Allmendinger, R.W., González, G

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ARTICLE IN PRESS TECTO-124586; No of Pages 18 Tectonophysics xxx (2009) xxx–xxx

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Tectonophysics

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Neogene to Quaternary tectonics of the coastal Cordillera, northern Chile

Richard W. Allmendinger a,⁎, Gabriel González b a Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, NY, USA b Departamento de Ciencias Geológicas, Universidad Católica del Norte, Antofagasta, Chile article info abstract

Article history: The extreme aridity of the northern Chilean Coastal Cordillera enables the complete preservation of Received 10 July 2008 permanent deformation related to the coupling between Nazca and South America. The region between Received in revised form 6 February 2009 Antofagasta and Arica is characterized by four types of Late Cenozoic structures: EW-striking reverse faults; Accepted 19 April 2009 ~NS-striking normal faults; sparse WNW and NNW-striking right-lateral strike–slip faults; and unimodal and Available online xxxx bimodal populations of surface cracks. The EW reverse faults occur only between 19° and 21°40' S latitude and have been active since at least 6 Ma to present. A March 2007 earthquake demonstrates that this Keywords: Subduction deformation can occur during the interseismic phase of the plate boundary seismic cycle. The NS-normal Tectonics faults are most abundant near Antofagasta and Mejillones and diminish in signiﬁcance northward. Several of Structural geology these faults, in both the Antofagasta and the Salar Grande area, have been reactivated as reverse faults Chile causing minor topographic inversion. We suggest that normal faults move during both interseismic and Coseismic coseismic deformation and are fundamentally very weak. Surface cracks are ubiquitous throughout the Interseismic region and form by both non-tectonic and tectonic mechanisms. A signiﬁcant proportion forms during coseismic deformation and thus may be used to identify long-lived rupture segments on the plate boundary. Overall, forearc deformation is very slow, with permanent strain rates of 1–5 nstrain/year and fault slip rates less than 0.5 mm/year. © 2009 Elsevier B.V. All rights reserved.

1. Introduction between Nazca and South America plates. A major question regarding these faults, however, is whether the structures move during the The forearc of the Chilean Andes provides an unparalleled interseismic or coseismic/post-seismic part of the plate boundary opportunity to study the processes that affect the leading edge of a cycle. Additionally, surface cracks of both tectonic and non-tectonic modern continent–ocean convergent plate boundary. This plate origins are ubiquitous. For the subset of cracks that are tectonic, we boundary periodically generates massive subduction zone earth- can, likewise, pose the question: are they related to interseismic or quakes which not only affect local communities but also produce coseismic plate boundary deformation? Long term and large scale tsunamis that impact the entire Pacific Ocean rim. The rupture zone uplift of the coastal line is marked by flights of marine terraces which for large and great earthquakes lies beneath the coastal areas in are particularly obvious on the Mejillones Peninsula, as well as the northern Chile and southern Perú (Dorbath et al., 1990; Tichelaar and coastal escarpment, itself. Ruff, 1991). The earthquakes are the culminations of cycles in which One of the great advantages of the forearc in northern Chile is the elastic strains build up over a 100–150 year period as convergence hyperarid climate: there is no vegetation to obscure subtle surface continues but the plate boundary is locked (Comte and Pardo, 1991). features and no running water to wear them away. The exquisite During the interseismic part of the cycle the Coastal Cordillera is preservation of these surface deformation features is virtually unique uplifted while the peninsulas of the littoral zone subside; this pattern in the pantheon of modern forearcs. It is likely that the structures is reversed during the coseismic part of the cycle (Klotz et al., 1999; described here are present to varying degrees in all tectonically Pritchard et al., 2002; Chlieh et al., 2004; Pritchard et al., 2006). eroding forearcs; only here, however, are they preserved for study. Deformation features in northern Chile are spectacularly exposed In this paper, we focus on the Coastal Cordillera between 24°S and along the Coastal Cordillera which occupies a position above the 18°S latitude (Fig. 1), a region roughly between 800 and 2000 m seismogenic zone of the Central Andes (Figs. 1, 2). elevation, bounded to the west by the coastal escarpment and, locally, Linear topographic scarps mark the traces of young faults and folds a narrow marine terrace, and to the east by the slightly lower elevation that overlie, and are spatially correlated with, the zone of coupling longitudinal valley. We explore two discrete geological aspects resulting from plate coupling: (1) the relationship between surface deformation features and the subduction seismic cycle and (2) the fl ⁎ Corresponding author. in uence in the upper crustal deformation regime of the map view E-mail address: [email protected] (R.W. Allmendinger). curvature of the plate coupling zone.

Please cite this article as: Allmendinger, R.W., González, G., Neogene to Quaternary tectonics of the coastal Cordillera, northern Chile, Tectonophysics (2009), doi:10.1016/j.tecto.2009.04.019 ARTICLE IN PRESS

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2. Tectonic setting the subducted plate. This proximity requires that about 200 km of forearc have been removed since the start of the Andean convergent Nearly three decades of intense geophysical investigation has margin by subduction erosion, an observation that has been made western South America one of the best known convergent conﬁrmed by many subsequent studies, which demonstrate the lack margins on Earth (Bevis et al., 2001; Buske et al., 2002; Götze et al., of any accretionary prism and document that normal faults are 1994; Husen et al., 1999; Kendrick et al., 2001; Khazaradze and Klotz, present almost all the way to the inner trench wall (e.g., von Huene 2003; Klotz et al., 1999; Oncken et al., 1999; Pritchard et al., 2002; and Scholl, 1991; von Huene and Ranero, 2003). Sobiesiak et al., 2007; Sobiesiak, 2000; von Huene and Ranero, 2003; Following a relative lull in convergence rate in the Oligocene and Wigger et al., 1994). The current cycle of ocean–continent conver- early Miocene, probably caused by very oblique subduction, orthogo- gence began in the Jurassic following the break-up of the Gondwana nal convergence peaked in the early–mid Miocene and has been supercontinent (Mpodozis and Ramos, 1990) and has been continuing decreasing ever since (Pardo-Casas and Molnar, 1987; Somoza, 1998). ever since with varying degrees of obliquity (Pardo-Casas and Molnar, The current, GPS determined rate of ~6.5 cm/year (Angermann et al., 1987). The remnants of the Jurassic volcanic arc currently compose 1999; Kendrick et al., 2003) is less than half of the Miocene rate of much of the basement of the Coastal Cordillera. This ancient arc lies ~15 cm/year (Pardo-Casas and Molnar, 1987; Somoza, 1998). too close to the modern subduction system (Rutland, 1971); in the During the last two decades, the position of the seismogenic zone Antofagasta area, it is just 75 km east of the trench and 35 km above in northern Chile has been constrained by using teleseismic data

