First seismic record for intra-arc strike-slip tectonics along the Liquiñe-Ofqui fault zone at the obliquely convergent plate margin of the southern Andes

Abstract

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_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_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.

Introduction

Bulk transpressional deformation is expected at continental margins where the convergence vector is oblique with respect to the plate boundary zone (e.g. Sanderson and Marchini, 1984, D et al., 1998, F and T, 1998). Kinematic models show that transpressional deformation arising from oblique convergence is accommodated by distinctive structural styles along and across different plate boundaries, which mostly depend on the angle of obliquity, defined as the angle between the convergence vector and the normal to the trench (e.g. J, 1986a, McCaffrey, 1992). For small angles of obliquity, transpression is homogeneously distributed as in the case of the Australian–Pacific plate boundary in New Zealand (Teyssier et al., 1995). For large angles of obliquity, complete partition of transpression is expected. This is the case of the Pacific–North America plate boundary of the western US, where the San Andreas Fault takes up most of the simple shear components (Teyssier et al., 1995). The general case will be that of heterogeneous transpression in which discrete domains across the plate boundary accommodate wrench-dominated or pure-shear dominated transpression (e.g. F, 1972, F et al., 1994, Tikoff and G, 1997). However, the nature and degree of deformation partitioning will not only depend on the angle of obliquity. For instance, thermally weak intra-arc shear zones can accommodate a significant part of the bulk transpressional deformation arising from oblique convergence and affecting the predictions of kinematic models (e.g. De Saint Blanquat et al., 1998). The southern Chilean Andes provides a natural laboratory to examine the nature of long- and short-term transpressional deformation across an obliquely convergent continental margin because of its very well-constrained plate kinematic history. This shows steady right-lateral oblique subduction of the Nazca plate beneath South America since 48 Ma with the exception of nearly orthogonal convergence from 26 to 20 Ma (Pardo-Casas and Molnar, 1987, Somoza, 1998). At present, the angle of obliquity of the Nazca–South America plate convergence vector is, with respect to the orthogonal to the trench normal, approximately 18° for southern Chile (Angermann et al., 1999). The inclination of the Wadati–Benioff zone is about 30° (H et al., 2006, L et al., 2007) in southern Chile and the age of the subducting Nazca plate decreases from 30 Ma at 38°S to virtually 0 Ma at 46°S (Müller et al., 1997), where the Chile ridge is currently subducting (e.g. Herron et al., 1981, C and L, 1986).

Available structural and thermochronological data documents that the intra-arc Liquiñe-Ofqui fault zone (LOFZ) in the Patagonian Cordillera has accommodated concomitant strike-slip, oblique-slip and reverse-slip ductile to brittle deformation over the last 6 Ma (H et al., 1994, L and C, 1999, C et al., 2002). However, the current deformation state of the arc and the fault zone is still poorly known because of the lack of systematic seismic studies in this remote area of the Andes. A recent event on April 21, 2007 with M_w 6.2 near Puerto Aysén, in the Fjordo Aysén, shows that the LOFZ is also a capable of large magnitude earthquakes and therefore imposes a significant seismic hazard to this region. In this work we present and discuss the first systematic local seismological study of a 130 km long segment of the LOFZ in the Southern Andes between 41.5°S and 43.5°S. Inversion of moment tensors from body waves yield to focal mechanisms which help to constrain the current stress field along the zone. Furthermore, we compare and combine the seismic data with previously published long-term structural data.