Tectonic evolution of the Engi Slates, Glarus Alps, Switzerland

We present a geological map, profiles and the results of a detailed structural analysis of the Early Oligocene Engi Slates southwest of the village of Engi in the Sernft Valley in canton Glarus (Switzerland). In this area, the Engi Slates are folded on a deca- to hectometer scale into tight NW-vergent folds with sharp hinges. This took place during the Plattenberg F1 folding phase. No axial plane foliation was formed. The F1 folds are unconformably cut-off by a 16-25° NE dipping thrust, along which Eocene marls were emplaced onto the folded Engi Slates. We refer to this thrust as Riedboden Thrust and correlate it with the base of the upper, chaotic part of the Wildflysch Nappe of Oberholzer (1942). This is a (probably tectonic) mélange of rocks from the underlying Sardona, Blattengrat and North Helvetic Flysch Units. It lies as a 5 to 100 m thick, more or less continuous nappe below the Glarus Thrust. A younger, SE-dipping tectonic foliation (Plattenberg F2 foliation) cuts both through the folded Engi Slates, the Riedboden Thrust and the Eocene marls. This foliation is the axial plane foliation to meter-scale open F2 folds. It developed parallel to the overturned limbs of the F1-fold

The brittle evolution of Western Norway – A space-time model based on fault mineralizations, K–Ar fault gouge dating and paleostress analysis

Basement fracture and fault patterns on passive continental margins control the onshore landscape and offshore distribution of sediment packages and fluid pathways. In this study, we decipher the spatial-temporal evolution of brittle faults and fractures in the northern section of the passive margin of Western Norway by combining field observations of fault mineralizations and K–Ar fault gouge dating with different paleostress approaches, resulting in the following model: (1) High-T fault mineralizations indicate Silurian NW-SE compression followed by NW-SE extension in the Early to Mid-Devonian. (2) Epidote, chlorite and quartz fault mineralizations indicate a dominant strike-slip stress field in the Late Devonian to early Carboniferous. (3) E-W extensional stress fields which could be related to Permo-Triassic or Late Jurassic rifting are not prominent in our data set. (4) K–Ar fault gouge ages indicate two extensive faulting events under a WNW-ESE transtensional stress regime with related precipitation of zeolite and calcite in the mid (123-115 Ma) and late (86-77 Ma) Cretaceous. Our results show that the brittle architecture of the study area is dominated by reactivation of ductile precursors and newly formed strike-slip faults, which is different from the dip-slip dominated brittle architecture of the southern section of the West Norway margin.publishedVersio

Constraining the tectonic evolution of rifted continental margins by U–Pb calcite dating

We employ U–Pb calcite dating of structurally-controlled fracture fills within crystalline Caledonian basement in western Norway to reveal subtle large-scale tectonic events that affected this rifted continental margin. The ages (15 in total) fall into four distinct groups with ages mainly ranging from latest Cretaceous to Pleistocene. (1) The three oldest (Triassic-Jurassic) ages refine the complex faulting history of a reactivated fault strand originated from the Caledonian collapse and broadly correlate with known rifting events offshore. (2) Two ages of ca. 90–80 Ma relate to lithospheric stretching and normal fault reactivation of a major ENE-WSW trending late Caledonian shear zone. (3) We correlate five ages between ca. 70 and 60 Ma with far-field effects and dynamic uplift related to the proto-Iceland mantle plume, the effect and extent of which is highly debated. (4) The five youngest ages (< 50 Ma) from distinct NE–SW trending faults are interpreted to represent several episodes of post-breakup fracture dilation, indicating a long-lived Cenozoic deformation history. Our new U–Pb data combined with structural and isotopic data show that much larger tracts of the uplifted continental margin of western Norway have been affected by far-field tectonic stresses than previously anticipated, with deformation continuing into the late Cenozoic.publishedVersio

Tectonic evolution of the Engi Slates, Glarus Alps, Switzerland

ISSN:1661-8734ISSN:1661-872

The behaviour of monazite from greenschist facies phyllites to anatectic gneisses: An example from the Chugach Metamorphic Complex, southern Alaska

