Implication of Dynamic Recrystallization Mechanism for the Exhumation of Lower Crustal Rocks: A Case Study in the Shear Zones of the Ambaji Granulite, NW India

Exhumation of granulites from middle to lower crustal depths to the surface involves different exhumation mechanisms (for example, see reviews by [1, 2] and references therein). One of the most important mechanisms is exhumation along shear zones. Shear zones experience simple shear deformation with various proportions of pure shear ([3], which vary with different mineral assemblages that depend on PT condition and fluid composition. Quartz-rich shear zones are specifically characteristic for their variations in dynamic recrystallization processes that include bulging (BLG, [4, 5]), subgrain rotation (SGR), and grain boundary migration (GBM, [6]) depending on temperature flow stress and strain rate. Hence, studies of shear zones lead to our understanding of how flow stress and strain rate variations account for the exhumation of the granulite at different levels of the crust.

The low- to medium-grade Aravalli-Delhi mobile belt of NW India exposes several isolated exhumed mid to lower crustal rocks. The Ambaji granulite (Figure 1) is one of such exhumed crustal bodies, and it belongs to the South Delhi terrane of the Aravalli-Delhi mobile belt. Several shear zones are responsible for the exhumation of the granulite [7–9]. Detailed structural study and strain analysis of the shear zones were done previously, but the temperature and flow stress conditions during exhumation are still unknown. We investigated quartz microstructures and grain boundary mechanisms to provide constraints on the deformation temperature, flow stress, and strain rate conditions during exhumation. Our study deciphers the role of dynamic recovery processes in controlling the nature of the flow of motion at different depths.

The ADMB was formed during the continental collision of the Marwar and Bundelkhand cratons through several orogenic cycles. During these orogenic cycles, several terranes were tectonically accreted along NE-SW trending shear zones (Figures 1(a) and 1(b); [3, 10–21]). The Mangalwar-Sandmata-Hindoli-Jahazpur terranes, previously grouped under Bhilwara supergroup (Figure 1(b)), went through the Bhilwara orogenic cycle, which ended with the intrusion of the Berach granite at 2.6 Ga [22]. The Mangalwar and Sandmata terranes, earlier referred to as banded gneissic complex (BGC) [23], consist of tonalite-trondhjemite-granodiorite gneisses formed from 3.3 Ga until 2.5 Ga ([24] and reference therein). The Hindoli-Jahazpur terrane hosts Archean greenstone belts [22, 25] in which rhyolite and volcanic tuffs intruded through arc related magmatism at 1.8 Ga [26].

The Aravalli and North Delhi terranes formed during the Aravalli orogenic cycle, which ended 1.7 Ga with the obduction of the ophiolites along the Rakhabdev shear zone [10]. The Archaean continental and oceanic basements of the Aravalli terrane is covered by 2.5 Ga old conglomerate and palaeosol that are overlain by shallow water stromatolite-bearing rocks in the east and carbonate and pelitic rocks in the west [27, 28]. The North Delhi terrane consists of quartzite, calc-schist, and mica-schist that occur in the fault-bounded Khetri, Alwar, and Lalsot-Bayana subbasins that also have an Archean basement [29]. The terranes have gone through two metamorphic cycles at 0.9 Ga [30] in the North Delhi terrane and at 1.7 Ga in the Sandmata terrane [31].

The Sirohi terrane and South Delhi terrane (SDT) formed during the South Delhi orogeny in response to westward subduction along the Kaliguman shear zone [10, 20, 32]. The Sirohi terrane consists of the large Erinpura granite intrusion, and the Malani igneous suite was emplaced ca. 1.0 Ga to ca. 765 Ma [33–37] with isolated pockets of low-grade metasediments that were deposited ca. 992 Ma [38]. In the north, the SDT consists of low-grade metamorphic rocks, intruded by the Sendra and Sewariya plutons resting unconformably on the gneissic-granulitic basement of the Mangalwar and Sandmata terrane [23]. Exotic granulites, emplaced between 1.7-1.5 Ga and 1.0 Ga, occur at Pilwa-Chinwali [39, 40]. The central part of the SDT is dominated by low-grade metasediments and metavolcanics (ophiolites [22]) that were intruded with diorite (ca. 1.0 Ga) and granites (840 Ma, [15]). The southern part of the SDT comprises quartzites and calcareous rocks that were affected by several stages of granite intrusions (ca. 960 Ma-759 Ma, [10]). Granulite in the SDT exposed along shear zones of Ambaji granulite underwent metamorphism between 875 and 857 Ma [11] or more exact around 860 Ma [10].

