the migration history of the nazca ridge along the peruvian active margin.pdf

The migration history of the Nazca Ridge along the Peruvianactive margin: a re-evaluation

Andrea Hampel

GEOMAR Research Center for Marine Geosciences, Wischhofstr. 1^3, 24148 Kiel, Germany

Received 21 March 2002; received in revised form 9 July 2002; accepted 23 July 2002

Abstract

The collision zone of the 200 km wide and 1.5 km high Nazca Ridge and the Peruvian segment of the convergentSouth American margin between 14S and 17S is characterized by deformation of the upper plate and severalhundred meters of uplift of the forearc. This is evident by a narrowing of the shelf, a westward shift of the coastlineand the presence of marine terraces. As the Nazca Ridge is oblique with respect to both trench and convergencedirection of the Nazca Plate, it migrates southward along the active plate boundary. For reconstructing the migrationhistory of the Nazca Ridge, this study uses updated plate motion data, resulting from a revision of the geomagnetictime scale. The new model suggests that the ridge crest moved laterally parallel to the margin at a decreasing velocityof V75 mm/a (before 10.8 Ma), V61 mm/a (10.8^4.9 Ma), and V43 mm/a (4.9 Ma to present). Intra-platedeformation associated with mountain building in the Peruvian Andes since the Miocene reduces the relativeconvergence rate between Nazca Plate and Peruvian forearc. Taking an intra-plate deformation at a rate ofV10 mm/a,estimated from space-geodetic and geological data, into account, does not significantly reduce these lateral migrationvelocities. Constraining the length of the original Nazca Ridge by its conjugate feature on the Pacific Plate yields alength of 900 km for the subducted portion of the ridge. Using this constraint, ridge subduction beganV11.2 Ma ago at11S. Therefore, the Nazca Ridge did not affect the northern sites of Ocean Drilling Program (ODP) Leg 112 located at9S. This is supported by benthic foraminiferal assemblages in ODP Leg 112 cores, indicating more than 1000 m ofsubsidence since at least Middle Miocene time, and by continuous shale deposition on the shelf from 18 to 7 Ma,recorded in the Ballena industrial well. At 11.5S, the model predicts the passage of the ridge crestV9.5 Ma ago. Thisagrees with the sedimentary facies and benthic foraminiferal stratigraphy of ODP Leg 112 cores, which argue fordeposition on the shelf in the Middle and Late Miocene with subsequent subsidence of a minimum of several hundredmeters. Onshore at 12S, the sedimentary record shows at least 500 m uplift prior to the end of the Miocene, also inagreement with the model.? 2002 Elsevier Science B.V. All rights reserved.

Keywords: Nazca Ridge; oblique subduction; plate reconstruction; forearc; Peru

1. Introduction

Seamount chains, submarine ridges and otherbathymetric highs on oceanic plates entering sub-duction zones will, in general, laterally migratealong the active margin, unless they are parallel

0012-821X / 02 / $ ^ see front matter ? 2002 Elsevier Science B.V. All rights reserved.PII: S 0 0 1 2 - 8 2 1 X ( 0 2 ) 0 0 8 5 9 - 2

* Present address: GeoForschungsZentrum Potsdam, Tele-grafenberg, 14473 Potsdam, Germany.Tel. : +49-331-288-1376; Fax: +49-331-288-1370.E-mail address: [email protected] (A. Hampel).

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Earth and Planetary Science Letters 203 (2002) 665^679

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to the convergence direction (e.g. [1,2]), and mayaect the sedimentological and tectonic evolutionof the forearc system signicantly. The lateral mo-tion of such features can lead to a temporal se-quence of uplift and subsidence of the forearc,frequently accompanied by enhanced surface andtectonic erosion as well as steepening of the innertrench wall and faulting in the upper plate (e.g.[3^8]). These eects are generally recorded in themorphology and sedimentary facies of the forearcand in uplifted coastal shorelines. As a conse-quence, models resolving the history of forearcand arc systems must account for these three-di-mensional eects and their development throughtime.The velocity at which a bathymetric high moves

along an active margin is controlled by three pa-rameters : the convergence velocity vc and the twoangles a and P, dened by the orientation of thebathymetric high relative to convergence directionand trench, respectively (Fig. 1). The lateral ve-locity vlat of a bathymetric high parallel to theplate boundary is then:

vlat vcsinasinP

Even if the convergence velocity is constant, acurvature of the trench line, i.e. a variable angle P,

would result in a variable lateral migration veloc-ity.The fate of bathymetric highs during subduc-

