Marine Geophysical Researches20: 141–156, 1998.© 1998Kluwer Academic Publishers. Printed in the Netherlands.

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Morpho-tectonic analysis of the Azores Volcanic Plateau from a newbathymetric compilation of the area

N. Lourenço1, J. M. Miranda1, J. F. Luis2, A. Ribeiro3, L. A. Mendes Victor1, J. Madeira3 andH. D. Needham4

Received 4 May 1998; accepted 17 February 1999

Key words:Azores bathymetry, morpho-tectonics, transtension

Abstract

The existing studies of the Azores triple junction, although based on specific geological or geophysical data, largelyrely upon morphological considerations. However, there is no systematic bathymetric coverage of this area, andthe quality of the available bathymetric charts does not allow consistent morpho-structural analysis.

In this work we present a new bathymetric grid elaborated with all the available data sources in an area com-prised between 24◦ W to 32◦ W and 36◦ N to 41◦ N. The basic data set corresponds to the merge of NGDC datawith new swath profiles. This new map, included as an Appendix, combined with other results from seismologyand neotectonics, is the basis for the study of the morpho-structural pattern of the Azores area, the present daystress field and its implications on the current view of the Azores geodynamics.

As a major result, we conclude that the Azores region is controlled by two sets of conjugated faults with 120◦and 150◦ strikes that establish the framework for the onset of volcanism, expressing as linear volcanic ridges oras point source volcanism. This interaction develops what can be considered as the morphological signature of theAzores Spreading axis segmentation. We argue that the Azores domain, presently in a broad transtensional regime,is acting simultaneously as a ultra slow spreading centre and as a transfer zone between the MAR and the dextralGloria Fault, as it accommodates the differential shear movement between the Eurasian and African plates.

Introduction

The Azores volcanic plateau is a first-order morpho-logical feature in the Atlantic basin. It has an overalltriangular shape corresponding to a surface area ofapproximately 400 000 km2 of elevated oceanic crust,roughly underlined by the 2000 m isobath. The plateaucrosses to the west the Mid-Atlantic Ridge (MAR),and is limited to the south by the East Azores fractureZone (EAFZ). Its northern limit follows a complexlineation that trends WNW–ESE from the MAR to the

1UCTRA, Campus de Gembelos, 8000 Faro, PORTUGAL (E-mail:nuno.lourenco@fc.ul.pt; jmiranda@fc.ul.pt)2UCEH, Campus de Gambelas, 8000 Faro, PORTUGAL (E-mail:jluis@mozart.si.ualg.pt)3Departamento de Geologia e Laboratorio de Tectonofisica eTectonica Experimental; Fc. Ciências, U. Lisboa, 5◦ Piso, C2Campo Grande, 1700 Lisboa, PORTUGAL4IFREMER, DRO/GM, BP70 - Plouzane, FRANCE

western limit of the Gloria Fault (Argus et. al., 1989)(Figure 1).

Between 37◦ N and 41◦ N the topography of theMAR flanks shows an attenuated roughness whencompared to a ‘normal’ ridge segment: the ridge axis,itself, is shallower or subdued and the crust is believedto be 100% thicker than in the surrounding oceanicarea (Luis, 1996). This is consistent with the influenceof a mantle plume acting under the plateau, clearlyidentified by geochemistry (Morgan, 1971; Schilling,1975; White et al., 1976; Bougault and Treuil, 1980;Bougault and Cande, 1985; Dosso et al., 1996). In arecent study Silver et al. (1998) demonstrated that theMAR moved westward some 200 km relative to thehot spot reference frame, nevertheless its relationshipwith the Azores archipelago is still largely unknown.

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Figure 1. Regional setting of the Study Area. Inset: simplified bathymetry of the Azores Plateau (contours each 1000 m).

The Azores plateau is the meeting point of threedifferent lithospheric plates: the American plate to theWest, the Eurasian plate to the NE and the Africanplate to the SE. The global kinematic models likethe RM2 (Minster and Jordan, 1978), the NUVEL 1(DeMets et al., 1990) and the local ‘model’ fitted tothe MAR near the Azores Triple Junction (Luis et al.,1994), imply for the motion in the Azores boundary,a right lateral transtensional regime along the Eu/Afboundary. The extensional component ranges between0.3 cm yr−1 (RM2 instantaneous rotation pole) and0.4 cm yr−1 (NUVEL 1 rotation pole) full spreading

rates. This implies that this domain is set in the fieldof the ultra-slow spreading ridges (i.e. spreading rates<10 mm yr−1).

The nature of the boundaries between the Amer-ican plate and the Eurasian and African plates is thedivergent MAR. The nature of the third arm is howevermuch more controversial. Between magnetic anom-alies 5 and 21, a highly diffuse Azores magnetic trendhas been identified (Searle, 1980 and Luis et al., 1994).This trend presents high intensities that contrast withMAR trend in the surrounding areas and implies a

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recent age for the Azores spreading system (Mirandaet al., 1991 and Luis et al., 1994).

Several authors have interpreted the Azores do-main as a spreading centre (Krause and Watkins,1970), or an oblique spreading centre (McKenzie,1972; Searle 1980), whereas others (Madeira eRibeiro, 1990) favour a leaky transform model. Thesemodels rely on different approaches but are largelybased upon analysis on sea floor morphology of theAzores domain.

Krause and Watkins (1970) presented the firstbathymetric chart of this area. Since then, only onemore chart concerning specifically the plateau waspublished by Searle (1980) (using also data fromLaughton et al., 1975) but all these maps relied on avery sparse bathymetric coverage.

