Lower crustal thinning in the Rhinegraben - AGU Publications

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part (Figure 2), together with the shoulders in the Vosges and. Black Forest, was uplifted and eroded [Villerain et al., 1986]; uplift was accompanied by only ...
TECTONICS,

VOL. 13, NO. 2, PAGES 342-353,

APRIL

1994

Lower crustal thinning in the Rhinegraben: Implications for recent rifting Helmut Peter Echtler • Geologisches Institut, Universit/itKarlsruhe,Karlsruhe,Germany

Ewald Liischenand GunterMayer Geophysikalisches Institut, Universit/itKarlsruhe,Karlsruhe,Germany

Abstract..Seismicnear-verticalandundershooting experiments carriedoutin thesouthern upperRhinegraben area between1984and 1991 showpronounced lateralvariationsof deepcrustalproperties.Significantdifferences in theapparent thickness of thereflectivelowercrustandthetransparent upper crustappearto be relatedto differentstructuralsettings. A 12to 14-km-thick

reflective lower crust beneath a 15-km-thick

transparent uppercrustof theeasterngrabenshoulder(Black Forest)probablyoriginatedduringPermo-Carboniferous reequilibration of thickenedVariscanorogeniccrust.Thinning of thislowercrustby about5 km beneaththe grabenandrifted domainstransitionalbetweengrabenandshoulder(the Dinkelbergblock)is interpretedto be relatedto Cenozoic extensional faulting.A discrepancy betweenmoderate extensionof uppercrustandlowercrustalgeometryindicates mechanical decouplingat depthduringextension. Congruent modificationof otherphysicalpropertiesis suggested by strongsinglereflectiveelementsin the topmostpartsof the thinnedlowercrust.In the transitionalDinkelbergblocksuch an anomalously strongreflectoroccursat a depthof 20 km belowa pronounced localmaximumof earthquake activity;it is interpreted to be thepresentlyactivezoneof decoupling which in time shifted from the rift axis to the eastern transition

INTRODUCTION

The UpperRhinegraben is a majorsegmentof theCentral EuropeanRift Zone (Figure1) whichdevelopedin Tertiary timesnearthe front of the Alps. Crustalstructures of thenorthnortheasttrendingcontinentalrift resultedfrom moderatecrustal extension(10-15%);thesestructures werethetargetof many seismicandgeologicalinvestigations duringthelastdecade. JointGerman (DEKORP, DeutschesKontinentales Reflexionsseismisches Programm)andFrench(ECORS,Etude Continentale et OctaniqueparReflexionet Refraction Sismique)deepseismicreflectionprograms providedtwo profilesacrossthe northern[Wenzelet al., 1991;Meier and Eisbacher,1991] andsouthern Rhinegraben (Figures2 and3) [Brunet al., 1991]. They revealedasymmetryin termsof crustalandsedimentary thicknesses.The lowercrustasa decoupled low-strength layerduringasymmetric extension alonga low-angle,ductileanddeep-reaching detachment is one

1Now atGeoForschungsZentrum Potsdam, Potsdam, Germany. Copyright 1994 by the American GeopysicalUnion Paper number 93TC02193 0278-7407/94/93TC-02193510.00

possibilityto interpretethesefeatures[Brunet al., 1991].The imageof thereflectivelowercrustin theseprofilesdisplays variationsalthoughnear-vertical reflectiondatado notreveal significant reflectivitybeneaththeRhinegraben fill (Figure3). Thuscontinuityof thehighlyreflectivelowercrustacrossthe grabenis not substantiated by thesedata. In thispaperwe compileintegrateddataandinterpretations (1) fromsupplementary undershooting andwide-angle experiments withintheremaininggapson the southern ECORS-DEKORPline andcomparethemwith measurements of theKTB (Kontinentales Tief-BohrProgramre) deep reflectionprofileson theeasternRhinegraben shoulder [Liischen et al., 1987]and(2) froma recentnear-vertical deep reflectionprofilecombined withdetailedstructural analysis in thesouthernmost graben-to-shoulder transitionarea(the Dinkelbergarea).Thesedatarevealmodifications of thelower crustbasedon reflectivecharacter whichwe interpretto be resultof extensionandgrabenformation. SETTING

