Evolutionary cycles during the Andean orogeny: repeated slab breakoff and flat subduction?

Evolutionary cycles during the Andean orogeny: repeated slabbreakoff and flat subduction?

M. R. Haschke,1* E. Scheuber,2 A. Gunther2 and K.-J. Reutter2

1Department of Geophysics and Planetary Sciences, Tel Aviv University, 69978 Tel Aviv, Israel; 2Institut fur Geologie, Geophysik und

Geoinformatik, FU Berlin, Malteserstr. 74–100, 12249 Berlin, Germany

Introduction

Variations in the distribution andchemistry of Neogene magmatic rocksin north Chile (22–33°S) tend to reflectchanges in dip of the subductingNazca plate and thickness of theoverlying lithospheric mantle andcrust (Kay et al., 1987, 1991; Coiraet al., 1993). Eastward migration ofsubduction-related magmatism fol-lowed by compressive deformationand subsequent termination of main-arc andesitic volcanism by » 10 Ma,and virtual cessation of volcanic ac-tivity by � 5 Ma (Kay et al., 1999)are diagnostic for the present flat-slabregion between 28° and 33°S (Baraz-angi and Isacks, 1976; Cahill andIsacks, 1992). Cessation of volcanismin the flat-slab region is attributed todisplacement and/or suppression ofthe asthenospheric wedge. Geochemi-cal characteristics accompanying thisNeogene tectonomagmatic evolutioninclude increasing La/Yb ratios (i.e.steepening REE patterns) and increas-ing ‘crust-like’ initial Sr- and decreas-ing initial Nd-isotopic signatures ofarc magmatic rocks (Leeman, 1983;Kay et al., 1987, 1991; Davidsonet al., 1988; Hildreth and Moorbath,

1988; McMillan et al., 1993). Thepresent contribution shows that thesetectonomagmatic and geochemicalfeatures are part of a larger scalecyclic evolution that is diagnostic ofthe entire north Chilean Andean oro-geny, and offer an integrative dynamicmodel with which to explain thiscyclicity.

Geological framework

In northern Chile (21–26°S), repeatedeastward stepwise shifts of main-arcmagmatism between Jurassic and Pre-sent have generated a collage of fourlargely parallel, eastward-youngingmagmatic arcs, each separated by£ 100-km-wide gaps with no or onlysparse magmatic rocks (Coira et al.,1982; Scheuber et al., 1994): the lar-gely Jurassic arc along the presentCoastal Cordillera, the mid-Creta-ceous arc aligning the LongitudinalValley, the late Cretaceous–Eocenearc largely seated in the Chilean Pre-cordillera and the Neogene and mod-ern central volcanic zone (CVZ)including the Western Cordillera(Fig. 1). A compilation was made ofgeochemical, magmatic and tectonicdata from pre-Neogene magmaticrocks (data from Rogers andHawkesworth, 1989; Maksaev, 1990;Dobel et al., 1992; Williams, 1992;Heumann et al., 1993; Mpodoziset al., 1993; Lucassen and Franz,1994; Pichowiak, 1994; Trumbullet al., 1999), and data from late Cre-

taceous–late Eocene magmatic rocksas published and discussed in Haschkeet al. (2002). The palaeo-arc studiedherein represents the base for themodern Andean evolution, and it isstressed that its tectonomagmatic andgeochemical features are integral partsof repeated evolutionary patterns af-fecting the entire north ChileanAndean orogeny (200 Ma–Present).Like the present subhorizontal sub-

duction segments in northern Chile,subduction from late Cretaceous–lateEocene was influenced by substantiallyincreasing convergence rates betweenthe Farallon (Nazca) and SouthAmerican Plate (Isacks, 1988). Mostof the subduction-related magmatism(� 78–49 Ma) occurred during lowconvergence rates (< 5 cm yr)1; Par-do-Casas and Molnar, 1987; Somoza,1998), at a highly oblique subductionangle within an intra- and back-archorst-and-graben type extensional tec-tonic setting in the upper plate (Char-rier and Reutter, 1994). According toPardo-Casas and Molnar (1987),reconfiguration of SE-Pacific oceanicplates at � 49 Ma led to almost arc-normal convergence and more thandoubling convergence rates, coevalwith termination of intra- and back-arc extension (� 44 Ma; Charrierand Reutter, 1994). Subsequentlyhigh convergence rates are consistentwith late Eocene Incaic horizontalshortening (� 15 km; Gunther et al.,1998) and crustal thickening in theChilean Precordillera (� 38.5 Ma;