Here we present a new bathymetric compilationconcerning the Azores plateau. It comprises all thedata that has been made available to us, for a regionlocated between 24◦ W, 36◦ N and 32◦ W, 41◦ N.This data set gives a new insight for the study of themain morpho-structural features of the Azores TripleJunction (ATJ) third arm and, together with the nowavailable geophysical compilations of the area, allowsthe establishment of a new tectonic interpretation.

The main question that we address can be sum-marised as follows: can the Azores be consideredas a spreading centre? How does the stress distribu-tion relate with the observed tectonics? In what wayis the Azores morphology coupled with the in-depthstructure of the Azores Plateau?

The new bathymetric chart

The Azores Plateau has never been the subject of a sys-tematic bathymetric survey, because of its huge area.Nevertheless due to the port facilities provided by theislands, many geophysical cruises crossed this zone,in the past decades, and acquired some bathymetricprofiles. Most of those profiles are presently availableat the NGDC database.

The onset of international programs on ridge re-search (e.g. FARA – French American Ridge Atlantic,RIDGE – Ridge Inter-disciplinary Global Experimentsand INTER-RIDGE) has allowed the detailed survey,with swath bathymetry systems, over the Mid-AtlanticRidge domain. FARA programme also made a highquality survey of the MAR near the Azores Plateau(Needham et al. 1991).

During MARFLUX/ATJ (MAR Fluxes/AzoresTriple Junction) project (Bougault et al., 1996) twocruises were held: the HEAT (Hydrothermal Explo-ration at the Azores Triple junction) cruise (Germanet al., 1997) which provided some swath profiles ofpreviously uncovered areas and, in 1995, The ES-CAPE (European Surface Cruise for the Azores PlumeExploitation) cruise which allowed the collection ofnarrow beam echosounder data over the plateau, inpre-selected areas, to best fill the gaps in the existingcompilation.

The plot of this data (Figure 2) shows the hetero-geneity of its sources, where the ridge axis presentsa very dense coverage and the Azores domain revealsprofiles rather concentrated on some lanes but never-theless with a good spatial coverage for regional scalestudies.

The recent swath data has been acquired with a13 kHz SIMRAD EM12 dual bathymetry and sidescan system, with a depth accuracy estimated to be±0.1% of the water depth for the inner beams increasingto ± 0.25% for the outermost beams (Detrick et al.,1995). However, most of the previously acquired dataresults from heterogeneous sources and were obtainedwith a multitude of devices, with varying spatial reso-lution and location accuracy. This resulted in the poorquality of some of the points in the compilation andimplied that the first contour maps we made displayednumerous artefacts, usually related with a single shiptrack, as well as spikes in poorly covered zones or inship tracks intersections.

The processing of the data set consisted mainly onthree steps. The first step corresponded to data normal-isation: in this stage the data were reduced to the sameco-ordinates (geographic), depth units (m), and filestructure. The final grid spacing was set to be 400 mand redundant points were discarded. The second stepcorresponded to data cleaning of the non-swath data:this was the most critical and time-consuming stage ofthe whole process. The third step corresponded to datainterpolation.

The subsets that resulted from the previous stepwere merged with the bathymetry from ESCAPEcruise and the swath data from the ridge axis. Areasremaining with a poor coverage where filled with pre-dicted bathymetry from Smith and Sandwell (1997).Due to its lower resolution we cross checked it withavailable cruise tracks and confirmed the good qualityof the data.

Finally, the resulting file was triangulated and theresulting digital terrain model was converted to a

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Figure 2. Distribution of points used in the compilation. Coverage from swath surveys around the MAR axis (SIGMA cruise, FARA Pro-gramme). Ship tracklines are from NGDC database, echo-soundings are from Gebco charts, and regular spaced grid on the background consistson predicted bathymetry from Smith & Sandwell (1997) in areas with poor or absent coverage (projection Mercator).

grid (spacing of 400 m) using a bicubic interpolationmethod. The resulting bathymetric map along withproposed toponimy designating the main morpholog-ical features in the Azores plateau is presented as anappendix to this work.

Morpho-structural analysis

The morpho-structural analysis was made in two mainways. In good quality areas, shaded relief images (Fig-ure 3), false colour images (Figure 3a) and large-scalemaps (grids 100× 100 m, contours each 50 m) wereconstructed for analysis. In zones of poor coverage wetried to isolate main morphological trends and mainscarps as fault presence indicators. Seamounts, mor-phological highs and lows individualisation (Figure 4)

was accomplished using criteria by Gracia (1996)modified from the work of Batiza (1982) and Smithand Cann (1992). These features were not digitizedat a specific contour level, but rather by drawing acurve starting at the base of the slope break and con-tinued until the feature was circumscribed. Given thedata scarcity in some areas which affect both slopegradients and slope geometry we used additional sis-motectonic data (Hirn, 1980; Grimison and Chen,1986; Buforn et al., 1988), islands neotectonic data(Madeira and Ribeiro, 1990; Madeira, 1998) and mag-netic anomalies map (Luis, 1996) to constrain ourinterpretation in the Azores Plateau.

Figure 3 displays the off axis domain between theMAR axis and the Faial island. The area adjacentto the MAR, covered by swath bathymetry, extends

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Figure 3.

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Figure 3. Top: Shaded relief image from the Azores Triple Junction Area, illumination from Northwest. Inset: area covered by figure 3A.Bottom: structural interpretation. Note the development of NW-SE and WNW-ESE horsts and grabens and volcano-tectonic lineations relatedwith Azores deformation. Figure 3A - False colour image from the same area. Red corresponds to scarps facing the sun and blue to scarpsfacing away from the sun. It is possible to observe the Azores trend clearly disrupting the MAR generated morphology.

to crustal ages of approximately 5.5 Ma (Luis et al.,1994), and allows the observation of the early stages ofthe Azores development and its interaction with MARfabric.

