TECTONICS, VOL. 16, NO.6, PAGES 855-878, DECEMBER 1997

Controls on accretion of flysch and melange belts at convergent

margins: Evidence from the Chugach Bay thrust and Iceworm

melange, Chugach accretionary wedge, Alaska

Timothy M. Kusky

Department of Earth Sciences, Boston University, Boston, Massachusetts

Dwight c. Bradley, Peter J. Haeussler, and Sue Karl

u. s. Geological Survey, Anchorage. Alaska

of the accretionary wedge. including exhumation of themetamorphosed McHugh Complex, and its emplacement overthe Valdez Group. The Iceworm melange formed in a zone offocused fluid flow at the boundary between the McHughComplex and Valdez Group during this critical taper adjustmentof the wedge to these changing boundary conditions.

Abstract. Controls on accretion of flysch and melangeterranes at convergent margins are poorly understood.Southern Alaska's Chugach terrane forms the outboardaccretionary margin of the Wrangellia composite terrane, andconsists of two major lithotectonic units, including Triassic-Cretaceous melange of the McHugh Complex and LateCretaceous flysch of the Valdez Group. The contact betweenthe McHugh Complex and the Valdez Group on the KenaiPeninsula is a tectonic boundary between chaotically deformedmelange of argillite, chert. greenstone, and graywacke of theMcHugh Complex and a less chaotically deformed melange ofargillite and graywacke of the Valdez Group. We assign thelatter to a new, informal unit of formational rank, the Icewormmelange, and interpret it as a contractional fault zone(Chugach Bay thrust) along which the Valdez Group wasemplaced beneath the McHugh Complex. The McHughComplex had already been deformed and metamorphosed toprehnite-pumpellyite facies prior to formation of the Icewormmelange. The Chugach Bay thrust formed between 75 and 55Ma, as shown by Campanian-Maastrichtian depositional agesof the Valdez Group, and fault-related fabrics in the Icewormmelange that are cut by Paleocene dikes. Motion along theChugach Bay thrust thus followed Middle to Late Cretaceouscollision (circa 90-100 Ma) of the Wrangellia compositeterrane with North America. Collision related uplift anderosion of mountains in British Columbia formed a submarinefan on the Farallon plate, and we suggest that attemptedsubduction of this fan dramatically changed thesubduction/accretion style within the Chugach accretionarywedge. We propose a model in which subduction of thinlysedimented plates concentrates shear strains in a narrow zone,generating melanges like the McHugh in accretionarycomplexes. Subduction of thickly sedimented plates allowswider distribution of shear strains to accommodate plateconvergence, generating a more coherent accretionary styleincluding the fold-thrust structures that dominate the outcroppattern in the Valdez belt. Rapid underplating and frontalaccretion of the Valdez Group caused a critical taper adjustment

IntroductionI.

Many contemporary and ancient accretionary prisms aredominated by two fundamentally different types of structuralbelts -melange and relatively coherent flysch terrains. Therelati ve importance of different accretionary mechanisms i nforming these different types of accreted terrain are not wellknown. The Mesozoic part of south central Alaska'saccretionary prism, the Chugach terrane, is divided intolandward and seaward belts along a major thrust fault that canbe traced discontinuously for > 1500 km. On Kodiak Island,the thrust fault is known as the Uganik thrust [Connelly,1978; Sample and Moore, 1987; Sample and Fisher, 1986],and on the Kenai Peninsula (Figure I), it is known as theChugach Bay thrust [Cowan and Boss, 1978; Kusky et at.,1993]. From there, it continues northward (Figure I)becoming the Eagle River thrust on the northern KenaiPeninsula and western Chugach Mountains [Magoon et at.,1976; Pavlis, 1982; Winkler, 1992], and then it continues asthe Tazlina fault through the northern Chugach mountains[Winkler et at., 1981; Nokleberg et at., 1989; Plafker et at.,1989]. A number of other thrusts separating similar landwardand seaward belts occur discontinuously through SE Alaska andBritish Columbia [Plafker et at., 1994]. These related thrustfaults separate units characterized by grossly different rocksuites and structural styles. The landward belt consists of amelange of mafic igneous rocks, Triassic, Jurassic, andCretaceous chert, graywacke, argillite, plus rareCarboniferous, Permian, and Triassic limestone, and a fewmassifs of ultramafic igneous rocks, assigned to the McHughComplex and its correlatives. The seaward belt consists ofrelatively coherent Late Cretaceous turbidites, assigned to theValdez Group and its correlatives.

Here we report the results of new geologic mapping on thesouthern Kenai Peninsula (Figure 2) bearing on the nature,location, and significance of this important fault. In theSeldovia quadrangle, Magoon et at. (1976] delineated the

Copyright 1997 by the American Geophysical Union

Paper number 97TCO2780.

0278-7407/97/97TC-O2780$12.00

855

856 KUSKY Ef AL.: CONfROLS ON ACCRETION OF FL YSCH AND MELANGE

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KUSKY Er AL.: CONTROLS ON ACCREnON OF FL YSCH AND MELANGE 857

contact between McHugh Complex and Valdez Group as aseaward directed thrust fault. Cowan and Boss [1978] namedthis structure the Chugach Bay fault and traced it with

considerably greater accuracy than Magoon et at. [1976]. 0Jrmapping refines the location of the Chugach Bay thrust andshows that the McHugh Complex is far more extensive in thenorthern part of the quadrangle than was indicated by Magoonet at. [1976]. In the southern part of the quadrangle, we have

generally confirmed but locally modified the fault trace asmapped by Cowan and Boss [1978]. A second purpose of thispaper is to describe and discuss the origin of a type I melange

[Cowan, 1985] consisting entirely of disrupted Valdez Groupalong most of the fault trace [Cowen and Boss, 1978]. Wehave found this melange to be a mappable, regionallyextensive unit, and we informally name it the Icewormmelange after excellent exposures on Iceworm Peak (SeldoviaC3 1:64,000 quadrangle). The presence of a thick melangezone suggests that the fault formed when relatively coherentValdez Group turbidites were underplated beneath the McHughComplex between deposition during Campanian-Maastrichtiantime and intrusion by Paleocene igneous dikes. A third andfinal aim of this paper is to consider the tectonic significanceof the Chugach Bay and correlative faults with associated typeI melange. We propose a structural model to explain thedramatic change in accretion style from melange of theMcHugh complex to coherent turbidites of the Valdez Group,and we relate this change to specific tectonic events i nMesozoic and Cenozoic Cordilleran tectonics. Melanges ofthe McHugh complex were generated during subduction of athinly sedimented Farallon plate beneath the Chugach terrane,which formed an accretionary wedge outboard of the

