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Geol Rundsch (1994) 83: 832-852 © Springer-Verlag 1994

J. H. Behrmann S. D. Lewis S. C. CandeODP Leg 141 Scientific Party

Tectonics and geology of spreading ridge subductionat the Chile Triple Junction: a synthesis of resultsfrom Leg 141 of the Ocean Drilling Program

Received: 18 May 1994/Accepted: 5 August 1994

Abstract An active oceanic spreading ridge is beingsubducted beneath the South American continent at theChile Triple Junction. This process has played a majorpart in the evolution of most of the continental marginsthat border the Pacific Ocean basin. A combination ofhigh resolution swath bathymetric maps, seismic reflec-tion profiles and drillhole and core data from five sitesdrilled during Ocean Drilling Program (ODP) Leg 141provide important data that define the tectonic, structuraland stratigraphic effects of this modern example ofspreading ridge subduction.

A change from subduction accretion to subductionerosion occurs along-strike of the South American fo-rearc. This change is prominently expressed by normalfaulting, forearc subsidence, oversteepening of topogra-

phic slopes and intensive sedimentary mass wasting,overprinted on older signatures of sediment accretion,overthrusting and uplift processes in the forearc. Datafrom drill sites north of the triple junction (Sites859 — 861) show that after an important phase of forearcbuilding in the early to late Pliocene, subduction accretionhad ceased in the late Pliocene. Since that time sedimenton the downgoing oceanic Nazca plate has been subduct-ed. Site 863 was drilled into the forearc in the immediatevicinity of the triple junction above the subducted spread-ing ridge axis. Here, thick and intensely folded andfaulted trench slope sediments of Pleistocene age arecurrently involved in the frontal deformation of theforearc. Early faults with thrust and reverse kinematicsare overprinted by later normal faults.

J. H. Behrmann (E3)Institut für Geowissenschaften und Lithosphárenforschung,Justus-Liebig-Universitàt Giessen, Senckenbergstrasse 3,D-35390 Giessen, Germanyemail: Jan.Behrmann@geo.uni-giessen.de

S. D. LewisUS Geological Survey, MS 999, 345 Middlefield Road,Menlo Park, CA 94025, USA

S. C. CandeScripps Institution of Oceanography, University of California, SanDiego, La Jolla, CA 92093, USA

ODP Leg 141 Shipboard Scientific Party: J. H. Behrmann (Co-ChiefScientist), Institut für Geowissenschaften und Lithosphárenfor-schung, Universitàt Giessen, Senckenbergstrasse 3, D-6300 Gies-sen, Germany; S. D. Lewis (Co-Chief Scientist), Branch of PacificMarine Geology, US Geological Survey, MS 999, 345 MiddlefieldRoad, Menlo Park, CA 94025, USA; R. Musgrave (Staff Scientist),Ocean Drilling Program, Texas A & M University Research Park,1000 Discovery Drive, College Station, TX 77845-9547, USA; N.Bangs, Institute for Geophysics, 8701 Mopac Boulvard, Austin, TX87859, USA; P. Bodén, Department of Geology and Geochemistry,Stockholm University, 106-91 Stockholm, Sweden; K. Brown,Scripps Institution of Oceanography, University of California, SanDiego, La Jolla, CA 92093, USA; H. Collombat, Mission OR-STOM, Apartado postal 17-11-06596. Quito, Ecuador; A. N.Didenko, Institute of Physics of the Earth, USSR Academy ofScience, st. Bolshaya Gruzinskaya 10, Moscow 123810, Russia;

B. M. Didyk, Empresa Nacional del Petróleo, Refineria de Petróleosde Concón SA, Casilla 242 Concon, Chile; P. N. Froelich, Lamont-Doherty Geological Observatory, Columbia University, Palisades,NY 10964, USA; X. Golovchenko, Lamont-Doherty GeologicalObservatory, Palisades, NY 10964, USA; Randy Forsythe, Depart-ment of Geography & Earth Sciences, University of North Carolina/Charlotte, Charlotte, North Carolina 29223, USA; V. Kurnosov,Geological Institute, USSR Academy of Sciences, Pyzhevsky Per., 7,Moscow 109017, Russia; N. Lindsley-Griffin, Department of Geo-logy, 214 Bessey Hall, University of Nebraska, Lincoln, NE68588-0340, USA, K. Marsaglia, Dept, of Geological Sciences,University of Texas at El Paso, El Paso, TX 79968-0555, USA;S. Osozawa, Institute of Geology and Paleontology, Faculty ofScience, Tohoku University, Aoba, Sendai, 980, Japan; D. Prior,Department of Earth Sciences, Liverpool University, Liverpool L693BX, UK; D. Sawyer, Department of Geology and Geophysics,Rice University, PO Box 1892, Houston, TX 77251, USA; D. Scholl,US Geological Survey, MS 999, 345 Middlefield Road, Menlo Park,CA 94025, USA; D. Spiegler, GEOMAR, Wischhofstrasse 1 -3 ,D-2300 Kiel 1, Germany; K. Strand, Department of Geology,University of Oulu, Linnanmaa 90570 Finland; K. Takahashi,Department of Geology and Geophysics, Woods Hole Oceanogra-phic Institution, Woods Hole, MA 02543, USA; M. Torres,GEOMAR, Wischhofstrasse 1 - 3 , D-2300 Kiel 1, Germany;M. Vega-Faundez, Departamento Ciencias Geológicas, Universi-dad Católica del Norte, Antofagasta, Chile; H. Vergara, Departa-mento de Oceanografia, Servicio Hidrografico de la Armada, Casilla324, Valparaiso. Chile; A. Waseda, J APEX Research Center, 1-2-1Hamada, Chiba 260, Japan

(Reprinted by permission from Geologische Rundschau, 83:832-852, 1994, Springer-Verlag)

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The Chile Triple Junction is also the site of apparentophiolite emplacement into the South American forearc.Drilling at Site 862 on the Taitao Ridge revealed anoffshore volcanic sequence of Plio-Pleistocene age asso-ciated with the Taitao Fracture Zone, adjacent to expo-sures of the Pliocene-aged Taitao ophiolite onshore.Despite the large-scale loss of material from the forearc atthe triple junction, ophiolite emplacement produces alarge topographic promontory in the forearc immediatelyafter ridge subduction, and represents the first stage offorearc rebuilding.

Key words Spreading ridge subduction Chile TripleJunction ODP Leg [4] Tectonics • Pacific Ocean • SouthAmerica Chile forearc ophiolite sediment deforma-tion

Introduction

The accretion of sediment during subduction at oceanictrenches is an important process in the evolution ofcontinental margins that can lead to mountain buildingand the growth of continents. Since the origin of ancientmelange terranes such as the Franciscan assemblage ofthe California Coast Ranges (Bailey et al. 1964) wasrecognized to be related to subduction accretion (Hsü1971), understanding the processes and mechanisms thatare active during the formation of accretionary wedgesand forearc terranes has been the focus of many researchprograms, both on land and offshore. The aim ofstudying the tectonic and hydrogeological processesassociated with the deformation and accretion of sedi-ment has motivated ODP drilling in the Barbadosaccretionary wedge (Leg 110; Mascle et al. 1988) and,more recently, in the Nankai Trough (Leg 131; Taira et al.1991) and the Cascadia convergent margin (Leg 146;ODP Leg 146 Scientific Party 1993).

As the number of trench — forearc systems that havereceived careful study has grown, it has become evident thatmany convergent margins have undergone the removal offorearc material through both processes of subductionerosion (Honza et al. 1989; von Huene and Culotta 1989;von Huene and Lallemand 1990; Hibbard and Karig 1990 a,b) and by the lateral motion of terranes by strike-slipfaulting (Howell 1980). Quantitative estimates of the rockmasses subducted or accreted at active plate margins areusually made under the assumption of long-term steady-state behaviour (von Huene and Scholl 1991) of the platemargin. Temporal and spatial changes are undetected, yetthey are critical in shaping the architecture of forearc regionsand in determining the history of mountain building andmountain destruction. One important mechanism to bringabout a change from subduction accretion to subductionerosion is the impact of topographic features, such asseamounts, aseismic ridges and active oceanic spreadingridges (e.g. von Huene and Scholl 1991; Cloos 1993) intoa convergent margin.