The Ambaji granulite is situated in the southern part of the SDT bounded by the Kui-Chitraseni in the west and the Surpagla-Kengora faults in the east (Figure 1(c)). The fold pattern [10], shear kinematics and vorticity of the shear zones [3], magma pressure measurements of granite veins [19], paleostress reconstructions of faults [21], and the geochemistry [20], metamorphic grades, and geochronology of the granites and strain fabrics [10, 11] have all been studied (Table 1). The Ambaji granulite consists of pelitic, calcareous, and mafic granulite and four phases of granite intrusions have been identified (⁠$G0=960Ma$⁠, $G1=860Ma$⁠, $G2=840Ma$⁠, and $G3=759Ma$⁠, Table 1, [7, 8, 10]). Contractional deformation (D₁ and D₂) in the NW-SE direction during the South Delhi orogeny resulted in isoclinal-recumbent F₁ folds and upright NE-SW trending F₂ folds [10]. The Balaram shear zone, SZ-I, SZ-II, SZ-III, and Surpagla-Kengora shear zones surrounding or internally affecting the Ambaji granulite are formed during this D₂ deformation phase. Folds and shear zones are superimposed by the NW-SE trending upright F₃ folds formed in response to the NE-SW-oriented compressional deformation phase D₃ [41]. This resulted in strike and dip variations in the north and south of SZ-I [3].

An electron microprobe analysis of the coexisting mineral phases in pelitic granulite yielded peak pressure-temperature (PT) conditions of 5.5–6.8 kbar/≥850°C, which corresponds to deformation at ~25 km depth with a clockwise exhumation path using the NaKFMASH system [10, 42]. Vorticity analyses using rigid grain net and Rs/θ methods showed that deformation occurred in two distinct stages: low-grade and high-grade shearing of a continuous ductile deformation [20]. The low-grade shearing event was dominated by simple shear, but relicts of high-grade metamorphic events were found within the low-grade rocks of the shear zones. Vorticity analysis of high-grade metamorphic rocks showed that this deformation was dominated by pure shear deformation [3]. The difference in deformation types (simple vs. pure shear) is appointed to a first, granulite-grade (high-grade) deformation event under contractional event at 834 Ma, overprinted by a second, greenschist-grade (low-grade) deformation event under strike-slip regime at 778 Ma. The Ambaji granulite was first thrusted upwards until it reached the BDT, after which strike-slip shearing continued the deformation at a lower metamorphic grade [3]. Final extrusion of the Ambaji granulite occurred along the major normal fault with NW-SE extension [19, 21].

We mapped the shear zones in the Ambaji granulite and added them to the already existing structural geological map made by [10, 11] (Figure 1(c)). The samples (locations in Figure 1(c)) of the mylonite were collected from quartz-rich parts of the Balaram shear zone (BSZ, samples BL1 and BL2), SZ-I (samples PG1, AJ, and AJ1), SZ-II (samples SR1 and SR2), SZ-III (samples GH3 and GH4), and the Surpagla-Kengora shear zone (SKSZ, samples K1 and K2). The samples were oriented with respect to the north direction by marking the strike and dip of mylonitic foliations and the plunge of stretching lineations. The thin sections were prepared by cutting the samples perpendicular to foliation and parallel to lineation (i.e., XZ plane of strain ellipsoid).

The microstructural analysis was carried out on thin sections using a high-resolution optical microscope (Leica 2000). Microphotographs were captured for data analysis with a standard image analyzer (LAS V4.5). Recrystallized quartz grains of every sample were then subjected to a grain size analysis using “ImageJ” (https://imagej.nih.gov/ij/) software. We manually calculated the area of each grain individually using the above-mentioned software. The entire exercise is a 2D analysis of the grain size in the XZ plane of a thin section. To calculate the approximate size of each recrystallized grain, we used the methodology described by [43], where the area of each recrystallized quartz grain is considered equivalent to area of a circle (with diameter $d=2R$⁠, where $R$ is the radius). The diameter (⁠$d$⁠) of that circle was calculated by following the above process. An average (⁠$D$⁠) of the $d$ values for the entire grain population for each sample was calculated from the data set of approximated grain sizes (Supplementary Figure S1 and Table S1).

