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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 1936
Complex holocene deformation in the Lower Tagus Valley, Portugal, with
probable tectonic origin
Dr. Joao Fonseca
IASPEI
Susana Pires Vilanova, Pedro Costa, Sandra Heleno, Vittorio Bosi
Palaeoseismological investigations on the Lower Tagus Valley, Portugal, led to the identification of
holocene deformation, dated with C14 and archaeological artefacts. The deformation consisted on a set
of NW-SE dipping planes with variable dip angles, with reverse N-S slip. The deformation was
interpreted as indicative of compressive active tectonics. Alternatively, other authors attributed to the
same structures a gravitic origin. This research was recently resumed, with resource to boreholes and
improved outcrop exposure due to new roadcuts. This new phase led to the following additional results:
1) a gravity-driven landslide, with a sub-horizontal sliding plane, can in fact be identified in the new
exposure, close to the trenches. 2) However, a sub-vertical contact surface striking about N50E and
showing normal slip can also be identified in the new outcrop, directly underneath the old trenches. The
surface remains steep down to a depth of 9 meters at least, making it difficult to reconcile with a
gravitic origin. NW-SE extension, as implied by the new structure now discovered, is at odds with the
usually adopted stress field for Western Portugal. This suggests that the structure is a secondary fault,
with small dimension. Our preferred interpretation is that it belongs to a more complex pattern of
tectonic deformation yet to be constrained by further study, of which the landslide is also a secondary
effect. Such further study is key to the assessment of the contribution of intraplate seismicity to the
seismic hazard in Portugal, and in particular in the Greater Lisbon region.
Keywords: paleoseismology, active fault, seismic hazard