Chilean Forearc Research Articles

Tracing the orogenic sulfur cycle in the Andes using stable isotope composition of dissolved sulfate in thermal springs

The cycling of sulfur (S) to the upper crust and surface via thermal springs at convergent margins has not been explored outside areas with active arc volcanism, even though subduction plays a key role in the Earth's long-term S cycle. To address this knowledge gap, we analyzed stable sulfur and oxygen isotope compositions (δ34S and δ18O values) of dissolved sulfate (SO42−) in 55 thermal springs from five distinct settings in the Andean orogen. These regions are the Peruvian flat slab and backarc, transition between these two, Argentinian backarc, and Chilean forearc. Although the flat-slab settings had lower SO42− concentrations (<2000 mg/L) compared to the steep-slab settings (<12,700 mg/L), there was no significant relationship between isotope composition of SO42− and slab geometry. The δ34S and δ18O values of SO42− varied widely across the studied areas (+0.2 to +23.5 ‰ and − 3.3 to +16.0 ‰, respectively) and reflected the isotope compositions of local bedrock endmembers from dissolution of marine evaporites (+5 to +25 ‰ and + 10 to +20 ‰, respectively) and oxidation of magmatic and/or hydrothermal S and ore sulfide minerals with variable δ34S (0 to +16 ‰). The δ18O and δ2H values of thermal spring water (−18.5 to −3.3 ‰ and − 141.1 to −23.7 ‰, respectively) were consistent with meteoric precipitation, and in most cases decreased with increasing altitude following precipitation in the Andes. Generally, our isotope results do not support the direct transfer of slab-derived S/SO42− to thermal springs in the investigated settings. Rather, the δ34S and δ18O of SO42− in the thermal springs are a sensitive indicator of local water-rock interactions that remobilize bedrock S originating from a complex orogenic cycle reflecting tectonic uplift, erosion, weathering, and exhumation history across the duration of Andean Mountain building.

The Northern Chile forearc constrained by 15 years of permanent seismic monitoring

In this review article, we compile seismological observations from the different constituent parts of the Northern Chile forearc: the downgoing Nazca Plate, the plate interface, the upper South American Plate as well as the mantle wedge beneath it. As Northern Chile has been monitored by a network of permanent seismic stations since late 2006, there is a wealth of observations that enables us to characterize the structure as well as ongoing processes in the forearc throughout the last 15 years. We put an emphasis on the analysis of seismicity, for which we have extended a massive earthquake catalog that now contains >180,000 events for the years 2007–2021. Moreover, we draw on published results for earthquake mechanisms, source properties, seismic velocity structure, statistical seismology and others, and discuss them in context of results from neighboring disciplines. We thus attempt to provide a comprehensive overview on the seismological knowledge about the structure and ongoing processes in the Northern Chile forearc, a breviary of which is found in the following: The Northern Chile megathrust hosted two major earthquake sequences during the analyzed time period. The 2007 Mw 7.8 Tocopilla earthquake broke the deep part of the megathrust just north of Mejillones Peninsula, whereas the 2014 Mw 8.1 Iquique earthquake ruptured the central segment in the north of the study region. The latter event has a highly interesting preparatory phase, including a significant foreshock sequence as well as aseismic slip transients. Besides these large events, background seismicity elsewhere on the megathrust may be helpful for characterizing the earthquake potential and locking state in the remaining seismic gap. The downgoing Nazca Plate in Northern Chile exhibits very high seismicity rates, with the vast majority of earthquakes occurring at depths of ∼80-140 km with downdip extensive mechanisms. While seismic tomography shows no sudden changes in slab geometry along strike, seismicity describes peculiar offsets that may be linked to subducted features on the oceanic plate. Upper plate seismicity likewise shows strong variations along strike, with the north and south of the study area showing only weak activity, whereas the central segment shows pervasive microseismicity throughout the upper plate, all the way to the plate interface. These earthquakes have thrust and strike-slip mechanisms with P-axes striking roughly N-S, indicating margin-parallel compression that may be connected to the concavity of the margin.

