A scar on the face of the Andes
Extending for roughly 1,200 kilometers along the Northern Patagonian Andes, the Liquiñe-Ofqui Fault System (LOFS) is a massive fault in the Earth's crust. This major geological feature is a dextral intra-arc strike-slip fault. This means the land on the western side of the fault is constantly moving north relative to the eastern side. The fault's existence is a direct consequence of the tectonic arrangement here, where the Nazca Plate subducts, or dives, beneath the South American Plate at an oblique angle. The LOFS absorbs a significant portion of this sideways motion, with different segments of the fault slipping at rates estimated between 11 and 25 millimeters per year.
This immense fault zone is not a single, clean break. It is a complex network of parallel and interconnected fault strands that run through the main axis of the Andes. The fault's movement shapes the landscape, carving deep valleys and defining the paths of rivers. It also is a conduit for processes deep within the Earth's crust. Most of the major stratovolcanoes in the Southern Volcanic Zone of the Andes are aligned with the fault. The fractures and weaknesses created by the fault provide pathways for magma to ascend to the surface, feeding some of Chile's most active volcanoes.
A landscape of fire and water
The connection between the Liquiñe-Ofqui Fault System and volcanism is central to the region. The fault controls the surface location of many volcanoes, including historically active ones like the Hudson volcano. In 2021, geologists even discovered a previously unknown active volcano, named Gran Mate, situated directly in the middle of the fault zone. The volcano is estimated to be less than 5,000 years old.
This same system of deep fractures allows for significant geothermal activity. Groundwater percolates deep into the crust, where it is heated by underlying magma before rising to the surface through fissures created by the fault. This results in numerous hot springs and geothermal fields all along the fault's length. The chemistry of these thermal springs shows the deep water-rock interactions and magmatic gases present.
The fault also generates significant seismic activity. While it has not produced the mega-quakes seen in Chile's subduction zone, it generates notable crustal earthquakes. On April 21, 2007, a magnitude 6.2 earthquake struck the Aysén Fjord, directly related to movement on a portion of the LOFS. This earthquake triggered hundreds of landslides along the steep fjord slopes. The impact of rock and debris into the fjord generated localized tsunamis with waves up to fifty meters high, causing ten fatalities and destroying local salmon farms.