tectonic erosion of the peruvian forearc, lima basin,

7/29/2019 Tectonic Erosion of the Peruvian Forearc, Lima Basin,

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Tectonic erosion of the Peruvian forearc, Lima Basin,

by subduction and Nazca Ridge collision

Peter D. Clift

Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA

Ingo Pecher

Institute of Geological and Nuclear Sciences, Lower Hutt, New Zealand

Nina Kukowski and Andrea Hampel

GeoForschungsZentrum Potsdam, Potsdam, Germany

Received 18 March 2002; revised 11 December 2002; accepted 27 February 2003; published 4 June 2003.

[1] Subsidence of Lima Basin, part of the Peruvianforearc, is controlled by tectonic erosion by thesubducting Nazca plate. Multichannel seismic

reflection data coupled with age and paleowater depthconstraints derived from Ocean Drilling Program(ODP) coring now allow the rates of erosion to bereconstructed through time. In trenchward locations theforearc has experienced limited recent relative uplift(700850 m) likely due to preferential basal erosionunder the center of Lima Basin. Long-term subsidencedriven by basal tectonic erosion dominates and is fastestclosest to the trench. Since 47 Ma (Eocene) up to 148km of the plate margin have been lost at an average rateof up to 3.1 km myr1. Appoximately 110 km of thattotal appears to be lost since 11 Ma, implying muchfaster average rates of trench retreat (10 km myr1)

since collision of the Nazca Ridge with the Lima Basinat 11 Ma. Although there is no clear subsidence event atODP Site 679 during the time at which Nazca Ridgewas subducting beneath this part of the forearc (411Ma), the more trenchward ODP Sites 682 and 688 showsignificant deepening after 11 Ma indicating thatsubduction of the ridge accelerates tectonic erosion.Long-term rates of crustal erosion in the region of LimaBasin are greater than estimates of regional arcmagmatic productivity, implying that such marginsare net sinks of continental crust. INDEXTERMS: 3025Marine Geology and Geophysics: Marine seismics (0935); 3040

MarineGeology and Geophysics: Plate tectonics(8150, 8155, 8157,

8158); 8015 Structural Geology: Local crustal structure; 0935Exploration Geophysics: Seismic methods (3025); KEYWORDS:

Peru, subduction, tectonics, subsidence. Citation: Clift, P. D.,

I. Pecher, N. Kukowski, and A. Hampel, Tectonic erosion of the

Peruvian forearc, Lima Basin, by subduction and Nazca Ridge

collision, Tectonics, 22(3), 1023, doi:10.1029/2002TC001386,

2003.

1. Introduction

[2] The tectonic erosion of crust in the forearc of con-vergent plate margins represents an important part of the

mass budget within subduction environments. Understand-ing the fate of the sedimentary cover and oceanic crust of asubducting plate is important if global geochemical cyclesare to be understood. Does material extracted from theupper mantle get recycled back into this reservoir throughdeep subduction, or is this material merely reworked alongconvergent margins, either being off-scraped within accre-tionary complexes, or re-melted and incorporated into thearc magmatism itself? Although large accretionary com-

plexes formed along the frontal edges of continental litho-spheric plates are known from many margins (e.g.,Barbados) [Nankai, Makran and Cascadia [Moore and

Biju-Duval, 1984; Davis and Hyndman, 1989; Moore etal., 1990; Minshull and White, 1989], there are moresignificant lengths of modern convergent margin, mostlylocated around the periphery of the Pacific, including large

parts of the Peruvian margin, where minor or no accretion isobserved [Rutland, 1971; Hilde, 1983; von Huene andScholl, 1991]. In these areas it is often unclear as to whetherthe sediment on the oceanic plate is being subducted deepinto the mantle, or if accretion is occurring by basal under-

plating under the forearc, but at a depth that is not readilyimaged by seismic reflection surveys. Study of the verticaltectonics in forearc basins in such areas can help to estimatethe rates of subduction erosion or accretion by underplating

because the character of the sedimentary record can con-strain the bathymetry of the forearc over long periods ofsubduction. Although forearc basins do not cover the entireforearc, they do provide information from areas lying tensof kilometers landward of the trench region.

[3] In this study we quantify the rates of tectonic erosionof the Peruvian forearc in the area of Lima Basin (Figure 1)in order to understand the mass budget of this convergentmargin over significant lengths of geologic time. To do thiswe use seismic and drilling data to reconstruct the sub-sidence and uplift history of the forearc (Figure 2). In

particular, we examine the rates of vertical motion duringnormal subduction of oceanic crust and compare this withthe dynamics related to subduction of the Nazca Ridge

TECTONICS, VOL. 22, NO. 3, 1023, doi:10.1029/2002TC001386, 2003

Copyright 2003 by the American Geophysical Union.0278-7407/03/2002TC001386$12.00

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[Pilger, 1981], an aseismic volcanic edifice that is in

oblique collision with the South American margin and thecrest of which is considered to have been subducted belowthe Lima Basin after 11 Ma [Hampel, 2002].

