GP23A-01 INVITED
The Beginning of Paleomagnetism in Brazxil in the Early Seventies
The first polar wander curves were drawn by Ken Creer, using rocks of different ages and the important question then was whether this polar wander was real or apparent, which would support the ideas of continental drift. The question could be answered if polar wander curves for different continents could be compared. Ted Irving went to Australia and Ken Creer went to South America in 1956-57, when he collected samples in Brazil and Uruguay. Ken returned to South America several times . In the early 1960's he took some instruments to Curitiba, Brazil and tried to set up a paleomag laboratory there but the sudent who was involved decided to do something else and the instruments were left there. Then he met Daniel Valencio in Argentina and a long story of collaboration and friendship began. Valencio was invited to stay at Ken Creer's laboratory in Newcastle-upon-Tyne where he prepared the installation of a Paleomag laboratory in the University of Buenos Aires. In the middle 1960's, Prof. P.M.S.Blackett gave a conference on Continental Drift at the University of São Paulo and by that time some geologists and physicits at the University were interested in developping research in Geophysics. In 1969-70 geologist Umberto Cordani told me about the attempt that Creer had made to install a paleomag lab at Curitiba and after an exchange of correspondence Creer visited us and we decided to bring the instruments to the Institute of Physics to start a Paleomag lab. Ken Creer and some Brazilian geologists who knew Daniel Valencio suggested that he could be invited by the Institute of Physics to give a course on Geomagnetism and help us to set up the Paleomag Lab .Daniel finally went to São Paulo, where he stayed for nearly six months. At the same time, the University of São Paulo was experiencing deep changes in its structure and one of he consequences was the creation in 1972 of a new Institute including Astronomy, Geophysics and Meteorology . There was some tradition only in Astronomy but the opportunities were open for the development of Geophysics and Paleomagnetism was the first research activity in the new department. The fruitful collaboration with Ken Creer, Daniel Valencio and members of his group leke Vilas and Mendia continued and the first publications began to appear in the early 1970's.
GP23A-02
Revisiting the Meso-Neoproterozoic dykes of Bahia, Brazil
Mafic dykes exposed along the Bahia coastline of Brazil have yielded paleomagnetic and geochronological results that are important for Rodinia reconstructions and Precambrian geodynamo behavior. Previously published data have been interpreted in terms of four paleomagnetic components residing within these dykes: a south-down "C" direction associated with a U-Pb baddeleyite age of ca. 924 Ma, an east-up "B" direction of unknown age, and two nearly antipodal "A" components that are associated with Ar/Ar ages of ca.1080 Ma (the east-steeply up "A-reversed" direction) and ca.1020-1010 Ma (the northwest-steeply down "A-normal" direction). Literal reading of these data, particularly the high-latitude positions inferred for the 1080-1010 Ma interval, have supported exclusion of the Sao Francisco - Congo craton from Rodinia. However, an additional U-Pb baddeleyite age of ca. 920 Ma, on a dyke with "A-normal" direction in Salvador, has prompted our return to Bahia with the goal of obtaining precise U-Pb ages for each of the paleomagnetic components, thus testing the reliability of the Ar/Ar ages. Following a sampling trip in September, 2006, we have completed the paleomagnetic analyses and confirmed the published directions for the "A-normal" dykes at Ilheus, both the "A-normal" and "A-reversed" dykes at Olivenca, and the "B" and "C" dykes in Salvador. Our new direction for the ca.920-Ma "A-normal" dyke in Salvador differs slightly from the published direction, but it does not resolve the large discrepancy between that direction and the coeval (ca. 924 Ma) "C" component. We are currently processing samples for U-Pb geochronology, from Ilheus "A-normal", Olivenca "A-normal", and Salvador "B" dykes. To complement our study of Rodinian paleogeography, we are also beginning a paleointensity study of these dykes in order to characterize mid-Proterozoic geomagnetic field variations.
