Seismic Evidence for Proterozoic Collisional Episodes along Two Geosutures within the Southern Granulite Province of India

The Southern Granulite Province of India had witnessed episodes of multiple tectonic activities, leading to sparsely preserved surface geological features. The present study is focused on unraveling the geodynamic evolution of this terrain through measurement of Moho depth and Vp / Vs ratio using data from a large number of broadband seismic stations. These results unambiguously establish three domains distinct in Moho depth and crustal composition. An intermediate to felsic crust with a 7 – 10 km step-in-Moho is delineated across the Moyar – Bhavani region. Anomalously high felsic crust with abrupt jump in Moho ( ~ 8 – 10 km) together with a dipping feature at deeper level characterizes the transition from eastern to southern segments of the Jhavadi – Kambam – Trichur region. By contrast, the central zone hosting the Palghat – Cauvery shear zone records uniform felsic crust and ﬂ at Moho. Drawing analogy from similar results in di ﬀ erent parts of the globe, juxtaposition of petrologically dissimilar crustal blocks characterized by varied depths to the Moho is argued to point towards unambiguous presence of two distinct geosutures in the study area: one along the Moyar – Bhavani region and the other across the Jhavadi – Kambam – Trichur. This inference is corroborated by the presence of layered meta-anorthosite, related rock suites, and ma ﬁ c-ultrama ﬁ c bodies, supporting the view of a suprasubduction setting in the Moyar – Bhavani region. The Jhavadi – Kambam – Trichur area is marked by operation of the Wilson cycle by way of sparsely preserved geological features such as the presence of ophirags (ophiolite fragments), alkali syenites, and carbonatites. Geochronological results suggest that the suturing along Moyar – Bhavani took place during the Paleoproterozoic and that along Jhavadi – Kambam – Trichur was during the late Neoproterozoic.


Introduction
The Archean Dharwar Craton represents an inclined crosssection of the crust in southern India such that the greenschist facies rocks are exposed on the surface towards north, amphibolite facies in the middle, and granulite facies rocks to the south [1,2]. The transition from amphibolite to granulite facies is marked by occurrences of (i) pseudo charnockites that resemble charnockites in terms of greasy appearance but devoid of hypersthenes and are actually diopsidic gneisses [3], (ii) incipient or arrested charnockite patches, and (iii) basic granulite dykes with plagioclase clouding [4]. Figure 1(a) shows the generalized geological map of the region (after [5]). The transition boundary from amphibolite to granulite facies is popularly known as the "Fermor line" ( [6]; see Figure 1(a)). The south of this transition boundary forms the Southern Granulite Province (SGP). Two major aspects of the Southern Granulite Province that are central and have attracted the attention of geoscientists from across the world are (i) to provide a suitable model for the genesis of granulite facies rocks [7][8][9] and (ii) to understand the role played by the Southern Granulite Province during the amalgamation and break-up of the Gondwanaland [10][11][12]. Numerous studies encompassing a wide variety of tools that include geological, geochemical, geophysical, and geochronological studies were therefore conducted by many researchers from different countries to understand the genesis and evolution of this province (e.g., [12][13][14][15][16][17][18][19][20][21]; [11,[22][23][24][25][26][27]). Based on some of the significant studies, it is now broadly understood that the Gondwana assembly is characterized by late Neoproterozoic collisional orogeny and deformation marked by operation of Wilson cycle during the geological past. However, the location of the geosuture/geosutures within the highgrade rocks of southern India is highly controversial and debatable [4,11,13,22,[24][25][26]. This is because the province is punctuated by several crustal-scale megashear zones ( Figure 1) such as the Moyar shear zone (MSZ) and the Bhavani shear zone (BSZ) together forming the Moyar-Bhavani shear zone (MBSZ), the Palghat-Cauvery shear zone (PCSZ), the Karur-Kambam-Painavu-Trichur shear zone (KKPTSZ), and the Achankovil shear zone (AKSZ). Evidently, these shear zones form the key tectonic elements of the Southern Granulite Province. However, the nature and evolution of these megashear zones of the Southern Granulite Province, viz., MBSZ, PCSZ, and KKPTSZ, are ambiguous in the absence of information related primarily to their depth disposition. In other words, the basic question related to disposition of suture(s) in three dimensions with their location needs to be addressed and established unambiguously.
In a recent study, the crustal structure beneath a prominent and crucial segment of the Southern Granulite Province using passive seismological experiment was investigated by us (Das Sharma et al. [28]). Based on the Ps receiver function analyses and H-κ stacking [29] results, it was demonstrated that the seismic structures of the crust across the Jhavadi-Karur-Kambam region within the Southern Granulite Province are distinct. Their dissimilar nature is manifested in terms of differences in the estimated average crustal compositions (Vp/Vs ratios) and Moho depths across this boundary. Our seismological results, when integrated with available geochronological data and preserved geological features such as presence of ophirags (ophiolite fragments)   [5]). To the south of Archean Dharwar Craton, rocks are metamorphosed to granulite facies [6]. This transition from amphibolite to granulite facies is popularly known as the "Fermor line" and is marked TZ (transition zone) in the figure. Major shear/fault zones (after [41]) are demarcated using diverse-colored broken lines. Thick red line represents a geosuture (after Das Sharma et al. [28]). Various crustal blocks are after Ramakrishnan [4] and Brandt et al. [22]. (b) Location of broadband seismic stations. Red inverted triangles represent stations sited during the course of this work. White inverted triangles are stations (see Appendix) reported by Rai et al. [45]. The distribution of alkaline rocks and carbonatites (star) and anorthosites (diamonds) is shown. Suture/shear/fault zones are same as in (a). Thick black line represents seismic profile along the Kolattur-Palani transect [20,64]. and occurrence of alkali syenites and carbonatites in the area, could establish unambiguous presence of a geosuture along the Jhavadi-Karur-Kambam region.