OF THE UPPER RHINEGRABEN

The TertiaryupperRhinegrabentrendingroughlynorthnortheast betweenBasel,Switzerland,andFrankfurt,Germany, hasa lengthof about300 km andan averagewidthof 40 + 5 km. A northernanda southernsegmentcanbe definedon the basisof asymmetric grabenfill anddifferentialsubsidence. Horizontal extension doesnot exceed 10-15% [Meier and Eisbacher, 1991] and affects a continentalcrust which attained

its structureduringthepolyphasetectonometamorphic and magmaticeventsof thelatePaleozoicVariscanorogeny. Crustalconvergence andsubsequent extensionled to a reequilibration of thethickened crustandproducedcrustal-scale structures withina highlydifferentiatedcrystallinebasement [Eisbacheret al., 1989; Echtler and Chauvet, 1993]. Late

Variscanto post-Variscan (Permo-Carboniferous)extensional basinssuperimposed on high-gradegneisses, developed simultaneously with the reductionof crustalthicknesses to normal values at that time.

Subsidence in theRhinegrabenstartedduringthe lateEocene andcontinuedto be activeduringOligoceneandlowerMiocene [Rothe and Sauer, 1967, Illies, 1974, 1975]. Afterward

subsidence occurred onlyin thenorth,whereasthesouthem part (Figure2), togetherwith the shoulders in theVosgesand BlackForest,wasupliftedanderoded[Villerain et al., 1986]; upliftwasaccompanied by onlylimitedvolcanicactivity (Kaiserstuhlvolcano,16-17 Ma, lower-middleMiocene). Interdisciplinary programs for theGermandeep-drilling programKTB (1984-1985)andtheFrench-German ECORSDEKORPdeepreflectionseismicprofiling(1988)provided insightson theregionalcrustalfabricsof the grabenarea. Reflectionseismicsurveysof theeasterngrabenshoulder (BlackFores0revealeda stronglydifferentiated crust[Lfischen et al., 1987] with a relativelytransparent uppercrust(0-15 kin)

Echtleret al.:LowerCrustalThinningin theRhinegraben

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undedain by a highlyreflectivelowercrust(15-28kin).This crustalpatternextends morethan120km to theeastof the Rhinegraben [Bartelsen etal., 1982].Theuppercrustcontains discontinuous dippingreflectors interpreted asrelictVariscan shearzonesrelatedto bothorogenicoverthrusting andlate

about24 km in theapex,compared to ambientnormal thicknesses of 28-30km in theadjacentareas.Therefraction seismicexperiments alsoinferreda topof thelowercrust

withinthegrabenatabout20-kmdepthin average. Coincidentnear-vertical andwide-anglereflection

orogenic extensional faulting.Thesubhorizontally layered lowercrustappears to crosscut andoverprintthesestructures

experiments ontheeastern shoulder of thegraben demonstrate

andis inferredto be theresultof lateVariscanto post-Variscan mechanical recoupling[Eisbacher et al., 1989].This interpretation complements thosefor otherWestEuropean Variscansegments [e.g.,Boiset al., 1989;BoisandECORS ScientificParty, 1991]. Refractionseismicprofilingin the BlackForestarea

anabruptdecrease in lowercrustal reflectivity [Lilschen et al.,

[GajewskiandProdehl,1987; Gajewskiet al., 1987;

coincidence with theMohodiscontinuity corresponding with 1987].Thusthe lowercrustcanbe definedby a strongly reflectivefabriccontrasted by a transparent uppermantleand

uppercrust.Trueamplitude representations andamplitude decayanalysis of thevarious datasetsrevealanabruptdecrease in seismicamplitudes at thebottomof thereflectivelower crust,indicatingthatthiseffectcannotbe an artifactof

Holbrooket at., 1988] revealeda 7-8 km thick low-velocity zoneabovethelowercrustalongtheRhinegraben shoulder. The bottomof thislow-velocityzonecoincideswith thetopof thereflectivelowercrustat 14 to 15 km depth[Ltischenet al.,

insufficientsignalenergy. On thebasisof combinedP andS wavewide-angle observations, SandmeierandWenzel [ 1990]emphasized lower

1987](Figure3 to theright).Fuchset at. [1987]interpreted thisvelocitystructure to betheresultof fluidsrelatedto riff formationanduplift.In thesouthern partof theRhinegraben anupwarping of theMohodiscontinuity neartheKaiserstuhl volcanoindicates significant crustalthinningin thisarea.

