Arauco Peninsula Research Articles

Marine terraces of the last interglacial period along the Pacific coast of South America (1° N–40° S)

Abstract. Tectonically active coasts are dynamic environments characterized by the presence of multiple marine terraces formed by the combined effects of wave erosion, tectonic uplift, and sea-level oscillations at glacial-cycle timescales. Well-preserved erosional terraces from the last interglacial sea-level highstand are ideal marker horizons for reconstructing past sea-level positions and calculating vertical displacement rates. We carried out an almost continuous mapping of the last interglacial marine terrace along ∼ 5000 km of the western coast of South America between 1∘ N and 40∘ S. We used quantitatively replicable approaches constrained by published terrace-age estimates to ultimately compare elevations and patterns of uplifted terraces with tectonic and climatic parameters in order to evaluate the controlling mechanisms for the formation and preservation of marine terraces and crustal deformation. Uncertainties were estimated on the basis of measurement errors and the distance from referencing points. Overall, our results indicate a median elevation of 30.1 m, which would imply a median uplift rate of 0.22 m kyr−1 averaged over the past ∼ 125 kyr. The patterns of terrace elevation and uplift rate display high-amplitude (∼ 100–200 m) and long-wavelength (∼ 102 km) structures at the Manta Peninsula (Ecuador), the San Juan de Marcona area (central Peru), and the Arauco Peninsula (south-central Chile). Medium-wavelength structures occur at the Mejillones Peninsula and Topocalma in Chile, while short-wavelength (< 10 km) features are for instance located near Los Vilos, Valparaíso, and Carranza, Chile. We interpret the long-wavelength deformation to be controlled by deep-seated processes at the plate interface such as the subduction of major bathymetric anomalies like the Nazca and Carnegie ridges. In contrast, short-wavelength deformation may be primarily controlled by sources in the upper plate such as crustal faulting, which, however, may also be associated with the subduction of topographically less pronounced bathymetric anomalies. Latitudinal differences in climate additionally control the formation and preservation of marine terraces. Based on our synopsis we propose that increasing wave height and tidal range result in enhanced erosion and morphologically well-defined marine terraces in south-central Chile. Our study emphasizes the importance of using systematic measurements and uniform, quantitative methodologies to characterize and correctly interpret marine terraces at regional scales, especially if they are used to unravel the tectonic and climatic forcing mechanisms of their formation. This database is an integral part of the World Atlas of Last Interglacial Shorelines (WALIS), published online at https://doi.org/10.5281/zenodo.4309748 (Freisleben et al., 2020).

On the Origin of Postseismic Deformation Processes in the Region of the Maule, Chile Earthquake of February 27, 2010

Abstract—For the first time, we present a rupture surface model of the 2010 Maule earthquake, Chile, based on the combination of satellite geodesy, InSAR, and satellite gravimetry data. The regularization method used for constructing the model allowed us to find a uniform displacement field on rupture surface provided that a slip rake is close to a given one. On average, the displacements on the rupture surface are about 5 m with a maximum displacement of 13.1 m. The rupture zone extends south of the Arauco Peninsula and reaches a depth of 42 km along the plate surface. Using the constructed seismic rupture model, we have modeled the process of viscoelastic relaxation of stresses that emerged in the lithosphere and upper mantle as a result of the earthquake in order to estimate the contribution of this process in the observed postseismic displacements. Surface displacement velocities mainly depend on the viscosity value adopted for the asthenosphere. The comparison of the calculated and measured displacements at low viscosity of the asthenosphere shows that when the displacements are measured far from rupture surface as it is the case with the ocean–ocean subduction zone earthquakes, the observed displacements can be explained by the process of viscoelastic relaxation with a low viscosity of the asthenosphere. In the cases when there are data on the displacements above rupture surface, e.g., for the 2010 Maule earthquake, explaining the observed displacements by stress relaxation process in the near zone of the rupture is not possible at any viscosity: the displacements substantially differ both in amplitude and direction. At the same time, the postseismic creep models fairly well agree with the entire set of the existing data. Therefore, there is no need to accept the hypothesis of a low-viscous asthenosphere in the region of the 2010 Maule earthquake. Previously, we arrived at the similar conclusion considering the modeling results for the Sumatran earthquake of 2004, Simushir earthquakes of 2006, and a number of other large earthquakes in the subduction zones.

