Three-dimensional lithospheric electrical structure of Southern Granulite Terrain, India and its tectonic implications

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH: SOLID EARTH, VOL. 119, 71 82, doi: /2013jb010430, 2014 Three-dimensional lithospheric electrical structure of Southern Granulite Terrain, India and its tectonic implications Prasanta K. Patro, 1 S. V. S. Sarma, 2 and K. Naganjaneyulu 3 Received 13 June 2013; revised 26 August 2013; accepted 15 November 2013; published 13 January [1] The crustal as well as the upper mantle lithospheric electrical structure of the Southern Granulite Terrain (SGT) is evaluated, using the magnetotelluric (MT) data from two parallel traverses: one is an ~ 500 km long N-S trending traverse across SGT and another a 200 km long traverse. Data space Occam 3-D inversion was used to invert the MT data. The electrical characterization of lithospheric structure in SGT shows basically a highly resistive (several thousands of Ohm meters) upper crustal layer overlying a moderately resistive (a few hundred Ohm meters) lower crustal layer which in turn is underlain by the upper mantle lithosphere whose resistivity shows significant changes along the traverse. The highly resistive upper crustal layer is interspersed with four major conductive features with three of them cutting across the crustal column, bringing out a well-defined crustal block structure in SGT with individual highly resistive blocks showing correspondence to the geologically demarcated Salem, Madurai, and Trivandrum blocks. The 3-D model also brought out a well-defined major crustal conductor located in the northern half of the Madurai block. The electrical characteristics of this south dipping conductor and its close spatial correlation with two of the major structural elements, viz., Karur-Oddanchatram-Kodaikanal Shear Zone and Karur-Kamban-Painavu-Trichur Shear Zone, suggest that this conductive feature is closely linked to the subduction-collision tectonic processes in the SGT, and it is inferred that the Archean Dharwar craton/neoproterozoic SGT terrain boundary lies south of the Palghat-Cauvery shear zone. The results also showed that the Achankovil shear zone is characterized by a well-defined north dipping conductive feature. The resistive block adjoining this conductor on the southern side, representing the Trivandrum block, is shown to be downthrown along this north dipping crustal conductor relative to the Madurai block, suggesting a northward movement of Trivandrum block colliding against the Madurai block. The lithospheric upper mantle electrical structure of the SGT up to a depth of 100 km may be broadly divided into two distinctly different segments, viz., northern and southern segments. The northern lithospheric segment, over a major part, is characterized by a thick resistive upper mantle, while the southern one is characterized by a dominantly conductive medium suggesting a relatively thinned lithosphere in the southern segment. Citation: Patro, P. K., S. V. S. Sarma, and K. Naganjaneyulu (2014), Three-dimensional lithospheric electrical structure of SouthernGranuliteTerrain, Indiaanditstectonicimplications, J. Geophys. Res. SolidEarth, 119, 71 82, doi: /2013jb Introduction [2] The Southern Granulite terrain (SGT) represents a unique and one of the largest granulite terrains of the Earth exposing a wide range of deformed and retrograded hard crystalline rocks. The crystalline rocks of the region are 1 Magnetotellurics and Deep Resistivity Sounding Division, CSIR- National Geophysical Research Institute, Hyderabad, India. 2 Chief Scientist (Retired), CSIR-National Geophysical Research Institute, Hyderabad, India. 3 Engineering Airborne Geophysics Division, CSIR-National Geophysical Research Institute, Hyderabad, India. Corresponding author: P. K. Patro, Magnetotellurics and Deep Resistivity Sounding Division, CSIR-National Geophysical Research Institute, Uppal Road, Hyderabad, AP , India. (patrobpk@ngri.res.in) American Geophysical Union. All Rights Reserved /14/ /2013JB derived through a complex evolutionary history marked by at least two major tectonic events during Achaean and Proterozoic times with multiple deformations, anatexis, intrusions, and polyphase metamorphic events, and a series of major shear zones traversing across the region impart a highly complex structural fabric to the region. Earlier geological, geochemical, and geochronological observations suggest that in the SGT region, major tectonic processes were active from mid-archean to Late Proterozoic times which might have affected the physical nature of the crust and upper mantle underneath, making it significantly heterogeneous and structurally complex. [3] Several geophysical studies were carried out during the last three decades in the SGT including gravity [Subrahmanyam, 1978;Narain and Subrahmanyam, 1986; KrishnaBrahmam, 1993; Mishra and Rao, 1993; Singh et al., 2003], aeromagnetics [Reddi et al., 1988], seismic tomography 71

