Geology of the Lago Fagnano area (Fuegian Andes, Tierra del Fuego Island).

MENICHETTI, M.; TASSONE A.; LIPPAI, H,; LODOLO E.

BOLLETTINO DI GEOFISICA TEORICA ED APPLICATA

ISTITUTO NAZIONALE DI OCEANOGRAFIA E DI GEOFISICA

Lugar: Trieste; Año: 2010 vol. 51 p. 178 - 179

0006-6729

Lago Fagnano represents the Earth’s southernmost lake, it is elongated in E-W direction for 105 km, with an average width of 8 km, and a maximum depth of 210 m. It’s hydrographic basin covers an area of about 1900 km2 and has an irregular shape, especially in its eastern part, where the water divide between the Atlantic and Pacific/Magallanes Strait is located. The complex morphostructural evolution of the region, especially in the Quaternary, strongly influenced the regional water divide and the main drainage tributaries that run along the N-S and E-W directions. In the east, the catchment boundary runs very close to the lake shore while in other zones it is difficult to identify due to flat topography with large ponds and peat bogs. Located along the southern and western lake shores are outcrops of the Upper Jurassic volcanoclastic metasediment of the Rocas Verdes marginal basin with basalts of the Lemaire Fm. rich in acidic volcanic fragments in the Sierra Valdivesio Paso Garibaldi, and rhyolites of the M.te Buckland Fm in the Sierra de Alvear. Andesitic volcanoclastic turbidites of the Lower Cretaceous Yahgán Fm outcrop in the lake area east side, in the Sierra Lucas Bridge. Sediments pertaining to the Cenozoic Magallanes foreland basin outcrop in the north side of the basin. In the Sierra Inju Gooiyin, metasediments of dark slates, marls and tuffs of the Beauvoir Fm. are present, while to the east, in the Sierra de las Pinturas, outcrops of marls, sandstones and siltstone of Upper Cretaceous of the Cerro Matrero Fm. can be seen. Two Upper Cretaceous intrusive plutons are known to be located south of Tolhuin, Co. Jeujepen and Cerro Krund in the southern slope of Sierra de Inju Gooiyin. Glaciofluvial and Quaternary sediments with moraine, glacio-fluvial and glacio-lacustrine facies cover all the southeastern sectors of the Lago Fagnano area. Exposed along the river incisions and scarps are several, tens of meters thick sediments related to the glacial moraines recoding different ice advances probably linked to the Cordillera Darwin ice cap that flowed eastward with an ice thickness of a few thousands meters. The infill of Lago Fagnano, highlighted by geophysical surveys and core samplings, is composed of at least two seismic stratigraphic units related to the sedimentary deposits resting on a deformed basement.  The lake is located in the frontal part of the main stack of the Fuegian Cordillera basements sole thrust front, at the boundary with the Magellan foreland. A system of low angle, NNE verging thrusts constitute the main compressional structures related to the Upper Cretaceous Andean collisional tectonic phase. A thick-skinned tectonic geometry with arrangements of imbricate fan thrust systems characterize the internal zones of the Sierra del Alvear and Sierra Valdivieso, where basement sole thrusts can be observed. Thin-skinned tectonics with thrust duplex geometries prevail in the foreland north of the Lake. Several Oligocene E-W left-lateral strike-slip faults that define the principal deformation zone of the Magallanes-Fagnano transform faults, superimpose and possibly reactivate older compressive structures. The most prominent structure of the area is the Co.Hope-Catamarca-Knokeke fault, running E-W through the lake area over a distance of more than 100 km. Several outcrops allow the characterization of the geometries and the kinematics of this left lateral, transtensional, subvertical, south dipping structure. Furthermore, the fault trace from the eastern arms of the Magallanes Strait to the Atlantic off-shore shows strong morphostructural evidence. Two other faults, along both the north and the south lake edges, the Rio Turbio-Las Pinturas and the S. Rafael, respectively, form a releasing step step-over within the principal deformation zone. The kinematics of these subvertical E-W faults is mainly transcurrent with an important extensional component. The fault arrays form complex structures that could derive from deformational partitioning involving the distribution of strain orientation or intensity in various domains. An angle of a few tenths of degrees between the thrusts and wrench faults strikes is sufficient for the reactivation of pre-existing structural weaknesses. The geodynamic evolution of the Lago Fagnano pull-apart basin can be related to the westward migration of the fault step-overs along two bounding strike–slip faults.