Feb 15, 1994 - Bruce G. Bills, â¢'2 Shanaka L. de Si!va, 3 Donald R. Currey, 4 Robert S. ..... Oviatt, C.G., D.R. Currey, and D. Sack, Radiocarbon chronology of ...
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GEOPHYSICAL RESEARCH LETTERS, VOL.21,NO.4, PAGES 293-296, FEBRUARY 15,1994
Hydro-isostatic deflection andtectonic tiltingin thecentral Andes:Initial resultsof a GPSsurveyof Lake Minchin shorelines
Bruce G. Bills,•'2Shanaka L. deSi!va, 3Donald R. Currey, 4Robert S.Emenger, 4 KarlD. Lil!quist, 4'sAndreaDonnellan, 6andBruceWorden 7 Abstract. Sufficiently largelakeloadsprovide a meansof probing to 3790m. Thespatialpatternof netverticalmotions (accumulated since rheo!ogical stratification of the crustanduppermantle.LakeMinchin wasthelargestof thelatePleistocene pluviallakesin thecentralAndes. Prominent shorelines, whichformedduringtemporary still-stands in the climaticallydriven lake level history, preserverecordsof lateral
the shoreline wasformedroughly17,000yearsago),contains important new informationaboutthe response of the crustand uppermantleto normal loadsfrom two sources(variationsin the lake load itself, and
fluctuationsin sea level during the last deglaciation) and places
variations in subsequent netverticalmotions.At its maximumextentthe lakewas 140 m deepand spanned400 km N-S and 200 km E-W. The
constraintson the venica! tectonismassociatedwith subductingthe Nazcaplateunderthe SouthAmericaplate. load of surficial water contained in Lake Minchin was sufficient to Depositional shorelines of largeendorheic lakeshavethreeessential depress thecrustandunderlying mantleby 20-40m, depending on the propertiesthat make them usefulfor geodynamicstudies: they form subjacent rheology.Any otherdifferentialverticalmotionswill alsobe quickly,theydisappear slowly,andtheyare initially horizontal.As a recorded asdepartures fromhorizontality of theshorelines. We recently result,the climaticallyforcedoscillations in depthand areal extentof conducted a surveyof shorelineelevationsof Lake Minchin with the majorlakesin hydrologically closed basinsthroughout theworldprovide express intentof monitoringthe hydro-isostatic deflection and tectonic a uniqueopportunity to investigate theEarth'sresponse to normalloads tilting.Using real-timedifferentialGPS, we measured topographic on lengthscalesof 1O'sto 100'sof kilometersandtimescalesof decades profiles acrosssuitesof shorelines at 15 widelyseparated locations to millennia.Gilbert's(1890) pioneeringwork on Lake Bonneville(in throughout the basin. Horizontaland vertical accuracies attainedare westernUtah and parts of Nevada and Idaho) establishedthe basic roughly 30 and 70 cm, respectively. Geomorphic evidence suggests that framework for limnologic neotectonics andhasexerteda majorinfluence thehighestshorelinewasoccupied onlybriefly(probablylessthan 200 on subsequentstudies (Crittenden, 1963; Currey, 1982, 1990; years)and radiocarbon dateson gastropod shellstbundin association NakibogluandLambeck,1982;Bills andMay, 1987,Bills and Currey, withthe shoredepositsconstrainthe age to roughly17 kyr. The basin1993). widepatternof elevationsof the highestshorelineis composed of two Thoughmuchof the literatureon rheologyof the crustand upper distinctsignals:(27 + 1) m of hydro-isostatic deflectiondue to the lake mantleis concerned with response to majorglacialloads(Peltier,1974,
load,anda planartilt witheastandnorthcomponents of (6.8_+0.4) 10-5
1976; Wu and Peltier, 1983; Mitrovica and Peltier, 1991, 1992;
and(-5.3+ 0.3) 10-5. Thisrateof tiltingis toohighto be plausibly Lambeck,1990),lake loadshaveseveraladvantages overglacialloads. attributed to steadytectonism, andpresumably reflectssomeunresolved Primaryamongthemis thatthecomplexspatio-temporal patternof the combination of tectonism plusthe effectsof oceanicandlacustrine loads load can be easilyand accuratelyreconstructed from two ingredients: on a laterally heterogeneous substrate.The history of lake level present-daytopographyand a history of lake level fluctuations.In fluctuations is still inadequately knownto allow detailedinferences of contrast,reconstruction of an ice sheetload requiresat least three ingredients: geologicconstraints on the temporalevolutionof the ice crestandmantlerheology.However,it is alreadyclearthat the effective elasticplate thicknessis closerto 40 km than the 60-70 km crustal margin, observations (or guesses) to constrainthe variableboundary thickness in the centralAndesand the effectiveviscosityis lessthan 5 conditions at theice-tillinterface,anda complexsimulation of the non1020Pa s. linear flow of ice (Payneeta!., 1989; MacAyeal, 1989; Boultonand Introduction Clark, 1990).Lakesare alsomuchbetterrecorders of climatichistory thanareglaciersandice-sheets (Currey,1990). The altip!anoof Bolivia is presentlya cold high desert,but at The early work of Agassiz(1876), Steinmannet al. (1904) and elevationsbelow 3800 m the landscapeis dominatedby coastal Bowman(1909)established thata seriesof largelakeshave,at various geomorphic features.During colderand/orwetterepisodes in the past, timesthroughout thePleistocene, occupied thepresently semi-aridbasins the hydrologically closedbasinsof the altiplanohave contained of the Bolivianaltip!ano.The northernbasin,with a total area of 57.1 substantial lakes.Thelargest ofthesewaslakeMinchin,whichextended 103km2, presently contains lakeTiticaca (8.7 !03km2 in area,3810m 200km E-W and 400 km N-S, andwas 140 m deep.Despitehaving surfaceelevation)and previouslyheld a somewhatlargerlake, (13-14 formedas a level surface,the highestshorelineof lake Minchinis 103km2 area,,-,3900 m surface elevation) whichBowman (!909)called presently foundat orthometric (geoidal)elevations thatrangefrom3750 lake Bal!ivian.The southern basin,with a totalareaof 134 103km2,
