Active tectonics in Southern Portugal (SW Iberia) inferred from GPS data. Implications on the regional geodynamics

Accepted Manuscript Title: Active tectonics in Southern Portugal (SW Iberia) inferred from GPS data. Implications on the regional geodynamics Authors: Jo˜ao Cabral, Virg´ılio Brito Mendes, Paula Figueiredo, Ant´onio Brum da Silveira, Joaquim Pagarete, Ant´onio Ribeiro, Ruben Dias, Ricardo Ressurreic¸a˜ o PII: DOI: Reference:

S0264-3707(16)30258-7 https://doi.org/10.1016/j.jog.2017.10.002 GEOD 1508

To appear in:

Journal of Geodynamics

Received date: Revised date: Accepted date:

28-12-2016 16-9-2017 5-10-2017

Please cite this article as: Cabral, Jo˜ao, Mendes, Virg´ılio Brito, Figueiredo, Paula, Silveira, Ant´onio Brum da, Pagarete, Joaquim, Ribeiro, Ant´onio, Dias, Ruben, Ressurreic¸a˜ o, Ricardo, Active tectonics in Southern Portugal (SW Iberia) inferred from GPS data.Implications on the regional geodynamics.Journal of Geodynamics https://doi.org/10.1016/j.jog.2017.10.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Active tectonics in Southern Portugal (SW Iberia) inferred from GPS data. Implications on the regional geodynamics

João Cabrala, Virgílio Brito Mendesb, Paula Figueiredoc, António Brum da Silveirad, Joaquim Pagaretee, António Ribeirof, Ruben Diasg, Ricardo Ressurreiçãoh a

(Corresponding author) Instituto Dom Luiz, Faculdade de Ciências, Universidade de Lisboa, 1749-016 Lisboa, Portugal. [email protected] b

Instituto Dom Luiz, Faculdade de Ciências, Universidade de Lisboa, 1749-016 Lisboa, Portugal. [email protected] c Instituto Dom Luiz, Faculdade de Ciências, Universidade de Lisboa, 1749-016 Lisboa, Portugal. (Present address) Quaternary and Anthropocene Research Group, University of Cincinnati, USA. [email protected] d Instituto Dom Luiz, Faculdade de Ciências, Universidade de Lisboa, 1749-016 Lisboa, Portugal. [email protected] e Departamento de Engenharia Geográfica, Geofísica e Energia, Faculdade de Ciências, Universidade de Lisboa, 1749-016 Lisboa, Portugal. [email protected] f Instituto Dom Luiz, Faculdade de Ciências, Universidade de Lisboa, 1749-016 Lisboa, Portugal. [email protected] g Laboratório Nacional de Energia e Geologia, Apartado 7586-Alfragide, 2610-999 Amadora, Portugal. [email protected] h Laboratório Nacional de Energia e Geologia, Apartado 7586-Alfragide, 2610-999 Amadora, Portugal. [email protected]

Abstract A GPS-based crustal velocity field for the SW Portuguese territory (Algarve region, SW Iberia) was estimated from the analysis of data from a network of campaign-style GPS stations set up in the region since 1998, complemented with permanent stations, covering an overall period of 16.5 years. The GPS monitoring sites were chosen attending to the display of the regional active faults, in an attempt to detect and monitor any related crustal straining. The residual horizontal velocities relative to Eurasia unveil a relatively consistent pattern towards WNW, with magnitudes that noticeably increase from NNE to SSW. Although the obtained velocity field does not evidence a sharp velocity gradient it suggests the presence of a NW-SE trending crustal shear zone separating

two domains, which may be slowly accumulating a slightly transtensional right-lateral shear strain. Based on the WNW velocity differential between the northeastern block and the southwestern block, a shear strain rate accumulation across the shear zone is estimated. This ongoing crustal deformation is taken as evidence that a nearby major active structure, the São Marcos – Quarteira fault, may be presently accumulating strain, therefore being potentially loaded for seismic rupture and the generation of a large magnitude earthquake. Further inferences are made concerning the interseismic dynamic loading of other major onshore and offshore active structures located to the west.

Key words: crustal deformation; Global Positioning System (GPS); plate boundary; slow tectonics environment; seismotectonics; SW Iberia

1

Introduction In the present work we estimate a spatially dense, GPS-based crustal velocity field for the SW

Portuguese territory (Algarve region, SW Iberia), through the analysis of data from a network of campaign-style GPS stations set up in the region since 1998, complemented with several permanent stations, covering a period of up to 16.5 and of more than ~7 years, respectively. The geographical distribution of the GPS monitoring sites in the study region was primarily chosen considering knowledge of the regional neotectonics, particularly the display of the known major active or potentially active faults, in an attempt to detect and monitor any related crustal straining. The specific location for most of the monitoring sites was later selected based on the existence of stable monuments for installing the antennas, preferably geodetic benchmarks, and a few marks were also implemented on rock outcrops when monuments were not available.

