Introduction

Neoproterozoic granites in this study popularly referred to as Godhra granite, constitute a part of major syn- to post-orogenic granitic phase of southeastern Aravalli domain, western India. The Godhra granite have emplaced regionally along NW–SE trend, which splayed further SE-producing sporadic plutons (Mamtani et al. 2001; Mamtani and Greiling 2005). Emplacements of these plutons have locally deformed as well as generated contact metamorphism within Mesoproterozoic Champaner metasediments (Mamtani et al. 2001; Das et al. 2009; Limaye and Joshi 2016). The role of sporadic plutonic activity, however, induced structural complexity (Jambusaria and Merh 1967; Srikarni and Das 1996; Karanth and Das 2000). Doubly plunging Narukot dome, a part of Champaner Group forming southern extension of Aravallis in Gujarat, is one such feature that gives an opportunity to study the relationship of pluton and associated deformation. Deciphering subsurface morphology of pluton becomes vital.

Globally, the plutons are understood emplacing country rock with several geometric shapes, viz. circular, thick disk, sheet-like, hockey puck, flat-floored, wedge-shaped and many other discrete forms (McSween and Harvey 1997; Benn et al. 1998; Vigneresse et al. 1999) that in turn depend on the heterogeneity of magmatic activity, depth, and their degree of isolation as well as volume, strength and density difference between the plutonic melt and the country rocks (Bott 1955; Pitcher 1979; Vigneresse 1995; Benn et al. 1998; Stevenson et al. 2006; Cruden 2008). Several geophysical methods are deployed to study plutons, viz. gravity (Bott 1955; Vigneresse 1990; Singh et al. 2004; Rao et al. 2006; Cruden 2008; Singh et al. 2014), magnetic (Mamtani and Greiling 2005); aeromagnetic (Sahu 2012) magnetotelluric (Sastry et al. 2008); deep resistivity soundings (Singh et al. 2008); and deep seismic methods (Kaila et al. 1981; Dixit et al. 2010).

We apply a cost-effective microtremor technique to map subsurface pluton covering a large area at a prerequisite terrain-specific resolution from 250 m to 1 km interval. The assessment was quicker than the conventional indirect methods. The microtremor method has been used successfully to map subsurface rheological boundaries based on strong acoustic impedance along contrasting density at sediment/rock interphases at shallow depths and across fault zones (Kanai 1957; Yamanaka et al. 1994; Ibs-Vonseht and Wohlenberg 1999; Delgado et al. 2000a, b; Parolai et al. 2002; Garcia-Jerez et al. 2006; Guéguen et al. 2006; Zhao et al. 2007; Dinesh et al. 2010; Rošer and Gosar 2010; Sukumaran et al. 2011; Paudyal et al. 2013).

The present maiden attempt is to record a shallow seismic profile along doubly plunging Narukot dome and its western extension incorporating both microtremor method and field evidences. This enabled us (1) to delineate morphology of an independent granite pluton underneath the Narukot dome, (2) to determine the thickness of the Mn-bearing rocks of the Champaner Group, and (3) to infer implication towards syntectonic deformation of the Champaner Group.

Geology and structures

The vast area E and SE of Narukot dome has a rolling topography with isolated highs that exposes Jambughoda Granite (1050 ± 50 Ma: Sm–Nd method, Shivkumar et al. 1993); Chhota Udepur Granite (1168 ± 30 Ma: Rb–Sr method, Srimal and Das 1998) and Godhra Granite (950 Ma, Rb–Sr method assuming an initial Sr ratio of 0.700, Crawford 1975; Rb–Sr method 955 ± 20 Ma, Gopalan et al. 1979; Rb–Sr method 938.8 ± 20 Ma, Srimal and Das 1998; Rb–Sr method 965 ± 40 Ma, Goyal et al. 2001) (Fig. 1a). Negative Bouguer gravity anomaly (−40 to −20 mgal) substantiates granites in the region (Fig. 1b; Sandwell et al. 2014). However, the structure and tectonic regime under which the granite emplaced remain indeterminate. The sporadic granite pluton under present study emplaces within Champaner metasediments comprising intercalated sequence of quartzites and phyllites (Narukot Formation) exposed in the eastern portion of the dome. This is followed by polymict conglomerate with lithicwacke (Jaban Formation) and Mn-bearing phyllites and quartzites (Shivrajpur Formation) in the central part, whereas thin phyllite–quartzite bands with dolomitic limestone (Rajgarh Formation) characterize the western extension (Fig. 1c; Table 1; Gupta et al. 1992, 1997). These sequences are regionally metamorphosed up to greenschist facies (Jambusaria and Merh 1967) and preserve relic primary sedimentary structures (Srikarni and Das 1996). Further, isolated development of hornfelses and skarn zones are observed close to the granitic body (Das et al. 2009). The extreme WNW portion of the Narukot profile under present study exposes Mesozoic sedimentaries and the Deccan basalts.

