Abstract
<p>Southwest mainland Portugal is located close to the Eurasia-Nubia plate boundary and is characterized by moderate seismicity, although strong events have occurred as in 1755 (Mw≥8), 1969, (Mw 7.9), and more recently in 2007 (Mw 5.9) and 2009 (Mw 5.5), all located in the offshore. No historical earthquakes with onshore rupture are known for this region. At the coastline, high sea cliffs, incised drainages, emergent marine abrasion platforms and paleo sea cliffs indicate that this region is undergoing uplift, although no morphological features were found that could be unequivocally associated with the 1755 mega earthquake. To better understand the recent tectonic activity in this sector of Iberia, it is necessary not only to analyze active structures on land, but also to search for evidence for deformation that may relate to inferred offshore active structures. We thus conducted a study of marine terraces along the coastline to identify regional vertical crustal motions. Several poorly preserved surfaces with thin sedimentary deposits, comprising old beach sediments, were recognized at elevations starting at 2 m elevation and rising inland up to a regional abrasion platform situated at about 120 m a.s.l.. We identified distinct paleo sea level references at several locations at consistent elevations. This terrace sequence is likely Late Pleistocene in age, with individual platforms correlative to MIS 5 high stands and is coherent with a long-term slow uplift of the littoral zone for the southwest of Portugal. Although dating of discrete platforms is an ongoing and difficult task, preliminary correlations of paleo-shoreline elevations suggest that the uplift rate is in the range of 0.1-0.2 mm/yr.</p>
Highlights
The littoral section of southwest Portugal corresponds to the onshore area located close to the main known regional offshore active structures and to the inferred source area of the 1755 earthquake Mw ≥ 8
Vertical movements for the southwestern Portuguese coastal region were previously recognized as the result of a Pliocene-Pleistocene long term uplift [Feio 1951], and the first attempt to quantify the vertical rate was conducted by Cabral [1995] who estimated it to be in the range of 0.1 to 0.2 mm/yr, using an assumed Pliocene or Pleistocene age for the displaced geological markers
São Teotónio-Aljezur-Sinceira left-lateral fault system (STASFS) trends parallel to the western coast, corresponding to the nearest known inland active brittle structure that may be related to the ongoing plate boundary deformation observed in the offshore
Summary
The littoral section of southwest Portugal corresponds to the onshore area located close to the main known regional offshore active structures and to the inferred source area of the 1755 earthquake Mw ≥ 8 The Southwest Portuguese Margin is located in the northwest sector of the Gulf of Cadiz, and is characterized by NNE-SSW to NE-SW striking thrust systems, such as the previously referred Marquês de Pombal, Horseshoe and Gorringe Bank faults Some of these structures in this sector extend onshore, such as Alentejo-Placencia fault system [Pereira and Alves 2013] where a minor sinistral strike-slip component has been recognized offshore and the São Teotónio-Aljezur-Sinceira fault system [Terrinha 1998, Lopes 2002], where a sinistral component was recognized but inland. STASFS trends parallel to the western coast, corresponding to the nearest known inland active brittle structure that may be related to the ongoing plate boundary deformation observed in the offshore This fault zone deforms a ca. km wide regional abrasion platform that is considered to be originally of late Miocene age, and which has been reoccupied or recut during the Pliocene and the Pleistocene, implying that the capping sediments are of Quaternary age. This is very similar to the middle Pleistocene Linda Vista terrace set of southern California, which transitions from a very broad (>10 km) abrasion surface at San Diego to a sequence of discrete, narrow (50-200 m-wide) marine terraces northward towards Los Angeles, primarily as a function of bedrock resistance to erosion [Kern and Rockwell 1992]
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