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Ridge Subduction, and Feedback Controls on the Andean Margin Developmentat the Chile Triple Junction Area (45-48Øs)

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Ridge Subduction, and Feedback Controls on the Andean Margin Developmentat the Chile Triple Junction Area (45-48Øs) JOURNALOF GEOPHYSICALRESEARCH, VOL. 105,NO. B4,PAGES 8355-8386, APRIL 10,2000 Glacial-interglacial trench supply variation, spreading-ridge subduction, and feedback controls on the Andean margin developmentat the Chile triple junction area (45-48øS) JacquesBourgois,• Christhie Guivel,2 Yves Lagabrielle,3Thierry Calmus,4 JacquesBoulbgue,S and Valerie Daux6 Abstract. Duringthe Chile triple junction (CTJ) cruise (March-April 1997), EM12 bathymetry andseismic reflection data were collected in thevicinity of theChile triple junction (45-48øS), wherean activespreading ridge is beingsubducted beneath the Andean continental margin. Resultsshow a continentalmargin development shaped by tectonicprocesses spanning a spectrum from subduction-erosionto subduction-accretion. The Andeancontinental margin and the Chile trenchexhibit a strongsegmentation which reflects the slabsegmentation and the Chiletriple junctionmigration. Three segments were identified along the Andeancontinental margin: the presubduction,the synsubduction,and the postsubductionsegments, from northto south.Both climate-inducedvariations of the sedimentsupply to the trenchand the tectonic reorganization at the Nazca-Antarcticaplate boundary involving postsubduction ridge jump arethe two main factors that controlthe tectonicregime of this continentalmargin. Along the surveyarea we infer the successionof two differentperiods during the lastglacial-interglacial cycle: a glacialperiod with ice-rafteddetrital discharges restricted to the shorelinearea and low river outputand a warmer periodduring which the Andeanice capretreat allowed the Andes to be drainedoff. Duringthese warmperiods, rapid increase in trenchdeposition caused the margin to switchfrom subduction- erosionor nonaccretionto subduction-accretion:(1) along the presubduction segment after the last deglaciationand (2) alongthe postsubduction segment after the interglacialepisode at 130-117ka. Conversely,a nonaccretionor subduction-erosionmode characterized the presubduction and postsubductionsegments during glacial maximums. The majoreffects of subductionof the buoyantChile ridgeinclude a shallowtrench which diverts trench sediment supply and tectonic instabilitiesat the Nazca-Antarcticaplate boundary. We suggestthat a postsubductionwestward jump of the Chile ridgeoccurred during the past780 kyr. It producedslab fragmentation and individualizationof an ephemeralmicroplate north of the Taitaofracture zone: the Chonos microplate.In 780 kyr, two episodesof subduction-accretionseparated by an episodeof subduction-erosionoccurred in relationwith the Chonosmicroplate individualization and subduction.The currentnorthward migration of thetriple junction along the Chonosmicroplate- SouthAmerica plate boundary introduces a sharpchange in the tectonicmode from subduction- erosionto the northto subduction-accretionto the south.The datacollected along the Taitaoridge haverevealed the complexthree-dimensional structure of an accretionarywedge which includes a midslopethrust sheet exhibiting the characteristicsof an ophiolite:the TaitaoRidge ophiolite. No connectionexists between the Taitao Ridge ophiolite and the BahiaBarrientos ophiolite cropping out onlandin the Taitao peninsula. 1CentreNational de la RechercheScientifique, Universit6 Pierre et 1. Introduction Marie Curie, Laboratoire de G6odynamique,Tectonique et Environnement,Paris. The Chile triple junction (Figure 1) is the site wherethe 2Laboratoirede Plan6tologieet G6odynamique, Universit6 de Antarctica,the Nazcaand the SouthAmerica plates meet Nantes, Nantes, France. [Candeand Leslie, 1986; Cande et al., 1987;Behrmann et al., 3Institutde Recherchepour le D6veloppement,Noumea, Nueva Caledonia. 1994]. At 46ø09'S, the active spreadingcenter at the 4InstitutoGeologico, Universidad Autonoma de Mexico,Estacion Antarctic-Nazcaplate boundary is being subductedbeneath Regionaldel Noroeste,Hermosillo, Sonora, Mexico. theSouth America continental margin. The Chile margin triple 5Laboratoirede G6ochimieet M6tallog6nie,Universit6 Pierre et junction area providesthe opportunityto study the Marie Curie, Paris. petrologicaland tectonic effects of spreadingridge subduction 6LaboratoiredeG6ologie S6dimentaire, Universit6 Pierre et Marie alonga continentalmargin, a processthat has dramatically Curie, Paris. affectedthe geology of both North and South American Copyright2000 by the AmericanGeophysical Union. marginsduring the past 70 Myr [Atwater, 1970; Dickinson andSnyder, 1979; Ramos and Kay, 1992; Sisson and Pavlis, Papernumber 1999JB900400. 