Monazite is a common accessory mineral in various metamorphic and magmatic rocks, and is widely used for U-Pb geochronology. However, linking monazite U-Pb ages with the PT evolution of the rock is not always straightforward. We investigated the behaviour of monazite in a metasedimentary sequence ranging from greenschist facies phyllites into upper amphibolites facies anatectic gneisses, which is exposed in the Eocene Chugach Metamorphic Complex of southern Alaska. We investigated textures, chemical compositions and U-Pb dates of monazite grains in samples of differing bulk rock composition and metamorphic grade, with particular focus on the relationship between monazite and other REE-bearing minerals such as allanite and xenotime. In the greenschist facies phyllites, detrital and metamorphic allanite is present, whereas monazite is absent. In lower amphibolites facies schists (~. 550-650. °C and ≥3.4. kbar), small, medium-Y monazite is wide-spread (Mnz1), indicating monazite growth prior and/or simultaneous with growth of garnet and andalusite. In anatectic gneisses, new low-Y, high-Th monazite (Mnz2) crystallised from partial melts, and a third, high-Y, low-Th monazite generation (Mnz3) formed during initial cooling and garnet resorption. U-Pb SHRIMP analysis of the second and third monazite generations yields ages of ~. 55-50. Ma. Monazite became unstable and was overgrown by allanite and/or allanite/epidote/apatite coronas within retrograde muscovite- and/or chlorite-bearing shear zones. This study documents polyphase, complex monazite growth and dissolution during a single, relatively short-lived metamorphic cycle

The behaviour of monazite from greenschist facies phyllites to anatectic gneisses: An example from the Chugach Metamorphic Complex, southern Alaska

AbstractMonazite is a common accessory mineral in various metamorphic and magmatic rocks, and is widely used for U–Pb geochronology. However, linking monazite U–Pb ages with the PT evolution of the rock is not always straightforward. We investigated the behaviour of monazite in a metasedimentary sequence ranging from greenschist facies phyllites into upper amphibolites facies anatectic gneisses, which is exposed in the Eocene Chugach Metamorphic Complex of southern Alaska. We investigated textures, chemical compositions and U–Pb dates of monazite grains in samples of differing bulk rock composition and metamorphic grade, with particular focus on the relationship between monazite and other REE-bearing minerals such as allanite and xenotime. In the greenschist facies phyllites, detrital and metamorphic allanite is present, whereas monazite is absent. In lower amphibolites facies schists (~550–650°C and ≥3.4kbar), small, medium-Y monazite is wide-spread (Mnz1), indicating monazite growth prior and/or simultaneous with growth of garnet and andalusite. In anatectic gneisses, new low-Y, high-Th monazite (Mnz2) crystallised from partial melts, and a third, high-Y, low-Th monazite generation (Mnz3) formed during initial cooling and garnet resorption. U–Pb SHRIMP analysis of the second and third monazite generations yields ages of ~55–50Ma. Monazite became unstable and was overgrown by allanite and/or allanite/epidote/apatite coronas within retrograde muscovite- and/or chlorite-bearing shear zones. This study documents polyphase, complex monazite growth and dissolution during a single, relatively short-lived metamorphic cycle

Large-scale, short-lived metamorphism, deformation, and magmatism in the Chugach metamorphic complex, southern Alaska: A SHRIMP U-Pb study of zircons

We present sensitive high-resolution ion microprobe (SHRIMP) U-Pb geochronology and trace-element chemistry of zircons from the Chugach metamorphic complex, southern Alaska, which allow us to precisely constrain the age and duration of peak metamorphism

Tectonic evolution of syn- to late-orogenic sedimentary- volcanic basins in the central Norwegian Caledonides

We present new structural, geochemical and U–Pb zircon data from syn- to late-orogenic sedimentary–volcanic basins in the southwestern part of the Trondheim Nappe Complex, central Norwegian Caledonides. In this area, a succession of enriched mid-ocean ridge basalt type metabasalt, jasper, ribbon chert with associated sandstone and conglomerate, and green siltstone is interpreted to represent volcanism and sedimentation in a hitherto little-known spreading-dominated tectonic environment. This environment is different from the suprasubduction-zone ophiolite setting dominating the Iapetus rock record elsewhere in the Scandinavian Caledonides. This volcanic and sedimentary succession was overturned and isoclinally folded in a pre-427 Ma orogenic phase. Post-427 Ma cross-bedded sandstones were deposited on the eroded surface of the previously deformed rocks, representing a rare example of a late Silurian or younger sedimentary basin within the Scandinavian Caledonides. The cross-bedded sandstones are intercalated with and/or overlain by post-427 Ma intermediate volcanic or subvolcanic rocks of calc-alkaline composition, representing a hitherto unknown volcanic phase within the Trondheim Nappe Complex and elsewhere within the Scandinavian Caledonides. Their particular geochemical signature could be the result of late-stage subduction-zone volcanism just prior to the onset of continent–continent collision between Baltica and Laurentia, or much younger post-collisional extensional melting with inherited subduction signatures