Pre-collisional high pressure metamorphism and nappe ...jupiter.ethz.ch/~tgerya/reprints/2005_TerraNova_Alps.pdfPre-collisional high pressure metamorphism and nappe tectonics at active

Pre-collisional high pressure metamorphism and nappe tectonicsat active continental margins: a numerical simulation

Bernhard Stockhert1 and Taras V. Gerya1,2,31Institute of Geology, Mineralogy, and Geophysics, Sonderforschungsbereich 526, Ruhr-University, Bochum, Germany; 2Institute of

Experimental Mineralogy, Chernogolovka, Moscow district, Russia; 3Present address: Geologisches Institut, ETH Zurich, Zurich, Switzerland

Introduction

Studies on high pressure (HP) andultra-high pressure (UHP) metamor-phic rocks exposed in collisional beltshave shown that these units (i) arederived from both continental andoceanic crust, being intermingled onthe length scale of nappes, (ii) arefrequently associated with hydratedperidotites, (iii) reveal a variable P–T–t record, with (iv) narrow time con-straints indicating that exhumationrates can be on the order of platevelocity. The P–T paths, the restrictedsize of the UHP metamorphic slices,and the narrow time constraints fa-vour tectonic models that involveexhumation by forced flow in a sub-duction channel. Using a set of appro-priate flow laws for different crustaland mantle materials, and includingprogressive hydration of the mantlewedge, we have extended our previousmodel of a subduction channel (Geryaet al., 2002) to an active continentalmargin.It seems to be tacitly assumed by

many authors (e.g. Hacker and Liou,1998; Chopin, 2003) that UHP meta-

morphic continental crust is derivedfrom the downgoing plate and thusgenerally indicates collision. Deepburial during collision is accordinglysimulated in analogue (e.g. Chemendaet al., 1995) and numerical studies(e.g. Burov et al., 2001). On the con-trary, geochronology suggests thatUHP metamorphism may have takenplace prior to collision (e.g. Eide andLiou, 2000). Furthermore, the limitedsize of UHP metamorphic slices, forexample, in the Alps, and the overallcrustal volume available in orogenicbelts bearing UHP metamorphicrocks do not support the hypothesisof burial and exhumation of coherentcontinental crust. The volumetricproblems inherent in the palinspasticreconstruction of the Alps, with indi-vidual microcontinents separated byoceanic branches, have been lucidlyportrayed by Polino et al. (1990).Motivated by these apparent inconsis-tencies, we present an alternativehypothesis supported by the numer-ical simulation, which accounts forUHP metamorphism prior to colli-sion, limited size of UHP metamor-phic units, and the problem of storageof an appropriate volume of contin-ental crust in present day orogenicbelts, as in the Alps.

Numerical approach

We use a 2-D numerical model(Fig. 1) with a kinematically pre-

scribed subducting plate, a surfaceundergoing erosion and sedimenta-tion, and progressive hydration ofthe mantle wedge (Gerya et al., 2002;Gerya and Yuen, 2003a). A set of bestguess flow laws is used to describe themechanical behaviour of the materialsas a function of depth and tempera-ture. For solving the momentum,continuity and temperature equations,we have employed a 2-D numericalthermomechanical code I2VIS basedon finite differences and marker-in-celltechnique (Gerya et al., 2000; Geryaand Yuen, 2003b). Details are given inthe Supplementary Material, hyaa,Table S1.

Results

The results of the numerical simula-tion for one of our reference models(hyaa; Table S1) are visualized inFig. 2 as a sequence of time slabs:within a few million years from theonset of subduction, subduction ero-sion starts to remove continental crustfrom the front of the upper plate.Former upper and lower continentalcrust becomes wound up in a marblecake-like manner forming a widewedge beneath the frontal part ofthe forearc reaching a depth ofapproximately 50–70 km. Minoramounts of continental crust are car-ried further down, to depths of100 km and more, into the narrowingsubduction channel and partly return