Ridge —trench collisions have punctuated the subduc-tion history around the Pacific Ocean (Grow and Atwater

1970; Atwater 1970; Uyeda and Miyashiro 1974) and area potentially important mechanism of ophiolite emplace-ment. The principal geological consequences are rapiduplift and subsidence of the margin, regional metamor-phism with high geothermal gradients, a cessation of arcmagmatism and anomalous magmatic activity near theoceanic trench. With the passage of the spreading ridgebeneath the continent, there is extensional deformation,collapse of the margin and perhaps anomalous backarcvolcanism.

One important effect of spreading ridge subduction arechanges in the style of forearc deformation. To maintaina critical taper (Davis et al. 1983; Platt 1986) the rocks inan accreting forearc wedge will be shortened horizontally.Structural patterns of frontal imbrication or out ofsequence overthrusting, folding and distributed horizon-tal ductile shortening will result (e.g. Needham and Knipe1986; Brown and Westbrook 1987; Behrmann et al. 1988;Lewis et al. 1988; Needham 1993). If material is removedfrom the front of the forearc and moved down into thesubduction zone attached to the downgoing plate, thewedge will be oversteepened and will regain its criticaltaper by horizontal extension. Near-surface normal fault-ing, as recorded in the forearc off the Pacific coast ofCentral America (Aubouin et al. 1984; Mclntosh et al.1993), and megascopic slumps, as seen off the coast ofPeru (von Huene et al. 1985 a, 1989; Bourgois et al. 1993)are two obvious phenomena generated in this process. Wehave to note, however, that only the frontal part ofa forearc wedge has Coulomb-type rheological beha-viour, and those parts underlain by thick continental orarc basement may respond in a very different fashion.

A second effect of spreading ridge subduction isvertical motion in the forearc. Removal of material fromthe front or the base causes subsidence. Frontal imbrica-tion and large-scale underplating (cf. Moore et al. 1991)causes uplift. A record of vertical motions may bepreserved in paleo-bathymetric data from benthic for-aminifer assemblages both in accreted and slope sedi-ments.

The impact of a spreading ridge with a continental forearccan produce a distinct thermal pulse as the hot, young andbuoyant oceanic crust is subducted beneath the continentalmargin (Marshak and Karig 1977; DeLong and Fox 1977;DeLonget al. 1979; Thorkelson and Taylor 1989). Theoreti-cal models (DeLong et al. 1979) and heat flow determina-tions from the southern Chile margin (Cande et al. 1987)suggest that heat flow within a few kilometers of the trenchincreased to 300 — 400 mW m~2, indicating a geothermalgradient roughly eight times higher than normal. Furtherlandward on the continental slope heat flow is lower, but theinferred geothermal gradient is still increased from normalvalues by a factor of two. Consequently, substantialvariations in the thermal regime are present across theforearc in the region of ridge subduction. Apatite fissiontrack data and the thermal maturity of organic carbon in theforearc sediment can be used to quantify thermal overprin-ting and diagenesis. Any differences between the instanta-neous geothermal gradients determined from borehole

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Fig. 1. Tectonic —geographical sketch map of the Chile TripleJunction region. The locations of ODP Sites 859 — 863 are shown

measurements and those inferred from the state ofdiagenesis in the recovered sediment can be used to detectnon-steady-state thermal conditions in the forearc.

The objective of this paper are to review the regionaltectonic setting of the Chile Triple Junction and tosummarize and synthesize geophysical, structural, sedi-mentological and paleontological data from shipboardand post-cruise research related to Leg 141 of the OceanDrilling Program. The aims of the drilling were toelucidate the tectonic and geological consequences of thesubduction of an active oceanic spreading ridge beneatha continental forearc. We discuss the implications of thesedata for models of the along-strike changes from subduc-tion accretion to subduction erosion, for the interpreta-tion of sediment distribution patterns and for models ofophiolite emplacement into forearcs.

Three drill sites occupied during Leg 141 of the OceanDrilling Program form a downdip transect across theforearc about 40 km north of the Chile Triple Junction(Fig. 1). Site 859 was drilled about 4 km landward of thetoe of the trench slope, with Sites 860 and 861 5 and 12 kmfurther landward, respectively. Site 862 was drilled nearthe crest of the Taitao Ridge to investigate a setting ofpotential ophiolite formation and emplacement, and Site863 was drilled into the toe of the landward trench slopeimmediately above the subducted ridge axis.

Regional setting

Plate kinematic and tectonic framework

The Chile Rise spreading ridge is being subducted beneaththe Peru — Chile trench convergent margin in the region ofthe Taitao Ridge and the Golfo de Penas at about 46° S.latitude (Fig. 1; Herron et al. 1977,1981). The three platesthat define the Chile Triple Junction are the Antarctic plate,the Nazca plate and the South American plate. The Nazcaand Antarctic plates are moving away from each otheracross the Chile Ridge spreading center at a rate of about60 — 70 mm/a (Chase 1978) and the continental SouthAmerican plate is overriding both along the Peru —Chiletrench convergent plate margin. The Nazca plate is beingsubducted beneath South America in a direction slightlynorth of east at a rate of about 80 — 90 mm/a north of thetriple junction. The Antarctic plate is being subducted ina direction slightly south of east at a rate of about 20 mm/asouth of the triple junction. Thus there is a substantialchange in the plate convergence rate on either side of thetriple junction, but the direction of plate convergencechanges very little (Pilger 1978; Cande and Leslie 1986).

The Chile Ridge first collided with the Chile Trenchabout 14 Ma ago near the latitude of Tierra del Fuego(Cande and Leslie 1986). Since then, the overall motion ofthe triple junction has been northward migration alongthe margin. However, the relative orientations of theChile Ridge, the fracture zones associated with the ChileRidge and the Chile trench result in rapid northwardmigration of the triple junction when the ridge itself isbeing subducted, and slow southward migration whena fracture zone is being subducted. A long ridge segmentwas subducted between Tierra del Fuego and the Golfo dePenas from 14 Ma to 10 Ma; the triple junction reachedthe Golfo de Penas about 6 Ma and another ridgesegment reached the region of the Taitao Peninsula about3 Ma. The segment of the Chile Ridge between theDarwin Fracture Zone to the north and the TaitaoFracture Zone to the south is now being subducted. Thetriple junction is presently in a ridge — trench — trenchconfiguration (McKenzie and Morgan 1969), with theactual triple junction now located at 46° 12' S.

Patagonian forearc

The basement of the Pacific continental margin in theSouthern Andes of Chile consists partly of metasedimen-tary and metavolcanic rocks of probable Paleozoic age(e.g. Pankhurst et al. 1992). These rocks were intruded byCretaceous and Tertiary, predominantly acidic, I-typeplutonic rocks of the Patagonian Batholith. The meta-morphic rocks were mainly formed from pillow lavas,radiolarites, limestones and siliciclastic turbidites (e.g.Hervé et al. 1981). Glaucophane-bearing rocks have beenrecognized in the area (Hervé et al. 1987), indicating thatthese rock associations may have formed part of pre-Jurassic subduction complexes. The continental base-ment has encountered a stepwise exhumation during the

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Tertiary and Quaternary. On the Tres Montes Peninsulasouth of the Chile Triple Junction (Fig. 1), an ophiolitecomplex of Pliocene age was emplaced into the forearcand intruded by granodioritic intrusive rocks very close tothe Chile Trench (e.g. Mpodozis et al. 1985; Keadinget al. 1990). Ophiolite accretion and anomalous near-trench magmatism most likely relate to the passage of theChile Triple Junction about 3 Ma at the latitude of theTres Montes Peninsula.