Fig. 1. Shaded relief map of northern Chile and adjacent areas; international boundaries shown with long-short dashed line. The long curving lines are contours on the depth to the Wadati–Benioff zone from Cahill and Isacks (1992), modiﬁed in the zone of interplate coupling by local network and seismic reﬂection data (Buske et al., 2002; Husen et al., 1999; Oncken et al., 1999). The heavy dashed line is the axis of Andean topographic and WBZ symmetry (Gephart, 1994). The labeled boxes show areas described in the text.

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Fig. 2. Generalized topographic cross-section showing the relationship of the Coastal Cordillera to the zone of seismic interplate coupling. Modiﬁed from Buske et al. (2002).

(Tichelaar and Ruff, 1991) and local networks (Comte et al., 1994; terminating in the present day endorheic regime of the western Delouis et al., 1996, 1997; Husen et al., 1999). Recent large thrust slope of the Andes, as well as the formation of the Central Depression. earthquakes, including the 1995 Mw 8.1 Antofagasta and the 2007 Long term hyperaridity may produce intensified plate coupling by Mw 7.7 Tocopilla events and their after shocks, have shown that the reducing the amount of water saturated sediment in the trench (Lamb seismogenic zone extends down-dip to a depth of 50±5 km below and Davis, 2003). This history begs the question of whether the uplift the Coastal Cordillera (Husen et al., 1999, 2000; Buske et al., 2002). of the Coastal Cordillera is a consequence of increased plate coupling The seismogenic zone dips approximately 12°–14° to the east (Fig. 2). and therefore if the present day position of the seismogenic zone has The Iquique rupture segment, which extends from Ilo, Peru to been relatively stable since Oligocene–Miocene time. Mejillones, Chile, has not had a great earthquake since 1877. Modeling of regional campaign GPS data shows that the plate boundary is 4. Regional description largely or completely locked during the interseismic part of the earthquake cycle (Norabuena et al.,1998; Bevis et al., 2001; Klotz et al., In this section, we describe the Late Cenozoic structures of the 2001). InSAR and GPS data combined with elastic dislocation model northern Chilean Coastal Cordillera. For the two southern segments, suggest a fully locked zone up to 35 km and a transition zone between Paposo and Mejillones, we update the observations made by Delouis 35 and 55 km depth (Chlieh et al., 2004); the depth of locking et al. (1998) with observations of our own; for the rest of the area, we probably varies along strike (Khazaradze and Klotz, 2003). update our own work (Allmendinger et al., 2005; Carrizo et al., 2008; Dunai et al., 2005; González et al., 2003a,b, 2006, 2008; González and 3. Initiation of modern deformation cycle Carrizo, 2003; Loveless et al., 2005). The Late Cenozoic deformation is overprinted on a Mesozoic and Paleozoic basement. The most Fault line scarps disrupt a smooth and, at one time, more common rock types include highly altered Jurassic volcanics, as well continuous topography. This modern deformation regime began as red clastic deposits and limestones of Cretaceous age that appear, at after a long period erosion that culminated in a regional peneplain least locally, to have been deposited in extensional half-graben surface (the Tarapacá peneplain of Mortimer and Saric, 1972). (Allmendinger et al., 2005; Anonymous, 2003). There are also Recently, surface dating based on cosmogenic nuclides (Carrizo numerous Mesozoic intrusive igneous rocks and, locally, Paleozoic et al., 2008; Dunai et al., 2005) confirms that this peneplain surface metasedimentary rocks also occur. was probably formed approximately 22–25 Ma before present. The consolidation and preservation of the peneplain surface is relatively 4.1. Paposo segment close in time to the onset of the hyperaridity in the Central Andes forearc (Rech et al., 2006). Remnants of the Tarapacá surface can be The Paposo segment (Figs. 1, 3) is one of the topographically found in the highest parts of coastal escarpment suggesting that the highest parts of the Coastal Cordillera. Within this segment, several Coastal Cordillera once extended some distance westward from the fault zones have clear late Cenozoic activity: the Paposo segment of present day coastline. A U–Th/He transect up the coastal escarpment the Atacama fault zone and the much less significant Barazarte and near Tocopilla demonstrates cooling through 40–70 °C at 45±5 Ma Salar de Navidad faults (Arabasz, 1971; Delouis et al., 1998). The late (Juez-Larre et al., 2007). Cenozoic Paposo fault zone is exposed near its southern limit of The more recent evolution of the Coastal Cordillera has been exposure where it dips steeply (~75°) eastward and juxtaposes basin characterized by coastal escarpment retreat and surface uplift, fill to the east against basement rocks, or older Cenozoic basin fill to

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The Salar de Navidad fault (Fig. 3) strikes 290°–305° and has also been described by Delouis et al. (1998) and mapped by González and Niemeyer (2005). The dip is not well determined but fractures along the main fault trace where it crosses a large valley suggest that it probably dips 45–60°NE. The 6 m scarp is uplifted on the NE side, indicating a component of reverse movement. The fractures along the fault trace cut the youngest drainages, so there is evidence of relatively recent motion. The most recent, and primary, motion on the fault is right-lateral based on consistently offset drainages, small pressure ridges along minor left stepping segments of the fault, and the orientation of open fractures in the hanging wall, with strikes between 339° and 355°.

