Co-axial horizontal stretching within extending orogens: the exhumation of HP rocks on Syros (Cyclades) revisited

Abstract

Although the role of extensional tectonics in the exhumation of high-pressure metamorphic terranes is widely established, the kinematics of such deformation remains ambiguous. This paper outlines new field data from the Attic-Cycladic blueschist belt that suggest that distributed ductile strain plays a significant role in the extension and that, consequently, the role of major detachment faults may have been over-emphasized in previous studies. The high-pressure blueschist terrane (Ermoupolis Unit) of Syros shows abundant evidence of subhorizontal extension, manifest as layer boudinage and ductile thinning without the development of significant internal detachments. The deformation approximates to pure shear stretching that was heterogeneously distributed in space and time. Minor zones of asymmetric shear are interpreted not as through-going extensional shear zones but as structures that maintain compatibility between zones of differential stretching. The progression of deformation is charted through the systematic development of increasingly lower-pressure metamorphic assemblages. However, most of the decompression (potentially from 20 kbar to 6 kbar) occurred within the blueschist stability field, as the rocks were actively extending. Heterogeneous retrogression and concomitant deformation are believed to relate to the local chemistry and availability of hydrous fluids.The relative importance of broadly distributed, approximately pure shear deformation and strongly localized, approximately simple shear through the crust is a recurrent debate in continental tectonics. Mike Coward was at the vanguard of early attempts to explain highdeformation zones in the continental crust, chiefly in terms of simple shear, through his work on basement structures in Africa and NW Scotland. As focus shifted to basin dynamics in the 1980s, debates polarized between homogeneous rifting (sometimes called 'pure shear'; McKenzie 1978) and whole lithospheric shear zone models (e.g. Wernicke 1985). Common ground was found through the development of 'depth-dependent' stretching models (see Roberts & Yielding 1992, for review), including the heterogenous rifting model of Coward (1986). Yet the whole lithosphere shear models have retained much influence, especially in tectonic scenarios for the exhumation of high-pressure metamorphic rocks. Drawing on models from the Basin-and-Range Province that inspired Wernicke's (1985) lithospheric models, Lister et al. (1984) proposed that blueschists of the Attic-Cycladic belt in the Aegean had been exhumed in the footwall to major low-angle, crustal-scale normal faults. This model has since seen widespread use, not only in the Aegean but also in other parts of the Mediterranean (e.g. Jolivet et al. 1998) and further afield. With these continuing studies it is pertinent to examine the structural geology of these settings carefully.To what degree are the high-pressure (HP) rocks in the footwalls to large faults merely passive passengers back to upper crustal conditions, or do they contain significan...

Sequential development of interfering metamorphic core complexes: numerical experiments and comparison with the Cyclades, Greece

“…This interpretation may apply to most islands (e.g. Wijbrans et al 1990;Parra et al 2002;Bond et al 2007), Ios included (Forster & Lister 1999). As a result, the inference that no wedge of upper crustal rocks exists in the Cyclades (Gautier & Brun 1994b) remains probably valid, which, in turn, supports the view that detachment zones and associated MCCs do interfere with one another in this region.…”

“…In this particular case, the extent of retrogression is apparently linked with the intensity of shearing during greenschist facies metamorphism (Jolivet & Patriat 1999;Parra et al 2002). The same may hold for Syros (Bond et al 2007) and, eventually, Sifnos (Wijbrans et al 1990). Rosenbaum et al (2002) also consider that, in northern Syros, at high levels of the metamorphic pile, greenschist facies overprint is localized into top-to-NE shear zones that are contemporaneous with Miocene extensional shearing in neighbouring islands.…”

“…In addition, the topmost detachment fault seen in southeastern Syros, with Cretaceous metamorphic rocks in the hanging wall (Maluski et al 1987) and well preserved high-pressure rocks in the near footwall, should represent a relatively old structure. However, in the case of Syros, Bond et al (2007) claim that the intermediate extensional shear zones do not exist and, like other authors have argued for Sifnos and Tinos Islands (Schliestedt & Matthews 1987;Bröcker 1990;Ganor et al 1996), consider that the degree of preservation of the high-pressure assemblages reflects primarily the extent of fluid infiltration during greenschist facies retrogression. Limited fluid infiltration and deformation in the least retrogressed rocks may also account for the preservation of older ages by the time rocks passed through P-T conditions of the greenschist facies, as proposed by Wijbrans et al (1990) in the case of Sifnos (see however Wijbrans et al 1993).…”

“…While agreeing with a continuum of extensional deformation from blueschist to greenschist facies conditions, Bond et al (2007) recently questioned the existence of these prominent shear zones on Syros and argued that extensional deformation was dominantly coaxial throughout the synmetamorphic exhumation history. Kinematic data reported by Gautier (1995) and Trotet et al (2001a) do not show a dominant sense of shear on the scale of Syros Island (apart from dominantly top-to-NE/ENE shearing in the eastern part), apparently supporting the hypothesis of Bond et al (2007). Finally, the southwestern chain of islands, from Kea to Sifnos, is the least known of the Cyclades (Sifnos excluded).…”

Cenozoic geodynamic evolution of the Aegean

“…High-pressure units were thus exhumed with little internal deformation (Keiter et al 2004), except in the lowermost unit, in the southwest of the island, where greenschist facies deformation is predominant. This is at variance with Rosenbaum et al (2002a) and Bond et al (2007), who argued for HP rocks exhumation resulting from distributed coaxial stretching at crustal-scale. In summary, the Vari detachment can be considered as the roof of the exhuming Cycladic Blueschists extruding wedge (Ring and Layer 2003;Ring et al 2007), or subduction channel .…”