Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

A. B. Roy ¹, P. R. Golani ², B. R. Bejarniya ¹

Affiliations
1 Department of Geology, University of Rajasthan, Udaipur 313 001, IN
2 Geological Survey of India, Assam-Meghalaya Circle, Shillong 793003, IN

Abstract

The Ahar River granite near Udaipur city, considered by Heron as intrusive into the Aravalli rocks of early Proterozoic age, is a coarse-grained, massive, leucocratic rock except along the outcrop margins. The granite ranges in composition from alkali granite to granodiorite and tonalite, with about 5% ferromagnesian minerals. The enveloping Aravalli metasediments form a continuous sequence starting with greenschist (metavolcanics) and quartzite (locally conglomerate) along the granite contact. The stratigraphic position of the Ahar River granite in relation to the surrounding metasediments suggests that the granite is the basement rock. The granite characteristically shows post-crystalline deformation of the constituent grains, specially quartz and feldspar; and the granite-cover contacts are marked by ductile shear zones developed during the earliest deformation of the Aravalli rocks.

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B. R. Bejarniya ¹, A. Joshi ¹

Affiliations
1 Antarctica Division, Geological Survey of India, NH-5P, N.I.T., Faridabad - 121001, Haryana, IN

Abstract

Considered earlier to be very little defonned, the granulite facies rocks of the Petennann Ranges, East Antarctica, reveal three phases of folding and two episodes of ductile shearing. These metamorphic rocks preserve the F₁ folds as small scale, ischolar_mainless, intrafolial features with tight to isoclinal profiles, and F₂ folds as mesoscopic features with close to tight profiles. F₁ and F₂ folds are coaxial and broadly coplanar having axial surfaces marked by pervasive NW-SE striking foliation. F₃ folds are open, upright with N-S to NNW-SSE axial planes and impart local swings in the regional foliation. The early ductile shears produce a strong mylonitic fabric and are synchronous to F₁, while occurring as thin wavy zones, the later shears are post F₂.

Shape and geometric analyses of representative folds show that F₁ folds belong to class 2 and 1C at low and high limb dips respectively and are fonned dominantly by flexural flow. F₂ folds exhibit geometric features of class 1C at low as well as high limb dips and owe their origin to buckling with a flexural slip component and flattening.

Keywords

Folds, Mylonitisation, Petermann Ranges, Antarctica.

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