The Malpica-Tui complex (NW Iberian Massif) consists of a Lower Continental Unit of variably deformed and recrystallized granitoids, metasediments and sparse metabasites, overridden by an upper unit with rocks of oceanic affinities. Metamorphic minerals dated by the Ar-40/Ar-39 method record a coherent temporal history of progressive deformation during Variscan metamorphism and exhumation. The earliest stages of deformation (D1) under high-pressure conditions are recorded in phengitic white micas from eclogite-facies rocks at 365-370 Ma. Following this eclogite-facies peak-metamorphism, the continental slab became attached to the overriding plate at deep-crustal levels at ca. 340-350 Ma (D2). Exhumation was accompanied by pervasive deformation (D3) within the continental slab at ca. 330 Ma and major deformation (D4) in the underlying para-autochthon at 315-325 Ma. Final tectonothermal evolution included late folding, localized shearing and granitic intrusions at 280-310 Ma.Dating of high-pressure rocks by the Ar-40/Ar-39 method yields ages that are synchronous with published Rb-Sr and Sm-Nd ages obtained for both the Malpica-Tui complex and its correlative, the Champtoceaux complex in the French Armorican Massif. The results indicate that phengitic white mica retains its radiogenic argon despite been subjected to relatively high temperatures (500-600 degreesC) for a period of 20-30 My corresponding to the time-span from the static, eclogite-facies M1 peak-metamorphism through D1-M2 eclogite-facies deformation to amphibolite-facies D2-M3. Our study provides additional evidence that under certain geological conditions (i.e., strain partitioning, fluid deficiency) argon isotope mobility is limited at high temperatures, and that Ar-40/Ar-39 geochronology can be a reliable method for dating high pressure metamorphism. (C) 2003 Elsevier B.V. All rights reserved.

Southeastern sectors of the Bohemian Massif are locally transected by two generations of lamprophyric dykes which post-date internal Variscan deformation of the variably metamorphosed constituent nappe units. Dykes of the first generation trend ESE, are locally weakly foliated and have been variably affected by low grade metamorphism. This generation is interpreted to have been emplaced during final WNW-ESE shortening of both external and internal nappe complexes. A 323 +/- 3 Ma Ar-40/Ar-39 biotite plateau age from a representative unmetamorphosed dyke of the first generation is interpreted to reflect cooling after magmatic crystallisation. Second generation dykes are unfoliated and unmetamorphosed. These follow a major NNE-trend, and were emplaced into all nappe complexes of internal sectors of the orogen after regional Variscan deformation and metamorphism. Three biotite concentrates from the younger dykes record Ar-40/Ar-39 plateau ages of 315-306 Ma. Chemical compositions of second generation dykes vary from monzogabbroic, potassic mafic to trachyandesitic of a high-K series. They are interpreted to have originated from a crust-contaminated mantle source which likely resulted from post-collisional remelting of subducted lithosphere. Emplacement was related to late stage orogenic extension which allowed ascent of magmas generated by post-collisional remelting of subducted lithosphere or within the asthenosphere following lithospheric break-off. The new data constrain an interval of some 10-20 My between Variscan plate collision and subsequent late-stage extension.

Regional cooling in the course of Neoproterozoic core complex exhumation in the Central Eastern Desert of Egypt is constraint by 40Ar/39Ar ages of hornblende and muscovite from Meatiq, Sibai and Hafafit domes. The data reveal highly diachronous cooling with hornblende ages clustering around 580 Ma in the Meatiq and the Hafafit, and 623 and 606 Ma in the Sibai. These 40Ar/39Ar ages are interpreted together with previously published structural and petrological data, radiometric ages obtained from Neoproterozoic plutons, and data on sediment dynamics from the intramontane Kareim molasse basin. Early-stage low velocity exhumation was triggered by magmatism initiated at ∼650 Ma in the Sibai and caused early deposition of molasses sediments within rim synforms. Rapid late stage exhumation was released by combined effect of strike-slip and normal faulting, exhumed Meatiq and Hafafit domes and continued until ∼580 Ma. We propose a new model that adopts core complex exhumation in oblique island arc collision-zones and includes transpression combined with lateral extrusion dynamics. In this model, continuous magma generation weakened the crust leading to facilitation of lateral extrusion tectonics. Since horizontal shortening is balanced by extension, no major crustal thickening and no increase of potential energy (gravitational collapse) is necessarily involved in the process of core complex formation. Core complexes were continuously but slowly exhumed without creating a significant mountain topography.

