Granitic and magmatic bodies in the deep crust of the Son Narmada Region, Central India: constraints from seismic, gravity and magnetotelluric methods
© 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 2010
Received: 14 October 2009
Accepted: 29 October 2010
Published: 26 January 2011
Magnetotelluric (MT) data at 24 locations in the Son Narmada region, Central India, were collected across the Tapti North Fault and Son Narmada Fault along the Chinchpada–Godhra profile (220 km). MT impedance tensors were then estimated in the period range 0.001–1, 000 s using robust processing codes. The N70°E geo-electric strike direction was obtained by multi-site, multi-frequency analysis. The data were modeled using non-linear conjugate gradient scheme taking both apparent resistivity and phase into account. The two-dimensional MT model obtained (after static shift correction) represents resistive bodies (1, 000–3, 000 ohm-m) and conductive bodies (<20 ohm-m) in the deep crust. The resistive bodies in the lower crust are interpreted to be granitic intrusive complexes. The conductor on the south of Son Narmada Fault is attributed to the presence of magmatic bodies due to underplating, and the conductor on the north as due to the presence of fluids. The highly resistive (>2, 000 ohm-m) upper crust is interpreted to comprise felsic rocks of granitic composition, and the low-resistive (<100 ohm-m) deep crust as being composed of dense mafic granulites. The Domal upwarp structure near Son Narmada Fault, with a thick felsic crust sandwiched between the mafic and intermediate crust, can be explained by underplated sediments/felsic crust which jacked-up the lithounits above.
In the eastern and central part of the Son Narmada region, the processes of uplift and erosion of upper crustal layers have been inferred from seismic (Kaila et al., 1989; Tewari and Kumar, 2003) and magnetotelluric studies (Gokarn et al., 2001). In contrast, the more westerly region (west of Barwani; Fig. 1(a)) has been interpreted as a graben structure (Tewari and Kumar, 2003).
In the study reported here, I provide the first detailed description of the deep crustal geo-electric structure for this unexplored region from Chinchpada to Godhra using the magnetotelluric (MT) method. The MT method is considered to be one of the best methods for imaging models that envisage the presence of fluids, underplated material, and melt at the deep crustal depths (for a review, see Jones, 1992). Consequently, the use of MT in delineating deep crustal structures, as discussed here is a well-accepted and popular method that is applied world-wide. For example, MT studies (Wannamaker and Doerner, 2002) conducted in the Ruby mountains imaged resistive structures close to deep crustal, steeply dipping fault zones. It has been inferred that the zones acts as conduits for upper crustal-induced electric current flow and, therefore, that the lower crust is heterogeneous. It has also been inferred that the crustal breaks would decrease the effective elastic thickness and represent the deep crustal fault zones in which major earthquakes nucleate (Wannamaker and Doerner, 2002). Some of the more recent studies have linked high-conducting anomalies in the deep crust with the presence of graphite and resistive anomalies in the upper-middle crust and with igneous complexes of variable age (Almeida et al., 2005). MT studies (Bologna et al., 2005) from the Alto Paranaiba igneous province identified silicic dry rocks in the lower crust (20–40 km) based on high resistivities (1, 000–10, 000 ohm-m).
The objective of the study reported here was to image the electrical properties of the upper crustal and the lower crustal features in the Son Narmada region in order to identify the diverse processes of underplating, fluid-rich regimes, and zones of partial melt, if any, along the Chinchpada-Godhra profile. These results have been integrated with results from other studies and inferences on rock types drawn.
2. Magnetotelluric Data and Methodology
Data along the Chinchpada-Godhra profile were collected at 24 stations with wide-band digital MT systems in single-site mode in the period range of 0.001–1, 000 s. This 220-km-long profile, which traverses through the Tapti North Fault (TNF) and Son Narmada Fault (SNF), provides an opportunity to delineate the deep-crustal electrical resistivity structure over the region of the Alluvium and Deccan Trap (also known as Deccan Basalts) (Fig. 1(a)). The time-series data were processed to obtain both the impedance tensors and induction vectors using a robust processing code (Ellinghaus, 1997). Because the induction vector data were not of high quality, impedance data in the period range 0.01–1, 000 s were used in the modeling.
3. Summary and Conclusions
Several geophysical studies have been carried out in the complex Son Narmada region. The results of the MT modeling study reported here have been integrated with gravity, seismic, and regional heat flow studies to explain the deep crustal properties. The Bouguer gravity anomaly of the region shows a high gravity (approx. −10 mgal) between stations 9 and 13. Other regions show gravity lows ranging from −20 to −50 mgal. Several similarities between the results obtained using this MT model and those from earlier gravity, seismic (Tewari and Kumar, 2003), and MT models (Gokarn et al., 2001) are discussed in the following section.
