LONG-TERM AND SHORT-TERM KINEMATIC HISTORY OF THE LIQUIÑE-OFQUI FAULT ZONE, SOUTHERN CHILE: A REVIEW AND IMPLICATIONS FOR GEOLOGIC HAZARD ASSESSMENT

The Liquiñe-Ofqui fault zone (LOFZ) is a 1200 km long intra-arc fault system that runs from 38ºS to t 47ºS in the Patagonian Andes. Early recognized as a strike-slip fault, more recent studies have shown a more complex kinematics of partitioned transtension and transpression over the last 25 Ma. The last episode of ductile deformation, as recorded in regional dextral and dextral-reverse mylonitic zones, has been dated as ca. 4 Ma, and is overprinted by widespread brittle deformation. Kinematic analysis of fault-slip data throughout the LOFZ documents a long-term NE-trending maximum horizontal compression axis, consistent with the short-term instantaneous compression as indicated by shallow crustal earthquakes.

Absolute displacement along the LOFZ has been difficult to establish because of the lack of good geologic markers within the mostly plutonic and volcanic rocks. Available estimates for horizontal separation are in the range of a few tens of kilometers. Although these figures are controversial, they place an upper limit of ca.100 km offset along the LOFZ for the last 25 Ma. This relatively modest displacement is consistent with a buttressed geometry leading to north-south shortening of the adjacent forearc sliver.

Pleistocene-Holocene stratovolcanoes and/or minor eruptive centers within the LOFZ are roughly organized into NNE-, NW- and NE-trending alignments at the regional scale. The former are typically clusters of small holocene pyroclastic cones sitting on top of the master faults. The NW-trending volcanic alignments consist mostly of stratovolcanoes controlled by deep-seated, long-lived basement structures. In contrast, the NE-trending clusters of volcanic vents, minor eruptive centers and stratovolcanoes can be regarded as en échelon tension fracture arrays, compatible with the long- and short-term NE-trending maximum horizontal stress axis.

The recent Mw 6.2 dextral strike-slip crustal earthquake in the Aysén region (46ºS) shows that the fault is currently active. Evidence of massive landslides along NNE-trending glacial valleys and fjords representing main branches of the LOFZ, suggests a strong link between climate fluctuations, fault activity and landscape development. Hazard assessment along the LOFZ is then a major pending issue.

This research is being funded by Fondecyt 1060187.