Fig. 1
figure 1

a Regional geological map showing extension of Aravalli Supergroup in Gujarat (after Mamtani et al. 2001). NW-trending batholith (Godhra Granite) constitutes the most conspicuous feature that demarcates the Lunawada Group at ENE and the Champaner Group at WSW. b Regional Bouguer gravity map showing extension of Aravalli Supergroup in Gujarat (Sandwell et al. 2014). c Geological map of study area (modified after Gupta et al. 1997). Oval structure along the E margin represents the Narukot dome with N95° axial trace. Dotted line across the dome and further W shows location of stations (1–32) for microtremor measurements

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Table 1 Lithostratigraphy of southern Aravalli, Gujarat, W India after Gupta et al. (1992)
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The deformation pattern of southern Aravalli domain comprising Lunawada and Champaner Group are not comparable to the main Aravalli domain. The main Aravalli domain shows two deformation phases (AD1 and AD2). AD1 exhibits W trending rootless reclined, inclined, and rarely upright isoclinal folds. On the other hand, AD2 are coaxial isoclinal folds with widely dispersed axial planes (Naha et al. 1966, 1969). Further south, the Lunawada Group displays AD3 deformation comprising LF1 and LF2 coaxial folds (L: Lunawada) with NE-trending axial planes. LF3 folds are open with E- and NW-trending axial planes (Mamtani et al. 2001). Additionally, the Champaner Group demonstrates AD4 deformation developing upright folds with E-trending axial traces (CF1) followed by open upright cross folds with N–S axial traces (CF2) emerging as large domal structures in Narukot and Poyali areas (Jambusaria and Merh 1967; Gopinath et al. 1977; Srikarni and Das 1996; Gupta et al. 1997; Karanth and Das 2000).

The domal character at Narukot is well preserved by quartzites that skirt the dome (Fig. 2a, b). Quartzite rimming N, E and S portion of dome shows discordant relation, steep dip, steep/vertical foliation and strong annealing. On the other hand, quartzites and phyllites in core region and towards the western margin show concordant relations, gentle westerly dip and regional metamorphism. Phyllites exposed adjacent to Narukot dome preserve S–C fabric (Passchier and Trouw 2005; Mukherjee 2011a, 2012, 2013a, b, 2014, 2015; Mukherjee and Kovi 2010a,b). Fieldwork did not reveal any visual effect of shear heating (Mukherjee and Mulchrone 2013; Mulchrone and Mukherjee 2015, 2016) The S- and C-planes meet at ~24°. The dip direction of both S–C fabrics is parallel to the plunge of open folds that characterizes western portion of the Narukot dome (Fig. 2c–e). Stereonet of lower hemisphere equal area projection containing n = 67 foliations have been plotted. Beta intersection diagram represents superimposition of N–S axial plane over the E–W trends. Beta-1 and Beta-2 are respective fold axes of CF1 and CF2 producing dome and basin geometry (Fig. 2f).

