The processes of underthrusting and underplating in …crowe/downloads/Meneghini...trending geological sub-parallel domains are generally recognized from E to W, on the eastern side

The processes of underthrusting and underplating in the geologicrecord: structural diversity between the Franciscan Complex(California), the Kodiak Complex (Alaska) and the Internal

Ligurian Units (Italy)

F. MENEGHINI 1,2*, M. MARRONI 1,3, J.C. MOORE 2, L. PANDOLFI 1,3 and C.D. ROWE4

1Dipartimento di Scienze della Terra, Universita di Pisa, Pisa, Italy2Earth and Planetary Sciences Department, University of California at Santa Cruz, CA., USA

3C.N.R., Istituto di Geoscienze e Georisorse, Pisa, Italy4Department of Geological Sciences, University of Cape Town, South Africa

Existing studies on active subduction margins have documented the wide diversity in structural style between accretionaryprisms, both in space and time. Together with physical boundary conditions of the margins, the thickness of sedimentarysuccessions carried by the lower plate seems to play a key role in controlling the deformation and fluid flow during accretion. Wehave tested the influence of the subducting sedimentary section by comparing the structural style and fluid-related structures offour units from three fossil accretionary complexes characterized by similar physical conditions but different subductingsediment thicknesses: (1) the Franciscan Complex of California, (2) the Internal Ligurian Units of Italy and (3) the KodiakComplex, Alaska.Subducting plates bearing a thick sedimentary cover generally result in coherent accretion through polyphase deformation

represented by folding and thin thrusting events, while underplating of sediment-starved oceanic sections results in diffusedeformation and melange formation. These two structural styles can alternate through time in a single complex with a longrecord of accretion such as Kodiak.The parallel analysis of the selected analogues show that although the volume of sediments carried by the lower plate

determines different structural styles, deformation is strongly controlled by injection of overpressured fluids during under-thrusting and accretion. Transient hydrofracturing occurs through the development of a system of dilatant fractures grosslyparallel to the decollement zone. Copyright # 2009 John Wiley & Sons, Ltd.

Received 30 January 2008; accepted 30 October 2008

KEY WORDS subduction; underplating; melange; sediment thickness; hydrofracture; Franciscan Complex; Ligurian Units; Kodiak Complex

1. INTRODUCTION

The study of active convergent margins reveals that accretionary prisms are continuously shaped by varioustectono-metamorphic processes, in a complex circulation of masses and fluids comprising accretion, the growing ofthe prism by transfer of material from the downgoing to the upper plate; tectonic erosion, through which materialpreviously accreted is removed from the prism base and underthrust at depth and, finally, exhumation thatcompletes the cycle transferring the material in the prism interior to the surface.

Since the advent of plate tectonic theory, different models have been proposed to explain the deformationprocesses active during accretion. Seismic reflection imaging and ocean drilling data from modern margins, as well

GEOLOGICAL JOURNALGeol. J. (2009)Published online in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/gj.1144

*Correspondence to: F. Meneghini, Dipartimento di Scienze della Terra, Universita di Pisa, via S. Maria, 53, 56126 Pisa, ITALY.E-mail: [email protected]

Copyright # 2009 John Wiley & Sons, Ltd.

as field studies on ancient accreted complexes suggest an imbricate-thrust model for shallow accretion andhorizontal prism growing (Karig and Sharman 1975 and references therein) and, at increasing depths, a verticalthickening of the prisms through underplating of internally coherent duplexes (Silver et al. 1985; Sample and Fisher1986;Moore and Sample 1986; Kusky et al. 1997; Hashimoto and Kimura 1999) or by formation of a melange (Hsu1968; Cowan 1974, 1985; Cloos 1982; Moore and Sample 1986; Moore and Byrne 1987; Kusky et al. 1997).The data available for modern margins have demonstrated that there is a wide structural diversity along the same

margin both in space and time, both along and across the deformation front (von Huene 1984; vonHuene and Scholl1991; Le Pichon et al. 1992; Shipley et al. 1995; Maltman et al. 1997; Moore et al. 1998; Clift and Vannucchi2004). A single margin can show accretionary and non-accretionary features on different transects across it (i.e.Middle America Trench, Aleutian margin), or can show different accretionary styles and different prism dimensions,depending on the distance from sedimentary sources (i.e. Lesser Antilles margin). Both types of variations can beobserved in the same margin when changing the scale of observation (von Huene 1984) or through time.Other than convergence rate, that alone could not explain variations across a single margin, some other factors

might interplay to cause structural diversity along subduction boundaries (von Huene 1984; Clift and Vannucchi2004): (1) lower plate topography (subduction of positive or negative sea-floor relief); (2) changes in dip andconfiguration of the Benioff zone; (3) type and volume of incoming sediment pile on the subducting plate; (4) depthof accretion and (5) volume of fluids migrating into the prism and related pore-pressure conditions.It has been shown that while prisms can grow preferentially through accretion of both coherent units and

melanges, most of the natural exhumed examples display both types at various times in their history (Fisher andByrne 1987; Kusky et al. 1997; Hashimoto and Kimura 1999). What controls the accretion of thick coherentterranes and melange terranes is still poorly understood. Some authors have tried to solve this problemhypothesizing an important control of the volume rate of sedimentary input (a function of convergence rate and rateof deposition) on the coherent or diffusive style of accretion (Moore and Sample 1986; Kusky et al. 1997; Sampleand Reid 2003), suggesting that subduction of slabs with a thin veneer of sedimentary cover generally lead to theformation of melanges, while subduction of thickly-sedimented slabs result in large-scale accretion of relativelycoherent packages, separated by sharp zones of shearing or type I melange (sensu Cowan 1985). Supporting thishypothesis are numerous observations and models of active and exhumed accretionary prisms (Sample and Fisher1986; Sample and Moore 1987; Moore et al. 1988; Kimura and Mukai 1991; Taira and Ashi 1993; Kimura 1994;Plafker et al. 1994; Kusky et al. 1997; Ujiie 2002), which testify that rapid growth of accretionary wedges is oftenassociated with input in the subduction system of a large amount of sediments.Clift and Vannucchi (2004) presented a quantitative comparison of mass flux for 30 active convergent plate

boundaries, in an attempt to relate the accretionary (or erosive) character of a margin with various boundary conditions.We have tried to extend these observations at underplating depths by studying and comparing the structural style ofancient exhumed prism complexes. In particular, we have investigated the possible role of the thickness of thesubducting sedimentary section in controlling structural style and fluid flow by comparing the accretion-relatedstructural features of different units from three fossil accretionary complexes characterized by similar physicalconditions but with different sedimentary input. The selected complexes (Figure 1) span from the Franciscan Complexof California, the source of one of the first definition of melange (Hsu 1968; Cowan 1974), to the Ligurian Units of theNorthern Apennines, where the concepts of underplating in an accretionary prism were first applied in the Italian belt(Principi and Treves 1984; Marroni and Pandolfi 1996), through the Kodiak Complex of Alaska, one of the moststudied prisms, famous for the wide exposure of both melange and coherent formations (Connelly 1978; Sample andMoore 1987; Fisher and Byrne 1987). In each complex the units have been selected on the basis of:

- good exposure and documented geological background,- age control on sediments involved in underplating, in order to check the different nature of the sedimentary inputto the three systems at the time of subduction,

- comparable metamorphic conditions during accretion from units of different complexes. All the selectedexamples span between the zeolite and the prehnite-pumpellyite facies, representative of depth of accretionranging between 8 and 13 km (Figure 1)

Copyright # 2009 John Wiley & Sons, Ltd. Geol. J. (2009)

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- weak post-accretion deformation or, at least good cross-cutting relationships between different phases.

Although a quantitative determination of thickness of sediments is intrinsically difficult in the fact of studyingexhumed remnants of accreted subducting plates, the detailed geological mapping of the three complexes availableallows, especially for the coherent units, a reconstruction of the stratigraphy of the subducting sequence, makingpossible assumptions on the relative thickness of sediments carried to the system in the three analysed complexes.

In the paper, we also characterize the fluid flow regimes, through a detailed description of the vein systems,aiming to examine the control of sediment input on the nature of fluid pathways through the system during bothunderthrusting and underplating.

