Turbidite paleoseismology along the active continental margin of Chile – Feasible or not?

Quaternary Science Reviews 120 (2015) 71e92

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Turbidite paleoseismology along the active continental margin of Chile e Feasible or not? Anne Bernhardt a, *, Daniel Melnick a, Dierk Hebbeln b, Andreas Lückge c, Manfred R. Strecker a €t Potsdam, Karl-Liebknecht-Strasse 24-25, 14476 Potsdam, Germany Department of Earth and Environmental Sciences, Universita MARUM e Center for Marine Environmental Sciences, University of Bremen, Leobener Straße, 28359 Bremen, Germany c Bundesanstalt für Geowissenschaften und Rohstoffe (BGR), Stilleweg 2, 30655 Hannover, Germany a

b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 21 July 2014 Received in revised form 20 March 2015 Accepted 2 April 2015 Available online

Much progress has been made in estimating recurrence intervals of great and giant subduction earthquakes using terrestrial, lacustrine, and marine paleoseismic archives. Recent detailed records suggest these earthquakes may have variable recurrence periods and magnitudes forming supercycles. Understanding seismic supercycles requires long paleoseismic archives that record timing and magnitude of such events. Turbidite paleoseismic archives may potentially extend past earthquake records to the Pleistocene and can thus complement commonly shorter-term terrestrial archives. However, in order to unambiguously establish recurring seismicity as a trigger mechanism for turbidity currents, synchronous deposition of turbidites in widely spaced, isolated depocenters has to be ascertained. Furthermore, characteristics that predispose a seismically active continental margin to turbidite paleoseismology and the correct sample site selection have to be taken into account. Here we analyze 8 marine sediment cores along 950 km of the Chile margin to test for the feasibility of compiling detailed and continuous paleoseismic records based on turbidites. Our results suggest that the deposition of areally widespread, synchronous turbidites triggered by seismicity is largely controlled by sediment supply and, hence, the climatic and geomorphic conditions of the adjacent subaerial setting. The feasibility of compiling a turbidite paleoseismic record depends on the delicate balance between sufficient sediment supply providing material to fail frequently during seismic shaking and sufficiently low sedimentation rates to allow for coeval accumulation of planktonic foraminifera for high-resolution radiocarbon dating. We conclude that offshore northern central Chile (29e32.5
S) Holocene turbidite paleoseismology is not feasible, because sediment supply from the semi-arid mainland is low and almost no Holocene turbidity-current deposits are found in the cores. In contrast, in the humid region between 36 and 38
S frequent Holocene turbidite deposition may generally correspond to paleoseismic events. However, high terrigenous sedimentation rates prevent high-resolution radiocarbon dating. The climatic transition region between 32.5 and 36
S appears to be best suited for turbidite paleoseismology. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Turbidite paleoseismology Chile convergent margin Earthquake Seismoturbidites

1. Introduction Recently, detailed paleoseismic records inferred from vertically displaced Sumatran corals have revealed that great earthquakes in subduction zones have variable recurrence periods (e.g., Sieh et al., 2008). In fact, variable recurrence and magnitudes are apparently a

* Corresponding author. E-mail address: [email protected] (A. Bernhardt). http://dx.doi.org/10.1016/j.quascirev.2015.04.001 0277-3791/© 2015 Elsevier Ltd. All rights reserved.

hallmark of many subduction zones and have been deciphered by using different types of archives found at Nankai (Ando, 1975), Tohoku (Sawai et al., 2012), Hokkaido (Sawai, 2001), south-central Chile (Cisternas et al., 2005; Garrett et al., 2014; Moernaut et al., 2014), EcuadoreColombia (Satake and Atwater, 2007), Sumatra (Sieh et al., 2008; Patton et al., 2013), and Cascadia (Goldfinger et al., 2003a, b; Witter et al., 2012). At the two latter sites, shallow-marine and terrestrial archives have been complemented by a long (6e12 kyrs) record inferred from turbidites recovered from offshore coring of the continental slope. It has been commonly

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accepted that the most reliable indicators of pre-historic great (M 8e9) and giant (M > 9) subduction-zone earthquakes include land-level changes as well as tsunami deposits, which are found at specific coastal sites where the preservation potential for such deposits is high (Nelson et al., 2006). Estimating the earthquake rupture length and magnitude using such records requires correlation among distant sites, which are often subjected to different geomorphic and climatic conditions. Additionally, in highly dynamic coastal depositional environments, terrestrial archives may be incomplete and sometimes even selective in the preservation of past seismic events (Nelson et al., 2009) as coastal landforms and sediments associated with seismic events may be eroded over time. Moreover, coastal archives are usually restricted to the late Holocene when sea level stabilized after the last deglaciation (e.g., Bintanja et al., 2005; Nelson et al., 2006). In light of these difficulties, the use of turbiditic deposits to infer records of past great earthquakes offers two major advantages: 1) On a temporal scale, earthquake supercycles can only be understood if long-term paleoseismologic archives are analyzed that extend beyond instrumental records, the historical earthquake record, and most terrestrial archives. Marine turbidite records may provide such long-term archives that are key to assessing seismic risks (e.g., Goldfinger et al., 2003a, b). In this context, deciphering earthquake recurrence intervals is important for infrastructure planning and development and may provide crucial insight into the seismotectonic behavior of forearcs on centennial to millennial timescales, and beyond. 2) On a spatial scale, turbidites may be correlated over large distances to better assess the extent of paleo-rupture zones and, hence, long-term seismotectonic segmentation (e.g., Goldfinger et al., 2007). Turbidite frequency has been applied to reveal earthquake recurrence intervals along many seismically active continental margins, including the Cascadia margin of North rrezAmerica (Adams, 1990; Goldfinger et al., 2003a, b; 2007; Gutie Pastor et al., 2009; Nelson et al., 2012; Patton et al., 2013), the northern California margin (Garfield et al., 1994; Goldfinger et al., rrez-Pastor et al., 2009), the southern Cali2007, 2008; Gutie fornian Borderland (Gorsline et al., 2000), Taiwan (Huh et al., 2004), Japan (Nakajima and Kanai, 2000; Noda et al., 2008), New Zealand (Pouderoux et al., 2012), the Mediterranean (Kastens, 1984; Anastasakis and Piper, 1991; Polonia et al., 2013; Ratzov et al., 2015), the Marmara Sea (McHugh et al., 2006), offshore Portugal cia et al., 2010), and offshore Haiti (McHugh et al., 2011). (Gra Whereas turbidites can potentially provide valuable long-term paleorecords of great and giant earthquakes, not every earthquake of such magnitudes triggers widespread submarine slope failure, leading to incomplete records along some margins (e.g., Noda et al., 2008; Hunt et al., 2013; Sumner et al., 2013). A key task and requirement in turbidite paleoseismologic research is to distinguish between different potential trigger cia mechanisms of turbidity currents (e.g., Goldfinger, 2011; Gra et al., 2013; Talling, 2014). From a sedimentological perspective, deposition from all turbidity currents is controlled by the same general process: waning turbulent sediment gravity flow. As such the resulting sedimentary character of turbidites does not yield direct clues about the initiating process of the depositing turbidity current. Piper and Normark (2009) suggested three main mechanisms for the initiation of turbidity currents: (a) transformation from slumps; (b) hyperpycnal flow from rivers; and (c) stormgenerated flows from near the shelf edge. Seismically triggered turbidity currents fall into the first category. Earthquakes can initiate sediment failures on submarine slopes that can subsequently transform into turbidity currents. One of the best known examples is the turbidity current that followed the Grand Banks earthquake of NE Canada in 1929 (Heezen and Ewing, 1952; Piper et al., 1999). However, earthquakes are just one possible

mechanism that can cause sediment slumping, and other triggers such as sediment loading, destabilization of gas hydrates, and wave-induced slumping need to be considered as well (e.g., Talling, 2014). Several criteria have been inferred to be of use when gathering evidence for seismically triggered turbidites, including the style of submarine canyon erosion (Piper and Normark, 2009) and the great areal extent and larger depositional volume of individual turbidite events (e.g., Talling et al., 2007). Variable provenance, and multiple pulses of the coarse sediment fraction leading to an irregular sequence of sedimentary structures and multiple fining-upward cycles within one turbidite have been cited as well (Nakajima and rrez-Pastor et al., 2013), but Kanai, 2000; Shiki et al., 2000; Gutie are not considered to be strongly diagnostic of earthquake triggering (Talling, 2014). Recently, Van Daele et al. (2014) expanded this approach by showing not just multiple pulses, but also multiple flow directions of subdeposits within a single stacked turbidite, suggesting simultaneous triggering of turbidity currents along different slopes of the basin. The most convincing and probably exclusive criterion of recognizing seismically triggered turbidites in the depositional record are synchronous deposition and wide spatial extent (e.g., Goldfinger, 2011; Talling, 2014). Along the Chile convergent margin, historical seismic events are characterized by widespread ground shaking along regional rupture zones and in excess of at least 10s of kilometers in length or even several 100 km (see Fig. 1B, C and references therein). Seismic shaking along these rupture zones often spans multiple submarine drainage systems and seismically induced turbidity currents should occur synchronously along the slope and generate deposits of large areal extent and possibly with different provenance signals (Goldfinger et al., 2007). In contrast, other causes of slope failure and turbidity current triggers, including storms, river flood peaks, and hyperpycnal river discharge (as summarized in Talling, 2014), are thought to act on a more local scale and tend to affect smaller regions when compared to seismic ground shaking along a rupture zone. In this sense, synchronicity of turbidites over wide regions effectively precludes most non-seismic triggers as viable mechanisms. Synchronicity and spatial extent of turbidite events can be established by either widespread correlation of turbidites between widely spaced submarine depocenters or through the confluence test that requires an equal number of turbidites in upstream and downstream locations of canyon and channel confluences (e.g., Adams, 1990; Goldfinger cia et al., 2010). et al., 2003a, b; Gra Recently, the need for testing seismically hazardous margins for the feasibility of compiling a paleoseismic record based on turbicia et al., 2013; Sumner et al., 2013; dites has been stressed (Gra Talling, 2014). Furthermore, the details of site selection with regard to the specific depositional environment, sediment gravity flow paths, variations of sediment supply, and to the position within seismic segments or individual earthquake rupture zones are strongly debated (Goldfinger et al., 2012, 2014; Patton et al., 2013; Sumner et al., 2013, 2014; Atwater et al., 2014). Until the underlying controls of why a submarine slope is predisposed to widespread failure are better understood, turbidite paleoseismology should be applied with considerable caution (e.g., Sumner et al., 2013). Requirements for the successful application of turbidite paleoseismology to a seismically active margin are (1) sufficiently long earthquake recurrence intervals, because too frequent earthquakes and associated shaking may cause sediment consolidation rather than failure (Lee et al., 2004) and may not allow for sufficient datable material to accumulate between events (Goldfinger, 2011); (2) high sediment supply to ensure that there is sufficient material accumulated, so most great to giant earthquakes trigger sediment gravity flows (Goldfinger, 2011; St-Onge et al., 2012; Urlaub et al.,