Subsequent to these interesting seismological results (across the Jhavadi-Karur-Kambam region), a more detailed investigation was undertaken over a wide area of the Southern Granulite Province through installation of broadband seismic stations in a phased manner at other locations (total 29 stations; see Figure 1(b)). It is pertinent to mention that while data from mere 15 stations focusing on the Jhavadi-Karur-Kambam segment were published earlier by us (Das Sharma et al. [28]), the present research study is based on data from twice the number of stations, covering all the three major shear zones of Southern Granulite Province, viz., the Jhavadi-Karur-Kambam-Trichur shear zone (JKKTSZ), the Palghat-Cauvery shear zone (PCSZ), and the Moyar-Bhavani shear zone (MBSZ). The objectives of this study are therefore more rigorous and broad. Our aim is to evaluate the average seismological character (Moho depth and Vp/Vs ratio) of the crust across three major shear zones, viz., the Jhavadi-Karur-Kambam-Trichur shear zone (JKKTSZ), the Palghat-Cauvery shear zone (PCSZ), and the Moyar-Bhavani shear zone (MBSZ) through Ps receiver function analyses such as the H-κ stacking method. We also present CCP stack sections along select profiles, to confirm step-indepth to Moho across JKKTSZ, besides observation of a dipping feature at some stations, reinforcing our earlier inference about the presence of a suture across this boundary. The CCP stacks also cover two other prominent shear zones of Southern Granulite Province, viz., PCSZ and MBSZ. Together, these effectively offered unprecedented insights into the character, status, and evolution of all the megashear zones of the region in a comprehensive manner from the perspective of operation of the Wilson cycle in the Southern Granulite Province that were widely separated in space and time. Further, since the ratio of compressional to shear wave velocity of crustal rocks depends on the nature of the crust (i e., Vp/Vs ratio depends on the average wt% SiO 2 of the crust; [30,31]), the Vp/Vs ratio obtained from the H-κ stacking method is transformed to average wt% SiO 2 using a modified linear relationship between Vp/Vs and wt% SiO 2 . The method for the estimation of average SiO 2 content from the Vp/Vs ratio is detailed in a subsequent section. The ultimate goal of this study is to evaluate the disposition and average compositional variation of the crust in three dimensions across the above-mentioned megashear zones based on seismological imaging. Such an attempt over the entire Southern Granulite Province of India is first of its kind and expected to yield a clear understanding about the role played by these megashear zones during geodynamic evolution of the Southern Granulite Province of India. This would eventually help to establish firmly the correlation of Southern Granulite Province of India with other segments of eastern Gondwana, when similar results in those segments accrue. Further, we also address some of the major concerns raised in the literature [4,18,32] pertaining to the V-shaped pattern of KKPTSZ (see Figure 1) mapped by Ghosh et al. [13]. Whether the KKPTSZ should be considered a terrane boundary was questioned by Ramakrishnan [4,18] as well as Cenki and Kriegsman [32].

Geological Framework of the Study Area
The Southern Granulite Province of India, with an areal extent of~40,000 km 2 , is the third largest Precambrian granulite province of the world after Canada and Australia [33][34][35]. The "Fermor line" shown in Figure 1(a) represents the transition boundary across which metamorphism of rocks from amphibolite to granulite facies took place. The region south of the "Fermor line" represents the Southern Granulite Province of India. The granulite facies rocks (Figure 1(a)) occurring between the "Fermor line" and the E-W trending Moyar-Bhavani shear zone constitute the "Dharwar granulite belt" [4]. Drury et al. [36] were the first to propose a subdivision of the South Indian Shield into the Northern block and the Southern block, with the Palghat-Cauvery shear zone representing the tectonic boundary between them. In this subdivision, granulites of the Madras-Nilgiri block (Figure 1(a)) are included in the Northern block, whereas granulites occurring south of the Palghat-Cauvery shear zone constitute the Southern block. Subsequent to the subdivision proposed by Drury et al. [36], many researchers have used individual preferences to name different blocks of the South Indian Shield, leading to confusion among the readers. However, it is claimed by Ramakrishnan [4] that presently a consensus is arrived at regarding the usage of nomenclature of various blocks. According to this, the Southern Granulite Province is divided into four major blocks (e.g., [4]; see Figure 1), viz., the (1) Madras-Nilgiri block between the Fermor line and the Moyar-Bhavani shear zone, (2) Namakkal block lying between the Moyar-Bhavani shear zone and the Palghat-Cauvery shear zone, (3) Madurai block between the Palghat-Cauvery shear zone and the Achankovil shear zone, and (4) Kerala Khondalite block (also called the Trivandrum block), lying south of the Achankovil shear zone. Based on recent geochronological and geochemical studies, the Madurai block is further subdivided into western and eastern domains [22]. Likewise, the Kerala Khondalite block is also subdivided (not shown in Figure 1) into the northern Ponmudi block and the southern Nagercoil block [4].
The lithological association that is documented within the four major blocks of Southern Granulite Province is elaborated in detail by Ramakrishnan [4]. The granulites of the Namakkal block are products of 2.5 Ga metamorphic event with P-T-t path showing decompression of~3 kbar during the Neoproterozoic [37]. The northern Dharwar granulite belt and the Namakkal as well as Madurai blocks include numerous alkali syenite plutons and anorthosites (Figure 1(b)). Massif-type anorthosite bodies within Madurai block are documented at various places such as Oddanchatram, Kadavur, and Perinthatta [38]. Besides these, occurrences of a number of anorogenic igneous intrusive bodies including pink granites are also documented. Although the igneous emplacement ages are of Neoproterozoic times (~0.8 Ga), the granitic protoliths show evidences of incorporation of older crustal components. For example, while the detrital zircons are as old as~3 Ga, the younger granites 3 Lithosphere indicate that they were emplaced at~0.8-0.6 Ga [13,22,[39][40][41]. The P-T-t studies conducted on sapphirine granulites in the Madurai block suggest that ultrahigh-temperature (840-1070°C) metamorphism with pressure ranging from 8.5 to 9.6 kbar and multistage exhumation history are characteristic of this area [42][43][44].

