Abstract
Active tectonics within the Upper Tagus Basin is related to the lithospheric flexure affecting the Palaeozoic basement of the basin. This flexure displays NE-SW trending. Besides, this structure is in agreement with the regional active stress field defined by the maximum horizontal stress with NW-SE trending. In this tectonic framework, irregular clusters of instrumental seismicity (Mw< 5.0) fade in the zone bounded by the Tagus River and the Jarama River valleys. These clusters are related to major NW-SE trending faults of suspected strike-slip kinematics. Moreover, reverse faults with NE-SW trending are affected by the strike-slip system as well. Despite the reverse faults are in agreement with the present SHMAX orientation, though, they apparently are blocked as seismogenic sources (scarce instrumental seismicity recorded today). In addition, we have determined the regional and local stress/ strain fields and two different fracture patterns were observed. Hence, we have divided the area in two zones: (1) the lateral bands of the basin, defined by reverse faulting (NE-SW trending) and strike-slip faulting (NW-SE trending) and (2) the central zone of the basin characterized by shallow normal faulting and NE-SW trending strike-slip faults. Furthermore, surface faulting and liquefaction structures are described affecting Middle to Late Pleistocene fluvial deposits, suggesting intrabasinal palaeoseismic activity (5.5 < M < 6.5) during the Late Quaternary. The obtained structural and tectonic information has been used to classify and characterize the Upper Tagus Basin as a semi-stable intraplate seismogenic zone, featured by Pleistocene slip-rates < 0.02 mm/yr. This value is low but it affords the occurrence of Pleistocene paleoearthquakes.
Highlights
The Upper Tagus Basin (UTB) is located at the central part of the Iberian Peninsula, and include the province of Madrid, parts of the provinces of Guadalajara, Toledo, Cuenca and Segovia, and the major mountain range constituted by the Spanish Central System (SCS)
Both strain patterns are in agreement with the strain regime obtained from the structural analysis: (A) the regional strain tensor with SHMAX NW-trending, activating NE-reverse faults and NW-strike-slips faults, and (B) the local strain tensor defined by SHMAX NE-trending
Basement flexure determined by NE-trending axis and located underneath the Tagus Basin (TB)
Summary
The Upper Tagus Basin (UTB) is located at the central part of the Iberian Peninsula, and include the province of Madrid, parts of the provinces of Guadalajara, Toledo, Cuenca and Segovia, and the major mountain range constituted by the Spanish Central System (SCS). Quaternary fluvial deposits of Middle to Late Pleistocene age, display evidence of strong fracture density, tectonic deformation and a wide variety of liquefaction structures related to synsedimentary faulting and collapse of the underlaying Neogene evaporites (Silva et al, 1988; Giner, 1996; Silva et al, 1997; Silva, 2003) Under these assumptions, the integration of both new and old data as well as the implementation of new techniques are required to perform an updated seismic hazard analysis for the UTB
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