Tectonic erosion of the Peruvian forearc, Lima Basin, by subduction and Nazca Ridge collision

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1 TECTONICS, VOL. 22, NO. 3, 1023, doi: /2002tc001386, 2003 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 [1] Subsidence of Lima Basin, part of the Peruvian forearc, is controlled by tectonic erosion by the subducting Nazca plate. Multichannel seismic reflection data coupled with age and paleowater depth constraints derived from Ocean Drilling Program (ODP) coring now allow the rates of erosion to be reconstructed through time. In trenchward locations the forearc has experienced limited recent relative uplift ( m) likely due to preferential basal erosion under the center of Lima Basin. Long-term subsidence driven by basal tectonic erosion dominates and is fastest closest to the trench. Since 47 Ma (Eocene) up to 148 km of the plate margin have been lost at an average rate of up to 3.1 km myr 1. Appoximately 110 km of that total appears to be lost since 11 Ma, implying much faster average rates of trench retreat (10 km myr 1 ) since collision of the Nazca Ridge with the Lima Basin at 11 Ma. Although there is no clear subsidence event at ODP Site 679 during the time at which Nazca Ridge was subducting beneath this part of the forearc (4 11 Ma), the more trenchward ODP Sites 682 and 688 show significant deepening after 11 Ma indicating that subduction of the ridge accelerates tectonic erosion. Long-term rates of crustal erosion in the region of Lima Basin are greater than estimates of regional arc magmatic productivity, implying that such margins are net sinks of continental crust. INDEX TERMS: 3025 Marine Geology and Geophysics: Marine seismics (0935); 3040 Marine Geology and Geophysics: Plate tectonics (8150, 8155, 8157, 8158); 8015 Structural Geology: Local crustal structure; 0935 Exploration 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: /2002tc001386, Copyright 2003 by the American Geophysical Union /03/2002TC Introduction [2] The tectonic erosion of crust in the forearc of convergent plate margins represents an important part of the mass budget within subduction environments. Understanding the fate of the sedimentary cover and oceanic crust of a subducting plate is important if global geochemical cycles are to be understood. Does material extracted from the upper mantle get recycled back into this reservoir through deep subduction, or is this material merely reworked along convergent margins, either being off-scraped within accretionary complexes, or re-melted and incorporated into the arc magmatism itself? Although large accretionary complexes formed along the frontal edges of continental lithospheric plates are known from many margins (e.g., Barbados) [Nankai, Makran and Cascadia [Moore and Biju-Duval, 1984; Davis and Hyndman, 1989; Moore et al., 1990; Minshull and White, 1989], there are more significant lengths of modern convergent margin, mostly located around the periphery of the Pacific, including large parts of the Peruvian margin, where minor or no accretion is observed [Rutland, 1971; Hilde, 1983; von Huene and Scholl, 1991]. In these areas it is often unclear as to whether the sediment on the oceanic plate is being subducted deep into the mantle, or if accretion is occurring by basal underplating under the forearc, but at a depth that is not readily imaged by seismic reflection surveys. Study of the vertical tectonics in forearc basins in such areas can help to estimate the rates of subduction erosion or accretion by underplating because the character of the sedimentary record can constrain the bathymetry of the forearc over long periods of subduction. Although forearc basins do not cover the entire forearc, they do provide information from areas lying tens of kilometers landward of the trench region. [3] In this study we quantify the rates of tectonic erosion of the Peruvian forearc in the area of Lima Basin (Figure 1) in order to understand the mass budget of this convergent margin over significant lengths of geologic time. To do this we use seismic and drilling data to reconstruct the subsidence and uplift history of the forearc (Figure 2). In particular, we examine the rates of vertical motion during normal subduction of oceanic crust and compare this with the dynamics related to subduction of the Nazca Ridge

2 7-2 CLIFT ET AL.: TECTONIC EROSION OF THE PERUVIAN FOREARC Figure 1. Bathymetric map of South American forearc and Pacific Ocean showing location of Lima Basin and major bathymetric features discussed in the text. Water depth is in meters. [Pilger, 1981], an aseismic volcanic edifice that is in oblique collision with the South American margin and the crest of which is considered to have been subducted below the Lima Basin after 11 Ma [Hampel, 2002]. 