Data and Methods
The network of broadband seismic stations that was installed in the Southern Granulite Province of India for this study is of semipermanent nature. Installation of seismic stations at various locations was carried out in four separate time windows. Among the first group, ten broadband instruments were installed during 2011 (Table 1). However, two stations (DDG and HSR in Figure 1(b); see also Table 1) were discon-tinued in 2012 due to unforeseen problems, while the rest continued to operate. Seven more stations were sited during early 2013. Out of these seven stations, five seismometers were freshly acquired while two seismometers from DDG and HSR of the first phase were shifted to new locations. In the third phase, two more new stations became operational during 2014 (see Table 1). During the last phase, one new station (DHMP) was installed during January, 2015. Further, eight stations were relocated to new sites by shifting six seismometers of the first phase and two from the second phase. This was done during 2015. Seismological data from these 28 stations and a permanent broadband station PCH operated by CSIR-NGRI are processed and utilized in this study. These station locations (red inverted triangles) are shown in Figure 1(b). Data from 29 stations sited in this study are supplemented by similar published data from 26 other stations  [45]). In Figure 1(b), white inverted triangles represent the station locations reported by Rai et al. [45]. Thus, data pertaining to a total of 55 stations are utilized in this study during interpretation of the results. Five stations reported by Rai et al. [45], which fall in distinctly different lithological association constituting the Kerala Khondalite belt (viz., MVK, NGC, TKS, TRB, and TYD; not shown in Figure 1(b)), were not considered in our analysis.

Receiver Function Computation.
It is well known that large velocity contrast across a discontinuity (e.g., Moho, 410 km or 660 km) causes a part of the steeply incident teleseismic P wave to convert into an SV wave. Besides the direct P-to-s (Ps) conversions, there are also many multiple reflections and conversions that occur between the surface and the interface (e.g., Moho). While the P wave and its multiples dominate the vertical component, Ps-converted waves and their multiples are prominently registered on the horizontal component (SV). Therefore, an appropriate component rotation into ray coordinate system isolates the Ps energy from that of P. The effects related to source, mantle propagation and instrument response, are suppressed by deconvolving the P waveform from the SV component, to obtain what are called receiver functions. The crustal multiples, designated as Pps and Pss, together with the direct conversion from the Moho boundary (Pms), contain a wealth of information concerning the average crustal properties such as the Moho thickness (H) and Vp/Vs value (related to Poisson's ratio, σ) in a well-constrained manner. Therefore, these useful parameters were evaluated and utilized in this study. Teleseismic events with magnitudes greater than 5.5 Mb and epicentral distance range 30°-90°were used. The network of 29 stations recorded a total of 14582 events. Seismograms, with clear P wave registrations, recorded at each station, were selected for the receiver function analysis. A high-pass filter with corner frequency of 0.02 Hz was applied to the data. Radial and tangential components were obtained by rotating the horizontal components of the seismogram using corresponding back-azimuths. Receiver functions were computed using the radial and vertical components by applying a time-domain iterative deconvolution procedure [46]. To obtain better resolution of crustal layers, a Gaussian filter of width 2 was chosen during the iterative deconvolution. Thereafter, these receiver functions were visually inspected, and any conspicuously bad receiver functions were discarded. The number of receiver functions used for each station in final analysis are listed in Table 1. Such a processing procedure renders easy comparison of our receiver functions with the published data of Rai et al. [45].

Crustal Parameter Estimation by H-κ Stacking. Zhu and
Kanamori [29] proposed a grid search method for the estimation of the Vp/Vs ratio (κ) and the crustal thickness (H). In this method, the weighted amplitudes of the converted waves at the Moho and corresponding multiples in crust recorded on radial receiver functions are stacked at their respective predicted arrival times using the standard IASP91 velocity model.
Assuming a homogeneous P wave velocity in the crust, we therefore applied the stacking technique which sums the receiver function amplitudes at the predicted arrival times of the crustal phases, Pms, Pps, and Pss, for a combination of crustal thickness (H) and Vp/Vs (κ) values. The values of H and κ for which all the three phases stack coherently and the sum reaches a maximum value were taken to be the estimates of the crustal thickness and Vp/Vs. In the present study, a Vp of 6.3 km/s was used and a grid search was performed over the H and κ ranges of 20 to 60 km and 1.5 to 2.0, respectively. The average crustal Vp/Vs ratio is a measure of crustal elastic property. It is related to Poisson's ratio in accordance with the following equation: The advantage of the Zhu and Kanamori [29] method is that it excludes the need to pick the arrival times of seismic phases. Therefore, large amounts of teleseismic waveforms can be processed easily. By stacking receiver functions from various azimuths and distances, one gets the average structure of the crust where the effects of lateral structure variation are suppressed [29].

Common Conversion Point (CCP)
Binning. In order to get adequate insight into the nature of the underlying crust in the study area, we also carried out 2D common conversion point (CCP) stacking at certain critical segments using the FuncLab toolbox [47,48]. The toolbox implements the CCP stacking method of Dueker and Sheehan [49]. Receiver functions from 29 stations totaling to~3000 were handpicked by visual examination of the computed receiver functions. All these selected receiver functions display clear Moho arrivals and a good signal-to-noise ratio.
