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Paloma Lafuente, Universidad de Zaragoza (Spain)
Luis E. Arlegui, Universidad de Zaragoza (Spain)
Carlos L. Liesa, Universidad de Zaragoza (Spain)
José Luis Simón, Universidad de Zaragoza (Spain)
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We present the paleoseismological study of an ancient railway trench that crosses the southern sector of the Concud normal fault. The aim of this study is the identification of Quaternary seismic events linked to the fault movement. The Concud fault constitutes the southeastern boundary of the Jiloca semigraben (central Iberian Chain, Spain). It is a 13 km-long normal fault with an overall NW-SE strike, that has been active since Late Pliocene, with calculated slip rates between 0.07 and 0.23 mm/y, potential magnitudes between 6.37 and 6.78, coseismic displacements from 0.35 to 2.02 m and inferred recurrence periods between 1.1 and 28.9 ka. The studied trench (about 13 m high), shows Upper Miocene lacustrine carbonates on the footwall and Pleistocene fluvial and alluvial materials on the hanging wall.
We have identified a minimum of 6 large single events (probably 7). The first one is represented by the rupture and displacement that affects fluvial deposits outcropping at the lower part of the trench. Although more than one paleoearthquake would be probably related to the deformation of those deposits (its lower part seems to be cut by previous ruptures), only one has been clearly distinguished. This event took place soon before the deposit of a sedimentary level (dated on 71.6±5 ka) that overlies the terrace. The second event is represented by a colluvial wedge of grain supported pebbles with sandy matrix. This colluvial wedge is broken and displaced almost a metre by a new event (the third one). After that, there is a little change on the fault behaviour, so that the 3-4 last paleoseisms involve both downthrow displacement and fissure opening. During these last events, some sedimentary units and structures on the hanging wall undergo tilting, and then the fissure is filled by falling materials from the footwall. Subsequently, slope deposits overly the deformed materials. Those fissure fills are massive, one of them including a chaotic ensemble of conglomerate blocks of a Middle-Pleistocene fluvial terrace, and are bounded by fault planes. The last fissure fill represented at the trench provided an OSL age of 32.1±2 ka. If there is a minimum of 5 events between 71.6±5 and 32.1±2 ka, we obtain an average recurrence interval of about 8 ka. The sedimentary thickness accumulated during this time lapse (9-10 m) probably approaches the total coeval fault throw, which provides an estimate of around 2 m for the average coseismic displacement. However, the geometric, retro-deformational reconstruction of the events does not explain all the displacement. This suggests the occurrence of either a number of events not recorded in the trench or creep episodes between seismic periods.
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