Earth and Planetary Science Letters 337–338 (2012) 85–92

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Recently active contractile deformation in the forearc of southern Peru S.R. Hall a,n, D.L. Farber a,b, L. Audin c, R.C. Finkel d a

University of California, Santa Cruz, Earth & Planetary Sciences, 1156 High Street, Santa Cruz, CA 95064, USA Lawrence Livermore National Laboratory, Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA ´veloppement, ISTERRE, Grenoble, France Institut de Recherche pour le De d Department of Earth and Planetary Sciences, University of California, Berkeley, CA 95064, USA b c

a r t i c l e i n f o

abstract

Article history: Received 30 October 2011 Received in revised form 3 April 2012 Accepted 5 April 2012 Editor: T.M. Harrison Available online 21 June 2012

Geomorphic and structural features of southern Peru (14–181S) provide strong evidence for distributed crustal deformation along range-sub-parallel contractile structures. We use in situ produced cosmogenic radionuclides, in conjunction with field and remote mapping, to determine the ages of geomorphic features and find (1) ancient surfaces (41 Ma) preserved as a result of very low surface erosion rates, (2) young ( 30 ka) low-relief pediment surfaces developed during recent landscape modifications, (3) active tectonic structures accommodating compressional stresses, and (4) Pleistocene river incision rates of  0.3 mm/yr consistent with longer-term rates. In this region of southern Peru, the steep western wedge (trench to arc area) of the Andean margin presently maintains the high topography of the Altiplano through a combination of uplift and contractile deformation along steep east-dipping faults and isostatic responses to the focused removal of large amounts of crustal material through canyon incision. & 2012 Elsevier B.V. All rights reserved.

Keywords: cosmogenic 10Be deformation forearc incision erosion

1. Introduction While conceptually simple, models of Andean tectonics remain controversial particularly concerning the forces responsible for and the timing of range uplift. Recently, there has been renewed interest in the active tectonic and climatic processes along the Andean orogen (Allmendinger et al., 2005a; Gonza´lez et al., 2006; Kober et al., 2007; Schildgen et al., 2007, 2009; Jordan et al., 2010; Saillard et al., 2011). The canonical model for the western Andean margin suggests that low-relief surfaces within the Atacama Desert are ancient relict surfaces that were abandoned 47 Ma due to incision caused by periods of intense surface uplift (Tosdal et al., 1984) and are preserved by the hyperarid climate and an eastward migration of the active deformation belt through time. In this view, the Western Cordillera formed early on in the sequence of Andean uplift and today the western slope of the Altiplano is a passive monocline that produces no significant Neogene deformation (Isacks, 1988). However in the last decade, with the refinement of remote sensing and absolute dating methods, new data has called into question our present understanding of the rates, timings, styles, and locations of active deformation within the western Andean margin. Work in northern Chile highlights active structures in the forearc leading to refinement and enhanced detail of Isacks’ (1988) ¨ model of the western margin. For example, Worner et al. (2002) n Corresponding author. Present address: McGill University, Earth & Planetary Sciences, 3450 University Street, FDA 238, Montreal, Que., Canada H3A 2A7. Tel.: þ1 11 514 398 2722; fax: þ1 11 514 398 4680. E-mail address: [email protected] (S.R. Hall).

0012-821X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.epsl.2012.04.007

related contractile and extensional structures in the Precordillera to giant gravitational block rotation and oversteepening of the western margin, which formed in response to the monoclinal warping ˜oz and Charrier suggested by Isacks (1988). Alternatively, Mun (1996), Victor et al. (2004), and Farias et al. (2005) suggest a westvergent thrust system active between  30 and 6 Ma. Still other studies document more recent (Plio-Pleistocene) deformation associated with fault zones within the Coastal Cordillera and Precordillera (Armijo and Thiele, 1990; Allmendinger et al., 2005a; Gonza´lez et al., 2006; Audin et al., 2008) resulting in multiple models of forearc uplift through, at least in part, crustal deformation along steeply dipping faults. Others have stressed the importance of regional post-10 Ma forearc incision in response to regional uplift related to monocline growth (Schildgen et al., 2007, 2009; Hoke et al., 2007; Jordan et al., 2010) and, in the absence of evidence for abundant horizontal shortening in the western margin (GregoryWodzicki, 2000), have called on additional mechanisms for uplift that include subduction erosion (Hartley et al., 2000), lower-crustal ductile flow (Husson and Sempere, 2003) or lithospheric delamination (Garzione et al., 2006). One of the strongest arguments for an active forearc during the last  10 My comes from Allmendinger et al. (2005b) who show that the modern-day magnitude and direction of forearc rotations observed in GPS data are the same as the Miocene average of the time integrated vertical axis rotations observed in paleomagnetic data. While other studies of paleomagnetic rotations in the forearc region do not see significant post  15 Ma rotations (Roperch et al., 2006), some of this may be the lack of suitable units to measure as most of the forearc basin is filled with Miocene-age and older conglomerate and volcanic rocks. Taken