Fig. 2
figure 2

a Structural map of the study area (modified after Gupta et al. 1997). b Geo-eye image of Narukot Dome. N–S axial trace overlay over WNW–ESE trend; discontinuous lines: shear in the region; P1, P2 and P3: locations for field photographs. c–e Top-to-E ductile shear along vertical section. S schistosity fabric dipping steeper than the C-plane. f Foliation surfaces as great circles (n = 67, S0, S1). g Beta intersection diagram representing superimposition of N–S axial plane over E–W (2211 intersections of 67 planes). Beta 1 and Beta 2 are the respective fold axes of CF1 and CF2 producing dome and basin geometry. h Pie diagram (n = 67, S0 and S1) showing similar fold axes of CF1 (i.e., N275°). i Contoured pie diagram; 2, 4, 8 and 16% contours per 1% area

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Microtremor studies

Studies reveal that microtremors are activated by ambient noise that encapsulates the fundamental resonant frequency of near surface sediment horizons (Ohta et al. 1978; Celebi et al. 1987; Lermo et al. 1988; Nakamura 1989; Field et al. 1990; Hough et al. 1991; Yamanaka et al. 1994; Konno and Ohmachi 1998; Ibs-Vonseht and Wohlenberg 1999; Delgado et al. 2000a, b; Aki and Richards 2002). These resonating frequencies derived from microtremors strongly correlate with the velocity of seismic wave as well as the sediment thickness (Ibs-Vonseht and Wohlenberg 1999; Parolai et al. 2002). To characterize amplification of seismic wave for a given site, Nogoshi and Igarashi (1971) proposed a technique to normalize the source effect by taking the ratio of the horizontal (NS + EW component) and vertical component (H/V) of the noise spectrum. Nakamura (1989) further popularized the method and its applications. The merits and demerits of this method are discussed by several workers and has been used extensively as a low cost tool for site characterization in estimating the resonant frequency and thickness of sedimentary layers, viz. Field and Jacob (1993), Parolai and Galiana-Merino (2006), Bonnefoy-Claudet et al. (2006), Garcia-Jerez et al. (2006), Zhao et al. (2007), Nakamura (2008), Bard (2008), Pilz et al. (2009), Lunedei and Albarello (2010), and Sánchez-Sesma et al. (2011).

We deployed a Lennartz seismometer (5 s period) and a City Shark-II data acquisition system to acquire ambient noise in forms of three components, viz. NS, EW, and vertical directions. The recording was carried out for 40 min at the rate of 100 samples/s per site (Sukumaran et al. 2011, fig. 3). All the 32 geophysical stations (Fig. 1c) arrayed for measurement run almost parallel to the axial trace (N95°) of the Narukot Dome (Fig. 1c). The station interval was decided considering topography along the profile line. The region with rolling topography from station 1–13 (Fig. 1c) was surveyed at 1 km interval, whereas the rugged terrain, stations 13–32 (Fig. 1c), was surveyed at 500 m interval.

The ratio between the Fourier amplitude spectra of the horizontal to the vertical (H/V) components of persisting Rayleigh waves were calculated from the ambient noise vibrations acquired from 32 stations using the GEOPSY (SESAME European Project 2004). The H/V spectral ratios were plotted between 0.2 and 25 Hz encompassing the complete range of resonating frequencies recorded within the study area (Fig. 3). These H/V ratios were further processed individually to identify statistically significant spectral peaks using custom-written Matlab code. The statistically significant peaks were taken to be those peaks that were at least one standard deviation greater than the baseline activity. These peaks then correspond to significant fundamental resonating frequencies for each station. The significant fundamental resonating frequencies f 0, f 1 and f 2 were singled out for individual stations quantifying their amplitudes (Fig. 3; Table 2). Figure 3 illustrates a series of H/V spectral frequency plots recorded from the study area. Station 22, 30 and 31 show the peaks at fundamental frequency (f 0). Station 2, 3 and 4 show dual frequency (f 0, and f 1) with representing the boundary at both deeper and shallower levels. Station 15 and 29 too display dual frequency (f 0, and f 1) but at different frequencies that correspond to the boundary at moderate to shallower depth level. However, station 32 represent three frequencies (f 0, f 1, f 2) incorporating three boundaries at shallow, moderate and deeper levels.

