Vale de Santarém is a small valley near the Tagus River, 60 km NNE of Lisbon, in an area of important seismicity. It is located in the Lower Tagus Cenozoic Basin, astride a narrow WNW-ESE tectonic trough where a thick Pliocene sedimentary sequence is preserved. Tectonic control is evidenced by faults bordering the trough, which have controlled sedimentation. It is aligned with the Rio Maior Pliocene basin, located 20 km NW, suggesting a WNW-ESE fault zone beneath the Neogene sedimentary cover, which was reactivated in Plio-Pleistocene (?) times, generating the small tectonic basins at the southern and northern tips. The interference of this structure with the active NNE-SSW faults of the Tagus Valley may cause segmentation of these faults, with implications for the expected seismogenic sources and the regional seismic hazard.