with the Wm

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estimates that resulted from the Rs/θ method (Fossen and Tikoff, 1993; Bailey et al., 2004;

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Zhang et al., 2009; Faleiros et al., 2016). The strain ratio (Rs) is calculated using the Rf/Φ method

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(Ramsay, 1967; Lisle, 1985) and the θ angle is obtained by measuring the angle between S- and

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C- bands of the S-C fabric. However, there are also uncertainties in this method. The angle θ is a

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measure of instantaneous strain and represents the last instantaneous strain if the flow is non-

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steady (Wallis, 1995; Xypolias, 2010). Further, the quartz grains used for analysis tend to have a

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more equant geometry by the end of the deformation process as the recrystallization takes over

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mechanical deformation underestimating the real strain ratio (Xypolias et al., 2013). Lastly, the

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angle θ changes when there is a change in volume. Negative dilation reduces the angle θ and

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with it the Wm values (Ramsay and Huber, 1983). Despite these uncertainties we compare the

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Wm values of both methods to get more reliable Wm estimates.

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The equation used to calculate Wm with the Rs/θ method for general shear is

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Wm = cos [tan-1{1- Rstan2θ / (1+Rs) tanθ}] (Xypolias, 2009).

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The Wm values for simple shear are higher than for general non-coaxial shear, because the

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magnitude of the angle θ for general non-coaxial shear is smaller than for simple shear (Ramsay

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and Hubber, 1983, 1987; Tikoff and Fossen, 1995; Grasemann et al., 1999; Xypolias, 2010). We

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thus compared the Wm values of the 12 selected samples to identify the type deformation of the

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shearing.

(eq. 1)

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3.2. Working methodology and assumptions for Ambaji granulite

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For the vorticity analysis of the Ambaji granulite, we made several assumptions:

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i) We assumed that for the 2D vorticity gauges plane strain is needed (e.g., Lin and Jiang, 2001).

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Therefore, we obtained vorticity estimates along NNW-SSE striking straight segments of the

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shear zones. Asymmetric fabrics such as S-C fabric and porphyroclasts are best observed in the

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XZ plane, parallel to the deformation direction (Fig. 3a, c, d). In the YZ plane, perpendicular to

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the transport direction, the S-C fabric and porphyroclasts are mostly symmetrical in shape (Fig.

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3b). In general, the shear zones are monoclinic (e.g., Lin et al., 1998; Lin and Jiang, 2001; Forte

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and Bailey, 2007).

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(ii) The coarse-grained Ambaji granulite experienced granulite facies metamorphism, during

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which a pervasive S1 fabric developed (Fig. 4a, b). For the vorticity analysis we used individual

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quartz, feldspar and garnet grains that had euhedral shape because we assumed those minerals to

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have crystallized at lower crustal levels under high temperature and pressure condition with less

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differential stress. In this case, the minerals are less affected by previous orientation patterns

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caused by granulite facies metamorphism (Fig. 4b).

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iii) For a correct Wm estimate, we assume there was no volume change during deformation.

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Although the assumption of constant volume is difficult to prove, the absence of P-bands (no

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volume loss) and fissure veins (no volume gain) indicate that the volume didn’t change (e.g.,

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Ramsay and Lisle, 2000). Constant volume is further supported by the observation that garnet in

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the pelitic mylonite maintains an equant shape (Fig. 5c, d) and that the fractures inside the garnet

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do not undergo dilation or translation (Fig. 6a). The retrograde mineral phases replaced the

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garnet instead of filling the open fractures (e.g., Mahan et al., 2006).

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(iv) For the clast- based method, we used garnet grains to obtain the vorticity for high

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temperature shearing and feldspar grains for low temperature shearing. In both cases, the clasts

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have a large range of aspect ratios and are widely spaced (Fig. 3c, d; Fig. 5b, c). This means they

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did not experience any interference from other grains during rotation inside the homogeneous

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matrix. Furthermore, the clasts do not have a mantle rim that could interfere with rotation (Fig.

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5d, Fig. 6c). The porphyroclasts show no evidence for internal plastic deformation or reactions

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that distorted the original shape.

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(v) We assume the rotation of grains took place only around the axis normal to the XZ plane

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(vorticity normal plane, Passchier and Trouw, 2005) and there is no evidence for rotation in the

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YZ plane.

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undergone a large amount of finite strain as indicated by the feldspar delta porphyroclasts (Fig.

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6c).

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(vi) A large variety of aspect ratios (< 5) were included in the analysis (Fig. 5c, d, Fig. 6b).

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Grains with higher aspect ratios (> 5.0) are less frequently observed because the samples used for

Therefore, the influence of 3D rotation is minimal (Fig. 3b). The rocks have

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the analysis are from mylonites and ultramylonites. We used the following formula to calculate

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the aspect ratio (B*):

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B* = Mx2 - Mn2 / Mx2 + Mn2

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where Mx is the long axis and Mn is the short axis of the porphyroclast. The aspect ratio, Wm and

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the percentage of pure shear were calculated following Jessup et al., (2007). The calculated Wm

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values (Fig.7) were then plotted against θ (Fig.8a) (Tikoff and Fossen, 1995).

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(vii) For Rs/θ method, we measured the dynamically recrystallized quartz grains on the same thin

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sections as for the RGN analysis. In most cases, the grains are stretched and elliptical showing no

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signs of shape change after deformation. This is possible when the exhumation from the ductile

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to the brittle domain is very fast. For the Ambaji granulite, ductile shearing along the shear zones

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ended by 778 ± 8 Ma and brittle shearing began at 764 ± 9 Ma (Tiwari and Biswal, 2019a). The

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granulite terrane was exhumed quickly to shallower levels and thus cooled faster below the

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recrystallization temperature of quartz. We assume, therefore, that the shape ratio does not differ

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too much from the strain ratio. The Rs values were plotted against θ to solve for Wm, (Fig.8b,

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Ramsay and Huber, 1987; Tikoff and Fossen, 1995). The Wm obtained by both methods, RGN

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and Rs/θ, were then compared (Table 1, Fig.8c).

