JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. B12, PAGES 29,095-29,112,DECEMBER 10, 1999
Life cycle of the East Carpathian orogen: Erosion history
of a doubly vetgent critical wedge assessed by fission
track thermochronology
CarloA. E. Sanders,
• P. A.M. Andriessen,
andS. A. P. L. Cloetingh
Instituteof EarthSciences,Vrije Universiteit,Amsterdam,Netherlands
Abstract. Apatitefissiontrack(b-T)thermochronology
appliedto the EastCarpathianfold-andthrustbelt in Romaniaconstrains
the interactionbetweenthe erosionhistoryand the tectonicevolutionof the orogen.The long-livedconstantasymmetrictopographyof the orogenandthe erosion
patternasinferredfrom apatiteFT dataare in agreementwith the styleof evolutionpredictedby
doublyvergentcriticalCoulombwedgemodels.During the Miocene,up to 4 km of erosiontook
placeovertheactivelydeformingorogen,andthe erosionpatternsformsa mirroredimageof the
subsurface
deformingwedge.The erosionproductsare depositedin flankingmolassebasinswhich
subsided
contemporaneously
dueto the growingtopographic
weightof thewedge.Deformationof
the wedgewaspresumablyactivefrom the early to late Miocene,but apatiteFT agesbetween14
and9 Ma andtracklengthmodelingpredictan accelerationof erosionrates(0.5-0.3 mm/yr) initially in the middleto late Miocene(• !3_+2Ma). The onsetof erosioncoincideswith the last
phaseof convergence
anda climaxin thedeformationhistory.Thesefeaturesareinterpretedto have
beencausedby underplating
of theEuropeancontinentalmargin.Convergence
ceasedbeforethe
Plioceneaccordingto the activityof thrustfaults.During the PlioceneandQuaternary,up to 2 km
of overburden
wasremovedby erosion,leadingto isostaticrockuplift of theentireregion.The use
of apatitefissiontrack"minimumages"supports
the generalideathatthe orogenhasa diachronous
evolutionfrom northto south.The orogenin the southernBendZone hasa morejuvenile tectoniccharacter,andthemainerosionphasetookplaceduringthePlioceneandQuaternary
(•5-0 Ma). The integratedapproachof largescalemorphologyandfissiontrackanalysisconfirms
the applicabilityof the theoreticalCoulombwedgemechanicsto a naturalcompressive
fold-andthrustbelt.The studycomplements
the geometricfeaturesof the orogenicwedgewith quantified
erosionhistoriesandillustratesthe effectsof orogenevolutionon verticalmovementsin the entire
convergentsetting.
but the infinite height of the experimentalbackstopwas an
unrealisticassumptionfor naturalorogens.
Frontal parts of oceanicaccretionarywedgesand fold-andThe useof backstopswith finite height resultedin signifithrustbeltsare well understoodthroughthe mechanicsof criticantly different wedge geometries[e.g., Malavieille, 1984;
cal Mohr-Coulomb wedges [Chappie, 1978; Davis et al.,
Byrne et al., 1993; Willet et al., 1993; Koons, 1989, 1990],
1983; Dahlen et al., 1984]. The fundamentalprinciplesare ilalthough the deformation mechanism remained the same
lustratedin Figure la [e.g.,Davis et al., 1983] in a simplified
(Figure lb). The frontal part (or prowedge)still behavesas in
modelbasedon analoguesandboxexperiments:Sedimentsin
the minimumtaperedbulldozermodel (Figure la), but differfront of a rigid backstopof infinite height are scrapedfrom a
encesoccurto the rear above the tip of the backstop.The tip
continuouslysubductingplate and stackedin a wedge. The
of the backstopis a velocitydiscontinuumwhich in numerical
wedgeattainsa minimumcritical taper (c•+•) and grows in a
simulationsis termeda singularitypoint [Willet et al., 1993].
self similar way [Dahlen, 1990] by a processof alternating
Above the singularity point forms an outer-arc-high,and in
frontal accretion and internal deformation. The ratio of interthe samezonethe stressregime flips orientation.To the rear,
nal friction coefficient(•)in the wedge and friction coeffian oppositelytapering inner deformationbelt (or retrowedge)
cient alongthe decollement(•xb)are the main factorscontrolis formed,characterizedby a reversalin thrustvergencewith
ling the angleof the critical taper.The wedge-formingmecharespect to the prowedge (Figure lb). This retrowedge apnism is independentof scale, and the "bulldozer"concept proachesa maximum critical taper becausematerial is essenshowedsatisfyinggeometricresemblances
with natural examtially addedto its back [Wang & Davis, 1996]. As a conseples like Taiwan [e.g., Dahlen et al., 1984]. These studies _ quence,the prowedgeand retrowedgehave different heightprovidedvaluableinsightson the mechanismof deformation, width ratios, creating the generally recognizedasymmetric
geometryof the combineddoublyvergentwedge.
Byrne et al. [1993] have shown that the key features de•Nowat MidlandValleyExploration
Limited,Glasgow,
Scotland,
UnitedKingdom.
scribedaboveare insensitiveto the geometryof a backstopas
long as it is situatedat depth.The doubly vergentwedge has
Copyright1999by theAmericanGeophysical
Union.
beenrecognizedonly in modernactive orogens[Koons, 1995;
1.
Introduction
Papernumber1998JB900046.
0148-0227/99/1998JB 9ff1)46509.00
Beaumont et al., 1996; Torrini and Speed, 1989; Silver and
Reed, 1988].
29,095
29,096
SANDERS ET AL.: LIFE CYCLE OF THE EAST CARPATHIAN OROGEN
"bulldozer"
new
backstop
toe
thrust
front
decollement
outerarc-
detachment
wedgehigh
pro-wedge
foreland
zone
B
stress
shadow
The EastCarpathians,on the otherhand,form a closernatural analogueof a critical Coulombwedge.It is a classical thinskinned fold-and-thrust belt, with a relatively shallow basal
decollement(maximum of 10 km deep) and is therefore supposedto deform entirely according to the (frictional) Mob_rCoulombcriterion. Substantialstrike-slip displacementshave
neverbeenidentifiedin this part of the orogen. Moreover, the
effectsof the orogenformation on the subsidenceand sedimentationhistory of its adjacentbasinscan be taken into account becausethey are presently exposedand well studied.
With this new approach the tectonic history of the East
Carpathianscan be interpretedin the context of the critical
wedge theory, in contrastto the descriptiveconceptspublishedsofar. We concentrateon the Neogenetectonic history
of the RomamanEast Carpathiansand use the doubly vetgent
critical wedgeconceptas a working hypothesis.
2. Regional Geological Setting
gularity
point
The Carpathian orogen is part of the European Alpine
chain. It was formed by convergenceof allochtonous microct r
min.
continents(intra-Carpathianplate) with the Europeanforeland
critical
critical
during the Cretaceousand Neogene.The Paleogenewas a period of tectonicquiescence.During the Neogene the microcontaper
taper
tinentscollideden bloc with the Europeancontinentalmargin
Figure 1. Diagram explaining mechanismof critical Coulomb [Burchfiel, 1980; Sandulescu,1988; Csontos, 1995]. In
wedges.(a) Bulldozer model: Sedimentsin front of a backstop
Romania the collision formed an orocline comprisingthe East
(black) are scrapedfrom a subductingplate (gray). The wedge
grows in a self-similar way (stippled)accordingto a minimum Carpathiansand SouthernCarpathians,which at presentreach
criticaltaper(ix+It).Here [.ti is internalfrictioncoefficient;[.tb is maximumelevations of approximately 2000 m above sea
max.
••
ap
friction along basal decollement. (b) Doubly vergent critical
wedge. See text for explanation.Horizontal hatchingis unde-
level.
In the East Carpathians,three structuralunits are distin-
formedsediments;
arrows
illustrate
opposite
thrustvergence;
tXp guishedaccordingto their tectogenetic history (Figure 2).
and • are surfaceslopesof prowedgeand retrowedge,respec- From youngto old (mainly after $andulescuet al. [1981a, b])
tively; [•r is angle betweenhorizontaland detachmentzone they are the Moldavides (Neogene thrust belt), the Outer
(retrowedge),
and[•pis anglebetween
horizontal
andbasaldecollementof the prowedge.
The fundamentalconceptof the doubly vergent wedge is
theoretically well analyzed, but still needstesting and validation againstthe complexityof naturalexamples.We testedthe
doublyvetgentcritical wedgeconceptagainst observationsin
the EastCarpathianfold-and-thrustbelt in Romaniausing the
large-scalegeomorphologyand structuralfield studies.An extra tool we presentis the erosionpattern of the mountainbelt
as reconstructedwith the use of apatite thermochronology
data, which can potentially distinguish between proposed
geophysicalmodels. Kamp et al. [1989], for example, have
shownthat cooling histories of rocks form information critical to constrainingthe underlying deformationmechanicsfor
the SouthernAlps in New Zealand. The SouthernAlps are a
well-known natural example of a doubly vetgent orogenic
wedge[Koons, 1994; Beaumontet al., 1996]. However, the
SouthernAlps are a compressionaloblique-sliporogenwhere
deformationin the deepercrustis no longercontrolledby friction but by time-dependentviscousbehavior which influences
the morphology of the orogen [Beaumont et al., 1996].
Moreover,amountsof uplift and erosionrateshave high values (18 km and 2.:5 mm/yr, respectively)[Kampand Tippett,
1993; Tippet and Kamp, 1993], which greatly influencesthe
thermal structureof the orogen [Koons, 1987; Stiiwe et al.,
1994] and complicatesinterpretationof cooling data [Brown
and Summerfield,1997].
Dacides (Cretaceous thrust belt), and the Middle Dacides
(allochtonouscrystalline basement).
