Synkinematic White Mica Research Articles

Cenozoic evolution of the Tripolitza carbonate platform in the Tethyan realm: new age constraints on deposition, diagenesis, metamorphism and nappe emplacement based on U-Pb and Rb-Sr dating (External Hellenides, Crete)

AbstractWe present kinematic, radiometric, geochemical and PT data, which help to constrain the tectonometamorphic evolution of the Tripolitza Unit (TPU). The age of both the metamorphic peak (P = 0.4 ±0.2 GPa, T = ca. 310 °C) and top-to-the WNW mylonitic thrusting, attributed to the emplacement of the hanging Pindos nappe, has been constrained at 19 ±2.5 Ma using Rb-Sr on synkinematic white mica of a basal mylonite of NW Crete. This early tectonic event is also documented by the oldest generation of veins, which cut through less metamorphic (T = 240 ±15 °C) late Bartonian/Priabonian Nummulite limestone exposed as olistolith in TPU flysch of central Crete. Calcite of these veins yielded a similar U-Pb age at 20 ±6 Ma. U-Pb dating of matrix calcite, on the other hand, reflect the time of sedimentation (38.4 ±5.7 Ma and 37.6 ±1.2 Ma), which is in line with the faunal content of the black limestone. Geochemical data and U-Pb calcite ages of fibres of the Nummulite test (32.3 ±3.1 Ma and 34.6 ±0.9 Ma) suggest unexpected pseudomorphic fibre replacement during late Priabonian/early Rupelian diagenesis. Additional calcite veins, which developed at ca. 10–11 and 7 – 9 Ma (U-Pb on calcite), are attributed to top-to-the S thrusting and subsequent extension, respectively. The resulting anticlockwise rotation of the shortening direction within the TPU from WNW-ESE at ca. 20 Ma to N-S at ca. 10 Ma has significant implications for the geodynamic evolution of the External Hellenides.

Cenozoic kinematics of the Wenchuan-Maoxian fault implies crustal stacking rather than channel flow extrusion at the eastern margin of Tibetan plateau (Longmen Shan)

The Longmen Shan, eastern and steepest margin of the Tibetan plateau, is often seen as the archetype example of an orogenic system built by crustal channel flow extrusion since the Miocene. This model is controversial as other studies propose an accretionary prism mechanism. A key difference between these models resides in the kinematics proposed for the Wenchuan - Maoxian (WM) fault zone, a major tectonic structure of the Longmen Shan. We constrain the Cenozoic kinematics of the WM fault zone by combining structural observations, fault gouge K/Ar dating and 40Ar/39Ar dating of syn-kinematic white mica. Normal / right lateral ductile deformation occurred at 28.0 ± 0.9 Ma while top-to-the-east reverse deformation at 15.4 ± 0.2 Ma. K/Ar ages of authigenic illite from two fault gouges show that brittle right-lateral / reverse deformation was active at 6.9 ± 2.9 Ma. These ages are consistent with the relative vertical motions across the fault zone deduced from thermochronology. Three deformation phases can be identified: right-lateral / normal in the Middle Oligocene (∼30–25 Ma), reverse in the middle Miocene (25–15 Ma), and right-lateral / reverse since the upper Miocene (since ∼6 Ma). The WM fault zone never experienced pure normal motion, and only shortening since the Oligocene, in contrast to predictions of lower crustal channel-flow extrusion models. These results are in favor of a crustal accretionary prism model for the Cenozoic building of the eastern Tibetan plateau.

Dating recurrent shear zone activity and the transition from ductile to brittle deformation: White mica geochronology applied to the Neoproterozoic Dom Feliciano Belt in South Brazil

Argon geochronology of white mica has a good potential of recording recurrent dynamic recrystallization in shear zones in the transition from ductile to brittle deformation, as temperature ranges for mineral crystallization and isotopic closure of different grain-size fractions overlap between ca. 275–425 °C. This study presents over 40 K-Ar ages of coarse-grained muscovite and multiple clay-sized white mica fractions extracted from mylonites and fault gouges in southernmost Brazil, complemented by detailed structural and XRD characterization. The widespread obtained ages are best explained in the context of recurrent deformation under retrograde conditions during progressive strain localization and transition from low-grade ductile deformation into brittle faulting. The onset of the Ibaré Shear Zone is constrained in the Tonian, followed by reactivation during the Ediacaran evolution of the adjacent Dom Feliciano Belt, which records a remarkable long-term history of recurrent deformation for over 100 Myr. New data extends the geochronological constraints of the Dorsal do Canguçu Shear Zone into low-temperature conditions, estimating the onset of brittle activity at 525-470 Ma by dating early fault gouges formed shortly after final syn-kinematic white mica crystallization in the hosting mylonites. Dating of recurrent faulting events during the Paleo-Mesozoic complement the regional sedimentary and thermochronological record.

