recent intraplate stress field in the eastern duero basin (n spain)

Recent intraplate stress field in the eastern Duero Basin (N Spain)

Angel L. CorteÂ s* and Adolfo MaestroDepartamento de GeologõÂa, Universidad de Zaragoza, Plaza San Francisco s/n, E-50009 Zaragoza, Spain

Introduction

Recent tectonic stress in the northernIberian Plate is not well defined. Theinterpolation of present-day stress or-ientation data from the World StressMap (MuÈ ller et al., 1992;Zoback, 1992)suggests aNW±SE compression inwes-tern Europe which is related to twotectonic forces: the ridge-push fromthe Mid-Atlantic ridge and the plate-collision in the Mediterranean area(MuÈ ller et al., 1992; GoÈ lke and Co-blentz, 1996). Nevertheless, geologicaland geophysical data in the northernpart of the Iberian Plate show stressorientations that differ from the ex-pected directions in this region of Eur-ope. Depending on the age of materials(upperMiocene toQuaternary), type ofanalysed data (geological or geophysi-cal) and structural characteristics of thestudied area, several tectonic regimesrelated to the neotectonic stage havebeen inferred by different authors (San-tanach et al., 1980; SimoÂ n, 1989; Philipet al., 1992; RebaõÈ et al., 1992; Juradoand MuÈ ller, 1997).In this work we present new palaeos-

tress indicators obtained from fault andjoint population analysis in upper Mio-cene±Quaternary rocks of the easternDuero Basin. This region is located inthe northern part of the Iberian Penin-sula and presents neither a significantseismic record nor large-scale deforma-tions in recent deposits (CorteÂ s andMaestro, 1997). The main objectives ofthis work are: (i) the characterization ofthe recent tectonic stress field in theeasternDueroBasin; (ii) the comparison

with recent and present-day stress indi-cators in neighbouring areas of the Iber-ian Plate and (iii) the determination ofpossible stress sources responsible forthe orientation of the late Neogene±Quaternarymaximumhorizontal stress.

Geological setting

The Duero Basin is a Tertiary basinlocated in the northern part of theIberian Peninsula (Fig. 1a). It occupiesan area around 50,000 km2, boundedby the Cantabrian Mountains to thenorth, the Hercynian Massif to thewest, the Iberian Chain to the east andtheSpanishCentral System to the southand south-east. The easternmost partof the Duero Basin corresponds withthe Tertiary AlmazaÂ n Basin. The fillingof the Duero Basin consists of 1000±3500-m thick Palaeogene and Neogenesediments (Bond, 1996; Santisteban etal., 1996). The Tertiary units comprisecontinental alluvial deposits, with con-glomerates and sandstones near theborders and lacustrine limestones andgypsum toward the central-southernpart of the basin. Clastic depositiondominated the Palaeogene, whereasNeogene units are characterized by lu-tites and lacustrine limestones.The area studied in this work corre-

sponds to the eastern part of the DueroBasin, including the AlmazaÂ n Basin. Itis located between threemajor structur-al units uplifted during the Tertiary(Fig. 1b):1 The Cameros Massif, located to thenorth, consists of Jurassic marine lime-stones and Upper Jurassic±Lower Cre-taceous terrestrial sandstones, shalesand limestones. They overthrust theTertiary deposits of the Duero and

AlmazaÂ n basins to the south. The strikeof thrusts changes from WNW±ESEdirection in the western part to E±Wand NE±SW near Soria.2 TheAragonianBranchof the IberianChainbounds theAlmazaÂ nBasin to theeast. It consists of Cambrian and Or-dovician shales and sandstones uncon-formably overlain by Jurassic and Cre-taceous sandstones and limestones,folded with a NW±SE trend duringthe Palaeogene compression.3 The Castilian Branch forms thesouthern border of the AlmazaÂ n Basin.It consists mainly of Triassic sand-stones and limestones and Jurassiclimestones with very gentle folds trend-ingWNW±ESE andNE±SW.TheCas-tilian Branch joints to the west with theSpanish Central System which ismainly formed by Palaeozoic meta-morphic and igneous rocks. Their con-tacts with the Duero and AlmazaÂ n ba-sins are usually unconformities of theNeogene strata upon the Palaeozoicand Mesozoic rocks.The Duero Basin shows a wide-