Apart from measuring the diameter of the grains, we also identified the different deformation mechanisms for dynamic recrystallization (i.e., change in grain size and shape) and orientation of the mineral as a result of material displacement [44–47] for every measured quartz grain (Figures 2 and 3). Two distinct grain boundary processes are vital during dynamic recrystallization: (1) displacement along existing grain boundaries and (2) the development of new grain boundaries (e.g., [6, 48, 49]). There are three different mechanisms of recrystallization that can operate during deformation depending on temperature, flow stress, or both [50–52] and initial grain size (e.g., grain size sensitive (GSS) creep, [53]). GSS diffusion creep does not significantly contribute to the dynamic recrystallization mechanisms in our samples because the grain size in our samples from the granulite and granitic host rock is quite coarse, on an average 70 μm.

Dynamic recrystallization processes with increasing temperature and decreasing flow stress are (1) bulging recrystallization (BLG), (2) subgrain rotation recrystallization (SGR), and (3) high-temperature grain boundary migration recrystallization (GBM) [6, 45–47, 54]. Bulging recrystallization (BLG) occurs at low-temperature (280-390°C) and higher differential stress when the grain boundary protrudes or “bulges” into the quartz crystal forming new, independent small crystals [4–6, 47]. Recrystallization through subgrain rotation occurs at a higher temperature (420-490°C) than bulging [47] and higher differential stress. When the angle between the crystal lattice of the subgrain boundaries increases enough, the subgrain can no longer be part of the same grain and a new grain develops. Recrystallization through grain boundary migrations occurs at even higher temperatures (>530°C), but with low differential stress. In this case, grain boundary mobility increases, and the grain boundary can sweep through an entire crystal, thereby removing dislocations and subgrain boundaries, producing new, larger grains [6, 45, 47, 55].

Estimation of temperature during deformation is the key requirement in many structural studies that analyze strain rates during major orogenic events (e.g., [56–58]). We use the quartz recrystallization thermometer of naturally deformed and dynamically recrystallized quartz grains established by [6, 50] to measure the range of deformation temperature (Figure 4). The thermometer is based on microstructures and their inferred dynamic recrystallization mechanisms (low-temperature grain boundary bulging and subgrain rotation and high-temperature grain boundary migration).

A detailed structural study of all five shear zones (Balaram, SZ-I, SZ-II, SZ-III, and Surpagla-Kengora shear zone) shows that the shear zones consist of protomylonite and mylonite in the granitic host rock and ultramylonite in the pelitic and mafic granulitic host rocks. The shear zones are dominated by low-temperature retrograde shearing, but evidence for high-temperature shearing is preserved in some places. The stretching lineations of low-temperature shearing are expressed by quartz ribbons and biotite (Figure 5(a)) and of high-temperature by granulite minerals such as garnet, cordierite, spinel, and pyroxene that retrograded to biotite (Figure 5(b)). The low-temperature mylonite contains feldspar porphyroclasts that are surrounded by stretched quartz grains. The mylonitic foliations strike in NNW-SSE direction and dip towards the ENE; the stretching lineations are horizontal to gentle plunging (10-15^o) towards the SSE (stereoplots in Figures 1(e)–1(n)). Stretching lineations are marked by stretched quartz, feldspar, and spinel grains (Figures 5(a) and 5(b)). Shear kinematics such as rotated porphyroclasts, S-C fabrics, and asymmetric folds indicate a top-to-NW sinistral sense of shear (Figures 5(c) and 5(d)). The high-temperature shearing is marked by a steep to the ENE dipping mylonitic foliation and down-dip stretching lineations (Figure 5(b)). The S-C fabric indicates a top-to-NW sense of shear (Figure 5(d)).