2. GEOLOGICAL SETTINGS

2.1. The Marin Headlands Terrane, Franciscan Complex, Northern California

The Franciscan Complex belongs to the California Coast Ranges that constitute, together with the Sierra Nevada,the two big mountain chains of central California (Wahrhaftig 1989). In central California, four north-southtrending geological sub-parallel domains are generally recognized from E to W, on the eastern side of the SanAndreas Fault (Blake et al. 1984; Wakabayashi 1992): the Sierra Nevada batholith, the Great Valley Sequence, theCoast Range Ophiolite and the Franciscan Complex. These domains are interpreted as different components of asubduction complex associated with the underthrusting of Farallon oceanic lithosphere, under the western NorthAmerican plate margin during the mid-Mesozoic to mid-Cenozoic until the initiation of the San Andreas transformsystem.

The accretionary Franciscan Complex is composed of a series of thrust sheets, referred to as tectonostratigraphicterranes (Figure 2), comprising sequences of massive and pillow basalt, radiolarian chert and deep sea fan andtrench deposits, accreted at different times and depths to the North American margin (Wakabayashi 1992 andreferences therein). These sequences crop out as coherent bodies surrounded by thick highly disrupted units, mostlyreduced to tectonic melanges, defined by various exotic blocks, embedded in a highly sheared matrix (Cowan 1974;Cloos 1982; Blake et al. 1984; Wakabayashi 1992; Jeanbourquin 2000).

The largest melange zones (up to 1500m thick) mark thrusts classically interpreted as analogues of thick plate-boundary zones (Wakabayashi 1992). Depending on the depth of accretion, the units have experienced asubduction-related metamorphism ranging from zeolite to blueschist, up to early eclogite facies.

Figure 1. (a) Location of analysed complexes: FC!Franciscan Complex; ILU! Internal Ligurian Units; KC!Kodiak Complex and (b) P/Tconditions of metamorphic climax of analysed complexes.


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processes of underthrusting and underplating

Near the structural base of the Franciscan Complex, the Marin Headlands Terrane is formed by imbrication andrepetition of an oceanic crust which was originally basalt overlain by chert and capped by greywacke (a ridge-pelagic-trench progression). The section analysed (Rodeo Cove Thrust of Meneghini and Moore 2007) is a 200mthick melange zone that is interpreted as a ‘sediment-poor’ stage in the decollement evolution during Franciscansubduction.

2.2. The Internal Ligurian Units, Northern Apennines, Italy

The Northern Apennines (Figure 1) is a fold and thrust belt built during the Late Cretaceous-Eocene subduction ofthe Ligure-Piemontese oceanic basin that then evolved in the collision between the European and Adria plates.Remnants of the Ligure-Piemontese ophiolites and associated sedimentary sequences are grouped into the Liguranunits, cropping out at the top of the western sector of the Apenninic belt (Figure 3). The Internal Ligurian Unitsexpose the most complete and best preserved section of the Ligure-Piemontese oceanic basin lithosphere (Decandiaand Elter 1972; Abbate et al. 1980), comprising a well-preserved ophiolitic sequence, a Middle Jurassic/LateCretaceous pelagic and hemipelagic sequence and a complex, thickening- and coarsening-upward turbiditic

Figure 2. The tectonostratigraphic terranes of the Bay area of San Francisco. FromMeneghini andMoore (2007). Letters K and J in the terranes’names refers to Cretaceous and Jurassic, respectively.


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system, ranging in age from Campanian to Early Paleocene. The stratigraphic section reconstructed in various unitsreveals ca. 2500 km thick turbidite and debris-flow deposits on top of the Ligure-Piemontese oceanic basin (Nilsenand Abbate 1983-1984; Marroni and Pandolfi 1996; Marroni et al. 2004), testifying to the sediment-rich nature ofthis basin as it became involved in subduction processes.

The Internal Ligurian Complex is affected by complex deformation developed during the Late Cretaceous-Eocene subduction and accretion of the Ligure-Piemontese basin, followed, during the Late Eocene, by thecollision between the Adria microplate and the European continental margin. The subduction-related deformationhistory includes folding and faulting events, each representing a different stage in the accretionary history andsubsequent exhumation towards the surface (Marroni and Pandolfi 1996; Marroni et al. 2004).

The complex tectonic stack is affected by subduction-related metamorphism ranging from the low-T blueschistto zeolite facies (Leoni et al. 1996; Ducci et al. 1997; Ellero et al. 2001), suggesting that accretion occurred bysampling of coherent slices of the lower plate from different depths.

Among the Internal Ligurian units, the Gottero Unit has been selected because it shows the widest and bestpreserved sedimentary succession, characterized by a thickness of up to 2500m (Decandia and Elter 1972; Nilsenand Abbate 1983-1984; Marroni et al. 2004). The stratigraphic succession, detached from an ophiolitic basement,includes hemipelagic shale that grades upward to a complex turbidite fan system (Nilsen and Abbate 1983–1984),topped by trench debris flows and slide deposits (Marroni and Pandolfi 2001).

Figure 3. The Internal Ligurian Units (ILU) of Northern Apennines. In the inset, location of the described sector is shown.


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2.3. The Kodiak Accretionary Complex, Kodiak Island, Alaska

The Kodiak Accretionary Complex of south-central Alaska (Figure 1) exposes a tectonic stack of discrete units ofoceanic igneous and sedimentary rocks, deposited and accreted to the North American continent during Mesozoic–Cenozoic subduction (Figure 4). The complex has its modern analogues offshore, in the Eastern Aleutian Trench(Byrne 1982; Moore 1969; Sample and Moore 1987; Fisher and Byrne 1987; Plafker et al. 1994). The entirecomplex shows a mean NE–SW structural trend and decrease in age and metamorphic grade towards the southeast(Moore 1969; Connelly 1978; Moore and Allwardt 1980; Roeske 1986; Sample and Moore 1987).Internally coherent and melange units alternate in NE-trending, parallel slices. Each unit has his correlative unit

along strike in the Kenai Peninsula and Anchorage area (Plafker et al. 1994). Each terrane is fault-bounded andrecords a polyphase history of subduction and accretion (Sample and Reid 2003); those classified as melanges havebeen previously interpreted as palaeo-decollement zones (Byrne 1984, Byrne 19851985; Vrolijk et al. 1988). Fromnorthwest to the southeast, the metamorphic grade ranges from epidote amphibolite and blueschist, found in theoldest rock units of the northwestern side of the island, to the zeolite facies (Roeske 1986).The coherent Kodiak Formation and the melange terranes of the Ghost Rocks Formation and the Uyak Complex

analysed in this study are composed of variable amounts of ocean floor basalt and associated sediments, reflectingthe thickness of the incoming sediment pile (Connelly 1978; Vrolijk et al. 1988; Fisher and Byrne 1987; Byrne andFisher 1990).The Kodiak Formation encompasses almost 70% of the exposures on Kodiak Island (Figure 4), representing a

period of voluminous accretion of trench sediments (Sample and Reid 2003) that occurred in a relatively short timespan of between 2 and 4m.y. (Fisher and Byrne 1987). The Kodiak Formation is an extensive turbidite sequenceinterpreted as deposited in an oceanic trench along the margin of southwestern Alaska in Late Cretaceous time(Sample and Moore 1987). The thickness of the Kodiak Formation, entirely made up of thin-bedded turbidites andthick massive sandstones, is only locally disrupted, and duplexes, where observed, involve narrow discrete faultsrather than thick melanges (Sample and Fisher 1986).In contrast to the Kodiak Formation, the Paleocene-Eocene Ghost Rocks Formation is a largely disrupted

melange unit "4 km thick (Byrne 1984). The Ghost Rocks Formation consists primarily of tightly folded tostratally-disrupted thin- to medium-bedded sandstone and shale with occurrences of pillow basalt. Lenticular slicesof coherent material occur within the Ghost Rocks Formation and the degree of disruption increases towards the

Figure 4. The Kodiak Accretionary Complex of Kodiak Islands.


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basal contact. The Cretaceous Uyak Complex, structurally overlying the Kodiak Formation, is a 2 km thick unitcomposed of ocean floor sediments and basalts with only minor clastic turbidites, representing, as suggested byConnelly (1978), a low stratigraphic and structural level on the subducting plate. This stratigraphy is disrupted to amelange in which the main lithologies crops out as up to metre-scale blocks in a shale matrix.

