Deciphering the structural and metamorphic history of the Balsfjord Series in the Upper Allochthon of the Scandinavian Caledonides in northern Norway

<p>Deciphering the structural and metamorphic history of the Balsfjord Series in the Upper Allochthon of the Scandinavian Caledonides in northern Norway</p><p>Höpfl Stephan<sup>1</sup>, Konopásek Jiří<sup>1</sup>, Stünitz Holger<sup>1,2</sup> Bergh G., Steffen<sup>1</sup></p><p>UiT Norges arktiske universitet, Institutt for geovitenskap, [email protected]</p><p> </p><p><sup>1</sup>Department of Geosciences, UiT The Arctic University of Norway, Tromsø 9037, Norway</p><p><sup>2</sup>Institut des Sciences de la Terre (ISTO), Université d’Orleans, Orleans 45100, France</p><p> </p><p>The Balsfjord Series is located in the central part of Troms–Finnmark County, northern Norway, and is part of the upper allochthon of the Scandinavian Caledonides. It consists of an Ordovician–Silurian metsedimentary sequence lying on top of the mostly gabbroic Lyngen Magmatic Complex (LMC). The unit exhibits an inverted metamorphic gradient, where the metamorphic conditions increase from the base to the top, from very low grade in the southeast to medium grade in the west and northwest. The Balsfjord Series is sandwiched between two high-grade units, the Nakkedal + Tromsø Nappe Complex in the hanging wall and the Nordmannvik Nappe as the top part of the Reisa Nappe Complex (RNC) in the footwall. The Nakkedal + Tromsø Nappe Complex features metamorphic peak ages of ca. 455–450 Ma and the Nordmannvik Nappe of ca. 430 Ma. The peak metamorphism of the Balsfjord Series has never been dated and the role of the inverted metamorphic gradient is not yet understood. One of the main motivations in this project is to resolve the Caledonian deformation history in the Balsfjord Series, ideally leading to a regional tectonic model explaining the tectonostratigraphic and metamorphic relationships between the abovementioned units.</p><p>The Balsfjord Series features two main discernible folding phases. The earlier phase displays tight to isoclinal folds with flat lying axial surfaces parallel to the penetrative foliation. Observed fold axes are parallel with the stretching lineation. These folds are best preserved in the northwestern, upper part of the unit and are syn-metamorphic in certain areas, as they fold original bedding (transposed foliation). A later folding phase is represented by mainly open folds with inclined to steep axial surfaces. Their fold axes are gently plunging with a predominant NE–SW orientation. We interpret these two folding events to be genetically related but slightly diachronous. The earlier folding phase with flat lying axial surfaces was likely generated during nappe thrusting and peak metamorphism of the Balsfjord Series. The subsequent open folding with inclined to steep axial surfaces is explained as a result of continued shearing and shortening of the weaker metapelitic Balsfjord Series against the more rigid gabbroic part of the LMC during the late stages of the Caledonian nappe thrusting.      </p><p>Observed thrust kinematics and penetrative retrogression at the bottom of the Nakkedal + Tromsø Nappe Complex suggest that its final exhumation took place during prograde metamorphism of the underlying Balsfjord Series. The ongoing dating of the prograde metamorphism in the Balsfjord series will provide important information about a possible continuity between the timing of peak metamorphism in the Nakkedal + Tromsø Nappe Complex, the Balsfjord series and the underlying RNC.</p>

What slates can tell us about strain localisation and fluid flow in accretionary wedges: a microstructural analysis of deforming foreland basin sediments

