metamorphic characters of the lower continental crust in Phanerozoic ...

TECTONICS,VOL. 12, NO. 2, PAGES580-590,APRIL1993 SEISMIC AND TECTONOMETAMORPHIC CHARACTERS OF THE LOWER CONTINENTAL CRUST IN PHANEROZOIC AREAS: A CONSEQUENCE OF POST-THICKENING

an alternation of low andhighseismic velocities laminaebest

explains thehighreflectivity of thelowercrust[Sandmeier and

Wenzel,1986;Paul andNicollin,1989].However,the geological significance ofthislayering remains enigmatic. In Europe, it hasbeensuggested thatthelayering of the

lowercrustformedoverwideareasbetween thePermo-Triassic

EXTENSION

andtheearly-middle Jurassic andwasassociated withhigh

Patrice Rey1,2

Scientific Party, 1990]. The DEKORP Team, however, consider thattheBlackForestmayhaveacquired itslaminated

regionalheat flow [Boiset al., 1989, Bois and ECORS Laboratoire desSciences dela Terre, EcoleNormale Sup6rieure deLyon,France

lower crustabout40 Ma ago duringthe heatingevent associated withtheformation of theRhinegraben andthatthe lowercrustallayeringin Urach(northeastern BlackForest) mayresultfroma younger magmatic eventsome14 Ma ago

[DEKORPResearchGroup,1990a].For theBIRPS team,the similaritybetweenseismicprofilesrecordedfrom British tectonicprovincesthat rangefrom Archaean(Laurentian forelandof northwest Scotland) to Mesozoic(NorthSearift thelowercrustcanbeattributed to thetectonic transposition system)in age, suggeststhat mostlower crustalreflectors of various petrological heterogeneities in themainflowplane. wereformedduringthe last majorthermaleventrelatedto TheVariscan provinces of western Europehavebeenaffected, Mesozoicextension[Matthewsand Cheadle,1986]. More during Phanerozoictimes, by several extensionaland recently,it hasbeenarguedfromdeepseismic profilesacross compressional events. Thegeometrical relationships between theIapetussuturethatthereflectivitydistribution withinthe seismic andgeological structures indicate thatthelayering of crustis, at leastlocally,directlycontrolled by Caledonian thelowercrustwasacquired duringtheLateCarboniferous to structures and that the reflectivelower crustmay be no youngerthanEarly Devonian[Freemanet al., 1988;Matthews Permian whenthethickened Variscan crustwasaffected by gravitational collapse.Petrological and geochronological andBIRPSGroup,1990;Klemperer et al., 1990].

Abstract.Deepseismicprofilesof Phanerozoic continental crust commonly show a highly reflective lower crust. Rheological considerations suggest thattheseismic fabricof

analyses of deepcrustal rocks(xenoliths andexposed sections) indicatethatthelowercrusthasrecorded a majorhigh-T/ medium-Pgranulitefaciesmetamorphism duringthe late Variscanextension,whereason the surfaceUpper Carboniferous to Permianbasinswerebeingdeposited. A similarscenario characterizes otherPhanerozoic orogenic belts. In theCaledonian provinces of theBritishIslands,thelower crustis seismically reflective;it hasundergone medium-P granuliticmetamorphism duringthedeposition of Devonian sedimentary basins, at theendof theCaledonian orogeny. In the sameway, collapseof the Mesozoicbelt in the western partof NorthAmerica isresponsible, duringtheCenozoic, for pervasive crustalextension whoseconsequence is a seismic layering of thelowercrustaccompanied bya low-Pgranulite grademetamorphic event,whilein themid-anduppercrusts, low-angle,ductile,normalfaultsgiveriseto theBasinand RangeProvince.Therefore,it is proposedthat thereis a geneticrelationship between(1) post-thickening crustal extension, (2) low- to medium-P granulite facies metamorphism of thelowercrustand(3) seismic layering of the lower crust.

