Saillard 2011 Coastal Uplift

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Andean coastal uplift and active tectonics in southern Peru: 10Be surfaceexposure dating of differentially uplifted marine terracesequences (San Juan de Marcona, ~15.4S)

M. Saillard a,b,c,, S.R. Hall d,1, L. Audin a,b,c,2, D.L. Farber d,e, V. Regard a,b,c, G. Hrail a,b,c

a Universit de Toulouse ; UPS (OMP) ; LMTG ; 14 Av. Edouard Belin, F-31400 Toulouse, Franceb IRD ; LMTG ; F-31400 Toulouse, Francec CNRS ; LMTG ; F-31400 Toulouse, Franced University of California, Santa Cruz, Department of Earth Sciences, Santa Cruz, CA 95060, USAe Lawrence Livermore National Laboratory, LLNL, Livermore, CA 94550, USA

a b s t r a c ta r t i c l e i n f o

Article history:

Received 14 April 2010Received in revised form 3 January 2011Accepted 11 January 2011Available online 19 January 2011

Keywords:

Marine terrace10-BerylliumCRN datingUplift rateSlip ratePeru

Nazca Ridge

Along the San Juan de Marcona Bay of southern Peru, two spectacular sequences of preserved marine terracesrecord net Quaternary uplift. Previous geomorphic analysis of these paleo-shorelines has revealed evidence ofupper plate deformation and regional uplift. However, in the absence of a robust absolute dating method,these studies contain substantial uncertainties concerning the numerical dating of these marine markers andthus the corresponding calculated surface uplift rates. However, eld mapping, surveying the neotectonicfeatures and 10Be dating of abraded surfaces contained within two sequences of marine terraces along thismargin should allow for the robust calculation of Pleistocene uplift rates.The San Juan de Marcona Bay lieson the southernank of the subducting and south-migrating aseismic NazcaRidge where the maximum rates of coastal uplift are expected. In this locality, we measure high uplift ratesranging from 0.4 m/ka to 0.9 m/ka during the Pleistocene. Margin-parallel normal faults displace severalmarine terraces and inuence the development of bays, thereby contributing to the conguration of paleo-and present-day coastlines. The faults have relatively low slip rates, b0.1 m/ka over 400 ka, and have been

inactive forthe last 80 ka. Thepresence and activity of these normalfaults can be directly linked to subductionzone processes, with the release and accommodation of short-term coseismic compressionduring megathrustsubduction-zone events.In contrast, the regional permanent upliftis probably controlled by post-seismic and/or interseismic strain accumulation over longer time-scales due to inelastic behavior of the upper plate. Sinceat least the latest Pliocene, the San Juan de Marcona area has experienced long-term regional tectonic upliftthat hasincreased since about 800 ka due to thesouthward migrationof thesubducting Nazca Ridge. Based onmigration velocity and geometry of the ridge, the inuence of the Nazca Ridge on the uplift of the forearcshould account for ~0.4 m/ka within 145 km south of the ridge axis. Hence, most of the post-400 ka meanuplift rate of the San Juan de Marcona area could be explained by the Nazca Ridge subduction.

2011 Elsevier B.V. All rights reserved.

1. Introduction

Marine terraces are important markers for quantifying coastaluplift along active margins over ~1 Ma; individual marine terraces areeroded during sea-level highstandsand then preserved by subsequentcoastal uplift (e.g.,Bloom et al., 1974; Chappell, 1974; Pillans, 1983;

Bull,1985;Lajoie, 1986; Ota, 1986; Merrittsand Bull, 1989;Lajoie etal.,1991; Pirazzoli et al., 1991, 1993; Zazo, 1999). In forearc basins,sequences of marine terraces preserve cumulative records of eustaticuctuations and tectonic histories of coastal forearcs (Radtke, 1987;Muhs et al., 1990; Goyet al., 1992; Hsu, 1992; Leonard andWehmiller,1992; Machar and Ortlieb, 1992; Ota et al., 1995; Ortlieb et al., 1996a;Cantalamessaand Di Celma,2004;Marquardt et al., 2004; Pedojaet al.,2006a,b; Saillard et al., 2009).As the present day sea-level isone of thehighest levels recorded for Quaternary time span, every emergedmarine terrace is preserved by coastal uplift. These marine markersarereference surfaces that record a spatial and temporal history of pastsea-levelhighstands. When theterraces areof Quaternary age, as is thecase in this study, these marine markers allow a calibration of coastaluplift rates based on their current elevation above modern sea level.

Geomorphology 128 (2011) 178190

Corresponding author at: Universit de Toulouse ; UPS (OMP) ; LMTG ; 14 Av. EdouardBelin, F-31400 Toulouse, France. Tel.: +33 5 61 33 25 85; fax: +33 5 61 33 25 60.