Wrangellia composite terrane. Rapid sedimentation of theoceanic Farallon plate following collision of Wrangellia withNorth America led to subduction of a thick sedimentarypackage beneath the Chugach terrane and a change fromaccretion of melanges to accretion of relatively coherent thrustsheets of the Valdez Group. The Chugach Bay thrust andIceworm melange mark the contact between previouslyaccreted melanges of the McHugh Complex and thrust sheets ofthe Valdez Group accreted during the episode of subduction of athickly sedimented plate. We suggest that the switch i naccretion style may be analogous to similar changes in otheraccretionary prisms in the current and past plate mosaics.

variably disrupted greenstone (basalt and gabbro), chert,argillite, graywacke, conglomerate, outcrop-scale melange,and rare limestone (Figures 3a and 3b). Several ultramafic

complexes, including the large Red Mountain complex, occuras fault-bounded massifs, and have been variously interpretedas accreted ocean floor fragments [Bradley and Kusky, 1992;Kusky, 1997] or as deep fragments of basement to theWrangellian arc [Burns, 1985]. Bradley and Kusky [ 1992]provided descriptions of each of the component rock types,which are distributed in innumerable strike-parallel belts,bounded by mainly contractional faults, that vary in widthfrom a few centimeters to one or more kilometers. The McHughComplex has been metamorphosed to prehnite-pumpellyitefacies [Clark, 1973; Bradley and Kusky, 1992]. Stratifiedoceanic-plate rocks that now comprise the McHugh Complexrange in age from Triassic to mid-Cretaceous and wereprogressively off scraped and (or) underplated to form anaccretionary prism seaward of the Wrangellia compositeterrane during much of Mesozoic time [Bradley and Kusky,1992]. Some limestone blocks in melange have yieldedPermian Tethyan fusilinids [Stevens et al., in press, 1997], buttheir significance is unclear.

3. Coherent Strata of the Valdez Group

The Valdez Group [Tysdal and Plajker, 1978; Nilsen andZuffa, 1982] constitutes the seaward part of the Chugachterrane. Correlatives of the Valdez Group are known as theKodiak Formation in the Kodiak Islands [Moore, 1969], theShumagin Formation on Shumagin Island [Burk, 1965], theYakutat Group in the Gulf of Alaska and St. Elias Mountains[Plajker et al., 1977], and the Sitka graywacke and the FordArm Unit in southeastern Alaska [Loney et al., 1975; Decker,1980]. In the Seldovia quadrangle, we have broken out (anddiscuss separately) the Iceworm melange from the main,coherent part of the Valdez Group. Coherent turbidites of theValdez Group (Figures 3c and 3d) include most of the turbiditefacies recognized by Multi and Ricci-Lucchi [1978], which arediscussed below in alphabetical order. Massive conglomerates(facies A) have been recognized in several places along thesouthern coast but are not particularly abundant. The mostcommon conglomerates contain abundant intraformationalrip-up clasts of black mudstone (now state) supported in agraywacke matrix. Less common are conglomeratescontaining various extraformational clasts, including quartziteand granitic rocks. The conglomerates occur in massive oramalgamated beds up to a few tens of meters thick. Facies A i sinterpreted as having been deposited in submarine fanchannels. Massive, amalgamated graywacke sandstones offacies B also are widespread throughout the quadrangle and areprominent in outcrop because they resist erosion. Thesandstones are up to a few tens of meters thick and areinterpreted as channel deposits. Facies C, consisting ofclassical turbidites beginning with the Bouma Ta division, are

2. McHugh ComplexThe McHugh Complex fonns the hanging wall of the

Chugach Bay fault in the Seldovia quadrangle. It is equivalentto the Uyak Complex and Cape Current terrane on KodiakIslands [Moore and Connelly, 1977; Connelly, 1978] andKelp Bay Group of southeastern Alaska [Decker and Plajker,1982]. Rock types, which we interpret as the basement andsedimentary cover of a now subducted oceanic plate, include

Figure I. (a) Map of southern Alaska showing terranes south of the Denali fault system (inset shows locationof map). (b) Map of south-central Alaska [modified after Plajker et al., 1994] showing the distribution of theUganik-Chugach Bay-Eagle River fault system. A-A. corresponds to line of seismic section shown in FigureIc. (c) Interpretation of deep crustal structure along section A-A. [modified from Moore et al., 1991] based onEDGE seismic reflection data.

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860 KUSKY Ef AL.: CONTROLS ON ACCREfION OF FL YSCH AND MELANGE

4. Chugach Bay Fault Zone andAssociated Iceworm Melange

minor saddles and valleys. On the basis of the map patternand the orientation of fabric elements, we have dividedstructural data from the fault zone and adjacent rocks into 10domains, as shown in Figures 5,6, 7, 8, 10, and 14. The faulthas been strongly folded in domains 0 and I, moderately foldedin domain 2, and only weakly folded, if at all, in domain 3.

5.1. McHugh Complex Fabrics

Primary layering within the McHugh Complex, defined byflattened pillow lavas, cumulate layering in mafic-ultramafic

complexes (Figure 5a), basalt/chert bedding contacts (Figure5b), bedding in chert units (Figure 5c), bedding in argillite andgraywacke (Figures 5d and 5e), and bedding in massivegraywacke (Figure 51) is variable but shows a preference fornortheast strikes. Bedding and other primary fabrics appear tobe folded about northeast to north-northwest, gently plungingaxes (Figure 5). Macroscopic folds in the McHugh Complexare rare in the field, however, suggesting that the dispersion oflayering attitudes may have a different origin and significance,such as warping around large boudin structures, which arecommon in outcrop. Tectonic layering in the McHugh

Complex (Figure 6), defined by clast orientation, extendedlithological layers, early tectonic contacts, and phacoidalcleavages, also shows highly variable attitudes, alsosuggesting boudinage about northeast striking, gentlyplunging axes (Figure 6). Large-scale pinch and swellstructures are visible on the map (e.g., McHugh complexgraywacke, Figure 2) and outcrop scale, supporting thisinterpretation of layer orientation.