INTRODUCTION

In manydeepseismicprofilesof Phanerozoic continental

lithosphere, thelowercrustdisplays, onlargeareas, numerous subhorizontal anddiscontinous seismicreflections.Numerical

modeling concludes thata subhorizontal layered structure with

In NorthAmerica, thebestlowercrustlayering isrelated to widespread crustalextension, exemplified in theBasinand RangeProvince. Fromthisobservation, manyauthors have proposed a relationship betweenpervasive crustalextension andlowercrustseismic layering [e.g.,Klemperer et al., 1986; McCarthy,1986;Allmendinger et al., 1987;Goodwinand Thompson, 1988;McCarthy andThompson, 1988;Valasek andBranch, 1987;Valasek etal., 1989;Nelson,1991a].Other

mechanisms havebeeninvoked toexplaintheoriginof the lower crustalseismicfabric, amongthem are crustal thickening [Boiset al., 1988;1990;Cazeset al., 1985,1986; MatteandHirn, 1988]or crustalstretching duringocean openingand continental rifting [Boiset al., 1988, 1990; Matthews andCheadle, 1986;Klemperer, 1989]. As onecanobserve, extensional tectonics andplate-scale thermalanomalies are commonly invokedto explainthe layeringof the lowercrust.Because bothprocesses are involvedwithin post-thickening crustalextension,this tectonicsettingis one of the bestworkinghypotheses concerning the originof the lowercrustallayering[e.g., McCarthyandThompson, 1988;Fountain,1989;Nelson, 1991a,b; Rey, 1991].However, considering onlyseismic data,it is difficultto expandthishypothesis. Onemethodto

constrain theageandthegeodynamic environment responsible forthelowercrustal layering istocompare, in thesamearea, thegeometrical relationships between seismic andgeological structures,taking into account the petrologicaland

geochronological dataobtained ondeepcrustal rocks exposed to the surfaceby tectonism or xenoliths broughtup by

volcanism.This paperdealsprimarilywith the Variscan

1 AlsoCentre G6ologique etG6ophysique, Universit6 des Sciences etTechniques duLanguedoc, Montpellier, France.

provinces of Western Europe, a suitable areatodevelop this discussion because (1) thereis a greatdiversity of welldocumented Phanerozoic geological events,(2) therearea

2 Nowat Department of Geology andGeophysics, number of published deepseismic profiles,and(3) thereis a

University of Wyoming, Laramie,WyomingUSA.

wealthof geologicaldataavailableon the lowercrust.There aregoodgeologicalarguments torelatethelowercrustseismic

Copyright 1993by theAmerican Geophysical Union.

layering to lateVariscan, mediumto low-Pgranulite facies metamorphism of thelowercrust,aswellasstretching of the

Paper number92TC01568.

post-thickening crustalextension.We will first review the

0278-7407/93/92TC-01568510.00

various hypotheses putforward, concerning thegeological

uppercrustaccompanied by sedimentary basindeposits and

Rey: SeismicandGeologicalCharacters of theLowerCrust

significanceof lower crustalseismicreflectorsand seismic layering.

581

tectonicprocesses, duringwhichthe lower crusthada very low viscosity[e.g.,PhinneyandJurdy,1979;Meissner,1986; Fuchs, 1986; Wever et al., 1987; Meissnerand Kuznir, 1987;

SEISMIC DATA ON PHANEROZOIC LITHOSPHERE AND GEOLOGICAL INTERPRETATIONS

SeismicStructureof Phanerozoic and ShieldLithospheres Nationaland internationalprojectsof deepseismicprofiles (BIRPS, COCORP, DEKORP, ECORS, etc.) have given evidenceof a relativelycommon,but certainlynot universal, seismic structure, with a highly reflective lower crust occurring betweentheuppermantleandtheuppercrustwhich are both relatively seismicallytransparent[e.g., Klemperer, 1987]. In Phanerozoic areas, the thicknessof the reflective zonecommonlyvariesbetweenthe lowerthirdandthe lower half of thecrust,with a rapidincreasetowardtheMohoof the quantity and the quality of seismicreflections [Meissner, 1986]. This distribution of seismic reflection contrastswith

that of shieldareaswherereflectivitydecreases towarda deep and ill-defined Moho, as hasoften beendescribed(Figure 1) [Meissner,1986; Trappeet al., 1988; Matte and Hirn, 1988; Cook et al., 1988]. This contrastcan be exemplifiedon the "Nord de la France" ECORS profile (Figure 3f) or more recently by the lithoprobetransecton southernCanadian Cordillera [Cook et al., 1992]. However, I emphasize,as mentionedby Nelson [1991b], that thereare many different typesof seismicstructures betweenthesetwo end members, becauseof thegreatdiversityof crustalevolution.