The data presented in this study, compiled over a denser array of monitoring sites and a longer observation period relatively to previous studies (e.g. Palano et al., 2015), are particularly relevant for analyzing crustal deformation occurring at low tectonic rates, as is the case of SW Iberia. We therefore discuss the relevance and applicability of this type of study for a region of low tectonic strain rate and where the crustal deformation is poorly constrained, but which is located close to a plate boundary zone where large earthquakes have been generated, such as the Mw >8 1755 Lisbon earthquake (Johnston, 1996; Martinez-Solares and López Arroyo, 2004).

2

Tectonic setting The study region, corresponding to the southern area of the Portuguese mainland

territory, is located in SW Iberia, near the West-Iberian continental margin and the Eurasia-Nubia plate boundary (eastern sector of the Azores – Gibraltar fracture zone) (Fig. 1A). Regional geodynamics has been driven by a NW-SE convergence of Eurasia and Nubia, which has occurred at an average rate of 4-5 mm/yr for the past 3 Myr, according to global geological models of plate motions, such as NUVEL-1A (DeMets et al., 1994) or MORVEL (DeMets et al., 2010). Spacegeodetic observations indicate a similar current convergence rate though rotated anticlockwise relatively to the NW direction of geological models (Calais et al., 2003; Fernandes et al., 2003; Nocquet and Calais, 2004; Serpelloni et al., 2007; Palano et al., 2015; among others).

Fig.1 – A – Location of the study region (framed area) in the context of the Azores – Gibraltar plate boundary. GBF, Gorringe Bank fault, HSF, Horse Shoe fault, MPF, Marquês de Pombal fault, PBF, Portimão Bank fault. Circles are epicenters of major regional earthquakes. B – Simplified regional geological map (modified from LNEG, 2010).

The geometry, kinematics and dynamics of the Eurasia-Nubia plate boundary have been the subject of focused research for decades and are still matter of debate. The plate boundary is clearly discernible at the western and central parts of the Azores – Gibraltar fracture zone, but east of the Gloria transform fault (~20° W) it is poorly established, the interplate deformation being distributed across a broad area, a few hundred kilometres wide.

In an effort to locate the plate boundary related structures, as well as to identify the source of the great MW 8.5-8.7, 1755 “Lisbon earthquake” (Johnston, 1996; Martinez-Solares and López Arroyo, 2004), a large amount of multi-channel seismic reflection and high-resolution multibeam swath bathymetry data have been acquired at the SW Iberian margin and the Gulf of Cadiz. The performed studies (e.g. Zitellini et al., 1999, 2001, 2004; Gràcia et al., 2003; Terrinha et al., 2003) revealed a complex structural pattern, with discrete active reverse faults trending NE-SW to NNE-SSW, such as the Gorringe Bank, Marquês de Pombal and Horseshoe faults, and E-W to WNW-ESE, such as the Portimão Bank fault (Fig. 1A). Based upon detailed seafloor morphology, earthquake distribution and seismic profiles, Gutscher and others (e.g. Gutscher, 2004; Gutscher et al., 2002, 2009) propose the occurrence of active subduction in the Gibraltar Arc as a consequence of the westward roll-back of an old (Miocene?) plate. Pedrera et al. (2011) question this model arguing that the eastward Gibraltar Arc oceanic subduction system is inactive probably since the Late Miocene and that the current tectonic framework in the Gibraltar Arc domain is of continental collision. High-resolution multi-beam bathymetry data of the Gulf of Cadiz also revealed several WNW–ESE trending lineaments in seafloor sediments which have been interpreted as the morphological expression of ongoing right-lateral strike-slip reactivation of WNW–ESE pre-existing faults (Rosas et al., 2009, 2012; Zitellini et al., 2009). These authors argue that these faults, distributed along a narrow band of deformation over a length of 600 km, correspond to the present Nubia–Europe plate boundary in this region. Duarte et al. (2013) claim that two tectonic mechanisms are currently operating in the southwest Iberia margin, namely westward migration of the Gibraltar Arc and oblique convergence between Nubia and Iberia, inducing compressive stresses sufficient to overcome the lithospheric strength in the southwest Iberia margin and thus making this region a locus of subduction zone initiation leading to closure of the Atlantic.

