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Tectonophysics 570–571 (2012) 151–162
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Tectonophysics
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Connecting onshore structures in the Algarve with the southern Portuguesecontinental margin: The Carcavai fault zone
João Carvalho a,⁎, Hugo Matias c,d, Taha Rabeh b, Paulo T.L. Menezes e, Valeria C.F. Barbosa f,Ruben Dias a, Fernando Carrilho g
a Laboratório Nacional de Energia e Geologia, Alfragide, Portugalb National Research Institute of Astronomy and Geophysics, Helwan, Cairo, Egyptc Repsol-Ypf, Madrid, Spaind Instituto Dom Luis, Lisbon, Portugale DGAP/FGEL/UERJ, Brazilf MCT-ON, Brazilg Instituto de Meteorologia, Rua do Aeroporto, Lisboa, Portugal
⁎ Corresponding author at: Estrada da Portela, ApartaAmadora, Portugal. Tel.: +351 21 470 5515; fax: +351
E-mail addresses: [email protected] (J. Carvalho(H. Matias), [email protected] (T. Rabeh), ptarsom(P.T.L. Menezes), [email protected] (V.C.F. Barbosa), [email protected] (F. Carrilho).
0040-1951/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.tecto.2012.08.011
a b s t r a c t
a r t i c l e i n f oArticle history:Received 21 December 2011Received in revised form 22 June 2012Accepted 8 August 2012Available online 21 August 2012
Keywords:Carcavai faultSeismic reflectionAeromagneticGravimetrySeismicityActive fault
The Algarve is located a few hundred kilometres north of the crossing of the E–WEurasia–Africa plate boundaryand is characterised by a moderate seismicity, with some important historical and instrumental earthquakescausing loss of lives and significant material damages. The area is affected not only by plate boundary generatedearthquakes but also by local events capable of generating moderate to large earthquakes. The assessment ofonshore local sources and its connections with the plate border is therefore of vital importance for an evaluationof the regional seismic hazard. This paper discusses the application of geophysical data to study a large fault zonewhich is the offshore prolonging of the Carcavai fault zone (CF), an onshore outcropping structure more than20 km long which is seen to deform sediments of Plio-Quaternary age. Offshore and onshore aeromagneticdata, offshore gravimetric and seismic reflection data shows the existence of a long (over 200 km) WSW–ENEtrending fault zone affecting the Palaeozoic basement with a normal geometry which is probably segmentedby NNW–SSE to N–S faults. Seismic data shows that this fault zone has been reactivated as a left-lateralstrike-slip fault and inverted in the Cenozoic with the upthrust of the northwestern block, in agreement withthe onshore CF characteristics. Recent work carried out onshore and offshore near the coastline that showsdeformation of Plio-Quaternary sediments suggests that this is an active fault. Some of the faults segmentshave instrumental seismicity associated. Though faults very rarely rupture along its entire length, several faultsegments have a length of about 30 km and may produce an earthquake of magnitude about7. The proximityof the onshore segment to the city of Faro and of the offshore segments to the main population centres of theAlgarve makes it a serious threat to the Algarve.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
The Algarve province of Southern Portugal is located close to the ap-proximately E–WEurasia–Africa plate boundary (about 200 km)whichwesternmost part extends from the Azores islands to Tunisia, from30°W to 10°E. The Azores region forms a triple junction with ridgestructure and oblique spreading at each three branches (e.g. Bufornand Udias, 2010), while East of the Açores until a longitude of about
do 7586- Alfragide, 2610-99921 471 9018.), [email protected]@[email protected] (R. Dias),
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12°W, where the Gorringe Bank is located, the movement betweenthe two plates is accommodated by the dextral strike-slip GloriaFault (e.g. Buforn et al., 1988). To the East of the Gorringe Bank till theWestern part of Algeria (3.5°E), the plate boundary becomes more dif-fuse and forms a wider area of deformation (e.g. Buforn et al., 1988,2004; Sartori et al., 1994). Gutscher et al. (2002) have proposed an activesubduction zone below the Gibraltar Arc, while Zitellini et al. (2009)have postulated a transpressive limit – the SWIMFault system–betweenthe Gloria Fault and nortwestern Marocco where they meet thetranspressive, seismogenic limit of the orogenic Rif-Tell system (Moreland Meghraouie, 1996).
In this sector between the Gorringe Bank and the Arc of Gibraltar(5°W), where the study area is located, the general stress pattern cor-responds to an approximate NW–SE compression caused by the con-tinental collision of Africa and Iberia (Buforn et al., 1988, 1995;
152 J. Carvalho et al. / Tectonophysics 570–571 (2012) 151–162
Grimson and Chen, 1986; McKenzie, 1972; Rosas et al., 2008; Udias etal., 1976; Zitellini et al., 2009). Here, the plate boundary correspondsto an ~E–W trending, dextral transpressive deformation zone associ-ated with an oblique convergence of the plates at approximately4 mm/yr in the NW–SE direction (DeMets et al., 1990, 1994;Jimenez-Munt and Negredo, 2003).
According to other models, the Western Iberian margin may be ina transition state from a passive to compressive, eventually associatedwith the nucleation of a subduction zone (e.g Ribeiro, 2002; Ribeiro etal., 1996) but no evidence of this subduction zone has been found sofar (Borges et al., 2001; Stich et al., 2005 in Buforn and Udias, 2010).
This tectonic setting is responsible for an important regionalneotectonic and seismic activities that are producing Pliocene to Pleis-tocene deformation and a significant seismicity (Fig. 1), presenting ascenario of important seismogenic potential (Dias, 2001; Dias andCabral, 2002). Several important earthquakes affected some of themajor cities of this portuguese province (Carrilho et al., 1997), causingthe loss of lives and significant material damages. These earthquakes,which occur both inland and offshore, have been reported since 63 BC.Offshore, tsunamigenic earthquakes have been reported several times
Fig. 1. a) Study area location and seismicity for the period 1958–1998 (source: Instituto deMetAljezur–Sinceira–Ingrina fault; SQF: S. Marcos–Quarteira fault; PF: Portimão fault; LF: Loulé fafault; 5 — fold axis. b) Schematic geological map of Algarve (adapted from Oliveira et al., 19926— Paleozoic; 7 —Monchique intrusive massif; 8— dyke; 9 — fault.