2. Controls on Subduction Erosion [4] The factors governing accretion versus tectonic erosion along a given margin are understood in outline, but not in detail. Rates of accretion and brief phases of tectonic erosion have been estimated in margins where an accretionary complex juxtaposed against much older rock is preserved as a record of the margin s development [e.g., von Huene et al., 1994, 1996]. In non-accretionary margins the removal of the forearc sediment record has resulted in estimates of tectonic erosion rates that vary widely. In the central Andes at 21 S, average rates of trench retreat have been estimated at km Myr 1 [Scheuber and Reutter, 1992]. Ballance et al. [1989] suggested that tectonic erosion in Tonga has been generally minor, except during the subduction of major seamount features, most notably the Louisville Ridge. Conversely, Lallemand [1998] has argued that strong coupling between the down-going and overriding plates around the Pacific results in rapid trench retreat of 4 10 km Myr 1 in such settings. As a result, several hundred 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 the subsidence history of Tonga forearc from sedimentary and structural data, concluding that long-term tectonic erosion of the forearc was mostly by slow removal of material from the base of the forearc crust, causing the trench to retreat at <1.5 km Myr 1. Trenchward tilting of the forearc suggested that the strongest tectonic erosion occurred closest to the trench. These authors also noted that collision of the Louisville Ridge with the Tonga Trench caused significant shortening of the forearc (80 km removed), mostly by a major steepening of the trench slope, accounting for 60% of the trench retreat since 35 Ma. 3. Geologic Evolution of the Lima Basin [5] The Peruvian margin is generally recognized as a long-lived convergent margin with the Nazca Plate subducting approximately eastward below South America. Compared to classic accretionary margins, such as the Nankai, Makran, Barbados or Cascadia margins, the Peruvian margin was early identified as not being marked by strong compressional deformation or the development of a major accretionary complex, but instead by extensional basins such as the Talara forearc basin [Rutland, 1971; Moberly et al., 1982; Scholl et al., 1970]. In addition, the forearc to

3 CLIFT ET AL.: TECTONIC EROSION OF THE PERUVIAN FOREARC 7-3 Figure 2. Bathymetric map of Lima Basin showing location of multichannel seismic reflection profiles and Ocean Drilling Program (ODP) sites considered in this study. Water depth is in meters. the Mesozoic arc appeared to be missing, implying a substantial loss of crust since that time. The moderate accretionary complex that is noted trenchward of Lima Basin 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 prediction confirmed by bathymetric mapping [Kukowski et al., 2001]. Such a rough subducting plate would promote tectonic erosion of the overriding forearc. [6] A number of sedimentary basins are recognized along the margin, whose development can be used to understand the tectonic evolution of the subduction zone. The Lima Basin forms in the forearc between 12 and 10 S and is separated from the adjacent Salaverry Basin, located on the shelf, by a basement high [Thornburg and Kulm, 1981]. Seaward it is separated from the steep lower slope by a structural high that permits turbidites to pond [Hussong et al., 1988]. The Lima Basin is inferred to be underlain by metamorphic continental crust [Suess et al., 1988], although there has been debate about how far seaward this extends, some workers [e.g., Hussong and Wipperman, 1981] suggesting that the lower slope comprises sedimentary rocks accreted from the subducting plate. Indeed the lower slope is marked by landward dipping reflectors that have been interpreted as imbricated sedimentary rocks off-scraped from the subducting plate [von Huene et al., 1996]. The tectonic structure of the Peruvian margin has been affected not only by the subduction of normal oceanic crust but has also 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 plate margin. von Huene et al. [1996] used the plate model of DeMets et al. [1990] to calculate that Nazca Ridge was passing the Lima Basin portion of the margin at 4 Ma. The passage of Nazca Ridge is normally considered to cause significant tectonic erosion of the forearc. Subsequently, i.e., after 4 Ma, accretion under the outer forearc is believed to be responsible for the Pleistocene-Recent uplift of the outer trench slope stratigraphy [von Huene and Pecher, 1999]. In contrast, Hampel [2002], in a revised reconstruction, indicated initial collision of Nazca Ridge with the Peruvian forearc at 11.2 Ma at 11 S, i.e., in the vicinity of the study area considered here. In this model the ridge moved at 6 cm/yr obliquely along the margin. Although the width of the Nazca Ridge at the modern trench is 200 km, the reconstruction of Hampel [2002] indicates a wider ridge colliding with the trench at that time, making the 3.3 Myr estimate of collision duration at any given point of the forearc a minimum. [7] The stratigraphy of the Lima Basin has already been described by Ballesteros et al. [1988] using multichannel seismic lines by which they identified 11 different sediment packages and noted the rather dramatic seaward thickening of the stratigraphy, as well as the abrupt truncation of very young reflectors against the seafloor on the lower midslope, which they inferred to be due to the action of fast flowing contour currents. We base our revised stratigraphy on this study, but converted travel time picks to depth using newly obtained velocity functions (Figures 3 and 4). We also incorporated new multichannel seismic data collected in 2000 [Bialas and Kukowski, 2000].