The dynamic recrystallization mechanisms have been described for various samples for all five shear zones. The BSZ (sample BL1, Figure 1(c)) contains quartz clasts that show both BLG and SGR recrystallization along the recrystallized quartz grain margins (Figure 2(a)). In some places, we observed relicts of high-temperature GBM recrystallization (Figure 2(b)). The GBM recrystallization is associated with ductile deformation in feldspar and chessboard twinning in quartz (sample CBT, Figure 2(c)). The SZ-I (Figure 1(c)) is characterized by BLG and SGR recrystallization (samples PG1, AJ, and AJ1). The sample PG1 consists of a pelitic granulite with quartz-rich bands. Individual quartz grains are rectangular and along the margin of the grains in which BLG recrystallization is the dominated recrystallization mechanism (Figure 2(d)). The samples AJ and AJ1 are characterized by SGR recrystallization. Small grains are formed around the quartz clasts (Figure 2(e)). In some places, the coarser quartz grains are completely recrystallized with a shape-preferred orientation (SPO) (Figure 2(f)). These grains define the S-C fabric in the rock (Figure 2(f)). SZ-II (Figure 1(c)) is dominated by SGR recrystallization, and in addition, extensive biotite growth is observed parallel to the C-fabric (sample SR1, Figure 2(g)), and these are rare relicts of GBM recrystallization which are overprinted by SGR (as in sample SR2, Figure 2(h)) in this domain of the study shear zone. Deformed twinned lamellae (DTL) (Figure 3(a)) and myrmekitic (MYR) (Figure 3(b)) intergrowth in plagioclase (Figures 3(a) and 3(b)) provide other evidence for high-temperature deformation in sample SR2. SZ-III (Figure 1(c)) is dominated by SGR recrystallization (samples GH3 and GH4) where the recrystallized quartz grains have an equigranular geometry and individual grains have a smaller size. The quartz grains have an anhedral shape with a shape-preferred orientation (Figures 3(c) and 3(d)). The biotite minerals define the S-C fabric. Some of the grains are equidimensional and affected by BLG recrystallization. In some places, the samples of the SKSZ (Figure 1(c)) contain relicts of GBM recrystallization that are overprinted by SGR (sample K1, Figure 3(e)), whereas in other sections, SGR recrystallization is the dominant recrystallization mechanism (sample K2, Figure 3(f)).

In this section, we couple the dynamic recrystallization mechanisms to the various grain sizes of our samples. By using the grain size plot of [50], who identified temperature conditions for specific grain sizes, we are able to identify temperature conditions for the dynamic recrystallization mechanisms (Figure 4). The calculated grain sizes vary between 46 and 119 μm. The samples PG1 and GH3 show both BLG and SGR recrystallization, which means that the deformation temperature was between 390 and 420°C (Figure 4 and Table 2). Most of the other samples (BL1, AJ, AJ1, SR1, GH4, and K2) only show SGR recrystallization with a deformation temperature between 420 and 490°C. The samples SR2 and K1 show both SGR and GBM recrystallization, for which the deformation temperature was between 490 and 530°C. The sample BL2 is the only sample that shows GBM recrystallization, and the deformation temperature of this sample lies between 530 and 600°C.

Our estimated flow stress (Table 2) for the BSZ (samples BL2 and BL1), varies from 18 MPa (diameter $D=71.00±24.48$ μm, number of grains $N=481$⁠) for SGR recrystallization to 12.0 MPa (⁠$D=119±44.6$ μm, $N=393$⁠) for GBM recrystallization. SZ-I (samples PG1, AJ, and AJ1) flow stress estimates for the measured recrystallized quartz grains (⁠$D=46.00±15.67$ μm, $N=399$⁠) yield an average flow stresses of 26 MPa for sample PG1 with BLG-SGR recrystallization. For AJ and AJ1 with SGR recrystallized grains (⁠$D=62±24.90$ μm, $N=390$ and $D=65±24.6$ μm, $N=458$⁠), the flow stress conditions are 21.0 MPa and 20.0 MPa. The flow stress for SZ-II (samples SR1 and SR2) varies from 23.0 MPa to 14.0 MPa (⁠$D=55.00±21.29$ μm, $N=451$ and $D=96.00±32.09$ μm, $N=460$⁠). The samples with GBM recrystallization have lower values for the differential flow stress (sample SR2) of 14.0 MPa. SZ-III (samples GH3 and GH4, $D=51.00±19.4$ μm, $N=251$ and $D=54.00±18.86$ μm, $N=444$⁠) has very similar differential flow stress values for both samples of 24.0 MPa and 23.0 MPa. Both samples show similar dynamic recrystallization mechanisms (predominantly SGR, with some bulging). However, to the far east of the SKSZ, the differential flow stress (samples K1 and K2, $D=91.00±35.7$ μm, $N=417$ and $D=62.00±27.08$ μm, $N=325$⁠) varies between 15 MPa and 21 MPa due to a few relicts of high-temperature shearing.

