The Kalak Nappe Complex is composed of variably retrogressed, middle Proterozoic basement and imbricated, polydeformed, metasedimentary cover successions which, at least in part, appear to have originated within the late Proterozoic-early Palaeozoic Baltoscandian miogeocline. Mafic-alkaline plutons (Seiland Igneous Suite) occur within uppermost structural units of the nappe complex. 40 Ar/ 39 Ar incremental-release mineral ages suggest local differences in tectonothermal evolution within the Kalak Nappe Complex. Post-metamorphic cooling dates of c. 490 Ma are locally recorded by hornblende within the structurally highest tectonic unit suggesting significant orogenic activity occurred during and/or immediately prior to the Arenig. A distinctly later metamorphic overprint occurred in the Silurian and was of sufficient magnitude to everywhere rejuvenate Ar systems in muscovite and nepheline and locally in hornblende. This was followed by relatively rapid post-metamorphic cooling through hornblende and muscovite Ar closure temperatures between c. 425 and 415 Ma. The complex tectonic history suggested for the Kalak Nappe Complex is generally similar to that previously outlined for the central Swedish Caledonides. Together, 40 Ar/ 39 Ar mineral ages from these widely separated areas indicate that significant tectonothermal activity occurred along the Baltoscandian margin prior to the Silurian. This activity appears, at least in part, to have been associated with formation of a Late Cambrian-Early Ordovician accretionary wedge over a westward-dipping subduction zone during initial closure of the Iapetus Ocean.
The Nimrod Group in the central Transantarctic Mountains and the Lanterman Metamorphic Complex in northern Victoria Land of Antarctica constitute internal metamorphic basement terrains of the Ross orogen that share many first-order similarities in lithology, structure, and metamorphism. However, new $$^{40}Ar/^{39}Ar$$ cooling ages determined for hornblende and muscovite from tectonites in the Geologists (Nimrod Group) and Lanterman ranges (Lanterman complex) indicate that these terrains experienced different post-kinematic cooling histories. Nimrod ductile L-S tectonites derived from igneous and sedimentary protoliths yield $$^{40}Ar/^{39}Ar$$ hornblende cooling ages of 524-495 Ma and muscovite cooling ages of 499-496 Ma. These ages are interpreted to date the cooling following syn-metamorphic ductile deformation. Combined $$^{40}Ar/^{39}Ar$$ and existing U-Pb age data yield an average post-kinematic cooling rate for the Nimrod tectonites of ~10°C/m.y. Metasedimentary L-S tectonites of the Lanterman complex yield $$^{40}Ar/^{39}Ar$$ cooling ages of ca. 486 Ma (hornblende) and ca. 482 Ma (muscovite). These cooling ages indicate an average post-kinematic Lanterman cooling rate of ~30°C/m.y. We interpret the contrasting thermal histories of these terrains to be the result of different modes of accretion along the orogen, due in part to oblique subduction of paleo-Pacific lithosphere beneath the East Antarctic craton. In the central Transantarctic Mountains, Nimrod cooling rates indicate relatively modest post-tectonic denudation rates (~0.4 mm/a), resulting from crustal thickening in an upper plate-margin setting. Contraction of adjacent continental-margin supracrustal sequences did not involve significant under-thrusting, possibly as a result of a high component of margin-parallel translation. In contrast, markedly faster Lanterman cooling rates indicate more rapid denudation (~1.2 mm/a) of thickened crust in northern Victoria Land. Kinematic and cooling-rate data from the Lanterman complex indicate that accretion of outboard lower Paleozoic volcanic arc and continental-rise assemblages involved near-orthogonal underthrusting beneath crystalline basement, leading ultimately to rapid tectonic denudation.