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5 CLIFT ET AL.: TECTONIC EROSION OF THE PERUVIAN FOREARC Seismic and Drilling Data [8] Lima Basin makes a good location to examine tectonic erosion processes because it is relatively well surveyed by multichannel seismic profiles, most notably by a University of Hawaii, Institute for Geophysics (HIG), cruise in 1986 and in 2000 by R/V Sonne cruise 146 GEOPECO, which increased the density of the seismic reflection grid and provided ties with the existing Ocean Drilling Program (ODP) sites drilled during Leg 112 (Figure 2). The HIG data were recorded by R/V Moana Wave using a partially detuned air-gun seismic source and a 1600-m-long streamer [Moore and Taylor, 1988]. This source-receiver offset was sufficient for seismic velocity analyses. The GEOPECO data 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 excellent resolution, in most cases reaching the crystalline basement. With its relatively short streamer, the GEOPECO survey was 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 these time sections, together with the new GEOPECO data, to pick stratigraphic horizons. We then determined intervalvelocities in major stratigraphic units from the pre-stack migrated seismic data along four HIG transects across Lima Basin (including HIG 13 and HIG 14; Figures 3c and 4c). [10] Three ODP sites are located within the area of study ( , and 688). ODP Site 679 is especially useful because it lies in shallower water at the landward edge of the survey, and is a relatively complete, well-dated section, with one major hiatus between 7.2 and 11 Ma and two shorter ones at Ma and Ma. The seismic reflection data was loaded on to a Unix workstation running Schlumberger s Geoframe 2 interpretation package and a series of depositional packages were identified, similar to those defined by Ballesteros et al. [1988]. By mapping horizons in three dimensions, along strike variability was assessed and a greater level of confidence in the interpretation was achieved through the need to match reflectors in series of profiles. After interpretation of the seismic profiles the seismic stratigraphy was converted to a depth section using the stacking velocities derived from the seismic profiles. Two of the most representative sections across Lima Basin HIG-13 and 14 are shown in Figure 3 and Subsidence Reconstructions [11] The subsidence history of Lima Basin is considered here using both a one-dimensional backstripping analysis of the stratigraphy of the ODP drill sites, and a two-dimensional backstripping of the interpreted seismic profiles. The one-dimensional method allows the vertical motions of the basement at the drill site to be reconstructed with the detailed temporal resolution derived from the biostratigraphy, within the uncertainty of the water depth estimates derived from sedimentary and benthic foraminifer studies. The two-dimensional method is limited by the resolution of the seismic data, but allows wide areas of the basin, away from the drill sites to be examined. [12] In the one-dimensional approach we used the backstripping subsidence method of Sclater and Christie [1980], in which lithology and age information taken from the cored material are used to calculate a depth to basement, after removing the loading effects of sediment and water, allowing a residual, tectonically driven subsidence of the basement to be isolated. The backstripping is done in a series of stages, controlled by the number of dated sedimentary packages. At each stage the youngest sediment package is removed, and the underlying sediments are decompacted to restore them to the thickness that they originally had before deposition of that youngest package. In addition, the depth to basement can be calculated for each time period, with a correction made for the weight of sediment at any given time. [13] For each time period two possible depths to basement are calculated, representing minimum and maximum estimates of the water depth at the time of sedimentation. The true loading corrected subsidence pattern of the basement must lie between these two estimates. The Sclater and Christie [1980] backstripping method assumes an empirical porosity-depth curve that is based on lithology, but this can be corrected to match the measured porosity values from recovered core material. The porosity-depth curve of Sclater and 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 Basin appear to be consistently higher than the Sclater and Christie [1980] model for shale, at least above 400 m below seafloor (mbsf), representing 40% of the total sediment thickness in Lima Basin. This is unlikely to be a major source of errors, because it introduces only 60 m of uncertainty into the tectonic subsidence, which is calculated at m in the outer Lima Basin. Nonetheless, in this study we use the porosity-depth relationship found in the ODP wells when performing the backstripping reconstruction. When making the unloading correction the density of the lithospheric mantle is assumed to be 3330 kg/m 3 [Oxburgh and Parmentier, 1977] Paleowater Depths [14] When dealing with continental margin sediments deposited in significant water depths, such as the Lima Basin, estimates of paleowater depth are crucial to a meaningful result and represent the single largest uncertainty. Variations in the degree of sediment compaction are of a magnitude smaller than any possible errors in the water depth. No attempt has been made to correct for fluctuations in 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 model derived from the stacking velocities, and (d) interpreted stratigraphy after depth conversion. See color version of this figure at back of this issue.

