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Electrical signatures of the Earth’s crust in central India as inferred from magnetotelluric study
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2011
Received: 27 January 2010
Accepted: 4 August 2011
Published: 21 February 2012
The tectonic scenario of the Narmada-Son Lineament (NSL) zone has been the subject of debate for the last few decades. It is characterized tectonically as a highly-disturbed zone in Indian geology since Precambrian times. A magnetotelluric (MT) study has been carried out across the NSL zone along a 270-km-long N-S-trending traverse, extending from Hoshangabad in the North to Ner in the South. As a part of the present study, 25 magnetotelluric soundings have been collected and the data rotated to N70°E after removing local distortions, arising from 3D galvanic effects, using the Groom-Bailey decomposition technique. 2-D inversion has been carried out using an NLCG scheme. The results derived from the 2-D inversion have brought out the highly conductive nature of the mid-lower crust at places coinciding with the known faults. The significant high heat flow and seismicity in the region associated with these faults may be caused by tectonic activity and the highly conductive nature of the mid-lower crust. This may be due to the partial melting of subsurface rocks resulting from the high temperature caused by mantle upwarping in the region. The results are also compared with the gravity and a nearby Deep Seismic Sounding (DSS).
The Narmada-Son Lineament (NSL) is a late Archean mega geo-fracture extending for about 1200 km across the Indian peninsula in the E-W/ENE-WSW direction. Tecton-ically, this lineament is highly disturbed and reactivation of this lineament is believed to be due to the collision of the Indian plate with the Eurasian plate. As a result, the rigid continental crust has broken down along the pre-existing fracture system (Jain et al., 1984). Detailed geological studies of the region have been reported (West, 1962; Wellman and Mc Elhinny, 1970; Ghosh, 1976 etc.). The origin of the lineament is believed to have been formed in two distinctly different periods—after the commencement of the eruption of the Deccan traps, and the period prior to it. From various geological studies, this lineament is shown to be situated close to the zones of fractures belonging to early Precam-brian, Cretaceous and post-Deccan trap period. The Nar-mada valley dates back predominantly to the pre-Deccan trap period. Although the tectonic movements began significantly before the Deccan traps, it is still active as is attested by the number of earthquakes reported along the lineament (Ramalingeswara Rao and Sitapathi Rao, 1984).
2. Data Analysis
3. 2-D Inversion Results
The 2-D inversion has been carried out using the Rodi and Mackie’s (2001) RLM2DI code, as implemented in the WinGLink software package. This finds regularized solutions (Tikhonov Regularization) to the two-dimensional inverse problem for MT data using the method of nonlinear conjugate gradients (NLCG) to minimize an objective function that penalizes data residuals and second spatial derivatives of resistivity.
The trade-off parameter represents a measure of compromise between data fit and model smoothness. To find a suitable value of , the inversion procedure is repeated with different values of 5, 10, 15....... 50, 100 and an L curve (Hansen, 1998) is plotted between model roughness and rms error. The value corresponding to the corner of the curve (in our case, ) is considered to be the most appropriate for the model shown in Fig. 3(b). The inversion process was repeated with different starting models (homogeneous half space) of 50, 100, 500 and 1000 ohm-m and, for the final model, a resistivity of 100 ohm-m was set. Error floors of 20% for the apparent resistivity and 5% (1.5°) for the phase were assigned, thus down-weighing the apparent resistivity with respect to phase in minimizing the static shift effect on apparent resistivity. The root-mean-square (RMS) fit indicates the misfit between the observed and modeled data as a percentage. Here, the value of the misfit is 2.25% for the derived model after 100 iterations. Beyond this, the RMS misfit remained constant.
4. 2-D Geo-electric Crustal Structure
Towards the north, the thin high-resistive (500–5000 ohm-m) upper crust and the anomalously conductive (130 ohm-m) nature of the mid crust (A) in Fig. 4(b) and the seismicity near the Narmada south fault are pointers to the tectonic activity in the region. The two other conductive features, B and C (Fig. 4(b)), are also observed between the Tapti and Gavligarh faults as well as the Gavligarh and Purna faults. The lower crust is characterized by a conductive (1 –30 ohm-m), to a moderately resistive (50–200 ohm-m), nature. The results of the MT study in the form of the geo-electric section are compared with nearby Deep Seismic Sounding (DSS) and the regional Bouguer gravity data (NGRI, 2006) as shown in Fig. 4. The gravity values are taken for each station by digitizing the gravity map (NGRI, 2006). A maximum value of −25 mGal at station 9 is observed and maintains a value of about −40 mGal towards the southern end of the traverse, while small highs and lows are caused by local features (Fig. 4(a)). A steep fall in the gravity is observed near station 7, where it has a minimum value of − 70 mGal. A small gravity high is observed between stations 4 and 6. This is probably caused by a local feature. The Narmada South fault (NSF) is located between stations 3 and 4, whereas the Tapti North fault (TNF) is located close to station 7. We can find a minimum gravity value of about −70 mGal at both the faults. The MT traverse passes nearer to the earlier Deep Seismic Sounding profile (Kaila et al., 1985) from Khajuria Kalan (23.5°N, 77°E) to Pulgaon (20.75°N, 78.25°E). The seismic section along the MT traverse is presented in Fig. 4(c).