The receiver functions from all the stations were transformed into the depth domain by back-projecting the energy to their corresponding conversion points along their paths at every 1 km, using the IASP91 standard velocity model. To enhance the spatial coherence of the converted phases, these amplitudes corresponding to different depth-offsets are subsequently projected onto a 2D reference plane. The optimal choice of this plane is based on the subsurface geometry, station disposition, and pierce point distribution. The backprojected amplitudes of the RFs falling in a spatial grid are stacked using move-out corrections and plotted using a color scheme where red represents a positive polarity and blue indicates a negative one. In this study, we used a grid size of 0:5 × 3 sq: km in the depth range 0-100 km to obtain a relatively high-resolution image for each selected profile. Such depth-migrated receiver functions produce 2D CCP stack cross-sections. 5 Lithosphere

Estimation of Average SiO 2 Content from the Vp/Vs
Ratio. Using pulse transmission technique, Christensen and Mooney [31] and Christensen [30] measured the compressional (Vp) and shear (Vs) wave velocity of mineral and rock samples that essentially comprise the important constituents of the crust globally. Then, the Poisson ratio (σ) was calculated using the standard relation shown in equation (1). As a second step in the analyses, the wt% SiO 2 in each sample was determined. Finally, a relationship between the wt% SiO 2 and Poisson's ratio σ was arrived at (see Table 3 and Figure 13 of [30]). Based on the results, it was pointed out by Christensen [30] that there exists a good agreement between the experimentally determined Poisson's ratios of monomineralic rocks and those calculated from singlecrystal elastic constants. For igneous and metamorphic rocks, he observed that Poisson's ratios correlate qualitatively with mineralogical changes. The overall trend is nonlinear [30] in the binary plot between weight percent SiO 2 and Poisson's ratio (σ). However, σ increases with decreasing SiO 2 content and exhibits a linear correlation between 55 and 75 wt% SiO 2 . Based on such an analysis, Christensen [30] proposed that measurements of Poisson's ratio might provide valuable information on average crustal chemistry of a region. The validity of the relationship proposed by Christensen [30] was subsequently verified by a large number of workers covering diverse geological terrains having variable antiquity (e.g., [50][51][52][53]).
The relationship between wt% SiO 2 and Poisson's ratio (σ) obtained by Christensen [30] was reevaluated by us (see also [28]). We used the data given in Christensen [30] for 18 different rock types of metasedimentary and igneous origins that essentially comprise the important constituents of crustal geology. These include quartzite, slate, metagraywacke, paragranulite, felsic and mafic granulites, granite, basalt, diorite, and anorthosite. We then explored the relationship between Vp/Vs and SiO 2 . This exercise yielded an excellent linear relationship between Vp/Vs and SiO 2 (with R 2 = 0:88) for the entire compositional range of the crust (Figure 2). Such a good correlation is in concert with the documented decrease in compressional wave velocities with concomitant increase in shear wave velocities as a function of SiO 2 content [30]. The estimated linear relation ( Figure 2) is used to compute the average crustal wt% SiO 2 using the Vp/Vs ratio obtained from the H-κ stacking method beneath each station shown in Figure 1 It is therefore important to note that the SiO 2 values presented as a linear function of Vp/Vs ( Figure 2) could represent a general case for average crust globally. The SiO 2 content obtained in this study using the measured Vp/Vs values (from the H-κ stacking method) correspond to the study area, i.e., the Southern Granulite Province of India.

Results and Discussion
The results pertaining to 29 stations that were sited within the Southern Granulite Province of India for the present study are presented in Table 1. The measured parameters include the average crustal depth (Moho) and Vp/Vs ratio pertaining to each independent broadband seismic station. Standard bootstrap technique (see [28]) was used to estimate the uncertainty in the H and Vp/Vs values.

Observations from the Receiver Function Stack Sections
and H-κ Stacking Results. Figure 3 presents Ps receiver function distance stack sections from the study region corresponding to representative broadband seismic stations plotted against the back-azimuth (BAZ) of each event. The first positive arrival on each trace at time zero is the direct P arrival, and the next highest positive amplitude arrival is the P-to-s-converted wave signal from the Moho (Pms). A major observation from Figure 3 is that the sum traces record large variations in the delay times of the Pms arrival. The Moho depth varies from 30.1 to 49.0 km (Table 1). Similar large variations (see Appendix) in Moho depth were also documented at other stations sited within the Dharwar Craton and Southern Granulite Province of India by Rai et al. [45]. Figure 4 shows H-κ stacking results at nine typical stations obtained by adopting the approach of Zhu and Kanamori [29]. The obtained Vp/Vs and crustal thickness (H) values with error bounds for each station from the receiver functions are presented in Table 1 Figure 2: Relationship between average SiO 2 content and corresponding Vp/Vs ratio. All common crustal rock types are shown in the diagram (data from [30]). Excellent linear correlation (R 2 = 0:88) can be observed between the plotted parameters. This diagram forms the basis for estimation of average crustal SiO 2 content of the study area and is presented in Figure 8(b) using Vp/ Vs ratios recorded at various stations. values beneath each station from recorded seismograms (see Table 1 and Figure 4). The result pertaining to Moho depth (H) is shown in Figure 5 using different-colored    Figure 5). In this central core region, the average crustal depth (Moho) is typified mostly by uniform values ≤45 km. The Moho depth beneath the region across the meridional arm of the Jhavadi-Karur-Kambam-Trichur shear zone along the Jhavadi-Kambam arm as well as the southern arm (thick red line marked 2) varies significantly. Compared to the central core region, the Moho depth beneath stations sited in the eastern and southern segments of JKKTSZ is shallower (≤40 km) over a large area ( Figure 5).