spread cover of Neogene horizontalbeds which present only brittle defor-mation (faults and fractures) and somekilometric structures consisting of gen-tle folds and faults. Themost importantNeogene macrostructure in the studiedregion [trendingE±W, 15km long and2kmwide (10±158S dipping limb)] corre-sponds to the Langa de Duero mono-cline, affectingMiddle±UpperMiocenerocks. This fold is located over an E±Wfault-related anticline in Mesozoic±Pa-laeogene materials which has been in-ferred by means of geophysical data(Philips Oil Co.-EspanÄ a, 1982 in CorteÂ set al., 1999). The Langa de Dueromonocline and someminor folds affect-

*C 1998 Blackwell Science Ltd 287

ABSTRACTPalaeostresses inferred from brittle mesostructures in theeastern Duero Basin show a recent stress field characterized byan extensional regime, with local strike-slip and compressionalstress states. Orientations of the maximum horizontal stress(SHmax) show a relative scattering with two main modes: NNE toNE±SW and NW±SE. These orientations suggest the existence oftwo stress sources responsible for the dominant directions of themaximum horizontal stress in northeastern Iberia. Extensionalstructures within a broad-scale compressional stress field can berelated to both the decrease in relative stress magnitudes from

active margins to intraplate regions and rifting proccessesocurring in eastern Iberia. Stress states with NW±SE-trendingSHmax are compatible with the dominant pattern established forwestern Europe. NE±SW orientations of SHmax suggest theoccurrence of tectonic forces coming from the Pyrenean zone.Geological and geophysical data indicate the existence of bothorientations from the upper Miocene to the present-day inNE Iberia.

Terra Nova, 10, 287±294, 1998

AhedBhedChedDhedRef markerFig markerTable markerRef endRef start

*Correspondence: Tel. + 34/976 762127;

E-mail: [email protected]

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ing Neogene rocks in the Aranda deDuero-Burgo de Osma area are prob-ably linked to reverse or oblique-re-verse reactivation of Palaeogene struc-tures. Close to the western limit of theCameros-Demanda Massif and nearthe Aragonian Branch, gentle NW±SE-trending folds affect the Neogenerocks (LendõÂ nez and Ruiz, 1991; Maes-tro andCasas, 1995). In the central partof the Duero Basin, movements ofbasement faults during Neogene timesare hardly apparent at the surface butproduce differential subsidence or in-duce the development of small faults(Santisteban et al., 1996).

Palaeostress analysis

Palaeostress analysis was performedusing brittle mesostructures, mainlystriated fault planes and tension joints,measured at 31 sites in subhorizontalupperMiocene±Quaternary rocks (Fig.2). Three hundred and twenty-eightstriated faults (22 sites), around 500extension fractures (9 sites), tensiongashes (1 site) and stylolytes (1 site)were measured and analysed. Normalfaults with small offset are dominant atthe exposure scale although strike-slip

faults with near-vertical planes can berecognized. In both cases, the slip onthe fault planes varies between centi-metres and a few metres. Tensiongashes at site 13 show an importantseparation between blocks (up to 30cm) displaying fibrous calcite crystalsfilling them. In the westernmost part ofthe studied area (sites 1±12) and SSE ofAlmazaÂ n (sites 22±29), measured faultsand joints are located in lacustrine lime-stones. The remaining sites are situatedin detrital rocks.The majority of measured joints cor-

respond to extension fractures formednormal to the directionofs3 (Hancock,1985; Hancock and Engelder, 1989).The direction of systematic vertical ex-tension joints is parallel to the greatesthorizontal stress (SHmax). Non-sys-tematic cross-joints linked to the prin-cipal sets are roughly parallel to theminimum horizontal stress (Fig. 3).Arlegui (1996) considers 50 measuredjoints as the minimum data number forcharacterizing the reliable fracture pat-tern in each site.At present there are several methods

for fault population analysis. We havechosenthecombinedapplicationof threemethods (Casas et al., 1990; Casas and

