Along strike variations in the geo-electric structure of ...

IAGA WG 1.2 on Electromagnetic Induction in the Earth 20th Workshop Abstract Giza, Egypt , September 18-24, 2010

Along strike variations in the geo-electric structure of the Central Indian Tectonic Zone (CITZ) and origin of elevated conductivity 1

K. K. Abdul Azeez1, Martyn Unsworth2, T. Harinarayana1 Magnetotelluric Division, National Geophysical Research Institute, Hyderabad, India 2 Department of Earth and Atmospheric Sciences, University of Alberta, Canada

SUMMARY The Central Indian Tectonic Zone (CITZ) is a major tectonic element extending more than 1000 km east-west across the Indian sub-continent. It marks the boundary between the southern and northern blocks of the Indian peninsula. The geoelectric structure of several regions of the CITZ has been studied with single-profile magnetotelluric (MT) studies by a number of authors. This abstract describes the first attempt to use multiple MT profiles to systematically study west-east variations in the structure of the CITZ. The conductivity models on the five profiles all show a low resistivity layer (10 - 80 Ωm) in the middle to lower crust beneath the CITZ. The crustal conductance in this region is in the range 300-800 S with isolated locations with conductance exceeds 1000 S. The depth to the top of conductive zone is shallow (~ 10 km) at the western end of the CITZ and deepens to 18 - 22 km in the east. The upper crust in the area has a high resistive (> 1000 Ωm) structure consistent with the occurrence of gneissic basement rocks in the region. The medium to high resistive (300 - 10000 Ωm) upper mantle zones imaged indicate relatively cool lithosphere underneath. What is the origin of the elevated conductivity in the middle and lower crust in the region? Rao et al. (2004) suggested that partial melt associated with magma bodies was present beneath the CITZ. This suggestion is inconsistent with the low-moderate heat flow and the absence of any significant seismic P-wave attenuation (Roy and Rao, 2000; Kaila et al., 1981 &1989). Graphite mineralization is unlikely due to the high oxygen fugacity (Dessai et al., 2009) and active tectonics noticed in the form of deep and micro earthquakes occurring in this region (Rao et al., 2002; Gahalaut et al., 2004). The presence of 0.6 – 1.8 % pore fluids could produce the observed conductivity of the mid-lower crust. Hydrous and dry mineral conduction is also possible source as the xenoliths samples from the region show the presence of hydrous mineral phlogopite and dry metallic minerals, such as titanomagnetite and magnetite. The CITZ is an ancient suture that was reactivated during the Late Cretaceous - Early Tertiary period. This was associated with the huge Deccan volcanic eruptions, and may have been attributed to the fluids generation in the crust. The complex geochemical processes in the deep crust, catalyzed by the major tectonothermal events, may have generated conductive minerals in the mafic granulite middle to lower crust under the region. These factors have contributed to the elevated conductivity observed for CITZ deep crust. Keywords: Central Indian Tectonic Zone, electrical resistivity, magnetotellurics, fluids, hydrous and metallic minerals.

INTRODUCTION The shield region of the Indian subcontinent is an assemblage of various Precambrian tectonic provinces (Fig. 1) with ages varying from early Archaean to late Proterozoic that were sutured together along younger mobile belts (Naqvi et al., 1974). The ENE-WSW trending Central Indian Tectonic Zone (CITZ) is considered to be the tectonic boundary that separates the Indian subcontinent into the Southern Peninsular block and northern foreland block (Jain et al., 1991; Acharyya, 2003). It is believed to have formed during Middle to Late Archaean and was

subsequently reactivated during the Proterozoic (Acharyya and Roy, 2000). The Cretaceous Deccan flood basalts that cover the western part of the CITZ are considered to have originated from the interaction between the Indian plate and the Reunion mantle plume. It is inferred that the majority of the Deccan basalt was erupted through the major tectonic lineaments that define the CITZ and formed extensional tectonics/rift structure (Bhattacharji et al., 1996). Nevertheless, the widely accepted mantle plume interaction model as the source of Deccan volcanism and related continental rifting in the region is questioned by some authors (Sheth, 2005, 2007).