Unexpected coseismic surface uplift at Tirúa-Mocha Island area of south Chile before and during the Mw 8.8 Maule 2010 earthquake: a possible upper plate splay fault

The Tirúa-Mocha Island area (38.2°-38.4° S) in southern Chile has been affected by two megaearthquakes in only 50 years: the 1960 Mw=9.5 Valdivia earthquake and 2010 Mw=8.8 Maule earthquake. We studied in the field the vertical ground movements occurred during the interseismic period between both earthquakes and the coseismic period of 2010 Maule earthquake and 2011 Mw=7.1 Araucanía earthquake. During the 1960 earthquake, vertical coseismic ground movements are typical of subduction related earthquakes with Mocha Island, located close to the trench, experienced bigger ground uplift (150 cm) than that occurred in Tirúa (-20 cm), place located in the continental margin at the latitude of Mocha Island. Then during the 1960-2010 interseismic period, the 1960 coseismic uplift remained at Mocha Island unlike the normal interseismic subsidence that occurred northward at Arauco Peninsula and Santa María Island. Also Tirúa experienced the biggest interseismic uplift (180 cm) in all the area affected later by 2010 Maule earthquake. Then during the 2010 Mw=8.8 Maule earthquake an anomalous vertical coseismic ground uplift occurred in the study area, opposite to that of 1960 since Mocha Island experienced lower (25 cm) ground uplift than Tirúa (90 cm). Subsequently, during the Araucanía 2011 earthquake a ground uplift in Mocha Island (50 cm) and subsidence at Tirúa (20 cm) occurred. These unexpected vertical ground movements can be explained by the existence of an upper plate splay fault located below the sea bottom between Tirúa and Mocha Island: the Tirúa-Mocha splay fault. Considering the last seismic cycle, the activity of this fault would have started after the 1960 Valdivia earthquake. During 2010 Maule earthquake, the main slip occurred at Tirúa Mocha splay fault. Finally during 2011 Araucanía earthquake, the slip occurred mainly at the updip of Wadati-Benioff plane with probable normal activity of Tirúa-Mocha splay fault. Simple elastic dislocation models considering the Wadati-Benioff plane and the Tirúa-Mocha splay fault activity, can account for all the vertical ground movements observed during 1960 earthquake, the 1960-2010 interseismic period, the 2010 Maule earthquake and the 2011 Araucanía earthquake.

The 21 May 1960 Mw 8.1 Concepción Earthquake: A Deep Megathrust Foreshock That Started the 1960 Central-South Chilean Seismic Sequence

AbstractOne of the most notable seismic sequences in modern times was recorded in May 1960 along the southern Chilean subduction zone. The sequence started on 21 May with the Mw 8.1 Concepción earthquake; 33 hr later the Mw 9.5 Valdivia megathrust earthquake occurred, the largest ever recorded in the instrumental period. These events changed the geomorphology of the coast along more than 1000 km, generated extensive structural damage in the main cities of central-south Chile, and triggered a Trans-Pacific tsunami. Observed land-level changes due to both earthquakes were reported in 1970. These observations were ascribed to both events but have been used to study only the general source properties of the 22 May Valdivia mainshock. Here, we separate these data to constrain for the first time the slip distribution of the 21 May Concepción earthquake, applying a Bayesian approach that considers uncertainties in the data. Our results show that the Mw 8.1 Concepción earthquake ruptured a deep segment of the megathrust, concentrated in a compact zone below the Arauco peninsula between depths of 20 and 50 km. Tsunami generation from this deep source agrees well with the tsunami arrival times and small amplitudes recorded by tide gauges along the Chilean coast. Our study highlights the importance of the 21 May 1960 Concepción earthquake in the context of large historical Chilean earthquakes.