(eq.2)

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4. Results

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4.1. Lithology and structure of the Ambaji granulite

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The Ambaji granulite consists of pelitic, calcareous and mafic granulites (Biswal et al., 1998a,b;

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Singh et al., 2010) with elongated bodies of schist and amphibolite alongside the shear zones 11

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(Fig. 1c). The pelitic granulite is composed of migmatites with thick layers of melanosomes and

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leucosomes (Fig.4a). The melanosome consists of spinel, cordierite, sillimanite, garnet and

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biotite and the leucosome contains quartz, plagioclase and K-feldspar (Fig. 4b). The garnet,

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spinel, cordierite, quartz and feldspar are equiaxed and typically form 120° triple junction. The

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reaction that forms garnet from cordierite and spinel indicates pressure-temperature (PT)

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conditions for peak metamorphism of ca. 6-7 kbar and ≥ 850 °C for the pelitic granulite. This

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corresponds to a depth of ~25 km (Singh et al., 2010). The calcareous granulite consists of an

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alternation of coarse carbonate and calc-silicate layers with calcite, diopside, scapolite,

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plagioclase and wollastonite mineral assemblage. The mafic granulite includes a mineral

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assemblage of orthopyroxene, clinopyroxene, plagioclase, and hornblende arranged in distinct

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layers. Decompression texture is observed around garnet in pelitic granulite (Fig.4c) and

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orthopyroxene in mafic granulite. Also kyanite and andalusite that resulted from sillimanite are

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observed (Fig. 4d). PT-conditions of 4 kbar and 450 °C have been estimated for these reactions

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(Sarkar, 2006), corresponding to 15 km depth.

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The granulites are deformed by four stages of deformation, D1-D4. During D1

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deformation a set of isoclinal, recumbent/reclined F1 folds were produced. A pervasive S1-fabric

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runs parallel to the axial plane of the F1-folds defined by migmatite layers and granulite facies

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mineral assemblages in different rock types (Fig. 4a, b). During D2 deformation, the S1 fabric

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and F1 folds are refolded, by open upright F2 folds that are coaxial with F1 folds along NE-SW

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axis. The fold axes and intersection lineations of F1-F2 folds are generally plunging gently

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towards NE and SW. F2-axial planar crenulation cleavage and shear bands form the S2 fabric in

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the rock. Large D2 shear zones run parallel to the axial plane of the F2 folds. The shear zones

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host schist and amphibolite bodies containing quartz-biotite schist, albite-biotite-hornblende

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schist, granitic mylonite and mylonitised pseudotachylite. During D3 deformation, NW-SE

263

striking F3 folds were produced that brought variation in strike of the litho units and produced

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several interference patterns. The D4 deformation includes strike-slip and normal faults that cross

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cut the terrane and structures described above.

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4.2. Field description and mesoscopic structures of the D2 ductile shear zones

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We mapped five shear zones in the Ambaji granulite (Fig. 2a-d). The shear zones are up to 500

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meters wide and several kilometers long. The shear zones developed mainly along the contact

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between G2 granite intrusion and the mafic- and pelitic granulites. F1 and F2 fold-axes and

271

lineations rotated to shear direction along the shear zone. The mylonitic foliations of the shear

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zones vary in dip. Towards the SW in the Balaram shear zone and in SZ-I (Fig.2e,f), the dip is ~

273

45 degrees (Fig. 2j,l), in SZ-II and SZ-III the foliations are near vertical to the SE (Fig.2c,d, g,h,

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n,p) and in the Surpagla-Kengora shear zone the foliations dip 50 degrees towards NE (Fig.2d, i,

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r). The description of every individual shear zone can be found in supplementary Table S1. Most

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of the shear zones contain only low-grade metamorphic features which have overprinted the

277

deformation fabrics formed during high-grade metamorphism.. Exception is some parts along the

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Balaram shear zone and SZ-I (Fig. 1c, 2) where high-temperature deformation features are

279

preserved. In these high grade metamorphic parts, the stretching lineations are nearly vertical

280

(Fig.2k,m) and dominated by stretched quartz and feldspar minerals (Fig. 4e). Garnet and spinel

281

porphyroclasts in these parts deformed only in a brittle way. The feldspathic aggregate and fine-

282

grained garnet indicate a top-to-NW sense of shear (Fig. 4f, see photomicrograph Fig. 5d). The

283

low temperature parts are marked with low plunging stretching lineation (Fig.2o,q,s), defined by

13

284

biotite and stretched quartz grains (Fig.4g). The rotated sigmoidal feldspar porphyroclasts

285

deformed in a brittle phase and like the S-C fabric, they indicate a top-to-NW sinistral sense of

286

shear (Fig. 4h).

287 288

4. 3. Microscopic description of D2 shear zones

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4.3.1. Protomylonite and mylonite

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Protomylonites and mylonites are found in the low grade metamorphic part of the shear zones, in

291

the granitic host rocks G1 and G2. These consist of a large number of feldspar porphyroclasts in a

292

quartz-biotite matrix (Fig. 3c, 5a, b). The mylonites have a higher amount of biotite and fine to

293

medium sized feldspar porphyroclasts (> 50 %) than the protomylonite. Feldspar porphyroclasts

294

with aspect ratio as high as 3.0 are observed in the mylonites (Fig.5b). S-C fabric, sigmoidal

295

lenses, θ and δ type porphyroclasts indicate top-to NW sinistral sense of shear. Quartz lenses and

296

ribbons show undulose extinction, bulging (BLG) and sub-grain rotation (SGR) dynamic

297

recrystallization.

298

4.3.2 Ultramylonite

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Ultramylonite is found in the pelitic granulite, mafic granulite and pseudotachylite host rock. A

300

mylonitic foliation overprinted the coarse banding of migmatite in the high temperature part.

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Garnet grains, both rounded to elongated, are oriented parallel to mylonitic foliation (Fig.5c),

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and indicate top-to-NW sense of shear (Fig.5d). The garnets have fractures inside the crystal.