shoulder, compatible witha verticalvariationin felsicto maficcomposition of thelowercrust.Ltischenet al. [1990] presented observations of near-vertical P andS waveswhich provideevidencefor anisotropy of thelowercrustpossibly

Seismicrefractiondata [Edel et al., 1975;Proriehlet al. 1976; Zucca, 1984;Zeis et al., 1990] indicatea crustalthicknessof

crustal variations of the Poisson's ratio beneath the eastern

relatedto strain-relatedmetamorphictextures.From twodimensional elasticfinite-difference modelingof thewide-angle data,Sandmeier[1990]interpreted thelowercrustasa layerof

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additionalexperimentsconsideredin thisstudy (subsurfacecoverage,fig. 4,9) wide-angleexperimentswith subsurface coverage (fig. 5, 6, Liischenet al., 1987,Damotteet al., 1987) Fig. 2. Geologicmapof thesouthern upperRhinegraben andlocationof seismicreflectionlines.K, Kaiserstuhl Miocene volcano.

randomly distributed bodies,about100m thickwithlengths of severalhundredmeters(averagevalues),andcharacterized by seismicvelocitiesabout10% higherthanthebackground. RHINEGRABEN

UNDERSHOOTING

EXPERIMENTS

In 1988an attemptwasmadeto tracethereflectivelower crustfrom the easternshoulder[Liischenet al. 1987] toward

the weston the southernECORS-DEKORP Rhinegraben traverses[Wenzel et al., 1991; Brunet al., 1991]. In both continuous near-verticalsurveys,thereis a gapbeneaththerift axiswherethelowercrustalimagewasexpected(Figure3). This is mostlikely not dueto geologicalstructures but to energylossdueto'multiplereflectionsin thesedimentary fill. Complementary to thestandard Vibroseisnear-vertical profiling,a stationary14-kin-longreceiverarmyon theeastern

Echtleret al.:LowerCrustalThinningin theRhinegraben

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Fig. 3. Geologicalcrosssectionandinterpreted line drawingof theECORS-DEKORPsouthernupper Rhinegraben seismicprofile,modifiedafterBrunetal. [ 1991]andBoisandECORSscientificparty [1991].P-M, Permo-Mesozoic cover;C, Cenozoicgrabenfill. shadedareasindicatereflectivelowercrust. Dottedareamarksthegeometry of thelowercrustbeneaththegrabeninferredby conplementary wideanglemeasurements of thiswork.Boxesindicatecoverage positionof Figures4 and6. Righthand portion showsone-dimensional velocitydepthfunctionfromcoincident near-verdcal andwide-angleprofilesin the Black Forest [from Sandmeierand Wenzel, 1990].

shoulder (BlackForest)recorded fiveexplosive shotsdistributed alongtheseismiclineat a meanspacing of 15km.Figure4 showstheresultingsingle-foldrecordsectionafternormal moveout(NMO) correction. Exceptfor gapsa few kilometers long,thesubsurface coveragelinkstherift axiswith the eastern shoulder wherethelowercrustallayeringwasimaged well [Liischenet al., 1987; Brunet al., 1991]. This section

showslateralcontinuityof thereflectivelowercrustbeneath therift axis,althoughit appearsto be modifiedandthinned towardthewest.The depthof thisreflectivezonehasto be regardedwith an errorof +/- 1 km beneaththerift becauseof thepoorlyconstrained normalmoveout velocity(maximum offset60 km), whichhasbeenusedfor thecorrectionto zerooffsetrepresentation. The existence of lowercrustalreflections in theRhinegraben wasdemonstrated firstby Dohr[1970]ona shorttestprofilenearRastattin thecentralpartof thesouthern upperRhinegraben.

