Richard F. Wendlandt ... Museum, University ofOslo. â¢bstrac. t. We document a systematic inverse .... gabbroic compositions (Kern and Richter, 1981; Fountain.
GEOPHYSICAL RESEARCH LETTERS, VOL.18,NO.9, PAGES 1759-1762, SEPTEMBER 1991
MODIFICATIONOF LOWERCRUSTBY CONTINENTAL RIFT MAGMATISM Richard F. Wendlandt
Dept.ofGeology andGeological Engineering, Colorado School of Mines W. ScottBaldridge
LosAlamos National Laboratory E.-R. Neumann
Mineralogical-Geological Museum, University of Oslo •bstrac. t. We document a systematicinverse relationship betweenthe volumeof eruptedvolcanicrocks associated with
continental
rifts
and
the
seismic
compressional wavevelocitycontrastacrossthe Moho. This relationship is interpreted as evidence that
16:1 (Rambergand Morgan,1984;Mohr, 1983;Crisp, 1984;Shaw,1985;Riciputiand Johnson,1990). Althoughcontinentalrift environments are regionsof variedand often complexmagmaticinteractionsamong mantlesources,crust,and partial melt, consideration of
magmatism significantly modifiesthecomposition(s) ofthe the generalspectrumof likely igneousprocesses leadsto lowercrust(and uppermantle)duringriftingprocesses. recognitionof two fundamentaleffectsof magmatismon The relationshipis not influencedby the currentthermal lithospherecomposition:(1) Extractionof basalticmelt stateof the lithosphere,being observedfor both modern from a peridotiticmanfiecanproducea residuumthat is rifts(Rio Grande,Kenya,Rhine) andpalcorifts(Oslo), asmuchas2.5-5%lessdensethanthesourcecomposition and may also characterize continental flood basalt (e.g.,Boydand McCallister,1976). Moreover,intrusion provinces (ColumbiaRiverandSnakeRiverplain). The of basalticmelts into the uppermostmantle can further trendisreconciledbyigneousandmetamorphic processes reducedensitiesimmediatelybeneaththe crust. (2) The thatcollectively modifythe lowercrustto moremaficbulk compositionof the lower crustbecomesmore marie and compositions, resultingin highercompressional velocities, its densityincreasesas a result of intrusionof mantleand that decreasethe density of the residual mantle, resulting in lower compressional velocities.Theseresults supportthe view that most mantle-derivedmagma in continental rift settingsis trappedin the crust.
derived basaltic melts, accumulation of mafic cumu!ates,
extraction of felsic melts, high-grade metamorphic reactions(includingdehydration),and assimilationof felsicandvolatilecomponents by ascending magmas. The resultsof theseprocessesare that basalticmelt Introduction sourceregionsin the mantlebecomeincreasingly depleted and refractory,the lower crustbecomesmore mafic (the Currentunderstandingof the evolutionof continental upper crustpresumablybecomesmore felsic), and the boundarybecomespetrologically gradational. riftsis limitedby a restrictedperceptionof the physical crust-mantle andchemicalpropertiesof the lower crustand howthese These compositionalmodificationsshould produce changesin the compressional wave velocity propertiesevolve during rifting. That the lower crust systematic participates in, and is modifiedby, volcanismassociated structureof the lithosphericmantle and lower crust. In with continental extensional deformation is a reasonable the mantle,the decreasein densitywill result in reduced expectation. Diversegeophysical andgeological datafrom compressional velocity,while in the lower crust, the increasein densitywill producea velocityincrease(e.g., numerous rift environments indicate the presenceof Kern and Richter, 1981). Accordingly,with increasing intrusivebodies and crustal magma chambers. The abundance of crust-derived felsicmeltsin manyriftsalso degreesof magmaticactivity,a trend towarddecreasing velocitycontrastacrossthe Moho is predicted. in this supports the premisethat the crustevolves physically and paper,we attemptto testthismodel. We assumethatthe chemically duringrifting. Centralto an understanding of petrologic crust-mantle boundaryandthe seismicMoho lowercrust evolutionis the ability to estimatethe are coincident;Griffin and O'Reilly (1987) and Wilshire percentage of generatedmagmathat doesnot reachthe surface. These volumes are difficult to determine (1990), however,have argued convincinglythat this is probablynot the casein magmatically activesettings. unequivocally. The consensus,however,is that most mantle-derived magmadoesnot makeit to the surface. Results
In the Oslorift, for example, greaterthan80% of all
magmaticproductsmay be deep crustalmafic and We havetabulatedvolumesof igneouseruptives(Table ultramaficintrusivesand cumulates(Ramberg1976; wavevelocitiesfor crustandmantle Neumann et al., !986). Estimates of intrusive:extrusive1) andcompressional acrossthe Moho (Table 2) from four continentalrifts and ratiosfor rifts and other occurrences rangefrom 2:1 to two flood eruptiveprovinces(possiblyassociated with continentalrifting;Catchingsand Mooney,1988). These occurrences wereselectedon the basisof data availability
Copyright 1991 by the American Geophysical Union.
