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Earth and Planetary Science Le

Magnetic reorientation induced by pressure solution: A potentialmechanism for orogenic-scale remagnetizations

Belén Oliva-Urcia a,b,⁎, Emilio L. Pueyo c, Juan C. Larrasoaña a,d

a Departamento de Ciencias de la Tierra, Universidad de Zaragoza, Pedro Cerbuna 12, 50009, Spainb Department of Geological Sciences, University of Michigan, 1100 N University Ave. Ann Arbor, 48109-1005 MI, USA

c Unidad de Geología y Geofísica. Instituto Geológico y Minero de España, Paseo Fernando el Católico 59-4C, 50006 Spaind Institut de Ciències de la Terra ‘Jaume Almera’, Solé i Sabarís, s/n, 08028 Barcelona, Spain

Received 29 October 2006; received in revised form 30 September 2007; accepted 21 October 2007

Available onlin

Editor: G.D. Price

e 26 October 2007

Abstract

New paleomagnetic analyses of Triassic, Cretaceous and Eocene strata in the south-central Pyrenees show evidence for awidespread remagnetization, located along the southern border of the Axial Zone, the Internal Sierras, and the northern part of theJaca-Pamplona basin. This remagnetization, always reversed in polarity, was acquired after an extensive period of Late Eocene–Early Oligocene folding and tilting in the area, and affects limestones, sandstones, marls and red beds. In addition, a characteristicprefolding component was identified in 30% of Upper Cretaceous and Triassic red beds. These results, together with a revaluationof previously published paleomagnetic data from the central Pyrenees, indicate that the spatial distribution of the postfoldingremagnetization coincides with that of a domain of pressure solution cleavage. A relationship between the intensity of theremagnetization and the characteristic (prefolding component) with respect to the density of cleavage surfaces, leads us to proposea mechanism for the remagnetization related to the development of pressure solution cleavage that is framed within the tectonicevolution of the central Pyrenees. Partial dissolution of rock under tectonic compression leads to the liberation and subsequentaccumulation of insoluble minerals in cleavage planes. Magnetic grains are part of the relatively insoluble residue, and they reorientin the presence of the ambient field after they are freed during dissolution of the rock matrix. Chemical reequilibrium (dissolutionand/or neoformation of magnetic carriers) during this process cannot be excluded. The remagnetization mechanism we propose canhelp to explain widespread remagnetizations in low to moderately deformed rocks without the need of large-scale migration oforogenic fluids.© 2007 Elsevier B.V. All rights reserved.

Keywords: widespread remagnetization; pressure solution cleavage; Pyrenees

⁎ Corresponding author. Departamento de Ciencias de la Tierra,Universidad de Zaragoza, Pedro Cerbuna 12, 50009, Spain.

E-mail addresses: boliva@unizar.es (B. Oliva-Urcia),unaim@igme.es (E.L. Pueyo), jclarra@unizar.es (J.C. Larrasoaña).

0012-821X/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.epsl.2007.10.032

1. Introduction

Pervasive and widespread remagnetizations arecommon in many orogens and provide additional con-straints on the geometry, kinematics and thermo-chemical evolution of fold–thrust belts. Mechanisms

