Low velocity layers in the subcrustal lithosphere beneath the Deccan Traps region of western India

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Physics of the Earth and Planetary Interiors, 67 (1991) 288—302 Elsevier Science Publishers B.V., Amsterdam

Low velocity layers in the subcrustal lithosphere beneath the Deccan Traps region of western India V.G. Krishna, K.L. Kaila and P.R. Reddy National Geophysical Research Institute, Hyderabad 500007, India (Received 4 May 1990; revised and accepted 3 January 1991)

ABSTRACT Krishna, V.G., Kaila, K.L. and Reddy, P.R., 1991. Low velocity layers in the subcrustal lithosphere beneath the Deccan Traps region of western India. Phys. Earth Planet. Inter., 67: 288—302. Travel times and relative amplitude modeling of a seismic record section from the Koyna DSS profile in the Deccan Traps region has yielded a velocity model 1forand the using Indiandigitized continental lithosphere. The sounding observed (DSS) record recorders, section, assembled data subcrustal from analog deep seismic reveals a with a reduction velocity of 6 km s prominent wave group with an apparent velocity similar to but about 3 s after the reflected P phase from the Moho (PMP), at a reduced time of 7.5 to 6.5 s and recording distance range from 70 to 95 km. We interpret this strong amplitude phase as the reflection from an interface in the subcrustal lithosphere. Comparison of synthetic seismograms for a range of velocity models with the observed record section shows that the observed phase under investigation cannot be explained either by a reflected phase from a subcrustal velocity discontinuity with a single velocity increase or by the P-to-S converted phase (PMS) from the Moho. Subcrustal velocity models with a single low velocity layer (LVL) do not seem to give a satisfactory fit to the amplitude observations on the field record section. We prefer a subcrustal velocity model with alternating LVL (at least two with a velocity of 7.0—7.4 km s1) separated by a thin high-velocity layer. In this model, the synthetic amplitudes of the reflected phase from the top of the deeper LVL at 56 km depth well match those of the observed reflection phase. It is inferred that the continental subcrustal lithosphere in this region of the Indian shield has a Iamellar structure with significant vertical variation of structural and mechanical properties. The alternating LVL, occurring at relatively shallow depths below the Moho (velocity decreasing from 8.3 to 7.4 km s 1), may be associated with zones of weakness and lower viscosity. The LVL in the subcrustal lithosphere, as well as the LVL inferred in the upper and the lower crust (Krishna et al., 1989) together with the observed depth distribution of seismicity in this region, suggest a well-defined rheological stratification of the continental lithosphere with varying material properties.

1. Introduction The structure and physical properties of the continental crust and the subcrustal lithosphere are directly related to many dynamic processes observed at the surface of the Earth. The controlled source long-range seismic refraction experiment is one technique for investigating the structure and stratification of the subcrustal lithosphere. A large number of such experiments, using powerful sources and observations by densely spaced mobile stations (5—10 km apart) out to 1000 km and beyond have already been conducted 0031-9201/91/$03.50

© 1991

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Elsevier Science Publishers B.V.

in the USSR, western Europe and the United States. These studies have revealed significant fine layering (with thicknesses on the order of a few kilometers) in the subcrustal lithosphere, changing drastically the standard Earth models consisting of a rather homogeneous uppermost mantle exhibiting only slight variation in its physical properties with depth. The most important recent findings on the structure of the subcrustal lithosphere are the following. (1) Occurrence of low velocity layers (LVL) at shallow depths below the Moho in the continental upper mantle, with velocity contrasts at their boundaries as large as those found