Eocene extension and meteoric fluid flow in the Wildhorse detachment, Pioneer metamorphic core complex, Idaho

The relationship between microstructure and fluid flow traced by hydrogen isotope ratios (δD) is examined within the Wildhorse detachment system of the Pioneer metamorphic core complex in south-central Idaho. Within the detachment footwall, 100-m-thick mylonitic quartzite containing minor white mica and K-feldspar displays a NW-trending stretching lineation and consistent top-to-the-NW sense of shear criteria. Microstructures within the detachment footwall comprise two groups: quartz ribbons and relict quartz grains flattened within the foliation, with porphyroclastic white mica fish; and intensely deformed and recrystallized quartz with high-aspect-ratio white mica arranged within C′ shear bands. White mica δD values are highly negative and cluster around −145‰ in high-aspect-ratio white mica and around −120‰ in porphyroclastic white mica fish. The most negative values are interpreted to reflect interaction with meteoric fluids from a high-elevation catchment (3000−4000 m), and the less negative values are interpreted to represent incomplete hydrogen isotope exchange between the meteoric fluid and the pre-extensional metamorphic fluid δD values in the white mica porphyroclasts. A suite of tightly clustered 40 Ar/ 39 Ar ages from synkinematic white mica in the detachment footwall dates deformation, recrystallization, fluid-rock interaction, and therefore the presence of high topography at 38−37 Ma; these ages are consistent with the cooling/exhumation history of the high-grade core of the Pioneer metamorphic core complex in the late Eocene. The 38−37 Ma 40 Ar/ 39 Ar ages are substantially younger than previously published ages of high topography in British Columbia to the north (49−47 Ma), in line with the hypothesis that high topography propagated from north to south in the northern segment of the North American Cordillera through Eocene time.

Meteoric water circulation in a rolling-hinge detachment system (northern Snake Range core complex, Nevada)

Combined petrofabric, microstructural, stable isotopic, and 40 Ar/ 39 Ar geochronologic data provide a new perspective on the Cenozoic evolution of the northern Snake Range metamorphic core complex in east-central Nevada. This core complex is bounded by the northern Snake Range detachment, interpreted as a rolling-hinge detachment, and by an underlying shear zone that is dominated by muscovite-bearing quartzite mylonite and interlayered micaschist. In addition to petrofabric, microstructural analysis, and 40 Ar/ 39 Ar geochronology, we use hydrogen isotope ratios (δD) in synkinematic white mica to characterize fluid-rock interaction across the rolling-hinge detachment. Results indicate that the western flank of the range preserves mostly Eocene deformation (49–45 Ma), characterized by coaxial quartz fabrics and the dominant presence of metamorphic fluids, although the imprint of meteoric fluids increases structurally downward and culminates in a shear zone with a white mica 40 Ar/ 39 Ar plateau age of ca. 27 Ma. In contrast, the eastern flank of the range displays pervasive noncoaxial (top-to-the-east) fabrics defined by white mica that formed in the presence of meteoric fluids and yield Oligocene–Miocene 40 Ar/ 39 Ar ages (27–21 Ma). Evolution of the Oligocene–Miocene rolling-hinge detachment controlled where and when faulting was active or became inactive owing to rotation, and therefore where fluids were able to circulate from the surface to the brittle-ductile transition. On the western flank (rotated detachment), faulting became inactive early, while continued active faulting on the eastern flank of the detachment allowed surface fluids to reach the mylonitic quartzite. The combined effects of synkinematic recrystallization and fluid interaction reset argon and hydrogen isotope ratios in white mica until the early Miocene (ca. 21 Ma), when the brittle-ductile transition was exhumed beneath the detachment.

