Haroldo Vizan

Paleomagnetism of Jurassic and Cretaceous rocks in central Patagonia: a key to constrain the timing of rotations during the breakup of southwestern Gondwana?

Abstract A paleomagnetic study in Jurassic and Cretaceous rocks from the Canadon Asfalto basin, central Patagonia, indicates the occurrence of about 25–30° clockwise rotation in Upper Jurassic–lowermost Cretaceous rocks, whereas the overlying mid-Cretaceous rocks do not show evidence of rotation. This constrains the tectonic rotation to be related to a major regional unconformity in Patagonia, which in turn seems to be close in time with the early opening of the South Atlantic Ocean. The sense and probably the timing of this rotation are similar to those of other paleomagnetically detected rotations in different areas of southwestern Gondwana, suggesting a possible relationship between these and major tectonic processes related with fragmentation of the supercontinent. On the other hand, the mid-Cretaceous rocks in the region yield a paleopole located at Lat. 87° South, Long. 159° East, A95=3.8°. This pole position is consistent with coeval high-quality paleopoles of other plates when transferred to South American coordinates, implying it is an accurate determination of the Aptian (circa 116 Ma) geomagnetic field in South America.

New Late Permian paleomagnetic data from Argentina: Refinement of the apparent polar wander path of Gondwana

The Late Paleozoic–Early Mesozoic apparent polar wander path of Gondwana is largely constructed from relatively old paleomagnetic results, many of which are considered unreliable by modern standards. Paleomagnetic results derived from sedimentary sequences, which are generally poorly dated and prone to inclination shallowing, are especially common. Here we report the results of a joint paleomagnetic-geochronologic study of a volcanic complex in central Argentina. U-Pb dating of zircons has yielded a robust age estimate of 263.0 +1.6/−2.0 Ma for the complex. Paleomagnetic analysis has revealed a pretilting (primary Permian) magnetization with dual polarities. Rock magnetic experiments have identified pseudo-single domain (titano)magnetite and hematite as the mineralogic carriers of the magnetization. Lightning-induced isothermal remagnetizations are widespread in the low-coercivity magnetic carriers. The resulting paleomagnetic pole is 80.1°S, 349.0°E, A95 = 3.3°, N = 35, and it improves a Late Permian mean pole calculated from a filtered South American paleomagnetic data set. More broadly, this new, high-quality, igneous-based paleomagnetic pole falls between the previously distinct Late Permian segments of the Laurussian and Gondwanan apparent polar wander paths, suggesting that the long-recognized disparity between these large paleomagnetic data sets may be primarily due to the inclusion of low-quality or systemically biased data.

ANALYSIS OF A LOWER JURASSIC GEOMAGNETIC REVERSAL BASED ON A MODEL THAT RELATES TRANSITIONAL FIELDS TO VARIATIONS OF FLUX ON THE CORE SURFACE

SummaryFor the last 12 Myr the transitional virtual geomagnetic poles (VGPs) of different reversals lie close to two preferred and practically antipodal longitudinal paths. In spite of some controversies about these transitional paths, it has been pointed out that they are linked to geomagnetic phenomena. Jurassic transitional VGP paths are quite similar to those of the last 12 Myr. Paleomagnetic data recorded in Stormberg Lavas (195 ± 5 Ma) belonging to two sampling localities of South Africa have been rotated according to an absolute palaeoreconstruction of Africa for the lower Jurassic. In order to avoid the hypothesis about dipolarity implicit in the VGPs calculations, the transitional directions recorded in the lavas were compared with others that were simulated on the basis of a model that relates transitional fields to variations of flux on the Earths core surface. They were quite similar. For both, recorded and simulated data, the VGPs showed similar paths. Similar conditions could thus have driven both late Cenozoic and Jurassic reversals.

Early Cretaceous Volcanism in Central Argentina

The main exposed site of Early Cretaceous volcanism in central Argentina is located in Sierra Chica of Cordoba Province (SCC), within the Central Rift System. Also to the south, in Levalle basin, a thick Early Cretaceous volcanic pile lies buried in the subsurface. Other localities where volcanism is exposed are Sierra de las Quijadas and Cerrillada de las Cabras of San Luis Province. In SCC, as in the other mentioned localities, a volcanic–sedimentary complex was developed under rifting tectonics. Lava flows are frequently associated with scoria fall, pyroclastic and phreatomagmatic breccias within a strombolian-type volcanism. A new 40Ar/39Ar dating performed on sanidine phenocrysts of a trachyte from Almafuerte locality indicated an age of 129 ± 1 Ma. Diverse groups of rocks, mainly of potassic character, were distinguished: (1) alkali basalt—trachyte suite, (2) transitional basalt—latibasalt suite, (3) basanite—phonolite suite and (4) ankaratrites. Magma evolution must have taken place at crustal level(s) from distinct parental melts, mainly through fractional crystallization in an open-system magma chamber. Mantle source composition supports residual garnet and phlogopite, it does not exhibit features related to slab-derived metasomatism despite its location over Pampean mobile belt, and bears a lithospheric nature. SCC volcanism is of high Ti, display similarities with potassic Brazilian localities around Parana basin as Alto Paranaiba and Goias, pointing out analogies in their mantle sources.

Early Cretaceous Volcanism in Central and Eastern Argentina During Gondwana Break-Up

During the Early Cretaceous, a reorganization of lithospheric plates led to one of the major volcanic processes: the Paraná Magmatic Province (PMP) that covered an extensive region of Brazil, Paraguay, Uruguay and north-eastern Argentina. It was a tholeiitic event constituting a large igneous province (LIP). By contrast, an alkaline volcanism also occurred but it is volumetrically restricted, has a peripheral location with respect to this LIP and took place either prior as well as contemporary and posthumously to that large tholeiitic episode. Both volcanic events are Early Cretaceous andmust be linked to the break-up ofWestern Gondwana. In the Province of Córdoba (central Argentina), alkaline volcanic rocks outcrop in Sierra Chica (SCC), andwere generated under extensional conditions integrating a systemof rifts. They are located about 150 km west of PMP basalts that are lying in the subsurface of Chaco-Paraná basin. Recent radiometric dating provided by other authors revealed ages of 134.7 ± 1 and 131.6 ± 2.3 Ma for PMP basalts, whereas interbedded acid rocks show 137.3 ± 1.8 and 134.3 ± 0.8Ma, respectively. A new Ar/Ar dating of 129 ± 1Ma is here presented for the volcanism of SCC, indicating that it is slightly younger than that of the PMP. Both volcanic events must have had different origins in the context of reorganization of geological plates during the Early Cretaceous.