(in Carrilho et al., 2004), such as the well known 1755 Lisbon earth-quake, which occurred S or SW of Algarve. Inland, earthquakes haveproduced intensities up to XMM (ModifiedMercalli scale). The historic1856 Loulé earthquake epicentre is currently located close to the studyarea.
Though the present-day instrumental seismicity indicates that themain seismogenic sources are located offshore (Carrilho et al., 2004),some onshore zones present an important seismicity, such as theMonchique-Portimão, the Albufeira-Faro-Loulé and Tavira-V.R. Sto.António areas (Dias, 2001). While the prolongation offshore of majorland fault zones till the plate boundary has been suggested by severalauthors (Borges et al., 2001; Buforn et al., 1988; Jimenez-Munt andNegredo, 2003; Moreira, 1985) to explain the observed seismicity, theconnection between the plate border and the inland regional structuresas the Portimão or S. Marcos–Quarteira fault, have been recently beenthe focus of several studies (e.g. Terrinha et al., 2009). The Carcavaifault zone (Fig. 1a), a regional onshore geological structure in the areaand a probable source of the Loulé earthquake (e.g. Carvalho et al.,2012; Ressureição, 2009; Ressurreição et al., 2011), was recently con-firmed offshore close to the coastline (Noiva, 2009; Noiva et al., 2010),
eorologia) overlaid to an active fault map (after Dias and Cabral, 2002). SASIF: S. Teotónio–ult; CF: Carcavai fault. 1 — hidden fault; 2 — thrust fault; 3 — strike-slip fault; 4 — normal). 1 — Quaternary; 2 — Pliocene–Pleistocene; 3 — Miocene; 4 — Paleogene; 5 — Mesozoic;
153J. Carvalho et al. / Tectonophysics 570–571 (2012) 151–162
This paper presents results from a study of the offshore prolongingof the Carcavai fault zone in the Portuguese Southern margin and itsconnection with the plate boundary using seismic reflection, gravimet-ric and magnetic data. The study area location is shown in Fig. 1a.
The seismic reflection method is probably the most suited geo-physical method for geological fault mapping and has been appliedto fault location and characterisation several times (e.g. Benson andMustoe, 1991; Carvalho et al., 2006; Harris, 2009; Mair and Green,1981; Miller et al., 1990; Shtivelman et al., 1998; Treadway et al.,1988; Wang, 2002; Williams et al., 2001). Potential Field data hasalso often been used for identifying faults at depth (e.g. Carvalho et al.,2008; Carvalho et al., 2011; Matias, 2007; Matias et al., submitted forpublication; McPhee et al., 2004; Rabeh et al., 2009; Valentino et al.,2012).
In this work, the TGS-NOPEC offshore gravimetric data recentlyacquired in 2002 and a reprocessed aeromagnetic survey from 1969have been used for the above mentioned purpose. Gravimetric 3D and2D3/4 modeling calibrated by well and seismic data were performed(Matias, 2007; Matias et al., submitted for publication) and later, mag-netic 2D Euler deconvolution and the horizontal gradient methodshave been applied to confirm the previous results. Afterwards, offshoreseismic reflection data from the oil industry acquired from 1974 till2002 was used to confirm the location, imaging the structure andcharacterise it. The assessment of the seismogenic potential of the off-shore Carcavai's fault zone is carried out for future seismic hazard analy-sis. The connection of this structure in the Portuguese Southern marginwith the Azores-Gibraltar plate boundary is also investigated usingbathymetric data and its relation with other recently proposed offshorefault zones is discussed.
2. Geological setting
The regional geology of the Algarve is comprised of Palaeozoicbasement rocks, flysch sequences of slates and graywackes (foldand faulted during the Variscan orogeny) that outcrop in the northernarea, and Mesozoic and Cenozoic rocks of two superposed sedimenta-ry basins in the south (Fig. 1b). The Palaeozoic basement is intrudedin northwestern Algarve, at Monchique, by an igneous intrusive mas-sif of Upper Cretaceous age (Fig. 1b). It shows an approximately ellip-tical shape in outcrop, elongated in the E–Wdirection, about 16 km inlength and 6 km in width and rising as an inselberg above the upliftedregional erosion surface (Terrinha, 1998).
TheMesozoic rocks, dating fromUpper Triassic to Lower Cretaceous,are comprised mainly of continental siliciclastic and marine carbonatesediments that are deposited in a basin developed in a transtensional re-gime related to the opening of the Tethys Sea and the Central AtlanticOcean (Terrinha et al., 1998). The Cenozoic basinwas formed by flexuralprocesses associated with the collision of Africa and Iberia (Terrinha,1998; Terrinha et al., 1998) and ismainly composed ofmarine carbonateand siliciclastic sediements.
The regional neotectonic activity is evidenced by vertical crustalmovements, as well as by brittle deformation structures (comprisingseveral macroscale and mesoscale active faults and a large number ofjoints), by ductile deformation structures (represented by folds), andby soft sediment deformation structures affecting the Pliocene toQuaternary sands (Dias, 2001; Dias and Cabral, 2002).
The Algarve region is cut by several regional active faults. TheS. Teotónio–Aljezur–Sinceira–Ingrina fault, the Portimão fault, theS. Marcos–Quarteira fault, the Loulé fault and the Carcavai fault areamong the most important (Fig. 1b). The main populated areas,Faro and Portimão, are located tens of kilometres apart, close to theS. Marcos–Quarteira and Portimão faults, respectively (Fig. 1b). Theoffshore prolonging of the first three structures has been recognisedby a few authors (Borges et al., 2001; Noiva, 2009; Noiva et al.,2010; Terrinha et al., 2009).