ABSTRACT

The evolution of a subduction channel and orogenic wedge issimulated in 2D for an active continental margin, with P-Tpaths being displayed for selected markers. In our simulation,subduction erosion affects the active margin and a structuralpattern develops within a few tens of millions of years, withfour zones from the trench into the forearc: (i) an accretion-ary complex of low grade metamorphic sedimentary material,(ii) a wedge of nappes with alternating upper and lowercrustal provenance, and minor interleaving of oceanic orhydrated mantle material, (iii) a megascale melange com-posed of high pressure (HP) and ultra-high pressure (UHP)

metamorphic rocks extruded from the subduction channel,and (iv) the upward tilted frontal part of the remaining lid.The P–T paths and time scales correspond to those typicallyrecorded in orogenic belts. The simulation shows that HP/UHPmetamorphism of continental crust does not necessarilyindicate collision, but that the material can be derived fromthe active margin by subduction erosion and extruded fromthe subduction channel beneath the forearc during ongoingsubduction.

Terra Nova, 17, 102–110, 2005

Correspondence: Bernhard Stockhert,

Institute of Geology, Mineralogy, and

Geophysics, Sonderforschungsbereich 526,

Ruhr-University, Bochum, Germany. Tel.:

+49 (0)234 3223227; fax: +49 (0)234

3214572; e-mail: bernhard.stoeckhert@

ruhr-uni-bochum.de

102 � 2005 Blackwell Publishing Ltd

doi: 10.1111/j.1365-3121.2004.00589.x

Fig.1

Initialconfiguration,startingandboundary

conditionsofournumericalsimulationofanactivecontinentalmargin.Thecomputationaldomain

isregionalin

character,and

kinem

aticboundary

conditionsare

imposed.Theinitialtemperature

fieldin

thesubductingplate,withagiven

uniform

descendingrate,isdefined

byanoceanic

geotherm

T0(z)

withaspecified

age.

Initialtemperature

distributionin

theoverridingplate

T1(z)correspondsto

theequilibrium

thermalprofile

with0�C

atthesurface

and1350�C

at90km

depth.Infinity-likethermalcondition¶2T/¶z2

¼0simulatingcontinuityofthetemperature

fieldin

averticaldirectionisusedalongthelower

boundary

(Geryaet

al.,2002).The

initialstructure

ofthe7km

thickoceanic

crust

istaken

asfollows(topto

bottom):sedim

entary

rocks¼

0.5

km;basaltic

layer

¼2.5

km;gabbroic

layer

¼4km.Theinitial

thicknessofthecontinentalcrust

¼40km.Thehydrationrate,v h,isapproxim

atedbyalinearfunctiondependingonthehorizontaldistance,x,measuredfrom

thetrench.

Terra Nova, Vol 17, No. 2, 102–110 B. Stockhert and T. V. Gerya • High pressure metamorphism and nappe tectonics

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� 2005 Blackwell Publishing Ltd 103

by forced flow, mixing with materialderived from the subducted oceaniccrust and the hydrated mantle at thehangingwall. These returning HP orUHP metamorphic slices becomeextruded from the subduction channelforming a mega-scale melange at thelandward side of the marble cakewedge. Controlled by the strength ofthe overlying continental lid, both thewedge and the landward megascale

melange progressively warp up andeventually become exposed by ero-sion. After 30 Myr of subductionwith a rate of 3 cm year)1 an activecontinental margin structure withfour distinct zones has developed,which are (landwards from thetrench):

1 An accretionary complex of at bestlow grade metamorphic sedimen-

tary material (poorly developed inthe model shown here).

2 A wedge of deformed continentalcrust derived from the front of theoverriding active margin by sub-duction erosion, with medium gradeHP metamorphic overprint, woundup and stretched in a marble cakefashion to appear as metamorphicnappes with alternating upper andlower crustal provenance.

Fig. 2 Evolution of the active margin (model �hyaa�, parameters see Supplementary Material, Table S1) shown in six time slabs(1.9 Myr, 5.1 Myr, 10.2 Myr, 12.6 Myr, 16.5 Myr and 20 Ma). Rock types and materials are as follows: 1, weak surface layer (air,sea water); 2, sediments; 3, upper continental crust; 4, lower continental crust; 5, upper oceanic crust; 6, lower oceanic crust; 7, drymantle; 8, serpentinized mantle; 9, hydrated mantle.