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3. Methodology

Trujillo

Forearc BRAZIL

Average Elevation >3 Km

Huarmey

Locations of Study (Figure 2)

10°S

Lima

PERU

Lake Titicaca

15°S

Ri dg

e

Chala

a

Locumba

Na

zc

Ilo

Pacific Ocean

BOLIVIA

Tacna Arica CHILE

80°W

75°W

20°S

70°W

Fig. 1. Location map of study are in southern Peru. The inset box corresponds to Fig. 2.

together, these recent studies yield an emerging view of a western margin that is more active than previously recognized. In order to refine existing models, we must understand how significant and how consistently the deformation can be observed along the strike of the margin. However, this remains largely unknown. Here, we use regional mapping and geochronologic data from neomorphic features to investigate the style and magnitude of neotectonic activity along the western margin of the Altiplano in southern Peru (Fig. 1).

2. Forearc geomorphology Since at least 3 Ma, the coastal Atacama Desert has been situated in a zone of hyperaridity that has resulted in a high degree of geomorphic surface preservation (Hartley, 2003). The low-relief surfaces of the Atacama are primarily developed on alluvial fan conglomerates of the Late Oligocene–Miocene Moquegua Fm. (and correlative units), which reach thicknesses of hundreds of meters. These surfaces have been abandoned through river incision in response to changes in regional or local base-level (Mortimer, 1973; Tosdal et al., 1984; Sebrier et al., 1988) plausibly in conjunction with episodic higher precipitation events. They have been preserved as result of very low surface erosion rates that are characteristic in this region. Erosion rates on these surfaces derived from in situ produced cosmogenic isotope concentrations, are consistently less than 0.5 m/ Ma (Kober et al., 2007; Nishiizumi et al., 2005; Hall et al., 2008) and are broadly similar over 111 of latitude. Offset and abandoned geomorphic features such as the Pleistocene-aged pediment surfaces and fluvial terraces that exist along rivers and structures provide useful markers of base level change that can be exploited to determine the rates of river incision and thus local-regional uplift during the Pleistocene. While on short timescales (101–104 yrs), changes in precipitation may enhance or dampen incision rates and result in complex sediment storage throughout a river system, the magnitude of long-term potential base-level change is fixed either by surface uplift or sea level change. We note that incision rates along a river reflect both any regional uplift signal as well as local rock uplift along discrete structures. By calculating incision rates in various locations, we attempt to identify and quantify localized uplift associated with specific faults as well as any background regional signal.

Using Landsat and Advanced Spaceborne Thermal Emission and Reflection. (ASTER) satellite imagery and aerial photographs, we identified sites of potentially active deformation by mapping locations of abrupt changes in topography or incision and by conducting field mapping of abandoned low-relief surfaces as well as structural features. The extents and elevations of geomorphic features were measured using a theodolite and handheld GPS. For surface exposure dating using in situ produced cosmogenic 10 Be, we collected both surface and depth-profile samples from wellpreserved pediments and fluvial terraces. To avoid possibly disturbed areas, we took care to collect samples away from any significant drainages or cliffs. At each site (listed in Table 1 and locations shown in Fig. 2), we collected at least 4 quartz-rich surface samples from within a 5 m  5 m area or at least 6 samples at known depths in trenches dug into the terrace or pediment surface. The surface samples are generally cobbles ( 7 cm  7 cm  o5 cm) where 5 cm is the thickness of the sample; we did not collect samples with a thickness of more than 5 cm. We collected composite sand and pebble samples from trenches at a uniform elevation below the terrace surface from a layer that had a thickness (depth below surface) of no more than 5 cm. We performed the quartz purification and the separation of beryllium according to the methodology of Kohl and Nishiizumi (1992). The 10Be/9Be ratio measurements were made at the Center for Accelerator Mass Spectrometry (CAMS) of the

Table 1 Ages, offsets, and incision rates for sites in Fig. 2. Site

Terrace

Height (m)a

Surface exposure age (ka)

Incision rate (mm/yr)

52 71 59 71 78 71

1267 21.8 1497 17.8 3497 23.8

0.417 0.07 0.40 7 0.05 0.227 0.02

Pampa Gallinazos 3 A 4 A 4 B 4 C

 50 7 10 37 0.5 107 0.5 207 0.5

150 7 14.1y 35.9 7 9.0 1837 17.3y 4827 66.1

0.337 0.07 0.08 7 0.03 0.05 7 0.01 0.04 7 0.01

Rio Sama 5 5 5 5 6 6 6 7 7 7 8 8 9 10 11

12 72 22 72 32 72 41 72 39 72 47 72 58 72 107 2 29 72 33 72 24 72 46 72 37 0.5 14 7 0.5 307 5