In sandstone-rich portions of the McHugh Complex, acomplex cataclastic composite fabric disrupts originallayering and appears in many cases to have accommodated alarge amount of layer-parallel extension, accounting for nearlycomplete obliteration of all primary textures, forming, i nessence, a cataclasite. This "web structure" [see Cowan, 1982]is best observed in massive graywacke and greenstone unitsand may contribute to their characterization as "massive"through total destruction of all primary (80) features.Individual cataclastic surfaces are typically less than I mm i nthickness but occur as an anastomosing network of shearsurfaces [see also Cowan, 1985; Byme, 1984]. In~ividual orgroups of these shear surfaces typically segment the rock intoblocks with similar shapes and sizes to those of inclusions i nnearby melange belts. In thin section, web structures are seento be zones of grain size reduction through grain breakage

(cataclasis), although enhanced concentrations ofphyllosilicates along these zones suggests that diffusionalmass transfer (pressure solution) was also an importantdeformation mechanism involved in their genesis [e.g.,Knipe, 1989]. Web structure typically is the oldest tectonicdeformation structure observed in graywacke of the McHughCompex, and its presence indicates that the graywackes werealready lithified when subjected to layer-parallel extension. Inunlithified sand, extension would have been accommodated bygrain boundary sliding, leaving no petrofabric record.

The Chugach Bay fault is a wide zone of type I [see Cowan,1985] melange; its upper boundary is a tectonic contact withthe McHugh Complex, where the melange is repeated bylocally numerous thrust imbricates, and thrust horses arecommon along some sections of the fault trace (Figure 2). Thelower boundary is a gradational structural contact withrelatively coherent turbidites of the Valdez Group. This belt ofmelange is extensive, distinctive, and significant enough towarrant mapping as a separate unit. We refer to it informallyas the Iceworm melange and treat it as a lithodemic unit offormational rank within the Valdez Group. It takes its namefrom striking cliff exposures at Iceworm Peak (Seldovia C3

quadrangle, Figure 4a), where its genetic relationship to theChugach Bay fault zone is clear. A more accessible referencearea for the melange is along the northern shoreline of RockyBay (Seldovia A4 quadrangle, Figure 2). Similar belts ofmelange occur along the McHugh/Valdez contact alongTurnagainArm, southeast of Anchorage [Kusky etal., 1997],in the western Chugach Mountains north of the Knik River

[Pavlis, 1982], and also beneath the Uganik thrust on KodiakIsland [Fisher and Byrne, 1987]. On the Kenai Peninsula, themelange occurs in a fairly continuous belt that extends fromChugach Bay and East Chugach Island in the south, to at leastas far north as the northern boundary of the Seldoviaquadrangle. The outcrop belt varies in width from O to 2.5 km.Structural thickness of the melange zone is up to 1.8 km.Because it is a structurally complex lithodemic unit (Figures 4cand 4d), a type section is not appropriate.

For mapping purposes (Figure.2), we drew the Chugach Bayfault at the contact between the McHugh Complex and eitherIceworm melange or coherent turbidites of the Valdez Group

(Figure 4b). Locally, where the transition between McHughand Iceworm melange is gradational, we have drawn the fault atthe structurally lowest or farthest outboard occurrence ofmelange blocks of McHugh aspect (i.e., chert, volcanic, orlimestone blocks). The attitude of the fault ranges fromgently to steeply west or north dipping, and in several placesthe fault zone is cut by swarms of late brittle faults. Thesefaults belong to a regional orthorhombic set [e.g., Kusky etal., 1997], which locally reorient fabric~ from the earliermelange forming event.

5. Structural Geology

The regional map pattern and detailed structural analysistogether indicate that the Chugach Bay fault originated as aseaward directed thrust fault which was later folded about axesat a high angle to regional strike. The deeply fiord-incisedcoastline from the Chugach Islands to Rocky Bay mimics thefolded fault trace (Figure 2). The fault follows Port Dick, a deepeast-west fiord and the major north-south topographic lowoccupied by Nuka Passage and Yalik Glacier. The fault lackstopographic expression only for short sections where i tcrosses low mountains between Rocky Bay, Port Dick, andNuka Passage. From Iceworm Peak (area a on Figure 2)

northward, however, the fault bears little relationship to majortopographic features, although locally it does correspond to

5.2. Coherent Valdez Group fabrics

Coherent Valdez Group sedimentary rocks crop out only indomains 2, 3, 5, and 6 (Figure 7). Where tops could be

KUSKY Ef AL.: CON1ROLS ON ACCREnON OF fL YSCH AND MELANGE 861

Figure 4. Representative photographs of the Chugach Bay thrust and Iceworm melange. (a) View of ChugachBay thrust and Iceworm melange on the flanks of Iceworm peak (location a on Figure 2). Mm is McHughComplex, Kvm is Iceworm melange, arrowhead along Chugach Bay fault. (b) View of Chugach Bay thrustplacing McHugh Complex melange at top of cirque wall over Iceworm melange, which forms the dark shaleyrocks in the lower part of the cirque (from location b on Figure 2). (c) Blocks of semi coherent Valdez turbiditesin the Iceworm melange. (d) Completely disrupted blocks of graywacke in an argillaceous matrix, formingtypical Iceworm melange.

recognized, most beds are upright, dip landward (Figure 7), andare folded about north-northeast trending, gently northplunging fold axes (Figures 7a, 7c and 7f). Structural trends inthe Valdez Group are relatively coherent for many kilometersalong strike, with individual panels up to several kilometersthick, bounded above and below by thin thrust fault surfaces.Thrust ramps and flats (Figure 3d) are both found in outcrop.Some areas contain numerous coherent panels of Valdez Groupturbidites apparently repeated along thrust faults, whereasother areas show tight to isoclinal folds within these panels.Figure 8 shows the orientation of slaty cleavage in domains O ,2, 3, 5, and 6. In outcrop, bedding/cleavage angles aretypically less than 25°, reflecting cleavage fans in tight toisoclinal folds. On a regional scale, bedding and cleavage areparallel, and the cleavage is distributed along great circlescompatible with folding about northeast axes but i sconcentrated in point maxima along these circles suggestingthat its regional orientation reflects cleavage fanning withinlarge-scale folds as observed in outcrop (Figure 9) and notfolding of the cleavage. The sequence of folding and cleavagedevelopment is discussed in section 5.5.

