JOURNAL OF GEOPHYSICAL
RESEARCH, VOL. 100, NO. B7, PAGES 9949-9967, JUNE 10, 1995
Landscapeevolutionof the Dry Valleys,TransantarcticMountains:
Tectonicimplications
David E. Sugden
Department
of Geography,
Universityof Edinburgh,Edinburgh,Scotland
GeorgeH. Denton
Department
of GeologicalSciences
andInstitutefor Quaternary
Studies,Universityof Maine,Orono
DavidR. Marchant
•
Department
of Geography,
Universityof Edinburgh,
Edinburgh,
Scotland
Abstract. Therearedifferentviewsabouttheamountandtimingof surfaceupliftin the
Transantarctic
Mountainsandthegeophysical
mechanisms
involved. Our new interpretation
of
the landscape
evolutionandtectonichistoryof theDry Valleysareaof theTransantarctic
Mountains
isbased
ongeomorphic
mapping
of anareaof 10,000km2. Thelandforms
aredated
mainlyby their association
with volcanicashesandglaciomarinedepositsand thispermitsa
reconsmaction
of the stagesandtimingof landscapeevolution. Followinga loweringof baselevel
about55 m.y. ago,therewasa phaseof rapid denudationassociated
with planationand
escarpment
retreat,probablyundersemiaridconditions.Eventually,downcutting
by rivers,aided
in placesby glaciers,gradedvalleysto nearpresentsealevel. The mainvalleyswerefloodedby
the seain the Mioceneduringa phaseof subsidence
beforeexperiencinga final stageof modest
upwarpingnearthe coast. Therehasbeenremarkablylittle landformchangeunderthe stable,
cold,polarconditionsof thelast 15 m.y. It is difficultto explainthe SiriusGroupdeposits,which
occurat highelevationsin thearea,if theyarePliocenein age. Overall,denudationmay have
removeda wedgeof rock with a thicknessof over4 km at the coastdecliningto 1 km at a point75
km inland,which is in goodagreementwith the resultsof existingapatitefissiontrack analyses.
It is suggested
thatdenudation
reflectsthedifferencesin baselevel causedby highelevationat the
time of extensiondueto underplating
and the subsequent
role of thermaluplift andflexural
isostasy.Most crustaluplift (2-4 km) is inferredto haveoccurredin theearlyCenozoicwith 400
m of subsidence
in the Miocenefollowedby 300 m of uplift in thePliocene.
Introduction
debate,we do not considerthe issuein this paper(but seeDenton
et al, [ 1993]).
The aim of this paperis to relate geomorphological
evidence
Extending acrossAntarctica, the TransantarcticMountains
of landscape
evolutionto thetectonicprocesses
in theDry Valleys
rise to over 4000 m in the Royal SocietyRange. The mountains
of the Transantarctic Mountains. Like the Drakensberg
consistof gentlytilted blocksof sedimentaryrocks(DevonianMountains in southernAfrica and the Great Dividing Range in
TriassicBeacon Supergroup)overlyingPre-Cambrian-Devonian
Australia
the Transantarctic
Mountains
consist of an eroded
basementand are intrudedand cappedby Jurassicdoleritesand
plateauedge characteristicof high-elevationpassivecontinental
basalts. The mountainscomprisethe uplifted rift flank of the
margins[Gilchrist and Summerfield,1990]. Understandingthe
West AntarcticRift System[Behrendtet al., 1991], which has
landscapeevolutionof the mountainsis importantbecauseit can
been periodicallyactive since the separationof Antarcticaand
be used to constrainvariousgeophysicalmodelsof the tectonic Australia [Tessensohnand Worrier, 1991].
processes
associated
withsuch
margins.
Thetectonic
history
of
There
is considerable
uncertainty
about
themountand
theTransantarctic
Mountains
isintimately
involved
ina debate
timing
ofupliftandthetectonic
processes
involved.
Apafite
about
thestability
oftheEast
Antarctic
IceSheet.
Although
ourfission
track
analysis
indicates
that
there
has
been
about
5 kmof
new interpretationof landscapeevolutionhas implicationsfor the
1Now
atInstitute
forQuaternary
Studies,
University
ofMaine,
Orono.
Copyright
1995by theAmerican
Geophysical
Union.
Papernumber94JB02875.
0148-0227/95 / 94JB-02875$05.00
9949
denudationin the Dry Valleys area since the early Cenozoic;
denudationrateswerehigh in the early Cenozoicbeforetailing off
[Gleadowand Fitzgerald, 1987; Fitzgerald, 1992; Brown et al.,
1994]. This impliesthat significantlocalrelief existedaround55
m.y. ago and that the rift flank was alreadyelevatedwith respect
to base level, either as a consequenceof thermal uplift or
underplatingor as a result of the rifting of a preexisting,highelevationsurface. This interpretationagreeswith evidenceof
limited recent surfaceuplift as inferred from the elevation of
9950
SUGDEN ET AL.: DRY VALLEY LANDSCAPE EVOLUTION
undisturbedsubaerialPliocenevolcanicconesin Taylor Valley
[Wilchet al., 1993a]. A differentline of evidencesuggests
that
there has been substantialsurface uplift of parts of the
Transantarctic
Mountainsof 1-2 km over the past 2-3 m.y.
[McKelveyet al., 1991; Webbet al., 1984; Webband Harwood,
denudationprofilesfrom the northernsidesof Ferrar andWright
valleys[Fitzgeraldet al., 1986] imply that the blockhasbehaved
as one unit sincethe early Tertiary, thus holdingconstantone
variable in landscapeevolution. Second,the block contains
outcropsof SiriusGroupdepositsat high elevations(2650 rn on
1987]. This view is based on the existenceof the remains of
Mount Feather), critical evidence which must eventually be
in any explanation. Third, the valleys containa
temperatevegetationof presumedPlioceneage(2-3 Ma) in Sirius accommodated
Group depositsat high elevations. There are some30 Sirius terrestrial record of surface deposits extending back to the
long window into
Groupoutcrops
in theTransantarctic
MountainsflankingtheRoss Miocene, thus providingan unprecedentedly
of a studyrestrictedto one
Ice Shelfbetweenlatitudes86øSand78øSandfurtheroutcrops
in mountainevolution. A disadvantage
cannotnecessarily
be extrapolatedto
the highmountainsof SouthernVictoriaLand in the Dry Valleys blockis thatanyconclusions
and the Prince Albert Mountains [Denton et al., 1993; Verbers
other areas of the Transantarctic
Mountains.
There have been two main phases of interest in the
andVan der Wateren,1992]. The datingof the deposits
depends
evolution of the Dry Valleys. The first
criticallyon the age and origin of reworkeddiatomsthoughtto geomorphological
have originatedin marine basinsin East Antarcticaand to have involved geologistsof Scott's and Shackleton'sexpeditions,
on the great age of the landscapeand
been subsequently
transportedto the mountainsby ice. The whoseinitial observations
biostratigraphic
ageof certainof the diatomshas been confirmed the modestimpactof presentglaciersmake relevantreadingto
by isotopicdatingof a volcanicashlayerin theoffshoreCIROS-2 this day [Ferrar, 1907; David and Priestley,1914; Wright and
core [Barrett et al., 1992]. Severalother supportinglines of Priestley,1922; Taylor, 1922]. The secondphaseaccompanied
evidencefor recentsurfaceuplift havebeencollatedby Behrendt U.S. and New Zealand activities after the International
andCooper[1991],includingthe"youthfulappearance"
of therift Geophysical
Year in 1957. In numerous
paperselaborating
the
shoulderand the presenceof offshoreHolocenefault scarps. geologicaland glacialhistoryof the area, a generalconclusion
Thesevariousviewswere synthesized
by Fitzgerald[1992] who was that the main valleysowed their origin to earlier stagesof
suggestedthat crustaluplift of the TransantarcticMountains in glaciationundermore temperateconditions[e.g., Taylor, 1922;
the Dry Valleys area has occurredthroughoutthe Cenozoicbut Bull et al., 1962; Calkin, 1974a; Denton et al., 1971, 1984;
with two accelerated
pulsesat 55-40 Ma and 10-0 Ma.