We calculated the strain rate using the values of the differential flow stresses (Table 2). In the Kui-Chitraseni shear zone, BL1 strain rate varies from $2.91×10−13$ to $3.39×10−14$⁠. The BL2 strain rate varies from $1.20×10−12$ to $2.39×10−13$⁠. For the case of SZ-I shear zone (samples PG1, AJ, and AJ1), the sample PG1 strain rate is estimated at $1.06×10−13$ to $3.68×10−14$⁠. The calculated AJ strain rate ranges from $4.69×10−13$ to $5.47×10−14$⁠. The sample AJ1 has strain rate values for the flow stresses that vary from $4.03×10−13$ to $4.70×10−14$⁠. The strain rate for SZ-II (samples SR1 and SR2) for SR1 varies from $6.22×10−11$ to $7.26×10−12$⁠, whereas the strain rate varies from $3.85×10−13$ to $1.33×10−13$ for SR2. For the SZ-III shear zone, the GH3 samples have strain rate values ranging from $8.28×10−14$ to $2.87×10−14$⁠. The GH4 strain rate values range from $6.22×10−13$ to $7.26×10−14$⁠. However, to the far east of the SKSZ, the K1 samples have strain rate values that vary between $4.77×10−13$ and $1.65×10−13$⁠. Strain rate values of K2 range from $4.69×10−13$ to $5.47×10−14$⁠. All these separate strain rate calculations lead to a general estimated strain rate between $1.20×10−12$ and $7.26×10−14$/s, which is similar to those of natural shear zones [50, 61–63].

The high-temperature shearing took place at ~22 km depth, and the low-temperature shearing took place at ~16 km depth (Figures 6(a) and 6(c)). Previous studies of the exhumation of the Ambaji granulite suggested that high-temperature shearing took place in the ductile field of the quartz-feldspar aggregate at ca. 834 Ma. The low-temperature shearing took place in the brittle feldspar and ductile quartz field at 778 Ma [11]. On the basis of microstructures and field relations, we have concluded the shift of flow from vertical to horizontal during exhumation as lineations represent the final strain direction during the ductile shearing. The switching of vertical to horizontal flow during exhumation was given on the basis of vertical lineations associated with high-temperature mylonites which shows GBM type of dynamic recrystallization and horizontal lineations of low-temperature mylonites which shows BLG-SGR type of dynamic recrystallization [3, 11].

The calculated differential flow stresses for the high-temperature deformation are between 12 and 15 MPa and for the low-temperature deformation between 18 and 26 MPa (Figure 6(a), Table 2). The strain rate is between $1.20×10−12$ and $7.26×10−14$/s (Figure 6(b)), comparable to strain rates of natural shear zones (for example, the Lachlan orogen, Australia [64], the Palmi shear zone, Italy [65], the Tinos metamorphic core complex, Greece [66], the Whipple Mountains, California [56], the Alpine Fault, New Zealand [67, 68], the Ailao Shan-Red River (ASRR), Southwest China, and the Karakorum, Northwest India [69]). These results lead to three points of discussion.

First, differential flow stress and grain size (Figure 6(a)) control the dynamic recrystallization of quartz. The two factors are inversely proportional: the differential flow stress is higher when the grain size is smaller and vice versa. Second, the differential flow stress versus the temperature (Figure 6(b)) does not fit into one single strain rate curve, because the deformation becomes increasingly localized with time during exhumation. Our strain rates between $1.20×10−12$ and $7.26×10−14$/s fall within the limitation of natural strain [70], but the dynamic recrystallization mechanism varies from GBM to SGR-BLG with decreasing strain rate (Figure 6(b)). Third, in the differential flow stress versus depth (Figure 6(c), for our temperature-to-depth conversion, we considered a normal geothermal gradient 25°C/km after [71]) shows that deformation mechanisms of the ductile shear zone progressively evolved from GBM recrystallization at ~22 km depth to BLG-SGR recrystallization at ~16 km depth near the brittle-ductile transition.

In the case of the extrusion of the Ambaji granulite in a compressive tectonic setting, the larger quartz grains of the shear zones are formed at lower differential stresses, higher strain rates, and deeper levels, whereas smaller grains are developed at higher differential stress, lower strain rate, and shallower depths. The GBM recrystallization mechanism is only active at greater depth (e.g., larger grains and lower differential flow stress), whereas the conditions for BLG and SGR recrystallization (e.g., higher differential flow stress to reduce the grain size) only occur at shallower levels, just below the BDT. Ductile extrusion above the BDT is by definition not possible, which is why the differential flow stress increases at shallower depths, forcing the material laterally once it reaches these shallower depths and at the same time reducing the grain size that needed for the SGR and BLG dynamic recrystallization mechanisms to become the dominant recrystallization mechanism. Hence, the ductile extrusion of a lower crustal body can happen in two different regimes where (1) the differential flow stress is low and allows the material to move in (semi-)vertical direction, accommodated only by GBM recrystallization, and (2) the differential flow stress is high and forces the material to move only in the horizontal plane through BLG and SGR recrystallization.

