Paleomagnetism and Tectonics in Latin America I
Presiding: M Ernesto, Universidade de Sao Paulo; A Rapalini, Universidad de Buenos Aires
GP33A-01 INVITED 13:30h
Paleomagnetism and the Tectonic Evolution of the Central Andes
The curved shape of the Central Andes has long been attributed to oroclinal bending, the so-called Bolivian Orocline. Refined oroclinal models do not consider a former straight orogen but an inherited curvature that was enhanced to accommodate along strike gradients in Late Cenozoic horizontal shortening. They predict regional counterclockwise rotation in the northern branch and regional clockwise rotation in the southern branch of the arc. Although paleomagnetically detected rotations show an overall consistency with this arrangement, a closer inspection indicates that they record a deformation history that is more complex than a simple bending. The orogen evolves since the Mesozoic. The present forearc (northern Chile and coastal areas of Peru) was the locus of the early contractional phases in Late Cretaceous to Paleogene times. Building of the modern Central Andes occurred in the Late Cenozoic when the locus of shortening shifted eastward to the present backarc region (Bolivia and NW Argentina). Paleomagnetic studies in the modern forearc have shown widespread rotations in Mesozoic-Paleogene rocks and no rotation in Neogene units. In contrast, rotation in Neogene rocks is widespread in the modern backarc. This points out that the timing of rotation roughly follows the overall eastward shift of deformation. On the other hand, recent paleomagnetic results seem to favor an Oligocene age for the rotations in the southern Peruvian forearc, whereas rotations in northern Chile seem to be mainly Eocene. This latter suggests that the timing of rotations also changes along-strike in the forearc. Tectonic rotations in the Central Andes are time-transgressive, constituting a characteristic component of the deformation during, at least, the compressive stage of the orogenic development. It is likely that the pre-orogenic shape of the South American margin is one of the factors in controlling the along-strike change in sense of rotations. The fact that Neogene rocks along the curved forearc do not show paleomagnetically resolvable rotation suggests that the prominent curvature delineated by the external Neogene fold-thrust belt, the Central Andean salient, is a primary arc.
GP33A-02 13:45h
A Paleomagnetic Test of the Patagonian Orocline, Thirty Years Later: New Data, New Insights, New Problems.
The Andean Chain undergoes a 90 degrees bend at around 52 S, from N-S to the north to E-W to the south. This feature is called the Patagonian orocline and its origin and evolution have been much debated. Since the first paleomagnetic test performed by Dalziel et al. in the early seventies that suggested a secondary bend, several paleomagnetic studies have been carried out in the region. However, density of studies is still very low compared to other oroclines. Despite numerous uncertainties, including in cases experimental reliability, ages of magnetization and paleohorizontal control, some preliminary conclusions can be drawn from the available distribution of paleomagnetic declinations along the curved orogen. A systematic pattern of ccw rotation is apparent along the orogen that in a broad sense is consistent with a secondary origin for the bend. Largest values of rotation, 90 or more degrees, on the ocean side of the inverted Rocas Verdes marginal basin, suggest that its closure was probably an important event in the development of the orocline. To the foreland, rotations are smaller, with ca. 30 degrees in the Fueguian Andes (south of 52 S), which suggest either magnetization ages younger than most of the oroclinal bending or a complex development with local patterns of rotations. To the north, anomalous ccw rotations of ca. 50 degrees have been observed along the front of the Magallanes thrust and fold belt and an identical declination anomaly has been found along the whole Sarmiento ophiolite, although in the latter case an alternative explanation to crustal block rotation around a vertical axis is possible.
GP33A-03 14:00h
Discrimination of Multiple Magmatic Episodes in a Dike Swarm Along the Brazilian Southeast Coast
Integrated paleomagnetic, geochemical and geochrological data allowed the identification of four magmatic episodes (Jurassic to Tertiary) within the NE-SW trending dike swarm that extends from Santos to Rio de Janeiro cities, along the southeast Brazilian border. The Early Cretaceous, Late Cretaceous and Tertiary rocks were already known in literature and are discriminated by their tholeiitic (Early Cretaceous), and alkaline (Late Cretaceous and Tertiary) character, as well as by the magnetic polarities. The Jurassic dike group, however, has not yet been dated, and was recognized among the tholeiitic rocks by their distinct chemical signature and paleomagnetic characteristic magnetization. The corresponding paleomagnetic pole correlates with South American Early Jurassic poles putting some new constraints on the tectonic history of the South American margin before the Atlantic opening.