2. Controls on Subduction Erosion

[4] The factors governing accretion versus tectonic ero-sion along a given margin are understood in outline, but notin detail. Rates of accretion and brief phases of tectonicerosion have been estimated in margins where an accre-tionary complex juxtaposed against much older rock is

preserved as a record of the margins development [e.g.,von Huene et al., 1994, 1996]. In non-accretionary marginsthe removal of the forearc sediment record has resulted inestimates of tectonic erosion rates that vary widely. In thecentral Andes at 21S, average rates of trench retreat have

been estimated at 1.5 2.0 km Myr1 [Scheuber and Reut-ter, 1992]. Ballance et al. [1989] suggested that tectonicerosion in Tonga has been generally minor, except duringthe subduction of major seamount features, most notably theLouisville Ridge. Conversely, Lallemand[1998] has arguedthat strong coupling between the down-going and over-riding plates around the Pacific results in rapid trench retreatof 410 km Myr1 in such settings. As a result, severalhundred kilometers of forearc material would have been lost

since the initiation of subduction in the western Pacific at$45 Ma. Clift and MacLeod [1999] reconstructed thesubsidence history of Tonga forearc from sedimentary andstructural data, concluding that long-term tectonic erosionof the forearc was mostly by slow removal of material fromthe base of the forearc crust, causing the trench to retreat at


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the Mesozoic arc appeared to be missing, implying asubstantial loss of crust since that time. The moderateaccretionary complex that is noted trenchward of LimaBasin has a steep taper that analog modeling suggests as

being indicative of a high basal friction [Kukowski et al.,1994], consistent with high degrees of basal friction driven

by a rough topography on the subducting plate, a predictionconfirmed by bathymetric mapping [Kukowski et al., 2001].Such a rough subducting plate would promote tectonicerosion of the overriding forearc.

[6] A number of sedimentary basins are recognized alongthe margin, whose development can be used to understandthe tectonic evolution of the subduction zone. The LimaBasin forms in the forearc between 12 and 10S and isseparated from the adjacent Salaverry Basin, located on theshelf, by a basement high [Thornburg and Kulm, 1981].Seaward it is separated from the steep lower slope by astructural high that permits turbidites to pond [Hussong etal., 1988]. The Lima Basin is inferred to be underlain bymetamorphic continental crust [Suess et al., 1988], althoughthere has been debate about how far seaward this extends,some workers [e.g., Hussong and Wipperman, 1981] sug-gesting that the lower slope comprises sedimentary rocksaccreted from the subducting plate. Indeed the lower slopeis marked by landward dipping reflectors that have beeninterpreted as imbricated sedimentary rocks off-scrapedfrom the subducting plate [von Huene et al., 1996]. Thetectonic structure of the Peruvian margin has been affectednot only by the subduction of normal oceanic crust but hasalso been disrupted by collision of the Nazca Ridge. Cande[1985] calculated that the Nazca Ridge first collided with

the Peru Trench at 8 Ma, and migrated south along the platemargin. von Huene et al. [1996] used the plate model of

DeMets et al. [1990] to calculate that Nazca Ridge waspassing the Lima Basin portion of the margin at$4 Ma. Thepassage of Nazca Ridge is normally considered to causesignificant tectonic erosion of the forearc. Subsequently,i.e., after 4 Ma, accretion under the outer forearc is believedto be responsible for the Pleistocene-Recent uplift of theouter trench slope stratigraphy [von Huene and Pecher,1999]. In contrast, Hampel [2002], in a revised reconstruc-tion, indicated initial collision of Nazca Ridge with thePeruvian forearc at 11.2 Ma at$11S, i.e., in the vicinity ofthe study area considered here. In this model the ridgemoved at 6 cm/yr obliquely along the margin. Although thewidth of the Nazca Ridge at the modern trench is 200 km,the reconstruction ofHampel [2002] indicates a wider ridgecolliding with the trench at that time, making the 3.3 Myrestimate of collision duration at any given point of theforearc a minimum.

[7] The stratigraphy of the Lima Basin has already beendescribed by Ballesteros et al. [1988] using multichannelseismic lines by which they identified 11 different sediment

packages and noted the rather dramatic seaward thickeningof the stratigraphy, as well as the abrupt truncation of veryyoung reflectors against the seafloor on the lower midslope,which they inferred to be due to the action of fast flowingcontour currents. We base our revised stratigraphy on thisstudy, but converted travel time picks to depth using newlyobtained velocity functions (Figures 3 and 4). We alsoincorporated new multichannel seismic data collected in2000 [Bialas and Kukowski, 2000].

Figure 2. Bathymetric map of Lima Basin showing location of multichannel seismic reflection profilesand Ocean Drilling Program (ODP) sites considered in this study. Water depth is in meters.

CLIFT ET AL.: TECTONIC EROSION OF THE PERUVIAN FOREARC 7 - 3


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4. Seismic and Drilling Data

[8] Lima Basin makes a good location to examine tec-tonic erosion processes because it is relatively well surveyed

by multichannel seismic profiles, most notably by a Uni-versity of Hawaii, Institute for Geophysics (HIG), cruise in1986 and in 2000 by R/V Sonne cruise 146 GEOPECO,

which increased the density of the seismic reflection gridand provided ties with the existing Ocean Drilling Program(ODP) sites drilled during Leg 112 (Figure 2). The HIG datawere recorded by R/V Moana Wave using a partiallydetuned air-gun seismic source and a 1600-m-long streamer[Moore and Taylor, 1988]. This source-receiver offset wassufficient for seismic velocity analyses. The GEOPECOdata were collected with a 105/105 cubic inch generator/injector (GI) gun and a 225-m-long 36-channel streamer[Bialas and Kukowski, 2000]. This setup yielded excellentresolution, in most cases reaching the crystalline basement.With its relatively short streamer, the GEOPECO surveywas not optimized for a study of sub-surface velocities.

[9] The HIG data were stacked and time-migrated by

earlier investigators [Ballesteros et al., 1988]. We used thesetime sections, together with the new GEOPECO data, to

pick stratigraphic horizons. We then determined interval-velocities in major stratigraphic units from the pre-stackmigrated seismic data along four HIG transects across LimaBasin (including HIG 13 and HIG 14; Figures 3c and 4c).