tion to greater depth has long been subject tocontroversy. While some authors note the tempo-rally irregular occurrence and reduced number oflarge earthquakes in the vicinity of such features(e.g. [10]), others argue that subducting sea-mounts and ridges form asperities, at which earth-quakes may nucleate [11] and increase seismiccoupling [12]. In addition, the buoyancy of sub-ducted bathymetric highs may decrease the dip ofthe subducting slab and thus may terminate themagmatic activity in the overriding plate [7,10,13^15].An outstanding example of a subducting bathy-

metric high migrating along an active plateboundary is the Nazca Ridge, which has aectedthe Peruvian portion of the long-lived Andeansubduction zone. Due to southward migration ofthe ridge, the Peruvian margin displays, fromsouth to north, dierent stages of its tectonic evo-lution during and after ridge passage. Variousfeatures in the oshore and onshore geology ofthe Peruvian margin, such as uplift and subsi-dence of forearc basins, tectonic erosion of thelower continental slope and uplift of marine ter-races have been attributed to ridge subduction[16^21]. Moreover, the coastal area above thesubducting ridge was ruptured by two shallowthrust earthquakes with magnitudes of Mw = 8.1and Mw = 7.7 in 1942 and 1996, respectively [22].The downward continuation of the ridge has beenrelated to a zone of reduced intermediate depthseismicity and to the southern boundary of thelow-angle subduction segment beneath SouthernPeru [23^25], which coincides with the terminusof the Quaternary volcanic arc [14,26]. To corre-late these dierent observations with the subduc-tion of the Nazca Ridge, it is crucial to constrainboth the rate of its lateral movement along themargin and the original length of this feature.The rst part of this study calculates the migra-

tion velocity of the Nazca Ridge and yields a sig-nicantly slower lateral motion than previouslyinferred [16,18^21,25,27,28], with the consequencethat ages at which the ridge passed specic sitesincrease signicantly. The second part species

Fig. 1. Geometric relations between the lateral migration ve-locity vlat of a bathymetric high parallel to an active plateboundary, the plate convergence velocity vc, and the orienta-tion of the bathymetric high relative to convergence directionand trench [9].


A. Hampel / Earth and Planetary Science Letters 203 (2002) 665^679666

the onset of ridge subduction, assuming that theoriginal length of the Nazca Ridge approximatesthat of its conjugate feature on the Pacic Plate[18,25,27,28].

2. Geodynamic setting

The Nazca Ridge is a more than 1000 km longand 200 km wide aseismic submarine ridge, whichformed at the Pacic^Farallon/Nazca spreadingcenter in the early Cenozoic [25,29,30] (Fig. 2).The linear crest of the ridge is elevated 1500 mabove the surrounding sea oor and trendsN42E. The average crustal thickness of the ridgederived from the analysis of Rayleigh waves is18R 3 km [30]. Where the ridge descends beneaththe South American Plate, the trench does notshow a pronounced deviation from its lineartrend, but the water depth along the trench lineshoals from 6500 m south of the ridge to 4000 mat the ridge crest. In bathymetry and side-scansonar images, features indicating ongoing surfaceerosion and faulting have been identied on thecontinental slope [32,33]. Landwards, the recentcollision zone is expressed by a narrowing of theshelf, a seaward shift of the coastline and thepresence of raised marine terraces at the coastbetween 13.5S and 15.6S [19,20]. Above thenorthern ank of the subducted ridge, the recentsubsidence of the marine terraces, which had beenuplifted earlier by the ridge until the passage of itscrest, illustrates its southward movement [19,20].Further inland, the Abancay Deection (Fig. 2),which marks the northern boundary of the zoneof active arc volcanism and separates segments ofcontinental crust diering in geochemical compo-sition, has been related to the continuation of theNazca Ridge [34].North of the collision zone, a small accretion-

ary wedge may have begun to grow in the wake ofthe ridge [18]. Further north, o Central andNorth Peru, the absence of a large accretionaryprism and tectonic erosion as the dominant masstransfer regime have been recognized [35,36].Along this part of the margin, long-term tectonicerosion since at least the Middle Miocene has ledto rapid subsidence of the forearc and to an east-

ward shift of the trench and the magmatic arc[27]. However, interpretations of seismic dataand ODP cores, in particular in the Lima Basinat 11.5S, indicate that during some periods, theforearc subsided at a lower rate than during timesof prevailing long-term tectonic erosion or haseven been uplifted [17,37].Regarding the temporal evolution of the colli-