All units were metamorphosed at prehnite-pumpellyite facies conditions (Figure 1).Intrusions at 60–55Ma cross-cut the accretion-related deformation on Kodiak Island and post-date the

metamorphism, strictly constraining the age of deformation of the complex, from burial, to underthrusting andaccretion (Fisher and Byrne 1987).

3. ACCRETION OF COHERENT TERRANES

3.1. The coherent units of the Ligurian Apennines

The Gottero Unit (Figures 3 and 5) has been selected as a key area for this study because it preserves a thickness ofup to 2500m of the sedimentary succession topping the ophiolitic basement of the Ligure-Piemontese basin(Decandia and Elter 1972; Nilsen and Abbate 1983-1984; Marroni et al. 2004). The stratigraphic succession(Figure 5a) comprises a complex series of siliciclastic to mixed siliciclastic-carbonate coarsening-upward basinplain deposits that reflect the trenchward motion of an area belonging to the Ligure-Piemontese oceanic lithospherein a sediment-dominated subduction zone (Marroni and Pandolfi 2001 and references therein). This succession isreconstructed in detail due to the coherent style of deformation that allowed preservation of internal stratigraphy.

The Gottero succession is affected by a polyphase deformation history, developed under very low-grademetamorphism peak conditions (Pertusati and Horremberger 1975; van Zutphen et al. 1985; Van Wamel 1987;Marroni et al. 1988;Marroni 1991;Marroni and Pandolfi 1996;Marroni et al. 2004;Meneghini et al. 2007). Amongthese phases, the D1 event is correlated with underthrusting and underplating at low structural level in anaccretionary wedge and can be subdivided in different sub-phases of veining, folding and thrusting (Marroni andPandolfi 1996; Marroni et al. 2004).

Figure 5. Deformation style of the coherent Gottero Unit: (a) Internal coherence is preserved so that bedding can be followed at map-scale. Thehill shown in the picture is 35m high. (b) F1 isoclinal folding. S1 cleavage axial planar to folds is also shown. Coin diameter is 2.5 cm. (c) S1 slatycleavage, axial planar to F1 folds. (d) D1-related cataclastic shear zones. Shear zones are marked by highly disrupted competent ribbons in a

shaly foliated matrix. Coin diameter is 2.5 cm.


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The D1 phase is characterized by strongly non-cylindrical, subisoclinal to isoclinal F1 folds with a similargeometry (Figure 5b). The F1 folds, though they can vary in morphology, have been recognized throughout all theformations from the Gottero Unit. The limbs are generally affected by boudinage and necking. An axial-planefoliation (S1) is found in the shale as a continuous and penetrative slaty cleavage (Figure 5c), with an associatedmetamorphic mineral assemblage of quartz# calcite# albite# chlorite#white mica# Fe-oxides.Northeast striking, 1–1.50m thick shear zones cut the structures related to the D1 folding phase and are in turn

folded by the D2-related folding event (Figure 5d). They represent the boundaries of the main units of the LigurianComplex and are responsible for emplacement and internal imbrication of the complex (van Zutphen et al. 1985;Marroni et al. 1988, 2004; Marroni 1991; Marroni and Meccheri 1993; Marroni and Pandolfi 1996). These sharpsurfaces are marked by thin zones of highly disrupted fault rocks featuring tabular fragments of siltstone andcarbonate blocks, chaotically mixed with shaly turbidite intervals (Figure 5d) and locally comprising foliatedcataclasites. In thin section they show a well-developed foliation (S foliation), represented by a compositionallayering made by alternating mica-rich and calcareous mm-size layers. Grains are elongated along this foliation andsurrounded by thin layers of very fine-grained mica-rich material. Thin regular seams of opaque minerals alsodefine the S foliation, testifying that pressure solution was an important deformation mechanism. The foliation isassociated to deformation bands and shear surfaces with an orientation corresponding to R surface of Riedel shears.The analysis of kinematic indicators, after removing the effects of later deformation, suggests a top-to-the-NWsense of shear, consistent with the vergence inferred for the D1-related mega-structures (Marroni and Pandolfi 1996and references therein).According to illite and chlorite crystallinity and illite b0 parameters (Leoni et al. 1996), the peak of the

metamorphism developed during P/T (pressure/temperature) conditions of 0.4GPa/210–2708C (Figure 1). Thecalculated T ranges are in agreement with the vitrinite reflectance (Bonazzi et al. 1987) and twins in calcite fibres,corresponding to type II of Burkhard (1993), developed in the range of temperature spanning from 180 to 3008C(Meneghini et al. 2007).

3.2. The Kodiak Formation: coherent accretion of sediments in the Kodiak Complex

The Kodiak Formation occupies the majority of the entire accretionary complex and consists of three structuralbelts (landward, central and seaward belts, see Sample and Moore 1987) that experienced similar structuralhistories though exhibiting slightly different structural styles (Figure 4).This formation features deep-water turbidites comprising black shales and argillite, quartzitic to lithic greywacke

sandstones, rare conglomerates and pebbly mudstones. The sedimentary facies indicate rapid deposition along thetrench axis (Sample and Reid 2003). Except for zones of stratal disruption, which account for approximately 20%of exposures (Sample and Moore 1987), bedding throughout the formation is mainly coherent (Figure 6a), with thedominant outcrop scale structural features being a northwest-dipping cleavage, SW–NE folds and south-eastverging thrust faults (Figure 6). Infact, the deformation history of the Kodiak Formation, reconstructed in detailduring more than a decade of field campaigns (for a general review see Fisher and Byrne 1987; Sample and Moore1987; Fisher and Byrne 1992) is comparable with that observed in the Gottero Unit of the Italian complex, with bothbeing characterized by stages of folding and thrusting.In particular, five major deformation events are recognized, the first two being interpreted as related to the

underthrusting and underplating stages. The D1 event as defined by Fisher and Byrne (1987, 1992) is characterizedby layer-parallel extension and layer-parallel shear (underthrusting). The D2 event (see D1 phase of Sample andFisher 1986; Sample and Moore 1987) features seaward verging fold and thrust deformation characterized by F2isoclinal folds with variable geometries, from similar to kink-like, and interlimb angles ranging from 08 to 908(Figure 6b). However, due to the variable character of deformation, F2 folds style is also variable through theformation and according to the Ramsay classification, type 1B, 1C, 2 and 3-fold styles have been reported in severalstudies across the formation (Sample and Moore 1987). In particular, a complex history of strain variation has beenreconstructed across the three structural belts forming the unit and related essentially to the distance from thedecollement of the analysed sector (Fisher and Byrne 1992). Fold structures from all belts show strong asymmetry


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and a southeast vergence as testified by southeast-facing short limbs. Sample andMoore (1987) also report the axesof folds that define a separation arc trending north-northwest, suggesting fold formation in a high strain regime.

Axial planar to F2 folds is a penetrative, well-developed northwest dipping S2 foliation, the dominant feature atoutcrop scale (Figure 6c), associated with a down dip stretching lineation. In the fine-grained lithologies it is a slatycleavage defined microscopically by pressure solution seams, marked by insoluble material and preferredorientation of platy minerals. Dynamic re-crystallization of quartz is locally associated with pressure solution.

Northwest dipping thrust faults accommodate much of the Kodiak Formation shortening. The thrusts are narrow,spaced and separate packages of folded sediment (Figure 6d). Similar to what is described for the Ligurian units,thrust zones thickness is on the orders of a few metres and is frequently marked by quartz-filled shear veins.Southeast vergence for these thrusts is indicated by slickensides and sigmoidal shear veins adjacent to the faults andby dragging of folds and cleavage. The consistent vergence of all structures indicates that the formation underwenta bulk shear related to northwest-directed shortening during underthrusting.

Re-crystallization of prehnite and pumpellyite during the D2 event has been identified in volcaniclastic wackesof the Kodiak Formation (Sample and Moore 1987). Fluid inclusion measurements and stable isotopes analysessuggests that vein precipitation associated with deformational fabrics occurred at 200–2508C and 250–300MPa

Figure 6. Deformation style of coherent Kodiak Formation: (a) Internal coherence is preserved. The formation is a monotonous turbiditicsequence. Geologist is 1.65m. (b) F2 isoclinal folding with associated S2 axial plane. Chisel in the centre of picture is 30 cm long. (c) S2 slatycleavage, axial planar to F2 folds. Pen is 15 cm long. (d) D2-related narrow shear zones (white line). Associated isoclinal folding and penetrative

slaty cleavage (parallel to the white line) also visible. Pencil is 15 cm long.