Architecture of the Southern Chile trenchand forearc around 46° S: evidence from bathymetryand seismic reflection investigations

The collision zone of the Chile Rise and the Southern Chilecontinental forearc is shown in the processed image ofSeaBeam bathymetry data (Fig. 2). The subducting Nazcaand Antarctic plates show low relief in water depthsexceeding 3 000 m. The rough topography of the spreadingaxis between the Darwin and Taitao fracture zones depictsa series of small volcanic centers. On the western side of thespreading ridge a series of NNW trending normal faults withclear scarps is seen. One of the normal faults appears totruncate a seamount, half of which sits atop the upliftedwestern block adjoining the fault. The overriding SouthAmerican forearc displays extremely irregular seafloortopography. In the southern part a deep submarine canyoncrosses the slope from the outer continental shelf anddebouches onto the trench floor at the base of thecontinental slope. Small fault scarps are present along thetrench slope. Most are discontinuous along-strike, traceablefor only 1 — 5 km. The faults typically strike north — south,parallel to the regional bathymetric trend of the continentalmargin, and not parallel to the structural fabric of thespreading ridge or the subducting oceanic crust. The mostprominent exception to this pattern is the large normal faultscarp present about 5 km landward of Site 863 (Figs 2 and3). This fault scarp has accommodated about 1 200 m ofapparent down to the trench relative motion of the westernblock and is oriented roughly parallel to the subductedspreading ridge. The bathymetric expression of the trench isinterrupted south of the triple junction by the Taitao Ridge(Fig. 2). This is a north-east trending submarine mountainrange rising almost 2 000 m above the surrounding abyssalplain of the Antarctic plate. South of the Taitao Ridge, thetrench does not become a well defined morphologicalfeature until the latitude of the Golfo de Penas (Fig. 1).

Reflection seismic data from the collision zone reveallarge differences in the structure of the overriding forearcalong the strike of the Chile Trench. In the following, linedrawings of seismic profiles 745, 750, 751 and 762 (Fig. 2)and their geological interpretations will be discussed. Theoriginal data were recorded on cruise RC 2901 of R/VConrad in 1988 and, with the exception of Line 750, werepublished by Bangs et al. (1992).

Along Line 745, collision of the Chile Ridge with theSouth American forearc will occur in about 100000 years.At present, the rift valley lies outboard of the trench axis

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towards the West (Fig. 3). The top of the basaltic oceaniccrust, offset by numerous normal faults (see also Fig. 4), istraceable as a strong band of reflectors for about 15 kmbeneath the forearc. It is covered by a thin layer of sedimentsthat is currently being underthrust. The toe region isinterpreted as a small accretionary prism, with a signature ofsmall, discontinuous, but generally landward dippingreflectors that could represent boundaries of lithologicalunits or thrust faults. The accreted rocks appear to beoverlain by a thin (about 200 m) sequence of seismicallytransparent slope sediments (Fig. 4). The accretionarywedge can be traced beneath a reflector that is interpreted tobe the top of the South American continental basement. Thecontinental basement is overlain by a sequence of sedimentsup to 1 500 m thick, locally offset by West-dipping normalfaults. A well developed bottom-simulating reflector (BSR)appears at a depth of about 200 m on the middle trenchslope and can be traced westward, shallowing to about100 m near the trench. The different depth to the BSR isthought to represent marked regional differences in heatflow (Cande et al. 1987). An extensive description andinterpretation of this BSR is given in Bangs et al. (1995).

Line 750 (Figs 2 and 3) crosses the trench axis at thepresent collision site of the spreading ridge. A prominentbasement reflector (which can be tied to the basementreflector on Line 745 using seismic lines parallel to the trendof the forearc) can be followed downslope to within 2 km ofthe forearc toe, and less than 0.5 s TWT above the top of thesubducting slab. Above the forearc basement reflector thereare approximately 600 — 1000 m of relatively undeformedsediments that comprise a series of flat-lying or landwarddipping units. Each of these units may be rotated faultblocks, but such faults are not imaged on Line 750.Alternatively, the geometry of these sediment piles may bethe result of sediment deposition at the base of smallsubmarine canyons or forearc basins. On the uppercontinental slope and shelf there is as much as 1.5 s TWT ofstratified sediment above the forearc basement reflector.

Line 751 (Figs 2 and 3) crosses the trench at 46C15', justsouth of the triple junction. Here the subducted spreadingaxis is located beneath a wedge of sediments that shows littlestructural coherence. One of the most prominent features inthis profile is the large displacement normal fault atCDP 1100, displacing the forearc toe approximately1000 m down towards the trench. This fault is also visible asa prominent escarpment in the SeaBeam map (Fig. 2). Thetop of the oceanic crust appears as a strong reflector and canbe traced approximately 5 km beneath the overridingforearc. The subducted rift axis forms a distinct structuralhigh beneath a sediment wedge on the lower trench slope.The rocks landward of the steep escarpment on the middletrench are stratified, supporting the notion that they mayrest on rigid, undeformed forearc basement.

Line 762 (Figs 2 and 3) traverses the northern slope ofthe Taitao Ridge that extends westward from the TaitaoPeninsula along the oceanic Taitao Fracture Zone (Fig. 1)and is surrounded by the abyssal plain of the Antarcticplate. The major structures imaged on Line 762 are theAntarctic oceanic crust covered by at least 0.5 s TWT of

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Fig. 2. Image of SeaBeambathymetry data (after Bangset al. 1992) from the collisionzone of the Chile Rise and theSouth American forearc.Illumination is from the south-east. Drill sites of ODP Leg 141and seismic Lines 745, 750, 751and 762 are shown

45°40' S

' S

46°20' S

46°40' S

Site 859

Line 745

Line 750

Line 762

7rj°U0' W 75°40' W

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0 km

Line 745pre-stack depth migrated

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Riftvalley

Accretionarywedge

Slope sediments

Site 859

Oceanic crustBasal

décollement

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Oceanic crust

i i i i i i i i i i i i r2000 1500 1000

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500 1000 1500

Line 762unmigrated time section

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CMPr i i r r

1000 500

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Fig. 3. Line drawings andgeological interpretations ofreflection seismic sectionsalong Lines 745, 750, 751and 762 recorded on the RC2901 cruise of R/V Conrad.Line 745 data were pre-stackdepth migrated usingMIGPACK at Geomar,Kiel (Germany); Lines 750and 751 are post-stackmigrated time sections usingDISCO at USGS, MenloPark (USA); Line 762 is anunmigrated time section

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slope sediment coveraccreted sediments

oceanic basement

Fig. 4. Seismic section detail and line drawing interpretation showing the toe of the South American forearc at Line 745 (after Behrmannet al. 1993)

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undeformed sediments on the north-west side of thesection. The top of the oceanic crust is not well imaged,but apparently dips beneath the Taitao Ridge, which isrepresented by the acoustically unstratified, 10 km widemound on the south-east side of the section. The TaitaoRidge is virtually free of sediments and the lack of onlapstructures on its north-west flank suggests that it is not anautochthonous magmatic feature on the Antarctic sea-floor. Large free-air gravity and linear magnetic anoma-lies also suggest that the Taitao Ridge is composed of highdensity and magnetic material.

From the bathymetric and seismic data it is evident thatthe architecture of the South American forearc changesradically along-strike. At the latitude of Line 745 anaccretionary wedge and continental basement covered byforearc slope sediments appear to be present in a 'stan-dard' geometrical configuration (cf. Seely 1979). As theChile spreading ridge approaches the trench and issubducted (Lines 750 and 751), large volumes of rockappear to be missing from the leading edge of the forearc,and there is little doubt that they have been removed byunderthrusting analogous to the fashion proposed byScholl et al. (1980) or Hilde (1983). The Taitao Ridge ismost likely a promontory of the South American forearcthat was created during or shortly after the subduction ofthe Chile Ridge about 3 Ma (Cande and Leslie 1986).