20th IAGA WG 1.2 Workshop on Electromagnetic Induction in the Earth Giza, Egypt, September 18-24, 201

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Abdul Azeez et al., 2010, Geoelectric structure of CITZ

The CITZ has been a major zone of differential crustal movement since the Neoarchaean (Acharyya and Roy, 2000). The Narmada-Son Lineament (NSL) marks the northern boundary of the CITZ; while the Tapti-Purna lineament is considered to be the southern limit of the CITZ in the west (Acharyya and Roy, 2000). The region records a complex tectonic history and has seen episodic reactivation since the Proterozoic. In the present study, the deep crustal electric resistivity (conductivity) structure in the CITZ is studied using magnetotelluric imaging. Data on five profiles traversing the CITZ is used to understand the deep crustal structure variations along strike. This is the first attempt, with multiple MT profiles, to study the along strike variation of crustal electric structure in the CITZ.

20 km on profiles PK and KE. The conductance values shows variation between 300 – 800 S with occasional higher values in excess of 1000 S (Figure 4). The inversion model for the AB profile that show discrete conductors in the upper crust, coinciding with the surface position of major geological faults, is different to those obtained for the other profiles. The conductors on this profile appear to be connected to a comparatively thin and weak lower crustal conductive zone.

MT PROFILES AND MODELING Broad-band MT measurements were made on five profiles crossing the western to central part of the CITZ (Fig. 1). The station distribution was planned to study the middle-lower crustal electric resistivity structure beneath the area. However, uniform spacing was not always possible due to access problems. Times-series data were acquired using Metronix (Germany) MT systems (GMS 05 and ADU-06), in the period range 0.0001 – 4000 s . The time variations of the two orthogonal horizontal electric and magnetic field variations were recorded at each MT location along with the vertical magnetic field variations.

Figure 2: Examples of MT data.

The MT time-series were processed using single site robust algorithms to compute estimates of the impedance tensor (Figure 2). The impedance data showed low skew values and tensor decomposition indicated a preferred geo-electric strike direction, between E-W (N90OE) and N74OE (Fig. 3), consistent with the geological trends in the region. The data were then rotated to the strike directions for each profiles, i.e., N90OE (DS), N85OE (PK), N90OE (KE), N74OE (AB) and N82OE (HB). Two-dimensional inversions of the of TE, TM and Hz responses was used to generate a resistivity model. RESISTIVITY SECTIONS The inversion models for the five profiles all show a low resistivity zone (10-80 Ωm) in the middle to lower crust (Figure 4). This low resistivity zone is observed as an almost continuous feature under the CITZ, and is bounded by the NSL in the north and Tapti lineament in the south. This correlation is most obvious in the three western most profiles, especially profile KE. The depth to this conductive zone increases to east from a depth of 10 km on profile DS to around

Figure 3: Rose plot illustrating the strike angles determined from multi-site, multi frequency tensor analysis.


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Gahalaut et al., 2004) The other main feature in the models is the image of high resistivity (> 1000 Ωm) Precambrian basement in the upper crust. This high resistive Precambrian crust is disturbed with the presence of less resistive/conductive zones almost beneath the surficial trace of some major tectonic lineaments mapped in the region. The region outside the CITZ, in general, is characterized by a highly resistive and uniform crustal layer of thickness 35-40 km. The robustness and necessity of this resistive separation was confirmed through sensitivity tests. The constrained inversions suggest medium to high resistive (300 - 10000 Ωm) upper mantle in the region. DISCUSSION AND TECTONIC INTERPRETATION A number of factors have been proposed to explain the low resistivity crust observed in the CITZ. Rao et al. (2004) proposed magma bodies. Interconnected melt fractions would produce low resistivity in the lower crust and a melt fraction of about 2% would be sufficient to explain the low resistivity in the CTIZ deep crust (Schilling et al., 1997). The heat flow reported by Roy and Rao (2000) is 45 - 62 mW/m2 and predicts Moho temperatures in the range 500 – 635 OC (Rai and Thiagarajan, 2006; Mall et al., 2008), which is too low for partial melting. The P-wave velocities observed at mid-lower crust of the region were 6.6-7.2 km/s (Kaila et al., 1981, 1985 and 1989) and do not show anomalies that could indicate partial melting in the deep crust. Magma was likely present in the past since the CITZ was a major conduit for the Deccan lava eruption 65 Ma ago. However any thermal anomalies associated with crustal magmatism would decay in 10 Ma (e.g. Sass and Lachenbruch, 1979). Crystallization and cooling to the ambient crustal temperature takes only about 5 - 10 Ma to (Pitcher, 1993). In addition, 3He degassing that is characteristic of cooling magma is not reported (Minissale et al., 2000). Hence, magmatism can be excluded as a possible explanation for the elevated conductivity of mid-lower crust underneath CITZ. Graphite is a common explanation for elevated conductivity in the middle and lower crust. Analysis of xenoliths from the CITZ show the presence of magnetite indicating relatively high oxygen fugacity (Dessai et al., 2009) that is inconsistent with the presence of graphite in the crust. Moreover, graphite has not been observed in lower crustal xenoliths (Dessai et al., 2009). Also, the required interconnection of the graphite film is unlikely to be stable under a tectonically active and strained region (Rao et al., 2002;