E-mail address:[email protected](M. Saillard).1 Now at McGill University, Earth and Planetary Sciences, FDA 238, 3450 University

Street, Montreal, QC H3A 2A7 Canada.2 Now at IsTerre, Universit Joseph Fourier, Maison des Gosciences, Bote postale 53,

38041 Grenoble cedex 9, France.

0169-555X/$ see front matter 2011 Elsevier B.V. All rights reserved.

doi:10.1016/j.geomorph.2011.01.004

Contents lists available at ScienceDirect

Geomorphology

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Thanks to a hyper-arid climate and very low erosion rates(b0.5 m/Ma; Alpers and Brimhall, 1988; Kober et al., 2007; Hallet al., 2008), marine terraces along the Pacic rim of South Americahave beenremarkably well preserved. These climatic conditionsmakegeomorphic markers such as these a perfect candidate for theapplication of cosmogenic radionuclide (CRN) dating method. Indeed,in-situproduced CRNs, here we used10Be, have been demonstrated tobe a robust tool for the dating of marine terraces (Perg et al., 2001;

Kimand Sutherland, 2004; Quezada et al., 2007; Alvarez-Marrn et al.,2008; Saillard et al., 2009). A numberof previous authors have studiedthe marine terrace sequences of the San Juan de Marcona Bay (15.4S)using various chronological methods including amino-acid racemiza-tion, Electron Spin Resonance and U-series dating of shells (Fig. 1;Broggi, 1946; Regg, 1956, 1962; Hsu, 1988a,b, 1992; Hsu et al., 1989;

Ortlieb and Machar,1990; Machar and Ortlieb, 1992). However, dueto the small number of shells in the deposits accurate age determina-tions were insufcient to propose robust conclusions concerningvertical rock-uplift rates, as discussed inGoy et al. (1992).

The objective of this study is to quantify the timing of Andeanforearc deformation and to evaluate how uplift and fault activityinteract along the active margin where the Nazca Ridge is currentlysubducting. Here, we present a geomorphic analysis, including new

tectonic mapping and new absolute dating of marine terraces in theSan Juan de Marcona Bay. We use these measurements to quantifygeodynamic processes (by calculating uplift and slip rates) in thisregionof theAndean forearc activeover thelast ~0.5 Ma.We comparethe CRN ages of marine terraces to marine isotopic stages (MIS) usingthe eustatic curve ofSiddall et al. (2006 and references therein). To

Fig. 1.A: Geodynamic setting of the Central Andes in southern Peru. The base map is produced using bathymetric data from the Geosat and ERS-1 spacecraft (Smith and Sandwell,1997) and elevation data from NASA SRTM GTOPO30. Velocity is calculated from NUVEL-1A plate motion model ofDeMets et al. (1990).B: The San Juan de Marcona Bay (black andwhite square, mapped inFig. 2) is located in southern Peruvian forearc, above the south-eastern ank of the subducting Nazca Ridge (Machar and Ortlieb, 1992). Bathymetric and

topographic digital elevation models have been drawn with GMT software.

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assign ages to each terrace we correlated marine terraces to sea-levelhighstands (odd MIS) based on their measured exposure age and theages determined by Siddall et al. (2006). Combined with themeasured marine terrace elevations from kinematic GPS proles, weuse the tentative age assignments to calculate surface uplift rates.Uncertainties associated with uplift rates are due to uncertainties ontentative age assignments, marine terrace elevations and elevationsreached by past sea-level highstands. We also calculate fault slip rates

between the two differentially uplifted sequences of marine terraces.In this case, elevations of the eustatic uctuations are not needed aswe study two marine terraces corresponding to the same sea-levelhighstand. Uncertainties associated with faults slip rates are thus dueonly to uncertainties on marine terrace ages and measured offsets.

2. Geologic and tectonic setting

In southern Peru, Central Andes are related to the subduction ofthe Nazca Plate beneath the South America Plate with a convergencevelocity of 75 mm/yr and a convergence obliquity of N77 (DeMetset al., 1990;Fig. 1). The forearc of southern Peru is characterized bythe subduction of the Nazca Ridge, an aseismic ridgethat is more than1000 km-long, 200 km-wide, elevated 1500 m above the surroundingsea oor, and trends N42. It has been suggested that subduction ofthis buoyant topographic anomaly has led to the formation of a atsubduction segment (Gutscher et al., 2000; Gutscher, 2002; Espurtet al., 2008). Subduction of the Nazca Ridge beneath southern Perubegan in the Miocene (11.2 Ma; Hampel, 2002) and has laterallymigrated south-east along the Peruvian coast from 11S to 15S (vonHuene et al., 1996). The recent collision zone is expressed by anarrowing of the shelf, a seaward shift of the coastline (e.g., ParacasPeninsula at ~14S) and many uplifted geomorphologic featuresreecting accelerated coastal uplift (von Huene and Suess, 1988; Hsu,1992; Machar and Ortlieb, 1992; von Huene et al., 1996; Gutscheret al., 1999; Hampel, 2002; Clift et al., 2003; Wipf et al., 2008; Regardet al., 2009). North of the subducting Nazca Ridge (around Lima,~11S), the coast is currently subsiding (le Roux et al., 2000), possibly

related to the transient response following the passage of oceanicridge subduction.

The San Juan de Marcona Bay is located above the south-easternank of the subducting segment of the Nazca Ridge at ~15.4S. Thecoastal uplift recorded at this location is the maximum measuredalong the entire Peruvian coast and is directly related to thesubduction of the ridge (Machar and Ortlieb, 1992; Hampel, 2002;Fig. 1). Fauna of thehighest marineterrace of theSan Juan de Marcona

area at + 780 m have been assigned to the Plio-Pleistocene boundary,however, most of theSan Juan de Marcona terraces that aredevelopedon two hills in the bay are of Pleistocene age (Machar,1987;Macharand Ortlieb, 1992). With up to 20 levels of stair-cased marine terracespreserved below 500 m, San Juan de Marcona is one of the mostspectacular examples of marineterrace sequences in theworld. At thislocality, the bay is bound by two isolated hills that are separated fromone another by about 8 km: the Cerro El Huevo (CEH, 492 m asl) andthe Cerro Tres Hermanas (CTH, 380 m asl;Fig. 2). The two marineterrace sequences are developed on the anks of these hills (Figs. 2and 3). The Cerro El Huevo marine terrace sequence is developed onSW ank of the hill with 8 terraces spanning 200 m in elevation. Theterrace sequence at Cerro Tres Hermanas is best developed on the NWank and comprises 7 major terraces up to 175 m in elevation (Fig. 3).Extending laterally between the two hills, the +105 m (CEH) and+80 m (CTH) marine terraces are morphologically connected aroundthe San Juan de Marcona Bay (Figs. 2 and 3).