N.D.y N.D.y 201 7 32z N.D.y 66.1 7 6.0 144 717z N.D.y 177 723z 216 728z N.D.y 45.07 21.0 60.5 7 5.7y 24.2 72.6y 1018 7 381z 179 7 17.0y

N.D.y N.D.y 0.167 0.04 N.D.y 0.597 0.06 0.337 0.04 N.D.y 0.06 7 0.01 0.137 0.02 N.D.y 0.53 7 0.25 0.767 0.07 0.127 0.02 0.01 7 0.01 0.177 0.03

Rio Locumba 1 A 2 A 2 B

A B C D A B C A B C A B A A A

Note: Surface samples calculated with an erosion rate of 2E 5 cm/yr and depth profiles with a maximum erosion bound of 0.5 m/Ma. Both are calculated using a time-varying production rate (Lal, 1991/Stone, 2000). Where no symbol follows age, the error is the standard deviation from the mean age. Incision rate errors include the error on the age and error on distance above channel measurement. a

Distance between the basal contact of river sediments and the active channel. Error includes only the analytical error. z Depth profile sample: age is the lowest w2 value and the error reflects the largest possible deviation (maximum or minimum). y N.D.¼not determined. y

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87

˜ as Fault (AF), Fig. 2. (A) ASTER image of the study location and field sites discussed in the text. Red lines show location of structures, named ones include: Agradon Gallinazos Fault (GF), and Calientes Fault (CF). This is a false color image where the red areas in the river valleys reflect vegetation. A partial geologic map is shown along the main Rio Sama based on maps prepared by the Instituto Geolo!gico Minero y Metalu!rgico (INGEMMET, 1998). Units include the Cretaceous Toquepala Fm. metavolcanic and metasedimentary rocks (Kt), unconformably overlain by the Miocene Moquegua Fm. (undivided, although in this location it is mainly the upper unit—Moquegua C) portion of the megafan conglomerates, which are conformably overlain by the Miocene Huaylillas Fm. ignimbrites. The focal mechanism corresponds to an earthquake of 17 km depth, 5.6 MW. The cross-section line (A–A0 ) corresponds to Fig. 3. (B) Shuttle Radar TM (SRTM) Digital Elevation Model hillshade image showing major landslide scarps and deposits (maroon line), abrupt changes in incision (dashed white lines), drainages showing: headless channels (orange), deflected channels and drainages of the PCdV catchment being captured by larger rivers (yellow), major drainages capturing PCdV drainage in uplifted areas (blue), and locations of surveyed strath terraces (purple). (C) Strath terrace along the Rio Locumba near site 1. (D) Strath terrace along the Rio Sama near site 8. The yellow dashed line on C and D shows the basal unconformity with river deposits above and the Moquegua Fm (C) and Toquepala Fm. (D) below. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Lawrence Livermore National Laboratory and normalized to LLNL standards. Using a sea level high-latitude 10Be production rate of 4.84 70.41 10Be atoms g  1 yr  1 as calculated by Balco et al. (2008), we scaled this production rate for latitude and altitude according to the time-dependent Lal (1991)/Stone (2000) scaling scheme and for shielding and sample thickness as described by Balco et al. (2008). Along the coast of Peru an atmospheric inversion perturbs the air density altitude function and thus, the production rate in this region. We make a correction to the standard atmosphere according to Farber et al. (2005) for the sites in this study as they are at elevations within the inversion layer

(  800–2500 m; Houston and Hartley, 2003; Johnson, 1976). We used the CRONUS web-based calculator (version 2.2) to determine ages (sample parameters listed in Table A1). Age calculations based on other scaling schemes are provided in Tables A2– A5. Depth profile nuclide concentrations were used to calculate the surface age according to the parameters outlined in Table A6 following the method of Hidy and Gosse (2010) using an attenuation length of 177 g/cm2 as measured by Farber et al., 2008. Except where noted, the ages presented in the text and in Table 1 are average ages with errors reflecting the standard deviation; they have been calculated with an erosion rate

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S.R. Hall et al. / Earth and Planetary Science Letters 337–338 (2012) 85–92

(0.2 m/Ma) as limited by depth profile analysis in this study (yields a rate of 0.2–0.3 m/Ma) and by a 10Be concentration from a genetically similar forearc surface (Hall et al., 2010; yields a rate of 0.1 m/Ma). Additional information concerning the 10Be methodology, sample parameters, and zero-erosion ages are provided in the Supplemental Material.