The Carcavai fault zone, which is the focus of our work, is a NE-SWtrending outcropping complex structure extending between S. Brás deAlportel (North) and Quarteira (south), with a total length of about20 km, presenting reverse fault geometry with left-lateral strike-slipcomponent (Dias, 2001; Dias and Cabral, 2002; Ressureição, 2009;Ressurreição et al., 2011). It is located close to the city of Loulé anddifferent sectors have been identified from surface geological studies(op. cit.). The northern sector is characterised by several faults withNE–SW to E–NE–E–SW segmented by NW–SE trending faults.According to some authors (Manuppella et al., 1992), the fault may pro-long further northeast then the presently mapped course, extendingfrom S. Brás de Alportel to the Guadiana river region (the border withSpain, see Fig. 1). In the southern sector, trendingNE-SE, there are severalevidences of neotectonic deformation in sediments of Pio-Quaternaryage. It is therefore considered to be an active fault zone and a probablesource of the 1856 Loulé earthquake (intensity of VIII-MM). Itsprolonging offshore has recently been studied close to the coastline(Noiva, 2009; Noiva et al., 2010).
The offshore prolonging of other major onshore structures in Ibe-ria has also been proposed, like the Messejana Fault, the Nazaré faultor the Lower Tagus Valley Fault (e.g. Buforn et al., 2004; Moreira,1985). In the Algarve, as referred above, several onshore faults havebeen prolonged offshore but their relation with the plate border isstill to be determined.
Further offshore, in the western and southern Portuguese margins,several large fault zones have been recognised in the last years frombathymetric and seismic reflection data (Duarte et al., 2010; Rosaset al., 2009; Terrinha et al., 2009; Zitellini et al., 2009). The Marquêsde Pombal fault, the Horseshoe Thrust Fault, the Gorringe Fault, theCoral Patch Fault, the SWIM faults and the Accretionary WedgeDefomation Front (AWDF) are the major structures identified (op.cit.). The structures agree with focal mechanisms and other stress in-dicators determined for the area (e.g. Borges et al., 2001; Buforn et al.,2004; Geissler et al., 2010).
3. Identification of an offshore large fault zone inaeromagnetic data?
3.1. Available aeromagnetic data
In 1969 Fairey Surveys Ltd. flew an aeromagnetic survey over thewest and south portuguese margins for the oil industry with a flightaltitude of 600 m. The flight lines had an E–W orientation and anapproximate 2 km spacing, with some trend lines randomly orient-ed. This survey was only available in map form and has beendigitised from the crossings of the flight lines with the 2.5 nT inter-val contour lines. Therefore we cannot expect a precision higherthan 5 nT for this survey. The original IGRF model was replaced bythe 1990 model and the data was reduced to the pole (seeCarvalho (1995) for a full description of the digitising andreprocessing of this dataset).
This data set was merged with another aeromagnetic survey flownin 1991 by Geoterrex for the mining company Rio Tinto at a flight al-titude of 90 m (Carvalho, 1995). The flight lines were spaced 500 mand SW–NE oriented with perpendicular tie lines. The Fairey surveywas then analytically prolonged downwards using the software pack-age of Cordell et al. (1992) to the Geoterrex flight altitude, DC sift wascorrected and the both surveys adjusted by comparing data pointsand grids in the overlapping areas of the two surveys (id). Theresulting survey covers the Algarve totally and the adjacent offshoreregion (Fig. 2).
We can observe that the possible offshore extension of theCarcavai fault zone onshore course (marked CF in Fig. 2) is coincidentwith a clear alignment of the magnetic anomalies (dash line in Fig. 2),suggesting a possible extension of the fault zone for several tens ofkilometres. The prolonging of the fault zone offshore close to the
Fig. 2. Aeromagnetic anomaly map of southern Portugal (including Algarve andadjacent offshore area) resulting from the joining of the Fairey Surveys and Geoterrexsurveys (see text) overlaid by a fault map shown in Fig. 1b (after Oliveira et al., 1992).Anomalies have been reduced to the pole. Dash line indicates possible prolongingoffshore of the Carcavai fault. The location of the magnetic profiles shown in Fig. 3 isindicated by black heavy lines. SASIF; MF; PF; SQF; CF — please see Fig. 1 caption.The back ellipses indicate location of faulting associated with the offshore prolongingof the Carcavai fault shown in Fig. 3.
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coastline has recently been confirmed with seismic reflection data(Noiva, 2009; Noiva et al., 2010).
3.2. 2D Euler deconvolution and horizontal gradient analysis
This possibility leads us to further interpret the aeromagnetic datato confirm the fault zone. Four 2D profiles were selected, two offshoreover the suspected course of the fault zone and two other onshore(see location in Fig. 2). The corresponding 2D Euler deconvolutioncross sections were produced (Fig. 3) and the horizontal gradientwas also calculated for these profiles.
The Euler deconvolution method (Thompson, 1982) attempts toestimate the magnetic sources and contrasts depths and locationseither in profile (id) or gridded data (Reid et al., 1990). The measureof the rate of change of the magnetic field with distance is indicatedby a structural index. Several structural indexes can be used, dependingon the geological structures that we are looking for in our data. As thevalue of N increases it produces progressively deeper solutions thatmust be analysed in conjunction with available geological data. A stepindex represents well the sharp magnetic contrasts produced by adip-slip fault. In the Euler deconvolution interpretation carried out inthis work a magnetic step indexes of 0.5 km were used.
Gradient analysis is also often used to locate sharp changes in thedepth of the magnetic sources and/or the magnetic properties thatcan be produced by geological faults (e.g. Hansen and deRidder,2006). Here we have applied the horizontal gradient to the reducedto the pole magnetic total field, which is quite effective for faultdetection (e.g. Grauch and Hudson, 2007; Phillips et al., 2007).
The results of these interpretations are shown in Fig. 3. Major faultzones are marked where the discontinuities of Euler solutions and thepeaks of the gradient curves are coincident. All three profiles showseveral important discontinuities which we can relate with faults origneous bodies. A large discontinuity(ies) in the Euler solutions canbe observed at the expected location of the Carcavai fault zone in allprofiles (signalled with CF in Fig. 3). In profile 1, the Carcavai faultis located offshore in the southeastern point of the profile. The profile
Fig. 3. Four magnetic profiles (see location in Fig. 2) interpreted with horizontal gradient medeconvolution method, using a step index of 0.5 km (bottom panels). All profiles show the pand other well known fault zones (see caption of Fig. 2). Profiles 1 and 2 show CF offshoreplease see Fig. 1 caption; HVSC — Hettangian volcano–sedimentary complex.
shows an uprise of the Euler solutions and two discontinuities at bothends of the area where the possible location of the Carcavai fault isexpected. Two peaks of the horizontal gradient are also visible inthe same area. The profile also shows other important discontinuitiesand coincident peaks of the horizontal gradient curve: the central partof the profile depicts the most important discontinuities that are veryprobably associated the onshore presence of the S. Teotónio–Aljezur–Sinceira–Ingrina fault system (SASIF, Figs. 2 and 3). The presenceoffshore of the Messejana Fault is also recognizable close thenorth-western part of the profile (MF, Figs. 2 and 3).