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2012; Sumner et al., 2013); (3) a margin morphology that allows for the development of isolated slope basins and discrete channel systems (Goldfinger, 2011); and (4) high biogenic productivity providing sufficient datable calcareous microfossils (Goldfinger, 2011). Along the Chile convergent margin, recurrence times of major to giant earthquakes as well as the nature and extent of seismotectonic segments are still highly uncertain, despite recent advances in retrieving paleoseismological records from terrestrial archives (Cisternas et al., 2005; Bookhagen et al., 2006; Bertrand et al., 2008; Dura et al., 2014; Garrett et al., 2014; Moernaut et al., 2014) and new compilations of historical records (Cisternas et al., 2012; Udías et al., 2012). However, the Chile margin combines distinct characteristics that may promote it as a suitable candidate for expanded paleoseismoturbidite analysis. (1) It has been repeatedly struck by megathrust earthquakes of variable magnitude and rupture length during the 500-yr-long historical record (e.g., Lomnitz, 2004). Postulated recurrence times differ between seismotectonic segments depending on the timescale and type of archive investigated. (2) Terrestrial erosion rates and related sediment flux to the Pacific Ocean from the Andean orogen are thought to be primarily controlled by climate on timescales of 104e105 yrs, with the gradual, but high-amplitude northesouth precipitation gradient being the most important factor (Fig. 1A; Scholl et al., 1970; Bangs € lker and Cande, 1997; Hebbeln et al., 2007; Rehak et al., 2010; Vo et al., 2013). On glacialeinterglacial timescales, the strong precipitation gradient has frequently been shifted north-/southward by about 5
of latitude (e.g., (Heusser, 1989; Lamy et al., 2001; Mohtadi and Hebbeln, 2004; Romero et al., 2006). The general spatial pattern of high rainfall in southern Chile, however, remained stable. (3) The submarine accretionary forearc offshore Chile is characterized by an irregular marine slope with small intraslope plateaus, basins, ridges, and several submarine canyons that extend from the shelf to the trench (e.g., Thornburg et al., 1990; Laursen and €lker et al., 2006; Geersen et al., 2011a) and Normark, 2002; Vo that serve as independent sediment conduits or isolated sediment traps, respectively. (4) The PerueChile Current (PCC; or Humboldt Current) extends along the Chilean and Peruvian coast over 40
of latitude and is characterized by strong coastal upwelling resulting in high biological productivity (>200 g C m2 yr1) (Berger et al., 1989). Hence, the Chile convergent margin is an ideally suited place to investigate the feasibility of turbidite paleoseismology on a regional spatial scale and a centennial to millennial temporal scale (and possibly beyond). In this study, we address two study sites along the Chile margin and one intermediate area in between both sites that are characterized by distinct climatic onshore conditions: semiarid and year-round humid (Fig. 1A). Both regions are tested for the feasibility of establishing a turbidite paleoseismologic record by identifying synchronous turbidites within separate depositional settings that finally will be compared with the historical record and available onshore paleoseismic archives. 2. Regional setting 2.1. Geologic and climatic background The Chile convergent margin results from subduction of the oceanic Nazca plate below the South American continent at ~7 cm/ yr (Somoza, 1998; Angermann et al., 1999). The oceanic Juan ndez Ridge constitutes a major bathymetric anomaly and Ferna tectonic discontinuity of the Nazca plate and is located between the northern and southern study area and interrupts the south to € lker north-directed trench-parallel sediment transport (Fig. 1A; Vo et al., 2006). The continental slope between 29 and 32
S is

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characterized by a relatively smooth slope plateau that gently inclines toward the trench until ~4000 m of water depth, where it is dissected by small-scale gullies in the upper reaches (Fig. 2A). The plateau is bounded to the north at 29.3
S by an unnamed submarine canyon (Fig. 2A, B). Offshore Valparaíso at 33
S, the upper slope is characterized by a crescent-shaped intraslope forearc basin of ~50 km northesouth width, the Valparaíso Basin (Fig. 2A, C; Laursen et al., 2002). South of 33
S, the Chile onshore margin is morphotectonically segmented into the Coastal Range of the forearc, the Central Depression, and the Main Cordillera (Fig. 1A). The upper continental slope between 33 and 43
S is relatively smooth and inclined at low €lker angles (2e4
) to a water depth of 2000 m (Figs. 1 and 2D; Vo et al., 2014). Below 2000 m water depth, the slope morphology has steep segments alternating with approximately trench-parallel slope basins. This irregular morphology is caused by the continuous deformation of the relatively young (~4 Ma) accretionary prism that € lker et al., 2014). constitutes the lower continental slope (Vo The climatic conditions in this region of Chile are characterized by a high-amplitude but gradual precipitation gradient over >2000 km from the hyper-arid subtropical Atacama Desert with year-round aridity in the north, semi-arid climate zones with winter rain in the central part, to year-round humid regions in the south (Fig. 1A). Between 18 and 33
S, the Andes are located in the subtropical belt of deserts with little to no precipitation on the western side of the mountain range. To the south, the Southern Hemisphere Westerlies provide abundant moisture that precipitates at the western slopes of the Andes, resulting in a significant increase in mean annual precipitation (Fig. 1A). The region between 28 and 34
S is subjected to episodic variations in rainfall amount due to increased moisture transport from the south controlled by a northward shift of the Westerlies during glacial periods (e.g., Heusser, 1989; Lamy et al., 2000; Heusser et al., 2006; Romero et al., 2006; Hebbeln et al., 2007; Kaiser et al., 2008). 2.2. Seismicity The Chile convergent margin is frequently struck by megathrust earthquakes (Fig. 1B; e.g., Lomnitz, 2004). Several patterns of seismotectonic segmentation along the Chile margin have been put forward based on the extent of the rupture zones of historic earthquakes, the degree of modern interseismic plate locking inverted from Global Positioning System (GPS) velocities, and Quaternary deformation patterns of the forearc, (e.g., Rehak et al., 2008; Jara et al., 2015; Melnick et al., 2009; Moreno et al., 2011, tois et al., 2012). Segmentation is manifested by the Val2012; Me divia segment to the south (rupture zone of the 1960 event (Mw n segment in Central Chile (rupture zone of the 9.5)), the Concepcio 2010 Maule event (Mw 8.8)) and the Valparaíso or Metropolitan segment to the north (rupture zone of the 1730 event) (Fig. 1B; tois et al., Jara et al., 2015; Melnick et al., 2009; Farías et al., 2010; Me 2012). Postulated recurrence intervals differ between these segments and on the timescale and type of investigated archives, which are influenced by their magnitude threshold for recording a seismic event. n segment the historic record reaches back to In the Concepcio 1575 AD, with six M > 8 events attributed to the megathrust with a recurrence interval of ~88 years (Lomnitz, 2004). Even though this interval might suggest periodic recurrence, it includes earthquakes of variable magnitudes (M ~ 8e9) and rupture lengths (several tens to several hundreds of km), with the AD 1730 (M > 9) event extending into the neighboring segment to the north (Udías et al., 2012). A paleoseismologic record compiled from uplifted beach berms at Santa María Island (Fig. 1A) suggests a recurrence time of strandline-forming earthquakes of 180 ± 65 years (Bookhagen

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Fig. 1. A) Overview of the studied area including the rupture zones of historic subduction earthquakes. The marine part of the overview map shows the offshore bathymetry n. An intermediate core compiled from several research cruises including the core locations. The investigation was focused on the area offshore La Serena and offshore Concepcio offshore Valparaíso (GeoB 3304-5) was selected in order to document general changes in the sedimentology between the northern and southern study area. The terrestrial part of the overview map shows a shaded relief map draped with the mean annual rainfall distribution derived from the Tropical Rainfall Measurement Mission (TRMM) satellite. Rainfall ~ ez (1992) according to methods described in Bookhagen and Burbank (2006). B) Megathrust amounts were calibrated with ground-control stations reported in Bianchi and Yan earthquakes (black) and one intra-oceanic plate earthquake (blue) and their geographical range are indicated in the right and were compiled from Beck et al. (1998), Campos et al. (2002), Cifuentes (1989), Cisternas et al. (2012), Comte et al. (1986), Farías et al. (2010), Kelleher (1972), Lomnitz (2004), Udías et al. (2012). Dashed lines show estimated rupture length based on historical accounts. C) Distribution of co-seismic slip (Moreno et al., 2010) and shaking intensity along the rupture zone of the 2010 Maule earthquake (see also Fig. 1A for location of the rupture zone). Plots deliniate the maximum and mean co-seismic slip and intensity. The white area denotes the extent of the rupture zone. High shaking intensity exceeds the limit of the rupture zone at its northern boundary. However, the extent of shaking can exceed the rupture zone by ~20% with peak intensity values at the inferred segment boundary, as may be observed by a direct comparison of coseismic slip (Moreno et al., 2010) and instrumental intensity (USGS Shakemap, 2010, http://earthquake. usgs.gov/earthquakes/shakemap) for the 2010 Maule event. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Fig. 2. Detailed 3D-view of the bathymetry offshore Chile showing the core locations. Elevation is 6 times exaggerated.