[10] Three ODP sites are located within the area of study(679 682, and 688). ODP Site 679 is especially useful

because it lies in shallower water at the landward edge ofthe survey, and is a relatively complete, well-dated section,with one major hiatus between 7.2 and 11 Ma and twoshorter ones at 4.25.3 Ma and 1.93.75 Ma. The seismicreflection data was loaded on to a Unix workstation runningSchlumbergers Geoframe2 interpretation package and aseries of depositional packages were identified, similar tothose defined by Ballesteros et al. [1988]. By mappinghorizons in three dimensions, along strike variability wasassessed and a greater level of confidence in the interpre-tation was achieved through the need to match reflectors inseries of profiles. After interpretation of the seismic profilesthe seismic stratigraphy was converted to a depth sectionusing the stacking velocities derived from the seismic

profiles. Two of the most representative sections acrossLima Basin HIG-13 and 14 are shown in Figure 3 and 4.

5. Subsidence Reconstructions

[11

] The subsidence history of Lima Basin is consideredhere using both a one-dimensional backstripping analysis ofthe stratigraphy of the ODP drill sites, and a two-dimen-sional backstripping of the interpreted seismic profiles. Theone-dimensional method allows the vertical motions ofthe basement at the drill site to be reconstructed with thedetailed temporal resolution derived from the biostratigra-

phy, within the uncertainty of the water depth estimatesderived from sedimentary and benthic foraminifer studies.The two-dimensional method is limited by the resolution ofthe seismic data, but allows wide areas of the basin, awayfrom the drill sites to be examined.

[12] In the one-dimensional approach we used the back-stripping subsidence method of Sclater and Christie [1980],in which lithology and age information taken from the coredmaterial are used to calculate a depth to basement, afterremoving the loading effects of sediment and water, allow-ing a residual, tectonically driven subsidence of the base-ment to be isolated. The backstripping is done in a series ofstages, controlled by the number of dated sedimentary

packages. At each stage the youngest sediment package isremoved, and the underlying sediments are decompactedto restore them to the thickness that they originally had

before deposition of that youngest package. In addition, thedepth to basement can be calculated for each time period,with a correction made for the weight of sediment at anygiven time.

[13] For each time period two possible depths to base-ment are calculated, representing minimum and maximumestimates of the water depth at the time of sedimentation.The true loading corrected subsidence pattern of the base-ment must lie between these two estimates. The Sclater andChristie [1980] backstripping method assumes an empirical

porosity-depth curve that is based on lithology, but this canbe corrected to match the measured porosity values fromrecovered core material. The porosity-depth curve of Sclaterand Christie [1980] lies within 15% of the values measured

by the ODP scientific party [Shipboard Scientific Party,1988a, 1988b] (Figure 5), but the values in the Lima Basinappear to be consistently higher than the Sclater andChristie [1980] model for shale, at least above 400 m below

seafloor (mbsf), representing$

40% of the total sedimentthickness in Lima Basin. This is unlikely to be a majorsource of errors, because it introduces only $60 m ofuncertainty into the tectonic subsidence, which is calculatedat 25003200 m in the outer Lima Basin. Nonetheless, inthis study we use the porosity-depth relationship found inthe ODP wells when performing the backstripping recon-struction. When making the unloading correction the den-sity of the lithospheric mantle is assumed to be 3330 kg/m3

[Oxburgh and Parmentier, 1977].

5.1. Paleowater Depths

[14] When dealing with continental margin sediments

deposited in significant water depths, such as the LimaBasin, estimates of paleowater depth are crucial to a mean-ingful result and represent the single largest uncertainty.Variations in the degree of sediment compaction are of amagnitude smaller than any possible errors in the waterdepth. No attempt has been made to correct for fluctuationsin eustatic sea level, as current predictions of rates and

Figure 3. (opposite) (a) Multichannel seismic reflection profile HIG-13, with (b) interpreted structure, (c) velocity modelderived from the stacking velocities, and (d) interpreted stratigraphy after depth conversion. See color version of this figureat back of this issue.



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magnitudes of eustatic sea level remain controversial andproduce unlikely, saw tooth-like subsidence curves when

taken into account [Wood, 1982]. Although short period sealevel fluctuations predicted by reconstructions like Haq etal. [1987] are difficult to account for, long-term variationsmay be incorporated without producing geologicallyimprobable subsidence histories. Nonetheless, the magni-tude of the sea level variations remains contentious. Sealevel variations based on oxygen isotope work predict

fluctuations of 3050 m above modern levels in the EarlyTertiary [Miller et al., 1985], compared to $150 m for the

Haq et al. [1987] model. Haq et al. [1987] predict sea levelat 4 Ma (the end of Nazca Ridge collision) to be $80 mhigher than present day and 140 m above present day at13.6 Ma, the start of the drilled sediment record at ODP Site679. If these figures are correct then a failure to account forthem will lead to an underestimate of the amount of tectonicsubsidence of this magnitude. Even if this is not accountedfor the uncertainty introduced is small compared to the totalsubsidence predicted ($2.5%).

[15] Water depth controls are much better for ODP Site679 than Sites 682 and 688 because it is located inshallower water, where benthic foraminiferal depth zonesare closely spaced. All water depth assignments are derivedfrom the sedimentology and paleontology report of theShipboard Scientific Party [1988a, 1988b, 1988c] and from

Resig [1990]. At ODP Site 679 outer shelf sediments(200500 m water depth) in the Middle Miocene show ashallowing upward into Upper Miocene midand inner shelfenvironments (1500 m

by the Late Miocene. A lower boundary of the present waterdepth is placed on the site assuming that long-term sub-duction erosion has resulted in progressive deepening. As

explained by von Huene and Pecher [1999], the landwardtilting of strata in the outer Lima Basin suggests somerelative uplift of this area where ODP Sites 682 and 688 arelocated. The tilting affects strata dated as Pliocene andolder, but does not affect the Quaternary. Although thedeformation seen in the seismic profiles suggests recentuplift of 700800 m along the western edge of Lima Basin,

Figure 4. (opposite) (a) Multichannel seismic reflection profile HIG-14, with (b) interpreted structure, (c) velocity modelderived from the stacking velocities, and (d) interpreted stratigraphy after depth conversion. See color version of this figureat back of this issue.