presently contains onlytheshallow lakePoopo (34 103km2 area,3660 m surfaceelevation) andtheplayasof Empexa,Coipasa andUyuni,but
t Geodynamics Branch, NASAGoddard Space FlightCenter previously helda single largelake(48-50103km2 area,3760m surface Dept.ofEarthandPlanetary Science, Johns Hopkins University elevation)which Steinmannet al. (1904) calledlake Minchin. It is this
Dept.of Geography andGeology, Indiana State University Dept.of Geography, University ofUtah Dept.ofGeography andGeology, DrakeUniversity JetPropulsion Laboratory, California Institute ofTechnology Seismological Laboratory, California Institute ofTechnology
Copyright 1994bytheAmerican Geophysical Union. Papernumber93GL03544 0094-8534/94/93GL-03544503. O0
southern basinlakewhichis thefocusof ourpresentstudy. The chronology of fluctuations in the level of lake Minchin is still only poorly known. Early workerssuggested that the large altiplano lakesmay haveresultedfrom meltingof glaciersin the surrounding mountains.However,Hastenrath andKutzbach(1985) showedthat there wasinsufficient waterstoredin theglaciersto be a majorsourcefor the lake. The work of Servant and Fontes (1978) providedthe first radiocarbon dateson shorelines of lakeMinchin.Theyestimatedthatthe higheststandof the lake (at closeto 3800 m) occurred prior to 28 kyr ago.Morerecentwork,summarized byLavenuet al. (1984),suggests an agerangeof 22-27kyr. Servantand Fontes(!978) alsopointedout that there was a late episodeof pluvialactivity,whichthey calledthe Taucastage,that
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Billsetal.:Rebound andTiltingin Central Andes
culminated near3720m elevation at about12 kyr BP. This roughly coincides in agewith the YoungerDryasclimaticevent,whichwasa briefreturnto full glacialconditions, andis seenin icecorerecords at bothpoles(Jouzel etal., 1987;Dansgaard etal., 1993)andin globalsea level variations(Fairbanks,1989). The Gilbert shorelineof Lake Bonneville alsoreflects thissameglobalclimaticevent(Currey,1990;
topographic antenna were traverse, obtained real-time usingtheestimates L1 signals offrom theposition 5-8 GPS of satellites the"rover" and
Oviattet al., 1992).
respectively. Distance between "base" and"rover" neverexceeded 2 km
Duringmuchof thelastdeep-lake cycleonthealtiplano, globalsealevelstood -•120m lowerthanat present (Fairbanks, 1989).Thelargescaleandlong-term neteffectof a---120m deglacial risein sea-level is to depressthe oceanbasinsrelativeto the continents by --,40m. Interpretationof relative sea-levelhistoriesrequiresaccurate computation of deflectionat the coastline(Peltier, 1976; Lainbeck, 1990),but thereare relativelyfew directobservational constraints on howthesea-level induced deflection decays withdistance fromthecoast. The spatialandtemporalscalesoverwhichthisdeformation occursare determined by the strength of the crustanduppermantle.For any significant oceanloadingsignalto be presentin the LakeMinchin
duringthesesurveys.
shorelines,which lie at distancesof 100-300 km from the coastline,
wouldrequireaneffective lithospheric thickness of order100kin. Thoughmanymodelsof the tectonicevolutionof the centralAndes exist (Lyon-Caenet al, 1985; !sacks,1988; Kono et al., 1989; Wdowinski andO'Connell,1991;Babyet al., 1990),understanding of the spario-temporal pattern of vertical motionsassociated with the subduction of theNazcaplateandtheaccompanying deformation of the SouthAmericaplate is really quitelimited.Existingobservational constraints are few and mostlyindirect.One of the mostcompelling observations is that the dip of the subducting slab (as indicatedby patterns of earthquake hypocenter depths) variessignificantly alongthe strikeofthetrench(james,1971;Barazangi andIsacks,1979;Hasegawa andSacks,1981),andthepatternof reliefis significantly differentover the steeplyand shallowlydippingsegments of the slab(Jordanet al., 1983; Dewey and Lamb, 1992). Marine terracesalongthe Pacific coastlines of ChileandPeruprovidesomeconstraints on upliftnearthe trench,and indicatethat ratesare spatiallyand temporallyvariable (DeVries, 1988; Hsu et al., 1989;Machareand Ortlieb, 1992). Fission track recordsof exhumationratesof plutonicrocksfartherfrom the trenchindicatetemporalvariability(Crough,1983; Benjaminet al., 1987),but are oftenquotedas thoughtheywereusefulmeasures of the uplift rate of the centralAndes,ratherthan singlepointsamplesof a complexspatialpattern.
Risacherand Fritts(1991) recentlypresented evidence for regular variationsin thickness of the uppersalt layer in the Salarde Uyuni, whichpresentlycontains mostof thesaltdeliveredto lakeMinchinby its tributaryriversduringthe lastdeep-lake cycle.Thetopof thesaltunit is maintainedas a levelsurfaceby annualepisodes of rewettingand hydroaeolianplanation(Ericksenet al., 1978;Currey,1990).The bottomof the saltunit occursat depthsthat increase nearlylinearly from