Late Palaeozoic to Mesozoic kinematic history of the Talas–Ferghana strike-slip fault (Kyrgyz West Tianshan) as revealed by 40Ar/39Ar dating of syn-kinematic white mica

The NW-trending Talas–Ferghana Fault (TFF) in Kyrgyzstan, Central Asia, is one of the largest intracontinental strike-slip faults in the world. It extends over a distance of more than 2000km from southern Tourghai to western Tarim and exhibits a maximum dextral offset of ∼200km during the late Palaeozoic to present. The history of the fault provides important insights for the understanding of the evolution of southern Central Asia but remains poorly constrained due to lack of reliable geochronological data. We present new Ar–Ar ages and structural data from the Kyrgyz West Tianshan, that elucidate the kinematic history of the TFF in the Palaeozoic and Mesozoic. 40Ar/39Ar ages on mylonitic white micas document a deformational history consisting of several episodes. A late Carboniferous age of 312±4 Ma point to initiation of top-to-the-south and dextral transpressional deformation during a metamorphic overprint in Precambrian and Palaeozoic rocks along the northern compartment of the TFF. The main phase of dextral motion along the entire fault occurred in the Permian as suggested by minimum ages of 260–290 Ma obtained at two different locations in the NW and central parts of the TFF. Partial isotopic resetting occurred between 240 and 210Ma and younger ages of <200–210Ma are ascribed to late brittle reactivation and hydrothermal fluid flow in the Late Triassic to Early Jurassic times. The Jurassic trans-tensional phase is featured by emplacement of 195±3Ma pegmatitic dykes.The Ar–Ar mineral ages and structural data argue for a major phase of dextral shearing, affecting the entire region from the West Tianshan to Mongolia in the late Permian and leading to formation of almost equally spaced major NW-trending dextral strike-slip faults. The uniform character of this deformation indicates that the process of amalgamation in this part of the CAOB ended prior to the late Permian, and Central Asia evolved as a single coherent continental block since that time.

Formation of phyllosilicates in a fault zone affecting deeply buried arkosic sandstones: their influence on petrophysic properties (Annot sandstones, French external Alps)

The Restefond fault, located in the Late Eocene-Early Oligocene Alpine foreland basin, affects the well lithified and low porosity Grès d’Annot. The fault core zone is characterized by the occurrence of highly deformed sandstone lenses. Deformation inside the lenses corresponds to mm to sub-mm-spaced cleavage planes rich in phyllosilicates and up to cm-thick and dm-long quartz-calcite pure extensional veins. The cleavages are mostly composed of newly-formed synkinematic white mica and chlorite. By using thermodynamic thermometers based on the chemical composition of chlorite, a temperature of 200 ± 20 °C of fault activity was computed. This temperature shows that the Restefond fault was active at burial conditions comprised between 6.5 and 8 km, assuming a mean geothermal gradient between 25 and 30 °Ckm−1. The petrophysic properties of sandstones from the core zone and in the hanging and foot wall of the fault were determined on drilled plugs following three spacial directions. The permeability of the highly deformed sandstone from the core zone is about one order of magnitude higher than in the host rock. This increase in permeability occurs in the direction parallel to the S–C structures and is explained by the occurrence of well-connected micropores localized between platy phyllosilicates. This study shows that the fault petrophysic properties are mostly controlled by the precipitation of synkinematic phyllosilicates under deep burial conditions.

Dating low-temperature deformation by 40Ar/39Ar on white mica, insights from the Argentera-Mercantour Massif (SW Alps)