Maestro, 1996): (i) the Right Dihedramethod (Angelier and Mechler, 1977);(ii) they±Rdiagram(SimoÂ n, 1986,1989);and (iii) the numerical method of Etch-ecopar (Etchecopar et al., 1981; Etche-copar, 1984).Methods (ii) and (iii) allowthe determination not only of stress axisorientation but also the stress ratio in-dicated by R=(sz±sx)/(sy±sx) (Bott,1959) andRe= (s2±s3)/(s1±s3) (Etche-copar et al., 1981), respectively.Some sites present two or even three

different stress tensors calculated froma single fault population. Using (iii),taking into account the whole of faults,several solutions depending on thenumber of the explained faults can befound. After this preliminary analysis,we chose the solution that explained themajority of faults with a good histo-gram of angular discrepancies and agood distribution of explained faultson the Mohr circle. Faults explainedby this solution are eliminated andremaining faults are analysed again(Fig. 4). Reliability of calculated solu-tions can be tested using random sub-sampling analysis (Arlegui and SimoÂ n,1998). Occurrence of different stresstensors with similar orientations in thesame site could be related to local per-turbations of the regional stress field.To define the relative quality of the

calculated stress tensors, the Quality In-dex (Q), defined by SimoÂ n et al. (1996),was used. This index can be applied tonumerical solutions obtained from anymethod of fault population analysiswhich provides angular discrepanciesbetween measured and calculated slipvectors and distribution of explainedfaults in the Mohr circle representation.

Quality index ��7

a

��

t

n� t

��1ÿ 4

t

��c

t

�where a corresponds to the averagedeviation in degrees (optimized duringthe calculation) between theoreticaland measured striations. Solutionswith a 4 158 were usually rejectedduring the calculation of the tensor axes(Casas et al., 1990). t is the number offaults defining the stress tensor and n isthe number of faults which are notexplained by any stress tensor in thesite. c is the number of faults explainedby theMohr±Coulomb criterion. Thesefaults must be situated over the failureenvelope, considering 308 as the coeffi-

288 *C 1998 Blackwell Science Ltd

Terra Nova, Vol 10, No. 5, 287±294 Intraplate stress field, Duero Basin . A. L. CorteÂ s and A. Maestro. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Fig. 1 (a) Geological sketch of the Iberian Peninsula and location of the studied zone.(AB, AlmazaÂ n Basin; BC, Betic Chain; CM, Cantabrian Mountains; CR, CatalonianRanges; CS, Central System; DB, Duero Basin; EB, Ebro Basin; GB, GuadalquivirBasin; HM, Hesperic Massif; IC, Iberian Chain; MB,Madrid Basin; PY, Pyrenees). (b)Geological map of the eastern Duero and AlmazaÂ n basins and sourronding areasdisplaying main large structures affecting Neogene materials.

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cient of static friction (mean value fordifferent lithologies determined byBar-ton and Choubey, 1977). Occasionally

fault planes present slickensides (or ob-lique stylolites) as slip indicators. Inthese case, we assume that planes are

near perpendicular to s1 and located atthe right part of the Mohr circle belowthe failure envelope (Fig. 4a). If fieldobservations indicate planes with slick-ensides, we do not reject this solutionalthough the Quality Index decreases.An arbitrary scale from A to D has

been associated with this parameter toease comparison between different sites(see Table 1). Stress tensors of highquality (A) correspond toQ larger than0.7,B corresponds toQbetween 0.4 and0.7, C corresponds to Q ranging from0.1 to 0.4 and D to Q smaller than 0.1.Tensors of good quality (A, B, C) wereused for regional synthesis. Tensors oflow quality (D) were used dubiously.Results of palaeostress analysis are

shown in Table 1. The graphical repre-sentation of orientation of the principalstress axes is displayed by means of they±R diagram (SimoÂ n, 1986) showing


Intraplate stress field, Duero Basin . A. L. CorteÂ s and A. Maestro Terra Nova, Vol 10, No. 5, 287±294. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Fig. 2 Geological map of the studied area showing SHmax directions obtained from palaeostress analysis. Stereoplots correspond tostriated faults in each site. The size of arrows depends on theQuality Indexdetermined from fault population analysis.SHmax directionsobtained from joints, tension gashes and stylolites are represented in a different style.