sion zone between the Nazca Ridge and the Pe-ruvian margin, current models dier in the lateralmigration velocities, in the ages of ridge passageassigned to dierent latitudes and in the predictedlength of the original Nazca Ridge. The followingreconstructions cover the migration history of theNazca Ridge along the entire Peruvian margin:Pilger ([25] ; his gure 4) shows that the ridge rstcame in contact with the Peruvian trench at 5S inthe Middle Miocene and later passed 10S atV9Ma. Other studies [16,18,27], based on plate re-constructions [28] and the NUVEL-1A conver-gence rate [38], inferred that the Nazca Ridge be-gan to subduct 8 Ma ago at 8S and was locatedat 9S and 11.5S at 6^7 Ma and 4^5 Ma, respec-tively. Three other reconstructions concentrate onthe migration of the ridge from the end of theMiocene to the present: Based on the plate mo-tion data by Pardo-Casas and Molnar [39], Hsu[19] infers a lateral migration velocity of V71mm/a. Machare and Ortlieb [20] use the platemotion data by Pardo-Casas and Molnar [39] todeduce a passage of the ridge crest at 13S at4 Ma, i.e. a lateral velocity ofV64 mm/a. Le Rouxet al. [21] suggest that the ridge crest was locatedat V13.5S at 5.3 Ma and thus o Lima (12S)before the end of the Miocene, i.e. laterally mov-ing at a velocity ofV42 mm/a, derived from con-vergence rates given by Stein et al. [40]. Thesedierences in the inferred migration rates of theNazca Ridge underline the importance of the re-evaluation presented here.

3. Reconstruction of the migration history

3.1. Lateral migration velocity

Unraveling the migration history of subductingridges, seamount chains and other submarine


A. Hampel / Earth and Planetary Science Letters 203 (2002) 665^679 667

Fig. 2. Map [31] showing the location of the Nazca Ridge, the spatial distribution of seismicity and active volcanoes (black trian-gles; from the Smithsonian Global Volcanism Program). ODP Leg 112 sites and two industrial wells (Ballena, Deln) are markedby white circles. The Peruvian low-angle subduction segment is located between 5S and 14S. Note the gap in the intermediatedepth seismicity (70^300 km) (dotted line) and the presence of deep seismic events (500^650 km) beneath Brazil (dashed line).(Earthquake data from 1973 to 2002; US Geological Survey^National Earthquake Information Center.)



bathymetric highs requires knowledge of pastplate motions, which can be obtained by twotypes of data sets. Plate motions averaged overthe last 3 Ma are provided by the NUVEL-1Amodel, based on evaluation of spreading rates,transform fault azimuths and earthquake slip vec-tors [38]. On longer time scales, paleo-plate posi-tions and motions can be reconstructed by ana-lyzing the magnetic anomalies of the oceaniccrust. This method yields average velocity vectorsfor dierent time intervals (e.g. [28,41]).This study uses updated Nazca (Farallon)^

South American relative motions [42] which takeinto account a revision of the global geomagnetictime scale [43]. This data set provides constantconvergence velocities and directions for dierenttime intervals for the last 40 Ma at dierent lat-itudes, of which the values given at 12S are ap-plied (Table 1). The convergence rate of 75 mm/afor the last 5 Ma [42] agrees well with the NU-VEL-1A prediction [38]. Both estimates are higherthan the current convergence rate determined byspace-geodetic measurements, i.e. 61R 3 mm/a at12S [44,45]. Since the convergence rate may beslowing with time, the space-geodetic values areless relevant for this reconstruction.Using the average convergence velocities and

directions for the three latest time intervals, threedisplacement vectors and respective paleo-posi-tions of the Nazca Ridge relative to a xed SouthAmerican Plate are constructed (Table 1 and Fig.3a). The resulting time path allows one to deter-mine when the ridge crest passed a specic pointon the trench line, assuming a linear continuationof the ridge towards the trench, as suggested bythe shape of the present ridge, and a paleo-trenchposition similar to the present trench line [18^21,25,27,28] (Fig. 3b).Uncertainties in the convergence velocities are

not specied [42], but may be of the order of 10%[44]. Since the errors are likely to be smaller in thelatest time interval, as suggested by the errors ofthe NUVEL-1A convergence rates [38], and maybe larger in the earliest time interval, this studyassigns uncertainties of 5%, 10% and 15% to theconvergence velocities of the 0^4.9 Ma, 4.9^10.8Ma and 10.8^16 Ma time intervals, respectively(Table 1). Using these error limits, the uncertain-ties in the ages of ridge passage with respect to theconvergence rates of the three time intervals aregiven in Table 1. Potential errors of the geomag-netic time scale and of the convergence azimuthsfor the dierent time intervals have not been tak-en into account.An implicit assumption of this reconstruction is