Segmentation in continental forearcs: Links between large-scale overriding plate structure and seismogenic behavior associated with the 2010 Mw 8.8 Maule, Chile earthquake

Subduction along the active margin of a continental plate occurs in a context where the overriding plate's crust and lithospheric mantle may contain inherited structures significantly predating the present tectonic conditions of the margin. These structures are persistent over very long-term time scales (>105 to >106 years) and are thought to play an important role in both seismogenic processes on the megathrust and development of topography along coastlines. We use receiver functions calculated from broadband seismic data collected along the Chilean forearc between ~33°S and 38.5°S in the vicinity of the 2010 Mw 8.8 Maule earthquake to determine the structure of the overriding South American continental plate and subducting Nazca oceanic plate along and inboard of the seismogenic portion of the megathrust. We show that the Chilean forearc is divided into three structurally distinct zones: a northern zone where the continental crust intersects the subducting plate well inboard of the coast at ~35–40 km depth, a central zone where the continental crust tapers to <20 km thickness at the coast before intersecting the subducting plate at 15–20 km depth, and a southern zone corresponding approximately to the rapidly uplifting Arauco Peninsula where we find that the continental crust forms a wedge-shaped feature ~35–40 km in thickness which intersects the subducting plate well inboard of the coast. The thin crust of the central zone is associated with a dense, high velocity mantle body (the Cobquecura anomaly) that may have played an important role in stabilizing this segment of the Chilean forearc since the late Paleozoic.

Landslides induced by the 2010 Chile megathrust earthquake: a comprehensive inventory and correlations with geological and seismic factors

The 2010 Mw = 8.8 Maule earthquake, which occurred in the subduction contact between the Nazca and the South American tectonic plates off the coast of Chile, represents an important opportunity to improve understanding of the distribution and controls for the generation of landslides triggered by large megathrust earthquakes in subduction zones. This paper provides the analysis of the comprehensive landslide inventory for the Maule earthquake between 32.5° S and 38.5° S. In total, 1226 landslides were mapped over a total area of c. 120,500 km2, dominantly disrupted slides. The total landslide volume is c. 10.6 Mm3. The events are unevenly distributed in the study area, the majority of landslides located in the Principal Andean Cordillera and a very constrained region near the coast on the Arauco Peninsula, forming landslide clusters. Statistical analysis of our database suggests that relief and lithology are the main geological factors controlling coseismic landslides, whilst the seismic factor with higher correlation with landslide occurrence is the ratio between peak horizontal and peak vertical ground accelerations. The results and comparison with other seismic events elsewhere suggest that the number of landslides generated by megathrust earthquakes is lower than events triggered by shallow crustal earthquakes by at least one or two orders of magnitude, which is very important to consider in future seismic landslide hazard analysis.

Subduction zone slip variability during the last millennium, south-central Chile

The Arauco Peninsula (37°-38°S) in south-central Chile has been proposed as a possible barrier to the along-strike propagation of megathrust ruptures, separating historical earthquakes to the south (1960 AD 1837, 1737, and 1575) and north (2010 AD, 1835, 1751, 1657, and 1570) of the peninsula. However, the 2010 (Mw 8.8) earthquake propagated into the Arauco Peninsula, re-rupturing part of the megathrust that had ruptured only 50 years earlier during the largest subduction zone earthquake in the instrumental record (Mw 9.5). To better understand long-term slip variability in the Arauco Peninsula region, we analyzed four coastal sedimentary sections from two sites (Tirúa, 38.3°S and Quidico, 38.1°S) located within the overlap of the 2010 and 1960 ruptures to reconstruct a ∼600-year record of coseismic land-level change and tsunami inundation. Stratigraphic, lithologic, and diatom results show variable coseismic land-level change coincident with tsunami inundation of the Tirúa and Quidico marshes that is consistent with regional historical accounts of coseismic subsidence during earthquakes along the Valdivia portion of the subduction zone (1960 AD and 1575) and coseismic uplift during earthquakes along the Maule portion of the subduction zone (2010 AD, 1835, 1751). In addition, we document variable coseismic land-level change associated with three new prehistoric earthquakes and accompanying tsunamis in 1470–1570 AD, 1425–1455, and 270–410. The mixed record of coseismic subsidence and uplift that we document illustrates the variability of down-dip and lateral slip distribution at the overlap of the 2010 and 1960 ruptures, showing that ruptures have repeatedly propagated into, but not through the Arauco Peninsula and suggesting the area has persisted as a long-term impediment to slip through at least seven of the last megathrust earthquakes (∼600 years).