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that have frequently been called upon to explainpervasive remagnetizations of low to moderatelydeformed rocks in tectonic regimes include thermalmetamorphism, migration of orogenic fluids, maturationof organic matter, and diagenetic mineral transforma-tions (McCabe and Elmore, 1989; Jackson, 1990;Menard and Rochette, 1992; Banerjee and Elmore,1997; Katz et al., 1998). Interestingly, the developmentof pressure solution cleavage has only occasionally beenconsidered as a remagnetization mechanism. Forexample, neo-crystallization of magnetite in strataaffected by pressure solution cleavage has beenproposed to account for remagnetizations in weaklydeformed mudrocks (Housen et al., 1993) and lime-stones (Lewchuk et al., 2003; Zegers et al., 2003; Evansand Elmore, 2006). On the other hand, the way in whichpreexisting magnetic grains respond to the formation ofpressure solution planes has been largely ignored andthe role of preexisting magnetic grains as possibleagents of remagnetization due to grain rotation has beenseldom suggested (Zwart and Oele, 1966; Lewchuket al., 2003; Evans et al., 2003; Cioppa and Kodama,2003). The main factors controlling the development ofpressure solution cleavage are horizontal shortening,tectonic and/or sedimentary load, thermal gradient,rheology and composition of the rocks and fluidpressure (Mattauer, 1976). Critical values of ∼170 °Cfor limestones and ∼300 °C for siliciclastic rocks, and5–6 km of tectonic load are typically needed to triggerthe development of pressure solution cleavage. Al-though cleavage can develop throughout the evolutionof a fold–thrust belt, the conditions mentioned above areusually met at the intermediate stages of the evolution oforogenic wedges, when rapid uplift in the internal partsof the orogens leads to maximum tectonic loading andsubsequent compression causes the outward propaga-tion of the deformation front and denudation of cleavedrock. Since tectonically driven pressure solution is aprevalent deformation mechanism active at intermediateto high structural levels in many orogens, we suggestthat the magnetic reorientation induced by the pressuresolution cleavage development, accounts for widespreadremagnetizations. The new equilibrium of the magneticcarriers within the cleavage planes may be physical(rotation of preexisting magnetic grains due to themagnetic ambient field) and/or chemical (dissolutionand/or neoformation of magnetic carriers).

In order to explore the viability of pressure solutioncleavage to explain widespread and pervasive remagne-tizations due to reorganization of preexisting magneticgrains, we carried out a paleomagnetic study ofsedimentary rocks affected by pressure solution cleavage

that outcrop at the southwestern part of the centralPyrenees. Our results, which show the occurrence of apervasive remagnetization, together with a revaluation ofstructural data and other paleomagnetic results from thesouthwestern Pyrenees, allow us to establish a closerelationship between remagnetized rocks and the occur-rence of a pressure solution cleavage front developedduring the structural evolution of the southern Pyrenees.

2. Geological setting

The Pyrenean orogen is an asymmetric mountain beltformed between the Eurasian and Iberian plates duringCretaceous to Miocene times (Muñoz, 1992). Located inthe southwestern part of the central Pyrenees, the studyarea includes the southern part of the Axial Zone, theInternal Sierras, and the northern sector of the Jaca-Pamplona basin (Fig. 1). The region is characterized bya set of south verging, imbricated thrust sheets thatdeveloped in a piggyback fashion as deformation pro-gressed towards the foreland (Teixell, 1998). The Larra-Monte Perdido cover thrust system was formed duringMiddle–Late Eocene. Further deformation during theLate Eocene caused the emplacement of the underlyingGavarnie basement thrust sheet and resulted in anepisode of major folding and increased tectonic loading.Subsequent deformation during the latest Eocene andMiocene moved the Guarga thrust sheet to the south,affecting the overlying thrust sheets, and exhumed theinternal portions of the orogen. As a result, the defor-mation front migrated both laterally and towards theforeland such that a domain of pressure solution cleav-age, with a general fan-shaped geometry, exists in theAxial Zone, the Internal Sierras, and the northern part ofthe Jaca-Pamplona basin (Fig. 1) (Choukroune 1976).The strain increases from the south to the Axial Zonewithin the cleavage front (Choukroune 1976; Holl andAnastasio, 1995). The burial temperatures of the rockswhere cleavage domain developed did not exceed 250°–300 °C (Travé et al., 1997; McCaig et al., 2000). Thetemporal and geometrical relationships between cleav-age and folding allow associating the main cleavagefront with the latest stages of emplacement of theGavarnie thrust sheet, which is thought to have occurredfrom Late Eocene to Miocene (Labaume et al., 1985).