MAR generated morphology expresses both bypervasive seamount development and a characteristicabyssal relief trending parallel to the ridge axis (N–S to 20◦ N). Superimposed and clearly disrupting thistrend (Figure 3A), it is possible to observe two distinctvolcano-tectonic directions strictly connected with theAzores domain, extending from 38◦20 N until at least39◦ N (the limit of swath coverage).

The main trend consists of a series of parallelfaults striking 110◦–120◦ N, defining horst and grabentopography. This direction corresponds to the Azoresvolcanic emplacement, and the observed features arerather elongated seamounts with high length/width ra-

tios, that will be designated in this work, by LinearVolcanic Ridges (LVR). An example is given in fig-ure 3, there are two twin LVR present in the immediatevicinities of the MAR axis (at 38◦45′ N, 30◦ W and39◦ N, 29◦55′W) and around Faial Island further east.

The other ‘Azores trend’, less defined, is oriented140◦–150◦ N and consists on several lineations, bestvisible between 29◦20′W and 29◦50′W at an approx-imate latitude of 38◦50′ N. The interaction betweenthese two fault systems promotes the development of adistinct v shaped bathymetric signature (e.g. 38◦50′ N,29◦25′ W). All the above mentioned morphologi-cal features, as we shall see, may correspond to anearly analogue of similar features, but more evolvedobserved in the Azores plateau (Lourenço, 1997).

The analysis of the bathymetric map, in the AzoresPlateau reveals characteristics that were partially de-

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Figure 4. Morpho-structural interpretation of the Azores Plateau bathymetry. PA – Princesa Alice Bank; AC–Açor Bank; BN- Big North; MB- Monaco Bank; DJC – D. João de Castro Bank;WGB – West Graciosa Basin; EGB – East Graciosa Basin; NHB – North Hirondelle Basin; SHB – South Hirondelle Basin; PB – Povoação Basin; RF – Ribeira Seca Fault (Madeira & Ribeiro,1990); LF – Lajes Fault and CG- Congro Graben. A marked clockwise rotation is evident, from west to east in the main plateau features.

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Figure 5. Histogram of the azimuth frequency distribution of allmapped scarps, lineations and LVR axis. Bins each 10◦, n=142.Non linear Gaussian fit allows to constrain peaks at 117◦ ±1.34 and145◦ ± 1.45 azimuths (95% degree of confidence). Also note that89◦ of population falls between azimuths 100◦ and 160◦.

scribed by Searle (1980) from its interpretation ofGLORIA imagery. It is clear (Figure 4 and map) thatthe Terceira axis consists of a morphological pattern ofen echelon, fault controlled, rhombic basins, present-ing a characteristic interval of 20 to 30 km, alternatingwith v-shaped seamounts, some of which reachingsurface and forming the islands. These basins are con-trolled by the above mentioned sets of conjugatedfaults striking between 150◦ N and 110◦ N, the lat-ter better developed in the Terceira Axis. This patternis dominant between the Formigas and the West Gra-ciosa Basin and gradually loses importance towardsthe outer areas of the plateau.

The new bathymetric map allows some clarifica-tions of the Azores morphology (cf. Appendix): (1) thebasins although best developed along theTerceira axiscan be traced to the SW of the plateau, even if theyare not so well developed. (2) The area comprisedbetween São Jorge Island and the Açor bank (alsocalled ‘Faial Ridge’) presents a distinct bathymetricsignature from the rest of the plateau. (3) Two types

of volcanic constructions can be observed: circularseamounts and volcanic ridges which present a markedclockwise rotation, running from Northwest to South-east, and (4) there are V shaped bathymetric highsrelated with S. Miguel and Terceira islands (Figure 4).In this V shaped seamounts, the N120 arm is betterdeveloped than the N150 arm and the islands are es-sentially located in the tip of the V- or along the N120arm.

The spatial distribution of the volcanism can bestudied from the close analysis of the bathymetric fea-tures. Volcanism is barren or absent in the basins floor,generally covered with pelagic sediment and volcanicsediments (Miranda et al., 1996). Around these basins,considering the distribution and orientation of the vol-canic features in the plateau, we can identify twodifferent types of volcanism: Point Source Volcanism(PSV) and Linear Volcanic Ridges.

PSV features are widespread through out theplateau. The volcanic seamounts are preferentially lo-cated in the vicinities of the troughs especially inits western and southern limits. This type of volcan-ism expresses by the development of circular volcaniccones with heights ranging from 200 m to 1000 m,sometimes rising to very shallow depths where theydefine submarine banks like D. João de Castro, Formi-gas banks or Grande Norte (37◦20′ N 25◦ W). Theirshape and location suggest that they originate fromthe intersection of the two main tectonic fabrics. Thisis evident in the João de Castro Bank. Some of theIslands like Graciosa, Santa Maria and Terceira rep-resent extreme cases of this situation of seamountdevelopment.

LVR are the most pervasive form of volcanismin the Azores Plateau. They mark the regional struc-tural pattern of the Plateau, probably built by fissuralvolcanism along the fault system. Islands like SãoJorge or Faial-Pico system, are extreme cases of LVRdevelopment.