Megathrust friction determined from mechanical analysis of the forearc in the Maule earthquake area

The seismogenic potential of a given fault depends essentially on its frictional properties and on the mechanical properties of the medium. Determining the spatio-temporal variations of frictional properties is therefore a key issue in seismotectonics. This study aims to characterize the friction on the South America megathrust in the 2010 Mw 8.8 Maule earthquake area from mechanical analysis of the forearc structure and morphology. Based on the critical taper theory, we first show that the rupture area of the Maule earthquake, also shown to be locked in the interseismic period, coincides with the stable part of the wedge. In the surrounding area, the wedge is critical, a finding consistent with various evidence for active deformation there. This is in particular true for the Arauco Peninsula area which seems to have arrested the Maule earthquakeʼs rupture to the South. This observation lends support to the view that the seismic rupture is inhibited when propagating beneath a critical area. The geometry of the critical portion of the wedge suggests a standard internal friction (μint=0.7±0.13) and a hydrostatic pore pressure within the wedge. The effective friction beneath the critical outer wedge is estimated to be μbeff=0.27±0.03. This could be related to intrinsically low friction minerals (clay) or high pore pressure along the megathrust. We next use the limit analysis approach to constrain the variation of the effective friction along the megathrust based on the location and geometry of internal faulting within the forearc. A low effective friction is found within the rupture area (μbeff⩽0.14) to explain the reactivation of thrust fault such as the Santa Maria, updip of the coseismic rupture, or the activation of normal splay faults downdip of the rupture area. The low effective friction found there could reflect strong dynamic weakening.

Coseismic and postseismic slip associated with the 2010 Maule Earthquake, Chile: Characterizing the Arauco Peninsula barrier effect

AbstractObservations of coseismic and postseismic deformation associated with the 2010 Mw = 8.8 Maule earthquake in south‐central Chile provide constraints on the spatial heterogeneities of frictional properties on a major subduction megathrust and how they have influenced the seismic rupture and postseismic effects. We find that the bulk of coseismic slip occurs within a single elongated patch approximately 460 km long and 100 km wide between the depths of 15 and 40 km. We infer three major patches of afterslip: one extends northward along strike and downdip of the major coseismic patch between 40 and 60 km depth; the other two bound the northern and southern ends of the coseismic patch. The southern patch offshore of the Arauco Peninsula is the only place showing resolvable afterslip shallower than 20 km depth. Estimated slip potency associated with postseismic slip in the 1.3 years following the earthquake amounts to 20–30% of that generated coseismically. Our estimates of the megathrust frictional properties show that the Arauco Peninsula area has positive but relatively low (a−b)σn values (0.01 ~ 0.22 MPa), that would have allowed dynamic rupture propagation into this rate‐strengthening area and afterslip. Given the only modestly rate‐strengthening megathrust friction in this region, the barrier effect may be attributed to its relatively large size of the rate‐strengthening patch. Coseismic and postseismic uplift of the Arauco Peninsula exceeds interseismic subsidence since the time of the last major earthquake in 1835, suggesting that coseismic and postseismic deformation has resulted in some permanent strain in the forearc.

