Liquiñe-Ofqui’s fast slipping intra-volcanic arc crustal faulting above the subducted Chile Ridge

Abstract

The southernmost portion of the Liquiñe-Ofqui fault zone (LOFZ) lies within the proposed slab window which formed due to oblique subduction of the Chile Ridge in Patagonia. Mapping of paleo-surface ruptures, offsets, and lithological separations along the master fault allowed us to constrain geologic slip rates for the first time with dextral rates of 11.6–24.6 mm/year (Quaternary) and 3.6–18.9 mm/year (Late-Cenozoic) respectively. We had trouble mapping the LOFZ in one local because of a partially collapsed and previously undiscovered volcanic complex, Volcan Mate Grande (VMG: 1,280 m high and thus Vesuvius-sized) that grew in a caldera also offset along the LOFZ and has distinct geochemistry from adjacent stratovolcanoes. Besides the clear seismic and volcanic hazard implications, the structural connection along the main trace of the fast slipping LOFZ and geochemistry of VMG provides evidence for the slab window and insight into interplay between fast-slipping crustal intra-arc crustal faults and volcanoes.

Large landslides cluster at the margin of a deglaciated mountain belt

“…The influence of major fast slipping faults (esp. Liquiñe-Ofqui Fault 30 ) on the distribution of large landslides in Patagonia is negligible and large landslides cluster in the semiarid piedmont of mountains characterized by rather low recent uplift rates 36 . This landslide pattern differs in comparison with other reported deglaciated orogens.…”

“…1 ), which was affected by the 1960 Mw 9.5 Valdivia megathrust earthquake. The northern half of the Patagonian Andes is dominated by the fast-slipping (~ 11.6–24.6 mm/yr) dextral Liquiñe-Ofqui fault 30 ; the source of the 2007 Mw 6.2 Aysén Fjord earthquake 31 . The major tectonic structure in the southern region is the Magellanes-Fagnano fault system representing a sinistral boundary between the South American and Scotia Plates, with estimated movement rates ~ 7.8–10.5 mm/yr 32 .…”

Climatic control on the location of the Southern Andes volcanic arc

“…1) without requiring a contribution by northward migration of the CTJ nor a change in slab dip angle, although we do not dismiss these factors. We thus propose that, following the Miocene fragmentation of the Farallon slab and associated topographic growth 20,22,23,[37][38][39][40][41] , a Plio-Quaternary trench-ward migration of the strain and the volcanic arc south of ~40°S is fostered jointly by the established orographic erosion gradient, the approach of the CTJ to its present position 28,41,43 and possibly a slab steepening event 19,40 . In this view, heat advection from the approaching CTJ 28,39 was enhanced by higher orographic erosion on the windward side of the orogen, weakening the crust and fostering the activation the LOFZ and westward volcanic arc migration.…”

“…South of ~40°S, strain localization along the transpressive Liquiñe-Ofqui Fault Zone (LOFZ) since at least the Pliocene 15,28,42 is related to the oblique convergence of the Nazca Plate and/or the CTJ approaching its current position 15,16,28,[41][42][43] . Trench-ward migration of the Late Miocene volcanic arc to its current position is ascribed to the activation of the LOFZ which accommodates magma upwelling 15,38,40 (Figs.…”

“…Results, however, further show that also an orographic shift of the topographic load during magma upwelling can foster the formation of localized strain structures toward regions of enhanced erosion and reduced topographic load. If these structures serve as a main path for the magma ascent as suggested for the LOFZ [15][16][17][18]28,[40][41][42][43] , then a leeward orographic shift of the topography can facilitate an upwind migration of the volcanic arc (Fig. 4).…”