The complete morphology of each marine terrace is composed of aformer sea cliff, a planar surface (wave-cut surface) and a shorelineangle (Fig. 4). The shoreline angle is the elevation of the major changein slope between the planar terrace surface and the former sea cliff ofthe next higher marine terrace. The spatial location of the shorelineangle marks the position of the former coastline and its elevationcharacterizes each marine terrace (Lajoie, 1986). The name given toeach marine terrace here refers to the elevation of its shoreline anglemeasured with kinematic GPS (for example, shoreline angle elevationof ~+150 m marine terrace is about 153 4 m;Fig. 3andTable 1).Geomorphologic and tectonic mapping were performed from eld

Fig. 2. Map ofthe SanJuan de Marcona area. Redlinesare normalfaults striking NWSE.Red squares show thedownthrown block andred dashedline is theinferredextension of LF.LF Lomas Fault. EHF El Huevo Fault. THF Tres Hermanas Fault. SJF San Juan Fault. Blue frames indicate the two marine terraces sequences developed in the CEH and CTHanks

(seeFig. 3). Inset shows location of the study area (white square) in Peru.

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investigations and satellite images analysis. The marine terraces hereare mainly wave-cut platforms directly eroded into the bedrock(mostly intrusive rocks) with very few overlying younger depositsexcept near the shoreline angle. In this particular region, marineterracesare mostly isolated from any continental deposits post-datingtheir formation. Only a thin layer of sand (b10 cm) irregularly coversthe abraded bedrock surface. Some of the marine terraces preserve

rocky paleo-sea stacks reaching up to a few meters in height (Fig. 3).Considering that each marine terrace is related to one sea-levelhighstand, our data reects uplift rates over time spans of ~100 kyr.Even shorter time spans (~20 kyr) are represented by terracesthat formed during Marine Isotopic Sub-Stage (MISS) highstands.This corresponds to the periodicity of marine terrace formation, aban-donment during regression and preservation by coastal uplift. It is

Fig. 3.Marine terraces sequences of the San Juan de Marcona area. A Panorama of the Cerro El Huevo sequence. B Panorama of the Cerro Tres Hermanas sequence. CD Cartography of marine terraces of the Cerro El Huevo sequence (C) and of the Cerro Tres Hermanas sequence (D) from Google Earth images ( http://earth.google.fr). Copyright:Terrametrics images 2008, DigitalGlobe images 2008, Europa Technologies image 2007. Red lines are normal faults and red squares indicate the downthrown block. Black shadedareas are former sea cliffs of marine terraces. Black numbers indicate shoreline angle (cliff foot) elevations above mean sea-level. Shoreline angle elevation measurements were

performed by kinematic GPS proles. Black arrows associated to white numbers are names given to studied marine terraces. Blue stars are dated marine terraces with10Be method.Stars 1, 2, 3 and 4 correspond to +150, +190, +220 m (CEH) and +160 m (CTH) marine terraces, respectively. Black framed numbers in D are shoreline angle elevations fromOrtlieb and Machar (1990). The small asperities on the planar surface of marine terraces in A are sea-stack relicts.

Fig. 4.The complete morphology of marine terrace. MT I: The oldest marine terrace; MT II: the youngest marine terrace.

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of the marine terrace or from the smallest sea-stack are plausiblycloser to the real age of abandonment.

Samples were also collected from quartz-rich igneous clasts setinto the thin veneer of sediment deposited on the terrace. Thesesamples may contain an inherited nuclide concentration from a pre-deposition transport history. Assuming that all of the deposits weredelivered to the terrace surface shortly after the terrace wasabandoned from the sea, multiple clast samples collected from a

given terrace should have the age of the marine terrace in commonbut will have different inherited concentrations. In this case, theyoungest sample reects the sample with the least inheritance.Considering the sediment layer is very thin, it is not likely that clastshave been shielded by sediment burial.

We sampled three terrace surfaces on CEH and onesurface on CTH.At each site, we collected and analyzed 3 samples within an area of~6 m in diameter. We made sure to sample locations that wereexceptionally planar without visible relief due to surface incision.Samples were prepared at University of California Santa Cruz (UCSC)and measured at the Center for Accelerator Mass Spectrometry of theLawrence Livermore National Laboratory (details in Table 1 andcaption).

3.2. Results

As the Atacama Desert experiences extremely low erosion rates(Alpers and Brimhall, 1988; Hartley, 2003; Clarke, 2006; Kober et al.,2007; Hall et al., 2008), we present model ages calculated with zeroerosion (Table 1). Where samples from a given terrace yield a tightcluster of ages, we calculate a mean age. From the Cerro El Huevoank, we sampled the +150 m, +190 m and +220 m marineterraces (Fig. 3 and Table 1). For the +150 m and the +220 mmarine terraces, we collected the top ~37 cm from angular quartz-rich igneous clasts (~1040 cm in diameter)set into thethin veneerofsediment deposited on the terraces (mostly sand and colluvium). Forthe +190 m marine terrace, we collected samples from the top 3 cmof the abraded planar bedrock surface and from the top 45 cm of twodifferent sized sea-stacks (1 and 2 m). Sampled clast ages from the