The Carcavai fault in profile 2 is still located offshore, in thenorth-western part of the profile close to its centre. It correspondsagain to a region where the Euler solutions are shallower and thelimits of the area marked by strong discontinuities of the Euler solu-tions and coincident peaks of the horizontal gradient. The northwest-ernmost is particularly striking. Further northwest, two other faultsmarked in the profile coincide approximately with two cartographedfaults.
Profile 3 shows theCarcavai fault zoneonshore, approximately in thecentre of the profile. Again the fault location zone corresponds toshallower Euler solutions but only the southeastern limit of the areacoincides with a sharp change of the horizontal gradient. Two other sig-nificant discontinuities in Euler solutions that are coincident with im-portant changes in the gradient curve (in particular the northwesternone) are visible at the centre of the northwestern and southeasternsections of the profile. The northwesternmost does not correspond toany known outcropping fault but the southeasternmost correlateswith an approximately NNE–SSW trending fault.
The last profile covers an area where the position of the Carcavaifault is not known but its prolonging further to NE is suspected(Ressureição, 2009; Ressurreição et al., 2011). The northwestern sec-tion of the profile shows three possible faults whose location matchan E–Wand two NW–SE oriented faults outcropping in the Palaeozoicterrains (Oliveira et al., 1992). The central part of the profile is locatedin an area where a system of NE–SW trending faults affecting thePalaeozoic basement outcrop (only part of the faults that constitutethis fault system are drawn in Fig. 2). Most of these structures aresub-vertical strike-slip faults that cross a relatively homogenous col-umn of schists and greywackes (e.g. Oliveira et al., 1992) and thereforepresent a reducedmagnetic signature. For this reason some of them arenot recognizable in the gradient curve but their presence is identified bydiscontinuities in the Euler solutions.
One of these faults relates to a significant change of the gradientcurve (close to the centre of the magnetic profile) because it was in-truded by basic igneous rocks (Manuppella et al., 1992). Furthersoutheast, three structures are interpreted in profile 4. The southeast-ernmost is located close to the coastline and is a hidden unknownfault. Further northwest, the interpreted structure corresponds tothe Hettangian volcano–sedimentary complex that borders the Algar-ve Basin (id.).
The last of these three structures corresponds to the location ofthe above mentioned NE-SW fault system; it is associated to a cleardiscontinuity in the Euler solutions and a peak of the horizontal gra-dient. We interpret this fault as a possible prolonging of the Carcavaifault zone due to the similarity of the Euler solutions that also areshallower in this area and also by topographic indications but furtherstudies era required to confirm this possibility. The results obtainedhere encouraged us to examine other available data sets, gravimetricdata and seismic reflection profiles, and look for the signature of thefault zone offshore, characterise it and compare it with onshore out-crop data of the Carcavai fault zone.
thod (top panels, solid line — total field; dash line — horizontal gradient) and the Eulerresence of a possible fault zone at the expected location of the Carcavai fault zone (CF)while 3 and 4 cross the fault onshore. Only in profile 3 the fault outcrops. SASIF; MF —
156 J. Carvalho et al. / Tectonophysics 570–571 (2012) 151–162
4. Gravimetric and seismic reflection data interpretation
4.1. TGS Nopec survey acquisition parameters
During 2002 TGS Nopec acquired a large gravimetric and 2D seis-mic reflection survey offshore Portugal (PDT00 and PD000 cruises),including the Algarve basin. This was an excellent data set to provideus new insights into the Carcavai fault zone prolongation offshore.The quality of the seismic data could provide vital informationabout the period of the faults activity and possibly estimate throwsin the Quaternary.
The gravimetric dataset used in this study consists of a high reso-lution marine gravity survey. This dataset was acquired simulta-neously with the seismic reflection data. A total number of 19,894line km of data, along 270 lines, were acquired over the offshorePortuguese platform area. The dataset used herein corresponds to asubset of 58 lines, with a grid spacing of around 8×4 km and a sam-pling distance of 25 and 30 m. TGS-NPEC provided the gravity data inthe form of Bouguer anomalies in xyz ASCII format, later gridded witha 2 km grid spacing cells. The residual–regional Bouguer anomalymap is shown in Fig. 4a.
Fig. 4. a) Residual of the Bouguer anomaly map of offshore Algarve calculated from theTGS-Nopec Survey (after Matias, 2007; Matias et al., submitted for publication) withidentification of the magnetic lineament that could correspond to the offshoreprolonging of the Carcavai fault zone (dash line). Symbols indicate deep well location.PF, SQF and CF see Fig. 1 caption. b) Top of basement map obtained after 3D inversionand 2.5D forward gravimetric modelling calibrated by seismic and well data afterMatias (2007) and Matias et al. (submitted for publication). The offshore prolongingof the Carcavai fault zone is also shown in thick solid line after aeromagnetic data in-terpretation (aeromagnetic anomaly contours −10 nT — shown in black thin lines).Lines A, B, and C indicate 2.5D modelled profiles used to confirm the basement of the3D inversion.
4.2. Gravimetric data interpretation
Several interpretation techniques, such as vertical derivatives andanalytical signal, were previously carried out to this map with thepurpose of identifying major fault zones (Matias, 2007; Matias et al.,2005, submitted for publication). To refine the techniques of detectingfaults, the data was processed to highlight subtle lineaments. Thisapproach aimed at providing some insight on a) the fault pattern;b) distribution of gravity anomalies and sources and geometry of thestructures; and c) allowing the differentiation of salt and basement“signatures”. After this semi-quantitative analysis, 3D gravity inver-sion was performed to estimate the basement relief. Finally, a moredetailed gravimetric modelling was carried out, through 2 3/4D for-ward modelling, along three regional profiles in the basin. This proce-dure led to a better understanding of the fault pattern, salt distributionand basement structure within the offshore Algarve Basin (Matias,2007; Matias et al., 2005).