Figure 5. Porosity-depth measurements made in ODPHole 679 and 688 in the Lima Basin compared to thecompaction model for shale from Sclater and Christie[1980]. Note that while they form a linear array the porosityvalues in the Lima Basin appear to be consistently higherthan the Sclater and Christie [1980] model.



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the uncertainties on the water depth estimates from Pleisto-cene sediments at ODP Sites 682 and 688 do not allowresolution of such an uplift event.

[17] A crucial assumption in one-dimensional backstrip-ping calculations is that of local isostasy. Local isostaticcompensation treats the lithosphere as though it has zerolateral strength, so that any applied load is supported by the

buoyancy of the underlying mantle rather than laterally bythe strength of the lithosphere. Although this assumption ofa zero strength lithosphere is clearly an approximation, it isreasonable where the lithosphere is weak or the applied loadis wide and evenly distributed [Sclater and Christie, 1980].The strong deformation and faulting observed in the Peru-vian forearc is compatible with this region being one of verylow flexural rigidity, similar to other tectonized forearcs(e.g., Tonga [Clift and MacLeod, 1999]). Studies of theforearc in areas where seamounts are in collision, show thatdeformation is focused close to immediate vicinity of thecollision (e.g., Costa Rica [von Huene et al., 2000]). Ifflexural rigidity were high in the forearc, uplift would bemore broadly distributed.

[18] Age control for the drill sites considered is derivedfrom the nannofossil biostratigraphy produced by Ship-board Scientific Party [1988a, 1988b] and Martini [1990].The nannofossil zone assignment is then converted to anumerical age using the timescale ofBerggren et al. [1995].These sites were chosen because of their location within theseismic survey and because of the semi-complete stratig-raphy since $14 Ma, allowing a detailed reconstruction ofthe basement subsidence history since that time.

6. Results of One-Dimensional Backstripping

[19] The results of the unloading calculations at ODPSites 679, 682 and 688 are shown in Figure 6. Table 1summarizes the amounts of vertical tectonic motion linkedto Nazca Ridge subduction. At ODP Site 679 there is strongevidence for a tectonically driven uplift event following the7.211 Ma hiatus, culminating in a peak at 7.2 Ma andagain at 4.2 Ma, followed by a rapid collapse. The 7.211Ma hiatus broadly correlates with the passage of the NazcaRidge as dated by Hampel [2002], with the tectonic sub-sidence between 7.2 and 5.3 Ma reflecting collapse of theforearc basement after the passage of the main section of theridge. The second phase of tectonic uplift at 4.2 Ma appearsto be short lived, being finished by 4.0 Ma and may reflectthe subduction of an additional basement feature on the

Nazca Ridge. In the model of Hampel [2002] the width ofthe Nazca Ridge colliding with Lima Basin was much

broader than it is in the modern day, a prediction basedon the assumption that the Nazca Ridge mirrors the Tuo-motu Ridge, since it is produced from the same hot spot.Consequently, we interpret the second uplift event at ODPSite 679 to reflect subduction of a secondary ridge, but still

part of Nazca Ridge.[20] There is evidence that regardless of the activity at 7.2

and 4.2 Ma there has been a long-term descent in the forearcbasement, totaling at least 130 m since 13.6 Ma. This maybe interpreted to reflect long-term tectonic erosion of the

forearc basement under ODP Site 679. The sedimentaryhiatus at 1.93.75 Ma does not correspond to a phase oftectonic uplift and represents a period of non-deposition,

possibly driven by bottom current activity.[21] The backstripping results from ODP Sites 682 and

688 show that there is relatively little long-term permanentsubsidence of the site between 50 Ma and 16 Ma. Sub-sequently, ODP Site 682 showed strong subsidence after11.3 Ma, while Site 688 suffered a significant deepening

between 16 and 12 Ma, suggesting a period of enhanced

Figure 6. Sediment-unloaded, backstripped subsidencereconstructions of depth to basement at ODP Sites 679,682 and 688 derived from one-dimensional backstripping ofthe drilled section using the methodology of Sclater andChristie [1980]. Vertical range show uncertainties in

paleowater depth estimates, accounting for the vast majorityof the uncertainty in the calculated depth.



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tectonic erosion, likely linked to the start of Nazca Ridgesubduction after 11 Ma. Because of the uncertainties in thewater depths the amount of this subsidence is not wellconstrained, especially at ODP Site 688, where it couldtheoretically be almost zero, although the sharp change insediment facies argues against this. Temporary uplift ofODP Sites 682 and 688 during passage of Nazca Ridge isnot resolved, meaning that it was less than the uncertaintiesin the water depths, i.e., 1788 m and 1600 m respectively. Alarge hiatus spanning 5.311.0 Ma was recognized at ODPSite 688 coincident with the deepening phase. This suggests

that the hiatus was generated by submarine erosion orslumping during the period of ridge passage. ODP Site682 has a poorly defined hiatus at 910 Ma and a secondhiatus at 5.3 6 Ma associated with a zone of slumpedsediment dated at 69 Ma. The slumping, hiatus and strongtectonic subsidence after 11.3 Ma are consistent with the

passage of the Nazca Ridge at that time.[22] Although there is evidence at all three drill sites to

indicate accelerated basement subsidence coincident withand following Nazca Ridge collision, the backstrippingmethods reveal anomalous activity extending far beyondthe 3.3 Myr minimum duration of ridge passage predictedfrom the current width of the Nazca Ridge at the trench andthe 6 cm/yr lateral migration rate of Hampel [2002].

