Age resetting of hanging wall rocks above a low-angle detachment fault: Tinos Island (Aegean Sea)

The central Aegean region has been affected by extensional tectonics since at least the Oligo-Miocene. On Tinos Island (Cyclades, Greece), a low-angle normal fault places low-grade metamorphic rocks (the Upper Unit) above high-pressure rocks (the Cycladic Blueschist Unit). K-Ar and 40Ar/39Ar ages were obtained by us on white-micas, amphiboles, and whole-rock samples separated from various metamorphic rocks representing different structural levels of the hanging wall. The hanging wall Upper Unit is commonly thought to have resided at shallow crustal levels since Late Cretaceous times, but our geochronological data, supported by previously published Rb/Sr dating [Brocker, M., Franz, L., 1998. Rb-Sr isotope studies on Tinos island (Cyclades, Greece): additional time constraints for metamorphism, extent of infiltration-controlled overprinting and deformational activity, Geol. Mag., 135 (3), 369-382], indicate severe age resetting of the base of the hanging wall during Tertiary times. Amphiboles from the top of the hanging wall section yielded Late Cretaceous ages which correlate well with other Cretaceous Upper Unit rocks in the Cyclades. Their preservation implies that Tertiary age resetting was mainly confined to the lower part of the Upper Unit, while upper parts retained older ages. The base of the hanging wall section, where age resetting and ductile deformation are most pronounced, is taken to represent a part of a thick extensional shear zone that operated in low-greenschist-facies conditions during Tertiary time. The results of one-dimensional thermal modeling suggest that warm footwall rocks, emerging from great depth subsequently to HP/LT metamorphism, may have supplied the heat required for the metamorphism and ductile deformation at the base of the hanging wall. The model also indicates that only minor cooling of the footwall during decompression is allowed if the footwall is to warm up the hanging wall. This is a priori consistent with the isothermal decompression path of footwall blueschists and eclogites deduced from previous petrological studies on Tinos. (c) 2005 Elsevier B.V. All rights reserved. The central Aegean region has been affected by extensional tectonics since at least the Oligo-Miocene. On Tinos Island (Cyclades, Greece), a low-angle normal fault places low-grade metamorphic rocks (the Upper Unit) above high-pressure rocks (the Cycladic Blueschist Unit). K-Ar and 40Ar/39Ar ages were obtained by us on white-micas, amphiboles, and whole-rock samples separated from various metamorphic rocks representing different structural levels of the hanging wall. The hanging wall Upper Unit is commonly thought to have resided at shallow crustal levels since Late Cretaceous times, but our geochronological data, supported by previously published Rb/Sr dating [Brocker, M., Franz, L., 1998. Rb-Sr isotope studies on Tinos island (Cyclades, Greece): additional time constraints for metamorphism, extent of infiltration-controlled overprinting and deformational activity, Geol. Mag., 135 (3), 369-382], indicate severe age resetting of the base of the hanging wall during Tertiary times. Amphiboles from the top of the hanging wall section yielded Late Cretaceous ages which correlate well with other Cretaceous Upper Unit rocks in the Cyclades. Their preservation implies that Tertiary age resetting was mainly confined to the lower part of the Upper Unit, while upper parts retained older ages. The base of the hanging wall section, where age resetting and ductile deformation are most pronounced, is taken to represent a part of a thick extensional shear zone that operated in low-greenschist-facies conditions during Tertiary time. The results of one-dimensional thermal modeling suggest that warm footwall rocks, emerging from great depth subsequently to HP/LT metamorphism, may have supplied the heat required for the metamorphism and ductile deformation at the base of the hanging wall. The model also indicates that only minor cooling of the footwall during decompression is allowed if the footwall is to warm up the hanging wall. This is a priori consistent with the isothermal decompression path of footwall blueschists and eclogites deduced from previous petrological studies on Tinos. (c) 2005 Elsevier B.V. All rights reserved. The central Aegean region has been affected by extensional tectonics since at least the Oligo-Miocene. On Tinos Island (Cyclades, Greece), a low-angle normal fault places low-grade metamorphic rocks (the Upper Unit) above high-pressure rocks (the Cycladic Blueschist Unit). K-Ar and 40Ar/39Ar ages were obtained by us on white-micas, amphiboles, and whole-rock samples separated from various metamorphic rocks representing different structural levels of the hanging wall. The hanging wall Upper Unit is commonly thought to have resided at shallow crustal levels since Late Cretaceous times, but our geochronological data, supported by previously published Rb/Sr dating [Brocker, M., Franz, L., 1998. Rb-Sr isotope studies on Tinos island (Cyclades, Greece): additional time constraints for metamorphism, extent of infiltration-controlled overprinting and deformational activity, Geol. Mag., 135 (3), 369-382], indicate severe age resetting of the base of the hanging wall during Tertiary times. Amphiboles from the top of the hanging wall section yielded Late Cretaceous ages which correlate well with other Cretaceous Upper Unit rocks in the Cyclades. Their preservation implies that Tertiary age resetting was mainly confined to the lower part of the Upper Unit, while upper parts retained older ages. The base of the hanging wall section, where age resetting and ductile deformation are most pronounced, is taken to represent a part of a thick extensional shear zone that operated in low-greenschist-facies conditions during Tertiary time. The results of one-dimensional thermal modeling suggest that warm footwall rocks, emerging from great depth subsequently to HP/LT metamorphism, may have supplied the heat required for the metamorphism and ductile deformation at the base of the hanging wall. The model also indicates that only minor cooling of the footwall during decompression is allowed if the footwall is to warm up the hanging wall. This is a priori consistent with the isothermal decompression path of footwall blueschists and eclogites deduced from previous petrological studies on Tinos. (c) 2005 Elsevier B.V. All rights reserved.