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(Myers and Vrolijk 1986; Paterson and Sample 1988; Sample and Moore 1987; Brantley et al. 1998). Theseconditions are consistent with peak metamorphism at high-zeolite facies, to low prehnite-pumpellyite facies(Paterson and Sample 1988), pointing to an accretion depth of 8–12 km (Figure 1).As already mentioned this D2 event predates the intrusion of Paleocene-Eocene (ca. 60–55Ma) plutons.

4. ACCRETION OF MELANGE TERRANES

4.1. The melange along the Rodeo Cove Thrust, Franciscan Complex

Except for some pervasive strong deformation near the San Andreas Fault, the Bay area of San Francisco is the leastaffected by Neogene deformation, providing the best field area to examine the Franciscan structural evolution tosubduction and accretion. The unit stack crops out as NW–SE striking Early Jurassic to Early Tertiarytectonostratigraphic terranes (Figure 2), with varying lithology and metamorphic conditions, separated byextensive low-angle melange zones (Blake et al. 1984; Wahrhaftig 1984; Wakabayashi 1992; Blake et al. 2000).The selected area for this study is theMarin Peninsula (Figure 2), just North of the Golden Gate of San Francisco,

where the Marin Headlands Terrane is well exposed. The terrane is composed of a complex array of SSE dipping,ENE-WSW–striking disrupted tectonic slices, 300–500m in thickness, bounded by a similar thickness of melangeshear zones (Figure 7a). The studied melange crops out along the Rodeo Cove Thrust and has been previouslyinterpreted as a palaeo-decollement zone (Meneghini and Moore 2007).A low-grade metamorphism (Figure 1) of prehnite-pumpellyite facies has been estimated for the entire terrane

(Wahrhaftig 1984; Wakabayashi 1999). Although the main structural trend for Franciscan terranes strikes NWanddips NE (Figure 2), the Marin Headlands terrane strata and internal shear zones dip S to SSE, due to a 908 to1308clockwise rotation of the Marin Headlands block, possibly associated with motion on the San Andreas Fault(Blake et al. 1984; Curry et al. 1984; Wakabayashi 1999).The reconstructed stratigraphy in the less disrupted terranes of the area testifies to the sediment-starved nature of

the oceanic lithosphere when involved in subduction: after restoring the thrust-related thickening, the trench

Figure 7. Deformation style in the Marin Headlands Terrane: (a) N–S trending schematic geologic cross-section across the terrane. Terrane ismade up by thin slices, repeating a basalt-chert-sandstone sequence, separated by thick melange zones. Melanges are then interpreted aspalaeodecollements. (b) Basalts in the less deformed slices usually preserve primary pillow structures. Lens cap diameter is 4.5 cm. (c) Chert

sequence is extensively crumpled by kink and chevron folds. Folds do not display lateral continuity.


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sediments are highly subordinate in volume with respect to the basalts. In particular, the stratigraphic successionconsists of coherent pillow lava bodies, thinly bedded Jurassic to Cretaceous radiolarian cherts, and subordinateturbiditic sequences of Albian to Cenomanian age (Wahrhaftig 1984). The stratigraphic succession is repeatedmany times by the thick low-angle shear zones (Figure 7a), as demonstrated by repeated biostratigraphic horizons(Murchey 1984), to form a duplex structure. Each thrust slice repeats an upright basalt-chert-sandstone sequencerepresentative of fragments of the Pacific oceanic crust and pinches out laterally or break into blocks in themelange: there are no thrust slices extending for the entire length of the terrane.

Syn-tectonic re-crystallization of chlorite and pumpellyite has been detected, together with pumpellyite andlaumontite veinlets in basalts. This mineral association is typical of the prehnite-pumpellyite facies (Figure 1),close to the zeolite facies, at P values of around 2.5 kbar and T values between 200 and 3008C (Frey and Robinson1999 and references therein).

The thrust slices within the Marin Headlands Terrane display a complete range of structural disruption from nooutcrop-scale visible deformation, to broken formation containing isolated folds, to complete stratal disruption. Inthe less disrupted slices, primary structures, such as originally oriented pillows and sedimentary structures in theturbidites and igneous textures in basalts, are usually preserved (Figure 7b). The most prevalent deformation at theoutcrop scale is the pervasive crumpling in chert units by kink and chevron folds (Figure 7c). Despite poor lateralcontinuity and changes in morphology, there seems to be a gross consistency in fold axis orientation throughout allthe thrust slices. In fact, when restored to pre-rotation geometry, the folds display a marked asymmetry consistentwith the NE-directed subduction direction postulated by Engebretson et al. (1985) and the fold axis trend is normalto a SW-directed transport-induced shear (see also Wahrhaftig 1984; Wakabayashi 1999).

Well-exposed cross-sections of melange shear zones can be found along cliffs and beaches, because the coastlineruns often normal to the fault strike. One of these exposed sections can be found at Rodeo Cove, in the hills just NWof the Golden Gate in the Marin Peninsula (Meneghini and Moore 2007). The Rodeo Cove melange shear zone(Figure 8) is made up of fragments of basalt (even comprising entire, deformed pillows, as in Figure 9a), chert andminor sandstone, from the metre- and millimetre-scale, enclosed in a fine pelitic matrix affected by penetrative,typical scaly foliation (Figure 9b). Disruption occurs through progressive localization of deformation by a variablydistributed complex network of discrete shear zones of concentrated deformation that isolates relativelyundeformed competent blocks (Figure 8). The shear zones range in thickness from the millimetre- to the decimetre-scale and reveal a top-to-the-NW sense of shear with two conjugate systems, making angles of 158–258, that havebeen interpreted as R and P fractures of the Riedel shear model (Figure 8; see Meneghini and Moore 2007).

Figure 8. Schematic reconstruction of melange fabric as observed in the Rodeo Cove melange: (a) Disruption occurs through localization ofdeformation by a complex network of discrete shear planes arranged as R and P Riedel surfaces isolating basalt and chert blocks. S foliation onblocks grossly parallels the scaly foliation of the matrix. Straight lines also represent pressure-solution surfaces. (b) Schematic close-up ofdiscrete shear surface showing scale invariant cataclastic aspect made up by clasts wrapped by a very fine matrix, showing scaly fabric and S–C

structures.


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Each discrete surface is composed of millimetre to decimetre elongate and polished basalt fragments enclosed ina greenish or reddish matrix composed of very fine siltstone and shalewith abundant chlorite (Figure 9b). The fabricis mainly characterized by extreme grain size reduction through cataclasis with a large range in grain size and sharpand angular clasts. The matrix shows a penetrative scaly foliation characterized microscopically by cleavagelamellae arranged in an anastomosing web of discontinuous layers bounding the fragments. A strong preferredorientation of platy minerals, mostly syn-deformational chlorite, together with pressure seams, defines thecleavage. Brittle S–C structures are widespread (Figure 9c). C-type planes are represented by the fracture surfacesdefining the scaly fabric, and their mean attitude approximate to a plane consistent with the average strike of both Rand P fractures. This S-type foliation is approximately parallel to the spaced cleavage of the competent, large basaltblocks, and it always makes a very low angle with the C-type planes. Syn-tectonic chlorite and acicular pumpellyiterecord the opening and development of fractures through shear, by growing in S–C structures (Figure 9c). The shearsense of all these structures is always top-to-NW, compatible with that inferred in the available structural studies.

4.2. The melange units of the Kodiak Complex

Twomainly melange units are mapped in the Kodiak Complex: the Ghost Rocks Formation and the Uyak Complex.The Ghost Rocks Formation is composed of marine sedimentary and volcanic rocks deposited on a ridge flank in

the latest Cretaceous to earliest Paleocene (Byrne 1982, 1984; Rowe et al. 2005) and crops out at present for a

Figure 9. Deformation style in the Marin Headlands melange zones: (a) in the highly disrupted sections blocks are deformed and elongatedalong the primary shear-related foliation. Locally, even entire pillows are stretched and sheared. Lens cap diameter is 4.5 cm. (b) Melange matrixdevelops a penetrative scaly foliation. Pencil is 15 cm long. (c) Brittle S–C fabric. S foliation defined by syntectonic chlorite (chl) and by re-

orientation of fragments and clasts. Chlorite also along C-planes.