+220 m terrace are tightly clustered at 44256 ka, 40049 ka, and42049 ka, and yield an average age of 41930 ka (Table 1).Individual sample ages from the + 150 m and + 190 m terraces arenottightly clusteredand thus present a more complicated scenario fordetermining the actual age of the surface. Forthe +190 m terrace, thetwo ages from the sea-stacks (high=200 26 ka, low=255 29 ka)are both much younger than the age of the one sample from theplanar bedrock surface (318 37 ka; Table 1). Possibly, the sea-stackshave been subjected to higher surface erosion resulting in apparently

younger ages as discussed above (cf. 3.1) as ages decrease from thehighest sea-stack to the planarsurface.We propose that age of samplefrom the planar surface, the oldest age, is the most representative ofthe real age of the marine terrace, i.e. ~318 ka. The calculated ages forthe three clasts sampled from the sediments on the +150 m terracehave a high degree of scatter: 22828 ka, 27332 ka, and 33640 ka. When 10Be exposure ages have a high degree of scatter,calculating an average age has little meaning for the actual age of the

surface. As these samples are clasts unconformably deposited on theterrace bedrock, the large scatter is likely due to diverse inheritedconcentrations. Based on our model described above, we take thesample with the lowest nuclide concentration (lowest inheritance) asthe age of the surface, i.e. ~228 ka.

At the Cerro Tres Hermanas, we sampled the +160 m marineterrace by collecting the top 4 cm of three quartz veins within theabraded bedrock (Fig. 3). Thesample ages from this surface aretightlyclustered at 35441 ka, 34640 ka, and 35942 ka, and yield anaverage age of 35224 ka. As the three samples are all from theabraded planar bedrock surface and likely have experienced similarsurface erosion, wetakethe average age asthe age of thissurface to be~352 ka.

4. Discussion

4.1. Marine terraces correlations to marine isotopic stages (MIS)

Comparing our 10Be ages to the sea-level variations on the eustaticcurve ofSiddall et al. (2006 and references therein), it appears that the+150 m terrace age of ~228 ka corresponds to the marine isotopicsubstage (MISS) 7e (232.52.5 ka;Fig. 5, Table2). The+ 190 m terracedated to ~318 ka was likely last eroded during MISS 9c (3216 ka;Fig.5) and the +220 m terrace age of~ 400 kacorrespondsto theMIS11(40510 ka;Table 2,Fig. 5). For the +160 m marine terrace dated to~352 ka, the correlation is less straightforward. It could have last beeneroded during MIS 11 (405 10 ka) or MISS 9c (321 6 ka;Fig. 5).However, based on previous studies and lateral extents of all terraces,we assign the + 160 m marine terrace to MISS 9c as discussed below.

We combine our new age assignments of each marine terrace to a MISwith those in the literature in order to assign each undated marineterrace to a MIS. To do this, we made some assumptions: 1) a given MISis necessarily recorded on both hills of the bay, the Cerro El Huevo andTres Hermanas; 2) marine terraces are all assigned to sea-levelhighstands (Lajoie, 1986); and 3) only the highest sea-level highstandsarerecorded(i.e.,5a,5c,5e,7c,7e,9a,9c,11)asmarineterracesarelargeand well-developed in the landscape. In the Table 2, we report andcompared our data to data existing in the literature where age

Fig. 5.Correlation of marine terrace ages to marine isotopic stages (MIS) from the eustatic curve ofSiddall et al. (2006 and references therein).+150, + 160, +190, +220 m arenames of the dated marine terraces of the San Juan area and their respective age. Mean ages and associated errors of +220 m and +160 m marine terraces are weighted by inversevariance (1/). Ages of +190 m and +150 m marine terraces come from one sample analyzed and associated errors are the sum of internal and external uncertainties (seeTable 1

and text for further explanations).



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assignments were made onthebasisof amino-acid racemization studies(Hsu, 1988a, 1988b; Ortlieb and Machar, 1990).Ortlieb and Machar(1990)assigned the +105 m (CEH) and +80 m terrace (CTH) to MISS5e, and the +160 m terrace(CTH) toMISS9c.As the+105 m (CEH)and+80 m (CTH) marine terraces are laterally connected around the bay,we agree withOrtlieb and Machar (1990)in assigning the +105 m(CEH) and+80 m (CTH) marineterraces to theMISS 5e (seediscussioninOrtlieb and Machar (1990, pp 100101)). The two levels (+40 mand +70 mat CEH and +40 mand+55 m atCTH) lower thanMISS 5elevel, are therefore assigned to MISS 5a and 5c, respectively. Based onour assignment of the ~150 m and 130 m terraces to MISS 7e age, weassignthe higher+ 170 m (CEH) and+ 145 m marineterraces(CTH)toMISS 9a. The +190 m (CEH) and thus the +160 m marine terraces(CTH) are likewise assigned to MISS 9c. Finally, the +220 m (CEH) and+177 m marine terraces (CTH) are assigned to MIS 11 (Table 2).

4.2. Coastal uplift: uplift rates of the San Juan de Marcona area

We calculate the mean uplift rate over a time interval tit0 (i0)between each marine terrace (i) and present sea level (0), applyingthe following equation:

UpliftRatei0 = ShAiEi = Agei 1

where ShAiis the present-day elevation of the shoreline angle of themarine terrace (time ti), Eiis the sea-level elevation at t icompared tothe present sea level, and Ageiis the MIS age of the marine terraceibased on age assignments discussed in the previous section.

