Liquine-Ofqui Research Articles

Co- and postseismic subaquatic evidence for prehistoric fault activity near Coyhaique, Aysén Region, Chile

Abstract. Chilean Patagonia is confronted with several geohazards due to its tectonic setting, i.e., the presence of a subduction zone and numerous fault zones, e.g., the Liquiñe-Ofqui Fault Zone (LOFZ). This region has therefore been the subject of numerous paleoseismological studies. However, this study reveals that the seismic hazard is not limited to these large tectonic structures by identifying past fault activity near Coyhaique in Aysén Region. Mass-wasting deposits in Lago Pollux, a lake located ca. 15 km SW of this region's capital, were identified through analysis of reflection-seismic data and were linked to a simultaneous event recorded in nearby Lago Castor. Furthermore, a coeval ∼50-year-long catchment response was identified in Aysén Fjord based on the multiproxy analysis of a portion of a sediment core. Assuming that this widely recognized event was triggered by an earthquake, ground-motion modeling was applied to derive the most likely magnitude and source fault. The model showed that an earthquake rupture along a local fault, in the vicinity of Lago Pollux and Lago Castor, with a magnitude of 5.6–6.8, is the most likely scenario.

A comparative machine learning approach to identify landslide triggering factors in northern Chilean Patagonia

Worldwide landslides correspond to one of the most dangerous geological events due to their destructive power and unpredictable nature. In the Chilean Patagonia, the SERNAGEOMIN (Chilean Geological Survey Service) has detected 2533 landslide events in the Northern Patagonia (42. 7∘S, 72. 4∘W) alone, a small area compared to the whole Chilean Patagonia. However, only 11 evens have known date. Consequently, it is not possible to associate temporal triggers and mechanisms that control such events, resulting in a lack of understanding of the factors that enable landslides. This work aims to detect landslides and identify the main environmental variables (climatic and geomorphological) that explain their occurrence using machine learning methods. We will address the following research questions: 1) How can a temporal landslide dataset be built using Landsat images and Google Earth Engine in Northern Patagonia? 2) Once the landslides and their timing have been detected, what are the main variables that condition the landslide processes? In our work, we developed a temporal dataset of landslides for the northern Patagonia of Chile. We used three machine learning approaches, where it was possible to identify the main environmental variables that allow us to predict their generation. Statistical models show that during the last 19 years, there has been complex interaction between different environmental variables that have influenced the activity of landslides. Climatic indices, indicators of extreme events, have a high incidence in the events’ predictive capacity. However, the most important are those linked to Patagonia’s tectonic context. In particular, the time elapsed after the eruptive event of the Chaiten volcano. Finally, the Liquine-Ofqui Fault System’s presence extending throughout the entire north of Patagonia has generated discontinuities at a general level, causing significant geomorphological instability. The study area has a relief in evolution and reactive to climatic conditions. Therefore, we highlight the need to understand better the interaction between geological and climatic processes and the future impact of these natural hazards. We emphasize the importance of analyzing landslide controls, considering both geomorphological variables and sporadic geological events.

Orogen‐Parallel Transition From a Decoupled Fore‐Arc Sliver to Andean‐Type Mountain Chain: Paleomagnetic and Geologic Evidence From Southern Chile (37–39°S)

AbstractThe Chiloé fore‐arc sliver is an approximately N‐S elongated crust block detached from South America along the dextral intra‐arc Liquiñe‐Ofqui fault zone (LOFZ). The sliver is internally dissected by active NW‐SE sinistral faults whose relations with the LOFZ are speculative, also due to widespread fluvioglacial and volcanic blanket hiding the substratum. We focus on the northern LOFZ end and on the Biobio fault, supposedly the northernmost of the sinistral fault set, reporting on the results from field investigation and paleomagnetism of 48 (mostly Oligo‐Miocene) volcanic sites. We find that the Biobio fault is an old inherited crust discontinuity that did not yield significant block rotation and deformation during the Cenozoic, thus testifying the end of sinistral shear at about 38°S. At the same latitudes, a northward transition from pure strike‐slip to transpressive LOFZ deformation occurs. Intense tectonic deformation and >90° clockwise rotations characterize the main LOFZ strand. Conversely, a supposedly western LOFZ strand displays counterclockwise rotations, similar to the pattern previously documented in the forearc; thus, it does not represent a LOFZ segment. LOFZ and sinistral fault kinematics must be related, and we suggest that crust pushed northward west of the LOFZ escapes laterally toward the trench along the sinistral faults. We also speculate that the northward increasing age of the subducting Nazca plate implies a concomitant decrease of heat transfer on the upper plate, thus an increasing crust rigidity that eventually inhibits strain partitioning and sliver decoupling from 38°S.