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7 CLIFT ET AL.: TECTONIC EROSION OF THE PERUVIAN FOREARC 7-7 Figure 5. Porosity-depth measurements made in ODP Hole 679 and 688 in the Lima Basin compared to the compaction model for shale from Sclater and Christie [1980]. Note that while they form a linear array the porosity values in the Lima Basin appear to be consistently higher than the Sclater and Christie [1980] model. magnitudes of eustatic sea level remain controversial and produce unlikely, saw tooth-like subsidence curves when taken into account [Wood, 1982]. Although short period sea level fluctuations predicted by reconstructions like Haq et al. [1987] are difficult to account for, long-term variations may be incorporated without producing geologically improbable subsidence histories. Nonetheless, the magnitude of the sea level variations remains contentious. Sea level variations based on oxygen isotope work predict fluctuations of m above modern levels in the Early Tertiary [Miller et al., 1985], compared to 150 m for the Haq et al. [1987] model. Haq et al. [1987] predict sea level at 4 Ma (the end of Nazca Ridge collision) to be 80 m higher than present day and 140 m above present day at 13.6 Ma, the start of the drilled sediment record at ODP Site 679. If these figures are correct then a failure to account for them will lead to an underestimate of the amount of tectonic subsidence of this magnitude. Even if this is not accounted for the uncertainty introduced is small compared to the total subsidence predicted (2.5%). [15] Water depth controls are much better for ODP Site 679 than Sites 682 and 688 because it is located in shallower water, where benthic foraminiferal depth zones are closely spaced. All water depth assignments are derived from the sedimentology and paleontology report of the Shipboard Scientific Party [1988a, 1988b, 1988c] and from Resig [1990]. At ODP Site 679 outer shelf sediments ( m water depth) in the Middle Miocene show a shallowing upward into Upper Miocene midand inner shelf environments (<200 m). There is a major unconformity noted in drill and seismic data at 255 m below seafloor (mbsf) separating Upper and Middle Miocene strata, and constrained as spanning Ma. Within the shelf sediments at the top of the drilled section a second subaerial exposure horizon was dated at 4.2 Ma on the basis of the nannofossil biostratigraphy and the Berggren et al. [1995] timescale. Subsequently the site rapidly regained outer shelf depths before subsiding to the present water depth of 450 m. [16] ODP Site 682 was drilled in 3788 m of water and recovered sedimentary rocks dating back to the Eocene. Water depth constraints from the benthic foraminifers indicate upper to midbathyal conditions ( m) for much of the Oligocene and Early Miocene, shallowing to outer shelf to upper midbathyal depths ( m) in the Middle Miocene. A sharp deepening to lower bathyal depths is recorded after 11.3 Ma. ODP Site 688 now lies in 3835 m of water. The oldest drilled sediment is Eocene and is assigned to a midshelf-upper bathyal range by Shipboard Scientific Party [1988c], i.e., m. Much of the Cenozoic is missing in this area, but there is a semicontinuous record after 16 Ma. Starting at upper middle bathyal ( m) water depths at 16 Ma the site deepened to midbathyal or deeper depths, i.e., >1500 m by the Late Miocene. A lower boundary of the present water depth is placed on the site assuming that long-term subduction erosion has resulted in progressive deepening. As explained by von Huene and Pecher [1999], the landward tilting of strata in the outer Lima Basin suggests some relative uplift of this area where ODP Sites 682 and 688 are located. The tilting affects strata dated as Pliocene and older, but does not affect the Quaternary. Although the deformation seen in the seismic profiles suggests recent uplift of m along the western edge of Lima Basin, Figure 4. (opposite) (a) Multichannel seismic reflection profile HIG-14, with (b) interpreted structure, (c) velocity model derived from the stacking velocities, and (d) interpreted stratigraphy after depth conversion. See color version of this figure at back of this issue.

8 7-8 CLIFT ET AL.: TECTONIC EROSION OF THE PERUVIAN FOREARC the uncertainties on the water depth estimates from Pleistocene sediments at ODP Sites 682 and 688 do not allow resolution of such an uplift event. [17] A crucial assumption in one-dimensional backstripping calculations is that of local isostasy. Local isostatic compensation treats the lithosphere as though it has zero lateral strength, so that any applied load is supported by the buoyancy of the underlying mantle rather than laterally by the strength of the lithosphere. Although this assumption of a zero strength lithosphere is clearly an approximation, it is reasonable where the lithosphere is weak or the applied load is wide and evenly distributed [Sclater and Christie, 1980]. The strong deformation and faulting observed in the Peruvian forearc is compatible with this region being one of very low flexural rigidity, similar to other tectonized forearcs (e.g., Tonga [Clift and MacLeod, 1999]). Studies of the forearc in areas where seamounts are in collision, show that deformation is focused close to immediate vicinity of the collision (e.g., Costa Rica [von Huene et al., 2000]). If flexural rigidity were high in the forearc, uplift would be more broadly distributed. [18] Age control for the drill sites considered is derived from the nannofossil biostratigraphy produced by Shipboard Scientific Party [1988a, 1988b] and Martini [1990]. The nannofossil zone assignment is then converted to a numerical age using the timescale of Berggren et al. [1995]. These sites were chosen because of their location within the seismic survey and because of the semi-complete stratigraphy since 14 Ma, allowing a detailed reconstruction of the basement subsidence history since that time. 6. Results of One-Dimensional Backstripping [19] The results of the unloading calculations at ODP Sites 679, 682 and 688 are shown in Figure 6. Table 1 summarizes the amounts of vertical tectonic motion linked to Nazca Ridge subduction. At ODP Site 679 there is strong evidence for a tectonically driven uplift event following the Ma hiatus, culminating in a peak at 7.2 Ma and again at 4.2 Ma, followed by a rapid collapse. The Ma hiatus broadly correlates with the passage of the Nazca Ridge as dated by Hampel [2002], with the tectonic subsidence between 7.2 and 5.3 Ma reflecting collapse of the forearc basement after the passage of the main section of the ridge. The second phase of tectonic uplift at 4.2 Ma appears to be short lived, being finished by 4.0 Ma and may reflect the subduction of an additional basement feature on the Nazca Ridge. In the model of Hampel [2002] the width of the Nazca Ridge colliding with Lima Basin was much broader than it is in the modern day, a prediction based on the assumption that the Nazca Ridge