ABSTRACT

Tectonomagmatic similarities between the modern Chileanflat-slab region and pre-Neogene magmatic episodes suggestthat they represent analogues to flat subduction. Evolutionarypatterns in each magmatic suite include (i) increasing La/Ybratios and Sr-and Nd-isotopic enrichment through time, (ii)eastward-migration of magmatism after periods of transpres-sional/transtensional intra-arc deformation, and (iii) sub-sequent termination and virtual absence of main-arc activityfor 5–10 Myr. These patterns may reflect slab shallowingfollowed by flat subduction and thickening of the overlyingcrust. If repeated, they require interchanging episodes of slab

steepening. Increasing convergence rates force slab kinkingand eventual failure of the oversteepened slab, followed byrebound of the slab tip (owing to lack of further slab pull),flat subduction and termination of subduction-related mag-matism. Rapid subduction leads to shallow overriding of thedetached slab fragment. Eclogitization of the graduallysteepening slab tip at depth and subsequent slab pullpermits asthenospheric corner flow and subduction-relatedmagmatism.

Terra Nova, 14, 49–55, 2002

*Correspondence and present address:

M. R. Haschke, Institut fur Geowissen-

schaften, Universitat Potsdam, 14476 Pots-

dam, Germany. Tel.: +49 331 977 2483;

fax: +49 331 977 5060; e-mail:

[email protected]

Ó 2002 Blackwell Science Ltd 49

Evolutionary cycles during the Andean orogeny • M. R. Haschke et al. Terra Nova, Vol 14, No. 1, 49–55.............................................................................................................................................................

50 Ó 2002 Blackwell Science Ltd

Dobel et al., 1992). Following Incaictranspressional deformation, arcmagmatic activity between 20 and26°S terminated and convergencerates decreased substantially, beforethey increased again in late Oligo-cene (� 26 Ma, Somoza, 1998)coeval with resuming arc magmatismin the area of the present WesternCordillera.Establishing reliable convergence

parameters for older Andean mag-matic episodes is more difficult be-cause of larger uncertainties involvedin plate motion reconstructions. Ear-lier studies proposed a sinistral ob-lique convergence for the Jurassic arcto explain the transtensional defor-mation and left-lateral offset alongthe Atacama fault zone (135–130 Ma), and higher convergencerates and less oblique subduction forthe mid-Cretaceous arc to accountfor the Peruvian shortening andthickening of the crust (90–80 Ma;Scheuber et al., 1994) and off-set along the San Cristobal fault(Fig. 1). Like the late Cretaceous–Eocene arc, magmatic activity in theolder palaeo-arcs terminated aftertranspressional or transtensional tec-tonic activity along the arc, andresumed after 5–10 Myr of relativemagmatic quiescence up to 100 kmfurther east.

Geochemical evolution

In the northern Chilean palaeo-arcsthe gradual increase of La/Yb ratios isthe result of gradually decreasingheavy REE contents (such as Yb).Decreasing Yb and Y (and otherheavy rare earth elements) and in-creasing Sr contents (as well as in-creasing Na2O and Al2O3) aresensitive to monitor changing lowerpressure plagioclase-and clinopyrox-ene-dominated residual mineralogies

towards higher pressure, amphibole-and garnet-bearing mineral assem-blages, when migrating through thegarnet stability field at > 12 kbar,or > 40 km crustal thickness (Rush-mer, 1993; Rapp and Watson, 1995;Petford and Atherton, 1996).La/Yb ratios in late Cretaceous–

Eocene arc magmatic rocks risegradually (from < 10 in late Creta-ceous dacites and rhyolites to 34in late Eocene syn-/post-tectonicgranitoids) as a result of decreasingYb contents (2.2–4.2 p.p.m. to 0.6–

1.5 p.p.m.), and Sri values increase(0.7039–0.7045) and Ndi values de-crease (0.51276–0.51265) with decreas-ing age. The small contrast betweenlate Cretaceous and late Eocene main-arc melts in terms of Sri and Ndiisotopic ratios rule out major compo-sitional, structural or age changesduring petrogenesis. Moreover, thepresence of less-enriched back-arc al-kaline rocks (Sri ¼ 0.7036–0.7046,Ndi ¼ 0.51280–0.51286) argue againstisotopic enrichment by recycling ofold mantle lithosphere during east-

Fig. 1 Simplified geological map of theAndes in northern Chile, schematicallyshowing the distribution of pre-Neogeneand Neogene magmatic rocks in thesouthern Central Andes. Black linesmark trench-linked strike slip faultzones in each arc. Inset: Plot of age ofAndean magmatic rocks of the NorthernCentral Andes vs. longitude (after Sche-uber et al., 1994).