Digital Rock Approach to Model the Permeability in an Artificially Heated and Fractured Granodiorite from the Liquiñe Geothermal System (39°S)

The Southern Volcanic Zone of the Andes has a high potential in terms of geothermal resources and is an exceptional and poorly explored natural laboratory to study the interplay between tectonic stresses, thermal damage, low-permeable crystalline rocks, and fluid flow. Permeability is mostly related to the damage zones associated with the faults controlling regional tectonics, namely, the Liquine–Ofqui Fault System and Andean Transverse Faults. This research presents a laboratory approach comprising a characterization of the analogue host rock from a shallow, low-to-medium temperature geothermal system surrounding the Liquine area in Southern Chile (39°S) to better constrain intrinsic and extrinsic factors which allow permeable pathways to exist. We analyse the effect of thermal stress at 25, 150, and 210 °C in a granodiorite, measuring some petrophysical properties before and after applying thermal damage, and then loaded the samples until failure. We also compared petrophysical properties with the fracture network characterization using X-ray microcomputed tomography imaging, segmentation, and fluid flow computational simulations. The results show that thermal stress produces intercrystalline microcracks, which result in: (1) an increase in capillary absorption; (2) a decrease in ultrasonic wave velocities; (3) a decrease in compressive strength; (4) a decrease in fracture aperture, and (5) fluid flow simulations indicate that permeability is similar at different temperatures. We conclude that for the granodiorite host rock of the Liquine geothermal system, the combined effect of thermal stress, even at low temperature, may constitute an effective mechanism for sustaining permeability at shallowest depths.

Holocene Event Record of Aysén Fjord (Chilean Patagonia): An Interplay of Volcanic Eruptions and Crustal and Megathrust Earthquakes

AbstractIn the first months of 2007, the Aysén region in southern Chile was affected by a crustal seismic swarm. Its largest earthquake (Mw 6.2) occurred in April and had its epicenter in Aysén Fjord. Seismic intensities became so high that hundreds of onshore mass movements were triggered, several of which entered into the fjord, resulting in mass transport deposits (MTDs) preserved at the fjord bottom. Here we present a Holocene record of paleo‐earthquakes in the previously unstudied Patagonian fjordland based on MTD stratigraphy. High‐resolution seismic data retrieved using two different seismic systems (sparker and TOPAS) reveal multiple older MTDs on different stratigraphic levels. Correlation of the seismic stratigraphy with sedimentological data obtained from a long Calypso core (MD07‐3117) allows conclusion on the seismic origin of these deposits. Additionally, radiocarbon dating permits constructing an age model, validated by tephrochronology, providing an age for the different MTD levels. We thus present a highly detailed paleoseismological history of the Aysén region, including at least six major Holocene earthquakes, one of which is likely related to a known megathrust earthquake. Other earthquakes are related to activity of the Liquiñe‐Ofqui Fault Zone (LOFZ), forming the main source of seismic hazard in the area. We can infer a general average recurrence time for LOFZ earthquakes of ~2,100 years in the vicinity of Aysén Fjord with clustered events during the early and late Holocene. Finally, we argue that cascading events (causal link between volcanic and seismic events) may be a frequent phenomenon along the LOFZ.

Depth‐Dependent Geometry of the Liquiñe‐Ofqui Fault Zone and Its Relation to Paths of Slab‐Derived Fluids

AbstractUsing a nonhomogeneous elastic model for the south Chile subduction zone, we calculate the depth‐dependent geometry of the Liquiñe‐Ofqui Fault Zone (LOFZ), considering that faults develop where shear stress is maximum. Shear stress develops due to the oblique subduction process, depending on shear modulus which varies as a function of the amount of fluids within the overriding plate. Regions with different values of shear modulus are obtained by the geometries of isotherms calculated from a thermal model. Based on the principle that fluids move from higher to lower pressure regions, we calculate paths of fluids from the subducting slab toward the free surface. In the vicinity of the volcanic arc, the obtained fluid paths agree with the geometry of the LOFZ, suggesting that margin‐parallel strike‐slip faults could serve as pathways for fluids through the overriding plate in oblique subduction zones.