Nichols,1971;Selbyand Wilson,1971]. This view is consistent
In view of the uncertaintyaboutthe timingof the uplift, it is with the arguments
of Van der Waterenand Verbers[1992] in
difficult to relate the tectonic evolution of the mountains to the
widerprocesses
of platetectonics.At leasttwoperiodsof rifting
have been inferred from study of the Ross Sea embayment
[Tessensohn
and Worner,1991;Cooperet al., 1991;LeMasurier
andRex, 1991]. The first phasebeganin the Mesozoic(possibly
linked to the separationof Antarcticaand Australia) and was
associatedwith widespreadgraben downfaultingand crustal
thinning. The secondbeganin the Eoceneand was associated
northernVictoria Land. In addition, there is recognitionof the
particularrole of cold desertagentsof erosion. For example,
Selby [1971, 1974] recognized the prevalence of straight
rectilinearslopesand tentlikeridgeswhichhe attributedto salt
weatheringandwind deflationcharacteristic
of this type of frigid,
arid environment. The characteristicslope angles were also
related to rock strengthcharacteristics
[Augustinusand Selby,
1990]. All these slopeswere thoughtto be wearing back the
with uplift and tilting of the TransantarcticMountains. One initial, steeper,glacialtroughsidesandcirques.
This paper puts forward a new interpretationof the
interpretation
of theTransantarctic
Mountainsis thattheyformed
by asymmetric
rifting associated
with extensionandsubsidence
in geomorphology.The key conclusions
relevantto the landscape
the adjacentRossembayment
[Fitzgerald,1992;Fitzgeraldet al., evolution are as folows.
1986]. Accordingto this structuralinterpretationthe mountains
1. The landscape
is relict, andwith the exceptionof a few
representan upperplate margin [Lister et al., 1986, 1991]. Al•o,
glaciallandformstherehasbeennegligibleslopeevolutionsince
flexural effects related to the strong lithospherein East the Miocene.
2. The landscape
is essentially
fluvial, with escarpments
and
Antarctica,togetherwith lateral heat flow from the adjacent
extendedWest Antarcticlithosphere,may have played a role planafionsurfacesat high elevationsand fiver valleys(the Dry
[Sternand Ten Brink, 1989]. Whicheverprocesses
are the most Valleys)dissectingthe mountains.
important, one particular problem is why, if there has been
3. Glacial modificationof the landscapehas beenrelatively
significantlate Cenozoicuplift, it shouldhaveoccurred>50 m.y. minor and selective.
after the late Mesozoic-earlyCenozoicextensionalevent which
Thisnew interpretation
givesan insightinto the longhistory
formed
themarginandtheadjacent
Rossembayment.
Hereis a of baselevel and environmentalchange,which in mm constrains
suite of problemsconcerningthe tectonicprocessesassociated modelsof thetectonicevolutionof thispartof theTransantarctic
with passivecontinentalmarginswheregeomorphology
can help Mountains.
constrainthe varioushypotheses.
The Dry Valleyscomposeoneof the manyblockswithin the Approachand Assumptions
Transantarctic Mountains [Fitzgerald, 1992].
Bounding
The crux of our approachwas landformmappingat a scaleof
transformfaultsunderlieMackay Glacier in the north and Ferrar 1:250,000. The resultingmap hasbeenpublished,togetherwith
Glacier in the south(Figure 1). There are severaladvantages
in
an explanationof the methodsand landforms,by Denton et al.,
studyingthe Dry Valleys block. First, similar fission track [1993]. A samplecoveringWright Valley, Taylor Valley, andthe
SUGDEN ET AL.' DRY VALLEY LANDSCAPE EVOLIZHON
0
10
20
30
' '"'"'""!'i'!'!:i?i'!':':':':""
'
•
....................
•
•
I
Scale (kin)
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9951
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MACKAY
,Shapeless
........
'-'"-'"-'"'"'-'"'""'"'"'-'""ß
ß
Doorly
ßß. :.:.:':::::'-:¾::•/• . ..-.....:.:.:.:::..'::":'"-'""
'•
,..................-.............................
•, •.....:.:
' S.....:.:.:::::::::::'"'
.. !:!:!:i:!:!:!:!:!:i:i:!:!:!
.0..
Mr.
Fleming
Key
::Mt.::::::::::::::• :':':':':':':':':
Newall
:::::::::::::::::::::::
.........................
Labyrir
..........
Lake
Vanda
:.!.!.!:!:!::.:-:
:1"'
============================
N •.:
:Sessrumnir:.:.:.
..
I IGlacier
:.'-'.:.'"...:•
Ice
Free
•...... IceFree(over
2000m)
As
Lake
Bonne
'....:::::::i
GLA C
Quartermain
Mountains
Mount
Feather
Figure1. Location
mapshowing
themainvalleys
andbounding
transform
faults
intheDryValleys,
Antarctica.
The
faultsaretakenfromFitzgerald[ 1992].
9952
SUGDEN ET AL.: DRY VALLEY LANDSCAPE EVOLUTION
(7..,0
c)
S
H
E
E
T
SUGDEN ET AL.: DRY VALLEY
LANDSCAPE
EVOLUTION
9953
-.
Figure 3. Rectilinearslopestoppedby an uppercliff on the flanks of Arena Valley. The patchof remnantregolith
mantlein the upperleft containsburiedventifactsandstoneswith desertvarnish. The crosson the slopeto the right
showsthe site of an ash fall depositwhich occursin the regolith. The ash (Table 1, site 2) is 8.5 Ma and has
survivedon a slopeof 28ø.
QuartermainMountainsis reproducedin Figure 2.
A key
assumptionin any suchmorphologicalanalysisis that different
slopeformsand associations
can be recognizedand usedto infer
past environmentalconditions. For example,the implicationis
that a landscape
of buttescomprising
rectilinearslopestoppedby
steepcliffsandrisingabruptlyabovea flat-lyingplain reflectsnot
only structuralcontrol but also the operationof a particular
assemblage
of processes
(Figure3). Rectilinearslopesand cliffs
reflect the balancebetweenrock strengthand processes
which
are ableto removeanyweatheredmaterialmadeavailable[Selby,
1993]. The surrotmding
plain is theresultof processes
whichare
sufficientlypowerfulto removeany weathereddebris supplied
from the adjacentslopes. The sharper the angle between
rectilinear slope and plain, the more effective the contrast
betweenratesof slopeweatheringand the transportprocesses
on
possible that similar slope forms may result from different
processes. For example, whereas rectilinear slopes are best
developedin arid and semiarid environments,there is no reason
why a glaciershouldnot maintainan existingslopeif it is able to
carry away any debristhat falls down the slope. Again, it is easy
to confusethe arcuate head of a canyon formed under desert
conditionswith a glacial cirque when, in the case of the Dry
Valleys, glaciersmay partly obscurethe foot of the cliff. Such
limitationsmust be borne in mind in the subsequentanalysis.