and quality, To facilitatecomparison, the volumesare normalizedby the lengthof the eruptiveprovince. The
Paper number 91GL0188!
0094-8534/91 / 91GL-0! 88153. O0 1759
1'/60
Wendlandt et al.: Modification of Lower Crust
TABLE
1- Volumes
Rift
of Volcanic Rocks
Volume
(km
Volume/Length
(km3/)
Oslo Kenya #
24,300' 230,000
122 230
Rhine Rio Grande Snake River Columbia R.
4,000 25,000 < 70,000 170,649
6.7 25 127 284
characterizedby lithospherethat is not in thermal equilibrium.The Oslorift, in contrast, is a paleoriftand haslithosphere that is thermallystabilized(Morganand
Ramberg, 1987).TheColumbia Riverbasalt province,
whicheruptedbetween17.5and 6 Ma, is presumed to havelithosphere that is not in thermalequilibrium. In Figure 1, an inverserelationship betweenvolcanic discharge andcompressional wavevelocityis evident.At high length-normalizeddischargevolumes,the trend
asy •mptotically approaches a velocity contrast of0.9kin/s, which isanappropriate contrast formantleperidotitC and
gabbroic compositions (KernandRichter, 1981; Fountain andChristensen, 1989).Wepropose thatthisrelationship indicates compositionalmodification of the lower
Includes extrusive and shallow intrusive rocks
# Includes Kenya, E. Uganda, N.Tanzania
crust/upper mantleto anextentthat'isproportional tothe volumeof magmathat haspassedthroughthe crust.
Sources:Ramberg,1976;Neumannet al., 1986;Williams, !982; Lippard,1973;Barberiet al., 1982;Lipmanet al., 1989;Greeleyand King, 1977;Reidel et al., 1987.
1) rangefrom a low of 1.0km/s for the ColoradoPlateau
normalization lengthsfor the rifts,takento be parallelto the axisof rifting,are 200 km for the Oslorift, 600km for
the RhineGraben,and 1000km eachfor the Kenyaand the Rio Grande rifts. The lengthof the SnakeRiver Plain volcanicprovinceis 550 kin. The ColumbiaRiver provinceis estimatedto be 600kin. Normalized magma discharge volumesvary by more than a factorof 40. Seismic profilelines,withoneexception, cross volcanic terrainwithintheriftsandflooderuptive provinces (Table 2). Alongthe Rio Granderift line, however,thevolcanic rocksare concentrated to thewestof the centralgraben. The absolutevariationin lowercrustvelocities (6.25-7.5
Velocitycontrasts for tectonically stableareas(Figure to a highof 1.5km/sfor the Wyoming Basin(Smithet al., 1989). Velocitycontrasts for averageshieldand average midcontinental interiorare 1.26and1.29kin/s, respectively (Braile, 1989). This variationis believedto
reflectprimarilydifferences in lowercrustcomposition. 300
'
I
ß
I
,i
I
i
I
•
!
,
C
, 1.6
.