4.2. Mejillones segment

The normal faults of the Mejillones segment (Fig. 6) have already been well described by numerous authors (Armijo and Thiele, 1990; Niemeyer et al., 1996; González et al., 2003a,b; González and Carrizo, 2003; González et al., 2006). Here, we emphasize new observations and analyses that have not received wide distribution.

4.2.1. Faults of the Mejillones Peninsula Normal faults of the Mejillones Peninsula strike generally NS and dip to the east, except for the minor Bandurias fault (Fig. 6), which dips to the west. In general the faults have a listric geometry with gentle rollover anticlines in the hanging walls and unrotated footwalls. The rollover geometry on the south side of the peninsula is clearly marked by a gently westward tilted Pleistocene terrace. The Caleta Herradura normal fault and half graben is the best exposed of any in the coastal region of northern Chile (Fig. 7). 60–70 high sea cliffs exposed the Lower Miocene to Pleistocene strata (Ibaraki, 1990; Koizumi, 1990)whichfill the half graben and demonstrate a protracted history of extension. The westward tilting and thickening of these strata, combined with the lack of rotation of the well preserved Pliocene and younger wave cut terraces and strata in the footwall require that the fault has a listric geometry and shallows with depth. Shear oblique to bedding in the hanging wall (Dula, 1991; Rowan and Kligfield, 1989) is recorded by normal faults that dip both east and west in similar numbers and with similar slip magnitudes (Fig. 8A). We model kinematics of hanging wall deformation with both vertical and synthetic simple shear. With this Fig. 3. Map of the Paposo segments, showing some of the faults and localities described interpretation, the Caleta Herradura listric normal fault should in the text. Shaded relief and topographic contours the same as in Fig. 1. Dashed white become horizontal at less than 2 km depth (Fig. 8B). Given about line is the 50 km contour of the depth to the subducted plate. Stars show the location of 500 m of slip on the Caleta Herradura fault and the fact that the fi subsequent gures. growth strata are lower Miocene and younger, we calculate a long term slip rate of ~0.025 mm/year. Less is known about the geometry of the Mejillones half graben the west, indicating normal faulting (Fig. 4A). The topographic scarp is because of almost non-existent vertical exposure of the hanging wall. very smooth suggesting no significant recent movement in this area. The fault also appears to be listric, but it probably shallows to Farther north, incision is insufficient to expose the fault plane but a horizontal at a deeper crustal level given that the half graben is much clear topographic scarp can be seen on the hillside 50–100 m above wider than the Caleta Herradura half graben. The fault plane, exposed the valley bottom (Fig. 4B). The scarp, which cuts the heads of the in at least two places, dips ~63° to the east and has down-dip alluvial cones, forms a topographic bench on the side of the hill; the slickensides. It is likewise imaged on high resolution bathymetry and detailed morphology of the cone heads indicates that the valley has shallow reflection seismic data (Vargas et al., 2005). Alluvial fans from been uplifted relative to the mountains, by 2±1 m along a reverse the Morro Mejillones form growth strata in the hanging-wall of this fault. fault, indicating that the most recent continental deposition has been The right lateral, N 20–30° W striking, Barazarte fault has already accommodated by vertical slip on the Mejillones Fault. Inactive been well described by Delouis et al. (1998). New high resolution alluvial surfaces, dated at 47,000 years, have been offset across the imagery permits a more accurate determination of displacement. fault by ~13 m (Marquardt, 2005), yielding a short term slip rate of Headless gullies that truncate against the fault document the ~0.3 mm/year and a horizontal extension rate of ~0.11 mm/year. progressive abandonment of channels on the downstream side of the main gully as displacement accrued (Fig. 5). Total minimum right 4.2.2. Coastal Cordillera domino blocks, including the Salar del Carmen lateral displacement is about 250 m, with 60–100 m of fault offset segment of the Atacama Fault Zone before the abandonment of each gully. Ridge crests are also offset East of the Mejillones Peninsula, several well known (Niemeyer across a subsidiary fault trace. Unfortunately, the landforms are et al., 1996) and relatively well exposed normal faults dip 60–68° to currently undated. the east. These faults strike more northeasterly than many structures

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Fig. 4. (A) Outcrop of the Paposo normal fault in late Cenozoic basin ﬁll. (B) Topographic bench produced by reverse fault reactivation of the Paposo normal fault. in the area. The conventional interpretation is that the faults are Niemeyer et al., 1996); if we assume an age of initiation of extension simply reactivating the Mesozoic Atacama fault zone, but some between early Pliocene and early Miocene, the resulting horizontal authors attribute this change to a change in depth of interseismic rate of extension is 0.18–0.06 mm/year. coupling from 35 km to the south to 50 km farther north (Khazaradze The easternmost fault in this set of domino blocks is the Salar del and Klotz, 2003; Loveless, 2007). Carmen fault (Armijo and Thiele, 1990; Delouis et al., 1998; González Unlike the faults of the Peninsula, both the hanging wall and the et al., 2003a,b, 2006). The northernmost end of the fault (Fig. 6) footwall blocks are tilted 4–6° to the west, suggesting a domino exhibits a morphology similar to that seen along the Paposo fault in normal fault model (Fig. 9). Using the relationship between tilt and which the scarp forms a subtle bench on the hill slope about 20–40 m fault dip for horizontal extension from domino blocks (Wernicke and above the topographic break in slope at the base of the hill. This bench Burchﬁel, 1982), we calculate about 4% horizontal extension–or about has accumulated a number of small dark clasts that have moved down 1200 m across the current horizontal distance of 30 km–for this part of slope and thus forms a dark band across the hill side (Fig. 10A). the Coastal Cordillera. The deposits cut by these normal faults are Outcrops in a number of the small, steep gullies that cut the scarp poorly dated in the region of study (Naranjo and Paskoff, 1985; show that the fault dips SE at 65–70° and uplifts a sliver of basement