In order to obtain better insight into depth disposition of the Moho boundary across MBSZ and PCSZ (based on new data obtained from stations that were installed later), the sum traces of the receiver functions recorded at stations along four select profiles are presented ( Figure 6 Figure 6) and those reported earlier across Jhavadi-Karur-Kambam-Trichur shear zones are significant. Therefore in this study, in order to further comprehend the disposition of Moho depth across these two megashear zones, we transformed this information into depth domain through 2D migration and present the CCP stack images. Similarly, the CCP stack images across the Palghat-Cauvery shear zone are also obtained. The CCP stack sections along five select profiles (a-e) across MBSZ, JKKTSZ, and PCSZ are presented in Figure 7. While profile (a) cuts across the Jhavadi-Karur segment of JKKTSZ (Figure 7(a)), profile (b) traverses through two shear zones, namely, the southern arm of JKKTSZ and the PCSZ (Figure 7(b)). Two profiles (c and e) pass through both MBSZ and PCSZ (Figures 7(d) and 7(e)), and finally profile (d) transects mostly the PCSZ.  8 Lithosphere The CCP stack images along profiles (a) and (b) across JKKTSZ document a clear step in depth-to-Moho (Figures 7(a) and 7(b)). Significantly, a prominent SE-NW dipping feature between stations HRR and KGR is distinctly observed at greater depths (Figure 7(a)). Across PCSZ, four profiles are considered (Figures 7(b)-7(e)). It can be seen that there is no noticeable variation in depth-to-Moho across PCSZ along any of these profiles. The CCP stack images across PCSZ are therefore in conformity with those observed in Figure 6 indicating a flat Moho. In this context, it may be mentioned that similar results across PCSZ are well documented independently by other researchers along multiple CCP stack profiles oriented approximately N-S (e.g., see Figure 10 of [54]). Therefore, the results of a flat Moho presented by us in Figures 6 and 7 across PCSZ of the Southern Granulite Province of India are in concert with those obtained using similar data and techniques. Finally for stations sited across MBSZ, we present CCP stack images along two profiles (Figures 7(c) and 7(e)). These images exhibit a prominent step-in-Moho depth across MBSZ and are in perfect agreement with those presented in Figure 6. Therefore, combining the results from Figures 6 and 7, a distinct stepin-Moho depth is characteristic across JKKTSZ and MBSZ, whereas no such break is apparent across PCSZ. Additionally, a SE-NW dipping feature at greater depth is also seen in the CCP stack image across JKKTSZ (Figure 7(a)) reinforcing our earlier findings of a suture.
To summarize these results in the study area, variations in depth-to-Moho in 3D are presented as Figure 8(a). This enabled us to visualize the disposition of crustal thickness across the three major shear zones of the Southern Granulite Province of India. The station locations shown in the figure provide information on the control points related to Moho depth variations. It is interesting to note that a vast region bounded by MBSZ and PCSZ, and which forms the central core region of Figure 5, is characterized mostly by uniform Moho depth extending from west to east (Figure 8(a)). This region known as the Palghat gap coincides with a geomorphic low on the surface. Therefore, the low-surface topography (Figure 1(b)) and uniform Moho depth (Figure 8(a)) over the vast area of Palghat gap are in good agreement. In this context, we would like to emphasize that there exists a strong seismological gradient (linear break) in the eastern part of PCSZ. However, this gradient should not be mistaken to have originated from suturing across the PCSZ boundary. We wish to explicitly point out that this observed linear break can be traced to the sutured SE-NW trending arm of JKKTSZ. This suture is also manifested by way of step-in-Moho depth along six profiles across JKKTSZ (see Figure 3 of [28]) which is further reinforced by the CCP stack images along two transects across the JKKTSZ (Figures 7(a) and 7(b)) in this study. Further, the three-dimensional modeling of magnetotelluric data [24] presents unprecedented insights into the subsurface electrical structure of the major shear    Figure 5. Note that each profile (1-4) traverses through PCSZ and MBSZ. Seismic station names of Rai et al. [45] are shown in italics. Consistency in Pms arrivals from this study and Rai et al. [45] can be noted. Location of the shear zones (bottom panel) between stations is indicated with their colored abbreviations. Black arrow indicates a clear offset in Pms arrivals. A step-in-Moho depth corresponding to~7 to 10 km across MBSZ can be observed along each profile (1)(2)(3)(4). Such offset in Pms arrivals is not discernible at stations sited across PCSZ. The implications of these results are discussed in the text. 10 Lithosphere conductance of about 600 S. The close spatial correlation of this major conductor with structural feature such as the Karur-Kamban-Painavu-Trichur shear zone (KKPTSZ) assumes importance. It shows resemblance with some of the major conductive features associated with Proterozoic subduction zones [55]. Thus, it may be inferred to represent the electrical signatures of such subduction-collision zone in the Southern Granulite Province. Contrastingly, the 3D results reveal that a thick highly resistive crustal block underlies the PCSZ and a portion of the northern part of Madurai block to suggest that it is indeed a typical stable cratonic block unlike the KKPTSZ which witnessed significant reworking. These findings from 3D modeling of magnetotelluric (MT) data are consistent with our 2D-migrated seismological images that document dipping features along several profiles across the Jhavadi-Karur-Kambam-Trichur shear zone (JKKTSZ).

Composition of Average Crust of the Study Area.
Utilizing the measured average crustal Vp/Vs values (data from Table 1 and Appendix) beneath each station (Figure 1(b)) and the linear relationship between SiO 2 wt% and Vp/Vs (Figure 2), we estimated the average crustal SiO 2 content in the study region. Figure 8(b) presents average crustal composition in three dimensions corresponding to the entire study area. It is interesting to note that akin to Moho depth (Figure 8(a)), the variation in average composition of the crust is apparently prominent in this diagram across the crustal-scale megashear zones. The average silica content of the crust corresponding to stations sited north of Moyar-Bhavani shear zone is characterized by variable composition of intermediate to felsic nature (SiO 2 content centered around 62-66 wt%). In the central core region, the average SiO 2 content is typified mostly by uniform values in the range 67-68 wt%. This uniform felsic nature of the crust is therefore characteristic of the entire central core region. The average silica content across the meridional arm of the Jhavadi-Karur-Kambam-Trichur shear zone, i.e., along the Jhavadi-Kambam arm, varies significantly. While the eastern segment exhibits exceptionally high average crustal SiO 2 in excess of 70 wt% (highly felsic nature), the western counterpart is typified broadly by uniform felsic composition of 67-68 wt% (Figure 8(b)).