5. Discussion and Conclusions
There have been few MT studies carried out in central India over the Deccan volcanic province across the major fault zones. A recent study by Naganjaneyulu and Santosh (2010) identified the highly-conductive bodies as mafic/ultra mafic layered intrusives in the central India tectonic zone (CITZ), and also they have proposed a double-sided subduction history along the CITZ. Patro and Sarma (2009) identified a two-layered lithospheric mantle structure of this region describing a highly-resistive nature of the lithospheric structure on the top, and a low resistive nature at the bottom. In addition, other studies by Naidu and Harinarayana (2009), Gokarn et al. (2001), and Rao et al. (2004) have identified anomalous conductive features at mid-lower crustal depths, through various factors such as partial melts, fluids, etc., in the same region. However, anomalous conductive features associated with significant seismicity is not discussed in detail in the previous studies. Hence, the present study confirms anomalous conductive features in the region, and also proposes that the relation between seismicity and conductivity is of relevance to understand the earthquake mechanism.
The present MT study infers that a basin-like structure with thin trap and buried sediments is identified at shallow crustal depths, confined between the stations 12 (close to the Tapti fault) and 20 (close to the Purna fault) in the geo-electric section. The results clearly infer that the faults are close to the anomalous conductive features (A, B and C). This is more prominent near the Narmada south fault towards the northern part, as compared with the southern part of the traverse at mid-lower crustal depths. The conductive features at upper-mid crustal depths obtained from the geo-electric section spatially correlates with the absence of reflections and an indication of low seismic velocity in the Deep Seismic Sounding section (Fig. 4(c)). The gravity is low near the anomalous conductor A (Fig. 4(b)). Though the Deep Seismic Sounding study does not provide any suggestions regarding low gravity and high conductivity, it can be assumed to be of low seismic velocity. Together, these three observations suggest that the reasons for the anomalous conductive features (A, B and C) can be attributed to any one of the factors such as graphite, minerals, fluids or partial melt (Patro et al., 2005) as explained below.
5.1 Graphite boundary films
Graphite boundary films are often a probable candidate for high electrical conductivity in the lower crustal depths (Frost et al., 1989). Graphite is stable only at low oxygen fugacity in the stable continental regions. However, the present study region (NSL) is not a stable zone; it is a rift zone and active since Precambrian times. Hence, the existence of graphite in the NSL zone is not certain.
5.2 Conducting minerals
Metallic ore minerals, such as iron and copper sulphides, can produce high electrical conductivity (100 ohm-m) at mid-crustal depths, while they often occur in discrete ore bodies and also disseminated over a large volume (Li et al., 2003). A large-scale mineral deposit would probably produce detectable gravity and magnetic anomalies. However, in the absence of evidence for the occurrence of such large-scale mineral deposits in the study area, we can rule out this factor as a reason for the high electrical conductivity in the NSL zone.
In active subduction zones, at lower crustal depths, saline fluids can produce high electrical conductivity (Hyndman and Hyndman, 1968; Brace, 1971). A large amount of water is available in subduction zones at depths less than 40 km, from the expulsion of pore waters and from CH4–H2O fluids produced by diagenetic and low-grade metamorphic reactions (Peacock, 1990). However, the NSL zone is a rift zone and not a subduction zone. Hence, fluids are unlikely to be a probable candidate for producing the high electrical conductivity in the NSL zone.
5.4 Partial melting
Dry rocks begin to melt at around 1200°C and would produce high electrical conductivity. Partial melts, in most cases, are associated with a low seismic velocity and low gravity (Schilling et al., 1997). Hence, the low crustal seismic velocity in the study region with the absence of reflections starting from shallow depths to mid-lower crustal depths, and the low gravity anomaly, could be evidence to infer that the presence of partial melting could be a favorable source of observed crustal conductors. The partial melting of subsurface rocks may be due to high temperature and pressure conditions at mid-lower crustal depths that could be caused by mantle upwarping due to tectonic activity in the NSL zone. Additionally, this zone is considered to be a high heat flow zone (Ravi Shanker, 1988).
MT modeling results brought out three high conductive structural features (A, B and C) extending from mid to lower crustal depths. These features can be interpreted as the electrical signatures responsible for the geologically-mapped major tectonic faults, namely, the Narmada South, Tapti, Gavligarh and Purna faults as they occur in their vicinity. The distribution of the epicenters (>M 5.0) along the faults in the area suggests that the origin of the earthquakes are probably related to the faults which are conductive in nature. Rapid crustal erosion must have been taken place due to a high-temperature partial melt (where conductivity is high) in the northern part which causes faulting along pre-existing zones of weakness. The block movement along the faults could generate an earthquake. The Narmada South fault is more prominent and active compared with the other fault features, and is associated with the M 6.0 Ja-balpur earthquake on 22 May, 1997 (Rastogi, 1997) in the study region. Thus, the present study helps in understanding the tectonic features and their nature in the NSL region.
We thank Dr. V. P. Dimri, former Director, and Dr. Y. J. Bhaskar Rao, the acting Director, of NGRI for giving permission to publish this work. The authors are thankful to Prof. Heinrich Brasse and an anonymous reviewer for their critical comments and valuable suggestions on earlier versions of the manuscript which has greatly helped to improve the manuscript. We are also thankful to Makoto Uyeshima, editor of the journal for his support during the editorial process. We also would like to thank Shri. D. N. Murthy for useful discussions and all the help regarding the modeling work. G. Dhanunjaya Naidu thanks CSIR for the Senior Research Fellowship.
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