Deformation and metamorphic evolution of a large-scale fold in the lower crust: the Dumbara synform, Sri Lanka

In the granulite facies Highland Complex of Sri Lanka, large scale folds are formed after the peak of metamorphism (800-850°C, 8-9kbar) at slightly lowered temperatures (700-750°C). The folds formed with axes parallel to a marked stretching lineation which is a continuous element throughout the whole deformational sequence. In the central parts of the Highland Complex a lithostratigraphic sequence derived from shelf-type sediments can be established. Numerous laccolithic or sill-like intrusions with granitic to basic composition thickened the pile. A pronounced foliation and stretching lineation ( S₁/ L₁) parallel to compositional layering formed during peak metamorphic conditions and continued towards lower temperatures. Isoclinal folding with axes parallel or normal to this stretching lineation locally complicates the lithological sequence ( D₂/ D₃). Deformation involving grain-size sensitive mechanisms led to an equalizing of viscosities between layers of differing composition, and thus to very homogeneous deformation during the highest temperature stage. The Dumbara Synform is one of several large scale fourth folds which formed at slightly decreased temperature. The fold mechanism changed from the outer, deeper level towards the core of the fold. Stretching parallel to fold axis continued during folding causing small scale extensional shear zones, which formed after return to marked viscosity contrasts between compositionally different layers. Deformation ended in the higher amphibolite facies and was followed by static annealing and slow strain-free exhumation.