TSK 11 Göttingen 2006 Müller et al. Extensional crustal-scale shear zones in the Western Cyclades (Kea, Greece) Poster Monika Müller1 Bernhard Grasemann1 Michael A. Edwards1 Erich Draganits3 Klaus Voit1 Christoph Iglseder1 An- drás Zámolyi1 Konstantin Petrakakis2 Intense seismicity and intensely devel- oped active and ancient fault systems are common to the Aegean Region. Ex- tending/thinning crust involves a com- plex interplay of (1) Gulf of Corinth rift- expansion, (2) west- and south-ward re- treat of the Hellenic Trench, (3) west- ward impingement of the Anatolian Platen, and/or (4) propagation of the Anatolian Fault system into the Aegean. New geological/structural investiga- tions on Kea (also known as Tzia), in the Western Cyclades reveal a low angle crustal-scale, detachment-type ductile shear zone probably formed during Miocene extension and thinning of the continental crust. The area of interest, which lies in north- western Kea, comprises a large-scale de- tachment shear zone. Brittle deforma- tion and lithospheric failure in the re- gion includes at least two failure phases: (1) multiple low angle cataclastic fault zones formed within, and parallel to, a regional mylonitic ductile foliation and (2) a widespread system of (sub)vertical cross-cutting steep faults. For low angle extensional faulting, a priori co-seismic deformation (i.e. pseudotychylites) is 1 Department of Geodynamics and Sedimen- tology, Structural Processes Group, University of Vienna, A-1090 Vienna, Austria 2 De- partment of Geodynamics and Sedimentol- ogy, University of Vienna, A-1090 Vienna, Austria 3 Institute for Engineering Geology, Vienna University of Technology, A-1040 Vi- enna, Austria absent and therefore aseismic creep is suggested, is overprinted by younger steep/vertical fault zones. A several meter thick low angle cataclas- tic fault zone, with interestingly devel- oped S–C fabrics, separates (i) steeply dipping, minimal deformation-related microstructure, ankeritised dolomite in the hanging wall from (ii) folded (ultra- )mylonitic marbles in the footwall and is regarded as the upper crustal ex- pression of the failure of regionally thinning crust. The locally more than 10m thick brittle fault zone com- prises numerous generations of catacl- asites ranging from foliated protocat- aclasites with brittle/ductile overprint, incohesive coarse grained fault breccias and partly graphitic fine grained fault gouges. This brittle fault zone lo- cally includes a 2m thick serpentinite- talc zone, fractured boudin lenses of opalescent serpentine associated with partly ankeritised (mega-) boudins of dolomites. In the northern part of Kea, this brittle fault zone can be mapped over several kilometres. The faults dip at low angle towards the NNW. Slick- enlines on brittle faults show consis- tent NNE–SSW orientations. Shear sense indicators including scaly fab- rics and Riedel geometries of secondary fractures consistently indicate south- directed hanging wall displacement di- rection. The footwall of the brittle fault zone consists of a several tens of meters thick ultramylonitic shear zone, mainly comprising marbles, phyllites, gneisses and quartzitic schists. The mylonites have a pronounced stretching lineation that has maximum plunge gently to- wards NNE parallel to the brittle kine- matics. Countless textbook examples of a broad range of shear sense indi- 1 Müller et al. TSK 11 Göttingen 2006 cators (flanking structures, asymmet- ric boudinage, stable porphyroclasts with monoclinic symmetry, rotated and boudinaged veins) consistently indicate a south-directed, non-coaxial shearing. Beside a pronounced stretching lin- eation towards NNE specially in one area distinctive lineations and recurva- ture is observed. However, the most striking structural observation is the up- right non-cylindrical folding of the my- lonites with fold axes parallel to the stretching lineation. Shearing of these folds into tubular/sheath folds suggests that folding occurred during shearing due to shortening perpendicular to the stretching lineation. The same short- ening direction is associated with gen- tle buckling of the structurally overlying cataclastic zones suggesting that the W– E shortening component accompanied deformation persisting from ductile, to brittle/ductile to brittle conditions. Several generations of extension gashes filled with calcite, quartz and actino- lite are widespread throughout the my- lonitic rocks. Locally, some extension gashes with associated flanking folds are rotated into the shearing direction de- veloping trains of elongated boudins. Quantitive kinematic flow analyses sug- gest an effective shear strain in the or- ders of several tens of gamma support- ing the interpretation of a high-strain shear zone corroborating with the ob- servation of sheath folds. The low angle cataclastic fault zones are regarded as the upper crustal expres- sion of the failure of regionally thin- ning crust. Further investigations will reveal whether steep faults suites may be related to more than one tectonic event and show, for example, a regional genetic link with the actively widening Gulf of Corinth. In summary, lithological and structural investigations on Kea indicate that the island is a further example of crustal scale shear zone. Preliminary obser- vations suggest that the shear zone bends around the whole island forming a dome-shaped antiform. In analogy to Serifos, a metamorphic core complex to the S of Kea, we speculate that, comple- mentary to the N directed shear zones of Naxos and Paros, the mapped shear zone on Kea is part of an extensional S-directed detachment system. 2