3. Paleomagnetic data

The samples were collected with a gas-powered drillat 114 paleomagnetic sites located within the pressuresolution cleavage domain (grey dots in Fig. 1). Ten tofifteen cores per site were taken. Seventeen sites are

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located in Triassic red beds that directly overlie thePaleozoic rocks of the Axial Zone, eighty-eight sites arelocated in Upper Cretaceous limestones and sandy-limestones from the Internal Sierras, and nine sites arelocated in Lower–Middle Eocene sandstones andmarls of

Fig. 1. Geological map (Choukroune and Séguret, 1973), stratigraphy, and b(Teixell and García Sansegundo, 1995; Millán, 1996; Oliva et al., 1996; Teixelare shown in grey (black) dots.

turbiditic origin from the northern part of the Jaca-Pamplona basin (Gállego river). After the removal of alow-temperature component that is either randomlyoriented or parallel to the present day field direction forthe study area, stepwise thermal demagnetization reveals

alanced cross-section across the southern part of the central Pyreneesl, 1998). Mean of remagnetized (nonremagnetized) paleomagnetic sites

Fig. 2. Paleomagnetic results from the studied rocks. Component B is shown in grey in the orthogonal diagrams. A) On the left, fold test andstereographic projection of the Ch1 component; on the right, Ch1 component after bedding correction combining normal and reverse polarities for theUpper Cretaceous rocks. B) Fold test and stereographic projection for the Ch2 component on the left, and on the right the same Ch2 component afterbedding correction combining normal and reverse polarity for the Triassic red beds. C) Stereograph, intensity decay and orthogonal plot of one redbed sample, with the fold test of the component B for the Triassic sediments. D) The same for two samples of Upper Cretaceous age, and the fold testfor the B component. E) Paleomagnetic data of one sample from Eocene turbidites and the fold test for the B component.

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the occurrence of two stable components in many of thestudied rocks. At high temperatures, a characteristicremanentmagnetization showing both normal and reversepolarities has been isolated in ∼30% of the UpperCretaceous rocks, in some specimens (9%) of the Lower–

Notes to Table 1:Method: PCA: principal component analysis or STACK, using stacking routine (& inclination; bbc: before bedding correction; abc: after bedding correction. Tuplane (strike and dip).

Middle Eocene turbidites (component Ch1), and in∼40%of the Triassic red beds (component Ch2). ComponentCh1 unblocks below 550 °C, which suggests that iscarried bymagnetite in the Upper Cretaceous and Lower–Middle Eocene rocks. Component Ch2 unblocks below

Pueyo et al., in review). n/N: number of samples per site,D&I: declinationnb: unblocking temperature. α95 and k statistic parameters. So: bedding

Table 1Paleomagnetic data, both components (B and Ch) are shown in situ and bedding corrected

“In situ” Bedding corrected

No Site Component Method n/N D&I(bbc)

Tunb(°C)

α95 k So(Str, dip)

D&I(abc)

19 AÑI-8 B STACK 9/10 168, −16 500 10 – 104, 18 S 164, −349 ACU-1 B ACP 9/10 223, −59 350 12 21 88, 78 S 333, −3226 IP-1 B ACP 9/10 199, −45 400 9 27 125, 43 S 131, −783 ARG-7 B ACP 10/10 194, −49 400 8 32 140, 18 S 175, −6227 AGU-1 B ACP 6/8 191, −29 450 7 63 40, 15 W 185, −2118 OTA-6 B ACP 7/10 207, −34 450 13 19 118, 68 N(i) 32, −3325 TOR-1 B ACP 6/8 202, −34 450 15 13 110, 50 S 217, −8415 TEN-2 B ACP 3/5 175, −24 450 41 7 90, 57 S 151, −8014 ASN-3 B ACP 9/10 207, −32 450 10 23 96, 45 S 256, −7121 PIN-1 B STACK 7/10 163, −19 430 17 6 134, 36 S 148, −3323 TOU-4 B ACP 3/6 160, −41 550 11 84 80, 89 N(i) 0, −3913 ELE-1 B ACP 7/13 193, −40 500 8 56 88, 59 S 309, −742 ARG-6 B ACP 8/10 205, −44 450 12 20 155, 25 W 179, −596 ARA-5 B ACP 14/14 189, −43 450 6 44 114, 20 S 181, −6220 MAR-2 B STACK 7/10 178, −42 450 10 25 124, 17 S 165, −558 ARA-8 B ACP 4/9 205, −48 400 19 18 110, 25 S 227, −7611 PIE-4 B ACP 4/6 206, −34 400 45 4 3, 35 S 220, −1616 TEN-3 B ACP 5/10 225, −53 430 10 47 74, 65 S 302, −3710 ACU-2 B ACP 10/10 212, −62 400 6 54 88, 4 N 208, −5812 PIE-3 B ACP 10/10 189, −57 400 9 30 87, 45 S 330, −761 ARG-4 B ACP 9/10 216, −50 450 12 21 40, 12 N 103, −485 TOR-2 B ACP 6/9 211, −47 450 28 5 110, 22 N 208, −2522 ELE-5 B ACP 8/11 209, −40 460 13 16 104, 84 S 352, −5317 OTA-2 B ACP 6/8 155, −39 450 21 10 82, 44 S 105, −754 ARG-8 B ACP 8/9 217, −43 450 10 26 115, 25 S 229, −67