Anomalous intraslab structure revealed by the analysis of aftershocks of the Mw 6.7 Coquimbo-La Serena earthquake of 20 January 2019

Seismograms of about 12,000 aftershocks of the Mw6.7 Coquimbo-La Serena earthquake of 20 January 2019 that were recorded during six months by an array of 33 short-period seismic stations are used to generate wavespeed images of the subduction wedge beneath the Chilean forearc between 28.0°-30.6°S. This part of the margin is near the northern terminus of the 2015 Mw8.4 Illapel earthquake and the southern terminus of the 1922 Mw8.5 Copiapo earthquake. The distribution of hypocenters and wavespeeds at the slab interface and within the subduction wedge, are like those determined by Comte et al. (2019) in the Illapel aftershock region, and we infer that the same processes of subduction erosion and re-accretion inferred by them are active here. Moreover, anomalous wavespeeds are found in the immediate vicinity of a dense cluster of aftershocks located in a well-defined double seismic zone, within the subducting Nazca plate. Vp/Vs ratios in the upper seismic zone are uniformly high (∼1.9), those in most of the lower seismic zone are average to low, and those in the intraslab region of the aftershock cluster are exceptionally low (∼1.6). These values suggest dehydration of the slab interior and hydration of the mantle near the slab interface. While the origin of this feature is enigmatic, its limited spatial extent suggests that it most likely is due to an inherited structure, such a seamount or petit-spot volcano, in the Nazca plate, rather than a feature that developed in situ. The high rate of activity coupled with the low wavespeeds suggests a region of local weakness that could eventually evolve into a tear or hole in the slab, and its location at the bounds of two megathrust earthquakes suggests that it plays a significant role in delimiting the rupture of such events.

Automated Earthquake Detection and Local Travel Time Tomography in the South‐Central Andes (32–35°S): Implications for Regional Tectonics

AbstractIn the South‐Central Andes, the crustal structures driving the tectonic evolution of the Andean Cordillera remain unresolved. So far, most seismological studies focused on the subduction interface, leaving crustal seismicity and its relationship with crustal deformation and Andean volcanism mostly unconstrained. However, because of their large number compared to higher magnitude events, the characterization of small‐magnitude crustal earthquakes is key to identify active structures and better constrain the tectonic models. In this work, we exploit 53 months of continuously recorded, three‐component waveforms from the permanent seismic network in central Chile using a deep‐learning approach to improve the detection of small‐magnitude earthquakes. To increase station coverage, we also use the seismic phases obtained from a previous temporary seismic deployment. We use the obtained seismicity catalog to refine tomographic models of that region, revealing a more detailed architecture of the Chilean forearc. Travel times calculated in the new 3‐D velocity model allowed us to locate ∼14,000 earthquakes. Refined double‐difference relocations of ∼4,900 events located beneath the West Andean Thrust suggest a large‐scale, west‐dipping structure which, together with the west‐verging tectonic front, likely contributed to the uplift and crustal deformation during the past ∼20 Myr.

Fore-to-retroarc crustal structure of the north Patagonian margin: How is shortening distributed in Andean-type orogens?

