IASPEI - Picture Gallery

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS006 Poster presentation 1949
First paleoseismological evidence at the Plio-Quaternary Munbrega Halfgraben
(Iberian Chain, NE Spain)
Dr. Francisco Gutirrez
Earth Sciences Department University of Zaragoza
Eullia Masana, Lvaro Gonzlez, Jess Guerrero, Pedro Lucha
The Iberian Chain, in the NE of Spain, is an intraplate Alpine orogene created by the tectonic inversion
of Mesozoic basins (orogenic stage, late Cretaceous-early Neogene). In the central sector of the
orogene, westward propagation of a rifting process has given rise to Mio-Pliocene grabens and later
Plio-Quaternary half-grabens locally superimposed on the western margins of the pre-existing basins
(postorogenic stage). The Munbrega Plio-Quaternary Half-graben is a 19 km long and up to 3 km wide
NW-SE trending neotectonic depression superimposed on the western margin of the Calatayud Neogene
Graben. The poorly exposed fill of this recently captured basin consists of more than 30 m of finegrained
and gravel sheetflood alluvial fan facies capped by a petrocalcic horizon that displays the stage
V of Machettes sequence. The NE active margin of the fault-angle depression corresponds to a
prominent horst structure flanked by the Munbrega E and Munbrega W Faults. The Munbrega W Fault,
which controls the development of the Munbrega depression, has generated a well-defined mountain
front with triangular facets. In the north-western sector of the half-graben, the master Munbrega W
Fault offsets an Upper Pleistocene mantled pediment creating a straight uphill-facing scarp. This
pediment deposit grades distally into a 20 m thick terrace of the Jaln River whose aggradation surface is
located at 45 m above the current channel. A 40 m long trench was dug perpendicularly to the 7.5 m
high antislope fault scarp in order to conduct a paleoseismological investigation. This is the first trench
ever dug across an active fault in the Iberian Chain. The sediments exposed in the trench walls show
numerous deformational structures within a 25 m wide band, including fissure fills, cross-cut synthetic
and antithetic normal faults, grabens, a roll-over, a monoclinal flexure, and a reverse fault that may
correspond to an oversteepened normal fault. The higher thickness of the alluvial sequence in the
downthrown sector of the fault zone suggests that deposition has been controlled by the fault activity
(syntectonic sedimentation). A cumulative vertical displacement of 7.45 m has been estimated on the
deformed mantled pediment deposit. Retro deformation of the trench units suggests three
paleoearthquakes post-dating 70 ka sediments (OSL dating). This investigation demonstrates the
seismogenic potential of the Munbrega W Fault. The mappable length of the structure (ca. 20 km)
indicates that earthquakes Mw as large as 6.5 might be expected on this fault.
Keywords: iberian chain, normal faults, trenching paleoseismology