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ABSTRACT Detrital fission-track studies on sedimentary basins surrounding eroding mountain belts provide a powerful tool to reconstruct exhumation histories of the source area. However, examples from active arc-trench systems are sparse. In this study, we report detrital apatite fission-track (AFT) data from Holocene and Pleistocene turbiditic trench and modern river sediments at the Chilean margin (36°S-47°S). Sediment petrography and detrital AFT data point to different major sediment sources, underlining the need for multidisciplinary studies: whereas sediment petrography indicates the erosion of large volumes of volcanic detritus, no such volcanic signal is seen in the detrital age pattern. Areally subordinate plutonic units are identified as the main, often unique sources. This result has important implications for studies of fossil systems, where the feeder areas are eroded, and where the youngest age population is often interpreted to indicate active volcanism. For the southernmost part of the study area in the Patagonian Andes, where the source area is mainly composed of granitoids, the sediment is derived from only small portions along the main divide, pointing to focused glacial erosion there. Our detrital AFT data show no exhumational signal that could be related to the subduction of the actively spreading Chile Ridge at c. 47°S and to the opening of a slab window beneath the South American Plate. [ABSTRACT FROM AUTHOR]

We relocated the six large-magnitude (5.2 < M < 6.2) earthquakes of the destructive, tsunamigenic Aysen seismic swarm, which occurred from 2007 January-October in Patagonian Chile. We used P and SH arrival times from near-source stations of a temporary seismic network fortuitously deployed in the area when the swarm began, and also traveltimes to stations of the permanent global networks, to locate the 2007 January 23, M 5.2 earthquake, the first of the six large magnitude events. This earthquake's hypocentre lies at shallow depth (<10 km) on the eastern strand of a major intraarc shear zone, the dextral Liquiñe-Ofqui fault zone. Using the hypocentre of the January 23 earthquake as a fixed location, we relocated the five other large magnitude Aysen earthquakes by joint hypocentral determination. Four of these five events also occurred at shallow depth on the eastern strand Liquiñe-Ofqui fault, whereas the 2007 April 2, earthquake occurred some 45 km to the west on the Aysen fault, a strike-slip duplex fault that segments the area between the eastern and western Liquiñe-Ofqui fault strands. The five earthquakes on the Liquiñe-Ofqui fault were all produced by dextral slip on ∼N-S nodal planes approximately parallel to the mapped trace of the fault. The April 2 earthquake resulted from normal slip on the Aysen fault. Modelling of Coulomb stress changes on the nodal planes of the April 2 earthquake shows that the cumulative slip on the Liquiñe-Ofqui fault strand could have triggered the April 2 earthquake. Similarly, the April 2 earthquake may have triggered the M 6.2 April 21 earthquake, which caused mass wasting into Aysen Fjord, generating a destructive tsunami. The system of channels and fjords in the study region is a major shipping route around South America, and therefore careful evaluation of the seismic hazard is warranted. [ABSTRACT FROM AUTHOR]

We used cross-correlation of ambient seismic noise recorded in the Chile Triple Junction (CTJ) region to estimate interstation surface wave time-domain Green's functions, and then inverted traveltimes to obtain crustal surface wave velocity models. Interstation distances within the Chile Ridge Subduction Project (CRSP) temporary seismic network ranged from 40 to ∼100 km. We selected 365 d, and cross-correlated and stacked 24 hr of vertical component data at 38 stations pairs, resulting in nominally 703 traveltimes along assumed-straight interstation paths. Velocities in 2-D cells of 30 km × 30 km were calculated using a linear least-squares inversion of the Rayleigh wave group velocity traveltimes. Furthermore we performed a Rayleigh wave group velocity dispersion analysis to estimate the sensitivity of different period waves at depth and to calculate a 3-D shear velocity model of the Patagonian crust. The process was applied to cross correlation pairs determined in two period bands, 5–10 s, corresponding to shallow crustal velocities down to approximately 10 km depth, and 10–20 s, for velocities down to around 20 km. Our results show that cell velocities correlate well with known geological features. We find high-crustal velocities where the Patagonian Batholith outcrops or is likely present at depth, and low velocities correlate with the active volcanic arc of the Southern Volcanic Zone and the subducted Chile ridge in Taitao peninsula, where thermal activity of hot springs is present. High velocities in the mountainous portions of the southeastern study area appear to correlate with outcropping older metamorphic units. Low velocity in the east correlate with sequences of volcaniclastic deposits. [ABSTRACT FROM AUTHOR]

SUMMARY On 2007 April 21, a Mw= 6.2 earthquake shook the Aysén Fjord, Southern Chile in an unprecedented episode for this region characterized by low seismicity. The area is intersected by the Liquiñe-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 Aysén 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. [ABSTRACT FROM AUTHOR]