4. Results 4.1. Erosion rates At each of the 5 trench locations we collected 6 samples of sediment down to a maximum depth of  1.5–2.0 m. We collected 750–1000 g of sand-coarse gravel and small pebbles at each depth. As the dominant lithology is rhyolite–andesite, many trench samples did not yield enough recoverable quartz for analysis. From 2 of the trenches, SA06-T4 and SA06-T5 at site 7, we were able to measure a 10Be concentration for 5 samples. For the remaining 3 trenches, SA06-T1, SA06-T3, and SA06-TP, only 4 samples contained a sufficient amount of quartz. We use the Hidy and Gosse (2010) Monte Carlo simulator to model the profiles and identify the most likely values for the surface exposure age, the erosion rate, and the inherited nuclide concentrations. Trenches T4 and T5 also returned the best fits with the most number of useable data points. For these 2 trenches we were able to place an upper bound on the surface erosion rate as well as the age of the surface. For both samples we calculate a maximum surface erosion rate of 0.5 m/Ma and an average of 0.3 m/Ma70.2 m/Ma. As the degree of surface preservation and the composition of the material comprising each strath is the same, we then fit the nuclide concentrations for the remaining 3 trenches (each with only 3 useable data points) based on this erosion rate bound (maximum of 0.5 m/Ma). Finally, the depth profile, SA06-TP, was treated separately as it is sampled from the large pediment surface south of Sama, not from a fluvial strath terrace. This profile is much older than the other terraces ( 10177 380 ka) and yields a maximum erosion rate of 0.2 m/Ma70.1 m/Ma. Ages from these depth profiles are discussed in Section 4.2 according to site location. 4.2. Surface exposure ages The Rio Locumba and the Rio Sama are 2 major exoreic rivers in southernmost Peru bounding the elevated low-relief region of Pampa Cabeza de Vaca (PCdV; Fig. 2). Strath terraces preserved along both of these rivers reflect river incision. At each specific site location we refer to the terraces from the lowest (closest to the river and youngest) as ‘‘A’’ and each subsequently higher terrace as ‘‘B’’, ‘‘C’’, etc. The ‘‘A’’ terrace in 1 location does not necessarily correspond in time or space with the ‘‘A’’ terrace in another location. Multiple levels of strath terraces cut on the Moquegua Fm. conglomerates span the entire length of the Rio Locumba. At site 1, 1 prominent terrace is preserved (the 52-m ‘‘A’’ terrace), where 10Be concentrations yield an average age of 126721.8 ka based on 5 surface samples. At site 2, located  3.8 km downstream of site 1, 2 terraces are preserved (the 59-m ‘‘A’’ terrace and the 78-m ‘‘B’’ terrace; Fig. 2C). The lower ‘‘A’’ terrace comprises 15 m of river deposits overlaying the bedrock notch. Three samples from this terrace surface yield an average surface exposure age of 1497 17.8 ka. The upper ‘‘B’’ terrace has 10 m of river deposits overlain on the eroded bedrock notch and yields an average surface exposure age based on 3 surface samples of 349723.8 ka. In the Pampa Cabeza de Vaca (PCdV) region, between the Rio Sama and Rio Locumba drainages, monoclines trending east–west correlate with breaks in topography on pediment surfaces (Fig. 2A). The Rio Sama (to the east) and the Rio Locumba (to the west) have

