D. S. Ramesh

Images of possible fossil collision structures beneath the Eastern Ghats belt, India, from P and S receiver functions

The Proterozoic Eastern Ghats belt of India is often believed to be the ancient analogue of the present-day Himalayas. However, geological and geophysical signatures that can be traced and linked to the Eastern Ghats belt orogen due to a Precambrian collisional episode are sparse and evidence of such a geotectonic process in the deep lithosphere remains elusive. Utilizing the P and S receiver function imaging technique, we present depth signatures of this convergence event and its lateral extent. Approximately 2000 P and S receiver functions that predominantly sample the Eastern Dharwar craton–Eastern Ghats belt reveal the presence of two distinct westerly dipping interfaces at depths centered on 150 km and 200 km in the study region. Drawing analogy from similar tectonic settings of Proterozoic age and younger Paleozoic times around the globe, we interpret these boundaries to represent remanent structures fashioned by the collisional processes that affected this region. Recent geological, geochemical, and geochronological evidence from the region strongly favors interpretation of our delineated dipping structures as possible vestiges of a Proterozoic collision event that are preserved due to their coherent translation with the overlying lithosphere. Due to this long-lasting record of Proterozoic tectonics, our results add a complication to simple models of the Indian subcontinent in which relatively thin lithosphere underwent rapid transit during the Cretaceous.

Receiver function images of the central Chugoku region in the Japanese islands using Hi-net data

Crustal configuration of the central Chugoku region with disposition of the Philippine Sea Plate (PHS) in this area are investigated through the receiver function approach using short-period Hi-net data. Images of the upper mantle discontinuities are also obtained. Restituted short-period receiver functions bring out discernible variations in average composition of the crust and its thickness in the study region. The Vp/Vs values in the study area are generally high, reaching values in excess of 1.85 at a few places. The central part of the study region showing the highest Vp/Vs values is coincidentally a subregion of least seismicity, possibly bestowed with special subsurface structure. Migrated receiver function images, both Ps and Pps images, unambiguously trace the NW subducting PHS taking a steeper plunge in the northwest part of the Chugoku region reaching depths of 70 km from its low dip disposition in the southeast. An excellent correlation of the subducting PHS with the hypocenters is also seen. We demonstrate that short-period data after restitution and application of appropriate low pass filters can indeed detect presence of the global 410-km and 660-km discontinuities and map their disposition reasonably well. Our migrated receiver functions image the deflections in the 410-km and 660-km discontinuities in an anti-correlated fashion on expected lines of Clapeyron slope predictions induced by subduction of the Pacific plate (PAC) beneath Japanese islands, though PAC itself is feebly traced but shows good correlation with slab seismicity.

Propagation of Crustal-Waveguide-Trapped Pg and Seismic Velocity Structure in the South Indian Shield

Short-period vertical-component digital recordings of the seismic wave field generated by mine tremors and explosions recorded at the Gauribidanur array (GBA) reveal relatively high-frequency secondary energy, with a group velocity of about 6 km/sec accompanied by a long, incoherent coda. We interpret this part of the wave field including the coda as resulting from a crustal-waveguide-trapped Pg , and it is observable from a distance of about 150 km behind the first arrival Pn phase. These observations and extensive modeling results reveal the fine structure of the crust beneath the Archaean eastern Dharwar craton in the south Indian shield. A waveguide with random successions of high- and low-velocity layers in the upper crust, possibly with considerable lateral extent beneath the Closepet granite, explains the main features of the waveguide Pg , such as coda length and group velocity. Reflectivity synthetic seismograms for 1D lamellae structures in the upper crust are found to reproduce the observed features of a record section assembled from GBA seismograms extending to a distance of 240 km. The preferred models of P - and S -velocity structure reveal lower than normal V P / V S and Poisson9s ratios: respectively, 1.63–1.65 and 0.20–0.21 in the upper crust, including the lamellae structure, and 1.70 and 0.235 in the lower crust. A Q P / Q S ratio nearly equal to, or even slightly less than, 1 in the upper crust is found to generate the synthetics consistent with the observed seismogram section. F-K spectral analysis of the observed P coda following the waveguide Pg , for travel paths mostly through the Closepet granite, convincingly reveals two dominant energy peaks, at near-on-azimuth as well as relatively off the source azimuth, both with an apparent velocity of 6.1 km/sec appropriate to the upper crustal depths. The inferred upper crustal structure modeled by alternating high and low velocity lamellae, together with the derived physical properties and the F-K analysis of the P coda, suggests a scattering waveguide at 5–15 km depth in this region. The scattering waveguide appears to be related to the large Closepet granite intrusion, which is believed to be a Precambrian suture between the western and eastern Dharwar cratons in the south Indian shield. Possible lateral extent of the modeled lamellae structure receives added support from a recently proposed hypothesis of a late Archaean mantle plume centered below the Closepet granite. The inferred crustal P -velocity model for the eastern Dharwar craton, revealing a transitional Moho at a shallow depth of 34–36 km, is consistent with the models for other Archaean cratons. The observed Pn velocity of 8.2 km/sec and its gradient 0.013 sec –1 compare fairly well with similar values found for other shield regions. V P / V S and Poisson9s ratios are found to increase respectively from 1.71 and 0.24 below the Moho to 1.78 and 0.27 at about 60 km depth in the upper mantle.