that the decreasing relative convergence rate be-tween the Nazca Plate and stable South Americaover the last 15^20 Ma, as derived from platereconstructions, equals the amount of relative mo-tion between the Nazca Plate and the Peruvianforearc. This assumption has also been the basisfor all previous reconstructions of the NazcaRidge motion [16,18^21,25,27,28]. However, thepresence of the Andean mountain belt east ofthe forearc demonstrates that, strictly speaking,this assumption is not correct, since some of therelative plate motion is taken up by intra-platedeformation within the South American Plate.Obviously, this intra-plate deformation tends toreduce the relative motion between the NazcaRidge and the Peruvian forearc system. Atpresent, a rigorous assessment of the amountand the direction of shortening accommodatedin the Peruvian Andes is dicult due to the lackof sucient geological data. Nevertheless, space-geodetic measurements [44] and geological proles([46^48] and references therein) across the Andescan be used to estimate the present-day and past

Table 1Relative plate motion between Nazca and South American plates at 12S [42]

Time interval Convergence velocitya Convergence direction Length of displacement vector Age uncertainty[Ma] [km/Ma] [km] [Ma]

0^4.9 (chrons 0^3) 75R 4 77 368R 20 R0.34.9^10.8 (chrons 3^5) 106R 11 82 625R 65 R0.610.8^16 (chrons 5^5C) 123R 18 84 640R 94 R0.8a Errors are assumed to be 5%, 10% and 15% for the latest, intermediate, and earliest time interval, respectively.



shortening rates across the Eastern Cordillera andthe Subandean belt [49]. These data show thatgeologic and space-geodetic displacement ratesare generally consistent and that directions ofshortening in the Eastern Andes are approxi-mately parallel to the Nazca^South America con-vergence vector. The data have been interpreted interms of a two-stage model with rates of short-

ening across the Eastern Andes of 5^8 mm/a forthe last 25^10 Ma and of 10^15 mm/a for the last10 Ma [49]. In order to account for the Andeanintra-plate deformation, the lateral migration ve-locity of the Nazca Ridge is also presented for ascenario in which an average Andean shorteningrate of 10 mm/a for the last 16 Ma is subtractedfrom the relative convergence velocity between

Fig. 3. (a) Three paleo-positions of the Nazca Ridge and displacement vectors for the present intersection point of ridge andtrench. Gray lines represent the assumed linear continuation of the ridge. Inset (b) shows diagram in which the latitudinal posi-tion of the linearly continuing ridge crest on the trench line and the migration velocity of the ridge parallel to the plate boundaryare plotted versus time. The two black lines are derived by using the relative plate motion data as given in [42]. The black arrowmarks the onset of ridge subduction inferred by this study (Section 3.2). The two gray lines refer to a scenario in which a smallamount of intra-plate deformation (10 mm/a) accommodated in the Peruvian Andes is subtracted from the convergence rates of[42].



Nazca and South American plates (gray lines inFig. 3b). Considering the intra-plate deformationtends to slightly increase the ages of ridge passageassigned to specic latitudes, in other words, thelateral migration velocity of the ridge slightly de-creases. However, the geological implications ofthe model (see below) remain valid, even if theintra-plate deformation is taken into account.To allow a straightforward comparison of themodel with previous reconstructions of the NazcaRidge motion, the following discussion uses themodel-curve neglecting intra-plate deformation(black lines in Fig. 3b). Once more detailed infor-mation on Andean shortening rates and directionsin Peru becomes available, it should be incorpo-rated into the model.In summary, the rst part of the reconstruction

demonstrates that the ridge moved signicantlyslower parallel to the margin than inferred byprevious studies [16,18^21,25,27,28]. In particular,a ridge of sucient length would have passed theODP Leg 112 sites in the Trujillo/Yaquina (9S)and Lima basins (11.5S) at V14.5 Ma and atV9.5 Ma, respectively. Apart from this migrationhistory, deducing the onset of ridge subductionrequires an estimate of the length of the originalridge.

3.2. Original length of the Nazca Ridge and onsetof ridge subduction

The preservation of oceanic ridges and plateausin the Southeastern Pacic oers the possibility toconstrain the shape of already subducted parts ofbathymetric highs on the Nazca Plate by theirmirror images on the Pacic Plate (Fig. 4). Asthese pairs of conjugate highs have formed simul-taneously at the Pacic^Farallon/Nazca spreadingcenter (e.g. [29,52]), they are thought to have asimilar length and shape assuming symmetricspreading [25,52].The Nazca Ridge has a common origin with the

Tuamotu Plateau at the Pacic^Farallon/Nazcaspreading center [25,29,30] and the pre-conditionof symmetric spreading seems to be met, since therespective segments of the Nazca and Pacicplates between chrons 13 and 23 have similarwidths (see Figs. 4b and 5).