Palano et al. (2015), based on a large GNSS dataset covering approximately 15 years of observations, recently proposed a clockwise rotation of the Iberian Peninsula with respect to stable Eurasia at a rate of 0.07°/Myr, with a nearly continuous straining of the ductile lithosphere in some sectors of South and Western Iberia due to viscous coupling of Nubia and Iberian across the plate boundary in the Gulf of Cadiz area. As a result of the regional geodynamic setting, mainland Portugal and the nearby Atlantic area experience moderate levels of seismicity, characterized by the occurrence of M6+ interplate earthquakes nucleated in the SW offshore region, such as the Horseshoe Abyssal Plain events in 1969 (MS 7.9, Fukao, 1973) and 2007 (Mw 6.0, Stich et al., 2007), and the above referred 1755 great Lisbon earthquake (Fig. 1A). Important historical earthquakes have also occurred in the nearoffshore and onshore southern and central Portuguese territory, as in 1722 (estimated M6.5, probably located close to the eastern Algarve coast, according to Baptista et al., 2007), in 1858 (estimated M6.8-7.2, located near the coast south of Lisbon), and in 1531 and 1909 (estimated M6.5-7.1 and 6 respectively, located in the Lower Tagus Valley, near Lisbon) (Custódio et al., 2015, and references therein) (Fig. 1A). These last earthquakes correspond to some of the largest European intraplate events, being the current expression of a significant seismotectonic activity.

3 Regional geological setting and neotectonics The study area is located in southern Portugal, covering the Province of Algarve. The northern part comprises Paleozoic schists and greywackes of the Iberian Hercynian Massif while the Meso-Cenozoic Algarve basin is located southward, encompassing two superimposed sedimentary basins where Mesozoic and Cenozoic sediments crop out (Manuppella, 1988, 1992; Kullberg et al., 1992; Terrinha, 1998; Ramos et al., 2015) (Fig. 1B). The Mesozoic basin was formed within a sinistral transtensional regime during the opening of the Tethys Sea and the North

Atlantic Ocean, from the late-Triassic to the Early Cretaceous (Terrinha, 1998; Ramos et al., 2015), and comprises predominantly carbonate rocks. In the Upper Cretaceous, an alkaline magmatic body intruded the Paleozoic basement in the southwestern-most area, known as the Monchique massif (Miranda et al., 2009 and references therein). The Cenozoic basin is a flexural feature related to the convergence between Nubia and Iberia, including mostly Miocene limestones as well as Pliocene to Pleistocene fluvial and marine detrital sediments. That convergence started in late Cretaceous and conditioned a polyphase tectonic inversion of the previously extensional Mesozoic basin, with a major fini-Cretaceous to Paleogene inversion phase due to N-S to NW-SE compression, followed by a less expressive Miocene to Present NW-SE compressive phase (Terrinha, 1998; Ramos et al., 2015). Due to the current geodynamic setting, Algarve experiences neotectonic activity as well as significant seismicity, with local historical and instrumental earthquakes of M5+ magnitude (Martins and Mendes Victor, 2001; Mezcua, 1982; Martinez Solares and Mezcua, 2002; Carrilho et al., 2004; Custódio et al., 2015) (Fig. 2). Regional neotectonics (upper Pliocene to Present) is evidenced by vertical movements at the regional scale (Dias and Cabral, 1997; Dias, 2001; Dias and Cabral, 2002a; Cabral, 2012; Figueiredo et al., 2013; Figueiredo, 2015; among others) and by several active faults. These are characterized by low, poorly constrained slip rates (0.02–0.1 mm/yr) and a generally rather subtle geomorphic expression, evidencing prevalence of reverse or strike-slip movement component (Dias, 2001; Dias and Cabral, 2002a,b; Cabral, 2012; Figueiredo, 2015; among others) (Fig. 2). The distribution of earthquakes in the study region evidences a scattering of epicentres, with a notorious concentration in the southern offshore. Several clusters and alignments may however be identified onshore (Carrilho et al., 2004; Custódio et al., 2015), although the correlation with the regional geological structure, particularly with the major mapped faults, is not

