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We document a crustal-scale structural model for the central Chile Andes based on seismicity and surface geology, which consists in a major east verging ramp-detachment structure connecting the subduction zone with the cordillera. The ramp rises from the subducting slab at ~60 km depth to 15-20 km below the western edge of the cordillera, extending eastward as a 10 km depth flat detachment. This structure plays a fundamental role in the Andean orogenesis because most of the shortening has been accommodated by structures rooted in it and allows the distribution of crustal thickening in a 'simple shear deformation mode.' Indeed, despite shortening distribution being very asymmetric (~16 km versus ~70 km in the western and eastern side, respectively), the western side is higher and thicker than what is expected. Yield strength envelopes show strong rheological control on this structure. [V.sub.p] and [V.sub.p]/[V.sub.s] variations in the upper mantle and in the deepest limit of the seismogenic interplate contact mark the intersection of the ramp with the slab, which coincides with the blueschist-eclogite transition. Therefore, subduction processes would control the depth where the major east verging structure may merge with the slab. Such a ramp-flat structure is observed in other parts of the Chilean margin; hence, it seems to be a first-order feature in the Andean subduction zone. This structure delimitates upward the rocks, transmitting part of the plate convergence stress from the plate interface, and controls mountain-building tectonics, thus playing a key role in the Andean orogeny. citation: Farias, M., D. Comte, R. Charrier, J. Martinod, C. David, A. Tassara, F. Tapia, and A. Fock (2010), Crustal-scale structural architecture in central Chile based on seismicity and surface geology: Implications for Andean mountain building, Tectonics, 29, TC3006, doi: 10.1029/2009TC002480.

We have compiled, collected, and analyzed 31 temperature depth profiles from boreholes in the Atacama desert in central and northern Chile. After screening these profiles, we found that only 9 profiles at 4 different sites were suitable to invert for ground temperature history. For all the sites, no surface temperature variations could be resolved for the period 1500-1800. In the northern coastal region of Chile, there is no perceptible temperature variation at all from 1500 to present. In the northern central Chile region, between 26° S and 28° S, the data suggest a cooling from ~ 1850 to ~ 1980 followed by a 1.9 K warming starting ~ 20-40 years BP. This result is consistent with the ground surface temperature histories for Peru and the semiarid regions of South America. The duration of the cooling trend is poorly resolved and it may coincide with a marked short cooling interval in the 1960s that is found in meteorological records. The total warming is greater than that inferred from proxy climate reconstructions for central Chile and southern South America, and by the PMIP3/CMIP5 surface temperature simulations for the north-central Chile grid points. The differences between different climate reconstructions, meteorological records, and models are likely due to differences in spatial and temporal resolution between the various data sets and the models. [ABSTRACT FROM AUTHOR]

SUMMARY The space-time-size distributions of seismicity during the last four decades (1973-2010) are used to identify megathrust asperities, the degree of plate coupling (hence potential future large earthquakes) and slab dehydration below the Andean margin (15°-48°S). This seismicity displays globally a typical magnitude distribution with a b value of 1.04 ± 0.024, for a magnitude of completeness of 4.5 and without considering aftershock sequences. However, this unitary b value of the entire catalogue masks large variations (0.6-1.7) in space and time, interpreted as related to different tectonics contexts or different states of the seismic cycle. The study has been performed for different regions (upper continental plate, upper and lower parts of the subducting oceanic plate) with respect to a 3-D slab geometry model all along the Andean margin. Low b values (0.6-0.8) are observed in areas that were ruptured by large Mw≥ 7.5 megathrust earthquakes. A temporal analysis of these features show that b values where low before these earthquakes and remain low after the relaxation of the aftershock phase, suggesting that they are related to time-persistent asperities within the seismogenic zone of the megathrust. Interestingly, a persistently low b value was observed from 2001 to the end 2009 within the southern limit of the Maule Mw= 8.8 earthquake, 2010 February 27. Our results also identify other low b asperities below the Chilean margin that have not ruptured during the last 40 yr and are places of potentially large megathrust earthquakes in the future. In contrast, large b values (1.1-1.7) are observed at seismicity clusters occurring inside the subducted slab beneath the Central Volcanic Zone (CVZ) of the Andes and below the forearc near the subduction point of aseismic ridges and fracture zones. Clusters below the CVZ are spatially related to relative large intermediate-depth earthquakes (like the Mw= 7.8 Tarapaca 2005 earthquake) and may reveal extensive thermally-driven dehydration of the oceanic lithosphere that favours wet mantle melting and therefore magmagenesis feeding the volcanic chain. [ABSTRACT FROM AUTHOR]