In order to date low-temperature deformation, intensely strained muscovite porphyroclasts and neocrystallized shear band phengite from greenschist-facies shear zones have been dated by 40Ar/39Ar method in the Argentera-Mercantour massif. Shear zones are featured by gradual mylonitization of a Variscan granite, gneiss and Permian pelite protolith (300–315Ma) during the Alpine orogenic event. Mineralogical and textural observations indicate that phengites and chlorites developed from biotite and plagioclase in fluid system during deformation following dissolution–transport–precipitation reactions of the type biotite + plagioclase + aqueous fluid = chlorite + albite + phengite + quartz + titanite + K-bearing fluid in the granite-gneiss mylonite. Contrariwise, phengite developed at the expense of clays following substitution reaction in pelite mylonite. Based on conventional thermobarometry on phengite and chlorite and Pressure–Temperature-aqueous fluid (P–T-MH2O) pseudosections calculated with shear zone bulk compositions, the conditions during shear deformation were estimated at 375±30°C and 4.8–7±1kbar in an H2O-satured system. In this low temperature environment, 40Ar/39Ar analysis of the Variscan muscovite for various grades of ductile strain intensity shows a limited 40Ar/39Ar isotopic resetting, all ages scattering between 296 and 315Ma. Under conditions of intense ductile deformation and large-scale fluid circulation, muscovite grains formed during the Variscan retain their much older ages. 40Ar/39Ar dating of very fine grained synkinematic phengite grains, neoformed during the Alpine history, give consistent plateau ages (34–20Ma) for each shear zone. In detail, 40Ar excess can be detected in the pelite mylonitic sample where phengites crystallized by substitution process while the other mylonitic samples where phengites grow from fluid-induced reactions do not evidence any 40Ar excess. These results demonstrate that the 40Ar/39Ar dating of neocrystallized synkinematic white mica allows the determination of precise ages of deformation and fluid activity. Together with precise thermobarometry undertaken on the basis of mineral chemistry and whole-rock composition, 40Ar/39Ar dating of white mica leads to the reconstitution of precise depth-deformation history of low-grade (<400°C) metamorphic units. At the Argentera-Mercantour massif scale, several stages of shear zone development at 15–21km depth are dated between 33 and 20Ma. In the SE part of the massif shear zone ages are well constrained to be either (1) 33.6±0.6Ma or in the range (2) 26.8±0.7Ma–26.3±0.7Ma. In the West of the massif, younger shear zone ages range between (3) 22.2±0.3Ma and (4) 20.5±0.3Ma.

Geochronological evidence for continuous exhumation through the ductile‐brittle transition along a crustal‐scale low‐angle normal fault: Simplon Fault Zone, central Alps

Major low‐angle normal faults juxtapose different structural levels of the crust that record both brittle and ductile deformation. Field relationships alone cannot establish whether these different responses to deformation represent (1) parts of a single process of exhumation along the detachment or (2) two separate events, with the later, more discrete brittle detachment exhuming a fossil ductile shear zone from depth. These two general models are critically assessed for the low‐angle normal Simplon Fault Zone (SFZ) in the central Alps. The SFZ shows a spatial transition from a broad ductile mylonitic shear zone to a discrete brittle detachment with identical kinematics. The age of the ductile shear zone and ductile‐brittle transition is controversial. We present a detailed geochronological study based on fission track, 40Ar/39Ar, and Rb/Sr microsampling dating, coupled with structural, petrological, and chemical analyses that provides tight constraints on SFZ timing. Discontinuous mineral cooling ages over a broad range of temperatures across the fault zone argue for fault activity between 20 and 3 Ma. On the basis of synkinematic white mica in low‐temperature shear zones and necks of foliation boudinage, the brittle‐ductile transition in the footwall could be dated at ∼14.5–10 Ma. Overall, the data presented here are consistent with a continuous transition from ductile shearing to a more localized zone of brittle deformation within the same geological framework, over a period of ∼15 Ma. The SFZ is therefore an example of a telescoped crustal section within a single major low‐angle fault, involving a continuous period of exhumation rather than a two‐stage structure.

Constraining deformation stages in brittle–ductile shear zones from combined field mapping and 40Ar/ 39Ar dating: The structural evolution of the Grimsel Pass area (Aar Massif, Swiss Alps)

40Ar/ 39Ar dating of syn-kinematic white mica (phengite) and biotite is used to constrain the age of shear zones within the granodiorites of the Grimsel Pass area (Aar Massif, Central Alps). Three sets of steeply dipping shear zones have been mapped. Stage 1 shear zones are very broad (∼200 m wide) zones of biotite-bearing rocks, with a progressive increase in intensity of ductile deformation from rim to core, and have dip-slip kinematics. Subsequent Stage 2 shear zones are narrower, contain phengite-rich mylonites, and show a gradual change from dextral strike-slip to dip-slip kinematics across strike. Strain localisation during the formation of Stage 2 shear zones is interpreted to have been aided by reaction-weakening during extensive fluid–rock interaction. Brittle precursors of Stage 2 shear zones are marked by biotite-rich cataclasites preserved along the northern contact of the overprinting main Stage 2 shear zone. Biotite from a Stage 1 shear zone yields an age of 21.1 ± 0.2 Ma. Phengites in Stage 2 mylonites in the Grimsel Pass area have a very narrow age range (13.8–12.2 Ma), which is interpreted to bracket the duration of ductile deformation during Stage 2 deformation at mid-crustal levels. However, the biotite-rich cataclasites at the margin of a Stage 2 shear zone may have localised fluid flow up to ∼2 Ma prior to the onset of main Stage 2 ductile deformation. Stage 3 shear zones are brittle strike-slip faults containing cataclasites, breccias and clay fault gouges. Earlier studies have established that these late faults formed at <9 Ma and were active until at least 3 Ma.