Fig. 3 Schematic line drawing of the fracture pattern observed at site 23 and the inferredpalaeostress directions. Rose diagram shows the measured directions of joints.

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the orientation of the maximum hori-zontal stress axes and their associatedstress regime (Fig. 5).

Orientations of SHmax show twomain modes trending NNE to NE±SW and NW±SE. They correspond to

extensional (19 ellipsoids) and strike-slip and compressional (11 ellipsoids)stress tensors obtained from fault po-pulation analysis. Analysis of joints,stylolytes and tension gashes may in-dicate the orientation of SHmax but notthe associated tectonic regime. The dis-tribution ofSHmax orientations inferredfrom brittle mesostructures in each siteis shown in Fig. 2. The relative chron-ology between extensional and com-pressional structures is not clear. Onlytwo sites (8 and 12) show normal faultscross-cutting strike-slip faults, but thenumber of observations is not signifi-cant. In the same way, chronologicalrelationships between NE±SW andNW±SE SHmax orientations cannot beestablished since both directions havebeen inferred from upper Miocene andPlio-Quaternary rocks.

Data comparison and discussion

The existence of several orientations ofSHmax within the Iberian Plate from theupperMiocene has given rise to severalinterpretations (i.e. Casas and Maes-tro, 1996; CorteÂ s et al., 1996; Juradoand MuÈ ller, 1997); similarly, relation-ships between local tectonic stressesand the broad-scale European stressfield during the Pliocene and Quatern-ary are ambiguous. The overall exten-sion regime of the northeastern part ofIberia is a complex geodynamic scheme(SimoÂ n, 1989; De Vicente et al., 1996;Sanz de Galdeano, 1996). In order toexplain the tectonic stress states ob-tained in this paper within an intraplatedeformation context we show the re-cent and present-day stress fields estab-lished in neighbouring areas of theIberian Plate.

Recent stress field

The palaeostress evolution of north-eastern Iberia since earliest Miocenehas been interpreted to result from anextensional regime with minor com-pressional episodes (Santanach et al.,1980). Palaeostress indicators in thePyrenees area suggest a N±S to NNWcompression (Philip et al., 1992; RebaõÈet al., 1992). In the eastern IberianChain and central Ebro Basin, the LateCenozoic stress field is characterizedfrom fault and joint population analy-sis by a N±S compression and a subse-quent multidirectional tension field,probably Quaternary in age (SimoÂ n,



Fig. 4 Process followed for determining several stress tensors from a single faultpopulation bymeans of Etchecopar's method. (a) Best solution explaining the majorityof measured faults. Faults not compatible with the Mohr±Coulomb criterioncorrespond to planes with slikensides or oblique stylolites. (b) Second solution obtainedfrom the remaining faults of (a). (c) Third solution obtained the remaining faults of (b).

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1989; Arlegui, 1996). In central andsouthern Iberia the analysis of striatedfaults in upper Miocene to Quaternarydeposits shows preferred directions ofthe maximum horizontal stress (SHmax)around NW±SE and NE±SW in theMadrid Basin (De Vicente et al., 1996)and a compressional regional stressfield with s1 subhorizontal trendingNNW±SSE around the Betic cordil-leras (Galindo-ZaldõÂ var et al., 1993).

Present-day stress field

The present-day tectonic stress in theIberian Peninsula has been inferredmainly from earthquake focal mechan-isms and borehole breakout analysis.Focal mechanisms in the Pyrenees(Gallart et al., 1982, 1985) suggest sev-eral orientations ofs1 corresponding tothrust (s1: WNW±ESE) and strike-slipfaults which indicate a NW±SE com-

pressive regime to thewest (Gagnepain-Beyneix et al., 1982) and a NE±SWcompressive regime to the east (Rigoet al., 1997). Analysis of focal mechan-isms in central Iberia indicate a rela-tively homogeneous stress field withSHmax trending NW±SE (De Vicenteet al., 1996), although there are faultplane solutions indicating NE±SW or-ientations of SHmax. Borehole breakoutanalysis in the southern Pyrenees, Ebro