GP33A-04 14:15h
Exploring the Tectonic Behavior of Patagonia During the Breakup of Gondwana: A Paleomagnetic Approach
Recent paleogeographical models for southwest Gondwana place the Malvinas/Falkland islands in connection with the Cape Fold Belt in South Africa. This reconstruction is further supported by a large clockwise rotation (ca. 100°) paleomagnetically detected in Early Jurassic dolerites from the islands. The regional extent of this rotation is, however, still unknown. This led us to investigate the pre-Jurassic paleomagnetic record of the Deseado Massif, the closest continental region to the Malvinas/Falkland Islands, to determine whether it too records a Jurassic-Cretaceous rotation history. We sampled Upper Triassic and Jurassic units, both sedimentary and intrusive. The results suggest the occurrence of clockwise vertical-axis rotations comparable in both magnitude and sense to those previously detected farther north in Upper Jurassic-Lower Cretaceous rocks from the North Patagonian Massif but having considerably smaller magnitude than those in the Falklands/Malvinas. This latter contrast suggests that the intervening region between the Deseado Massif and the islands, in the present Argentine shelf, would has been a zone of major structural discontinuity, although offshore exploration did not identify any such zone yet. Alternatively it may have been a zone of distributed deformation with rotation decreasing westward. In any case, the continuity of rotations within Patagonia requires further investigation to determine its pre-breakup configuration and later kinematics. This may contribute in clarifying the microplates' puzzle of southwest Gondwana.
GP33A-05 INVITED 14:30h
Mountain Building in the Central Andes: Paleomagnetic Constraints on Temporal and Spatial Crustal Rotations.
More than 25 years of paleomagnetic investigations have revealed vertical-axis rotations in the Central Andes synchronous with the development of the mountain chain. The large paleomagnetic database now available for the Central Andes enables a better description of the magnitude, timing and driving mechanism of these rotations. The forearc of northern Chile between ~23-29°S records large (up to 50°) paleomagnetically detected clockwise rotations. Such rotations are observed in Mesozoic and Paleocene to Eocene rocks. The few Neogene paleomagnetic data do not show evidence of rotation. Eocene red beds sediments covering the forearc of southern Peru record a gradient of counterclockwise rotations increasing northward from 0 near Arica at 18-19°S up to -50° at 15°S. Oligocene sediments record about half of the rotations recorded by Eocene sediments. Although secular variations are not fully averaged, available paleomagnetic results in Miocene ignimbrites do not show rotation. Large counterclockwise rotations are observed in pre-Neogene rocks of the northern Altiplano and the Eastern Cordillera of Peru and northern Bolivia while no more than 10° of counterclockwise rotations have occurred during the last 10My. Clockwise rotations are observed within the southern Bolivian Andes and the Puna, especially within dextral transfer zones indicating that rotations are mainly associated with shortening gradient during the Neogene. Clockwise rotations along the Chilean margin correspond to a period of Eocene dextral oblique convergence between the Farallon and the South American plate. Northward displacements along the Chilean forearc were likely accommodated by counterclockwise rotations of the Peruvian margin and shortening in the Abancay-Cuzco deflection. The breakup of the Farallon plate resulted in a more E-W normal relative convergence direction between the Nazca and South American plates during the Oligocene. The increase in rate of convergence, about twice the present-day rate, is associated with an eastward shift of the front of the Andean deformation to the Eastern Cordillera of Bolivia and the Puna. Large late Oligocene-early Miocene shortening within the Eastern Cordillera achieved the bending of the Andean forearc. During the late Miocene-Pliocene, the 10° counterclockwise rotations within the northern Altiplano and the clockwise rotations within the southern subandean belt of Bolivia are related to the propagation of the curved Andean front toward the east. Changes in relative plate convergence and the major role of inherited structures in the propagation of Andean deformation explain the pattern and timing of rotations within the Central Andes. The paleomagnetic data indicate that the Bolivian orocline was formed during a long period from the Eocene to middle Miocene. This observation contradicts recent claims that deformation within the Andes was increasing with decreasing relative convergence during the last 10My.
GP33A-06 14:45h
Rock Magnetic and Oxide Microscopy Studies of two South American Iron-Ore Deposits
Microscopy and rock-magnetic studies of the iron oxide-ore and host rocks in the Cristales-Pleito Melon (Chile) and Jacupiranga (Brazil) deposits were carried out to characterize and compare the magnetic mineralogy and the processes that affected the natural remanent magnetization (NRM) during emplacement and evolution of the iron-ore deposits. The microscopy study under reflected light shows that magnetic carriers are mainly magnetites, with minor amounts of ilmenite-hematite minerals. Titanomagnetite, shows trellis texture, which is compatible with high temperature oxy-exsolution processes. Grain sizes range from a few microns to >100 æm, and dominant magnetic state pseudo-single-domain, in agreement with hysteresis measurements. Thermal spectra, continuous susceptibility measurements, and isothermal remanent magnetization (IRM) acquisition suggest a predominance of some spinels (titanomagnetite or titanomaghemite) with low-Ti content as magnetic carriers. These data help to investigate the magnetic domain states and the remanence acquisition processes, and to assess their significance as a source of magnetic anomalies.