Along the Mahan–Ujjain profile (200 km further east of the present profile), domal structures with a velocity (6.6 km/s) in the upper crust near Narmada river are present (Figs. 2(a) and 5(a)). The crustal velocities here are generally ≤7.2 km/s for the crust. The gravity model along the Mahan–Ujjain profile also shows a 10- to 15-km-thick low-density (2.7 gm/cc) layer trapped between high-density (2.9 gm/cc) layers. Based on these models, Tewari and Kumar (2003) inferred the presence of mafic intrusives in this region. Using earlier MT models on the eastern part of Narmada Son region, Gokarn et al. (2001) reported a relatively low resistivity of 200 ohm-m and 6.5 km/s in the upper crust. A similar low-resistive upper crustal structure near the Narmada river was also observed in the present study. Similarly, the MT model used here and earlier MT model reported by Gokarn et al. (2001) both found deep crustal conductors (C1 and C2 in Fig. 5(a)).
A general density (2.70 gm/cc), velocity (6.0 km/s), and high resistivity (2, 000–10, 000 ohm-m) of the upper crustal rocks are to be inferred as granites, extending to about 12 km south of the Narmada river and up to about 22 km to the north. Earlier studies by Verma and Banerjee (1992) also share such an inference.
The results reveal that the region below the Tapti river shows anomalous high resistivity (1, 000–3, 000 ohm-m) in the deep crustal depths of 20–40 km (R1 in Fig. 5(a)). The region near SNF also shows anomalous high resistivity in the deep crust from 10 to 30 km (R2). This is an unexpected result since one would expect a conductor in MT models near a vertical Moho penetrating fault (Kaila et al., 1985). However, a gradual increase of apparent resistivity up to 300 s confirms this result. If the results, namely a density of 2.9–3.1 gm/cc and a velocity of 6.8–7.2 km/s, of the Ujjain–Mahan profile can be validated, then R1 and R2 may represent high-density mafic granulites devoid of any fluids.
In the absence of detailed gravity and seismic modeling studies along this profile, the MT model presented here can be explained taking clues from other MT studies carried out elsewhere. Both R1 and R2 can be explained by igneous intrusive, such as granitic complexes, and the crustal structure can be compared to Ruby Mountain, Nevada (see Wannamaker and Doerner, 2002). Bologna et al. (2005) inferred the high-resistive (1, 000–10, 000 ohm-m) rocks in the lower crust as silicic dry rocks in the central Brazil region. For the Son Narmada region, the estimated temperatures are about 300°C at a depth of 20 km (see Rai and Thiagarajan, 2006). Free water will evaporate from the rocks at a temperature around 250°C, implying that the inferred igneous granitic complexes in the lower crust is consistent with the observed high resistivities (R1 and R2) in the study region. At temperatures around 350°C, felsic material turns ductile (Burgmann and Dresen, 2008). Therefore, in the neighboring zones, earlier interpreted Moho reaching near vertical faults (TNF and SNF) turns out to be listric around 20 km and for this reason, no conductors extend to Moho below these faults.
In general, the deep crust (20–40 km) in the northern part of the SNF shows low-resistivity values of < 100 ohm-m. For the Mahan–Ujjain profile, seismic velocity and density in the deep crust from 20 to 40 km ranges from 6.8 to 7.2 km/s and from 2.9 to 3.1 gm/cc, respectively. These values indicate that the lower crust is composed of mafic granulites with only a small fluid content. Based on a high gravity value (−10 to −20 mgal), a 5- to 20-ohm-m low-resistive body (C1) near station 9 between the depths of 15 and 25 km is interpreted to be an mafic intrusive formed due to underplating. The presence of mafic intrusives in the Son Narmada region has been inferred in several studies (e.g., Verma and Banerjee, 1992). Another important feature is the presence of a low-resistive body (C2) below station 18. C2 is not well resolved and is based on only TE data. However, earlier MT studies also observed similar conducting features in the lower crust in the Son Narmada Region extending north of Narmada River (Gokarn et al., 2001). Hence, C2 is inferred to be a fluid-rich region; this inference is also supported by a low gravity (−20 to −40 mgal; Fig. 1(b)). The porosity values for the observed conductivity anomalies in the deep crust are calculated using the Hashin and Shtrikman (1963) approximation. For about 20-ohm-m conductors (C1 and C2), a porosity value of 7–8% is required, whereas for the 100-ohm-m conducting lower crust, a porosity value of 2% is required for a brine resistivity of 0.01 ohm-m.
An interpretation of the resistivity model is presented in Fig. 5(b) where a domal structure develops through the underplating of sediments which ‘jack-up’ the units above. The presented model is consistent with tectonic extrusion and the subsequent exposure through extensive erosion. The mafic and felsic layers are markedly thicker in the domed-up portion in Fig. 5(b). A thicker mafic crust can be explained by younger mafic intrusions related to the Deccan volcanic activity. More geophysical and geological data are needed to better constrain the model for this region.
I thank Prof. Makoto Uyeshima for his editorial comments and Dr. Tada-nori Goto and Prof. A. Adam for excellent and helpful reviews which improved the quality of the manuscript. Prof. M. Santosh is thanked for his helpful discussions on the geological model presented in this paper. I thank Prof. Alan Jones for providing the multi-site, multi-frequency MT tensor decomposition code, Dr. R. K. Chadha for all his encouragement, and Dr. V. P. Dimri, the Director, NGRI, for his permission to publish this work.
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