Fig. 2 Plot of age (Myr) of Andean orogeny: (a) vs. La/Yb ratios, (b) vs. initial87Sr/86Sr ratios, and (c) vs. 143Nd/144Nd-isotopic ratios. The highest La/Yb ratios andmost enriched Sr- and Nd-isotopic values in each magmatic episode correlate withtranspressional/transtensional intra-arc deformation. Periods without magmatismcorrelate with flat-slab type subduction followed by stepwise eastward arc migrationof up to 100 km. Alkaline magmatism in each magmatic episode occurred after onsetof arc magmatism in the back-arc and is not part of the main-arc geochemical trendof increasing La/Yb ratios.

Terra Nova, Vol 14, No. 1, 49–55 M. R. Haschke et al. • Evolutionary cycles during the Andean orogeny.............................................................................................................................................................


propagating Neogene Andean arcmagmatism, as proposed by Rogersand Hawkesworth (1989). In addition,late Cretaceous–Eocene magmaticrocks lack correlation between in-creasing Sri and Ndi isotopic enrich-ment and increasing SiO2 anddecreasing Sr content, in contrast tomost younger CVZ silicic volcanicrocks erupted through thicker crust(e.g. Rogers and Hawkesworth, 1989;Trumbull et al., 1999). This correla-tion is attributed to upper crustalcontamination during low-pressuredifferentiation. Such evolutionary An-dean series correlating with crustalthickening are reported through timefrom late Oligocene to late Miocenelavas in the Maricunga belt (26–28°S,Kay et al., 1994), and along the south-ern volcanic zone (Hildreth andMoorbath, 1988) (Fig. 2a–c). There-fore, the moderate Sr- and Nd-isotopicenrichment in late Cretaceous–Eocenemain-arc magmatic rocks is probablya consequence of higher pressure(deep crustal) enrichment. Otherwiselate Cretaceous–Eocene main-arc vol-canic and intrusive rocks in northernChile show strong evolutionary simi-larities in geochemical and isotopicsignatures through time with thoseseen in Neogene magmatic rocks(Fig. 2a–c), such as the small-volumeof alkaline magmatic rocks in the arcto back-arc transition (67–66 Ma,Cerro Quimal and Cerro Colorado;Maksaev, 1990) during the early stageof magmatic activity (see alkalineSegerstrom and Maquinas lavas inKay et al., 1994). These chemical andisotopic evolutionary trends are char-acteristic of Andean palaeo-and mod-ern arcs in northern Chile (insets ofFig. 2a–c). Moreover, the La/Yb in-crease tends to accelerate progres-sively to higher values each timemagmatism shifted stepwise eastwardand establishes on thicker crust. Thecorrelation of increasing La/Yb ratiosin palaeo-arc magmatic rocks andcrustal thickening can be verified bytemporal correlation of highest La/Ybratios with shortening and thickeningof arc crust (about 90–80 Ma: Peruviantectonic phase, about 38.5 Ma: Incaictectonic phase) in each magmatic epi-sode, except for the Jurassic arc.Structural studies on the oldest An-dean magmatic arc show no evidenceof contractional deformation (Scheu-ber et al., 1994), consistent with a lack

of significant increase in La/Yb(Fig. 2a).Of importantance for the present

model is that each palaeo-arc mag-matic episode was characterized bytermination and lack of subduction-related magmatic activity for5–10 Myr after crustal thickening, fol-lowed by stepwise eastward migrationof the main-arc front, similar to themodern north Chilean flat-slab region.