Orogenic belts like the Andes experience several changes during long-term evolution of the subduction margin; however, the degree to which their different components (the fore-to-retroarc domains) are jointly or separately deformed is not well understood. To investigate this problem, we provide new field and seismic data of the Chilean forearc at the Chiloé island latitude (~41-44°S) integrated with the retroarc structure, in order to obtain a clear visualization of the crustal architecture in the north Patagonian margin. Field work in the western Andean flank and Coastal Cordillera along with 2D multichannel lines, basement isobaths and borehole data in the forearc basins are used to build a structural framework and stratigraphic correlations between the offshore and onshore areas. Forearc syntectonic strata associated with basement faults show a change in the tectonic regime from late Oligocene-early Miocene extension to middle-late Miocene contraction, coincidently to the previously determined deformational stages in the main cordillera and retroarc regions. This coupled fore-to-retroarc behavior ceased during the Pliocene when glaciations covered the North Patagonian Andes, providing abundant sedimentary supply to the forearc and promoting the growth of the accretionary wedge. During this period the retroarc fold-thrust belt growth stagnated and a strike-slip regime was activated in the western Andean flank. Different mechanisms controlled the forearc basins during these late Cenozoic changing tectonic conditions; whilst the external depocenter at the oceanic platform is part of a west-vergent wedge affected by a major splay-like thrust beneath the Coastal Cordillera, the internal depocenter next to the main orogen is overthrusted by the steep western Andean slope through a major west-directed fault. We refer to this latter structural system as the “Western Patagonian Thrust” and find that positive tectonic inversion is the main deformation mechanism. These west-directed crustal faults are part of the orogenic prowedge that is associated with considerably less crustal shortening (~4.2 km) than the retroarc fold-thrust belt (~18 km), part of the retrowedge. By placing these values in a global context through a comparison with other segments of the Andes and with type-examples of collisional orogens, we find that shortening along the Andes is preferentially accommodated in the retrowedge, while shortening in collisional orogens is mainly absorbed in the prowedge. These results have implications for models of subduction orogeny and the crustal architecture of the Andes.

The role of interplate locking on the seismic reactivation of upper plate faults on the subduction margin of northern Chile

Quaternary deformation in the northern Chile forearc is controlled by trench parallel shortening along reactivated Mesozoic faults. Dextral strikes-slip is expressed in NW–SE striking faults of the Atacama Fault System, and reverse displacement dominates in E–W faults. This deformation results of the convergence in a concave-seaward continental margin. On September 11th, 2020, a Mw 6.3 earthquake and its subsequent aftershocks took place in the coastal region of northern Chile, revealing the reactivation of the deepest segment of a WNW–ESE striking upper plate fault. The reactivation of this fault occurred after the Mw 8.1 Iquique earthquake, and it seems to be connected to a N–S interplate locking segmentation of the plate margin, which is clearly shown by the locking pattern before the Iquique earthquake. This poses the question of how heterogeneous locking influences upper plate seismicity and how it relates to trench-parallel shortening.

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.

Seismicity Structure of the Northern Chile Forearc From &gt;100,000 Double‐Difference Relocated Hypocenters

AbstractIn this study, we present high‐resolution seismicity images of the northern Chile subduction zone forearc. We used 8 years of continuous seismic waveform data from the Integrated Plate Boundary Observatory Chile network and auxiliary stations to produce an extensive earthquake catalog containing 101,601 double‐difference relocated earthquake hypocenters using automatic event detection and phase picking routines. The minimum magnitude of retrieved events is &lt;2, and the catalog is estimated to be complete at magnitudes above ∼2.8. Intraslab seismicity makes up the majority of detected earthquakes. Where the seismogenic zone of the megathrust is active, a clear separation of seismicity into three distinct planes can be observed. The uppermost plane corresponds to the plate interface, which is observed to terminate downdip at a depth of 50–55 km. The other two planes, located ∼7 and ∼26 km below the slab surface, dip at a constant angle of about 20° until they are absorbed by a 25 km thick highly active cluster of intermediate‐depth seismicity at depths of 80–120 km. Downdip of this cluster, the slab steepens and lower plate seismicity is considerably sparser, even absent in the northern part of the study area. Upper plate seismicity is also considerable, with a segment between 21 and 21.6°S standing out for featuring pervasive activity occurring all the way down to the plate interface. Here the seismicity resembles a wedge in west‐east profile view and occurs where the upper plate crust is coldest based on thermal models.

