Paleomagnestism of the Santa Victoria Group in the Mojotoro Range, Salta: Contributions to the paleogeographic position of Pampia in Early Paleozoic

Sedimentologic magnetofabric paleomagnetic studies were carried out on the early Ordovician La Pedrera Formation (Santa Victoria Group) in the Mojotoro Range Salta province. These studies aim at contributing to a better knowledge on the paleogeographic position of the Pampia block in the Early Paleozoic in relation to the Gondwana supercontinent. Oriented samples were collected from 12 sites distributed in three localities. These outcrops consist of middle to proximal shelf deposits of mainly quarzitic composition. Studies on the anisotropy of magnetic susceptibility (AMS) suggest a depositional fabric in one of these localities meanwhile some tectonic influence cannot be ruled out in the magnetic fabric found in the other two. The paleomagnetic analysis permitted to determine a characteristic magnetic remanence carried by hematite at all twelve sites. From the virtual geomagnetic poles computed at each site a paleomagnetic pole was obtained for the Santa Victoria Group at these localities which is located at 340,4°E 38,3°N A95=8,8°. This pole position suggests that any of the three localities underwent significant tectonic rotations due to the Andean orogeny or pre-vious tectonic events. However, it presents a somewhat lower than expected paleolatitude in comparison to coeval reference poles for Gondwana. Studies of oriented isothermal remanent magnetization suggest that this difference should not be due to inclination errors. On the other hand, the paleomagnetic pole for the Santa Victoria Group disagrees with those previously obtained for the middle to upper Cambrian Mesón Group and the lowermost Ordovician Santa Rosita Formation, to the north of our study localities but still in the same region. These discrepancies can be explained either by the presence of Andean local tectonic rotations around vertical axes in the localities to the north of our study zone or by considering that the hypothetical displacement of Pampia along the Río de la Plata craton margin was already over, or nearly so, by the Early Ordovician

Age and setting of Permian Slide Mountain terrane ophiolitic ultramafic-mafic complexes in the Yukon: Implications for late Paleozoic-early Mesozoic tectonic models in the northern Canadian Cordillera

The Yukon Tanana (YTT) and Slide Mountain terranes (SMT) of the Cordillera in Canada and Alaska were interpreted in terms of opening and closing of a Late Devonian-Permian Japan Sea-style backarc basin behind a continental arc built upon YTT, which rifted from Laurentia during the Famennian-early Mississippian. Formation of Famennian transitional oceanic lithosphere supports rifting, but a combination of existing and new data on the setting and age of SMT ophiolites do not support the Japan Sea model. The studied Clinton Creek and Midnight Dome complexes represent suprasubduction zone ophiolites formed at ca. 265 Ma, consistent with analyses of other SMT ophiolites. Ultramafic rocks dominate most ophiolites. They lack sheeted dikes and contain relatively minor volumes of mafic plutonic and volcanic rocks, suggesting they formed in oceanic core complexes characterised by slow spreading and low magma productivity. The Permian ophiolites formed during or immediately after eclogite formation in YTT, coeval with or immediately preceding emplacement of orogenic peridotites into YTT due to hyperextension. Several tectonic scenarios are discussed. We propose that YTT is a composite terrane comprising a continental block and an oceanic arc-backarc complex with the latter obducted onto the former during the middle Permian-early Triassic Klondike orogeny. Obduction may have come from the west or east, but east-directed obduction is most consistent with geological constraints. Obduction was followed by initiation of west-dipping subduction east of the composite YTT; slab roll back causing extension in the composite upper plate, leading to exhumation of orogenic peridotites. Tectonic relationships show many analogies to the collision between Australia and the New Britain arc, in which collision in the Huon Peninsula of New Guinea is contemporaneous with extension in Australian crust in the adjacent Woodlark basin. Syn-orogenic Permian Klondike calc-alkaline magmatism is attributed to extension in a Woodlark basin-like setting rather than a representing a continental arc

Age and setting of Permian Slide Mountain terrane ophiolitic ultramafic-mafic complexes in the Yukon: Implications for late Paleozoic-early Mesozoic tectonic models in the northern Canadian Cordillera

The Yukon Tanana (YTT) and Slide Mountain terranes (SMT) of the Cordillera in Canada and Alaska were interpreted in terms of opening and closing of a Late Devonian-Permian Japan Sea-style backarc basin behind a continental arc built upon YTT, which rifted from Laurentia during the Famennian-early Mississippian. Formation of Famennian transitional oceanic lithosphere supports rifting, but a combination of existing and new data on the setting and age of SMT ophiolites do not support the Japan Sea model. The studied Clinton Creek and Midnight Dome complexes represent suprasubduction zone ophiolites formed at ca. 265 Ma, consistent with analyses of other SMT ophiolites. Ultramafic rocks dominate most ophiolites. They lack sheeted dikes and contain relatively minor volumes of mafic plutonic and volcanic rocks, suggesting they formed in oceanic core complexes characterised by slow spreading and low magma productivity. The Permian ophiolites formed during or immediately after eclogite formation in YTT, coeval with or immediately preceding emplacement of orogenic peridotites into YTT due to hyperextension. Several tectonic scenarios are discussed. We propose that YTT is a composite terrane comprising a continental block and an oceanic arc-backarc complex with the latter obducted onto the former during the middle Permian-early Triassic Klondike orogeny. Obduction may have come from the west or east, but east-directed obduction is most consistent with geological constraints. Obduction was followed by initiation of west-dipping subduction east of the composite YTT; slab roll back causing extension in the composite upper plate, leading to exhumation of orogenic peridotites. Tectonic relationships show many analogies to the collision between Australia and the New Britain arc, in which collision in the Huon Peninsula of New Guinea is contemporaneous with extension in Australian crust in the adjacent Woodlark basin. Syn-orogenic Permian Klondike calc-alkaline magmatism is attributed to extension in a Woodlark basin-like setting rather than a representing a continental arc