Previous studies explained the exhumation tectonics of the Ambaji granulite through mineral assemblages and structural relations [3]. We identified the temperature-dependent dynamic recrystallization mechanisms of quartz grains from the shear zones. We correlate the microtectonics as an additional factor in controlling the flow direction of the material. GBM recrystallization at depth probably plays a role in vertical extrusion of the material, and SGR-BLG at BDT accounts for the lateral extrusion.

D₁ and D₂ are the two major contractional phases of deformation in South Delhi orogeny. Peak granulite facies metamorphism occurred during D₁ deformation. The D₂ shear zones exhumed the granulite to the BDT, which means that the granulite underwent a high-temperature shearing followed by low-temperature shearing on its continuous exhumation path. While high-temperature shearing was characterized by vertical flow (22 km depth), the low-temperature shearing indicates lateral flow (16 km depth). The change in kinematics is associated with the different dynamic recrystallization mechanisms: GBM was the dominant recrystallization mechanism at depth, while SGR-BLG was predominant at the BDT (Figure 7).

In Phanerozoic mobile belts like the Himalayas, thrust tectonics played a major role in the exhumation of lower crustal rocks, which was only followed by lateral extrusion tectonics when strike-slip shear zones developed and became the dominant exhumation mechanism (for example, [72–74]). It is, however, still unclear if in general, exhumation through thrusting occurs only through GBM, while exhumation through strike-slip shear zones is facilitated by SGR-BLG, similar to Ambaji granulite, or if this process is more gradual.

The results of our microstructural analysis and temperature, flow stress, and strain rate calculations conclude that the exhumation of Ambaji granulite from deeper crust to the BDT was not only manifested in terms of change in kinematics but also in microtectonics and dynamic recrystallization mechanisms.

• (1)

  At lower-middle crustal levels, where feldspar and quartz deform in a ductile way, the dominant dynamic recrystallization mechanism is GBM, which indicates a deformation temperature range of 490-600°C. The granulite underwent a vertical flow and rose to the BDT zone. The paleopiezometric calculations indicate that the differential flow stress values were between 12 and 15 MPa, as the confining stress was higher at deeper level. At the BDT, SGR and BLG recrystallization were the dominant dynamic recrystallization mechanism, and the granulite underwent a retrogression at 390-420°C, accompanied by lateral flow. Lateral flow did not account for vertical uplift, but it assisted in strain accommodation. The differential stress was 18 to 26 MPa, whereas the confining stress was lower, but still relatively high at shallower level. Finer grain size and an SGR-BLG dynamic recrystallization mechanism probably assisted lateral flow to take place under higher differential stress. Strain rate estimates are between $1.20×10−12$ and $7.26×10−14$/s, comparable to other natural shear zones

• (2)

  We show that the different deformation stages through which the shear zones went can be correlated to the different dynamic recrystallization mechanisms. At depth, when the shear zones were subjected to compressional forces, quartz recrystallized through GBM that facilitated the vertical flow of the material until it reached the BDT. At the BDT, strike-slip faulting affected the shear zones, and deformation happened through SGR and BLG recrystallization. From this point onwards, the material migrated laterally. The change from GBM to SGR-BLG recrystallization coincides with a change from vertical to horizontal flow of the material. We argue that (1) the temperature drop with decreasing depth during exhumation in combination with the decreasing lithostatic pressure increases the flow stress of material at shallower levels (e.g., at the BDT), (2) the change from ductile to near brittle-ductile state from lower to upper crust forces the exhumation of material to change its vertical flow direction to lateral flow of the material, and (3) due to the constraints of the lithostatic pressure and decreasing temperature, in this case, the GBM recrystallization accommodates the vertical flow of material and the SGR and BLG recrystallization the horizontal flow of material

All data related to the present research is available in this manuscript and in the supporting information Figure S1 and Table S1.

The authors declare that they have no conflicts of interest.

The authors sincerely thank the Indian Institute of Technology Roorkee for funding the FIG (IITR/SRIC/2385/FIG) project. The authors are thankful to Subhajit Ghosh for the useful discussion during the preparation of the manuscript.