Nevertheless,each landformis part of a coherentassemblageof
landforms,and this helpsto constrainthe interpretation.Also, in
the Dry Valleys where particular landformsare relatively well
dated, the approachcan place firm constraintson the histow of
changesin baselevel andsurfaceprocesses.
theplain.Theform'ofsuchlandscapes
is influenced
byclimate,
lithology,and structure. Climatic conditionsmost favorablefor Landforms of the Dry Valleys
suchlandformsare semiaridwhereepisodicfloodsand the lack of
Severallandformassemblages
canbe recognized(Figure4),
deep absorbentregolithsmean that transportprocessesdominate and it is useful to summarize the main characteristics.
over weathering. In terms of lithology,the presenceof a rock
Planation Surfaces
such as sandstone,which breaks down into granularmaterial,
favorsa sharpcontrastbetweenratesof weatheringand rates of
Therearethreemainsurfaces.The uppersurface,formedon
transport.In addition,geologicalstructurecan influencedirectly Jurassic dolerim, occurs at altitudes of 2000-2400 m and
the formof cliffsandplains;it canalsoencourage
springsapping composes
thehighgroundmostdistantfromthe coast(Figure5).
whichinfluencesthe locationand clarityof the junctionbetween It surrounds
the headsof the main valleysandis boundedon its
valley sideandplain. Slopesin morehumidenvironments
tendto coastwardside by an escarpment
500-1200 m high, the prime
have more concave and convex elements. This is because of the
featureof the landscape
(Figure6). Largeinselbergs,
oftenflatpresenceof more vegetationand a deeperregolithand a whole toppedand boundedby rectilinearslopes,rise 400-700 m above
rangeof differentprocesses
whichoccurin the regolith[Carson the surfaceto form the highestsummitsin the area, such as
and Kirkby, 1972].
MountFeather(3011 m) andShapeless
Mountain(2739 m). The
It is important to be aware of the limitations of such an
surfacehas a relief of 10-30 m. The lower, intermediatesurface,
approach,the most importantof which is equifinality;it is - cut mainlyin Beaconsandstone
at altitudesof 1700-1850m, is
9954
SUGDEN ET AL.: DRY VALLEY LANDSCAPE EVOLUTION
$hapeles•
Doorly
Mountain
Newall
Mt.
%%%%
•,C..•,,',• / •/.,,.:.,,
,'...'...':•t' /
,••
Mt. Feather
0
i
10
i
20•
•.1 •-'-----•Up•,
/' .•
•surr•c•
/• ' F •'
.•
!
,,v.•.. •
..:.•lntermediate
!:':':':':':! trace
Mountain
"-'-Ml, i•dmoat
./-•
Y•N.•,..•
su
I• •
•co•t•
i
km
.,L•• Rectilinear
landscapes
ßi•?•Rolling
slopes
and
Valley benches
valleys
(• Sirius
depositer
ned
river
valleys
Figure4. Map showing
thedistribution
of themainlandscape
typesandtherelationship
to theintegrated
patternof
rivervalleys.Theuppersurfacein thewestandits frontingscarpgivewayto dissected
mountain
landscapes
toward
the coast.
patchyandoccursmostclearlyin the vicinityof the Asgardand
OlympusRanges,whose summitsare remnantsof the upper
surface.Theserangescompriseinselbergs
andbutteswhichshow
a progressive
changefromplateauremnantnearthemainscarpto
isolatedbuttesfartheraway. A lessextensivesurfacefringesthe
coastandrisesgentlyfromsealevel to altitudesof lessthan200
m. It is separatedfrom the intermediatesurfaceby a major
escarpment
andis cutmainlyin granite. Inselbergs
with rounded
tops,alsocutin granite,rise abovethesurface.
upstanding
inselbergof Mount Feather. Also, a remnantof the
intermediatesurfacecuts acrossgranite basementand dolerite
immediatelywestof Mount Newall. However,structureinfluences
the morphology
of the surfaces.For example,the uppersurfaceis
underlainby a prominentdoleritesill. Also, irregularities
in the
surface,for examplesurrounding
the buttesof the OlympusRange,
arethe resultof planarioncrosscutting
the gentledip of underlying
thin beds of sandstone.
Thesurfaces
and
associated
inselbergs
and
buttes
areerosional
Valleys
ratherthan tectonicfeatures. This can be illustratedin the caseof
The valleys are the main landformsof the area and extend
the QuartermainMountainswherean tininterrupted,
layeredrock from west to east (Figure 4). There are severalnoteworthy
structure extends beneath both the upper surface and the features.
SUGDENET AL.: DRY VALLEY LANDSCAPEEVOLUTION
w
9955
E
m
Feather
3000--• Shapeless
•
Obelisk
Mt.Newall
1000-1
-
-
'
t •
N
Quartermain
Mts.
Asgard
Mts
Olympus
Mts
Taylor
• .............
• ....
McKelv
Barw
..
Glacier
•
•
.......
•
wngnt •
•
Valley-
Valley•
'mJa
Uppersurface
k-,x--xx-x•
Rolling
slopes
•,,',.',.',.',.',,',,',.',,'q
Intermediatesurface .•:'•:'-"•-'-'-q'
Valleybenches
•,,'/'.,'.,"• Coastalpiedmont
Figure 5. SchematicCrosssectionshowingthe relationshipbetweenthe variouslandscapetypes:(top) from the
inlandupperplanationsurfaceto the seaand (bottom)from southto northacrossthe main valleys.
1. The valleys from an arborescent
network indicativeof
fluvial action. The bestexampleis the Victoria Valley System,
but the patternis alsoclearin the caseof the Wright and Ferrar
Valleys. All continueto the coast,the Wright and Victoria
ValleysextendingbeneaththeWilsonPiedmont[Calkin, 1974b].
2. Several valleys are sinuous. The best examplesare
centralTaylorValley (Figure7) andlowerWrightValley.
3. The valleysare cut down to near, or below, sea level.
The Ferrar, Taylor, Debenharnand Mackay Valley mouthsare
belowsealevel, while part of the sinuouscentralTaylorValley is
90m belowsealevel [MudreyandMcGinnis,1975].
4. Several deeper valleys have gently sloping, elevated
valleybenches.The bestexamplesoccuron eithersideof central
Taylor Valley where the bench is often terminatedby a cliff
Figure6. Theupperplanation
surface
bounded
by a prominent
escarpment
in theOlympus
Range.Theinselbergs
andbuttesof theOlympus
Range(foreground)
areremnants
of theuppersurface.Shapeless
Mountain(background)
risesabovethe uppersurface.
9956
SUGDEN ET AL.: DRY VALLEY LANDSCAPE EVOLUTION
Figure 7. CentralTaylorValley viewedtowardthe sea showingthe valleybenchesmmcatedin placesby glacial
cliffs. The sinuous
natureof the innervalleyis broughtout by the shapeof LakeBonneywhichextendsfromthe
snoutof Taylor Glacier(foreground).A darkvolcanicconecanbe seenon the benchto the left.
(Figure 7). Other benchesflank Ferrar and Mackay Valley
systemsandpartsof Wright andVictoriaValleys. The benchcut
by the Labyrinthat the headof WrightValley is a particularly
goodexample(Figure8).
5. All majorvalleyshavehigh-leveltributaries. Someof
theseare gradedto the intermediatesurface,as in the Asgard
Range,and othersare gradedto the level of the valley benches.