4.2.1. 2D map view techniquesGravity imaging of subtle potential field signatures of faults and
fractures requires detailed data processing, using a wide variety ofanomaly-enhancement techniques and display parameters. We haveused here previous work carried out by Matias (2007) and Matias etal. (submitted for publication). The processing included, the residu-al–regional separation, application of vertical derivatives transforma-tion and analytical signal method (Bastani and Pedersen, 2001;Debeglia and Corpel, 1997; Emilia, 1973; Gunn, 1975; Hsu et al.,1996; Hsu et al., 1998; Nabighian, 1972; Thurston and Smith, 1997;Thurston et al., 2002; Zhang, 2001) using INTREPID © processing soft-ware (id).
4.2.2. 3D gravimetric inversionIn order to obtain a structural of the top of Paleozoic metamorphic
and igneous rocks, to identify Hettangian salt trends and thereforelocate major fault zones in the basement we have used 3D and 2 gravi-metric modelling also carried by Matias (2007) and Matias et al.(submitted for publication). This modelling was calibrated by seismicreflection data and well data (see following section for seismic data in-terpretation). The basement paleo-relief has certainly conditioned theMeso-Cenozoic evolution and reactivation of major fault zones. Forfull details of the gravimetric inversion please see above references.
To confirm the relationship between the basement as interpretedfrom the 3D gravity inversion and the presence of diapirs as interpretedfrom seismic data, a preliminary 2.5D gravity forward modelling wasperformed for three NW–SE striking profiles (see location A–A′, B–B′and C–C′ in Fig. 4b; Matias (2007)) that have shown that the top ofthe basement controls the gravity response and corroborate the pres-ence of several salt diapirs previously suggested by vertical derivativeand analytical signal maps (id). The obtained basement relief mapfrom the 3D inversion (Fig. 4b) shows an elongatedWSW–ENE trendingdepocentre. Other less striking depocentres can be seen in the NE andSW part of the survey area. These depocentres are bounded to thesouth by a series of highs. A more detailed interpretation of this mapis presented in Section 4.2.3.
4.2.3. Integrated gravimetric interpretationThe result from the integrated structural interpretation of the
gravimetric data (vertical gradient map, analytical signal and inver-sion) is shown in Fig. 4b. We can observe that in the offshore AlgarveBasin, in the surveyed area, the basement is strongly controlled byseveral faults (id.). These faults were interpreted by analyzing subtlegradient changes in the estimated relief map. The dominant faultpattern consists of two sets of faults (id):
1) NW–SE faults; these faults segment the Algarve basin in differentsub-basins. They were inferred from strong lineaments from the
Fig. 5. a) Location of available seismic reflection profiles offshore Algarve overlaid tothe map shown in Fig. 4b. Arrows point to location of the examples profiles shownin Fig. 5b and c (thick black lines). b), c) Examples of interpreted stacked migratedseismic reflection sections, showing a complex fault system associated with theCarcavai fault zone (CF). Black solid lines: interpreted major faults. Dash line: neartop of basement. Dash–dot line: top of salt. Normal faults are seen to affect the base-ment while a faults with reverse (and also normal) geometry cuts across Cenozoicformations. This geometry is in agreement with the expected kinematic of the Carcavaifault zone onshore.
157J. Carvalho et al. / Tectonophysics 570–571 (2012) 151–162
shelf extending to the Guadalquivir bank, where they also sepa-rate different basement highs. This set of faults is consistent withthe geological information reported by Terrinha (1998);
2) ENE–WSW faults; these faults are inferred from the gradientchange around the 5500 m and 7500 m contour lines. They areprobably associated with Mesozoic extensional episodes and arebisected by the NW–SE set of faults.
Fig. 4b also shows the aeromagnetic contour lines (with a 10 nTinterval spacing) overlaid to the gravimetric interpretation.We can ob-serve that the alignment of the magnetic anomalies (continuous blackcurves) is coincident with the alignment of the Bouguer anomalies.Both alignments relates to a large gradient in the basement, possiblycorresponding to a fault zone with the downwards block located tothe southeast. One of the ENE–WSWfaults interpreted fromgravimetricdata is coincident with the possible offshore prolonging of the Carcavaifault as interpreted from aeromagnetic data (see Fig. 4b). This encour-aged us to analyse previously acquired seismic reflection data offshoreand verify the presence of a fault zone at the suspected locationsaccording to gravimetric and aeromagnetic data interpretations.
4.3. Seismic reflection data interpretation
The release in the early 1990s of seismic data acquired for offshorehydrocarbon exploration concessions has triggered several studiesfocusing on the Meso-Cenozoic offshore Algarve Basin (Lopes, 2002;Lopes et al., 2006; Roque, 2007; Terrinha, 1998). These studies wereconcentrated on the shelf and slope of the central and eastern areaof the basin due to limited dataset quality and coverage. In thispaper we used this and older 2D seismic dataset (Fig. 5a) to detectthe presence of a fault zone coincident the gravimetric and aeromag-netic lineaments that may correspond to the prolonging offshore ofthe Carcavai fault zone in the shelf and slope of the deep basin.
We started by inspecting the seismic reflection profiles at the ap-propriate place and looked for the presence of a larger fault zone.From the analysis of the basement (top of Paleozoic) map producedby gravimetric inversion constrained by seismic, well and dredgedata it was evident that the potential-field lineament correspondedto a major gradient in the basement depth. The upraised part of thebasement was located to the west of the lineament.
Therefore we looked for a fault zone with a western–northwesternupthrust block of the basement in the seismic profiles. It is generallydifficult to identify faults in metamorphic or igneous units that com-pose the basement with seismic reflection data in particular when thebasement is several km deep. In previous work (Matias, 2007; Roque,2007) such a fault was not identified, but three disconnected faultsegments with an upthrust block to the north with a similar trendof the expected fault zone were found affecting Middle Jurassic andoverlying units.