Based on the results presented in Figures 5-8, several inferences can be distilled:     (1 and 2). Such a feature across the Palghat-Cauvery shear zone is absent. The nature of the crust and Moho depth in the Palghat gap are largely uniform. The thick red lines (1 and 2) are inferred based on the distinctive character of the depth-to-Moho and average composition of the crust to represent two geosutures in the study region. Note their close correspondence with MBSZ and JKKTSZ on the surface. Each degree in the diagram corresponds to~111 km. Our results demonstrate that the status of PCSZ continues to remain as a crustal-scale megashear (see text for more details). Therefore, the dashed line representing PCSZ in this figure is shown only as a reference. 12 Lithosphere boundary on the map by Ghosh et al. [13]. These authors demarcated the said zone based on observed lithological differences, structural studies, and geochronological data across this boundary. They attributed regional refolding to have imparted the delineated V-shaped pattern. However, several researchers do not reconcile that such a sharp feature could represent a "terrane boundary." Thus, such a peculiar disposition became contentious. Further, they do not find any field evidence to merit it as a terrane boundary (e.g., [4,18,32]). Based on results from the individual station locations depicting step-in-Moho, two thick red lines numbered 1 and 2 are inferred ( Figures 5-8 Figure 8(b)). Although the reasons remain unclear at this stage, a focused study is warranted to explore the causal mechanism in this regard.

Comparison of Results across the Globe with Those of the Southern Granulite Province of India and Their Geodynamic
Implications. At several geologic and geodynamic settings of the globe, juxtaposition of distinct crustal blocks as observed across the Moyar-Bhavani shear zone as well as the Jhavadi-Karur-Kambam-Trichur boundary in our study region is often viewed as a consequence of suturing between two geologically diverse terranes [56][57][58][59][60][61][62][63]. For example, during the period between 1.8 and 1.0 Ga, the North American Archean Craton and the Proterozoic North America got amalgamated due to suturing. Along the Abitibi-Grenville transect, results from radial receiver functions reveal abrupt variations in Moho topography on the order of 5-10 km at latitude 46.6°N close to surface expression of the Grenville Front [62]. Across the Grenville Front, a thicker crust to the south can be distinctly distinguished from thinner crust to the north. Rondenay et al. [62] ascribed the observed tapered crustal thickening followed by abrupt thinning across the Grenville Front to a suture between two colliding plates. Further, the step-in-Moho is inferred to be a relict subduction feature. The timing of collision is inferred to be Mesoproterozoic (1.71-1.23 Ga), which coincides with the timing of subduction along southeastern Laurentia [62].
Using receiver function stacked images across the Cheyenne belt, Crosswhite and Humphreys [58] recognized Proterozoic accretion (ca. 1.8 Ga old) of active arcs onto the 3.2 Ga Wyoming Craton in the central United States region. Their seismological images revealed a 100 km wide zone of 50-60 km thick crust south of the Cheyenne belt, a set of predominantly south-dipping conversions within this thick crust, and a step-in-Moho towards south near the Uncompahgre uplift. The thick crust is construed to represent a remnant from the original 1.8 Ga suturing event.
As part of the joint Lithoprobe-IRIS Canada Northwest Experiment (CANOE), Mercier et al. [61] deployed broadband seismometers in Northwestern Canada to image the crustal features associated with Wopmay Orogen. They recorded a huge jump in Moho depth (~20 km) in the radial component of receiver functions. This jump, which stretches over a horizontal distance of~70 km on the surface, increases from~30 km to~50 km. The recorded anomalous jump in Moho depth is attributed to a suturing event associated with Wopmay Orogen fossil subduction during the Paleoproterozoic (1.8 Ga).
Brennan et al. [57] presented results from receiver function analyses along different transects across the central Alaska Range. They identified three distinct crustal sections based on differing crustal thickness and Vp/Vs ratios, besides the presence of intracrustal discontinuities. The documented variability in the crust across the Alaska Range was interpreted to represent amalgamation between a former continental margin and allochthonous oceanic terrane with the intermediate zone characterizing the suture zone.
Thurner et al. [63] used receiver function technique to examine the deep crustal structure in the Trans-Hudson Orogen that represents Proterozoic suturing event during 1.71-1.68 Ga between the Wyoming and Archean Superior province. The CCP stacked images exhibit evidence of massive thrusting of the Wyoming province in the west over the Superior province to the east. Besides this crustal-scale thrusting, a relic subduction feature is also documented, which is associated with the Yavapai-Superior boundary [63].
The Trans-European Suture Zone (TESZ) represents a significant lithospheric boundary in Europe that marks the transition between the Proterozoic lithosphere of the East European Craton (EEC) and the Phanerozoic lithosphere of Central and Western Europe. Using a dense network of passive seismic stations, Alinaghi et al. [56] reported results of crustal structures from (i) northern Germany to southern Sweden (TOR profile) across the Trans-European Suture Zone (TESZ). They also reported similar data from SVEKA-LAPKO network in Finland that gave crustal character across the Archean-Proterozoic suture. Receiver functions along the TOR profile exhibit pronounced crustal thickening north of the TESZ, besides documenting intracrustal Ps conversions. Likewise, data from SVEKALAPKO network indicates that towards south of the line of Archean-Proterozoic suture, a sudden deepening of the Moho by more than 20 km occurs. The receiver function data reported by Alinaghi et al. [56] could therefore confirm that sudden jump in Moho depth is characteristic of subduction zone. More importantly, in both these cases, the features associated with subduction survived through geological times despite a large difference in the geological antiquity of the regions of study (~0.9 and 1.9 Ga, respectively). 13 Lithosphere Based on P-and S-receiver function results, Knapmeyer-Endrun et al. [60] could delineate a number of tectonic subdivisions within central Europe that evolved through geologic time. These results show a sharp stepwise change in the Moho depth, on the order of 15 km (from~30 km to more than 45 km), between the Phanerozoic Europe and the East European Craton coinciding with the Trans-European Suture Zone. The average crustal Vp/Vs ratio is also distinct across these terranes. In addition to this feature, another ancient tectonic event that took place~1.7 Ga ago affecting the crustal configuration within Lithuania is also documented by them. For example, the crust in the eastern portion of Lithuania is on an average 5 km thicker than its western counterpart. These results have been correlated with the crustal subdivision of Lithuania along a terrane boundary of Paleoproterozoic antiquity. Distinctly different metamorphic grades, magnetic and gravimetric properties are also characteristic of the region across this terrane boundary [60]. Therefore, collectively, these results point towards a complex tectonic evolution of Europe.