The Moldavidesoccupythe main part of the orogenand are
situatedon its mosteasternside. The nappe systemsare made
up of flysch sedimentswith stratigraphicages ranging from
late Early Cretaceous in the internal nappes to middle
Miocene in the more external nappes.The most external and
youngest nappe consists mostly of molasse sediments.
According to Sandulescu[1988], the nappes were progressively emplacedon the Europeanforeland from 22 to 10 Ma
(early to late Miocene).
The
Outer
Daddes
lie to the
west
of
the
Moldavides.
Stratigraphicagesof the flysch sedimentsrange from Early to
LateCretaceous
[Sandulescu
et al., 1981a]. The main phaseof
thrusting in the Outer Dacides took place during the Late
Cretaceous.The third and most internal unit is formed by the
Middle Dacides,consistingof crystallinebasementrocks with
Mesozoic and Cenozoic sedimentarycover. The nappe structure wasformedin the early Late Cretaceous.
The observed present-day configuration was basically
achievedby a three-stepevolution(Figure3). Closureof the
Outer Dacidian flysch trough during the late Early to Late
Cretaceous(130-66 Ma) by subductionof oceanic(?)crustunder the allochtonous microcontinent (Middle Dacides) resulting in the formation of an (old) accretionarywedge (Outer
Dacides).A period of tectonicquiescencefollowedduringthe
Paleogene(Figure 3b) with extensive flysch sedimentation
from forelandand hinterlandsourcesto the Moldavide trough.
Renewedconvergence
duringthe Neogeneby westwardsubduc-
SANDERS ET AL.: LIFE CYCLE OF THE EAST CARPATHIAN OROGEN
24 ø
2
29,097
2
N
c
B
D
Transylvanian
basin
w
-
South Carpathians
!
1-2
5-6
7
8
9 ! reset
central
age
reset
minimum
age
1Gnon
reset
minimum
age
Figure 2. Tectogeneticmap of Romaniaafter Sandulescuet al. [1978]. Legend: 1 and 2, allochthonous
plate: 1,
sedimentarycover (Senonian-Neogene);
2, Middle Dacides(crystallinebasement);3, Outer Dacides (flysch); 4,
Moldavides(flysch);5, Neogenevolcanics(extrusive);6, intrusive;7, Miocene-Quaternary
molasse;8, retrovergent
thrusts(see discussion).Romannumbersare localitiesreferredto in text; A-A' to D-D' are profiles from Figures4
and 5.
tion and underplatingof the Europeancontinentalmarginled
to the closureof the Moldavideflyschtroughand formationof
a fold-and-thrustbelt (Figure3c). Balancing cross sectionsby
restoration of thrust sheets [Roure et al., 1993; Ellouz and
Boca, 1994; Morley, 1996] showedthat a minimum shortening of 130-220 km occurredduringthe Neogene,indicatinga
subduction
rate in the order of 10-15 km/Ma.
During the final phase of convergence,molasse basins
formedon both sidesof the orogen.A late Neogenecalc-alka-
line volcanic arc formed in the allochtonousplate, intruding
both the molassebasin and the flanking areasof the orogen
(Figure2 and 3). Convergencefinally stoppedwith collision
(docking)of the allochtonousplate againstthe Europeanforeland duringthe late Miocene.
Tectonicactivity is presumablydiachronousfrom north to
southalongthe orogen,as indicatedby both the last activity
of the thrust front [Sandulescu
et al., 1981a, b] and the eruption agesof the volcanic chain [Pecskayet al., 1995]. The
29,098
•
SANDERS ET AL.: LIFE CYCLE OF THE EAST CARPATHIAN OROGEN
Middle
Dacides
Outer Dacidesbasin
130-60
E
A
Ma
Notethatin thispaperwe usethe termsrock uplift, surface
uplift, andexhumationasdid EnglandandMolnar [ 1990]; that
is rockand surfaceuplift are with respectto a referenceframe
(geoidor meansealevel), and exhumationis rock uplift with
respectto the local mean surfaceelevation [see also Brown,
1991]. When no information of the local Earth surface is
available,the generalterm "erosion"is usedfor all removal of
overburden.
M.D.
t O.D. [
'-"•'•
4. OrogenMorphology and the Subsurface
Wedge
Moldavides
basin
B
60-25
Ma
M.D
tO.D.tMoldavides
molasse
ß
(Figurelb) [Koons,1990;Byrneet al., 1993]: 1) Doublyvergentwedgesare asymmetric
with shortand steepretrowedges
opposinglong and gentleinclinedprowedges.2) The dominantthrustshaveoppositevergencein the two wedges.3) The
molasse
c
25-0 Ma
1
2
3
4
5
All criticaldoublyvergentCoulombwedgeshavea number
of fundamentalgeometricfeaturesin commonthat form the
maincriteriato recognizea doublyvergentwedgein nature
{5
7
Figure 3. Diagramsketchingthe tectonichistoryof the EastCarpathians.Europeanforelandis to the east.(a) Cretaceousconvergenceby subductionof oceaniclithosphere:formationof Middle
and Outer Dacides(MD and OD), (b) Paleogeneflysch sedimentation in Moiravide troughin front of a passivemargin,and (c)
renewedconvergencein Neogeneforming East Carpathianorogen, resulting in thrustingof Moldavides and underplatingof
European continental margin. Legend: 1, allochthonouscontinental crust; 2, European continental crust; 3, Outer Dacides
flysch; 4, Moldavides flysch; 5, Neogenevolcanicchain;6, oceanic crust; 7, molasse.
agesof both processesshow a trend from late Miocene in the
north to Pliocene-Quaternary
in the Bend Zone. Thereforethe
northern and central East Carpathians ("East Carpathians
sensu stricto") are treated as one unit separately from the
southernsector("Bend Zone") throughoutthis paper.
3. Approach to Investigation
We first identify the surfaceexpressionof the doubly vergent wedge by its present day topography.Subsequentlywe
makean inventoryof the information that is available on the
known subsurfacegeometryand active structures.With the integratedinformation an attempt is made to constrainthe outline of the deformingorogenicwedgein the East Carpathians.
We next determinethe erosionhistory of the area with the apatite fission track thermochronologymethodand evaluateif
the erosion history is in accordancewith theoretical predictions. Finally, we revisit and evaluateour working hypothesis
andcomparewith othermodelsafter we have shownthe validity of the conceptfor the East Carpathianorogen.We subsequently integratethe observationson erosion, deformation,
and sedimentationin the critical wedge conceptto discussthe
evolution of the East Carpathianorogen.
"outerarc high" representsthe highestpeaks and coincides
with the maindrainagedivide;it formsthe transitionzone betweenthe two opposingwedgeswhichis situatedroughlyon
top of the singularitypoint. 4) The retroforelandabove the
backstop
is protectedagainstdeformation
by a stressshadow.
Also,theperipheralforelandbasinin frontof the prowedge
is
notdeformedsincenodifferentialmovement
takesplacealong
thebasaldecollement
here.By analyzingthestructures
andthe
large-scale
geomorphology
of the EastCarpathian
orogenwe
wantto seeif thesefeaturesareretainedin anancientorogen.
Four topographicprofilesin the EastCarpathiansshow a
similarasymmetry
(Figure4). Theeastern
partof theprofile is
formed
bya gently
inclined
mean
surface
slope
oferp= 0.8øto
1.4ø (regression
fit overtopographic
summits),representing
the surfaceslopeof the prowedge.
The main drainagedivide
(outer-arc-high)reachespeaksof up to 1900 m above sea
level.The westernflank of the orogenis formedby a relatively steepsurfaceslope(err = 2.8ø to 4.8ø) with a sharp
boundary(topographickink) to the retroforeland.This is the
surface
slopeof the retrowedge.
The ratiobetweenthe length
of the prowedge
andthe lengthof the retrowedge
is approximately3:1 (exceptfor profile B-B'), which is closeto obser-
vationsfrom othernaturaldoublyvergentwedges[Silverand
Reed,I988]. The retroforeland
(Transylvanian
Basin)andthe
proforeland
areessentially
undeformed
[Ciupagea
et al., 1970;
Huismans et al., 1997] and have a semihorizontal surface
slope.
The subsurface
outline of the prowedgeis availablefrom
publishedgeologicalsections[Stefanescu,1985], where the
basaldecollementis a clear reflectoron seismic profiles
[Mocanu
et al., 1996].Overall,decollement
angles
([3p;see
Figure5) vary between4ø and 6.5ø. The total taper of the
prowedge
(erp+[3p)
is fairlyconstant
in the EastCarpathians
and varies between 4.8 ø and 7.4 ø, which coincides well with
critical tapersof other natural settings[Davis et al., 1983;
Dahlen, 1990].
The detachment
zone is a lineamentfrom the singularity
pointto the toe of the retroslopewhichseparates
deforming
materialin the retrowedge
from essentiallyundeformed
material below.Frommodelsandnaturalexamples(e.g.,Southern
Alps,NewZealand)weknowthatthelineament
is expressed
as
a localization of strain which we will call here the "detachment
zone." A detachment zone has never been identified as such in
the EastCarpathians,probablybecauseits existencewas not
SANDERS ET AL.: LIFE CYCLE OF THE EAST CARPATHIAN OROGEN
Ma
Ma
1oo
1oo
50
50
A
(m)
dd o
- 2000
at=2.8ø
Ciuj
t
=0.8ø
!
0
2000
Tirgu Ocna
Reghin
f
I
(•p=0.8
o
Radauti - 1000
165
km
1 ooo
130 km
1:3,3
1:3.3
Ma
Ma
t O0
B
29,099
.....
