DEFORMATION PARTITIONING DURING UPDOMING OF THE SONNBLICK AREA IN THETAUERN WINDOW (EASTERN ALPS, AUSTRIA)

The Sonnblick Dome forms a NE-vergent dome structure cored by basement gneisses within the southeastern Tauern Window of the Eastern Alps (A ustria). A succession of ductile and brittle deformation stages docume nts doming and exhumation subsequent to the thermal peak of metamorphi sm. Contrasting deformation geometries within internal parts and along the margins of the dome are explained in terms of deformation partiti oning. Subhorizontal shortening is documented by subvertical en-echelo n extensional gashes within central parts of the dome. Subhorizontal a s well as subvertical flattening is also documented by fold structures . During dextral transpression strike-slip is accommodated along the N E margin (Sonnblick Lamella, Moll Valley Fault) and the southern margi n (Moser Fault) of the Sonnblick Dome, while vertical thickening occur red within the interior of the dome. Crustal thickening triggered unro ofing and extension parallel to the dome axis which is documented by d uctile low-angle normal faults in the uppermost structural sections of the dome. This normal fault system contributed to footwall uplift and exhumation of the dome structure. The Sonnblick Lamella, associated w ith the dextral Moll Valley Fault, forms a stretching fault where defo rmation is concentrated along a potential zone of weakness. This fault is interpreted to represent the transition from vertical thickening w ithin the dome to vertical thinning at the dome margins. During upbend ing, the dome structure passed the isotropic stress surface that is ch aracterized by equality of sigma(1) and sigma(2). This is documented i n a perturbation of the orientations of principal stress axes sigma(1) and sigma 2, while sigma(3) remains constant in orientation. Transpre ssion contributed substantially to updoming and to rapid, nearly isoth ermal, exhumation subsequent to the thermal peak of metamorphism. Copy right (C) 1996 Elsevier Science Ltd