High pressure metamorphism and nappe tectonics • B. Stockhert and T. V. Gerya Terra Nova, Vol 17, No. 2, 102–110

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3 A megascale melange composed ofHP and UHP metamorphic oceanicand continental crust, and hydratedmantle, intermingled and subse-quently extruded from the subduc-tion channel.

4 An upward tilted frontal part of theremaining lid, with exposed deeperlevels of the overriding continentalcrust juxtaposed against zone C.

The predicted P–T time paths forrepresentative volumes of crust,displayed for each time slab inFig. 2, are compiled in Fig. 3. Asimple scheme visualizing the char-acteristic trajectories, lithology andinferred large scale structures for thezones A to D is shown in Fig. 4.

Discussion

Structure and particle trajectories

Limitations in the simulation arebecause of the grossly simplified struc-ture and the neglected inhomogenei-ties, particularly in the continentalcrust. In fact, the structure of a realorogenic belt may be largely con-trolled by prescribed heterogeneitiesand pre-existing zones of weakness, asfor instance by faults developed pre-viously during rifting of the continen-tal margin. In addition, newdiscontinuities – as large scale faults– cannot develop in the present simu-lations. For instance, the boundarybetween zones C and D, as definedabove and shown in Fig. 4, is likely todevelop more realistically as a faultsimilar to the so-called backthrust inthe Alps (e.g. Escher and Beaumont,1997; Ellis et al., 2001).In the simulation shown here, con-

tinental material removed from thelandward slope of the trench is con-tinuously carried down beneath theforearc. This type of subduction ero-sion is documented at active contin-ental margins showing little accretionworldwide (von Huene and Scholl,1991). In our simulation, the process isenhanced by the chosen �weak� rheol-ogy, supposed to be a fair approxima-tion to the behaviour of continentalcrust in a forearc with continuoussupply of fluid from the dewateringsubducted crust underneath. Subduc-tion erosion affects both upper andlower continental crust of the forearc,and material derived from both levels

(discriminated by colour in Fig. 2)becomes stretched out and wound upas schlieren in the broad wedge (zoneB). The schlieren on the crustal scalemay be compared with metamorphicnappes, both with respect to thicknessand aspect ratio. The characteristiclength scale of intermingling betweenunits of different provenience has beenfound to depend on the viscosity ofthe materials (Gerya et al., 2002), withhigher viscosities leading to moreextensive coherent nappe structures,and lower viscosities to a mega-scalemelange-type pattern, as developed inthe subduction channel and exposedin zone C.After circulation in the broad con-

volute wedge (zone B) and extrusionfrom the deep-reaching subductionchannel (zone C) the structural grainis predominantly horizontal or shal-low-dipping. Steep zones develop onthe flanks of the marble cake wedge,with a convolute structure on a lengthscale of tens of kilometres, while thematerial extruded from the subduc-tion channel spreads horizontally onthe landward side of the convolutemarble cake wedge and protrudesbeneath the rear part of the lid. Suchstructures may be compared with thestructures exposed in mountain belts.

Time scales

The time scales inherent in the simu-lated evolution are governed by theimposed plate velocity. For the chosenmoderate convergence rate of3 cm year)1, the structure of the activemargin develops within c. 20 Myr and(U)HP metamorphic rocks becomeexposed at the surface a few millionyears later. Exposure at the surface isgoverned by the mechanical strengthof the overriding continental crust,which – in the simulations – is con-trolled by the prescribed pore pressurecoefficient. In the model shown here(Fig. 2) this is defined as 0.9, makingthe lid rather weak. A stronger lid,obtained by imposing a lower value fork, would prevent the rapid uprise andrelatively early surface exposure of themetamorphic wedge of zone B.

P–T paths

The shape of the predicted P–T pathscorresponds to those typically inferred

from thermobarometric analysis ofexhumed HP and UHP metamorphicrocks (e.g. Spalla et al., 1996; Duch-ene et al., 1997; Carswell and Zhang,1999; Ernst and Liou, 1999; Ernst,1999), although temperatures achievedin the simulation tend to be lower byapproximately 100 �C. Notably, someof the particle trajectories imply loopsin the P–T paths, for which anunequivocal record in natural rocksremains to be identified.The position of the markers, for

which the P–T paths are shown inFigs 2 and 3, is chosen in such a wayso as to represent a series of rocksclosely spaced in a horizontal level,envisaged to represent a future post-collisional erosional land surface to besampled by a geologist. This choicevisualizes the contrast in the P–Tpaths, as well as in the provenienceof the protolith and the timing of peakmetamorphism, which can be realizedin immediately adjacent tectonic unitsor �nappes� in an orogenic belt.