Holocene tephrochronology of the lower Río Cisnes valley, southern Chile

Sediment cores from lakes and bogs in the Río Cisnes valley contain tephra from explosive eruptions of volcanoes in the southern part of the Andean Southern Volcanic Zone (SSVZ). These tephra, which thicken and coarsen to the west, are attributed to eruptions from Melimoyu, Mentolat, Hudson, and potentially either Macá, Cay or one of the many minor eruptive centers (MEC) located both along the Liquiñe-Ofqui Fault Zone (LOFZ) and surrounding the major volcanoes. Correlation of the tephra between two new cores in the lower Río Cisnes valley, and amongst other cores previously described from the region, and source volcano identification for the tephra, has been done using lithostratigraphic data (tephra layer thickness and grain size), petrography (tephra glass color, vesicle morphology, and type and abundance of phenocryst phases), and by comparison of bulk tephra trace-element characteristics with previously published whole-rock and bulk tephra chemical analysis. Four tephras in these cores are attributed to eruptions of Mentolat, four to eruptions from Melimoyu, one possibly to Hudson, and six cannot be assigned to a specific source volcano. Some of these tephra correspond to pyroclastic tephra fall deposits previously observed in outcrop, including the MEL2 eruption of Melimoyu and the MEN1 eruption of Mentolat. However, others have not been previously observed and represent the products of newly identified small to medium sized eruptions from volcanoes of the SSVZ. These results provide new information concerning the frequency and magnitude of explosive eruption of SSVZ volcanoes and contribute to the evaluation of volcanic hazards in the region.

An Mw = 7.7 slow earthquake in 1960 near the Aysén Fjord region, Chile

We re-examine the source characteristics of an M_s = 6.9 earthquake in Chile which occurred on 1960 June 6, near the Aysen Fjord region where a remarkable earthquake swarm occurred in 2007 with more than 7000 earthquakes and hundreds of landslides. The June 6 event occurred during the aftershock activity of the 1960 May 22, M_w = 9.5 great Chilean earthquake. A recently found well-calibrated strain seismogram of the June 6 event recorded at Isabella (ISA), California, is the impetus to this study. We confirm that this event is a slow earthquake caused by a source process extending at least 190 s with a seismic moment of M_0 = 4.5 × 10^(20) N⋅m (M_w = 7.7). Although the mechanism cannot be uniquely determined, an NS trending right-lateral strike-slip mechanism is consistent with the ISA record. The depth cannot be constrained well, but the slowness of the event suggests that it may have occurred in a somewhat deeper high-temperature ductile environment caused by nearby subduction of the Chile Rise. The mechanism and the proximity of this event to the Liquine-Ofqui fault (LOF) which extends north–south over 1000 km along the Chilean coast suggest that the June 6 event represents a slip on this fault. The large moment of the June 6 event indicates strong interaction between the Nazca-South American plate boundary and the LOF with significant slip partitioning.

A geochemical approach to distinguishing competing tectono-magmatic processes preserved in small eruptive centres

Small eruptive centres (SECs) representing short-lived, isolated eruptions are effective samples of mantle heterogeneity over a given area, as they are generally of basaltic composition and show evidence of little magmatic processing. This is particularly powerful in volcanic arcs where the original melting process generating stratovolcanoes is often obscured by additions from the down-going slab (fluids and sediments) and the overlying crust. The Pucon area of southern Chile contains active and dormant stratovolcanoes, Holocene, basaltic SECs and an arc-scale strike-slip fault (the Liquine Ofqui Fault System: LOFS). The SECs show unexpected compositional heterogeneity considering their spatial proximity. We present a detailed study of these SECs combining whole rock major and trace element concentrations, U-Th isotopes and olivine-hosted melt inclusion major element and volatile contents to highlight the complex inter-relations in this small but active area. We show that heterogeneity preserved at individual SECs relates to different processes: some start in the melting region with the input of slab-derived fluids, whilst others occur later in a centre’s magmatic history with the influence of crustal contamination prior to olivine crystallisation. These signals are deduced through the combination of the different geochemical tools used in this study. We show that there is no correlation between composition and distance from the arc front, whilst the local tectonic regime has an effect on melt composition: SECs aligned along the LOFS have either equilibrium U-Th ratios or small Th-excesses instead of the large—fluid influenced—U-excesses displayed by SECs situated away from this feature. One of the SECs is modelled as being generated from fluid-enriched depleted mantle, a source which it may share with the stratovolcano Villarrica, whilst another SEC with abundant evidence of crustal contamination may share its plumbing system with its neighbouring stratovolcano Quetrupillan, showing that polygenetic–monogenetic connections are unpredictable. Such marked preservation of individual magmatic histories highlights the isolation of individual melting events even in complex and highly volcanically active areas.