In this work we used the seismic-well tie of Matias (2007) andRoque (2007). The former tied the seismic horizons to the availableoffshore wells using sonic logs and checkshots and a revised stratigra-phy of the well's geological logs. Roque (2007) carried out a detailedstratigraphic analysis which helped to better understand and con-strain the existing seismic-to-well tie. This seismo-stratigraphic inter-pretation is overlaid to the stacked sections in Fig. 5b and c. Only themajor epoch interfaces are shown: the Cenozoic–Mesozoic andMesozoic–Paleozoic boundaries. The Quaternary–Pliocene bound-ary, important for seismic hazard purposes, could not be identifiedin the geological well logs.
We have examined several migrated stacked seismic sections inthe area where the extension of the Carcavai falt zone was expected.Fig. 5(b and c) shows two examples of seismic stacked sections whoselocation is indicated in Fig. 5a. A large fault zone with an upwardblock consistent with the Carcavai fault zone geometry at depth(northern block) is observed. This fault zone is sometimes associated
with Hettangian salt bodies which make it more difficult to identify inthe seismic profiles. Together with its location close to the end of thecoverage of the seismic profiles helps to explain why it has not beenidentified previously. However, guided by the gravimetric and mag-netic lineaments the fault zone is clearly recognizable in the seismiclines.
We can also observe that the geometry of this fault zone is normalin the Mesozoic sediments and the basement (Paleozoic rocks). In theCenozoic units a reactivation of the fault zone is observed in most of
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the profiles with a reverse geometry. This behaviour is in agreementwith that of the onshore Carcavai fault zone (Carvalho et al., 2012;Dias, 2001; Ressureição, 2009; Ressurreição et al., 2011). The faultpropagates upwards close to the surface but its throw is difficult toestimate from seismic data. Geological and geophysical estimates ofthe fault's throw in onshore sediments of Pliocene-Quaternary agepoint to a value around 14 m (op. cit.).
If we consider that the vertical seismic resolution of the seismicsections according to the λ/4 criteria is around 17 m and that theslight throw observed is of the order of the vertical resolution, we ob-tain a similar value for the fault's throw offshore to the value that hasbeen estimated onshore for Upper Neogene sediments (14 m, op. cit.).
5. Instrumental seismicity
The Instituto deMeteorologia, the official institution responsible forthe seismic network and earthquake data, has located all earthquakesfor the period 1958–1998 and recently relocated events for the period1961–2000. Instrumental seismicity and relocated epicentres , wherethe average error (90% confidence level) in the epicentral location is5 km, were used to see if the proposed location of the Carcavai faultzone has associated seismicity. Epicentres were relocated with the soft-ware Hypocent (Lienert and Havskov, 1995; Lienert et al., 1986), whichimplements a Geiger iteration scheme, with a centred, scaled and adap-tively damped least squares technique, to find the solution. The 1D ve-locity model is the one routinely used for locating hypocentres inPortugal mainland, which is based in deep refraction data (e. g. Mendes-Victor et al., 1980; Moreira et al., 1980; Mueller et al., 1973). Fig. 6ashows the seismicity over an onshore neotectonic map (after Dias,2001), continental platform neotectonic map (after LNEG, 2010) andour proposed course of the Carcavai fault zone offshore and onshore(signalled CF). Fig. 6b shows an enlarged study areawith the same tecton-ic information together with offshore neotectonic structures overlaid toa bathymetric map (see Discussion and seismic hazard implicationssection) and the relocated seismicity.
Association of earthquakes and fault zones in Portugal is a difficulttask due to errors in epicentres locations on one side and on the other,low slip rates that generate a low to moderate seismicity and theerosion/burry of surface ruptures. To this problem we should addthe fact that earthquakes occur at various depths in the seismogeniccrust, many times in blind or hidden faults. However, the associationbetween earthquakes and groups of fault zones is possible in someareas of Portugal, such as the Lower Tagus Valley area in centralPortugal (Carvalho et al., 2008).
In the Algarve, this association is also problematic but some clus-ter of seismicity can be associated with known structures. The mostnotorious is the cluster over the Monchique massif and smaller onescan be seen around the Portimão fault and its extension offshore, inthe Faro-Loulé region and also the Guadiana river region. The clusterover the Faro-Loulé area corresponds to the Carcavai and Loulé faults.Offshore, several faults are located close to seismicity clusters. Themost striking examples are the Gorringe Bank and the HorseshoeFault. We can also notice that the Carcavai fault zone course onshoreand offshore can be correlated with several epicentres, which arespread close to the course of the fault zone.
The study of available focal mechanism in the area is thereforequite important to determine if the earthquakes located close to thefault that is very possibly the offshore prolonging of the Carcavaifault zone have a geometry compatible with the Carcavai fault kine-matic onshore. There are not many earthquakes with focal mecha-nisms determined suitable for this purpose in the literature, due torelatively low instrumental seismicity and lack of azimuthal coverage.
We have used four events whose parameters can be found inTable 1 and whose location and focal mechanism is presented inFig. 6a. Event 1 has been calculated by Moreira (1985, in Borges etal., 2001), event 2 has been studied by Buforn et al. (1995, 2004),
the third by Borges et al. (2001) and the forth by Buforn et al.(1995, 2004). Events 1 to 3 are located offshore while the forth is lo-cated onshore where the fault's location is not known in outcrop.With the exception of event 3, which has not been attributed to anyfault know in outcrop, all earthquakes have been attributed byBorges et al. (2001) to N–S trending faults. The fault for event 4 isnot known in outcrop and has been postulated in agreement withthe seismicity for the period 1958–1998.
However, the calculated focal mechanisms are also compatiblewith a WSW–ENE oriented fault with a major strike-slip componentand a minor reverse component, in agreement with the Carcavaifault kinematic established onshore from geological and seismic re-flection data (Carvalho et al., 2012; Ressureição, 2009; Ressurreiçãoet al., 2011). The location of the epicentres also has a better matchwith the course of the Carcavai fault, suggesting that this fault zone,in agreement with onshore data (op. cit.) is probably active.
