Geometry and Kinematics of Bivergent Extension in the Southern Cycladic Archipelago: Constraining an Extensional Hinge Zone on Sikinos Island, Aegean Sea, Greece

We report the results of a field study on Sikinos Island in the Aegean extensional province of Greece and propose a hinge zone controlling incipient bivergent extension in the southern Cyclades. A first deformation event led to top-S thrusting of the Cycladic Blueschist Unit (CBU) onto the Cycladic basement in the Oligocene. The mean kinematic vorticity number (W-m) during this event is between 0.56 and 0.63 in the CBU, and 0.72 to 0.84 in the basement, indicating general-shear deformation with about equal components of pure and simple shear. The strain geometry was close to plane strain. Subsequent lower-greenschist-facies extensional shearing was also by general-shear deformation; however, the pure-shear component was distinctly greater (W-m = 0.3-0.41). The degree of subvertical pure-shear flattening increases structurally upward and explains alternating top-N and top-S shear senses over large parts of the island. Along with an increased coaxial deformation component, the strain geometry became oblate. Published quantitative data from nearby Ios Island are similar and both data sets define an extensional hinge zone between top-N extensional deformation across large parts of the central and northern Cyclades and top-S extensional deformation at the southern and western fringe of the archipelago. This extensional hinge zone is an important large-scale structure forming early in the history of lithospheric extension due to southward retreat of the Hellenic slab.

Automated summary

The study in the Aegean Sea region of Greece revealed the presence of a hinge zone controlling extensional structures, with distinct characteristics of deformation events and evolution of strain geometry. This early-formed extensional structure is deemed significant in the history of lithospheric extension in the region due to the southward retreat of the Hellenic slab.