Late Panafrican Geodynamical Evolution of the Tuareg Shield: An AMS Study of Neoproterozoic Granitoids Intrusions around the Raghane Mega Shear-Zone
Magnetic fabric studies have been carried out in three plutons emplaced during Late Panafrican times (600 - 540 Ma) around the Raghane N-S mega-shear zone in southeastern Algeria. This shear zone, which can be observed across all the Aïr and Hoggar shield, is one of the most important in Africa. It separates two metacratons (LATEA and eastern Saharan), which present very different degree of deformation and of metamorphism. The studied plutons are apparently undeformed, except close to NNE-SSW and WNW-ESE faults. No clearly oriented magmatic structures have been observed in all these intrusions, except in only few sites with green lengthened enclaves. This study represents the last part (southern area) of an extensive work made along the Raghane shear zone in Algeria. In most previously studied plutons, the magnetic fabric has the same dominant characteristics, with a sub-horizontal magnetic lineation. This lineation is mainly N-S to NNE-SSW, underlying the dominant role of the Raghane shear zone and of its associated NNE-SSW structures during and after the emplacement of the plutons. The new study of three plutons (Abdou, Hanane and Yvonne) points that they are all with a composite character, only part of each of them showing sub-horizontal lineation related to stresses around the shear zones during the late-magmatic period. Their other part has a clearly different magnetic fabric. In fact, the southern part of the Hanane pluton presents fabric clearly due to post-magmatic deformation along a reactivated NNE-SSW fault. For the Abdou and Yvonne plutons, this other part represents other intrusive episodes having occurred in different geological contexts. The granodioritic part of the Yvonne pluton has a deformational magnetic fabric very similar to that of the relatively old (594 Ma) neighboring granodioritic Ohergehem pluton and could belong to this generation of plutons emplaced during the main northward movement of the Assodé-Issalane terrane along the western boundary of the Saharan metacraton. On the contrary, the northern part of Adjou present a fabric still related to the magma emplacement, with magnetic lineation close to the direction of lengthening of the enclaves. Like the other plutons of this region, the northern part of Adjou emplaced in a context of local extension related to movements along the Raghane shear zone, but, contrary to the others, it was not submitted during its late magmatic period to compressive context due to new movements along the shear zone. Similar magnetic fabric, with sub-horizontal lineation related to shear zones, has been already obtained in all the other intrusions of different parts of the Hoggar shield. All these magnetic fabric data therefore show that homogeneous compressive conditions continued in whole Hoggar during a very long period after the Panafrican orogeny.
Paleomagnetic Study of a Sub-Aerial Volcanic Ridge (S. Jorge Island -Azores Archipelago). Implications on the Island Construction and the Cobb Mountain Event
The Azores Archipelago is located in the North-Atlantic Ocean, near the triple junction between the American, the Eurasian and the Nubian lithospheric plates. It comprises nine active volcanic islands which have developed on the eastern and western flanks of the Mid-Atlantic Ridge (MAR). The area is affected by active faults responsible for recurrent high-magnitude earthquakes. Present-day regional stress field yields the preferential construction of the islands parallel to the WNW-ESE structural direction. S. Jorge Island, especially, has a peculiar shape, resembling a ridge-like volcanic edifice highly elongated along the WNW-ESE direction. The eruptive system is cut by numerous faults and dykes dominantly parallel to the main axis of the island, except in the eastern area where they trend along the NNW-SSE direction. From geochronological and geochemical studies, Hildenbrand et al. (2008) have recently shown that the sub-aerial development of the S. Jorge ridge encompassed the last 1.3 Myr and was overall characterized by a progressive migration of the volcanic construction towards the west. The present work has two main objectives: i) to bring new insights regarding the spatial-temporal construction of the island and ii) to achieve a paleomagnetic characterization of the first stages of the Cobb Mountain Event. To reach such objectives, paleomagnetic and rock magnetic studies from 42 lava flows and 17 dykes from the various sectors of S. Jorge were carried out. Standard progressive alternating and thermal demagnetization procedures display similar results, mainly defining accurate Virtual Geomagnetic Poles (VGP) for each lava flow and dyke. Our new paleomagnetic data show that: (i) the secular variation is not perfectly averaged, though agreeing with the G model proposed by MacFadden et al. (1991). This result is interpreted as indicating volcanic events that occurred massively during distinct and brief periods; ii) The VGP records of the old S. Jorge volcanic units (dated between 1.32 ± 0.2 Ma and 1.21 ± 0.2 Ma) are similar to those observed for Northern Atlantic and Pacific ODP drills as well as for Tahiti volcanic island, showing a character nearly dipolar through the polarity reversal; iii) Dykes and lava flows from the eastern part of S. Jorge display VGP contemporaneous of the first stages of the Cobb Mountain Event while samples from the western sections only display normal polarity acquired during the Brunhes Geochron. These results confirm a progressive westward multi-stage migration of the volcanic construction at S. Jorge that was accompanied by a significant change of the regional stress field.