6. Discussion and seismic hazard implications
We have shown above that aeromagnetic, gravimetric data andseismic reflection data suggests that a major fault zone offshoremight join the onshore course of the Carcavai fault zone. Recentwork (Noiva, 2009; Noiva et al., 2010) has also shown that the faultzone extends offshore at least close to the coastline and probably off-sets another major fault zone: the S. Marcos–Quarteira fault. Onshore,aeromagnetic data suggests the possibility that the course of the faultzone prolongs approximately with a NE to E–NE direction for severaltens of kilometres.
The seismo-stratigraphic and structural interpretation of the off-shore seismic reflection profiles based on available well data showsthat a complex fault system corresponds to the alignment of the mag-netic and gravimetric anomalies. It also shows that this fault zone hasa normal geometry in the basement and Mesozoic units and has beenreactivated in Cenozoic times, in agreement with geological and geo-physical onshore studies of the Carcavai fault zone (Carvalho et al.,2012; Dias, 2001; Ressureição, 2009). In the Cenozoic, onshore evi-dences of reverse faulting with an important strike-slip componentcharacterise the fault zone. Offshore, seismic data suggests a similarbehaviour in the Cenozoic.
Deformation of Plio-Pleistocene sediments has been observed ingeological outcrop and geophysical studies (Carvalho et al., 2012;Dias, 2001; Ressureição, 2009) but offshore the dating of the sedi-ments of this age is problematic (Noiva, 2009). Noiva (2009) hasinterpreted deformation of sediments of Lower Pliocene-Mioceneage offshore close to the coastline in the area of influence of theS. Marcos–Quarteira fault. According to the same author, deformationof units of probable Plio-Quaternary and Holocene age also occurs,though the faults throw could not be measured due to the lack ofseismic-horizon markers. The kinematic of this fault zone is reversebut the upraised block is the south-eastern one, in contradictionwith what seismic and gravimetric data presented here shows furtheroffshore. Our data suggests that it is rather the southern branch of theCarcavai fault zone that prolongs offshore and the upraised block isthe north-western one.
Two possibilities may explain this apparent discrepancy. The firstis that since the area studied by Noiva (2009) is in the confluencewith the S. Marcos–Quarteira fault, the deformation zone is quitelarge at this point and a particular situation. The second one, whichwe favour most, is that the Carcavai fault has an important compo-nent of strike-slip movement and it is well known that this type offaults can produce areas of rise and collapse in the same block ofthe fault.
Concerning the distribution of instrumental seismicity (Fig. 6a),several comments can be made. First, the lack of a clear correlationbetween seismogenic sources and earthquake locations is commonin Iberia. As stated above, this results from two factors: relatively
Fig. 6. a) Course of the offshore Carcavai fault zone after seismic reflection, gravimetric and aeromagnetic data interpretation derived in this work, overlaid to an onshore (after Dias,2001) and continental platform (adapted from LNEG, 2010) neotectonic maps; seismicity for the period 1958–1998 is also shown (source: Instituto de Meteorologia); Black squaresindicate location of earthquakes which hypocentral data and fault-plane solutions are presented in Table 1 (see text). b) Neotectonic and bathymetric map of the south and westportuguese margins (modified from Duarte et al., 2010) overlaid by the onshore (Dias, 2001) and continental platform (adapted from LNEG, 2010) neotectonic maps shown ina), the relocated seismicity for the period 1961–2000 and the offshore course of the Carcavai fault derived in this work (marked CF). The Carcavai fault's signature on thebathymetry suggests it might be part of the Horseshoe fault system (dotted line).SASIF-S. Teotónio–Aljezur–Sinceira–Ingrina fault; PF — Portimão fault zone; SQF-S. Marcos–Quarteira fault zone; CF — Carcavai fault zone.
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poor epicentre locations caused by geographical network constraintson one side and diffuse intraplate seismicity and deep hiddenunknown seismogenic sources on the other. Nevertheless, clusters
Table 1Hypocentral data and fault-plane solutions (ϕ, δ, λ: strike, dip and rake, respectively) ofthe four earthquakes located over the Carcavai fault used in this study but calculated byother authors. A — Buforn et al. (2004); B — Moreira (1985); C — Buforn et al. (1995);D — Borges et al. (2001).
Date Lat. N Lon. E Depth M ϕ δ λ NF Ref.
25/09/86 36.8 −8.9 – 4.3 7 70 −10 1 B20/10/86 36.9 −8.6 37 4.8 180 37 3 2 C, A02/11/89 36.8 −8.7 40 4.5 180 75 8 3 D20/12/89 37.3 −7.4 23 5.0 351 77 10 4 C, A
of seismicity can be associated with several known neotectonic struc-tures. Another reason for the non existence of a clear lineament in theseismicity maybe that ductile deformation is taking place in the faultplane, which is being lubricated by the Hettangian salt as observed inthe seismic profiles and potential field interpretation.
However, recently relocated epicentres for the period 1961–2000show a clearer correlation between the geological structures and seis-micity. In Fig. 6b we have plotted by magnitudes this relocated earth-quake data, superimposed to a neotectonic map including a simplifiedcompilation of the faults recently mapped offshore by differentauthors using multi-channel seismic reflection and bathymetric data(after Duarte et al., 2009, 2010; Terrinha et al., 2009; Zitellini et al.,2009), an onshore neotectonic map (after Dias, 2001), a simplifiedtectonic map of the south and western Portuguese continental
160 J. Carvalho et al. / Tectonophysics 570–571 (2012) 151–162
platform (after LNEG, 2010) and the offshore course of the Carcavaifault interpreted in this work. This Figure also displays offshorebathymetric data using SWIM compilation (Zitellini et al., 2009) com-pleted by GEBCO (2003, in Duarte et al., 2010).
We can observe in Fig. 6b, 4 major clusters of seismicity. Cluster Ais related with the hanging wall block of the Gorringe Fault. Furthersoutheast, cluster B is associated with the footwall block of the Horse-shoe fault and probably also the SWIM1 and SWIM2 faults. The SWIMfaults have recently been proposed as right-lateral strike slip faultsystem which was evidenced by bathymetric and seismic reflectiondata (Terrinha et al., 2009; Zitellini et al., 2009). To the East of thesetwo clusters, Cluster C is associated with the confluence of three im-portant faults: the São Vicente Fault, the Horseshoe Fault and the off-shore prolonging of the Carcavai fault zone derived in this work.
