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SUMMARY The upper Jurassic El Quemado Complex was sampled at 36 sites from five localities in the cordilleran foothills of southern Patagonia between Lago Argentino and Lago Posadas–Sierra Colorada, and the middle Jurassic Marifil Formation at 12 sites in the Somuncurá Massif near Camarones. The main lithology was ignimbrite, with minor tuff and lava. Petrographical and SEM observation show that the El Quemado rocks suffered an intense, high‐temperature alteration which resulted in transformation of most primary Ti‐magnetite in pseudobrookite, rutile and minor Ti‐haematite and Fe hydrated oxides. A similar, less pronounced alteration occurred in the Marifil rocks. 40Ar/39Ar dating of El Quemado was possible for one sample from Sierra Colorada and yielded an age of 156.5 ± 1.9 Ma. Magnetic mineralogy measurements (isothermal remanence, hysteresis loop, Curie balance) show that the remanent magnetization is dominated by PSD low‐Ti magnetite, often associated to a minor high‐coercive mineral (haematite). Secondary magnetization components are usually absent or weak at El Quemado sites, strong at Marifil. They were completely erased by thermal and AF demagnetization and a characteristic remanence (ChRM) stable up to temperatures higher than 550°C or peak‐field values of 100 mT was successfully isolated. The virtual geomagnetic pole (VGP) from the Marifil Formation (83°S, 138°E) is in agreement with the literature data for Jurassic rocks from stable South America. The El Quemado VGPs fall in two groups. The localities to the north of latitude 48°S (Lago Posadas, Sierra Colorada) yield a VGP (81°S, 172°E) close to that of Marifil, whereas those south of latitude 49°S (Lago San Martín, Lago Argentino) show a highly elongated VGP distribution consistent with counter‐clockwise block‐rotation about vertical axes. These rotations were likely caused by thrust sheets which were rotating counter‐clockwise at the same time they were advancing towards the foreland. The amount of rotation varies according to the location of the sampling sites in the thrust and fold belt.

The Gondwana megacontinent was composed of different domains separated by self-lubricated weak lithospheric zones, two of which could have extended into Laurasia. Displacement vectors determined through three consecutive paleomagnetism-constrained paleogeographic reconstructions (Early Pennsylvanian-early Guadalupian, ca. 320-270 Ma; late Guadalupian-Middle Triassic, ca. 260-240 Ma; and Late Triassic-early Late Jurassic, ca. 230-160 Ma) show similar orientations to coeval tectonic stresses along Gondwana. Triggered by slab pull at the northern subduction margin of the Paleotethys Ocean, differential displacements between the Gondwana domains caused localized deformation along their borders, reactivating old weak lithospheric zones (e.g., Ventana fold belt south of Buenos Aries province, Argentina; basins such as Cuvette in central Africa; and Neuquén on the Pacific margin of Gondwana). We propose that the wide extent of these structures was possible due to the transmission of mantle toroidal flow induced by strike-slip movements along these focused self-lubricated weak lithospheric zones, along with the northward drift of Pangea. These processes occurred simultaneously with a major mantle reorganization from a huge cold downwelling to a hot upwelling event caused by thermal energy storage beneath Pangea.

S U M M A R YA palaeomagnetic study of Upper Palaeozoic rocks was performed on Paganzo basin, centralwestern Argentina and its western extension, Río Blanco subbasin. The volcanic-volcaniclastic Río del Peñón and marine sedimentary Punta del Agua formations were sampled in the Rincón Blanco syncline. Both units represent the Upper Carboniferous-Lower Permian transition according to palaeontological and geochronological evidence. The Chancaní and Cerro ColoradoCaminiaga formations were sampled in their type localities, in the eastern Paganzo basin. All-reversed, pre-tectonic characteristic magnetizations were isolated in all three localities, spanning a wide lithological variation. Particularly in the Rincón Blanco syncline, the same reversed magnetization is observed in the entire 1500 m-thick sequence in volcanic and sedimentary rocks; at least part of the deformation there is thought to be Permian in age, related to the San Rafael orogenic phase (SROP). The palaeomagnetic poles are: Rincón Blanco (RB) Lat. 77 • S, Long. 294 • E, N = 19, A 95 = 4.9, K = 47; Chancaní (CH) Lat. 85 • S, Long. 359 • E, N = 3, A 95 = 8.8, K = 196; Cerro Colorado-Caminiaga (CC) Lat. 79 • S, Long. 291 • E, N = 6, A 95 = 8.0, K = 71.Neither of the poles is coincident with expected Late Carboniferous-Early Permian directions according to accepted apparent polar-wander paths (APWPs) from South America and from other plates forming Pangaea in Late Palaeozoic times. This discrepancy could be the result of: (i) polar wander affecting the western Gondwana margin, probably related to SROP; (ii) tectonic rotations related to strike-slip faults active in the area since the Palaeozoic and controlling the position of the main depocentres; (iii) incorrect age assignment for the sedimentary sequences or for the remanence acquisition, i.e. local remagnetizations. Tectonic rotations and local remagnetization have both been proven in particular localities and further work needs to be done to establish the Late Palaeozoic APWP for South America, however, SROP seems to have played a major role controlling both processes, rotation and remagnetization.

We present here new paleointensity and geochronology results from Early Cretaceous volcanic rocks of Sierra Chica de Cordoba (Argentina). The new K-Ar isotopic ages of 5 samples range from 136 to 122 Ma. Twenty five samples from 7 individual flows yielded acceptable paleointensity estimates. The mean paleointensity values per flow are ranging from 53.0 ± 1.9 to 25.4 ± 2.6 μT and the corresponding Virtual Dipole Moments (VDMs) are ranging from 9.3±1.3 to 4.6±0.5 (10 22 Am 2 ). This corresponds to the mean value of 7.3±1.7×10 22 Am 2 , which is compatible to the present geomagnetic axial dipole. Currently available selected paleointensity data from 80 to 130 Ma suggest that geomagnetic field strength frequently fluctuated before and during the Cretaceous Normal Superchron while the magnetic polarity maintained stable. The mean paleointensities derived from Córdoba lavas agree remarkably well with those obtained from the Paraná Magmatic Province (133-132 Ma). This reinforces the hypothesis about the unreliability of 'Mesozoic Dipole Low'.