Deformation Mechanisms of Blueschist Facies Continental Metasediments May Offer Insights Into Deep Episodic Tremor and Slow Slip Events

Abstract Exhumed fossil subduction zones are archives of the deformation conditions and mechanisms from depths not directly accessible. Microstructural analysis of samples exhumed therefrom offers insights into the micromechanics and deformation processes associated with subduction such as earthquakes, slow earthquakes, and aseismic creep. Subducted and exhumed continental metasediments of the Tuscan Metamorphic Units of the Italian Northern Apennines contain a mylonitic foliation and quartz and carpholite dilational hydroshear veins with crack-and-seal textures, both developed at blueschist facies conditions (350?400°C, ?1 GPa). As shown by overprinting relationships and mineral assemblages, these two structure types formed broadly coeval within the stability field of carpholite. Metaconglomerates and metaquartzarenites deformed mainly by dissolution-precipitation creep and secondary by dislocation creep. Microstructural and electron backscatter diffraction analyses of the veins suggest only limited recrystallization of quartz fibers by subgrain rotation recrystallization, with adjacent metapelite bands acting as decollement horizons, likely by slip on the basal plane of phyllosilicates. We estimated differential stresses of 43?55 MPa and strain rates between 10?13 and 10?14 s?1 from the vein recrystallized quartz fibers. We propose these microstructures and deformation mechanisms to represent a geological evidence of deep episodic tremor and slow slip events in subducted continental metasediments. Pore pressure cyclically reached supralithostatic values triggering tremors causing fracturing of all involved lithotypes. Likely, slow slip was accommodated preferentially by slip on phyllosilicate bands. Aseismic creep occurred mainly by dislocation creep with subgrain rotation recrystallization in vein quartz, slip on the basal plane of phyllosilicates, and dissolution and precipitation creep in the host rock

Tectonomagmatic evolution of the Sveconorwegian orogen recorded in the chemical and isotopic compositions of 1070–920 Ma granitoids

The Sveconorwegian Province in Southern Norway and Sweden hosts at least four granitoid suites, representing apparently continuous magmatism at the SW margin of the Fennoscandian Shield between 1070 and 920 Ma. This study presents a compilation of published and new zircon LA-ICP-MS U-Pb geochronology, whole-rock and zircon geochemistry and Sm-Nd isotope data for the granitoid suites and demonstrates the granitoids’ ability to record changes in the tectonomagmatic evolution of this orogenic Province. The Sirdal Magmatic Belt (SMB, ca. 1070–1010 Ma) represents the earliest magmatism, west in the Province, followed by two hornblende-biotite granitoid suites (HBG, ca. 1000–920 Ma) and the Flå–Iddefjord–Bohus suite (FIB, ca. 925 Ma), in central and eastern parts of the Province, respectively. The SMB and the HBG bodies located outside of the SMB (referred to as HBGout) are chemically similar, whereas the HBG bodies located in the same region as the SMB (referred to as HBGin) are more ferroan, enriched in incompatible elements and have higher zircon saturation temperatures. Isotopically, the SMB and both HBG suites fall on an evolutionary trend from widespread 1.5 Ga crust in the region, suggesting this was the dominant crustal contribution to magmatism. The FIB suite is more peraluminous, rich in inherited zircon, and has isotopic compositions suggesting a more evolved source than both the HBG suites and the SMB. Trace element modelling shows that the SMB and HBGout suites could have formed by 50% partial melting of 1.5 Ga crust, whereas 5–10% remelting of the dehydrated and depleted SMB residue accounts for the geochemical composition of the HBGin suite. The available data suggest a scenario where the 1.5 Ga lower crust underwent melting due to long-lived mafic underplating giving rise to the SMB suite. After ca. 1000 Ma, regional-scale extension may have led to more widespread mafic underplating causing remelting of the residue following SMB melt extraction, forming the HBGin suite, with lower-crustal melting farther east forming the HBGout suite. Changes in melt composition over this 150 Myr time interval may thus be ascribed to an evolving melt source rather than fundamental changes in tectonic regime. Deep continental subduction at ca. 990 Ma, east in the orogen, provided an isotopically evolved crustal source for the FIB suite. The data underline the difference in tectonic processes across the orogen, with long-lived, high temperatures in the western and central parts and colder, high-pressure events in the eastern parts of the orogen

Evolution of the Gállojávri ultramafic intrusion from U-Pb zircon ages and Rb-Sr, Sm-Nd and Lu-Hf isotope systematics