captured many of the stream channels originating in the northern portion of the PCdV (Fig. 2B). An incised low-relief surface at site 3, now stranded  50 m above the active channel, yields a surface abandonment age of 150714.1 ka. Hall et al. (2008) mapped a monocline in the northern part of the PCdV and interpreted this fold to be related to a blind reverse fault at depth (site 4). 10Be ages from the terraces abandoned on the up-thrown block of the reverse fault, which we refer to as the Gallinazos Fault, were presented by Hall et al. (2008), but have been recalculated here with an erosion rate of 0.2 m/Ma as C¼482766.1 ka, B¼183717.3 ka and A¼35.97 9.0 ka (Balco et al., 2008). In the southern portion of the PCdV, a southwest plunging anticline has offset surfaces, diverted streams, and is currently being incised by the Rio Sama (Fig. 2B). In the central portion of ˜ as the Rio Sama, a steeply dipping (87–901) fault (the Agradon Fault) produces south-side-up vertical motion and locally forms the contact between the Jurassic–Cretaceous Toquepala Fm. and the Miocene Moquegua Fm. (Fig. 2A). Where the strath surface is carved on the meta-volcanic and metasedimentary rocks of the Toquepala Fm., the river deposits of the strath terraces are clearly visible (usually 3–5 m thick; Fig. 2D). At other locations along the river, an incised low-relief surface carved into the Moquegua Fm. is overlain by fan and/or river deposits (site 11). In most locations there are 2 clear sets of strath terraces along the river, however at other locations, up to 4 terraces are preserved (site 5). Three strath terrace sites (sites 5–7) are located near the Gallinazos Fault, with sites 5 and 6 on the uplifted side while site 7 is located on the down-dropped side. Site 5, the northernmost site along the river, has 4 prominent terraces at 12 m, 22 m, 32 m, and 41 m (A, B, C, and D respectively) above the channel. The surface exposure age based on a depth profile of terrace C yields an age of 201732 ka. At site 6, which is nearest to and on the uplifted side of the Gallinazos Fault, 3 terraces are suspended above the active channel at 39 m, 47 m, and 58 m (terraces A, B, and C, respectively). Terrace A yields a 10Be surface exposure age based on surface samples of 66.1 76.0 ka and the B terrace yields a depth profile surface exposure age of 144717 ka. Just south of the Gallinazos Fault at site 7, 3 terraces have heights of 10 m, 29 m, and 33 m above the active channel. The A and B terraces at site 7 yield exposure ages based on depth profiles of 177723 ka ˜ as Fault and 216 728 ka respectively. Just north of the Agradon (site 11), one prominent strath terrace (A) at 23 m above the active channel is carved on Moquegua Fm. bedrock Based on one surface sample, this terrace yields a surface exposure age of 179 þ/  17.0 ka. At the southern end of the river, site 8 is located on the uplifted block of the Calientes Fault. Two strath terraces are preserved at 24 m and 46 m above the active channel in this location. The A terrace at this site yields an average exposure age based on surface samples of 45.0721.0 ka and the B terrace yields an age based on a surface sample of 60.575.7 ka. Along a small tributary valley carved through the folded Moquegua Fm., a recent strath terrace exists at 3 m above the current channel and yields a surface exposure age based on a surface sample of 24.272.6 ka (site 9). Finally, the southernmost site along the Rio Sama, site 10, is located on the edge of the broad pediment extending south of the Calientes Fault and suspended 14 m above the active channel. A depth profile from this surface yields an exposure age of 10187331 ka.

5. Discussion 5.1. Evidence for contractile deformation The presence and position of a regional system of steeply dipping reverse faults located at the southwestern edge of the

S.R. Hall et al. / Earth and Planetary Science Letters 337–338 (2012) 85–92

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ion ss e r p ult lera ill De a Fa ordil al r ord o n e i i l C l C qu ud ordi tal t pu stern git c as as a n e o c o C Lo C In We NE Pr 0 era

ch

n Tre

SW 0 20

20

40

40

60 120 Alluvium, QAL Huaylillas Fm, TH Moquegua Fm, TM Toquepala Fm, KT and underlying bedrock ~Site Location #

80

0

40

120

80

40

60

Distance from coast (km)

A 2.0 1.0 0 -1.0

8

0

9

5

11

10

3

15

7

6

20

4

25

A’

30

Fig. 3. (A) Model western margin cross-section showing inferred offsets in basement rocks related to a thrust system in the Precordillera and Western Cordillera regions (modified after Victor et al., 2004). This cross-section is meant to illustrate an idea of how the surface deformation identified through geomorphic mapping plausibly reflects buried active structures at depth. (B) Local schematic cross-section through the Sama region, A–A0 as shown in Fig. 2A. Units include: Quaternary Alluvium (QAL), Miocene Huaylillas Fm. (TH; ignimbrites and some conglomerates), Miocene Moquegua Fm. Undivided (TM; conglomerates with some interbedded ignimbrites), and the Cretaceous Toquepala Fm. (KT; metaigneous and metasedimentary rocks). The approximate site positions are indicated with numbers and correspond to the site numbers in the text and in Fig. 2.

Precordillera (Fig. 3) is supported by a range of geophysical and geological observations (1) offset youthful geomorphic features on the southern margin of the Precordillera, (2) seismicity data reflecting contractional focal mechanism solutions, and (3) GPS data suggesting contractional strain axes of  0501. Multiple flexures trending sub-parallel to the coast and Western Cordillera dissect the 3 major drainages of southernmost Peru. In many cases, these flexures correspond to abrupt changes in river incision and topography (Fig. 2B). Southeast of Sama, north of the city of Tacna, the Calientes Fault breaks the surface and can be observed in outcrop (Audin et al., 2006). Near the Rio Sama, the fault does not break the surface but is manifested by flexures, the largest of which is produced by a propagating hanging-wall anticline above the blind Calientes reverse fault (Fig. 2A). The youthfulness of this feature is suggested by the deflection of active channels around the propagating tip of the anticline, by young (  24–560 ka) surface exposure ages on strath terraces suspended above those active channels, and by deformed Pliocene–Pleistocene deposits on the PCdV. Seismicity data supports the notion of the Calientes Fault as an active structure. Tavera et al. (2007) present an analysis of a recent seismic event (5.4 MW, 17 km depth, November 20, 2006) in the Sama area. Based on the epicenter location, the fault plane solutions and the fault geometry, they conclude that the event occurred on a fault dipping either at  751S or 151N in the Calientes Fault system. We agree with Tavera et al. (2007) that the most plausible interpretation is that the fault associated with this event is a low-angle northward-dipping ramp of the Calientes Fault system that becomes much more steeply dipping near the surface. The multiple contractile structures in the Locumba-Sama region trend  300–3301 (NW–SE), with principal stress axes trending 030–0601 (NE–SW). While this is sub-parallel to the convergence direction ( 0701; Norabuena et al., 1998), it is roughly the same direction as the maximum principal contraction rate axes ( 046–0501; Allmendinger et al., 2007; Dewey and Lamb, 1992). Further, this orientation is consistent with the coseismic principal stress axes ( 0451) for the coastal region near 17–181S as calculated by Loveless et al. (2009) based on the

