Tectonic evolution of an intraplate basin: the Lower Tagus Cenozoic Basin, Portugal

Abstract

This article focuses on the reinterpretation of well, seismic reflection, magnetic, gravimetric, surface wave and geological surface data, together with the acquisition of seismic noise data to study the Lower Tagus Cenozoic Basin tectono‐sedimentary evolution. For the first time, the structure of the base of the basin in its distal and intermediate sectors is unravelled, which was previously only known in the areas covered by seismic reflection data (distal and small part of intermediate sectors). A complex geometry was found, with three subbasins delimited by NNE‐SSW faults and separated by WNW‐ESE to NW‐SE oriented horsts. In the area covered by seismic reflection data, four horizons were studied: top of the Upper Miocene, Lower to Middle Miocene top, the top of the Palaeogene and the base of Cenozoic. Seismic data show that the major filling of the basin occurred during Upper Miocene. The fault pattern affecting Neogene and Palaeogene units derived here points to that of a polyphasic basin. In the Palaeogene, the Vila Franca de Xira (VFX) and a NNE‐SSW trending previously unknown structure (ABC fault zone) probably acted as the major strike‐slip fault zones of the releasing bend of a pull‐apart basin, which produced a WNW‐ESE to NW‐SE fault system with transtensional kinematic. During the Neogene, as the stress regime rotated anticlockwise to the present NW‐SE to WNW‐ESE orientation, the VFX and Azambuja fault zones acted as the major transpressive fault zones and Mesozoic rocks overthrusted Miocene sediments. The reactivation of WNW‐ESE to NW‐SE fault systems with a dextral strike‐slip component generated a series of horsts and grabens and the partitioning of the basin into several subbasins. Therefore, we propose a polyphasic model for the area, with the formation of an early pull‐apart basin during the Palaeogene caused by an Iberia–Eurasia plates collision that later evolved into an incipient foreland basin along the Neogene due to a NW‐SE to WNE‐ESE oriented Iberia–Nubia convergence. This convergence is producing uplift in the area since the Quaternary except for the Tagus estuary subbasin around the VFX fault, where subsidence is observed. This may be due to the locking or the development of a larger component of strike‐slip movement of the NNE‐SSW to N‐S thrust fault system with the exception of the VFX fault, which is more favourably oriented to the maximum compressive stress.

Crustal seismic structure beneath Portugal and southern Galicia (Western Iberia) and the role of Variscan inheritance

“…It is associated with several significant historical earthquakes (Cabral et al, 2013(Cabral et al, , 2004Carvalho et al, 2008;Custódio et al, 2015), from which a major tectonic fault is deduced, the LTV fault system in Fig.2b. The sediment thickness can reach several km's (Carvalho et al, 2016), making difficult a proper study of the crustal structure. To the NW the LTV is bounded by the uplifted Mesozoic Estremadura region, whereas to NE and SE is bounded by the CIZ and OMZ.…”

“…More recently several studies have been performed at smaller scales, mainly in the area of the Lower Tagus Valley (Borges et al, 2016;Carvalho et al, 2016Carvalho et al, , 2014Carvalho et al, , 2008Ghose et al, 2013), due to this region being a potential seismogenic area for Lisbon. These studies show the presence of an uplifted area to the NW of the Tagus Valley and a thick sedimentary basin to the SE.…”

Los terremotos del Bajo Valle del Tajo (Portugal): Lisboa 26 de Enero de 1531 y Benavente 23 de Abril de 1909

“…2000 m of Tertiary sediments consisting of alluvial fan deposits of Eocene-Oligocene age 200-400 m thick, overlain by Miocene fluvial and shallow marine sediments showing a variable width that surpasses 1000 m in depocenters. Fluvial sands and gravels of fini-Zanclean to Piazencian age, generally less than 100 m thick, unconformably overlie the Miocene deposits (Cabral et al 2003, Carvalho 2003, Carvalho et al 2016, Pais et al 2012, Rasmussen et al 1998. The culminating surface of the Pliocene sedimentation is still preserved as a plateau in some areas of the LTB, reaching an altitude of ca.…”

“…It later evolved to a transpressive foredeep basin during the Miocene, under a NNW-SSE to NW-SE compression that caused tectonic inversion of the extensional Mesozoic Lusitanian Basin (LB) located to the W (Fonseca 1989, Ribeiro et al 1990, Cabral 1995, Rasmussen et al 1998, Cabral et al 2003, Carvalho 2003, Carvalho et al 2016 (Figure 2).…”

“…These are linked by WNW-ESE to NW-SE faults also accommodating significant vertical offsets, which together constrain a few Neogene depocentres and structural highs in the basin. (Fonseca 1989, Rasmussen et al 1998, Cabral et al 2003, Carvalho 2003, Carvalho et al 2016.…”