et al., 2006). Within the Valdivia segment, the historically recorded recurrence interval is inferred to be ~128 years (Lomnitz, 2004), but longer-term sedimentary evidence from tsunami deposits and coastal subsidence suggest a recurrence interval of 285 ± 15 years during the last 2000 years (Cisternas et al., 2005) or ~270 years during the last 1000 years (Garrett et al., 2014). These historical and

paleoseismic estimates are both in agreement with a recent compilation of seismically triggered turbidites from several lake records that indicate a recurrence interval of 280 yrs for giant M ~9.5 events suggesting a recurrence of 140 yrs for M  7.75 events for the past 900 yrs (Moernaut et al., 2014). Along the Valparaíso segment, historical earthquakes occur within a consistent

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recurrence interval of ~80 yrs (Comte et al., 1986; Barrientos, 2007). However, this record includes events of very variable magnitude (M 8e9), and a mid-Holocene coastal record of paleo-tsunamis and land-level changes suggests a mean recurrence interval of great and giant earthquakes and associated tsunamis of ~500 years (Dura et al., 2014). The persistence of the boundaries between such segments is still poorly understood (e.g., Ely et al., 2014) and giant seismic supercycle events might rupture more than a single segment (McCaffrey, 2008; Shennan et al., 2009). Moreover, turbidity current generation is sensitive to shaking intensity during a seismic event as opposed to coastal paleoseismological sedimentary archives that are associated with coseismic land-level changes or tsunami inundation. Archives of land-level changes are thus restricted to the region that directly overlies the rupture zone of an earthquake (e.g., Bookhagen et al., 2006; Farias et al., 2010; Dura et al., 2014). However, the extent of shaking can exceed the rupture zone by ~20% with peak intensity values at the inferred segment boundary, as may be observed by a direct comparison of coseismic slip (Moreno et al., 2010) and instrumental intensity (USGS Shakemap, 2010; http://earthquake.usgs.gov/earthquakes/ shakemap) for the 2010 Maule event (Fig. 1C). Furthermore, the comparison between the 2010 event suggests shaking intensity can be distributed asymmetrically along a rupture zone, and even peak toward the segment boundaries (Fig. 1C). Hence, we refrain from using an a priori model of segmentation in the selection of core sites. In fact, the nature of seismotectonic segmentation of convergent margins and the persistence of these segments is still an unresolved issue that turbidite paleoseismology, if applied properly, may be capable of addressing (Goldfinger et al., 2007). 2.3. Turbidite deposition and previous turbidite paleoseismology studies offshore Chile Previous turbidite studies offshore Chile (~38e40
S) have recognized a decrease in the frequency of large turbidity current events after the last glaciation induced by climate change when € lker et al., sediment availability decreased (Blumberg et al., 2008; Vo 2008). Moreover, turbidite layers deposited on seamounts suggest the occurrence of very large turbidity currents during the Last €lker et al., 2008). Due to extensive repeated AnGlacial period (Vo dean glaciations, meltwater runoff, and increased precipitation during the Pleistocene, sediment supply to the marine realm was high (Hebbeln et al., 2007), providing sufficient material for gravitational failure, finally resulting in the generation of turbidity currents. In this climatic setting, associated sea level lowstands facilitated sediment transport into deeper sectors of the oceanic basin. Three pioneering attempts have been made to use the turbiditic record offshore Chile as a proxy for earthquake recurrence times during the late Pleistocene and Holocene. Blumberg et al. (2008) calculated a Pleistocene turbidite recurrence time of 100e200 years for earthquakes based on the sequence of turbidite events in two sediment cores recovered from the PerueChile trench in the Valdivia segment. For the first time, these rates elucidate the general patterns of turbidite deposition over glacial to interglacial timescales offshore Chile that are interpreted to be influenced by climatically induced changes in sediment supply, sea level change, and drainage inversion related to onshore tectonic uplift (Blumberg et al., 2008). These authors conclude that only during cold stages sediment availability and slope instability were sufficiently high to record a complete archive of great earthquakes. The use of Pleistocene turbidite recurrence in terms of earthquake recurrence times is ambiguous, however, because the association of turbidite layers with seismic triggers is assumed to be robust and was based

on two sediment cores. To rigorously establish recurring seismicity as a trigger for such turbidite events, synchronous deposition of turbidites in widely spaced and isolated depocenters has to be demonstrated (e.g., Goldfinger, 2011). In another study located on the same seismotectonic segment, Heberer et al. (2010) estimated Holocene turbidite recurrence within a similar range (192e477 years), having high associated errors (±105e346 years), because these intervals are based on the assumption of a mean sedimentation rate obtained from other cores and the thickness of the interbedded hemipelagites. Finally, € lker et al. (2008) related the frequency of outsized turbidite Vo events deposited on seamounts or large slide blocks (Geersen et al., 2011b) up to 300 m high to the recurrence of great Pleistocene earthquakes along the Valdivia segment, with a recurrence interval of some centuries. Taken together, these studies are promising and suggest that turbidites offshore Chile may be used as paleoseismological indicators along the Valdivia segment during the Holocene (Heberer et al., 2010) while deposits in more distal and/or € lker et al., elevated settings may record Pleistocene events (Vo 2006; Blumberg et al., 2008). However, to link such turbidite sequences unambiguously to seismic triggers a large spatial coverage of individual events embedded in a sufficient geochronologic framework has to be established. 3. Data and methods 3.1. Core selection A wealth of gravity cores (110) between several decimeters and 10 m length was collected during the RV Sonne cruises 102, 156 and 161 (Hebbeln and Shipboard Scientists, 1995, 2001; WiedickeHombach and Shipboard Scientific Party, 2002). Gravity cores provide a sediment record that can extend back for several thousand years; however, they do not preserve the water sediment interface and may therefore be unsuitable for the analysis of the most recent turbidite record, because sediment can be lost from core tops. Onboard core descriptions and bathymetric data sets were evaluated to identify turbidite depositional centers along the Chile slope. Trench locations were excluded because of great water depth below the carbonate compensation depth (CCD) (~4500 m; Hebbeln et al., 2000; Heberer et al., 2010) preventing radiocarbon dating (e.g., Patton et al., 2013). Additionally, south-north lateral transport along the trench axis complicates the stratigraphic record €lker et al., 2006). Sediment cores with abundant of the trench (Vo turbidites were chosen in two study areas offshore La Serena from  n between 36 and 38
S (Fig. 1A) 29 to 31
S and offshore Concepcio to test turbidite synchronicity and spatial extent on the same convergent margin, but across a strong precipitation gradient. An intermediate core offshore Valparaíso (Figs. 1 and 2A, C) was selected to document general changes in the sedimentology between the northern and southern study area. Gravity cores are located in separated slope basins that should not experience sediment exchange and are located preferably within the deep and flat part of the basin to represent most of the turbidity current input. Submarine canyons were excluded to minimize the risk of direct terrigenous sediment input by storms and river discharge (e.g., Piper and Normark, 2009; Sumner et al., 2013). The advantages and disadvantages of canyon/channel confluence tests, where the same number of turbidites should be recognized in the upstream canyons and downstream of the confluence if they were deposited by synchronous turbidity currents, were recently summarized and discussed by Atwater et al. (2014) and Talling (2014). The test was not applied in this study due to the morphology of the continental slope of Chile, where canyon confluences are rare and canyon

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mouths and submarine fans are located within the trench and below the CCD, hampering any stratigraphical interpretation of turbidite occurrence. Cores utilized for analysis are summarized in Table 1. 3.2. Study areas, core locations, and depositional environment We have chosen two main study sites and one intermediate location in areas where great historic earthquakes occurred (i.e., 1960, 2010, 1730, and 1943). These study sites are located in three distinct climatic zones and are thus ideal to test the ‘boundary conditions’ for turbidite generation during great and giant earthquakes. The northern study area includes sediment cores from 29 to 31
S (Figs. 1A and 2AeC; Table 1). The area is separated from the ndez Ridge bathymetric high southern study area by the Juan Ferna (Fig. 1A), with up to 3 km of relief 100 km westward of the trench and ~180 m of relief within the trench. The four northern cores (GeoB 7136-2, 7138-2, 3368-2, 3369-1) are located within the rupture zone of the 1943 earthquake and the core between the two study areas (GeoB 3304-5) in the rupture zone of the 1985 and 1906 earthquakes (Fig. 1A, B; Bilek, 2010; and references therein). The great 1730 event ruptured the entire length along the location of all five of these cores (Fig. 1A, B; Udías et al., 2012). Between 28.5 and 33
S, rivers originating in the semi-arid Andes generally cut through the Coastal Cordillera, but modern riverine discharges are low (0.15e1 km3/a; Milliman and Farnsworth, 2011), and mean annual precipitation ranges from ~0.08 to 0.37 m in the catchments (Department of Geophysics of the Universidad de Chile, Santiago de Chile, http://www.atmosfera.cl). The northernmost core GeoB 7136-2 was recovered from the flank of a submarine canyon and downslope of a small gully that may pirate the upper parts of very large turbidity currents exceeding 300 m in thickness (Fig. 2A, B). GeoB 7138-2 is located downslope of two small gullies on a slope plateau (Fig. 2A, B). GeoB 3369-1 was recovered 5.5 km downslope of GeoB 3368-2 on the same slope plateau. These cores are separated from the more northern GeoB 7138-2 core by a gullydissected, eastewest directed slope (Fig. 2B). Core GeoB 3304-5 is located within the large intraslope Valparaíso Basin of ~50 km northesouth width (Fig. 2C; Laursen et al., 2002). To the north, the basin is fed by two submarine canyons, however, GeoB 3304-5 is located within the southern part of the basin, which is separated into two subbasins by the ~50-m-high Aconcagua sediment ridge (Laursen and Normark, 2003). From the southern study area (36e38
S), three sediment cores have been selected (SL 21, SL 113, SL 112; Figs. 1 and 2DeG; Table 1). These are located within the rupture zone of the great 2010 Maule event (Fig. 1A, B; Farías et al., 2010) and at the northern tip of the rupture zone of the giant 1960 earthquake (Fig. 1A, B). The rupture zones of both events (1960 and 2010) overlapped by ~ 90 km. Therefore, seismic events of both rupture zones may be recorded in these sedimentary successions. Climatically, the southern study