The Modoc fault zone is a prominent zone of simple shear that has been mapped for 250 km from near Columbia, SC to the Ocmulgee River, in central GA. The steeply northwest-dipping fault zone is up to 5 km wide and contains variably mylonitic paragneiss and synkinematic sheets of mylonitic granite. Rotated tension gashes, reverse-slip-slip-crenulations, and asymmetric porphyroclasts in the fault zone are interpreted to indicate oblique dextral and normal movement. U/Pb zircon ages of 315--300 Ma yielded by some of these granite sheets are interpreted to date the time of movement on the Modoc fault zone, relatively early during the Alleghanian orogeny (ca 330--265Ma). Concurrent with movement along the Modoc fault zone, granite bodies (dated at 320--300 Ma) were intruded into both the hangingwall and the footwall sides of the fault. Cooling ages of ca 308 Ma (U/Pb monazite) and ca 305--288 Ma (40Ar/39Ar hornblende) from footwall rocks near the Savannah River indicate rapid cooling from temperatures above 700 starting with movement along the Modoc fault zone. Published geobarometry results suggest that footwall rocks were uplifted from depths of ca 29km and juxtaposed next to hangingwall rocks at depths of ca 11km by movement along the Modoc fault zone.more » Taken together, the crustal omission, uplift and rapid cooling of the footwall blocks, and the oblique normal sense of shear indicate at least a component of crustal extension along the Modoc fault zone. Intrusion of granite into and adjacent to the fault indicates magmatism accompanied movement on the fault at ca 315--300 Ma. Regardless of tectonic mechanism, extension associated with either crustal delamination or dextral transcurrent motion of accreted terranes, it is clear that crustal extension and magmatism was important during early phases of the Alleghanian orogeny in this part of the orogen, and it may have also been important elsewhere.« less
Abstract The Protogine Zone comprises a system of anastomosing deformation zones which approximately parallel the eastern boundary of the Sveconorwegian (1200–900 Ma) province in south‐west Sweden. Ages of granulite facies metamorphism in the Sveconorwegian province require exhumation from c . 30 to 35 km crustal depths after 920–880 Ma. 40 Ar/ 39 Ar cooling ages are presented for muscovite from high‐alumina rocks formed by hydrothermal leaching associated with the Protogine Zone. Growth of fabric‐defining minerals was associated with a ductile deformational event; muscovite from these rocks cooled below argon retention temperatures ( c . 375 ± 25° C) at c . 965–955 Ma. Muscovite from granofels in zones of intense alteration indicates that temperatures > 375 ± 25° C were maintained until c . 940 Ma. Textural relations of Al 2 SiO 5 polymorphs and chloritoid suggest that dated fabrics formed during exhumation. The process of exhumation, brittle overprint on ductile structures and hydrothermal activity along faults within the Protogine Zone tentatively are interpreted as the peripheral effects of initial Neoproterozoic exhumation of the granulite region of south‐western Sweden. Muscovite in phyllonites associated with the ‘Sveconorwegian thrust system’cooled below argon retention temperatures at c . 927 Ma. Exhumation associated with this cooling could have been related to extension and onset of brittle‐ductile deformation superimposed on Sveconorwegian contraction.
40 Ar/ 39 Ar and Rb–Sr mineral ages have been determined from various lithologies exposed in the Caledonian foreland and structurally overlying thrust nappes of north Sutherland, Scotland. Rb–Sr muscovite ages of c . 428, c . 421 and c . 413 Ma obtained from Moine Thrust Zone mylonites are interpreted to date closely regional thrusting during the Late Silurian to Early Devonian. 40 Ar/ 39 Ar muscovite ages within the lower parts of the Moine nappe are mostly anomalously old with respect to Rb–Sr analyses of muscovites from the same samples; it is likely that this discrepancy results from a component of extraneous or ‘excess’ argon. 40 Ar/ 39 Ar hornblende ages and Rb–Sr and 40 Ar/ 39 Ar muscovite ages obtained from structurally higher metamorphic units in the Caledonian thrust nappes generally range between c . 440 Ma and c . 410 Ma. These ages are interpreted to date cooling during and following ‘D 2 ’ regional thrusting and folding within internal sectors of the nappe sequence. A possible tectonic model involves the Silurian collision of Baltica with Scottish segments of Laurentia resulting in the Scandian orogeny and broadly coeval Moine Thrust Zone. D 2 structures were superimposed on structures and metamorphic fabrics formed during a regional Mid-Ordovician tectonothermal event dated previously at c . 470–460 Ma. Syn-D 2 temperatures were generally >600°C and sufficient to achieve more or less complete thermal rejuvenation of Rb–Sr and 40 Ar/ 39 Ar isotopic systems in muscovite and hornblende, even in areas of low D 2 strain.
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