5.3. Iceworm Melange Fabrics

On an outcrop scale, fabric elements of the Icewormmelange are dominated by extended bedding and phacoidill toslaty cleavage (Figures 10 and 11 ). A scaly or phacoidalfoliation is present in most of the argillaceous and mudstone-matrix parts of the Iceworm melange (Figures 4d, 11 b, andlId), and this is generally subparallel to primary layering.Although in detail the cleavage locally anastomoses and cutsacross the extended bedding, on an outcrop to regional scalethe two fabric elements are statistically parallel (compareFigures 7 and 10). We assign SI nomenclature to this layeringbut recognize that it is truly a composite So + SI planar fabric.In the rare instances where So (original layering) i srecognized, it is described separately from S 1. A linear fabric(Ll) within SI is defined by the elongation of inclusions and,more rarely, by a lineation on the scaly shaley units. In caseswhere the scaly or phacoidal cleavage transects primarylayering, it extends these layers parallel to the cleavage.Various stages of disruption from continuous So layering cutby an anastomosing scaly fabric to a scaly anastomosing

863KUSKY ffi' AL.: CON1ROLS ON ACCRIillON OF FL YSCH AND ~ANGE

Along Turnagain Ann ear Anchorage (Figure I ), a type Imelange structurally below he Eagle River thrust is similar tothe Iceworm melange on th southern Kenai Peninsula [Kuskyet al., 1997]. In the melang along Turnagain Arm, bedding islargely overturned, suggesti 9 that some of the type I melangemay be related to overtu ing and severe extension of the

bedding during overthrustin by more competent rocks of the

McHugh complex. I

fabric with inclusion trains of a different rock type are present(Figure II). The scaly cleavage typically is a compositeplanar fabric, with two or more orientations present, but sincethese tend to anastomose they are not assigned a C-S type of

terminology.Type I melange consists of originally interbedded

sandstones and shales which have experienced layer parallel

extension, forming discontinuous blocks of sandstones in anargillite matrix [Cowan, 1985]. Figure I I illustrates fourstages in evolution of the Iceworm melange from the ValdezGroup. The Valdez Group grades from nearly continuous to"broken" [cf Hsii, 1968, 1974] sandstone/shale beds (FigureIla) to a unit (Figures lib, Ilc, and lid) which has been bothextended along brittle faults and ductily thinned by pinch andswell processes to a true type I melange. Pinching andswelling and breakage is common in boudin necks, and shaleis in many cases injected along fractures in the more

competent graywacke blocks (arrow in Figure lid). Withcontinued extensional disaggregation, the broken formationgrades into an entirely disrupted rock type we map as theIceworm melange. The melange consists of blocks ofgraywacke enclosed in a matrix of phacoidally cleavedargillite. Both graywacke and argillite are indistinguishablefrom comparable rock types within the coherent parts of theValdez Group. The graywacke blocks are typically lozengeshaped (Figure lid) and range in length and thickness from afew centimeters to several meters.

Plots of extended bedding (fragment foliation), phacoidalcleavage, and bedding in the Iceworm melange from domainsI, 2, and 3 are consistent with folding of these fabric elementsin the Iceworm melange about north-northeast trending,gently north plunging fold axes, approximately parallel to theregional strike (Figure 10). In rare cases, the Iceworm melangeshows seaward vergent folds that fold the fragment foliationand the scaly cleavage. In Rocky Bay (Figures 2 and 12), mostbedding fragments are elongate parallel to the melangefoliation but are overturned and young trenchward, and severalsmall-scale (parasitic?) fold closures were observed. These F2folds deform both the extended graywacke beds and theargillite matrix showing that the folds formed after themelange was generated. Byrne [1984] described similar swirlsand folds in argillite matrix of the Ghost Rocks Formation onKodiak Island. He interpreted these structures as one of theearliest deformation structures in that location. <AIr datasupport Byrne's interpretation that these are early structures,but we can demonstrate that layer-parallel extension at least

locally preceeded folding because boudin trains can be tracedaround fold hinges.

5.4. Mechanisms of frlelange Formation inIceworm Melange I

We attribute formatio of the Iceworm melange tothrusting of the McHugh C mplex over initially coherent butonly partly lithified Valde Group sedimentary rocks becausethe melange is located alon the contact between the two rockunits, and all stages of fo ation of the melange, betweenundisrupted beds. through broken formation, and into truemelange are found in this nit. The presence of boudinage,pinch and swell structures, rittle fractures, and shale injectionveins in the graywackes sho that they were stronger than theargillites and shales durin deformation. This rheologicalcontrast could have been ind ced by high fluid pressures in theargillites, related to either uid entrapment when dewateringthe accretionary prism sedi ents that were emplaced beneaththe impermeable McHugh h nging wall, or by the incompletelithification of the argillite at the time of deformation. Thestructural differences betwe n the argillites and graywackesonly tell us the relative co petence contrasts not whether oneof the rocks was truly' wet" or "soft" at the time ofdeformation. The presence f rare clastic dikes, including somein Rocky Bay (Figure 12) d another along Turnagain Armnear Anchorage (Bradley et al., 1997) supports the idea thatValdez Group rocks were o Iy partly lithified at the time ofdeformation.

Pinch and swell as ell as boudinage are commonmanifestations of inferred layer-parallel extension in theIceworm melange. Some g aywacke boudins show complexstructural histories. For ex mple, the boudin in Figure 13 i selongate with the long axis parallel to the LI lineation in theargillite matrix. The boudi is cut obliquely by many calciteveins which are perpendi lar to faint stylolites, both ofwhich form significant angl s with the boudin long axis. Theprominent fracture cutting t e boudin is a shale injection vein,indicating that some defo ation occurred while the shaleswere relatively "soft" an or under high fluid pressures.Although not visible on the slabbed sample, the boudin is not

Figure 5. Structural domains of the southern Kenai Peninsula and lower hemisphere qual-area projections ofstructural data showing primary layering in the McHugh Complex. (a) Igneous I yering from domain 10including pillow lava flattening planes (circles, n=20), overturned pillow lavas ( pen squares, n=3), andcumulate layering in ultrmafic complexes (crosses, n=9). Layering is folded about axi 029 34° (best fit greatcircle 119 56°S). (b) Contacts between basalt and chert in domain 10, show folding bout axis 304 27° (bestfit great circle 034 63° SE). (c) Poles to chert beds in domain 10, which are fol ed about 043 44° axis(cylindrical best fit 133 46°S). (d) Poles to beds in McHugh complex in domain 9. (e) oles to beds in McHughcomplex in domain 8. (f) Poles to beds in massive graywacke unit of McHugh comple in domain 7. These arefolded about axis 351 49° (cylindrical best fit great circle 081 41°S). Mm is McHugh Complex. Large shadedsquares in Figures 5a and 5b are poles to visually best fit great circles to data, wherea large shaded squares i nFigures 5c and 5f are eigenvectors based on a cylindrical best fit to data.