,o
ø
100-
B'(m)
dd
er=4.8ø
- 2ooo
Sovata
D
-
,o
ø
I
dd
Odorhei C•r=3.1,
J
J
150km
Sarat
0
I: 5.4
2000
Rornnicu_1000
jPascani
- 1000
I
--
=1.4 ø
110km
•,1
1:2.7
Figure 4. Morphologicalprofilesandfissiontrackagepattern(33x verticalexaggerated):
outlineof bestfit surface
slope
({xp
and%)overpeaks;
volcanics
areblack;
open
circles
arefission
track
(FT)sample
locations
projected
on
profile hne with respectto main drainagedivide (dd). Plotteddirectly above the samplelocationsare the correspondingFT centralages(solidcircles)with two sigmaerror bars.
anticipated(seebelow).We inferredthe position of the singularity point at the contact between the subductingfrontal
decollement(foreland plate) and the tip of the crystalline allochtonous plate on the basis of sections by Stefanescu
[ 1985]. It coincideswith the deepestpoint of the sedimentary
wedge (approximately 10 km under the outer-arc-high).
Constructeddetachmentzone dips (13r)vary between 13ø and
19ø for the sectionsin the EastCarpathians(Figure 5). The total taper of the retrowedge(%+13r)(although less well constrainedthen the prowedge)variesbetween15ø and 22ø.
•.
Structural
Control
An essentialaspectof the doublyvergentwedgesis that the
prowedge and retrowedge have opposite vetgent dominant
thrust systems[e.g., Malavieille, I984; Koons, I990]. The
prowedgeof the EastCarpathianshas predominantlyeast vetgent thrusts. In the retrowedge the vergence of Miocene
thrustsis difficult to identify becauseof relict (both east and
west vergen0 faults causedby older (Cretaceous)deformation
phases. Because of the relatively old age of the rocks
(Palaeozoic to Cretaceous)it is difficult to date Miocene activity of the structuresin this area.
Publishedgeologicalmapsand sections[Stefanescu,I985]
of the East Carpathians,in general, show a remarkable increase in the areal density of retrovergent faults as the
retrowedgeis approached.Those occurring in the Middle
Dacides crystalline basementhave formerly been interpreted
as folded faults originating from Cretaceous deformation
phases. Sandulescuet al., [1981b], however, already mentioned a possiblereactivatedcomponentduringthe Miocene.
Field studies(near the villagesof Pojorita (I), Iacobeni (II),
and Voslabeni and Lazarea (III) (see Figure 2), maps
[Balintoni et al., 1982; Bercia et al., 1975; Krautner et al.,
1975, 1978; Muresan et al., 1986; Sandulescuet al., 1975],
and sections[Stefanescu,1985] on the retrowedgeof the East
Carpathiansreveal a numberof importanteast dipping faults
with cleareast-side-updisplacements
(Figure2) andthus representa retrovergentthrust system,oppositeto the thrust system in the prowedge.The brittle characterof these faults (in I,
II and III) indicatesthat they deformedunder low-temperature
conditions and together with overprinting relationshipssuggeststhat they were active in a late phaseof the deformation
history.
Despite these indications it is difficult to prove that a
retrovergentthrust systemwas dominantin the retrowedgeduring the Miocene orogenesis.The structuralapproachalone is
not sufficientto identify an active retrowedgeand the criterion
of oppositethrust vergencein the opposingwedgesremains
inconclusivefor the EastCarpathians.A main boundarythrust
as in the SouthernAlps in New Zealand [Kamp et al., 1989]
29,100
SANDERS ET AL.: LIFE CYCLE OF THE EAST CARPATHIAN OROGEN
A
OAH
A'
I
•
cM
_ _ .j.
- 8kin
Radauti
I
4
0
I
I_
••.:..':'.:.
:•'i'[
•'•decollement
detachment
pr--'o
B
OAH
Sovata
B'
I
Pascani- 8 km
detachment
•::.:.'•:•i•
,• _•½o
fir__13
o :::::•• vp
=•.o c
OAH
[
C•
Tirgu
Ocna
Reghin
-'8 km
•
13r=16ø
-
:•
tip=4ø
OAH
D
Odorhei
t
•
• km .
>km erosion
estimates
D'
Romnicu
_ 8 km
Sarat
'"'"'"'"•'••ec••x•nt
'•'
<km
veNical5 x
Figure5. Geologyandoutlineof subsurface
wedgecombined
witherosioncumulatives:
Profilesarethesameasin
Figure4, butverticalexaggeration
is less(5x).Thebasaldecollement
andinternalgeologyarereconstructed
from
published
sections
(colorschemes
areasin Figure
2). [•pand13
r areoverall
angles
of decollement
anddetachment
surfaces with the horizontal. Arrows indicate relative movements.The cumulative amounts of erosion in kilometers
(hatched)arebasedonestimates
in Table2. Cumulativesedimentation
thickness
of molasse
(M) sincetheBadenian
(17 Ma) is from literature.
whicharguesagainstthedoublyvergentwedgeconcept,or it
mightbe dueto thelackof a significantstrike-slipcomponent
in the Carpathians
and/orthe widespread
posttectonic
volcaniccover(Figure2). In the BendZone a majorretrovergent
faultis presentat theinferredlocationof the detachment
zone
tinct Miocene thrustingepisodeshave been recognizedby
Sandulescu
[1988]. An early eventduringthe early Miocene
[20-18 Ma] recordsthe onsetof Neogeneplateconvergence.It
is followedby a distinctearly middleMioceneevent(15 ma).
The third and last phaseof frontal thrustmovementin the
northernandcentralpartsof the EastCarpathians
occurredin
(Figure 2).
the Sarmatian(13-11 Ma) [Sandulescu,1981b]. The last out-
has never been identified as such. Either it does not exist,
deformation
withinthe wedgeis recordedin early
Constraints
on the timingof the structures
in the prowedge of-sequence
are basedon (overthrust-)stratigraphic
arguments.
Threedis- Pannonian sediments (11-7 Ma) of the Comanesti Basin
SANDERS ET AL.: LIFE CYCLE OF THE EAST CARPATHIAN OROGEN
[Dumitrescuet al., 1970]. In the Bend Zone the last deforma-
tion is documentedin the Pliocene-Quaternary,where largescale folding and shorteningof at least 22 km took place
[Ellouz et al., 1994; Hyppolyteand Sandulescu,1996].
6. Erosion of Critical Wedges
29,101
spreadof individual grain ages,particularlywhen samplesreside in the temperaturerange between90øC and 120øC.The
spreadin agesis statisticallydefinedas the amountof dispersion [GalbraithandLaslett,1993]. High dispersion(>30%) is
takenasevidencethatthe grainagesof a sampledo not represent a uniform grain age population.Note that in sediments,
high dispersioncanalsobe causedby different FT provenance
The critical taperof an activeCoulombwedgeis determined
ages.
by the balance between vertical (lithostatic) and horizontal
Chemicalvariationsamongapatite grainswithin a sample
forcesin every column of the wedge [Dahlen, 1990]. Ideally,
canbe recognizedby studyingthe effectsof etching.Apatites
any materialerodedfrom a columndecreases
the vertical force,
are chemicallyetchedwith HNO3 to make the tracks microwhich triggersinternaldeformationin the wedgeto restorethe
scopicallyvisible for routineFT age determinationprocedures
critical taper. The critical geometrical form of the wedge is
[Ravenhurstand Donelick, 1992]. CI apatites (and the more
thus unaffectedby erosion, but its size is dependent on the
rareOH apatites)have higher etch rates and thereforedisplay
mass outflux by erosion. It is argued by Dahlen and Barr
larger etch pits than F apatitesunder the sameetch conditions
[1989] that an active wedge eventually attains a steady state
[Burtneret al., 1994]. Etch pit dimensionsparallel to the cryswidth whenthe influx of material at the toe is balancedby the
tallographicc axis have been estimatedunderthe microscope
outflux of materialby erosion. A wedge in a destructivestate,
(in binsof 0.5 gm) simultaneously
with FT age determination
on the other hand, is reshapedby erosion processes[Kooi and
(magnification 1000x). On the basis of etch pit dimensions
Beaumont, 1996].
we classifiedF apatites(•1.5 [tm) and F-C1-OH apatites(>2
Erosionratesare stronglydefinedby the activity of weath[tm; in Table 2 labeled "CI") after the methoddescribedby
ering andtransportprocessesat the Earth's surface,and these
Burtner
et al. [1994]. As we shall see later, chemistry variaprocessesare mainly controlled by precipitation [Beaumontet
al., 1992; Kooi and Beaumont, 1996]. Therefore, the amount
and distributionof precipitation over an active critical wedge
is a major factor controlling where most erosion takes place.
Because removal
of overburden
causes vertical
advection
and
thus cooling of rocks, temperaturehistories as assessedwith
thermo-chronologicaltechniquesgive important information
about the erosion history [Brown and Summerfield, 1997;
Englandand Richardson,1977].
tions are crucialwhen interpretingthermochronologicalhistoriesof sampleswhich residedbetween90øC and 130øC.
7.1.
Results
Fission track analyseson apatite were carried out using
standard
procedures
[e.g., Rohrman, 1995; Andriessen,1995]
and the external detectormethod [Hurford and Green, 1982,
1983]. Four topographicprofiles were sampledperpendicular
to the strikeof the orogen(Figures4 and 6). Most samplesan7. Apatite Fission Track Thermochronology
alyzed comefrom approximatelythe sameelevation(600+_200
Thermal historiesfor the East Carpathiansare reconstructed m) andconsistof clastic sediments.The samplingof vertical
using the apatite fission track thermochronology (AFTT).
sectionswasprecludedby the lack of outcrop.Apatite fission
AFTT is a well-establishedlow-temperature
thermochronology track agesof 51 samplesare presentedas central ages (Ma)
method [e.g. Wagnerand Van den Haute, 1992; Andriessen, with one sigma error (Ma) and degree of dispersion (%)
[Galbraith and Laslett, 1993] in Table 1.