orientation of coseismic surface cracking. This suggests that both the longer-term contractile structures of the Precordillera and the short-term surface cracks of the coastal region reflect a similar regional stress field. We note that Loveless et al. (2009) suggest a coseismic stress field for the Precordillera that is oriented  345– 3601, inconsistent with our newly mapped structures, however this coseismic stress would be quite low in magnitude and would therefore not contribute to the growth of any of the structures studied here. Similar contractile structures have been mapped in northern Chile; however, they are not widely recognized to be recently ˜ oz and Charrier (1996) and Farias active. Victor et al. (2004), Mun et al. (2005) all describe a west-vergent thrust system where activity migrated westward during  30–6 Ma. Zeilinger et al. (2005) documents the growth of the Oxaya anticline between 7.5 and 2.7 Ma, a structure similar in style to the Sama anticline, along the Precordillera of northern Chile. This structure is also inferred to be the product of a blind fault at depth related to parallel buckling of the western margin in response to regional compression; the margin is buckling (shortening)  parallel to the ¨ coast/trench by a series of contractile structures. Worner et al. (2002) relate recently active shallowly-rooted contractile structures in the western Precordillera to the toes of massive rotated landslide blocks due to oversteepening of the western margin through monoclinal warping during the late Miocene ( 12– 10 Ma). These authors do note the recent activity along westvergent reverse faults since 2.7 Ma, however they conclude that these faults are not significantly contributing to uplift of the Western Cordillera and that most of the uplift is associated with lower crustal ductile flow from the east consistent with the Isacks (1988) model. Deep-seated Quaternary landslides are common in the Sama region as well with similar oversteepening enhanced by very low precipitation, and seismic activity along Precordilleran faults (Audin and Bechir, 2006). While these landslides are beyond the scope of this paper, a large landslide in this region can be observed on satellite imagery in the area just southeast of the town of Sama (Fig. 2B). Jordan et al. (2010) compile and describe the

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structural and temporal constraints on trench-parallel monocline formation in the Precordillera region of northern Chile. They document surface uplift related to large-scale active faults since  11 Ma with elevated uplift rates  11–6 Ma and continued uplift through the Quaternary which they attribute to long-wavelength monoclinal warping following Isacks (1988). We note that the monoclinal warping described by Jordan et al. (2010) is different than the shorter-wavelength style described here and in Victor et al. (2004). Similarly oriented structures and fractures penetrating Cretaceous–Miocene bedrock, Plio-Pleistocene pediment surfaces, and modern river channels are regionally widespread and plausibly reflect both active and ancient deformation. We suggest that the style and rate of deformation in this region has been similar and ongoing at least on a timescale of  1 Ma. 5.2. Incision rates From the measured ages and offsets of the strath terraces we calculate river incision rates (Leland et al., 1998) with the assumptions that (1) the timescale of strath abandonment is relatively short compared to the age of the surface and (2) the surfaces have not undergone significant post-depositional modification (such as removal or addition of material through mass-wasting events, or other such major events). However we do account for very low rates of surface erosion in our calculations (0.2 m/Ma). This erosion rate is similar to multiple cosmogenic measurements made on similar geomorphic surfaces (Nishiizumi et al., 2005; Kober et al., 2007; Hall et al., 2008, 2010) ranging from  0.1 to 0.7 m/Ma. As the surface erosion rate is incredibly low in this region, and the surfaces we are dealing with are quite young (100 s kyr), whether we choose an erosion rate of 0.1 or 0.7 m/Ma makes very little difference on the calculated age and thus incision rate. Additional information and discussion on this topic can be found in the Supplemental Material. Incision rates along the northern segment of the Rio Sama range from  0.1–0.8 mm/yr with the highest rates occurring closest to and on the uplifted side of the Gallinazos Fault (Table 1). Just upstream of the main Calientes Fault (site 8), the age and offset of the A and B terraces yield incision rates of 0.537 0.25 mm/yr and 0.7670.07mm/yr respectively. However, we note that these terrace ages are based on just 3 samples, 2 from the A terrace and 1 from the B terrace due to poor surface preservation, difficulty of access to the site, and low quartz yields. The 2 samples from the A terrace have ages of 59.9 and 30.2 ka while the 1 sample from the B terrace has an age of 60.5 ka. If we assume that the B terrace is 61 ka and the A terrace is 30 ka (with the 59.9 ka sample being contamination from the higher surface), we calculate an incision rate of 0.8 mm/yr from both terraces. Regardless of the exact rate, the youthfulness of these terraces reflects the recent uplift associated with discrete faults in the Precordillera. The one dated ˜as strath terrace from the Moquegua Fm. (just north of the Agradon Fault), yields an incision rate of 0.17 þ / 0.03 mm/yr. This rate is similar to the background incision rate we measure from strath terraces cut on the Moquegua Fm. along the Rio Locumba as described below. The extensive  1 Ma pediment surface located just south of the Calientes Fault near the town of Sama is stranded just 14 m above the current river. This corresponds to a very low river incision rate of 0.0170.01 mm/yr reflecting subsidence in the footwall of this fault. Along the Rio Locumba, incision rates range from 0.2 to 0.4 mm/yr (Hall et al., 2008). While we cannot separate incision related to a local or regional base-level change without a direct correlation to a distinct structure, we note that many monoclines cross this river valley, some of which are plausibly related to blind faults at depth. In particular, on the topographically and stratigraphically higher PCdV, folded Miocene–Pliocene deposits typically correspond to locations of abrupt changes in topography or