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area is located in the humid climate zone, which is characterized by a dense river system of perennial rivers with high present-day riverine discharge (up to 33 km3/a, Biobío River; Milliman and Farnsworth, 2011) and mean annual precipitation rates of approximately 1.11 m (Fig. 1A), about one order of magnitude higher than mean precipitation in the northern study area. Core SL 21 is located in a small slope basin that is fed by a few gullies that incise back to the shelf edge without extending farther coastward (Fig. 2D, E). The basin is enclosed to the north and south, but is drained to the west by a lower-slope gully or small canyon that leads down to a small trench fan (Fig. 2E). Core SL 113 is located within an 18  11 km enclosed slope basin, which is bounded by a downstream topographic high with a minimum elevation of ~40 m (Fig. 2D, F). Upslope, the basin is connected to a few slope gullies and/or small submarine canyons, that do not affect the shelf edge and thus, do not directly connect the small basin to riverine sediment supply. Core SL 112 is located within an enclosed slope basin that is conn Canyon (Fig. 2D, G). The canyon head nected to the paleo-Pellahue is located about 30 km westward of the present-day shoreline at about 200 m water depth. Tectonic uplift in the coast-proximal area n River and between 37 and 39
S forced deflection of the Pellahue may have left this submarine canyon inactive since the Pleistocene/ n Canyon is early Holocene (Rehak et al., 2008). The paleo-Pellahue not connected directly to the slope basin of SL 112, but a smaller canyon has been eroded into the outer side of a canyon meander bend (Fig. 2G), probably pirating flows traveling down the main canyon and effectively funneling them into the SL 112 slope basin. The basin is bounded by an outer morphologic high of up to 250 m (Fig. 2G) that effectively traps turbidity currents. Such a small, restricted slope basin connected to submarine conduits that funnel sediment gravity flows, but do not receive any direct riverine input that would “dilute” the record, may be an ideal location to record seismically triggered turbidites along this margin. 3.3. Turbidite recognition In the northern study area, turbidites can be easily identified visually due to a pronounced color difference between turbidites and hemipelagic clay and were confirmed by magnetic susceptibility measurements (Fig. 3A). In the south, turbidites are mud-rich and there is no pronounced color difference between hemipelagic clay intervals and turbidite mud. Hence, visual identification of turbidites is based on X-ray radiographs (Fig. 3B). X-ray radiographs were taken of 1-cm-thick sediment slices sealed in plastic boxes of 20 cm length and 10 cm width. A Philips 80 KV energy source was used, mounted at a distance of 1.2 m above the sample; the radiographs were fabricated of Agfa D4 Structurix film using an exposure time of 30 s. The film negatives were scanned at high resolution with a standard PC scanner and then color-inverted with a standard graphic program. The resulting X-ray photographs reveal high-resolution images of internal sediment structures.

Table 1 Sediment core locations. Name Northern study area GeoB 7136-2 GeoB 7138-2 GeoB 3368-2 GeoB 3369-1 GeoB 3304-5 Southern study area SL 21 SL 113 SL 112

Latitude 29
43.000 30
07.990 30
21.600 30
21.600 32
53.400

Longitude S S S S S

36
07.130 S 36
58.390 S 38
04.850 S

Water depth (m)

Sediment recovery (m)

Cruise

W W W W W

3188 2733 3238 3457 2411

7.4 6.4 4.6 5.5 9.1

SO SO SO SO SO

73
48.850 W 74
10.000 W 74
29.810 W

2468 3187 4125

9.5 4.8 5.7

SO 161 SO 161 SO 161

72
03.970 71
52.130 71
57.500 72
01.000 72
11.500

156 156 102 102 102

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Fig. 3. Turbidite recognition in an examplary core of A) the northern study area where turbidite can be dentified visually and B) the southern study area, where turbidites are predominiantly muddy and were identified with the help of X-radiographs and magnetic susceptibility measurements. The X-ray radiograph shows a siltyemuddy Td (plane lamination) and Te divisions of the Bouma-Sequence. The magnetic susceptibility curve shows an increase due to increased portion of terrestrial sediment. The base of this muddy turbidite deposited by a dilute low-density turbidity current does not appear to be erosional.

Light-colored sections of the photographs are related to sediment layers with high density and/or coarse sediment components, whereas gray and dark sections depict homogenous hemipelagic mud. Where necessary, turbidite thickness was confirmed by magnetic susceptibility measurements acquired with a GEOTEK Multi-Sensor Core Logger (Fig. 3).

3.4. Radiocarbon dating and age modeling During this study, 61 new radiocarbon ages were acquired based on planktonic foraminifera and 5 radiocarbon ages previously obtained by the Bundesanstalt für Geowissenschaften und Rohstoffe (BGR) in 2004 were recalibrated (Table A1). In the northern study area, where planktonic foraminifera are abundant, monospecific samples of Globigerina bulloides were used for radiocarbon analysis. In the south, where planktonic foraminifera are rare, mixed planktonic foraminifera were used (Table A1). Samples were taken within hemipelagic mud intervals directly below the turbidite base or above the turbiditic mud cap. The choice of the age modeling technique is of utmost importance when synchronous turbidite emplacement over a widespread area is to be demonstrated. In typical marine clastic stratigraphic successions comprised of sand, silt, and mud deposited from turbidity currents intercalated with hemipelagic mud, the stratigraphic record is characterized by very abrupt changes in sedimentation rates from several millimeters per second for turbidite sands (e.g., Sumner et al., 2008) to a few centimeters per 1000 years for hemipelagic material. Additionally, hiatuses may result from non-deposition but are most likely especially abundant at the bases of turbidite layers, where the same turbulent sediment gravity flow first eroded some of the underlying section before deposition.

Algorithms of widely used Bayesian age modeling techniques often introduce a memory value that provides a degree of smoothness along the core avoiding extreme variations in sediment accumulation rates as well as extremely high or low rates (e.g., Blaauw and Christen, 2011). This may be the most suitable approach for radiocarbon chronologies of homogenous sediment successions, e.g., pure hemipelagic clay. In the case of turbidite successions, smoothing of the age model should be avoided because of the expected variations in sedimentation rates and in order to not obscure the presence of hiatuses. We employed a novel probabilistic age-modeling technique that accounts for abrupt changes and extreme variations in sedimentation rates (Trauth, 2014). The algorithm provides independent means of determining the position and temporal length of even small-scale hiatuses by the evaluation of the overlap of two independent ages, one derived from characteristic sedimentation rates for each sediment type, and one derived by the age differences of the radiocarbon ages throughout the section. Additionally, the positioning of hiatuses can be justified by core analysis, e.g., in turbidite-rich cores hiatuses are most likely positioned at the erosive base of a turbidite layer. Appendix A contains the detailed documentation of individual radiocarbon ages, age calibration, marine reservoir correction, treatment of age reversals, the details of age model parameters, and individual age models for each core. To date, this technique (Trauth, 2014) has been tested and applied for age data with Gaussian errors, whereas radiocarbon ages often show more complex probability density functions. After calibration to the MARINE 13 calibration curve (Reimer et al., 2013), the radiocarbon ages used here are sufficiently normally distributed in order to apply the technique (Appendix A, Fig. A2). For simplicity, radiocarbon age differences are reported as 2s ranges, which is permissible due to their close-to-normal distribution

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(Fig. A2). Several hiatuses were detected in the age models for the cores (see Appendix A for details). The existence of additional small-scale hiatuses cannot be excluded following this approach. Some turbidites (e.g., in SL 21, Appendix A) reveal an extended period of deposition from 60 to 100 years, probably representing an overestimation of the deposition time of a single turbidite. These longer-than-expected deposition times may result from a smallscale hiatus at the turbidite base on the order of a few decades. These hiatuses may be present at the base of almost every turbidite, but are generally to short to be detected during age modeling. The non-detection of these small-scale hiatuses shows that the best-fit age model established for any one core is never a 100% true age model (e.g., Telford et al., 2004). Hence, the correlation of possibly synchronous turbidite events needs to be supported by minimum and maximum radiocarbon ages and age errors for the modeled and radiocarbon ages need to be properly documented. 4. Results Turbidite grain size and sedimentology differ substantially between the northern and southern study area. Based on the sedimentologic analysis of the cores, turbidite bases in the north are often erosive and the coarsest grain size reaches up to medium and coarse sand (Fig. 3A). Hemipelagic clay is light brown and turbidite sand, silt, and mud show much darker colors (Fig. 3A). Turbiditic mud caps can be identified clearly in many cases by dark to light brown color grading. In contrast, in the south, turbidite beds consist of silt to clay-sized particles, with almost no intercalated sand (Fig. 3B). Turbidite layers are dark olive-green and show almost no color change when compared to the surrounding hemipelagic material (Fig. 3B). Due to these fine grain sizes and lack in color contrast, turbidites are often difficult to identify visually and X-ray radiographs and physical properties, such as magnetic susceptibility measurements are required (Fig. 3B). The transition from turbiditic to hemipelagic mud is difficult to determine, even on Xray radiographs, especially in cores with very high sedimentation rates. In the following section, the Holocene and late Pleistocene turbidite events of the two study areas are investigated regarding synchronous deposition and spatial extent. Furthermore, they are compared to (1) the historical earthquake record for the specific seismotectonic segments and (2) the late Holocene paleoseismic record from terrestrial archives compiled by Cisternas et al. (2005), Moernaut (2010), Moernaut et al. (2014), and Wallner (2008). In this context the term ‘turbidity current’ refers to a turbulent sediment gravity flow, a ‘turbidite’ is a single deposit of a turbidity current, and a ‘turbidite event’ refers to the widespread occurrence of individual synchronous turbidites that can be correlated between several cores. 4.1. Northern area offshore La Serena and Valparaíso 4.1.1. Historical record None of the analyzed cores contains a turbidite event that is synchronous with any of the major or great earthquakes (M  7) of 1575 AD and later (Fig. 4A, C). Age modeling suggests that several decimeters are missing from the top of core GeoB 7136-2 and, therefore, it cannot be related to the historical record (Fig. 4A). This also might apply to (some of) the other cores as missing core tops often can result from gravity coring, which cannot always be detected with certainty during age modeling (Appendix A). However, as virtually no turbidite deposition has been recorded for the entire Holocene, although Holocene sediments are preserved (see below), it is unlikely that important historical events are missed only due to coring disturbance at the core tops.