1995] with a closuretemperature[Dodson, 1973] of approxiApatite FT central ages range from 5 to 170 Ma. The
mately 120øCover geologicaltimescales(106-108 Ma).
Fissiontracksare not entirely stableand are healed by a proyoungestages (14-5 Ma) prevail in a zone along strike over
cesscalledannealing[Greenet al., 1986]. The nonlinearchar- the outer-arc-high of the East Carpathians and in the
acterof the annealingprocesshasin practiceled to a subdivi- Maramuresregion (Figure 6). These sampleshave FT central
agesand singlegrain ages all much youngerthan their stratision in the annealingranges.Over geologicaltimescales,annealing rates are very slow in apatite at temperaturesbelow
graphicages (Table 1). They have low dispersion,representing a homogeneous
age population(Figure 7e and 7f), includ60øC, and insignificantreductionin fissiontrack lenthsis observed. Mean fission track lengths in apatite are typically
ing the sampleswherewe documentedsignificantvariation in
14.5_+0.5grn[Gleadowet al., 1986]. Annealing rates acceler- the chemical composition using the etch pit dimensions
(Table 2 and Fig 70. These samplesare referredto as totally
ate in the range from 60øC to 120øC, defining the Partial
AnnealingZone (PAZ), which is characterizedby mean track resetsamples.Mean track lengthsare typically 13_+1.5[•m.
The totally resetsamplesare alternatedwith and flanked on
lengthsbetween9 and 14 [tm [e.g., Gleadow and Fitzgerald,
1987]. Above 120øC, no tracks are preserved,defining the
either side by zoneswith increasinglyolder FT ages (12-37
by high grain age dispersion.FT central
Total Annealing Zone (TAZ). Track length reduction affects Ma), characterized
measuredagesdirectly [Green, 1988], and therefore the geo- ages and most single grain ages are younger than the stratilogical significanceof measuredages has to be evaluated graphicage.A minor amountof singlegrainagesis older than
the stratigraphicage, indicatingpartial annealing(Figures7c
againsttrack lengthdata.
The annealing behavior of fission tracks in apatites is
and7d). For some samplesthe high dispersionis due to high
partly dependentupon the chemicalcompositionof the host C1 content of the apatite grains since the older single grain
mineral (especiallythe fluorine-chlorineratio) [Green et al.,
agesgenerallyhave etch pits > 1.5 [tm (Figure7d).
1986]. There is good evidence that F apatites anneal more
On the most externalslopesof the mountainrange and on
readily than CI apatites[Greenet al., 1986]. Conservative es- the rim of the TransylvanianBasin are sampleswith central
timatesindicatethat tracksin F apatitesare totally annealedat
ages and most single grain ages equal to or older than the
stratigraphicage (37-155 Ma) (Figure 7b). These samples
110øCwhile Cl-rich apatitespreservetracks up to 130øC at a
timescaleof 10 Ma [Crowley et al., 1991; Burtner et al.,
have not been thermally disturbedsincetheir deposition and
1994]. The differential annealing behavior leads to a wide havehigh dispersionbecauseof different provenanceagesof
29,102
SANDERS ET AL.: LIFE CYCLE OF THE EAST CARPATHIAN OROGEN
XI
N
c
Transylvanian
basin
D
'Apuseni
Mts
l
SouthCarpathians
..,
d
sø
I
a!lochton
a
b
Figure 6. Tectogenetic
mapof RomaniaafterSandulescu
et al. [ 1978]withFT centralages.A-A' to D-D' areprofile
linesof Figures4 and5. Legend:1 and2, allochthonous
plate:1, sedimentary
cover(Senonian-Neogene);
2, Middle
Dacides;3, Moldavides;4, Outer Dacides;5, Miocene-Quaternary
molasse;6, Neogene(a) volcanicsedimentsand
(b) intrusives;7, FT samplelocality,samplenumber,andcentralage.
the singlegrains.Two tuffaceouslayers (samples88-89) have
FT centralagessimilarto their stratigraphicage (Table I) and
meantracklengthof 14_+0.7pan,reflectinga thermally undisturbeddepositionalage.
The crystallinebasementhas FF central ages ranging from
9 Ma near the outer-arc-high to 134 Ma next to the
Transylvanian Basin, and dispersionis generally high. The
meanlengthassociated
with the older FT agesvariesbetween
11.2_+
1.6 and 13_+
1.5 ttm and indicatesslow cooling.
It is remarkablethat no obvious relation of FT ages with
the tectonic units can be recognized.When the FT ages are
plotted as a function of topography (Figure 4), the profiles
showa roughlysimilartrendlinked to the morphologyof the
doubly vetgentwedge.On the prowedge,FT centralages decreasegraduallyto the outer-arc-high,wherethey attainlowest
values.The retrowedge
hasvariableFT ages,but in •he retroforeland,FT agesrapidly increaseagain.The trend in profile
A-A' and B-B' is somewhatblurredbecauseof the different pre-
SANDERS ET AL.: LIFE CYCLE OF THE EAST CARPATHIAN OROGEN
Table
29,103
I. Fission Track Results
Lithology
Strat.Age
Sample
Sandstone
Miocene
3'
Sandstone
Sandstone
Dej tuff
Amphibolite
Sandstone
Schistone
Quartzite
Idem
Sandstone
Sandstone
Sandstone
Sandstone
Sandstone
Sandstone
Sandstone
Sandstone
Sandstone
Sandstone
Schistone
Sandstone
Schistone
Sandstone
Sandstone
Sandstone
Sandstone
Sandstone
Schist
Sandstone
Sandstone
Schist
Schist
Sandstone
Sandstone
Sandstone
Sandstone
Sandstone
Sandstone
Sandstone
Sandstone
Sandstone
Sandstone
Sandstone
Schist
Schist
Schist
Schist
Sandstone
Tuff
Tuff
Sandstone
Sandstone
Oligo-Miocene
Oligo-Miocene
Badenian
Barr-Albian
Cret•infi
idem
Cret.
Cret.
Oligocene
Cret.inf.
Eocene
Eoceneinfi
Oligocene inf.
Oligocene
Miocene inf.
Cret. sup.
Oligocene
Vrac-Cenom.
Miocene
Eocene
Miocene inf.
Oligocene inf.
Paleo-Eocene
Oligocene
Paieocene
Eocene
Oligocene
Eocene
Albian
Paleogeneinfi
Miocene inf.
Cretaceous
Albian
Eocene
Oligocene
Cret. inf.
MioceneM+infi
Oligo-Miocene
Paleogene
Paleogene
4*
5*
6*
15
16
17
18(I)
18(II)
I9
19b
21
23
24
25
26
28
30
31
32
33
34
45
47
48
50g
5I//
52#
56
57
58
60
64g
65g
66#
67#
68g
69#
70ti
72#
73#
77#
7841
80
81
82
85
86
88
89
106
107
Elev.,
N
Rho s (Ns)
Rho i (Ni)
m
Grains
x I06/cm2
xI06/cm2
500
400
400
600
800
600
800
1000
1000
700
700
600
600
600
600
400
400
380
700
700
700
700
800
700
1280
600
590
2130
450
530
770
830
430
500
620
630
680
370
250
520
680
600
600
835
1240
620
620
640
380
600
900
1030
20
0.3382 (224)
20
20
20
31
39
22
27
11
19
42
15
48
34
14
14
22
64
29
22
32
25
81
I00
84
72
45
51
18
7
45
46
92
55
100
115
44
77
53
75
52
73
45
44
27
38
41
9
36
8
48
86
0.0506 (38)
0.2042 (134)
0.1286 (80)
0.3877 (545)
0.1151 (117)
0.0307 (19)
0.0075 (9)
0.0350 (16)
0.0796 (61)
0.0660 (145)
0.0914 (25)
0.0535 (63)
0.0604 (53)
0.0710 (37)
1.2991 (417)
0.2069 (113)
0.2813 (745)
0.0351 (53)
0.2893 (175)
0.5923 (529)
0.7503 (1067)
0.3916 (1947)
0.6456 (2273)
0.0692 (206)
0.5254 (1250)
0.1641 (358)
0,0124 (41)
0.9222 (640)
0.9291 (194)
0.0040 (10)
0.0068 (16)
0.1993 (660)
0.2684 (487)
0.1955 (938)
0.1302 (533)
0.1191 (351)
0.0950 (854)
0.5087 (2044)
0.0768 (282)
0.0630 (164)
0.0732 (291)
0.1080 (266)
0.3434 (896)
0.2472 (511)
0.1484 (386)
0.0616 (I67)
0.7294 (439)
0.1272 (343)
0. i076 (67)