The N70W trending, elongated Tuamotu Pla-teau is a composite feature consisting of islandchains and oceanic plateaus with volcanic edicesthat once were subaerial and today form atolls[54], whereas the Nazca Ridge is characterizedby smaller, but similar submarine volcanic fea-tures [32]. Despite these dierences in their topog-raphy, both ridges have an overall linear trend.Therefore, the 4000 m water depth contour lineof the Tuamotu Plateau has been used to approx-imate the outline and total length of the originalNazca Ridge [18,25,27,28]. To estimate the lengthof the subducted part of the Nazca Ridge, how-ever, it has to be taken into account that thenorthwesternmost part of the Tuamotu Plateauformed on 10^20 Ma old oceanic crust of thePacic Plate, indicating an origin 600 km o thespreading center [54]. The hotspot that generatedthe northwesternmost part of the Tuamotu Pla-teau [55] most likely had no eect on the NazcaPlate [54]. For this reason, the northwestern endof the plateau probably does not have a counter-part on the Nazca Plate. Another assumptionmade to specify the onset of ridge subduction isthe use of the present trench line as the paleo-trench position [18^21,25,27,28].To estimate the length of the original Nazca

Ridge, a mirror image of the Tuamotu Plateauis created using its 4000 m contour line. To ndthe correct position of the mirror image on theNazca Ridge, magnetic anomaly lineations ofthe surrounding sea oor are tted, using a globaldata set [51,52] together with specic data for theTuamotu Plateau region [53]. Chrons 15^20 arethe oldest magnetic anomalies common to thesea oor close to both features (Fig. 4b).To t these chrons north and south of the Tua-

motu Plateau to the ones on the Nazca Plate, noscaling of the mirror image is needed, which in-dicates symmetric sea oor spreading. On theNazca Plate, the trends of the chrons are betterconstrained north than south of the ridge andappear to be roughly parallel to each other (Fig.4b). In contrast, the same magnetic lineations areat an angle with each other north and south of theTuamotu Plateau. As a consequence, tting thechrons leads to two endmember positions (Fig.5). Matching chrons 19 and 20, located south of



the ridges, with chrons 18^21 being parallel, leadsto an abrupt bend of the original Nazca Ridgewhich results in a N16W trend and a length ofabout 1000 km corresponding to an onset of sub-duction V10.0 Ma ago at 8.5S (Fig. 5). Adjust-ing chrons 16 and 18, located north of the bathy-metric highs, with chrons 15^20 being parallel,leads to the position of the mirror image preferredby this study, because in that case the NazcaRidge continues linearly beneath South Americafor 1100 km, suggesting that the rst contact ofridge and trench occurred V12.5 Ma ago at alatitude of 10S (Fig. 5). Regarding the locationof chron 18, it should be noted that the spatialextent of its magnetic signal allows dierent

phases to be picked as chron 18. Since the detailsof the picking procedures are not available for allpublications ([51^53] and references therein), thisstudy uses the locations of chron 18 as shown inthe published maps. Given that the preferred re-construction is additionally constrained by chrons15 and 16, the possible non-unique identicationof chron 18 by dierent authors is considered tohave only a minor eect on the reconstructedlength of the Nazca Ridge.The values of 1000 km and 1100 km for the

original length of the Nazca Ridge, as inferredabove, are maximum values. Taking into accountthat theV200 km long northwesternmost part ofthe Tuamotu Plateau most likely does not have a

Fig. 4. (a) Bathymetric map [31] of the South Pacic showing the Pacic^Nazca spreading center and the conjugate features Naz-ca Ridge and Tuamotu Plateau. (b) Outlines of the Nazca Ridge and the Tuamotu Plateau are shown by their 4000 m waterdepth contour lines. The global age grid [50] of the oceanic crust interpolated from magnetic anomalies is shown by color code.Selected magnetic anomaly lineations are represented by black [51], blue [52] and red [53] lines.



counterpart on the Nazca Plate yields the pre-ferred scenario of this study, in which the originalridge continues for V900 km beneath SouthAmerica and entered the trench V11.2 Ma agoat 11S (Fig. 5).