Thereis someasyrnmetry
in that tributariesform a subparallel
patternon thesouthern
sideof upperWright andTaylorValleys.
6. Taylor, Wright, and Victoria Valleys have a reverse
slope,andpresent-daymeltwaterstreamsdrain inland into lakes.
In thecaseof WrightValleytheamplitude
of thisreverseslopeis
to be less steepand to lead down to concaveslopesin the valley
bottom.
Rolling Topography
This landscape type, comprising convex-concaveslopes,
occurs in two situations.
The first is on the flanks
of the inner
Taylor and Victoria Valleys, whereit lies below 1800 m and the
intermediate surface and yet above the valley benches. The
secondsituationcomprisesthe low-lying hills betweenthe main
mountainfront andthe coastalplain.
Glacial
Landforms
at least 147 m betweenthe exposedlower valley and the surface
Themajorglacialfeatures
arestraight,
cliffedtroughs,
such
of Lake Vanda(Figure8).
as thoseoccupied
by the Mackayand Miller glaciersand the
Rectilinear Slopes
Straight slopescutting bedrockat angles of 26-36ø are
characteristic and occur in three situations. The first includes the
slopesof inselbergs,buttes,canyons,and the main escarpment
(Figure3). Here,thefootof therectilinearslopeis markedby a
sharpbreak as it givesway to a flat surfaceor valley floor. A
secondassociationis with dissectedmountainridges and the
mountainfront. Ridgecrestsare typicallystraightwith angular
plan viewsandboundedby rectilinearslopesoftenleadingdown
to glaciersin the interveningvalleys. Tors top someridges.
inlandflanksof FerrarGlacier. Otherglacialcliffsramcatethe
valleybenches
inTaylor,Wright,andlowerVictoriaValleys.All
are markedby their large scale,association
with straightened
valley sectionscompletewith mmcatedspurs,and the cliffed
termination
of the upperrollingor rectilinear
ridgelandscapes
(Figure8). CentralWrightValley,whichis straightandcliffed,
hasa broadU-shaped
valleycrossprofile. Landscapes
of areal
scouring
whichhavebeenroughened
by glacialerosion[Sugden,
1974] occurin threemain locations:the upper surface,the valley
benches,andtherolling slopesandlOw-lyingplain near the coast.
Cirques and couloirsoccupiedby small glaciersoccur patchily
Wheneverremnantsof the intermediatesurfaceoccur,the altitude throughoutthe area. Typically, they form small arcuatebasins
of the adjacentridgecrestsis alwayslower than the surface,for excavatedinto a rockrectilinearslopebeneathan upperfree face.
examplein the Kukri Hills, easternAsgardRange,andthe Saint Impressivechannelsystems,sometimesover 50 m deep, occur
John'sRange(Figure1). A thirdassociation
of rectilinearslopes throughoutthe area. The anastomosingpattern, up-and-down
is with the main valleys in the area. The sidesof the Ferrar long profiles,ungradedconfluences,and associationwith gigantic
Glaciervalleyseawardof the pointof flotation,Taylor,Wright, potholes all point to the role of subglacialmeltwater in their
and the Victoria/Barwick/McKelvey
Valleys are rectilinear. The formation[Sugdenet al., 1991]. They occurnear the headsof the
maindifferencewith elsewhereis that the rectilinearslopestend main valleys,for example,the Labyrinthin upperWright Valley
SUGDEN ET AL.: DRY VALLEY
LANDSCAPE
EVOLUTION
9957
Figure 8. View up Wright Valley showingLake Vanda(foreground),the valleybenchincisedby the channelsof the
Labyrinth,and the overdeepened
troughof Wright Valley. The flanks are boundedby the intermediatesurface,
bearingthe inselbergs
andbuttesof theOlympusandAsgardranges.U.S. Navy photograph
TMA 1564.
(Figure
8),theheadofMcKelvey
Valley,andtheMillerglacialevolution.The agesof the oldestsediments
and their
trough.Theyarealsoassociated
withcolsin theAsgard
Range,stratigraphic
settings
are summarized
in Table1, whilethe
notably
inSessrunmir
Valley.
They
arecutintoboth
dolerite
and relationship
ofthedeposits
tothelandformes
isshown
inFigure
2.
sandstone.
TheDryValleys
themselves
arelongknown
tohavebeenrelict
Glacialdeposits
mantlemanyof the gentlerslopes
and since
glacial
andmarine
sediments
in theirseaward
mouthsare
reflecta remarkably
complex
storyof deposition,
subsequent
Miocene
in age.At themouth
ofFerrar
Glacier
thebaseofthe
erosion,
andtransport
[Marcham
etal., 1993b,
c]. A particularlyCIROS-2 core revealed glaciomarinesedimentsdated on
significant
deposit
istheSirius
Group.WithintheDryValleysbiostratigraphicgroundsas older than 4.5 Ma [Barrett and
areait is knownin fourlocations
(Figures
2 and4). Threeof Hambrey,1992]. Microfossils
at the baseof the Dry Valleys
thesearerestricted
to theflanksof inselbergs
whichriseabove Drilling Project(DVDP 11) core drilled into sedimentsat the
theupper
surface,
namely,
MountFeather
[Brady
andMcKelvey,mouthof Taylor Valley are late Miocenein age [McGinnis,
1981'lshmanandRieck,1992]. The 87Sr/86Sr
ratioson shellsin
1979], MountFleming,
andShapeless
Mountain
[McKelvey,
1991].Thefourthoccurs
onTableMountain,overlooking
the the Jasonglaciomarinediamictonin central Wright Valley
southsideof FerrarGlacier[BarrettandPowell,1982]. The suggest
agesof 9 + 1.5 Ma, while in situ shellsin the Prospect
deposits
consist
mainlyof tillstypicalof warm-based
glaciers,Mesa gravelsare aged5.5 + 0.4 Ma [Prenticeet al., 1993]. All
containing
facetedandstriatedstones
withina fine-grained
thesedatessuggest
thatthevalleyswerealreadyexcavated
before
matrix.
beingfloodedby theseaduringtheMiocene.
The flanksof the valleyscan alsobe shownto be old. In
Age of the Landforms
Taylor Valley there are numeroussmall volcanic cones and
Dating of rocks and organicremainsassociatedwith the associated
scotiadepositswhichhave eruptedon to the valley
variouslandforms
placesconstraints
on thesequence
of landscape benchesand the lower glaciallymouldedslopes(Figure 7).
9958
SUGDEN ET AL.: DRY VALLEY LANDSCAPEEVOLUTION
o
+1
a:5 a:5
+1
+1
a:5
+1
+1
c•
+1
•
+1
c•
+1
+1
•
SUGDEN ET AL.: DRY VALLEY LANDSCAPE EVOLUTION
•
o
9959
9960
SUGDEN ET AL.: DRY VALLEY LANDSCAPE EVOLUTION
Fourteeneruptionsrange in age from 1.5 to 3.89 Ma and
demonstrate
thattheunderlyingbedrockslopesare olderthanthis
[Wilch et al., 1993b]. Details of three eruptionsare listed in
Table 1 (sites 16-18). In Wright Valley there are terminal
morainesassociatedwith tributary glaciersforming loops on its
southernflank, which are >3.7 Ma in age [Hall et al., 1993].
Their preservation demonstratesthat there has been little
morphologicalchangesubsequently.