0.0604 (193)
0.1597 (938)
1.3060 (865)
1.5620 (1174)
1.5605 (1024)
0.8020 (499)
0.8984 (1263)
0.9169 (932)
0.2184 (135)
0.2260 (272)
0.3140 (124)
1.3950 (535)
1.1368 (2497)
0.6181 (169)
0.2556 (301)
0.9204 (807)
0.8582 (447)
1.2710 (408)
0.2228 (11o)
0.9826 (2602)
1.1012 (1661)
0.3257 (197)
0.9909 (885)
2.0208 (2874)
0.8223 (4089)
1.1072 (3898)
0.9160 (2728)
1.7111 (4071)
2.6352 (5749)
0.1449 (478)
1.1153 (774)
1.1780 (246)
0.0535 (135)
0.0675 (159)
1.0593 (3508)
1.6385 (2973)
1.1586 (5558)
1.3900 (5689)
1.0055 (2963)
0.8019 (4085)
1.4264 (5732)
0.9685 (3609)
0.9855 (2566)
0.6646 (2643)
1.8400 (4531)
1.9314 (5040)
0.6985 (1444)
0.3422 (890)
1.1130 (2868)
0.6347 (382)
1.3840 (3731)
0.5926 (369)
0.8840 (3147)
1.7888 (10507)
Rho d (Nd)
xI06/cm 2
0.0264 (3290)
0.0264 (3290)
0.0264 (3290)
0.0264 (3290)
ff0278 (2068)
0.0261 (1944)
0.0261 (1944)
0.0277 (1975)
0.0261 (1944)
0.0261 (1944)
0.0261 (1944)
0.0261 (1944)
0.0261 (1944)
0.0261 (1944)
0.0278 (2068)
0.0278 (2068)
0.0261 (1944)
0.0278 (2068)
0.0278 (2068)
0.0255 (2273)
0.0255 (2273)
0.0255 (2273)
0.0255 (2273)
0.0255 (2273)
0.0255 (2273)
2.5342 (4611)
2.5342 (46II)
2.5342 (4611)
0.0277 (1975)
0.0277 (1975)
0.0260 (2009)
0.0260 (2009)
2.5342 (4611)
2.5342 (4611)
2.5342 (4611)
2.5342 (4611)
2.5342 (4611)
2.5342 (4611)
2.5342 (4611)
2.5342 (4611)
2.5342 (4611)
2.5342 (4611)
2.5342 (4611)
0.0260 (2009)
0.0260 (2009)
0.0260 (2009)
0.0260 (2009)
0.0260 (2009)
0.0260 (2009)
0.0260 (2009)
0.0353 (2460)
0.0353 (2460)
C.A.+1o, Disp., Mean L+1o
1Vm
37_+4
7_+2
25•-5
23_+3
71+_6
19_+4
27_+9
5+-2
25_+9
26_+6
9_+1
30+-8
36+6
10+2
14_+3
103+-24
155-+30
47_+4
5•1
134+-18
88+8
56_+4
72+-5
85-+6
12+_1
38_+10
9_+2
12_+4
126_+18
115+_22
11+4
17+-5
27+-7
21_+6
24_+6
12+3
16_+4
30_+8
52+13
11+-3
9_+2
16+_4
8_+2
28_+2
53+4
50-+9
9_+1
76+_31
14+1
27_+5
13+_1
22+2
N,L
%
14
81
73
8
16
75
69
14
75
77
14
49
49
14
11
73
58
44
24
31
18
14
17
31
14
67
12
16
49
37
14
89
43
84
48
99
49
52
34
16
11
37
15
30
14
78
47
100
21
31
26
78
13.0_+1.4
10.2+2.3
21
6
12.1_+0.9
13.0+_1.5
13
100
13.5+2.3
28
13.9+0.8
10
12.7+2.3
17
11.2+1.6
100
8.1+_2.8
4
13.1+-I.82
47
12.7+1.5
12.8+2.1
47
45
13.7_+1.0
11.7+-2.0
11.2+1.7
19
57
84
14.0_+0.7
7
,
Personal• usedof C. Sandersis 11733+-589; • of P. Andriessen(sampleslabeledwith an asterisk)is 11,134 +-334, bothwith Fish Canyonstandard
apatiteanddosimeterglass
963. For FishCanyonapatiteanddosimeterCN-2 (labeledwith #) (=111_+22for C. Sanders.
Irradiationtookplaceat thelow flux
reactor(fluence5x 10•5) at EnergieCentrumNederlandin Petten(NL). N is the numberof grainscounted;Rho s, Rho i, and Rho d are spontaneous,
induced,anddosimetertrackdensities(tracksx I06/cm2)with numberof tracksin brackets.Centralage(C.A.) is in Ma, dispersion
(Disp.) is in percentages,
meantracklengthis in gm_+1o,andnumberof measuredconfinedtracksN,L.
Miocene FT history of the various samples(volcanic tuff versus clastic sedimentsof various stratigraphic ages), or an
anomalousbut poorly constrainedFT age (profile C-C').
7.2.
Thermal
History
and Timing
of Cooling
Phase
Thermal historiescan be evaluatedby combining apparent
apatiteFr centralagesand track length distributionsmeasured
(in [xm)on horizontally confinedtracks[Gleadowet al., 1986;
Lastett et al., 1987; Greenet at., 1989; Crowleyet at., 1991].
We usedthe Monte Trax programwith the Laslett Durango annealing model [Laslett et al., 1987; Laslett and Galbraith,
1996] to constrainthe thermalhistorieswith the inversegenetic algorithm model [Gallagher et al., 1991; Gallagher,
1995].
A disadvantage
of mostEastCarpathiansamplesis the lack
of large numbersof fossil track lengthsneededfor detailed
thermal history analysis.Also, samplesdisplaying a high
amountof grainage dispersioncannotbe thermallymodeled
because
the inversemodeldemandsthat all apatitegrainsannealsimilarlyunderuniformT conditions(chemistry)and experiencedthe sameFT history(provenance).
The total-resetsamplesare usedto reconstructthe postburial coolinghistory.In the northernand centralsegments
29,104
SANDERS ET AL.' LIFE CYCLE OF THE EAST CARPATHIAN OROGEN
I0 øC
,
,//!!1
60øC
C
D and E
I20 øC
F
Temp.
time
15
burial
0
Ma
erosion
Ma
N
sample47: C.age85+-6Ma
disp.3t%
2
0
10
-2
B
<60 øC
o
0
50
100 150 200
lO
20
Precision
Ma
3C
X
25
,
/
20•
sample
64: C.age
27+7Ma
disp.
43%
C 10
,0
ß
F
o
Tmax.
90+10
-2
50 I00
150 200
o
o
t.•t,.•,
20
lO
30
40
øC
10
/ sample
107:C.age22+-2Ma
] gl
disp.78%
D •0111 N=86
L._.R_.
I•'
.....r•.....
o.,•
o
o ,•
•.
2
-.
-.;
• sample
106:C.age13+_1
Ma
t•
di•p.26%
7o
•o
o so
•oo
0
1
Tmax.
110-•-_10
øC
........
?•z•ziii•
25 Tmax.
9b:
C.age9•lMa :• •,•7o
disp.14%
N•2
•
......... 30
Tm•.
o........=•b::•:::¾60
•:
• •o • •o •o ;o
Figure 7. Figure7a Thermalhistories
fromtheEastCarpathians.
Figures7b-7fFive samples
representing
thermal
historiesfrom Figure7a. The degreeof annealing
increases
fromtop to bottom.Left columndiagramsare single
grainagedistribution
histograms.
Grayshading
marksF apatite;
hatching
marksF-C1-OHapatite,basedonetchpit
dimensions.
Rightcolumndiagramsare radialplotsof the samesamples.
Gray shadingmarksstratigraphic
age;
solidcirclesmarkF apatite;opencirclesmarkF-C1-OHapatitegrainages.The dashedline is FT centralage.Open
bar represents
minimumage_+20.
SANDERS ET AL.: LIFE CYCLE OF THE EAST CARPATHIAN OROGEN
29,105
in Figure 8e), but also higher paleotemperatures,
which is in
contradictionto the observationthat F apatites anneal more
Samplet Etch, T Etch, EtchPits, ChemistryMax PaleoT, Paleodepth,
readily. Thereforepreferenceis given to the well-established
s
øC
I•m
øC
km
LaslettDurangomodel[Laslettand Galbraith, 1996] for our in3
+25
20-25
no
?
60
2.5
terpretation.
Besidesa Miocene phase,a well-constrainedLate
4
+25
20-25
no
?
110-+10
5M).5
Cretaceouscoolingphaseis recordedin the thermal history of
5
+25
20-25
no
?
90-+10
4-+0.5
6
•:.25
20-25
no
9
80ñ10
3.5:e0.5
the crystallineschistof the Outer Dacides(Figure8 f).
Table 2, Etch conditionsandPalcotemperature
Estimates
15
16
17
181
1811
19
19b
21
23
24
25
26
28
30
31
32
33
34
45
47
48
50
51
52
56
57
58
60
64
65
66
67
68
69
70
72
73
77
78
80
81
82
85
86
88
89
106
107
25
25
25
25
32
25
25
25
25
25
25
25
25
25
25
20
23
20
20
20
23
20
23
23
25
25
32
32
20
21
20
20
23
20
20
23
23
20
22
32
30
32
30
32
25
30
32
32
20-25
22
22
22
23
19
22
21
22
22
22
20-25
22
20-25
22
25
25
25
25
25
25
25
25
25
21
21
23
23
25
25
25
25
25
25
25
25
25
25
25
23
23
23
23
23
23
23
22
22
0.5-1.5
0.5-3.0
0.5-3.0
no
no
no
0.5-3.5
no
0.5-3.5
0.5-2.5
0.5-2.5
no
no
0.5-1.5
1.0-2.5
0.5
0.5-1.5
0.5-1.5
0.5-1.5
0.5-1.5
0.5-2.5
0.5-1.5
0.5-2.5
0.5-1.0
0.5-1.5
0.5-1.0
no
no
0.5-2.5
0.5-2.5
0.5-2.5
0.5-2.5
0.5-1.5
0.5-1.5
0.5-2.5
0.5-1.5
0.5-1.0
0.5
0.5-2.0
0.5-1.5
1.0-1.5
1
1
0.5-1.5
0.5-2.0
0.5-2.5
0.5-1.5
0.5-2.5
F
mix F-C1
mix F-CI
?
?
?
mix F-CI
?
mix F-CI
mix F-CI
mix F-CI
?
?
F?
mix F-CI
F
F
F
F
F
mix F-CI
F
mix F-CI
F
F
F
F?