Table 1 Summary of stress tensors and stress orientations obtained from mesostructural analysis. Sites are located in Fig. 2. Age:age of rocks (M, Miocene; UM, upper Miocene; Pl, Pliocene; P-Q, Plio±Quaternary). TD, type of analysed data (F, faults; J, joints;S, stylolytes; T, tension gashes). ND, number of data measured in each site. Values of both principal stress axes and stress ratiohave been estimated from results of Etchecopar's method. SH, maximum horizontal stress. eSH, error in SH azimuth. Re, stress ratioafter Etchecopar et al. (1981)= (s2±s3)/(s1±s3). Rb, stress ratio after Bott (1959)= (sz±s73x)/(sy±sx). s, average deviationbetween theoretical and real striations. n/N, number of data defining each stress tensor/number of data in the fault population. n.e.,number of unexplained faults in each site. Q, quality index

Site Age TD ND s1 s2 s3 SH eSH Re Rb aa n/Nn/N n.e.n.e. QQ

1 UM F 11 146/78 301/11 032/05 301 28 0.07 14.28 7.3 10/11 1 0.52

2 UM F 18 215/85 324/01 054/04 324 18 0.15 6.66 9.2 15/18 3 0.47

3 UM F 12 263/80 142/05 052/08 142 39 0.05 20 10.7 10/12 2 0.33

4 UM F 7 149/80 286/07 017/06 286 3 0.23 4.34 1.5 7/7 0 1.93

5 UM F 8 276/89 114/01 024/00 114 20 0.02 50 9.7 7/8 1 0.27

6 UM F 12 089/77 216/08 307/10 216 60 0.02 50 9.6 11/12 1 0.42

7 UM F 15 193/83 325/05 055/05 325 40 0.02 50 11.1 13/15 2 0.38

8a UM F 40 138/08 317/82 048/00 138 15 0.93 0.93 10.9 28/40 1 0.34

8b UM F 40 284/33 075/54 185/14 284 15 0.52 0.52 13.1 6/12 1 0.12

8c UM F 40 204/54 338/27 080/22 338 1 0.7 1.42 0.8 5/6 1 1.19

9 UM F 20 339/87 232/00 142/02 232 8 0.18 5.55 8.6 14/20 6 0.34

10 UM F 17 222/04 125/78 312/11 222 7 0.09 0.09 9.3 14/17 3 0.44

11a UM F 20 142/05 232/03 353/85 142 56 0.01 ±0.01 8.8 13/20 2 0.48

11b UM F 20 242/10 106/77 334/09 242 3 0.32 0.32 2.1 5/20 2 0.37

12a UM F 32 187/02 277/05 074/84 187 85 0 0 8.8 20/32 7 0.40

12b UM F 32 344/19 181/70 075/05 344 3 0.89 0.89 9.9 5/12 7 0.05

13a UM F 6 144/86 237/00 327/04 237 6 0.34 2.94 11.2 6/6 1 0.20

13b UM T 37 130/00 40 15

14a Pl-Q F 18 136/67 350/19 256/12 350 20 0 0 9.0 9/18 2 0.35

14b Pl-Q F 18 139/02 231/40 047/50 139 6 0.11 0.11 11.1 7/9 2 0.21

15 P-Q J 53 085/00 175 5

16 Pl J 54 115/00 25 5

17a UM J 59 085/00 175 10

17b UM F 6 033/07 300/20 141/68 33 19 0.37 ±0.58 15.5 5/6 1 0.07

18 Pl J 53 140/00 50 10

19a Pl F 14 206/00 319/89 116/01 206 10 0.14 0.14 10.3 8/14 1 0.25

19b Pl F 14 105/85 202/01 292/05 202 20 0.01 100 12.3 5/6 1 0.07

20 M-UM J 53 135/00 45 10

21 M-UM J 72 160/00 70 10

22 Pl F 12 197/78 086/04 356/11 86 20 0 0 5.5 10/12 2 0.63

23 UM J 51 130/00 40 5

24 Pl J 53 110/00 20 10

25 M-Pl F 5 018/76 112/01 202/13 112 4 0.12 8.33 8.3 5/5 0 0.10

26 Pl F 5 311/84 131/06 041/00 131 20 0 0 18.5 5/5 0 0.07

27 Pl F 5 064/04 267/86 154/02 64 20 0.42 0.42 3 5/5 0 0.46

28a UM S 19 030/00 30 15

28b UM F 11 018/82 004/08 094/02 4 20 0.16 6.25 6 8/11 3 0.42

29a M-UM F 24 101/86 328/03 238/03 328 20 0.02 50 10.1 16/24 3 0.43

29b M-UM F 24 288/63 045/13 140/23 45 52 0.11 9.09 10.4 5/8 3 0.08

30 UM-Pl J 49 140/00 50 10

0 31 UM-Pl F 5 108/66 292/24 201/02 292 70 0.16 6.25 18.5 5/5 0 0.07

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Basin and northwestern ValenciaTrough indicates a NE±SW to ENE±WSWmaximum horizontal stress (Jur-ado and MuÈ ller, 1997), suggesting anapparently different source of stressnonparallel to the direction of Eur-opean plate motion.

Geodynamic model

