Juan F. Vilas

Palaeomagnetism and K-Ar ages of triassic igneous rocks from the Ischigualasto-Ischichuca Basin and Puesto Viejo Formation, Argentina

Abstract The palaeomagnetism of Middle Triassic (224 ± 5 m.y.) igneous rocks from the Ischigualasto-Ischichuca Basin (67°40′W, 30°20′S) was investigated through 86 oriented hand samples from 11 sites. At least one reversal of the geomagnetic field has been found in these rocks. Nine sites yield a palaeomagnetic pole at 239°E, 79°S (α95 = 15°, k = 13). The K-Ar age determinations of five igneous units of the Puesto Viejo Formation give a mean age of 232 ± 4 m.y. (Early Triassic). The palaeomagnetism of six igneous units of the Puesto Viejo Formation (68°W, 35°S) was investigated through 60 oriented samples. These units, two reversed relative to the present magnetic field of the Earth and four normal, yield a pole at 236°E, 76°S (α95 = 18°, k = 14). Data from the Puesto Viejo Formation indicate, for the first time on the basis of palaeomagnetic and radiometric data, that the Illawarra Zone, which defines the end of the Kiaman Magnetic Interval, extends at least down to 232 ± 4 m.y. within the Early Triassic. The palaeomagnetic poles for the igneous rocks of the Ischigualasto-Ischichuca Basin and Puesto Viejo Formation form an “age group” with the South American Triassic palaeomagnetic poles (mean pole position: 239°E, 77°S; α95 = 6.6°, k = 190). The Middle and Upper Permian, Triassic and Middle Jurassic palaeomagnetic poles for South America would define a “time group” reflecting a quasi-static interval (mean pole position: 232°E, 81°S; α95 = 4°, k = 131).

Palaeomagnetism and KAr age of Mesozoic and Cenozoic igneous rocks from Antarctica

Abstract A new analysis of palaeomagnetic data for igneous rocks from Deception Island, 25 de Mayo Island (King George Island) and Cape Spring, are given. K-Ar age determinations indicate that most of the igneous samples from 25 de Mayo Island included in the palaeomagnetic study are of Late Mesozoic/Early Tertiary age. The significance of these palaeomagnetic-radiometric data on the hypothesis of oroclinal bending of the Antarctic Peninsula and on the apparent polar movement of Antarctica is discussed. The positions of palaeomagnetic poles for the Andean igneous complex indicate that there has not been any apparent post-Late Cretaceous/Early Tertiary oroclinal bending in the Antarctic Peninsula from 74°S to 62°S. A comparison of the positions of palaeomagnetic poles for Antarctica and Australia suggests that the direction of apparent polar movement relative to Antarctica reversed after the Miocene.

Palaeomagnetism of Late Palaeozoic and Early Mesozoic rocks of South America

Abstract Further geological evidence and radiometric studies of South American igneous rocks suggest, (i) a Middle Permian age for geomagnetic poles until now quoted as Permo-Triassic or Lower Triassic, and (ii) a Middle-Upper Triassic age for a geomagnetic pole previously quoted either as Middle or Upper Triassic. The palaeomagnetic and radiometric data from igneous rocks of the Quebrada del Pimiento Formation of Argentina, define a period of normal polarity within the Late Palaeozoic Interval of reversed geomagnetic polarity (Kiaman Magnetic Interval); the possibility of using this short normal period for geological correlation is suggested. The Late Palaeozoic geomagnetic poles now available for South America define the polar path relative to this continent fairly well; it is suggested that this polar path is also valid for Africa, South America and Africa having been joined by their Atlantic margins throughout the Late Palaeozoic and Triassic. The discrepancy between the positions of Permian geomagnetic poles of South America and Australia is pointed out. It could represent different polar shifts relative to South America and Australia and could imply that the fragmentation of Gondwana started in Early Permian times with the formation of two super-continents: South America-Africa and Australia-Antarctica-India.

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

Late Cenozoic Paleomagnetic Studies in Patagonia

Publisher Summary This chapter focuses on the Late Cenozoic Paleomagnetic studies in Patagonia. Paleomagnetism is a geoscience that incorporates aspects of geomagnetism, rock magnetism, and geology. paleomagnetic studies have applications to a wide variety of geological problems. In paleogeographic studies, paleomagnetism is providing evidence about movement of large crustal blocks and localized rotation of smaller tectonic blocks. Also, paleomagnetism can be used for stratigraphic correlation and geochronological calibration of paleontological zonation. Paleomagnetic studies of Late Cenozoic rocks in Patagonia have been made from the early times of this discipline. So far an important number of studies have been carried out in Late Cenozoic basaltic lava flows, glaciolacustrine sediments, lake sediments, and sediments from archeological and paleontological locations. Paleomagnetic studies carried out in basalts have allowed determining the secular variation of the geomagnetic field, measured by the angular dispersion of the Virtual Geomagnetic Pole (VGP) around the Earths rotation axis.