Aqueous fluids are thus the most likely explanation for the elevated conductivity. Archie’s law with a cementation factor of m = 1.5 and saline fluid resistivity value of 0.01 - 0.05 Ωm (Nesbitt, 1993) requires only 0.6% - 1.8% of interconnected pore fluid to explain the minimum bulk resistivity of 20 Ωm observed beneath the CITZ. Among the above possible fluid sources, diffusion/expulsion of fluids from magma emplaced into the deep crust and dehydration of hydrated minerals are the most probable mechanisms that suits the tectonic history of the region. The hydrous mineral phlogopite is reported in the lower crustal xenoliths from CITZ (Dessai et al., 2009) and increase the conductivity. Dry mineral conduction is possible as the xenoliths data represent appreciable quantities of paramagnetic minerals (titanomagnetite and magnetite) which could be taken into account for the lower crustal conductivity. It is difficult to conclude whether the low resistivity is solely due to the presence of fluids or hydrous/metallic minerals, or a combination of both. The origin of possible fluids in the deep crust beneath the CITZ, could be related to the modern reactivation of a Proterozoic suture. Also, conductive minerals could have formed during the crystallization and slow cooling of magmatic material injected to the crust during Deccan volcanism. The moderate to high resistivity mantle do not suggest any anomalous hot upper mantle conditions at present. CONCLUSIONS The resistivity models derived along five MT profiles across CITZ show low resistive (10 - 80 Ωm) middle to lower crust underneath CITZ. The crustal conductance in this section show 300-800 S with occasional high values (> 1000 S) at tectonic fault locations. The depth to the top of conductive zone is shallow (~ 10 km) at the western end; while it show much deeper depth range (18 - 22 km) towards east. The elevated conductivity can be explained with 0.6 – 1.8 % fluids in the deep crust. Hydrous and dry mineral conduction is another possibility as the xenoliths samples from the region show the presence of hydrous mineral and metallic minerals. RFERENCES Acharyya, 2003. Gondwana Res., 6, 197-214. Acharyya and Roy, 2000. JGSI, 55, 239-256. Bhattacharji et al., 1996. J. Geology, 104, 379-398. Dessai et al., 2010. Gondwana Res., 17, 162-170. Gahalaut et al., 2004. Geophys. J. Int., 156, 345-351. Jain et al., 1991. GSI, special publication, 10, 1–154. Kaila et al., 1981. Tectonophysics, 76, 99-130. Kaila et al., 1989. AGU Monograph, 51, 113-127.


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Kaila et al., 1985. J. Geol. Soc. India, 26, 465-480. Mall and Sharma, 2009. J AES, 34, 450 - 457. Minissale et al., 2000. EPSL, 181, 377-394. Naqvi et al., 1974. Precambrian Res., 1, 345-398. Nesbitt, 1993. J Geophys. Res., 98, 4301-4310. Pitcher, 1993. Blackie Academic and Professional, London, p. 187. Rai and Thiagarajan., 2006. JAES, 28, 363-371. Rao, et al., 2004. Earth Planet Space, 56, 229-238.

Rao, et al., 2002. GJI, 148, 132-138 Rodi and Mackie, 2001. Geophysics, 66, 174 – 187. Roy and Rao, 2000. JGR, 105, 25587-25604 Sass and Lachenbruch, 1979. The Earth: Its Origin, Structure and Evolution. 301-351. Schilling et al.,1997. PEPI., 103, 17-31. Sheth, 2005. GSA Special Paper, 388, 477-501. Sheth, 2007. GSA Special Paper, 430, 785-813.

Figure 1: Location of MT sites (solid rectangles) along the five profiles is shown over the major geologic and tectonic features in the area. Also shown are the previous MT study locations and seismic profiles.

Figure 4: Resistivity models obtained from joint inversion of TE, TM and Hz data using the algorithm of Rodi and Mackie (2001). Right panel shows the conductance calculated for the crust.


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