The Atacama Fault Zone is a major Mesozoic structure that trends along the Coastal Batholith of northern Chile. Part of the fault zone underwent a kinematic change from dip-slip to strike-slip displacement during the Early Cretaceous. The Las Tazas complex intruded the fault zone during this change. New analyses of country rock protomylonites from the edge of the complex firmly constrain the age of the change to 130 Ma and confirm that the complex was emplaced during active displacement along the fault zone. The intrusion heated its immediate country rocks and allowed localised ductile shearing during emplacement. Upper crustal intrusions like the Las Tazas complex are ideal targets for geochronological studies of major shear zones.

Isotopic and geochemical data indicate that intrusions in the eastern Creignish Hills of central Cape Breton Island, Canada represent the roots of arcs active at similar to 540-585 Ma and similar to 440 Ma. Times of intrusion are closely dated by (1) a nearly concordant U-Pb zircon age of 553 +/- 2 Ma in diorites of the Creignish Hills pluton; (2) a lower intercept U-Pb zircon age of 540 +/- 3 Ma that is within analytical error of Ar-40/Ar-39 hornblende plateau isotope-correlation ages of 545 and 550 +/- 7 Ma in the River Denys diorite; and (3) an upper intercept U-Pb zircon age of 586 +/- 2 Ma in the Melford granitic stock. On the other hand, similar to 441-455 Ma Ar-40/Ar-39 muscovite plateau ages in the host rock adjacent to the Skye Mountain granite provide the best estimate of the time of intrusion, and are consistent with the presence of granitic dykes cutting the Skye Mountain gabbro-diorite previously dated at 438 +/- 2 Ma. All the intrusions are calc-alkaline; the Skye Mountain granite is peraluminous. Trace element abundances and Nb and Ti depletions of the intrusive rocks are characteristic of subduction-related rocks. The similar to 540-585 Ma intrusions form part of an extensive belt running across central Cape Breton Island, and represent the youngest Neoproterozoic are magmas in this part of Avalonia. Nearby, they are overlain by Middle Cambrian units containing rift-related volcanic rocks, which bracket the transition from convergence to extension between similar to 540 and 505/520 Ma. This transition varies along the Avalon are: 590 Ma in southern New England, 560-538 Ma in southern New Brunswick, and 570 Ma in eastern Newfoundland. The bi-directional diachronism in this transition is attributed to northwestward subduction of two mid-ocean ridges bordering an oceanic plate, and the migration of two ridge-trench-transform triple points. Following complete subduction of the ridges, remnant mantle upwelling along the subducted ridges produced uplift, gravitational collapse and the high-temperature/low-pressure metamorphism in the are in both southern New Brunswick and central Cape Breton Island. The similar to 440 Ma are magmatism in the Creignish Hills extends through the Cape Breton Highlands and into southern Newfoundland, and has recently been attributed to northwesterly subduction along the northern margin of the Rheic Ocean.

Ar-40/Ar-39 mineral dating has been carried out within the amphibolite-facies metamorphic basement and greenschist-facies metamorphic cover of various tectonic units within the Kaintaleck Nappe, Eastern Alps, Austria, to evaluate the age of pre-Alpine metamorphism. Hornblendes display discordant Ar-40/Ar-39 apparent age spectra, minimum ages recorded in medium- to high-temperature gas release steps are ca. 430-405 Ma. White mica from micaschist record discordant Ar-40/Ar-39 age spectra with ages of ca. 350-379 Ma in medium- and high-temperature increments. White mica from discordant aplite and pegmatite record Ar-40/Ar-39 plateau ages of 375.4+/-0.4 Ma and 364.0+/-0.8 Ma respectively. The new isotopic ages indicate that (1) mid-Paleozoic (e.g. 430-380 Ma) tectonornetamorphic activity is recorded within the basement of the Kaintaleck Nappe; (2) this basement cannot represent the metamorphic basement for Ordovician to Late-Cretaceous sedimentary sequences of the Noric-Tirolic nappe complex (within uppermost units of the Austroalpine nappe complex); (3) the tectonometamorphic evolution of this basement unit contrasts with that of other basement units exposed in the Eastern Alps, where predominantly "late Variscan" (e.g. 330-300 Ma) tectonometamorphic events are recorded in Silurian to Early Carboniferous passive continental margin sequences; and therefore (4) at least two contrasting terranes comprise the Austroalpine basement.

