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PHYSICS O F T H E EARTH ANDPLANETARY INTERIORS

ELSEVIER

Physics of the Earth and Planetary Interiors 97 (1996) 27-41

Teleseismic tomographic evidence for contrasting crust and upper mantles in south Indian Archaean terrains D. Srinagesh, S.S. Rai * National Geophysical Research Institute, Hyderabad-500007, India Received 20 March 1995; revised 5 March 1996; accepted 20 March 1996

Abstract We compare the velocity structure of the crust and upper mantle of the Archaean granulite terrain of southernmost India with that of the Archaean granite-greenstone (Dharwar craton) terrain just to the north, through P-wave teleseismic travel time measurements. The teleseismic rays recorded by a coarse network of 11 portable seismic stations show an anomalous pattern of late arrivals (delays) over the granulite terrain in contrast to fast arrivals on the Dharwar craton. Such a pattern of time residuals amongst these Archaean terrains, which have remained inert in the last 2 Ga, may indicate the presence of compositional heterogeneity within the crust and upper mantle beneath them. Three-dimensional P-wave velocity tomography using teleseismic rays from a variety of azimuths indicates the existence of contrasting P-wave crust and upper-mantle velocity patterns: (1) in the crust (0-40km) the western Dharwar craton and the granulite terrain have lower velocity (up to - 3 % ) compared with the higher velocity (1-3%) observed over the eastern Dharwar craton, and (2) in the upper mantle (40-177km), there is 2-3% lower velocity beneath the granulite terrain compared with the western Dharwar craton. The existence of such lateral velocity variation in the crust and upper mantle, and its preservation since late Archaean times, points to the presence of possibly thick and chemically distinct lithospheres that did not participate in mantle convection.

I. Introduction Archaean continents are largely composed of granite-greenstone (low-grade) and granulite-charnockite (high-grade) rocks. These rocks evolve under distinct processes, with the pressure-temperature conditions playing a vital role. The granite-gneisses are formed under low P - T conditions whereas the granulites require high P - T conditions. The highest-grade metamorphic rocks (granulite facies) generally originate at the mid to lower ( 2 0 - 3 0 k m )

* Corresponding author.

continental crust (Christensen and Fountain, 1975; Fountain and Salisbury, 1981; Kay and Kay, 1981; Newton and Perkins, 1982) at an average formation temperature range of 700-1000°C and a pressure range of 6 - 1 0 k b a r . These rocks are later exhumed onto the surface by tectonic processes. Evidently, the crustal blocks composed of low- or high-grade rocks should have formed under distinct physico-chemical conditions and possibly still retain their generic signatures. In southern India, two such crustal blocks have coexisted since Archaean time without being affected by any major tectonothermal event. In this paper, we present evidence for crust and upper-mantle dissimilarities amongst these Archaean terrains through three-dimensional (3-D) mapping of P-

0031-9201/96/$15.00 Copyright © 1996 Published by Elsevier Science B.V. All rights reserved. PH S 0 0 3 1 - 9 2 0 1 ( 9 6 ) 0 3 167-6

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D. Srinagesh. S.S. Rai / Physics of the Earth and Planetary Interiors 97 (1996) 27-41 17

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The south Indian granulite terrain (SIGT in Fig. 1) is mainly composed of highland massifs with elevations reaching a maximum of 2.6km. The isotopic ages indicate that the granulitic event occurred around 2.6Ga in a time span of 100m.y. (Harris et al., 1982; Bemard-Griffiths et al., 1987). The granulites were formed at average temperature-pressure of 700800°C and 7-10 kbar corresponding to burial depths of about 20-25 km. The processes for exhumation of south Indian granulites from depth levels of about 20-25 km remain a matter of speculation (Newton, 1987). The Proterozoic Cuddapah basin is the another notable geological feature in south India.