4. Discussion

The new model presented for the kinematic

evolution of the Nazca Ridge predicts, for bothendmember positions described above (Fig. 5), alateral migration history that diers signicantlyfrom previous studies [16,18^21,25,27,28]. Withrespect to the two possible positions of the mirrorimage of the Tuamotu Plateau, this study preferstting the magnetic anomaly lineations 16 and 18north of the plateau instead of chrons 19 and 20south of it for the following reasons: First, withthis t, the straight Nazca Ridge continues with-

Fig. 5. Migration history of the Nazca Ridge on the assumption that the mirrored Tuamotu Plateau resembles the subductedpart of the Nazca Ridge. The magnetic anomalies on the Nazca Plate are marked in black and blue. The magnetic lineationsnorth and south of the Tuamotu Plateau have red and green colors, respectively (see inset). At the present collision zone, twoendmember models for the continuation of the Nazca Ridge are shown: Adjusting chrons 15^20, located north of both features,yields the red mirror image of the plateau. Fitting chrons 18^21, located south of both ridges, leads to a position of the mirroredTuamotu Plateau shown as the green mirror image. Both mirror images are plotted without consideration of the variable dip ofthe subducting plate. For both mirror images, the lighter colors at their northeasternmost ends mark the V200 km long part ofthe Tuamotu Plateau which most likely does not have a counterpart on the Nazca Plate (see text for details). Thus, the red mir-ror image with a linear continuation of V900 km is the preferred scenario of this study. Note the coincidence of the preferredred mirror image with the reduced intermediate depth seismicity (dotted line) and with the presence of deep seismic events be-neath Brazil (dashed line). For the onset of ridge subduction, three dierent scenarios are presented: Using the preferred congu-ration, the original Nazca Ridge entered the trench V11.2 Ma ago at 11S (red). If the original Nazca Ridge continues for 1100km, its subduction began V12.5 Ma ago at 10S (light red). The northernmost possible contact of ridge and trench at 8.5S cor-responds to a mirror image adjusted to chrons 19^21 (green).



out a bend. Second, the outline of the northernarm of the Tuamotu Plateau resembles the mod-ern Nazca Ridge, in agreement with their prob-able alignment during their common origin [30].Apart from that, the uncertainties in the directionof the magnetic anomalies 16 and 18 are consid-erably smaller than those of the shorter chrons 19and 20, which, like in the Tuamotu Plateau re-gion, might not be parallel to chrons 16 and 18.Dierent orientations of chrons 18 and 19 arealso suggested by the magnetic anomalies of theNazca Plate south of the Nazca Fracture Zone(Fig. 4). Another argument is that a linear con-tinuation of the Nazca Ridge coincides well withthe zone of reduced intermediate depth seismicityand the southern boundary of the segment of low-angle subduction beneath South Peru (Figs. 2, 5).The predicted northeastern end of the ridge cor-relates with the cluster of deep seismic eventsbeneath Brazil between V8.5S and V10.5S(compare Figs. 2 and 5). This agrees well withinterpretations of the deep seismicity that proposean association of the southern earthquake clusterwith the subducted part of the Nazca Ridge[56,57]. Moreover, a VN42E trending ridge co-incides with the northern boundary of active vol-canism and the Abancay Deection [34,15]. Pilger[25], however, argued for the position tted tochron 19, because the ridge then extends fartherto the north and thus can explain the at slabbeneath Northern Peru. This northern at slab,however, may be caused by the subducted partof the Carnegie Ridge o North Peru/Ecuadoror by another, completely subducted oceanicplateau [58]. Taken together, these argumentsstrongly support a linear continuation of the ridgeof V900 km and an onset of ridge subductionV11.2 Ma ago at 11S.If the Nazca Ridge, continuing with a linear

trend, had entered the trench at 8S [16,18,27,28], its subduction would have begun V16 Maago and the original ridge would have to be atleast 1500 km long. Such a length is not sup-

ported by the conjugate feature of the NazcaRidge on the Pacic Plate, since the entire Tua-motu Plateau is at most 1100 km longer than themodern Nazca Ridge. Thus, although the trenchhas probably been shifted eastward for at least 20Ma due to tectonic erosion [27], a linearly trend-ing original Nazca Ridge could not have reachedthe trench north of 10S.The new reconstruction has signicant implica-