The great age of the slopesof the Quartermainand Asgard
rangescanbe establishedby their associationwith in situvolcanic
ashestrapped in frost cracks,tills, regolith and ash avalanche
cones[Marchant et al., 1993a, b, c). The asheshave been dated
by laserfusion4øAr/39Ar
analysis
of singlefeldspar
crystals.
o
oc: 5
There is goodevidencethat the ashesaccumulatedas a result of
directairfall or by avalanching
associated
with the settlingof the
ash,ratherthanby desertaeolianprocesses.This is demonstrated
by the physical characteristicsof the ash, notably the
concentration,coarse grain size, poor sorting (bimodal) and
presenceof angularshards,all characteristics
of primary ashfalls
with limited transport [Fisher and Schminke,1984]. It is also
confirmedby the distinctgeochemistry
of individualdepositsand
the consistency
of age of individual crystalswithin a deposit
[Marchantet al., 1995]. The ashesare remarkablyunweathered
and contain less than 10% clay-sizedgrains, while individual
volcaniccrystalsshowfew signsof chemicalweathering.Dating
of unweathered
grainsshowsthatthedepositsareMiocenein age.
Table 1 lists the detailsof 15 sitesand revealsa rangeof ages
from 3.9 Ma to 15.25 Ma.
The ashesoccur on high-altitudevalley floors, rectilinear
slopes,and even in minor lake hollows. One significantsite is
shown in Figure 3, where ash has survived in regolith on a
rectilinear slopeof 28ø since it was depositedat the time of an
eruption8.5 m.y. ago. Otherashesare preservedin thin regolith
or in ash avalanchecones. The implicationis that the slopesin
the Asgardand Quartermainrange had achievedtheir present
detailedform by the mid-Mioceneand haveremainedessentially
unchangedfor 15 m.y. This conclusionis consistentwith the
presence of Pliocene ash in the CIROS-2 borehole in Ferrar
Glaciervalley, asrecordedby Barrett et al., [ 1992].
It is important to note the associationbetween Pliocene
Sirius Group depositsand the higher surfacesand associated
inselbergs.There is an implicationthat thesehigh-levelsurfaces,
like the overlyingdeposits,may be Pliocenein age,as arguedby
Van der Wateren and Verbers [1992] in northern Victoria Land.
This posesan immediateproblem in that they are distinctly
youngerthan the depositsat lower altitudesin the Dry Valleys.
At this point in the argumentit is worthrecallingthat the dating
of the SiriusGroupdepositsrelies on biostratigraphic
correlation
of diatoms,supported
by the datingof volcanicashin oneoffshore
borehole[Barrettet al., 1992]. At presentthereareno published,
direct exposure age or ash dates from the deposit itself.
Moreover, there is debateaboutthe age and significanceof the
Nothofagusfossilsin the deposit[Burckleand Pokras,1991], the
significanceof the diatoms [Burckle, 1995] and the climatic
conditionsin the areaat the time of volcaniceruptions3 m.y. ago
[Marchantet al., 1993a].
There is limited but useful informationabout the age of
SUGDEN
ET AL.: DRY VALLEY
LANDSCAPE
EVOLUTION
9961
progressively
offshoresedimentswhich relatesto the story of landscapeinterpretation.First, the presenceof escarpments
consuming
higher surfacesreflectsdenudationby parallel slope
retreat. Sucha patternof denudationis well knownin semiarid
environments
wherethe key processis the powerof surfacewash
in creatingpediplainsand removingregolith from upstanding
slopes[Carsonand Kirkby, 1972]. Second,the detailedform of
the slopesis characteristicof semiaridconditions,notably the
buttesboundedby rectilinearslopes,for examplein the Olympus
earlyMiocene.The implication
is thatthe FerrarGlaciervalley Range,and the presenceof box canyonsat the head of a dendritic
wasin existenceat the time [Barrettet al., 1989]. Presumably, fiver valley system, as, for example, in Beacon Valley,
the lower, equallythick depositsreflectdepositionduringthe QuartermainMountains and the Asgard and Olympus Ranges.
Eocene. There are minimal thicknesses of Miocene and Pliocene Third, buried desertsurfaceventifactsoccurwithin the regolith
sediments.The presenceof basementgranitefrom the earliest found on somerectilinear slopes(Figure 3), a finding which is
Oligocenehas been interpretedto mean that some 2.5 km of consistentwi.than origin under semiaridconditions[Marchant et
BeaconSupergroup
hadbeenerodedfromthe adjacentmountains al., 1993b,c].
by about36 Ma.
Semiaridconditionsmay alsohavemarkedthe initial stages
Finally,it is worthnotingthe oldestdepositsassociated
with of downcuttingas suggestedby the rectilinearridge landscapes
the presentlandforms. Jurassicbasalts(Kirkpatrickbasalt) and rectilinear slopesof the mountainfront. Possibly,further
eruptedinto a depression
on a land surfacewhich is preserved downcuttingsaw a change to more humid conditions which
todayon an inselbergrisingabovetheuppersurfacein the Allan allowedthe creationof the rolling slopesin the inner reachesof
Hills [Ballance and Watters, 1971]. The Allan Hills are the the main valleys and the gentleslopesof the valley benches,but
equivalent of inselbergssuch as Mount Feather and Mount this morphologicalchangecould be related to lithology as the
Flemingandlie on the northernflank of UpperMackayGlacier valleys increasinglycut into basementrocks. The rivers were
evolution. Seismic surveysof the offshorebasins reveal the
presenceof gentlyseawarddippingsedimentsthoughtto have
been mainly derived from the rising TransantarcticMountains
[DaveyandChristoffel,1986;Cooperet al., 1991]. The CIROS-1
drill hole penetrated700 m into deltaicsedimentsassociated
with
the FerrarGlacier. It reachedonlyhalfwayto the basementbut
revealedsedimentsdatingfrom the early Oligocene(36 Ma) to
(Figure 1).
able to grade their main valleys to near sea level some distance
from thecoast. In the caseof Lake Bonney,whichtodayoccupies
LandscapeEvolution
a fiver-cut,s'muous
section
of TaylorValley,thevalleywasclose
The nature, spatial relationships,and age of the various to sealevelsome35 km from thecoast(Figure7).
A change to cooler conditionsis indicated by evidence of
landform associationsprovide the basis for reconstructingthe
glaciation
at a wide variety of altitudes. Cold polar conditions
sequenceof landscapedenudationand the climatic environments
have
existed
since the main elementsof the landscapewere
involved. We assumethat the high upper and intermediate
formed.
This
is
implied by the preservationof the relict regolith
planationsurfacesarethe oldestelementsof the landscape.Since
on
slopes
of
26-30
ø for millionsof years[Marchantet al., 1995].
inselbergsassociated
with the intermediatesurfacecompriseparts
It
is
also
demonstrated
by the minimal erosionaleffectsof valley
of the uppersurface,it is temptingto suggestthat the lower of the
two surfacesis younger. However,it is quite possiblethat both glaciersflowing into the main valleys.For example,the glaciers
surfaces formed contemporaneously
but with their detailed originatingin the AsgardRangeand flowing into Wright Valley
morphologyinfluencedby lithology. The valleys and dissected have failed to excavatetroughs,suggestingthat their baseshave
rectilinearridge landscapes
are cut into the two surfaces.This is remainedbelow the pressuremelting point sincetheir formation.
illustratedby the way the surfacesare progressivelydissected They are also fringed by relatively small morainesderived from
towardthe coastandby the way the remnantinselbergsandpeaks valley side debris,and theserepresentthe total load over at least
lie at comformableor slightly lower altitudesthan the adjacent 3.7 m.y. [Hall et al., 1993]. Suchsmalldebrisloadsare typicalof
glaciersin a cold polar environment.
surfacefrom whichtheyhavebeeneroded.