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1.2. Overview of previous geophysical studies

LONGITUDE (E) Fig. 1. Geological framework of the Precambrian south Indian craton. WDC, western Dharwar craton; EDC, eastern Dharwar craton; SIGT, south Indian granulite terrain; EGGT, Eastern Ghat granulite terrain; CPG, Closepet granite; CB, Cuddapah Basin; PC, Phanerozoic sedimentary cover. Dotted line indicates Fermor's line (boundary between Dharwar craton and south Indian granulite

terrain).

velocity variations using relative teleseismic traveltime residuals.

1.1. Geology The south Indian craton is a juxtaposition of Archaean nuclei, Proterozoic sedimentary basins and mobile belts. Primarily, the Archaean block is composed of a low-grade granite-greenstone terrain (Dharwar craton) surrounded by high-grade metamorphic terrains (granulites) to the south and east (Fig. 1). The Dharwar craton is divisible into the western Dharwar craton (WDC) and eastern Dharwar craton (EDC), separated by the north-south trending 2.5Ga Closepet granite (Swaminath et al., 1976). Geochronological studies indicate distinct ages for WDC and EDC crustal blocks in the Dharwar craton. The oldest ages (3.5-3.0Ga) are observed in the WDC, whereas the EDC records ages between 3.0 and 2.6 Ga leading to large-scale plutonism (Harris et al., 1982; Drury et ai., 1983; Taylor et al., 1984). The granites-gneisses in the Dharwar craton were evolved at about 3 kbar pressure (Harris et al., 1982).

Through the study of teleseismic P - S converted phases, Peseckis and Burdick (1982) obtained a crustal thickness of 40 km and average crustal velocity of 6.0 km s- ~ beneath World Wide Standardized Seismograph Network (WWSSN) station Kodaikanal (KOD; 10.2°N, 77.5°E) located in the granulite terrain at an elevation of 2.34km. Arora (1971), using local earthquake data, obtained a two-layer crustal model with layers of 16 km and 19 km thickness and corresponding velocities of 5.7 km s- i and 6.5 km s followed by an 8.0km s-z substratum beneath the Dharwar craton at Gauribidanur seismic array (GBA; 13.6°N, 77.4°E) at an elevation 0.7 km. Considering the above crustal velocity models and near-vertical incidence angles of teleseismic rays, a delay of 0.31 s ( 4 0 / 6 . 0 - 1 6 / 5 . 7 - 1 9 / 6 . 5 - 5 / 8 . 0 ) would be observed at KOD relative to GBA owing to different crustal thicknesses. Dziewonski and Anderson (1983) reported time delay of 0.89 s at KOD in contrast to faster arrivals of about - 0.1 s of teleseismic rays at GBA for their azimuth-independent A o term. As crust beneath KOD can account for only 0.31 s of delay, the remaining 0.58 s could be attributed to the low-velocity upper mantle beneath it. Deep seismic sounding studies (Kaila et al., 1979) in the Dharwar craton indicate intra-terrain variations in the crustal parameters with a thick crust of 40 km beneath the WDC compared with about 35km in the EDC. Bouguer gravity studies reveal that the granulite terrain is characterised by a gravity low ( - 8 0 to - 9 0 m g a l ) as compared with the Dharwar craton

D. Srinagesh, S.S. Rai / Physics of the Earth and Planetary Interiors 97 (1996) 27-41

( - 6 0 m g a l ) . The observed difference of - 2 0 to -30mgal Bouguer gravity anomaly between the two terrains has been interpreted in terms of 4-5 km crustal thickening beneath the granulite terrain (Subba Rao, 1988; Mishra, 1990). These geological and geophysical investigations indicate differences existing in the crust and upper mantle beneath the WDC, EDC and SIGT. These inferences, however, remain largely qualitative. Detailed knowledge of the crust-mantle velocity image of these contiguous yet dissimilar terrains is likely to provide new glimpses into the structure and evolution of continental lithosphere. We investigate here the deep structural seismic dissimilarities amongst these Archaean terrains and possible geodynamic consequences, based on the 3-D velocity images reconstructed from azimuthally varying teleseismic travel times recorded over a temporary network of seismographs.