F?
mix F-CI
mix F-C1
mix F-CI
mix F-C1
F
F
mix F-CI
F
F
F
mix F-CI
F
F
F
F
F
mix F-C1
mix F-CI
F
mix F-C1
60
110-+10
110+20
110+20
110+__20
110-x10
130
110-+20
100+10
120
120
60
60
80
120
60
60
60
60
60
110-+10
60
120
110_+10
60
60
110-+10
110_+10
90_+10
1•
10
100_+10
110• 10
100+10
100•10
60
110
110
100•10
120
90+ 10
90-+10
80-+20
110
80-x20
70_+10
80
110
11• 10
2.5
5_+0.5
5+1
5+ 1
5ñ1
5-+0.5
6
5_+1
5_+0.5
5.5
5.5
2.5
2.5
3.5
5.5
2.5
2.5
2.5
2.5
2.5
5•-_0.5
2.5
5.5
5+0.5
2.5
2.5
5-+0.5
5-+0.5
4-•.5
4.5-+0.5
4.5_+0.5
5-+0.5
4.5ñ0.5
4.5•0.5
2.5
5
5
5-x0.5
5.5
4_+0.5
4-+0.5
3.5+1
5
3.5•1
3-+0.5
3.5
5
5+0.5
Apatitemountswereetchedwith 5N HNO3 (etchtime in seconds)
to
maketracksmicroscopically
visible(EtchantTemperature
in øC).A mean
valueof thelongaxesof etchpitswasestimatedfor eachgrain(in binsof
0.5 gm). Presentedetch pit valuesare variationsamonggrainsin a
sample.Chemistryis basedon Etchpits.Etchpits<1.5 aregroupedasFrich apatites.Palcodepth
is basedon paleogeotherm
of 20 øC/krnand a
surfacetemperature
10 øC.
7.3. Minimum
Populations)
In the case of partially annealedsamples with high grain
agedispersion,inverse modeling could lead to misinterpretation of thermal histories.We use insteadthe grain age distributionwithin thesesamplesto obtainthe onsetof the cooling
event. For samplesresidingat temperaturesbetween 100ø and
120øC (which is the case for many samples of the East
Carpathians;seebelow), the track recordof the F apatiteswas
totally erased,while C1 apatitesstill preserveda significant
part of the record of an older history [Green et al., 1986;
O'Sullivanand Parrish, 1995]. Upon cooling the once totally
annealedF apatitesstart to retain tracks again, and their Fr
age representsa minimum estimatefor the onsetof cooling.
The grain age populationof a samplewith high dispersion
canbe describedby a numberof statisticallydistinct components (usually two or three) [Galbraith and Green, 1990].
Becausethe differential annealing behavior of apatites is not
only dependenton the C1-F ratio [e.g., Ravenhurst et al.,
1992] and not completelyunderstood,it is useful to decompose high-dispersionpopulations irrespectiveof a demonstrable relation
with the CI-F content.
We calculatedthe number of componentsand the age of
each component using the Fat Mix program based on
Sambridgeand Compston[1994] (seeFigure 9). It is preferred
to have analyzed40-100 apatitegrainsto get statistical meaningful resultsand to preventincidentalfeaturesbasedon a few
grains. Samplesyielding significantly different components
are presentedin Table 3. The youngestcomponentis called the
minimum age [see also Galbraith and Laslett, 1993] and the
older componentsare ignored for interpretation.
In the northernand centralpart of the orogenthe minimum
agescorrespondwell to the lowestFr centralages(as would be
expected since both are presumablyreset ages) clustering
around 11 (+3) Ma (Figure 2). In the Bend Zone, however
(profile D), the well-definedminimum agesdecreasegradually
from the toe of the prowedgetowardthe outer-arc-high,where
they are as young as 2 Ma (see Figure I0). Although these
minimumages are not track length correctedthey indicate an
approximatePliocenecooling age. This is confirmed by track
length modeling of the totally reset sample 31 (Figure 10),
which yields a near-linearPliocenecooling phase.
7.4.
(profile A-A' to C-C') of the mountain chain these samples
have similar agesbetween8 and 14 Ma. Six of these samples
with sufficient track lengths gave mean lengths from
12.7_+1.5to 13.5_+2.3!xm(Table 1) and wide track length distributions(inset Figure 8) yielding roughly identical thermal
histories (Figures 8a-8d). The onset of cooling from 120ø130øCstartedduringlate Badenianto Sarmatian(15-11 Ma)
with moreor lessconstantcoolingratesuntil the presentday.
Modeling with the CrowIcy F apatitemodel yielded somewhat
youngeronsetsof cooling and faster cooling rates (illustrated
Ages (Multicomponent
Maximum
Temperature
One of our goals is to determinethe amount of erosion
acrossthe orogenin order to study the relationship between
the erosion pattern and the wedge geometry. This involves
two assumptions.First, cooling of rocks took place entirely
by erosion.This is a reasonableassumptionsinceno evidence
for tectonicunroofing[cf. Platt, 1986] is found in the East
Carpathians.Second,we assumethat the geothermalgradient
was constantand uniform in time and space.We discussthis
assumptionin more detail later. We determinethe amount of
sectionremovedby estimatingpalcotemperatures
from the FT
data. In the case where track length measurementsare not
29,106
SANDERS ET AL.: LIFE CYCLE OF THE EAST CARPATHIAN OROGEN
?(øc)sample 19b
L•smt
Durango
0 20
T(øC)sample51
•,,• Durango
0
20
4O
i:40
100
100
120
120
i
30
24
18
12
Time(Ma)
T(øC)sample72
6
0
L•mtr•r•o
0 -
•
[
30
!
24
[
i
18
12
Time(Ma)
T(øC)sample73
•.•tt Durango
0
60
60
80
80
6
9M
i
30
24
18
12
6
0
30
24
18
Time (Ma)
T(øC)sample 72
0
12
6
Time
crow• F
T(øC) sample 15
L,,• Durango
_
40
,o F
100
lOO
120
30
' 2•
' 1•
' 1•
' 6'
•.
120
' 0'
.....
160
128
96
Time (Ma)
64
32
0
Time (Ma)
Figure 8. Thermalhistoriesof selectedsamples.Dark grayis Badenian(17-12 Ma); grayis Sarmatianto Pannonian
(12-5 Ma); and light gray is Pliocene-Quaternary
(5-0 Ma). Figures8a-8d are sedimentswith thermalhistoriesbased
on the Durangoannealingmodel;Figure8e is samesampleas Figure8c but with Crowleyannealingmodel;Figure
8f is thermalhistoryof crystallinebasementin the EastCarpathians.Lengthdistributionhistogramsand FT central
ageareplottedin the left uppercorner(verticalreferenceline is thethermallyunaffectedlengthof 14.5 gm)
100
b
a
N=
/
72
lO
77 Ma ageill
/
8
/
N6
50
2
4
$e
8 Ma
0
5 Ma
-2
0 10
60
_
110 160 Ma
16+3 Ma (prop. 49+20 %)
35+9 Ma (prop. 40+20 %)
6 Ma[ min.aõeI
77+t 5 !vta(prop. t t+20%)
_o
I
I
22
11
7
5
4
Error in Age
Figure 9. Example(sample50) of grainagedistribution
histogram(Figure9a) subdivided
in threecomponents.
Componentagesandrelativeproportions
arenotedbelowthe plot. Figure9b showstheradialplot of the samesample. The ageis in Ma on radialaxes.Individualgrainagescanbe readby a line from origin,throughblacknotation
to theradialaxes.Horizontalaxesareerrorin agein Ma for eachgrain.Verticalaxesarestandard
deviation(o) applicableto eachgrain.
SANDERS ET AL.' LIFE CYCLE OF THE EAST CARPATHIAN OROGEN
Table 3. SignificantResultsof CalculationsCarriedOut
With the Fat MIX Program
Sample
Min Age 1,
Ma
Age 2,
Age 3,
Ma
Ma
(66+__37)
cool [cf. Desegaulxet al., 1991]. Today a fairly uniformheat
flow (q0) of 30-60 mW/m2 exists over the entire East
% 1-2-3
16
14+2
23
45
14+5
50_+4
25x-8
73+_3
47
44_+4
84_+4
132+13
31-57-12
50
16+_3
29_+6
67_+10
44-36-20
64
14_+2
26+_3
52-48
27-73
65
6-+2
15•_2
66
67
68
4-+ I
2-+1
7+_3
20-+ I
6-+1
13_+7
•Q
I 1.4-1
'•/"•.4-•
69
70
8-+2
33_+4
17-+2
47_+4
77
107
11+1
11_+2
(17+_58)
17+_3
__
97-03
56-44
31-69
23_+5
19_+5
/!1.4-1"7
42_+8
(77_+7)
29,107
Carpathians[DOvenyi and Horvath, 1988' Cranganuand
Demirig, 1996; Veliciu, 1987], with the exceptionof the volcanicchain.Assuminga generalthermalconductivity(•:pcv)
for schist,sandand argillite of 3.5 W/(møC) [Barr and Dahlen,
1989] and usingthe relationship
qo- •CpCvT'0
this coincideswith a presentgeotherm(T'0) of 10ø-20øC/lcm.
Because
of the uncertainties
in the thermalconductivity(2-7
I 1-89
17-18-65
39-35-26
W/(møC) and the presence of a few isolated measured
geothermsaround20øC/km [DOvenyiand Horvath, 1988;
A•I/•_
Veliciu_
1 1
12-62-26
30-70
99-01
62-31-07
19R'71
we.
nqe.
a
nnifcwm
nnloc•ooc•fhorrn
20(_+5)øC/kmto yield estimatesof erosion.
The resultsare shownin Figure 5 wherethe erosionenve-
lopeisreconstructed
byadding
thepaleo-overburden
(zp)to
thesampleelevation(Tables1 and2) andinterpolating
a curve
Only
samples
withdispersion
>30%
andcontaining
more
than
40 between
theresults.