Coseismic extension from surface cracks reopened by the 2014 Pisagua, northern Chile, earthquake sequence

The A.D. 2014 Pisagua earthquake sequence reactivated ancient surface cracks along the entire rupture length in the northern Chilean forearc. These subtle brittle strain features that are ∼50 km above the subduction zone interface in the hyperarid Atacama Desert record deformation from the single earthquake sequence. In this study we document how ancient cracks, formed during thousands of plate boundary earthquake cycles, were reopened during the 2014 earthquake sequence. We show that crack orientations along the rupture length reflect deformation from the M w 8.1 mainshock and from an M w 7.7 aftershock 100 km to the south, as documented by displacements calculated from continuous geodetic observations. We suggest that cracks form during the passage of surface waves, and repeated opening and closing enhance crack aperture. The orientation and opening of the oldest cracks in the forearc are indicative of the modal or most common rupture area of major megathrust earthquakes in the region. While the long-term preservation of cracks may be limited to northern Chile, similar features likely form during strong earthquakes at other subduction zones and represent permanent forearc deformation.

Paleomagnetic rotation pattern of the southern Chile fore‐arc sliver (38°S–42°S): A new tool to evaluate plate locking along subduction zones

AbstractThe Chile fore arc at 37°S–47°S represents the coseismic deformation zone of the 1960 Mw 9.5 Valdivia earthquake. Here we report on the paleomagnetism of 43 Oligocene‐Pleistocene volcanic sites from the fore‐arc sliver between 38°S and 42°S. Sites were gathered west of the 1000 km long Liquiñe‐Ofqui dextral fault zone (LOFZ) that represents the eastern fore‐arc sliver boundary. Nineteen reliable sites reveal that the fore arc is characterized by counterclockwise (CCW) rotations of variable magnitude, except at 40°S–41°S, where ultrafast (&gt;50°/Myr) clockwise (CW) rotations occur within a 30 km wide zone adjacent to the LOFZ. CCW rotation variability (even at close sites) and rapidity (&gt;10°/Myr) suggest that the observed block rotation pattern is related to NW‐SE seismically active sinistral faults crosscutting the whole fore arc. According to previously published data, CW rotations up to 170° also occur east of the LOFZ and have been related to ongoing LOFZ shear. We suggest that the occurrence and width of the eastern fore‐arc sliver undergoing CW rotations is a function of plate coupling along the subduction zone interface. Zones of high coupling enhance stress normal to the LOFZ, induce high LOFZ strength, and yield a wide deformation zone characterized by CW rotations. Conversely, low coupling imply a weak LOFZ, a lack of CW rotations, and a fore arc entirely dominated by CCW rotations related to sinistral fault kinematics. Our locking inferences are in good agreement with those recently derived by GPS analysis and indicate that seismotectonic segment coupling has remained virtually unchanged during the last 5 Ma.

Investigating multiple fault rupture at the Salar del Carmen segment of the Atacama Fault System (northern Chile): Fault scarp morphology and knickpoint analysis

This study presents a new geomorphological approach to investigate the past activity and potential seismic hazard of upper crustal faults at the Salar del Carmen segment of the Atacama Fault System in the northern Chile forearc. Our contribution is based on the analysis of a large set of topographic profiles and allows extrapolating fault analysis from a few selected locations to distances of kilometers along strike of the fault. We detected subtle changes in the fault scarp geometry which may represent the number of paleoearthquakes experienced by the structure and extracted the cumulative and last incremental displacement along strike of the investigated scarps. We also tested the potential of knickpoints in channels crossing the fault scarps as markers for repeated fault rupture and proxies for seismic displacement. The number of paleoearthquakes derived from our analysis is 2–3, well in agreement with recent paleoseismological investigations, which suggest 2–3 earthquakes (Mw = 6.5–6.7) at the studied segments. Knickpoints record the number of events for about 55% of the analyzed profile pairs. Only few knickpoints represent the full seismic displacement, while most retain only a fraction of the displacement. The along-strike displacement distributions suggest fault growth from the center toward the tips and linkage of individual ruptures. Our approach also improves the estimation of paleomagnitudes in case of multiple fault rupture by allowing to quantify the last increment of displacement separately. Paleomagnitudes calculated from total segment length and the last increment of displacement (Mw = 6.5–7.1) are in agreement with paleoseismological results.