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4.1.2. Holocene record Cores GeoB 7138-2 and 3368-2 do not record any turbidite deposition during the entire Holocene (Fig. 4B). A single turbidite event with a maximum age of 4378 ± 256 (2s error) cal BP is present in GeoB 7136-2. Two successive events are recorded in GeoB 3369-1 at 0.67 m and 0.75 m core depth. A radiocarbon age in between the two events reveals a maximum age of 7137 ± 215 cal BP for the 0.67 m event and the minimum age for the underlying 0.75 m event (Fig. 4B). 280 km farther to the south, GeoB 3304-5 also does not record any turbidity current deposition during the Holocene (Fig. 4C, D). Hence, no contemporaneous Holocene turbidite events are recorded in the five cores.

4.1.3. Pleistocene record Numerous late Pleistocene turbidites are recorded in each core of the northern study area. For the first-order identification of possibly synchronous events across the four cores, turbidite beds were plotted against age (Fig. 5). Eleven turbidite events were identified that extend across two or more cores (T1e11, Table 2, Fig. 5). For a more detailed investigation of the turbidite event bed ages and their synchronicity, the modeled turbidite event bed ages and the measured radiocarbon ages were plotted against core depth (Fig. 6). T1 is well constrained in GeoB 7136-2, 7138-2, and 3368-2 by maximum radiocarbon ages measured in the hemipelagic interval right below the turbidite base. T2 is the only turbidite event that can be correlated across all four cores and is well constrained in GeoB 7136-2 and 3369-1 by maximum and minimum radiocarbon ages, however, the modeled ages show large 75% error bounds (Fig. 6). Turbidite layer T2 in GeoB 7138-2 and 3368-2 is less well constrained but the modeled ages and maximum and minimum radiocarbon ages are consistent with the correlation (Fig. 6). Turbidite-event beds T3eT5 are exclusively present in GeoB 7136-2 and 7138-2 (Figs. 5 and 6). T3 is well constrained in both cores, in GeoB 7136-2 by minimum and maximum radiocarbon ages and a modeled age with narrow error bounds and in GeoB 7138-2 by a maximum radiocarbon age. T4 is well constrained in GeoB 7136-2 by its modeled age with narrow error bounds and a minimum radiocarbon age and in GeoB 7138-2 by a minimum radiocarbon age. T5 is constrained in GeoB 7136-2 by a minimum and maximum radiocarbon age, but is poorly constrained only by a modeled age with narrow error bounds and a maximum radiocarbon age that is located 41.5 cm below the turbidite base and a minimum radiocarbon age located 27.5 cm above. Due to poor age constraints, error bounds of the T4 and T5 events partly overlap (Fig. 6). T6A is present in GeoB 7136-2, 7138-2, and 3368-1. The event is well constrained in GeoB 7136-2 by a maximum radiocarbon age and a modeled age with narrow error bounds. In GeoB 7138-2, the T6A turbidite event is constrained by a modeled age with narrow error bounds and a maximum age 12 cm below the base of the turbidite. The event is poorly constrained in GeoB 3369-1 by a modeled age with large error bounds. A second event (T6B) right below the T6A turbidite in Geob 7138-2 can be correlated with GeoB 3369-2, but is poorly constrained in GeoB 3369-2 by a modeled age with large 75% error bounds. T7 turbidite-event beds are present in core GeoB 7136-2, 7138-2, and 3368-2 (Figs. 5 and 6). In GeoB 7136e2, T7 is relatively poorly constrained by maximum and minimum radiocarbon ages, but has a modeled age with narrow error bounds. T7 is relatively well constrained in GeoB 7138-2 by minimum and maximum radiocarbon ages and a narrow error range in the modeled age. The T7 turbidite in GeoB 3368-2 is constrained by a maximum radiocarbon age below the turbidite base (Fig. 6). However, constraints are moderate due to the reversal in mean radiocarbon ages and their overlap within 2s error bounds. These

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Fig. 4. Historical and Holocene turbidite record of the northern study area. Earthquake magnitude classes are defined as strong (M ¼ 6e7), major (M ¼ 7e8), great (M ¼ 8e9), giant (M > 9). (A) Regional record of historical earthquakes in the La Serena region compiled from Comte et al. (1986), Beck et al. (1998), Cisternas et al. (2012), and Lomnitz (2004) compared to the core logs of the northernmost cores. (B) Holocene and Late Pleistocene core record of the northernmost cores. (C) Regional record of historical earthquakes in the Valparaiso seismotectonic segment with magnitude  7 (Comte et al., 1986; Lomnitz, 2004) compared to the core log of GeoB 3304-5. (D) Holocene and Late Pleistocene core record of GeoB 3304-5. (E) Late Pleistocene core record of GeoB 3304-5.

ages were corrected as described in Appendix A and they both represent maximum ages of the T7 turbidite bed. T8 event beds are present in cores GeoB 7136-2, 7138-2, and 3369-1. In GeoB 7136-2, the T8 turbidite is poorly constrained by a

maximum radiocarbon age 64 cm below the turbidite base and a minimum radiocarbon age with large 2s errors. The modeled age of the T8 turbidite in GeoB 7136-2, however, is consistent with the modeled age and maximum radiocarbon age of the well-

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Fig. 5. Simplified core logs plotted versus age including the identification of eleven possibly synchronous turbidite events (indicated in red) that can be correlated over two or more cores. In GeoB 3368-2, ages indicated in green represent original radiocarbon ages, whereas the adjacent gray ages are the corrected radiocarbon ages (see Appendix A, Fig. A1). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

constrained T8 event bed in GeoB 7138-2 (Fig. 5). In core GeoB 3369-1, age modeling suggests a hiatus of 1.2 kyrs (see Appendix A) below its base at 3.335 m of core depth (Fig. 5). Hence, the T8 turbidite is poorly age-constrained by a modeled age with large error bounds and a maximum age below its base. T9 turbidite event beds are present in GeoB 7136-2 and 7138-2 (Fig. 5). The T9 turbidite is well constrained in the two cores by a maximum radiocarbon age below its base and a narrow range modeled age (Fig. 6). T10 turbidite event beds are present in GeoB 7138-2, 3368-2, and 3369-1. GeoB 7136-2 does not cover this age range (Figs. 5 and 6). The T10 turbidite is well constrained in all three cores by a maximum radiocarbon age below its base. T11

turbidite-event beds are present in GeoB 7138-2 and 3369-1 (Fig. 5). Age constraints are rather poor, because the modeled ages show a large spread in their third quartiles, but a minimum radiocarbon age in GeoB 7138-2 and a maximum radiocarbon age in GeoB 3369-1 are consistent with the coeval deposition of T11 (Fig. 6). In summary, many of the detected turbidite layers in the individual cores are well constrained by radiocarbon dates just above or below them (or both). For others, the assigned ages result from the age modeling with varying uncertainties expressed by the width of the error bars in Fig. 6. However, comparing the ages of the individual turbidites in the four cores that are finally based on the age

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Table 2 Turbidite events in the northern study area and their occurrence across cores. Turbidite event

GeoB 7136-2

Core depth (m)

GeoB 7138-2

Core depth (m)

GeoB 3368-2

Core depth (m)

GeoB 3369-1

Core depth (m)

T1 T2 T3 T4 T5 T6 A T6B T7 T8 T9 T10 T11

x x x x x x

0.798 1.36 1.95 2.645 3.07 3.71

0.945 1.535

x

2.3

x

3.12

4.715 5.96 6.38

1.18 1.675 1.865 2.1 2.22 2.53 2.605 2.805 3.2 3.44 4.135 4.5

x x

x x x End of core End of core

x x x x x x x x x x x x

x

2.305 x

3.33

x x

3.985 4.13

modeling allows for a clear correlation of some of them between the cores (Figs. 5 and 6). 4.1.4. Late Pleistocene recurrence intervals of turbidites and turbidite events For the late Pleistocene record in the northern sector, turbidite events, which can be correlated between two or more cores (GeoB 7136-2, 7138-2, 3368-2, and 3369-1), recur within a temporal range between 500 and 3450 years with an average of 1530 ± 1000 years (Fig. 9A). Turbidite events that can be correlated between three or four cores recur between 1425 and 4675 years. Between 26 and 16 kyrs, turbidite events occur more regularly with recurrence times of 1110e1250 yrs for events correlatable between two or more cores. Recurrence times of individual turbidites vary greatly between cores, from 547 yrs (GeoB 7136-2) to 2569 yrs (GeoB 3368-2; Fig. 9A). GeoB 3304-5 is located ~280 km to the south in the adjacent rupture zone of the 1985 earthquake (Fig. 1A: Valparaíso segment; e.g., Beck et al., 1998). However, the late Pleistocene recurrence interval for the deposition of individual turbidite beds is approximately 113 years, averaged from ~12 to 24 kyrs BP, about a magnitude lower than in most northern cores (Figs. 4 and 9A), but well within the recurrence of historical earthquakes. n 36
e38
S 4.2. Southern area offshore Concepcio Gravity cores SL 21, SL 113, and SL 112 yield abundant turbidites and a limited number of radiocarbon ages. However, the age models of SL 113 and SL 112 are each constrained by only two radiocarbon ages (Figs. A24, A27). Additionally, the generally high sedimentation rates in the southern study area vary from north to south with 0.9 m/ka in SL 21 over 2.5 m/ka in SL 113 to as high as 5.6 m/kyrs in SL 112. 4.2.1. Historical record Age modeling suggests a missing core section of 470 years at the top of SL 113 (in total 522 yrs: 470 BP þ 52 yrs until 2002, when the core was recovered) and a missing section equivalent to 160 yrs (in total 212 yrs: 160 BP þ 52 yrs until 2002) at the core top of SL 21, limiting the use of these cores for correlation with further historic events (Fig. 7A, Appendix A). Whereas for the last 400 years three distinct turbidite beds (T1 to T3) are preserved in core SL 112, two turbidite beds stretch over comparable long time periods in core SL 21 (Fig. 7A). Turbidite deposition lasting for 100 yrs (possibly equivalent to T2) or 60 yrs (possibly equivalent to T3) (Fig. 7A) is an artefact resulting from age modeling that is rather poorly constrained for the historic section of both cores as no minimum or maximum radiocarbon ages exist and possible hiatuses at the base of turbidites are not detected.