mirrors the Tuomotu Ridge, since it is produced from the same hot spot. Consequently, we interpret the second uplift event at ODP Site 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 forearc basement, totaling at least 130 m since 13.6 Ma. This may be interpreted to reflect long-term tectonic erosion of the Figure 6. Sediment-unloaded, backstripped subsidence reconstructions of depth to basement at ODP Sites 679, 682 and 688 derived from one-dimensional backstripping of the drilled section using the methodology of Sclater and Christie [1980]. Vertical range show uncertainties in paleowater depth estimates, accounting for the vast majority of the uncertainty in the calculated depth. forearc basement under ODP Site 679. The sedimentary hiatus at Ma does not correspond to a phase of tectonic 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 permanent subsidence of the site between 50 Ma and 16 Ma. Subsequently, ODP Site 682 showed strong subsidence after 11.3 Ma, while Site 688 suffered a significant deepening between 16 and 12 Ma, suggesting a period of enhanced

9 CLIFT ET AL.: TECTONIC EROSION OF THE PERUVIAN FOREARC 7-9 Table 1. Amounts of Vertical Uplift and Subsidence at ODP Drill Sites Within the Lima Basin Linked to Subduction of the Nazca Ridge Drill Site Tectonic Erosion Since 11 Ma, m Uplift During Nazca Ridge Subduction, m Subsidence Immediately After Nazca Ridge Subduction, m ODP Site to to to 170 ODP Site to to to 2625 ODP Site to to to 1625 tectonic erosion, likely linked to the start of Nazca Ridge subduction after 11 Ma. Because of the uncertainties in the water depths the amount of this subsidence is not well constrained, especially at ODP Site 688, where it could theoretically be almost zero, although the sharp change in sediment facies argues against this. Temporary uplift of ODP Sites 682 and 688 during passage of Nazca Ridge is not resolved, meaning that it was less than the uncertainties in the water depths, i.e., 1788 m and 1600 m respectively. A large hiatus spanning Ma was recognized at ODP Site 688 coincident with the deepening phase. This suggests that the hiatus was generated by submarine erosion or slumping during the period of ridge passage. ODP Site 682 has a poorly defined hiatus at 9 10 Ma and a second hiatus at Ma associated with a zone of slumped sediment dated at 6 9 Ma. The slumping, hiatus and strong tectonic 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 with and following Nazca Ridge collision, the backstripping methods reveal anomalous activity extending far beyond the 3.3 Myr minimum duration of ridge passage predicted from the current width of the Nazca Ridge at the trench and the 6 cm/yr lateral migration rate of Hampel [2002]. Sedimentation and subsidence at ODP Site 679 is anomalous between 11 and 4 Ma, spanning 7 Myr, reflecting the greater width of the Nazca Ridge entering the trench at that time. 7. Two-Dimensional Backstripping [23] The subsidence of the margin can also be reconstructed using a two-dimensional backstripping approach. In this method the subsidence of a paleosubaerial surface is reconstructed. We choose to focus on the hiatus surface between the tilted and eroded Middle Miocene and the overlying, undeformed Upper Miocene sequence because this surface is recognized over the entire Lima Basin and it has been drilled and dated ( Ma) at ODP Site 679. Major deepening at ODP Site 682 is seen to postdate 11.3 Ma, constraining formation of this hiatus surface to being after this time, likely representing shoaling during the initial collision of Nazca Ridge. Water depth constraints at ODP Site 688 are less clear for this interval, since Resig [1990] showed that while the Lower Miocene was deposited in upper middle bathyal conditions ( m), the Upper Miocene is barren of benthic foraminifers. We assign an age of 11 Ma to the hiatus surface as the time after which strong subsidence at the drill site can be identified. [24] Although ideally we would like to reconstruct the subsidence of the subaerial exposure surface in three dimensions (Figure 7), a good impression of across margin variability can be derived from examining a series of twodimensional profiles, since most of the depth variability is expressed across the strike of the trench, not along it. For this purpose we choose lines HIG-13 and HIG-14 (Figures 3 and 4) because these cross the entire basin, are well dated by the ODP wells, and show good definition of the seismic reflectors above Eocene basement. The approach used is similar to that in the one-dimensional case in that the dated sedimentary layers are progressively removed from the section, allowing the underlying layers to decompact and then adjusting the entire section for isostatic equilibrium. We employ the program FLEX-DECOMP TM developed by N. Kusznir and A. M. Roberts to backstrip the Lima Basin. This program has been used to successfully backstrip basins in passive margin and intra-continental settings [Kusznir et al., 1991, 1995], as well as in transform margins [Clift and Lorenzo, 1999]. Although this approach can account for the effects of flexural rigidity, the strong faulting and deformation of the forearc, coupled with the pinching out of the mantle lithosphere under the forearc as the trench is approached suggests very low rigidity under Lima Basin. Consequently, we use a zero strength crustal model. Because the crust under the profile is continental we make no correction for tectonically induced thermal subsidence. 8. Results of Two-Dimensional Backstripping [25] The results of unloading and decompacting profiles HIG-13 and 14 to the 11 Ma reflector are shown in Figure 8. What is apparent is that this surface, which we and earlier workers [e.g., von Huene and Pecher, 1999] consider to have been at sea level at this time, does not restore to sea level after the removal of all the younger sediments and allowing local isostatic compensation to be achieved. The implication is that subsidence due to basal subduction erosion has affected the margin, causing the mismatch in observed and reconstructed water depths at 11 Ma. In other words, the current depth of Lima Basin cannot be achieved simply by the loading of sediments on to the subaerial surface present at 11 Ma. The backstripping also clearly shows that the reconstructed profiles are deeper at the SW end than the NE, indicating greater net subsidence, and thus more tectonic erosion closer to the trench. [26] In practice the reconstructed basin profiles can be used as a measure of net tectonic subsidence since 11 Ma (Figure 9). The interpretation is slightly complicated by the recognition that the most trenchward portions of the profiles