Considering the LVR orientations, three differentzones can be roughly individualised from Northwestto Southeast:

The first domain (Figure 4) is the shallowest one.From North to South we can individualise severalridges (e.g. ridge A, São Jorge Island, Faial-Pico Sys-tem, ridge B and ridge C) defining a horst-grabentopography. These volcanic ridges have a character-istic spacing of approximately 22 km, lengths of 54to 60 km and directions ranging from 110◦ to 120◦.Although present, the 150◦ fault trend is here poorlydeveloped.

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The second domain corresponds to a set of parallelvolcanic ridges with an orientation rotated clockwisefrom the previous one. Volcanic ridges have a widerspacing (from 32 to 50 km) and strike from 135◦ to145◦.

From Northeast to Southwest (cf. figure 4) one candistinguish 3 rhombic shaped basins: the Hirondellesystem (North and South Basins), Basin 1 and Basin 2.Further SW, in the Princesa Alice and the Açor banks,the morphology becomes again more complex and thestructural control more evident. Both banks present aseries of elevated crustal blocks with a roughly squaresection, which indicates control by the two tectonic di-rections. Between Princesa Alice and the Mid-Atlanticridge, in the crust younger than anomaly 5 (10 My),a basin trending along the 145◦ direction (B3 in Fig-ure 4), in the continuation of the Princesa Alice bank,disrupts the ‘normal’ MAR trend. Its morphology issimilar to the one observed along the Azores domain,with square sections and faults controlling the basinflanks.

Finally, the third domain, corresponding toMonaco Bank area, shows a similar configuration ofalternating volcanic ridges and poorly defined basins,probably a result of the lack of bathymetric data. Nev-ertheless it is still visible, particularly in its easternpart, that the strike of the volcanic ridges rotates to150◦ N direction (e.g. Monaco bank) before interact-ing with the Azores trend towards East. This domaingradually deepens to South towards the East AzoresFracture Zone.

The azimuths of all lineations, fault scarps andLVR axis mapped on Figures 3 and 4 were measured,considering small trend changes along each feature.The plot of these data as a histogram is presented infigure 5. 89% of the azimuth population fall withinthe azimuth range 100◦ to 160 ◦ N. The data ismainly grouped in the 120◦ N, 140◦ N and 150◦ Nazimuth bins. Performing Gaussian multi-peaks curvefitting, using chi-square statistical test as fitting con-trol, we resolved two peaks with a confidence levelof 95%, one at 117.7◦ ± 1.34 and the other at 145±1.45. This values are in close accordance with activefault orientation measurements in the Central GroupIslands (discussed later). They reflect the prevalenceof these two directions in what concerns both tectonicdeformation and linear volcanic ridges emplacementthus implying that high control of magma injection bypre-existing faults must occur.

The morphological interpretation that we presenthere is not typical of a spreading axis. There is no

continuity between adjacent segments, and even if weconcentrate only on the Graciosa – São Miguel axisand assimilate the island – and neighbouring bathy-metric ridges as segment equivalents, neither the mor-phological features, nor the magnetic signature (Luis,1996) points towards a typical axial segmentationpattern.

Generally, triple junctions present some degree ofdistributed deformation. In the Pacific, Triple Junc-tions commonly show microplates (Lonsdale, 1988;Larson et al., 1992), while the Bouvet shows smalldisruption (Ligi et al., 1997, Mitchell and Livermore,1988) and Rodriguez shows more distributed rifting(Munschy and Schlich, 1989; Mitchell, 1991). TheATJ, seems a limiting case, as deformation extendsthrough a NW–SE band nearly 190 km wide, withno simple plate boundary geometry. We believe thatthis fact results from a combination of two main fac-tors: (1) hot spot activity, which reduces lithosphericstrength, allowing more aerial extent of deformationand volcanism. (2) Highly complex evolution of theATJ and its present instability. Luis et al. (1994) pre-cludes in the past 10 Ma a northward migration ofthe ATJ. This setting implies that some of the reliefis inherited from a previous geodynamic environmentwhere blocks of Eurasian plate are incorporated intothe African plate.

Despite of its specific deformation pattern, somesimilarities arise when we compare the Azores mor-phological features with other oblique ridge systemslike: Eastern Bransfield basin (Gracia et al., 1996),South West Indian Ridge (Rommevaux, 1994; Grind-ley et al., 1996) and Mohn’s ridge in the North At-lantic (Géli et al., 1996; Dateuil and Brun, 1993). Inthe Mohn’s Ridge these authors observe a series ofoblique highs altering with rhombic deeps, defining anen-echelon array. Like in the Azores, volcanism seemsto be associated with the highs, and in both areas thetectonic control seems to be the main factor regulatingthe spatial distribution of volcanism.

The Azores Stress Regime

The different morphological features of the Azoresplateau must be the result of the different deformationand accretionary mechanisms that shaped the area.The enormous amount of data that was gathered, theavailability of high resolution swath data in criticalareas, reinforce the conclusion that the now avail-able bathymetric map does reflect the main morpho-

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Figure 6. Seismicity map (M>4) of the Azores plateau from 1928 until the present, retrieved from the USGS database. The focal mechanisms are extracted from the same file (black symbols),from Buforn et al. (1988) (dark grey) and from Grimison and Chen (1986) (light grey).

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Table 1. Tension axis average oerientation and standard deviation of the events from the three different groupsthat we have considered. Only the normal and strike slip faulting mechanisms were considered.