2 boundary. Toward this, the available magnetotelluric data in the region acquired till 2004 over different segments covering a 500 km long near N-S trending traverse across the Southern Granulite terrain as also from a parallel but shorter traverse (see Figure 1) are reanalyzed and remodeled using the recent 3-D inversion schemes [Siripunvaraporn et al., 2005] in order to take into account the 3-D complexity of the subsurface structure of the region as well as to account for the effects, if any, due to the ocean which surrounds the SGT on its three sides. The MT data have been inverted on all the four impedances, using the data space Occam inversion to get a 3-D model that provides a more realistic representation of the crustal as well as upper mantle electrical structure for the entire SGT. Figure 1. Geological map of Southern Granulite Terrain showing various crustal blocks such as Salem block, Madurai block, and Trivandrum block (modified from Santosh and Sajeev [2006]). MT stations recorded during different field campaigns since 1998 till 2004 are plotted as red stars. Geoelectric section obtained from 3-D inversion along the cyan + green line (N-S) is presented in Figure 6. Green color profiles (Traverses I and II) are discussed in Figure 9 for 2-D versus 3-D models. PCSZ: Palghat-Cauvery Shear Zone; CSZ: Chennamalai Shear Zone; ACSZ: Achankovil shear zone. 2. Geology, Structure, and Tectonics [4] The Southern Granulite region located adjacent to and south of the Dharwar craton is basically divided into three blocks from north to south, viz., Salem, Madurai, and Trivandrum blocks. Figure 1 shows the geological and tectonic map of the southern peninsular shield. The Salem block exposes a wide range of deformed and retrograded hard crystalline rocks, comprising of Archean/Proterozoic orthogneiss, charnockite, mafic granulite, and ultramafic intrusives in association with metasedimentary units [Santosh et al., 2009]. These are generally believed to be metamorphosed equivalents of Dharwarian rocks in the adjacent north. The block was subjected to metamorphic processes at least three times, viz., the metamorphic cycle of granulite facies during Late Archean to Early Proterozoic as revealed from isotopic studies [Peucat et al., 1993] and a high-pressure metamorphism during early Neoproterozoic [Bhaskar Rao et al., 1996] followed by a high-grade granulite metamorphism during Ediacaran- Cambrian period particularly in the southern half of the block. [5] The Madurai block is mainly occupied by Charnockites showing ages ranging from 2200 to 3170 Ma [Bartlett et al., 1998; Bhaskar Rao et al., 2003] such as those exposed in [Rai et al., 1993, 2003], deep seismic sounding [Reddy et al., 2003], and magnetotellurics [Harinarayana et al., 2003, 2006; Naganjaneyulu and Harinarayana, 2003; Naganjaneyulu and Santosh, 2010, 2011; Naidu et al., 2011]. Some of the geophysical responses observed in this part of the Indian plate such as the nature of teleseismic velocities, electrical structure models from MT studies, and crustal thickness variations modeled from deep seismic and gravity observations provided valuable insights into such crustal heterogeneities and structural complexities in different segments of the SGT. Many of these studies have focused mainly on the crustal structure of the SGT region and in particular the Palghat-Cauvery shear Zone (PCSZ), believed to be a Late Neoproterozoic- Cambrian crustal-scale suture zone and considered to be a tectonic boundary between the SGT on the south and Dharwar craton on the north. While many of the earlier studies focused on individual segments of the SGT, the present study is an effort aimed at obtaining a 3-D model of crustal as well as the upper mantle lithospheric electrical structure of the entire SGT to gain further insights into the geotectonic setting of this important lithospheric segment of the Indian plate and also to examine the Archean Dharwar-Neoproterozoic SGT terrain Figure 2. Available periods for each site. Fifteen periods from 0.01 s to 546 s were considered for 3-D inversion. The range is marked by a rectangle. 72