<p>During the accretion of foreland basin sediments into an accretionary or orogenic wedge, the sediments dehydrate and deform. Both dehydration and deformation intensity increase from the outer to the inner wedge and are a function of metamorphic processes and strain. Here, we study the microstructural evolution of slates from the exhumed Flysch units making up a paleo accretionary wedge in the European Alps. With classic SEM imaging and synchrotron X-ray fluorescence microscopy, we document the evolution of slate fabrics and calcite veins and aim at understanding the role of the evolving slate fabrics for strain localisation and fluid flow at the micro-scale.</p><p>The investigated slate samples are from a NW-SE transect covering the outer and inner wedge from 200 to 330 °C. The metamorphic gradient directly correlates with an increasing background strain gradient. With the use of the autocorrelation function, we quantify the evolution of the microfabrics along the metamorphic gradient and document deformation stages from a weakly deformed slate without foliation in the outer wedge to a strongly deformed slate with a dense spaced foliation in the inner wedge. The foliation mainly forms by dissolution-precipitation processes, which increase with increasing metamorphic gradient.</p><p>The slate matrix reveals two main sets of veins. The first vein set includes micron-scaled calcite veinlets with very high spatial densities. The second vein set includes layer parallel calcite veins that form vein-arrays (couple of metres thick) in the inner wedge. Both vein sets could have moved large amounts of fluids through the wedge. The spatial distribution of the micron-veinlets reveals that fluids were moved pervasively. In the case of the layer parallel veins forming vein-arrays, fluid flow was localized, supported by the dense spaced foliation formed in the slate matrix in the inner wedge. This way we now establish a direct link between the microstructural evolution in the slate matrix and associated dehydration, where fluids become increasingly channelled towards the inner wedge. Knowing that the vein-arrays have length dimensions in the order or hundreds of metres to kilometres, these structures are important for larger-scale fluid flow, the feeding of fluids into megathrusts and for related seismic activity in the wedge.</p>

Investigating heat transfer through the Lepontine Dome (Central European Alps) with a combined petrological, structural, dating and modelling approach

<p>Heat transfer during and after the emplacement of tectonic nappes within an orogeny is controlled by three fundamental processes: advection, diffusion and production of heat. Production is mainly caused by radioactive decay and shear heating. The relative importance and timing of these processes is often contentious. For example, in the Lepontine Dome of the Central European Alps the timing of the thermal evolution and the relative importance of advection, diffusion and shear heating is disputed. To better constrain and understand heat transfer in the Lepontine Dome, we apply a combined approach of petrological and structural analysis, zircon dating of migmatites and theoretical modelling.</p><p>We use data from an almost vertical transect (in the Ticino’s valleys) cutting from bottom-to-top the Simano, Cima Lunga and Maggia gneissic nappes. These nappes show an extremely pervasive mineral and stretching lineation (NW-SE directed) indicating non-coaxial deformation during shearing at amphibolite facies metamorphic conditions. The transition from the Simano to the Cima-Lunga nappe is marked by a progressive change in the texture of gneisses, in which the porphyroblasts become more stretched from the bottom to the top. Locally, at the tectonic contacts, syn-tectonic migmatites have been found. Their leucosomes contain metamorphic zircons with ages spreading from 40 to 31 Ma (U-Pb dating). <br>The widespread paragneisses frequently contain garnets of different sizes and internal microstructure. Published and own petrological data of these garnet-bearing rocks attest an inverted metamorphic gradient from ca. 700°C to 650-600 °C at intermediate pressures below the Cima Lunga unit during the peak-T amphibolite facies condition.</p><p>Overall, the field data depict a major km-scale shear zone that generated an inverted metamorphic gradient during the peak-T amphibolite facies condition between 40 and 31 Ma. These results hint that fast advection of heat or shear heating (or both component contempraneously) contributed to imprint the regional amphibolite facies metamorphism during nappe emplacement.</p><p>To take another step towards unravelling the controlling heat transfer processes in the Lepontine Dome and to test the relative importance of production, diffusion and advection, we employ three theoretical approaches with increasing complexity. First, we perform a dimensional analysis estimating dimensionless numbers, such as Peclet and Brinkman, for a range of reasonable parameters for the Lepontine Dome. Second, we apply numerical 2D thermo-kinematic simulations of trishear thrust-ramp evolution to test, for example, the impact of temperature-dependent viscosity and the geometrical relationship between temperature isogrades and nappe boundaries. Third, we apply state-of-the-art numerical 2D thermo-mechanical simulations of subduction and collision to investigate heat transfer and the resulting metamorphic facies distribution during the formation of an orogenic wedge.</p><p>Finally, we combine our modelling results with the available structural, age and metamorphic results to discuss potential scenarios for the heat transfer through the Lepontine dome.</p>