GP23A-03
Paleomagnetism of the Late Proterozoic Sierras Bayas Group and the Ediacaran-Cambrian Apparent Polar Wander Path of the Rio de la Plata Craton
The apparent polar wander path (APWP) of the Rio de la Plata craton (RP) for the Late Proterozoic and Early Paleozoic is of significance to reconstruct the tectonic processes that led to the amalgamation of Gondwana. In order to define the Ediacaran-Cambrian APWP for RP, a systematic paleomagnetic study was carried out in the Sierras Bayas Group exposed in the province of Buenos Aires, Argentina. 328 samples from 44 sites were collected in the Cryogenian to Ediacaran Villa Mónica Fm., the Ediacaran Cerro Largo, Loma Negra and Las Aguilas Fms. and the Cambrian (?) Cerro Negro Fm. Sampling included mainly limestone and dolomite, red claystone and marls. Although most sites presented secondary magnetizations of likely recent and Permian (?) age, a few sites from the Cerro Largo and uppermost Villa Mónica Fms. carry a pre-tectonic magnetization that yielded a preliminary paleomagnetic pole at 34.6°S, 211.3° (6.4°/10-9°; n: 18 samples), while a pre-tectonic remanence from the Cerro Negro Fm. yielded a paleomagnetic pole at 10.9°N, 323.5°E (9.7°/13.3°, n:17 samples). These two preliminary paleomagnetic poles for the Sierras Bayas Group together with previously published ones permit to define with confidence the APWP of RP from ca. 600 to ca. 500 Ma.
GP23A-04
New Early Paleozoic Paleomagnetic Poles From NW Argentina: a Reappraisal of Tectonic Models
A paleomagnetic study carried out on Early Ordovician volcanic units in the Famatina Ranges of NW Argentina yielded a pre-tectonic paleomagnetic pole at 32.7°S, 4.3°E, (5.6°/ 8.6°, N=14 sites) that is consistent with four previous Early Ordovician poles from the Famatina - Eastern Puna Eruptive Belt of NW Argentina. However, these five poles are rotated around 50° clockwise respect to the coeval reference pole of Gondwana. Our new results seem to confirm previous models of this belt as a paraauthocthonous rotated terrane on the southwestern margin of Gondwana. However, a recent paleomagnetic pole from the Late Cambrian Mesón Group, at the Eastern Cordillera of NW Argentina, corresponding to the Gondwana foreland (4.5°S, 359.0°E, dp=5.5°, dm=8.8°, n=26 samples) and preliminary paleopoles obtained from the same unit and the latest Cambrian - Early Ordovician Santa Victoria Group at other three localities in the same region, also indicate an anomalous pole position rotated some 40° clockwise respect to the reference pole for Gondwana. These results suggest that the postulated model of a rotated terrane for the Famatina-Eastern Puna belt must be reconsidered. Different alternative scenarios including the possibility of an Early Paleozoic displacement of the whole basement of the Eastern Sierras Pampeanas of Argentina ("Pampia") will be explored.
GP23A-05 INVITED
Paleomagnetism and Tectonics: Latin American Perspectives
Since the early paleomagnetic studies in Latin America by K. Creer in the 50s, many hundreds of paleomagnetic studies in this region have been carried out. Because plate tectonic movements induce structural displacements in the crust near plate boundaries, remanent magnetizations therefore carry information about both types of displacements: plate displacements and structural displacements. The challenge for paleomagnetic researchers is to distinguish the structural ('local') effects from the plate-tectonic ('global') effects. On a global scale, plate motions are recorded in geomagnetic polar wander paths, and structural movements induce scatter about that path. On a local scale, structural displacements can be deciphered from the patterns of scatter of the paleomagnetic directions, leading to inferences about diverse structural and tectonic processes. Progressing from a local (i.e. near-site) to a regional scale, paleomagnetic patterns reveal structural events involving rotation on folds and faults, distortion in plate-boundary zones, accretion of allochthonous terranes, oblique subduction and strike-slip effects, tilting of large crustal blocks, and 'oroclinal' bending. Examples of such mixed signals are presented from the Latin American region, and some outstanding problems for paleomagnetic studies in Latin America are summarized.