KUSKY Ef AL.: CONlROLS ON ACCRImON OF R.. YSCH AND MELANGE 865

outcrop-scale overprinting relationships in the Icewormmelange. Nearly all of the disruption in the Iceworm melangeappears to be tectonic in origin, although we have evidence fora few early slump folds. These are irregular in appearance, withvariably oriented hinges and axial surfaces, and havesedimentary welded contacts with surrounding layers. Weassign these folds a DO and FO nomenclature. The firsttectonic deformation appears to be layer-parallel extension viaboudinage and structural slicing of bedding within originallycoherent or slump-folded Valdez sedimentary rocks. Thislayer-parallel extension (Dl) is constrained to have occurredprior to north-northeast striking, gently north plungingisoclinal folding (described here as D2 and Fz), becauseoutcrop-scale isoclinal folds of this generation warppreviously extended layering and individual boudin trains canbe traced around fold hinges. Cross-folds (F3) with northwest-southeast to east-west axial surfaces refold the earlier folds onboth the outcrop and map scales (Figures 2 and 14). Thesefolds are broad open structures with wide hinge zones. Yetanother style group of folds (F4?) locally present in the PortDick area includes north-south trending chevron folds withsteeply plunging axes associated with thrust faults thattransported the hanging walls to both the east and the west.The relative order of F3 and F4? could not be determined in thefield.

regularly bedded but is entirely disrupted by a networkcomprising web structure, cut by the shale injection vein.Thus, for this boudin, we can infer a structural historybeginning with cataclastic flow forming web structure, fluidoverpressure injecting shales into the boudin, rotation of theboudin with respect to the stress system, then brittleextension accompanied by dissolution along styloliticsurfaces. Many other boudins show curved terminations,indicating that ductile pinching and swelling were also

important mechanisms of layer parallel extension at somepoint prior to brittle extension or that deformation occurred i nthe brittle-ductile field.

Deformation mechanisms in the Iceworm melange includepressure solution and cataclasis of quartz, feldspar, and calcitegrains along with the development of a planar preferredorientation of chlorite and sericite along the fault zone[Hawley, 1992]. Two alternative hypotheses can account forthe generation of extensional phacoidal cleavages like thatfound in the Iceworm melange. In the first model, cleavagesare inferred to form perpendicular to or at a high angle to themaximum principal stress (0"1). Because 0"1 is presumablysubhorizontal in most accretionary wedges, the anastomosingscaly cleavage may have formed after the bedding was rotatedinto a nearly vertical attitude. In the second model, thecleavage is interpreted to have formed in a wide, downwardpropagating shear zone at the base of the accretionary prism[e.g., Silver et al., 1985; Moore and Byrne, 1987; Moore etal., 1986; Moore and Lundberg, 1986]. The second model forthe formation of the scaly melange fabric is favored by therelative timing of fabric development and overprintingrelationships as discussed in section 5.5., and by Bradley and

Kusky (1992).

5.6. Late Faults

The entire Seldovia quadrangle, including areas borderingthe Chugach Bay thrust and Iceworrn melange, is cut by amyriad of late brittle faults interpreted by Kusky et at. [ 1997]to be a consequence of Paleocene ridge subduction (e.g., Figure12). These faults do not affect the regional map pattern, andthey are not discussed further here. A more completedescription of late faults is given by Bradley and Kusky [1992]and Kusky et at. [1997].

5.7. Age of Displacement on the Chugach BayFault

Crosscutting relations bracket the timing of movement onthe Chugach Bay fault as Maastrichtian and (or) Paleocene i n

age. Crosscutting intermediate and silicic dikes truncatemelange fabrics in both the McHugh Complex and Icewormmelange (Figure 15) and cleavage in the coherent part of theValdezGroup. One such dike has yielded a 4OAr/39Ar isochronage of 57.0 :t 0.22 Ma and a plateau age of 55.8 :t 1.4 Ma(Figure 15) [Clendenin, cited in Bradley et al., 1994]. Theolder age limit is established by the fact that the Icewormmelange is composed of disrupted Valdez Group turbidites ofpresumed Campanian to Maastrichtian age [e.g., Jones andClark, 1973; Budnick, 1974], but as discussed above, fossilages of the Valdez Group are sparse. Within the Campanian toPaleocene age range, the older end is more consistent with thenotion that the Valdez Group was deposited wholly or partly ina trench [Nilsen and Zuffa, 1982]. The sedimentary fill of sucha trench would have been off scraped and (or) underplated at orbeneath the adjacent accretionary prism during sedimentation,and the farthest landward thrust fault would be the oldest.

5.5. Fold Orientation and Overprinting

Relationships

The orientation of the fabric elements described aboveclearly shows folding about north-northeast trending, gentlynorth plunging axes. On stereonet plots of mesoscopicallyvisible fold axial surfaces and planes, most folds belong tothis northeast trending set, but several outcrops in domain 2show northwest to west trending folds refolding the earliernorth-northeast trending fold set (Figures 14d and 14e).Northwest trending minor folds are also found in bedded chertsof the McHugh Complex of domain 10 (Figure 14c), and the

regional map pattern shows an open, northwest trendingcurvature that may be a large-scale fold. In addition, the map

pattern (Figure 2) indicates that the Chugach Bay thrust i sfolded about northwest-southeast axes. Folding of theChugach Bay thrust is most evident on a detailed map of theWindy Bay -Chugach Bay area (Figure 12). Here the thrust isfolded about WNW axes, controlling the shape of the coastlinein this area. Additionally, the foliation in the McHughComplex in this area is truncated by the Chugach Bay thrust,but the foliations in the Iceworm melange closely mimic thetrace of the folded thrust. From this, we conclude that the

McHugh complex had already been deformed (andmetamorphosed) at the time of formation of the Iceworm

melange.Data bearing on the relative timing of foliation

development and successive generations of folds come from

KUSKY El' AL.: CON1ROI.S ON ACCREI1ON OF FL YSCH AND MELANGE 867

6. DISCUSSION 6.2. Significance of I Folding Generations

All units show folding about subhorizontal axes roughlyparallel to regional strike. These folds are likely related to theoff scraping/accretion mechanism, such as incorporation ofthe accreted packages into duplex structures or tighter folds i nthrust and fold nappe packages [e.g., Sample and Fisher, 1986;Sample and Moore, 1987; von Huene et al., 1997]. Theyounger cross-folding ab~ t NW trending axes does not havesuch an obvious tectonic explanation, but we suggest thatthese folds may be relat to the post-Paleocene oroclinalrotations described by several authors from southern Alaska

[Boland Coe, 1987; Coee~al., 1989; Packer and Stone, 1972;Panuska, 1987; Stone, 1989].