A fourth cluster (D) cover the area between meridians 9°W and7°W, with a V shape. The eastern branch correlates with the CadizFault and the northern part of the Accretionary Wedge Thrust (AWT).The northern part of the western branch of the V shaped cluster corre-lates well with the N–S trending faults like the S. Teotónio–Aljezur–Sinceira–Ingrina and Portimão Faults and also the offshore prolongingof the Carcavai fault. There are also several epicentres on the onshorecourse of the Carcavai fault, including the northeastern extension pro-posed here based on aeromagnetic and geological data.
The existence of an active structure with the orientation and kine-matic suggested here for the Carcavai fault zone (offshore and on-shore) is also in agreement with the present-day stress field (Borgeset al., 2001; Ribeiro et al., 1996). Left-lateral strike-slip faults with areverse component and with a NNE–SSW to ENE–WSW trend whichrespond to a NW–SE to WNW–ESE oriented main compressive stressfield are common in Portugal mainland. The Azambuja (Cabral et al.,2004), V. F. Xira (Carvalho et al., 2009) or the Vilariça fault zones(Rockwell et al., 2009) are a few examples. At least in the LowerTagus Valley (LTV) area, these NE–SW to NNE–SSW faults thatconcentrate most of the seismicity (Carvalho et al., 2011; Pinto,2011) are constrained by WNW–ESE to NW–SE trending faults(Cabral et al., 2003).
In the south Portuguese margin this pattern of approximatelyoriented NE–SW thrust faults linked by NW–SE striking faults is alsopresent, with minor differences. The NW–SE to WNW–ESE strikingfaults are in the here the S. Marcos–Quarteira and the SWIM faults,and also other N–S striking faults as the S. Teotónio–Aljezur–Sinceira–Ingrina and Portimão faults. Similarly to the LTV region,these faults generate less seismicity, which also concentrates offshoreAlgarve in NE–SW thrusts. In the southern margin these are theGorringe, Horseshoe, Coral Patch, Marquês de Pombal and the easternpart of the Cádiz Fault. The offshore Carcavai fault presented here,with a major left-lateral strike-slip component and a reverse compo-nent with an upthrusted NW block fits into this seismotectonicpattern.
The Accretionary Wedge Thrust is a structure that does fit into thispattern and results from the very high stress-deformation originatedat the centre of the collision between the two plates (Rosas et al.,2009), which at distances of 300–400 km from the plate boundary,such as the LTV region, do not occur. The distance to the collisionfront may also explain why in the southern portuguese margin thehanging wall block of the thrust faults is the SE one, contrarily tothe LTV area, where the hangingwall block is the NW.
Bathymetric data suggests the possibility that the offshoreCarcavai fault maybe connected to the Horseshoe Fault and thispossibility is also suggested by the existence of cluster of seismicityC that is located at the confluence of these two faults and the SãoVicente Fault. However, the Horseshoe Fault and the offshoreprolonging of the Carcavai fault have opposite upthrusted blocks; fur-thermore, at least onshore, the major component of the Carcavai faultis strike-slip, what causes a problem to its connection to the Horse-shoe Fault.
Independently of its relation with other geological structures inthe area, the presence of the offshore Carcavai fault zone is solidlyestablished accordingly to seismic reflection, magnetic, gravimetricand seismicity data. Implications of the existence of this new struc-ture to seismic hazard analysis are relevant even if, according to theinterpretation of potential-field data, the fault zone is segmented.Interpretation of gravimetric and aeromagnetic data presented here,suggests the fault zone is segmented by approximately NW–SEtrending faults affecting the basement but with no surface expressionaccording to LNEG (2010), and by N–S oriented faults that havebeen charted by LNEG(2010) using seismic reflection data (seeFigs. 4b and 6, respectively).
The length of the Carcavai fault zone segments range between 10and 40 km. If we use a similar throw to the one estimated for the on-shore Carcavai fault zone at the base of the Plio-Quaternary Ludo for-mation (about 12 m, Carvalho et al., 2012), a segment length (L) of30 km, we will obtain, using the censored relations of Stirling et al.(2002), a Mw of 7.06 for the maximum expected earthquake and anaverage co-seismic displacement of 2.1 m.
Usually, faults rupture by segments only rarely the whole length ofthe fault breaks. It is not uncommon however, that ruptures propa-gate across two or three segments. This will produce a rupture lengthof at least 40 km in the Carcavai fault zone, producing earthquakes ofvery large magnitude.
7. Conclusions
The integrated interpretation of the seismic reflection, gravimetricand aeromagnetic data show the presence of a fault zone offshoreAlgarve that is part of an onshore structure: the Carcavai fault zone.The geometry and cinematic of the offshore fault inferred from geo-physical data is compatible with surface geological studies of theCarcavai fault zone: a previous normal fault reactivated as a left-lateralstrike-slip fault with a reverse component in Cenozoic times. Epicentredata, with a concentration of seismicity on the upthrusted fault blockwhere most of the deformation is expected, also suggests that the faultis active.
The total fault length based on potential-field data presented hereis over 200 km. The Carcavai fault offshore is apparently segmentedby several N–S to NW faults but some segments may reach 30 kmand a maximum expected earthquake magnitude above 7. Togetherwith vertical offsets estimates obtained, this data set will allow an im-provement of the seismic hazard of the area, namely providing morerefined estimates of co-seismic rupture, maximum expected earth-quake and return periods. Further studies are required to understandthe tectonics and structure of the area.
Acknowledgments
The authors are particularly grateful to TGS-NOPEC and DPEP(Divisão para a Pesquisa e Exploração de Petróleo) for allowing pub-lication of seismic and gravimetric data, the portuguese Foundationfor Science and Technology for funding the NEFITAG (PTDC-CTE/GIX/102245/2008) project, the ANPC (National Civil Protection Agen-cy) for financing the ERSTA (Study of the Tsunami and Seismic Risk ofthe Algarve) project and to Fernando Carrilho and the Instituto deMeteorologia for allowing publication of epicentre location data. Thefirst author acknowledges Pedro Falé, who recently passed awayand Jorge Saraiva for computer and graphical assistance.
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