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structural thickness of 5–10 km (Figure 4). In reference to the regional structure, the Ghost Rocks Formation wasthe last material preserved in the wedge prior to the subduction of a spreading ridge (Haeussler et al. 2003). Vrolijket al. (1988) estimated for this formation metamorphic conditions corresponding to the prehnite-pumpellyite facies.On a formation scale (Figure 10a), the Ghost Rocks Formation is a melange composed of blocks of coherent bedded

Figure 10. Deformation style in Ghost Rocks Formation: (a) Rounded sandstone blocks in shaly matrix. Pen is 15 cm long. (b) Locally, pillowbasalts are embedded in melange. Lens cap diameter is 4.5 cm. (c) Extreme localization of deformation in the Ghost Rocks Formation occurs as

highly sheared cataclastic shear zones. Hammer is 30 cm.


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turbidites and sandstones separated by bands of highly disrupted strata (Byrne 1982, 1984). Pillow basalts,hyaloclastites and local hypabyssal intrusions are interbedded with siliciclastic sediments (Figure 10b, see alsoByrne 1984). Rare limestones occur in interstices of the pillow basalts.Similar to what was observed in the Franciscan studied unit, the Ghost Rocks Formation structural style range

from coherent bedded portions to zones of disruption and melange formation. The upper part of the formationfeatures coherent terranes that reach 100’s m thickness and some are traceable for 10’s km along strike, althoughlocally diverse lithologic blocks are predominantly fault bounded, evidence that lithologic contacts are used as slipsurfaces. Towards the base of the formation, disruption intensifies, with coherent blocks reduced in thickness, strikelength and frequency. Shear and resultant melange development was concentrated in units with substantial shalecomponents, as visible in the outcrops analysed for this study and located in the Pasagshak Peninsula (see locationin Figure 4). Massive sandstone and volcanic blocks survive as mega- to micro-boudins with pinched tips andstructural signs of internal extension consistent with overall shear indicators. The basal melange of the Ghost RocksFormation is over 2 km thick and characterized by zones of extreme cataclastic shear (Figure 10c), even ifvolumetric shear at plate convergent rates is accommodated by pressure solution. Multiple cataclastic shearsurfaces have been observed within the basal melange, showing evidence of growth and reactivation duringdisplacement and demonstrating continued re-localization of shear surfaces even at the depth of underplating(Rowe et al. 2005).Although the Uyak Complex was not thoroughly analysed during this study, we report here a brief description of

the main deformations reconstructed in this Cretaceous unit by Moore (1978), Connelly (1978) and Byrne andFisher (1990). All these studies show how the reconstructed stratigraphy for this unit reveal a clastic sedimentarycomponent subordinated to the ocean floor sediments and basalts, indicative of a different thickness of thesubducting sediment section with respect to the Kodiak Formation. Fluid inclusion analyses also suggest, for thiscomplex, deformation under prehnite-pumpellyite metamorphic conditions (Vrolijk et al. 1988). The outcropsalong Uyak Bay (Figure 4) show how the stratigraphy of ocean floor basalts, pelagic cherts and subordinatedvolcaniclastic sandstones and shales is disrupted from metre- to centimetre-scale blocks in a shale matrix melange.The melange itself is imbricated by three northwest-dipping thrust faults. Progressive deformation, as reconstructedby Byrne and Fisher (1990), occur through several events of pervasive layer-parallel shear extension fracturing andcataclasis, with associated development of a penetrative S1 cleavage associated with asymmetric, southeast vergingstructures such as asymmetric folds and tails around rigid blocks. Deformation then evolves into more localized setsof extensional shear bands, still defined by the alignment of lensoidal-shaped blocks. This series of deformation isinterpreted as related to underthrusting. Imbrication mainly occurred through thrust faulting with associatedpressure solution cleavage and folding of the S2 cleavage.

5. FLUID MIGRATION AND MINERALIZATION ACROSS ANALYSED COMPLEXES

Conspicuous mineralization, in the form of complex systems of quartz- and calcite-filled veins, characterizes all thethree accretionary complexes. A brief description of the main features of the veins systems, contemporaneous to theunderthrusting- and underplating-related deformation events, follows.Consistency exists between vein systems across coherent units, while disrupted units show a different vein

distribution. Although the fluid budget is expected to be different in each unit, possibly depending on the amount ofclastic sediments carried to the system, in both the melange and coherent terranes the vein geometries and featuressuggest fluid flow associated with compaction, increasing cohesion and dewatering of shales. Moreover, all theanalysed examples, no matter the style of deformation, show evidence of high fluid pressure and episodic fluidpumping, according to what is widely observed in modern margins since the 1980s (Moore 1989; Gieskes et al.1990; Kastner et al. 1991; Moore and Vrolijk 1992; Shipley et al. 1995; Moore et al. 1995; Saffer and Bekins 1999;Bangs et al. 1999; Bourlange et al. 2003, Ujiie et al. 2003).


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5.1. Fluid flow across the coherent Gottero and Kodiak units

In the Gottero Unit, an important event of calcite vein development occurred prior to F1 folding (Figures 11–12, seeD1a phase of Marroni et al. 2004). This F1-folded vein system can be subdivided in two different sets (Meneghiniet al. 2007), distinguished by geometrical relationships with bedding.

The first set consists of thick crack-seal veins filling Mode I fractures with calcite and subordinate quartz. Theydevelop parallel to bedding (Figure 11a) for thicknesses up to 10 cm and lateral continuity of metres. The veinsdisplay tabular shapes and develop in the shaly intervals along the interface between sandstone and shales. In thinsection, they appear as crack-seal veins characterized by mosaic textures made up from irregular arrangements ofeuhedral calcite crystals of variable size (Figure 11b) and contemporaneous crystals of quartz. Vein boundaries arecontinuous and indented, without grain breakage involvement, possibly indicating that the sediments were weakenough for the veins to open by grain-to-grain separation, suggesting that compaction and cementation were poorlydeveloped. Moreover, veins infilling is ‘dirty’ due to the occurrence of variable sized inclusions of dark-green toblack shaly material from the host rock (Figure 11) As shown in Section 3.1, parallel to bedding and to thealternations of calcite and wall rock fragments is also the development of later folding-related pressure solution andS1 cleavage (Figure 11b), suggesting possible alternation of dilation and compaction in the same direction(Meneghini et al. 2007).

Layer-parallel veins are interconnected by a set of blocky veins normal or at high angles to bedding, developed inthe more competent layers of the turbidite sequences (‘V2’ in Figure 12) and branching from the layer-parallelsystem (Figure 12b). These veins, up to 5–6 cm thick, are generally arranged in two conjugate systems making anangle of 308–508 in a plane normal to bedding mean strike. These veins show a clear infilling with a mosaic fabricdefined by irregular arrangements of calcite crystals, up to 2–3 cm in size, with an almost total absence of wall rock

Figure 11. Pre-folding, layer-parallel veins in the coherent Gottero Unit: (a) Veins occur parallel to bedding at the sandstone/shale interface.Note typical crack-seal structure and dirty aspect due to incorporation of wall-rock material. Lens cap diameter is 4.5 cm. (b) Filling ischaracterized by blocky calcite (Cal) and quartz (Qtz). Development in poorly lithified sediments is suggested by dirty aspect of veins and by theirregular margins that follow grain boundaries, as visible in the lower left corner of image. Parallel to bedding (S0!S1) are also subsequent

pressure solution seams.


