Journal of Coastal Research, Special Issue 56, 2009 Journal of Coastal Research SI 56 688 - 692 ICS2009 (Proceedings) Portugal ISSN 0749-0258 The Use of the GPS in the Identification of Fossil Shore Platforms and its Tectonic Deformation: an Example from the Northern Portuguese Coast. M. A. Araújo and A. Gomes Geography Dept. Porto University, Porto 4150-564, Portugal m.a.araujo@netcabo.pt atgomes@netcabo.pt ABSTRACT ARAÚJO, M. A. and GOMES, A., 2009. The use of the GPS in the identification of fossil shore platforms and its tectonic deformation: an example from the Northern Portuguese coast. Journal of Coastal Research, SI 56 (Proceedings of the 10th International Coastal Symposium), 688 – 692. Lisbon, Portugal, ISSN 0749-0258 Since more than 20 years we have the strong impression that recent tectonics may be active in the northern coastal zone of Portugal. This assumption is compatible with several recent studies on this matter. However, to prove the incidence of recent tectonics, we must find unequivocal deformation marks on the geomorphologic surfaces and on the Pleistocene deposits. Moreover, the deformed surfaces/deposits must be dated. This poses several difficulties as the marine deposits could not be dated by TL and OSL is quite problematic due to the small thickness of the beds and its iron cemented character. In order to overcome these issues, the study of Quaternary sedimentary deposits of the coastal zone of Porto was based upon a very detailed fieldwork and the accurate altitude definition with a GPS (Leica SR20). The use of a static reference to correct the data obtained with the mobile receptor (rover) allowed us to obtain a good precision on the altitude of significant geomorphologic points. This procedure allowed us to characterize the several marine platforms along this coastline and perform some detailed profiles of rock outcrops emerging from sand beaches. Studying the geomorphologic development of present day platforms and its relationship with tidal levels, it was possible to identify some rare tiny platform remains standing above actual platforms (possible Flandrian) and also the more frequent and generally deposit bearing Eemien platform. This work suggests that the last interglacial marine deposits have suffered some tectonic disturbance as they appear at quite different altitudes along this coastal zone. Furthermore, its altitude accompanies the general trend of geomorphologic features: the higher coastal sectors generally bearing higher coastal deposits and higher fossil platforms. ADDITIONAL INDEX WORDS: shore platforms, last interglacial, neotectonics, Porto, Northern Portugal, Vila Chã beach INTRODUCTION There are much more studies about sand beaches than about rocky coasts, whose studies are a bit neglected (STEPHENSON, 2000). This happens partly because its slow evolution is hardly compatible with model use and experimentation (SUNAMURA, 1992). In the last years, the studies were more frequent: see TRENHAILE, (1997, 2005), STEPHENSON (2000), KENNEDY and DICKSON, (2006), TRENHAILE. and KANYAYA, (2007), NEVES, (2004), BLANCO CHAO et al., (2007). Most of the studies focusing on the erosion processes generally explain the development of shore platforms cut into sedimentary rocks, whose evolution is relatively fast. The crystalline bedrock of the northern Portuguese coastal zone is a difficult field work for finding rapid changes and measuring rock degradation. On the other hand, rock resistance allows the preservation of relict forms (shore platforms and notches), sometimes conserving old Eemien deposits. These forms and deposits are key information for assessing ancient sea levels and evaluating possible land movements from relative sea-level evolution. The total studied area lies between Rio Ave mouth and Espinho (fig. 1), and it corresponds to the coastal area surrounding Porto city. It is a structurally heterogeneous area (ARAÚJO et al., 2003). Several studies, including very recent ones confirm the existence of neotectonic movements acting upon the geomorphologic surfaces and late Cenozoic deposits (GOMES et al., 2007; GOMES, 2008). Its situation as a borderland between different structural regions and terranes can explain part of these movements. The possible emergence of a collision zone between North Atlantic sea floor and Iberian plate could reactivate some late variscan strike slip faults (CABRAL, 1995; RIBEIRO, 2002; DE VICENTE et al., 2008). This borderland tectonic situation provides a higher tectonic activity than should be expected from a passive margin. Most probably it also has disturbed the eustatic marine platforms that can be assigned to at least 3 levels (ARAÚJO, 1991). The crystalline Precambrian and Paleozoic rocks are frequently covered with beach sands. However, locally, a seasonal beach erosion reveals rock outcrops buried under the sand beaches. These outcrops generally correspond to notches and shore platforms. The old ideas about a stable and rigid staircase of “raised beaches” (TEIXEIRA, 1979) have been already completely 688