straightforward (Fig. 2). The dispersion of epicentres in the observed clusters and alignments may partially reflect uncertainty in the location of the earthquakes, although they clearly indicate a link to specific seismogenic sources which may be working as zones of distributed deformation. Most of the major regional faults that have been identified as active by geological evidence show poor correlation with the instrumental seismicity, apparently behaving as low activity, “silent” faults. The most conspicuous seismicity cluster has an approximately E-W trending elliptical shape coincident with the outcropping area of the Monchique intrusive massif, from where two lineaments extend, one towards SSW and another towards WNW (Fig. 2). Only the southern lineament may be correlated with a known major structure, the Portimão fault, where however Late Pleistocene to Holocene deformation has not been identified so far (Terrinha et al. 1999) (Fig. 2). The Monchique cluster is still poorly understood, though it may be explained by some rheological contrast that concentrates stress and promotes brittle failure, or by the interference of fluids in the crust (there is presently hydrothermal activity in the area). The SSW-NNE lineament apparently crosses the intrusive massif northwards and is interrupted by the proposed trace of the São Marcos-Quarteira fault (hereafter referred to as SMQF). To the NE of this fault there is another elongated cluster located in the Paleozoic basement, which may be correlated with a NE prolonging of the Ribeira de Telhares fault system (González-Clavijo and Dias, 2003) (Fig.2). The SMQF is a NW-SE trending structure generally described in the literature as extending for approximately 40 km from São Marcos da Serra (at the NW, in the Paleozoic basement) to Quarteira (at the SE, in the Meso-Cenozoic Algarve basin) (Figs. 1B, 3 and 5). It is interpreted as a major regional fault zone inherited from the Variscan Orogeny, which conditioned the evolution of the Algarve transtensional basin during the Mesozoic, separating it in two blocks that evolved distinctly. The fault was reactivated in the Cenozoic controlling the evolution of the regional landscape, as described below (Dias and Cabral, 1997; Terrinha, 1998; Dias, 2001; Carvalho et al.,

2006; Lopes et al., 2015; Ramos et al., 2015). Structural and geomorphic evidence point to PlioQuaternary reactivation, probably as a predominantly right lateral strike slip fault zone (Dias, 2001; Dias and Cabral, 2002a,b; Carvalho et al., 2006; Lopes et al., 2015).

Fig. 2 – Map of active faults and earthquake distribution in the study region for the period 1996– 2013. Seismicity according to Custódio et al. (2015). APF, Alentejo – Plasencia fault, CF, Carcavai fault, PF, Portimão fault, SMQF, São Marcos – Quarteira fault, STASF, São Teotónio – Aljezur – Sinceira fault, TF, Ribeira de Telhares fault system.

Although this structural feature shows a relatively apparent imprint on satellite imagery, it is difficult to recognize and to follow as a discrete fault trace on the ground, evidencing deformation distributed over a wide band (of circa 2 km) in the southern Mesozoic basin. Its

southeastwards prolonging to the offshore has been interpreted on a few seismic reflection lines for an additional 70 km, though its degree of activity and relationship with the onshore section are not yet clearly understood (Lopes et al., 2006, 2007; Terrinha et al., 2006, 2009; Noiva et al., 2010; Carvalho et al., 2012). At the NW, where it crosses the Paleozoic basement, it trends approximately parallel to the Variscan structural pattern rendering difficult its recognition at the outcrop scale. Despite the lack of outcrop evidence in this sector, its presence is also supported by a sharp lineament and related high magnetic gradient evidenced on a recently published geomagnetic map (Represas et al., 2016). The NW sector of the SMQF is thus poorly constrained, particularly at its northernmost part where it apparently interacts with the NE-SW trending Alentejo – Plasencia fault (Dias, 2001; Villamor et al., 2012; Figueiredo, 2015). However, the location and trend of the Mira River, flowing northwestwards approximately aligned with the SMQF and crossing the Alentejo – Plasencia fault, suggests that the river valley is a morphological evidence of the continuation of the former structure to the NW of the Alentejo – Plasencia fault (Fig.3). This rather speculative premise has long been matter of debate (e.g. Freire de Andrade, 1937; Feio, 1951). The weak geomorphic evidence supporting the crossing of the Alentejo – Plasencia fault by the SMQF and its prolonging northwestwards may be explained by the predominant strike-slip kinematics with no significant vertical displacement, a low rate of activity and/or possibly a rather young reactivation. Considering that the SMQF extends northwestwards to the Alentejo shoreline (Fig 3) it presents an additional 45 km length, thus totaling 85 km onshore. This, added to the 70 km in the southeastern offshore totalizes a length of 155 km for the fault zone. Three segments may thus be considered for this fault, namely a northwestern most segment, extending beyond the Alentejo – Plasencia fault, a central segment, and a southeastern, offshore segment, beyond the crossing with the NE-SW Carcavai fault (Ressurreição et al., 2011).

Fig. 3 – Hypsometric map of the study region with active faults (as in Figure 2). CR, Caldeirão Range, ECMR, Espinhaço de Cão – Mesquita Range

Regional uplift, as an expression of neotectonic deformation, is evidenced in the Algarve relief by a higher inland area which comprises two ranges: the Espinhaço de Cão – Mesquita range at the west, roughly consisting of a domed Neogene to Quaternary erosional surface cutting the Variscan basement and culminating at an altitude of around 400 m, above which rises the Monchique intrusive massif (top at 903 m), and the Caldeirão range at the east, also consisting of an elevated Cenozoic erosional surface carved on the Variscan basement (top at 589 m). These two mountains are separated by a NW-SE elongated depression controlled by the SMQF (Fig 3).

