P. Senthil Kumar

High mantle heat flow in a Precambrian granulite province: Evidence from southern India

[1]xa0Twelve new heat flow values determined at nine sites and heat production estimated from radioelemental measurements at 330 sites in the southern granulite province (SGP) bring out contrasting crustal and subcrustal thermal characteristics between the SGP and the adjacent Archaean Dharwar greenstone-granite-gneiss province (DP) in south India. A two-layer granulitic crust of Late Archaean charnockites and gneisses characterizes the northern block (NB), north of the Palghat-Cauvery lineament (PCL). The heat production of the upper, 7–10 km thick, metasomatized granulitic layer ranges between 0.2 and 0.75 μW m−3 (mean 0.5 ± 0.3 (SD) μW m−3). This layer overlies radioelement-depleted granulites characterized by very low heat production ranging from 0.14 to 0.2 μW m−3 (mean 0.16 ± 0.07(SD) μW m−3). In a large sector of the NB, erosion of the upper metasomatized granulite layer has laid bare the depleted granulitic rocks, which represent one of the lowest heat-producing crustal sections. The mean heat flow in the NB is 36 ± 4 mW m−2 (N = 10). The southern block (SB), south of PCL, in contrast to the NB, comprises complexly interlayered charnockites, gneisses, granites, khondalites, and leptynites, which have variable and much higher levels of heat production ranging between 1.11 and 2.63 μW m−3. The heat flow in the SB is 47 ± 8 mW m−2 (N = 3). Overall, the range of heat flow values in the SGP is within the range for the DP. Mantle heat flow in the NB, both from the lowest heat-producing sector and other areas, is deduced in the light of heat production and heat flow data, at 23–32 mW m−2, whose values are distinctly higher than 11–16 mW m−2 for the adjacent DP. The higher mantle heat flow in the NB appears to be a consequence of higher heat production in the subjacent mantle.

Structural effects of meteorite impact on basalt: Evidence from Lonar crater, India

Received 31 January 2005; revised 17 August 2005; accepted 26 August 2005; published 8 December 2005. [1] Lonar crater is a simple, bowl-shaped, near-circular impact crater in the � 65 Myr old Deccan Volcanic Province in India. As Lonar crater is a rare terrestrial crater formed entirely in basalt, it provides an excellent opportunity to study the impact deformation in target basalt, which is common on the surfaces of other terrestrial planets and their satellites. The present study aims at documenting the impact deformational structures in the massive basalt well exposed on the upper crater wall, where the basalt shows upward turning of the flow sequence, resulting in a circular deformation pattern. Three fracture systems (radial, concentric, and conical fractures) are exposed on the inner crater wall. On the fracture planes, plumose structures are common. Uplift and tilting of the basalt sequence and formation of the fractures inside the crater are clearly related to the impact event and are different from the preimpact structures such as cooling-related columnar joints and fractures of possible tectonic origin, which are observed outside the crater. Slumping is common throughout the inner wall, and listric faulting displaces the flows in the northeastern inner wall. The impact structures of Lonar crater are broadly similar to those at other simple terrestrial craters in granites and clastic sedimentary rocks and even small-scale experimental craters formed in gabbro targets. As Lonar crater is similar to the strength-controlled laboratory craters, impact parameters could be modeled for this crater, provided maximum depth of fracture formation would be known. Citation: Kumar, P. S. (2005), Structural effects of meteorite impact on basalt: Evidence from Lonar crater, India, J. Geophys. Res., 110, B12402, doi:10.1029/2005JB003662.

An alternative kinematic interpretation of thetis Boundary Shear Zone, Venus : Evidence for strike-slip ductile duplexes

[1]xa0On Venus a 1000-km-long, 50- to 200-km-wide, anastomosing shear zone system (Thetis Boundary Shear Zone (TBSZ)) separates the eastern Ovda and northwestern Thetis Regiones. Previous workers mapped a part of this shear zone and interpreted it as a sinistral strike-slip shear zone containing an extensional jog. In the present study it is reexamined for a detailed structural and kinematic analysis, and an alternative kinematic interpretation is provided. The TBSZ is divided into western, central, and eastern segments. The western segment is narrow and is oriented in a ENE-WSW direction. Along its northern boundary the preexisting folds of tessera are cut by the shear zone, and the folds are dragged into parallelism with the shear zone in response to dextral strike-slip motion. Both the western and central segments contain a pair of conjugate deformation bands (sigmoidal folds and linear ridges), which appear similar to S–C-like structures of large-scale ductile shear zones. The acute angle between the S and C bands suggests dextral sense of displacement. The eastern segment forms the northeastern restraining bend of the TBSZ, where the shear zone is characterized by a wide, branching, fan-like anastomosing shear zone system with well-developed contractional strike-slip ductile duplexes. The internal fabric defined by the sigmoidal folds consistently shows dextral sense of displacement. Therefore the TBSZ is a dextral strike-slip shear zone showing S–C-like structures and contractional strike-slip duplexes, similar to the continental-scale ductile shear zones on Earth.

Ground penetrating radar for groundwater exploration in granitic terrains: A case study from Hyderabad

The Ground Penetrating Radar (GPR) is a newly developing geophysical tool for imaging the sub-surface and is potentially useful in groundwater exploration. We test its usefulness in characterizing a groundwater rich lineament near Gajularamaram in the Hyderabad granite terrain, where groundwater is limited to soil, weathering zone and lineaments. The lineament is 2 km long and 50–100 m wide, and oriented in WNW-ESE direction. It is characterized by many closely spaced sub-vertical fractures and faults, majority of which are parallel to the lineament. On either sides of the lineament, sub-horizontal sheet joints are abundant. The lineament is saturated with groundwater that discharge as springs at some places. About 450 m long, 400–100 MHz GPR data (∼5–30 m depth) were acquired along five profiles across the lineament. In the lineament, soil thickness varies from ∼0.5 m to 5 m, and is underlain by weathered granite. In the WNW part, a thick weathering zone (∼15 m) is present and a 10 m wide vertical anomaly zone (lineament) is also present. The presence of shallow reflectors at 1 m depth in the lineament is attributed to the groundwater surface. The GPR images reveal many sub horizontal to gently dipping reflectors, which are interpreted to be the sheet joints. The GPR data clearly reveal the saturated lineament, from which groundwater may migrate laterally to long distance through the sheet joints. We demonstrate the GPR as a rapid geophysical tool that can be used successfully to explore the nearsurface groundwater.