3 Figure 3. MT stations (red and blue stars) plotted over the topography map of the SGT region. The strike directions obtained from Smith [1997] are plotted as a rose diagram in the top right corner. Black dashed lines represent the shear/suture zones. Black dots with numbers represent the surface heat flow values in mw/m 2 [Ray et al., 2003; Roy et al., 2007]. Dharwarian/Neoproterozoic granulite boundary that lies further south of PCSZ. [7] The PCSZ and adjacent domains have also been in focus for petrological studies on Mg-Al granulites that contain diagnostic mineral assemblages indicating extreme crustal metamorphism associated with subduction-collision tectonics [e.g., Shimpo et al., 2006; Santosh and Sajeev, 2006; Tsunogae et al., 2008; Ohyama et al., 2008; Kanazawa et al., 2009; Sato et al., 2009; Tsunogae and Santosh, 2010; Clark et al., 2009]. [8] Another major shear zone in the south is the Achankovil shear zone (ACSZ). The ACSZ separates the Madurai Block from the Trivandrum Block to the south. The ACSZ shows a well-defined lithological and isotopic boundary [Harris et al., 1994; Bartlett et al., 1998]. Recent studies recorded C and 8.5 9kbar metamorphic P-T conditions [Ishii et al., 2006] from cordierite and orthopyroxene-bearing ultrahigh temperature granulites indicating an ultrahot orogen. Metamorphic ages obtained from the charnockites in the ACSZ are between 548 ± 2 Ma and 526±3Ma[Ghosh et al., 2004]. Kodaikanal massif, Archean Gneisses, metasedimentaries together with U-Pb electron probe micro-analyzer ages on zircon and monazites [Santosh et al., 2006] mainly occupying the southern and eastern parts of the block. The Trivandrum block mainly exposes the metasedimentaries including leptynites, khondalites, alkali granitoids, synites, and charnokites. The protoliths of the metasedimentary units belonged to either Paleoproterozoic or Neoarchean times. The Madurai block is extensively deformed and metamorphosed to granulite facies during the Neoproterozoic [Collins et al., 2010]. Prolific occurrence of ultrahigh temperature mineral assemblages in the block has led to the conclusion that the region had witnessed an ultrahigh temperature metamorphic event with temperatures in the range C and pressures 7 11 Kbar, sometime between 600 and 480 Ma [Collins et al., 2010] presumably related to the process of Gondwana amalgamation. [6] Several shear/fault zones cut across the SGT region, the major ones in the northern half are the Palghat-Cauvery shear zone (PCSZ), Moyar Bhavani shear zone, and Mettur shear zone. The E-W trending 100 km wide region between 11 N and 12 N is often referred to as PCSZ [Chetty and Rao, 2006]. The PCSZ shows significant differences in the structural patterns, lithology, Nd model ages, Rb-Sr mineral ages, and metamorphic P-T conditions relative to the Dharwar craton on the north and the SGT in the south [see Bartlett et al., 1998; Santosh and Collins, 2003; Ghosh et al., 2004; Santosh et al., 2005] and is generally considered as an Archean- Neoproterozoic terrain boundary [e.g., Harris et al., 1994]. But consideration of geochronological data particularly U-Pb zircon age determinations and the Sm-Nd ages for charnokites and migmatitic gneisses [Bhaskar Rao et al., 2003; Ghosh et al., 2004] and identification of significant major structural features like the Karur- Oddanchatram-Kodaikanal Shear Zone (KOKSZ) [Bhaskar Rao et al., 2003] and the Karur-Kamban-Painavu-Trichur Shear Zone (KKPTSZ) [Ghosh et al., 2004] led to a suggestion that these features may be related to the possible 3. MT Data and Analysis [9] MT data in the Southern Granulite Region were acquired during and in 2004 (see Figure 1). Earlier surveys were carried using Metronix data acquisition system GMS05, while the data for the 2004 field measurements ADU06 were used. All the MT data were acquired on a single-site mode covering a frequency range of 100 Hz to Hz and was processed using the available code, namely the Metronix robust processing code (PROCMT and MAPROS). During these