The Austroalpine basement nappe complex, Eastern Alps, resulted from Mesozoic A-subduction which predated Paleogene collisional tectonics in the Alps. We present a new model for the age and P-T conditions of eclogite formation in the Sieggraben unit of the Austroalpine nappe complex. This unit is a tectonic melange which includes eclogite-bearing metamorphic units with both ophiolite-like fragments (retrogressed N-MORB type eclogites and serpentinites) and supracrustal, probably pre-Alpine, continental rocks. Mineral chemistry and textural characteristics indicate a three stage metamorphic evolution of the eclogites: (1) Inclusions of hornblende and epidote in eclogite-facies garnets suggest that an epidote amphibolite facies assemblage dehydrated to eclogite; (2) P-T conditions maintained during eclogite formation were c. 670-750 degrees C and 14-15 kbar; and (3) retrogression of eclogite included replacement of omphacite by symplectite including sodic augite, sodic plagioclase and formation of epidote + hornblende-bearing assemblages (c. 500-600 degrees C, c. 6-10 kbar). Ar-40/Ar-39 analyses of hornblende concentrates within retrograde assemblages yielded internally-discordant age spectra in which intermediate-temperature increments record similar apparent ages and plateau isotope correlation ages between 136.1 +/- 0.5 Ma and 108.2 +/- 0.3 Ma. These date the last cooling through c. 500 degrees C. We interpret Late Jurassic to early Late Cretaceous eclogite metamorphism and deformation of the Austroalpine nappe complex as having resulted from subduction of continental crust after consumption of the Meliata/Hallstatt ocean.

A paradox in dating metamorphic events in low-grade, polymetamorphic terranes is exemplified by the eastern Lower Austroalpine nappes of the Eastern Alps. Here, the last metamorphic event, best recorded in post-Variscan cover rocks, is dated as Late Cretaceous in age (c. 80–85 Ma) by white mica Rb/Sr and 40 Ar/ 39 Ar systems. Within the underlying polymetamorphic basement, 40 Ar/ 39 Ar and Rb/Sr ages of phengitic white mica record only Early and/or Late Variscan ages (375–270 Ma), indicating that the Alpine greenschist facies metamorphic overprint virtually caused no rejuvenation of Variscan mineral ages. Based on these results, the timing of a penetrative, ductile, top–to-WNW simple shear deformation recorded within both basement and cover rocks was contradictory. Deformation within the post-Variscan cover rocks had to be Alpine in age, whereas phengite 40 Ar/ 39 Ar ages from basement mylonites yield Variscan ages. To date this deformation directly, we isolated different mineral size fractions (63–30 and 30–10 μm) from highly strained shearbands within the Wechsel basement nappe. A resulting Rb/Sr errorchron pointed to an Upper Cretaceous age (c. 85 Ma) for the deformation, consistent with the timing of Alpine metamorphism in the cover rocks. Coarser-grained white mica (150–300 μim) from similar basement mylonites do not reflect any Alpine overprint of either K/Ar and/or Rb/Sr systems. It follows that dynamic re-and/or neocrystallization induced by ductile deformation within the shearbands was the dominant process by which the Rb/Sr system locally virtually re-equilibrated. This is valid even for overprinting metamorphic conditions below the temperatures required for Ar diffusional loss in phengitic white mica (c. <350°C). The data suggest that mineral ages that date low-grade mylonitization (e.g., white mica 40 Ar/ 39 Ar) should be considered with caution.

