THE ROLE OF EXTENSION-PARALLEL FOLDING IN THE TECTONO-SEDIMENTARY DEVELOPMENT OF THE EARLY TO MIDDLE MIOCENE KYMI-ALIVERI BASIN, EVIA, GREECE

Extension-parallel folds are common within ductile footwalls of metamorphic core complexes. However, the record of strain in the corresponding upper plate is often poorly preserved due to erosion or burial of the hanging wall. This is the case for the Attic-Cycladic massif in the Aegean Sea, which exhibits extension-parallel folds in the footwall but preserves the hanging wall only in a few small, scattered tectonic slices and dismembered supradetachment basins. New data from the Kymi-Aliveri Basin suggests that Early to Middle Miocene extension-parallel folding affected the upper plate of the Cycladic detachment system and was important for basin formation. Geological mapping and stratigraphic and structural analyses show that the Kymi-Aliveri basin formed during NE-SW directed extension, which overprinted an Eocene-age fold and thrust belt. During this event, the uppermost crust was warped into large-wavelength (10-20 km), moderate amplitude (~2 km) folds; these structures are comparable in scale to those predicted by numerical models of extension affecting a rigid upper crust underlain by a low viscosity layer. Progressive deformation is recorded by internal unconformities and syn-sedimentary faults, the lateral distribution and vertical succession of sedimentary facies, and the existence of an unroofing assemblage consisting of three distinct sedimentary provenances of progressively higher metamorphic grade. Detailed magnetostratigraphy and high-precision U/Pb zircon dating of volcanics constrains the onset of deposition at ~17 Ma, persisting until after ~13 Ma. Ductile deformation at depth was characterized by constrictional strain, with development of extension-parallel folds at all scales that are well-exposed within the Attic-Cycladic massif. The South Evia-North Attica Fault, which separates the nonmetamorphic Sub-Pelagonian zone from the blueschists of the Attic-Cycladic Massif, initiated at ~14 Ma along the inflection line of a crustal-scale fold as a high-angle extension-parallel transfer fault that accommodated ongoing extension within the Attic-Cycladic massif. The formation of the Miocene Kymi-Aliveri basin was strongly influenced by the crustal architecture inherited from Eocene subduction and exhumation of blueschists to shallow depth beneath a fold and thrust belt.