x

3.28

Consequently, the correlation of T1 to the historic megathrust earthquake of either the 1960 (Mw 9.5; Plafker and Savage, 1970), or the intra-oceanic plate event of 1939 (Ms 7.9; Beck et al., 1998) remains elusive (Figs. 7A and 8). However, because the 1939 intraoceanic plate event affected a much smaller area with a lower magnitude (Ms 7.8), the megathrust event of 1960 (Ms 8.5, Mw 9.5) is more likely to have caused such a widespread turbidite event. Tentatively, the T2 and T3 events in SL 112 and SL 21 might be  n segment linked to the 1570 or 1751 earthquakes in the Concepcio and/or to the 1737 and the 1575 event in the Valdivia segment, respectively (Figs. 7A and 8). However, the wide age assignment of T2 in SL 21 also allows for a correlation with the 1657 event in the  n segment pointing to the fact that T2 in SL 21 and in Concepcio SL112 are not necessarily linked to the same seismic event (Fig. 7A). The correlation of modeled ages for turbidite events (Fig. 9) shows that turbidite events may correlate to some of the reported historical earthquakes without allowing for any clear discrimination due to poorly constrained age models in the three cores and to the potential influence of earthquakes occurring on different segments in a close timely succession (e.g., 1737/1751 and 1837/1835). 4.2.2. Turbidite recurrence times In the southern study area, only five turbidite events (T2eT6) can be correlated across two or more cores (Figs. 7 and 8). These correlations are highly tentative due to poorly constrained age models in SL 113 and SL 112 (Appendix A). Hence, we refrain from establishing recurrence times for these turbidite events. When recurrence times of individual turbidites are compared to the recurrence times of historical earthquakes, cores SL 112 and SL 21 furnish the closest match (Fig. 9B). Due to the short time period recorded in SL 112 and SL 113 turbidite recurrence times older than 2 kyrs rely on one core (SL 21). Recurrence times of individual turbidites in SL 21 averaged over 0e5 kyrs are remarkably similar to earthquake recurrence based on lacustrine turbidites (Moernaut et al., 2014) in the Valdivia segment. Recurrence times are consistently shorter in all three cores, when compared to the Rio Maullín tsunami deposits, the lacustrine seismite record, and the Lago Budi (38.9
S, Fig. 1A) tsunami deposits (Fig. 9B; Cisternas et al., 2005; Moernaut, 2010; Wallner, 2008). 5. Discussion 5.1. Correlation of turbidites with the paleoseismological record Turbidite events in the southern study area may be correlated with paleoseismic events determined from terrestrial archives. These paleoseismic records are located in the Valdivia segment and are based on submerged soils and tsunami deposits in the Rio Maullín estuary related to M > 9, 1960-type events (41.3
S;

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Fig. 6. Comparison of the age constraints of the individual turbidite beds that were identified as part of a correlatable turbidite event in the four cores of the northern study area offshore La Serena. Core logs and core photographs are shown. For a better assessment of the individual age constraints, modeled turbidite ages and their 75% error (3rd quartile) and the measured radiocarbon ages including 2s errors are plotted.

Cisternas et al., 2005), tsunami inundation at Lago Budi (Wallner, 2008), as well as turbidites and other resedimentation events ~ ihue, Villarrica, and Calafque n recorded in lakes Panguipuli, Rin (39e40
S; Moernaut, 2010; Moernaut et al., 2014). As the records of SL 113 and SL 112 span only the past 1000e2000 years, they are compared to historical and paleoseismic records during this specific time span. By spanning most of the Holocene, the turbidites identified in core SL 21 can be compared to available Holocene paleoseismological records, which exist for the Valdivia segment (see compilations by Cisternas et al. (2005), Wallner (2008), Moernaut (2010), and Moernaut et al. (2014)). These correlations have to be taken with caution because the SL 21 core is taken about 100 km north of the 1960 rupture zone. However, high shaking

intensities can exceed the length of the rupture zone by several 100 kms (Fig. 1C). Most of these paleoseismological archives (Figs. 7 and 9) record great to giant earthquakes that may well have triggered a turbidite as far north as at this core location (with the exception of Moernaut et al. (2014) were the threshold is inferred at Mw 7.75). The 1466 AD paleoseismic event documented by Moernaut et al. (2014) may correspond to one contemporaneous turbidite event bed (T4) in SL 21 and in SL 113 (Figs. 7B and 8). Whereas the T4 event bed is well constrained by a maximum radiocarbon age in SL 113, it is only poorly constrained by a modeled age with large error bounds in SL 21. The 1319 paleoseismic event of Moernaut et al. (2014) that is correlative to the 1335 ± 55 AD event of Cisternas

Fig. 7. Historical and Holocene turbidite record of the southern study area. Gray horizontal bars indicate possible correlations between individual turbidites, historical earthquakes, and paleoseismic events determined from terrestrial archives. (A) Regional record of historical earthquakes with magnitude  7 (compiled from Beck et al. (1998); Campos et al. (2002); Cifuentes (1989); Farías et al. (2010); Kelleher (1972); Lomnitz (2004)) compared to the turbidite record of the cores SL 21, SL 113, and SL 112. Historical and paleoseismic events in A and B are labeled with their AD ages. (B) Late Holocene turbidite records compared to the paleoseismic events documented by Cisternas et al. (2005), Moernaut (2010), Moernaut et al. (2014), and Wallner (2008). (C) Holocene core record of SL 21 back to 8 kyrs cal BP compared to paleoseismic events documented by Cisternas et al. (2005) (black symbols, gray error bars), Moernaut (2010) (purple), Moernaut et al. (2014) (red), and Wallner (2008) (green). Some turbidite layers show an extended period of deposition from 60 to 100 years, probably representing an overestimation of the deposition time of a single turbidite probably resulting from small-scale hiatuses at their bases on the order of a few decades (Appendix A). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Fig. 8. Comparison of the age constraints of the individual turbidite beds that were identified as part of a correlatable turbidite event in the three cores of the southern study area n. Core logs and X-ray radiographs are shown. For a better assessment of the individual age constraints, modeled turbidite ages and their 75% error (3rd quartile) offshore Concepcio and the measured radiocarbon ages including 2s errors are plotted. High sedimentation rates lead to poorly constrained age models and hence, T1 to T6 may correlate to some of the reported paleo-earthquakes without allowing for any clear discrimination.

et al. (2005) and the 1340 AD event of Moernaut (2010), seems to correlate to a turbidite layer in SL 21 with no equivalent in SL 112 or SL 113 (Figs. 7B and 8). A possibly age-equivalent T5 turbidite event bed is present in SL 113 and in SL 112 (or could be linked to a time-equivalent hiatus)

with two potentially correlative turbidites in SL 21 (Figs. 7B and 8). The T5 turbidite is moderately well constrained in SL 21 by a maximum radiocarbon age beneath both potential correlative turbidites and well constrained in SL 112 by minimum and maximum radiocarbon ages, and a modeled age with narrow third quartile

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Fig. 9. (A) Recurrence intervals of the individual turbidites and correlative turbidite events of the cores offshore La Serena averaged over the time periods indicated on the y-axis n compared to during the Latest Pleistocene. Cores are color-coded according to latitude. (B) Holocene recurrence intervals of individual turbidites of the cores offshore Concepcio average recurrence times of various paleoseismic records averaged over several time scales indicated on the y-axis. Cores are color-coded according to latitude. Terrestrial paleoseismic records are color-coded according to their sampling period. Note the latitudinal position of the plaeoseismic records. The uplifted beach berms of Bookhagen et al. (2006) are located offshore Concepcion, whereas the paleoseismic records of Wallner (2008), Moernaut (2010), Moernaut et al. (2014), and Cisternas et al. (2005) are located to the south and are attributed to the Valdivia seismotectonic segment.

error bounds in SL 113 (Fig. 8). T5 may correspond in age to the 1127 AD event of Moernaut et al. (2014) and the correlative 1050 ± 30 AD paleoseismic event documented by Cisternas et al. (2005; Figs. 7 and 9B). Older paleoseismic events recorded in the Rio Maullín estuary (Cisternas et al., 2005) are less well ageconstrained and their age ranges partly overlap. Hence, a clear correlation with specific turbidite event beds in our record is hampered (Figs. 7B and 8). The 580 AD paleoseismic event of Moernaut (2010) may correspond to the T6 turbidites in SL 21 and in SL 113 (T6; Figs. 7B and 8). T6 event beds are moderately constrained by a minimum radiocarbon age in SL 21 and poorly constrained by radiocarbon ages in SL 113 (Figs. 7B and 8). These detailed correlations between turbidite layers in three cores and paleoseismic events may be possible, however, due to high sedimentation rates the age constraints, especially in core SL113 and SL 112, are rather weak (see Appendix A). High sedimentation rates prevent sufficient accumulation of planktonic foraminifera in the hemipelagic intervals and, thus, hinder the establishment of a highly resolved geochronology. Correlation of paleoseismic events with turbidite beds that are older than 1600 yrs BP can only be made for turbidites in a single core (SL 21). Hence, widespread turbidite events, likely triggered by seismic events, cannot be established. However, all paleoseismic events determined from various lakes in the Valdivia segment (Moernaut, 2010) have one or more turbiditic counterparts in SL 21 (Fig. 7B), except the 450 AD (450 BC) event. Radiocarbon age control is limited between 1.3 and 3 kyrs cal BP and correlations need to be taken with caution. The 310 AD and 70 AD events are not reflected in distinct coeval turbidites in SL 113. The oldest paleoseismic event at Rio Maullín that is possibly age-equivalent to the 70 AD event of Moernaut (2010) (Fig. 7B) can be correlated within

error bounds with two turbidite beds in SL 21. The 1200 BC of Moernaut (2010) can be correlated with several turbidites and the 1600 BC event can be correlated with a single turbidite in SL 21 that is much better constrained by radiocarbon ages (Fig. 7B, C). Each tsunami deposit from Lake Budi, except the oldest one from 7.31 kyrs cal BP, may be correlated with one or more age-equivalent turbidites in SL 21. However, turbidite frequency is much higher and, therefore, average turbidite recurrence times are about four times lower in SL 21 than the tsunami deposits of Lago Budi (Fig. 9B). This might be a result of the different magnitude thresholds necessary to cause tsunami inundation at Lago Budi and turbidite deposition. The differences in the recurrence times of both archives once again highlight the necessity of areally extensive correlations to establish seismic triggering of individual turbidites in order to rule out those turbidites that were triggered by other processes. Whereas the recurrence rate of individual turbidites in individual cores closely resembles the recurrence rates of historical earthquakes and paleo-events of M  7.75 inferred from lake deposits (Fig. 9B), the limited geochronologic control hampers the reconstruction of an unambiguous record. Due to the high sedimentation rates, the resulting short-term turbidite records and low-resolution geochronology, the crucial correlations cannot be established in the southern study area (Fig. 8). 5.2. Controls on turbidite distribution along the Chile margin 5.2.1. Climatic conditions Increasing Holocene offshore sediment accumulation from north to south is mimicked by processes operating on decadal to ~ oz et al., 2004) and millennial timescales (Hebbeln centennial (Mun