Polyphase Alpine transcurrent faulting and thrusting in the Apuseni Mountains in Romania resulted in the juxtaposition of contrasting structural units that include both metamorphic assemblages and Permian‐Mesozoic cover. Resolution of the chronology of Alpine events and distinction from pre‐Alpine evolution has been possible by systematic 40Ar/39Ar mineral dating of medium‐ and low‐grade assemblages. Hornblende and muscovite within nonretrogressed northern sectors of the Someş gneissic assemblage record rapid postmetamorphic cooling between 317 and 300 Ma, following a regionally penetrative “Late” Variscan tectonothermal overprint. The Codru amphibolite‐granodiorite assemblage records markedly older 40Ar/39Ar mineral ages (400–365 Ma hornblende; 340–335 Ma muscovite) that date delayed postmetamorphic cooling following “Early” Variscan tectonothermal activity. Although variably overprinted during Mesozoic tectonism, “Late” Variscan tectonothermal activity (320–300 Ma) is also evident from analysis of muscovite concentrates from mylonitic granite and schist within low‐grade assemblages in the regional Highiş‐Biharia shear zone (HBSZ). Shallow‐dipping phyllonites throughout western sectors of the HBSZ record effects of Aptian to Albian (114–100 Ma) north‐vergent thrusting and concomitant low‐grade metamorphism. A penetratively sheared Triassic phyllonite in the footwall yielded a similar whole‐rock plateau, age of ca. 117 Ma. Southern sectors of the Baia de Arieş carbonate‐lense gneissic assemblage record both Jurassic (186–156 Ma) and Early‐Middle Cretaceous (124–111 Ma) ages in different structural units. The 40Ar/39Ar results suggest that the Apuseni Mountains experienced a complex tectonothermal evolution that included “Early” and “Late” Variscan events and polyphase Alpine tectonism within major shear zones developed in interior sectors of the Carpathian arc.

Polyphase Alpine transcurrent faulting and thrusting in the Apuseni Mountains in Romania resulted in the juxtaposition of contrasting structural units that include both metamorphic assemblages and Permian-Mesozoic cover. Resolution of the chronology of Alpine events and distinction from pre-Alpine evolution has been possible by systematic Ar-40/Ar-39 mineral dating of medium- and low-grade assemblages. Hornblende and muscovite within nonretrogressed northern sectors of the Somes gneissic assemblage record rapid postmetamorphic cooling between 317 and 300 Ma, following a regionally penetrative "Late" Variscan tectonothermal overprint. The Codru amphibolite-granodiorite assemblage records markedly older 40Ar/39Ar mineral ages (400-365 Ma hornblende; 340-335 Ma muscovite) that date delayed postmetamorphic cooling following "Early" Variscan tectonothermal activity. Although variably overprinted during Mesozoic tectonism, "Late" Variscan tectonothermal activity (320-300 Ma) is also evident from analysis of muscovite concentrates from mylonitic granite and schist within low-grade assemblages in the regional Highis-Biharia shear zone (HBSZ). Shallow-dipping phyllonites throughout western sectors of the HBSZ record effects of Aptian to Albian (114-100 Ma) north-vergent thrusting and concomitant low-grade metamorphism. A penetratively sheared Triassic phyllonite in the footwall yielded a similar whole-rock plateau, age of ca. 117 Ma. Southern sectors of the Baia de Aries carbonate-lense gneissic assemblage record both Jurassic (186-156 Ma) and Early-Middle Cretaceous (124-111 Ma) ages in different structural units. The 40Ar/39Ar results suggest that the Apuseni Mountains experienced a complex tectonothermal evolution that included "Early" and "Late" Variscan events and polyphase Alpine tectonism within major shear zones developed in interior sectors of the Carpathian are.