Understanding kinematics of intra-arc transcurrent deformation: Paleomagnetic evidence from the Liquiñe-Ofqui fault zone (Chile, 38-41°S)

The Liquine-Ofqui fault zone (LOFZ) is a major ~1000 km long dextral shear zone of southern Chile, likely related to strain partitioning of Nazca Plate oblique convergence with South America. To understand block rotation pattern along the LOFZ, we paleomagnetically sampled 55 sites (553 samples) between 38°S and 41°S. We gathered Oligocene to Pleistocene volcanics and Miocene granites at a maximum distance of 20 km from the LOFZ, and at both sides of it. Rotations with respect to South America, evaluated for 36 successful sites, show that crust around the LOFZ is fragmented in small blocks, ~1 to 10 km in size. While some blocks (at both fault edges) undergo very large 150°–170° rotations, others do not rotate, even adjacent to fault walls. We infer that rotations affected equidimensional blocks, while elongated crust slivers were translated subparallel to the LOFZ, without rotating. Rotation pattern across the LOFZ is markedly asymmetric. East of the fault and adjacent to it, rotations are up to 150°–170° clockwise, and fade out ~10 km east of fault. These data support a quasi-continuous crust kinematics, characterized by small rigid blocks drag by the underlying ductile crust flow, and imply 120 km of total fault offset. Conversely, crust west of the LOFZ is cut by seismically active NW-SE sinistral antithetic faults, and yields counterclockwise rotations up to 170° at 8–10 km from LOFZ, besides the unrotated blocks. Further data from the Chile fore arc are needed to understand block rotation kinematics and plate dynamics west of the LOFZ.

Primera datación radiométrica (U-Pb, LA-ICP-MS, en circones detríticos) de la Formación Punta Topocalma: observaciones sobre la sedimentación marina durante el Cretácico Tardío en Chile central.

Upper Cretaceous marine rocks crop out along the Pacific coast of central and south-central Chile between 33° and 37°S. These strata constitute an important reference for the Upper Cretaceous of South America due to their diverse fossil fauna and flora. The type unit of these deposits is the Quiriquina Formation, near Concepcion. This unit is considered Maastrichtian in age based on ammonites. Upper Cretaceous marine strata from other localities of central and south-central Chile are largely unstudied and their biostratigraphic ages are not precisely known. We present the first radiometric dating (U-Pb on detrital zircons) for Upper Cretaceous marine strata of the Chilean forearc at Punta Topocalma that indicates a probable depositional age of 71.9+0.9 Ma (latest Campanian-earliest Maastrichtian). Provenance analysis indicates that the source of sediments of the Punta Topocalma Formation was plutonic and volcano-sedimentary rocks from the Coastal Cordillera and the Central Depression of central Chile. The Lo Valle Formation, a volcano-sedimentary unit in the Central Depression, recorded deposition of the Upper Cretaceous volcanic arc that was coeval with marine sedimentation in the Topocalma area.

Selected contributions from the 11th Gas in Marine Sediments International Conference of September 2012, Nice: an introduction

This Introduction presents an overview of selected contributions from the 11th Gas in Marine Sediments International Conference held on the 4–7 September 2012 in Nice, France, and published in this special issue of Geo-Marine Letters under the guest editorship of Catherine Pierre, Patrice Imbert and Jean Mascle. These cover fluid seepage dynamics at widely varying spatiotemporal scales in a giant buried caldera of the Caspian Sea, mud volcanoes and pockmarks in the Mediterranean and adjoining Gulf of Cadiz, as well as Lake Baikal, pockmarks of shallower waters along the Atlantic French coast and in Baltic Sea lagoons, deepwater pockmarks and cold seeps on the Norwegian margin and the Hikurangi Margin of New Zealand, asphalt seepage sites offshore southern California, and the tectonically controlled southern Chile forearc. We look forward to meeting all again at the 12th Gas in Marine Sediments conference scheduled for 1–6 September 2014 in Taipei, Taiwan.

