Abstract
AbstractWe 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 (Wm) 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 (Wm = 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.
Highlights
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., Fassoulas, 1999; Grasemann et al, 2012; Jolivet et al, 2010; Lee & Lister, 1992; Lister & Forster, 1996). These studies focused on the sense of shear and the timing of extension (Brichau et al, 2010; Buick, 1991; Gessner et al, 2001; John & Howard, 1995; Soukis & Stöckli, 2013; Urai et al, 1990), potential displacements (Brichau et al, 2006, 2007; Ring et al, 2001), the relation of extensional shearing with metamorphism, partial melting and magmatism (Beaudoin et al, 2015; Cao et al, 2013; Keay et al, 2001; Kruckenberg et al, 2011; Rabillard et al, 2017; Ring et al, 2018; Vanderhaeghe, 2004), and how various single extensional shear zones and faults
We studied the northern part of the Cycladic Blueschist Unit (CBU) section, which is not dominated by marble
Summary
The Aegean Sea extensional province in Greece is an exceptional laboratory for studying extensional deformation above the southward retreating Hellenic subduction zone. The West Turkish detachments taper out to the west while the Aegean detachments have not yet laterally propagated into West Anatolia (Figure 1) In map view, this geometry defines two spatially disconnected extension provinces separated by the zone of old fission-track cooling ages (Figure 1). A simple three dimensional elastic model of two laterally tapering detachment fault systems should cause extension perpendicular to the slip directions of the two detachment systems (Bernhard Grasemann, written communication, 2011) Such Miocene E-W extension has been reported from Samos Island in the easternmost Aegean by Gessner et al (2011), Ring et al (1999) and Roche et al (2019). Extensional deformation starting in the early Miocene has been discussed in a West-Turkey context by Gessner et al (2013)
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