Early exhumation of high‐pressure rocks in extrusion wedges: Cycladic blueschist unit in the eastern Aegean, Greece, and Turkey

Abstract

Structural, metamorphic, and geochronologic work shows that the Ampelos/Dilek nappe of the Cycladic blueschist unit in the eastern Aegean constitutes a wedge of high-pressure rocks extruded during early stages of orogeny. The extrusion wedge formed during the incipient collision of the Anatolian microcontinent with Eurasia when subduction and deep underthrusting ceased and the Ampelos/Dilek nappe was thrust southward over the greenschist-facies Menderes nappes along its lower tectonic contact, the Cycladic-Menderes thrust, effectively cutting out a ∼30- to 40-km-thick section of crust. The upper contact of the Ampelos/Dilek extrusion wedge is the top-to-the-NE Selçuk normal shear zone, along which the Ampelos/Dilek nappe was exhumed by ∼30–40 km. Detailed Rb-Sr and 40Ar/39Ar dating of mylonites demonstrates that both shear zones operated between 42 and 32 Ma. There is no evidence for episodic motion during the ∼10 Myr life span of the shear zones, suggesting that both shear zones operated in a steady, nonepisodic fashion. Our data provide supporting evidence that simultaneous thrust-type and normal sense shearing can accomplish the early exhumation of deep-seated rock

No need for lithospheric extension for exhuming (U)HP rocks by normal faulting

“…The geochronological data indicate that the top-to-the-WSW thrust at the base of the Styra Nappe operated, within errors, at the same time as the topto-the-ENE normal fault at the top of the Styra Nappe. This conclusion is corroborated by the gradual reversal of the shear-sense data from top-to-the-WSW in the basal Styra Nappe to top-to-the-ENE in the upper Styra Nappe; there are no systematic cross-cutting relationships between the fabrics (Ring et al 2007a).…”

“…Shaked et al (2000) and Ring & Layer (2003) showed that the Styra Nappe was thrust onto the Basal Unit during top-to-the-WSW shortening. Ring et al (2007a) dated mylonites resulting from this process at 29.5 ± 0.3 Ma. This age is similar to the 40 Ar-39 Ar white mica ages from thrust-related mylonites of c. 35-30 Ma of Ring & Layer (2003).…”

“…The upper contact of the Styra Nappe is subparallel to the basal contact but has an opposite shear sense (i.e. top-to-the-ENE; Ring et al 2007a). Dating of mylonites from this contact yielded ages of 33.0 ± 1.0 Ma and 27.2 ± 0.9 Ma (Ring et al 2007a).…”

“…top-to-the-ENE; Ring et al 2007a). Dating of mylonites from this contact yielded ages of 33.0 ± 1.0 Ma and 27.2 ± 0.9 Ma (Ring et al 2007a). The geochronological data indicate that the top-to-the-WSW thrust at the base of the Styra Nappe operated, within errors, at the same time as the topto-the-ENE normal fault at the top of the Styra Nappe.…”

Dating deformation in the Gran Paradiso Massif (NW Italian Alps): Implications for the exhumation of high-pressure rocks in a collisional belt

“…Butler and Freeman, 1996) and vertical ductile thinning (Ring and Kassem, 2007) (Figure 11b). The extrusion wedge model has been shown as a viable exhumation mechanism for high-pressure rocks in the Aegean Sea region and the Tauern Window of the Eastern Alps (Ring et al, 2007a;Ring et al, 2007b;Ring and Glodny, 2010). It demands alternating shear sense across the extruding wedge, i.e.…”