GeologicalSignificanceof SeismicReflectors

Trappeet al., 1988; Reston,1988; Mainpriceand Nicolas, 1989].The lowercrustmaybe interpreted asa zonewhichhas undergone intense ductile deformation, with tectonic transposition into the main flow plane of variouscrustal heterogeneities and developmentof flat lying ductileshear zones,responsiblefor the subhorizontal and discontinuous characteristics of seismicreflectors.The rapid increasewith

depthof boththequantityandthequalityof seismicreflectors can be related to an increasing density of crustal heterogeneitiesand to an increasing ductile behavior, respectively.Only lithospheric-scale perturbations may be invokedto explainpervasive shearing of thelowercrust. AGE AND ORIGIN OF THE DEEP CRUSTAL IN WESTERN EUROPE

LAYERING

During Phanerozoictimes, the Variscan provinces of WesternEurope(Figure 2) were involvedin (1) thickening processesand related collapse extension (the Variscan orogeny);(2) thickeningprocesses withoutwidespreadpostthickeningextension(Alpine orogeny);and (3) stretching processesduring oceanopeningin Mesozoicand Cenozoic times or continental rifting in Cenozoic time. The areas affectedby thesetectoniceventshave been studiedby deep seismicprofiling. From geometricalrelationshipsbetween seismicand geologicalstructures, it is possibleto evaluatethe relativeageof thedeepcrustallayering. Relationships BetweentheReflectiveLower Crustand the Mesozoicand CenozoicGeologicalEvents

From numerical models, seismic reflections of the lower

Gueguen, 1989, Jonesand Nur, 1984; Oliver et al., 1983]; theseexplanations are notmutuallyexclusive.

1. In the ECORS/DEKORP "Graben du Rhin", deep seismicprofile (Figure3a), thinningandoffsetof the layered lower crust below the Rhinegrabensuggestthat layering existedprior to Cenozoicrifting [Brunet al., 1991]. 2. In the "Alpes II" and "Pyr6n6es"ECORS profiles (Figures3b and 3c) the reflectivelower crustis flexuredand faultedby Tertiarytectonicsandsubsequent stackingof crustal units [Bayer et al., 1987' ECORS Pyr6n6esTeam, 1988]. Therefore,in both areas,the lower crustallayeringis older than the Cenozoicthickeningprocess.Furthermore,despite activeerosionin mountainsbelts,resultingin the exhumation of deepzones,only five examplesof layeredlowercrusthave beendescribedthroughoutthe world [Fountainand Salisbury, 1981]. This stronglysuggeststhat crustalthickeningis not responsiblefor lower crustallayering, sinceerosionshould havecausedwidespread exhumation of thesezones. 3. The Atlantic continentalmargin is a useful area for studyingtheconsequences of continental stretching on seismic structures duringoceanopening.In theBIRPS-ECORSprofile WesternApproaches Margin(WAM) [HobbsandPeddy,1987] and in the ECORS profile "Baie de Biscaye" [Pinet et al., 1987a; 1987b], the transparentupper crust and the layered lower crustare thinnedbelow late Jurassicto Tertiary basins (Figures3d and3e), the sameholdstrueon the lithoprobeeast profile on the Canadiancontinentalmargin [De Voogd and Keen, 1987]. As a consequence, the origin of the lower crust layeringis olderthanthe openingof the northAtlanticOcean

DynamicSignificance of theSeismicFabric of theLower

[Matthews and Cheadle, 1986; Bois et al., 1988; Reston, 1988].

crustare interpretedin termsof constructiveinterferencesat the boundariesbetweenlayers of alternatinghigh and low seismic velocities [e.g., Fuchs, 1969; Phinney and Jurdy, 1979; Fountainand Salisbury,1981; Hurich and Smithson, 1987; Sandmeierand Wenzel, 1986; Wenzel et al., 1987; Paul

andNicollin,1989].Indeed,alternating layers,abouta quarter wavelength in thickness,best explain the high density of seismicreflectors,their high amplitudesand the lossof the high-frequency contentof seismicwaveswhichhavetraveled through the lower crust [Fuchs, 1969; Paul and Nicollin, 1989].