We also calculate the incremental uplift rate over a time intervaltitj (ij) between two successive marine terraces as described inSaillard et al. (2009). As the error bars are extremely large, weconsider this calculation to be more of an exercise and thecorresponding results should be considered a broad estimate:

UpliftRateij= ShAiEi ShAjEj

= AgeiAgej

2

where ShAiis the present-day elevation of the shoreline angle of theolder marine terrace (time ti), Ei is the sea-level elevation at ticompared to thepresent sea-level, ShAj is thepresent-dayelevation ofthe shoreline angle of the subsequent terrace (time tj), Ejis the sea-level elevation at tjcompared to the present sea-level, Ageiis the ageof the marine terraceifrom MIS correlations (ti) and Agejis the age ofthe marine terracej from MIS correlations (tj). Our use of the ageassignments to MIS is a direct product of the fact that the

geomorphologic model of marine terrace formation considers thatthe marine terraces were formed during sea-level highstands (Lajoie,1986).

Table 2

Marine terrace assignmentsto MIS usingchronological datafrom literatureand 10Be dating. These data areusedto assignundated marineterracelevels ofthe study area toMIS. Greyand bold lines are marine terraces dated by 10Be method.

Cerro El Huevo Cerro Tres Hermanas

Marine terraces

+40 m

+55 m+70 m

+105 m

150 m

+170 m

190 m

220 m

+40 m

+130 m

+175 m

+55 m

+80 m

+145 m

160 m

Hsu (1988 a, b)

MIS MIS MISMIS age (ka) MIS age (ka) MIS age (ka)

This studyOrtlieb and

Machare (1990)

10Be age (ka)

3 60

125

200

300

400

500

60

85105

125

195

220

330

85

105

125

195

220

330

80

100

122

280

232,5

405

321

80

100

122

232,5

280

321

405

228 28

318 37 352 24

419 30

5e

7a

9

11

13

3

5a5c

5e

7a

7c

9c

5a 5a

5c

5e

7a

7c

9c

5a

5c

5e

7e

9a

9c

11

5c

5e

7e

9a

9c

11

Marine terraces

MIS MISMIS age (ka) MIS age (ka)

This studyOrtlieb and

Machare (1990)

10Be age (ka)

Table 3

Uplift rates of the San Juan de Marcona area since the Plio-Pleistocene boundary. Uplift rate 1 column: cumulative uplift rates calculated for each marine terrace, using shorelineangle elevation, tentative age assignment and the respective sea-level highstand elevation of the stage (Cf. Eq. (1)andFigs. 6 and 8). Uplift rate 2 column: incremental uplift ratescalculated between two marine terraces, using shoreline angle elevation, tentative age assignment and the respective sea-level highstand elevation of the stage (Cf. Eq. (2)).Consideringuplift ratescalculated following Eq. (2), wenote upliftratesare especially high since ~230 ka(MISS7e), although error terms arelargeand make these incremental ratesmeaningless. Grey and bold lines are marine terraces dated by 10Be method. Uplift rates of previous studies are taken from Machar and Ortlieb (1992)and references therein.

Marine teraces Shorel ine angle

elevation (m)

MIS MIS Age (ka) Sea-level highstand

elevation (m)

Uplift rate 1(m/ka) Uplift rate 2 (m/ka)10Be Age (ka)

Cerro El

Huevo

Cerro Tres

Hermanas

Previous studies

San Juan de Marcona terraces (Cerro El Huevo)

Duration (Kyr) Uplift rate (m/ka)

+40 m

+40 m

+55 m

+80 m

+130 m

+145 m

+175 m

+70 m

+105 m150 m

+170 m

190 m

220 m

160 m

153 4

170 2

103 5

72 1

42 1 80 4 -15 5 0.71 0.08 0.71 0.08

1.50 0.90

0.59 0.71

1.32 0.31

0.70 0.08 0.70 0.08

0.65 0.71

0.23 0.78

0.61 0.15

0.27 0.25

0.22 0.22

0.69 0.08

0.61 0.09

0.61 0.03

0.54 0.03

0.44 0.03

0.87 0.07

0.82 0.08

0.63 0.03

0.70 0.04 0.57 0.16

0.24 0.46

0.41 0.29

0.59 0.03

0.55 0.04

0.49 0.02 0.10 0.38

-15 5

-15 5

-15 5

+3 3

-15 5

-15 5

+3 3

100 5

122 7

80 4

100 5

122 7

258 5

41 1

54 2

77 7

131 1

145 2

177 2

191 4

161 1 352 24

223 4

228 28 -10 5

-10 5

+2.5 5.5

+2.5 5.5

0 10

0 10

232.5 2.5

321 6

405 10

285 5

232.5 2.5

321 6

405 10

318 37

419 30

5a

5c

5e7e

9a

9c

11

5a

5c

5e

7e

9a

9c

11

130 0.70

0.47

0.40500

Plio-Pleistocene

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The calculated uplift rates are reported inTable 3.Errors on upliftrates include the errors involved in the age and elevation ofhighstands and shoreline angle elevation of marine terraces(Table 3). Except for the youngest terrace of the Cerro El Huevo(+40 m, MISS 5a), cumulative uplift rates (Eq. (1)) have progres-

sively increased since ~400 ka, from ~0.550.04 to 0.870.07 m/kaand 0.440.03 to 0.700.08 m/ka for the CEH and CTH, respectively(Fig. 6;Table 3). If uplift rates have continued to increase since ~80 ka(MISS 5a), uplift rates may be considerably higher at present. The twosequences of marine terraces show the same trend of increasing upliftrates since ~400 ka although the magnitude of the uplift rates areslight different. This may be explained by the presence of the activeLomas Fault which separates the two sequences (Fig. 7).