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Fig. 5. Google Earth image of the northern part of the Barazarte fault showing dextral offset of morphological features. (A) Older displaced gullies, (B) and (C) shutter ridges. on the southeast with respect to the topographically higher footwall. Aside from locally well-developed surface cracks, there are The major scarp at this locality appears to be inactive: unlike the Salar relatively few young structures in the region between the Mejillones del Carmen fault farther south, there are no fresh fractures, raw scarp and Salar Grande segments (Fig. 1). This area has the highest relief faces etc. This site, together with the Paposo segment, demonstrates anywhere north of Antofagasta even though the bedrock geology is that the east-dipping fault zones have experienced reverse reactivation not signiﬁcantly different from segments to the north or south. (Fig. 10B). The age of the reverse fault motion is younger than the offset volcanic ash; elsewhere in the region, similar volcanic ashes have 4.3. Salar Grande segment Pliocene ages (3.5 and 5 Ma, González et al., 2003a,b). The Salar Grande segment (Fig. 12) has received, perhaps, the most 4.2.3. Open tectonic cracks intense recent study of any part of the northern Chile Coastal Open surface cracks characterize much of the area and are Cordillera north of the Mejillones Peninsula (e.g., González et al., commonly, but not exclusively, associated with fault scarps as at 1997; Reijs and McClay, 1998; Chong et al., 1999; Oncken et al., 1999; Salar de Navidad. The ages of these cracks are not known and several González et al., 2003a,b, 2008; Allmendinger et al., 2005; Loveless lines of evidence reviewed below suggest that the cracks are et al., 2005; Carrizo et al., 2008). Here, the complete suite of Coastal reactivated multiple times. On the west side of Salar del Carmen, Cordillera structural styles is well represented, including EW reverse surface cracks were observed to have formed during the 1995 Mw 8.1 faults, NS normal faults and NNW trending strike slip faults, as well as Antofagasta earthquake (González and Carrizo, 2003). The cracks here the most extensive development of surface cracks anywhere in the strike about 020°, parallel to the edge of the salar and to Salar del region. Carmen fault. More recently, we have observed on the Mejillones Peninsula surface cracks that formed during the 2007 Tocopilla 4.3.1. Northern terminus of the Atacama fault zone and related structures earthquake. The northernmost on-shore segment of the Atacama fault zone in An area of extensive surface cracking occurs to the north– this area has a notable fault line scarp on both sides of the Río Loa, yet northwest of the Mantos Blanco mine. The cracks occur along, but displays little Late Cenozoic offset in the walls of the river canyon are much more regionally extensive than, an unnamed north– itself. The fault does not appear to produce any observable offset in northeast-striking fault. This fault is shown on Delouis and others' the EW striking Cerro Aguirre fault system to the south of the river map, but with the incorrect sense of vertical displacement (their Fig. 2, (Fig. 12). To the north, the Salar Grande segment of the Atacama fault Delouis et al., 1998); in fact, the southeast side is down-dropped zone produces a spectacular scarp up to 30 m high in the pure halite relative to the northwest side and the dip of the fault is unknown. surface of the salar (Fig. 13); in this area several faults of the EW Surface cracks in this area (Fig. 11) strike dominantly NNW but there is Chuculay system appear to truncate against the structure. To the a second set which strikes NE (Loveless et al., 2009). As elsewhere, the northwest of the salar, the fault produces consistent offsets of cracks are best developed in gypsum indurated saline soils (“gyp- drainages in a right lateral sense. Offset and abandoned alluvial fans crete”) but they can be found cutting bedrock as well (Fig. 11, inset). have surface ages as young as 2.17 Ma (Carrizo et al., 2008). To the

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Fig. 6. Map of the Mejillones segment, showing some of the faults and localities described in the text: A–Antofagasta; B–Bandurias fault; CH–Caleta Herradura fault; M–Mejillones fault; SdC–Salar del Carmen fault; nSdC–northern Salar del Carmen fault; MB–Mantos Blanco. Tick marks are on the downthrown side of faults. Shaded relief and topographic contours the same as in Fig. 1. White dashed line is the 50 km contour on the depth to the subducted plate. Black dashed line shows location of cross section in Fig. 9. Black dots are aftershocks of the 2007 Tocopilla earthquake.

Fig. 7. Panorama of the Caleta Herradura half graben, looking WSW. The sea cliffs are 60–70 m high.

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related to the stretching of the uplifted hanging wall block as it folds above a propagating tip line or when the fault ramps onto the surface.

4.3.3. North-striking normal and reverse faults West of the Salar Grande, young north-striking faults have either normal or reverse offset. The Punta de Lobos fault (Fig. 12), like most normal faults in the region, dips 78° E and has clear normal displacement. A 3.9 Ma tuff, embedded in alluvial fan gravels in the hanging wall, but resting on bedrock on the side of an active valley in the footwall, has been offset by ~1.5 m; thus most of the normal displacement predates the ash, but a small amount of normal displacement postdates the ash. The same fault produces a scarp, also down to the east, on the cliff face of the Coastal Escarpment (González et al., 2003a,b), indicating that it has remained active until very recently. These relations indicate either a very slow continuous slip rate or a very long recurrence interval for slip events on this fault. In contrast to the Punta de Lobos fault, both the Hombre Muerto and the Geoglifos faults exhibit a mountain bounding scarp morphology in which the basin is uplifted relative to the mountains on faults that dip towards the basin (Carrizo et al., 2008). Likewise, at the western border of the Salar Oﬁcina Gloria (Fig. 12), an N–S striking fault produces the uplift of the basin relative to the mountain front. This fault related morphology suggests that the ﬁrst order topography was produced by mountain bounding normal faults, and the most recent movements on those faults have been in a reverse sense.