tions for models of the tectonic, sedimentologicaland geomorphic evolution of the Peruvian forearcand arc systems. In particular, dierent seismicdata sets (e.g. [59,60]) and ODP Leg 112 coresin the Trujillo/Yaquina (9S) and Lima basins(11.5S) have been interpreted [17,18,60,61] inlight of previous reconstructions suggesting thatthe ridge crest passed these sites V6 Ma andV4 Ma ago, respectively [16,18,27,28]. Accordingto this study, however, the ridge was not su-ciently long to inuence the region at 9S, whileat 11.5S, it already caused maximum upliftV9.5Ma ago (Fig. 3). The marine and subaerial sedi-mentological record of the forearc, the onshoretectonic history, and the temporal and spatialevolution of volcanism of the Andean magmaticarc in Peru support the new model and will bediscussed in some detail.In marine sediments, uplift of the forearc region

can, in general, be derived from a trend to coarserdeposits, possibly accompanied by an increase inthe number of unconformities, and from benthicforaminiferal stratigraphy that gives informationon the water depth at which the sediment wasdeposited. At 9S, benthic foraminiferal assem-blages in ODP Leg 112 cores and dredge samplesindicate that the continental slope and shelf sub-sided V1500 m between the Middle Eocene toMiddle Miocene and experienced further subsi-dence of 1300 m since 12^13 Ma [37,62]. Apartfrom that, cores recovered during ODP Leg 112and two industrial wells are characterized by thedeposition of ne-grained material, while sandydeposits are missing in the Miocene (Fig. 6a)

6Fig. 6. (a) 9S: Lithology of ODP Leg 112 sites [60] and of the Ballena industrial well, located on the shelf, with ages of datedsamples (black circles) [63]. (b) 11.5S: Lithology of ODP Leg 112 sites that drilled into Miocene strata [60]. Paleo-bathymetry isderived from benthic forminiferal assemblages of site 679, located on the outer shelf [37].



[60]. Especially in the Ballena industrial well,located above the crest of the outer shelf high,continuous shale deposition between 18 and7 Ma [63] argues against the disturbance of thedeposition milieu due to the passage of a ridge(Fig. 6a). A comparison with the recent collisionzone shows that the shelf area is strongly aectedby the Nazca Ridge. Marine deposits of Eoceneto Upper Pliocene age that correlate with equiv-alent strata in submerged oshore forearc basinso Central Peru have been raised above sea level[19].At 11.5S, deposits at ODP Leg 112 sites be-

come coarser, with a decrease in mud and an in-crease in silt and sand during the Middle and LateMiocene (Fig. 6b). At site 679, a layer of con-glomerates has been deposited before the end ofthe Miocene. In cores recovered at ODP site 679,Middle and Late Miocene benthic foraminiferalassemblages reect deposition on the inner shelfin shallow water [37]. Following the hiatus at theend of the Late Miocene, deposition resumed atthe outer shelf in the early Pliocene. The nextforaminifers-bearing strata are of Quaternaryage, with deposition depth uctuating around400 m. At site 682, Middle to Late Miocene fora-miniferal assemblages have been deposited at mid-dle bathyal depths (500^1500 m), while the LatePliocene paleo-environment was lower bathyal(2000^4000 m) [37]. Site 688 is barren of LateMiocene foraminiferal assemblages, however, be-tween Early Miocene and Quaternary, the paleo-biotope changed from upper middle bathyal (500^1500 m) to lower bathyal depth (2000^4000 m)[37]. In addition, earlier investigations based ondredge samples indicate more than 2000 m subsi-dence for 6 Ma, since Late Miocene benthic for-aminifers, living at V500 m depth, were recov-ered in the Lima Basin at a water depth ofmore than 2600 m [62]. Based on these initialODP Leg 112 results, a phase of uplift and ero-sion at 11.5S was derived to begin at 11 Ma andlast until 7 Ma, while 6 Ma ago, a transition fromuplift to subsidence occurred [16]. The oshoregeological record of ODP Leg 112 as summarizedabove shows uplift of the forearc during Middleand Late Miocene and subsidence since the end ofthe Miocene. This correlates very well with the

age ofV9.5 Ma derived from the new reconstruc-tion for passage of the ridge crest.The new model is also compatible with the sed-

imentological record of the R|mac^Chillon riversat 12S, which eroded deep valleys on the Limacoastal plain during the Miocene. The alluvial fandeposited by these rivers experienced uplift of atleast 500 m, which is attributed to the passage ofthe Nazca Ridge [21]. Potential sea level changesduring the Quaternary and Pliocene are smallerthan V125 m and have been considered [21].The uplift maximum at 12S was attained beforethe end of the Miocene [21].Another piece of evidence in support of the

