DISS 3.2.GreDaSS: Seismogenic Source GRCS150 - Thrace Fault Zone

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COMMENTS The Thrace CSS (GRCS150) is probably more than 120 km-long showing a prevailing E-W to ENE-WSW trend. This major tectonic structure dips southwards and borders the Rhodope Mountain to the north with Kavala-Xanthi-Komotini basin to the south. Location and strike of the Composite Source is based on morphotectonic maps (Lyberis, 1984; Mountrakis and Tranos, 2004; Rondoyanni et al., 2004). Based on sea bottom seismic profiles (Martin, 1987), this Composite Source probably continues westwards within the Gulf of Kavala. According to Mountrakis and Tranos (2004) and Mountrakis et al. (2006), this Composite Source is divided in four major segments, but only the three eastern ones show evidence of recent activity. The fault barriers are of angular type producing differences in strike and dip and generating a zig-zag geometry. According to the descriptions of Mountrakis and Tranos (2004), Mountrakis et al. (2006) and Rondoyanni et al. (2004), the dip-angle progressively decreases westernwards. The maximum depth of the fault zone probably increases from west to east since the dipping angle of the faults becomes steeper. Since seismicity is very low in the area and seismological data are scant, maximum depth (and width) was/were estimated from the seismogenic layer thickness of the adjacent Mygdonia CS (GRCS100). The mean kinematics is dip-slip normal, though oblique-slip displacements locally prevail (Mountrakis and Tranos, 2004; Mountrakis et al., 2006; Rondoyanni et al., 2004; Lyberis, 1984). Taking into consideration the geometrical barriers and assuming a dynamic triggering model, as a worst case scenario more than one segment could rupture. The �softest� barrier is probably the one between Xanthi-GRIS152 and Iasmos-GRIS151 segments which some authors describe as a unique (Lyberis, 1984 and Rondoyanni et al., 2004). Assuming 44 km of cumulative length and using the empirical relationships of Pavlides and Caputo (2004) a magnitude of 7.0 is inferred. However, if the two longest adjacent segments will be reactivated contemporaneously (Chrysoupoli-GRIS153 and Xanthi-GRIS152 Faults), the cumulative length is 61 km and the expected magnitude would be 7.1. OPEN QUESTIONS In the frame of the Thrace CSS (GRCS150), are the segment boundaries between the three recognized ISSs 'soft' or 'strong'? Could these boundaries safely stop the co-seismic propagation starting from one of the segments? Or could the fault zone behave as a single major structure? Is the Chrysoupoli Fault segment (Mountrakis et al., 2006) an inactive old structure or could it be triggered and reactivated by a nearby fault? How far in the Gulf of Kavala does the zone extends? SUMMARIES Lyberis (1984); Lyberis and Sauvage (1985) The Kavala-Xanthi structure (corresponding to the Chrysoupoli-GRIS153, Xanthi-GRIS152 and Iasmos-GRIS151 faults of this database) has been investigated as part of a broader study for unravelling the tectonic evolution of the North Aegean. The estimated cumulative vertical offset is 7 km. According to field observations, the average strike of the Xanthi Fault segment considered as a unique structure (corresponding to the Xanthi-GRIS152 and Iasmos-GRIS151 faults of this database) is 40�-60� (i.e. NE-SW) and the dip less than 40�. Three generations of striations indicating three tectonic events have been recognized: i) a NW-SE extension of Late Miocene age associated with dip-slip normal kinematic indicators; ii) a NE-SW Pliocene dextral transtension; iii) a post-Pliocene to Present N-S extension associated with an oblique-slip kinematics (dip-slip and right-lateral components). They consider the Kavala-Xanthi Fault as an active structure inherited from at least Miocene. Mountrakis and Tranos (2004); Mountrakis et al. (2006) The authors carried out a geological-morphotectonic mapping at the scale 1:50.000 of the Kavala-Xanthi-Komotini Fault zone (corresponding to the Thrace-GRCS150 of this database). They investigated the geometry, kinematics and fault segmentation of the zone using also seismological data. The fault zone is subdivided into four main fault segments that show different orientation and architecture as well as geologic features that might be classified as �barriers� or �asperities�, generally related to fault segmentation. Among such features the authors observed minor fault bends, en echelon stopovers, changing relief along the fault, and cross-structures. The westernmost segment is the 35 km-long, ca. 55� striking Chrisoupolis-Xanthi Fault that separates the marbles of the Pangeon Unit in the footwall from the tectonically overlying migmatites and gneisses of the Sidironero Unit in the hanging-wall, along with overlying post-Alpine Tertiary molasse-type sediments. Triangular facets and fault scarps striking NE-SW with moderate angle dipping towards the SE were also observed. Three generations of slickenlines have been observed and measured on the outcropping fault plane. The Xanthi-Iasmos Fault (corresponding to the Xanthi-GRIS152 fault of this database) is the next eastern segment, striking WNW-ESE to E-W with a length of 27 km. According to the authors, this segment contains secondary subparallel fault branches forming several fault scarps, with the most basinward ones being the most impressive. The fault affects the metamorphic rocks of the Sidironero Unit and the Tertiary molasse-type sediments as well as the Upper Oligocene Xanthi granitoid, forming well-exposed fault surfaces steeply dipping southwards. Along this segment they observed two generations of oblique-slip slickenlines documenting a first left-lateral and normal movement and a younger right-lateral and normal movement. The 16 km-long Iasmos-Komotini Fault (corresponding to the Iasmos-GRIS151 fault of this database) is the third segment striking ENE-WSW and comprising two parallel left-stepping and overlapping fault strands: the 6 km-long Polyanthos-Mega Piston and 13 km-long Mega Piston-Aghiasma fault strands. These fault segments juxtapose basement with Pliocene-Quaternary fanglomerates and Holocene deposits that entirely cover the underlying Neogene sediments of the Komotini basin. The Iasmos-Komotini fault segment is characterized by multiple reactivations revealing slickenlines of right-lateral strike-slip movement overprinted by younger ones that indicate normal-sense reactivations. The easternmost segment is the Komotini-Sapes Fault (corresponding to the Komotini-GRIS150 fault of this database), a 30 km-long, geometrically complex structure consisting of several WNW-ESE and E-W synthetic minor fault strands less than 8 km-long that dip SSW to S at medium to high angles. According to the authors, WNW-ESE-striking branches are cut by, or merge into, E-W trending younger faults. The youngest activity occurs east of Polyanthos village, where a rectilinear fault line [scarp?], a few tens of metres long, with a vertical offset of less than a metre, is observed within the Pliocene-Pleistocene fanglomerates of the hanging-wall. They suggest that this morphotectonic feature is associated with the 1784 (M = 6.7) earthquake reported in the catalogue of Papazachos and Papazachou (2003). Rondoyanni et al. (2004) The authors performed geological mapping and meso-structural analysis of the Kavala-Xanthi-Komotini Fault. According to field observations, geometric and dynamic parameters (direction, dip and rake) vary along strike. They suggest that the fault consists of three segments (Kavala-Xanthi, Xanthi-Komotini and Komotini-Aissimi) and has a total length of 120 km. The first segment (western one) has a NE-SW average direction with dip ranging between 35� and 45�; the second (central one) an E-W average direction with dip ranging between 65� and 70�; the third segment (eastern one) outcrops in a more complex structure consisting of several smaller parallel faults that have formed the northeastern limit of Komotini basin. Two generations of slickenlines on polished fault surfaces both associated with oblique-slip movements (dextral and normal components) have been observed. Also the alignment and/or congruent displacement of streams and the displacement of recent Quaternary deposits are described.