Tectonic imprint in magnetic fabrics in sediments from the Central Andes
Magnetic fabrics recorded by continental sediments from the Central Andes were systematically measured for about 200 sites also studied for tectonic rotation. Most sediments of Cenozoic age are fine-grained red beds with a significant content of magnetite of volcanoclastic origine. 80 sites were collected in the Puna and Argentinan Andes, 40 sites in the Bolivian Altiplano, 63 sites in the Abancay- Cusco region of Southern Peru and 27 sites in the Moquegua basin in the forearc of southern Peru. In the Moquegua basin where the sedimentary beds are nearly horizontal, samples from the Eocene - Oligocene Moquegua formation have an oblate magnetic fabrics parallel to bedding and scaterred lineations. In contast, to the east within the Altiplano and Puna Plateaus, a triaxial ellipsoid with a well-defined magnetic lineation is usually recorded. At most localities, the orientation of the magnetic lineation correlates with the orientation of fold axes. Along the Central Andes from north to south, the orientation of magnetic lineations rotates from NW-SE to NE-SW. Compaction and tectonic strain appear to be the two main factors controlling AMS in these continental red beds. The information related to the hydrodynamic conditions acting when the sediments were deposited appears to be fully overprinted. Incipient tectonic strain is recorded during the early stages of deformation. When the sediments are not strongly deformed, the magnetic lineation behaves apparently like a passive marker recording tectonic rotations about vertical and horizontal axes. In most paleomagnetic studies applied to tectonics, tilted sedimentary beds are assumed to have been rotated around an horizontal axis. Without a detailed local structural study, the classic tilt correction leads to an apparent rotation when a possible plunge of the fold axis is not detected (MacDonald, 1980). Non-horizontal magnetic lineation suggests either non-cylindrical folding and/or interference of two phases of compressive deformation and tectonic rotation. Results from the Andes indicate that the magnetic fabric may be used to better constrain the deformation and the applied tilt correction to determine tectonic rotations around a vertical axis at regional scale.
Preliminary AMS Study in Cretaceous Igneous Rocks of Valle Chico Complex, Uruguay: Statistical Determination of Magnetic Susceptibility
The Valle Chico Complex, at southeast Uruguay, is related Paraná-Etendeka Province. The study involved basaltic lavas, quarz-syenites, and rhyolitic and trachytic dikes. Samples were taken from 18 sites and the AMS of 250 specimens was analyzed. The AMS is modeled by a second order tensor K and it graphical representation is a symmetric ellipsoid. The axes relations determine parameters which describe different properties like shape, lineation, and foliation, degree of anisotropy and bulk magnetic susceptibility. Under this perspective, one lava, dike, or igneous body can be considered a mosaic of magnetic susceptibility domains (MSD). The DSM is an area with specific degree of homogeneity in the distribution of parameters values and cinematic conditions. An average tensor would weigh only one MSD, but if the site is a mosaic, subsets of specimens with similar parameters can be created. Hypothesis tests can be used to establish parameter similarities. It would be suitable considered as a MSD the subsets with statistically significant differences in at least one of its means parameters, and therefore, be treated independently. Once defined the MSDs the tensor analysis continues. The basalt-andesitic lavas present MSD with an NNW magnetic foliation, dipping 10º. The K1 are sub-horizontal, oriented E-W and reprsent the magmatic flow direction. The quartz-syenites show a variable magnetic fabric or prolate ellipsoids mayor axes dispose parallel to the flow direction (10º to the SSE). Deformed syenites show N300º/11º magnetic foliation, consistent with the trend of fractures. The K1 is subvertical. The MSD defined in rhyolitic dikes have magnetic foliations consistent with the structural trend. The trachytic dikes show an important indetermination in the magnetic response. However, a 62º/N90º magnetic lineation was defined. The MSDs obtained are consistent with the geological structures and contribute to the knowledge of the tectonic, magmatic and kinematic events.