“In situ” Bedding corrected

Site Component Method n/N D&I(bbc)

Tunb(°C)

α95 k D&I(abc)

AÑI-8 Ch ACP + int 16/24 10, 32 580 9 18 8, 50ACU-1 Ch ACP 10/10 356, −38 550 6 64 356, 40IP-1 Ch ACP 7/10 19, 28 550 12 25 355, 67ARG-7 Ch ACP + int 13/15 18, 26 550 7 21 11, 41AGU-1 Ch ACP + int 33/36 32, 47 580 5 24 17, 43OTA-6 Ch ACP + int 16/23 207, 46 550 11 13 206, −66TOR-1 Ch ACP + int 18/37 19, 30 550 6 59 13, 80TEN-2 Ch ACP + int 12/30 165, 22 580 12 13 164, −33ASN-3 Ch ACP 9/10 189, 24 580 11 19 189, −26PIN-1 Ch ACP + int 13/20 37, 32 550 11 14 29, 67TOU-4 Ch ACP + int 10/14 23, −47 550 8 31 142, −36ELE-1 Ch ACP + int 6/13 355, −1 580 7 74 352, 58ARG-6 Ch ACP + int 8/10 352, 28 550 15 13 338, 32ARA-5 Ch ACP 6/9 25, 31 580 7 13 25, 51MAR-2 Ch ACP + int 11/24 13, 24 550 9 23 9, 40ARA-8 Ch ACP 9/9 17, 24 550 10 24 16, 49PIE-4 Ch ACP + int 16/30 10, 63 550 7 29 54, 44TEN-3 Ch ACP 5/9 339, −15 560 8 71 336, 50ACU-2 Ch ACP 8/10 1, 42 550 7 51 1, 38PIE-3 Ch ACP + int 26/55 14, 43 550 6 21 84, 77ARG-4 Ch ACP + int 8/12 23, 34 580 9 63 16, 30TOR-2 Ch ACP + int 17/25 18, 22 550 5 41 18, 0ELE-5 Ch ACP 6/9 14, −18 500 12 23 14, 66OTA-2 Ch ACP + int 20/45 350, 45 580 9 14 301, 88ARG-8 Ch ACP + int 21/26 17, 39 580 5 46 11, 64

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Table 2Samples with the two components, NRM and the intensity of B and Ch components are shown

Numbersite

Name NRM(10 exp-6 A/m)

IntensityB

IntensityCh

IntensityB/Ch

Group Average of B/Chfor each group

StandardDeviation

19 añi8 715 67 49 1.37 1 1.92 1.2419 acu1 1589 474 690 0.69 126 ip1 1017 224 109 2.06 13 arg7 2218 412 104 3.96 127 agu1 2208 1201 788 1.52 118 ota6 3075 300 135 2.22 2 2.03 0.73325 tor1 1965 66 32 2.06 215 ten2 388 157 67 2.34 214 asn3 1201 211 77 2.74 221 pin1 71 4 5 0.80 223 tou4 402 75 155 0.48 3 2.38 1.36713 ele1 1582 415 277 1.50 32 arg6 1895 317 91 3.48 36 ara5 1758 394 106 3.72 320 mar2 1492 81 30 2.70 38 ara8 2354 291 70 4.16 4 3.40 1.83511 pie4 1604 144 28 5.14 416 ten3 2665 3771 1547 2.44 410 acu2 2210 294 442 0.67 412 pie3 1059 318 69 4.61 41 arg4 1625 387 53 7.30 5 3.91 2.4865 tor2 1392 65 31 2.10 522 ele5 755 57 59 0.97 517 ota2 129 25 5 5.00 54 arg8 2875 517 123 4.20 5

They are separated in groups depending on the density of cleavage planes.