The downcuttingseemsto be fluvial in origin. This is
illustrated by the integrated pattern of the tributaries, the
sinuosityof the main valleys,the presenceof valley benches,and
theway thevalleysaregradedto nearsealevel. The main valleys
and their associated
rolling slopesand valley bencheshave been
incisedmostdeeply,as is illustratedby the way tributaryvalleys
are left hangingabovethe main valley floors. Finally, glacierice
has coveredall slopesfrom the highestuplandsurfaceto valley
Relationshipto Structure and Tectonics
There is good evidenceof differential denudation,with a
maximum
near the mountain front and less inland.
This is well
displayedby the tilt of the basementsurfaceof Ordovicianor
Devonianage,known asthe Kukri "peneplain",whichis assumed
to havebeen approximatelyhorizontalat the time of its formation
and is now overlain by Beacon Supergroupsedimentaryrock
floors and the coastal lowland.
Glacial
erosion has been
[Gunnand Warren, 1962]. It is exposedin the innermostreaches
significantin straighteningand deepeningpreexisting sinuous of Taylor andWright valleyswhereit is now at an altitudeof 700
valleysbut has been minimal on someof the higher interfluves, m and 900 m respectivelyand dips inland at 2-3ø. It rises in
suchas the AsgardRange,whereformsdue to glacialerosionare discreteblockstowardsthe coast,reachingaltitudesof 1800 m in
rare.
the vicinity of Mount Newall. The main mountainfront itself is
The intermediatesurfacereflects scarp retreat, probably downfaulted [Fitzgerald, 1992]. Uniform rectilinear slopes
undersemiaridconditions,as arguedby Dentonet al., [1993]. It
trimcatethe fault lines and are associatedwith valley incision of
is useful to summarizethe argumentsin favor of such an the mountainfront. Also, the samefault zone cuts acrossTaylor
9962
SUGDEN ET AL.: DRY VALLEY LANDSCAPE EVOLUTION
intermediatesurfacein the vicinity. At the coastwhere there is
w
1600 -
relief of 900 m in basement rocks, the denudation exceeds 4.0
1400-
km.
These different methods of calculating amounts of
denudationagreeclosely. Together,they suggestthat erosionhas
removedan irregularwedgeof rock more than 4 km thick at the
coastandtaperingto lessthan 1 km 75 km inland.
._. 1200-
•E1000z
o
800
< 6OO
Model of LandscapeEvolution
'" 400'
'" 200-
It is possible to bring together the various strands of
evidence
and to presenta hypothesisof the evolutionof the Dry
MT TERMINATION
Valleys
which
synthesises
geomorphology,
tectonicsand climate.
2000•
I BULLPASS•
I. I SPIKECAPE
We suggestthe area beginsas one flank of an extensionalrift
(Figure 10a). The continentaledgehas an altitudeof 1200 m.
COAST
Admittedly,
thisis arbitrary,but 1200 m seemsa realisticaltitude
Figure9. Crosssection
of theTransantarctic
Mountains
parallel
for
the
interior
of a large continent,especiallyin view of the
to WrightValleyshowing
differentialupliftsince55 Ma, after
presence
of
extensive
flood basalts which could indicate
Ten Brink and Stern [1992]. The uplift is represented
by a preupliftisochron
relatedto a coastal
datum.Theisochron
is derived underplating[Listeret al., 1991]. Also, assumingno differential
from apatitefissiontrack analysisand represents
the partial tectonicmovement,this figurehas beencalculatedto have been
SL-
/ MT
JASON
MT.
THESEUS
IMT.
DOORLY
I
10
O0
f:!•i•
:'''
"' ii•iiiiil
'-•ii•ii:::
:"
"'•'"
""
"'""'
'•1
annealing
zonebeforeuplift[GleadowandFitzgerald,1987].
the initial
altitude of western South Africa
on the basis of the
volumeof offshoresediments[Gilchristand Sutmru•rfield,
1994].
The 1200 m altitude is higher than the 500 m assumedby
Valley and tnmcatesthe valley benchesof the interior. These Gleadowand Fitzgerald [1987], but this latter value is admitted
relationships
suggest
thatthe faultingoccurred
beforeandduring to be poorly constrained;it is based on the view that the lowthephaseof valleydowncutting,
but not subsequently.
gradient floodplain beneath the Kirkpatrick basak would have
The overallpatternof the differentialcrustaluplift can also beencloseto sealevel and on analogous
situationsin Greenland.
be inferredfrom fissiontrack analysisand is shownin Figure9. Such argumentsdo not necessarilycarry weight in escarpment
The profile,whichrunsat right anglesto the coastthroughMount landscapes.Under suchcircumstances,
it is perhapshelpful to
Doorly, representsthe distortionof a preuplift, approximately bear in mind both estimates as an indicator of the level of
horizontal, isochronin relation to sea level at the coast [Gleadow uncertaintyinvolvedin thisinitial assumption.
and Fitzgerald, 1987]. The downthrow of the faults at the
Following extension, scarp retreat associated with
mountainfrontis of the orderof 1650 m, approximately
the same pediplanationextendedinlandfrom the new baselevel createdat
as the difference in altitude between the intermediate surface and
the new coastandalongthe main river valleys(Figure10b). The
the coastalpiedmont. In view of this approximatecoincidence, upperand intermediatesurfacesmay be the resultof scarpretreat
the coastal piedmont could be a downfaultedremnant of the from two differentbaselevels,perhapsreflectingtheoriginalbase
intermediate surface.
level at the time of rifting and a subsequent
baselevel fall as a
It is possibleto usethe tectonicand structuralrelationships resultof someuplift. Alternatively,theymay reflectthe dominant
to estimate the amount of denudation involved.
One estimate
role of a major dolerite sill in interruptingthe slope of a single
comesfrom the fissiontrack analysis. Sincethe originalisochron phaseof base level change. Either way, the pediplainseroded
is thoughtto haveformedin the partial annealingzoneat a depth downto granitebasementat the coastandremovedat least 3 km
of around4-5 km, its exposurealongthe crestof the mountain of coverrocksin this location. Modest lowering occurredon the
front implies denudationof around4-5 km, with progressively uppersurface,perhapssufficientto createinvertedrelief whereby
lesseramountsinland [Fitzgerald, 1992]. Anotherestimatecomes the Jurassicbasaks, originally extruded in depressions,now
fromtherelativeelevationof theKulcripeneplain.Assuming
that cappedsomeof the inselbergsrising abovethe surface,as in the
the Kukri peneplainwas horizontaland covereduniformlywith , caseof theAllanHillstoday. The contrast
betweenthehighrates
3.1 km of youngerrocks (2.5 km of the BeaconSupergroup of erosionthat would be expectedalong the scarpand the low
[Barren, 1981] and about 600 m of dolerite sills [Hamilton et al.,
1965;Gleadowand Fitzgerald,1987], thenonecanestimatethe
amountof denudationfrom placeto place. In Wright Valley the
Kukri peneplaindeclinesfrom 1650-1800m at the mountainfront
to 1050 m in the vicinityof Mount Jasonand 700 m in upper
Wright Valley where it disappears beneath dolerite.
