Pcri-TcthvanPlatforms 9_F.Rouie(Ed'l9r]andÉditionsTcchnip,Paris,1994,pp. 197to 210 2 7 r u cG i n o u x . 7 5 0 l 5P a r i s .

CqlciteTwins os q K"y To Pqleosfresses in Sedimentqry Bcrsini: PrelimincrryResultsfrom Drill Cores of the Pqris Bcrsin O. Lncorr,lBEl, P. LRURErur2, nruoJ. ArueeuERl

ABSTRACT Paleostress reconstructions basedon inversion of calcitetwin data rverecarriedout in Jurassic ooliticlimestones from drill coresof theParisbasin (Château-Thierry area).The computedpaleostress orientations are found to be similar to those reconstructed using fault slip data collectedin outcrops of neighbouring areas.Furthermore, they arecorrelablewith the main stagesof the late Mesozoic-Cenozoic tectonicevolutionof theV/est Europeanplatform. In addition, calcite twin analysis combinedwith paleodepth estimates allows determination of magnitudesof paleostresses that p r e v a i l e d . adt e p t h i n t h e P a r i s b a s i n d u r i n g uenozolc tlmes.

work of Turner(1953),it Sincethepioneering is knownthat analysingtwinningin calcitegrains mayleadto theorientationof theprincipalstresses r e s p o n s i b l ef o r t h e o b s e r v e d c r y s t a l l i n e deformation.The possibleuse of mechanical twinning in calcite as an indicator of stress m a g n i t u d ehs a s a l s o b e e n d i s c u s s e di n b o t h theoretical contexts(Friedmanand andexperimental Heard,1974;Jamisonand Spang,1976;Tullis,

1980;Spiersand Rutter,1984;Rowe and Rutter, 1990; Lacombeand Laurent,1992). Recently, (Etchecopar, inversionprocesses computer-based 1984;Laurentet al., 1981,1990)havesupported the dynamicanalysisof calcitetwinningand its interpretation in terms of stress.The analysisof calcitetwins in carbonatesamplesfrom the V/est Europeanplatform and the Pyreneanforeland, carriedout alongwith a detailedstudyof striated faults (Lacombe et â1., 1990, 1992) have demonstrated thevalidity andthe local andregional consistencyof the stresstensorsderivedfrom the inversionof calcitetwin datain polyphasetectonics setting,and emphasizedthe broad interestof paleostress reconstructions at the microscopicscale (Lacombeet al., 1993a). In this paper,we analysemechanicaltwin sets in calcitein drill coresfrom the Parisbasin(Fig.1), in order to checkthe possibilityfor determining paleostress tensors.fromlimited rock volumes collectedat depth.Further,this preliminarystudy aimsat demonstrating thatcalcitetwin analysismay providea new significantand very usefultool for reconstructing thetectonicevolutionof sedimentary basins,especiallywhereminor striatedfaults are numerousto allow computationof not suft'iciently paleostresses.

l. Laboratoirc deTcctonique U.R.A.l3l5 CNRS,UniversitéPieneet MarieCurie,4placeJussicu, Quantitativc, 75252Parisccdcx05. Francc. 2. Laboratoirc du Langucdoc, dc GéologicStructuralc, et Techniques U.R.A.l37l CNRS,UniversitéScicnccs 4 placcE. Bataillon,34095Montpcllicrccdcx05, Francc.

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PALEOSTRESSRECONSTRUCTONSFROM CALCIE TWINS IN CORESFROM TIIE PARISBASIN

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FIG. 1. Structuralframeworkof the Château-Thierryarea(Parisbasin). Sedimentaryformationsof the Paris basin : e-g : Eocene-Oligocene; C2 : Late Cretaceous;Cl : early Cretaceous;J3 : Late Jurassic;J2 : Middle Jurassic;Jl : early Jurassic;T : Trias. Crosspattem : crystallinebasementof the Massif Central. Triangle pattern: Oligoceneformationsof the Bressegraben.The frame I indicates the general location of the drill holes. The frames 2 and 3 show the areaswhere microtectonic data are available. (a) : dau from Coulon and Frizon de Larnotte, t988; (bl and b2) : data from Lacombe et al., 1990. I : Eocene "pyrenean" compression;II : Oligocene extension;III : Mio-Pliocene "alpine" compression.ô ratio, defined in text. Arrows on diagrams (lower hemisphere,equal area stereographicprojection) indicate the corresponding directons of compression (convergentarrows)and extension(divergentarrows).