303

Quartz grains form large ribbons with individual grains sharing low angle grain boundary

304

(Fig.5e). These grains also show chessboard undulose extinction and grain boundary migration

305

(GBM) recrystallization (Fig.5f). Smaller feldspar grains with elliptical and euhedral shapes are 14

306

also observed surrounding larger grains. The parent feldspar deformed through SGR-GBM

307

dynamic recrystallization (e.g., Tullis and Yund, 1985) into rounded to elliptical grains with

308

undulose extinction (Fig.5g). Feldspar ribbons with minor recrystallized grains were folded

309

during shearing (Fig.5h).

310

The pelitic ultramylonite that developed under low-temperature conditions consists of granulite

311

minerals that altered to biotite in the presence of low temperature fluid. Biotite crystals formed

312

both parallel to the C-fabric as well as in the intragranular faults inside the garnets (Fig.6a, and

313

insets). Quartz minerals show BLG- SGR dynamic recrystallization (Fig. 6b). In some places, the

314

ultramylonite is well banded (Fig.6c; over the parent rock was most likely a pseudotachylite).

315

Low-temperature mafic ultramylonite contains biotite and albite (Fig. 6d). Several kinematic

316

indicators in the low temperature ultramylonite, such as S-C fabrics, intra-granular faults within

317

garnet and feldspar crystals and asymmetric tails in porphyroclasts indicate a top-to-N-NW

318

sinistral sense of shearing (Fig. 6a-d).

319 320

4. 4 Vorticity analysis

321

The vorticity estimation for high-temperature deformation was done on garnet clasts and for low-

322

temperature deformation feldspar clasts were analyzed. Dynamically recrystallized quartz grains

323

were used for both high- and low-temperature deformation.

324

4.4.1 High-temperature shearing (Sample PG1, PG2)

325

The samples PG1 and PG2 were collected from the pelitic ultramylonite of the Balaram shear

326

zone (PG1) and SZ-I (PG2). The samples have a fine mylonitic foliation and host quartz and

15

327

feldspar minerals that show GBM and SGR-GBM dynamic recrystallization (Fig. 5e-h). The

328

grains are aligned at a very low angle with respect to the mylonitic foliation. Garnets have both

329

elongated and equiaxed shape (R > 2.0, Fig.5c, d). The more elongated garnet minerals aligned

330

parallel to foliation (Fig.5c). The equiaxed grains consist of asymmetric wings (Fig.5d). We

331

measured garnet porphyroclasts for the RGN analysis. The feldspar minerals show signs for

332

dynamic recrystallization and are thus not suitable for RGN analysis. The RGN analysis of the

333

garnet porphyroclasts yields a Wm estimate of 0.32 to 0.40 (73 - 79 % of pure shear), and Rs/θ

334

yields a Wm estimate of 0.45 to 0.70 (51- 71% pure shear) (Table 1, Fig. 7, 8a,b).

335

4.4.2. Low-temperature shearing (Sample BL1, BL2, AJ, AJ1, SR1, SR2, GH3, GH4, K1, K2)

336

The samples BL1, BL2, AJ, AJ1, SR1, SR2, GH3, GH4, K1 and K2 are the representative

337

samples for the vorticity analysis of the low-temperature sheared samples. The samples are

338

arranged from west to east (for sample location see Fig. 2). Wm values are obtained from RGN

339

diagrams of Figure 7, listed in Table 1 and plotted in Figure 8a and b. Samples BL1 and BL2

340

belong to Balaram shear zone. They contain feldspar porphyroclasts rotated within a quartz-

341

biotite matrix. The RGN and Rs/θ yield Wm estimates of 0.72- 0.87 (32-49 % pure shear) for

342

sample BL1 and 0.71-0.96 (16-50 % pure shear) for sample BL2. Samples AJ and AJ1 belong to

343

SZ-I. The RGN and Rs/θ yield Wm estimates of 0.64-0.77 (44-56 % pure shear) for sample AJ

344

and 0.85-0.95 (22-34 % pure shear) for sample AJ1. Samples SR1 and SR2 belong to SZ-II. The

345

samples are rich in biotite, that retrograded from hornblende. Albite and quartz minerals are

346

observed in the matrix. The RGN and Rs/θ yield Wm estimates of 0.72-0.82 (39-49 % pure shear)

347

for sample SR1 and 0.86 – 0.97 (11-25 % pure shear) for sample SR2. Samples GH3 and GH4

348

belong to SZ-III that formed in the G1 granite. Sporadic sillimanite and garnet minerals are found

16

349

in the matrix. We have used feldspar clasts for RGN analysis. The RGN and Rs/θ yield Wm

350

estimates of 0.66-0.77 (44-54 % pure shear) for sample GH3 and 0.52-0.88 (31-65 % pure shear)

351

for sample GH4. Samples K1 and K2 belong to the Surpagla-Kengora shear zone. The feldspar

352

porphyroclasts appear to have rotated freely within a quartz-biotite matrix. Quartz minerals show

353

BLG and SGR dynamic recrystallization. Rotated feldspars form delta type porphyroclasts. The

354

RGN and Rs/θ yield Wm estimates of 0.72-0.81 (40-48 % pure shear) for sample K1 and 0.89-

355

0.98 (8-30 % pure shear) for sample K2. In summary, our results show that the average RGN and

356

Rs/θ yield Wm estimates of 0.35 and 0.61 for high temperature shearing and 0.75 and 0.87 for

357

low temperature shearing.

358 359

5. Discussion

360

5.1 Progressive shearing

361

The field results and petrological analysis provide information about the relative timing, depth

362

and temperature conditions during the D2 shearing event that partly exhumed the Ambaji

363

granulites. There is a clear distinction between high temperature and low temperature shearing

364

events and we interpret them as two different stages of a continuous event: an early stage of high

365

temperature shearing and a later stage of low temperature shearing.

366

5.1.1. Timing of the deformation phases

367

From the structures and folds, we have interpreted four different deformation phases (D1-D4).