Rocks from ODP Sites 859-863:age, origin and sedimentary facies

The sediments intersected by the drilling are mainly siltyclaystones, clayey siltstones, siltstones and sandstones ofQuarternary and Pliocene age (Behrmann et al. 1992;Spiegler et al. 1994). Diagrams summarizing the informa-tion about biostratigraphic age, lithology and mineralogi-cal composition are shown in Fig. 5. There are intercala-tions of gravel layers and matrix-supported conglomera-tes at Sites 860 and 861. Compositional (Kurnosov et al.1995 a) and textural (Diemer and Forsythe 1995) varia-tions of the individual rock types between the sites arevery minor, indicating an absence of marked changes inthe sedimentary environment and the source characteris-tics of the sediment in both time and space. The sand-stones and siltstones contain quartz, non-weatheredfeldspar, detrital biotite and pyroclastic material as theprincipal components, with the subordinate occurrenceof other ferromagnesian minerals such as hornblende andclinopyroxene. The fresh feldspar and the unalteredferromagnesian minerals in the sediment are interpretedto show the predominance of physical weathering proces-ses and indicate the glaciated environment of the nearbyAndean crystalline basement and the arc volcanics assource area for the terrigenous component. The results ofa provenance analysis of Leg 141 sands and sandstones(Marsaglia et al. 1995) shows a variably dissected volca-nic arc sediment source (Fig. 6). The only exception is thelower half of section at Site 859, where the sedimentsource is more clearly dominated by quartz. Only the

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basal beds of the thin sedimentary blanket of the TaitaoRidge contain volcanic minerals that are possibly derivedfrom local submarine outcrops of basalts (Kurnosov et al.1995 a; Strand 1995). The volcanogenic component in thesandy turbidites cored at Sites 859, 860, 861 and 863 ismore abundant in the Pliocene and lower Pleistocenesections than in the upper Pleistocene (Strand 1995). Thisprobably relates to the end of arc magmatism at thelatitude of the drillholes in connection with the subduc-tion of the Chile Ridge. Silicic volcaniclastic material ismore abundant in the Pleistocene sands, which is consis-tent with evidence for silicic, subaerial volcanism in thenearby arc terrane (Fig. 6 a).

Clay mineralogy in the phyllosilicate-rich rock varietiesshows significant downhole changes, especially at Sites 859and 863 (Fig. 5). Changes in illite and chlorite are matchedby decreases in the smectite content. In the vertically beddeddomain at Site 863 these changes demonstrably occurindependent of stratigraphic age and the abrupt downholeincrease in smectite content is therefore interpreted asa diagenetic effect triggered by the migration of thermalfluids through the toe of the accretionary prism.

Most of the sections at Sites 859, 860 and 861 reflectmixed hemipelagic sedimentation and high or low density,mostly fine-grained distal turbidites. In the Pliocene sectionat Site 860 there is some evidence for traction transport andreworking by bottom current flow. The intercalations ofcoarse clastic layers at Sites 860 and 861 are mainly locatedwithin the Pliocene and indicate sedimentation moreproximal to either the source area or feeder canyons than inthe Pleistocene. There are important differences in thePleistocene sedimentary facies between Sites 859, 860 and861, located on the northern traverse across the forearc, andSite 863, located more proximal to the large land masses ofthe Taitao and Tres Montes peninsulas. In this area much ofthe section below 100 m below the seafloor consists ofbioturbated sandstones and siltstones of turbiditic origin.A thin, but seemingly continuous, sequence of silty clays andfine sands was recovered at Site 862 near the crest of theTaitao Ridge, with ages ranging from late Pliocene toPleistocene.

Sediment accumulation rates can be derived frombiostratigraphic age estimates and formation thicknessesin those sections that have not been overprinted bydeformation. For the Pleistocene part of the slope aproncover at Site 859 it is difficult to derive an accumulationrate. The cored section is only 10 m thick, and from thebiostratigraphic and paleomagnetic records and the oc-currence of slump folds (Behrmann et al. 1992) a majorstratigraphic hiatus cannot be ruled out. At Site 860 thereis an estimated accumulation rate of 47 m/Ma for thePleistocene and Pliocene slope cover of the accretionarywedge. Accumulation rates for the undeformed or weaklydeformed slope cover at Site 861 are estimated to be about100 m/Ma for the Pleistocene. Estimation of accumula-tion rates for the deeper sections of Pliocene age at Sites859, 860 and 861 is impossible because of the intensedeformation. 'Apparent' accumulation rates at these sitesbetween vary 200 and 290 m/Ma. These figures, however,

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CHILE TRIPLE JUNCTION LITHOLOGY AND STRATIGRAPHY

Site 859 Site 861

Age

ü

(m)

100

20(

300

400

Rock type/Lithologic

unit

HA

HB

clay mineralfractions (%)

smβcβlβ dkH chkxite

Age

Site 862

Age

1(m)

100

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unit

yyyyyy,

VVVVVV\VVVVVY1¥VVVVV\vvvvYy\vvvvvv>

III

Proportion of totalphases(%)

20 40 60 60

ROCK TYPES

Nannofossil orsiliceous ooze

Clay, claystone

Silty clay, clayey silt, silt

Silty claystone,clayey siltstone, silt

Sand, sandstone

Conglomerate, gravel

Volcanic basement

(m)

300

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unit

HA

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Upper Pleistocene

Lower Pleistocene

Upper Pliocene

Lower Pliocene

Site 860

Site 863

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clay mineralfractions (%)

Fig. 5. Logs showing biostratigraphic age, lithology and relative contents of clay mineral species for ODP Sites 859-863

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Fig. 6. a Normally graded layer of silicic volcanic ash in upperPleistocene siliciclastic sediments at Site 861 (Core 141-861C-8H-5,15 — 35 cm). Strong bioturbation at the top of the ash layer and in theoverlying tuffaceous sand, b Core photograph of the contact ofsediment with the glassy rim of a basalt flow at Site 862. Note normalfaults in the sediment (Core 141-862B-2X-CC. 19-36 cm)

take into account the section thickening by thrust imbri-cation or pervasive deformation within the Plioceneaccretionary prism. Anomalously slow sedimentationatop the Taitao Ridge proceeded at an estimated rate of10 m/Ma at Site 862. Sediment accumulation rates in theupper flat-bedded Pleistocene domains at Site 863 areestimated to be around 200 m/Ma, although here defor-mation may have significantly modified the initial forma-tion thicknesses. All these estimates do not account forvolume loss by compaction of the sediment piles.

The suite of igneous rocks intersected at Site 862 iscomposed of apparently intercalated submarine basalt,rhyolite and dacite flows with rare sediment interbeds.Poor recovery makes it very difficult to establish anunequivocal magmatic stratigraphy, although attempts

Chile Triple Junction: provenance of sandstones

CratonInterior

TransitionalContinental

Fig. 7. Ternary QFL (quartz — feldspar —lithic —fragments) plots ofsands and sandstones from the Chile Triple Junction. Ornamentedareas on the right-hand plot show the scatter of data from individualsites; points in the left-hand diagram show the mean compositions.Data from Marsaglia et al. (1995). Fields for tectonic settings ofprovenance areas after Dickinson et al. (1983)

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842

have been made (Behrmann et al. 1992). The depositionalcontact between sediments and the underlying glassy rim ofa basaltic lava flow was recovered in Core 141-862B-2X(Fig. 6 b). The uppermost Pliocene age of the basal sedimentand its close facies relation to a local volcanic source (Strand1994; Forsythe et al. 1994 a) constrain a slightly older agefor the volcanic rocks, probably uppermost Pliocene.4 0Ar/3 9Ar isotopic dating on hornblende separates fromtwo rhyolite samples yielded ages of 1.54 + 0.08 Ma and2.05+ 0.4 Ma (Forsythe et al. 1994 b). These ages arebroadly comparable with the age of the Taitao ophiolite onland (see Forsythe and Prior 1992) and lend some initialsupport to the hypothesis that the Taitao Ridge is itssubmarine continuation (Cande and Leslie 1986; Candeet al. 1987), or that the two structures have a similar andcontemporaneous origin.