Miocene foraminifera from the south-central coast of Chile

Foraminifera are abundant and diverse in the Neogene sedimentary units of south-central Chile (34–43ºS), but their age and depositional paleoenvironment have been points of contention for decades. The marine strata most often referred to the Navidad Formation have been a focal point of study in the region by geoscientists in academia as well as the Chilean oil industry. Most of the foraminiferal assemblages documented in this study are from three sedimentary basins isolated from each other by more than 400km. From north to south, they are (1) the Navidad Formation in the vicinity of the name-bearing town located southwest of Santiago, (2) the Ranquil Formation on the Arauco Peninsula southeast of Concepcíon, and (3) the Lacui Formation on and near Chiloé Island. Also included in this study are two assemblages from deposits of uncertain affinity in San Sebastián (northeast of the Navidad area and just north of San Antonio), and one from the Santo Domingo Formation in the vicinity of Valdivia (about midway between Arauco and Chiloé Island). The 27 localities reported in this study span nearly 1000km of the south-central Chilean margin. Most of the samples are siltstones from coastal bluffs and wave-cut platforms in which bedding is nearly horizontal, but often massive or indistinct. Regional workers have disagreed on which subepochs these units represent. Suggested ages based on macro- and microfossils have ranged from Late Oligocene to Early Pliocene. The planktic foraminifera identified in this study suggest that most, if not all, are Miocene. Excluding the two San Sebastián assemblages, which are slightly younger, one assemblage devoid of planktic foraminifera, and three with only long-ranging planktic foraminifera, all of the remaining 21 concurrent ranges begin or end in the Early Miocene. Nineteen of those ranges are restricted to that subepoch and two range into the early Middle Miocene. Seventeen of 18 localities had an 87Sr/86Sr age ranging into or within the Early Miocene. The exception yielded an isotopic age within the latest Oligocene. The majority of the analytical data presented in this report suggest that most of the assemblages are of late Early Miocene (Burdigalian) age. The most abundant macrofossils in the Navidad, Ranquil, Lacui, and Santo Domingo formations (collectively and informally referred to in this study as the Navidad group) are gastropods typical of neritic habitats, but specimens tend to be widely scattered in the outcrops and they are associated with mixed-depth assemblages of foraminifera. I therefore concluded that there had been significant downslope displacement. The upper depth limits of the deepest-dwelling benthic foraminifera in each assemblage indicate that final deposition occurred at bathyal depths, well below the neritic zone. All of the 358 taxa (336 benthics + 22 planktics) identified in this study are presented systematically and illustrated. New species described are Astacolus novambiguus, Cornuspira libella, Cristellariopsis petersonae, Fissurina ambicarinata, Globocassidulina chileensis, Karreriella biglobata, Percultazonaria encinasi, Percultazonaria obliquispinata, Pseudolingulina nielseni, and Pseudononion ranquilensis. In addition, I propose one new (substitute) name, Lenticulina neopolita, to resolve a synonymy. The presence of Neogene cosmopolitan deep-water benthic foraminifera in all of the assemblages supports the hypothesis that deep water masses derived from the Antarctic Circumpolar Current have enabled many Neogene deep-water foraminifera to disperse widely in the global ocean. This compendium is a useful guide to identification of Neogene foraminifera in subtropical/temperate regions in and beyond south-central Chile, and it also provides a regional dataset that can be incorporated into future studies on benthic foraminiferal biostratigraphy and biogeography.

Outer rise seismicity related to the Maule, Chile 2010 mega-thrust earthquake and hydration of the incoming oceanic lithosphere

Most of the recent published geodetic models of the 2010 Maule, Chile mega-thrust earthquake (Mw=8.8) show a pronounced slip maximum of 15-20 m offshore Iloca (~35°S), indicating that co-seismic slip was largest north of the epicenter of the earthquake rupture area. A secondary slip maximum 8-10 m appears south of the epicenter west of the Arauco Peninsula. During the first weeks following the main shock and seaward of the main slip maximum, an outer rise seismic cluster of >450 events, mainly extensional, with magnitudes Mw=4-6 was formed. In contrast, the outer rise located seaward of the secondary slip maximum presents little seismicity. This observation suggests that outer rise seismicity following the Maule earthquake is strongly correlated with the heterogeneous coseismic slip distribution of the main megathrust event. In particular, the formation of the outer-rise seismic cluster in the north, which spatially correlates with the main maximum slip, is likely linked to strong extensional stresses transfered from the large slip of the subducting oceanic plate. In addition, high resolution bathymetric data reveals that bending-related faulting is more intense seaward of the main maximum slip, where well developed extensional faults strike parallel to the trench axis. Also published seismic constraints reveal reduced P-wave velocities in the uppermost mantle at the trench-outer rise region (7.5-7.8 km/s), which suggest serpentinization of the uppermost mantle. Seawater percolation up to mantle depths is likely driven by bending related-faulting at the outer rise. Water percolation into the upper mantle is expected to be more efficient during the co-seismic and early post-seismic periods of large megathrust earthquakes when intense extensional faulting of the oceanic lithosphere facilitates water infiltration seaward of the trench.