368

The D1 phase caused the S1-fabric (Fig. 4a) to run parallel to the axial plane of F1-folds which

369

was produced by a simple shear. The F1 folding event and the accompanied granulite facies

17

370

metamorphism are constrained at ca. 860 Ma (Singh et al., 2010). During the D2 deformation

371

phase, the S1 fabric was folded by F2 folds that are now coaxial with the F1 folds (Fig. 4a). The

372

F2 folds were produced from NW-SE compression. Because the large shear zones are oriented

373

parallel to the axial plane of the F2 folds, they likely formed simultaneously during the D2

374

deformation phase (Fig. 1c). Compression during D2 deformation phase accommodated most of

375

the exhumation of the granulite along the shear zones. The F2 folding is constrained at ca. 840

376

Ma based on the age of G2 granite (Singh et al., 2010). D2 shearing was constrained between 834

377

± 7 to 778 ± 8 Ma (Monazite ages, Tiwari and Biswal, 2019a). During exhumation, the D1

378

garnet, pyroxene and sillimanite formed decompression textures (Fig. 4c-d). PT-conditions of 4

379

kbar and 450 °C have been estimated for this retrograde phase (Sarkar, 2006), which suggests

380

that the granulites moved to shallower levels around the BDT at 15 km depth. Pseudotachylites

381

were injected in the shear-zones which is also an indication for brittle deformation and thus

382

shallower deformation than D1. The D3 deformation occurred subsequently and the last

383

deformation phase, D4, occurred in the brittle domain and is reflected by the strike-slip and

384

normal faults that are dated at 764 ± 9 Ma (Monazite ages, Tiwari and Biswal, 2019a).

385

5.1. 2 Early stage high temperature shearing during D2

386

The high-temperature shearing is preserved in the shear zones of the western part of the area

387

(Balaram shear zone and SZ-I). The shear zones are characterized by subvertical mylonitic

388

foliation with down-dip stretching lineation, the mylonitic foliation overprinted the original

389

granulite facies S1 that formed at lower crustal level around 25 km depth (Fig.9a). The rotation of

390

fold axes close to the shear zones that run parallel to the stretching lineation indicate that the F1

391

and F2 folds transformed from low plunging NE and SW to almost vertical in response to the D2

392

shearing. Quartz and feldspar grains in the leucosome had euhedral shapes during D1 phase but 18

393

stretched during D2 phase. The quartz grains deformed by dislocation creep and completely

394

recrystallized into elongate grains with chessboard extinction by GBM dynamic recrystallization.

395

Feldspar grains dynamically recrystallized into elliptical to euhedral grains. Both the quartz and

396

feldspar minerals are indicative of a deformation temperature of about > 700 °C (Stipp et al.,

397

2002; Passchier and Trouw, 2005). This suggests as well that the high-temperature deformation

398

took place at lower-middle crustal levels (Fig.9b). During this high temperature deformation

399

phase, garnet suffered mainly brittle deformation. Both the rotated garnet and feldspar lenses

400

show a top-to-NW sense of shear. The upward movement of material, suggest that the shear

401

zones brought the granulite to shallower level around the BDT and thus acted as a thrust (Fig.9c).

402

Synchronous with this thrusting and exhumation, the rocks underwent decompression and

403

cooling.

404

5.1.3. Late stage low temperature shearing during D2

405

In the central and eastern parts of the granulite, the high temperature shearing domains have been

406

completely overprinted by low temperature mylonitic deformation. Deformation took place at the

407

brittle-ductile transition zone (Fig.9c) which occurs at 10 to 15 km depth, as indicated by

408

multiple pseudotachylite veins that were injected in the mylonite and are associated with

409

deformation at the BDT (e.g., Sibson, 1975; Schmid and Handy, 1991; White, 1996; Price et al.,

410

2012). Granulite grade minerals retrograded into biotite in the presence of fluids and formed SE

411

dipping mylonitic foliation and subhorizontal stretching lineation. The high temperature fabric in

412

this part of the granulite is completely overprinted by the low temperature deformation. The

413

vertical quartz-feldspar stretching lineations were replaced by subhorizontal quartz-biotite

414

stretching lineations. Quartz grains deformed into slivers and lenses by dislocation creep and

415

BLG-SGR recrystallization. Feldspar deformed in the brittle domain and rotated with a top-to-

19

416

NW sinistral sense of shear. These feldspar grains survived from the high temperature shearing

417

phase and acquired euhedral to elliptical shape with aspect ratio as high as > 3.0. Based on the

418

retrograde reaction of garnet into biotite, BLG-SGR dynamic recrystallization of quartz and

419

brittle beahaviour of feldspar, a temperature of around 450 °C has been assigned to this low

420

temperature deformation event (Sarkar, 2006 and similar studies by Stipp et al., 2002; Stipp and

421

Kunze, 2008, Faleiros et al., 2010).

422 423

5.2 Non-steady strain path

424

We reconstructed the strain path of the exhumation of the granulites (Fig.9a-c) based on the

425

kinematic vorticity analysis that was similar to other studies that used kinematic vorticity number

426

to reconstruct temporal and spatial variation of strain in shear zones (e.g., French Pyrenees,

427

France, Passchier, 1987; Sanbagawa belt, Japan, Wallis, 1995; Hellenides, Greece, Xypolias and

428

Doutsos, 2000; Arizona, USA, Bailey and Eyster, 2003; MCT in NW Himalayas, Grasemann et

429

al., 1999, 2003; India/Nepal Himalayas, Law et al., 2004, 2013; Raft River Mountains, USA,

430

Sullivan, 2008, Law, 2010; Attico-Cycladic Massif (Internal Hellenides), Greece,

431

Xypolias et al., 2010; Red River shear zone, China,Wu et al., 2017). Vorticity estimates using

432

the recrystallized quartz method yield higher values than the vorticity estimates obtained from

433

the clast method for both high and low temperature deformation.

434

The RGN-Wm estimate of 0.32 to 0.4 for high temperature shearing event pointed to pure

435

shear deformation in a compressional tectonic setting (Table 1, Fig. 8a-c). The Rs/θ-Wm

436

estimates are a little higher (0.4 to 0.7) and suggest a simple shear dominated deformation at the

437

end of the high temperature deformation phase. Both results indicate that the granulite was

20

438

horizontally compressed. This horizontal compression led to ductile thinning of the shear zones

439

and the upward escape of material from lower crust to shallower levels. Though we did not

440

measure the amount of shortening from vorticity data (e.g., Wallis et al., 1993; Law et al., 2004),

441

the narrow outcrop width of the high temperature part of the shear zone (northern part of the SZ-

442

I) reflects the ductile thinning.