The Taitao Ridge basalts recovered at Site 862 aresubalkalic to tholeiitic (Fig. 8 a and 8 b), with phenocrysts ofolivine, clinopyroxene and Plagioclase and dominantlyvitrophyric textures. In the dacite specimens primarymagmatic hornblende and Plagioclase phenocrysts arefound in a fine-grained groundmass mainly composed ofPlagioclase. Alteration is very minor or absent. As shown inthe Harker diagram in Fig. 8 c, the basalts, dacites andrhyolites clearly form a bimodal suite. The occurrence ofrhyodacites at oceanic spreading centers has been reportedbefore (Byerly et al. 1976). Their petrogenesis has beeninterpreted to reflect differentiation processes in tightlyconfined magma chambers behind propagating rifts (Perfitand Fornari 1983; Langmuir et al. 1986). Rare earthelement (REE) patterns of the basalt specimens arediagnostic of normal (N) or transitional (T) type mid-oceanridge basalts (MORBs) (see Fig. 8d). In contrast, thepatterns of the rhyolite specimens show strong light REEenrichment, a distinct negative Eu anomaly and heavy REEdepletion. These observations preclude a simple geneticrelation to N- or T-type MOR magmas by straightforwarddifferentiation (see discussion in Forsythe et al. 1995 a).Instead, contamination by a 'continental' source is likely,possibly in the form of intermittent stoping of sedimentaryrocks of continental provenance (turbiditic graywackes,shales) into a MOR magma chamber.

Deformation patterns and overprinting relationships

Drilling at Sites 859, 860 and 861 revealed a commontectono-stratigraphy (see Fig. 9). In all three instancessediments that show either no deformation, or evidence

Fig. 8. a AFM diagram of Taitao Ridge volcanic rocks (closedcircles) and specimens from the Taitao ophiolite. Data from Keadinget al. (1990) and Kurnosov et al. (1995b). b K 2 O + Na 2 O versusSiO2 variation diagram of Taitao Ridge volcanic rocks (circles) andspecimens from the Taitao ophiolite. Data from Keading et al.(1990) and Kurnosov et al. (1995b). c Harker variation diagram ofTaitao Ridge volcanic rocks (after Behrmann et al. 1992). d REEpatterns of Taitao Ridge basalts and rhyolites (data from Forsytheet al. 1995a)

TAITAO RIDGE AND TAITAO OPHIOLITE VOLCANICS

15

10

5

I I

- Φ ^°

* * o o44-

j β i

1

* Na2O

• K2O

4- MgO

1

1

*

|

1

CaO

o AI2O3

° Fe 2 O 3

—

50

• 7

Tholeiitic

Fe2O3

MgO

10

50

10

I I I I I ITAITAO RIDGE RHYOLITES

TAITAO RIDGE MOR

^T-TYPE

BASALTS

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for near-surface slump folding, are underlain by anintensely fractured and sheared 'broken formation' (cf.Hsü 1968; Cowan 1985). The undeformed or slumpedsediments are interpreted as a slope apron sequence ofPleistocene to upper Pliocene age (Fig. 10). The under-lying broken formation forms part of an accretionarywedge that was evidently built during the Pliocene byfrontal accretion of the hemipelagic sediment cover of theNazca plate to the South American forearc. The intensityof deformation throughout the section is at variance withthe structures observed in. other active accretionaryprisms (e.g. Behrmann et al. 1988; Moore 1989; Tairaet al. 1991), where strain is extremely localized in thrustsand basal décollement zones. Structures encountered inthe slope cover at Site 859 are recumbent and uprightfolds with NNW — SSE trending axes. The broken forma-tion is rich in breccia zones, mesoscopic arrays ofdeformation bands (cf. Moore and Lundberg 1986) andsections with a well developed fracture cleavage (Fig. 9).At Site 860, a flat-bedded domain of slope sediments withnear-surface slump deformation in the upper 100 mbelow the seafloor is underlain by a thrust stack of mildlyto moderately deformed slope apron sediments. Twothrusts at 240 and 310 m below the seafloor are defined bybiostratigraphic age reversals. Below 420 m below theseafloor to terminal depth, the broken formation wasintersected below a shear zone. Structural indications formajor faults or shear zones within the broken formationexist at 520 and 580 m below the seafloor. Deformationbands at Site 860 reveal a complex deformation history,defined by overprinting relationships (Behrmann et al.1992). Reverse offsets dominate, but data from individualdomains also give evidence for a phase of subhorizontalextension, bracketed by phases of horizontal contraction.The sediments at Site 861 can be subdivided into threestructural domains. Domain I (0 — 210 m below seafloor)shows no tectonic deformation, but the development ofan incipient bedding-parallel fissility below 150 m belowseafloor. Domain II (210-390 m below seafloor) hasgently to moderately inclined bedding and isolated defor-mation bands exist. Domain III (390-496.3 m belowseafloor) has the characteristics of the broken formation,with abundant deformation bands and stratal disruption,similar to the structural associations found in the lowersections at Sites 859 and 860.

The common occurrence of the broken formation inthe lower sections at all three sites means that the lower,middle and upper trench slope at Line 745 is underlain bystrongly deformed sediments that have most likely under-gone a process of tectonic accretion to the South Ameri-can forearc during the late Pliocene. Paleo-depth indica-tors (see later) for abyssal water depths of depositionsuggest a trench origin of the sediments. The seismicsignature of the forearc and its interpretation (Fig. 3), aswell as the drillhole data, constrain a seaward vergentimbricate stack for the region of Sites 859 and 860. Thebroken formation at Site 861 has probably been emplacedonto the forearc basement along a landward-vergentbackthrust (Fig. 10). The structural patterns recorded

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along Line 745 are similar to those found along the Peruconvergent margin (Kemp and Lindsley-Griffin, 1990).This comparison helps to show that the Southern Chileconvergent margin to the north of the Chile Ridgecollision point does not have a peculiar type of geometry,but is a rather more common case, with a accretionarywedge 15 — 20 km wide.

Deformation of the sediment section at Site 862 isdominated by normal faulting (Figs 6 b and 11). At leastsome of the faulting occurred early in the diagenetichistory, as faults are overprinted by zones of hydrother-mal alteration and mineralization near the base of thesedimentary pile. It is unclear whether normal faulting isrelated to gravity-induced detachment and sliding of thesediment cover on an unstable slope, or to horizontalextension accompanying tectono-magmatic processes ator near an active and sedimented oceanic spreading ridge.No samples of the igneous rocks were recovered in theiroriginal orientation, so no structural analyses could beperformed on these lithologies. In cores retrieved fromSite 863, important events of reverse faulting and normalfaulting can be recognized. Overprinting relationshipsdocument that reverse faulting pre-dates normal faulting(Fig. 9). Some of the faults contain secondary calcitemineralization bands, often showing slickenfibers.Quartz —pyrite mineralization was also observed alongfaults. These observations suggest that fault motion, fluidmovement and mineralization were synchronous. Incores that could be oriented relative to magnetic north,both bedding and faults strike N W - S E (Fig. 12). Thisstrike is clearly oblique to both the relative plate move-ment vector (e.g. Chase 1978) and the strike of the ChileTrench. These data may indicate that some of thetrench-parallel motion component between the Nazcaand South American plates is taken up by transpressivedeformation of the forearc. One of the most remarkablestructural features at Site 863 is a long interval of steeplydipping to vertical bedding between 265 m below seafloorand terminal depth (Fig. 9). The steep orientation may bethe result of folding near the leading edge of theaccretionary prism during the phase of lateral contractionthat created the reverse faults. Bedding orientation maylater have been modified again by block tilting associatedwith late stage normal faulting. Folding and reversefaulting is likely to relate to the frontal accretion of thePleistocene turbidites after their deposition in the ChileTrench. The later normal faulting reflects the extensionand subsidence of this part of the forearc immediatelyafter ridge subduction.