Depth-dependent geometry of margin-parallel strike-slip faults within oblique subduction zones

Based on the principle that faults develop where shear stress is maximum, we determine the depth-dependent geometry of margin-parallel strike-slip faults within oblique subduction zones. Using an elastic half-space model for the south Chile subduction zone, we show that the geometry of a margin-parallel strike-slip fault as the Liquiñe-Ofqui Fault Zone (LOFZ), is vertical near the free surface and curved downwards, until reaching the subducting slab. This geometry is compatible with the observations of reflectors on seismic data obtained from wide angle refraction studies in southern Chile. GPS measurements also support this curved geometry. We suggest that this curved pattern should occur on all margin-parallel strike-slip faults within oblique subduction zones worldwide.

Tectonic role of margin-parallel and margin-transverse faults during oblique subduction in the Southern Volcanic Zone of the Andes: Insights from Boundary Element Modeling

Obliquely convergent subduction margins develop trench-parallel faults shaping the regional architecture of orogenic belts and partitioning intra-plate deformation. However, transverse faults also are common along most orogenic belts and have been largely neglected in slip partitioning analysis. Here, we constrain the sense of slip and slip rates of differently oriented faults to assess whether and how transverse faults accommodate plate-margin slip arising from oblique subduction. We implement a forward 3D boundary element method (BEM) model of subduction at the Chilean margin evaluating the elastic response of intra-arc faults during different stages of the Andean subduction seismic cycle (SSC). Our model results show that the margin-parallel, NNE-striking Liquine-Ofqui Fault System (LOFS) accommodates dextral-reverse slip during the interseismic period of the SSC, with oblique slip rates ranging between 1-7 mm/yr. NW-striking faults exhibit sinistral-reverse slip during the interseismic phase of the SSC, displaying a maximum oblique slip of 1.4 mm/yr. ENE-striking faults display dextral strike-slip, with a slip rate of 0.85 mm/yr. During the SSC coseismic phase, all modeled faults switch their kinematics: NE-striking fault become sinistral whereas NW-striking faults are normal-dextral. Because coseismic tensile stress changes on NW faults reach 0.6 MPa at 10-15 km depth, it is likely that they can serve as transient magma pathways during this phase of the SSC. Our model challenges the existing paradigm wherein only margin-parallel faults account for slip partitioning: transverse faults are also capable of accommodating a significant amount of plate-boundary slip arising from oblique convergence.

Mafic monogenetic vents at the Descabezado Grande volcanic field (35.5°S–70.8°W): the northernmost evidence of regional primitive volcanism in the Southern Volcanic Zone of Chile

In the Andean Southern Volcanic Zone (SVZ), the broad distribution of mafic compositions along the recent volcanic arc occurs mainly south of 37°S, above a comparatively thin continental crust (≤~35 km) and mostly associated with the dextral strike-slip regime of the Liquine–Ofqui Fault Zone (LOFZ). North of 36°S, mafic compositions are scarce. This would be in part related to the effect resulting from protracted periods of trapping of less evolved ascending magmas beneath a thick Meso-Cenozoic volcano-sedimentary cover that lead to more evolved compositions in volcanic rocks erupted at the surface. Here, we present whole-rock and olivine mineral chemistry data for mafic rocks from four monogenetic vents developed above a SVZ segment of thick crust (~45 km) in the Descabezado Grande volcanic field (~35.5°S). Whole-rock chemistry (MgO > 8 wt%) and compositional variations in olivine (92 ≥ Fo ≥ 88 and Ni up to ~3650 ppm) indicate that some of the basaltic products erupted through these vents (e.g., Los Hornitos monogenetic cones) represent primitive arc magmas reaching high crustal levels. The combined use of satellite images, regional data analysis and field observations allow to recognize at least 38 mafic monogenetic volcanoes dispersed over an area of about 5000 km2 between 35.5° and 36.5°S. A link between ancient structures inherited from pre-Andean tectonics and the emplacement and distribution of this mafic volcanism is suggested as a first-order structural control that may explain the widespread occurrence of mafic volcanism in this Andean arc segment with thick crust.