presented model may be inferred from the corre-lation of the Nazca Ridge with the associated seg-ment of low-angle subduction and the cessation ofmagmatic arc activity. At present, the boundarybetween active and ceased volcanism in the southand in the north, respectively, is located in thelandward continuation of the ridge, but mayhave gradually propagated southward due to thelateral movement of the ridge. Oshore, volcanicash layers recovered during ODP Leg 112 havebeen interpreted to show higher activity of thePeruvian volcanic arc in the Late Miocene for9S than for 12S [64]. Onshore geochronologicaldata throwing light on a possible southward prop-agating zone, where volcanism has ceased, are,however, rather limited [65,66]. Pulses of Miocenevolcanic activity [65,66] have been interpreted incontext of the Quechua tectonic phases of theAndean orogeny in Peru during the Middle toLate Miocene [64,67]. The Quechua II (V10Ma) and Quechua III (V5 Ma) tectonic phases,which seem to be related to changes in the relativeplate motion of the Nazca and South Americanplates [65,39], have been correlated with uncon-formities in ODP Leg 112 cores at 11.5S [16].According to this study, the Nazca Ridge inu-enced this region V9.5 Ma ago, which seems tocoincide with the Late Miocene Quechua II tec-tonic phase. Despite this apparent correlation, itshould be noted that the concept of distinct tec-tonic phases in Peru has been criticized, as theavailable temporal constraints argue in favor ofprolonged periods of tectonic activity [68]. Never-theless, in the Ecuadorian Andes, subduction of



the Carnegie Ridge since the Middle Miocenemay be responsible for the development of a high-er topography, a compressional stress regime, andincreased crustal cooling and exhumation rates,deduced from ssion track data in the collisionzone [69].While, in summary, no single observation is

conclusive about its relation to the subductionof the Nazca Ridge, the combination of the argu-ments raised above strongly suggests that the newmodel is more compatible with the existing geo-logical and geomorphic data.

5. Conclusions

This new reconstruction of the migration historyof the Nazca Ridge along the Peruvian marginsuggests that the lateral motion of the ridge hasdecelerated through time. Considering that a smallamount of the relative convergence rate betweenthe Nazca and South American plates is taken upby intra-plate deformation in the Andean moun-tain belt results in slower lateral migration of theridge. However, this has no eect on the geolog-ical implications of the new model. On the as-sumption that the original Nazca Ridge has alength similar to its mirror image on the PacicPlate, it continues for V900 km beneath SouthAmerica. Therefore, the northeastern end of theNazca Ridge entered the trench V11.2 Ma agoat 11S. As a consequence, the ridge did nothave an impact on the region north of 10S, wherethe northern transect of ODP Leg 112 is located.The region at 11.5S o Lima has been aected byridge subduction V9.5 Ma ago. Support for themodel is provided by the sedimentological and pa-leo-bathymetric record in ODP Leg 112 and indus-trial well cores. At 9S, cores show mostly ne-grained sediments on the continental slope and,on the shelf, continuous shale deposition. At11.5S, the predicted age of the new model corre-lates well with a Late Miocene period of uplift anderosion followed by subsidence since V6 Ma.In light of this study, seismic and drilling data

sets acquired along the Peruvian margin in thelast decades oer the possibility to compare re-gions that have not been aected by the ridge

passage with regions that have been inuencedby the ridge, but otherwise share similar boundaryconditions. Such a comparison may enable a bet-ter quantication of the geodynamic inuence ofthe Nazca Ridge on the Peruvian margin in futurestudies. The case of the Nazca Ridge emphasizesthat models regarding the geodynamic evolutionof active margins have to take into account themigration history and three-dimensional eectsassociated with laterally migrating bathymetrichighs.

Acknowledgements

Helpful comments and discussions with NinaKukowski, Onno Oncken, Ulrich Riller andDavid Hindle are gratefully acknowledged. UdoBarckhausen and Garrett Ito are thanked fortheir help with the magnetic anomaly data anduseful comments. Many thanks to EdmundoNorabuena for his helpful comments on the platemotion data. The GMT [70] software was used tocreate Figs. 2^5. I thank the reviewers EmileOkal, Tim Dixon and Steven Cande for construc-tive comments that helped to improve the manu-script. Funding was provided by the GermanMinistry of Education, Science and Technology(BMBF) within the GEOPECO project (Grantno. 03G0146A).[AC]

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The migration history of the Nazca Ridge along the Peruvian active margin: a re-evaluationIntroductionGeodynamic settingReconstruction of the migration historyLateral migration velocityOriginal length of the Nazca Ridge and onset of ridge subduction

DiscussionConclusionsAcknowledgementsReferences

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