Paleo-bathmetry and vertical movementsof the forearc sediments

Benthic foraminifer assemblages are paleo-bathymetricindicators (e.g. Ingle et al. 1980) that can be used toestimate the original depositional depth of sedimentsfound in the forearc. Biotopes are divided (cf. Resig 1990)

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Site 859 Site 860 Site 863

Site 861

100

200

4QQ,

Age Structuraldomains

pt

i

Beddingdip n

>

Fractures(per m o( core)

Bedding Fracturesdip (°) (per m of core)

Fig. 9. Structural features versus drillhole depth and stratigraphicage in ODP Leg 141 cores. True bedding dips. Densities of faults,deformation bands and fractures are measured per meter of core(corrected for 100% recovery). The diagnostic features of structuraldomains are represented in the schematic drawings. Data fromBehrmann et al. (1992)

ENE

5 km SubductedPleistocene sediment

Fig. 10. Interpretative, crustal-scale section across the southern Chile forearc along Line 745, constrained by stratigraphic, structural andreflection seismic data (after Behrmann et al. 1993)

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845

Fig. 11. Scale model for normal faults in the sediment cover atSite 862 (after Behrmann et al. 1992)

into shelf (0 -150 m), upper bathyal (150-500 m), upperand lower middle bathyal (500 — 2000 m), lower bathyal(2000-4000 m) and abyssal (4000-6000 m) depth ran-ges. The database for the interpretations presented here isfound in Behrmann et al. (1992) and Schönfeld andSpiegler (1995). Benthic foraminifer abundances are lowin Leg 141 cores and some constraints have to be acceptedregarding the interpretative value of the data.

Nearly all of the sediment was deposited in a hemipela-gic turbidite environment (Strand et al. 1994) and asa result downslope transport and resedimentation ofbenthic foraminifers from their original habitats on theshelf and upper continental slope may render Paleo-bathymetric interpretations difficult. In fact, shelf andupper bathyal forms are found in a large number ofsamples analysed (Behrmann et al. 1992) and are particu-larily evident as 'yellow faunas' with poor preservation insamples from Site 861 (Schönfeld and Spiegler 1995).Taking this observation into account, we propose that thetaxon or community indicating the deepest depositionalenvironment in a particular lithological unit reflects itsdepth of deposition.

An important distinction has to be made between thosesediment piles that form part of the accretionary wedgeand the sediments of the slope cover (see sections onstructure and deformation). Paleo-bathymetric data fromthe accreted sediments may record the uplift involved inprogressive overthrusting and stacking of the trench fillturbidites and hemipelagic rocks. On the other hand, theslope sediments are capable of recording any verticalmovements that occurred after the build-up of theaccretionary wedge and the deposition of the sedimentaryblanket.

Sediment deposition in deep water is evident from thedata at all five sites (see Fig. 13, Table 1). At Site 859 benthic

a Oriented bedding b Oriented faults

Fig. 12a—c. Lower hemisphere, equal area projections of trueorientation of a bedding and b fault planes in cores 141-863A-4Hand -5H; data from Behrmann et al. (1992). c Direction of maximumhorizontal shortening (open arrows) and finite strain ellipse fordextral simple shear with magnitude y = 0.6. This is the approxi-mate shear strain integrated over 1 Ma, if the trench-parallelcomponent of the Nazca —South America plate motion is distri-buted within a 30 km wide forearc. For discussion, see text

foraminifer communities indicate a middle to lower bathyalenvironment (Fig. 13) for the whole pile of sediments (cf.discussion in Behrmann et al. 1992). Diagnostic taxa arelisted in Table 1. Thus the present depth of the sediments issimilar to the original depositional depth and there is noevident signal of large-scale uplift or subsidence. Theaccreted sediments in the lower part of Site 859 are bestinterpreted as an upper Pliocene trench fill that wasoffscaped and deformed shortly after deposition in a trenchenvironment that was similar to the present trench outboardof the South American forearc at the latitude of Line 745(Fig. 2). The lower to middle bathyal communities in theslope sediment suggest that the surroundings of Site 859formed the toe of the accretionary prism since the latePliocene, with no record of significant uplift or sub-sidence.

In the upper part of the drilled rock sequence (abovecore 860B-44X), Site 860 revealed tectonically imbricatedslope sediments that were deposited in a middle bathyalenvironment. The benthic foraminifer fauna is dominatedby Bulimina mexicana and Uvigerina perigrina, whichhave their main occurrence under less than 2000 m ofwater (cf. Ingle et al. 1980; Resig 1990). This indicatesthat some (of the order of 500 m) subsidence has occurredat Site 860 since the deposition of the slope sediments. Inthe 'broken formation' (see earlier) below core 860B-44X,the occurrence of Melonis pompilioides sphaeroides (vanMorkhoven et al. 1986) indicates abyssal water depths ofdeposition. This is clear evidence for a trench or abyssalplain origin of the sediments and uplift of the order of

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846

Fig. 13. Depths of depositionfor sediments at ODP Sites859-863 in relation to theirstratigraphic age

Age (Ma) Series

0

Site 859

2748 m

Site 860

2146 m

Site 861

1652 m

Site 862

1270 m

Site 863

2564 m

2 — ^

3 —

4 —

Table 1. Paleo-depth and diagnostic benthic foraminifer taxa fromODP Sites 859-863

Site

859

860(above 400 mbsf)

860(below 400 mbsf)

861(Pleistocene cores)

861(Pliocene cores)

862

863(above 85 mbsf)

863(below 200 mbsf)

Paleo-depth

Middle to lowerbathyal

Middle bathyal

Abyssal

Upper to middlebathyal

Abyssal

Middle to lowerbathyal

Upper to middlebathyal

Bathyal(> 2400 m)

Diagnostic taxa

Melonis pompilioidesCassidulina crassaEhrenbergina pupaBulimina mexicanaPullenia bulloidesUvigerina sp.

Melonis pompilioidesBulimina mexicanaPyrgo murrhinaUvigerina perigrina

Melonis pompilioidessphaeroides

Bulimina striatamexicana

Uvigerina perigrinaHoeglandia elegansUvigerina bifurcataOridorsalis umbonatius

Melon is pomp ilio idessphaeroides

Uvigerina perigrinaMelonis pompilioides

Planulina wuellerstorfiPyrgo murrhinaBulimina exilisSphaeroidina bulloides

Melonis pompilioidesUvigerina cf. scenticosc

2 000 m by processes of overthrusting and piggy-backimbrication in an accretionary wedge during the latePliocene.

As at Site 860, major uplift must be inferred for thedeeper part of the sedimentary column penetrated atSite 861 from the occurrence of M. pompilioides. Thetaxon is rare in Pliocene cores (see Behrmann et al. 1992;Schönfeld and Spiegler 1995) above 400 m below sea-floor, but its occurrence becomes more frequent in coresbelow 400 m below seafloor. In core 861D-38X thereliable indicator for abyssal water depths M. pompilio-ides sphaeroides is present. Analogous to the interpreta-tion of Site 860 we propose that at Site 861 accreted anddeformed (see earlier) strata of late Pliocene age witha lower bathyal to abyssal provenance provide a substra-tum for a Plio-Pleistocene slope cover sequence that wasdeposited in upper to middle bathyal water depths. Theonly observation contradicting this paleo-bathymetricinterpretation of the slope cover sequence is the rareoccurrence of U. scenticosa in most of the Pleistocenecover sequence (Schönfeld and Spiegler 1994). If thepaleo-bathymetric range interpretation of Boersma(1984) for the Peru —Chile Trench is correct, then theslope cover has been deposited under more than 2400 mof water. In this instance the area of Site 861 must haveexperienced a very young (Holocene) uplift of at least700 m.

Cores from Site 862 contain a poorly preserved benthicforaminifer fauna dominted by U. perigrina, which hasa broad bathyal range of occurrence (cf. Resig 1990). Inthe lowermost sediment core of Hole 861C the occurrenceof M. pompilioides indicates a possible lower bathyalsetting of the Taitao Ridge during the onset of sedimenta-tion during the late Pliocene. In this instance, a post-

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K47

Pliocene uplift in excess of 1000 m for the Taitao Ridgecan be inferred.

