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

Abstract

The Aegean Sea extensional province in Greece is an exceptional laboratory for studying extensional deformation above the southward retreating Hellenic subduction zone. Starting with the pioneering work of Lister et al. (1984), aspects of low-angle extensional shear zones and associated brittle detachments have been worked out in great detail (e.g.

Stretched Thin: Oligocene Extrusion and Ductile Thinning of the Basal Unit Along the Evia Shear Zone, NW Cyclades, Greece

“…A conspicuous map-scale pattern or geological rationale for its distribution is not easy to distinguish, with proposed exposures found on Tinos (Avigad & Garfunkel, 1989), Samos (Ring, Laws, & Bernet, 1999), and mainland Greece (Schermer, 1989). Exposures of the Cycladic Basement, which occupies a lower structural position equivalent to the Basal Unit, are likewise documented on islands recognized to have accommodated significant ductile thinning in the Oligocene or Miocene (Mizera & Behrmann, 2016;Ring & Glodny, 2021). Whereas an intuitive understanding of this relationship would suggest that greater magnitudes of exhumation are responsible for greater depths of exposure, and not vice-versa, the irregular occurrence of windows to subjacent structural levels remains poorly understood and this theoretical causal link merits further investigation.…”

“…A recent study from Ios has interpreted symmetrical N-S strain markers as a reflection of plane strain ductile thinning active under post-orogenic extension (Mizera & Behrmann, 2016). Ring and Glodny (2021) used the distribution of coaxial strain across the southern Cyclades to define a "kinematic hinge zone," a regional-scale feature that these authors speculate was either a byproduct of, or may even have directly controlled, bivergent extension in the Aegean.…”

“…Numerous island‐ and regional‐scale studies have established a two‐step exhumation history for the CBU, marked first by Eocene syn‐orogenic wedge extrusion (e.g., Brichau et al., 2008; Ring & Layer, 2003) and then by Oligo‐Miocene bivergent low‐angle detachment faulting (Gautier & Brun, 1994; Grasemann et al., 2012; Jolivet, Lecomte, et al., 2010; Jolivet & Patriat, 1999; Lecomte et al., 2010). Despite being reported from a few localities in the Cyclades (e.g., Kumerics et al., 2005; Ring & Kumerics, 2008), ductile thinning remains poorly integrated into geodynamic models for exhumation in the region, but may be an important mechanism governing the exhumation of units structurally below the CBU (e.g., Mizera & Behrmann, 2016) and perhaps even in determining the ultimate geometry of the major detachment systems (e.g., Ring & Glodny, 2021).…”

“…into geodynamic models for exhumation in the region, but may be an important mechanism governing the exhumation of units structurally below the CBU (e.g., Mizera & Behrmann, 2016) and perhaps even in determining the ultimate geometry of the major detachment systems (e.g., Ring & Glodny, 2021).…”

Coeval Miocene exhumation of the Cycladic Blueschist Unit and the Cycladic Basement in the southern Cyclades, Ios and Sikinos, Greece

“…Other studies propose that the contact represents an Eocene top‐to‐the‐S thrust fault (“South Cycladic Thrust”; Huet et al, 2009) that imbricated the CB and CBU during subduction (Augier, Jolivet, Gadenne, Lahfid, & Driussi, 2015; Glodny & Ring, 2022; Huet et al, 2009; Ring & Glodny, 2021; van der Maar & Jansen, 1983). The CB‐CBU contact has been regionally correlated with the Trans‐Cycladic Thrust (Grasemann et al, 2018) on Sikinos (Ring & Glodny, 2021) and Ios (Glodny & Ring, 2022). Another hypothesis suggests a parautochthonous nature of the CBU, and evidence for Oligocene static fluid alteration along the CB‐CBU contact argues against large‐magnitude displacement <20 Ma (Poulaki et al, 2019, 2021).…”