The Paleoproterozoic structural evolution of the Rio Itapicuru greenstone belt (RIGB, Bahia, Brazil) involves two tectonic events of different natures, both associated with magmatism. The first event ...(D1) developed in response to NW-SE shortening and was responsible for subhorizontal foliation, NW-SE trending lineation and SE-directed thrusts. D1 was coeval with the emplacement of rare 2130 Ma granodioritic plutons. The D2 tectonic event was characterized by left-lateral strike-slip tectonics, developed on steeply dipping D1 foliation and within numerous NS elongated ca 2080 Ma plutons. Quartz c-axis analysis within the granites shows that they experienced progressive left-lateral shearing from magmatic to solid-state conditions under decreasing temperatures. This fact, in addition to microstructural arguments, indicates syntectonic emplacement of these plutons. A zone of maximum deformation (the Main Shear Zone) has been identified in the centre of the RIGB on the basis of strain measurements. A tectonic model in which D1 thrusting event initiated basin closure before being replaced by D2 strike-slip motion is proposed. The Main Shear Zone corresponds successively to the main thrust zone and the main wrench fault during D1 and D2 events, respectively. Such a switch from orogen-normal thrusts to orogen-parallel transcurrent movements may represent a common progression during evolution of mountain belts in order to accommodate excess crustal thickness. However, because low-grade metamorphism and low intensity of the deformation demonstrate that excess crustal thickness was not attained, it is argued that granite emplacement and ascent played a significant role in triggering strike-slip tectonics in the RIGB.
A new aeromagnetic map together with new geological and geochronological data has led to a reinterpretation of the geological history of the Arabian Shield.
The magnetic anomalies outline an orogenic ...complex containing a network of mostly left‐lateral strike‐slip faults, including the Nabitah Belt and several peripheral mountain ranges. Oblique accretion resulted in obliteration of early volcanic‐arc magnetic fabrics, which were almost completely replaced by a NW–SE magnetic fabric in the northern Shield; the southern Shield, however, reveals extensive E–W anomalies related to post‐accretion magmatic intrusions. This complex web of orogenic zones is intimately associated with synchronous molasse basins that formed 680–610 Ma.
The distribution and chronology of orogenic zones, related to the closing of East and West Gondwana, brings into question several earlier assumptions, such as high continental growth rates, palaeogeodynamic reconstructions, the definitions of the Nabitah and Najd faults, and the significance of molasse basins.
The islands of St. Pierre and Miquelon comprise part of the Avalon Composite Terrane of the Appalachian orogen of North America. They are composed of several units including a plutonic metamorphic ...group (Cap de Miquelon Group) and a mainly ignimbritic volcanic group (St. Pierre Group). These two units have been dated by U-Pb and Pb-Pb methods on zircon and are clearly temporally distinct. The Cap de Miquelon Group consists of a clastic sedimentary sequence that was intruded and contact metamorphosed by granitoid plutons dated at 615 plus or minus 14 Ma. The St. Pierre Group is dominated by rhyolitic flows and ignimbrites; rhyolotic flows are dated at 581 plus or minus 19 and 584 plus or minus 12 Ma. The age data indicate that the Cap de Miquelon Group should be considered part of the ca. 630-550 Ma Avalonian orogenic cycle, rather than its crystalline basement. Although the Cap de Miquelon plutonic rocks were produced prior to the St. Pierre volcanic rocks, they are coeval with well-known Avalonian volcanic sequences of Marystown and Harbour Main Groups of southern Newfoundland. The St. Pierre Group may be the temporal equivalent of a ca. 580 Ma dykes that intrude the Harbour Main Group volcanic rocks. Similar-aged magmatic events with similar characteristics have also been described in Nova Scotia (south coast of Cape Breton Island) and in southern New-Brunswick, suggesting that St. Pierre and Miquelon magmatism may represent part of making of regional tectonothermal events within the Avalon Composite Terrane.
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