4.3. Coastal tectonics: slip rates along the Lomas Fault system

One of the aims of this project was to determine fault activity andcalculate the slip rate along the major margin-parallel normal faultobserved in theSan Juande Marcona coastalregion(Lomas Fault,Figs.2,

6and7). To do this, we used theMIS assigned ages and the elevation of

marine terraces on both sides of the Lomas Fault. Slip rates are obtainedalong the fault considering differential uplift of the same marineterraceover a time interval since the MIS age assignment of the marine terrace.As the activity occurred sometime after the abandonment of thesurface(MIS age assignment), this slip rate is a minimum rate:

SlipRateij = ShACEHiShACTHi ShACEHjShACTHj

= AgeiAgej

3

where ShACEHi,jis the present-day elevation of the shoreline angle ofthe marine terrace (time tiand tj) on the Cerro El Huevo, ShACTHi,jisthe present-day elevation of the shoreline angle of the marine terrace(time tiand tj) on the Cerro Tres Hermanas, and Agei,jis the MIS ageassignment of the marine terracei,j.

Differential uplift of marine terraces ranges from about 20 to 46 mbetween40510 ka(MIS11)and1005 ka(MISS 5c) and isnull since80 ka (MISS 5a;Figs. 7 and 8;Table 4). The last fault activity can bededucedfrom theage of theyoungest displacedmarine terrace (100 ka)

and the age of the oldest terrace that is not displaced (80 ka). In this

Fig. 6.Marine terrace shoreline angle elevation vs. last sea-level highstand ages for Cerro El Huevo (A) and Cerro Tres Hermanas (B). Left: solid black lines correspond to the upliftrates calculated foreachmarine terrace respect to present-day andrepresent thetiming of theuplift of theAndean coastal forearc in thestudyarea. Ages andmagnitudesof sea-levelhighstands used in the calculation of uplift rates are based onSiddall et al. (2006)and references therein,Fleming et al. (1998) and Lambeck et al. (2002) .



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locality, last fault activity probably occurred sometimebetweenMISS 5cand 5a (Fig. 7), however this does not preclude the fact that it mayrupture again. In contrast to the dominance of margin-perpendicularnormalfaultsin southernmost Peru (Audinetal.,2008), margin-parallelnormal faults are well developed in the vicinity of Paracas and San Juande Marcona in southern Peru, as well as along the Chilean coastassociated with the Bolivian Orocline (Hartley and Jolley, 1995; Otaet al., 1995; Delouis et al., 1998; Heinze, 2003; Marquardt et al., 2004 ).On the Mejillones Peninsula of northern Chile, rates of horizontalextension vary from 0.025 to 0.18 mm/a (Allmendinger and Gonzlez,

2010). At both San Juan de Marcona and the Mejillones Peninsula itseemslikely that thesemargin-parallelfaultsare sometimes reactivatedduring major plate boundary thrust events (e.g., Audin et al., 2008;Quezada et al., 2010). This type of fault activity is observed, in general,for the whole Neogene time span and it constitutes a local response ofthe upper plate to stress released during megathrust earthquakes inhighly coupled subduction zones (Loveless et al., 2009).

Since thetimeof the oldest dated marineterrace (405 ka), themeanslip rate along the Lomas Fault (46 m/405 kyr) is 0.110.02 m/ka(Table 4). If we look at only the time interval between 405 10 ka (MIS11) and 1005 ka (MISS 5c), the mean slip rate is 0.092 0.003 m/ka;between 1005 ka (MISS 5c) and 804 ka (MISS 5a), the mean sliprate is 0.85 0.23 m/ka and it is null since 80 ka (MISS 5a;Table 4). Asshown on Fig. 8B, slip rates arenoticeablyhigher since 1227 ka (MISS

5e). Wecalculate a slip rate of ~0.21 0.1 m/ka based on the 122 ka ageand the26 m offset of this terrace (Table 4).In fact, thisrate istwicetherate of the long-term average slip rate. Further, if we consider thatfaulting stopped by 80 ka, the slip rates between each highstand areeven higher. As theMISS 5e and 5c terraces are differentially uplifted by~25 m(=261)and ~17 m(=181) respect to MISS 5a,respectively,thesliprateontheLomasFaultis0.60.3 m/kabetween~122 ka(MISS5e)and 80 ka (MISS5a). Between~ 122 ka (MISS5e)and ~100 ka (MISS5c) the slip rate is 0.360.13 m/ka and 0.850.23 m/ka between100 ka (MISS 5c) and 80 ka (MISS 5a;Fig. 7andTable 4).

4.4. Implications for coastal geodynamics of Peruvian margin

Based on marine terraces ages and elevations, we have shown that

uplift rates have increased progressively since ~405 10 ka (MIS 11).