Previous Vibroseis undershooting experiments described by Damotteet al. [ 1987]madeit possibleto comparelower crustalfeaturesin thegrabenwiththoseof theeastern shoulder. The observationswere made with Vibroseis sources in one shoulderand receivers in the other shoulderand vice

versa.Thustheraypaths werenotaffected by thesedimentary grabenfill. Liischen etal. [ 1987]presented a comparison (Figure5) withwide-angle recordings alonga N-Sprofilein theBlackForest.Bothdatasetsexhibitinteresting detailsof lowercrustals.tructures. Beneaththegrabenthereflected wideanglesignals fromthelowercrustaredelayed by about0.3 s. Simplekinematic raytracingrevealsa 3-kmdeeper position of thetop of thelowercrust,in accordance with near-verticaltest experiments 30 km southof Karlsruhe[Dohr,1970].The thinner band of reflectionsindicatesa reducedlower crustal

thickness in thegrabenin comparison to theshoulder. The phasevelocityof 7.0 km/s of reflections fromthetopof the lowercrustin thegraben(markedin Figure5 witha solidline) is higherthanin theshoulder (6.65 km/s).Thisindicates that theuppercrustallow-velocityzonementioned aboveis more pronounced in theshoulder andsubstantiates possible presence of rift-relatedfluids[Fuchset al., 1987].

Observations of amplitiudes alsodemonstrate significant

differences inthephysical nature between shoulderandgrabenrelatedlowercrust.Reflective elements belowtherift appear muchstronger, continuous, andconcentrated withintheupper levelsof thelowercrust,thusindicating stronger contrasts in elasticrockpropertiesthanbeneaththeshoulders.

In Figure6 theundershooting wide-angle datafromthe Rhinegraben havebeenNMO corrected, similartoFigure4. Thisdemonstrates againanapparent subsidence of thetopof thereflectivelowercrustby 3 km. Thecrustaldivingwave(Pg,cristalline basement P wave)is notobserved hereabove70-kmdistance. ThePgdiesoutwhen reaching themidcrustal low-velocity zones. DINKELBERG

PROFILE

GeologicalSetting

TheDinkelberg areatothesoutheast of theRhinegraben (Figure 2) represents a crustal block,intermediate in position betweentherift andtheeasternshoulder.This blockis bounded by high-angle crustalfaultswhichaccommodated subsidence

andslighttiltingduringlateEocene-Miocene extension. A

moderate rollover structure ofthePermo-Mesozoic sedimentary cover wascontrolled by normaldisplacement on a N-S

trending, steeply westdipping master fault(Figure 7). The northern andsouthern borders of theDinkelberg blockactedas transferfaults associatedwith the E-W extension.

Structural analysis alongtheeastern andnorthern faultzones

reveals a blockgeometry whichisfundamentally controlled by preexisting VariscanandlateVariscancrustal-scale shearzones

[Echtler andChauvet, 1993].Theeastern master faultparallels a zoneof synthetic andantithetic blockfaulting[Lutz,1964]. Thecrystalline basement exhibits a prominent ductilenormal shear zonein high-grade metamorphic rocksthatdipstothe west and relatesto intenselate Variscan crustalextension

(Figure7). Syntectonic granites andradiometric dataindicate a

lateCarboniferous ageforthisstructure whichdeveloped in association withupliftandexhumation of thickened orogenic crust.Subsequent brittlenormal faulting during progressive

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Fig. 4. Single-foldseismicreflectionsectionof theundershooting experiment of theECORS-DEKORP southern Rhinegraben traverse1988.Toppanelshowssimplifiedlinedrawingwithconfiguration of the experiment. On thetopof eachpanelisa simplified geological crosssection. Processing includes a normalmoveoutcorrection usingreasonable velocitiesandmutingafterthefirstarrivalsof thedirect wave.Trueobserved amplitudes havehorizontal traceenergybalancing.

extensioncontrolledtheformationof an Early Permian sedimentary pull-apartbasin.The northerneast-west trending Dinkelbergborderfaultactedasa dextralsubvertical strike-slip fault with an offsetof 4-5 km (Figure7b). DuringCenozoic extensionthesestructureswereclearlyreactivated(Figure7c). Reference horizons indicatesubsidence alongtheeastern master faultof lessthan1 km duringCenozoicextension. Permian subsidence is moredifficultto quantify,buta thickness of

sediments of 200m indicates moderate extension in postVariscanandpre-Tertiary times.

TheDinkelberg areaisalsocharacterized bya peculiar seismotectonic setting.Seismological observations reveala heterogeneous distribution of low-magnitude earthquakes in the studyarea [Bonjeret al., 1989;Faberet al., 1993]. In the Rhinegraben seismicactivityis restricted to shallowcrustal levels(