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 portion of the Wanni Complex (WC), is exposed at five doubly plunging synforms (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 sequences from the earliest to the latest: Dn-2, Dn-1, Dn, Dn + 1, and Dn + 2 deformations. The Dn-2 structures include the major compositional banding, pinch-and-swell and boudinaged structures formed by compression normal to the banding. Mesoscopic intrafolial isoclinal to tight folds (Fn-2) were formed synchronologically with Dn-2 deformation. During the Dn-2 stage, the AG thrusted over the HC with top-to-the-southward movement. The asymmetric structures such as asymmetric foliation boudinaged structure, E-W-trending asymmetric folds (Fn-1b) and asymmetric megacrysts, and N-S-stretching lineations were formed by this simple shear deformation. This transport of the AG caused the N-S shortening and produced macroscopic gentle folds (Fn-1a) with mostly E-W-trending upright axial surfaces. During the Dn-1 stage, the eastward movement of the AG took place. This movement was associated with N-S-trending asymmetric folds (Fn-1c) and microfabrics observed in the AG, as well as the eastward vergences of related folds in the HC. N-S-trending upright linear folds (Fn) and related structures such as parasitic folds 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 planes 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 with 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 Sri Lankan geochronological data (e.g. Hölzl et al., 1994), the first thrusting event might have taken place around 700 Ma and the second displacement probably occurred around 570 Ma.