29

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Fig. 2. Location of temporary seismic network. KOD is a WWSSN station.

and upper mantle. We operated 11 vertical-component seismic stations (Teledyne Portacorders (Teledyne Geotech, USA) with S-13, 1 Hz seismometers) at 240 mm min-i dram speed and 72 dB gain with their internal clock synchronised at least twice daily with an externally broadcast signal. Locations of these stations (Table 1) are depicted in Fig. 2. The 11 station network was operated in two phases each comprising six stations. We ensured that only one station was shifted at a time and that the stations were evenly distributed in different geological ter-

2. Experimental design In seismic tomography one reconstructs images of heterogeneities through inversion of perturbations of seismic waves which pass through them. Teleseismic travel time residuals, which are essentially a form of such perturbations, formed the basic input for our study. Teleseisms (earthquakes greater than 25 ° from the receiver) provide well-constrained ray paths and steep angles of incidence through the modelled crust

Table I Location of seismic stations with corresponding elevations Station

Shimoga (SMG) Mysore (MYS) Gauribidanur (GBA) Tirupati (TPT) Bangalore (BNG) Salem (SAL) Coimbatore (COM) Tiruchinapalli (TCY) Rajapalayam (RIP) Kalakkadu (KLK) Idayangudi (IDG)

Latitude (N)

82

LONGITUDE (E)

Longitude (E)

Deg

Min

Deg

Min

14 12 13 13 12 11 Il 10 09 08 08

04.50 18.59 36.25 35.30 55.33 43.27 02.38 45.31 28.00 30.25 19.33

75 76 77 79 77 78 76 78 77 77 77

22.80 37.24 26.17 20.18 30.10 04.89 52.94 49.38 32.00 34.84 53.33

Elevation (m)

Geologic province

700 235 700 180 840 275 300 090 182 121 015

WDC WDC EDC EDC EDC SIGT SIGT SIGT SIGT SIGT SIGT

WDC, Western Dharwar craton; EDC, eastern Dharwar craton; SIGT, south Indian granulite terrain.

D. Srinagesh, S.S. Rai / Physics of the Earth and Planetary Interiors 97 (1996) 27-41

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rains. Direct P phases were used in this study. One cycle waveform was visually correlated and multiple readings (i.e. first arrival, trough, peak and zero cross) of events were used to ensure better accuracy in the computation of travel time residuals. We believe that this enables us to achieve an accuracy of 0.05-0.1 s in reading the arrival times of the wave phases. Travel time residuals are the differences between observed and calculated travel times using Herrin's standard Earth model (Herrin, 1968a):

where T,~ is the observed travel time and T,H is the theoretical travel time for the ith station and jth event. Residuals represent the sum total of all deviatoric effects caused by source mislocation, uncertainity in origin time and presence of heterogeneities in the source-receiver path. To minimise the source and path effects and isolate the residual component caused by heterogeneities present beneath the seismic network, travel time residuals are reduced to relative mean travel time residuals RRij. T h e relative mean residual at station i for event j is obtained by subtracting the jth event mean residual recorded at N stations from the Herrin residual at station i, where R R i j = Rij -- ( l / N )

3. Qualitative interpretation of residuals Interpretation of travel time residuals is essentially based on the analysis of its magnitude and spatial variation. The magnitude of residuals reflects the magnitude and depth extent of the anomaly, whereas their azimuthal variation is controlled by the geometry of the inhomogeneity. The travel times at TPT, located on the southern fringe of the Proterozoic Cuddapah Basin, are affected by the Eastern Ghat granulite terrain (EGGT) for the eastern azimuths and by the Cuddapah basin for western azimuths. Therefore, travel time residuals at TPT were not considered for qualitative interpretation. Fig. 3 depicts station anomalies that are different in these terrains. The SIGT is characterised by slower arrivals ranging between 0.0 and 0.2 s whereas the Dharwar craton exhibits residuals in the range - 0 . 5 s to 0.04 s. We compare our station anomaly results (Fig. 3) with those of Dziewonski and Anderson (1983) at GBA and KOD. As we have no measurement at KOD we compare results from the nearest stations, COM and RJP. The A 0 values at GBA and KOD are - 0 . 1 s and 0.89s. As our observations are reduced to MSL, the necessary travel time correction in the A 0 at GBA and KOD owing to elevation effect (velocity 5.3 km s- ~) would be approximately 0.13 s