LOW VELOCITY LAYERS IN THE SUBCRUSTAL LITHOSPHERE BENEATH THE DECCAN TRAPS REGION OF WESTERN INDIA

at the Moho (Fuchs and Vinnik, 1982; Prodehi, 1984; Fuchs, 1986; Fuchs et a!., 1987). (2) Observations of unexpectedly high P wave velocities of up to 8.6—8.9 km s and high velocity gradients of 0.02—0.04 s~ at depths of 10—30 km below the Moho, providing indirect evidence for the continuation of elastic anisotropy with depth in the uppermost mantle (Fuchs and Vinnik, 1982; Fuchs, 1983, 1986; Fuchs et al., 1987; Bean and Jacob, 1990). The Deccan Traps region in the Indian shield is the largest continental volcanic area, covering more than 500000 km2. Recent studies based on 4°Ar— 39Ar yield age bounds of 69—65 Ma (Courtillot et al., 1988) and 68.5—66.6 Ma (Duncan and Pyle, 1988) for the Deccan Traps. In this region there is a broad gravity low which encompasses the whole of the south Indian shield, with superimposed shorter wavelength features. Kailasam et al. (1972, 1976) have interpreted the pronounced gravity anomalies in the Koyna-Karad region as having deep-seated causes, possibly uplifts and subsidences within the deep crust and upper mantle, while Guha et al. (1974) suggested that the negative gravity anomaly might be owing to an upward 73°C’

80

projection of low density material below the Deccan volcanics. The presence of the long wavelength gravity low in this region suggests a relatively low density region at sub-crustal depths. Using teleseismic P-wave residual tomography, Iyer et al. (1989) delineated a prominent low velocity zone in the westernmost part of the Deccan volcanic province at depths less than 100 km, and a 1—4% high velocity anomaly in the 100—400 km depth range They concluded that the low velocity zone might be related to the rift systems which provided the channel for the outpouring of Deccan basalts at the close of the Cretaceous period. Heat flow in the Deccan Traps region appears to be normal and the data currently available do not provide any significant information on the deep structure in this region. Although numerous deep seismic sounding (DSS) wide-angle reflection/refraction profiles with offsets of 200—250 km have been covered in India, refraction experiments on long-range profiles with larger offsets are yet to be accomplished. Nevertheless, the Indian DSS observations on some profiles provide important data on subcrustal reflectors, thereby constraining the struc.

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tural properties of the subcrustal lithosphere. These data deserve particular attention since they provide the first models of the structure of the Indian continental subcrustal lithosphere based on explosion seismology. We present here a detailed modeling study of travel times and relative amplitude data from one such seismogram section well recorded on the Koyna I DSS profile (Fig. 1). Recent modeling studies (Krishna and Kaila, 1986; Krishna et al., 1989) of a number of DSS record sections in this region provide the necessary crustal velocity control to place constraints on the subcrustal velocity structure.

2. Modeling and interpretation The analog DSS records in the Koyna region, acquired by continuous profiling with a 200 m geophone spacing and a high cut frequency of 15 Hz, have been digitized and plotted as a trace-normalized record section. The record section thus obtained for SP bA, shown in Fig. 2, is plotted with a reduction velocity of 6 km s~ and trace interval of about 1.5 km. The travel time curves of correlatable crustal P-wave phases shown in this record section correspond with those computed for the velocity model inferred from seismogram modeling by Krishna et al. (1989). The travel time curves of the phases designated as P5P, P6P and P7P, P8P correspond to reflections from the top and bottom of the two LVL in the subcrustal lithosphere inferred in the present study. The travel time curve of the PMS phase is computed for the same crustal model for a P-to-S converted phase at the Moho. In this study we focus on the prominent refleclion phase P7P, observable about 3 s after the PMP phase at a reduced time of 7.5 to 6.5 s, especially at 70—95 km recording distance. We interpret this prominent phase as the reflection from an interface at 56—57 km depth in the subcrustal lithosphere, with the assumption of a layered model. We consider this a valid assumption for the P7P reflection event under study which is modeled at a relatively small source—receiver offset comparable with the depth of the reflector. At source—receiver offsets beyond about

V.G. KRISHNA ET AL.

