Beschreibung: A temporal seismic network recorded local seismicity along a 130 km long segment of the transpressional dextral strike-slip Liquiñe-Ofqui fault zone (LOFZ) in southern Chile. Seventy five shallow crustal events with magnitudes up to M(tief)w 3.8 and depths shallower than 25 km were observed in an 11-month period mainly occurring in different clusters. Those clusters are spatially related to the LOFZ, to the volcanoes Chaitén, Michinmahuida and Corcovado, and to active faulting on secondary faults. Further activity along the LOFZ is indicated by individual events located in direct vicinity of the surface expression of the LOFZ. Focal mechanisms were calculated using deviatoric moment tensor inversion of body wave amplitude spectra which mostly yield strike-slip mechanisms indicating a NE–SW direction of the P-axis for the LOFZ at this latitude. The seismic activity reveals the present-day activity of the fault zone. The recent M(tief)w 6.2 event near Puerto Aysén, Southern Chile at 45.4°S on April 21, 2007 shows that the LOFZ is also capable of producing large magnitude earthquakes and therefore imposing significant seismic hazard to this region.

Beschreibung: We examine the intra‐arc crustal seismicity of the Southern Andes Volcanic Zone (SVZ). Our aim is to resolve inter‐seismic deformation in an active magmatic arc dominated by both margin‐parallel (Liquiñe‐Ofqui fault system, (LOFS)) and Andean transverse faults (ATF). Crustal seismicity provides information about the schizosphere tectonic state, delineating the geometry and kinematics of high strain domains driven by oblique‐subduction. Here, we present local seismicity based on 16‐months data collected from 34 seismometers monitoring a ~200 km long section of the Southern Volcanic Zone, including the Lonquimay and Villarrica volcanoes. We located 356 crustal events with magnitudes between Mw 0.6 and Mw 3.6. Local seismicity occurs at depths down to 40 km in the forearc and consistently shallower than 12 km beneath the volcanic chain, suggesting a convex shape of the crustal seismogenic layer bottom. Focal mechanisms indicate strike‐slip faulting consistent with ENE‐WSW shortening in line with the long‐term deformation history revealed by structural geology studies. However, we find regional to local‐scale variations in the shortening axes orientation as revealed by the nature and spatial distribution of microseismicity, within three distinctive latitudinal domains. In the northernmost domain, seismicity is consistent with splay faulting at the northern termination of the LOFS; in the central domain, seismicity distributes along ENE‐ and WNW‐striking discrete faults, spatially associated with, hitherto seismic ATF. The southernmost domain, in turn, is characterized by activity focused along a N15°E striking master branch of the LOFS. These observations indicate a complex strain compartmentalization pattern within the intra‐arc crust, where variable strike‐slip faulting dominates over dip‐slip movements.

Beschreibung: The late Tertiary tectonic and geological evolution of the Southern Andes at 33–34°S has been strongly controlled by the Challenger–Juan Fernández–Maipo (CH–JF–M) structural system. The present configuration of a flat slab between 28–33°S may be explained by a series of favorable conditions evolving with time since the breakup of the Farallon Plate at 25 Ma. The dramatic shift of the pole of rotation and the rapid eastward propagating rift along the Challenger Fracture Zone induced an unbalanced slab pull force, south of the CH–JF–M, that may have triggered the detachment of the subducting slab. The upwelling of a warmer asthenospheric material and the partial melts of the slab are likely consequences that are consistent with the anhydrous tholeiitic late Oligocene volcanism and the anomalous adakite-type magmatism of the early Miocene, respectively. The present seismogenic zone across the CH–JF–M tectonic boundary shows a continuity for more than 600 km along the flat slab segment, in contrast with the much shorter slab southward. Such a tectonic configuration is probably a quasi-steady condition since 25 Ma. Gravity modeling along the JF chain and the broadly located focal mechanisms at the locus of the already subducted JF chain indicate a thick (〉25 km), wide (100 km), and continuous belt of lighter oceanic crust, which is the major contribution to the positive buoyancy of the slab. The decoupling of the subducted slab at intermediate depths further contributes to the flattening of the slab, focusing the buoyancy forces associated with the thickened oceanic crust along the already subducted JF chain. The absolute plate reconstruction during the Miocene shows that the JF collision against the margin migrates southward, in agreement with geological and tectonic observations that further support the causative relationship between the flattening of the slab and the subduction of the JF chain. Preliminary deformation models for the indentation of the JF ridge against the continental lithosphere is consistent with the particular east–west trend of the Maipo deformation zone, connecting in this way the CH–JF–M major tectonic boundary in the ocean-continent lithosphere system.