Uplift is also evidenced in the SW coastal area by raised planation surfaces and stepped marine terraces, with coastal cliffs exceeding 100 m in height, pointing to an uplift rate of ca. 0.1 mm/yr in the Pleistocene (Dias, 2001; Figueiredo et al., 2013; Figueiredo, 2015).

4

GPS data The GPS data used in our analysis are a combination of data acquired in episodic

campaigns and data from permanent stations. The campaigns were conducted for the entire Algarve region in 1998, 2000, 2001, 2002, and 2004, mainly in the scope of the GEOALGAR project (Carrilho et al., 2004), and more recently within the study area, in 2011 and 2015 (Fig. 4). The observations were carried out at specifically established markers installed either on bedrock outcrops or at monuments belonging to the Portuguese national geodetic network. In this case, a forced-centring piece was adapted to the top of the monument, in order to warrant proper centring and minimization of systematic errors due to antenna setup. The selection of the monuments and installation of the new markers was made taking in consideration the regional active geological structures, resulting in a dense network across those structures. Unfortunately some markers could not be reoccupied in the latest campaigns, as they were destroyed or became unusable due to urban expansion. For all the campaigns, with rare exceptions, a consistent set of geodetic-quality receivers and antennas was used.

Fig. 4 – Map of the GPS monitoring sites, with actives faults (as in previous figures).

Permanent stations have been installed in mainland Portugal by the Centro de Informação Geoespacial do Exército (CiGeoE, formerly Instituto Geográfico do Exército) and Direção Geral do Território (DGT). CiGeoE is responsible for a network of currently 28 stations designated as SERVIR, initially established in 2006, composed of a consistent set of receivers and antennas. DGT maintains a network of 41 stations for mainland Portugal, designated as ReNEP. The hardware used in this network is more heterogeneous. In total, there are 12 continuous monitoring stations covering the study area, operating for more than 7 years (indicated in Figure 4). The spatially dense network resulting from the combination of data from continuous and temporary stations

allows the determination of a more robust velocity field. The regional dataset used in the present study is specified in Table 1. Table 1 – Regional data used in the study (# - number of monthly solutions; start – date of first solution; end – date of last solution)

station SCAC BEJ0 BEJE MESS CERC MERT ODEM STEO PEFU ESPA FOIA PELA ZIMB CHLB MALH BENA CTEL MTLD GUIN ALFE MTQM FONT TVRA CATA DELG BARR MOSQ ALPO LAGO ATAL CERR SAGR FARO PTAT

long 351.3074 352.1272 352.1346 351.7553 351.2868 352.3400 351.3687 351.2717 351.2602 351.1531 351.4038 352.0383 351.6932 351.2000 351.1452 351.8733 351.4455 351.3711 351.7527 351.9388 351.1331 351.5024 352.3591 351.2516 351.4319 351.8374 351.7704 351.6148 351.3316 351.2812 351.0624 351.0404 352.0625 351.0685

lat 38.0188 38.0129 37.9982 37.8346 37.7898 37.6465 37.5987 37.5476 37.3675 37.3399 37.3158 37.3071 37.2401 37.2360 37.2349 37.2324 37.2066 37.1835 37.1786 37.1588 37.1578 37.1575 37.1323 37.1268 37.1252 37.1243 37.1167 37.1044 37.0989 37.0870 37.0476 37.0220 37.0164 37.0023

# 84 96 111 87 102 85 92 16 5 3 6 4 4 5 4 84 4 5 4 3 4 5 97 3 5 3 3 4 181 4 4 82 105 4

start 2008.91 2008.08 2006.83 2008.83 2007.25 2009.08 2008.25 1997.25 2001.83 2002.75 2001.83 2001.83 2001.83 2001.83 2001.83 2009.08 2001.91 2001.91 1998.75 1998.75 2001.91 2001.83 2008.08 2001.83 2001.83 1998.75 1998.75 2001.83 2000.33 2001.91 2001.91 2008.50 2007.33 2001.91

end 2016.00 2016.00 2016.00 2016.00 2016.00 2016.00 2016.00 2011.75 2015.25 2015.25 2015.25 2015.25 2015.25 2015.25 2015.25 2016.00 2011.75 2015.25 2015.25 2015.25 2011.75 2011.75 2016.00 2011.75 2015.25 2015.25 2015.25 2015.25 2016.00 2011.75 2011.75 2016.00 2016.00 2011.75