twice the target depth, waves scattered from a random zone may produce the appearance of horizontal layering in wide-angle data, as shown by Gibson and Levander (1988). However, at smaller offsets, such as those considered in the present modeling study, the scattered wave field (if any) is not likely to have the same lateral coherence as that observed for the P7P phase. We used computational methods for travel times in media with continuous velocity—depth functions and in stacks of homogeneous layers (MUller, 1970), and for synthetic seismograms based on the zero-order asymptotic ray theoretical method (Cerveny et al., 1977). These computational methods, applicable to laterally homogeneous models, have been found suitable for modeling the observed record section in this region and inferring a velocity model for the Indian continental subcrustal lithosphere. The lateral homogeneity of the crustal velocity structure in the Koyna region is well established by a recent modeling study (Krishna et a!., 1989). Since we focus on modeling the subcrustal P7P phase, we use the PMP amplitudes as a reference for a qualitative comparison with the P7P amplitudes predicted from ray theoretical synthetic seismograms. For each model giving an acceptable travel time fit, we computed the reflection response from only the Moho and the deeper structure to save computation time. In all our computations, S velocities have been obtained from P velocities (V,,) under the assumption that the Poisson ratio is 0.25, and the densities (p) followed according to the empirical relationship p 0.252 + 0.3788 V~, given by Birch (1964). We consider these relations as reasonable assumptions because the theoretical seismograms computed essentially depend on the P velocity structure. By extensive seismogram modeling we deduce that a subcrustal velocity structure (Fig. 3) with at least two LVL separated by a thin (about 1 km) high-velocity layer fits the travel times as well as amplitudes of the observed prominent P7P phase reasonably well. A thicker high velocity layer between the two LVL increases unacceptably the apparent velocity of the reflected phase from the top of the deeper LVL. In this model the two subcrustal LVL are bounded by 1 km thick transi=

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tion boundaries, except the top of the deeper LVL which is considered a sharp boundary. The reflection from this sharp boundary at 56 km depth has relative amplitudes similar to those of the P7P phase in the observed record section, as can be seen from the synthetic seismogram section cornputed for this model and shown in Fig. 3. In order to substantiate the plausibility of this model, we will present a range of subcrustal velocity models that only match the observed travel times of the P7P phase. Figure 4 shows a synthetic seismogram section computed for a simpler subcrustal velocity model that has a velocity increase of 0.6 km s - at a discontinuity at a depth of 58 km. The reflected phase computed for this discontinuity (P6P) fits the travel times of the observed P7P phase from 70 to 95 km distance, but predicts an unaccepta-

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observed and synthetic amplitude ratios of PMP and P7P phases for a range of subcrustal velocity models including one or two LVL. In the synthetic examples shown in this figure, P7P is the phase reflected from either the base of the single LVL or the top of the deeper LVL of the two LVL model of the subcrustal lithosphere. It can be seen from Fig. 5 that the subcrustal velocity models with single LVL (of velocity 7.4 km s~),with the base as a sharp (model 5) or 1 km thick transition (model 6) boundary, do not give a satisfactory fit of the PMP-to-P7P amplitude ratio, comparable with that observed (Curve 1). For the subcrustal velocity models with two LVL, we considered LVL velocities of 7.4, 7.0 and 7.8 km s’ (models 2, 3 and 4 respectively, in Fig. 5). While model 4, with LVL velocity of 7.8 km s’, does not give a good fit, both models 2 and 3 give a satisfactory fit of the PMP-to-P7P amplitude ratio, comparable with that observed (Curve 1). Although there may be random uncertainties in the amplitude ratios compared, it appears that the subcrustal velocity model with the two LVL of velocity 7.0— 7.4 km s~ is the most acceptable of the various plausible models considered in the present study. Figures 6a and 7a show the trace-normalized observed seismogram sections plotted in the distance ranges from 60 to 120 km and 70 to 100 km for the same data set but with trace intervals of about 800 m and 400 m respectively, illustrating the prominent P7P phase along with the PMP and other phases. It is clear from Fig. 6a that the P7P phase does not correspond to the PMS phase, computed for the crustal velocity model in this region. It may be seen from Fig. 7a that the computed travel time curve (P7P), from the top of the deeper subcrustal LVL at 56 km depth, appears to be early relative to the observed P7P phase from 89 to 95 km distance. In this distance range, the PMP phase also appears to be delayed relative to the computed travel time curve. Travel time computations shown in Fig. 7a reveal that both the PMP and the P7P reflectors are 1—2 km deeper in the Koyna Nagar region (see Fig. 1 for location), where the bounce points correspond to the delayed PMP and P7P reflected arrivals at 89—95 km distance. The synthetic seismogram sections for the preferred model given in Fig. 3 are

299

also plotted in Figs. 6b and 7b to the same scale as the observed sections shown in Figs. 6a and 7a. It can be seen from Figs. 6 and 7 that the relative amplitude match of the PMP and P7P phases obtained for the subcrustal velocity model with two LVL is fairly close, suggesting this as a plausible model. We thus infer that the subcrustal lithosphere in the Deccan Traps region of the Indian shield has a lamellar structure with two prominent LVL at shallow depths below the Moho.