Sedimentation and subsidence at ODP Site 679 is anom-alous between 11 and 4 Ma, spanning 7 Myr, reflecting thegreater width of the Nazca Ridge entering the trench at thattime.

7. Two-Dimensional Backstripping

[23] The subsidence of the margin can also be recon-structed using a two-dimensional backstripping approach. Inthis method the subsidence of a paleosubaerial surface isreconstructed. We choose to focus on the hiatus surface

between the tilted and eroded Middle Miocene and theoverlying, undeformed Upper Miocene sequence becausethis surface is recognized over the entire Lima Basin and ithas been drilled and dated (7.211 Ma) at ODP Site 679.Major deepening at ODP Site 682 is seen to postdate 11.3Ma, constraining formation of this hiatus surface to beingafter this time, likely representing shoaling during the initialcollision of Nazca Ridge. Water depth constraints at ODPSite 688 are less clear for this interval, since Resig [1990]showed that while the Lower Miocene was deposited inupper middle bathyal conditions (5001500 m), the UpperMiocene is barren of benthic foraminifers. We assign an ageof 11 Ma to the hiatus surface as the time after which strongsubsidence at the drill site can be identified.

[24] Although ideally we would like to reconstruct thesubsidence of the subaerial exposure surface in threedimensions (Figure 7), a good impression of across marginvariability can be derived from examining a series of two-dimensional profiles, since most of the depth variability isexpressed across the strike of the trench, not along it. Forthis purpose we choose lines HIG-13 and HIG-14 (Figures 3and 4) because these cross the entire basin, are well dated bythe ODP wells, and show good definition of the seismicreflectors above Eocene basement. The approach used issimilar to that in the one-dimensional case in that the dated

sedimentary layers are progressively removed from thesection, allowing the underlying layers to decompact andthen adjusting the entire section for isostatic equilibrium.We employ the program FLEX-DECOMPTM developed by

N. Kusznir and A. M. Roberts to backstrip the Lima Basin.This program has been used to successfully backstrip basinsin passive margin and intra-continental settings [Kusznir etal., 1991, 1995], as well as in transform margins [ Clift and

Lorenzo, 1999]. Although this approach can account for theeffects of flexural rigidity, the strong faulting and deforma-tion of the forearc, coupled with the pinching out of themantle lithosphere under the forearc as the trench isapproached suggests very low rigidity under Lima Basin.Consequently, we use a zero strength crustal model.

Because the crust under the profile is continental we makeno correction for tectonically induced thermal subsidence.

8. Results of Two-Dimensional Backstripping

[25] The results of unloading and decompacting profilesHIG-13 and 14 to the 11 Ma reflector are shown inFigure 8. What is apparent is that this surface, which weand earlier workers [e.g., von Huene and Pecher, 1999]consider to have been at sea level at this time, does notrestore to sea level after the removal of all the youngersediments and allowing local isostatic compensation to beachieved. The implication is that subsidence due to basalsubduction erosion has affected the margin, causing themismatch in observed and reconstructed water depths at11 Ma. In other words, the current depth of Lima Basincannot be achieved simply by the loading of sediments onto the subaerial surface present at 11 Ma. The backstrippingalso clearly shows that the reconstructed profiles are deeperat the SW end than the NE, indicating greater net sub-sidence, and thus more tectonic erosion closer to the trench.

[26] In practice the reconstructed basin profiles can beused as a measure of net tectonic subsidence since 11 Ma(Figure 9). The interpretation is slightly complicated by therecognition that the most trenchward portions of the profiles

Table 1. Amounts of Vertical Uplift and Subsidence at ODP Drill Sites Within the Lima Basin Linked to Subduction

of the Nazca Ridge

Drill SiteTectonic ErosionSince 11 Ma, m

Uplift During NazcaRidge Subduction, m

Subsidence ImmediatelyAfter Nazca Ridge Subduction, m

ODP Site 679 35 to 250 240 to 550 190 to 170ODP Site 682 1680 to 2680 0 to 1000 380 to 2625

ODP Site 688 140 to 1700 0 to 1600 0 to 1625



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have been slightly uplifted relative to the basin center.Evidence for relative uplift in the trenchward portions ofLima Basin is shown by the reversal of dip of Pliocene andolder sediments to a landward direction at the SW end of

both profiles. Figures 3 and 4 show the way in which theamount of this recent relative uplift can be estimated. Byassuming that the basement and strata would have continuedto deepen offshore, as they do in landward portions, it is

possible to estimate the amount of uplift generated by recentuplift under the trenchward edge of the Lima Basin. Thiscan then be subtracted from the net subsidence in thoseareas to estimate the total amount of subsidence in thetrenchward regions prior to the recent accretion event.

[27] The two-dimensional estimates for permanent netsubsidence due to basal subduction erosion can be matchedwith estimates derived from the ODP drill sites (Figure 9).Because the unloaded depth to basement at each drill sitewas calculated from the one-dimensional backstrippingmethod and the depth at 11 Ma is known to be subaerialat ODP Site 679, the total net subsidence can be measured.ODP Site 679 lies at the NE end of the profiles and matchesestimates derived from the seismic data, as might beanticipated. However, ODP Sites 682 and 688 lie trench-

ward of the profiles and help constrain degrees of tectonicerosion closer to the trench. HIG-13 shows $2500 m ofsubsidence driven by tectonic erosion at its trenchward end,while HIG-14 reaches almost 3000 m.