Y'~Rij

17

i

where N is number of stations recording an event j. All the data sets (residuals) in the present study have been corrected for the Earth's ellipticity and reduced to mean sea-level (MSL) using a surface P-velocity of 5.3 km s-1 (Kaila et al., 1979) for the corresponding station elevation (Table 1). The residuals at a station are then averaged over the entire azimuthal and distance range to assess the nature of inhomogeneity lying in the shallower part of the lithosphere directly beneath the station. This invariant part of the residual, or station anomaly, is represented by

15~ m-0"2

z

i E RRij J

where M i is number of events at the ith station.

• 2

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V. 5, for each of the above directions have better resolution. It is also observed that the peripheral blocks have unidirectional rays. Owing to

bias in ray directions the hit count projection is elongated along the eastern direction. A horizontal view of the hit count reveals that the Dharwar and granulite terrains could be properly resolved only to depth levels of 0-306 km (i.e. not Layer 4). Beyond these depths only the eastern segment of the granulite terrain could be resolved. Therefore, we shall restrict our interpretation of the anomaly to a depth level of 306 km and refrain from interpreting the last layer (306-519 km).

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LONGITUDE (E)

Fig. 9. Averaged P-wave velocity variation (%) at various depth levels in the Archaean terrains of south India. Velocity perturbations are averaged for nine inversion results with blocks offset by one-third of a block in each direction.

D. Srinagesh, S.S. Rai / Physics of the Earth and Planetary Interiors 97 (1996) 27-41

Because the block placement is only an ad hoc representation of the real Earth structure, the anomaly location inferred from the model may not depict the true location. To reduce the effect of block positions, the blocks in the lower three layers were horizontally shifted by one-third of a block width in nine directions. Though the first layer grid element was not shifted, solutions show leakage of velocity perturbations into the first layer. A similar observation was also made by Ellsworth (1977, p. 303). The velocity perturbations of the nine solutions were averaged for each block, resulting in a smoother velocity anomaly pattern. The velocity perturbations of this averaged model for the crust and upper mantle (to 306kin depth) are presented in Fig. 9. From the smoothened velocity image (Fig. 9) we attempt to correlate the P-velocity variation in the crust and upper mantle with surface geology and tectonics. Earlier seismic studies indicated thicker crust beneath the western Dharwar craton and south Indian granulite terrain than under the eastern Dharwar craton. To examine the mantle features, the crustal effects were removed by assuming a crustal layer of 40km thickness and assigning individual blocks to each station. Examination of the first layer results reveals that the stations over the WDC (SMG and MYS) are characterised by lower crustal velocities than for station over the EDC (GBA, BNG and TPT). This could be due to significant thickening of the crust (to 40km) as observed in deep seismic sounding results (Chowdhury and Hargraves, 1981). Also, a large part of the western Dharwar craton is marked by significant negative Bouguer gravity anomalies ( - 1 0 0 m g a l ) in contrast to the - 5 0 to - 60 regal observed in the rest of the WDC. Stations located in the eastern Dharwar craton (BNG and GBA) are characterised by smaller velocity perturbations (1.8% and 2.8%, respectively) signifying a normal shield-type crustal configuration, which is evident from the studies of Arora (1971) at the GBA array. Though the majority of seismic stations over the SlGT (SAL, TCY, RJP and IDG) exhibit lower velocity in the crust, COM and KLK are distinct by their high-velocity character. These two stations are placed over shear zones, and therefore, local inhomgeneity may have influenced the velocity. Analysis of the second layer (40-177km), corresponding to the upper part of the mantle, reveals

39

distinctly different velocity features amongst the Archaean terrains. The higher velocities (1.0-2.0%) observed in the western Dharwar craton change to lower velocities ( - 0 . 3 % to - 1 . 6 % ) beneath the SIGT. A 2-3% velocity contrast between the two terrains is observed in the upper mantle. Also, the EDC region (east of 78°E longitude) is conspicuous by the presence of a low-velocity feature ( - 0.4% to -

1.8%).