443

The low temperature sheared domains that dominate in all the shear zones of the Ambaji

444

granulite, show a similar sense of deformation direction as the high temperature part i.e. top-to-

445

NW. The RGN-Wm estimates are 0.64 - 0.87 and the Rs/θ-Wm estimates are around 1.0 (Table 1,

446

Fig. 8c). These results are consistent with that of other shear zones (e.g., Wallis, 1995; Law et

447

al., 2004, 2013; Sullivan, 2008; Johnson et al., 2009; Faleiros, 2016; Wu et al., 2017).

448

The higher Rs/θ -Wm number than the RGN-Wm values (Fig.8c), obtained from the

449

ductile strain fabric, was calculated from the inclination and aspect ratio of the dynamically

450

recrystallized quartz grains. The fabric represents the fossilized last instantaneous strain. Hence,

451

the Rs/θ-Wm estimate does not provide information about the progressive strain history but is

452

rather a measure of ultimate instantaneous strain. Oppositely, RGN-Wm estimate provides the

453

mean vorticity of progressive shearing as it is measured from the shape ratio and the inclination

454

angle of the rigid clasts that rotated with different rates in response to the changing strain pattern

455

(pure shear/simple shear ratio). The lower RGN-Wm values suggest an overall pure-shear

456

deformation regime for the earlier high-temperature deformation phase, whereas the high Rs/θ-

457

Wm number of the low temperature deformation event, shows that later in the ductile

458

deformation phase simple shear dominates. This shows that there is a temporal variation of strain

459

during deformation and thus a non-steady strain path during the D2 deformation phase. If the

21

460

shear zones would have undergone steady strain both the RGN and Rs/θ methods would produce

461

similar Wm estimates.

462

Only in the SZ-III the Wm estimates from both methods have similar values, implying

463

that some parts of the granulite underwent steady strain during this deformation phase. Although

464

the exact reason for this difference is unknown, the SZ-III developed in the G1-granite while the

465

other shear zones developed along the contact between different rock units. Lithology may have

466

a control on the strain behaviour. Another observation is that the shear zones, in particular, the

467

SZ-I yields lower RGN-Wm estimate (~0.8) towards the southeast, suggesting a spatial variation

468

of strain at the edge of granulite may also influence the vorticity.

469

Our vorticity analysis revealed that the exhumation of the Ambaji granulite occurred

470

along a non-steady strain path. Strain at depth and at high-temperature was mainly pure shear

471

(decelerating flow path, Fig.9a, b, Simpson and De Paor, 1993). Strain at shallower crustal levels

472

and low temperature changed from general non-coaxial shear to simple shear (accelerating flow

473

path, Fig.9c). This partitioning in strain during exhumation resulted in the first phase of ductile

474

thinning and vertical displacement of the granulite from the lower crust to shallower crustal

475

levels around the BDT (Fig.9b). During a second phase, the granulite was displaced laterally at

476

the depth of the BDT (Fig.9c), because the differential stress needed to enable thrusting in the

477

brittle crust was not high enough (Behr and Platt, 2011). As a result, the initial vertical flow of

478

material, dominated by pure shear through compression, transformed into a lateral flow of

479

material, dominated by simple shear and transpression, that was also assisted by fluid activity.

480 481

5.3. Regional tectonic implication

22

482

The vertical extrusion and lateral flow tectonics have been reported in the Phanerozoic orogens

483

(Xypolias and Koukouvelas, 2001; Law et al., 2004, 2010; Xypolias et al., 2010; Faghih and

484

Sarkarinejad, 2011). In the Himalayas, the Indo-Asian collision was accommodated by vertical

485

flow and crustal thickening in the Himalayan–Tibetan Plateau (England and Houseman, 1986).

486

In other parts, crustal blocks of the Asian continent extruded laterally along numerous strike-slip

487

fault systems (Tapponnier et al., 1982; Peltzer et al., 1989; Replumaz and Tapponnier, 2003).

488

Our study on the Ambaji granulite indicates a similar tectonic style which is an innovative of

489

vorticity analysis of shear zones in the lower crustal rocks of Precambrian orogen. Similar to the

490

Himalayas, the ADMB formed after multiple phases of subduction/collision between the

491

Bundelkhand and Marwar craton. The SDT resulted from this Neoproterozoic collision (Biswal

492

et al., 1998a, Singh et al., 2010). Crustal shortening in the SDT was partitioned between vertical

493

extrusion and crustal thickening, which exhumed the lower crustal rocks to the BDT, and strike-

494

slip shearing, which displaced material laterally to the south. Evidence for an early stage

495

thrusting phase has been reported elsewhere in the ADMB, for example in the Phulad thrust along

496

the western edge of the SDT (Sengupta and Ghosh, 2004; Chatterjee et al., 2017). Final

497

exhumation of the ADMB to the surface accompanied by tectonic denudation/erosion by

498

extensional faulting. In the SDT, this process thinned the brittle crust, exposed the granulites to

499

the surface (Tiwari and Biswal, 2019b). This final stage of extensional faulting thereby extruding

500

lower crustal rocks, has also been observed in other orogens (e.g., Stuwe and Powell, 1989;

501

Harley, 1989; Carson et al., 1997; Stuwe and Barr, 1998).

502 503

6. Conclusions

23

504

Based on the field work, the petrological analysis and the vorticity analysis of the D2- shear

505

zones in the Ambaji granulite, we have come to the following conclusions:

506

1) The Ambaji granulite has undergone four deformation phases (D1-D4), of which the D2

507

deformation phase was most significant for the exhumation of granulites in the ductile regime.

508

The shear zones record a non-steady flow path with an early phase of high temperature shearing

509

and a later phase of low temperature shearing. The high temperature deformation (> 700 °C)

510

yielded Wm estimates of 0.32-0.40 that indicate pure shear dominated deformation at lower

511

crustal levels. The low temperature deformation (450 °C) yielded Wm estimates of 0.64-0.87 that

512

points to a general non-axial dominated deformation with an equal or even larger contribution of

513

simple shear at shallower crustal levels around the BDT. The low temperature shearing phase

514

ended with simple shearing as indicated by Rs/θ-Wm estimate of almost 1.0. During the high

515

temperature deformation, the granulite was elevated from lower crustal levels to the BDT,

516

through pure shear deformation in compression. During the low temperature deformation, the

517

granulite was displaced laterally at around the BDT, through simple shear deformation that

518

resulted from strike-slip tectonics.