Extrapolatingthe samedip would suggestit is at an altitudeof
100 m beneathMount Fleming,an inselbergrisingto 2400 m. If
theseapproximate
figuresare usedandthe altitudeof the present
summits is allowed for, then there has been denudation of 0.6 km
aboveMount Fleming,1 km from the uppersurfaceand over 3.1
km
from
above the summits
of the mountain
front
and the
ratesof erosionon flat surfacesis well describedby Gilchrist and
Summedeld[1990].
Figure 10c showsthe situationfollowingdowncuttingof the
majorfiver valleys. Valleys have cut into the mountainscausing
most fretting towardthe sea. The intermediatesurfaceis tilted
back and has virtually no seawardslope, while the valleys are
graded to sea level. Apparently, much of the downcutting
accompanied
faultingandrockuplift of the mountainfront,which
wouldaccountfor a furtherfall in relativebaselevel. It is likely
that glacierscontributedto the widening and straighteningof
somemajorvalleys.
There followeda periodof apparentsubsidence
sufficientto
SUGDEN ET AL.: DRY VALLEY LANDSCAPE EVOLUTION
9963
W
m
Tectonics
Kirkpatrick
2000] basalt •
(a)
Sea
10000
u•er
s• "peneplain"
-1000•
--'• -- Kulcri
Age
(Ma)
extension
and new
base level
-3000
-
30004_
(b)
Denudation
process
. _.
original
s_ur•_a•ce
uplift,
2000 -- u••m•iint•ermedaaterock
faulting?
-"'uppe;
.........
.,..--"
0
Fleming
"-x
Obehsk
upper
m
2000-
Newall
-............... coastal
I'-x
Obehsk
upper
m
..............
_,000
;
coastal
valley downcuttingby
rivers
and glaciers
rock
fiords,
subsidence
polar desert,
>400m
cold-based
?->15
Obehsk
>9 - <3.5
glaciers
½/ I
Newall
(e) 2000
lOOO
renewed
polar desert,
uplift
no rivers,
< 3.5 - 0
cold-based
0
0
-1000
0
continued
rock uplift
and faulting
Newall
intermediate ß
2000
Fleming
55
intermediate,
(c)
Fleming
semi-arid
planation
10
20
30
40
50
60
glaciers
70
km
%XXx.
Kukri 'peneplain'
/•,
sill/ basalt
•
fjords
Figure10. Empirical
modelof landscape
evolution
in theDryValleysareaof theTransantarctic
Mountains
showing
thesituation
(a) afterinitialextension,
(b) dlxring
planation
witha lowerbaselevel,(c) aftervalleydowncutting,
(d)
duringa subsequent
phaseof subsidence,
and(e) followingfurtherupliftof 300 m at themountain
front. The
possible
agesof thestages
of evolution,
theassociated
climate,
andtectonic
environments
arealsoshown.
causethe main valleysto become'inundated
by the sea, even in
sinuous valley sections devoid of significant glacial
overdeepening,
such as inner Taylor Valley (Figure 10d). In
Wright Valley, morainemorphology
suggests
that the inundation
was300 m higherthansealevel at present[Hall et al., 1993]. If
one allowsfor the marinesedimentinfill in fluvial partsof Taylor
Valley, thenthe subsidence
amountedto >400 m.
Subsequently,
therewas a phaseof reneweduplift alongthe
mountainfront (Figure 10e). This was sufficientto drain the
fjords and to createreverseslopesin both Wright and Taylor
Valleys. Continuity of slopes acrossmountain front faults
suggests
thatthisrecentuplift hasnot reactivatedany faults.
The age of the various stagesof landscapeevolution is
constrained
on the onehandby apatitefissiontrackevidencethat
9964
SUGDEN ET AL.: DRY VALLEY LANDSCAPE EVOLUTION
rapid denudationbegan55 m.y. ago [Fitzgerald,1992; Gleadow
and Fitzgerald, 1987]. This implies that the planationsurfaces
andvalleysformedafter this date. On the otherhand,denudation
was largely completeby 15 Mal as indicatedby the age of the
surficialdepositson valley-sideslopes(Figure10c). The timing
of the subsidence
of the Dry Valleys block to an intervalbetween
>9 and <3.5 m.y. ago is constrainedby the age of marine
sedimentsin the mouthsof Wright, Taylor, and Ferrar Valleys
(Figure 10d).
The agesattributedto the stagesof landscapeevolutionin
the model raise one major difficulty, namely,the Sirius Group
explainthe minimumthicknessof late Mioceneand Pliocene
sediments in the CIROS-1 drill hole immediately offshore
[Barrett et al., 1989].
Wider Implications
Althoughthe model of landscapeevolutionis tentativeand
incomplete,it has the merit of integrating the sequenceof
landscapeevolution with tectonic evidence and producinga
combinedinterpretationwhich is consistent. There are several
importantimplicationsfor our understanding
of the evolutionof
passivecontinental
margins.
deposits
thought
tohavebeenemplaced
lessthan2-3m.y.agoby
The landformevidencesuggeststhat since continental
the mainice sheet. If so, thentheirpreservation
onlyon the separation,
denudation
hasremoveda wedgeof rockover4 km
oldestrelictelements
of thelandscape
impliesthattheplanation thickatthecoastand1 km thick75 km inland.At thesametime,
and valley incisionhas occurredsubsequently.However,the
presence
of olderdeposits
on theselowerdissected
slopesand
valleyspresentsa difficulty;it impliesthat there has beenno
significant
landscape
modification
in the last 15 m.y. So it is
necessary
to find an alternativeexplanation
for the age and
locationof the SiriusGroupdeposits.At presentwe haveno
satisfactory
explanation,
but it is usefulto highlighttwo further
arguments
suggesting
that the SiriusGroupdeposits
in the Dry
Valleys cannotbe Pliocenein age. First, we have shown
assumingan initial elevation of 1200 m, the land surface
elevationhasbeenloweredby 1 km in the downfaulted
coastal
zoneandhasbeenraisedby 0.8 km on the rift shoulderof the
mountainfrontandby 1 km at the headof the valleys75 km
inland.If theinitialelevation
is correct,thencrustal
upliftsince
riftinghasbeenat least3 km at the downfaulted
coast,4 km at
themountain
front,and2 km75 kminland.Theselatterfigures
agreewith the view that uplift has beengreateston the rift
shoulderandhas declinedprogressively
inland[Gleadowand
elsewherethatit is possibleto fix thelimit of thoseEastAntarctic Fitzgerald,
1987;Fitzgerald,1992;TenBrinkandStern,1992].
ice sheetoutletglaciersflowinginto TaylorValleyduringthe The smaller magnitudeof our figures is the result of the
Pliocene[Dentonet al., 1993;Marchantet al., 1994]. Well- difference in assumed initial elevation.
constrained
reconstructed
pi'ofilesbasedon theselimitsdo not
Our reconstruction
of landscape
evolutionplacessome
permitice to be sufficientlythick to coverthe adjacentSirius constraintson the tectonicprocessesinvolved. Rates of
Groupdeposit
onMountFeather.Thusit is difficultto envisage denudation
were high in the early Cenozoicand tailed off to
the glaciological
conditions
that couldexplainthis particular virtuallynothing
by 15Ma. Thechange
fromhightolowratesof
SiriusGroupdeposit.Second,
thelackof weathering
in Miocene denudation
duringtheCenozoic
is mostsimplyexplained
by a
volcanic
ashes,
their association
withcoldpolarsoilwedges,
and progressively
decreasing
fall in baselevel followingan initial
their preservation
in sensitivelocations
pointto continued
cold change.The initialrift allowedplanationto extendinlandfrom
desertconditions
sincethemid-Miocene
[Marchantet al., 1993c]. the new coast. Subsequent
valley downcutting
(and some
Thiswouldseemto preclude
thepossibility
of themild conditions planation?)