Comparison with principal features ofthe European Alps

Clearly, a 2D simulation starting witha simple lithospheric structure andwith the assumption of a constantkinematic framework can only verypoorly represent the actual evolutionof an orogenic belt, like the EuropeanAlps, in three dimensions. In nature,the structure of an orogenic belt isgenerally supposed to be controlled bya very wide spectrum of pre-existingtectonic features, crustal structure andinherited lithospheric properties.Amazingly, comparing cross-sectionsof the European Alps with the resultsof the simulations shown in Fig. 5nevertheless yield some similarity inthe principal features. The relativewidth and the spatial arrangement,the provenience, the inferred struc-tural and petrological record, and thesimulated P–T paths for materials inzones A to D, compare with therespective pattern in the Alps – withinthe limits outlined above.First, it should be emphasized that

the present day Alps are a collisionalorogen, and large part of their defor-mation and structures are ascribed tothe final collision between the passiveEuropean and the active overridingApulian margin (e.g. Coward and


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Dietrich, 1989). Our simulation is ter-minated in a pre-collisional stage, whiledeformation during collision – withreference to the Alps – has been simu-lated in other studies (e.g. Escher andBeaumont, 1997; Pfiffner et al., 2000;Ellis et al., 2001).In the Alps, the Penninic realm

comprises metamorphic nappes ofcontinental and oceanic material, witha broad variety of inferred palaeogeo-graphical settings. The originalarrangement of the Penninic units hasbeen reconstructed from their presentstructural setting, and a pattern ofoceanic troughs and microcontinentshas been inferred (e.g. Trumpy, 1960;Debelmas et al., 1983; Coward andDietrich, 1989; Schmid et al., 1996;Froitzheim, 2001). There is one majorproblem with these reconstructions,however. While oceanic lithospherecan be completely subducted, and onlysmall remnant slices may mark the

suture where a wide ocean has disap-peared, the volume of continentalcrust that once made up a microcon-tinent needs to be stored somewhere.As discussed by Polino et al. (1990),there is a tremendous contrast in scale:the thickness of the Alpine nappes istypically on the order of a few kilome-tres (e.g. Laubscher, 1988), and thenappes can be traced over some tens ofkilometres, with possibly further extra-polation to depth based on reflectionseismic profiles (e.g. Schmid et al.,1996). In contrast, the minimum hori-zontal dimensions of a microcontinentembedded in oceanic realms, with therespective passive margins, are likelyto be in the range of several tens tohundreds of kilometres. More import-antly, the thickness of the continentalcrust in such slivers embedded inoceanic crust can be expected to be atleast 20–30 km, which results in avolume of continental crust exceeding

that of the typical Alpine basementnappes by one to two orders of magni-tude (Polino et al., 1990). Thus, pal-aeogeographical restorations seem tobe at odds with the actual volume ofcontinental crust preserved in the Alps.Polino et al. (1990) have proposed

that this discrepancy can be resolvedwhen one accepts that the continentalparts of the Penninic nappes can bederived from the front of the overri-ding plate by subduction erosion (alsoreferred to as tectonic erosion), widelyestablished for active continental mar-gins (Von Huene and Scholl, 1991). Ifso, there would be no constraints onthe volume of the individual coherentnappes. In addition, the tectonic unitscan be derived from any level of thecrust of the overriding continent, notnecessarily in a systematic sequencerelated to palaeogeography. The con-tinent-derived Penninic nappes areseparated by oceanic slices, made upby both dismembered ophiolites withextensive volumes of serpentinizedmantle perdidotites (e.g. the ZermattSaas Zone), and nappes largely com-posed of metasediments (e.g. theCombin Zone), with widely varyingpeak metamorphic conditions rangingfrom blueschist facies to eclogite fa-cies, and with UHP metamorphismrecorded in both former oceanic (e.g.Reinecke, 1998) and continental crust(e.g. Chopin et al., 1991).Derivation of the continental slices

by subduction erosion, and pre-colli-sional circulation through either awide more frontal wedge (to finallyend up in zone B in our simulation) ora deep narrow subduction channel (toend up in zone C), leaves manydegrees of freedom for their intermin-gling with oceanic slices and sedimen-tary material deposited on bothoceanic or thinned continental crust,characteristic for the internal Penninicnappes in the Alps. Moreover,contrasting P–T paths with highly