6.1. Differences in Accretion Style Between theMcHugh Complex and Valdez Group

Outcrop-scale melanges within the McHugh Complexdiffer in several ways from the Iceworm melange and. moreobviously, from the belts of coherent Valdez Group rocks.Melanges of the McHugh Complex contain a polymictassemblage of graywacke, chert, greenstone, and limestoneset in a phacoidally cleaved argillaceous matrix (Figures 3a and3b) [Bradley and Kusky, 1992]. These are type II melangesaccording to the classification of Cowan [ 1985]. The Icewormmelange, which consists entirely of fragmented graywacke andargillite, is a type I melange according to the classification ofCowan [1985]. Both the McHugh complex and the Icewormmelange are characterized by intense early stratal disruptionwith layer-parallel extension but differ in that the McHughmelanges show significant mixing between individual rock

types (presumably initially layers), whereas the Icewormmelange does not exhibit much interlayer mixing. Cleavage i nargillites of the McHugh Complex is weak, probably owing tosilicification of matrix argillite. The presence of web structureis characteristic of McHugh Complex sandstones but is veryrare in the Iceworm melange. These differences may be causedby different depths of deformation of the McHugh Complexand Iceworm melange, by different amounts of simple and pure

shear in a general shear environment [see Simpson and dePaor, 1993], by relative differences in fluid pressures duringdeformation, by differences in the amount of lithification orstrength contrasts between units, or contrasts in the relativeabundances of argillite and sandstone. The McHugh Complex,coherent Valdez Group units, and the Iceworm melange allshow folding about north to NE trending, gently NE plungingaxes and a later folding about NW trending axes. All units arecut by late (Paleocene) faults and dikes.

The coherent Valdez Group shows a dramatically differentstructural style, presumably related to a different mechanism ofaccretion. Structures are dominantly contractional and includethrust flats and ramps repeating individual sections, and tightto isoclinal folds. The structural style is reminiscent of

contemporary accretionary complexes where the subductingplate is thickly sedimented, with kilometer-scale panels ofinternally coherent beds that are folded about margin-parallelaxes [e.g., McCarthy and Scholl, 1985; Scholl et al., 1987].

6.3. Broad Tectonic Implications for Alaska

The Chugach terrane was formed on the southern marginof the Wrangellia comp~site terrane by intermittent offscraping/underplating ever ts, related to the subduction ofoceanic plates (including the Izanagi, Kula, and Farallon

plates), during the Triassic, Jurassic, and Cretaceous [Atwater,1989; Plajker and Berg, 11994]. These events formed themelange fabrics within the McHugh Complex [Bradley andKusky, 1992] and were the setting for the regionalprehnite/pumpellite facies metamorphism (Figures 16a andl7a). Following COllisit n of the Wrangellia composite

terrrane with cratonic No h America in the Middle to Late

Cretaceous [Nokleberg et I., 1994; Plajker and Berg, 1994;Pavlis, 1989], uplift and unroofing in the zone of intensecontractional deformation [Hollister, 1979, 1982; Crawfordand Ho/lister, 1982] deposited large volumes of sedimentarymaterial on the subducting loceanic plates (Figure 16b). It hasbeen suggested that uplift (>f the Coast Mountains occurred i ntwo major pulses, including one in the Late Cretaceous and onein the Paleocene-Eocene [Ho/lister, 1979; Crawford and

Ho/lister, 1982].~The Valdez Group s ows a magmatic arc provenance

[Dumoulin, 1987, 1988; ilbert et al., 1992; Farmer et al.,1993 ], consistent with a source in the uplifted Coast

Mountains (K1uane arc of Plajker et al. [ 1994] (Figure16b), although some sedi~nt may have been contributedfrom uplifted mountains in central Alaska [Plajker et al.,1994]. Sediment transpod was dominantly toward the NW(reconstructed Cretaceous coordinates) along the continental

Figure 7. Lower hemisphere equal-area projections of structural data from I coherent Valdez Groupsedimentary rocks and from well-bedded blocks in Iceworm melange. Poles to beds with unknown youngingdirections are plotted as circles, poles to upright beds are plotted as shaded squares, and poles to overturnedbeds are plotted as open squares. (a) Data from coherent Valdez Group rocks in domain 6, which are folded aboutan axis 01421° (cylindrical best fit great circle 104 69°S). (b) Poles to beds from Valdez type I melange indomain 6. (c) Poles to coherent Valdez beds in domain 5, which are folded about an axis 016 11° (best fit greatcircle 106 79°S). (d) Poles to beds in Iceworm melange of domain I. (e) Poles to btds from coherent ValdezGroup rocks in domain 2. (f) Poles to coherent Valdez Group beds in domain 3, which are folded about an axis021 11° (best fit great circle III 79°S). KV is coherent Valdez Group, Kvm is Iceworm melange. Large solidsquares in Figures 7a, 7c, and 7f are poles to great circles, interpreted as fold axes, based on visual best fit todata.

KUSKY Ef AL.: CONlROLS ON ACCREnON OF FL YSCH AND MELANGE 869

Figure 9. Map-scale fold in Valdez Group (location c in Figure 2). Axial surface indicated by dashed line.

Another unresolved question relates to the relationshipbetween underplating of t~e Valdez Group and uplift of theChugach and Kenai Mo¥ntains. Several authors haveconvincingly shown that $ome uplift is related to motion onthe Border Ranges fault system, but the relative amounts ofuplift related to these tran~pressional events. to underplatingof the Valdez Group bene.th the McHugh Complex. and toother events such as ridge ~ubduction have not been resolved.Major uplift of the Chu~ach terrane in southern Alaskaoccurred in the 60-55 Mal interval but continued with other"localized" events includin~ one between 30 and 35 Ma andanother at 15-20 Ma [Little, 1988, 1990; Kveton, 1989;Clendenin, 1991; Little q11d Naeser, 1989; Plajker et al.,1989]. Hawley et al. [19~4] determined an apatite fissiontrack age of 65 :t 6 Ma frqm a granodiorite at 1865 m in thewestern Chugach Mountain$ and interpreted it as reflecting theage of underplating and ac~retion of the Valdez Group alongthe Eagle River fault. Little [ 1990] descibed stratigraphicevidence for Paleocene upJift of the northwestern Chugachmountains including Late Cretaceous and Tertiary strata of theCook Inlet -Matanuska V~lley forearc basin sequence restingunconformably on Lower Jurassic rocks of the southernPeninsular terrane. Li~tle [1988] also documented asyntectonic alluvial fan sequence- the Paleocene -lowerEocene Chickaloon Form~tion- that was deposited alongnorth-dipping normal faults associated with uplift of theChugach terrane to the south. Little [ 1988, 1990] and Pavliset al. [ 1988] have documented that this sedimentation wasrelated to strike-slip movements along the Border Ranges faultsystem. Sisson and Pavli~ [1993]. Bradley et al. [1994] and

margin [Nilsen and Moore, 1979; Nilsen and Zuffa, 1982;Decker and Plajker, 1983], but bimodal paleocurrent directionsfrom the Kenai Peninsula (D.C. Bradley and T.M. Kusky,unpublished data, 1994) show that some submarine fans shedturbidites in both directions (Figure 16). Subduction of thisthick sedimentary pile is here postulated to have caused thedramatic change in accretion style between the older melangeterranes of the McHugh Complex and the generally coherentturbidite thrust packages of the Valdez Group.