Heat flow in the southern Chile forearc controlled by large-scale tectonic processes

Between 33°S and 47°S, the southern Chile forearc is affected by the subduction of the aseismic Juan Fernandez Ridge, several major oceanic fracture zones on the subducting Nazca Plate, the active Chile Ridge spreading centre, and the underthrusting Antarctic Plate. The heat flow through the forearc was estimated using the depth of the bottom simulating reflector obtained from a comprehensive database of reflection seismic profiles. On the upper and middle continental slope along the whole forearc, heat flow is about 30–60 mW m–2, a range of values common for the continental basement and overlying slope sediments. The actively deforming accretionary wedge on the lower slope, however, in places shows heat flow reaching about 90 mW m–2. This indicates that advecting pore fluids from deeper in the subduction zone may transport a substantial part of the heat there. The large size of the anomalies suggests that fluid advection and outflow at the seafloor is overall diffuse, rather than being restricted to individual fault structures or mud volcanoes and mud mounds. One large area with very high heat flow is associated with a major tectonic feature. Thus, above the subducting Chile Ridge at 46°S, values of up to 280 mW m–2 indicate that the overriding South American Plate is effectively heated by subjacent zero-age oceanic plate material.

Late Miocene to Early Pliocene paleohydrology and landscape evolution of Northern Chile, 19° to 20° S

Northern Chile's forearc lies in the hyperarid Atacama Desert. The northern portion (~150km) of the forearc basin, known as the Central Depression, contains perennial streams that drain to the Pacific Ocean through canyons that are deeply incised within a pediplain. However, during the Late Miocene and Early Pliocene a lake existed in the western part of the Central Depression between 19.3°S and 19.7°S, indicating a forearc geomorphic landscape and paleohydrologic regime distinct from the present one. For approximately 2Ma, this lake was large (~375km2) and deep (up to ~120m). The catchment area sourcing the lake covered an area of 2900–5200km2. New 40Ar/39Ar ages of volcanic ashes intercalated with the lake deposits and the overlying fluvial deposits indicate that lacustrine diatomite-rich deposition had begun by 10.86±0.04Ma, and the final stage of incision of a canyon through the lake deposits is younger than 3.04±0.03Ma. Additional published age constraints demonstrate that the expanse of the lake shrunk shortly after 9Ma, yet the water balance in the western sector of the basin permitted lacustrine diatomite deposition until more recently than ~6.4Ma. Even after the sediment–hydrology balance shifted from balanced-fill to overfilled, a near-surface groundwater table led to accumulation of salt pan (salar) evaporites until more recently than ~3.5Ma. The existence of a large deep lake is consistent with, but not conclusive evidence for, a wetter than modern climate in the catchment region during the Late Miocene. The ultimate demise of the paleolake system is attributed to a kilometer-scale decline of the base level of drainages downstream of the paleolake. This study takes advantage of the unique characteristic of the drainage profiles in the study region, a spatial separation between areas of deep incision east and west of a prominent knickzone, to evaluate three end member scenarios of landscape evolution that could explain the base level fall event(s). The sedimentological, stratigraphic, and geomorphological data best fit the interpretation that forearc tectonic uplift and tectonically driven coastal retreat both contributed to base level fall.

The 2010 Mw 8.8 Maule, Chile earthquake: Nucleation and rupture propagation controlled by a subducted topographic high

Knowledge of seismic properties in an earthquake rupture zone is essential for understanding the factors controlling rupture dynamics. We use data from aftershocks following the Maule earthquake to derive a three‐dimensional seismic velocity model of the central Chile forearc. At 36°S, we find a highvp (&gt;7.0 km/s) and high vp/vs(∼1.89) anomaly lying along the megathrust at 25 km depth, which coincides with a strong forearc Bouguer gravity signal. We interpret this as a subducted topographic high, possibly a former seamount on the Nazca slab. The Maule earthquake nucleated at the anomaly's updip boundary; yet high co‐seismic slip occurred where the megathrust is overlain by lower seismic velocities. Sparse aftershock seismicity occurs within this structure, suggesting that it disrupts normal interface seismogenesis. These findings imply that subducted structures can be conducive to the nucleation of large megathrust earthquakes, even if they subsequently hinder co‐seismic slip and aftershock activity.