From three Rugen boreholes (G14, Rugen 5, Loissin I)across the Caledonian deformation front four Lower Palaeozoic slate samples and three detrital muscovite concentrates From associated sandstones have been analysed by Ar-40/Ar-39 analysis. Microprobe analysis of the detrital muscovites show homogeneous, bur distinct chemical compositions for each concentrate. Particulate organic matter reflectance and illite crystallinity indicate that all the analysed whole-rock samples have experienced anchizonal metamorphic conditions.Ar-40/Ar-39 analysis of the samples from the G14 and Loissin 1 wells give almost identical results. Muscovite concentrates define plateau ages of c. 830 Ma and show an isotopic rejuvenation of low-temperature increments around 440 Ma. Whole-rock slate samples give plateau ages of c. 440Ma and c.427 Ma. Isotopic rejuvenation in the low-temperature increments prior to 350 Ma in the Loissin 1 sample can be attributed to the Permo-Carbunifereous volcanism which is known From this borehole. In contrast, samples from the borehole Rugen 5 show a different isotope pattern. A muscovite concentrate defines a plateau age of 609 Ma. Two whole rock slate samples give internally discordant but identical Ar-40/Ar-39 spectra with consistent isotopic rejuvenation from high- to low-temperature increments from 510 Ma to 420 Ma.While for the G14 and Loissin 1 sandstone samples a Baltica-derived provenance can be assumed, greywackes of Rugen 5 indicate a Cadomian-influenced Perigondwanan detrital source. According to the results of whole-rock slate samples the main Caledonian tectonothermal event in NE-Germany occurred around the Ordovician-Silurian boundary or in the early Silurian (450 425 hla). From these data the Rugen Palaeozoic appears as an allochthonous Caledonian nappe complex which has been thrust much further onto the Baltic shield than previously thought.

In extensional tectonic settings, vertical movements of the earth's surface are largely controlled by rates of strain. Extensional strain rates within the Larderello geothermal area, located in inner parts of the Northern Apennines, have been evaluated together with uplift rates and 40Ar/39Ar cooling ages. Results indicate that extensional strain rates have been relatively low (5.8×10−17 s−1), since the middle Pliocene. Differential uplift rates are estimated to have been high (c. 0.2 mm/year). Strain and uplift rates within the Larderello region are consistent with the regional tectonic framework. The low strain rate, rapid uplifting and high heat flow within southern Tuscany can be explained by an eastward dipping lithospheric shear maintained since at least middle Pliocene times.

The Oki metamorphic complex exposed in the Oki-Dogo islands consists predominantly of psammitic and pelitic gneisses with subordinate amphibolite and rare calcareous gneiss. The Oki gneisses were regionally metamorphosed to general amphibolite facies conditions, with local development of granulite facies assemblages. Peak metamorphic conditions of c. 800 degrees C have been suggested. Hornblende concentrates from amphibolites collected within the Oki metamorphic complex record Ar-40/Ar-39 isotope correlation ages of 199-192 Ma. These are interpreted to date the post metamorphic cooling through temperatures required for intracrystalline retention of argon (c. 500 degrees C). Muscovite concentrates record Ar-40/Ar-39 plateau ages of 167-168 Ma. These are interpreted to date post metamorphic cooling through appropriate closure temperature of muscovite (c. 400-375 degrees C). Combined with the previously reported geochronological data, the Oki metamorphic complex appears to have experienced peak metamorphic conditions at c. 250 Ma. Subsequently, it cooled and was exhumed at the earth's surface at c. 90 Ma with cooling rate of c. 5 degrees C/Ma. The Oki metamorphic complex records a similar prograde metamorphic event as the Hida metamorphic complex exposed in central Japan. The cooling and exhumation rates of the Hida metamorphic complex were significantly more rapid compared with the Oki metamorphic complex, and they were exhumed with extensively intruded Jurassic granites (Funatsu granites). (C) 1998 Elsevier Science Ltd. All rights reserved.

40Ar/39Ar hornblende and muscovite ages indicate that basement units exposed within all major Alpine nappes of the Southern Carpathian orogen were penetratively deformed and metamorphosed during `late' Variscan (Carboniferous) tectonothermal events. Subsequent exhumation resulted in cooling from ca. 500°C (hornblende K–Ar retention temperature) to ca. 375°C (muscovite K–Ar retention temperature) between ca. 320 and 295 Ma. Late Variscan exhumation of middle crustal levels is interpreted to have resulted from intracontinental contraction and regional uplift subsequent to Variscan continental collision between Gondwana-derived tectonic elements (e.g., Africa) and Laurussia. Muscovite ages within mylonite zones suggest that late Variscan shear zones were probably associated with a regional shear system that developed along the southern margins of Laurussia. Alpine metamorphic effects within the Southern Carpathians are generally nonpenetrative, and are only significant within narrow zones of retrogression that are interpreted to have formed during initial attenuation of Variscan crust associated with formation of the Jurassic Severin rift.