519

2) The Ambaji granulite shows a strain partitioning between pure shear dominated deformation,

520

vertical displacement and crustal thickening in a large-scale thrust tectonic setting on one hand

521

and general co-axial simple shear to true simple shear dominated deformation and lateral

522

migration of the granulite in a large-scale strike-slip tectonic setting on the other hand. This is

523

comparable to Himalayan tectonics, that shows a crustal thickening in Tibetan Plateau and lateral

524

extrusion in SE Asian region along numerous strike-slip fault systems. This points to similar

525

crustal dynamics throughout the area during the Neoproterozoic and Phanerozoic periods.

24

526 527

Acknowledgments

528

The first author acknowledges the research funding from UGC- SRF fellowship from New Delhi.

529

The authors sincerely thank the Department of Science and Technology, New Delhi and Indian

530

Institute of Technology Bombay for funding the project. Prof. Arlo Brandon Weil, Prof. Jeremie

531

Lehmann, Prof. Helga de Wall, Prof. R. Harinarayan, Prof. T. Toyoshima, Prof. Frederico Meira

532

Faleiros and an anonymous reviewer have critically reviewed the earlier version of the

533

manuscript. Critical reviews by two anonymous reviewers greatly improved the manuscript. We

534

are extremely grateful to them and editorial handling by Prof. C. Passchier is greatly

535

acknowledged.

536 537 538 539 540 541 542

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619

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621

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629

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645

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729 730

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750 751 752 753

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771 772 773 774

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775

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781

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782

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783

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785

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786

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787

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788

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789

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790

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791

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792

Yunnan, China. Science in China Series D: Earth Sciences 52, 602-618.

793 794 795 796 797 798 799

33

800

Caption of the Figures

801

Fig. 1. (a) Location of the ADMB on the Indian Peninsula, (b) Simplified terrane map of the

802

ADMB, (c) Geological map of Ambaji granulite, modified after Singh et al. (2010), (d) NE-SW

803

trending cross-section A-B through the Kui-Chitraseni fault, the granulite block and the

804

Surpagla-Kengora fault, (e) E-W trending cross-section C-D including the Balaram shear zone,

805

SZ-I, SZ-II, SZ-III and the Surpagla-Kengora shear zone.

806

Fig. 2. Detailed geological maps of the (a) Balaram shear zone, (b) SZ-I shear zone, (c) SZ-II

807

shear zone, (d) SZ-III and Surpagla-Kengora shear zones, (e) E-W trending cross-section A-B

808

through the Balaram shear zone, (f) SW-NE trending cross-section C-D through SZ-I, (g) SW-

809

NE trending cross-section E-F through SZ-II, (h) SW-NE trending cross-section G-H through

810

SZ-III and SW-NE trending cross-section I-J through the Surpagla-Kengora shear zone, (j-s)

811

Stereoplots for mylonitic foliation and stretching lineation, (j-k) for the Balaram shear zone, (l-

812

m) for SZ-I, (n-o) for SZ-II, (p-q) for SZ-III, (r-s) for the Surpagla-Kengora shear zone. Index

813

and

814

Fig. 3. Hand specimen photographs of mylonite (3a, 3b length 5 cm). (a) XZ section with

815

asymmetric features e.g., sigmoidal feldspar (Fsp) porphyroclasts and oblique quartz (Qtz)

816

ribbons, (b) YZ section with no oblique fabric. The clasts are more symmetric, (c)

817

Photomicrograph of XZ section of the mylonite-ultramylonite, used for RGN analysis, with

818

widely spaced porphyroclasts, and forward (FW) and backward (BW) rotated grains. The strain

819

ratio of the dynamically recrystallized quartz grains (Rs) and the S-C angle (θ) are measured on

820

the same surface, (d) Schematic diagram for Fig. 3c, showing rotated porphyroclasts (FW, BW),

821

dynamically recrystallized quartz grains, the S-C fabric, the angle of inclination of clasts (θ) with

822

C-fabric. Brittle fractures (Fr) cross cut the mylonitic foliation.

scale

are

the

same

34

for

all

shear

zones.

823

Fig. 4. Structures in Ambaji granulite. (a) Outcrop photograph of migmatites showing the

824

upright open F2 fold. Alternating melanosome and leucosome bands define S1, (b)

825

Photomicrographs (Plane polarized light, PPL) of the mineral assemblage of the pelitic granulite,

826

sillimanite (Sil), spinel (Spl), cordierite (Crd) and garnet (Grt) in the melanosome, aligned with

827

S1. The leucosome consists of equiaxed quartz (Qtz) and feldspars (Fsp) with 120° boundary

828

angle, (c) Photomicrographs of smaller D2- cordierite (Crd) and spinel (Spl) crystals developed

829

as a result of decompression, surrounding a D1- garnet porphyroblast (Grt), (d) Photomicrograph

830

of andalusite (And) and kyanite (Ky) from D1- sillimanite (Sil), (e) Outcrop photographs of

831

stretching lineation (parallel to scale) on the mylonitic foliation in the high temperature pelitic

832

ultramylonite of Balaram shear zone, (f) XZ section showing feldspar lens indicating top-to-NW

833

sense of shear, (g) Horizontal stretching lineation on vertical section in a low temperature

834

mylonite, SZ-II, (h) Horizontal section of the low temperature mylonite, SZ-III, sigmoidal

835

feldspar porphyroclasts show top-to-NW sinistral shear.