Fig. 8. (A) Equal area, lower hemisphere projection of minor normal faults in the 4.3.4. Open tectonic cracks hanging wall of the Caleta Herradura half graben. Arrows on the great circles show the More than 50,000 open tectonic cracks have been measured on motion of the hanging wall. The equal numbers of synthetic (east-dipping) and 1 m resolution IKONOS imagery in the Salar Grande areas (González antithetic (west-dipping) faults indicates that a vertical simple shear (“Chevron”) kinematic model is most appropriate. (B) Listric rollover normal fault model ﬁt to the et al., 2008; Loveless et al., 2005). Those associated with Chuculay east top of the basement surface, dips of Miocene and Pliocene strata along the beach, and of the salar have already been described. The cracks west of the salar the dip of the main fault plane at sea level. Solid fault trace is for the vertical simple have a strong NNW preferred orientation with the mean direction shear model; dashed trace is for synthetic shear model. about 20° more northerly than the dominant NW strike of faults in the region. The extensional strain due to cracks is 1–2%, oriented west of the Salar Grande fault, the sub-parallel Chomache–Bahía statistically parallel to the direction of modern plate convergence. Blanca–Antenna fault system also displays evidence of right lateral Within a few hundred meters of fault scarps, extension due to displacement of modern drainages (González et al., 2003a,b; Carrizo cracking can increase to 5–10% (Loveless et al., 2005). et al., 2008). 4.4. Iquique segment

4.3.2. EW reverse faults Within the Iquique segment, the overall trend of the Coastal Reverse faults that strike approximately E–W and have dip slip Cordillera changes from an azimuth of ~000° to the south to ~340° to movement characterize the Coastal Cordillera from the Cerro Aguirre the north. This change in orientation occurs more-or-less where the structure just south of the Río Loa (21.6°S) to the Quebrada de topographic symmetry plane of the Central Andes crosses the Coastal Camarones at the north end of the Atajaña segment (19.2°S). These Cordillera (Fig. 1). To the north of this axis, the morphologic Coastal structures in the Salar Grande region were originally considered to be Cordillera becomes narrower as the structural province trends normal faults (González et al., 1997; Reijs and McClay, 1998), but offshore. The narrow, littoral, Pleistocene wave cut platform is almost subsequent ﬁeld work, particularly in the regions where the faults cut totally absent north of the city of Iquique. The north- to northwest- the Coastal Escarpment, demonstrated that they are reverse faults striking structures that control the morphology to the south are very which dip moderately to the north or south (Allmendinger et al., much diminished in importance in the Iquique segment and farther 2005). Several of the structures (Chuculay, Aguirre, Barranco Alto) cut north as the Atacama fault zone is here located offshore. and offset small, and now inactive paleodrainages; the channels are The Iquique segment (Fig. 14) does have abundant ~EW-striking offset vertically but not horizontally (Fig. 13), demonstrating that they reverse faults, including some of the best exposed of any in the region. have pure dip-slip motion which is consistent with fault slip data Multiple structures with EW fault scarps at the top of the coastal (Allmendinger et al., 2005; Carrizo et al., 2008). In general, the EW escarpment can be traced down to sea level where they deform the reverse faults cut and offset a peneplained surface that has yielded littoral platform on which the city of Iquique is built. For example, a surface ages between 10 and 20 Ma (Carrizo et al., 2008) so the faults 40 m high EW fault scarp separates the northern part of the city are post-early Miocene in age. Allmendinger et al. (2005) used growth (the “Zofri” zone) from downtown, and the Cavancha Peninsula is strata at Barranco Alto to show that the structure was growing by formed by another EW structure. But, the most spectacular exposure 5.6 Ma; it probably also offsets the Pleistocene wave-cut platform on (C. Marquardt, pers. comm. 2005) occurs at the southern entrance to the coast. Carrizo et al. (2008) dated an offset paleochannel in the the city where the “Los Pacos” reverse fault is well exposed in the 10 m eastern part of the Chuculay system at about 4 Ma. high sea cliff (Fig. 15). The 30° north-dipping fault places altered Chuculay, the largest EW reverse fault in the region, is notable for Mesozoic volcanic rocks over coarse grained beach deposits that have extensive development of surface cracks localized in the hanging wall yielded late Pleistocene fossils. As the fault plane exits the basement block. Using numerical modeling and mapping of ~30,000 cracks, rocks of the footwall, it splays upward into a triangular zone of González et al. (2008) demonstrate that crack formation is directly deformation reminiscent of trishear kinematics (Allmendinger, 1998;

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Fig. 9. Block diagram and cross-section (at 23°S latitude) looking south of the structures in the Mejillones segment. Topography above sea level is exaggerated by 2×. Section below sea level is 1:1. Red dots are aftershocks of the November 2007 Tocopilla earthquake. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

Fig. 10. (A) Topographic bench along the northern Salar del Carmen fault marked by white arrows. (B) Detail of reverse fault relationship showing folding of a Pliocene(?) tuff over a basement block.

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Fig. 11. (A) Image modiﬁed from Google Earth showing cracks northwest of Mantos Blanco. (B) Field photo of Jack Loveless in a 2 m wide crack in bedrock.