Date No. T axis T σ Zone Expansion direction∗θ φ

26.6.89 1 29◦ 76◦11.12.73 2 17◦ 80◦20.1.93 3 20◦ 74◦ 10◦ 22◦ Central 39◦ N N68◦ E

23.11.73 4 (−33◦) 89◦ 20◦1 6◦1 set 28◦ W

1.1.80 5 14◦ 88◦

[5pt] 12.2.81 6 44◦ 86◦6.9.64 7 46◦ 69◦ 42◦ 4◦ Hirondelle 38.1◦ N N72◦ E

21.1.89 9 39◦ 85◦ Set 26.5◦W21.11.88 10 37◦ 66◦

[5pt] 16.10.88 11 78◦ 90◦4.7.66 13 39◦ 84◦9.9.84 14 81◦ 81◦ 51◦ 32 S. Miguel 37.5◦ N N76◦ E

9.12.91 15 60◦ 90◦ 65◦2 172 set 25◦ W

8.3.39 16 (−5◦) 67◦

1Without even number 4:2Without even number 16:∗Calculated with pole 21◦N 20.6◦W (DeMets et al., 1990).

structural units and that the proposed models for thearea must be checked against this data set.

Féraud (1980) based on radiometric dating of rocksamples and a regional interpretation of the volcanicand tectonic activity proposed a model for the stresspattern (cf., Féraud, 1980 for description). Madeiraand Ribeiro (1990) proposed a stress field model in theAzores Plateau on the base of the islands tectonics, andavailable focal mechanisms of earthquakes. They de-scribe (Madeira, 1998) the presence of a complex sys-tem of active faults. One family is parallel to the lengthof the plateau, WNW–ESE; it is composed by twoconjugate sets of oblique dextral normal faults dip-ping to SSW and to NNE. The other family, orientedNNW-SSE, is composed by two conjugate sets ofoblique sinistral normal faults dipping to ENE and toWSW. Both faults show alignment of volcanic cones.The pattern of faulting can be easily explained bythree-dimensional strain field (Reches, 1983, 1988).The corresponding strain regime is transtensional withoblique dextral normal slip parallel to the WNW–ESEmain direction and oblique sinistral normal slip inthe conjugate direction. This strain regime has beenconfirmed by geodetic methods (Bastos et al., 1998;Pagarete et al., 1998.)

The Azores plateau is seismically active (Figure 6).The distribution of instrumental epicentres shows that

the present plate boundary is oriented WNW-ESEalong the São Jorge–São Miguel alignment. Thisis why some authors preferred the designation SãoJorge Leaky transform to the Terceira rift (Madeira &Ribeiro, 1990) The highest historical magnitude is 7.4in 1757/07/09; the highest instrumental magnitude is7.2 in 1980/01/01 (Hirn, 1980).

The focal mechanisms of seismic events are as fol-lows: dextral strike slip with variable components ofnormal dip-slip in WNW–ESE oriented fault planes;sinistral strike slip with variable components of nor-mal dip-slip in NNW-SSE oriented fault planes andpure dip-slip normal events in both directions (Fig-ure 6). This suggests decoupling in transtensionalregime; according to the position in time during theseismic cycle we can have pure dip-slip events, purestrike slip events and oblique slip events. No time pat-tern is discerned up to now but very sparse neotectonicobservations suggest that strike slip slickenlines aremore frequent on main faults and normal slickenlinesare more frequent in subsidiary faults (Madeira, 1998).These are recognised by fault scarps; offset streamsand cinder cones and dating of faulted materials. Thedata were compiled in 1/100 000 scale maps (Madeira,1998), see Figure 7.

Like the observations of bathymetry, volcanism inthe islands expresses by the presence of recent cinder

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Figure 7. Neotectonic maps from S. Jorge, Pico and Faial Islands.Note the presence of the two main tectonic directions in the threeIslands. In Pico and Faial Island, the intersection of the fault systemscorresponds to the location of both Islands main volcanoes.

cones aligned along both fault families, dextral WNW-ESE and sinistral NNW–SSE. This demonstrates thatthe stress field must change during seismic and mag-matic events. Maximum compressive stress increasesduring seismic cycle until a seismic event occurs, inthe fault family where the resolved shear stress is at amaximum. Stress drop during this event changes theorientation of maximum compressive stress that de-creases the acute angle with the fault family wherethe seismic event occurred, allowing the trapping ofmagma because the fracture can now open by obliqueslip with a tensile component larger than the shearcomponent. Obviously we can not reconstruct the

stress field in space and time in a very precise way,otherwise we would be able to predict both seismicand magnetic events. We approach this dynamic varia-tion of the stress field by estimating a mean orientationof the stress tensor for a domain larger than the sourcezone of the seismic or magnetic event, where stressfluctuates in space and time.

The decoupling of oblique slip transtensile regimein the strike-slip and dip-slip components is explainedby partitioning of deformation in the Azores Plateaubetween remote stress specified by boundary condi-tions related to plate kinematics and local stress due topresence of high topography in the plateau.

One important limitation of the previous stressmodels is the exclusion of the bathymetric data fromthe analysis. However, the use of morphological datafrom the sea floor to infer the geodynamic mechanismscan be done with good results in the oceanic domain,as the erosion processes are slower than in the Islands.So the bathymetric map can thus be used in tectonicinterpretation if we combine morphology and focalmechanism of earthquakes to access type and senseof fault displacements. To include the morphologicalpattern into the regional stress evaluation we assumethat:

(1) In the area between the anomaly 5 isochron andthe MAR there is a ‘normal’ spreading regime, charac-terised byσ3 approximately perpendicular to the ridgetrend andσ1 vertical;

(2) The seamounts are not considered to the re-gional stress distribution, due to the fact that the stressconditions favouring their emplacement results frominterference between the fault systems on a local scale.