campaigns remote reference mode of data acquisition could not be taken up due to nonavailability of required equipment and also due to logistic problems. The 2-D modeling results of the subsets of data presented earlier [Harinarayana et al., 2003, 2006; Naganjaneyulu and Harinarayana, 2003; Naganjaneyulu and Santosh, 2010, 2011; Naidu et al., 2011] were thus limited to different segments of the SGT. For the present 3-D analysis, which aims at providing a comprehensive 3-D model for deep electrical structure for the entire SGT, we have selected the stations falling along a 500 km long N-S traverse starting from Dharwar craton in the north to the southern edge of India near the coast. As the MT time series analysis in different field campaigns was carried out using different processing codes which used different sets of target frequencies, the frequencies of MT transfer functions differ moderately between 2004 and earlier campaigns. In order to arrive at a consistent frequency set for all the sites, interpolation of the MT response functions along with their errors from nearby frequencies (within 10% radius of the original frequency) has been carried out. [10] The set of frequencies, thus became available for further analysis for each site, is presented in Figure 2. Though a broad range of frequencies are available, considering the coherency and data errors, the data quality for most of the sites was good up to a period of 546 s and this has set the limit for selection of the longest period for the present analysis. Accordingly, for the purpose of 3-D inversion, 15 periods ranging from 0.01 s to 546 s are considered. 73

4 Figure 4. Phase tensor ellipse plots for different periods (0.256 s, s, s, and 38 s). These ellipses are filled in with skew values. [11] The strike analysis was done using Smith s [1997] procedure for different frequency bands. The directions (with 90 ambiguity) are plotted as a rose diagram (see Figure 3). Note that there is no single preferred strike angle evident from the rose diagram. It is known from geological and structural data that a number of shear zones of different orientations viz., E-W, N-S, NE-SW, and NW-SE cut across the region suggesting a possible 3-D subsurface geometry for the study region. Additionally, the area of study is surrounded by Bay of Bengal to the east, Arabian sea to the west, and Laccadive Sea to the south and these would naturally create further complication in the form of coastal effects and introduction of additional 3-D distortions which need to be accounted before retrieving a reasonable subsurface electrical model and hence the need to carry out a 3-D analysis of the data under consideration. [12] Further, the dimensionality of the MT data was also examined using phase tensor (Φ)analysis[Caldwell et al., 2004]. Phase tensor and other parameters such as Φ min, Φ max,andβ are computed from the following equations: " Φ ¼ Φ # 11 Φ 12 Φ 21 Φ 22 Φ 1 ¼ Φ 11 þ Φ 22 Φ 2 ¼ Φ 12 Φ 21 Φ 3 ¼ Φ 11 Φ 22 Φ 12 Φ 21 Φ min ¼ Φ 2 1 þ 1=2 Φ2 3 Φ 2 1 þ Φ 2 3 1=2 Φ2 2 Φ max ¼ Φ 2 1 þ 1=2 Φ2 3 þ Φ 2 1 þ Φ 2 3 1=2 Φ2 2 β ¼ 1 Φ 3 2 tan 1 Φ 1 [13] Generally, the phase tensor ellipses take a form of circle for all the periods if the subsurface is a uniform half space, which means Φ max Φ min. For 2-D cases, the principal 74

5 for all the sites (see Figure 4). These ellipses are filled with the skew angle ( β) values. The increasing ellipticities with increasing periods together with high β values suggest a complex subsurface structure favoring a 3-D analysis for interpretation of these data sets. Though there appears no distinct pattern in the phase tensor data for the sites along the traverse, the data tend to show a gradual change in the pattern from south to north along the traverse. 