I
,
ICRB nyl
200
kin/s)is greaterthanthatfor theuppermantle(7.6-8.4 kin/s). Thesevariationsare attributedto differences in
composition (particularly of thelowercrust)andthermal regime. All the occurrences, withthe exception of the Oslorift and ColumbiaRiver basaltgroup,havebeen volcanicallyactive within the past 100 ka and are TABLE 2: Compressional VelocityContrastat the Moho
O
lOO
RGR
o 0.6
-
,.. 0.8
CP
C 1.0
COMPRESSIONAL
Rift
Lower Crust
Mantle
(m/s)
(m/s)
Location
AS
,'O:: 1.2
, 1.4
VELOCITY
-
I
' 8
2.0
CONTRAST
(KIn/S)
Fig. 1. Volumeof eruptedmagmanormalizedby the lengthof theprovince isplottedversusthecompressional velocitycontrastacrossthe Moho. Abbrev.: RGR-Rio
Oslo
7.1
8.07
Kenya
6.7
Rhine
6.25
7.6 8.1
Main rift crossline: Fallmm-Orud Profile D - KRISP 90 RhenishMassif-Hessian
Rio Grande
6.4
7.7
N. Albuquerque-Be!en
Grande Rift, SRP-SnakeRiver Plain, CRB-Columbia
RiverBasalt,CP-Colorado Plateau,AS-Average Shield, AM-Average Midcontinent Interior,WB-Wyoming Basin.
Depression Basin
Snake River Columbia R.
6.8 7.5
7.9 8.4
E. Snake River Plain Columbia Plateau
Data Quality
The uncertaintiesin our analysisare not easily quantified.Compressional wave-velocities andestimates of eruptivevolumes canbe questioned.The tabulated Sources:Tryti and Sellevoll,1977;Olsenet al., 1979; volumes (Table1), however, wouldhaveto be in errorby Braileet al., 1982,1990;Mechieet al., 1983;Sinnoeta!., greaterthan25%to significantly disrupttherelationship 1986; Catchingsand Mooney, 1988; Aichbroth and presented in Figure1. An additionalerror couldresult Prodehl, 1988. fromnormalizing volumeofvolcanics bythelengthofthe
Wendlandtet al.: Modificationof LowerCrust
1761
rift feature. This approachis justifiedfor rifts, which
of lowercrustis estimatedfrom velocitycontrast.At one extreme,for the Kenya and Oslo rifts and the Columbia structural axisandhighlyvariableeruptivevolumes.The Riverprovince, thissimplemodelsuggests thatthecurrent approach is probablyalsovalidfor the SnakeRiver compositionof the lower crust is broadly gabbroic. province whichis characterized byanaverage length-to- Accordingly,the lower crust may be 75% convertedto widthratio of 5:!. The ColumbiaRiver provinceis less basaltcomposition beneaththe SnakeRiver Plain, 50% amenableto normalizationby length and the use of convertedbeneath the Rio Grande rift, and unmodified surface areamaybe moreappropriate for thisregion. beneaththe Rhine Graben. This approachis speculative but impliesthat large amountsof magmamay interact Effect of Temperature with, and be retained within, the lower crust. We assumethat lowercrustanduppermantlevelocities The use of velocitycontrastacrossthe Moho allows are for the sametemperatureand that velocitycontrasts riftsanderuptiveprovinces at differentstagesof thermal are controlledby compositionalvariation. The effect of evolutionto be comparedbecausetemperatureeffects a verticaltemperaturegradientacrossthe Moho will be apparently cancelout. The Oslo paleorifthas much to further reducethe velocitycontrastand, thus,to give
characteristically have volcanics concentrated alonga
higherseismic velocities for lowercrustandmantlethan do the modern rifts (Table 2), attestingto thermal reequilibration of thelithosphere undersouthern Norway. The lower crust and mantle velocities for the flood basalt
the appearancethat the lower crustis more mafic than it actuallyis. Separatingthe effectsof lateral temperaturegradients on velocityfrom thoserelated to compositional variation
provinces arealsohigherthanvelocities frommodern rifts is difficult. Olsen et al. (!987) report a large lateral (Table2), perhapsattestingto differences in mantleand velocityincreasefor the lower crust, from 6.4 to 6.7-6.8 lowercrustcomposition, or to differingtectonomagmatic km/s, with distanceawayfrom the axialportionsof the processes. However,because the Mohovelocitycontrast Rio Granderift. They suggestthat lateral temperature in thesetwo provincesis consistentwith that observedin gradients (300ø to 400øC)can adequately explainthis rifts,we believethat the relationshipin Figure 1 canbe appliedto magmaticregimesbesidescontinental rifts.