High-velocity anomaly under the Deccan Volcanic Province

Abstract Tomographic modelling of P-wave residuals at 22 vertical-component seismograph stations operated in the Deccan Volcanic Province (DVP) in west-central India and the Dharwar Craton point to the existence of a large, deep (at least 300 km), high-velocity anomaly (1–3% faster) directly beneath most of the DVP and Dharwar Craton. There is also marginal evidence that the westernmost part of the DVP may be characterised by low velocity (up to −1.5%) to a depth of about 200 km. This coherent high-velocity anomaly under the DVP and its extension into the south Indian shield is viewed as a lithospheric root, an architecture characteristic of Precambrian shields.

High velocity anomaly beneath the Deccan volcanic province: Evidence from seismic tomography

Analysis of teleseismicP-wave residuals observed at 15 seismograph stations operated in the Deccan volcanic province (DVP) in west central India points to the existence of a large, deep anomalous region in the upper mantle where the velocity is a few per cent higher than in the surrounding region. The seismic stations were operated in three deployments together with a reference station on precambrian granite at Hyderabad and another common station at Poona. The first group of stations lay along a west-northwesterly profile from Hyderabad through Poona to Bhatsa. The second group roughly formed an L-shaped profile from Poona to Hyderabad through Dharwar and Hospet. The third group of stations lay along a northwesterly profile from Hyderabad to Dhule through Aurangabad and Latur. Relative residuals computed with respect to Hyderabad at all the stations showed two basic features: a large almost linear variation from approximately +1s for teleseisms from the north to—1s for those from the southeast at the western stations, and persistance of the pattern with diminishing magnitudes towards the east. Preliminary ray-plotting and three-dimensional inversion of theP-wave residual data delineate the presence of a 600 km long approximately N−S trending anomalous region of high velocity (1–4% contrast) from a depth of about 100 km in the upper mantle encompassing almost the whole width of the DVP. Inversion ofP-wave relative residuals reveal the existence of two prominent features beneath the DVP. The first is a thick high velocity zone (1–4% faster) extending from a depth of about 100 km directly beneath most of the DVP. The second feature is a prominent low velocity region which coincides with the westernmost part of the DVP. A possible explanation for the observed coherent high velocity anomaly is that it forms the root of the lithosphere which coherently translates with the continents during plate motions, an architecture characteristic of precambrian shields. The low velocity zone appears to be related to the rift systems (anomaly 28, 65 Ma) which provided the channel for the outpouring of Deccan basalts at the close of the Cretaceous period.

Rupture histories of two stable continental region earthquakes of India

MS received 14 September 1998

Information theory–based measures of similarity for imaging shallow-mantle discontinuities