GP23A-06 INVITED
Contributions and Lessons of Paleomagnetic Studies in the Northern (tropical) Andes: Examples from Mesozoic and Cenozoic rocks in Colombia, South America
In the last 50 years few paleomagnetic studies have been carried out in the Colombian Andes. The first attempts were in Triassic and Jurassic igneous rocks and red beds (1960-1980), which results yielded: (1) an idea of complex vertical-axis rotations in blocks near to strike-slip faults, and (2) the difficulty of identifying Jurassic and younger magnetization components due to the same paleolatitudinal position of the South America plate since Jurassic. Paleomagnetism in Cretaceous volcanic and volcaniclastic rocks and Cenozoic intrusives to the west of the Romeral paleosuture have allowed to constrain paleolatitudinal translations of oceanic terranes and rotations associated to the complex array of strike-slip faults in the Romeral fault system. These works have been carried out by William MacDonald and co-workers in Colombia. In the last decade, paleomagnetic studies moved eastward of the Romeral paleosuture. The integration of paleomagnetic, magmatic, and stratigraphic constrains data have allowed (1) the proposition of along-margin northward translations of continental-affinity terranes along the northern margin of the South America plate during the Jurassic and (2) constrain rotation of fault-bounded blocks related to syn-extensional tectonism. These interpretations are supported by components of magnetization uncovered in Jurassic and Cretaceous rocks that passes several field tests to verify the near post-depositional age of magnetization. These rocks should be used for further paleomagnetic analyses. Magnetostratigraphy studies of sedimentary Paleogene rocks have been more challenging than in older rocks. Even though several of the sites collected have red beds or Fe-rich nodules and cementation, magnetization intensity is very low, the direction of uncovered component is parallel to the present magnetic field (with negative field tests), or new mineralization occur during heating of the sample. However, other demagnetizations techniques and rock magnetic studies should be applied in these rocks. Magnetostratigraphic studies need to be carried out in basins with volcaniclastic strata of Miocene or younger age to better constrain the uplift the Andes.
GP23A-07
Vertical-Axis Rotations Within the Peruvian Altiplano
During the last two decades, numerous paleomagnetic studies have reported tectonic rotations in the Central Andes but only few data are available for a key tectonic region like the southern Peruvian Altiplano. Clockwise rotations (> 25°) are recorded in Mesozoic to Early Paleogene rocks along the forearc of northern Chile (23-28°S) [Arriagada et al., 2006, Tectonics, doi:10.1029/2005TC001923]. Within the forearc of southern Peru, counterclockwise rotations recorded by flat lying red-beds (Moquegua Formation) increase from south to north from about -30° to more than -45° and rotations decrease with time from late Eocene to late Oligocene - early Miocene time [Roperch et al., 2006, Tectonics, doi:10.1029/2005TC001882]. Rotations in the forearc seems concomitant with deformation localized in the Eastern Cordillera. We speculate that the rotations within the forearc are related to shortening to the east of the Altiplano leading to the first stage of oroclinal bending. If this model is correct, rotations within the Altiplano should have similar magnitude than those of the forearc. We have undertaken a new paleomagnetic sampling (490 samples) along a transect from Nazca (8 sites in the early Miocene Nazca ignimbrites and 7 sites in Oligocene volcanics near Puquio, Western Altiplano) to Abancay (8 sites in Paleocene - Eocene red beds and dikes, Central Altiplano) and Cusco (20 sites in Eocene - Oligocene red beds, Eastern Altiplano). We will present the new results and discuss the spatial and temporal distribution of the tectonic rotations within the Andes of southern Peru.
GP23A-08
Tectonic Rotations in the Southern Central Andes
During the last ten years numerous paleomagnetic studies have reported large clockwise rotations (>25°) in Mesozoic and early Paleogene rocks along the forearc of northern Chile (Arriagada et al., 2003, doi:10.1029/2001JB001598; Arriagada et al., 2006, doi:10.1029/2005TC001923; Taylor et al., 2007, doi:10.1029/2005JB003950). Taylor et al (2007) have argued that these rotations occurred in response to rapid and highly oblique Nazca-South American plate convergence between 60-45 Ma, prior to later Incaic deformation mostly localized along the Domeyko fault system (DFS). However, the striking symmetry of the pattern of clockwise rotations in northern Chile and counterclockwise rotations in southern Peru (Roperch et al., 2006, doi:10.1029/2005TC001882) suggests that rotations likely occurred contemporaneously with the rotations in Peru and pre 20 Ma compressional deformation in the Bolivian Eastern Cordillera. Thus the rotations within the Chilean forearc appear to be not only the result of transpression associated to oblique convergence west of the DFS but also to deformation that should have occurred across the Puna region during the late Eocene - Oligocene. To test this hypothesis, we drilled 31 sites in late Paleozoic and Tertiary (pre Miocene) red beds in the southern Puna. Characteristic remanent magnetizations were determined after detailed thermal demagnetization. Preliminary results show a regional pattern of highly variable clockwise rotations. Measurements of anisotropy of magnetic susceptibility show that most sites record a well-defined magnetic lineation oriented either NS or NNE- SSW that we interpret as evidence for the record of Andean compression even at sites far away from major thrust faults. The deviation of the AMS lineation from a NS trend can be correlated with the tectonic rotation determined by the characteristic remanent magnetizations.