contrastand estimatethat only a small volume of mafic material(possibly10%) needsto be presentin the lower
FelsicIgneousEruptives
crust, in contrast to our estimate. For there to be more mafic material in the lower crust beneath the rift axis, a
Petrologicand geochemicalargumentscan be invoked that require the introductionof substantialamountsof maficconstituents into the lower crust. These arguments are particularlycogentin rift environmentscontaining largevolumes of silicic magmas. The Kenya rift, for
example, maycontain 50,000 km3ofphonolites and 30,000 km 3 oftrachytes (Lippard, 1973;Williams, 1982).Such largevolumesrequirea thermal"event"that is significant by virtue of its intensity,proximityto the surface,and lateralextent. If thesemelts are generatedin the crust, thethermaleventthat triggersthe meltingis likelyto be theintroductionof a largevolumeof maficmagmainto the lowercrust. If thesesilicicflood eruptivesoriginate byfractionalcrystallization of maficparents,the required volumeof postulated parentalcomposition wouldexceed thevolumeof differentiateby more than 1 to 2 ordersof-magnitude (Goles,1976;Bakeret al., 1978). By either mechanism, extractionof felsiccrustalcomponents or the formation of largevolumesof mafic/ultramafic cumulates in the lower crust,the composition of the lowercrust becomes significantly moremaficthanit wasoriginally. Lower Crust Evolution
The relationship in Figure1 canbe usedto estimate changes in bulkcomposition of thelowercrustresulting frommagmatism duringrifting.If it isassumed thatthe velocity contrast between mantleperidotite anda gabbroic
greaterlateraltemperature gradientand/ora hotterlower crust is necessaryto satisfy the velocity constraints. Alternatively,nonlinearmixingmodels(seeFigure1) may providebetter estimatesof lower crustmodification. In contrast,the absenceof a large lateral velocity variation within the lower crust (6.7-6.8 kin/s) of the SnakeRiver Plain accompaniedby a large lateral velocity
change(5.5-6.0kin/s) in the uppercrustpromptedBraile et al. (1982) to imply that rising basalt may not have significantly modifiedthe compositionof the lowercrust. A laterallyextensiveregionof lower crustmodification, perhapscoupledwith only slight lateral temperature variation might explain the lower crust velocities. Alternatively, the lowercrustvelocity,6.7 km/s,suggests an originallyrelativelymafic composition.Addition of a basalticmelt or mafic cumulatesto this mafic protolith couldoccurwithoutproducinga largechangein velocity. Conclusions
An inverserelationship betweenvelocitycontrastacross the Moho and the volume of erupted volcanic rocks in continental rifts is interpreted to be evidence of interactionbetweenmantle-derivedmagmasand the lower crustand,to a lesserextent,of changingmantleperidotite
composition duringrifting. The relationshipsupports viewsthat the physicaland chemicalpropertiesof the lowercrust/upper mantlecanbe significantly modifiedby continentalrifting.
lower crust at Moho conditionsis 0.9 km/s and the
Theauthors thanktheircolleagues contrast betweenmantleperidotiteand a silicic-to- Acknowledgrnen.?. and intermediate lowercrustis !.7 km/s (KernandRichter, in the CREST studygroup for helpful discussions reviews. This work was supportedby NSF grantsEARlinear mixing modelmaybeapplied whereby modification 9004368to RFW and EAR-8617315 to Larry Braile.
1981;Fountainand Christensen, !989),thenan arbitrary
1762
Wendlandt et al.' Modification of Lower Crust
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(Received: May 17, 1991 Accepted: June24, 1991)