Geochemistry of basalts from small eruptive centers near Villarrica stratovolcano, Chile: Evidence for lithospheric mantle components in continental arc magmas

In the Central Southern Volcanic Zone (CSVZ) of the Andes, the location of stratovolcanoes and monogenetic small eruptive centers (SEC) is controlled by the Liquiñe-Ofqui Fault Zone (LOFZ), a trench-parallel strike-slip feature of over 1000km length. The geochemistry of basalts from SEC is different from those of stratovolcanoes, and are termed Type 2 and Type 1 basalts, respectively. In the region of Villarrica stratovolcano, contemporaneous SEC are more MgO-rich, and have greater light rare earth element (LREE) enrichment, lower 87Sr/86Sr and 143Nd/144Nd, and lower ratios of large ion lithophile elements (LILE) to LREE and high field strength elements (HFSE). A unique finding in this region is that basalts from one SEC, San Jorge, has Type 1 character, similar to basalts from Villarrica stratovolcano. Type 1 basalts from Villarrica and San Jorge SEC have strong signals from time-sensitive tracers of subduction input, such as high 10Be/9Be and high (238U/230Th), while Type 2 SEC have low 10Be/9Be and (238U/230Th) near secular equilibrium. Based on new trace element, radiogenic isotope and mineral analyses, we propose that Type 1 basaltic magma erupted at San Jorge SEC and Villarrica stratovolcano forms by melting of the ambient actively subduction-modified asthenosphere, while Type 2 SEC incorporate melts of pyroxenite residing in the supra-subduction zone mantle lithosphere. This scenario is consistent with the close proximity of the volcanic features and their inferred depths of magma separation. The pyroxenite forms from arc magma produced during earlier episodes of subduction modification and magmatism, which extend back >300Ma along this segment of the western South American margin. Type 2 basaltic magmas may reach the surface during LOFZ-related decompression events, and they may also be a normal but episodic part of the magma supply to large stratovolcanoes, resulting in cryptic geochemical variations over time. The presence and mobilization of stored and aged subduction-related pyroxenite in the mantle lithosphere complicates the identification of subducted, mantle and crustal inputs to magma erupted in continental arcs.

Contrasting records from mantle to surface of Holocene lavas of two nearby arc volcanic complexes: Caburgua-Huelemolle Small Eruptive Centers and Villarrica Volcano, Southern Chile

Most of the small eruptive centers of the Andean Southern Volcanic Zone are built over the Liquiñe-Ofqui Fault Zone (LOFZ), a NS strike-slip (>1000km length) major structure, and close to large stratovolcanoes. This contribution compares textural features, compositional parameters, and pre- and syn-eruptive P,T conditions, between basaltic lavas of the Caburgua-Huelemolle Small Eruptive Centers (CHSEC) and the 1971 basaltic andesite lava of the Villarrica Volcano located 10km south of the CHSEC. Olivines and clinopyroxenes occur as phenocrysts and forming crystal clots of the studied lavas. They do not markedly show compositional differences, except for the more scattered composition of the CHSEC clinopyroxenes. Plagioclase in CHSEC lavas mainly occur as phenocrysts or as microlites in a glass-free matrix. Two groups of plagioclase phenocrysts were identified in the 1971 Villarrica lava based on crystal size, disequilibrium features and zonation patterns. Most of the CHSEC samples exhibit higher LaN/YbN and more scattered Sr–Nd values than 1971 Villarrica lava samples, which are clustered at higher 143Nd/144Nd values. Pre-eruptive temperatures of the CHSEC-type reservoir between 1162 and 1165±6°C and pressures between 10.8 and 11.4±1.7kb consistent with a deep-seated reservoir were obtained from olivine–augite phenocrysts. Conversely, olivine–augite phenocrysts of 1971 Villarrica lava samples record pre-eruptive conditions of two stages or pauses in the magma ascent to the surface: 1208±6°C and 6.3–8.1kb±1.7kb (deep-seated reservoir) and 1164–1175±6°C and ≤1.4kb (shallow reservoir). At shallow reservoir conditions a magma heating prior to the 1971 Villarrica eruption is recorded in plagioclase phenocrysts. Syn-eruptive temperatures of 1081–1133±6°C and 1123–1148±6°C were obtained in CHSEC and 1971 Villarrica lava, respectively using equilibrium olivine–augite microlite pairs. The LOFZ could facilitate a direct transport to the surface of the CHSEC magmas and explain the observed differences with the pre-eruptive conditions of the 1971 Villarrica lava.