An intriguing but unresolved question is the origin of theage gap between the McHugh Complex and the Valdez Group.The youngest fossils yet found in the McHugh Complex are

Albian-Aptian (D.C. Bradley and T.M. Kusky, unpublisheddata, 1995), yet the oldest fossils found in Valdez Group areCampanian-Maastrichtian, leaving a gap of no reported agesfor the early Late Cretaceous. It could be that this correspondsto a period of subduction erosion or that the unfossiliferousprotoliths of the Iceworm melange represent an older, muddierfacies of turbidites than preserved in more typical ValdezGroup. These could represent the first, distal deposits fromuplift and erosion in the Coast Mountains that would havebeen incorporated into the accretionary wedge before thethicker, more proximal part of the Valdez Group turbidite fan.Attempted subduction of the large amount of buoyant materialof the Valdez fan beneath the McHugh Complex would haveresulted in rapid frontal accretion [e.g., Byme and Fisher,

1987, Kimura, 1994] and may have activated the Chugach Bayand related thrusts, along which the already deformed and

metamorphosed McHugh Complex was emplaced over theValdez Group (Figure 17b).

Figure 8. Lower hemisphere equal-area projections of poles to slaty cleavage frqm Valdez Group rocks indomains 0, 2, 3, 5, and 6. The cleavage poles generally form two point maxima indIcating some variation i ncleavage orientations about a general nonh-nonheast strike. The cleavage from dom~n 2 appears folded aboutan axis 013 27° but may represent cleavage refraction around cleavage fans. Large sl\aded squares in Figure Saare eigenvectors based on cylindrical best fit to data.

KUSKYEf AL.: CONlROLS ON ACCREnON OF FL YSCH AND MELANGE871

the McHugh Complex over the Valdez Group and the formationof the Chugach Bay thrust and Iceworm melange may representa dramatic critical taper adjustment to these changingsubduction regimes. Davis et al. [1983] defined a popularColoumb wedge model for the mechanics of accretionaryprisms in which the stress is everywhere as large as possible.with the wedge thickening toward the rear because of theincreased strength of rock in this direction. For a wedge of

given strength (determined by rock rheology. basal resistance.pore fluid pressure. etc.). the forces that resist motion of thewedge are balanced by the forces that control the shape of thewedge (e.g.. the thickness of the wedge determines the cross-sectional area across which the principal horizontal stress canact). In this way. the combination of surface slope plus basaldecollement dip (critical taper) is maintained by deformationof the wedge. as material is 00<bl to the toe or base of the

wedge. Platt [1986] analyzed the dynamics of accretionarywedges with negligible yield strength and demonstrated thatunderplating can oversteepen the surface slope. causing

Kusky et at. [1997] have discussed several implications of

Paleogene subduction of the Kula-Farallon ridge beneath theChugach terrane. Kusky et at. [1997] presented structural dataand discussed a model that predicts that the wedge experiencedvertical extension as the triple junction approached any point,but at present there is not enough data to assess the relative

importance of ridge subduction, transpression, andunderplating in the uplift of the Chugach Mountains.

6.4. Relationship Between Subduction ZoneTectonism and Volume/Rate or SedimentInput: A General Model

We interpret accretion of the melanges of the McHughComplex as a normal response to subduction of oceanic crustbearing a thin pelagic sedimentary veneer (Figure 17a),whereas accretion of the coherent thrust packages of the ValdezGroup is interpreted as a normal wedge response to subductionof oceanic crust bearing a thick pile of loosely consolidated

submarine fan and trench sediments (Figure 17b ). Thrusting of

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Figure 13. Boudin of graywacke extracted from Iceworm melange in Windy Bay showing different fabricelements including boudin elongation parallel to L. lineation, web structure, calcite veins, and stylolites.

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..' F3 axial surfaces (2) .hinges (13)Figure 14. Lower hemisphere equal-area projections of fold data. Great circles represent fold axial planes;points represent fold hinge lines. Fold data plotted are F2 folds, except for dashed great circles, which are F3folds. (a) Folds in McHugh Complex in domain 3. (b) Folds in domain 3 Valdez Group. (c) Folds in domain IOMcHugh Complex. (d) Folds in domain 2 Iceworm melange. (e) Folds from domain 2 Valdez Group. Mm is

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874 KUSKY Ef AL.: CONIROLS ON ACCREnON OF FL YSCH AND MELANGE

.

Figure 15. (a) Dike cutting fabrics in Iceworm melange. Similar dikes belonging to a regional dike swarmhave yielded a 4OArf9Ar isochron age of 57.0 :t 0.22 Ma and a plateau age of 55.8 :t 1.4 Ma [W.S. Clendenin,personal communication cited by Bradley el al., 1994], as shown in the (b) age spectra. The dikes arecontemporaneous with a suite of near trench plutons that cut much of the Chugach terrane including Icewormmelange fabrics [Bradley el al., 1994].

160- 120 Ma 120- 66 Ma ba

~

Figure 16. Paleogeographic reconstructions for western North America for (a) 160-120 Ma period,reflecting time of formation of McHugh Complex, and (b) 120-66 Ma, during collision of the Wrangelliancomposite terrane with North America, and accretion of Valdez Group (modified after Plajker et al. [1994]).