Late Sinistral Shearing along Gondwana’s Paleo‐Pacific Margin in the Ross Orogen, Antarctica: New Structure and Age Data from the O’Brien Peak Area

New isotopic data from O’Brien Peak, Antarctica, provide constraints on the timing of deformation within the Ross Orogen in the Queen Maud Mountains. U‐Pb data, in conjunction with structural data from a mylonitic granite, indicate that sinistral, orogen‐parallel ductile shearing occurred after ∼489 Ma, the granite’s crystallization age, and $$\mathrm{before}\,\,478\pm 3\,\mathrm{Ma}\,\,\mathrm{to}\,\,473\pm 3\,\mathrm{Ma}\,$$, the 40Ar‐39Ar cooling ages of undeformed and synkinematic white mica, respectively. These data provide evidence for the youngest sinistral shearing event within the Ross Orogen to date and indicate that left‐oblique subduction possibly continued into the Early Ordovician along Gondwana’s paleo‐Pacific margin.

40 Ar/ 39 Ar dating of synkinematic white mica: insights from fluid-rock reaction in low-grade shear zones (Mont Blanc Massif) and constraints on timing of deformation in the NW external Alps

Abstract This paper highlights the use of synkinematic white mica, biotite and phlogopite for the dating of deformation in ductile shear zones within crystalline rocks under low-grade metamorphic conditions. The Mont Blanc shear zones range from 1 mm to 50 m in width and have localized intense fluid flow, resulting in substantial differences in mineralogy and whole-rock geochemistry. On the basis of their synkinematic alteration assemblages and geographic distribution within the Mont Blanc Massif, three main metamorphic zones are distinguished within the network of shear zones. These are: (i) epidote±white mica-bearing assemblages; (ii) chlorite–phlogopite-bearing assemblages; and (iii) white mica±biotite±calcite±actinolite±epidote- bearing assemblages. 40 Ar/ 39 Ar age spectra of biotite and phlogopite are complex, and reflect significant variations in chemical composition. In biotite, this is partly due to inheritance from precursor Variscan magmatic biotite. In contrast, new white mica grew at the expense of feldspar during Alpine deformation and its Ar spectra do not show any excess 40 Ar. On the SE side of Mont Blanc, ages of shear zone phengites have a narrow range of 15.8–16.0±0.2 Ma, which is in the same age range as 40 Ar/ 39 Ar ages of minerals from kinematically related veins. The top-to-SE sense of shear is consistent with initiation of a Mont Blanc flower-structure within a dextral transpressional system by 16 Ma. On the NW side, mini-plateaux ages of 14.5±0.3 and 23.4±0.4 Ma are preserved in the same sample, suggesting the possibility of two phases of deformation. This is also supported by partly preserved ages of 18–36.6 Ma in biotites and phlogopites. Ages between 36 and 18 Ma might reflect ongoing top-to-NW thrusting, following Penninic Front activation, in a context of nappe stacking and crustal thickening. NW-directed thrusting on the NW side of Mont Blanc continued after 18 Ma, synchronous with SE-directed thrusting on the SE side of the massif. These divergent movements produced the overall pop-up geometry of the Mont Blanc Massif, which may correspond to a positive flower structure developed within a zone of regional dextral transpression extending SW from the Rhone valley into the Mont Blanc area.

Comment on 40Ar/39Ar age constraints on low-grade metamorphism and cleavage development in the Transvaal Supergroup (central Kaapvaal craton, South Africa): implications for the tectonic setting of the Bushveld Igneous Complex (South African Journal of Geology, 109, 393 410), by Alexandre et al. (2006)