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680 °C, suggesting that is carried by hematite in theTriassic red beds. Field tests were conducted for allrock types and demonstrate that Ch1 and Ch2 compo-nents represent a dual polarity magnetization that wasacquired during, or shortly after deposition (Fig. 2,Table 1). The field test proves the prefolding character ofthe Ch1 and Ch2 directions (right side of Fig. 2A and B)and the reversal test (McFadden and McElhinny, 1990) ispositive for Ch1 (type C) and inconclusive for Ch2. Atlower temperatures, a second component (component B)has been isolated in about 80% of the Upper Cretaceousand Lower–Middle Eocene sites and in 100% of theTriassic sites (Fig. 2C, D and E). Component B unblocksbetween 300–450 °C in Upper Cretaceous and Lower–Middle Eocene rocks, suggesting that it is carried bymagnetite, and between 400–660 °C in Triassic redbeds, which indicates that it is carried by hematite.Component B displays a strikingly constant reversed-polarity direction (mean direction D&I=193, −43;α95=3°; k=22) regardless of rock type and age. Foldtest results demonstrate that component B represents aremagnetization acquired after folding throughout thestudy area, regardless of structural position (Fig. 2C, Dand E).

4. Discussion

A closer examination of the postfolding remagnetiza-tion reveals that all sample sites are located within thepressure solution cleavage domain in a band that extendsparallel to the strike of the orogen (grey dots in Fig. 1).Farther to the west, where intensity of the deformationdecreases, to the east and from the cleavage front to thesoutheast and to the south, no evidence for such apostfolding remagnetization has been reported in TriassictoMiocene sedimentary rocks. Fold and reversal tests androck magnetism data confirm the primary character of themagnetization out of central part of the cleavage domain(black dots in Fig. 1: Dinarès et al., 1992; Burbank et al.,1992; Meigs et al., 1996; Bentham and Burbank, 1996;Hogan and Burbank, 1996; Pueyo et al., 2002; Pueyoet al., 2003a,b; Larrasoaña et al., 2003a,b,c; Oms et al.,2003; Sussman et al., 2004; Rodríguez et al., 2006;Mochales et al., 2007;Muñoz et al., in preparation). To theeast (in the Cotiella Massif) but within the cleavagedomain, a secondary magnetization has been reported tobe prior to compression (Garcés et al., 2003; Fernández,2004; Garcés et al., 2005). In addition, a secondaryremagnetization but prefolding, has been reportedmore to

Fig. 3. A) Comparison between the intensity of the remagnetization, relatively to the intensity of component C, and the density of pressure solutioncleavage planes in the 5 groups. B) Qualitative comparison between the intensity of the remagnetization (in light grey) and of component Ch (in darkgrey) in three representative samples and their correspondent thin sections.

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the east, in the central part of the Pyrenees (Dinarès-Turelland García-Senz, 2000).

Within the study area, tectonic loading associated withthe final emplacement of the Gavarnie thrust sheet, andsedimentary loading due to the deposition of 2–3 km ofUpper Eocene marls and Lower Oligocene continentalsediments, induced pressure solution up to a structural

Fig. 4. Proposed model for the magnetic rearrangement after the libera

level marked by the Lower–Middle Eocene turbidites.Later emplacement of the underlying Guarga thrust sheetuplifted and denuded part of the cleaved rocks. Since theremagnetization always postdates the folding associatedwith the first stage of the emplacement of the Gavarniethrust sheet, and the cleavage development is related to thesame thrust sheet, we suggest that the remagnetization

tion of preexisting magnetic grains induced by pressure solution.