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et al., 2007). This pattern is in good agreement with the estimated Cenozoic sediment volumes in the Chile trench fill, which record increasing accumulation of Cenozoic terrigenous sediments from north to south (Scholl et al., 1970; Thornburg et al., 1990; Bangs and Cande, 1997), with some irregularities due to a combination of € lker et al., 2013). regional factors (Vo Turbidity current activity and frequency along the Chile margin also increases towards the south. The rainfall gradient results in an increase in river network density and higher river transport capacity towards higher latitudes, resulting in more efficient fluvial sediment transport towards the ocean (Rehak et al., 2010). For turbidity currents to recur frequently, the ultimate amount of sediment that reaches the ocean at a river mouth is crucial, even though centennial to millennial erosion rates in the high-relief Andean catchments do not necessarily correlate with southward increasing precipitation, but rather with mean catchment slope (Carretier et al., 2013, 2014). Availability of unconsolidated sediment in the Central Depression for remobilization in the southern study region may further increase sediment supply to the ocean (Fig. 1A). On glacialeinterglacial timescales, the northward displacement of the Southern Westerlies resulted in substantial paleoenvironmental changes in the northern study area. Changes include an s et al., increase in precipitation (e.g., Heusser, 1990; Valero-Garce 2005; Hebbeln et al., 2007) facilitating glacial conditions in the high sectors of the Andes (Haselton et al., 2002), which possibly resulted in increased erosion, denser river networks, higher river transport capacity, and more efficient riverine sediment transport to the ocean. The synergistic interplay of these factors is probably reflected in the higher frequency of turbidite deposition before 16 kyrs BP. 5.2.2. Sediment pathways and depositional systems The geomorphology of a particular turbidite depocenter can impact the sedimentation rates and frequency of turbidites recorded within it. In the northern study area, core GeoB 7136-2 is located in a small basin on the flank of a submarine canyon (Fig. 2B). The top turbidite deposit at 4.378 ± 0.123 kyrs BP in core GeoB 7136e2 may represent overspill deposition from turbidity currents flowing down the adjacent submarine canyon. However, the lack of additional overspill turbidite beds in GeoB 7136-2 does not support an exhaustive Holocene turbidite record even within the canyon. Holocene turbidite deposition, if any, is probably restricted to submarine canyons, and hence, inecanyon cores may be the only possibility to record Holocene turbiditic sedimentation in this region. SL 113 and SL 112 are located in enclosed slope basins (Fig. 2D, F, G), whereas the SL 112 basin is enclosed by higher morphologic bounds (Fig. 2G). Gullies leading into the SL 113 basin reach back to the shelf edge but do not to incise back into the shelf (Fig. 2D, F). In n Canyon connected to the SL 112 slope contrast, the paleo-Pellahue basin is incised into the shelf, but not back to the coastline (Fig. 2D). Hence, the SL 112 slope basin receives higher sediment supply and acts as a more efficient sediment trap, probably accounting for the doubled sedimentation rate in SL 112 and a higher turbidite frequency (Fig. 7) when compared to SL 113. Moreover, the paleon Canyon is not abandoned by onshore river deflection Pellahue (Rehak et al., 2008) but still acts as an active sediment conduit. 5.3. Impact of provenance on turbidite recognition A mineralogical investigation of modern surface sediment along the Chile margin showed that marine clastic sediment between 27 and 30
S is dominated by the plutonic source rocks of the Coastal Range (Lamy et al., 1998) as evidenced by a high content of quartz,

87

amphibole, K-feldspar, and mica. At 33
S, K-feldspar and mica contents decrease while plagioclase and pyroxene amounts increase. The change in mineralogy is interpreted to represent an increasing contribution of Andean igneous rocks as more rivers cut through the Coastal Range, whereas the source rock composition of the Coastal Range does not change significantly (Lamy et al., 1998). In the southern study area (>36
S), marine clastic sediments with terrigenous lithologic components are characterized by volcanic Andean source rocks with very high plagioclase and pyroxene contents and comparatively low quartz contents (Lamy et al., 1998). In the north, coarser grain sizes are supplied to the ocean due to prevailing physical weathering under semi-arid conditions and due to intermediate calc-alkaline mineralogy and coarser-grained primary texture of the plutonic rocks (Lamy et al., 1998) leading to coarser-grained turbidites (Fig. 3A). Volcanic source rocks mainly comprised of andesites and basaltic andesites result in the production of fine-grained sediment (Lamy et al., 1998) producing mud-rich turbidites in the south that are difficult to distinguish from the surrounding hemipelagic mud (Fig. 3B). These provenance changes lead to the described difficulties in turbidite recognition in the south, whereas the wider grain size distribution facilitates turbidite recognition in the northern cores. Thus, the nature of the sediment produced by onshore source rocks and their erodibility plays an important role in constraining the identification of the offshore turbidite record. 5.4. Turbidite paleoseismology along the Chile continental margin e feasible or not? 5.4.1. Holocene 5.4.1.1. Northern study area. Despite several megathrust earthquakes during historic times and six paleoseismic, tsunamigenic events recorded at Quintero Bay (32.5
S) between 6200 and 3600 yrs BP (Dura et al., 2014), virtually no Holocene turbidites were retrieved in cores along the submarine slope between 29 and 33
S from offshore La Serena to Valparaíso. Turbidite paleoseismology in slope basins along this sector of the margin is not feasible. Holocene turbidite activity, if any, may be restricted to submarine canyons. 5.4.1.2. Southern study area. High sediment input ensures that sufficient sediment is available for frequent failure, e.g., during great and giant earthquakes, resulting in frequent Holocene turbidite deposition. However, sediment failure does not occur in every case of seismic shaking as documented by the apparent lack of  n margin after the submarine landslide events along the Concepcio € lker et al., 2011). NeverMaule earthquake in 2010 (Mw 8.8) (Vo theless, the increase of sediment delivery to the south facilitates the creation of a turbidite record that mirrors paleoseismic activity, which may be established by careful correlation of coeval turbidite beds across several isolated slope basins. Conversely, voluminous sediment delivery can hinder paleoseismic record compilation. In the cores recovered from the slope basins south of the Biobío Canyon (SL 113, SL 112), where sedimentation rates are exceptionally high, age modeling suggests that mud intervals, which appear to be hemipelagic as deduced from Xray imagery and magnetic susceptibility, settle at the speed assumed for turbiditic mud (Appendix A). These high sedimentation rates suggest that mostly turbiditic mud accumulated resulting in thick mud caps to the turbidites that were identified in radiographs and magnetic susceptibility measurements. Hence, almost no true hemipelagic material accumulated. In such sedimentary sections, little to no planktonic foraminifera accumulate and radiocarbon dating presents a challenge, even though the coast of  n is a zone of intense coastal upwelling and related high Concepcio

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biogenic productivity (e.g., Djurfeldt, 1989; Daneri et al., 2000; Atkinson, 2002). Radiocarbon dating of individual turbidite layers in these cores is impossible, even with large sample amounts. In the few intervals from which sufficient planktonic foraminifera could be recovered, the likelihood of sampling remobilized material is high. Consequently, dating of remobilized planktonic foraminifera may result in an overestimation of the depositional age and/or in age reversals, such as the one in SL 112 (Appendix A) that will complicate the sparse age information even further. The homogenous fine-grained material supplied results in mud-rich turbidites and severs the recognition of true hemipelagic mud intervals needed for radiocarbon sampling. Hence, the ideal sites for the acquisition of cores suitable for Holocene turbidite paleoseismology may be situated just north of SL 21, where sedimentation rates are slightly lower to allow for the accumulation of sufficient planktonic foraminifera, but are still high enough to reflect high turbidite recurrence times. 5.4.2. Pleistocene 5.4.2.1. Northern study area. Increased precipitation and associated sediment delivery (Hebbeln et al., 2007) and the lowered sea level during the LGM enhance the possibility for the successful compilation of a turbidite paleoseismicity record in the northern study area. Biological productivity and related accumulation rates of planktonic foraminifera were generally higher during the Last Glacial when compared to the Holocene (e.g., Mohtadi et al., 2008), facilitating radiocarbon dating of individual turbidite events during the late Pleistocene. However, the large range in turbidite event recurrence times between 500 and 3450 years suggest that a sizeable part of plate-boundary seismicity is not preserved in the turbidite record. Hence, turbidites cannot be used to constrain seismic cycles in the realm of the northern study area. Even though sediment transfer to the ocean was much more efficient during the late Pleistocene compared to the Holocene (Hebbeln et al., 2007), sediment accumulation may not have been high enough to ensure widespread failure during each high-magnitude seismic event. Preliminary data of a single core farther south, GeoB 3304-5, offshore Valparaíso show that turbidite recurrence times of approximately 113 years are much lower between 12 and 24 kyrs BP and are thus comparable to the recurrence time of historical earthquakes. Sedimentation rates here do not overwhelm foraminifera accumulation and the section can be radiocarbondated by monospecific foraminifera samples (Table A1). 5.4.2.2. Southern study area. To assess the Pleistocene record in the southern slope basins, much longer cores would need to be acquired. However, because sedimentation rates were also much higher during the Pleistocene along this part of the margin €lker et al., 2008), (Hebbeln et al., 2007; Blumberg et al., 2008; Vo radiocarbon dating leading to sufficiently resolved age models that allow for the correlation of individual turbidite events may well be virtually impossible. Hence, the undertaking of compiling a paleoseismic record based on widespread turbidite events in n is unlikely to be Pleistocene slope sections offshore Concepcio successful. 5.5. Most suitable areas for paleoseismic investigations based on turbidites along the Chile margin Our analysis shows that the only seismotectonic segment that may be potentially suitable for the compilation of a marine Holocene paleoseismic record, which could be used to tie in and expand  n seismoterrestrial records, is the northern part of the Concepcio tectonic segment that ruptured during the Maule earthquake in 2010 (e.g., Vargas et al., 2011) between 34 and 36
S. Here, the local