Changing subduction geometryduring the Andean orogeny

In spite of some intra-arc variations inthe evolution of Andean magmaticarcs (21–26°S), on the larger scale ofthe complete Andean orogeny thesearcs show strikingly similar chemical,magmatic and tectonic evolutionarypatterns. The main issue is how tointerpret these repeated magmaticgaps of 5–10 Myr connecting theAndean arc magmatic episodes. Oneexplanation is a repeated and ex-tremely systematic subduction ofoceanic plateaus every 30–40 Myr atexactly the same location throughoutthe Andean orogeny (see Gutscheret al. 2000a). Alternatively, a geody-namic model is envisaged in whichtermination of subduction-relatedmagmatism reflects onset of flat sub-duction. Temporal magmatic gapsafter each magmatic episode representthe time needed to resteepen the slaband reshape a sufficiently hot astheno-spheric mantle wedge to induce arcmagmatism (Fig. 3a–d). One mechan-ism of slab steepening may be pro-gressive eclogitization of the slab tipand onset of subsequent slab-rollback.The effect of slab-rollback may bereflected by incipient back-arc riftingand alkaline magmatism during theearly stage of arc magmatism (as seenduring late Cretaceous and late Oli-gocene back-arc magmatism).Increasing convergence rates result

in slab kinking and bending betweenthe rapidly subducting hydrated shal-low slab and the deeper and alreadyeclogitized part of the slab. It isconsidered unlikely that this deepeclogite slab portion will resume toshallow depths after onset of rapidsubduction, as slab shallowing wouldrequire ‘squeezing out’ asthenosphericmaterial between the slab and theoverlying plate vs. an opposite direc-ted corner flow; the preferred model

herein predicts that rapid subductionand overriding of a shallow subduct-ing slab over a steepened and moredense (eclogitic) slab portion will re-sult in slab breakoff, and that lateEocene crustal shortening could even-tually reflect such an event. Rapidoverriding above the detached sinkingeclogite slab portion (Fig. 3c) mayhave prevented partial melts of theslab edge from rising up into theoverriding upper plate. Analogous tomodern flat subduction, the lack ofslab pull is responsible for flat sub-duction and the virtual absence of arcmagmatism between the late Eoceneand late Oligocene in northern Chile(Fig. 3c–d), and may reflect the timerequired for eclogitization and subse-quent resteepening of the slab. In fact,the westward migration of Neogenearc magmatism to the present WesternCordillera, north of the Chilean flat-slab region, may reflect the onset ofsuch a cycle, and slab-rollback mayexplain the scattered occurrence ofOligocene mafic alkaline back-arcflows at 26–28°S (Kay et al., 1994).

Discussion

Although the model presented hereincan account for many chemical, mag-matic and tectonic features, the ques-tion remains why the evolution of themodern Chilean flat-slab region wascharacterized by widening of the arc(i.e. slab shallowing), whereas the lateEocene arc narrowed prior to cessa-tion of arc magmatism (slab steepen-ing, Fig. 1 inset). One explanation isthat the concept is wrong and arbi-trary sampling simply reflects appar-ent narrowing of the late Eocene arc.Another possibility is that instead offlat subduction, subduction ceasedfrom late Eocene to late Oligocene.Both explanations are unsatisfying asthe dataset compiled and used toevolve the model is dense and leaveslittle room for additional magmaticgaps, and reconstructed convergencerates for the period from 40 to 26 Maare between 5 and 10 cm yr)1

(Somoza, 1998), which is sufficient toinduce (flat) subduction.Repeated stepwise eastward arc mi-

gration during the Andean orogenyand the actual eroded Jurassic arcalong the Coastal Cordillera are evi-dence of substantial forearc erosion,and the question arises where this



material has been transferred throughtime. Underplating such forearceroded material may account for sub-stantial forearc crustal thickening asseen beneath the Chilean Precordillera(� 70 km; Schmitz et al., 1999), but isunlikely to cause crustal thickeningbeneath the arc and thus to correlatewith increasing La/Yb ratios. Under-plating of material eroded off thecontinental edge is likely to be acompositional melange of variablecrustal components of different age,and is expected to cause considerablevariation in Sri and Ndi ratios throughtime; presumably more than the oneseen during the Andean orogeny.Another problem may be the lack

of adakites after slab failure resultingfrom plunging of the slab into arelatively hot asthenospheric mantle

wedge (suggested by Gutscher et al.,2000b). In fact, the chemistry of thesyn/post-Incaic El Abra–Fortunabatholith shows some resemblance to

rocks interpreted as adakites. Chem-ical criteria of typical slab-melts (asgiven in Table 1) show extremely lowYb (as low as 0.29 p.p.m.) and Y, and