Alexandre et al. (2006) recently presented the results of a detailed structural study of brittle-ductile deformation, which could, as they state, be seemingly coincident with regional low-grade metamorphism of the pre-2.15 Ga Transvaal Supergroup strata between the Vredefort impact structure of ~2.02 Ga age (Kamo et al. , 1996; Gibson et al. , 1997) and the Bushveld Complex that was intruded at ~2.06 Ga (Walraven, 1997; Buick et al. , 2001). Alexandre et al. (2006) also performed six 40Ar/39Ar step-heating dating experiments with synkinematic white mica specimens. The results comprise ages of ~2.15 Ga and 2042.1 ± 2.9 Ma, the latter derived from “plateaus and pseudo-plateaus, as well as cumulative probability statistics” ( ibid ). These authors suggest that “at ~2.04 Ga the rocks of the Transvaal Supergroup in the central [Kaapvaal] craton were a part of a more extensive fold-and-thrust belt, named here [ibid] the Transvaalide fold-and-thrust belt ”. Alexandre et al. (2006) admit the difficulties to explain the ~2.15 Ga phase (although they gave a try), but assign the time at ~2.04 Ga to “orogenic activity at the margins of the craton”. Alexandre et al. (2006) conclude (p.402) that “The age of 2042.1 ± 2.9 Ma is based on unambiguous plateau and pseudo-plateaus ([their] Figure 6), and therefore can be considered a robust and reliable one…”. They proceed to discuss that this age is clearly distinguishable from the ages of the Bushveld magmatic event and the Vredefort impact event, and that a definite event at that time was also supported by other chronological results for rocks from the entire region between the Limpopo Mobile Belt and the Vredefort impact structure. In the light of these authors’ conclusion based on the age of 2042.1 ± 2.9 Ma, it appears reasonable to scrutinize the argon chronological results …

40Ar/39Ar age constraints on low-grade metamorphism and cleavage development in the Transvaal Supergroup (central Kaapvaal craton, South Africa): implications for the tectonic setting of the Bushveld Igneous Complex

Rocks of the Palaeoproterozoic Transvaal Supergroup, situated between the Bushveld Igneous Complex and the Vredefort Impact Structure in the central parts of the Kaapvaal craton (South Africa), experienced a regional low-grade metamorphism synchronous with brittle–ductile deformation that formed small- to medium-scale folds, cleavages, monoclines and thrusts in the dolomites, slates, and phyllites. Single-grain 40Ar/39Ar step-heating laser probe dating of synkinematic white mica from the phyllites indicates ages of ~2.15 Ga and 2042.1 ± 2.9 Ma (plateaus and pseudo-plateaus, as well as cumulative probability statistics). These ages are clearly distinct from the emplacement age of the Bushveld Igneous Complex (~2.06 Ga) and from the Vredefort impact event (~2.02 Ga), both of which have previously been suggested as possible causes of the deformation. The new data suggest that at ~2.04 Ga, the rocks of the Transvaal Supergroup in the central craton were part of a more extensive fold-and-thrust belt, named here the Transvaalide fold-and-thrust belt. While it is difficult to explain the significance of the ~2.15 Ga ages, the ~2.04 Ga event may be related to orogenic activity along the margins of the Kaapvaal craton at this time.

Multistage exhumation of the Menderes Massif, western Anatolia (Turkey)

Structural analyses along selected transects across the Menderes Massif and incorporation of existing data have resulted in a synthetic cross section across southwestern Turkey. The tectonic interpretation has been combined with 40Ar/39Ar laser-probe experiments on two syn-kinematic white mica populations, which, respectively, predate and overprint documented Early Miocene deformational fabrics. Our results indicate that the regional extension had initiated by Eocene–Oligocene times. A 36±2 Ma 40Ar/39Ar age derived from white mica which formed during a regionally observed, northward-directed tectonic transport suggests that the extension was contemporaneous with the tectonic emplacement of mid-crustal continental basement. Alternatively, the age may relate to the regional cooling of the basement sequence, and thus postdates the tectonic emplacement. In the latter scenario, the analysis fails to solve the present ambiguity on the Pan-African or Alpine affinity of the northward-directed transport. Further exhumation of basement rocks is characterized by a dominant displacement along the north-dipping Gediz Detachment at the northern margin of the massif, which developed under ductile conditions in the Early Miocene by partial exploitation of older zones of weakness, which originated from an earlier phase of basement emplacement. Youngest (semi)ductile activity along the Gediz Detachment has been recorded as having a 7±1 Ma 40Ar/39Ar age derived from syn-kinematic white mica in the top of the detachment. Following the initiation of the ductile Gediz Detachment, an antithetic, semibrittle, and south-dipping Buyuk Menderes Detachment had developed in the center of the massif, which, together with the Gediz Detachment, accommodated further doming of the central part of the Menderes Massif and controlled the architecture of the surrounding supradetachment basins.