Mode conversions, such as the P-to-s (Ps) converted waves, are now employed in a routine manner to image the velocity boundaries in Earth9s interior. However, there exists an ambiguity in establishing shallow-mantle velocity discontinuities in the depth range 100–300 km through the Ps receiver function approach. This primarily stems from overlap in the time windows of arrivals of direct converted phases from the target depth boundaries (∼100–300 km depth) and reverberations originating from shallow structures (e.g., from crust and/or shallow lithosphere layers). Attempts have been made to address this problem. Classically, limited success has been achieved through methods such as the move-out, which are essentially performed in the measurement space. Recognizing this, we explore generic space-related information theory measures of similarity to extract diagnostics of discrimination between the primary converted waves and the multiply reflected arrivals in Ps receiver functions. Various measures of similarity, such as the mutual information (MI), and associated normalized distance measures, like the normalized variation of information (NVI) and normalized information distance (NID), are successfully applied to receiver function data sampling regions of different tectonic regimes of wide antiquity. Our seismological stations are located in the Archean-Proterozoic craton–mobile belt regions of SE India, Canada, and Phanerozoic United States. Significantly, at several locations in SE India, we interpret the unambiguous presence of midlithospheric discontinuities for the first time. We also either reconfirm or negate the presence of midlithospheric discontinuities beneath stations FFC (Flin Flon) and HRV (Harvard) located in North America. This study reinforces the presence of significant velocity contrast features related to the Lehmann discontinuity depth beneath the Precambrian cratonic stations of India (Hyderabad and Cuddapah) and the Phanerozoic Pasadena station in the United States. Further, multiple deep discontinuities of opposing velocity contrasts are delineated at depths of ∼275 km and ∼320 km beneath station PAS (Pasadena). Our results therefore show tremendous potential to unambiguously detect and distinguish between direct converted seismological phases and multiple reverberations. These information theory approaches discriminate the seismic phases unequivocally. This new approach thus complements the S-to-p (Sp) receiver function technique, which is suitable for detection of shallow-mantle discontinuities (midlithospheric discontinuities, lithosphere-asthenosphere boundary, Lehmann, etc.).

Shear wave splitting observations from the Indian shield

Measurements of shear wave splitting of the waveforms of SKS, SKKS phases recorded at all WWSSN stations (1977–1988) in the Indian shield located on diverse geotectonic units are used to retrieve the anisotropic properties of the sub-Continental lithosphere beneath these regions. The azimuth of fast polarization direction (FPD) ‘α’ and delay time ‘δt’ of the split shear waves with their uncertainties are estimated. Events well distributed in azimuth yield tightly constrained average splitting parameters of α, δt that are roughly:KOD (ENE. 0.50s); HYB (NNE, 145s); POO (N-S, 0.9s); NDI (NE, 0.95s). No consistent anisotropic direction was found at SHL, though the phenomenon of shear wave splitting was clearly observed. In order to test the utility of analog data to document such secondary effects and to authenticate our digitizing procedures, results from GEOSCOPE digital data at HYB were compared with analog data results from the same location. Presence of detectable anisotropy at all the stations is explained either in terms of past and present deformations by tectonic episodes or by plate motion related strain which forms the two end member models in interpreting the observed azimuthal anisotropy. Knowledge of surface geology and maximum horizontal compressive stress (MHS) orientations are invoked to constrain the most plausible hypothesis that explains the observed anisotropic signatures at each of these locations.

Deciphering shallow mantle stratification through information dimension

Applying the concept of cluster entropy, we demonstrate that the primary P-to-s (Ps) conversions originating from the well-recognized seismological boundaries (Moho, 410 km, and 660 km depth interfaces) in Earths interior and multiple reflections (reverberations) between the surface and the Moho—the prominent crust-mantle boundary of Earth—can be unambiguously identified. Their attendant information dimension can act as a discriminant. Documentation of shallow mantle stratification at depths ∼150–300 km (L/X layers) using Ps signals is scarce and is sometimes attributed to their weak registration in the seismograms together with interference by reverberations from shallow structure. The slopes of best fits to mean cluster information dimension (as function of epicentral distance) of Ps signals from target boundaries such as the Moho, 410 km, and L/X discontinuities are consistently gentle with a slight negative character. In contrast, those related to multiples show a steep positive behavior. This opposite nature can potentially discriminate between reverberations and direct converted waves, even when they tend to arrive in overlapping time windows. Our approach thus enables unequivocal identification of shallow mantle layering from Ps data recorded at diverse tectonic provenances of wide antiquity.

Seismological mapping of a geosuture in the Southern Granulite Province of India

The evolution of the Southern Granulite Province of India has remained a contentious issue due to its complex tectonic history and sparsely preserved surface geologic features. This terrain has attracted global attention because of its central role in Gondwanaland tectonics. The crustal structure and composition of this province are examined using passive seismological data recorded by a network of broadband seismic stations sited in the region. Our results clearly show that the composition and seismic structure of the crust across the northeast arm of the Karur-Kambam-Painavu-Trichur shear zone are distinct. This is pronounced even beyond Karur across the eastern and western segments of the Madurai block along its meridional arm as well as across the Namakkal block. These vivid depth images with differing crustal compositions across the blocks, together with occurrence of alkali syenites and other rock types reminiscent of collision, enable us to demarcate the unambiguous presence of a suture in the region.