Erslev, 1991). The 125,000 year old wave cut surface in the hanging earthquake was too small (M 5.7) to produce a surface rupture at wall is obviously folded and the beach deposits onlap the forelimb. that depth, but a month after the event, we observed in the field fine surface cracks with vertical walls in unconsolidated surface materials 4.5. Atajaña–Pisagua segment that could be traced for tens of meters and have apertures of a few millimeters. These new cracks commonly occurred in the surface The Atajaña–Pisagua segment (Fig. 14) has a few significant coast- depression of older cracks with very rounded and degraded walls, parallel normal faults, but the primary neotectonic features are a suite suggesting that new cracks are reactivating old crack traces. of large, widely spaced southward-dipping reverse faults. The faults strike ENE so that they remain perpendicular to the WNW-trending 5. Discussion: a taxonomy of structural styles Coastal Cordillera and die out at the western margin of the Central Valley. For more than 600 km along strike, the Coastal Cordillera The Atajaña scarp has more than 500 m of vertical relief. At the morphological province tracks the geometry of the subducted plate: intersection of the fault and the coastline, a narrow marine platform the eastern margin of the province is consistently located 60±5 km has been uplifted and tilted. This platform, like similar structures in above the subducted Nazca plate (Fig. 1). At Arica, the subducted plate the Iquique segment, attests to young motion on the Atajaña fault. The is more than 60 km deep and the Coastal Cordillera is entirely missing. fault probably originated as a Mesozoic normal fault that has One might argue that, because the inland border of the province is east subsequently been inverted in Late Cenozoic time: the hanging wall of the 50 km depth limit of significant interplate seismic activity, the is composed of Cretaceous redbeds whereas the footwall contains province is not related to coupling of Nazca and South America. Jurassic volcanic rocks (Anonymous, 2003). Caprio (2007) studied the However, modeling of the upper plate elastic strain field associated Falla Blanco structure, a subsidiary scarp to the south of the main with the interseismically locked plate boundary shows significant Atajaña scarp. She showed that Falla Blanco and others in the Atajaña strain accumulation extending well east of the zone of seismic family formed initially as small separate fault segments and only later coupling (Fig. 16). Likewise, GPS data demonstrates that significant in their history have they coalesced into a single structure, thus coseismic elastic strain release during the 1995 Antofagasta Mw 8.1 explaining the irregular surface traces of all of the faults. She also earthquake occurred as far east as the 75 km depth to subducted plate showed that Falla Blanco, as well as the eastern part of Atajaña and contour (Klotz et al., 1999). Thus, it is likely that all of the Late most of the other smaller scarps in the system, are fold scarps rather Cenozoic structures in the region are a direct result of the long term than scarps of emergent faults. coupling between the subducted Nazca Plate and the overriding South We first noted the importance of fold scarps in the regional American plate. morphology of the Coastal Cordillera while studying the eastern end of The Late Cenozoic structures of the Coastal Cordillera described the Pisagua scarp to the south of Atajaña (Fig. 14). The fold forms are above can be grouped into four distinct types of structures (Fig. 17): well exposed in the Quebrada Tiliviche and other drainages. Allmen- (1) margin perpendicular reverse faults that strike approximately dinger et al. (2005) dated a tuff that was folded over the scarp at east–west; (2) north-striking normal fault zones, some of which show 3.5 Ma; most of the relief of the scarp has accrued since that time. several meters of reactivation as reverse faults; (3) reverse faults Pisagua, like Atajaña, also has an isolated uplifted Pleistocene marine striking NNW to NW; and (4) surface cracks of both tectonic and non- terrace where the fault intersects the coast, indicating that movement tectonic origins. We have direct evidence from recent earthquakes has continued into recent times. A crustal earthquake occurred in that the reverse faults of group 1 form during the interseismic part of March 2007 at a depth of ~26 km, with an epicenter about 20 km south the plate boundary cycle and that some surface cracks form during the of the town of Pisagua; the south dipping nodal plane has the same coseismic part of the cycle. For the rest of the structures, only indirect strike as, and projects to the surface at, the Pisagua fault scarp. The evidence and numerical modeling point to their origins.

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Fig. 12. Map of the Salar Grande segment, showing some of the faults and localities described in the text: A–Aguirre fault; BA–Barranco Alto fault; Ch–Chuculay fault system; G–Geoglifos fault; ChM–Chomache fault; PL Punta de Lobos fault; SGF–Salar Grande fault; RL–Río Loa. Sawteeth shown on the upper plate of reverse faults Shaded relief and topographic contours the same as in Fig. 1. White dashed line is the 50 km contour on the depth to the subducted plate. Inset image modiﬁed from Google Earth shows an oblique view of the Oﬁcina Gloria salar bounded on the west by a normal fault reactivated as a reverse fault (X), an EW reverse fault (Y) which has offset and uplifted the paleochannels (Z).

Fig. 13. Shaded relief detail of the Chuculay and Salar Grande fault systems, based on a 20 m digital elevation model by Yu and Isacks (1999).

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Fig. 14. Map of the Iquique and Atajaña–Pisagua segments, showing some of the faults and localities described in the text: A–Atajaña fault; FB–Falla Blanco; LP–Los Pacos fault; P–Pisagua fault; Z–Zofri fault. Reverse faults shown with sawteeth on the upper plate. Double arrow indicates a fold scarp. Normal faults shown with tick marks on downthrown side. Shaded relief and topographic contours the same as in Fig. 1. NNW trending, labeled dashed lines are contours on the depth to the subducted plate. Focal mechanism plotted at epicenter of M 5.7 crustal earthquake at ~26 km depth which occurred on 24 March 2007.The heavy nodal plane has same strike as, and projects to the surface at, the Pisagua fault. Oroclinal axis from Gephart (1994). Inset map shows the structures that cut the littoral platform on which the city of Iquique is located. CP–Cavancha Peninsula; AH–Alto Hospicio.

5.1. Margin-perpendicular reverse faults and margin curvature are still active today, as shown both by the deformed Pleistocene wave cut terrace in several locations and by the March 2007 crustal The margin perpendicular reverse faults are present only between earthquake just south of Pisagua (Fig. 14). A very rough estimate of the 19°S and 21°40' S latitude and are symmetrically distributed on either rate of shortening due to these structures is 0.2–0.5 mm/year. side of the axis of the Bolivian Orocline (Allmendinger et al., 2005). The Pisagua earthquake showed that margin-parallel shortening The faults have pure dip slip motion and produce about 1% margin can occur during the interseismic part of the seismic cycle. parallel shortening. They are restricted to the Coastal Cordillera and Furthermore, elastic modeling (Bevis et al., 2001) demonstrates that die out into fold scarps on the west side of the central valley. These interseismic elastic strain related to a curved plate boundary which is structures have been active for at least the last 6 million years and they concave towards the subducted plate should produce a component of