incision (Fig. 2; Tavera et al., 2007; Hall et al., 2008). All the surface exposure ages from surfaces abandoned near-to these locations yield ages o600 ka. Some of these structures are plausibly related to recent or renewed deformation, which we interpret to be contractile in nature. Specifically, sites 2 and 3 are both situated along a clear break in topography and near to folds visible along the Rio Locumba. The age of the ‘‘B’’ terrace at site 2 and the abandonment age of the incised pediment surface at site 3 are the same:  150 ka—potentially related to a localized uplift event. Given the steep dips of the structures, it is possible that these monoclines are the surface expression of normal faults at depth. However, given that the major regional surface-breaking faults in the Precordillera are reverse faults, we think a more plausible explanation is that these are contractile structures. Similar terrace abandonment ages may also reflect the onset of a period of enhanced incision due to changes in stream discharge or sediment flux related to a climatic event. Along the Rio Sama, we see a similar surface abandonment age from the C terrace at site 5 and the B terrace at site 7 of  210 ka and a similar age from the A terrace at site 7 and the A terrace at site 11 of  178 ka. While the timing of incision may be related to a climate event, the ultimate amount of incision is related to changes in regional base level. The range of measured Pleistocene incision rates spans 0.1–0.8 mm/yr with the majority of rates clustering at  0.2–0.4 mm/yr. Previous studies based on (U–Th)/He and Ar–Ar thermochronology have documented periods of intense canyon incision (0.2–0.5 mm/ yr) along major rivers in the forearc of southern Peru ( 161S) beginning  13–8 Ma and ending 2.2–5 Ma (Schildgen et al., 2007, 2009, 2010; Thouret et al., 2007). The Pleistocene incision rates we determine here are similar to those determined for the Late Miocene–Early Pliocene. Considering the ages and offsets of pediments and marine terraces in this region, Tosdal et al. (1984) suggest uplift rates of 0.06–0.1 mm/yr for the Coastal Cordillera and 0.19 mm/yr for the Precordillera since  18 Ma. More recent datasets with tighter temporal control suggest similar rates of uplift during the Pleistocene. Saillard (2008) calculated surface uplift in the coastal region near Ilo (17.61S) of 0.270.04 mm/yr based on the 10Be chronology of marine terraces. Regard et al., (2010) use the Saillard (2008) dataset, along with others, to suggest ‘‘renewed’’ Quaternary uplift of the forearc following a tectonically quiescent Pliocene. 5.3. Forearc deformation and the Andean orogen Our new observations of active surface-breaking reverse faults bounding the Precordillera of southernmost Peru shows that recent uplift is produced by contractile structures in this area. Together with the hyperarid climate, continued uplift along steep contractile structures results in oversteepening of the Precordillera–Cordillera region. Removal of material in response to ongoing uplift occurs through mass-wasting events and river incision focused in the ¨ Precordillera and Cordillera region (Worner et al., 2002). While active structures in this region have largely gone unrecognized, previous authors have mapped a steep west-vergent thrust system along the ˜oz and western Chilean margin active between30 and 6 Ma (Mun Charrier, 1996; Victor et al., 2004; Farias et al., 2005). As the regional state of stress has not changed significantly since 6 Ma (James, 1971), it is possible that the recently active contractile structures mapped in this current study are related to a similar regional system of active thrusts at depth. Armijo et al. (2010) suggest that the presence of an active thrust system within the western margin (Precordillera) of central Chile is evidence for a doubly-vergent orogen. As we document similar structures in southern Peru, we generally agree with Armijo et al. (2010), however, this dataset does not confirm or deny the existence of a true doubly vergent orogen. As this region is the locus of maximum shortening (Isacks, 1988) and a region that is undergoing slightly oblique subduction

