María Elena Cerredo

Palaeomagnetism of Mesozoic magmatic bodies of the Fuegian Cordillera: implications for the formation of the Patagonian Orocline

Abstract It is not known whether the Patagonian Orocline, the major bend of the southern Andes at the southern tip of South America, is a primary or secondary feature. Palaeomagnetic data along the Patagonian Orocline are still too scarce to provide a reliable and unambiguous answer to this question. New palaeomagnetic results on Late Jurassic–Late Cretaceous magmatic units along the central segment of the Fuegian Cordillera are reported. Data from four Late Cretaceous small intrusions and three sites on Late Jurassic–Early Cretaceous metabasalts and metagabbros showed anticlockwise declination deviations between 21° and 46° with respect to South America. From these and previous data, a picture of a nearly homogeneous post-Late Cretaceous regional rotation of the central Fuegian Cordillera is suggested. This supports a model of nearly 30° of anticlockwise secondary bending of the Patagonian Orocline since the Late Cretaceous (72 Ma). Lack of rotation of post-50 Ma sedimentary rocks exposed to the north of our study region, and larger rotations (of c. 90°) reported to the south of it suggest that a geographical and/or temporal progression of rotation values from south to north in the Fuegian part of the Patagonian Orocline should be investigated.

Shallow architecture of Fuegian Andes lineaments based on Electrical Resistivity Tomography (ERT). Evidences of transverse extensional faulting in the central Beagle Channel area

The Fuegian Andes, in the southernmost tip of South America, display a stacking of NNE-verging thrusts related to the Meso-Cenozoic Andean Orogenesis which are superimposed by a later left-lateral strike slip tectonics. The structural imprint along the northern margin of the Beagle Channel is observed in the topography as intersecting arrays of linear features of diverse scale, both inland and offshore. The most conspicuous linear morphology is the Beagle Channel itself, a nearly E-W trending deep trough. Further features with similar orientation are recognized in minor valleys situated north of the channel. An oblique array of lineaments trends NW-SE and comprises secondary fjords, valleys and ridges. The shallow architecture of these morphological lineaments was investigated by means of several 2D electrical resistivity tomographies (ERTs). Further integration of the obtained results with outcrop data, geomorphic and bathymetric information allowed to achieve closer insights into the continuity of major structures and time relationships between the E-W and NW-SE sets of lineaments. The E-W oriented features were characterized by conspicuous sub-vertical resistivity contrasts, interpreted as fractures and faults associated with the strike-slip Beagle Channel fault system. These structures cross-cut gently dipping contractional faults and appear to be topped by undeformed glacial deposits indicating that the system postdates the Andean compression but are preglacial in age. The oblique NW-SE trending lineaments, in turn, show resistivity displacements with somewhat less abrupt contrasts and were interpreted as a system of extensional faults that offsets the bedrock and bounds small basins filled by modern deposits pointing out to a recent activity. Both geophysical and field evidence indicate a post-glacial deformation along the NW-SE extensional faults, which could represent a potential hazard for a populated area. Normal 0 21 false false false ES-AR X-NONE X-NONE