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.

Implications of Deep-Marine Miocene Deposits on the Evolution of the North Patagonian Andes

The North Patagonian Andes (38°–43°30′S) present topographic and geologic characteristics distinct from those of the Central Andes to the north, including a lower altitude, a predominance of plutonic rocks, and the presence of a major N-S-trending, dextral strike-slip, intra-arc discontinuity known as the Liquiñe-Ofqui Fault Zone (LOFZ). The timing for the uplift of the North Patagonian Andes remains poorly constrained principally because high exhumation rates resulted in the erosion of most of its volcano-sedimentary cover during the late Cenozoic. The strongly deformed Cenozoic marine deposits ascribed to the Ayacara Formation, which crop out on the western flank of this range at ∼42°S, can provide for a better understanding of the recent geologic evolution of this range. To discern this formation’s age and sedimentary environment, we have integrated sedimentology, ichnology, foraminiferal paleontology, and geochronology. Our results indicate that this unit was deposited in a deep-marine environment during the Early-Middle Miocene. Correlation of the Ayacara Formation with coeval deep-marine deposits in the forearc to the west indicate that deposition was caused by a major regional event of tectonic subsidence probably related to subduction erosion. An Early-Middle Miocene age for the Ayacara Formation is a reliable maximum age for the deformation and uplift of the western flank of the North Patagonian Andes as well as for the commencement of transpressional deformation associated with the LOFZ.

Widespread deformation of basin-plain sediments in Aysén fjord (Chile) due to impact by earthquake-triggered, onshore-generated mass movements

Abstract The 2007 Mw 6.2 earthquake in Aysen fjord caused widespread basin-plain deformation and has had prehistorical predecessors. Both superficial and buried deformed basin-plain deposits are mapped using multibeam bathymetry and seismic-reflection (sparker) profiling. The seismic signature of the sediment was ground-truthed with short cores on key locations. Deformed basin-plain deposits induced by the 2007 earthquake can be divided in frontally emergent and confined deposits, with both a deep and shallow basal shear surface. All deformed basin-plain deposits with a deep basal shear surface are induced by the weight and impact of a slope-adjacent mass-flow wedge. The frontally emergent – most mobile – basin-plain deformation is triggered by mass flows originating from onshore mass movements (i.e. debris flows, rock slides and avalanches) propagating into the fjord. This basin-plain deformation results in vertical seafloor offsets of up to 20 m. Therefore it might be even more important for far-field tsunami propagation than the impact of the onshore mass movements on the sea surface. In the depressions created by the basin-plain deformation, megaturbidites occur, while more distally, sandy density-flow deposits cover the seafloor. The data also indicates that these density flows propagate slower than the basin-plain deformation. Based on correlations with the two main eruptions of the Hudson Volcano, we hypothesize that during the Holocene three to four similar events have struck the fjord. The small variability of the structural characteristics of the Liquine-Ofqui Fault Zone in the northern Patagonian fjordland and historical seismic swarms in this area make us conclude that similar hazards should be taken into account for most of the fjords in this region.

Submarine earthquake rupture, active faulting and volcanism along the major Liquiñe-Ofqui Fault Zone and implications for seismic hazard assessment in the Patagonian Andes