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occurs simultaneous with, and as a result of, the pressuresolution. This interpretation is further strengthened by theobservation that the intensity of the remagnetization(relative to the intensity of the primary component Ch),corresponds to the density of the pressure solutioncleavage planes in thin section and at outcrop scale. Band Ch intensities were obtained by means of principalcomponent analyses at the sample scale (Table 2), siteaverages of the intensity were later on calculated. On theother hand, the density of cleavage planes (Durney andKisch, 1994) can be quantified by considering, in a sectionparallel to the shortening direction, the ratio between thethickness of the cleavage surfaces (selvages) and themicrolithons (undeformed rock volumes). We selectedstandard paleomagnetic cores displaying both magneticcomponents (around 30 samples), they were sorted andclassified in 5 groups as a function of their cleavage spacingand were compared with the correspondent ratio betweenCh and B intensities. (Fig. 3A, Tables 1 and 2). A few thinsections done on sister paleomagnetic samples were alsoused to this purpose (Fig. 3B).

A model that explains the relationship between thepressure solution and remagnetization in the centralPyrenees is shown in Fig. 4. Formation of pressuresolution cleavage planes involves dissolution of solublemineral and subsequent transport by diffusion or flow,this process may be local and not large fluid movementsare needed. Because magnetic grains and phyllosilicatesare less prone to dissolution than carbonates, they tendto accumulate in discrete, selvages planes once they arefreed during dissolution of soluble minerals. Rotation ofthese freed magnetic grains depends on pore space andfluid pressure in the cleavage planes. With further de-formation, the pore spaces may become smaller than thesize of the magnetic grains, thereby blocking remagne-tization under the ambient magnetic field. On the otherhand, the remagnetization orientation seems to beindependent of the cleavage plane supporting the ideathat the minerals can freely rotate and that the magneticfield is the main force in the system during the reequi-librium. Chemical reactions may modify the originalferromagnetic minerals depending on the Eh–ph–∂Oconditions, but new formation of magnetic mineralogyis not necessarily required in the process. While we areaware that formation of new magnetic minerals mightresult in a chemical remagnetization with identical direc-tional properties, indirect evidence supports the hypothesisthat preexistingmagnetic grains carry the remagnetization.For all rock types studied, themagnetic minerals that carrythe remagnetization are the same magnetic minerals thatcarry the characteristic component although the unblock-ing temperature of the remagnetization component is

lower than the characteristic component. Significantly andmost important, the correlation between increased densityof the pressure solution cleavage planes and enhancedintensity of the remagnetization, relative to the intensity ofcomponent Ch (Fig. 3A), suggests that larger volumes ofdissolved rock led to a simultaneous increase in the amountof freed magnetic grains (which accumulate in selvagesplanes).

Magnetic reorientation induced by pressure solutionprocesses provides a useful concept for explaining theoccurrence of widespread remagnetizations in orogenicbelts. This remagnetization mechanism only requiresmass transfer at a local scale, and is independent on thefate of the fluids responsible for the mass transfer. Assuch, our proposed mechanism represents an alternativeexplanation for remagnetizations whose origin has beenlinked to a large-scale migration of orogenic fluids.

5. Conclusions

New paleomagnetic data from the central Pyreneesshow the occurrence of a widespread postfoldingremagnetization. Along with previously published results,our data indicate that the spatial and temporal distributionof remagnetization coincideswith that of pressure solution.The reorientation of magnetic grains is accommodated atthe local scale when the grains accumulate in the cleavageplanes and large-scale flows of orogenic fluids are notessential. In contrast to previous mechanisms proposed inthe literature, the mechanism proposed here takes intoaccount the fate of preexisting magnetic carriers that arefreed in solution and subsequently reoriented under thenew ambient field. Chemical reactions may modify theoriginal ferromagnetic minerals depending upon the Eh–ph–∂O conditions, but new formation of magneticmineralogy is not necessarily required in the process.

Acknowledgements

We are grateful to R. Scholger (Gams PaleomagneticLaboratory, Austria) and M. Garcés (“Jaume Almera”Barcelona, Spain) for their technical support and discus-sions. Dr. R. van der Voo and Dr. A. Sussman are ac-knowledged for the first revision of the manuscript. Wethank the Spanish Ministry of Education and Science forresearch positions that allowed us to work on this project.Funding from the projects BTE 2002-04168, CGL-2006-2289-BTE (MEC) and the Geokin3D-Pyr (CTPR04/2005 -INTERREG IIIb-UE) are also acknowledged R. D.Elmore and two anonymous reviewers are also acknowl-edged for their comments that helped to improve themanuscript.

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