4. The 3-D Inversion [14] We have used data space Occam inversion [Siripunvaraporn et al., 2005] to invert the MT data. All the four impedances (Zxx, Zxy, Zyx, and Zyy) for 50 sites were inverted at 15 periods from 0.15 s to 546 s. The model was created with Nx = 69, Ny = 73, and Nz = 38 layers (plus seven air layers). The core of the model is described by 56 (N-S) and 60 (E-W) grids in the horizontal plane with spacing of 10 km. The model mesh was created with a vertical factor of 1.33 with the top layer thickness being 50 m, and the dimensions of the total model domain were km. The model grid is shown in Figure 5. The prior model was a 100 Ω m half space with all the three seas (0.3 Ω m) around included and also set fixed during inversion. For this inversion, considering the data errors we have set the error floor as 5% of pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi jzxy Zyxj. The bad data points were assigned with very large number in the error map so as to exclude them from inversion. The final RMS obtained was 4.94 (Model A), and the resulting model is shown in Figure 6. Observed and computed responses are presented in Figure 7. Figure 5. The model grid used for 3-D inversion. Details about the grid geometry are explained in the text. values are usually different (i.e., Φmax Φmin) and there will be two directions for which a linearly polarized magnetic field will give rise to a linearly polarized electric field. When the regional conductivity distribution is 1-D or 2-D, the phase tensor skew angle ( β) assumes a value close to zero due to symmetry in phase tensor. However, for 3-D cases, β 0 and Φmax Φmin. Phase tensor ellipses for four different periods are plotted Figure 6. N-S geoelectric section (see Figure 1 for the profile direction) plotted from the derived 3-D model of the SGT region. Bold white lines below SB and PCSZ indicate seismic moho derived from seismic refraction studies [Reddy et al., 2003]. To the south below MB and ACSZ, the moho was taken from velocity model by Behera [2011]. Bouguer gravity anomaly [Kumar et al., 2010] are plotted along a profile from north to south. The conductors C1 C4 and resistors R1 R4 are explained in the text. Elevation at each site is plotted at the top portion of the figure. 75

6 Figure 7. Observed (star) and computed (line) responses of Zxy(r, i), Zyx(r, i) and Zxx(r, i), Zyy(r, i) (scaled by T) at sites marked as blue star in Figure 2. The sites are presented from north to south. [15] The model shows a resistive crustal column as well as four major conductive features (C1 C4). To check the resolution of the conductors C1 to C4, sensitivity tests were performed and their significance was examined by F test (see Toh et al. [2006] for a similar study). The resistivity of the conductor C1 (Model B) from its initial value 100 Ω m was changed to 2000 Ω m corresponding to the surrounding medium. This removes the conductor C1; however, the RMS has increased from 4.94 to Similarly, the resistivity of the conductor C3 (Model C) was changed to 5000 Ω m from its initial value of 200 Ω m and the RMS increased from 4.94 to In the case of conductor C4 (Model D), the resistivity was changed to 5000 Ω m from its initial value of 100 Ω m and the RMS has increased from 4.9 to From Table 1, it is evident that the variance ratios are higher than the critical F value of 1.04 for the present data set. Therefore, the F test validates the necessity for the presence of conductors C1 to C4 in the 3-D model obtained. 5. The Subsurface Electrical Model for SGT [16] The crustal electrical structure of the SGT, as may be seen from the 3-D electrical model presented in Figure 6 is basically characterized by a very high resistive (>10,000 Ω m) upper crustal column overlying a relatively less resistive ( Ω m) lower crustal column. The model also brings out four major conductive features (C1 to C4) in the SGT, and three of these cut across the crustal column reaching the upper mantle depths. The 3-D model for this nearly 500 km long traverse that cuts across the entire SGT thus brings out a distinct block structure for the crust, and the SGT crustal column may be broadly divided into four highly resistive crustal blocks (R1 R4) separated by the major conductors (C1, C3, and C4). [17] At the northern end of the traverse close to the Dharwar craton, the resistive block R1 extending to a depth of about km corresponds to part of the Salem block. Southward, the second highly resistive block (R2) covers the PCSZ and a part of the northern part of the Madurai block. The third block (R3) covers the southern half of the Madurai block. The Madurai block is apparently divided into two parts with the northern one forming a part of the PCSZ zone. The fourth block (R4) is located in the southernmost segment of the SGT corresponding to the Trivandrum block. The R4 is relatively 76