Trench sedimentsand oceanic material from lower plate

Very low grade

Accretionary prism

Continentalcrust of upperplate

Continental crust of upper plate

No contemporaneousmetamorphism

Little deformation

Continental crust andmantle of upper plate,oceanic crustof lower plate

[U]HP + Barrovianoverprint

‘mega-melange’

HP + Barrovianoverprint

Basement andcover nappes

Fig. 4 Scheme visualizing the characteristic trajectories of geological units (zones A toD) in the subduction zone and position in the pre-collisional orogenic belt, withcharacteristic materials, metamorphism and structural record indicated.

Fig. 3 P–T–t paths of rocks indicated as markers in Fig. 2. The black marker represents oceanic crust that passes the trench at2.5 Myr (referring to the start of the simulation); the green marker oceanic or trench sediment situated at the trench axis at4.5 Myr; the orange marker mantle material situated at a depth of 70 km immediately atop the prescribed weak zone whichdetermines the future position of the slab. The other three markers represent continental crust of the overriding plate. Theturquoise marker represents continental crust situated at approximately 20 km depth, in a deep level of the upper crust, and issituated about 70 km from the developing trench at 1 Myr; the blue marker represents a shallow level of the upper crust situatedabout 100 km from the developing trench at 1 Myr; and the pink marker lower continental crust, situated at about 40 km depthand 130 km from the developing trench at 1 Myr. Note how the P–T paths converge towards time slab 20 Myr, corresponding tothe position of the markers being closely arranged in space in Fig. 2f. The arrangement is chosen to simulate a specific level oferosion that could be attained subsequent to an eventual continental collision shortly after the 20 Myr time slab.


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variable early HP metamorphic re-cords (e.g. Spalla et al., 1996) andrather uniform later greenschist faciesoverprint in adjacent units are welldocumented for the Alps, and are alsoobserved in the simulations. Based onthe simulation, a pre-collisional struc-ture similar to that shown in Fig. 5can be envisaged for the Alps, withzone A corresponding to the externallow grade metamorphic nappes (e.g.Arosa Zone), zone B to the blueschistand eclogite bearing Penninic nappes(e.g. Bernhard Nappe), zone C to themore internal (U)HP metamorphicoceanic and continental slices withintermingled serpentinites (e.g. Zer-matt Saas Zone and Dora MairaMassif; understood as a megascalemelange or involuted melange; Hsu,1989), and finally zone D to thesouthern Alps, with the northernmostsection warped up and an unmeta-morphic Permian to Mesozoic sedi-mentary cover preserved further to thesouth.

Conclusions

1 High pressure metamorphic contin-ental crust in orogenic belts needsnot be ascribed to the downgoingplate, but can be derived by sub-duction erosion from the activecontinental margin; hence, (U)HPmetamorphism of continental

material does not unequivocallyindicate collision.

2 Large parts of the material removedby subduction erosion becomewound up in a marble cake fashionin a broad wedge beneath the fore-arc, forming alternating nappes oflower and upper crustal provenance.

3 Material carried further down intothe subduction channel mixes upwith oceanic and hydrated mantlematerial and partly returns byforced flow. It becomes extrudedon the landward side of the marblecake wedge, forming a megascalemelange of HP and UHP nappeswith contrasting P–T paths.

4 The kinematic patterns arising inour simulations suggest that pal-aeogeographical reconstructionsbased on the vertical sequence ofmetamorphic nappes in orogenicbelts may need critical re-evalua-tion.

5 Derivation of slices of continentalcrust by subduction erosion fromthe upper plate margin, and notfrom microcontinents in the down-going plate, may resolve the prob-lem of the discrepancy between thevolume of continental crustrequired by palaeogeographicalreconstructions and the actual vol-ume stored in orogenic belts.