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particles in the veins (Figure 12b). This last observation, together with the occurrence of sharp vein walls, suggeststhat the coarser layers were fairly lithified and competent to deform in a brittle fashion.Syn-tectonic veins develop during the D1 phase of folding and incorporation in the Ligurian prism, as a result of

folding-related stretching (‘V3’ in Figure 12a), and as extensively described by Marroni and Pandolfi (1996) andMeneghini et al. (2007). These veins develop at high angles to bedding in the more competent layers and inconjugate sets making angles of 408–508 on a plane normal to bedding, striking preferentially in a directionapproximately parallel to the fold axis and concentrated on fold limbs. Veins are lozenge-shaped, generally 2–3mmthick with length normally less than the host rock layer thickness, although locally they are associated with beddingboudinage. Vein texture is fibrous, defined by antitaxial growth of calcite fibres with well-defined, discontinuous,dark median lines. Fibres direction represents an extension lineation oriented roughly normal to the fold axes (seeMeneghini et al. 2007, figure 5).Fisher (1996) has usefully summarized the vein systems recognized in both coherent and melange terranes of the

Kodiak Complex, during both underthrusting and duplex accretion. The turbiditic sequence of the KodiakFormation, as well as the disrupted turbiditic sequence of the Ghost Rocks Formation suffered an intense andcomplex veining history prior to folding and imbrication (Figure 13). Mineralization occurs as up to 1 cm thick,quartz- and calcite-filled extension veins in sandstone layers (see D1 phase of Fisher and Byrne 1987 or pre-D1phase of Sample and Moore 1987) and are always sub-perpendicular to bedding (Figure 13a). Vein texture is

Figure 12. Layer-normal veins in the coherent Gottero Unit: (a) Pre-folding veins (V2) occur subnormal to bedding in the sandstone layers andare clearly folded by isoclinal F1 folds. Also visible are thin veins on fold limbs (V3) due to fold-related limb stretching. Hammer is 30 cm. (b)Blocky calcite fills the pre-folding veins. This set of veins (V2) clearly originates from the layer-parallel system (V1) as suggested also by optical

continuity between vein fillings.


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typically blocky, with coarse-grained, undeformed quartz and intensely twinned calcite crystals. Fisher and Byrne(1987) noted that quartz might have been replaced calcite in many of these veins; a suggestion supported by ourobservations of solid calcite inclusions in quartz crystals. Veins appear dirty due to incorporation of matrix andwall-rock material and, as well as in the Apenninic example, cracking and fracturing occurred along grainboundaries rather than breaking through grains, suggesting an incomplete lithification of sandstones during veinformation. Fisher and Byrne (1987) also report the occurrence of quartz mineralized micro-normal faults, linkingpre-folding veins of different sand layers. These faults develop in the shale intervals at a low angle with bedding.They offset bedding and are in turn folded by duplex-related folds.

Two systems of quartz veins have been also recognized in the Kodiak Formation, as related to fracture-channelled fluid flow during duplex formation (Fisher and Brantley 1992; Fisher 1996). One system ischaracterized by bedding-parallel laminated veins with slickenlines and stepped margins with internal laminationdefined by long and thin stripes of quartz and locally calcite separated by dark layers of insoluble residues (seestriped veins of Koehn and Passchier 2000). These veins show evidences of dilation and collapse. A second set ofsteeper crack-seal veins allow interconnection with the low-angle system through quartz-filled hydrofracturescontaining crack seal bands of chlorite inclusions (see Fisher 1996, figure 3). Along the accretion-related shearzones, where scaly fabric and local disruption occur, thin calcite and quartz shear veins develop along scaly fabricsurfaces and sandstone boudins (Figure 13b). This vein system is particularly pervasive in the melange units of theKodiak Complex, and is described in the following paragraph. Fisher (1996) also provides a detailed description ofthis system.

5.2. Fluid flow across the Franciscan and Kodiak melange terranes

The zone of biggest concentration of discrete shear surfaces in the Rodeo Cove melange is also the locus ofextensive mineralization (Figure 14), with veins locally occupying the "80% of the outcrop area (Meneghini andMoore 2007).

The veins are found both in competent blocks and in the fine scaly matrix of the discrete slip surfaces. Two veintextures are generally recognizable, depending primarily on vein thickness. The two types of veins have grossly thesame geometries, are calcite and rarely quartz filled, but show different distributions and texture. The thickest veins(generally 1 cm thick) occur in the sandstone and basalt blocks of the melange and show a blocky texture defined byirregular arrangement of clear anhedral calcite crystals. They show variable thickness along strike, with sharp,pinched terminations and boudinage. The thinner veins (generally <5–6mm thick) develop along the discrete slipsurfaces, or in association with finer lithologies (Figure 14a), although, locally, they can be found in the highlydisrupted basalt blocks. They show well-developed fibrous texture with antitaxial straight, calcite fibres and

Figure 13. Mineralization in Kodiak Complex coherent and melange units: (a) Pre-folding, early veining in coherent Kodiak Formation.Penknife is 10 cm long. (b) Typical melange-related veining as found in Ghost Rocks Formation. Also found in thrusting-related disruption in

coherent Kodiak Formation. Pen is 15 cm long.


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scattered, irregular, dark median lines (Figure 14b) and a more continuous lateral extension, developing repeatedsets of parallel veins (Figure 14a). Both types of veins show sharp boundaries and ‘clear’ vein fillings, with a verylow percentage of wall-rock particles. Despite the slightly different distribution, thick and thin veins show similararrangements with respect to foliation and slip planes, lying parallel to both the anastomosing spaced foliation ofthe competent blocks and the S-planes in the scaly foliation of the matrix (Figure 14) and to discontinuous pressuresolution seams of opaque, residual minerals.Cross-cutting relationships between veins and foliation have been observed at meso- and micro-scale

(Figure 14b). Veins, as well as S-planes and pressure-solution seams, are often rotated and truncated by the C planesof scaly foliation. Moreover, veins also deform through isoclinal folding with tight limbs developing parallel to thediscontinuous chlorite layers that define the anastomosing foliation (see also Meneghini and Moore 2007, figure 8).Two generations of syn-deformational veins are recorded in the Ghost Rocks and Uyak melange units

(Figure 13). First of all, further extensional veins precipitate in boudin necks as boudins and are themselvesdisrupted and rotated as strata are dismembered during shearing. Moreover, quartz and carbonate blocky veins areobserved in the melange matrix as coatings on boudins and scaly fabric surfaces (Figure 13b). Similar to what wasobserved in the Rodeo Cove melange, and as summarized in Fisher (1996), veins are thicker within sand beds orblocks than in melange matrix. Different from the Franciscan case, but similar to the Apenninic complex, veinmargins are irregular and follow sand grain boundaries and infilling is ‘dirty’ due to floating chunks of wall rock.The same system of veins, as mentioned above, is observed also in the disrupted sheared portions of the KodiakFormation.Both types of veins suggest that dilation occurs at the interface between more-viscous and less-viscous

lithologies during melange deformation, consistent with what was observed in the Gottero Unit.The very high-strain cataclastic faults near the base of the Ghost Rocks melange contain only inherited veins and

no additional veins precipitated during cataclastic flow. This suggests that some deformation may not beaccompanied by significant fluid advection, or that the amount of vein material is not a direct reflection of thevolume of fluid migrating into the open fracture.

6. DISCUSSION

As previously mentioned in the Introduction section, many authors since the 1980s pointed out how the spatial andtemporal structural diversity between accretionary prisms is controlled by many different factors (e.g. von Huene1984; Moore and Sample 1986; Moore 1989; Taira and Pickering 1991; von Huene and Scholl 1991; Kusky et al.1997; Clift and Vannucchi 2004). Some of these studies have particularly shown the influence in active margins of

Figure 14. Syn-melange vein system in the Marin Headlands Terrane: (a) Shear zone melanges are characterized by impressive calcite-infusedveins that run parallel to S-foliation. Pencil is 15 cm long. (b) Veins and pressure solution (ps) seams are parallel to S-foliation and are in turn cut

and deformed by left-inclined C-planes.


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subducting sediment and fluid budget in controlling the hydrogeology of both prism and decollement zone,demonstrating the feedback between sediments, their physical properties, fluid circulation and deformation into thewedge during both underthrusting and shallow accretion. Von Huene and Scholl (1991) and, more recently, Cliftand Vannucchi (2004) have shown the positive correlation between thickness of sediment on the subducting plate atthe trench axis and the magnitude of accretion. Moreover, von Huene and Scholl’s (1991) mass balance acrossselected active accretionary margins illustrates how, even in margins bordered by large prisms, only 20–30% of theincoming sediment is offscraped, while the 70–80% is subducted beneath the wedge to be either underplated atdepth or recycled to the mantle. Although one could reasonably conjecture that a high sediment supply would resultin a higher degree of offscraping, data from modern margins indicate that there is still a high volume of sedimentsavailable to be accreted at depth. Accordingly, it is shown by Clift and Vannucchi (2004) that the rate of accretion isroughly inversely related to the rate of convergence, because faster rates of convergence result in less sedimentationin the trench, thus inducing the same effect, in thickness of sediment available for underplating, as a sediment-starved lower plate.