7 Figure 7. (continued) smaller in size and also less resistive compared to the other blocks (R1 R3). [18] The vertical conductor C1, which is moderately conductive at upper crustal levels, becomes highly conductive at mid-lower crustal levels and spatially correlates with the Moyyar-Bhavani shear zone. It borders the resistive block R1 on its southern side and separates it from the R2 block, which represents mainly the PCSZ. The conductive feature C2 that lies at mid-lower crustal levels is located underneath the southern half of PCSZ. The conductive feature C3, located south of PCSZ, separates the two blocks R2 and R3. This conductor C3 dipping southward continues downward consistently from shallow depths of less than 10 km to as much as more than 50 km deep into the upper mantle. This major conductor occupies a significant portion of the northern half of the Madurai block. The conductor C4 at the southern end of the traverse, dipping northward, extends down to deeper levels and spatially correlates with the Achankovil shear zone (ACSZ). [19] The resistive block R2 corresponding to the PCSZ, as seen along a section toward west of the profile in Figures 7 and 8, itself encloses a near E-W trending conductor located roughly in the middle of the block and extends to about 40 km thus limiting itself to the crustal column (see the horizontal sections in Figure 8). In contrast, the adjacent highly resistive block (R3) toward south (Figures 7 and 8) is devoid of any conductors. Farther south, the resistive block R4 is overlain by a thin (5 10 km) horizontal low resistive crustal layer. This block (R4) shows a downward displacement along the north dipping conductive feature C4 which itself shows a spatial correlation with the Achankovil shear zone. [20] Another feature of interest that the 3-D model brings out is a distinct variation in the lithospheric electrical structure along the traverse. The deeper structure, i.e., lithospheric upper mantle electrical structure of the SGT up to a depth of 100 km may be broadly divided into two distinctly different northern and southern segments. The northern half, over a major part, is characterized by a thick resistive upper mantle column, while the southern half is relatively conductive. The boundary between the two runs approximately through the middle of the Madurai block and shows a broad spatial correlation with the major conductor C3. [21] A comparison between 2-D and 3-D inversion models for two traverses (see Figure 1 for location), viz., 77

8 Figure 7. (continued) Figure 7. (continued) 78

9 Table 1. Summary of F Test RMS Variance Variance Ratio a Significance Model A Model B (B/A) S Model C (C/A) S Model D (D/A) S a Since degree of freedom of the data set used in this study is 5999, F value corresponding to 95% confidence level becomes Traverse I, the 240 km long traverse that cuts across the ACSZ at its southern end, and Traverse II, the 160 km long parallel traverse that cuts across the PCSZ, is presented in Figures 9a and 9b (Traverse I) and Figures 9c and 9d (Traverse II). Figure 9a presents a cross section through the 3-D inversion model along the Traverse I, and Figure 9b presents the 2-D inversion model for the same traverse from Naidu et al. [2011]. The main difference that may be readily seen between the two is that the 3-D model shows distinct crustal block structure, R3 and R4 (see also R3 and R4 in Figure 6), as well as a well-defined northward dipping conductor C4 corresponding to ACSZ. Similarly, Figure 9c shows a cross section through the 3-D inversion model along Traverse II, and Figure 9d shows a recontoured 2-D inversion model from Naganjaneyulu and Santosh [2010]. The main differences between the two are in the geometry of the conductive feature between 60 and 120 km along the traverse besides the less resistive nature of mid-lower crustal column in the 3-D model. [22] It may thus be seen that in contrast to 2-D studies, whicharelimitedtoindividual segments, the present 3-D analysis, besides providing a unified and comprehensive subsurface model for the entire SGT, takes into account effects of the sea that surrounds all the three sides of the SGT. This study also brings out several electrical features like C3 and C4 as well as the nature of lithospheric electrical structure, the distinct crustal block structure, and the nature of polarity of relative movements of the crustal blocks; all these possibly related to different phases of collision-subduction tectonic history of the region. 