(3) In the Azores domain, the LVR represent fis-sural volcanism along fault zones, in this senseσ1(maximum compression) stress direction would beslightly oblique to the LVR orientations.

Based on the orientations of volcanic ridges on thethree different domains, we observe a marked rota-tion from the WNW-ESE (magmatic injection alongthe 120◦ faults) in the northwestern domain to NNW-SSE (but in this case along the 150◦ faults) in theSouthern one. From West to East we then have aσ1trace defining a sigmoidal pattern, from near 120◦ nearthe ridge axis to nearly N–S in the southeastern partof the plateau. The stress pattern deduced from themorpho-structural analysis is displayed in Figure 8.

The above-described fault pattern and its failuremode, the marked deflection on the stress axes in theAzores domain as well as the acute angles of theσ 1trace with the dominant fault family in each area, sug-

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Figure 8. Schematic stress pattern of the Azores plateau as inferred from the morphological features. Dots represent the boundaries between the different LVRdomains. Thicker lines representmaximum compressive stress orientation (σ1), thinner lines represent the minimum compressive stress orientation (σ3). The intermediate compressive stress (σ2) is vertical. (Inset top) – Taxis calculated from events displayed in figure 6 along with calculated standard deviations, the kinematic orientation of the spreading axis calculated from Nuvel-1 model (DeMets, 1990) inthe three individualised areas is also shown for reference. (Inset bottom) – Proposed schematic regional tectonic model for the Azores domain.

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gest a viscoelastic response of the Azores domain,taken as a whole.

In order to test if this stress pattern (inferredfrom morphology) matches the one obtained from theseismo-tectonic orientations, we calculated the exten-sion axis (T) from the focal mechanisms presented inFigure 6, the results being displayed in Table 1 andFigure 8. The T axis orientation for the three seis-mic groups follows a path very similar to what weobserve from the LVR. The tensions associated withCentral set seismic events have an average orientationtoo northerly than we would expect, but if we removeevent number 4, then the remaining ones present avery small dispersion around the 20◦ ± 6◦ N. The fourevents on the Hirondelle set also present a remarkablecoherence (42◦ ± 4◦ N). Finally, in the S. Miguel setthe dispersion in the tension directions is bigger thanin the previous cases (51◦±32◦ N) even if we disregardevent 16 (65◦±17◦ N) which is of less quality than theprevious ones. Again, associated with this scattering inthe tension directions we can observe a more complexstructural pattern inferred from the bathymetry, spe-cially in the Easternmost part of the plateau around S.Miguel Basin. This scattering occurs in the transitionzone between the Azores domain and the Gloria faultand is interpreted as a consequence of the interferencepattern between the stress field acting upon this twodomains.

Constraints from the Global Kinematic Models

The above conclusions must be compared with theconstraints that are given by the global kinematic platemodels. To do so, we calculated the mean directions ofthe extension axis inferred from the relative movementin the EU/AF boundary along the three different sets,also displayed in Table 1 and Figure 8. The diffusecharacter of the Eu/Af boundary in this area impliesthat it is extremely difficult to establish with accuracythe position of the rotation pole that describes the rel-ative motion between those two plates. For instance, ifwe confront the pole calculated with the data presentin the Eu/Af (21◦ N 20.6◦ W) boundary with thoseobtained from the closure-fitting procedure for threeplates (19.5◦ N, 23.7◦ W) from the NUVEL 1 model(DeMets et al., 1990), the spreading axis would suffera 10◦ clockwise rotation.

The high proximity of the Eu/Af rotation pole im-plies that in a distance no longer than 300 km, thedirection in the relative movement rotates 8◦. This is

clearly less than the 30◦ to 40◦ rotation observed fromthe focal mechanisms or the volcanic ridges and faultsthat we have interpreted. This disparity probably re-flects the fact that the lithospheric response is slowerin depth – where the ductile behaviour dominates –from what is observed in the shallow crust – wherethe brittle fracturation mechanisms dominate – to ac-commodate the small changes of the regional stressfield.

In this sense the global models like NUVEL 1 andRM2 (Minster and Jordan, 1978) cannot describe ac-curately the bathymetric and seismic signatures of theplateau. However, the decreasing obliquity betweenthe seismotectonic and the kinematic directions im-plies that the Azores block is acting as a transfer zonethat accommodates the differential shear movementbetween the Mid-Atlantic ridge and the Gloria Fault.This present complex stress pattern associated withthe inherited structures from the past kinematic his-tory is probably inhibiting the development of typicalmid-ocean ridge geometry in this area.

Conclusions

Following the data here presented several conclusionscan be outlined:

(1) The Azores Triple Junction morphology is con-trolled by two sets of conjugated faults oriented 150◦and 120◦ that constrain the development and shape ofthe basins and constitute the framework for the on-set of volcanism, both at its intersections (seamountconstructions) or along the main accidents (volcanicridges development). This pattern is striking along theGraciosa – São Miguel axis but is well developed allover the Azores Plateau.

(2) The area comprised between the Açor bankand the volcanic ridge flanking the south of the EastGraciosa basin consists of a horst and graben system.This area is much more controlled by a high magmaticbudget where the 120◦ direction is clearly prevalent.

(3) There is a marked correlation between thevolcano-tectonic and the seismo-tectonic directions(as inferred by LVR and T axis, respectively). Theyare approximately oriented 90◦ apart in each definedgroup and both follow the same path of clockwiserotation along the three domains within the plateau.This allows to constrain the stress field in this area.The maximum and minimum compression directionsshow a marked bending in the Azores domain that itis not confined to the Terceira axis. The wideness of

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the disturbed zone and its characteristic morphologyimply that the deformation is not confined to a narrowzone as its postulated for classical extensional plateboundaries, and points for a non conventional plateboundary geometry. Furthermore it is possible to ob-serve from west to east a decreasing obliquity betweenthe kinematic and the seismo-tectonic directions.