These elevated rates may be related to the increasing effect of theNazca Ridge subduction (Figs. 6 and 7;Machar and Ortlieb, 1992;Hampel, 2002). Previous studies report increasing uplift rates for theSan Juan de Marcona Bay, especially since the last 130 ka, based onmarine terraces dating (Hsu, 1988a; Hsu et al., 1989; Ortlieb andMachar, 1990; Machar and Ortlieb, 1992). These previous studiesproposed mean uplift rates of 0.40 m/ka since the Plio-Pleistoceneboundary, 0.47 m/ka since 500 ka and 0.70 m/ka since the last 130 ka.Machar and Ortlieb (1992) have shown that theSan Juan de Marconaarea is located above the southern ank of the Nazca Ridge and at the

apex of the uplift induced by the subduction of the Nazca Ridge. Inorder to separate the effect of the Nazca Ridge subduction from themore general coastal forearc uplift, we compared upliftrates along theAndean margin. Mean uplift rates calculated in the San Juan deMarcona area since ~400 ka are higher than those calculated in mostplaces along the Andean margin from northern Peru to southern Chilein different tectonic settings (cf. Ortlieb and Machar, 1990; Hsu,1992; Machar and Ortlieb, 1992; Goy et al., 1992; Zazo et al., 1994;Ortlieb et al., 1995, 1996a,b,c; Pedoja, 2003; Marquardt et al., 2004;Cantalamessa and Di Celma, 2004; Pedoja et al., 2006a,b; Quezadaet al., 2007; Saillard, 2008; Melnick et al., 2009; Saillard et al., 2009;seeRegard et al., 2010for a complete review).

Andean forearc uplift seems to be related to processes occurringalong the megathrust subduction zone involving simultaneous

compression at the base of and within the Andean wedge (trench toCoastal Cordillera area) and extension at the surface. No observationsare reported on the co- and post-seismic deformation of the upperplate during earthquakes of 1942 or 1996 in Peru (Mw 8.1 and 7.7,respectively;Swenson and Beck, 1999). Perhaps these faults wouldonly be activated after several seismic cycles in order to accommodateaccumulated uplift (Briggs et al., 2006). Indeed, as for Sumatrasubduction zones,marine terraces of SanJuan de Marcona Bay and thegreater Andean coastal area are broad and mostly at, with littleevidence of tilting and internal topographic discontinuities (i.e.,secondary scarps;Fig. 8;Sieh et al., 1999). Long-term regional upliftalong these coastsis responsible for the formation and preservation ofmarine markers. This suggests that a signicant amount of thedeformation leading to permanent uplift in San Juan de Marcona Bay

occurs slowly and cumulatively during the interseismic period. The

Fig. 7.Uplift rates for the two sequences of marine terraces of the San Juan de Marcona area and slip rates calculated assuming that all differential uplift is a result of displacementalong the Lomas Fault that separates the two sequences.

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episodic brittle faulting in the upper plate in tandem with megathrustrupture on the interplate probably occurs, as discussed above, aftermultiple seismic cycles along the subduction zone (Rosenau et al.,2009). Given the pattern of the magnitudes of offset on multipleterraces (up to 40 m), offset along the fault seems to have beenproduced during several seismic events linked with subduction zone

activity. Indeed, previous authors suggest that the coastal tectonics inthe Central Andes reect the interaction of upper plate crustaltectonicswith the plate boundary seismic cycle (Audin et al., 2008).Inthe study area, an additional parameter is surely the high obliquity ofthe convergence vector (N30). Thedetachment of forearc blocks fromthe overriding plate in cases of oblique convergence along subduction

Fig. 8.A Lateral correlations of marine terraces assigned to the same MIS between the two sequences of the Cerro El Huevo and Tres Hermanas. Each color corresponds to a MIS orMISS indicated in black squares. Red framed numbers are names of marine terraces and black numbers are their respective shoreline angle elevation. Red lines are GPS pro le ofmarine terrace sequences. Black dashed lines are the simplied GPS prole before erosion and cliff foot depositional processes. B Offsets of marine terraces along the Lomas Faultwith their respective error bars vs. tentative age assignments of marine terraces (MIS). The regression curve gives a mean slip rate of about 0.1 m/ka since ~400 ka. C D: GPS trackson Cerro El Huevo (C) and Cerro Tres Hermanas (D) used in A from Google Earth images ( http://earth.google.fr). Copyright: Terrametrics images 2008, DigitalGlobe images 2008,Europa Technologies image 2007.

Table 4

Slip rates inferred from the differentially uplifted marine terrace sequences of San Juan de Marcona. Mean slip rates are in m/ka.

Marine terracesCEH

+42 m +41 m 5a

5c

5e

7e

9a

9c

11

0

4 2

0,11 0,02

0,092 0,003

0,85 0,230,21 0,10

0,36 0,13

0,60 0,313

12

5

4

5

6

1

18

26

22

25

30

46

5

7

3

5

6

10

80

100

122

233

285

321

405

+55 m

+80 m

+131 m

+145 m

+162 m

+177 m

+72 m

+105 m

+150 m

+170 m

+190 m

+220 m

Marine terracesCTH

MIS MIS Age (ka) Mean slip rate (m/ka)

since 405 ka since 122 kabetween405-100 ka

between100-80 ka

between122-100 ka

between122-80 ka

Offset (m)

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zones has been documented in other settings than the Andes (Fitch,

1972; Jarrard, 1986). McCaffrey (1996) suggestedthat about half of allmodern subduction zones should have mobile forearc blocks, amongthem the Peruvian part of the Central Andes. The extensional activestructure of the San Juan de Marcona Bay could be related to thecorresponding mobile block, however here we have only quantiedthe vertical component of the deformation.