6. Discussion and Conclusion [23] Most of the earlier geophysical studies in the SGT region [e.g., see Kumar et al., 2009a, 2009b; Singh et al., 2003; Behera, 2011; Naganjaneyulu and Santosh, 2010, 2011; Harinarayana et al., 2003; Naidu et al., 2011] discussed the nature of upper and lower crustal columns of the region. The MT studies, based on 2-D modeling, have focused mainly on the electric signatures of a few shear zones in the Figure 8. Horizontal sections of the 3-D geoelectric model presented at different depths ranging from 10 to 100 km. Black triangles are the MT stations. Bold white lines are the suture/shear zones. C1 C4 are described in the text. PCSZ: Palghat-Cauvery Shear Zone; KOKSZ: Karur-Oddanchatram-Kodaikanal Shear Zone; KKPTSZ: Karur-Kamban-Painavu-Trichur Shear Zone; ACSZ: Achankovil shear zone. 79

10 SGT, e.g., the Mettur shear zone and the Atchan kovil shear zone, and have directly related them to different models of collision tectonics. The gravity and seismic studies dealt mainly with the heterogeneous nature of the crustal column and discussed the variation of moho depths in different parts of the SGT. The 3-D electrical model obtained in the present study provides a more comprehensive view of the crustal column, and the nature of variation of subcrustal electrical lithospheric characteristics along the 500 km long traverse goes across the entire SGT. Besides delineating additional subsurface conductors and bringing out the crustal block structure, the 3-D model brought out a distinct electrical signature of the ACSZ in the form of a north dipping conductor in contrast to a diffuse conductive anomaly observed in earlier 2-D models. [24] The crustal electrical structure in SGT derived from the 3-D model (Figures 6 and 8) is thus characterized by a two-layered configuration with a highly resistive upper (several thousands of Ohm meters) layer overlying a moderately resistive (a few hundred Ohm meters) lower crustal layer. The highly resistive upper layer interspersed by three major conductive features along this nearly 500 km long traverse covering the entire SGT presents a well-defined block structure comprising of four highly resistive crustal blocks (R1 R4). The major deep-seated crustal-scale conductive features in the 3-D model, some of them extending to upper mantle depths, suggest large-scale thick skin tectonic disturbances that the SGT was subjected to, with the crust-penetrating conductive features primarily representing the manifestation of collision/subduction tectonics involving different tectonic blocks of the SGT. [25] The PCSZ is generally considered to be a suture zone that marks the boundary between the Dharwar craton and the Southern Granulite Terrain. Based on geological observations, such as occurrence of Ophiolites suits and oceanic plate stratigraphy, detection of several high-pressure and ultrahigh temperature rock assemblages, identification of accretionary orogenic magmatic arc environment toward south of PCSZ, and a plate tectonic model involving subduction-collision process, was postulated [Santosh et al., 2009] to explain the amalgamation of neoproterozoic SGT region with the Dharwar craton with the PCSZ as the suture zone. A conductive feature in the PCSZ identified earlier from the 2-D analysis of the MT data [Naganjaneyulu and Santosh, 2010, 2011] was interpreted as fragments of the subducted oceanic crust, eclogitized and exhumed partly. As observed in the 3-D model (Figures 6 and 8), this feature is however confined to that traverse only and does not appear on the adjacent traverse, thus indicating its limited extent. But the conductor C3, located south of PCSZ, delineated from the 3-D modeling is a major feature passing through the northern part of the Madurai block that separates the PCSZ (R2) and Madurai block (R3). The conductor C3 dipping southward extends from a shallow depth to much deeper levels of about 50 km into the upper mantle and with a moderate conductance of about 600 S and a length of about km shows resemblance with some of the major conductive features associated with Proterozoic subduction zones [Jones, 1993] and may thus be inferred to represent the electrical signatures of such subduction-collision zone in the SGT. Further, the close spatial correlation of this major conductor (C3) derived from the 3-D MT model with the structural features KOKSZ and