9. Long-Term Rates of Subduction Erosion

[28] Determining the long-term rates of tectonic erosionor accretion at the Peruvian margin is crucial to under-standing the mass budget in this subduction zone. In thisstudy we estimate the average rates of erosion and howmuch of this can be related to the subduction of the NazcaRidge. The modern ridge is currently moving at an obliquerate of 6 cm/yr, although the relative velocity of NazcaRidge and Lima Basin has changed through time due tovariable convergence [le Roux et al., 2000; Hampel, 2002].Backstripping subsidence analysis at ODP Site 679 indi-cates a 7 Myr collision in the study area, consistent with thereconstruction ofHampel [2002] based on a geometry of the

Nazca Ridge mirroring the Tuomotu Ridge.[29] An estimate of the rate of long-term tectonic erosion

to the Peruvian forearc can be derived from the subsidencereconstructions. In our models we assume that the paleo-

Figure 7. Map of the central Lima Basin showing the depth in two-way travel time (TWTT) measuredin milliseconds to the 11 Ma reflector, interpreted as being a product of subaerial erosion. Area covered

by map is shown in Figure 2. Note how the reflector is deeper toward the trench, reflecting greater

subsidence and thus basal tectonic erosion of the forearc crust in that direction.



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forearc was similar to the modern forearc in its bathymetry.In the absence of detailed bathymetric constraints across theentire forearc the assumption of similar forearc slope isrequired and allows first order estimates of trench retreatrates to be made. Because there is little long-term sedimentrecord trenchward of the Lima Basin it is doubtful whether amuch more detailed reconstruction of paleoforearc slopescould be made even if more drill sites were available.Indeed, because of the progressive removal of material fromthe forearc by the ongoing tectonic erosion much of theearly record of trench slope has been lost.

[30] Sedimentary facies and microfauna constrain ODPSite 688 to being in shelf water depths (


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this would imply that tectonic erosion rates before 11 Macould not have exceeded 1.1 km Myr1, and that theaverage rate of erosion increased sharply after 11 Ma. Anestimate of 110 km lost since 11 Ma must also be aminimum for the trench retreat since 47 Ma, indicating thatthe higher end of the range, 68148 km, derived from drillsite backstripping must be closer to the true figure of forearccrust lost since the Eocene. Because sea level was $60 mhigher at 11 Ma than it is today [Haq et al., 1987],correction for this effect would only increase the apparentrate of subduction erosion since that time.

[33] Average tectonic erosion rates may have increasedsince 11 Ma because of either faster background tectonicerosion rates, or because of the influence of Nazca Ridgecollision starting at 11 Ma. Because the average tectonicerosion rates after 11 Ma are so much faster even than forthe Tongan forearc, we prefer to attribute faster tectonicerosion to Nazca Ridge collision. Tectonic erosion ratesduring subduction of normal oceanic crust are unlikely to bemuch faster than Tonga, because in that area the rate of

convergence is very high, there is little sedimentary coverand the plate is thermally mature and thus breaks with largenormal faults as it flexes the trench [Wright et al., 2000],resulting in a rough and potentially very erosive surfaceagainst the base of the forearc. Plate reconstructions for

Nazca plate-South American motions also show that con-vergence was slightly slower, not faster, after 11 Ma [Pardo-Casas and Molnar, 1987; Somoza, 1998], also arguingagainst faster tectonic erosion unrelated to Nazca Ridgecollision. We prefer to explain the faster average rate from0 11 Ma compared to 4711 Ma as being related tocollision with the Nazca Ridge.

[34] Lower rates of tectonic erosion could be estimated forthe 011 Ma period if the forearc slope was steeper at thattime, i.e., that the 11 Ma erosion surface does not requireLima Basin to be 180 km from the trench at that time. Thewater depth information might suggest that the trench slopewas steeper at 11 Ma, perhaps due to compressional defor-mation or basal accretion. It is noteworthy that the sedimen-tary rocks underlying the 11 Ma unconformity at ODP Site

Figure 9. Across margin profiles of HIG-13 and HIG-14 showing the net permanent subsidence of themargin basement since 11 Ma after correcting for sediment loading effects. The continuous profiles are

derived from the mismatch between the observed subaerial character of the 11 Ma reflector and the depthto which this is restored after backstripping (Figure 8). Vertical bars show constraints derived from ODPSite 679 at the landward end of the profiles and from ODP Sites 682 and 688 at the trenchward end ofHIG-13 and HIG-14, respectively.



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679 were identified as being upper bathyal [Suess et al.,1988; Resig, 1990], implying that Lima Basin had beenflooded and was subsequently exposed, an observationincompatible with a steady progression of any given pointon the forearc toward the trench on a constant slope margin.If the Lima Basin transect was trenchward of the coast priorto 11 Ma then its exposure at 11 Ma requires a tectonicallydriven uplift event to steepen the forearc at that time. Thisuplift event does not seem to have affected sedimentation atODP Site 688, but the uncertainties in the water depth thereat 11 Ma would make resolution of such an event difficult.However, ODP Site 682 does show a shallowing of the waterdepths between 15.6 Ma and 11.3 Ma from upper middle

bathyal (500 1500 m) to outer shelf-upper middle bathyal(50 1500 m), consistent with a compression and steepeningof the margin at that time.