836

Fig. 5. Photomicrographs of low temperature mylonite (a-b) and high temperature mylonite, (c-

837

h). (a) Cross-polar view (XPL), ductile deformation in quartz (Qtz) with BLG and SGR

838

recrystallization, biotite (Bt) grows along the C-fabric, top-to-NW sinistral sense of shear, (b)

839

XPL, feldspar porphyroclasts (Fsp) with aspect ratio over 3.0, quartz ribbons with undulose

840

extinction and SGR, top-to NW sinistral sense of shear, indicated by feldspar porphyroclasts, (c)

841

High temperature mylonite with garnet porphyroclasts (Grt) (aspect ratio over 2.0) rich layer, (d)

842

Garnet porphyroclasts show top-to-NW sense of shear, (e) Finely laminated high temperature

843

mylonite with stretched quartz ribbons, (f) Close- up of quartz grains with chessboard undulose

844

extinction (CB) and GBM recrystallization, (g) Feldspar porphyroclasts (Fsp) in high temperature

35

845

mylonite with a rim of SGR-GBM recrystallized feldspar grains, (h) Folded feldspar ribbons

846

around the feldspar porphyroclasts (Fsp).

847

Fig. 6. Photomicrographs of low temperature ultramylonite (XPL). (a) Retrogression of garnet

848

(Grt) to biotite (Bt). The biotite (Bt) grew along the C-fabric and intragranular faults. Inset:

849

detailed view on the retrogression of garnet, (b) Pelitic ultramylonite with alternating quartz

850

(Qtz) (white) and biotite (Bt) (dark) rich layers. Inset: BLG in quartz (Qtz), (c) Banded

851

ultramylonite with alternating white and dark bands. Rotated porphyroclasts and S-C fabric

852

indicate top-to-NW sinistral sense of shear, (d) Mafic ultramylonite with albite (Ab) and biotite

853

(Bt), S-C fabric indicates top-to-NW sinistral sense of shear.

854

Fig. 7. (a-l) RGN plot (Jessup et al., 2007) of porphyroclasts from the mylonite and

855

ultramylonite. Each plot belongs to one sample. The sample locations are shown in Fig. 2. (a)

856

and (b) correspond to samples of the high temperature mylonite (c)-(l) belong to samples of the

857

low temperature mylonite. Sample Number and Wm are mentioned at the top of each figure. (N)

858

Number of grains is mentioned at the bottom right corner. B*= (long axis2 - short axis2) / (long

859

axis2 + short axis2). Wm defines the line from where θ sharply rises above the Rcrit value.

860

Fig. 8. (a) θ vs Rigid Grain Net rotation (RGN) - Wm plot. Wm values are taken from Fig. 7 and

861

Table 1, (b) Strain ration (Rs)/ S-C angle (θ) plot, mean kinematic vorticity (Wm) curves are

862

after, Ramsay and Huber, 1983; Fossen and Tikoff, 1993. The length of the bar indicates the

863

uncertainty in θ values. Samples SR2, K2, BL1, BL2, SR1, AJ1 and AJ have a Wm of 0.9-1.0,

864

GH3 and GH4 have a Wm of 0.7-0.8, PG1 and PG2 have a Wm of 0.5-0.7, (c) Graph illustrating

865

the variation of “RGN- Wm” and “Rs/θ - Wm” across the granulite block. The length of the bars

866

reflects the uncertainty in the estimation of Wm. Samples PG1 and PG2 belong to high-

36

867

temperature mylonite and BL1, BL2, AJ, AJ1, SR1, SR2, GH3, GH4, K1 and K2 belong to low-

868

temperature mylonite. Rs/θ -Wm exceeds the RGN- Wm for all samples except GH3 and GH4.

869

The scale relations between Wm and percentage of simple shear are according to Forte and

870

Bailey (2007).

871

Fig. 9. Schematic cartoon (a) The formation of the granulite occurred at 25 km depth at ca. 860

872

Ma, (b) The granulite exhumed to 15 km depth, below the BDT, through compression and

873

thrusting at ca. 834 Ma, (c) Lateral flow of material was accommodated by sinistral strike-slip

874

shearing, at the BDT at ca. 778 Ma (ages after Singh et al., 2010; Tiwari and Biswal, 2019a).

875

Caption of the Table

876

Table 1. Kinematic vorticity values based on RGN and (Rs/θ) techniques. The scale relations

877

between Wm values and % of simple shear are according to Forte and Bailey (2007). PG1, PG2

878

are from the high temperature mylonite, the other samples are from the low temperature

879

mylonite.

880

Caption of the Supplementary Table

881

Table S1. Lithological description of the shear zones, including the sense of shear.

882 883

37

Table 1. Kinematic vorticity values based on RGN and Rs/θ techniques. The scale relations between Wm values and % of simple shear are according to Forte and Bailey (2007). PG1, PG2 are high temperature mylonite and the rest low temp mylonite.

Sample Name

1

PG1

2

PG2

3

BL1

4

BL2

5

AJ

6

AJ1

7

SR1

8

SR2

9

GH3

10

GH4

11

K1

12

K2

Shear zones

Wm (RGN Method)

Pure (RGN) Shear %

Simple (RGN) Shear %

Rs

θ

Wm (Rs/θ Method)

Pure (Rs/θ ) Shear %

Simple (Rs/θ ) Shear %

0.32- 0.40

73-79

21-27

5.86

6.1±2

0.6 – 0.7

51-59

41-49

0.32- 0.35

77-79

21-23

5.78

5.3±3

0.45- 0.7

51-71

29-49

0.77-0.79

39-44

56-61

5.13

13.3±10

0.71-0.96

16-50

50-84

0.72-0.87

32-49

51-68

4.18

15.4±8

0.80-0.94

22-41

59-78

0.64-0.73

48-56

44-52

6.65

13±7

0.85-0.95

20-34

66-80

0.72-0.77

44-49

51-56

5.73

13.9±6

0.86-0.95

20-33

67-80

0.80-0.82

39-41

59-61

5.08

17.5±8

0.92-0.97

11-25

75-89

0.72-0.82

43-49

51-57

3.33

21.7±4

0.93-0.96

16-24

76-84

0.66-0.74

47-54

46-53

4.14

10±8

0.52-0.88

31-65

35-69

0.70-0.77

44-51

49-56

3.98

10.1±6

0.60-0.84

36-59

41-64

0.73-0.81

40-48

52-60

3.83

20.2±10

0.89-0.98

8-30

70-92

0.72-0.80

41-49

51-59

3.70

20±10

0.89-0.98

8-30

70-92

Kengora

SZ-III

SZ-II

SZ-I

Balaram

SZ-I

Sr No

SE

NW

(a)

