Coexisting ultramylonite and pseudotachylyte from the eastern segment of the Mahanadi shear zone, Eastern Ghats Mobile Belt

Abstract

Pseudotachylytes occur associated with mylonite and ultramylonite in the Mahanadi shear zone (MSZ) in the Eastern Ghats Mobile Belt (EGMB). The MSZ is about 200 km long curvilinear high strain zone trending WNW-ESE in its eastern part that splays out in the west. In Kantilo-Ganian segment of MSZ in northern EGMB, an interbanded sequence of granulite facies lithoassemblage has undergone ductile shearing. Kinematic studies of mylonite and ultramylonite indicate MSZ to be a NE-dipping, extensional type ductile shear zone. Non-coaxial metamorphic growth of garnet and presence of truncated sillimanite-fish in ultramylonite suggest high temperature regime during shearing. Pseudotachylytes in MSZ occur as millimetre thick layers to decimetre thick zones containing fragments of mylonite, ultramylonite and lithic clasts. Pseudotachylyte generation veins are mostly sub-parallel to C-planes and the injection veins cross-cut at high angle to these. The presence of an isotropic glassy matrix, injection features, corroded grains and dendritic microlites can be evidences for the existence of a melt phase. The composition of pseudotachylyte matrix (by EPMA) indicates silica deficiency with higher normative hypersthene, plagioclase and lower quartz compared with average whole rock composition for host. Absence of overprinting of mylonitic fabric on pseudotachylytes indicates their formation by brittle failure without passing through a plastic deformation and thus a two stage development for mylonite-ultramylonite and pseudotachylyte generation is suggested.

Restoration of Late Neoarchean–Early Cambrian tectonics in the Rengali orogen and its environs (eastern India): The Antarctic connection

“…The shear zone at Kantilo (80 km south of Angul) is a segment of the E-trending, Ndipping Mahanadi shear zone (Lisker and Fachmann, 2001;Mahapatro et al, 2009) arguably defining the southern boundary of the Grenvillian-age granulites of the Angul domain (Rickers et al, 2001). The extensional (Mahapatro et al, 2009) shear zone hosts intensely-deformed and mylonitized charnockite and enderbite gneisses, leptynites (definition after Mehnert, 1972), garnet-sillimanite gneisses, and garnetiferous mafic granulite (Mahapatro et al, 2009).…”

“…The extensional (Mahapatro et al, 2009) shear zone hosts intensely-deformed and mylonitized charnockite and enderbite gneisses, leptynites (definition after Mehnert, 1972), garnet-sillimanite gneisses, and garnetiferous mafic granulite (Mahapatro et al, 2009). Within occurrence of dynamically recrystallized orthopyroxene (T ≥ 750 ⁰C;Cannat and Casey, 1995) in the mylonite fabric.…”

Elevated Equilibrium Geotherm in Stable Continental Shield: Evidence From Integrated Structural, Hydrological, and Electromagnetic Studies on Nonvolcanic Hot Springs in the Eastern Ghats Belt, India

“…The Mahanadi Shear Zone has a general WNW‐ESE trend that appears to be associated with reorientation of the NE‐SW trend of the EGB (Bose & Gupta, ; Mahapatro et al, ). This is reported to be a high‐strain, noncoaxial shear zone associated with abundant mylonites and pseudotachylites (Chetty, ; Mahapatro et al, ); along the western extension of the Mahanadi Shear Zone, ductile fabrics are overprinted by cataclasites representing brittle faulting (Nash et al, ).…”

“…The present study is confined to the area south of the river Mahanadi, in and around the town of Khurda (Figure ), and represents the eastern extension of the Mahanadi Shear Zone (Mahapatro et al, ). It covers an area of ~1,500 km 2 .…”

“…As a result of this collision, which was followed by successive deformation events through the Neoproterozoic (Gupta, ), the EGB is characterized by penetrative planar fabrics and dissected by a number of shear zones and faults (Nash et al, ). The Mahanadi Shear Zone (MSZ) is one of the most important of these shear zones as it separates the southern part of the EGB from the north (Mahapatro et al, ). Since Paleozoic times, when Gondwana (Lower Permian) sediments were unconformably deposited on the EGB, this region has largely remained tectonically inactive and is considered part of the Indian shield.…”