10 7-10 CLIFT ET AL.: TECTONIC EROSION OF THE PERUVIAN FOREARC Figure 7. Map of the central Lima Basin showing the depth in two-way travel time (TWTT) measured in 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. have been slightly uplifted relative to the basin center. Evidence for relative uplift in the trenchward portions of Lima Basin is shown by the reversal of dip of Pliocene and older sediments to a landward direction at the SW end of both profiles. Figures 3 and 4 show the way in which the amount of this recent relative uplift can be estimated. By assuming that the basement and strata would have continued to deepen offshore, as they do in landward portions, it is possible to estimate the amount of uplift generated by recent uplift under the trenchward edge of the Lima Basin. This can then be subtracted from the net subsidence in those areas to estimate the total amount of subsidence in the trenchward regions prior to the recent accretion event. [27] The two-dimensional estimates for permanent net subsidence due to basal subduction erosion can be matched with estimates derived from the ODP drill sites (Figure 9). Because the unloaded depth to basement at each drill site was calculated from the one-dimensional backstripping method and the depth at 11 Ma is known to be subaerial at ODP Site 679, the total net subsidence can be measured. ODP Site 679 lies at the NE end of the profiles and matches estimates derived from the seismic data, as might be anticipated. However, ODP Sites 682 and 688 lie trenchward of the profiles and help constrain degrees of tectonic erosion closer to the trench. HIG-13 shows 2500 m of subsidence 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 erosion or accretion at the Peruvian margin is crucial to understanding the mass budget in this subduction zone. In this study we estimate the average rates of erosion and how much of this can be related to the subduction of the Nazca Ridge. The modern ridge is currently moving at an oblique rate of 6 cm/yr, although the relative velocity of Nazca Ridge and Lima Basin has changed through time due to variable convergence [le Roux et al., 2000; Hampel, 2002]. Backstripping subsidence analysis at ODP Site 679 indicates a 7 Myr collision in the study area, consistent with the reconstruction of Hampel [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 subsidence reconstructions. In our models we assume that the paleo-

11 CLIFT ET AL.: TECTONIC EROSION OF THE PERUVIAN FOREARC 7-11 Figure 8. Sediment-unloaded cross sections of HIG-13 and HIG-14 showing the restored depth of 11 Ma reflector, which is interpreted as being a product of subaerial erosion at this time. The decompacted sediment overlying basement in this figure is the Middle Miocene shown in Figures 3 and 4. The mismatch between the observed subaerial character of the reflector and the reconstructed depth increases toward the SW, in a trenchward direction, thus indicating faster basal tectonic erosion close to the trench, compared to more landward areas. forearc was similar to the modern forearc in its bathymetry. In the absence of detailed bathymetric constraints across the entire forearc the assumption of similar forearc slope is required and allows first order estimates of trench retreat rates to be made. Because there is little long-term sediment record trenchward of the Lima Basin it is doubtful whether a much more detailed reconstruction of paleoforearc slopes could be made even if more drill sites were available. Indeed, because of the progressive removal of material from the forearc by the ongoing tectonic erosion much of the early record of trench slope has been lost. [30] Sedimentary facies and microfauna constrain ODP Site 688 to being in shelf water depths (<500 m) at Ma, implying that this site was then km from the trench axis. Because ODP Site 688 is now only 32 km from the trench, this suggests km of plate margin has been lost since 47 Ma, an average trench retreat rate of km Myr 1. ODP Site 682 lay in a water depth of m until 11.3 Ma, implying a location km from the trench at that time, compared to 37 km from trench today, constraining trench retreat to a long-term average of km Myr 1 since 11.3 Ma. The difference between the two sites suggests an acceleration in trench retreat rates during the last 11.3 Myr. [31] The calculated rates of trench retreat can be compared to those of von Huene and Lallemand [1990], who estimated an average trench retreat rate of km Myr 1 since 20 Ma for the Peru margin. The new estimate for Lima Basin compares with an average rate of 3 km Myr 1 for the Costa Rican forearc since 17 Ma [Vannucchi et al., 2001], 3 km Myr 1 for the Chile Trench since 10 Ma [Laursen et al., 2002], 3 km Myr 1 for the Japan Trench since 16 Ma [von Huene and Lallemand, 1990], 4.7 km Myr 1 for the South Sandwich Islands [Vanneste and Larter, 2002] and an average rate of 3.9 km Myr 1 in the Tonga Forearc since 35 Ma. In the latter case much of the erosion is due to collision of the Louisville Ridge [Clift and MacLeod, 1999]. Tectonic erosion rates in Tonga unrelated to Louisville Ridge collision are <1.5 km Myr 1. [32] Erosion rates can also be assessed from the seismic profiles. The trenchward edge of profile HIG-14 is now 70 km from the trench, having experienced 3 km of subsidence since 11 Ma, when it was at sea level, i.e., at the coast. If the forearc at 11 Ma were comparable with the modern system in geometry, that would place the southwestern end of profile HIG km from the trench at 11 Ma. This implies 110 km of lost crust since 11 Ma at an average rate of frontal erosion of 10 km Myr 1. This is much faster than the long-term rate derived from ODP Site 688, but close to the rate since 11.3 Ma calculated at ODP Site 682. If 110 km has been lost from the plate margin since 11 Ma and km has been lost since 47 Ma then