10. Subsidence Due to Nazca Ridge

Subduction

[35] Determining how much of the net subsidence is dueto Nazca Ridge passage and how much is due to subductionerosion by normal oceanic crust is a key objective of thisstudy. In addition, we try to understand how Nazca Ridgesubduction affects the tectonic erosion, whether the erosionis increased to high values during the passage, or whetherthe passage of the ridge weakened the forearc, making itmore susceptible to later basal erosion during normalsubduction. Clearly, reconstructing the vertical motionsacross Lima Basin during the 4.011 Ma period of ridgesubduction is required to answer that question, and can only

be addressed from the well data because the seismic profileshave no water-depth information for that period.

[36

] At ODP Site 682 16802680 m of permanentbasement subsidence has occurred since 11 Ma, of which4002630 m is caused by Nazca Ridge subduction after11 Ma. This only loosely defines the subsidence attribut-able to normal subduction erosion of the forearc to

between a minimum of 50 m (1.9% of the total since 11Ma), and a maximum of 2280 m (85%). At ODP Site 688the water depth uncertainties do not allow any constraintsto be placed on the proportion of Nazca Ridge versuscontinuous subduction erosion. In contrast, at ODP Site679, temporary uplift of the site during Nazca Ridge

passage is calculated at 130 550 m. Permanent subsidenceat this site caused by ridge passage is defined by thedifference in basement depth at 4.0 Ma and 11.0 Ma, i.e.,

ranging from 170 m of subsidence to an uplift of as muchas 190 m. The basement depths at 7.2, 5.3 and 4.2 Ma arelikely affected by ongoing collision events and cannot beused to measure long-term permanent subsidence driven bythese same collisions. There is no unambiguous indicationof rapid tectonic erosion under ODP Site 679 during thecollision of Nazca Ridge. In contrast, 80230 m of thetotal subsidence at ODP Site 679 since 11 Ma (i.e., >36%of the total subsidence) has occurred after 4 Ma, after theend of resolvable collision events tied to passage of NazcaRidge.

[37] Because of the water depth uncertainties, it is notpossible to unambiguously define how much tectonic ero-sion and subsidence is directly related to Nazca Ridge andhow much to ongoing subduction since the start of ridgecollision. However, some clues are provided by comparisonwith other ridge-trench collision events. In the Tonga Arc-Louisville Ridge collision zone in the SW Pacific theerosive effect of ridge collision is mostly in the form offrontal erosion and steepening of the trench slope, with

basal erosion of the forearc limited to the region close to thetrench axis [Dupont and Herzer, 1985; Clift and MacLeod,1999]. Unlike the Tonga-Louisville system however, thePeruvian forearc is not noticeably steeper or narrower northof Nazca Ridge than it is south of that feature, although it issteeper and narrower immediately opposite the point ofridge collision. The lack of a clear change in forearcgeometry after ridge passage suggests limited frontal ero-sion of the plate during ridge passage. However, because theaverage rates of tectonic erosion since 11 Ma are approx-imately ten times faster than both those before 11 Ma in theLima Basin, we suggest that collision between the trenchand Nazca Ridge did significantly accelerate basal subduc-tion erosion. Much of the enhanced tectonic erosion may

postdate the actual collision itself. In this scenario the upliftand deformation of the forearc during ridge subductionweakened the plate margin and allowed steady state erosion

processes to much more rapidly remove material from thebase of the forearc crust than was the case before ridgesubduction. The dominant mechanism of erosion in thiscase would be the removal of lenses of material from the

base of the forearc in the manner described by Ranero andvon Huene [2000]. No significant faulting is seen in LimaBasin following ridge passage that would support the ideaof faulting and breaking up the entire forearc at that time.

[38

] Using an average arc continental crustal thicknessclose to the coast of 32 km, the rate of crustal loss in theforearc can be averaged at 109 km3 Myr1 per km of trenchaxis since 47 Ma. Bialas et al. [2001] used seismicrefraction techniques to measure the crustal thickness underthe Lima Basin as 25 km, consistent with our slightly higherfigure for the crust onshore. We know that the average rateof basal tectonic erosion was faster after 11 Ma than beforethat time, i.e., following initial Nazca Ridge subduction. If110 km of trench retreat has occurred since 11 Ma then thetrench retreat rate must average 10 km Myr1 since NazcaRidge collision, resulting in average crustal erosion rate of320 km3 Myr1 per km of margin. We can then calculate along-term average crustal erosion rate of 35.2 km3 Myr1

per km of margin before 11 Ma. In comparison, the rate ofcrustal loss due to tectonic erosion in northern Chile wasestimated at a rate of 3754 km3 Myr1 per km of margin[Scheuber et al., 1994; von Huene et al., 1999].

[39] Rates of crustal tectonic erosion under the forearccompare closely in magnitude to estimates of magmatic

productivity in active margin settings. Although Reymerand Schubert [1984] estimated that globally only 2333 km3 of new melt were added every 1 Myr per km ofactive margin, more recent estimates have pushed thisvalue up. Holbrook et al. [1999] estimated rates of 55



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82 km3 Myr1 per km of margin for the Aleutians, whileSuyehiro et al. [1996] indicated long-term average accretionrates of 66 km3 Myr1 per km of margin in the Izu Arc.However, for the Peruvian margin, Atherton and Petford[1996] suggested that 70,000 km3 of new crust has beenintruded along a 90 km long transect at 9S between 100and 3 Ma. This would result in a rate of only 8.0 km3 Myr1

per km of active margin, which is significantly less than inother arcs, and significantly less than material removed bysubduction erosion. If true this would imply that thedominant pattern at the Peruvian margin would be netcrustal loss, a finding which is similar to that for northernChile [Laursen et al., 2002].