input of the overwhelming sediment supply of the Biobío River wanes. Even though precipitation decreases northward, millennialscale erosion rates in the Andean catchments peak due to steep average hillslopes (Carretier et al., 2013, 2014). Steeper hillslopes in the north are attributed to the fact that elevation and relief along the transient landscape of the western Andean flank between 16 and 41
S decrease with increasing rainfall (Rehak et al., 2010). High local relief between 28 and 35
S has been attributed to glacial erosion during the Quaternary and has not yet been obliterated due to inefficient fluvial drainage networks and inefficient hillslope processes. Lower local relief in the southern regions between 35 and 40
S is due to higher sustained rainfall that diminished mean and maximum elevations (Rehak et al., 2010). Between 34 and 36
S, few smaller rivers (e.g., Maule and Mataquito rivers; Fig. 1A) deliver sediment to the ocean. Farther north, Holocene turbidite deposition is scarce to absent, mostly due to the onshore semi-arid climatic conditions. South of 36
S, sedimentation rates are too high to ensure adequate radiocarbon dating of turbidite events. Pleistocene paleoseismic investigations of turbidite cores may be feasible south of 32
S along the rupture zone of the 1985 earthquake (Comte et al., 1986) and/or the northern tip of the Maule rupture zone, where sediment delivery appears to have been sufficiently high during the Pleistocene to cause frequent sediment failure during high-magnitude seismic events, as indicated in the Pleistocene section of GeoB 3304-5. During the Last Glacial, paleoproductivity peaked at approximately 33
S, facilitating radiocarbon dating (e.g., Mohtadi and Hebbeln, 2004). The northernmost 1500 km of the Chile continental slope (18.5e32
S) are most probably not suitable for paleoseismic analysis using turbidites, because precipitation decreases to 0.5 mm/yr (modern mean annual precipitation in Arica, 18.5
S, www. atmosfera.cl). Hence, sediment supply to the ocean decreases but terrigenous sedimentation by hemipelagic settling still takes place at least to 24
S (Mohtadi and Hebbeln, 2004). The Southern Hemisphere Westerlies shifted northward by 5
during the LGM but never reached these latitudes (Fig. 1A) and the Atacama desert is considered to have been hyperarid at least since the Pliocene (Hartley, 2003; Amundson et al., 2012) and possibly earlier (Rech et al., 2006). Hence, terrestrial sediment input to the Pacific was too low during the Holocene and late Pleistocene to facilitate sediment failure during earthquakes. Along the same lines, sedimentation rates along the 560-km-long continental slope between 37 and 42
S were probably much too high during the Holocene and Pleistocene to generate a turbidite-based paleoseismic record. South of 42
S, coastal morphology changes into a fjorddominated environment, where most sediment is trapped in the € lker et al., 2013). Thus, prerequisites for turbidity fjords (e.g., Vo current generation, deposition, and dating are completely different along the southernmost Chilean coast and beyond the scope of this paper. However, promising attempts have been made in Chilean fjords to assess the feasibility of in-fjord paleoseismic records using turbidites and landslides (St-Onge et al., 2012; Van Daele et al., 2013, 2014). 5.6. General implications for turbidite paleoseismology As recently discussed by Sumner et al. (2013) and Talling (2014), it is crucial to define criteria that help to determine which continental margin settings are most suitable for marine turbidite paleoseismology. These authors rightly point out that it is very important to understand where and when great earthquakes fail to produce widespread turbidite events. Hence, for a more efficient future application of turbidite paleoseismology, not only the successful examples need to be discussed in detail in scientific publications, but also the settings that turn out to be unsuitable for the

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cia et al., 2013; Sumner et al., compilation of such records (e.g., Gra 2013). Qualifying characteristics of marine active margin settings for turbidite paleoseismology can be assessed along the Chile margin, where the location and timing of historical earthquakes are known and paleoseismic records exist for some seismotectonic segments. The comparison of the turbidite record of two study areas characterized by distinct onshore climatic conditions highlights, that for turbidite paleoseismology to be feasible, a sensitive balance between sufficient sediment supply and sufficiently moderate sedimentation rates allowing for radiocarbon dating is required. This can be achieved through the analysis of a few test cores along the length of the margin segment. Further details, including a sediment provenance that allows for sand production and straightforward turbidite vs. hemipelagite recognition as well as a sufficiently dissected slope providing several separate turbidite depocenters, facilitate compilation. When a new area along a seismically active continental margin is chosen for an assessment of paleoseismicity using turbidites, the sediment availability for frequent failure vs. the opportunity of establishing a firm geochronologic record should be evaluated first. Second, the workload in conjunction with practical concerns, such as the datasets required for turbidite identification, can be estimated and the risks and benefits of a costly coring expedition and subsequent laborious analysis of many turbidite cores can be better evaluated. After the initial tests on limited cores for the general feasibility and the ease of a record compilation, thorough comparison to known historical and paleoseismic events from other archives should be established to obtain a better understanding of the test core archive to capture seismically induced turbidites and/ or the probability of turbidite generation through great events. Depending on the data set, these tests may not lead to the same conclusions, even along the same margin (Goldfinger et al., 2012, 2014; Patton et al., 2013; Sumner et al., 2013, 2014; Atwater et al., 2014) and the entirety of the available data has to be regarded. Recently, it has been stressed by Goldfinger et al. (2014) that sample sites for paleoseismic records based on turbidites should be focused towards the center of seismotectonic segments. However, seismic turbidite generation is associated with shaking intensity, which is not necessarily the strongest in the segment center, but can peak even at the margins of a rupture zone and beyond (Fig. 1C; Goto et al., 2013). Furthermore, the persistence of seismotectonic segments over time is not well understood and, therefore, positioning sample sites for paleorecords in the center of a segment is not trivial. In fact, turbidite paleoseismology is supposed to help solving the problem (e.g., Goldfinger et al., 2007), and researchers are running the risk of being trapped in a circular argument. Only after the cautious evaluation of the following factors: (1) sufficient sediment input and reasonable turbidite recurrence times; (2) datability of sedimentary sections; (3) ease of turbidite identification with the tools available; (4) sufficient coring sites according to the developed coring rationale; and (5) frequent turbidite generation during past known earthquakes, the costly and work-intensive endeavor of compiling a paleoseismic turbidite record should be commenced.

contrast, frequent Holocene turbidite deposition that partly may correspond to seismic and paleoseismic events is documented from 36 to 38
S. However, high sedimentation rates complicate radiocarbon dating and, therefore, the thorough and cautious regional correlation of seismogenic turbidite events. During the late Pleistocene, turbidite deposition between 29 and 30
S became more frequent due to enhanced precipitation and erosion, and a possibly more effective river network, but turbidite event recurrence intervals are too high to provide valuable information about seismic cycles. Regions suitable for turbidite paleoseismic records need to be characterized by a balance between high sediment delivery during the time period investigated to ensure frequent sediment failure during great and giant seismic events, and moderate sedimentation rates that allow for the accumulation of planktonic foraminifera for high-resolution radiocarbon dating. The most suitable region for a Holocene marine paleoseismic record at the Chile convergent margin is probably situated between ~34 and 36
S. This region comprises the northern part of the 2010 Maule rupture zone n seismotectonic segment), where sediment input may (Concepcio be high enough to facilitate slope failure during great and giant events but sedimentation rates are not too high to suppress accumulation of material suitable for radiocarbon dating. The ideal area for a late Pleistocene paleoseismic record, and thus the extension of terrestrial paleoseismic records, is located around 32.5
S and areas farther to the south, where Pleistocene turbidites were frequent, but still datable. Hence, potential areas suitable for turbidite paleoseismology are restricted to 32.5e36
S, merely 400 km of a 2500 km long stretch of the Chile coast between 18.5 and 42
S. Consequently, future Holocene paleoseismologic studies relying on turbidites should be focused along the northern part of the segment that ruptured during the 2010 Maule earthquake (Mw 8.8). Studies focused on the late Pleistocene record should be carried out along the Valparaíso segment, the most densely populated area in coastal Chile.

6. Conclusions

Appendix A. Supplementary data

Sediment cores retrieved from sites along the central Chile active margin suggest that the ability for great and giant earthquakes to produce widespread, synchronous turbidity currents largely depends on the climatic conditions on the nearby subaerial regions and the related sediment input into the ocean. Between 29 and 33
S, almost no Holocene turbidite deposition is recorded and, hence, Holocene turbidite paleoseismology is not feasible. In

Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.quascirev.2015.04.001.

Acknowledgments A. Bernhardt was funded by grant BE 5070/1-1 of the German Research Foundation (DFG) and the Potsdam Research Cluster for Georisk Analysis, Environmental Change and Sustainability (PROGRESS) through a grant of the German Federal Ministry for Education and Research (BMBF) to M. R. Strecker. D. Melnick was funded by DFG grant ME 3157/2-2. M. Wiedicke-Hombach (BGR €hl, and V. Lukies (MARUM Hannover) and M. Mohtadi, U. Ro Research Center Bremen) provided invaluable help and guidance during core analysis and sampling. V. Spiess is thanked for providing bathymetric data from RV Sonne Cruise 211. The BMBF provided financial support for RV Sonne cruise 156 and 161 (prorico, E. Boehm, K. jects: PUCK and 03G0161A). We thank L. Ande Hoder, M. Nennewitz, C. Reger, and V. Viert from the University of Potsdam for their hard work and patience during preparation of radiocarbon samples. The editor Henning Bauch and two anonymous reviewers are thanked for their insightful and constructive comments.

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