Figure
5 shows
thattheerosion
envelope
grains
were
decomposed
using
the
Laslett
model
with
Gaussian
errors.
(aswell
asthetopography
inFigure
4)isroughly
adepressed
Generally,15 runs of 15 iterationswere performedto maximisethe
number
ofcomponents
and
tocheck
thereproducibility.
Inagood
sample,
mirrored
image
ofthesubsurface
deforming
wedge.
Mostmaterunsproduce
similar
components
withintheerrormargins
(except
samplerial is erodedaroundthe outer-arc-high
(5-6 km) abovethe
69).Components
withabundances
<!0%(e.g.,
77and16inparentheses)
deepest
partofthewedge.
Erosion
diminishes
gradually
to the
were
neglected
(because
ofthe
effects
ofafew
extreme
grain
ages).
peripheral
foreland
butrapidly
(stepwise)
totheretroforeland,
which is best illustrated in sections D-D'
available we use the relation betweenFT agesand the stratigraphicagesof the sediments.For the sedimentsin the flysch
basins,burial (andthereforeheating)took place by continued
sedimentation
and, subsequently,
by nappestackingin the deformingwedge(Figure7a). As long as sedimentsremainat low
T (<60øC), they contain FF grain agesolder than, or at best
equalto, their stratigraphicages (FT "sourceages"or "provenance ages";see Figure 7b). With increasingtemperaturesin
the PAZ the fission tracks are increasinglyannealed,the Frich apatitesmore readily than the C1 apatites.The FF ages
correspondinglydecreaseand eventuallybecomeyounger then
the stratigraphicage ("intermediateages"; see Figures 7c-7d).
Above 110øC no tracks are retainedin F apatites and the FT
age is completelyreset(seeFigure 7e). The C1 apatitesare resetat 130øC(seeFigure7f). Oncethe onsetof postburial cooling is known to be at _+12Ma (Figures7e-7f), we can run a
forwardmodel with the Monte Trax programand simulatethe
relationshipbetweenFF agesand the stratigraphicages. This
yieldsestimatesof the maximumtemperaturea samplehas experiencedduringthe Miocenewith errorsat bestof _+10øC. The
resultsare presentedin Table 2. We used stratigraphicages
publishedon geologicalmaps,recalibratedby Rdigl [ 1996] for
the Central Para-Tethys.The metamorphicrocks of basement
schisthaveno suchreferencepoint as a stratigraphicage, but
they ultimately always comefrom deeperin the crust (TAZ).
The maximum temperatureduring the Miocene is evaluated
from track length modeling or the timing of the previous resettingevent, which is Cretaceous(Figure 8f). The maximum
temperatureestimatesare convertedto a paleodepthby
and C-C'.
The
prowedgeof profileA-A' showsa stepwiseerosioncurve coincidingwith an abruptramp in the decollementwith a vertical
displacementof 2 km [see also Mocanu et al., 1996]. Profile
B-B' showsthe sameerosionpattern,but the ramp in the
decollement
fadeslaterallyto the southandis less abruptin
profile B-B'.
Duringtheperiodin whichthe EastCarpathians
were eroding,theperipheralforelandbasinandtheTransylvanian
Basin
were subsiding and receiving clastics (see
below).
Postdepositional
erosionin theseareasis on averagelessthan
1 km, on the basisof sonicanalysisandAFTT datafrom boreholes(G. De Broucker,personalcommunication,1996; R. A.
Donelick,personalcommunication,
1997) but possiblymore
at the basin margins where Miocene-Pliocenesedimentsare
tiltedandtruncated
[Stefanescu,
1985].This informationis incorporatedin the erosioncurvesof Figure5.
7.5.
From Cooling
Data to the Onset of Erosion
Erosionratesare relativelylow in the EastCarpathians(see
below),andthe sampleswereneverbuffeddeep below the critical isothermof 120øC.As a resultthe onsetof cooling can be
directly convertedto the onset of erosion [cf. Brown and
Summerfield,1997]. The main erosion phasein the northern
and central segments started during the late BadenianSarmatian(15-11 Ma) andit continuedapproximatelywith the
samerate until 5-0 Ma (Figures 8a-8d). Time-integratederosionratesuntil the presentare of the orderof 0.5_+0.1mm/yr.
For the BendZone the main erosionphasestartedin the late
Miocene-Pliocene (7-2 Ma; Figure 10). Time-integrated
Pliocene-recent
erosion rates for the Bend Zone are of the order
of 1 mm/yr. Barr and Dahlen [1989] have shownthat for the
relatively low erosion and plate convergencerates we obZp=(Tmax-Ts)/T'0
served in the East Carpathian wedge a uniform undisturbed
whereZpis palcodepth,
Traaxis themaximum
palcotempera- geothermis a valid assumption.
ture, Ts is the surfacetemperatureand T'0 is the applied
geotherm.Here we applied a paleogeothermT'0 of 20øC/km
7.6. Relation Between Erosion, Precipitation,
(for the formerflyschbasinandthe subsequent
thrust belt) and and Topography
a surfacetemperatureTs of 10øC.These estimatesare reasonErosionin an active critical Coulombwedgeessentially
able since geothermsof old passivemargins are generally takes place as exhumation.Barr and Dahlen [1989] have
29,108
SANDERS ET AL.' LIFE CYCLE OF THE EAST CARPATHIAN OROGEN
ma
lOO
50
lO
5
(m)
o
looo-D
o
T(øC)
31 Laslett
Durango
0
20
40
80
100
120
20
16
12
8
4
0
Time (Ma)
Figure 10. Profile D with calculatedminimumFT ages(opencircles)andtotallyresetcentralages(solidcircles).
Note the breakin agealongthe time axes.The insetthermalhistoryof sample31 is basedon LaslettDurangoannealingmodel.
shown that exhumationpatternsof rocks in such wedgesare
principally determinedby the erosionrate at the surface.The
erosionrate, in turn, is dominatedby local relief and precipitation. Local relief is highestin the coreof the East Carpathians
(vertical elevationof peaksto major river valleys reach up to
1000 m but averagesat 500 m) and decreasesgraduallyto the
forelands,which is a commonfeatureof orogens[e.g.,Ahnert,
1984].
At present (and, we assume,during its Neogene history)
bothleewardandwindwardsidesof the orogenreceive similar
amountsof precipitationof 800-1200 mm/yr, with the higher
values restrictedto the outer-arc-high [Steinhauser, 1970].
This precipitationpattern,combinedwith the local relief, predicts that the highestexhumationrateswill occur symmetrically aroundthe outer-arc-high.The FT data indeedshowthat
the highestamountsof erosion took place in this part of the
orogen.
8. Alternative
Models
The combinedapproachof geometricand erosionalconstraintsconfirm the doubly vergentcritical wedge conceptas
an appropriate orogen-forming mechanism for the East
Carpathianfold-and-thrustbelt. The erosionpattern coincides
well with the independentlyconstrainedoutlineof the deforming wedgeon the basisof topographyand structures(Figure
5). The erosionpattern is consistentwith the doubly vergent
critical wedge models incorporating precipitation on both
flanks of the orogen.Immediately neighboringareasare not
involvedin compressive
deformationandthusnot upliftedand
eroded(Figurelb). On the contrary,they subsideand are areas
of sedimentdeposition.Thus erosionpatternsbecomea useful
criterion for identifying deforming wedges, especially at
placeswhere other (geometric)methodsare inconclusive.An
actively deforming retrowedgefor example, could not have
been identified in the East Carpathianswithout the fission
trackresttits.The erosionpatternand timing of erosionas assessedwith fissiontrack analysisform a potential tool to distinguishbetweendifferentgeophysicalmodelsappliedto natural settings.
Featureslike the formation of the outer-arc-highabove the
plate boundary,the erosionpatternsrelatedto the wedgegeometry, the synorogenicerosion and sedimentationhistory,
the (possible)flip of thrustvergencefrom the prowedgeto the
SANDERS ET AL.' LIFE CYCLE OF THE EAST CARPATHIAN OROGEN
S.!.
f
22-15
Ma
%%%%
29,109
mainsbelow sealevel. The underlying lithosphereis thought
to be of oceanic origin with a substantialbathymetry [e.g.
Ellouz et al., 1994]. During the early active phase(20-15 Ma)
the East Carpathiansprobably form an oceanic accretionary
wedgewhichis in a constructivestate(Figure 1la).
Acceleration of erosion rates took place during the late
Badenianto Sarmatian(15-11 Ma). The suddenonset of high
erosion rates is attributedto a changein the general subduction configuration(Figure 1lb). The orogenicwedgeis thrust
over the threshold onto the European continental foreland
plate [cf. Jamiesonand Beaumont, 1988, Stockrnal et al.,
1986], causingthe wedgeto rise high above sea level (needed
to increasethe erosion rate [Ahnert, 1984]). The onset of ero-
15-5
Ma
sion evolves parallel to the diachronousevolution of the orogen from north to south, and always coincideswith the final
stageof deformationat its toe. It is documentedthat the last
(Sarmatian)deformationphasein the East Carpathians(s.s.) is
the most intensive of the three Miocene phases [Sandulescu,
I988; Roure et al., I993] and responsiblefor the main nappe
emplacementsand reactivation of out-of-sequencethrusts in
the more internal regions of the orogen, pointing to a major
rearrangement
of the wedge in order to adapt to the changein
the geometryof the frontal decollement.Becauseof the counterbalancingeffect of erosion, the wedge tends to achieve a
steadystate width [e.g., Dahlen and Barr, 1989] during the
Sarmatian to Pannonian (13-5 Ma).