time span 7.09 7.92 9.17 7.17 8.75 6.91 7.75 14.50 13.42 12.50 13.42 13.42 13.42 13.42 13.42 6.92 9.84 13.34 16.50 16.50 9.84 9.92 7.92 9.92 13.42 16.50 16.50 13.42 15.67 9.84 9.84 7.50 8.50 9.84

network ReNEP ReNEP SERVIR ReNEP SERVIR ReNEP ReNEP GEOALGAR GEOALGAR GEOALGAR GEOALGAR GEOALGAR GEOALGAR GEOALGAR GEOALGAR ReNEP GEOALGAR GEOALGAR GEOALGAR GEOALGAR GEOALGAR GEOALGAR ReNEP GEOALGAR GEOALGAR GEOALGAR GEOALGAR GEOALGAR ReNEP GEOALGAR GEOALGAR SERVIR SERVIR GEOALGAR

5

GPS data processing The GPS data were analysed using the GAMIT/GLOBK software package (Herring et al.,

2015a,b). In the analysis we also included data of more than 300 global tracking stations from the International GNSS Service (IGS) (Dow et al., 2009) and the EUREF (Bruyninx et al., 2012) permanent networks, spanning the period 1998-2016. GPS data analysis followed the three-step, general mathematical approach outlined in Dong et al. (1998), and described in detail by Mendes et al. (2013). For GAMIT processing, the main settings are presented In Table 2.

Table 2 – Main data processing settings used in GAMIT. Observations Elevation cut-off angle Observations weight Antenna Phase Center Variations Neutral atmosphere refraction

Solid Earth Tides Ocean Tide Loading Orbits Earth Orientation Reference Frame

Double-differenced, ionosphere-free linear combination of L1 and L2 carrier phases 10° Elevation-angle dependent IGS absolute models for satellite and receiver antennas A priori zenith delays from GPT2 model (Lagler et al, 2013), mapped with the VMF mapping functions (Boehm et al., 2006); station zenith delays corrections estimated at each station at one-hour interval; station gradients parameters in North–South and East–West directions at 24-hour interval IERS Conventions (2010) Computed using the FES2004 (Lyard et al., 2006) ocean tide model. ESA/ESOC orbits, expressed in ITRF2008 (Altamimi et al., 2011) International Earth Rotation and Reference Systems Service (IERS) Bulletin B ITRF2008 (Altamimi et al., 2011)

Daily solutions obtained from data combination were aggregated into monthly solutions. The frame realization used to estimate the velocity field was defined by using generalized constraints, through the minimization of the horizontal velocity residuals of a large selection of sites and estimation of a 7-parameter Helmert transformation. The velocities resulting from this process, expressed in the ITRF2008 reference frame, are presented in Table 3.

Table 3 – Velocities for the SW Iberia region (Algarve), expressed in ITRF2008. E, N, and U represent the East, North, and Up components of velocity, and E, N, and U are the respective uncertainties, at the one-sigma level (velocities and uncertainties are expressed in mm/yr). Stations in bold are continuous monitoring sites. station ALFE