Strong late Pleistocene to Holocene subsidence isdocumented in the upper 85 m of the cored section ofPleistocene sediments at Site 863. Benthic foraminiferassemblages here indicate an upper to middle bathyalenvironment and a deposition of this sequence on the upperto middle trench slope. Below 2748 m of water thesesediments have recorded a very young and rapid subsidenceof at leat 1000 m, which is consistent with the kinematicinterpretation of the large escarpment landward of Site 863as an active normal fault with at least 1000 m of verticaldownward displacement of the seaward block (see sectionon seismics and bathymetry). In the Pleistocene sedimentsection below 85 m below seafloor U. scenticosa andM. pompilioides indicate a lower bathyal environment. As atSite 859 this reflects water depths that are now recorded inthe vicinity of the trench axis and on the abyssal plainseaward of the deformation front and warrants aninterpretation of a near-trench origin of the sediments. If wesuppose that the biostratigraphically defined overthrustingand folding of the sediments was accomplished during thePleistocene, then the trench sediments found below 85 mbelow seafloor have had a history of frontal accretion,imbrication, folding and uplift, and subsequent majorsubsidence. All this reflects a cycle of forearc building bysediment accretion and subsequent destruction by subduc-tion erosion at Site 863 of less than 700 ka duration.

The record of vertical tectonic movements in thesouthern Chile forearc around 46° S can be summarizedand interpreted as follows (see also Fig. 14). The slopecover sediments in the forearc section north of the ChileTriple Junction (Sites 859, 860 and 861) document nodramatic changes in water depth as their deposition in thelate Pliocene and Pleistocene. This is an importantindication that here no large volumes of rock were addedto or removed from the forearc by subduction accretionor erosion for the past 2 Ma. The sediments deposited invery deep (lower bathyal to abyssal) water found at Site859, 860 and 861 always show signs if intense deforma-tion, probably acquired during processes of frontalaccretion to the forearc and imbrication during the latePliocene. The variable uplift of these rocks is a function ofthe position in the forearc beneath the lower, middle orupper trench slope. Major Pleistocene uplift is likely forthe Taitao Ridge at Site 862. Tentatively, this uplift can beattributed to tectonic accretion of a fragment of the ChileRidge to the leading edge of the South American forearc.As discussed earlier, the paleo-bathymetric evolution ofSite 863 is complex and may be representative of a veryshort-lived cycle of subduction accretion and subductionerosion in the late Pleistocene and Holocene.

Discussion

Here we discuss the constraints imposed on the interpre-tation of spreading ridge subduction tectonics by thegeophysical survey and drilling data. Four points will

2000

Water depth (m)4000

Fig. 14. Probable uplift and subsidence histories of the forearcsediments at ODP Sites 859-863

receive special attention. Firstly, the deformation styleand the modification of forearc geometry will be discus-sed in the light of the geophysical, structural and paleon-tological data. Secondly the time scale and length scale forthe change in tectonic regimes will be inferred. Thirdly,the impact of spreading ridge subduction on the forearcdepositional system will be assessed, and, lastly we willdeal with the question of whether the Taitao Ridge isa nascent ophiolite complex.

Deformation style and modification of forearc geometry

From Fig. 3 it is evident that pronounced changes in thegeometry of the Southern Chile forearc occur along-strikeof the convergent plate boundary. Along Line 745, wherethe seismic line drawing and tectonic interpretations(Figs 3 and 10) are corroborated and supported by thebiostratigraphic, lithological (Fig. 5) and structural(Fig. 9) data from the cores drilled, the leading edge of theforearc consists of a well defined accretionary prism ofPliocene age. The upper Pliocene broken formation stratafound at the toe of the accretionary prism (Site 859) makeit clear that no frontal addition of sediments to the forearcoccurred after the late Pliocene. This is in markedcontrast with the findings from the toes of other activeforearcs, e.g. Barbados (Behrmann et al. 1988), Nankai(Taira et al. 1991), or Cascadia (ODP Leg 146 ScientificParty 1993), where there is a documentation of conti-nuous growth by frontal accretion. If compared with thePacific forearcs of Peru (Suess et al. 1988), MiddleAmerica (Aubouin et al. 1982; von Huene et al. 1985b)and Japan (Scientific Party 1980), however, the architec-ture of the southern Chile forearc along Line 745 does notappear very different. In these examples a small(15 — 20 km) sediment wedge is accreted against a hardrock buttres. These 'Andean-type' forearcs are far fromconforming to the 'continuous growth paradigm' by

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sediment accretion. They represent a common type ofconvergent margin that shows episodic accretion inter-rupted by long periods of non-accretion or even subduc-tion erosion.

From the data collected along Line 745 and at Sites859 — 861 it is evident that all sediment on the downgoingNazca plate has been subducted for approximately thelast 2 Ma. Given the internal forearc structure at Line 745and the lack of young (Pleistocene) uplift documented inthe slope cover sediments (see Figs 13 and 14), thelarge-scale basal addition of material by underplating(e.g. Platt et al. 1985) can be ruled out at least for thisseaward portion of the forearc. At Line 745 the SouthernChile forearc is presently in 'non-accretion' mode, andhas been in this state for the last 2 Ma.

If the interpretative line drawing of Line 750 (Fig. 3) iscorrect, continental basement of the Southern Chileforearc underlies the slope sediments as near as 3 km fromthe Chile Trench. The basement wedge has a size andtaper almost identical to that seen in Line 745, andprobably represents its continuation along-strike. Thissuggests that the complete accretionary wedge and itsslope cover seen along Line 745, approximately 15 km2 ofsediment in cross-section, has been consumed by subduc-tion erosion at the latitude of Line 750.

Subduction of the spreading ridge axis at the latitude ofLine 751 (Fig. 3) is accompanied by pronounced extensio-nal deformation of the sedimentary cover and removal ofthe leading edge of the continental basement wedge seenin the seismic sections further north. If constant taper ofthe wedge is assumed, approximately 5 km2 of crystallineforearc basement was removed by ridge subduction. Asdiscussed earlier, normal faulting and subsidence ofsediments in the hangingwall appears to be an importantprocess of structural modification associated with ridgesubduction. Following the passage of the triple point,rebuilding of the forearc begins with the tectonic accre-tion of the pile of basalt, dacite and rhyolite flows at theTaitao Ridge. Tectonic stacking here results in markedPleistocene uplift from initial middle to lower bathyalwater depths to the present depth of 1270 m belowseafloor at Site 862.

Change from subduction accretion to subductionerosion: length scales, time-scales and episodicity

The Pliocene broken formation overlain by Plio-Pleisto-cene sediments at the toe of the accretionary wedge alongLine 745 proves that no frontal accretion of sediment hasoccurred to the southern Chile forearc at this latitudesince the late Pliocene. Earlier, there was an importantphase phase of forearc building, as indicated by thepresence of large volumes of accreted and uplifted brokenformation at Sites 859, 860 and 861. From the drillingresults it is not evident that Miocene or Paleogenesediments presently form part of the accretionary wedgeat the latitude of seismic line 745. This does not excludethe possibility that accreted material of this age can be

found by drilling deeper into the accretionary wedge.However, as accretionary wedges tend to grow by piggy-back imbrication mechanisms (e.g. Platt, 1988; Brownet al. 1990), the absence of Miocene or older sediments inthe thrust stack and broken formation at Site 860 near thebackstop of continental basement makes it likely that thephase of forearc building was restricted to a short periodin the early and late Pliocene.

The onset of subduction erosion can be located south ofLine 745 and north of Line 750. Within 15 km along-strike of the Chile Trench most, if not all, of the accretedsediments are removed by subduction erosion, and at thelatitude of Line 751, approximately 15 km south of thegeometrical plate triple junction, considerable volumes offorearc basement have been tectonically eroded from thefront and base of the South American forearc. Thus thetransition from the dominance of subduction accretion tosubduction erosion along the Chile Trench occurs over analong-strike distance of no more than 30 km, and withintime periods as little as 3 Ma.

Important information about shorter episodes of sub-duction accretion and erosion superimposed on thelong-term pattern is contained in the stratigraphic recordat Site 863. Here the Pleistocene below 200 m belowseafloor was deposited in deep (> 2400 m) water andthen uplifted by accretionary mechanisms. This indicatesthe possibility of short-term pulses of sediment additionto the forearc, even in a regime of long-term subductionerosion. At Site 863 the thickness of the accretedfold-fault packet is rather large (1000 m or more). Asupper Pleistocene stata are accreted, it is evident that thispulse of sediment addition can only have a maximumduration of less than 0.5 Ma.

