XII Congreso Geológico Chileno Santiago, 22-26 Noviembre, 2009 S9_053 Neotectonics along the eastern flank of the North Patagonian Icefield, southern Chile: Cachet and Exploradores fault zones Melnick, D.1, Georgieva, V.1, Lagabrielle, Y.2, Jara, J.3, Scalabrino, B.2, Leidich, J.4 (1) Institute of Geosciences, University of Potsdam, 14476 Potsdam, Germany. (2) UMR 5243 Géosciences Montpellier, Université de Montpellier 2, France. (3) Departamento de Ciencias de la Tierra, Universidad de Concepción, Chile. (4) Patagonia Adventure Expeditions, Casilla 8, Cochrane, Chile. [email protected] Introduction In the southern Andes, the North Patagonian Icefield (NPI) is a poorly-known region in terms of geology and neotectonics that marks a major topographic anomaly at the transition between the Austral and Patagonian Andes. The NPI is located immediately east of the Nazca-Antarctic-South America Triple Plate Junction, where the Chile Rise collides against the margin (Fig. 1A). Since 14 Ma, this Triple Junction has migrated northward as a result of oblique plate convergence, resulting in collision and subduction of two relatively short ridge segments in the Golfo de Penas region at 6 and 3 Ma, and at present of one segment immediately north of the Taitao Peninsula [1]. Oblique plate convergence in addition to collision of these ridge segments resulted in the formation of a forearc sliver, the Chiloe block, which is decoupled from the South American foreland by the Liquiñe-Ofqui fault zone. Here we present geomorphic and structural field evidence that indicates neotectonic activity in the internal part of the orogen, along the flanks of the NPI (Fig. 1A and 1B). We define two hitherto unrecognized fault systems, the Cachet and Exploradores Faults, which run along the eastern and northeastern foothills of the NPI, respectively. Based on the offset of deep glacial valleys, we suggest that both faults have been active at least during the past ~3 m.y. We integrate the kinematics of these two fault systems with data from the Liquiñe-Ofqui Fault Zone, to propose a simple neotectonic model, which may help to explain the topographic anomalies in the NPI region. Furthermore, we describe the occurrence of several glacial-dammed lakes that occur along the main trace of the Cachet Fault Zone caused by drainage capture and fragmentation associated with strike- slip deformation. Due to the unstable position of these lakes, we emphasize the hazard of outburst megaflood events. 1 XII Congreso Geológico Chileno Santiago, 22-26 Noviembre, 2009 Geologic and morphotectonic setting of the NPI The NPI is a flat-topped massif capped by an ice shield and bounded by steep flanks. The flat ice shield has a mean elevation of 1.500 m, and several nunataks emerging out of the ice cap reaching the highest elevation of 4.057 m at Cerro San Valentín. Immediately north of San Valentín, the mean elevation of the Andean Cordillera decreases to ~1.000 m, with a few outliers formed by recent volcanoes that occasionally reach ~2.000 m. The core and highest parts of the NPI are formed by Paleozoic metamorphic rocks intruded by Jurassic to Miocene plutons of the Patagonian batholith. To the east of the NPI, deformed Jurassic to Miocene units compose the Patagonian Fold-and-thrust Belt, which records phases of shortening and uplift during the Late Cretaceous to Eocene and Oligocene to Middle Miocene [1] (and references therein). Subsequently to this main phase of mountain building, only minor extensional deformation has been documented in the foreland adjacent to Lago General Carreras. Normal faulting in this region is associated with the effusion of alkaline basalts and formation of vast plateau surfaces referred to as ‘mesetas’. Both widespread extension and alkaline volcanism have been related to subduction of the Chile Rise resulting in opening of an astenospheric window and weakening of the lower crust [2] (and references therein). The Cachet and Exploradores faults The NPI is bounded along its eastern foothills by the Cachet Fault System, which consists of a few dextral strike-slip fault strands that strike north-south. Continuous fault segments extend between the headwaters of the Ventisquero River at the southeastern tip of the NPI northward crossing the Pared Norte, Colonia, and Nef Glaciers up to the Soler River. North of Soler, the fault system loses continuity and grades to NNW-SSE oriented, en échelon faults, which extend northward probably to Lago Norte and the southeastern sector of the Exploradores Glacier. Along the northern foothills of the NPI, the Cachet System is intersected by the Exploradores Fault System, which consists of northwest- southeast oriented, southwest-dipping