There is not a clear geodynamic modelfor explaining the extensional stressesrecorded fromupperMiocene andPlio-Quaternary rocks of the Duero andAlmazaÂ n basins. In neighbouring areascompressional mesostructures occurand focal mechanisms of earthquakesshow a general strike-slip regime. Im-portant rifting processes during Oligo-cene to middle Miocene were responsi-ble for the development of westernMediterranean extensional basins (Bur-

rus, 1984; Banda and Santanach, 1992).The final rifting stage comprises themiddle Miocene up to the present, re-sulting in the extensional structures ineastern Iberia (SimoÂ n, 1989; Vegas,1992; Sanz de Galdeano, 1996).Major activity in the Pyrenean bor-

der ceased in the late Oligocene±earlyMiocene (Srivastava et al., 1990),although thrusting continued in thewesternmost part until the upper Mio-cene (MunÄ oz-JimeÂ nez andCasas-Sainz,1997). From the early Miocene to pre-sent the plate boundary between Africaand Eurasia has been located at theBetic margin, and the origin of therecent tectonic stress field within theIberian Peninsula is related to the colli-sional resistance forces originated at theBetic margin. The predicted intraplatestress field presents a NW±SE trend forthe maximum horizontal stress, which

explains an important number of pa-laeostress data obtained in this work.Minor compressional structures

(foldsand faults) related to thePyreneancompression appear in Neogene rocksof northern Iberia (SimoÂ n, 1989; Arle-gui, 1996; Sanz deGaldeano, 1996). Theinferred orientation of s1 is N±S toNNE in the Ebro Basin and aroundNE±SWin the IberianChainandDueroBasin (Fig. 6a), probably due to thedeflection of stress trajectories by majorstructures (Casas and Maestro, 1996;CorteÂ s et al., 1996). Occurence of Plio-cene±Quaternary strike-slip and reversefaults (Philip et al., 1992) and focalmechanisms (RebaõÈ et al., 1992; SouriauandPauchet, 1998) suggest the existenceof recent compressional forces linked tothe Pyrenean border.Indicators of the `Pyrenean' stress

field during the Neogene seem to berestricted to the northeastern part ofIberia, whereas NW±SE maximumstress (Fig. 6b) became widespread inthe Iberian Peninsula (Galindo-ZaldõÂ -var et al., 1993; De Vicente et al., 1996).Chronological relationships betweenNE±SW and NW±SE stress directionsare not clear. These results indicate acomplex tectonic situation during thelate Neogene and probably during theQuaternary. Areas located between thetwo major plate boundaries recorded aresultant stress field with areal andtemporary variations of SHmax fromNE±SW to NW±SE (Fig. 6).The decreasing stress magnitudes at

the plate margins implies a coaxialchange from a reverse regime tostrike-slip and normal regimes with nochange in the maximum horizontalstress (Tapponier and Molnar, 1976;Sassi and Faure, 1997). Mechanicalcompatibility between extensional ten-sors in intraplate areas and regionalcompressional and/or strike-slip ten-sors occurs when SHmax coincides inboth ellipsoids. The Duero Basin isrelatively distant from the Neogene±Quaternary active plate margins (Pyr-enees and Betics). Occurrence of coax-ial extensional and strike-slip stresses inthe studied zone can be related to thissituation. Extensional proccesses ineastern Iberia could also explain theexistence of intraplate extensionalstress states in the northeastern part ofthe Iberian Plate. Decrease to the westof themagnitude of extensional stressespermits the formation of brittle exten-