12 7-12 CLIFT ET AL.: TECTONIC EROSION OF THE PERUVIAN FOREARC Figure 9. Across margin profiles of HIG-13 and HIG-14 showing the net permanent subsidence of the margin 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 depth to which this is restored after backstripping (Figure 8). Vertical bars show constraints derived from ODP Site 679 at the landward end of the profiles and from ODP Sites 682 and 688 at the trenchward end of HIG-13 and HIG-14, respectively. this would imply that tectonic erosion rates before 11 Ma could not have exceeded 1.1 km Myr 1, and that the average rate of erosion increased sharply after 11 Ma. An estimate of 110 km lost since 11 Ma must also be a minimum for the trench retreat since 47 Ma, indicating that the higher end of the range, km, derived from drill site backstripping must be closer to the true figure of forearc crust lost since the Eocene. Because sea level was 60 m higher at 11 Ma than it is today [Haq et al., 1987], correction for this effect would only increase the apparent rate of subduction erosion since that time. [33] Average tectonic erosion rates may have increased since 11 Ma because of either faster background tectonic erosion rates, or because of the influence of Nazca Ridge collision starting at 11 Ma. Because the average tectonic erosion rates after 11 Ma are so much faster even than for the Tongan forearc, we prefer to attribute faster tectonic erosion to Nazca Ridge collision. Tectonic erosion rates during subduction of normal oceanic crust are unlikely to be much faster than Tonga, because in that area the rate of convergence is very high, there is little sedimentary cover and the plate is thermally mature and thus breaks with large normal faults as it flexes the trench [Wright et al., 2000], resulting in a rough and potentially very erosive surface against the base of the forearc. Plate reconstructions for Nazca plate-south American motions also show that convergence was slightly slower, not faster, after 11 Ma [Pardo- Casas and Molnar, 1987; Somoza, 1998], also arguing against faster tectonic erosion unrelated to Nazca Ridge collision. We prefer to explain the faster average rate from 0 11 Ma compared to Ma as being related to collision with the Nazca Ridge. [34] Lower rates of tectonic erosion could be estimated for the 0 11 Ma period if the forearc slope was steeper at that time, i.e., that the 11 Ma erosion surface does not require Lima Basin to be 180 km from the trench at that time. The water depth information might suggest that the trench slope was steeper at 11 Ma, perhaps due to compressional deformation or basal accretion. It is noteworthy that the sedimentary rocks underlying the 11 Ma unconformity at ODP Site

13 CLIFT ET AL.: TECTONIC EROSION OF THE PERUVIAN FOREARC were identified as being upper bathyal [Suess et al., 1988; Resig, 1990], implying that Lima Basin had been flooded and was subsequently exposed, an observation incompatible with a steady progression of any given point on the forearc toward the trench on a constant slope margin. If the Lima Basin transect was trenchward of the coast prior to 11 Ma then its exposure at 11 Ma requires a tectonically driven uplift event to steepen the forearc at that time. This uplift event does not seem to have affected sedimentation at ODP Site 688, but the uncertainties in the water depth there at 11 Ma would make resolution of such an event difficult. However, ODP Site 682 does show a shallowing of the water depths between 15.6 Ma and 11.3 Ma from upper middle bathyal ( m) to outer shelf-upper middle bathyal ( m), consistent with a compression and steepening of the margin at that time. 10. Subsidence Due to Nazca Ridge Subduction [35] Determining how much of the net subsidence is due to Nazca Ridge passage and how much is due to subduction erosion by normal oceanic crust is a key objective of this study. In addition, we try to understand how Nazca Ridge subduction affects the tectonic erosion, whether the erosion is increased to high values during the passage, or whether the passage of the ridge weakened the forearc, making it more susceptible to later basal erosion during normal subduction. Clearly, reconstructing the vertical motions across Lima Basin during the Ma period of ridge subduction is required to answer that question, and can only be addressed from the well data because the seismic profiles have no water-depth information for that period. [36] At ODP Site m of permanent basement subsidence has occurred since 11 Ma, of which m is caused by Nazca Ridge subduction after 11 Ma. This only loosely defines the subsidence attributable to normal subduction erosion of the forearc to between a minimum of 50 m (1.9% of the total since 11 Ma), and a maximum of 2280 m (85%). At ODP Site 688 the water depth uncertainties do not allow any constraints to be placed on the proportion of Nazca Ridge versus continuous subduction erosion. In contrast, at ODP Site 679, temporary uplift of the site during Nazca Ridge passage is calculated at m. Permanent subsidence at this site caused by ridge passage is defined by the difference in basement depth at 4.0 Ma and 11.0 Ma, i.e., ranging from 170 m of subsidence to an uplift of as much as 190 m. The basement depths at 7.2, 5.3 and 4.2 Ma are likely affected by ongoing collision events and cannot be used to measure long-term permanent subsidence driven by these same collisions. There is no unambiguous indication of rapid tectonic erosion under ODP Site 679 during the collision of Nazca Ridge. In contrast, m of the total subsidence at ODP Site 679 since 11 Ma (i.e., >36% of the total subsidence) has occurred after 4 Ma, after the end of resolvable collision events tied to passage of Nazca Ridge. [37] Because of the water depth uncertainties, it is not possible to unambiguously define how much tectonic erosion and subsidence is directly related to Nazca Ridge and how much to ongoing subduction since the start of ridge collision. However, some clues are provided by comparison with other ridge-trench collision events. In the Tonga Arc- Louisville Ridge collision zone in the SW Pacific the erosive effect of ridge collision is mostly in the form of frontal erosion and steepening of the trench slope, with basal erosion of the forearc limited to the region close to the trench axis [Dupont and Herzer, 1985; Clift and MacLeod, 1999]. Unlike the Tonga-Louisville system however, the Peruvian forearc is not noticeably steeper or narrower north of Nazca Ridge than