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Fig. 15. Photograph of the Los Pacos fault at the southern entrance to Iquique. Note the onlap of the late Pleistocene beach deposits on the forelimb and the folding of the wave-cut terrace in the hanging wall. The white box shows the location of the detailed photograph to the right which shows the fault zone and the growth strata with clasts of the hanging wall lithologies. margin-parallel shortening. The ideal elastic model for a fully locked have been active throughout much of the Neogene. Some normal fault Andean plate boundary gives −3.3±0.15 nstrain/year margin parallel zones in the Antofagasta segment display incipient reactivation as shortening in the Coastal Cordillera (Bevis et al., 2001). Assuming very reverse faults (Paposo and northern Salar del Carmen faults) and more roughly that the margin parallel shortening due to the EW reverse extensive reactivation can be seen for normal faults in the Salar Grande faults started sometime between 15 and 3 Ma, the permanent longer area. We have insufficient data to determine whether the normal faults term margin parallel shortening rate is −2±1.3 nstrain/year; thus, convert permanently to reverse faults with time or if normal and reverse the calculated interseismic elastic strain and the long term permanent faulting occur repeatedly throughout the life of the structure, with finite strain are very similar. A similar pattern has been demonstrated in horizontal extension dominating overall. The fact that reactivation Cascadia, another concave convergent margin (Johnson et al., 2004; appears common in the region that is 35–50 km above the subducted McCaffrey et al., 2007) with little in common with northern Chile plate would appear to favor the former possibility. besides the curvature. The primary remaining question is why the How the normal faults relate to the seismic cycle is not so clear. margin-parallel elastic strain is not fully recouped during subduction Margin-perpendicular extension can occur during both the inter- zone slip events but instead is converted into permanent upper plate seismic elastic elastic deformation (Fig. 16) and the coseismic parts of deformation. the cycle (Klotz et al., 1999). In the Antofagasta/Mejillones area, the zone of extension extends from the Salar del Carmen fault on the east 5.2. Margin-parallel normal faults to just east of the trench (von Huene and Ranero, 2003). Only part of this transect lies within the regions of interseismic flexural extension, The margin-parallel normal faults are best developed in the vicinity regardless of whether one chooses 38 or 50 km as the extent of down- of Antofagasta and the Mejillones Peninsula, where horizontal extension dip locking for interplate seismic coupling. Thus, it seems likely that rates vary from 0.025 to 0.18 mm/year, and diminish in importance some normal faults move during major plate boundary thrust events north of Tocopilla. Significant normal faulting does not exist east of the and some move during the interseismic part of the cycle. 50 km depth contour. The majority of the normal faults dip to the east Some authors have suggested that normal faults are due to long and, where present, hanging-wall growth strata suggest that the faults term subduction erosion (Armijo and Thiele, 1990; von Huene and

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Fig. 16. Model of interseismic elastic strain accumulation for a convergent plate boundary which is fully locked to either 38 km (solid line) or 50 km (dashed line). Shaded areas show regions of extension which are due to interseismic elastic ﬂexure. Figure from Loveless (2007).

Ranero, 2003). If the Nazca and South American plates are mostly or 5.3. NW to NNW strike–slip faults completely locked during the interseismic part of seismic cycle, then subduction erosion could only happen coseismically (Fig. 17). In this Signiﬁcant strike–slip appears to be limited to the relatively small case, any normal faults that are produced by subduction erosion number of west–northwest to north–northwest striking faults would be predominantly coseismic as well. virtually all of which have right lateral displacement. The kinematics

Fig. 17. Taxonomy of forearc structural styles and their relations to the interplate seismic cycle. See text for discussion.

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Fig. 18. Image modified from Google Earth showing tectonic and non-tectonic cracks. The latter form the fine polygons, whereas the former can be traced for hundreds of meters in length and have a rough preferred orientation. The heavy dark lines are drainages; note that the tectonic cracks cross the drainages. of the WNW-striking Salar de Navidad and Barazarte faults southeast they reflect deep seated, crustal scale processes rather than surface of Antofagasta are inconsistent with interseismic shortening across effects. Commonly, tectonic cracks are filled with gypsum, windblown the margin but consistent with coseismic elastic rebound as well as sand, or both. The gypsum is usually vertically banded, with the margin parallel shortening. Nonetheless, we have no direct evidence of opening occurring in the center of the structure. Thus, we interpret when they last moved. North of the Río Loa, the Salar Grande, Chomache, that most cracks have a protracted history of opening and filling. and Antena faults strike NNW. They are nearly perpendicular to the Where exposures permit, the cracks can be seen to penetrate bedrock convergence direction but kinematically consistent with interseismic for at least 9–12 m and are probably considerably deeper. shortening parallel to the margin (Carrizo et al., 2008). We distinguish three types of tectonic cracks: (1) those that form during upper crustal faulting that is unrelated to the seismic cycle on the 5.4. Surface cracks plate boundary. The cracks associated with the Chuculay reverse fault system are an example (González et al., 2008). (2) Cracks that form In a hyperarid environment, surface cracks can form due to a variety during plate boundary earthquakes and are due to focusing of surface of mechanisms, both non-tectonic and tectonic (Fig. 18). The former waves by topographic scarps, basin margins, and zones of weakness like are those due largely to surface processes such as saline soil formation faults. Numerical modeling by Loveless (2007) has shown that the and mass wasting. Pedogenic cracks form due to salt wedging and focusing is only important within a few hundred meters of the fault or expansion and contraction of gypsum indurated crusts (Tucker, 1978). scarp. Cracks that opened on the western side of Salar del Carmen during Though ubiquitous in the field, cracks produced by this process are the 1995 Antofagasta earthquake were probably due to surface wave sufficiently small that they seldom can be resolved on the 1 m–2.5 m focusing due to the edge of the salar basin and the adjacent Salar del resolution imagery that we currently use to map the cracks. One Carmen fault. Cracks that lie at greater distances from known structures exception to this rule is the region south of the Río Loa where one can and scarps are considered to be (3) features that form coseismically, resolve both tectonic and non-tectonic cracks in the imagery (Fig. 18); probably due to extension during elastic rebound of the upper plate. the non-tectonic cracks tend to form equidimensional polygons Modeling by Loveless (2007) of dynamic wave propagation during an whereas the tectonic cracks are hundreds of meters long, have a Antofagasta-type event in a uniform half space shows that the dynamic preferred orientation, and cross paleovalleys. Non-tectonic cracks may principal horizontal extension axes and the static principal extension also form due to sliding on unstable slopes, especially near canyons and axes have the same orientation. steep hillsides. Soil cracks probably form slowly over long periods of Ample evidence confirms that coseismic processes are important time and thus form during interseismic and coseismic parts of the in crack formation. The most direct evidence is the observed cracking cycle. If is likely that cracks due to slope instabilities form primarily due that occurred during the 1995 Antofagasta, 2001 Arequipa (Keefer and to shaking during earthquakes (e.g., Marquardt et al., 2006). Moseley, 2004), 2005 Tarapacá, and 2007 Tocopilla earthquakes. Even All types of cracks, including the tectonic cracks described below, though the latter two were smaller than the two former ones, new can be enhanced by surface processes (Keefer and Moseley, 2004). fine cracks were observed in the center of old cracks after both events Water runoff during the infrequent precipitation events that affect the (Fig. 19). Because the cracks are long lived, and because of the region dissolve the thick salts filling the cracks, leaving meter scale extraordinary preservation of these delicate structures in the subsurface voids, and leading to roof collapse. Once a crack is formed, hyperarid Atacama Desert, their distribution may provide insight regardless of mechanism, soil salts fill the crack and contribute to into seismic segmentation of the plate boundary. further wedging. As documented by Loveless et al. (2009), the cracks display Most of the N55,000 surface cracks that we have mapped on regionally consistent orientation locally and vary systematically in IKONOS and Google Earth imagery (Carrizo et al., 2008; González orientation regionally. At the northern end of the area described here, et al., 2008; Loveless et al., 2005, 2009) are primarily tectonic; that is, the cracks from groups 2 and 3, above, are bimodal, striking NW and