Metamorphic constraints on the tectonic evolution of the High Himalaya in Nepal: the art of the possible

This review presents an objective account of metamorphic, microstructural and geochronological studies in the Greater Himalayan Sequence (GHS) and adjacent units in Nepal in the light of recent research. The importance of integrated, multidisciplinary studies is highlighted. A personal view is presented of strategies for determining pressure–temperature evolution, and of petrological processes at the micro scale, particularly in relation to departures from equilibrium and the behaviour of partially-melted rock systems. Evidence has accumulated for the existence within the GHS of a High Himalayan Discontinuity, marked by differences in timing of peak metamorphism in the hanging wall and footwall, and changes in P–T gradients and paths. Whether or not this is a single continuous horizon, it forms at each location the lower boundary to a migmatitic zone capable of ductile flow, and separates the GHS into an upper division in which channel flow may have operated in the interval 25–18 Ma, and a lower division characterized by an inverted metamorphic gradient, and by metamorphic ages that decrease downsection and are best explained by sequential accretion of footwall slices between 20 and 6 Ma. An overall model for extrusion of the GHS is still not resolved.

Blueschist-facies metamorphism in the Kaczawa Mountains (Sudetes, SW Poland) of the Central-European Variscides: P-T constraints from a jadeite-bearing metatrachyte

Sodic pyroxene is reported from an Ordovician metatrachyte of the Kaczawa Mountains, SW Poland. Its composition ranges from Jd0.98Ae0.02 to Jd0.15Ae0.85. Relict jadeite and phengite (up to 3.75 Si atoms per fomula unit) belong to the peak-pressure assemblage of an early HP-LT event. Later greenschist-facies stages are represented by riebeckite, biotite, chlorite, low-Si potassic white mica and actinolite. P-T pseudosections calculated for the range 200–450°C, 3–13 kbar allow evaluation of the conditions of formation of jadeite in the metatrachyte and derivaton of a P-T path. Considering the position of prograde, peak and retrograde metamorphic assemblages and respective mineral compositions, we can derive the following equilibration stages: 8.5±0.5 kbar, 270±20°C for the pressure maximum, 6.0±1.0 kbar, 310±20°C for the temperature maximum and 3.5±0.5 kbar, 280±20°C as well as <3.5 kbar, <280°C for the retrograde stages.The metamorphic gradient for the peak-pressure is estimated at ∼10°C/km, which is typical of a subduction setting involving subducted continental crust, in particular of an exhumation channel within a collision zone of a microplate. Based on earlier structural observations, the ESE-oriented subduction in the NE Bohemian Massif was confined with WNW thrusting and followed by extension and ESE backward normal faulting during Devonian–Early Carboniferous times.

Metamorphic Gradient: A Regional-Scale Area Selection Criterion for Gold in the Northeastern Superior Province, Eastern Canadian Shield

ABSTRACT One hundred years after the first gold discoveries in the Abitibi subprovince, the Archean James Bay region to the north is experiencing a major exploration boom. Poor geologic coverage in this part of the northeastern Superior province has hindered the application of traditional Abitibi exploration criteria such as crustal-scale faults and “Timiskaming-type” sedimentary rocks. New area selection criteria are needed for successful greenfield exploration in this frontier region, and the use of steep metamorphic gradients is presented as a possible alternative. The statistical robustness of the metamorphic gradient area selection criterion was confirmed by using the curve of the receiver operating characteristic (ROC) to estimate the correlation between metamorphic fronts and the distribution of known Abitibi orogenic gold producers. The criterion was then applied to the James Bay region during a first-pass craton-scale exploration program. This was part of the strategy that led to the discovery of the Eleonore multimillion-ounce gold deposit in 2004.