In Layer 3, the majority of blocks in the Dharwar and granulite terrains show positive velocity perturbations. No definite correlation of velocity variation with surface geology exists at this depth level.

5. Discussion

Tomographic images of various geotectonic blocks show that the surface geology has a definite correlation with the velocity image in the upper mantle to a depth of 177km. The low-grade granite-gneiss Dharwar craton is characterised by 1-2% higher mantle velocity whereas the highland exhumed metamorphic SIGT terrain has lower velocity down to a depth of 177km. Below that, the mantle is more homogeneous. The velocity variations in the crust beneath the WDC ( - 1 . 6 % to - 2 . 1 % ) and EDC (0.1-2.8%) may be genetically related to their crustal evolution. The Dharwar craton, both east and west, is largely composed of gneisses and supracrustals (schist belts). A major difference in the composition of schist belts between the two terrains is the presence of metamorphosed sedimentary rocks in the WDC whereas mafic meta-volcanics are present in the eastern part (Krogstad et al., 1989), suggesting their diverse evolutionary histories. The lateral velocity variation in the mantle is controlled by temperature, pressure, composition, partial melting and pressure-induced phase changes. The higher velocity in the upper mantle to 177 km beneath the Archaean Dharwar craton terrain to the west of 78°E longitude may be due to low temperature and a refractory but buoyant olivine-rich shield lithosphere as well as absence of partial melt (Anderson, 1987; Jordan, 1979a). The low-velocity upper-mantle feature adjoining the east coast of India may possibly be related to the rifting of the Indian plate from Antarctica during Gondwana times. The delayed travel time

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D. Srinagesh, S.S. Rai / Physics of the Earth and Planetary Interiors 97 (1996) 27-41

residuals and inferred lower-velocity lithosphere beneath the uplifted SIGT metamorphic terrain require further analysis. Isotopic ages of both low- and high-grade rocks reveal that the terrains are devoid of any major tectonothermal event after 2.0 Ga. The late Archaean stabilisation of this metamorphic terrain without any major subsequent event argues against purely thermal heterogeneity as the source for observed low velocity. It is possible that the lithosphere beneath the two terrains is compositionally distinct. The average continental garnet lherzolite and pyrolite (Ringwood, 1966) produce density (and velocity) variation of about 1% (Jordan, 1979b). Such a compositionally different lithosphere may be responsible for observed velocity variation. An alternative explanation for the low velocity could be related to the evolution o f the south Indian granulite terrain, from the viewpoint of Precambrian plate tectonics and considering possible convergence of the Dharwar craton with another terrain ( ' X ' ) . This might have led to a western Himalayan type orogeny and lowering o f seismic velocity in the lithosphere (Roecker, 1982). Such a hypothesis considers the subduction o f the Dharwar crust under the terrain ' X ' leading to crustal doubling. The existing knowledge o f P / T conditions shows evidence for 2 5 30 km o f erosion in the granulite terrain. The existing crust of 3 5 - 4 0 k m thickness beneath the granulite terrain therefore supports the idea o f Archaean crustal doubling. Subsequent uplift and erosion of thickened crust led to the exhumation of lower-crustal rock. Such an orogeny possibly brought the south Indian granulite terrain into existence. The presence of such long-lived regional heterogeneities also suggests that this thick lid was not involved in the mantle convection, and therefore supports a thick Archaean lithosphere concept.

Acknowledgements The financial support for field operation was provided by C S I R (EMR). W e are grateful to P.V.S.S. Rajagopal Sarma and Y. Satyanarayana, who were responsible for the operation of portable stations. The authors are extremely grateful to Professor V.K. Gaur for his constant encouragement and invaluable guidance during the course of this work. Critical

review o f this manuscript by the anonymous reviewers is gratefully acknowledged.

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