Qtz

Fsp

10 mm

NW

SE

(b)

Qtz

Fsp

10 mm

NW

SE

(c)

NW

BW

S Qtz C

FW 0.5 mm

0.5 mm

SE

(d)

NW

SE

(a)

0.5 mm

(b)

F2

S1 Grt Fsp+ Qtz

Crd

Spl

S1 Sil

15 cm

0.5 mm

(c)

0.5 mm

(d) And

Spl Crd Grt

SW

Ky

NE

(e)

NW

NE (g)

SE (f)

2 cm

10 cm

SW

Sil

NW

SE (h)

Stretching lineation

15 cm

15 cm

0.5 mm

Qtz NW

(a) SE

(b) SE

NW

SGR

C BLG

0.5 mm

R > 3.0

SGR S

Bt

C

S Fsp

0.2 mm

(c)

0.5 mm

(d) SE

NW Grt R > 2.0

0.5 mm

(e)

0.2 mm

(f)

GBM

CB

0.3 mm

(g)

0.5 mm

(h) Fsp clast

Fsp Fsp

SGRGBM Fsp

Fsp ribbon

0.3 mm

SE

NW

(a)

NW

Bt

(b)

SE

Qtz Bt

C

0.2 mm

Grt Bt

Qtz+Bt

Grt

0.5 mm

SE

NW

2 mm

(c)

(d) NW

δ

SE S

σ

Ab

S C C

Qtz

Bt 0.5 mm

0.3 mm

Shape factor (B*)

GH3 (SZ III)

(N=428)

Wm= 0.66-0.74

Shape factor (B*)

(N=176)

(f)

(j)

AJ1 (SZ I)

Wm= 0.72-0.77

Shape factor (B*) GH4 (SZ III)

(N=336)

Wm= 0.70-0.77

Shape factor (B*)

(N=173)

(g)

(d)

(h)

(N=189)

(N=159)

Wm= 0.77-0.79

Shape factor (B*)

SR1 (SZ II)

Angle bet. Long axis a and macroscopic foliation (Θ)

Wm= 0.64-0.73

(N=142)

BL1 (BSZ)

Angle bet. Long axis a and macroscopic foliation (Θ)

(i)

AJ (SZ I)

Shape factor (B*)

(c)

Angle bet. Long axis a and macroscopic foliation (Θ)

(e)

(N=135)

Wm= 0.32-0.35

Angle bet. Long axis a and macroscopic foliation (Θ)

Angle bet. Long axis a and macroscopic foliation (Θ)

Shape factor (B*)

PG2 (BSZ)

Angle bet. Long axis a and macroscopic foliation (Θ)

Rcrit line

(b)

Angle bet. Long axis a and macroscopic foliation (Θ)

Rcrit line

Angle bet. Long axis a and macroscopic foliation (Θ)

Wm= 0.32-0.40

Angle bet. Long axis a and macroscopic foliation (Θ)

PG1 (SZ I)

Angle bet. Long axis a and macroscopic foliation (Θ)

(a)

Angle bet. Long axis a and macroscopic foliation (Θ)

Angle bet. Long axis a and macroscopic foliation (Θ)

RGN plot for porphyroclasts from all shear zones

(N=280)

Wm= 0.80-0.82

Shape factor (B*) (k)

K1 (KSZ)

Wm= 0.73-0.81

Shape factor (B*)

BL2 (BSZ)

Wm= 0.72-0.87

Shape factor (B*)

SR2 (SZ II)

Wm= 0.72-0.82

Shape factor (B*) (l)

K2 (KSZ)

(N=202)

(N=169)

Wm= 0.72-0.80

Shape factor (B*)

(N=186)

Rs/ɵ plot

ɵ vs Wm-RGN plot (a) Pure shear

50 Simple shear

ɵ( )

40

Simple shear dominated Transtension Simple shear dominated Transpression

Pure shear dominated Transpression

30

e diat

e

ISA

10

rm inte

P G1

P G2 00 0.0 Pure shear

0.2

0.6

(b) A

ma

60 x

Wm

Sample and error bars

0.8

50

0.9 40 1.0

o

S R2 K2 K1 B L2 S R1 AJ AJ1 B L1 G H3 G H4 Sample plots 0.6

0.4

Mean kinematic vorticity value

0.6

0.7

30

20

0.8

0.8

20

K1

K2

0.7 0.6 0.4 0.2

10

1.0 Simple shear

SR2

0.9

SR1 BL2

AJ1

AJ

BL1

GH4

GH3

PG2 PG1

0 1

2

3

5

4 Rs

7

6

Sample Number K2

K1

GH4 GH3 SR2 SR1

AJ1

AJ

BL2

BL1 PG2

PG1 0% 100%

(c)

90%

0.95

20%

80%

0.90

30%

70%

0.80

40%

60%

0.60 0.50 0.40

0.00

K2

K1 GH4 GH3 SR2

70% 80%

0.20 0.10

60%

SR1 AJ1

AJ

BL2

BL 1

PG2 PG1

50% 40%

30%

20%

90%

10%

100%

0%

Pure shear dominated

Wm values- RGN Method Wm values- Rs/θ Method

0.30

50%

Percent of Simple shear

Percent of Pure shear

0.70

General shear

10%

Simple shear dominated

1.00

Kinematic vorticity number

o

IS

ɵ( )

dominated Transtension

a)

BDT

15 km 25 km

860 Ma

b)

BDT

15 km

25 km

Vertical extrusion of granulite 834 Ma

c)

BDT

15 km

25 km

Lateral flow of granulite 778 Ma

Mica schist Amphibolite Granulite Granite

Highlights

•

Exhumation of the Ambaji granulite happened in two phases with non-steady strain.

•

Wm estimates are 0.4 for high and 0.64-0.87 to 1.0 for low temperature shearing.

•

The first exhumation phase was by NW thrusting, the second by sinistral strike slip.

•

High temperature transpression brought the Ambaji granulite to upper crustal levels.

•

Low temperature, non-coaxial/simple shear led to lateral displacement in upper crust.

Conflict of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.