KKPTSZ further suggests that the C3 feature is closely linked to the collisional tectonic processes related to the formation of the Dharwar/Neoproterozoic terrain boundary. This is consistent with the geological and geochronological inferences by Ghosh et al. [2004]. [26] Farther south, the ACSZ that forms the boundary between the Madurai and Trivandrum blocks shows a well-defined expression in the 3-D electrical model represented by the major north dipping conductor, C4. This feature along with C3 located south of the PCSZ must have played a significant role in the subduction-collision tectonic processes in the SGT region. The Trivendrum block represented by R4 in 3-D model may be seen to have been downthrown relative to the Madurai block (R3) along the northward dipping conductor C4, suggesting a south to northward movement of Trivandrum block colliding against the Madurai block. [27] The deeper electrical structure up to depths > 100 km corresponding to the upper mantle lithosphere along the traverse shows a distinct difference between the northern and southern halves of the SGT. While the electrical character in this depth ranges in the northern segment of the traverse covering the region of PCSZ and the northern half of the Madurai block presents a thick resistive column indicating a relatively thicker lithospheric segment in contrast to the southern part of the traverse covering the southern half of the Madurai block together with the entire Trivandrum block, which is characterized by a dominantly conductive medium extending from lower crustal levels to as much as 100 km, thus pointing out to a thinner and conductive lithosphere in the southern part. [28] Geological, geochemical, and geochronological data also show distinct difference between the northern (Salem and Madurai blocks) and southern (Trivandrum) halves [e.g., Bhaskar Rao et al., 1996, 2008; Ghosh et al., 2004]. The Charnokites in the northern half show in general older ages and the southern ones dominantly younger ages, particularly those in the southernmost segments, the Trivandrum block. The southern part of the SGT (central and southern Madurai block) shows distinctly a higher degree of intensity of deformation and metamorphism compared to the northern part [Santosh et al., 2006; Collins et al., 2007, 2010]. Ultrahigh P-T conditions are inferred to prevail in the southernmost segment of the SGT during Pan-African orogenic episode facilitating formation of high-grade metamorphic rocks [Christensen and Fountain, 1975] like Charnokites at deeper levels mainly due to streaming of carbonic fluids [Cenki et al., 2002]. That the southern part of MB and Trivandrum blocks experienced such deformation, high-grade metamorphism and intense crust-mantle interaction extending to deeper levels in contrast to the northern part may indeed be also seen to reflect as a significant and conspicuous difference in several of the geophysical responses, besides those brought out in the deeper electrical structure seen in the 3-D MT model. Higher heat flow values in the southern SGT [Roy et al., 2007], enhanced mantle heat flow contributions in SGT, as also significant difference between the northern and southern halves in the estimated moho temperatures [Behera, 2011], the Trivendrum block in the south showing 650 C as against 590 C for the Madurai block in the north, and the well-defined upward gradient in the gravity field toward south are all 80

11 Figure 9. The 2-D and 3-D inversion models along ACSZ traverse and PCSZ parallel traverse (see Figure 1). (a) Cross section through the 3-D inversion model along ACSZ traverses. (b) The 2-D inversion model from Naidu et al. [2011]. The main difference between 2-D and 3-D inversion models are the well-defined resistive block (R3 and R4 in Figure 6, main text) together with northward dipping conductor C4 below ACSZ. (c) Cross section through 3-D inversion model along a profile parallel to PCSZ. (d) Recontoured 2-D inversion model from Naganjaneyulu and Santosh [2010]. The main differences are in the geometry of the conductive feature between 60 and 120 km. In addition to this, the crust until 30 km is less resistive. consistent with the 3-D electrical model, which is characterized by shallowing of the moderately conductive lower segment of the upper mantle lithosphere toward south. 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