Slip Distribution and Seismic Moment of the 2010 and 1960 Chilean Earthquakes Inferred from Tsunami Waveforms and Coastal Geodetic Data

The slip distribution and seismic moment of the 2010 and 1960 Chilean earthquakes were estimated from tsunami and coastal geodetic data. These two earthquakes generated transoceanic tsunamis, and the waveforms were recorded around the Pacific Ocean. In addition, coseismic coastal uplift and subsidence were measured around the source areas. For the 27 February 2010 Maule earthquake, inversion of the tsunami waveforms recorded at nearby coastal tide gauge and Deep Ocean Assessment and Reporting of Tsunamis (DART) stations combined with coastal geodetic data suggest two asperities: a northern one beneath the coast of Constitucion and a southern one around the Arauco Peninsula. The total fault length is approximately 400 km with seismic moment of 1.7 × 1022 Nm (Mw 8.8). The offshore DART tsunami waveforms require fault slips beneath the coasts, but the exact locations are better estimated by coastal geodetic data. The 22 May 1960 earthquake produced very large, ~30 m, slip off Valdivia. Joint inversion of tsunami waveforms, at tide gauge stations in South America, with coastal geodetic and leveling data shows total fault length of ~800 km and seismic moment of 7.2 × 1022 Nm (Mw 9.2). The seismic moment estimated from tsunami or joint inversion is similar to previous estimates from geodetic data, but much smaller than the results from seismic data analysis.

A new Pliocene mollusk fauna from Mejillones, northern Chile

A new Pliocene (3.4 Ma) mollusk fauna from Mejillones Peninsula, northern Chile is described and compared with the Pliocene La Cueva fauna of little constrained age from central Chile and some species from the Huenteguapi Sandstone overlying the Ranquil Formation on Arauco Peninsula, south central Chile. Preliminary correlation is based on faunal similarities. A total of 45 taxa were identified, of which Cyclocardia kieli sp. nov. is new to science. New combinations are Macron escalonia (Vermeij and DeVries, 1997), Austrofusus steinmanni (Moricke, 1896) and Leukoma antiqua (King, 1832). For several species, the oldest occurrences and range extensions are reported. Co-occurrence of warm water taxa, previously assigned to MIS 11, and typical Pliocene taxa on Mejillones cannot be confirmed. Pliocene and Pleistocene mollusk faunas from Mejillones are listed for comparison.

Coseismic Tectonic Surface Deformation during the 2010 Maule, Chile, Mw8.8 Earthquake

Tectonic deformation from the 2010 Maule (Chile) Mw8.8 earthquake included both uplift and subsidence along about 470 km of the central Chilean coast. In the south, deformation included as much as 3 m of uplift of the Arauco Peninsula, which produced emergent marine platforms and affected harbor infrastructure. In the central part of the deformation zone, north of Constitución, coastal subsidence drowned supratidal floodplains and caused extensive shoreline modification. In the north, coastal areas experienced either slight uplift or no detected change in land level. Also, river-channel deposition and decreased gradients suggest tectonic subsidence may have occurred in inland areas. The overall north-south pattern of 2010 coastal uplift and subsidence is similar to the average crestal elevation of the Coast Range between latitudes 33°S and 40°S. This similarity implies that the topography of the Coast Range may reflect long-term permanent strain accrued incrementally over many earthquake cycles.

Control of forearc density structure on megathrust shear strength along the Chilean subduction zone

Unveiling how the interaction of friction ( μ), pore pressure ( p) and normal stress ( σ n) controls the shear strength distribution along megathrust faults at convergent plate boundaries is crucial to our understanding of processes leading to the occurrence of devastating earthquake and the construction of mountains. An existing density model of the Chilean margin (18°–45°S) allows the computation of the so-called Vertical Stress Anomaly (VSA) that accounts for the component of normal stress due to the weight of the overlying crustal column. A spatial correlation between VSA, surface geology and several earthquake parameters suggests a significant control exerted by the geologically-dominated density structure of the crust on megathrust shear strength. The very seismogenic forearc in front of the high Central Andes (18°–34°S) has positive VSA values (up to 60 MPa) and a mafic-dominated crust, which suggest high values of shear strength at the megathrust that are partially controlled by the weight of a dense forearc crustal column, in addition to a likely dry subduction channel characterized by high- μ and low- p. In contrast, the Southern Andean forearc (34°–45°S) shows neutral-to-negative VSA (down to −30 MPa) correlated with a felsic-dominated crust, which added to a presumably wet subduction channel (low- μ and high-p) configures a rather weak megathrust below a light forearc. Earthquakes here are concentrated around the Arauco peninsula, a first-order morpho-structural anomaly characterized by comparatively higher values of VSA than the rest of this segment, which is the nest of the giant (Mw 9.5) Valdivia 1960 earthquake and the southern rupture termination of the recent Mw 8.8 Maule earthquake. The ultimately giant size of the 1960 earthquake resulted from rupture propagation along a 1000 km long region characterized by negative VSA values. The correlation between VSA and the along-strike segmentation of the Andean orogen indicates that the geologically-inherited density structure of the forearc also rules long-term mechanical coupling between converging plates and prescribes the possible structure of the orogen. Because Central and Southern Andes represent two extremes in terms of orogenic structure and seismogenic behavior, results presented here have significant implications for the general comprehension of processes occurring at short and long time scale at convergent margins.