875KUSKY Er AL.: CON1ROLS ON ACCRETION OF FL YSCH AND MELANGE

Triassic -Albian/ Aptian

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of McHugh Complex Chugach Bay thrust

nn mflange

I

/ --

V'

rapid frontal accretion.--ofValdezGroup

thick defonnation zone.accretion of thick thrustpackages. duplexingrapid underplating

of Valdez Groupuplift of high pnow Tmetamorphic rocks

Figure 17. Schematic diagram showing possible differentaccretion styles for (a) a thinly and a (b) thickly sedimentedoceanic plate, corresponding to formation of the McHughComplex and Valdez Group.

extension in the upper part of the rear of the wedge. Thisregains the equilibrium critical taper and causes uplift ofmetamorphic rocks in the rear of the accretionary wedge.These models are directly applicable to the Chugachaccretionary wedge in that the McHugh complex is interpretedas material off scraped intermittently during Triassic -EarlyCretaceous subduction, with the establishment of a critically

tapered wedge (Figure 17a). Attempted subduction of the thicksedimentary veneer of the Valdez Group during Maastrichtian -

Paleocene time dramatically upset the critical taper by bothrapid frontal accretion and increased underplating (Figure 17b ).The wedge's response to this episode of rapid accretion was todeform internally to regain the critical taper. Rapid frontalaccretion would cause the wedge to thicken to maintain thecritical taper, and in the case of the Chugach terrane much ofthis thickening was accommodated on the Chugach Bay thrust,emplacing the older part of the wedge (McHugh Complex) overthe younger part (Valdez Group). Formation of the Icewormmelange may have been aided by fluids released fromdewatering of the recently accreted Valdez Group escapingupward along the lower boundary of the relatively impermeableMcHugh Complex. The Chugach Bay thrust may also haveformed where it did because the protoliths of the Icewormmelange represent a muddy facies of the Valdez Group withconsiderably more pore fluids that in other parts of the ValdezGroup. Elevated pore fluid pressure within part of theaccretionary wedge decreases the effective strength of the rock,providing a horizon for enhanced localized deformation.

Increased underplating of the wedge during accretion of theValdez Group caused enhanced uplift of the rear of theaccretionary wedge, probably with associated extensionalfaulting, [e.g., Platt, 1986], providing a mechanism for theuplift and exposure of the metamorphosed McHugh Complex,and its emplacement over the Valdez Group on the ChugachBay thrust (Figure 17b).

Differences in accretion style analogous to theMcHugh/Valdez dichotomy are suggested to be characteristicof subduction of thinly versus thickly sedimented oceanicplates, and we cite here a few additional examples to emphasizethe generality of this model. Late Cretaceous uplift anderosion of the Mongolia-Okhotsk intracontinental collisionzone in Asia shed a large amount of sediments to the south andeast [Klimetz, 1983; Nanayama, 1992]. which triggered rapidfrontal accretion and growth of accretionary complexes alongthe northwest Pacific rim [Kimura, 1994]. Kimura [1994] alsodocumented that rapid underplating by attempted subduction ofthick sedimentary caps on the downgoing plates caused upliftof metamorphic rocks of the Kamuikotan complex ofHokkaido and the Susunai complex of Sakhalin. Taira et at.[1988] have documented rapid growth of the Nankaiaccretionary wedge in response to the lzu collision. Tertiaryuplift and erosion of the Himalayan mountain range from theIndia-Asia collision caused rapid growth of the Makran andSunda accretionary prisms from sediments supplied rapidly tothe trench by the Indus and Ghanges Rivers [Graham et al.,1975; Burbank et al., 1996; Sengar and Natal 'in, 1996].Similarities are also seen between the model presented here andthe ongoing collision of the Yakutat terrane with the southernAlaska margin [R. von Huene, personal communication,1997]. This collision has caused uplift and erosion of theChugach and St. Elias Mountains, deposition of a thicksequence of Miocene and younger clastic sediments in theAleutian trench. and rapid accretion at the toe of the Aleutianaccretionary prism [Plajker et al., 1994] .

Climate, as well as collisions, may playa significant rolein episodic growth and structural style of accretionary prisms[e.g., Hay, 1996; van Huene and Schall, 1991]. Bangs andCande [1997] have shown how a thickly sedimented segmentof the Chile trench south of the Chile triple junction in thearea strongly influenced by glacial climate corresponds torapid growth and accretion in the accretionary prism. Thesubducting Chile ridge acts as a topographic barrier tosediments shed into the trench by glacial erosion in the south,and growth of the prism north of the triple junction is lessrapid than that to the south of the triple junction. Periods ofrapid accretion were also noted during intervals whenglaciation extended farther north and erosion acceleratedtrench sedimentation north of the triple junction. Subductionof the Chile ridge causes significant erosion of theaccretionary complex, making the pre-ridge subduction recordsouth of the northward migrating triple junction difficult to

interpret.We suggest that, in general, subduction of slabs with a

thin veneer of sedimentary cover may lead to subductionerosion [e.g., van Huene and Schall, 1991] or to the formationof melanges in accretionary wedges, whereas subduction ofthickly sedimented slabs may generally result in large-scaleaccretion of relatively coherent packages separated by zones

876 KUSKY Ef AL.: CON1ROLS ON ACCRImON OF FL YSCH AND MELANGE

had a greater plate boundary length, with more, smaller, fastermoving plates than at present [ e.g., Pollack, 1997]. Oneconsequence of such a model is that subducting plates wouldhave tended to be thickly sedimented, with their proximity toactive collisions along numerous plate boundaries.Subduction of these thickly sedimented plates would tend toproduce Archean accretionary wedges dominated by relativelycoherent terranes and few melanges, consistent with thegeological record preserved in Archean granite greenstoneterranes [e.g., de Wit and Ashwal, 1997].

,

Acknowledgments. This work was supported by NSF EAR9304647 and NSFEAR 9706699 awarded to T. Kusky and by the U.S.Geological Survey. Field visits and stimulating discussion with A.B. Till,J. Dumoulin, S. Bloomer, R. Goldfarb, and D. Donley are gratefullyacknowledged. Bill Clendenin provided 40 Ar/39 Ar data for a key dike.

J. Sink of CalAlaska provided excellent helicopter services. This paperwas improved by reviews and comments by Terry Pavlis, Darrel Cowan,Warren Nokleberg, George Plafker, Tim Byrne, Roland von Huene, andTectonics Editor David Scholl.

of shearing and type I melange formation. Using a constant

slip-rate model, suduction of an oceanic slab with a thinsedimentary veneer will result in significantly higher shearstrains in the boundary layer between the upper and lowerplates than would subduction of an oceanic slab with a thicksedimentary cover. For example, if we assume 1 km ofthrusting per million years and we accommodate this strain i na lOOm thick sedimentary section, then shear strains of 10will be typical for the sedimentary cover and will likelyinvolve significant mixing between layers. If the subductingoceanic plate has a 1 km thick sedimentary cover, then shearstrains will be about I, and we may expect strains that arelower than in the melange and perhaps more typical of aforeland fold-thrust belt.

This model may also explain a fundamental temporalchange in the style of subduction zone tectonism. Manyworkers have noted the near absence of subduction melangesfrom Archean terranes, even those interpreted as ancient

accretionary wedges [e.g., Kusky, 1989, 1990, 1991]. Modelsfor Archean plate tectonics predict that the planet may have

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(Received April 29, 1997;revised September 19, 1997;accepted September 29, 1997.

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