With the proposed study we intend to extend these observations to underplating depths, testifying the complexfeedback between deformation styles, dewatering history and fluid migration in the selected fossil analogues,spanning from coherent (Ligurian and Kodiak units) to totally disrupted (Franciscan and Kodiak units) style andcharacterized by different thicknesses of clastic sediments carried by the lower plate. In Table 1 are summarized themain characteristics of both deformation and fluid flow in each of the analysed units.

6.1. The coherent transfer of sediment-rich sections

In the example of the Northern Apennines the reconstructed stratigraphic section entering the subduction zone ismore than 2 km thick. For this sector of Apenninic subduction, Marroni and Treves (1998) have calculated anextremely low convergence rate of around 0.5 cm/year. The Kodiak Formation is characterized by a deformedmonotonous sequence of turbidite deposits for which calculations of Sample and Reid (2003) have estimated across-sectional area of 876 km2, suggesting an enormous sediment input to the subduction system.

As classically modelled, the decollement zone initiates along low strength layers, generally represented bylithologic boundaries in the lower plate stratigraphy (Moore 1989; Le Pichon et al. 1992). As shown by drilling dataon modern margins, one designated site is the boundary between trench fill deposits and muddier hemipelagicdeposits (Moore 1989).

The dominantly coherent style of the two units seems to suggest that when thick sediments are available, thedecollement follows specific weak sedimentary layers detaching coherent thick sections from the lower plate in theform of duplex where polyphase deformation occurs through development of regional-scale folding and thin thrustszones (phases of folding, cleavage and thrusting, for both analysed terranes, Figure 15a). Both analysed coherentunits show how the presence of sediments, which convey a large volume of fluids to the trench, allows compactionand diagenetic processes to strongly control and influence dehydration and deformation along the plate boundary(Fisher 1996 and references therein; Meneghini et al. 2007). The fluid-induced deformation progressively changeswith variations in lithification degree and competency contrast as sediments reach deeper levels of the prism(Figure 15a).

In the Kodiak Formation, early deformational fabrics and veins demonstrate that compaction and bedding-parallel extension contributed to volumetric strain and dewatering. Most dewatering occurs prior to folding in theform of the early stage fluid pathways produced by bedding-parallel extension (Figure 15a), locally interconnectedby veins in shales at low angle with bedding. Thewell-bedded strata facilitated the development of localized shears.These shears propagated across strata to form duplexes, resulting in the rotation and folding of the sediments(Figure 15a). The rheologic effect was strain hardening of duplex horses, effectively protecting them fromsubsequent deformation. Because the sediment pile was so thick, weak, dewatering, intact strata were continuouslysubducting and available to develop new shear zones. The net effect was the migration of shear through jumpingaround coherent terranes, in a downward-younging sequence of narrow, discrete faults.


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Similarly, the Gottero Unit features a complex system of interconnected hydrofractures demonstrating, as shownin the model of Meneghini et al. (2007), that most sediment dewatering occurs prior to folding and incorporationinto the prism and develops through layer- and decollement-parallel fractures interconnected by high anglefractures (Figure 15a1). Hydrofractures occur in response to cyclic pulses of high pore pressure generated duringenhanced sediment compaction and lithification and in both units they develop at first in poorly lithified sediments.During progressive diagenesis, a difference in lithification state occurs between shales and sandstones, allowing theformer to be more ductile than the latter, which display brittle behaviour. This difference and the building of furtherhigh pore pressure causes upward fluid flow and layer-normal hydrofracturing of the sandstones, resulting in aramp-flat geometry of fluid pathways (Figure 15a).In both the Gottero and Kodiak units, long-distance dewatering is shut off as volumetric strain in the form of folds

and pressure solution cleavage begins to dominate. Subsequent deformation occurs by the development of smallthrust faults, duplexing, and sequential underplating. There is little evidence for fluid advection following thisstage. In fact, Fisher and Brantley (1992) document closed system vein formation dependent on far-field forcing,and Meneghini et al. (2007) report on folding-induced vein development focussed only on fold limbs as a result oflimbs stretching. This is evidence that the fluid flux of the subduction system migrated elsewhere following bothcoherent terranes underplating.

Table 1. Summary of main characteristics of underthrusting- and underplating-related deformation and fluid flow in eachselected unit.

AnalyzedUnit

Clastic vs. “oceanic”sequence

ratio

Structuralstyle

Deformationstructures and

fabricsGeometry of vein network Main characteristics of veins

Gottero U. (Ligurian

Apennines)

Kodiak Fm (Kodiak

Complex)

MarinHeadlands

Terr. (Franciscan

Complex)

Ghost Rocks Fm (Kodiak Complex)

Uyak Complex (Kodiak

Complex)

>> 1

>> 1

+/- 1

+/- 1

>/= 1

Coherentfolded and thrust unit

Coherentfolded and thrust unit

Duplex of thick mélange zones

isolatingrelatively

coherent lenses

Duplex of thick mélange zones

isolatingrelatively

coherent lenses

Thick mélange zones with

localized shear cataclastic zones

Folding, thrusting, pressure solution,

slaty cleavage

Folding, thrusting, pressure solution,

slaty cleavage

Boudinage,cataclastic flow, S-C structures,

scaly fabric, pressure solution





Pre-folding Fold-related Pre-folding

Pre-foldingPre-folding

Fold-related

Fold-relatedFold-related

Inter-connected extensional to shear veins at high and low

angle with bedding


angle with bedding


angle with bedding

Extensionveins in limbs

Blocky calcite and quartz. “Dirty”

infilling with host rock inclusions.

Crack-seal



Crack-seal



Crack-seal

Fibrous calcite. Crack-seal.

Inclusion free

Hydrofractures along scaly folia and cracking around clasts



Blocky to fibrous calcite. Crack-seal




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Figure 15. Schematic cartoons showing different deformation style and fluid migration during underthrusting and underplating of oceanic crustbearing a different thickness of sediments. (a) When a thick sediment pile is available, deformation is dispersed on big volumes resulting in afold-and-thrust belt style of accretion which preserve internal stratigraphic and structural coherency of units. Fluids migrate along dilatantfractures corresponding to the weak sedimentary layering. (b) When high shear strain at decollement is accommodated on thin veneer ofsediments, deformation occurs through progressive activation of multiple shear zones, resulting in disruption of stratigraphy and melange

formation. Fluid migration is fault-controlled occurring along the shear zones themselves.


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6.2. The transfer of disrupted sediment-starved sections

The lack of sediments in the oceanic sequence around the Rodeo Cove Thrust is reflected in the unit’s stratigraphywith a small volume of turbiditic sandstones compared to basalt and pelagic chert sequences. Although only thincoherent slices were accreted, the decollement cut deeply into basalts even at relatively shallow levels (8–12 km).As a consequence, most of the thick shear zones in the terrane have concentrated within basalts. Similarly, in theGhost Rocks Formation and Uyak Complex, the appearance of oceanic crustal lithologies in the melange suggeststhat the decollement thrust system was active through the subducting sedimentary pile into the upper oceanic crust.Therefore, where weak sedimentary layers are absent, the porous hydrated basaltic upper crust may represent theweakest layer in the subducting pile for the decollement to develop.All these units are characterized by a disrupted appearance and a block-in-matrix fabric (Figure 15b). Although