1 GEOMETRY OF CALCITE TWINNING Twin lamellaears a commonmicroscopicfeaturein calciteof any type and origin (Handinand Griggs, l95l; Friedman,1964).A twin lamellaresultsfrom the simple shearof part of the host crystalin a

particular sense and direction along sp6cific crystallographically definedplanes(Turneret al., 1954).The resultingtwinnedportionof the crysral bearsa mirroredcrystallographic orientationto the untwinnedportionacrossthe twin plane(Fig.2).At low pressureand temperature,calcite aggregates deformprimarilyby twinningon e [0112]plànes.

j

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198

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PALEOSTRESSRECONSTRTICTON.S FROM C^LCITDTWINS IN CORESFROM THE PARISBASIN

2 DATA INVERSION AND PALEOSTRESS DETERMINATION The methodwhich is usedhereinto determine principalstressorientationsand differentialstress (Etchecopar, magnitudes 1984)is supportedby a computer-based inversionprocess. FIG. 2. Schematicsketchof a twin lamella (01i2) in a calcitecrystal.The e rwin planeis horizontal and perpendicularto the planeof drawing. C and C' are the optic axesof the host grain and the twinned lamella, respectively.The twinning direction [el:r2] correspondsto the directionof motion of the part of the crystal located above the twin plane. The imposed sense of shear is indicateclby the arrow. A low pressureand temperature, the twin lamellais very thin and cannotbe confusedwith the host crystal(see Fig.3).

In thin section,the resultinge twin lamellaeare straightand very thin (abouta fractionof micron: Fig.3).Eachcrystalcontainsthreepotentiale twin planesthatarearrangedsymmetricallyaroundtheC axls. Twinning requiresa resolvedshearstress,15, thecomponentof appliedstressalongthetwinning direction,that exceedsa critical value,tn. This critical value doesnot dependon temperature and (Turneret al., 1954; confiningandfluid pressures Friedmanand Heard, I974), but does dependon grain size.(Roweand Rutter, 1990).Here, we adopteda value of tn = 10 MPa for samples displayinga nearlyhomogeneous grainsizeof 300500 pm. Thesecharacteristics indicatethat calcite twinning occursat low differentialstressesand independent andconlîningpressure, of temperature so that its analysisis suitablefor reconstructing paleostresses in very weakly defbrmedcarbonate coverrocks. Eachcarbonate samplebeingcaret'ullyoriented in the field, about 150-200calcitetwin data are collectedwithin threemutually perpendicular thin sectionsusing a polarizingmicroscopecombined with a 3-axisU-stage.In eachcrystalexamined,the spatialorientationsof the C axis and of the three potentiale twin planesare thus detjned,and the twinnedor untwinnedcharacterof eachtwin plane is opticallychecked.

2.1 Basic assumptions Assuminghomogeneous stressdistributionat the grain scaleand constantyield stressvalue for twinning,the inverseproblemconsistsof finding thestresstensorthat bestfits the distributionof the twinnedand the untwinnedplanesmeasuredin a sample.This tensor must additionallyfit the tbllowing requirements: . ts ) ta for all the twinnedplanes(or in practice,for the twinnedplanescompatiblewith this tensor,if thetwin datasetis polyphase); . 15( rn for all the untwinnedplanes. 2.2 Principle of inversion The stresstensorsolution is searchedas a reducedstresstensorsuchas (ol-o3) is scaledto I [(o1-o3)*= l]. The resolvedshearstressrs acting alongany twin planethereforevariesbetween-0.5 and 0.5. The first step of the inversionconsistsof obtainingan initial guess of the solution by applyinga numberof randomtensors.For each tensor,the stresscomponentsarecalculatedfor all the twin planes,and theseplanesare classified accordingto the (decreasing) resolvedshearstress rs actingon them (Fig.a).This classification is a usefulqualityestimatorwhich allowsto precisely determine the percentageof twinned planes with the tensorsolution. consistent 2.3 Optimization Becausein practicethe tensorsolutionmay inducealong someuntwinnedplanesa resolved shear stresst5 greaterthan that exertedalong twinnedplanescompatiblewith it, the secondstep of the inversionprocessconsistsof minimizingthe tunctionf (ideallyequalto 0) definedas :

199

PALEOSTRESSRECONSTRT.ICIONSmOM CALCITE TWINS IN CORtlSFROM TIIE PARISBASIN

N (l) f =I (tsj-ra'), j=1 where tn' is the smallestresolvedshearstress appliedon the twinnedplanescompatiblewith the ténsor,and t51 the resolvedshearstressesapplied on the N untùinnedplanesj suchas t5i ) tn' (for moredetails,refer to Etchecopar,1984).Note that thet4' value may be consideredas the yield stress value for the form of the tensor adoptedfor calculation(i.e., reducedstresstensorwith (olo3)* = 11.

2.4 Results procedure leadsto thestress This computerized tensorsolutionthataccountsfor the largestnumber corresponds of twinnedplanesand simultaneously to the smallestvalue of f. The orientationsof the oL,o2 ando3 (o1>o22o3, threeprincipalstresses positive)arecalculated,as well as of compression 0