Beschreibung: We present new field structural data from the Chilean Coastal Cordillera located above the northern and central parts of the interplate contact ruptured by the A.D. 2010 M w 8.8 Maule earthquake. The northern study area contains the northwest-striking Pichilemu normal fault, an intraplate structure reactivated after the megathrust event by crustal earthquakes up to M w 7.0. The structural style of this region is dominated by kilometer-scale normal faults that have been active at least throughout the Quaternary. The orientations of these main faults define three structural systems: (1) northeast- and (2) northwest-striking margin-oblique faults, and (3) north- to north-northeast–striking margin-parallel faults. From north to south, these three systems vary in their predominant occurrence, starting with bimodal orientations of groups 1 and 2, followed by predominantly single north to north-northeast orientations of group 3. Reverse faults coexist in time and space with the normal structures, but are scarce and display variable, apparently random orientations. The shallow crustal normal faults, including the Pichilemu fault, show a persistent kinematic history probably spanning thousands of subduction seismic cycles. Though historically smaller in magnitude than those of the triggered normal faults, interseismic forearc thrust events were recorded above the rupture area prior to the Maule earthquake. The Quaternary reverse faults identified in our study regions may be preserving interseismic, slow-strain-rate, permanent deformation signature in the structural grain. Analogous observations along the A.D. 2011 Tohoku earthquake rupture in Japan imply that such a link between the short- and long-term deformation patterns of the forearc is not exclusive of the Maule earthquake region.

Beschreibung: The Carboniferous–early Permian Santo Domingo complex in coastal Chile (33.5°S) preserves magmatic structures that allowed us to partially reconstruct and compare the deformation histories of two intrusive units within a mid-upper crustal zoned pluton. The oldest history is preserved in the Punta de Tralca tonalite, where microgranitoid enclaves record the emplacement and partial assimilation of mostly mafic magma into an intermediate host. Enclaves record early foliation development by a mechanical sorting and alignment of minerals during hypersolidus flow in melt-rich magma currents, followed by diffusion creep and sliding along melt-coated crystals. Structures in a weaker, tonalitic matrix record compaction, flattening, and near-solidus deformation as porous flow, aided by brittle deformation, drained residual melts. These processes produced penetrative S 〉 L fabrics (i.e., planar more dominant that linear fabric) in an increasingly viscous, crystal-rich mush and promoted folding, fracturing, shearing, and crystal-plastic deformation as the mush approached its solidus. The deformation disrupted igneous layering and helped mobilize and concentrate melt-rich aggregates, forming diffuse patches and dikes that intruded previously deformed enclaves and matrix and aided pluton differentiation. A different deformation history is recorded by the Estero Córdoba dike, which intruded and interacted comagmatically with the Punta de Tralca tonalite. The dike records how magma flow near stiff boundaries resulted in velocity gradients that drove deformation during magma replenishment. This deformation reset inherited enclave fabrics, increased ductile stretching and winnowing, and formed linear (L 〉 S) fabrics. This example illustrates how different styles of deformation assisted magma movement through a mid-upper crustal magma chamber and highlights the diverse origins and significance of structures generated by deformation in magmas of variable crystal-melt ratios.

Beschreibung: A temporal seismic network recorded local seismicity along a 130 km long segment of the transpressional dextral strike-slip Liquiñe-Ofqui fault zone (LOFZ) in southern Chile. Seventy five shallow crustal events with magnitudes up to M(tief)w 3.8 and depths shallower than 25 km were observed in an 11-month period mainly occurring in different clusters. Those clusters are spatially related to the LOFZ, to the volcanoes Chaitén, Michinmahuida and Corcovado, and to active faulting on secondary faults. Further activity along the LOFZ is indicated by individual events located in direct vicinity of the surface expression of the LOFZ. Focal mechanisms were calculated using deviatoric moment tensor inversion of body wave amplitude spectra which mostly yield strike-slip mechanisms indicating a NE–SW direction of the P-axis for the LOFZ at this latitude. The seismic activity reveals the present-day activity of the fault zone. The recent M(tief)w 6.2 event near Puerto Aysén, Southern Chile at 45.4°S on April 21, 2007 shows that the LOFZ is also capable of producing large magnitude earthquakes and therefore imposing significant seismic hazard to this region.