Fig. 3
figure 3

H/V spectral frequency plot recorded for the representative stations from the study area. Station 22, 30 and 31 show the peaks at fundamental frequency (f 0); station 2, 3 and 4 show dual frequency (f 0, and f 1) with representing the interphases at both deeper and shallower levels; station 15 and 29 also show dual frequency (f 0, and f 1) but at different frequencies that correspond to the interphases at moderate to shallower depth level. However, station 32 represent three frequencies (f 0 , f 1 , f 2) incorporating three interphases at shallow, moderate and deeper levels

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Table 2 Fundamental resonant frequency f 0, f 1 and f 2 for station 1–32 across Narukot Dome and in its western extension along a WNW profile
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The thickness (h) of soil/sediment layer over the bedrock can be related theoretically with the fundamental resonant frequency (f r) of H/V spectral ratio (Ibs-Vonseht and Wohlenberg 1999)

$$h = af_{r}^{b} ,$$
(1)

where a and b are obtained by nonlinear regression between the thickness and the fundamental resonant frequency. For a given fundamental resonant frequency, if the velocity of seismic waves (V s) for a given interphase is known, the depth of the interphases is given by Parolai et al. (2002):

$$h = \frac{{V_{\text{s}} }}{{4f_{\text{r}} }}.$$
(2)

On the other hand, if the depth of the interphase in known based on available core record, the velocity of seismic waves (V s) can be determined using Eq. (2).

In the present study, we used a record of a private borehole 300 ft (91.4 m) closer to station 29, in Hirapur Village, east of Narukot dome. The records suggest 7-ft (2.13-m) thick soil unit, followed by 15-ft (4.57-m) thick white fine-grained sand (alteration product of in situ granite); and 278 ft (84.7 m) of massive granite. In the present case, we categorized both the soil unit and altered granite unit under the regolith. Using the observed depth of regolith–granite boundary (6.70 m), we computed V s (227 m/s) for the regolith unit at station 29 using Eq. (2). The depth of regolith–granite boundary for stations 28, 30, 31 and 32 has been estimated using the above computed value of V s. In addition, substituting the value of V s in Eq. 2,

$$h = \, (56.8)f_{\text{r}}^{ - 1} .$$
(3)

Equation (3) derived from the study area is comparable to the equation derived for a granitic terrain around Bangalore (state Karnataka, India) decoding interphase of soil–regolith from that of granites (Dinesh et al. 2010), viz.

$$h \, = \, (58.3 \pm \, 8.8)f_{\text{r}}^{ - 0.95 \pm \, 0.1} .$$
(4)

In this context, we preferred the equation established by Dinesh et al. (2010) in this study to derive theoretical depths of interphases as they had established the relationship using a larger number of observed borehole logs.

Further, grouping fundamental resonating frequency, geology and structural data from the study area, we identify two distinct rheological boundaries, viz. 0.2219–10.364 Hz that is inferred to record boundary between Champaner metasediment and granites (C–Gr boundary) and 10.902–27.1119 Hz that differentiates phyllites from quartzites (P–Qr boundary) (Figs. 4, 5). The other boundaries identified along the W margin of the profile, viz. 0.7088–12.6896 Hz frequencies distinguish the boundary between the Champaner metasediments and the Mesozoic sediments. On the other hand, at stations 2 and 3, 18.0848 Hz frequency distinguishes thin Deccan traps from Mesozoic sediments.

Fig. 4
figure 4

Fundamental resonant frequency of 1–32 stations along WNW trending profile. The diameter of bubbles captures amplitude of fundamental resonant frequency. The blue color represent frequency for C–Gr boundary (L1) that ranges between 0.2219 and 10.364 Hz, whereas red color represents frequency for P–Qr boundary (L2) that ranges between 10.902 and 27.1119 Hz

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Fig. 5
figure 5

Layered model for the profile along Narukot dome and to its W. Subsurface interphases of C–Gr and P–Qr plotted with reference to the surface elevation. C–Gr boundary shows the granite pluton hump (from station 16 to 29) towards eastern part of the profile. The C–Gr interphase in W distinguishes a steep wall of the pluton (between stations 6 and 7) taking pluton further deeper to 243.64 m (station 6) and 232.82 m (station 1). The P–Qr boundary shows a steep plunge E of the granite pluton hump and 15° gentle plunge due W. The profile highlights subsurface extension of the Champaner Group further W overlain by Mesozoic sedimentaries and thin cover of Deccan basalt between stations 1 and 7. Numbers in the figure indicate (i) granite, (ii) quartzites, (iii) phyllites, (iv) conglomerate, (v) Mesozoic sedimentaries, and (vi) Deccan basalt