wasassociated
withcrustal
upliftandfaultingat the
necessary
to explainthe remainsof temperatevegetation
in the mountainfront. Uplift latergavewayto subsidence,
asreflected
SiriusGroupdeposits.
by the incursion of the Miocene/Pliocenesea into the lower
The climatic environmentassociated
with landscapevalleysto levels300 m abovethoseat present.The survivalof
evolution in our model is based on the dominant landform ancient
rivervalleys
graded
closetopresent
sealevelin thispart
associations.We have arguedthat planationand at least the of theTransantarctic
Mountains
suggests
thatsubsequent
uplift
initial valley cuttingprobablyoccurredin semiaridconditions hasbeenlimited.Thislatterconclusion
agrees
withWilchet al.,
withscanty
regolithandvegetation
andyetsufficient
flashfloods [1993a],
whousedthepresence
ofvolcanic
cones
in TaylorValley
to favor pediplanation
[Dentonet al., 1993]. Although to suggestthatsurfaceuplift canhavebeenno morethan300 m in
downcutting
mayhavebeenwhollyrelatedtobaselevelchangesthelast2.54 m.y.
asa resultof uplift,it is possible
thatit couldbepartiallycaused
Thispatternof denudation
anduplift is consistent
with the
bytheonsetof wetterconditions.
In sucha casethicker,
regolith evolution
of an upperplatepassive
mountain
range. The initial
wouldproduceconvex-concave
slopesandfavorincisionof fiver Cenozoic
baselevelchange
agrees
with theviewof Fitzgerald
valleys,aidedperhaps
by intermittent
glaciation.A morehumid [1992]thattheTransantarctic
Mountains
formtheupthrown
block
climateduringtheearlyOligocene
to earlyMioceneis consistentof anasymmetric
rift. Suchasymmetry
explains
theexistence
of
with the resultsof the nearbyCIROS-1drill hole whichfound anupliftedflankwhichcreates
a largeinitialbaselevelchange
leavesof Nothofagus
of Oligocene
age[Barrettet al., 1989]. The and permitsscarpretreatfrom the new coast. The evidenceof
coldpolardesertconditions
of the last 15 m.y. associated
with crustal
upliftsoonafterrifting,followed
by subsidence,
pointsto
little signof runningwaterhelpto explainthepreservation
and theroleof thermal
processes.
Typically,
thermal
upliftbylateral
association of volcanic ashes with cold climate landforms heatconduction
or secondary
convection
wouldbe expected
to
[Marchant
et al., 1993b].Thelackof waterwouldalsoexplain leadto initialupliftof 500-1500m [Summe•eld,1988]. Such
whythemainriverswereunabletoexcavate
theirvalleys
in spite upliftcouldfollowriftingaftera lagof 10-20m.y.anddecay
with
ofmodest
ratesof surface
upliftin thelast15m.y. It wouldalso time,eventuallyleadingto subsidence
[Steckler,1985;Fleitoutet
SUGDEN ET AL.: DRY VALLEY
al., 1986]. Such a processwould explain the crustal uplift
associated
with faulting in the early Cenozoicand the subsidence
LANDSCAPE
EVOLUTION
9965
Craddock,pp. 1059-1067, Universityof WisconsinPress,Madison,
1982.
Barrett,P.J., M.J. Hambrey, D.M. Harwood,A.R. Pyne and P.N. Webb,
of several hundred meters in the Miocene.
Synthesis,in Antarctic Cenozoic History From the CIROS-1
Anotherprocesslikely to haveplayeda role is flexural uplift
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[1990]notehow this canbe expectedto lead to long-termsurface
uplift, initially at the coast but migrating inland with the Barrett,P.J., C.J. Adams,W.C. Mcintosh,C.C. Swisher,and G.S. Wilson,
Geochronological
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progression
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millionyearsago, Nature,359,816-818, 1992.
to accountfor about 600 m of uplift along part of the western
marginof southernAfrica. In the Dry Valleys, it could account Behrendt,J.C. and A. Cooper,Evidenceof rapid Cenozoicuplift of the
shoulderescarpment
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for a comparableamountof uplift in the early Cenozoic. Yet
speculation
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anotherprocessof uplift couldbe glacio-isostatic
and could be
relatedto a forebulgebeyondthe marginof the East Antarcticice Behrendt,J.C.,W.E. Lemasuder,A.K. Cooper,F. Tessonsohn,A. Trehu,
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contributedto someof theuplift sincethe Miocene.
Conclusion
This view of modestmountainuplift sincethe Plioceneand
Universityof WisconsinPress,Madison,1982.
limited geomorphicactivitysincethe mid-Miocenediffers from Brady,H., and B. McKelvey, The interpretation
of a Tertiarytillire at
the alternativeview of 1-2 km of uplift since the Pliocene and
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ratherdynamicchangesin glaciationandvegetation,as suggested
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elsewhere [Webb and Harwood, 1987; Webb et al.,
1984;
Brown,R.W., M. A. Summerfield,
and A. J. W. Gleadow,Apatitefission
McKelveyet al., 1991;Barrett et al., 1992]. The apparentyouth
trackanalysis:Its potentialfor the estimationof denudationramsand
of the Siriusgroupdepositsis the onepieceof evidencethat does
implications
for modelsof long-termlandscape
development,in
not fit our geomorphological
reconstruction.We haveno accepted
ProcessModelsand TheoreticalGeornorphology,
editedby M. J.
explanationfor the apparentage of the Sirius Group deposits.
Kirkby,pp.23-53,JohnWiley, New York, 1994.
We would simply note that our geomorphicreconstructionis Bull, C.B.B.,B.C. McKelvey,andP.N. Webb,Quaternary
glaciations
in
underpinned
by a considerable
numberof absolutedatesand that
SouthernVictoriaLand,Antarctica,J. Glaciol.,4(31), 63-78, 1962.
it agreeswell with all otherknowngeophysical
evidence.While Burckle,L.H. A criticalreviewof themicropalaeontological
evidence
used
we recognizethat we have studied only one block of the
to infera majordrawdownof theEastAnatarctic
Ice Sheetduringthe
Transantarctic
Mountains,the conclusions
reachedhere may have
earlyPliocene,in Climateand Evolution,editedby E.S. Vrba,Yale
wider implications.
UniversityPress,New Haven,Conn.,in press,1995.
Burckle,L.H., and C.M. Pokras,Implicationsof a Pliocenestandof
Acknowledgments. The researchwas supported
by the Divisionof Polar
Programs
of theNationalScienceFoundation,
the U.K. NaturalEnvironment
Nothofagus(Southem Beech) within 500 kilometresof the South
Pole,Antarct. Sci., 3, 389-403, 1991.
Research
Council,theRoyalSociety,andtheRoyalSocietyof Edinburgh. Calkin,P.E., Processes
in the ice-freevalleysof southernVictoriaLand,
We are very gratefulto Paul Fitzgeraldand Michael Surmnerfield
for
Antarctica,
in
Research
in PolarandAlpineGeornorphology,
edited
valuablecomments
on an earlydraft,andto fourreferees
whoseconstructive
comments
improvedthemanuscript
markedly.
by B.D. Faheyand R.D. Thompson,
pp. 167-186, Departmentof
Geography,
Universityof Guelph,Ontario,Canada,1974a.
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