Comment to “Nature and tectonic significance of co-seismic structures associated with the Mw 8,8 Maule earthquake, central-southern Chile forearc” from

Comment to “Nature and tectonic significance of co-seismic structures associated with the Mw 8,8 Maule earthquake, central-southern Chile forearc” from

Nature and tectonic significance of co-seismic structures associated with the Mw 8.8 Maule earthquake, central-southern Chile forearc

The Mw 8.8 Maule earthquake on February 27, 2010 affected the central-southern Chilean forearc of the Central Andes. Here we show the results of field investigations of surface deformation associated with this major earthquake. Observations were carried out within three weeks after the seismic event, mostly in the central and northern part of the forearc overlying the rupture zone. We provide a detailed field record of co-seismic surface deformation and examine its implications on active Andean tectonics. Surface rupture consisted primarily of extensional cracks, push-up structures, fissures with minor lateral displacements and a few but impressive extensional geometries similar to those observed in analogical modeling of rift systems. A major group of NW-WNW striking fractures representing co-seismic extensional deformation is found at all localities. These appear to be spatially correlated to long-lived basement fault zones. The NW-striking normal focal mechanism of the Mw 6.9 aftershock occurred on March 11 demonstrates that the basement faults were reactivated by the Mw 8.8 Maule earthquake. The co-seismic surface ruptures show patterns of distributed deformation similar to those observed in mapped basement-involved structures. We propose that co-seismic reactivation of basement structures play a fundamental role in stress release in the upper plate during large subduction earthquakes. The fundamental mechanism that promotes stress relaxation is largely driven by elastic rebound of the upper plate located right above the main rupture zone.

Tectonic control on sediment accretion and subduction off south central Chile: Implications for coseismic rupture processes of the 1960 and 2010 megathrust earthquakes

Based on a compilation of published and new seismic refraction and multichannel seismic reflection data along the south central Chile margin (33°–46°S), we study the processes of sediment accretion and subduction and their implications on megathrust seismicity. In terms of the frontal accretionary prism (FAP) size, the marine south central Chile fore arc can be divided in two main segments: (1) the Maule segment (south of the Juan Fernandez Ridge and north of the Mocha block) characterized by a relative large FAP (20–40 km wide) and (2) the Chiloe segment (south of the Mocha block and north of the Nazca-Antarctic-South America plates junction) characterized by a small FAP (≤10 km wide). In addition, the Maule and Chiloe segments correlate with a thin (<1 km thick) and thick (∼1.5 km thick) subduction channel, respectively. The Mocha block lies between ∼37.5° and 40°S and is configured by the Chile trench, Mocha and Valdivia fracture zones. This region separates young (0–25 Ma) oceanic lithosphere in the south from old (30–35 Ma) oceanic lithosphere in the north, and it represents a fundamental tectonic boundary separating two different styles of sediment accretion and subduction, respectively. A process responsible for this segmentation could be related to differences in initial angles of subduction which in turn depend on the amplitude of the down-deflected oceanic lithosphere under trench sediment loading. On the other hand, a small FAP along the Chiloe segment is coincident with the rupture area of the trans-Pacific tsunamigenic 1960 earthquake (Mw = 9.5), while a relatively large FAP along the Maule segment is coincident with the rupture area of the 2010 earthquake (Mw = 8.8). Differences in earthquake and tsunami magnitudes between these events can be explained in terms of the FAP size along the Chiloe and Maule segments that control the location of the updip limit of the seismogenic zone. The rupture area of the 1960 event also correlates with a thick subduction channel (Chiloe segment) that may provide enough smoothness at the subduction interface allowing long lateral earthquake rupture propagation.