Forearc uplift rates deduced from sediment cores of two coastal lakes in south-central Chile

The present paper introduces a new approach for deriving information about local forearc tectonics and related uplift rates based on the study of lake sediments. We investigated two coastal lakes at the south-central Chile margin, lakes Lanalhue and Lleu Lleu, located south of the Arauco Peninsula (38°S). Both lakes developed within the valleys of ancient rivers that once drained to the Pacific Ocean, being subsequently dammed by rising sills in the late Pleistocene/early Holocene. Seismic profiling and sedimentological analyses of cores from both lakes revealed similar successions consisting of marine to brackish sediments covered by lacustrine deposits. Radiocarbon dating indicates that the marine–lacustrine transition occurred at 8 ka and 8.15 ka, respectively. The correlation of this transition with global sea-level curves allowed the calculation of local uplift rates for the Holocene. Uplift rates for the lake basins amount to 0.42 ± 0.71 and 0.49 ± 0.44 mm/a, respectively, which are consistent with rates determined from a late Pleistocene marine terrace in the area. However, conspicuously higher, though likely transient, vertical movements at 8.83 ± 2.7 mm/a and 11.36 ±1.77 mm/a, respectively, were calculated for the sills that block both lakes nowadays. These barriers are interpreted to be the surface expression of a blind reverse fault associated to the Morguilla fault system, a seismically-active structure that controls uplift and folding along the adjacent Arauco Peninsula.

Segmentation of megathrust rupture zones from fore‐arc deformation patterns over hundreds to millions of years, Arauco peninsula, Chile

This work explores the control of fore‐arc structure on segmentation of megathrust earthquake ruptures using coastal geomorphic markers. The Arauco‐Nahuelbuta region at the south‐central Chile margin constitutes an anomalous fore‐arc sector in terms of topography, geology, and exhumation, located within the overlap between the Concepción and Valdivia megathrust segments. This boundary, however, is only based on ∼500 years of historical records. We integrate deformed marine terraces dated by cosmogenic nuclides, syntectonic sediments, published fission track data, seismic reflection profiles, and microseismicity to analyze this earthquake boundary over 102–106years. Rapid exhumation of Nahuelbuta's dome‐like core started at 4 ± 1.2 Ma, coeval with inversion of the adjacent Arauco basin resulting in emergence of the Arauco peninsula. Here, similarities between topography, spatiotemporal trends in fission track ages, Pliocene‐Pleistocene growth strata, and folded marine terraces suggest that margin‐parallel shortening has dominated since Pliocene time. This shortening likely results from translation of a fore‐arc sliver or microplate, decoupled from South America by an intra‐arc strike‐slip fault. Microplate collision against a buttress leads to localized uplift at Arauco accrued by deep‐seated reverse faults, as well as incipient oroclinal bending. The extent of the Valdivia segment, which ruptured last in 1960 with anMw9.5 event, equals the inferred microplate. We propose that mechanical homogeneity of the fore‐arc microplate delimits the Valdivia segment and that a marked discontinuity in the continental basement at Arauco acts as an inhomogeneous barrier controlling nucleation and propagation of 1960‐type ruptures. As microplate‐related deformation occurs since the Pliocene, we propose that this earthquake boundary and the extent of the Valdivia segment are spatially stable seismotectonic features at million year scale.