The natureof thisdeepcrustallayeringcanbe ascribedto (1) lithological layering, related to tectonic transpositionand igneous processes (mantle intrusions, partial melting, anatexis, chemical differentiation, etc.) [Meissner, 1973; McKenzie, 1984; Cheadle et al., 1987]; (2) petrophysical layering related to tectonometamorphicprocessesand mylonitization,responsible for synkinematic crystallization of elasticallyanisotropicminerals(suchas micasor amphiboles) andlattice-preferred orientations [e.g.,FountainandSalisbury, 1981; Jones and Nur, 1984; Fountain et al., 1984; Hurich et al., 1985; Smithson et al., 1986; Fountain et al., 1987;

Reston, 1987; 1988; Mainprice and Nicolas; 1989]; and (3) overpressured fluidstrappedin intergraincracksand fractures [Matthews, 1986; Meissnerand Kuznir, 1987; Gavrilenko and

Crust

4. The Nord de la FranceECORS profile (Figure 3f) cuts across the Mesozoic

Comparisonof the seismic,petrological,and rheological layeringof the lithosphere(Figure 1), stronglysuggests that the layeredstructureof the lower crusthasbeen formedby

and Cenozoic

Paris Basin

which

is

claimedto be formedby lithosphericthermalsubsidence. The lower crustseismiclayering,whichis presentin the southern part of the profile,disappears rapidlytowardthe north,while

582

Rey:SeismicandGeological Characters of theLowerCrust

Stress,•trength (Mpa) 2

I

10

2

section

I

10

4

10

o

.•., ;.-:%.; :..-.•..-. :"i?•.'.?• I •.•: •..:.:-•,.

nitic rocks

Stressstrength(Mpa)

Seismic

Seismic section

4

10

+ + + '•,

+

Graniticrocks

+

•+.•.•,•/

Mafic granulites 20

, :: .-......'.•..: •...........

crocks

.................

::,-'.. ...... ,.•;:.•.•;:{.:.;.;•-.;E'..-%

M•tle

/ Møl'tø •lO

40

ß::•';.';?':,--"•:b; 2,,.;.'-: ..%.-;..'-.'• t(sec) 15

60

10-14 -1

• = 10'14S'1

-2

0

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/•

5

10

15

20

25

30

i

i

i

i

•

-2

N

0

200

300

N

Q = 55 mWm

o

5

5

10

10

•

100

RP-Wind River

•

20

BIRPS-WlNCH

Line

20

II Shield area

Phanerozoic

area

Fig. 1. Illustrationof theological,petrological andseismiccontrasts betweenPhanerozoic and shield areas.For thesetwo areas,deepseismicreflectionsections andhistograms of distribution of reflections with depthhaveto be comparedwith rheological andpetrological profiles.Phanerozoic areasare characterized by a relativelyhighheatflow anda 30-kin-thickcrustwhoselowerpartis heterogeneous, weaklyviscous, andhighlyreflective(adaptedfromMeissner[1986]andTrappeet al. [1988]).On the otherhand,shieldareasarecharacterized by a thickcrustwithanintermediate to maficcomposition and a low heatflow.Thisresultsin a brittlebehavior, consistent withtheemplacement of maficmagmas in formsof dike swarms(seediscussion by Nelson[1991b]).The complexseismicpatterncommonly observedin shieldareasis probablyrelatedto thebadresolutionof verticalseismicreflectionmethods appliedto complexstructures. However,thedecrease withdepthof thereflectivityobserved in some seismic sections of shieldareacanbeinterpreted asa decrease of lithological contrast towardthemoho. on the surface the basin is continuous.Therefore, there is no

Group, 1990b]. These areas belong tOtheexternal domain not

spacialcorrelationbetweenthe seismiclayeringof the lower crust and the basin. As a consequence, thermal subsidence

muchaffectedby Variscanthickening proccesses. The southern part of the Nord de la Franceprofileis typicalof mostdeep seismicprofilescompletedin the Variscaninternaldomain (e.g., DEKORP Black Forest, DEKORP 2S, DEKORP 4) characterized by a highlyreflectivelowercrust.This contrast between lower crustal reflectivity in internal and external domainsof the Vailscan orogensuggeststhat the Variscan orogenycould be at the origin of the seismicfabric of the

associated with Mesozoic and Cenozoic intracontinental basin

formationis notresponsible for theseismicfabricof thelower crust.

Relationsht•)s BetweentheReflectiveLowerCrustand the PaleozoicGeologicalEvents

lower crust.