The Liquine-Ofqui fault zone (LOFZ) in the Patagonian Andes is an active major transpressional intra-arc fault system along which Quaternary faulting and volcanism develop. Subaerial and submarine geomorphologic and structural characterization of latest Pleistocene-Holocene faults and monogenetic volcanoes allows us to assess geological cartography of active faults and the kinematic model for recent tectonics during postglacial times, since 12,000 cal. years BP. This allows increasing the basic geological knowledge necessary for determining the seismic hazard associated with cortical structures in the Aysen region in southern Chile. Fault cartography and field observations suggest dominant dextral-reverse strike slip along north-south and locally NNW-striking faults, dextral-normal strike slip along NE to NNE- striking faults, and sinistral strike slip along east-west faults. This kinematics is consistent with regional SW-NE shortening in the context of a major transpressional fault zone. Holocene and even historic monogenetic and sub-aquatic volcanism occurred in this tectonic setting in a close spatial relationship and probably favored by the activity and local architecture of faults. Submarine fault scarps and deformed sediments observed at the bottom of the Aysen Fjord were associated with the destructive April 2007 Mw6.2 earthquake located along the LOFZ. Our observations show that this earthquake occurred along dextral 15-20 km long N-S structure named Punta Cola Fault (PCF). This fault system is located some kilometres to the east of the main N-S Rio Cuervo Fault (RCF). Most of the epicentres of the seismic swarm during 2007 were located along or in between both structures. The study area is a transference zone between N-S regional branches of the LOFZ. The cartography of fault segments proposed here together with geophysical and geologic data suggest that large earthquakes Mw6.2-6.5 can be typically expected along most of the active faults. Besides, seismic hazard assessment should also consider the possibility of earthquake magnitude in the order of 7.1 along the main fault systems like the RCF.

Seismo-tectonic structure of the Aysén Region, Southern Chile, inferred from the 2007 Mw= 6.2 Aysén earthquake sequence

SUMMARY On 2007 April 21, a Mw= 6.2 earthquake shook the Aysen Fjord, Southern Chile in an unprecedented episode for this region characterized by low seismicity. The area is intersected by the Liquine-Ofqui Fault System (LOFS), a +1000-km-long strike-slip fault that absorbs part of the oblique convergence motion between Nazca and South America plates. To study the aftershock sequence of this main event, we installed a seismic network of 15 stations in the area for a period of nearly 7 months. We characterized the seismogenic structure of the zone by calculating a minimum 1-D local velocity model and obtaining precise hypocentral coordinates and uncertainty estimates by using a non-linear probabilistic approach. We also obtained fault plane solutions based on first motion polarities and SV/P amplitude ratios. The velocity model shows an average Vp/Vs ratio of 1.76 for the area and low shear wave velocity values for the upper 3 km of crust. The aftershock seismicity was located mainly between 4 and 10 km depth and disposed in (1) an ∼N–S trending alignment that follows the trace of the LOFS and (2) an E–W alignment at the East of the main fault. Furthermore, we re-analysed the previously published foreshock and early aftershock activity of the sequence including four of its largest events, improving considerably previous location estimates. Selected focal mechanisms show a strong strike-slip component that coincides with the nature of the LOFS. Based on our new analysis we conclude that the 2007 Aysen seismic sequence had a tectonic origin related to activity on the southern end of the LOFS, however not discarding the presence and potential action of fluids on the aftershock activity.

Azimuthal anisotropy in the Chile Ridge subduction region retrieved from ambient noise

In the southern Andes, the oblique convergence of the Nazca plate and the subduction of an active oceanic ridge represent two major tec- tonic features driving deformation of the forearc in the overriding continental plate, and the relative effects of these two mechanisms in the stress fi eld have been a subject of debate. North of the Chile triple junction, oblique subduction of the Nazca plate is associated with the Liquine-Ofqui fault zone, an ~1000-km-long strike-slip fault that is partitioning the stress and deformation in the forearc. South of the Chile triple junction, the Antarctic plate converges normal to the trench, and several ridge segments have been colliding with the overriding plate since 14 Ma. Proposed effects of the collision include episodes of uplift, extension, and formation of a forearc sliver. Using ambient seismic noise recorded by the Chile Ridge Subduction Project seismic network, we retrieved azimuthal anisotropy from inversion of Rayleigh wave group velocity in the 6-12 s period range, mostly sensitive to crustal depths. North of the Chile triple junction in the forearc region, our results show a fast velocity for azimuthal anisotropy oriented subparallel to the Liquine-Ofqui fault zone. South of the Chile triple junction, anisotropy is higher, and fast velocity measurements present clockwise rotation south of the subducted ridge and counterclockwise rota- tion north of the ridge. These results suggest the presence of two main domains of deformation: one with structures formed during oblique convergence of the Nazca plate north of the Chile triple junction and the other with structures formed during normal convergence of the Antarctic plate, coupled with collision of the Chile Ridge south of the Chile triple junction. Low velocities and high anisotropy over the sub- ducted Chile Ridge and slab window could be an indication of anomalously high thermal conditions, yielding a more plastic deformation compared with the north, where conditions are more cold and rigid.