He et al. [59] used H-κ stacking method to estimate the average crustal thickness (H) and the Vp/Vs ratio across the Yangtze block-Jiangnan orogenic belt-Cathaysia block in southern China. Their data revealed that the composition and seismic structure of the crust across the Jiangnan orogenic belt-Cathaysia block are identical, while these parameters are distinctly different between the two flanks of the Jiujiang-Shitai buried fault, which is delineated between the Yangtze block and Jiangnan orogenic belt. Based on these results, they proposed that the Jiujiang-Shitai buried fault defines a geosuture between the Yangtze and Cathaysia blocks.
Taking cue from all the above-mentioned studies and recognizing that distinctness in the nature of the crust induced by a past tectonic event is extensively used to delineate terrane boundaries of the geological past that survived through geological times, we prefer to interpret the results across the Moyar-Bhavani shear zone and Jhavadi-Karur-Kambam-Trichur boundary to constitute terrane boundaries (geosutures) within the Southern Granulite Province of India. The boundaries of these two sutures are marked as thick red lines (1 and 2) in Figures 5-8.
The inferences drawn from our results in the foregoing receive further support from controlled seismic source investigations carried out in the Southern Granulite Province of India [20,64]. Seismic reflection and refraction/wideangle reflection studies conducted along Kuppam-Palani [20] and Kolattur-Palani [64] transects of the Southern Granulite Province are reported. The Kolattur-Palani transect (Figure 1(b)) represents a segment of the Kuppam-Palani transect. This transect yielded vital information related to both MBSZ and PCSZ. Therefore, we intend to briefly highlight in the following some of their significant findings that are in good agreement with the results obtained from our receiver function analysis.
The deep seismic reflection and refraction/wide-angle reflection studies document oppositely dipping reflectivity patterns across MBSZ [20,64], akin to subduction-related reflection fabrics at depths similar to those observed in other geographical locations worldwide (e.g., [65,66]; see also [67] for a review). Such oppositely dipping reflectivity patterns are inferred to be signatures characteristic of collision that survived throughout the geological periods.
The velocity model of the study region points to thickening of crust across MBSZ and Dharwar Craton, which thins southward [20,64]. This is in excellent agreement with Moho depths derived from our receiver function analysis.
The findings from 3D modeling of magnetotelluric (MT) data [24] are consistent with our 2D-migrated seismological images that document dipping features along several profiles across the Jhavadi-Karur-Kambam-Trichur shear zone (JKKTSZ), a designated terrane boundary ( [28] and this study), and a flat Moho beneath the PCSZ. Together, the 3D MT and seismological results reiterate the interpretation that JKKTSZ is a candidate terrane boundary while PCSZ does not qualify as a suture.
Unfortunately, however, both the MT profiles of Patro et al. [24] terminate either south of the Moyar-Bhavani shear zone (MBSZ) or just at MBSZ. Hence, they do not provide any information about this yet another important megashear zone of SGP. Thus, related to MBSZ, we are unable to compare our findings with that of MT results.
Thus, the 3D magnetotelluric results lend unambiguous support to the seismological findings with the attendant interpretation on operation of the Wilson cycle in the Southern Granulite Province and accordingly designating some megashear zones in the study region as sutures and others as shear zones. Taken together, integration of seismic results with those obtained from other geophysical tools along the same transects point to the fact that the MBSZ and Karur-Oddanchatram shear zone (KOSZ, not shown in Figure 1 by us) could represent suture zones within the SGP [24,64].  Figure 1(a)) has been elaborated by Gopalakrishnan [68] and Leelanandam et al. [38]. While the former relates occurrence of alkali plutons in the region to abortive (unsuccessful) rifting, Leelanandam et al. [38] have attributed the presence of alkaline rocks and carbonatites (ARCs) and their deformed variants (DARCs) to unmistakable operation of rifting and collision processes in the region. Further, association of DARCs with ultramafic/mafic igneous bodies is also noted by them.

4.7.
Moyar-Bhavani Suture. Raith et al. [17] undertook an integrated study encompassing geological, petrological, geochemical, and isotopic studies of the Nilgiri and Biligirirangan granulite terrains and the adjacent Moyar and Bhavani shear zones. Their study reveals that the Nilgiri granulite terrain represents a metasedimentary allochthonous unit. This unit has been interpreted by them to have been thrusted onto the Dharwar Craton during the Paleoproterozoic collisional 14 Lithosphere tectonics. Such an interpretation receives further support from the structural studies in key areas [69] that exhibit near-vertical movements along the Moyar shear zone with upward, northerly thrusting of the Nilgiri granulite terrain against the western Dharwar Craton. Furthermore, the presence of layered meta-anorthosite and related rock suites of Sittampundi, Bhavani, Gobichettipalayam, and other ultramafic-mafic bodies around the Moyar-Bhavani shear zone support the view that this shear zone represents a suprasubduction setting of the geological past. The Sittampundi complex is considered to be a tectonic remnant of a layered igneous body that has been recrystallized and deformed [70]. Further, the Peralimala pluton within the Moyar shear zone is an elongated body~20 km long with an average width of about 4 km. The pluton is composed predominantly of sodic plagioclase, microcline, quartz, and hornblende with compositional variation ranging from quartz-syenite to leucogranite. In view of the possibility of Moyar shear zone representing a terrane boundary, this pluton sited within the Moyar-Bhavani Suture Zone has been interpreted to represent DARC [71]. Suturing of these two domains took place giving rise to ultrahigh-temperature metamorphic rocks exposed in the area [22]. The timing of this collision event can be placed at late Neoproterozoic [22]. All these results together with multiple but independent lines of evidences (seismological, active seismic, geological, geochemical, and geochronological) when viewed collectively are significant and lend unambiguous support to the occurrence of two episodes of convergence in the study region. One took place at~2.5 Ga between the Dharwar Craton and the central block (see  along the Moyar-Bhavani Suture Zone. The second suturing event took place during the late Neoproterozoic along the Jhavadi-Karur-Kambam-Trichur region in the form of an arc, similar to the present-day Himalayan orogeny (Figures 5-8).