1993; Kay et al., 1993]. Spreadingridge subductionleaves 0148-0227/00/1999JB900400509.00 specificstructural and stratigraphicsignatures, as recently 8355 8356 BOURGOIS ET AL.: CHILE TRIPLE JUNCTION 78øW 77øW 76øW 75øW 74øW 73øW 45øS CHILERIDGE 64 km/m.y. 859 86O 861 46øS SEGMENT' •63 78øW 76øW 74øW 72øW NAZCA PLATE 8,0•,mlm-¾' 47øS GP 48øS 48øS Figure 1. Bathymetricmap of the region surveyedduring the CTJ cruise of the R/V L',4talante in the Chile triple junctionarea, between45øS and 48øS.The main seafloormorphological features include the Chile ridge axesand fracturezones, the Chile trench, and the Andean continental margin.Location map is shown in the inset.Numbers 859 to 863 referto the ODP sites drilled during ODP Leg 141. Chile triple junction (CTJ); Chiloe block (CB); Esmeraldafracture zone (EFZ); Golfo de Penas(GP); Liquine-Ofqui fault system(LOFS); Rio Baker (RB); Tres Montes peninsula(TMP); Taitao peninsula(TP). shown along the Taitao peninsula transect, including (1) regime of the margin would have evolved from subduction- rapid uplift and subsidenceof the forearcdomain [Cande and erosion to subduction-accretion.Indeed, the rebuilding of an Leslie, 1986; Bourgois et al., 1992], (2) anomalous near- accretionary prism following the partial destruction of the trenchand forearcmagmatism [Mpodozis et al., 1985; Kaeding forearc [Behrmann et al., 1994; Bourgois et al., 1996] i s et al., 1990; Lagabrielle et al., 1994; Guivel et al., 1999], (3) associatedwith the passageof the Chile triple junction along removal of forearc material from the overriding plate [Cande the margin. This situation offersto addressthe question of and Leslie, 1986; Cande et al., 1987; Behrmann et al., 1992a relative weight of factors controlling the tectonic regime of and b; Bourgoiset al., 1996], and (4) extensionaltectonics in convergent margins (i.e. type I versus type 2 convergent relation with a subducted ridge segmentat depth [Murdie et margin [von Huene and Scholl, 1991]) and the evolution al., 1993]. Moreover, ophiolite emplacement[Forsythe and through spaceand time from one type to another. Although, Nelson, 1995], elevatedthermal gradient [Cande et al., 1987], the effects of subduction-erosion have been demonstrated at alteration, diagenesis, and mineralization [Haeussler et al., many convergent margins, the constraints of physical 1995] of forearcmaterials are expectedas a consequenceof hot conditionsthat apply at the front of convergentmargins have fluids venting from the subductingspreading ridge. been poorly investigated [Scholl et al., 1980; Hussong and Results of Ocean Drilling Program (ODP) Leg 141 Wipperman, 1981; Aubouin et al., 1984; von Huene and [Behrmannet al., 1992a; Lewis et al., 1995] suggestthat the Lallemand, 1990]. Parameters such as (1) the rate and subductionof the active spreadingcenter of the Chile ridge direction of relative plate motion, (2) the topography of the beneath the South American continent is associated with a subducting plate, and (3) the type and volume of sediment major changein the tectonicregime of the continentalmargin. need to be quantified [von Huene, 1986]. As the Chile triple junction migratedto the north, the tectonic During the CTJ cruise (March 13 to April 7, 1997) of the BOURGOIS ET AL.: CHILE TRIPLE JUNCTION 8357 78øW 77øW 76øW 46ø26'S to 47øS. (3) The typical "postsubduction segment", located south of 47ø10'S, exhibits a wide accretionary wedge (i.e., the Golfo de Penasaccretionary prism). Climate-induced variations of the sediment supply to the trench axis and the tectonic reorganization at the Nazca- Antarctica plate boundary involving postsubduction ridge jump are identified as two main factors that control the tectonicregime of the Andeancontinental margin in the Chile : [: _ triple junction area. A nonaccretion or subduction-erosion mode characterized the presubduction and postsubduction segmentsduring the two last glacial maximums,whereas warmer periods associated with rapid increase in trenct• deposition caused the margin to switch from subduction- erosion or nonaccretion to subduction-acccretion.The major effects of subduction of the buoyant Chile ridge include a shallow trench which divert trench sediment supply and tectonic instabilities at the Nazca-Antarctica plate boundary. A postsubductionwestward jump of the Chile ridge occurred during the past 780 kyr. It produced slab fragmentationand individualizationof an ephemeralmicroplate. In 780 kyr, two episodes of subduction-accretionseparated by an episode of subduction-erosion occurred along the synsubduction segment.The data collectedalong the Taitao Ridge revealsthe complexthree-dimensional structure of an accretionarywedge which includes a midslope thrust sheet exhibiting the characteristicsof an ophiolite:the Taitao Ridge ophiolite. The survey carried out by the R/V L'Atalante helps to better address the problem of mass transfer in the case of the subductionof an active spreadingcenter and examine the main _ factors determining the tectonic
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