11. Recent Seafloor Deformation and Erosion

[40] Deformation of the trenchward edge of Lima Basinin the form of relative uplift, affects all the dated sedimen-tary packages up to and including the Pliocene. Pre-Quater-nary strata in the Lima Basin are seen to thicken offshore,and indicate that the trenchward portion of the basin was nota paleohigh before the Quaternary. Clearly, some veryrecent tectonic mechanism has caused landward backtiltingof these strata in the trenchward part of the basin, althoughit is not possible to constrain what that might be. Upliftmight be caused by recent preferential underplating ofmaterial from the subducting plate under the trenchwardedge of the Lima Basin. Such basal accretion may haveaffected the more landward areas of Lima Basin too, but ifso then the whole section has been elevated and thatmagnitude is unknown, although it must be less than thewater depth uncertainties at ODP Site 679 for the Pleisto-

cene, as no such uplift event is resolved there. Alternatively,subduction of a seamount under this area might causetemporary uplift immediately above the edifice, althoughthere is no evidence for this in the seismic data. Thornburgand Kulm [1981] show that this upturning of strata extendsalong strike for 70200 km, requiring subduction of atrench parallel ridge of that length if that mechanism is toaccount for the uplift. A third and most likely possibility isthat this stratal geometry could reflect greater recent sub-duction erosion and subsidence under the center of LimaBasin compared to the trenchward edge. This model isconsistent with the long-term character of the margin andwith the evidence for extensional faulting in the seismic

profiles. All these explanations would require short-termchanges in the evolution of the forearc because subsidence,and thus subduction erosion, dominate and generallyincrease toward the trench.

[41] Relative uplift of the trenchward edge is marked bystrong seafloor erosion. Extrapolating the interpreted hori-zons that are now truncated by a sharp unconformity againstthe seafloor, it is possible to estimate the original basin formand the eroded thickness (Figure 10). Erosion must havetaken place in deep water, as there is no evidence from thesedimentology or benthic foraminifer data to indicate thedramatic uplift and subsidence that would be required to

produce this unconformity in a subaerial setting. Organic-rich diatomaceous sediments of late Pleistocene age recov-ered at ODP Site 688 argue for consistently deep water overthat area in the recent geological past. Although theseupwelling-related facies are usually only known from shal-low water environments less than 500 m deep, it is mostlikely that these sediments were redeposited downslope to

Figure 10. Map showing estimated eroded thicknesses of sediment in meters due to current-scouring ofthe seabed following folding of the Pliocene and older strata, prior to the Quaternary.



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the location of ODP Site 688 [Shipboard Scientific Party,1988c]. Although it is difficult to imagine such powerfulsubmarine erosion being caused by current activity weconclude that bottom current activity, most likely linkedto a strengthening of the Humboldt Current in this area[Garcia, 1994], is the most probable cause of the post-Pliocene erosion.

12. Conclusions

[42] A combination of drilling and multichannel seismicreflection data from the Lima Basin region of the Peruvianforearc was analyzed by backstripping methods to isolate theamount of subsidence caused by tectonic erosion since theEocene. Dated horizons originally formed at sea level are

particularly useful for measuring the total amount of erosionsince 47 Ma and 11 Ma. Up to 148 km of forearc materialappears to have been removed since 47 Ma, of which around110 km have been lost since 11 Ma, triggered by thesubduction of the Nazca Ridge under the area of study after

11 Ma. The lack of a clear net deepening of the basement atODP Site 679 immediately following passage of the NazcaRidge along the margin suggests that enhanced erosion mayincrease during and in the wake of ridge subduction as aresult of tectonic disruption of the forearc. Rates of trenchretreat driven by basal subduction erosion during subduction

of normal oceanic crust prior to 11 Ma are slightly less thanthose measured in the Tonga forearc, a moderate 1.1 kmMyr1 compared to 1.5 km Myr1, but this rate increasesapproximately ninefold during and immediately after NazcaRidge subduction (10 km Myr1). Average rates of trenchretreat since 47 Ma of 1.5 3.1 km Myr1 are similar to thoserecently determined from the Costa Rica forearc [Vannucchiet al., 2001]. The study reinforces the suggestion from otheractive margin systems that collision of aseismic ridges playsa key role in controlling the rates of mass fluxing through thesubduction system, and that rates of crustal erosion are closeto or even exceed rates of magmatic addition, implying thatthe central Andean margin is not a site of significantcontinental crustal growth.

[43] Acknowledgments. P. C. wishes to thank Bill Lyons and AileenMcLeod (MIT) for their help in loading seismic data on to the Geoframe2

system, and John Grotzinger for his generous access to that facility. P. C.would also like to thank Alan Roberts and Nick Kusznir for the use ofFLEX-DECOMPTM. I. A. P. would like to thank Greg Moore (U. Hawaii)for his help with reading and evaluating the HIG data as well as MarkWiederspahn (UTIG) for making it possible to load these data. We wish to

thank C. Huebscher (U. Hamburg) for use of the GEOPECO data. R/VSonne cruise 146 GEOPECO was funded by the German Ministry ofEducation and Science (MBMF grant 03F0146. The work benefited fromdiscussion with Joerg Bialas, Carolyn Ruppel, Rob Larter, and PaolaVannucchi, as well as reviews by Kelin Whipple, Dale Sawyer, and Rolandvon Huene. This work was supported by a grant from the National ScienceFoundation and is WHOI contribution 10789.

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Figure 3. (opposite) (a) Multichannel seismic reflection profile HIG-13, with (b) interpreted structure, (c) velocity modelderived from the stacking velocities, and (d) interpreted stratigraphy after depth conversion.

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Figure 4. (opposite) (a) Multichannel seismic reflection profile HIG-14, with (b) interpreted structure, (c) velocity modelderived from the stacking velocities, and (d) interpreted stratigraphy after depth conversion.

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tectonic erosion of the peruvian forearc, lima basin,

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