Reply to comment on 'Tectono‐sedimentary evolution of lower to middle Miocene half‐graben basins related to an extensional detachment fault (western Crete, Greece)'

Abstract

In their comment, van Hinsbergen et al. (2008) question the validity of the tectono-sedimentary model proposed by Seidel et al. (2007) for the Topolia type breccia deposits in western Crete. Apart from various sedimentological aspects that could probably be sorted out in the field, they argue that the proposed time brackets of c. 20–15 Ma for the formation of the Topolia breccia do not reflect the onset of extensional deformation in the overriding plate of the Hellenic subduction zone after collision. Instead, they propose that the onset of extensional deformation is reflected by the formation of a late Miocene (c. 12–11 Ma) sedimentary basin unconformably overlying the Topolia type breccia deposits. Notwithstanding the fact that the age of the breccia is otherwise not well constrained apart from being deposited prior to the exposure of the highpressure metamorphic units in western Crete, we do not concur with this view. Whatever van Hinsbergen et al. (2008) mean with terms syn-orogenic and post-orogenic in the present context, they claim that the onset of crustal scale, post-orogenic N–S extension is dated by the oldest Cretan supradetachment basin. They also state that this basin formed well after the deposition of the oldest Neogene sedimentary unit in western Crete – the Topolia breccia . First, it should be noted that Crete represents a narrow horst structure developed in the forearc of the active Hellenic subduction zone since the Pliocene. When dealing with crustal scale processes, the highly incomplete Neogene sedimentary record on the island is just one limited source of information. Moreover, the buoyant escape model proposed by Thomson et al. (1998, 1999) should not be misunderstood to invoke extrusion from a subduction channel , as quoted by van Hinsbergen et al. (2008), which may rather be a potential cause of the marked uplift of Crete since the Pliocene (Meier et al., 2007). In the buoyant escape model, the partly subducted microcontinent is proposed to return towards the Earth surface by filling the space created by continuous roll back as a more or less coherent slice, becoming exhumed beneath an extensional detachment fault (e.g. Fassoulas et al., 1994; Kilias et al., 1994; Jolivet et al., 1996). This process probably causes extensional deformation in the hangingwall to the detachment, which is represented by the unmetamorphic upper units on Crete; extensional deformation continues because of roll back into the present. Any record of normal faulting in the upper plate to the extensional detachment does not allow an unequivocal distinction to be made between the buoyant escape stage (Thomson et al., 1999) and a subsequent stage of more coaxial crustal extension (Thomson et al., 1999; Rahl et al., 2005), stages possibly referred to as syn-orogenic and post-orogenic by van Hinsbergen et al. (2008). The structural record of the high-pressure metamorphic Phyllite–Quartzite Unit shows an episode of intense extensional deformation in the brittle field (e.g. Seidel et al., 2005), which took place between c. 20 and 15 Ma, according to fission track thermochronometry (Thomson et al., 1998, 1999) and at a depth of less than about 10 km (Kuster and Stockhert, 1997). This extensional deformation therefore happened after the major part of rapid exhumation from more than 30 km depth, when the high-pressure metamorphic Phyllite–Quartzite Unit already resided in the upper crust. We suspect that this marked extensional deformation of the Phyllite–Quartzite Unit should have had dramatic effects also in the higher levels of the upper crust and consequently at the surface. We therefore envisage that the formation of fault scarps, which are required to produce the fault-bound proximal breccia deposits with a minimum thickness of several hundred meters, preserved in western Crete, could be contemporaneous with the buoyant escape stage as well as with this later stage of extensional deformation. The breccia deposits, as all other units of the upper plate to the extensional detachment, were juxtaposed with the high-pressure metamorphic Phyllite–Quartzite Unit along normal faults during ongoing extension. In our simple structural model (Seidel et al., 2007), we propose that the normal fault presently bounding the Topolia breccia towards the south could have evolved by progressive deformation (or by repeated reactivation) from the fault along which the breccia was deposited on the hangingwall block, the rocks of the Phyllite– Quartzite unit forming the footwall block being juxtaposed with the breccia at a late stage of deformation. The proposed half-graben geometry corresponds to that observed in other extensional tectonic settings (e.g. Davis and Coney, 1979; Wernicke, 1981; Davis et al., 1986) with a more complete structural record and is motivated by the clast size and roundness trends observed in the Topolia area. We feel that the interpretation in terms of a major half-graben bounding fault with progressive exhumation of the footwall block is consistent with Correspondence: Dr Markus Seidel, Am Lindbruch 29, D-41470 Neuss, Germany. E-mail: markus.seidel@gmx.net