A ductile extensional shear zone at the contact area between HP-LT metamorphic units in the Talea Ori, central Crete, Greece: deformation during early stages of exhumation from peak metamorphic conditions

Abstract

We report on an extensional ductile shear zone in central Crete at the contact area between the high-pressure low-temperature (HP-LT) metamorphic Phyllite-Quartzite unit sensu stricto (PQ s.str.) and the Talea Ori group (Plattenkalk unit). Mapping and microscopic analysis along the 20 km long contact reveal extensional shear bands, shear band cleavages (C′-type) and associated quartz veins in both units, occurring over a width of up to a kilometer. The shear offset along the shear bands is systematically perpendicular to the contact with the hanging block (PQ s.str.) being downfaulted. Abundant discordant quartz veins are associated to shear band boundaries, asymmetric boudinage and foliation boudinage, forming m-wide vein networks. These mesoscopic deformation structures together with related microstructures such as strain shadows, growth rims of albite porphyroblasts and stylolites indicate dissolution-precipitation creep as main deformation mechanism accompanied by vein formation. Temperatures during deformation are indicated to be close to peak metamorphic temperatures (≥ 300-350 °C) by the growth rims of albite porphyroblasts and micas present in the shear band cleavages, which are consistent with quartz vein microstructures showing subgrains and sutured high angle grain boundaries. Peak metamorphic temperatures inferred by the degree of graphitization of carbonaceous material are similar in the hanging wall and the footwall of the shear zone, which is consistent with fast and nearly adiabatic exhumation. Our study demonstrates the importance of ductile shear zones with high strain rate dissolution-precipitation creep and vein formation in HP-LT metamorphic rocks for the early exhumation history.

Back to Normal: Direct Evidence of the Cretan Detachment as a North‐Directed Normal Fault During the Miocene

“…; Jolivet et al, 1996;Seidel et al, 1982;Theye et al, 1992;Theye & Seidel, 1993;Thomson et al, 1998;Zulauf et al, 2002), and lacking in the overlying Tripolitza Unit, led to a proposal of a major north dipping extensional detachment, that is, the Cretan Detachment (e.g., Fassoulas et al, 1994;Jolivet et al, 1996;Kilias et al, 1994). The metamorphic conditions of the exhumed footwall decrease significantly from 1.0 ± 0.2 GPa and 400 ± 50°C in western Crete to 0.8 ± 0.3 GPa and 300 ± 50°C in eastern Crete (Franz et al, 2005;Seidel et al, 1982;Theye et al, 1992;Thomson et al, 1999), and therefore, the progressive development of the deformation mechanisms along the Cretan Detachment will also have experienced a different history in western Crete (Chatzaras et al, 2006(Chatzaras et al, , 2013Seybold et al, 2019) compared to the exhumation of the lower-grade metamorphic footwall in eastern Crete, which is the focus of this study.…”

New constraints from U–Pb dating of detrital zircons on the palaeogeographic origin of metasediments in the Talea Ori, central Crete

“…The Talea Ori group is structurally inverted, with the southern carbonate-dominated units forming the overturned limb of a large-scale south-vergent fold structure with eastward-plunging fold axis ( Fig. 1; Epting et al 1972;König & Kuss 1980;Hall & Audley-Charles, 1983;Richter & Kopp, 1983;Krahl et al 1988;Chatzaras et al 2006;Robertson, 2006;Kock et al 2007;Seybold et al 2019). The stratigraphic contacts are often overprinted by younger normal faults, but the regular dipping of the layering and an at least locally still assessable sedimentary nature of the contacts indicates the stratigraphic relation of the different formations (e.g.…”

“…The Talea Ori group in central Crete comprises platy marbles with chert in its stratigraphic top (e.g. Epting et al 1972;Kuss & Thorbecke, 1974;Hall & Audley-Charles, 1983;Krahl et al 1988) and structurally underlies the Phyllite-Quartzite unit sensu stricto (Zulauf et al 2016;Seybold et al 2019). Because of the similarities in lithofacies and structural position, the Talea Ori group is associated with the 'parautochthonous' Plattenkalk unit cropping out e.g.…”

“…Given the fact that in the Talea Ori the HP-LT metamorphic lowermost units of the Cretan nappe pile are exclusively exposed, it has been the subject of several petrological and structural studies to unravel the Alpine subduction and exhumation history (e.g. Seidel et al 1982;Richter & Kopp, 1983;Theye, 1988;Seybold et al 2019;. Furthermore, the Talea Ori is a key area for unravelling the palaeogeographic relationships and the large-scale tectonic development of the Eastern Mediterranean (e.g.…”

“…on Crete and the Peloponnesus (Dörr et al 2015;Zulauf et al 2015;Chatzaras et al 2016;Zulauf et al 2018) and the Talea Ori group in central Crete (Kock et al 2007;Zulauf et al 2016). A large amount of Variscan-aged zircons was reported from the metasandstones at the stratigraphic base of the Talea Ori group (Bali formation; Seybold et al 2019), in contrast to detrital zircons from the PQ s.str., which systematically do not show Variscan ages (Zulauf et al 2016). These findings challenge the interpretation that the Talea Ori group can be associated with the Plattenkalk unit and deposited at the northern margin of Gondwana (Dornsiepen et al 2001) or Cimmeria (Stampfli et al 2003;Kock et al 2007;Moix et al 2008).…”