Influence on depositional systems

In what way does spreading ridge subduction influencethe depositional systems in the immediate vicinity of theChile Triple Junction ? Sediment types in the drillcores arevery similar both in mineralogical composition and fades.Regionally, there is a complex control on sedimentaryfacies by sea-level lowstands, different amounts of Plio-Pleistocene glaciation in the Patagonian Andes, and theintensity of arc — forearc volcanism (see Strand et al.1994). The volcanogenic component in the sandy turbidi-tes cored at Sites 859, 860, 861 and 863 is more abundantin the Pliocene and lower Pleistocene sections than in theupper Pleistocene sections. This probably relates to theend of arc magmatism at the latitude of the drillholes inconnection with the subduction of the Chile Ridgespreading segment south of the Taitao Fracture Zoneduring the Pleistocene. Pleistocene strata are enriched insilicic volcaniclastic material, providing evidence forsilicic, subaerial volcanism in the nearby arc terrane(Fig. 7).

The positions of large depocenters at the SouthernChile margin are controlled by submarine canyons deeplyincised into the continental margin (Thornburg et al.

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1990). Several hundred kilometers north of the ChileTriple Junction, the Chacao and Calle-Calle fans repre-sent such modern trench fan lobes, one of them (theCalle-Calle fan) being located on a subducting oceanicfracture zone. Closer to the Chile Triple Junction in thesurroundings of Lines 745 and 750, the overriding SouthAmerican forearc shows rough topography (Fig. 2), butno development of large canyons. This setting of limiteddownslope sediment transport (e.g. Underwood andBachman 1984) corresponds to low (47 —100 m/Ma)sedimentation rates on the forearc slope and only slightlyincreased (200 — 290 m/Ma) apparent sedimentation ra-tes in the accreted strata originating from the trench. Inthe immediate vicinity of the Chile Triple Junction aroundLine 751 (Fig. 2) and Site 863, very young oceanic crust iscovered by at least 0.5 s TWT of sediment (Fig. 3),indicating sedimentation rates higher than 1000 m/Ma.High sedimentation rates here are probably related toincreased turbidite activity triggered by the oversteepen-ing of the forearc slope and the formation of a deeplyincised submarine canyon north of the Taitao Ridge (seeFig. 2). The very thin, but seemingly continuous, sedi-ment sequence at Site 862 (see Fig. 5) gives evidence thatthe Taitao Ridge was a prominent bathymetric high foralmost all of its history.

Generally, it can be said that tectonic movements,forearc deformation and subduction erosion processesinfluence the dynamics of sedimentation on a local ratherthan a regional scale by creating or suppressing topogra-phic features and oversteepened slopes, and thus deter-mining sediment distribution, volumes and types. Mark-ed changes in sedimentation patterns can occur on scalesof less than 30 km.

Taitao Ridge: a nascent forearc ophiolite

Large and coherent ophiolite complexes have been inter-preted as representing the obducted remnants of oceanicbasement in supra-subduction zone setting (e.g. Caseyand Dewey 1984; Thy and Moores 1988; Elthon 1991).Most occurrences of metabasic or ultrabasic rocks inforearcs are strongly dismembered, deformed and havetheir likely origin in rift or passive margin settings (cf.Boillot et al. 1988; Trommsdorff et al. 1993), transformfault areas or oceanic fracture zones (e.g. Ogawa andNaka 1984). They are incorporated into forearcs bysubduction accretion (Karig 1982), sometimes becomeinvolved into processes of forearc diapirism (Fryer et al.1985) and form part of cryptic sutures in collisionalmountain belts once the oceanic basins are closed comple-tely (see e.g. Steinmann 1906; or the review by Knipperet al. 1986).

Little is known about processes accompanying theearly stages of ophiolite emplacement into forearcs, andthe data from Site 862 on the Taitao Ridge can providesome key insights. Firstly, it is clear from the youthful ageof the Taitao Ridge volcanic rocks and from theirgeographical position that the origin of the volcanic pile is

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connected with an oceanic fracture zone (the TaitaoFracture Zone) and one of its corresponding rid-ge—transform intersections. The thin but stratigraphi-cally continuous sediment cover shows that it formeda bathymetric high from the beginning of its history,although major uplift by more than 1000 m has occurredafter the building of the volcanic edifice and sedimenta-tion of at least part of its cover (Fig. 14). Because of thefragmentary recovery of cores we are unable to saywhether there is significant deformation and tectonicdisruption of the volcanic rocks. The preserved ba-salt—sediment contact (Fig. 6 b) and the lack of chemicalalteration, or even devitrification, in the basaltic volcanicrocks suggests that the system was not open to pervasiveflooding by hydrothermal fluids, as would have been thecase if extensive shearing had taken place. The example ofthe Taitao Ridge suggests that forearc ophiolites do notsuffer the fate of dismembering and heavy alterationunless they are deeply underthrust, reheated and hydratedwithin their forearc tectonic positions. This may beespecially true for ophiolite accretion from young sub-ducting oceanic crust. The Taitao Ophiolite on land(Keading et al. 1990) shows that structural coherence maybe retained through a process of forearc accretion withoutdeep burial.

Conclusions

The combined interpretation of swath bathymetric data,seismic reflection profiles and ODP drilling results fromthe Chile Triple Junction document significant variationsin the structural style and deformation patterns along-strike within the South American forearc. North of thetriple junction the deformation is characterized by Plioce-ne subduction accretion of mostly land-derived sediment,whereas the region of the actual triple junction itself is thesite of large-scale extensional faulting, subsidence andremoval of material from the forearc. Microstructuraloverprinting relationships in the cores recovered fromSite 863 clearly document a period of compressivedeformation related to subduction accretion that wasfollowed later by a period of extensional deformationrelated to the subduction of the active spreading ridge.Also, a component of oblique motion is documented inthe cores, suggesting that trench-parallel shear is broadlydistributed across the forearc, rather than being focusedalong major trench-parallel strike-slip faults.

Most of the drilled sections at Sites 859, 860 and 861north of the triple junction indicate a combination ofhemipelagic and fine-grained distal turbidite sedimen-tation at low to intermediate rates (roughly50 —300 m/Ma). Pleistocene sedimentation in the triplejunction area reflects a more proximal facies and higher(> 1000 m/Ma) accumulation rates. The accumulationrate atop the Taitao Ridge at Site 862 is approximately10 m/Ma. The source region for most of the sediment atall sites is the volcanic Andean arc and the continentalbasement that outcrops on land. The lack of significant

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chemical weathering of detrital grains suggests rapiderosion and deposition in a glacio-marine environment.Paleo-water depth estimates from benthic foraminiferassemblages recovered from the cores document periodsof both rapid forearc uplift during subduction accretion,and subsidence as the spreading ridge is subducted.

The pile of volcanic rocks at the Taitao Ridge repre-sents a likely candidate to become an ophiolite bodyemplaced into the South American forearc. Its offshoreorigin is clearly associated with the oceanic TaitaoFracture Zone and it was probably formed at a ridge —transform intersection. The volcanic rocks have notundergone detectable hydrothermal alteration or meta-morphism. The Taitao Ridge volcanic rocks show thatforearc ophiolites may be emplaced at shallow structurallevels into a forearc and not undergo dismemberment,hydration, or metamorphism.

Acknowledgements The Scientific Party of ODP Leg 141 is gratefulfor the assistance given to them by the ODP Technical andEngineering Groups, and the SEDCO drilling and marine staff.GEOMAR (Kiel, Germany) is acknowledged for providing seismicprocessing facilities. The Ocean Drilling Programs is sponsored bythe participating countries under the management of Joint Oceano-graphic Institutions, Inc. The government of Chile kindly granted uspermission to conduct operations within their Exclusive EconomicZone waters. Reviews of this manuscript by Terry Pavlis and Rolandvon Huene were most helpful.

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