Fig. 5 Diagrams showing all the SHmax orientations determined from the analysis ofbrittle mesostructures. (a) y±R diagram displaying the orientation of SHmax and thestress ratio for each stress tensor (obtained from fault population analysis). (b)Orientations ofSHmax obtained from joints, stylolytes and tension gashes. (c) Frequencyhistogram of SHmax directions. (d) Number of stress directions associated with eachmode corresponding to extensional (s2) and strike-slip and compressional (s1) regimes.

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sionalmesostructures (faults and joints)but not significant macrostructures.The tectonic regime and orientation

of local stresses depend on both therelative magnitude of forces comingfrom the active plate margins and thelocal effects of possible subsidiarystress sources: isostatic compensation,lateral mass variation, topography andsedimentary load. The influence ofthese stress sources in the regionalstress field has not been determined.

.5Conclusions

Analysis of brittle mesostructures inupper Miocene±Quaternary sedi-ments allows the recent tectonic stressfield in the eastern Duero Basin to becharacterized. The majority of stresstensors correspond to extensional el-lipsoids although strike-slip and com-pressional tensors have been deter-mined. The SHmax orientation showsa relative scattering with two main

modes trending NNE to NE±SW andNW±SE.This widespread extensional regime

can be related both to the decreasingrelative stress magnitudes from activemargins (Pyrenees and Betics) to intra-plate regions (Duero Basin) and to therifting processes occurring in easternIberia during the Neogene.Stress states with NW±SE trend of

SHmax are compatible with the domi-nant pattern established for westernEurope as a result of both the Mid-Atlantic ridge push and collisionalboundary forces in the Mediterraneanarea. However, stress states with NNEtoNE±SWorientation ofSHmax are notconsistent with the western Europeanmean direction, suggesting differentstress sources from the broad-scaleforces controlling the prevailing stressfield. Compressional forces from thePyrenean border control the SHmax or-ientation in strike-slip and extensionalstress states within the Iberian Chainand Duero Basin. The regional NE±SW pattern is a consequence of thedeflection of stress trajectories (N±Sto NNE at the Pyrenees and Ebro Ba-sin) due to existence of previous NW±SE structures in the Iberian Chain.

Acknowledgements

We thank A.M. Casas Sainz (Universidadde Zaragoza) for their helpful commentsabout the original manuscript. We arethankful to C. Faccenna, J.F. Ritz and A.Nicolas for their reviews and importantcomments. This work has been partiallysupported by the projects SHISTO2-SIG-MA (CSN-ENRESA) and PB97±0997 ofthe Spanish Ministry of Education.

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Fig. 6 Orientation of the maximum horizontal stress trajectories in northeastern Iberiafrom the upperMiocene to present. (a)Mode I (N±S toNE±SWstress field). (b)Mode II(NW-SE stress field). Arrows: synthesis of dominant orientations of SHmax obtainedfrom geological palaeostress indicators (faults and joints) and geophysical data (in situstress measurements and focal mechanisms). Grey areas, Neogene±Quaternary rocks.BAR, Barcelona,MAD,Madrid, ZAR, Zaragoza. (Data from [1]: RebaõÈ et al., 1992; [2]:Jurado andMuÈ ller, 1997; [3]:Massana, 1995; [4]: Arlegui, 1996; [5]: SimoÂ n, 1989; [6]: DeVicente et al., 1996; [7] Paricio and SimoÂ n, 1986; [8]: this work).

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Received 3 July 1998; revised versionaccepted February 1998



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recent intraplate stress field in the eastern duero basin (n spain)

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