Subduction zones are constituded by seismic segments of different extension, which seems to persist at time-scales similar to the seismic cycle. These seismic segments are a manifestation of heterogeneities in the frictional structure of the megathrust and are reflected through the prevalence of seismic asperities and barriers. However, it does not exist a clear comprehension about the nature of these elements, neither the factors that are controlling the frictional structure, and how they may interact and operate during the seismic cycle.Chile is a perfect natural laboratory to study and unveil the behavior of these factors because of the high rate and large magnitude of megathrust earthquakes. Together with the largest great earthquake ever recorded, The Mw 9.5 Valdivia giant event, three great earthquakes (Mw > 8) have occurred here during the last decade (Maule 2010, Iquique 2014, Illapel 2015). The slip distribution of these earthquakes has been estimated with great detail using numerical models and geodetic and seismic constraints. Thus, coseismic asperities are well constrained, which give us good insight about the spatial rupture patterns in the megathrust. Here we attempt to characterize the spatial variability of the frictional structure of the Chilean megathrust in order to determine the role of different potential factors, such as the presence of sediments and fluids at the trench and subduction channel or the geological structure of both plates (crustal faults, lithology, geological units, basins, and acretionary prisms, seafloor roughness, seamounts, fracture zones and dip variations of the subducted slab)Currently we are generating a complete data base containing seismicity catalogs, geodetic models of interseismic coupling and coseismic slip distribution, residual gravity and topography anomalies, and digital geology. After integration and georeferencing all these data, we will generate several quantitative proxies of the frictional structure. As proposed by other authors, we expect that models, such as gravity anomalies, locking degree, or b-value derived from seismicity should independently point to variations in the frictional properties of megathrust. Thus, the integrated study of these proxies is appropiated to understand the seismogenic behavior better.In a way to accomplish these objetives, we will make a statistical analysis linking different methodologies, which will allow us to estimate the degree of correlation between different proxies to finally produce an integral representation of this frictional structure.We will use two different statistical analysis to obtain an integral representation from several proxies. The first one will be the decomposition in frequencies of the proxies to study the coherence between them and gain insight on how they may correlate each other in space using Fourier decomposition. In the other hand, applying principal component analysis (PCA) to the same proxies, we will decompose the same fields in normal modes to attempt to unveil which of those modes are related to the frictional structure. Finally through quantitative tools, this image will be compared with several geological features of fore-arc, aiming to understand how much the many geological elements are influencing such structure. [ABSTRACT FROM AUTHOR]

The mechanism by which large megathrust earthquakes, upper crustal faults and volcanoes interact at subduction zones at different spatial and time scales are still poorly understood. The Southern Volcanic Zone (SVZ) of the Southamerican Andes (33°-46°S) is an ideal place to study these connections because of the very active nature of this region, particularly during the last decade, and a well-documented link between long-term tectonics and magmatism. Along the southern SVZ (south of 38°S), extensional and compressional regimes are created at the intersection of a trench-parallel structural system (the Liquiñe-Ofqui Fault Zone, LOFZ) with oblique NE- and NW-oriented faults. Contrasting stress conditions at these intersections favor respectively the rapid ascent of basic magmas from lower crust (e.g. Llaima volcano LLV) or upper crustal stagnation and differentiation toward acidic magmas (e.g. Puyehue-Cordon Caulle Volcanic Complex PCCVC). Along the Northern SVZ, a dominance of intermediate-to-acidic volcanism is associated to prevailing compressive structures linked with Neogene fold-thrust belts (e.g. Laguna del Maule Volcanic Complex LMVC). This explains the first-order spatial distribution of volcanic products along the SVZ but is only valid under the stress regime imposed by the oblique convergence between Nazca and Sudamerica during the interseismic phase of the megathrust seismic cycle when both plates are coupled. However, eruptions are statistically more common after large megathrust earthquakes that presumably develop a transient but likely large-scale perturbation (even reversion) of the interseismic stress field. We recognize this apparent paradox and consider the occurrence of the great (Mw8.8) Maule 2010 earthquake as an opportunity to get deeper insights into the relationship between megathrust seismic cycle, active crustal faulting and volcanism at different spatio-temporal scales. Our research combines a) field-based kinematic and dynamic analysis of basement structure below 9 selected volcanoes and integrated at the scale of the entire SVZ, b) temporal evolution of deformation field by geodetic data (InSAR and GPS) and seismicity for the end-member cases of LMVC, LLV y PCCVC, and for the SVZ as a whole, c) characterization of the 3D compositional, thermal and mechanical structure of the Andean margin as derived from a compilation of geophysical data and models. After several years of research we are currently integrating these observations into a unified conceptual model on the relationship between megathrust earthquake cycle, crustal deformation, active tectonics and volcanism valid for the Southern Andes, which we hope would become significant for the understanding of these connections in other active subduction margins. We will present here a synthesis of this research and the conceptual model that we want to propose to the community. [ABSTRACT FROM AUTHOR]