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Champaner–granite boundary

The Champaner–granite boundary (C–Gr boundary) occurs at a shallower depth towards E than at the W margin of the profile showing an arched-up geometry (Fig. 5). The granite pluton attains shallowest depth calculated from surface underneath station 20 (35.69 m) and station 23 (32.42 m) followed by a significant depth, or a ‘low’, beneath station 6 (243.64 m) and station 1 (232.82 m) towards W. C–Gr boundary follows a steep slope between stations 7 (45.40 m) and 6 (243.64 m). The low along profile between stations 1 and 6 marks an extension of the younger Champaner rocks exposed around stations 7 and 8 (Rajgarh Formation) and is confirmed based on aeromagnetic data (Sahu 2012).

Phyllite–quartzite boundary

The phyllite–quartzite (P–Qr boundary) sequence of Champaner Group is well exposed in the western extension of Narukot dome. During the field studies, boundary of different lithology and their trends were recorded and mapped (Figs. 1, 2). Lithology and structural trends were plotted along the topographic profile, extrapolating their contact up to the C–Gr boundary (Fig. 5).

Other rheological boundaries

In the western portion of the profile, the C–Gr boundary is ~240 m deep. The Rajgarh Formation in this part directly overlies granites deduced from aeromagnetic data (Sahu 2012). The boundary between the Rajgarh Formation and Mesozoic sediments is ~70 m deep. The boundary between the Mesozoic sediments and Deccan basalt is ~1–2 m deep (Fig. 5).

Discussions

The microtremor study reveals Champaner–granite boundary as the most conspicuous rheological boundary that emphasizes the morphology of subsurface granite pluton (Fig. 5). The granitic pluton forms a hump between stations 29 and 16 followed by gentle westerly dip up to station 7. The profile between stations 6 and 7 highlights a steep wall of the granite pluton, with 230-m deep C–Gr boundary, thereafter follows a rolling topography till station 1. On the other hand, the Champaner metasediment terminates abruptly above granite plutons imparting a discordant relation. The sporadic granitic plutons emplaced in the terrain presumably uprooted the Champaner metasediments giving “rootless” characteristic especially at Narukot dome and to its West (Fig. 5). Further northeast of the Narukot dome, at Gol Dungari such rootless character can be deciphered (Limaye and Joshi 2016). The estimated vertical thickness of Champaner metasediments varies as: 30 m (station 20), 100 m (station 21) and goes to a maximum of 136 m (station 12) at the Shivrajpur Manganese Mine. In the W extension of Narukot dome, the estimated thickness of Rajgarh Formation is ~108 m followed by 70-m thick Mesozoic sediment capped by 1–1.5-m thick Deccan basalt.

To present the relation between the pluton and associated deformation, we draw a geological cross-section across Narukot Dome and its extension towards W, by applying standard method adopted in geological studies, extrapolating surface geology and structural trends up to regolith–granite rheological boundaries delineated by microtremor studies (Figs. 2, 5). The sporadic emplacement of plutonic bodies produced asymmetric plunge along the dome. The Champaner metasediments between stations 23 and 29, E of the pluton hump, are tightly folded and plunge steeply towards E (Fig. 2), whereas to the W of pluton hump (station 20) metasediments show open folds and plunge 15° due W (Fig. 2). However, the fold axis of both tight (towards E) and open folds (towards W) across the Narukot dome trends N95° signifying the same deformation phase (Fig. 2). The accompanied deformation in form of open folds with N and NW trends has further resulted into dome and basin geometry. A more detail mechanism of doming (such as Mukherjee 2011b; Mukherjee et al. 2010; Mukherjee and Mulchrone 2012) remains a subject of future research. Finally, pluton morphology, selective metamorphism and related deformations favor syntectonic granite emplacement. Similar observations have been made in the Lunawada region—further NE of the study area (Mamtani et al. 2001).

Conclusions

  1. (a)

    Microtremor method is a handy tool for geoscientists to infer morphology of subsurface plutons underneath meta-sedimentary sequence.

  2. (b)

    Microtremor method would update the results along with field records to estimate thickness and to further project subsurface attitudes of the country rock.

  3. (c)

    Country rock and pluton boundary, contact metamorphism and associated deformation connote syntectonic pluton emplacement.