The third phaseis characterizedby destruction.As soon as
Figure 11. Three-phasewedgeevolution(see text for explanation):(a) constructivewedgebelow sealevel, (b) activelydeformingwedgeunderinfluenceof erosion;subsiding
forelandbasins,(c) destructiveinactivewedge;generalisostaticuplift of the
region(notesealevel marker).Dark andlight grayareasare allochthonous
and Europeanlithosphereplates,respectively;
black
areasareoceaniclithosphere.
Closestippledareasare activelydeformingwedge;stippledareas(M) areforelandmolasse
basins.
retrowedge,and the relativelyundeformedforelandand retroforelandare all consistentwith the proposedconcept. While
individual featurescan be explainedby other tectonic processes, no other tectonic models are known that can account
for all of these features. Alternative models suggestedby
Artyushkovet al. [1996], the episodicaccretionmodel from
Sandulescu[1988], or the back-arc extension model from
Doglioni [1992] are all inconsistentwith the observedfeatures and/or the erosionpattern.
9. A Three-Phase Wedge Evolution
Integratingthe informationon active structures,the erosionhistoryfrom AFTT, and predictionsbasedon the critical
wedge concept,we can interpretthe evolution of the East
Carpathiansin threedistinctphases,where we focus on the
northernand centralpans of the East Carpathiansand ignore
the BendZone unlessspecificallystated.Thrustingin the East
Carpathians(ss) has been active since the early Miocene
[Sandulescu,
1988], which marks the onsetof Neogene plate
convergence.
No erosionis recordeduntil the late Badenian,
andthe wedgeprobablygrowsin a self-similarway. The most
likely reasonfor the absenceof erosionis that the wedgere-
plate convergenceceasesduring the late Pannonian,no more
materialis stackedat the toe of the wedge, and the mechanism
to maintain the critical taper no longer functions. Erosion,
however, continuesat approximately the same rate, and the
wedgeentersa stateof destruction.The wedge becomesa passive landform which is gradually worn down by erosion processes.The destructionof the East Carpathians(ss) continued
until the presentday.
The gradualtransitionfrom a constructiveto a steady state
and finally to a destructivewedgeis difficult to constrainwith
structuresalone but is confirmed by the evolution of the foreland
basins.
A causal
relation
between
foreland
basins
and
wedgeevolutionis inferredon the basisof the axiom that orogens rest on a viscoelasticlithosphereplate which, in turn,
floats on viscousasthenosphere[e.g., Beaurnont, 1981]. The
growing weight of the orogen downflexes the underlying
lithosphere, creating a foredeep. Becauseof the doubly vergent characterof the East Carpathians,foreland basins develop on either side of the orogen [e.g., Johnson and
Beaurnont,1995]. The proforelandbasin and the retroforeland
basin(TransylvanianBasin) rapidly subsideand receivelarge
amountsof molassesedimentsstarting in the late Badenian-
Sarmatian
[Meulenkarnp
et al., I996; Cranganu
andDern(ng,
1996]. The forelandbasinsthusrecordthe growthand onset of
rapid erosion of the East Carpathianorogen. As long as the
orogen grows, the foreland basins continue to subside
[Jarniesonand Beaurnont,1988, Peper et al., 1995]. No mass
is lost in the region becauseof erosion of the orogen because
massis redistributedonly over short distances(Figure 11b).
The situation changesduring the late Pannonian and the
Pliocene.Dockinghasslackenedthe convergencebetweenthe
two continental plates, causingthe influx of material at the
toe of the wedge to decrease.Erosion continuesat approximatelythe samepace,andthe orogenreducesin size while the
foreland
basins
are
filled
with
sediments
to
sea
level.
Subsequently,
erosionproductscan no longer be storedin the
29,110
SANDERS ET AL.: LIFE CYCLE OF THE EAST CARPATHIAN OROGEN
forelandbasinsand are transportedover largerdistancesout of
the region.The net masslossin the regioncausesisostaticrebound[e.g., Burbank, 1992]. Sedimentationrates in the East
Carpathianforelandbasinsareobservedto decreaseduringthe
Pannonian [Cranganuand Denting, 1996], and since the
Pontian (7-5 Ma) no sedimentshave been deposited.During
the Plioceneand Quaternarythe forelandbasinsare uplifted to
a meanelevationof up to 400 m above sea level (Figure 1lc).
The evolution of the foreland basins thus suggeststhat the
East Carpathian orogen was in a destructive state from
Pliocene to present. A three-dimensional (3-D) isostatic and
flexural
numerical
simulation
based on the above
described
scenarioconfirmedthe observedmagnitudeof the vertical motions in the retro-foreland basin [Sanders, I998].
The amount of erosion associated with the Pliocene
destruc-
tion can be approximated. River terraces in the East
Carpathiansrangeup to 1000 m above the presentriver beds.
Estimated incision rates (taken as erosion rates) are of the order of 0.3___0.2
mm/yr [Radulescuet al., I996; Artyushkov et
al., 1996], somewhatless then long-term erosion rates deduced from fission track analyses (0.5-1 mm/yr).
Extrapolatingtheseratesto the entire Plioceneperiod yields a
total amount of 1.5___1km of erosion.
If constant
erosion
rates
are assumedon the basisof the AFTT data, approximately2.5
km of erosion must have taken place over this period.
Approximately 4___
1 km of material is then left to accountfor
exhumationduring the active phasesof the wedge (Figures
1 lb-1 lc).
In the Bend Zone, the erosiontakes place simultaneously
with active deformationin the wedge and subsidenceof the
proforelandbasin(Focsanitrough).The doubly vergentwedge
in the Bend Zone is interpretedto be in a constructiveor
steadystatethroughoutthe Plioceneand Quaternary.
10. North-South
Trend
The combineduse of FT resetagesand reset FT minimum
ages suggeststhat the East Carpathiansand the Maramures
areain the north have a uniform cooling recordthat startsin
the late Badenian-Sarmatian(15-11 Ma). The Bend Zone in the
4). This is probably due to more evolved continent-continent
collision in the northernpart. In the Bend Zone the wedgeis
entirelymadeup of Neogeneflyschand molasse,with possible but insignificant deformationand erosion in the Middle
and OuterDacides,displayinga more juvenile orogen.It remainsunclearwhetherthe transitionto the morejuvenile and
youngerBendZone is continuousor abrupt(fault bounded).
11. Conclusions
The combineduse of structuralstudies,topography,and
erosionpatternsas reconstructed
with AFI'T suggests
that the
deformation
historyof the EastCarpathians
is dominatedby a
compressiveorogenicfrictional mechanismoperativein the
upper 10 km of the crust above a major decollementfault.
Deformationis restrictedto a doublyvetgentwedgewhosegeometry is boundedby critical tapers.Incidental nappe emplacementspreviously referred to as separate "tectonic
phases"are more likely causedby continuouslyconverging
platesduringthe Miocene.
Duringthe earlyphaseof Mioceneconvergence
and thrusting, no significanterosiontook place,andthe wedgewasbelow or close to sea level. Erosion rates accelerated in a late
phaseof plateconvergence
(15-11 Ma) andmarkthe climax of
the deformationhistory. The major reorganizationof the
wedgeis attributedto the arrival of the continentalmarginat
the subductionzone.The erosionhistory was dominatedby
the interaction
between the deformation
mechanism and the
external climatic conditions. As a result, substantial exhuma-
tion (upto 4 km) tookplaceonly over the activelydeforming
orogen.Neighboringareaswere subsidingsynchronously
and
receivedthe elasticsshedfrom the orogen.
Duringthe Pliocenethe wedgeis in a destructivestatebecauseplate convergenceceased.Up to 2 km of erosion took
place,leading to isostaticuplift of the region. The critical
wedgegeometryformedduringtheMiocenehasbeenwell preserveduntil the presentday and only slightly reshapedby
Pliocene-Quaternary
erosion.
TheBendZonein the southis characterized
by an actively
deformingwedgethroughout
the Pliocene.The erosionperiod
lastsonly 5_+2Ma when up to 5 km of materialhasbeen re-
south, however, is significantly younger, and erosion starts
duringthe latestPannonian-Pliocene
(7-2 Ma). Our FF results moved.
thussupportthe generalideaof a diachronous
evolutionof the
Comparedto otherwell-studiedexamplesof critical wedges
EastCarpathiansfrom northto south.
like TaiwanandNew Zeeland,the EastCarpathians
forma very
The most northernwedge is not only older but also wider
modestorogenwith moderatesubduction,accretion,and preand showsdifferentcharacteristics
comparedto the BendZone.
cipitation rates, finally resulting in moderate exhumation
The doublywedgein profile A-A' consistslargely of material rates. thereforethermal disturbancesin the wedge are small
from the allochtonousoverriding plate (see Figure 5). Unlike
[BarrandDahlen, 1989], the elevationof the wedgeis relacurrentopinionswherein each deformationevent is classified tively low andstructures
like the majordatachment
zone along
to a singletcctogcncticunit, it is clear that the Outer Dacidcs, the retrowedgeare not clearlydevelopedor recognized.
the Middle Dacides,andthe Moirarides are all intensively involved in the Miocene deforming and eroding wedge. Here,
Acknowledgments. C. Sandersis financially supportedby the
even the area west of the retrowedgeis significantlyeroded NetherlandsOrganisationfor Scientific Research (NWO-GOA). P.
(although the samplesdo not give an unequivocal signal), Kamp,H. Kooi, R. Huismans,andG. Bertottiarethankedfor their fruitand comments
on earlierdraftsof the manuscript.
F.
possiblypointing to the formationof a minimal taperedretro- ful discussion
Neubauer, M. Roden-Tice, and M. Ellis are thanked for critical reviews
foreland wedge, as shownby Willet et al., [1993] and Wang
that greatlyhelpedto improvethe manuscript.
The Universityof
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