THE STRUCTURAL EVOLUTION OF THE ARENA GNEISSES AND ITS BEARING ON PROTEROZOIC TECTONICS OF SRI-LANKA

The Arena Gneisses (AG), which is considered to form an eastern portio n of the Wanni Complex (WC), is exposed at five doubly plunging synfor ms (arenas) in central Sri Lanka, near the boundary between the WC and the Highland Complex (HC). Structural investigation based on detailed field work on two of the arenas classified the following structural s equences from the earliest to the latest: Dn-2, Dn-1, Dn, Dn + 1, and Dn + 2 deformations. The Dn-2 structures include the major composition al banding, pinch-and-swell and boudinaged structures formed by compre ssion normal to the banding. Mesoscopic intrafolial isoclinal to tight folds (Fn-2) were formed synchronologically with Dn-2 deformation. Du ring the Dn-2 stage, the AG thrusted over the HC with top-to-the-south ward movement. The asymmetric structures such as asymmetric foliation boudinaged structure, E-W-trending asymmetric folds (Fn-1b) and asymme tric megacrysts, and NS-stretching lineations were formed by this simp le shear deformation. This transport of the AG caused the N-S shorteni ng and produced macroscopic gentle folds (Fn-1a) with mostly E-W-trend ing upright axial surfaces. During the Dn-1 stage, the eastward moveme nt of the AG took place. This movement was associated with N-S-trendin g asymmetric folds (Fn-1c) and microfabrics observed in the AG, as wel l as the eastward vergences of related folds in the HC. N-S-trending u pright linear folds (Fn) and related structures such as parasitic fold s and axial surface foliations were formed by E-W shortening, which is normal to the axial surface of the macroscopic Fn fold, during the Dn deformational stage. Dextral shear deformation with vertical shear pl anes took place during Dn + 1 deformation. ENE-WSW- or NE-SE-trending folds (Fn + 1) were developed during Dn + 2 deformation. The southward thrusting of the AG indicates that the WC has displaced over the HC w ith top-to-the-southward movement. The eastward transportation of the AG and the HC may be due to the collision of the Vijayan Complex (VC) with the HC. Considering the above-mentioned tectonic elements with Sr i Lankan geochronological data (e.g. Holzl et al., 1994), the first th rusting event might have taken place around 700 Ma and the second disp lacement probably occurred around 570 Ma. Copyright (C) 1996 Elsevier Science Ltd