Taken all together these data suggest that theAzores domain is a diffuse plate boundary acting si-multaneously as a oblique ultra slow spreading centreand as a transfer zone that accommodates the differen-tial shear movement between the Eurasian and Africanplates. In this sense the Azores can be viewed as alimiting case of a ultra slow ridge where the tectonicheritage and the regional transtensional regime con-trols the magmatic feeding and the regular spreadingalong stable segments is replaced by a more chaotic,less confined, regime. Lying the accretionary axis be-tween the N120 and the N150 directions, as a responseto small changes in the regional stress field.

Acknowledgements

We would like to thank the Captain R. Bourne and theofficers and crew of the RRS Charles Darwin, whohave conducted the MARFLUX/ATJ ESCAPE cruise.We wish to thank Dr. Chris German, Chief Scientistof HEAT cruise, who provided data used in this work.We also would like to mention Dr. Henri Bougault forall the invaluable help during MARFLUX Project. Fi-nally thanks are also due to Valentina, for her help andpersistence during the final stages of the map editionprocess. We used GMT (P. Wessel and W. Smith) tomake some of the figures in this report. This work wassupported by MASTII EEC project MARFLUX/ATJ(contract n◦-CT930070) and by a PRAXIS XXI grantreference BM1297/94. We thank two anonymous ref-erees for comments that have greatly improved themanuscript.

References

Argus, D.F., Gordon, R.G., DeMets, C. and Stein, S., 1989, Closureof the Africa-Eurasia-North America plate motion circuit and thetectonics of the Gloria fault,J. Geophys. Res.94: 5585–5602

Bastos, L., Osório, J., Barbeito, A., Hein, G., 1998, Resultsfrom geodetic measurements in the western part of the African-Eurasian plate boundary.In Tectonophysics special issue294:3–4.

Batiza, R., 1982, Abundances, distribution and size of volcanoes inthe Pacific Ocean and implications for the origin of non-hot spotvolcanoes.Earth Planet. Sci. Lett.60: 195–206.

Bougault, H. and Treuil, 1980, Mid-Atlantic Ridge: zero age geo-chemical variations between Azores and 22◦ N. Nature 286:209–212.

Bougault, H. and Cande, S.C., 1985, Background, objectives andsummary of principal results: Deep Sea Drilling Project sites556–564, in Initial Reports of the Deep Sea Drilling Projectedited by H. Bougault and S. Cande, pp. 5–16 (U.S. Govt.Printing Office), Washington.

Bougault, H., German, C., Miranda, M. and the MARFLUX/ATJParticipants, 1996, Mid-Atlantic Ridge: Hydrothermal fluxes atthe Azores Triple Junction.InterRidge News5 2: 15–17.

Buforn, E., Udias, A. and Colombás, M.A., 1988, Seismicity,source mechanisms and tectonics of the Azores-Gibraltar plateboundary.Tectonophysics152: 89–118.

Dauteuil, O. and Brun, J.P., 1993, Oblique rifting in a slowspreading ridge.Nature361: 145–148.

DeMets, C., Gordon, R., Argus, D. and Stein, S., 1990, Currentplate motions.Geophys. J. R. Astr. Soc.,101: 425–478.

Detrick, B., Needham, H.D. and Renard, V., 1995, Gravity anom-alies and crustal thickness variations along the Mid-AtlanticRidge between 33◦ N and 40◦ N. J. Geophys. Res.100: 3767–3787.

Dosso, L., Bonnier, O., Bollinger, C., Bougault, H., Etoubleau,J. and LANGMUIR, C.H., 1996, Geochemical structure ofthe Mid-Atlantic Ridge between 10◦ and 41◦ N. Journal ofconference Abstracts1: 784.

Feraud, G., Kaneoka, I. and Allègre, J.C., 1980, K/Ar ages andstress pattern in the Azores: Geodynamic implications.EarthPlanet. Sci. Lett46: 275–286.

Géli, L., Renard, V. and Rommevaux, C., 1994, Ocean crust for-mation at very slow spreading centers: a model for the MohnsRidge, near 72◦ N, based on magnetic, gravity and seismic data.J. Geophys. Res.99: 2995–3013.

German et al, 1996, Hydrothermal exploration at the Azores Triple-Junction: tectonic control of venting at slow-spreading ridges?Earth. Planet. Sci. Lett.138: 93–104

Grácia, E., Canals, M., Farrán, L., Prieto, M., Sorribas, J. andGEBRA TEAM, 1996, Morphostructure and Evolution of theCentral and Eastern Bransfield Basins (NW Antartic Peninsula).Mar. Geophys. Res.18: 429–448.

Grimison, N. and Chen, 1986, The Azores-Gibraltar plate bound-ary: focal mechanism, depths of earthquakes and their tectonicimplications.J. Geophys. Res.92: 2029–2047.

Grindley, N.R., Madsen, J.A., Rommevaux, C., Sclater, J. andMurphy, S., 1996, Southwest Indian Ridge 15◦E–35◦E: a geo-physical investigation of an Ultra-Slow spreading Mid-OceanRidge System.InterRidge News5 2: 15–17.