at a regional scale the units all show a duplex structure, the internal coherency of the different sheets is almost lostand they are bounded by thicker thrust-bounding melanges compared to the coherent sections. This was alreadyreported by Byrne and Fisher (1990) for the Kodiak Complex, and extensively described in the Shimanto Complexof Japan (Ujiie 2002 and references therein). In both cases, this is interpreted as related to the high shear strain thatthe sequences experience during underthrusting below the decollement and final transfer to the prism. The fact thatthe oceanic crust brings a thin veneer of sediments make this deformation pervasive down to the top of the crustitself, as already postulated in the so-called ‘diffusive underplating’ by Moore and Sample (1986). The shearingoccurs essentially through localization of deformation into discrete shear zones, which migrate through space intime as demonstrated in the basal Ghost Rocks melange (Figure 15b). Deformation develops essentially throughcataclastic flow, which alternates with intragranular particulate flow and pressure solution, determining a cycling ofdeformation mechanisms, as shown in detail by Meneghini and Moore (2007). The alternation is controlledessentially by the changes in mechanical properties that each mechanism determines. Cataclastic flow causesdilation and loss of cohesion, therefore triggering intragranular particulate flow mechanisms (Knipe 1986). Thisfavours circulation of fluids and pressure solution phenomena, which in turn seals the previous structures and allowsan increase in lithification (Figures 15b1–b3). The cycling is accompanied by fluid migration and high porepressure fluctuations parallel to the shear zones (Figure 15b, see also Meneghini and Moore 2007). The result is analternating fracture permeability with the same fracture that transiently dilate and collapse, as demonstrated bycross-cutting, co-planar events of veining and scaly fabric development (Figures 15b1–b3). In each cycle theepisodes of high pore pressure cause hydraulic fracturing and permeability increase allowing fluids to move intonew open fractures or into old weak surfaces. The subsequent mineralization in open spaces increases cohesion,strengthens the fault surfaces and favours a new cycle of cataclastic flow and scaly fabric development, determiningstrain hardening and thickening of the melange zone (Moore and Byrne 1987) until a slice of it is transferred to theprism to form duplexes (Byrne and Fisher 1990; see also the regional reconstruction of Wakabayashi 1992, 1999).Localization of deformation and heterogeneous bulk shear during underthrusting and underplating have both

been widely described also in other accretion-related melanges such as the Shimanto Complex of Japan (Ujiie 2002and references therein).

6.3. Structural diversity, fluid pathways and subducting plate features: an attempt to summing up observations

As extensively shown by numerous studies in both the Kodiak (Byrne and Fisher 1990 and references therein) andShimanto complexes (Ujiie 2002 and references therein) a general tectonostratigraphy seems to be common in theunderthrust section of a subducting margin, featuring, immediately below the decollement, a tectonic melange (thedirect response to the decollement-related shear) that grades down-section into structurally more coherentsequences. Disruption is a mechanism that starts during early underthrusting, progressively thickening the melangezone as underthrusting proceeds (Moore and Byrne 1987). Since melange thickness seems to correlate linearly withdisplacement (Wojtal and Mitra 1988), at a given depth, the thickness of melange section is the same and theproportion between coherent and disrupted material depends only on how much of the sediments of the subductingplate is underthrust. Therefore, if only a thin veneer of sedimentary cover is underthrust, there will be a given depth,


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or a given amount of displacement, great enough to convert all the section into a melange, possibly includingfragments of oceanic crust as in the Franciscan and disrupted units of the Kodiak Complex. If the underthrustsequence includes several kilometres of turbidites on top of the pelagic section, as in the Kodiak Formation andGottero Unit, at the same depth the coherent versus melange ratio will be larger than in the previous case, with ahigher percentage of coherent sections to be transferred to the prism.

The depicted structural diversity can be observed even in a single margin through time. The Kodiak Complex ofAlaska is characterized by a dramatic change in accretion style from the folded, coherent turbidites of the KodiakFormation to the melange of the Ghost Rocks Formation and Uyak Complex. Kusky et al. (1997) reported a similarvariation in deformation style from melange to coherent units in the along-strike equivalent units of the ChugachTerrane of the Kenai Peninsula inland in Alaska. The authors interpreted theWrangellia collision to have caused therapid deposition, on the Farallon lower plate, of a huge submarine fan, whose subduction and accretion would havecaused the rapid growth of the prism and the dramatic change in a more coherent style. The Kodiak Formation wasdeposited during the Maastrichtian, in only four million years. Following Sample and Reid (2003) the estimateddepositional rate is comparable to that of the Bengal-Nicobar fan system, composed of erosional products of theHimalayan uplift. So, the coherent style of accretion of the Kodiak Formation can be interpreted as the periodicconsequence of this filling of the trench by turbidite sediments. In contrast, the melanges of the Ghost RocksFormation and Uyak Complex locally include pillow basalts interbedded with sediments (Connelly 1978; Byrne1982, 1984), then representing episodes in which the decollement cut at low stratigraphic and structural level in theoceanic sequence.

The collected data also testify the complex feedback between style of the coherent versus melange units,thickness and mechanical condition of the subducting plate, and fluid flow regimes in terms of fluid volume, flowgeometries and mechanisms of opening and closure.

In a thick sequence of coherent subducting sediment, fluids must migrate and flow within the thick slices ofsediment to reach the unit-bounding thrusts. Therefore, fracture-channeled fluid flow develops in bedding-parallelshear zones, stepping upward by means of the development of high-angle structures. As a result, fluids transitioningfrom gradual scaly pathways in shales to joints in sandstone encounter a low-pressure zone and veins areprecipitated. Bedding surfaces also create fluid pressure horizons. This style of vein formation exists in all thedescribed units as an early stage during subduction, at varied density. In all cases, up-dip transport of fluids is mostefficient by bedding-parallel flow at shallow levels. This tendency is confirmed by the development of regionaldilated hydrofractures along and parallel to the decollement zone that has been postulated after studies on modernprisms (e.g. Brown et al. 1994; Bourlange et al. 2003 and references therein).

At the other side, the thick and numerous shear zones of the melange units probably offer the best way for thefluid to flow up to the surface, representing possibly a more efficient dewatering vehicle.

The subduction of bedded sediments is initially conducive to fluid flow, but after the onset of volumetric strain byfolding and/or pressure solution, the sediment package becomes an impermeable block that is more prone tolocalized thrusting.

In all cases, and extending what was already summarized by Fisher (1996) for the sole Kodiak example, there isevidence that fluid flow is episodic and cyclic with the fracture and fault system behaving as either a conduit or abarrier for fluid flow, thanks to the feedback between pore fluid, pressure, hydraulic fracture and fluid flow insediment that while theymove to depth change continuously diagenetic and mechanical conditions (see Knipe et al.1991).

7. CONCLUSIONS

An investigation of accretion-related deformation and structural style in three different fossil accretionary wedgesdepicts similar depth of accretion but strikingly different sediment budgets at the time of subduction.

The abundance of stratally disrupted sequences in the ‘sediment-starved’ examples demonstrates that shearstrain experienced by the thin sequence underthrusting below the decollement is great enough at that depth to


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convert all bedded sequences into melange. In contrast, the coherent, thin thrust-bounded type of accretion recordedin the Ligurian and Kodiak units suggests that when the oceanic crust is covered by a thick sedimentary sequence itexperiences strains that are distributed on many slip surfaces over wide volumes. These observations fit well withwhat already was postulated previously (e.g. Moore and Sample 1986; Kusky et al. 1997).Early deformation during subduction is related to the development of fluid escape pathways. Hydrofracturing

occurs through development of a system of dilatant fractures parallel and perpendicular to the decollement zonethat experiences cyclic dilatancy and collapse. The sediment budget at subduction margins influences the interplaybetween this fluid flow and deformation. In the case of accretion controlled by wide shear zones the dewatering isessentially fault-controlled, while in sediment-rich examples fluids follow the sedimentary layers until beforereaching the unit bounding shear zones.

ACKNOWLEDGEMENTS

The authors are grateful to Don Fisher and Kotharo Ujiie, who both provided critical and helpful reviews.This work was supported by MIUR PRIN 2005 Grants to Professor M. Marroni, together with all the field

campaigns in the Apennines by M. Marroni, L. Pandolfi and F. Meneghini. Travel back and forth to the U.S. by F.Meneghini, as well as field campaigns in Kodiak Island by JC. Moore, C. Rowe and F. Meneghini have beensupported by National Science Foundation Grants OCE-0549017 to Professor J.C. Moore. Alex McKiernan, AkitoTsutsumi and Asuka Yamaguchi are thanked for logistic and scientific support to the 2004 and 2006 campaigns inKodiak. Rolan Ruoss from SeaHawk Air of Kodiak supplied transportation in Kodiak Island. Fruitful informaldiscussions in the past years with Giancarlo Molli, Gianni Musumeci, Gian Andrea Pini, John Wakabayashi, DonFisher and Tim Byrne greatly improved some of the proposed ideas.

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