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with dominantly sinistral motion along crustal faults oriented sub-parallel to the margin north of the Arica Deflection (Jacay et al., 2002; David et al., 2004), it is possible that transpression results in active contractile structures producing uplift and a small amount of crustal thickening of the western margin. While these structures are quite steeply dipping and accompany little horizontal shortening, they are likely important for uplift and therefore to the formation and maintenance of the western wedge (trench to arc area) morphology. Jacay et al. (2002) have previously suggested that various positive flower structures exist along the Incapuquio Fault System of southern Peru as result of the transpressional regime. While these authors do not suggest any of the faults discussed here have been active in the Quaternary, they do note that other parts of the system are indeed active. Schildgen et al. (2007) suggest that canyon incision at  161S is consistent with either the Isacks (1988) monocline hypothesis (due to lower crustal ductile flow) or distributed forearc deformation along multiple non-surface breaking, or unmapped faults. Schildgen et al. (2009) note that this canyon incision is not linked to deformation along faults active in the Quaternary. In contrast, while our observations alone cannot rule out a role for lowercrustal ductile flow, they do suggest contractile deformation is significant and contributes to uplift in this region. Given that active structures are not observed north of  17–201S, the Arica Deflection region, it is plausible that these structures are related to a localized phenomenon. In fact, somewhat similar to the Jordan et al. (2010) study, our data could be consistent with a modified version of the Isacks (1988) hypothesis that the western margin is generally a continental-scale monocline. In this view, steep surface breaking reverse faults produce some uplift ( 0.5–1 mm/yr) in the Arica Bend region as the western margin is warped in response to farther-field uplift to the east (Altiplano region). To reconcile observations that both upper crustal structures and lower-crustal ductile processes are important in forearc deformation, Tassara (2005) propose a model involving a pseudo-indenting forearc based on an analysis of elastic plate thickness (Te). In this model, upper-crustal contractile structures in the Central Andean Precordillera and Western Cordillera allow for the westward movement of the upper crust at low and consistent deformation rates along a west-vergent thrust system while simultaneously allowing for lower crustal accumulation of material through ductile mechanisms. Finally, we note that while active contractile structures in the Precordillera do produce a significant amount of uplift, we do not suggest that all of the ongoing and past uplift of the western margin is produced by discrete faults. There are plenty of observations that suggest regional uplift is important (while harder to quantify) as well: (1) strath terraces exist along the length of most of the major rivers, many of which are plausibly related to marine terraces near the coast, (2) regional low-relief surfaces have been uplifted (rather uniformly), incised, and abandoned throughout the Precordillera and longitudinal basin (Tosdal et al., 1984), and (3) incision rates along major rivers are similar in magnitude along the channels (0.2–0.4 mm/yr) suggesting uniform background uplift rates (Hall et al., 2008). Indeed, the growing body of evidence for active and/or contractile deformation within the western Andean margin suggests that regional far field forces are driving the deformation and these observations inspire refinements to our present models of Andean orogenesis.

6. Conclusions The geomorphic and structural features of southern Peru are evidence of distributed active crustal deformation along range-sub-

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parallel contractile and strike-slip structures. The observation that Pleistocene incision rates are comparable with Late Miocene and Early Pliocene rates supports the notion that the rates and style of surface uplift within the forearc of southern Peru has been roughly consistent since at least 9 Ma. We suggest, that in this region the steep western side of the Andean margin supports the high topography of the Altiplano through a combination of uplift along steeply dipping contractile structures and isostatic responses to the focused removal of large amounts of crustal material in the massive canyons of the Precordillera and Western Cordillera through masswasting events and valley incision. Finally, given the limited number of field sites in the Andean Precordillera that have been studied in detail, there is a high likelihood that the style of tectonism we document here may be more ubiquitous than previously thought throughout the western Central Andes.

Acknowledgments Support for this work came from NSF Grant 03455895, the IRD, and IGPP-LLNL. Many thanks to J. Berrospi, M. Saillard, and M. Davis for dedicated field support and P. Greene and K. Hodson for laboratory support.

Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.epsl.2012.04.007.

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