it is south of that feature, although it is steeper and narrower immediately opposite the point of ridge collision. The lack of a clear change in forearc geometry after ridge passage suggests limited frontal erosion of the plate during ridge passage. However, because the average rates of tectonic erosion since 11 Ma are approximately ten times faster than both those before 11 Ma in the Lima Basin, we suggest that collision between the trench and Nazca Ridge did significantly accelerate basal subduction erosion. Much of the enhanced tectonic erosion may postdate the actual collision itself. In this scenario the uplift and deformation of the forearc during ridge subduction weakened the plate margin and allowed steady state erosion processes to much more rapidly remove material from the base of the forearc crust than was the case before ridge subduction. The dominant mechanism of erosion in this case would be the removal of lenses of material from the base of the forearc in the manner described by Ranero and von Huene [2000]. No significant faulting is seen in Lima Basin following ridge passage that would support the idea of faulting and breaking up the entire forearc at that time. [38] Using an average arc continental crustal thickness close to the coast of 32 km, the rate of crustal loss in the forearc can be averaged at 109 km 3 Myr 1 per km of trench axis since 47 Ma. Bialas et al. [2001] used seismic refraction techniques to measure the crustal thickness under the Lima Basin as 25 km, consistent with our slightly higher figure for the crust onshore. We know that the average rate of basal tectonic erosion was faster after 11 Ma than before that time, i.e., following initial Nazca Ridge subduction. If 110 km of trench retreat has occurred since 11 Ma then the trench retreat rate must average 10 km Myr 1 since Nazca Ridge collision, resulting in average crustal erosion rate of 320 km 3 Myr 1 per km of margin. We can then calculate a long-term average crustal erosion rate of 35.2 km 3 Myr 1 per km of margin before 11 Ma. In comparison, the rate of crustal loss due to tectonic erosion in northern Chile was estimated at a rate of km 3 Myr 1 per km of margin [Scheuber et al., 1994; von Huene et al., 1999]. [39] Rates of crustal tectonic erosion under the forearc compare closely in magnitude to estimates of magmatic productivity in active margin settings. Although Reymer and Schubert [1984] estimated that globally only km 3 of new melt were added every 1 Myr per km of active margin, more recent estimates have pushed this value up. Holbrook et al. [1999] estimated rates of 55

14 7-14 CLIFT ET AL.: TECTONIC EROSION OF THE PERUVIAN FOREARC Figure 10. Map showing estimated eroded thicknesses of sediment in meters due to current-scouring of the seabed following folding of the Pliocene and older strata, prior to the Quaternary. 82 km 3 Myr 1 per km of margin for the Aleutians, while Suyehiro et al. [1996] indicated long-term average accretion rates of 66 km 3 Myr 1 per km of margin in the Izu Arc. However, for the Peruvian margin, Atherton and Petford [1996] suggested that 70,000 km 3 of new crust has been intruded along a 90 km long transect at 9 S between 100 and 3 Ma. This would result in a rate of only 8.0 km 3 Myr 1 per km of active margin, which is significantly less than in other arcs, and significantly less than material removed by subduction erosion. If true this would imply that the dominant pattern at the Peruvian margin would be net crustal loss, a finding which is similar to that for northern Chile [Laursen et al., 2002]. 11. Recent Seafloor Deformation and Erosion [40] Deformation of the trenchward edge of Lima Basin in the form of relative uplift, affects all the dated sedimentary packages up to and including the Pliocene. Pre-Quaternary strata in the Lima Basin are seen to thicken offshore, and indicate that the trenchward portion of the basin was not a paleohigh before the Quaternary. Clearly, some very recent tectonic mechanism has caused landward backtilting of these strata in the trenchward part of the basin, although it is not possible to constrain what that might be. Uplift might be caused by recent preferential underplating of material from the subducting plate under the trenchward edge of the Lima Basin. Such basal accretion may have affected the more landward areas of Lima Basin too, but if so then the whole section has been elevated and that magnitude is unknown, although it must be less than the water depth uncertainties at ODP Site 679 for the Pleistocene, as no such uplift event is resolved there. Alternatively, subduction of a seamount under this area might cause temporary uplift immediately above the edifice, although there is no evidence for this in the seismic data. Thornburg and Kulm [1981] show that this upturning of strata extends along strike for km, requiring subduction of a trench parallel ridge of that length if that mechanism is to account for the uplift. A third and most likely possibility is that this stratal geometry could reflect greater recent subduction erosion and subsidence under the center of Lima Basin compared to the trenchward edge. This model is consistent with the long-term character of the margin and with the evidence for extensional faulting in the seismic profiles. All these explanations would require short-term changes in the evolution of the forearc because subsidence, and thus subduction erosion, dominate and generally increase toward the trench. [41] Relative uplift of the trenchward edge is marked by strong seafloor erosion. Extrapolating the interpreted horizons that are now truncated by a sharp unconformity against the seafloor, it is possible to estimate the original basin form and the eroded thickness (Figure 10). Erosion must have taken place in deep water, as there is no evidence from the sedimentology or benthic foraminifer data to indicate the dramatic uplift and subsidence that would be required to produce this unconformity in a subaerial setting. Organicrich diatomaceous sediments of late Pleistocene age recovered at ODP Site 688 argue for consistently deep water over that area in the recent geological past. Although these upwelling-related facies are usually only known from shallow water environments less than 500 m deep, it is most likely that these sediments were redeposited downslope to