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Fig. 19. Reactivation of an older surface crack during the 2007 Tocopilla earthquake. The new crack is in the unconsolidated coarse sandy ﬁll of the older crack. The older crack walls have been partially highlighted with dashed white lines.

NE. In the middle of the area, they tend to be unimodal and strike 4. Upper plate fault zones are probably very weak and prone to slip in NNW, and at the south end of the area, they are, again, bimodal with different directions due to minor changes in the stress field. both NE and NW strikes well represented. Loveless et al. (2009) have 5. Forearc deformation rates are very slow, with average fault slip proposed that the coseismic cracks can be used to define long-lived rates less than 0.5 mm/year and strain rates on the order of 1– segments. Their inversion of crack orientations for a best fitting 5 nstrain/year. rupture zone for the Iquique segment indicates that most of the slip on the plate boundary should occur north of the Río Loa. There is a broad A great deal more will be learned about these processes when the zone of transition between Río Loa and Mejillones to the south where Coastal Cordillera is completely covered by a dense network of there is high topography and relatively few cracks and those that exist continuous GPS stations. This will allow us to understand much more display bimodal orientations. The large, but not great, November 2007 fully the role of interseismic deformation, particularly interseismic Tocopilla earthquake occurred in this gap, hinting at the intriguing flexure. Unfortunately, we will also learn much more about these possibility that this segment may not experience numerous great processes once the next great Iquique earthquake occurs. It has been earthquakes. Likewise there is a broad zone of transition at the north more than 130 years since the last great earthquake and recent earth- end of this segment in the Arica area, which also displays bimodal quakes to the north (2005 Arequipa) and south (1995 Antofagasta, orientations. 2007 Tocopilla) of the Iquique segment suggest that we will not have to wait long. 6. Conclusions Acknowledgments The Late Cenozoic tectonics of the Coastal Cordillera of northern Chile reflect processes related to the seismic coupling between the We are indebted to numerous colleagues and students who, over subducted Nazca Plate and the overriding South American Plate. the last decade, have shared their knowledge of the Coastal Cordillera, Although these processes probably occur in all eroding convergent as well as, more often then not, their good humor in the field. In margins around the globe, only in northern Chile is the record particular, we wish to thank our students Jack Loveless and Daniel preserved due to the hyperarid climate of the region. Our study of the Carrizo whose work we have drawn on heavily in this paper. Coastal Cordillera has yielded the following conclusions: Additionally, we are grateful to José Cembrano, Greg Hoke, Holly Caprio, Terry Jordan, Matt Pritchard, Bryan Isacks, Tibor Dunai, 1. Margin-parallel shortening on ~EW reverse faults is probably Alejandro Macci, Francisco Gomez, Joaquin Cortés and Jacob Espina. related to the curvature of the margin, which is concave towards We are grateful to Pia Victor and Jonas Kley for their careful reviews of the subducted plate (Bevis et al., 2001). Additional shortening an earlier version of this manuscript. Allmendinger and his students' could also occur due to oblique subduction (McCaffrey, 1996). work in northern Chile has been funded by the U. S. National Science 2. Margin-perpendicular extension probably occurs during both Foundation through grants EAR 0087431, EAR-0337496, and EAR- interseismic and coseismic parts of the plate boundary seismic 0738507. González and his students' work was funded by the cycle. The interseismic extension is due to flexure of the margin, FONDECYT projects 1040389 and 1085117. and coseismic extension is due to elastic rebound and subduction erosion. References 3. Surface cracks form due to a number of mechanisms, but a particularly important one is coseismic extension. Regional varia- Allmendinger, R.W., 1998. Inverse and forward numerical modeling of trishear fault- tions in orientations of the cracks suggest that rupture segments of propagation folds. Tectonics 17, 640–656. Allmendinger, R.W., González, G., Yu, J., Hoke, G.D., Isacks, B.L., 2005. Trench-parallel the plate boundary persist over long periods of time, perhaps shortening in the northern Chilean forearc: tectonic & climatic implications. millions of years (Loveless et al., 2009). Geological Society of America Bulletin 117, 89–104. doi:10.1130/B25505.1.

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Please cite this article as: Allmendinger, R.W., González, G., Neogene to Quaternary tectonics of the coastal Cordillera, northern Chile, Tectonophysics (2009), doi:10.1016/j.tecto.2009.04.019