Alpine deformation and 40Ar/39Ar geochronology of synkinematic white mica in the Siviez‐Mischabel Nappe, western Pennine Alps, Switzerland

We explore the timing of deformation and exhumation of the Siviez‐Mischabel Nappe (western Swiss Alps), which has been considered a classic example of a midcrustal crystalline nappe since the studies of Argand [1916]. This study presents 40Ar/39Ar ages obtained on both synkinematic white mica from Permo‐Triassic cover sediments and more complex white mica populations from basement gneisses of the Siviez‐Mischabel and middle Pennine Nappes. Primary foliation developed in cover units by nucleation, growth, and rigid rotation of mica grains during noncoaxial Alpine deformation. Although some samples show a crenulation of this primary foliation, mica growth appears to have occurred only during the development of primary foliation, the main phase of greenschist facies deformation related to imbrication of the Siviez‐Mischabel Nappe and other middle Pennine Nappes. Good agreement exists between independent estimates of the timing of deformation and reported 40Ar/39Ar white mica ages from cover units of the central and southern Siviez‐Mischabel Nappe. In cover units from the central and southern Siviez‐Mischabel regions of the study area, 40Ar/39Ar ages appear to date synkinematic white mica growth. Results suggest that the Siviez‐Mischabel Nappe was emplaced and developed foliation during a 5 m.y. period from 41 to 36 Ma. In cover units from the eastern Siviez‐Mischabel, however, 40Ar/39Ar white mica ages appear to date postkinematic thermal events. These thermal events may be related to Oligocene magmatic activity in the lower Pennine Nappes or to Miocene development of the Simplon fault zone. Variations in the relation between Alpine age and grain size for cover samples from the central, eastern, and southern Siviez‐Mischabel correlate well with the regional variations in temperature inferred from quartz microfabrics and the pattern of regional metamorphism. When considered in concert with other recent isotopic studies on the timing of major tectonic and thermal events in the western Swiss Alps, these data support arguments that the relative timing of events such as thrusting and back thrusting of crystalline nappes in hinterland units and exhumation of high‐pressure units in the suture zone of the western Alps are intimately related and synchronous on the scale of a few million years.

Geochronologic, petrologic and kinematic constraints on the evolution of the Err-Platta boundary, part of a fossil continent-ocean suture in the Alps (eastern Switzerland)

The boundary between the Austroalpine Err nappe and the Penninic Platta unit represents part of a Late Cretaceous continent-ocean suture preserved in the highest stockwerk of the Tertiary Alpine orogen. Subduction and nappe stacking (D1) and subsequent extensional uplift (D2) along part of this suture are dated with the K-Ar method on two generations of syn-kinematic white mica in the Err nappe and with Ar-Ar step-heating of syn- to post-kinematic riebeckitic amphiboles in the Platta nappe. The first generation of white micas associated with isoclinal folds and W-directed mylonitic thrusting along nappe contacts yields a 76 to 89 Ma age range, whereas the second generation of white micas that grew during tight folding and top-to-the-E extensional D2 shearing yields 67 to 80 Ma ages. The K-Ar white mica ages are interpreted as ages of formation at the approximately 300°C temperature of lower green-schist facies regional metamorphism in the southern part of the Err nappe. The two amphibole samples from the Platta nappe yield 69 and 73 Ma Ar-Ar ages, interpreted as syn-D2 formational ages. The Si-content of the white micas in chlorite-K-feldspar-quartz-bearing meta-radiolarites in the southern part of the Err nappe indicate a pressure decrease from D1 to D2 deformation, probably accompanied by an increase in temperature. This can be related to D2 extensional uplift accommodated along reactivated D1 nappe contacts in the footwall of the Julier-Bernina nappe. When combined with stratigraphic evidence, the radiometric ages and deformation-crystallization relations indicate clockwise P-T paths for the Err, Platta, and Margna nappes: Suturing of the Austroalpine continental and Liguro-Piemontese oceanic crust began with near surface thrusting and flysch sedimentation between 88 and 100 Ma and culminated with D1 folding and nappe stacking under high P/T greenschist facies conditions sometime during the period 80 to 88 Ma. The unroofing of this suture was a multi-stage process involving Late Cretaceous D2 extension under greenschist facies conditions, Early Tertiary D3 thrusting under lower anchizonal conditions, and mid-Tertiary D4 normal faulting above the uplifting Lepontine dome.