3. Discussion and conclusions Figure 8 shows a synthetic seismogram section computed in the 50 to 500 km distance range for our preferred model with two LVL (velocity 7.4 km s ~)below the Moho. The behaviour of this model, especially that of the P7P phase in the distance range up to 190 km, is utilized here essentially to interpret the observed prominent P7P phase and infer the subcrustal LVL. However, it can also be seen from this synthetic record section that observation and modeling of the phases like P5P, P6P and P7P, P8P at larger distances can provide better constraints on the subcrustal velocity structure. Nevertheless, in the absence of such long-range refraction data, the present modeling study provides interesting clues to a plausible model of the subcrustal velocity structure in the Deccan Traps region. Detailed studies of the upper mantle by explosion seismology have shown its fine layering, with LVL in several tectonic settings such as old Precambrian platforms, young continental platforms and the oceans (Him et a!., 1973, 1975; Kind, 1974; Ansorge, 1975; Vinnik and Egorkin, 1980; Yegorkin and Pavlenkova, 1981; Fuchs and Vinnik, 1982; Yegorkin et a!., 1984; Grad, 1989). The existence of LVL is often evident in various regions from the observed seismic record sections, although it is difficult to derive the velocity reduction and the thickness of the LVL independently. Modeling of both travel times and amplitudes could reduce this ambiguity considerably. Observations of these LVL are very important for studying geodynamics, because a lower velocity could be associated with lower viscosity and higher -

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mobility of the mantle material. The fine layering of the subcrustal lithosphere with alternating LVL may suggest rheologically weak zones at corresponding depth ranges within the lithosphere, although compositional changes can also explain such structures. It still remains difficult to delineate unambiguously a sharp boundary separating the lithosphere from the underlying asthenosphere, which is considered to be essentially a rheologically weak layer. Fuchs and Vinnik (1982) stated that it is difficult to decide from presently available data what is lithosphere and what is asthenosphere. According to them it is possible that the asthenosphere under old continental platforms is not imaged in the seismic velocities by a single, broad zone of low velocity but forms a

here with subcrustal LVL. Iyer et a!. (1989) stated that a low-velocity zone should be expected on the western margin of the Deccan volcanic province, representing the source of the Deccan basalts. The present study is not, however, intended to speculate on the tectonic processes and the source of the Deccan basalts. It may be concluded that, because of the possible existence of alternating LVL with large velocity contrasts at their boundaries, the conditions are generally favourable in this region of the upper mantle for the generation of nearvertical reflections. Thus a coincident seismic reflection and long-range refraction probing of the upper mantle should provide significant results concerning the deep structure, via the reflectivity and the velocity structure, required to understand

rather extended region where high and low veloc-

the tectonic processes as well as physical and

ity layers are intermingled, Pavlenkova (1988) suggested that the asthenosphere should be defined not as any weak upper

compositional properties of the subcrustal lithosphere in this region.

mantle layer, but as a zone of partial melting. In such a case the calculated temperatures, if they are

sufficiently accurate, may permit evaluation as to whether or not partial melting is possible in an upper mantle anomalous zone with low velocity or high electrical conductivity. Presently there are no means of describing the nature of weak zones

where partial melting of material is not justified. According to Pavlenkova (1988) two alternatives are possible in this case: either some phase transition of the material causes the formation of rheologically weak zones or they represent former asthenospheric layers of partial melting which have not yet crystallized fully. It may be surmised that the LVL in the subcrustal lithosphere, as well as the LVL inferred in the upper and the lower crust

in the Deccan Traps region, suggest a well- defined rheological stratification of the Indian continental lithosphere with varying material properties. The observed concentration of seismic activity at 4—5 km depth and an appreciable reduction in seismic activity at greater depths in this region is consistent with such a rheological stratification of the lithosphere (Krishna et al., 1989). The results from the seismic tomography experiment (Iyer et a!., 1989) also reveal a relatively low velocity anomaly at depths less than 100 km in this region, consistent with the model proposed

Acknowledgements

We are grateful to the Director, National Geophysical Research Institute, for kind permission to publish this paper. We also thank the anonymous reviewer for valuable suggestions for improving the presentation of this paper.

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