6 Finally, the metamorphic nappestructure in the internal realm of

an orogenic belt – like the Alps –may develop largely prior to col-lision. For the Alps, such apre-collisional mountain buildingprocess was previously discussedin much detail by Polino et al.(1990); their hypotheses are sup-ported by the results of our simu-lations.

Acknowledgements

Funding by the German Science Founda-tion within the scope of collaborativeresearch centre SFB 526 �Rheology of theEarth – from the upper crust into thesubduction zone�, by the Alexander vonHumboldt Foundation, by the RussianFoundation of Basic Research (grantnumbers 03-05-64633 and 1645-2003-5),and by ETH research grant TH-12/04-1to T.V.G. is gratefully acknowledged.Two anonymous referees are thankedfor their constructive comments.

Supplementary material

The following material is availableat http://www.blackwellpublishing.com/products/journals/suppmat/TER/TER589/TER589sm.htm:Appendix S1 Model design andnumerical technique.Figure S1 Initial configuration, start-ing and boundary conditions of ournumerical simulation of an activecontinental margin.

Fig. 5 Two simplified cross sections through the central (a) and western (c) Alps, slightly modified after Polino et al. (1990),compared with the final (28.6 Ma) time slab of numerical simulation �hydr� (e). The distribution of the four zones withcharacteristic deformation patterns and P–T–t paths recognized in the simulation (Figs 2 and 3) and schematically visualized inFig. 4 are tentatively marked in the unlabelled Alpine sections shown in (b) and (d). The green dashed line in the Alpine crosssections marks the base of the rigid upper portion of the overriding continental crust, referred to as lid, and the red dashed linethe upper boundary of the European continent that entered the scene upon collision. The pre-collisional structure developed inour simulation is to be compared with the Alpine crustal structure above the red dashed line. The boundary between zones Cand D may be compared with the backthrust of the Insubric fault (e.g. Ellis et al., 2001), separating the southern (innermost)Penninic units (compared with zone C) from the southern Alps (compared with zone D). Zone C comprises intermingled (on anappe scale) continental, oceanic, and mantle material with widespread HP and UHP metamorphic record, as observed in thesouthern Penninic Units, e.g. the Zermatt Saas Zone or the Dora Maira Massif. Zone B can be compared with the Penninicunits of the Bernhard Nappe, the Lepontine Nappes, or the central gneisses of the Tauern Window in the Eastern Alps, with anappe structure engulfing former deeper and upper continental crust (including a sedimentary cover) and early stages of highpressure metamorphism (e.g. Heinrich, 1982) overprinted by later greenschist or amphibolite facies metamorphism. Zone A,which is more extensively developed in the �hydr� simulation shown here, when compared with model �hyaa� shown in Fig. 2,may be compared with the external Penninic units exposed in the Arosa Zone or the Lower Engadine Window in the Centraland Eastern Alps respectively. Note that the simulation was neither designed to reproduce the Alpine history, nor can thestructure of the Alps be represented by a 2D model; also it is probably governed by pre-existing crustal heterogeneity, notaccounted for in the model. Labels in the Alpine cross sections (Polino et al., 1990) are as follows: LPN, Lower PenninicNappes; AD, Adula Nappe; SU, Suretta Nappe; MR, Monte Rosa Nappe; SB, Grand St. Bernard Nappe and Brianconnais;AU, Austroalpine (�LID�, thrusted Apulian basement and cover); SA, Southern Alps (Apulian basement and cover); CA,Canavese Zone; GO, TV, AAR, MB, Gotthard, Tavetsch, Aar, Mont Blanc Massifs (European basement); HE, Helvetic nappes(European cover); MO, Molasse, PA, MA, PI, AN, Platta-Arosa, Malenco-Avers, Piedmont, Antrona ophiolitic units; P,Prealpine decollement nappes; VA, Valais ophiolitic and flysch units; B, Bergell intrusives (Oligocene); PL, PeriadriaticLineament (Insubric fault).


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Table S1 Material properties used in2-D numerical experiment hyaa.

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Received 30 March 2004; revised versionaccepted 2 September 2004


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