Fig. 3 Model of repeated flat subductionduring late Cretaceous to late Oligocenearc magmatism in Northern Chile (a)78–48 Ma: onset and progressive eclog-itization at the tip of the slab leads toslab-pull, slab-rollback and successiveslab steepening followed by incipientback-arc rifting. These processes corre-lated with low convergence rates andhigh obliquity. (b) 48–39 Ma: continu-ous slab-rollback during increasing con-vergence rates and decreasingconvergence obliquity. Termination ofcrustal stretching and transitional tec-tonic regime. (c) 39–37 Ma: slab break-off after exceeding a critical anglebetween a less dense hydrated upperbasaltic slab portion and a denser eclog-itized lower portion of slab. Removal ofthe denser eclogite portion allows re-bound of the slab and subsequent flatsubduction. Lack of eclogite (i.e. lack ofsufficient slab-pull) is resonsible for flatsubduction after slab-breakoff, prevent-ing subduction-related magmatism. Thisepisode correlates with transpressionand crustal shortening (Incaic tectonicphase) at high convergence rates andalmost arc-normal convergence. (d) 37–26 Ma: virtually flat subduction andlack of subduction-related magmatismafter slab breakoff and rebound untilabout 26 Ma. Time needed to (re)initiateslab steepening (owing to onset of eclog-itization). Arc magmatism resumes asslab steepens to allow a sufficiently hotmantle wedge. Slab rollback causes in-cipient back-arc rifting and alkalinemagmatism (Segerstrom flows andMaquinas lavas).



very high Sr (as high as 2650 p.p.m.)contents (see Kay, 1978; Smith et al.1979). North Chilean late Eocene syn/post-tectonic granitoids show higherYb contents (0.6–1.5 p.p.m.), lowerLa/Yb ratios (18–34) and much lowerSr concentrations (Sr 302–612 p.p.m.),and most importantly they occur di-rectly above the thickened crustal keelof the late Eocene magmatic arc whereIncaic crustal shortening caused cru-stal thickening. Adakites tend to occuroff the main-arc axis and are rarelypart of temporally coherent main-arcmagmatic series.Evolution of these chemical charac-

teristics may not be representative forthe entire central Andean volcaniczone (CVZ) as sampling reveals aconsiderable range in La/Yb ratios;however, it is relevant for the presentstudy that there is an evolutionarycycle tying tectonic, magmatic andgeochemical features together, nomatter what interpretation is applied(see Fig. 2a–c). Based on results fromthe recent central Andean magmaticarc, this may be diagnostic of crustalthickening, followed by slab shallow-ing and subsequent cessation of arcmagmatism.A previous similar approach to

establish geochemical patterns duringalmost 200 Myr of Andean subduc-tion magmatism was conducted byRogers and Hawkesworth (1989), buttheir work addressed the issue of crustgeneration and concentrated on chem-ical constraints without taking chan-ging convergence parameters andassociated tectonic settings intoaccount.It is not suggested that the entire

Andean orogeny was as simple as themodel outlined here. In fact, eachpre-Neogene magmatic episode com-prises its own and far more complexevolution than could possibly have

been elaborated in the present study,but on a larger scale the evolution ofAndean palaeo-arc magmatic epi-sodes in northern Chile has generatedclear and distinct tectonic, magmaticand geochemical patterns, establish-ing palaeo-arc magmatic episodes asdistinct analogues of the modernNeogene sequence. Recognizing suchpatterns may help identifying howcyclic variations in subduction dy-namics modulate the nature andcomposition of the magmatism gen-erated there. This is particularly im-portant for the Andes, as they havebeen the model region for under-standing magmatic and tectonic ef-fects of subduction on modern andancient continental margins since theearliest days of plate tectonics.

Acknowledgments

Thorough and constructive reviews by JonP. Davidson, N. Petford, M.-A. Gutscherand S. Lallemand greatly improved anearlier version of this manuscript. Fundingfor this study was provided by the Son-derforschungsbereich 267 ‘Deformations-prozesse in den Anden’ at the FU Berlin.

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Table 1 Characteristics of magmas (> 60 wt% SiO2)

Slab melts (‘adakites’) Syn-/post-tectonic El Abra/Fortuna batholith

Kay (1978) Smith et al. (1979) Haschke et al. (2001) and previous authors

La/Yb 30–57 108–238 18–34

Sr/Y – 177–344 19–57

Sr 1783–2600 1520–2650 302–612

Yb 0.948–0.633 0.52–0.29 0.6–1.5

Y – 7.7–9.9 8.0–22

Na2O 3.22–3.7 2.23–5.34 4.0–5.5

Al2O3 15.2–15.5 13.7–18.4 16.5–17.8



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Received 17 May 2001; revised versionaccepted 17 November 2001



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Evolutionary cycles during the Andean orogeny: repeated slab breakoff and flat subduction?