The current locus of the Nazca Ridge crest subduction is situated110 km northwest of the study area. Due to the obliquity of theconvergence vector (Hampel, 2002; Regard et al., 2009;Fig. 9), thiszone migrates southeastward along the active plate boundary. Thecollision of the ridge causes additional local deformation of the upperplate and hundreds of meters of forearc uplift. Here we quantify theeffect of the Nazca Ridge subduction on the coastal surface uplift ratesby calculating the expected uplift rates based on the ridge and

subduction zone geometry. First, we determined the migration

velocity of the ridge crest. Considering a N42-trending ridge axis, a

N130-trending subduction trench, and the convergence vector (i.e.,75 mm/a, in a N77 direction; Nuvel-1A, DeMets et al., 1990), wecalculate a migration velocity of 43 mm/yr southeastward along thetrench (Hampel, 2002). Next, we have extracted topographic cross-sections of the ridge (Fig. 9B). The rst topographic expression of theridge on the sea oor is detected at 320 km to the south-east of theridge crest. This corresponds onshore to Atico (~16.2S73.6W), thenothernmost area where the ridge-related uplift has been observed(Regard et al., 2010;Fig. 9). At 145 km from the ridge crest, there is achange in the ridge slope angle splitting the zone into the twosegments. In the northern segment between the ridge crest and145 km to the southeast, the ridge slope angle is ~0.55, and in thesouthern segment between 145 km and 320 km away from the ridgecrest, the ridge slope angle is ~0.15 (Fig. 9). The location of San Juan

de Marcona (110 km from the ridge axis) is located near to the

Fig. 9.A Bathymetry of the Pacic Ocean offshore of the study area (cf.Smith and Sandwell, 1997, extracted from GeoMapApp software, lightest tones for greatest depths). TheNazca Ridge and trench are trending N42 and N130, respectively. Convergence velocity is 75 mm/yr in a N77direction (Nuvel-1A, DeMets et al., 1990). Migration velocity of theNazcaRidgeis 43 mm/yr(Hampel, 2002). B Topographiccrosssection acrossthe ridge andparallel to thetrench; it shows anaverage slope of~ 0.55 forthe rst145 kmand then of~0.15 upto 320 kmfromridgesummit.Note that thestudy area (San Juan de Marcona) issituatednear thetransitionbetween thelowerand higherslope. Farther than 320 kmfromtheridge(Atico),it canbe considered thereis no more ridge inuence andthe average slope is null. C Geometricrelationsbetweenthe expected coastal upliftrate(Ue) dueto ridgesubduction, the migration velocity VMof the Nazca ridge along the trench and the mean slope of the southern ank of the Nazca Ridge (S).



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position of the change in slope (145 km from the ridge axis;Fig. 9).Considering a migration velocity parallel to the trench and a meanridge slope angle, we can calculate the expected uplift, Ue, of theforearc above the Nazca Ridge with the following equation:

Ue = VMtanS 4

where VMis the migration velocity of the Nazca Ridge parallel to the

trench and S is the slope of the Nazca Ridge

ank (Fig. 9). Thisequation does not take into account the exure of the ridge when itsubducts. Considering a migration velocity parallel to the trench of43 mm/a and a mean ridge slope of ~0.55, we determined anexpected uplift rate of ~0.41 m/ka for the northern segment of theridge crest, which includes the San Juan de Marcona area. For thesouthern segment, between145 km and 320 km awayfrom the ridgeaxis, we determined an expected uplift rate of 0.11 m/ka. Hence,most of the calculated post-400 ka mean uplift rate the San Juan deMarcona area (0.55 m/ka for the CEH and 0.44 for the CTH) could beexplained by the subduction and migration of the Nazca Ridge. Wehypothesize that the difference between the calculated uplift ratesfrom marine terraces and the expected uplift due to subduction andmigration of the ridge could have the same origin as the regionaluplift observed along the Central Andes forearc to the south of thestudyarea(Regardet al.,2010). Consideringthe migration velocity ofthe Nazca Ridge along the trench (43 mm/a), the 35 km-distancebetween the San Juan de Marcona bay (110 km) and the onshorelocation of the changing slope of the Nazca Ridge (145 km), we inferthat the uplift rate has increased due to ridge subduction since about800 ka.

5. Conclusion

10Be surface exposure dating of differentially uplifted marineterrace sequences in the San Juan de Marcona Bay allows us toquantify the timing of forearc uplift in coastal Peru related to thesubduction of the aseismic Nazca Ridge. This study has shown thatvariations in local uplift patterns and uplift rates that have

progressively increased since at least the Middle Pleistocene, rangingfrom 0.44 m/ka to 0.87 m/ka. Slip rates along the Lomas margin-parallel normal fault between the two sequences of marine terracesare very slow, ~ 0.11 m/ka since 400 ka. If we consider that faultingstopped by 80 ka, differential uplift indicates a very slow rate ofdisplacement of ~ 0.09 m/ka between 400 and 100 ka, a high rate ofdisplacement of ~0.85 m/ka between 100 ka and 80 ka and noactivity for the last 80 ky. Fault activity seems to be triggered bythe release and accommodation of short-term coseismic compres-sion, whereas the regional uplift is probably controlled by post-seismic and/or interseismic strain accumulation at a longer timescale. The San Juan de Marcona area has experienced long-termregional tectonic uplift, since at least the latest Pliocene. In thisstudy,we suggest thatcoastaluplift rates have increased since about 800 ka

in relation to the subduction and southward migration of the NazcaRidge beneath the South America Plate. We calculate the expectedcoastal uplift rates of the coastal area within 145 km south of theNazca Ridge axis to be ~0.4 m/ka and diminish to 0.1 m/ka between145 and 320 km south of the axis. Hence, most of the post-400 kamean uplift rate of the San Juan de Marcona area is explained by theNazca Ridge subduction.

Acknowledgments

This research project thanks the Institut de Recherche pour leDveloppement (IRD), NSF EAR grant 0345895 and IGPP-LLNL. Specialthanks to Jos Machar and INGEMMET (Lima, Peru) for their supportin this study. We thank the three anonymous reviewers for the work

they did to considerably improve the manuscript.

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