Examining dispersal mechanisms for the translocation of chicken ( Gallus gallus) from Polynesia to South America

DNA sequencing and radiocarbon dates from the archaeological site of El Arenal-1 (Arauco Peninsula, south central Chile) suggest that Polynesians may have introduced chicken ( Gallus gallus) to South America prehistorically (A.D. 1300–1420). The Chilean specimen identified has the exact same amplified mtDNA pattern as samples recovered from archaeological sites in Samoa and Tonga, but is only one base different from prehistoric chicken samples from Tonga ( n = 1), Niue ( n = 1), Easter Island ( n = 1 of 6), and Hawai'i ( n = 1). To examine the possible dispersal mechanisms of chicken between Polynesian islands and the Americas, we conducted computer simulations of seafaring from these six island groups and the Tuamotus, as well as return voyages. Simulation results from sailed vessels suggest that in certain months, landfall on the Pacific Coast of South America from Samoa and Tonga was fairly successful at around 40% (particularly in March for the former and January/February for the latter). For Niue, sailed vessels were 21% successful in January with only 2% success in December. Sailed vessels from the Tuamotus (a proxy for closer island groups to the west with evidence for chicken such as the Cook Islands) were successful in January and December, with 15% and 23% of the vessels making landfall in South America. Vessels sailing from all simulations heading east from Easter Island resulted in landfall. Hawai'i had only a 1% success rate if sailing in January, but high numbers of vessels reached southern Mexico and Central America in August and September. Most landfalls occurred much farther north than Chile, in Ecuador and Peru. Unsuccessful voyages from all of the islands other than Easter Island and Hawai'i often made landfall on islands to the east. Return voyages from Chile were not possible or extremely rare during most times of the year. Overall, seafaring simulations suggest that Polynesians may not have initially landed in close proximity to El Arenal-1—contemporaneous or even earlier archaeological evidence of translocated chicken remains may indeed be found further north. In addition, if Polynesians found it difficult or impossible to return to their homeland, the possibility exists that co-mingling between Polynesians and South American groups resulted in offspring that carried an admixture of genetic traits.

Upper lithospheric structure of the subduction zone offshore of southern Arauco peninsula, Chile, at ∼38°S

A joint interpretation of swath bathymetric, seismic refraction, wide‐angle reflection, and multichannel seismic data was used to derive a detailed tomographic image of the Nazca–South America subduction zone system offshore southern Arauco peninsula, Chile at ∼38°S. Here, the trench basin is filled with up to 2.2 km of sediments, and the Mocha Fracture Zone (FZ) is obliquely subducting underneath the South American plate. The velocity model derived from the tomographic inversion consists of a ∼7‐km‐thick oceanic crust and shows P wave velocities typical for mature fast spreading crust in the seaward section of the profile, with uppermost mantle velocities &gt;8.4 km s−1. In the trench–outer rise area, the top of incoming oceanic plate is pervasively fractured and likely hydrated as shown by extensional faults, horst‐and‐graben structures, and a reduction of both crustal and mantle velocities. These slow velocities are interpreted in terms of extensional bending‐related faulting leading to fracturing and hydration in the upper part of the oceanic lithosphere. The incoming Mocha FZ coincides with an area of even slower velocities and thinning of the oceanic crust (10–15% thinning), suggesting that the incoming fracture zone may enhance the flux of chemically bound water into the subduction zone. Slow mantle velocities occur down to a maximum depth of 6–8 km into the upper mantle, where mantle temperatures are estimated to be 400–430°C. In the overriding plate, the tomographic model reveals two prominent velocity transition zones characterized by steep lateral velocity gradients, resulting in a seismic segmentation of the marine fore arc. The margin is composed of three main domains: (1) a ∼20 km wide frontal prism below the continental slope with Vp ≤ 3.5 km s−1, (2) a ∼50 km area with Vp = 4.5–5.5 km s−1, interpreted as a paleoaccretionary complex, and (3) the seaward edge of the Paleozoic continental framework with Vp ≥ 6.0 km s−1. Frontal prism velocities are noticeably lower than those found in the northern erosional Chile margin, confirming recent accretionary processes in south central Chile.

Radiocarbon and DNA evidence for a pre-Columbian introduction of Polynesian chickens to Chile

Two issues long debated among Pacific and American prehistorians are (i) whether there was a pre-Columbian introduction of chicken (Gallus gallus) to the Americas and (ii) whether Polynesian contact with South America might be identified archaeologically, through the recovery of remains of unquestionable Polynesian origin. We present a radiocarbon date and an ancient DNA sequence from a single chicken bone recovered from the archaeological site of El Arenal-1, on the Arauco Peninsula, Chile. These results not only provide firm evidence for the pre-Columbian introduction of chickens to the Americas, but strongly suggest that it was a Polynesian introduction.