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.
Highlights
IntroductionThere, siliciclastic rocks forming the base of the Talea Ori group, not known from other areas on Crete and the Peloponnesus, are in contact with the siliciclastic metasediments of the PQ s.str
The contact between the Talea Ori group and the PQ s.str. is characterized by shear bands, extensional shear band cleavages, kink bands, asymmetric boudinage and a high amount of quartz veins that are associated to these structures (Figs. 2, 3)
To distinguish between deformation related to the ductile shear zone and the deformation at peak metamorphic conditions, we describe the relation between porphyroblast growth, foliation
Summary
There, siliciclastic rocks forming the base of the Talea Ori group, not known from other areas on Crete and the Peloponnesus, are in contact with the siliciclastic metasediments of the PQ s.str. Rocks from the contact area show microstructures indicative of transient high stresses (few hundred of MPa) within metasediments deforming by dissolution–precipitation creep at high strain rates on long term (Trepmann and Seybold 2018) Such microstructures cannot be reconciled with low stress deformation at HP-LT metamorphic conditions in subduction zones as typically recorded from HP-LT metamorphic rocks in Western Crete (Greiling 1982; Schwarz and Stöckhert 1996; Stöckhert et al 1999), but they document the early exhumation history of the HP-LT metamorphic rocks.
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