Five muscovite concentrates from high-grade, pelitic metasedimentary basement rocks exposed in northwestern sectors of the Teplá-Barrandian zone (Czech Republic) record 40Ar/39Ar mineral plateau ages which range between ca. 376 and 362 Ma. Hornblende concentrates from metagabbro (Mariánské Lánzě complex) and fine-grained basement amphibolite display plateaux which define 36Ar/40Ar vs 39Ar/40Ar isotope-correlation ages of ca. 370 and ca. 375 Ma. The mineral ages are interpreted to date relatively rapid cooling through appropriate argon retention temperatures following early phases of Variscan (Early Devonian) regional metamorphism. A slate/phyllite basement sample collected within lower-grade metasedimentary rocks in southeastern portions of the Teplá-Barrandian zone is characterized by an internally discordant 40Ar/39Ar whole-rock age spectrum which suggests partial Variscan rejuvenation of intracrystalline argon systems which had cooled through appropriate argon retention temperatures following an initial regional metamorphism at or prior to ca. 500 Ma (Cadomian). Hornblende from undeformed diorite of the Kdynž complex records a well-defined 40Ar/39Ar age plateau which corresponds to an isotope-correlation age of ca. 516 Ma. This is interpreted to date post-magmatic cooling following emplacement.

The sequence of thrusting within a regional‐scale Alpine thick‐skinned tectonic wedge has been constrained by Rb‐Sr and 40Ar/39Ar dating of whole‐rock, constituent white mica and biotite from structural units of the Austroalpine Nappe Complex, Eastern Alps (Austria). Preservation of Variscan and pre‐Variscan isotopic signatures in pre‐Alpine basement units indicates a non‐penetrative metamorphic Alpine overprint. Alpine nappe assembly and accompanying ductile deformation occurred during maintenance of lower greenschist‐facies metamorphic conditions (c. 300°–450°C). Analyses from greenschist‐facies metamorphic mylonites and penetratively ductile deformed cover sequences display decreasing 40Ar/39Ar plateau ages structurally downward. Thrusting and accompanied ductile deformation commenced at c. 100–90 Ma in the uppermost structural levels. Footwall fault propagation effected internal deformation and out‐of‐sequence thrusting within intermediate structural levels at c. 95–80 Ma. This was contemporaneous with regional exhumation and cooling of deeper crustal basement units at c. 88–82 Ma and was associated with extension and strike‐slip faulting, leading to the formation of synorogenic sedimentary basins. Additional foot‐wall thrust propagation was associated with ductile deformation within lowermost structural units at c. 80–70 Ma. The structural and new geochronologic results presented herein suggest that (1) the Austroalpine Nappe Complex represents a forward propagating thrust complex that formed as a result of footwall propagation of a master fault and associated piggyback transport of hangingwall structural units; (2) most parts of the Austroalpine block occupied a lower‐plate tectonic setting during Cretaceous tectonics subsequent to subduction of the Meliata‐Hallstatt ocean; and (3) the Austroalpine Nappe Complex had already been formed when subduction of Penninic oceanic domains was active.