E 17.40

N 17.45

E

N

0.13

U -0.09

0.19

0.65

ALPO

17.18

17.56

0.22

0.15

-2.31

0.71

ATAL

17.40

17.59

0.29

0.18

-0.82

0.84

BARR

17.71

17.60

0.19

0.15

-0.83

0.75

BEJ0

17.74

17.48

0.09

0.05

0.07

0.22

BEJE

17.93

17.02

0.07

0.04

-1.24

0.18

BENA

17.36

16.62

0.11

0.06

-0.19

0.27

CATA

16.24

17.97

0.30

0.20

0.66

0.92

CERC

17.82

17.15

0.08

0.04

-1.69

0.20

CERR

17.70

17.22

0.28

0.17

-0.08

0.79

CHLB

17.10

16.34

0.20

0.15

0.84

0.72

CTEL

17.25

17.41

0.28

0.17

-0.24

0.77

DELG

17.59

16.48

0.21

0.14

-1.36

0.62

ESPA

17.29

17.63

0.30

0.24

-1.00

1.18

FARO

17.28

17.04

0.08

0.04

-1.35

0.20

FOIA

17.36

17.02

0.16

0.09

-0.09

0.44

FONT

17.31

17.22

0.26

0.16

-0.96

0.72

GUIN

17.55

17.57

0.19

0.14

0.32

0.67

LAGO

17.25

17.35

0.06

0.03

-0.55

0.15

MALH

18.11

16.62

0.24

0.15

0.61

0.66

MERT

17.62

17.57

0.21

0.11

-0.33

0.54

MESS

17.83

17.18

0.10

0.05

-0.56

0.26

MOSQ

17.03

17.89

0.20

0.16

-1.78

0.76

MTLD

16.20

16.66

0.20

0.14

0.12

0.65

MTQM

16.38

17.22

0.29

0.18

-2.46

0.84

ODEM

17.97

17.79

0.19

0.10

-0.88

0.48

PEFU

17.17

17.21

0.19

0.13

-1.09

0.63

PELA

17.94

16.98

0.23

0.18

0.58

0.90

PTAT

17.14

17.29

0.28

0.18

-0.47

0.86

SAGR

17.28

17.76

0.10

0.05

-2.18

0.26

SCAC

17.47

17.02

0.18

0.09

-0.81

0.46

STEO

17.15

17.11

0.14

0.07

2.67

0.36

U

TVRA

17.93

17.24

0.15

0.08

-0.72

0.38

ZIMB

17.14

17.05

0.21

0.14

-0.91

0.63

A matter of concern is the effect of temporal correlation in the estimates of the uncertainties of the derived velocity field (e.g. Mao et al, 1999; Santamaría‐Gómez et al., 2011). In order to judge their suitableness, we compared the uncertainties derived from our GLOBK solution with those obtained from the analysis of the daily time series of GPS continuous sites in the study area, using a combination of Generalized Gauss Markov (GGM) with white noise (WN) as stochastic model. On average, the uncertainties derived from GLOBK are similar to those obtained from the combination GGM+WN for the East component (a ratio of 0.95±0.36); as regards the North component, GLOBK uncertainties are higher (ratio of 1.46±0.55), showing therefore their adequacy to represent realistic uncertainties for our velocity estimates. Figure 5 shows the horizontal velocities relative to Eurasia as predicted by the ITRF2008PMM model (Altamimi et al., 2012), to be consistent with the reference frame used to generate the velocity field solution. The residual velocities unveil a relatively consistent pattern towards WNW, with magnitudes that noticeably increase from NNE to SSW. These results are generally coherent with the velocity field presented in Palano et al. (2015) for the study area, though the considerably larger number of stations of this study allows the detection of some regional trends that are not expressed in the aforementioned work.

Fig. 5 – Map of estimated horizontal velocities relative to Eurasia, as predicted by the ITRF2008PMM model (Altamimi et al., 2012). Shaded area, postulated right lateral shear zone; velocities in blue report to campaign-style GPS stations; velocities in black report to permanent stations; velocities in red are weighted average velocities for the sectors located NE and SW of the shear zone, based on the permanent stations; error ellipses at 95% confidence level.

6

Data interpretation and discussion The interpretation of the obtained velocity field map (Fig. 5) is somewhat biased by the

spatial distribution of the monitoring sites, which corresponds to a higher number of stations in the southwestern region of the studied area relatively to the few points located to the N and NE. This uneven distribution was due to several factors that conditioned the number and location of the stations for the timeframe of the present study – in addition to a higher number of recognized Quaternary active faults in the SW sector leading to design a denser GPS monitoring network for

their characterization, factors like site accessibility and logistics were also determinant on the stations distribution. Also, in spite of efforts to maintain stations functional throughout the period of time covered, several stations in the NE sector were lost due to residential and commercial development at the area. However, the consistency of the obtained GPS velocity results allows detecting some trends with significant meaning for the regional geodynamic framework. The resulting velocity dataset does not evidence a sharp velocity gradient, although the velocities on the SW sector are on average slightly larger and rotated counterclockwise (WSW trend) relatively to the fewer velocity vectors located to the N and NE, which are smaller and directed more northerly (NW to NNW trend). The difference of velocity between the NE and SW sectors becomes clearer when the average velocity for the NE sector is removed (Fig. 6) suggesting the presence of a NW-SE crustal shear zone separating the two domains, which may be slowly accumulating a slightly transtensional right-lateral shear strain.

Fig. 6 – Residual GPS velocities after removing the average velocity of the NE sector.

A transect line crossing orthogonally the suggested shear zone (Fig. 7) further supports the proposed transtensional right-lateral kinematics.

Fig. 7 – Residual GPS velocities (top) and respective profile of the parallel (middle) and perpendicular (bottom) components along a transect orthogonal to the postulated shear zone. Also shown the weighted least squares line fit to those components. Distances are plotted using point A as reference (positive towards the NE direction). Slope in mm/yr/km.