Hirn, A., Haessler, H., Hoang Tronc, P., Wittlinger, G., MendesVictor, L.A., 1980, Aftershock sequence of the January 1., 1980earthquake and present day tectonics in the Azores.Geophys.Res. Lett.7: 501–504

Krause, D.C. and Watkins, N.D., 1970, North Atlantic crustal gen-esis in the vicinity of the Azores.Geophys. J. R. Astron. Soc.19: 261–283.

Larson, R.L., Searle, R. C., Kleinrock, M.C., Schouten H., Bird,R.T., Naar, D.F. Rusby, R.I., Hooft, E. and Lasthiotakis, H.,1992, Roller-bearing tectonic evolution of the Juan Fernandezmicroplate.Nature356: 571–576.

Laughton, A.S., Roberts, D.G. and Graves, R., 1975, Bathym-etry of the Northeast Atlantic: Mid-Atlantic Ridge to South-westEurope.Deep-Sea Res.22: 791–810.

Ligi, M., Bonatti, E., Bortoluzzi, G., Carrara, G., Fabretti, P., Pen-itenti, D., Gilod, D., Peyve, A.A., Skolotnev, S. and Turko, N.,

156

1997, Death and transfiguration of a triple Junction in the SouthAtlantic. Science276: 243–245

Lonsdale, P., 1988, Structural pattern of the Galapagos Microplateand evolution of the Galapagos Triple Junctions.J. Geophys. Res.93: 13551–13574.

Lourenço, N., 1997, Morfo-tectónica da junção tripla dos Açores:estudo de pormenor do segmento Lucky Strike (37◦17′ N,32◦16′ W), a partir de dados de batimetria multi-feixe e sonarlateral de alta resolução.

Luis, J.F., Miranda, J.M., Galdeano, A., Patriat, P., Rossignol, J.C.,Mendes Victor, L.A., 1994, The Azores triple junction evolu-tion since 10 Ma from aeromagnetic survey of the Mid-AtlanticRidge.Earth and Planet. Sci. Lett.125: 439–459.

Luis, J.F., 1996, Le Plateau des Açores et le point triple asso-cié: analyse géophysique et evolution. PhD thesis, Paris VIIUniversity, Paris, France, 203 p.

Madeira, J. and Ribeiro, A., 1990, Geodynamic models for theAzores triple junction: a contribution from tectonics.Tectono-physics184: 405–415.

Madeira, J., 1998, Estudo de neotectónica nas ilhas do Faial, Pico eS. Jorge: Uma contribuição para o conhecimento geodinâmico daJunção Tripla dos Açores. PhD Thesis. Lisbon University, 481 p.

Mckenzie, D., 1972, Active Tectonics of the Mediterranean region.Geophys. J. R. Astron. Soc.30: 109–185.

Minster, J. and Jordan, T., 1978, Present day plate motions.J.Geophys. Res.83: 5331–5354.

Miranda, J.M., Luis, J.F., Abreu, I., Mendes Victor, L.A., Galdeano,A., Rossignol, J.C., 1991, Tectonic framework of the Azorestriple junction.Geophys. Res. Lett.188: 1421–1424.

Miranda, M., J.F. Luis, N. Lourenço and the Scientific Party of theESCAPE mission, 1996, Cruise Report. 40 p.

Mitchell, N.C., 1991. Distributed extension at the Indian OceanTriple Junction.J. Geophys. Res.96: 8019–8043.

Mitchell, N.C. and Livermore, R.A., 1998, The present configura-tion of the Bouvet triple junction,Geology26: 267–270.

Morgan, W., 1971, Convection plumes in the lower mantle.Nature230: 42–43.

Munschy, M. and Schlich, R., 1989, The Rodriguez Triple Junction(Indian Ocean): structure and evolution for the past one millionyears.Mar. Geophys. Res.11: 1–14.

Needham, H.D. and SIGMA Scientific Team, 1991, The Crest of theMid-Atlantic Ridge between 40◦ and 15◦ N: very broad swathmapping with the EM12 echo sounding system.EOS Trans.AGU, 72: 470.

Pagarete, J., Pinto, J., Mendes, V., Ribeiro, A. and Antunes, C.1998, The importance of classical geodetic observations in thegeodynamic behaviour of the Azores Archipelago. 294 (3–4):281–290.

Reches, Z., 1983, Faulting of rocks in three dimensional strainfields. I: failure of rocks in poliaxial, servo control experiments.Tectonophysics95: 111–132.

Reches, Z., 1988, Evolution of fault patterns in clay experiments.Tectonophysics145: 141–156.

Rommevaux, C., 1994, Etude Gravimétrique et magnétique del‘evolution de la segmentation des dorsales lentes. PhD thesis,Paris VII University, Paris, France, pp. 321.

Schilling, J.G., 1975, Azores mantle blob: rare earth evidence.EarthPlanet. Sci. Lett.25: 103–115.

Searle, R., 1980, Tectonic pattern of the Azores spreading centreand triple junction.Earth Planet. Sci. Lett.51: 415–434.

Silver P., Russo, R., Lithgow-Bertelloni, C., 1998, Coupling ofSouth American and African plate motion and plate deformation.Science279: 60–63.

Smith, D.K. and Cann, J. R., 1992, The role of seamount volcanismin crustal construction at the Mid-Atlantic Ridge (24◦–30◦ N). J.Geophys. Res.97: 1645–1658.

Smith, W. and Sandwell, D., 1997, Global seafloor bathymetry fromsatellite altimetry and ship depth soundings.Science277: 1956–1962.

White, W.M., Schilling, J.G. and Hart, S.R., 1976, Evidence for theAzores mantle plume from strontium isotope geochemistry of theCentral North Atlantic.Nature263: 659–663.