The Caledonian thrust nappes and basal decollement zone above the Baltic shield record penetrative Caledonian deformation related to ESE-directed nappe transport, and subsequent WNW-directed ductile shearing during extensional collapse of the orogen. Muscovite within Proterozoic quartzite conglomerates in the Lower Bergsdalen Nappe that record only ESE-vergent Caledonian deformation display internally concordant Ar-40/Ar-39 age spectra which yield plateau ages of 403 Ma and 398 Ma. Similar plateau ages (402 and 399 Ma) were obtained from muscovite from mica schists which record additional penetrative WNW-directed shearing, suggesting that muscovite intracrystalline argon systems did not cool below appropriate closure temperatures until during or after extensional WNW-vergent movements. Quartz microfabrics indicate that temperatures were higher than those required for argon retention in muscovite until the end of the WNW-directed nappe translation (extension), and that lower temperatures were maintained during subsequent movements along W- and NW-dipping extensional ductile shear zones. The Ar-40/Ar-39 muscovite ages are therefore interpreted to post-date the extensional WNW-directed shearing along the decollement zone, and closely date the initiation of W-and NW-dipping extensional shear zones. When compared to the age of contractional deformation, the results confirm that extension followed orogenic contraction very shortly after the main, Scandian orogenic phase. (C) 1998 Elsevier Science B.V. All rights reserved.

Multigrain concentrates of hornblende and muscovite together with whole-rock slate/phyllite samples have been dated (27 analyses) using Ar-40/Ar-39 incremental-release methods along a systematic traverse across the various lithotectonic structural elements which comprise northwestern sectors of the Variscan Iberian Massif. Hornblende concentrates from amphibolites in the allochthonous Ordenes Complex yield plateau isotope-correlation ages of 425 Ma and 377 Ma. Muscovite concentrates and whole-rock slate/phyllite from this and the Cabo Ortegal Complex yield plateau ages which range from 367 Ma to 295 Ma. Analyses of similar material from the relative autochthon yield plateau ages between 359 Ma and 275 Ma. Muscovite concentrates from three late-to post kinematic granitic stocks yield plateau ages between 309 Ma and 274 Ma.At least seven of the Ar-40/Ar-39 analyses from metamorphic rocks record variable thermal rejuvenation of intracrystalline argon systems associated with emplacement of proximal granitic stocks, The remaining analyses may be used to constrain the local age of various Variscan tectonothermal events. The oldest fabric ages are recorded in allochthonous units whereas the youngest fabric ages occur along the boundary between internal and external zones. Middle Devonian ages are recorded in the allochthon and suggest a chronological continuity with deformational events in the relative autochthon, where Variscan deformation initiated in the Upper Devonian and diachronously prograded eastward. The first deformational events recorded in the limit within the internal and external zones occurred ca. 20-25 Ma later (lower Namurian). Variscan deformation systematically prograded diachronously eastward across the orogen as new crustal material was added along the front of the developing orogenic wedge, However, the entire orogen remained tectonically active with different structural features forming at different times and at different places, An average propagation rate of ca, 5 km/m.y. is suggested by consideration of a 20-25 Ma difference in correlative fabric ages end present separations.

Five detrital white mica concentrates from very low-grade, metaclastic sequences within pre-Variscan basement and post-Variscan cover units of the Upper Austroalpine Nappe Complex (Eastern Alps) have been dated with 40Ar/39Ar incremental heating techniques to constrain the age of tectonothermal events in their respective source areas. Two samples from early Palaeozoic sandstone exposed within the same Alpine nappe record slightly discordant age spectra. The maximum age recorded in one is 562.2±0.7 Ma, whereas the other yielded a 40Ar/39Ar plateau age of 607.3±0.3 Ma. These results indicate a source area affected by Cadomian tectonothermal activity. Three detrital muscovite concentrates from post-Variscan, Late Carboniferous and Permian cover sequences exposed within three different Alpine nappes yielded 40Ar/39Ar plateau ages of 359.6 ± 1.1 Ma, 310.5±1.2 Ma, and 303.3±0.2 Ma. The contrasting detrital white mica ages are interpreted to reflect different source areas. Detrital muscovite from a post-Variscan Carboniferous molasse-type sequence and from a Permian Verrucano-type sequence record ages which indicate “late” Variscan (e.g. 330–300 Ma) metamorphic sources. By contrast, detrital white mica from another Permian Verrucano-type sequence suggests a source area affected by “early” Variscan (e.g. 400–360 Ma) metamorphism. These results help clarify palinspastic relationships and tectonic correlations between pre-Late Carboniferous metamorphic basement sequences and Carboniferous to Permian cover sequences.