Magmatic and Tectonic Structures from the Chimalpahad Layered Complex, Andhra Pradesh, India

Abstract The Chimalpahad Layered Complex (CLC) is the largest (>200 sq km) metamorphosed anorthosite complex in the Precambrian shield of Peninsular India. It exhibits spectacular magmatic layering together with superposed tectonic structures. Modal layering, with primary cumulus texture, is the most prominent magmatic structure displayed by the complex. A variety of magmatic structures (rhythmic mineral-graded layering, cyclic and cross layering, and troughs), practically unaffected (or weakly modified) by later tectonic overprinting, form the relict igneous structures in low strain zones within the CLC; they are described and depicted in sufficient detail, and the possible mechanisms for their formation are enumerated. The structural features in the CLC owe their origin to dual play of magmatic and tectonic processes. The original magmatic structures in high strain zones are variably overprinted by: (1) planar foliation (S1) and mineral lineation (L1) of the D 1 deformation, (2) the F1a and F1b folding event of the D 2 ductile deformation and (3) brittle-ductile fractures/faults of the D 3 deformation that are inferred to have developed during exhumation of the CLC by transpressional tectonism. Micro-textural features of the CLC provide useful criteria for identification of superimposition of solid-state recrystallization on magmatic fabrics. Simultaneous compression and strike-slip shear mechanism (transpression and transtension) have been proposed to account for the magmato-tectonic evolution of the CLC.