These inferences obviously demand an answer to another important question as to "what is the role of Palghat-Cau-very shear zone?" during the evolution of Southern Granulite Province of India. In the following, an attempt is made to address this based on the results from this study and few important field observations. 4.9. Status of Palghat-Cauvery Shear Zone. Based on the above inferences about the terrane boundaries, it is important to evaluate the status of Palghat-Cauvery shear zone, which has been argued recently to represent a terrane boundary by a number of researchers. Furthermore, this shear zone has been hypothesized to have developed as a consequence of closure of the Mozambique Ocean (e.g., [11,72] and references therein). As pointed out above, no observable anomaly is recorded in our seismological results across this boundary. Although an in-depth review and analyses are beyond the scope of the present study, we wish to highlight some crucial points. Firstly, our seismological results (see Figures 5-8) on the nature of the crust reveal (a) flat topography of the Moho and (b) uniform average crustal SiO 2 across this shear zone. Secondly, large number of field visits (N = 23) conducted by us during the six-year-long operation of the broadband seismic stations provided us with ample opportunity to examine critically the Palghat gap encompassing the Palghat-Cauvery shear zone. We noticed that the Palghat gap is a~60-70 km wide linear strip entity with an average elevation~200 m (see Figure 1(b) for surface topography and also compare with Figure 8(a), where Moho is flat and uniform in the Palghat gap). This gap therefore represents a geomorphic low. Thus, the above two observations present an anomalous situation for the PCSZ to merit as a Proterozoic collision zone. It is important in this context to note that the collision zones in different parts of the globe are either highlands or plateaus having an altitude in excess of 600-700 m. Therefore, proponents of the idea of PCSZ as a suture need to ponder over the perplexing fact that in spite of their spatial proximity, how and why the geologically older (~2.5 Ga) Nilgiri massif could still survive its topography, while a Neoproterozoic metamorphic belt like PCSZ withered away preferentially due to erosion.
Based on our seismological results and field observations, it is unambiguously demonstrated that the Palghat-Cauvery shear zone represents only a crustal-scale megashear zone, although the causal mechanism for formation of this shear zone remains unclear at this stage. Besides the geosutures delineated in this study, there could be other fragments that might have collided with the Southern Granulite Province through geological time during its evolution. In this context, it is significant to note that a microterrane hypothesis for the Southern Granulite Province of India is already proposed [68]. Close-spaced geologic structure and geophysical crustal mapping of the entire region is therefore essential to examine this complex terrain so that further details related to its evolution could be unraveled. Special focus is warranted across the Achankovil shear zone, which could not be taken up in this study. However, it is pertinent to note that our general interpretation that large-scale thick skin tectonic disturbances have affected the SGP in the geologic past also receives unambiguous support from the presence of a well-defined north dipping conductive feature (50 to 100 Ohm m) beneath 15 Lithosphere the Achankovil shear zone that forms the boundary between the Madurai and Trivandrum blocks towards farther south of our study region [24]. It is therefore interesting to note that two major features alluding to subduction-collision tectonics in the SGP, about 300-350 km apart, are delineated in the 3D models of MT. These findings are in line with the proposed microterrane hypothesis for the Southern Granulite Province of India [68]. Therefore, our seismological findings across JKKTSZ and MBSZ recognizing them as terrane boundaries receive support from field geological, petrological, and isotopic evidences. The location of two sutures at crustal depth, viz., MBSZ and JKKTSZ, is marked using thick red line based on our data from individual station locations (Figures 5-8).
In the absence of seismologically discernible diagnostics and based on above cited compelling field geologic evidences with constraints from the 3D images of MT study, in the context of PCSZ, it is represented in Figure 8 as a dashed line only for purposes of reference alone.
Hence, another important outcome of this study is the inference that PCSZ represents merely a megashear zone. This shear zone is unlike the JKKTSZ and MBSZ that document seismologically delineated geosutures across them. This is a major breakthrough in our research related to Southern Granulite Province evolution. Prior to this effort, a comprehensive study of all the megashear zones of Southern Granulite Province based on judicious experimental design and integration of the apparently disparate evidences available in terms of seismic/seismological results till crustal depths, geological, geochronological, and petrological evidences reflecting the P-T processes of their deeper domains was never attempted. In our research, harnessing all these information, we arrive at a cogent model depicting the status, character, and evolution of these crustal-scale features in the context of Southern Granulite Province and place them in an appropriate geodynamic perspective.

Conclusions
Based on P-to-s (Ps) receiver function analyses, we presented depth images and crustal character across three prominent megashear zones of the Southern Granulite Province of India that are observed on the surface. These crustal-scale shear zones include (i) the Moyar-Bhavani shear zone, (ii) the Palghat-Cauvery shear zone, and (iii) the Karur-Kambam-Trichur shear zone. Our results yielded an unambiguous, comprehensive, and mutually consistent model of the study region. These results are capable of explaining observations