The postulated shear zone shares a similar trend with the SMQF and roughly coincides with its northern sector, suggesting that strain accumulation on the shear zone may dynamically affect the fault promoting its tectonic activity. Despite the fact that there are no known major historical earthquakes associated with the fault, it shows some control on the regional seismicity pattern, apparently working as the NE boundary of an area of higher seismicity in western Algarve (Fig. 2). As such, it seems to play a significant role in the regional seismicity and very likely in the regional stress accommodation. Based on the computed velocity component parallel to the shear zone (strike component) and considering the slope of the corresponding best fit line (Fig. 7), a shear strain rate accumulation

across the shear zone of ca. 0.93–1.09 x 10-8 yr-1 can be estimated. For the

perpendicular component, a strain rate of 4.3–5.6 x 10-9 yr-1 can similarly be obtained, which is one order of magnitude smaller than the shear strain. This is in good agreement with strain rates typical of intraplate environments (on the order of 10-8 yr-1) and one order of magnitude lower than typical interplate strain accumulation rates (of 10-7 yr-1) (Kanamori and Brodsky, 2001). The small strain rate accumulation on the shear zone, which is located at a near plate boundary environment, may be explained by the relatively low Iberia-Nubia convergence velocity in the Gulf of Cadiz region and the spreading of deformation across a wide plate boundary zone.

7

Conclusions The GPS velocity pattern obtained in the present study indicates a relatively consistent

WNW motion, generally coherent with the velocity field presented for the study area by Palano et al. (2015). However, the considerably larger number of velocity points and the longer observation period considered in the present work allowed detecting additional regional trends that were not

expressed in past studies, which carry relevant significance for the understanding of the regional tectonics. The increment of GPS data points in the SW area of the Iberian Peninsula in comparison to the database used by Palano et al. (2015) revealed a significant velocity gradient between the southwestern-most area of the Portuguese territory and the region located to the NE, which was not previously expressed, suggesting the presence of a shear zone between these two regions. The crustal straining inferred from the GPS data across the proposed shear zone will presumably promote rupture of any favorably oriented pre-existent crustal discontinuity located along it or in its vicinity, and most likely of the SMQF. Although aseismic, creep like slip cannot be discarded, some considerations can be made concerning the seismogenic potential of the SMQF. Applying a magnitude-length scaling regression, as by Wells and Coppersmith (1994) or Wesnousky (2008), we estimate that this major regional structure has the capacity to generate an earthquake with surface rupture and magnitude ranging from Mw 6.9 to 7.5, depending if it ruptures one or more segments. Considering that the coseismic strain drop associated with large earthquakes has been estimated in the range of ca. 3 x 10-5 to 3 x 10-4 (Kanamori and Brodsky, 2001), we may thus infer that for the above referred strain rate accumulation in the shear zone, a recurrence of about 103-104 years is expected for a large earthquake generated by a newly reactivated structure, or by the SMQF in particular. The ongoing subtle crustal deformation, as evidenced by the GPS data presented in this study thus supports that the SMQF fault is slowly accumulating strain, therefore corresponding to a major regional active structure with the potential of generating a large magnitude earthquake. Considering that there are no large earthquakes that can be associated to this fault in the historical records (which spans a period of ca. 2,000 years; Custódio, 2015, and references

therein), we may conclude that the fault zone has been dynamically charged for a relatively long period increasing the hazard level for a possible near future seismic rupture. As Palano et al. (2015) pointed out when discussing their model of clockwise rotation of the Iberian Peninsula with respect to Eurasia, the WNW differential motion of a southwestern rigid lithosphere block, which we propose to be partially accommodated by the postulated shear zone, implies significant shortening (and some left-lateral shear) along the southwestern Portuguese margin. This shortening is likely being distributed across major regional faults located to the W, as the São Teotónio – Aljezur – Sinceira fault near the western shoreline (Figs. 2 and 3) (Figueiredo, 2015, Figueiredo et al., 2010, 2013, 2016), and offshore, as the Gorringe Bank and the Marquês de Pombal faults (e.g. Zitellini et al., 1999, 2001, 2004; Gràcia et al., 2003; Terrinha et al., 2003) (Fig. 1A), which would be currently accumulating significant inter-seismic strain. Another relevant consequence of the WNW motion of a SW lithosphere block is a foreseen increase of SHmax offshore SW Portugal, which may promote subduction initiation in the area, as has been proposed several authors, based on the pattern of deflection of stress strain trajectory around the rolling buffer micro-plate of Iberia (Ribeiro, 2002; Duarte et al., 2013, and references therein). The present work and the obtained results highlight the relevance of using GNSS-based studies to constrain tectonic activity in slow deforming regions as the southern Portuguese mainland. Although requiring relatively long-time series, these studies allow inferring relevant kinematic data for the characterization of the regional geodynamical setting, with implications on the regional fault activity and seismic hazard. Acknowledgments This study was supported by Fundação para a Ciência e a Tecnologia (FCT) through project PTDC/GEO-GEO/2860/2012 (FASTLOAD), and by Instituto Dom Luiz (Associate Laboratory, IDL). Figures 4 and 5 were created using Generic Mapping Tools (Wessel et al., 2013). The HPC facilities

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