Deep crustal structure in central India using magnetotelluric studies

Abstract

SUMMAR YMagnetotelluric studies over the Damoh±Jabalpur±Mandla±Anjaneya pro®le in central India have delineated Vindhyan sediments which are about 5 km thick in the Damoh± Katangi region. The crust below the Vindhyan sediments shows the characteristics of lower crust, as observed from the relatively lower resistivity of about 200 V m and high seismic velocities (P-wave velocities of 6.5 km s x1 compared with 5.8±6.2 km s x1 in the surrounding region). It is conjectured that the upper crust may have been completely eroded in the uplift and erosion process and thus the Vindhyan sedimentation has occurred directly over the lower crust. An anomalous conductivity is observed at depths of 10±12 km in the Vindhyan crust. The conductance of more than 1200 S observed here may be due to either the serpentinization of the ma®c and ultrama®c rocks or the presence of grain boundary graphites. The thickness of the Deccan traps is about 100 m near Jabalpur and decreases near Mandla. On the south of Mandla, the Archaean crust is exposed. Two crustal conductors are delineated below the Deccan volcanics with a resistivity of about 30 V m. The ®rst on the immediate south of Jabalpur seems to mark the southern boundary of the Jabalpur horst block. The second conductor was delineated about 40 km southeast of Jabalpur, coincident with a positive gravity anomaly of about 30 mGal. Deep seismic sounding studies do not show any signi®cant density contrast associated with this conductive feature. It is proposed that the gravity high may be due to the upwarp of the Moho. The high electrical conductivity is attributed to the¯uids in the upper crust.

Granitic and magmatic bodies in the deep crust of the Son Narmada Region, Central India: constraints from seismic, gravity and magnetotelluric methods

“…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)).…”

“…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.…”

“…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.…”

“…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.…”

“…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.…”