Litho-tectonostratigraphy of the Dhanjori Basin, India: A fold-thrust sequence and its tectonic relation with the Singhbhum Shear Zone

Abstract

Based on direct field evidence, litho-tectonostratigraphy of the Neoarchaean-Palaeoproterozoic Dhanjori Basin, situated at the northeastern fringe of the Singhbhum Craton (SC), is being appraised for the first time. Entire sequence of the Dhanjori Basin is presently interpreted as a single-stack of volcano-sedimentary assemblage of terrestrial clastic metasedimentary rocks of conglomerate-quartzite-phyllite in the lower part followed by meta-volcanic-volcaniclastic sequence. Singhbhum Shear Zone (SSZ) marks the eastern boundary of the Dhanjori Basin with Singhbhum Group representing North Singhbhum Mobile Belt (NSMB). In the western and southern margin, the Dhanjori sequence exhibits angular unconformity with the Palaeoarchean Iron Ore Group (IOG) represented by the Badampahar-Gorumahisani Belt and nonconformity with the Singhbhum Granite Complex and Mayurbhanj Granite-Gabbro which also indicates its post Iron Ore Orogenic development along SC margin. The rocks of the Dhanjori Basin and NSMB have undergone the same progressive deformational event of the North Singhbhum Orogeny but the Dhanjori Basin escaped the initial phase of it. SSZ, developed during later phase of this progressive deformational event, affected both of these packages. Prior to this, the Dhanjori Basin was undeformed. Four splays of SSZ transects the Dhanjori Basin longitudinally. These splays are characterized by development of two sets of mylonitic fabrics with moderate to low east-northeasterly dip and downdip mineral stretching lineation. They have resulted in thrust related repetition of the Dhanjori sequence within interior part of the basin. The third set of planes having same trend as mylonitic fabric but dipping steeply in opposite direction, is present in the form of spaced cleavage, kinks and fractures. These planes are formed out of stress release after the episodes of shearing-mylonitisation-thrusting. Older quartzites are thrusted over younger metavolcanics in all along the eastern margin and central part. Isolated overthrusted units of NSMB also overlie the Dhanjori package as thrust klippe. Mafic-Ultramafic rocks with plutonic fabric, present in the southeastern and northern part of Dhanjori Basin, exhibits intrusive relation with the Dhanjori package.