reverse faults with a minor strike-slip component. The Exploradores Fault System controls the occurrence of the Exploradores-Bayo Valley, the first to cut through the entire Andean Main Cordillera north of the NPI. The Cachet Fault marks a major axial drainage at the foothills of the NPI. Dextral slip along the Cachet Fault resulted in the progressive deflection of the Nef, Colonia, and Pared Norte Glaciers and consequently in the formation of several beheaded glacial valleys. These valleys are located in a hanging position along the eastern fault block (Fig. 1B and 1C). Glacial erosion along large-drainage glaciers as Soler and Colonia has outpaced strike-slip motion of the Cachet Fault resulting in deflection of the glacier but continuous drainage across the fault. On the other hand, strike-slip motion across glaciers with smaller drainages resulted in the formation of hanging valleys and drainage captures by the stream parallel to the fault (Fig. 1B and 1C). Based on correlation of the beheaded valleys floors and ridge crests, we estimate the total dextral offset along the Cachet Fault, which decreases northward from ~6 to 4 km between the Pared Norte and Soler Valleys. 2 XII Congreso Geológico Chileno Santiago, 22-26 Noviembre, 2009 In outcrops it is possible to recognize that some faults of the Cachet System have minor tensional offsets of a few hundred meters (Fig. 1B and 1C). However, subhorizontal striations suggest that along the main trace of the Cachet Fault the latest incremental strain has been mainly transcurrent. Glacial-dammed lakes as a result of strike-slip deformation along the Cachet Fault As a result of dextral strike-slip deformation, several glacier-dammed lakes occur along the axial valley controlled by the Cachet Fault. The largest of these lakes are the Cachet 2 and Arco Lakes, which are dammed by the deflected Colonia Glacier. The Colonia Glacier has the largest drainage area along the eastern NPI and dams the Cachet 2 Lake (Fig. 1B). Presumable due to enhanced melting of the Colonia, three sudden jökulhlaups, or glacial lake outburst floods occurred in April, October, and December 2008. The unstable position of Soler, Cachet 2, Arco, and Guillermo Lakes, all associated with the main trace of the Cachet Fault, which are dammed either directly by ice or by lateral moraines of the major glaciers, makes this region particularly prone to the occurrence of outburst flooding events. Discussion We interpret margin-parallel strike-slip deformation along the eastern flank of the NPI to arise from localized, oblique collision of three segments of the Chile Rise at 6, 3, and 0 Ma in the Golfo de Penas region. This resulted in decoupling of the NPI from the Patagonian foreland. We suggest that northward motion of the NPI is accommodated by the Exploradores and other reverse fault systems along its northern and northeastern flanks, which act as crustal ramp. This would explain the highest topography at the northern edge of the NPI. The minor margin-normal extension observed locally along the Cachet Fault System may be interpreted as a result of lower crustal weakening by subduction of slab windows after collision of Chile Rise segments or from flexure arising from postglacial rebound. Our integrated geomorphic and structural observations revealed the existence of neotectonic features at the foothills of the NPI; some of these faults are potentially active and we expect that work currently in progress (thermochronology, structural analysis, paleomagnetism) will help to quantify their deformation rates and tectonic significance. References [1] Lagabrielle, Y. et al. (2004) Neogene to Quaternary tectonic evolution of the Patagonian Andes at the latitude of the Chile Triple Junction. Tectonophysics, vol. 385, 211-241. [2] Lagabrielle, Y. et al. (2007) Pliocene extensional tectonics in the Eastern Central Patagonian Cordillera: Geochronological constraints and new field evidence. Terra Nova, vol. 19, 413-424. 3 XII Congreso Geológico Chileno Santiago, 22-26 Noviembre, 2009 Figure 1. A: Regional location map and major fault zones. LOFZ: Liquiñe-Ofqui Fault Zone; EFZ: Exploradores Fault Zone. NPI: North Patagonian Icefield. B: Geological map of the eastern NPI flank. Pz: Paleozoic metamorphic rocks (metasediments and ultramafics); Jrg: Jurassic granites (after geologic map of Chile); KMg: Cretaceous- Miocene intrusives; Qs: Quaternary sediments. C: Panoramic view of the deflected Nef Glacier across the Cachet Fault. Mount San Lorenzo can be seen in the background. A hanging beheaded valley is observed, with about 850 m of relief between its floor and adjacent ridge crest. Valley abandonment resulted from strike-slip offsets after ~3 Ma. 4.