1. TheParisBasinwasdeposited on a composite Paleozoic basement,the Bray fault separatingthe London-Brabant Proterozoic block to the north from a Variscan internal domain

2. In the deep seismicprofiles SWAT 4 (Figures2 and Figure3), Nord de la France,Pyr6n6es,andBalede Biscaye, intracrustal reflectors whichhavebeeninterpreted asVariscan thrusts[Bois et al., 1987, 1988; Choukrouneet al., 1990] are

to the south.As indicatedby the very low-gradePaleozoic sedimentarysequence,the London-Brabantblock suffered moderateVariscan shorteningwhoseintensitydecreases northward.It is worthnotingthatnorthof theBrayfault,the lowercrustallayeringthinsout towardthe north.A poorly reflectivelowercrusthasbeenalsoreportedin the northern part of the North GermanBasin[Trappe,1989] and in the

be related to the thermal and mechanical relaxation of the

northern part of the Rhenish Massif [DEKORP Research

Variscan thick crust.

offsetby the lowercrustseismiclayering.This geometrical relationship hasbeentakenasevidencethatthe originof the seismic layering took place after the Variscan orogeny [Meissner et al., 1986; Bois et al., 1988, 1990]. However,

evenif layeringis youngerthanVariscanthickening,it could

Rey:Seismic andGeological Characters of theLowerCrust

583

Suture zones

•

Mainthrusts Deep seismicprofiles

........ :--•'• Precambrian London-Brabant Block with • .......... Caledonian Provinces Variscan Provinces Alpine Provinces moderate Caledonian and Variscan deformation

Baltic

Shield

CF: Caledonian Front VF: Variscan Front VGR

AF: AlpineFront 500 Km I

•

(•f•'Alpesll Pyr. ,

Fig.2. Sketchmapof western Europeshowing theCaledonian, Variscan, andAlpineprovinces withtheir mainthrustsandsuturezones.The deepseismic profilesshownaretheonesdiscussed in thetext.Key: VosgesGrabenduRhin(VGR), Norddela France(NdF),Pyr6n6es (Pyr),andBaiede Biscaye(Ba.Bi.). Other abreviations as in the text.

In summary,it appearsthat neither crustal thickening processesduring Cenozoicorogeniesnor crustalstretching related to Mesozoic intracontinental basin formation, crustal

rifting or ocean opening, can be invoked to explain the seismicfeaturesof the lower crust.Lower crustallayering seems to be younger than Variscan thickening, yet the associationbetween substantiallayering in the internal Variscanzonesand the absenceof layeringbelow foreland domainssuggestthat the lower crustal fabric is Variscan. Therefore, it is suggestedthat the origin of the seismic

reflectivityof the lowercrustis relatedto the thermaland mechanical reequilibration of Variscan thickened crust. It has been proposed that in the Variscan belt thermomechanical relaxation occurred mainly during extensionaltectonicsrelated to gravitational collapseof thickened crust[e.g.,WickhamandOxburgh,1987;M6nard andMolnar,1988;Dewey,1988;KroheandEisbacher;1988, Mattauer et al., 1988]. A numberof regional studieshave documented the magnitudeof this event,especiallyin the internal Variscan domains [Faure, 1989; Van Den Driessche

and Brun, 1989; Echtler and Malavielle, 1990; Malavielle et

al., 1990; Faureand Pons,1991, Rey et al., 1991, 1992]. The consequences of post-thickening extensionaltectonicson the bulk structureand main metamorphicfeaturesof the crustare considered belownotonlyin theEuropeanVariscanprovinces but also in the Caledonian and in the Basin and Range Province.

METAMORPHIC CHARACTERS AND RADIOMETRIC AGES OF LAYERED LOWER CRUST IN SOME PHANEROZOIC

AREAS

Variscan Provinces

In the upper crust of Variscan internal domains,postthickeningextension is nowwell documented. Extensionwas accommodated by kilometer-thick, ductile,normalshearzones and lateral ramps [Van Den Driesscheand Brun, 1989; Malavieille et al., 1990;Rey et al., 1991] whichoverprinted Eo-Variscan

thrust structures. These extensional

structures

584

Rey:Seismic andGeological Characters oftheLower Crust

a

NW

e

Lorraine Basin

Vosges

Rhinegraben

SE

SSW

Parentis Basin

NNE

Bale de Biscaye

20 krn ,

b

NW

Penninic

•-___•...•

f

Insubria n

ECM Front

ICM Line' PoPlain SE ....... _-.•--:• - _.•,-.____. ........ '-_.•_--..-•.--•...-, , _.._•..._....., .

NE

LondonBrabantblock

Bray Fault

•-•_.,•-,•.•._ .............. ............ •.