The Karasjok–Central Lapland Greenstone Belt is one of the largest Palaeoproterozoic greenstone belts in the Fennoscandian Shield and includes multiple (ultra)mafic intrusions, some with notable ore reserves, formed during three episodes at 2.44, 2.22 and 2.05 Ga. This study presents new mineralogical, geochronological and isotopic data for the Gállojávri ultramafic intrusion, in the Karasjok Greenstone Belt, northern Norway. Previous petrogenetic modelling suggests that the intrusion was emplaced as a conduit system open for influx of melt with signs of polybaric fractionation and assimilation. Zircon U-Pb geochronology yields an age of 2051 ± 8 Ma, interpreted to reflect magmatic crystallisation. Large variations in isotopic signature over decimetres to metres indicate incomplete magma mixing. In bulk samples, εNd(t) ranges from −15 to 4. Zircon εHf(t) ranges from −14 to −1. Bulk 87Sr/86Sr(t) shows an apparent range from 0.5041 to 0.7072: the anomalously low values and general alteration indicates that 87Sr/86Sr is non-primary, whereas the less mobile Sm-Nd/ Lu-Hf systems are interpreted to represent primary magmatic signatures. We ascribe the large variations in the Nd and Hf isotopic signatures to local melting or dissolution of xenoliths and influx of variably contaminated melt into the semi-consolidated Gállojávri magma chamber, consistent with a conduit model involving variable replenishment and crustal interaction. The most evolved isotopic signatures cannot be accounted for by interaction with the local Archaean basement, indicating the presence of unidentified crustal components at depth. The Gállojávri intrusion shows many petrogenetic similarities to other c. 2.05 Ga (ultra)mafic intrusions in the Central Lapland Greenstone Belt

Metamorphic refinement of quartz under influence of fluids during exhumation with reference to the metamorphic/metasomatic evolution observed in amphibolites: a detailed field, microtectonic and geochemical study from the Bamble sector, South Norway

Outline of the thesis This thesis is comprises three papers that are intended for journal publication. The version provided in the thesis is a bit longer than they will be at final publication. However it was the desire to include some additional documentation that may be omitted in the final versions. Each paper in the manuscript is written as an independent paper. Because all the papers are strongly tied together there is significant repetition partly because the documentation and interpretation of the results in on paper depends on the results documented in another paper. The essence of the three papers are summarised below: Paper 1: Sørensen, B.E. and Larsen R.B. 2007. “Fluid induced multistage recrystallisation microstructures in Quartzites and Quartz veins from the Bamble shear zone complex” Paper 1 deduces basic principles of infiltration, physiochemical alteration processes and deformation mechanisms in quartzites and in quartz veins from the Froland area, the Bamble sector. SEM-CL is used to define generations of recrystallised quartz. A progressive quartz purification process ending with high purity quartz through recrystallisation is documented through LA-ICP-MS. The study demonstrates how fluids provokes recrystallisation and trace element mobilisation from the quartz lattice and at fluid-absent conditions, quartz is resistant to retrograde recrystallisation hence preserving its early high grade metamorphic trace element signature. Evidence of retrograde fluid flow is preserved in fluid channel textures intersecting quartz and documented by SEM-CL images as well as changes in the trace element distribution. Quartz recrystallisation is associated with metasomatic processes in the quartzites such as for example the replacement of ilmenite by rutile and replacement of biotite by muscovite. The strong focus of fluid flow along narrow pathways intersecting the quartzites generates mm-scale differences in the rheological properties of the quartz grains with increased ductility and recovery in recrystallised grains. Therefore, the study demonstrates a strong coupling between strain softening, fluid flow and mass transfer in shearzones in quartz rich rocks. Paper 2: Sørensen, B.E. and Larsen R.B. 2007. “The fluid evolution of the Froland area in the Bamble sector from peak P-T through cooling and uplift: implications for retrograde mineral paragenesis and PT evolution of the Bamble sector” Paper 2 comprises a detailed characterisation of the fluid evolution during cooling and uplift through fluid inclusion studies compared with thermodynamic modelling of calc-silicate volatile dependant mineral equilibria. Fluids are documented to be brines throughout the cooling and uplift path. The paper also includes a detailed discussion of the chemical properties of the aggressive brines and their implications for mineral equilibria during cooling and exhumation. The study also document which fluids that were in equilibrium with quartz recrystallisation and purification. The study focussed on fluids that could be related to the quartz SEM-CL textures documented in Paper 1. Had the main purpose of this thesis been to understand calc-silicate equilibria in the Bamble sector fluid inclusions in the calc-silicates would have been in the focus. Paper 3: Sørensen, B.E, Larsen R.B. and Austrheim, H. 2007. “Metasomatic evolution of the Froland amphibolites during cooling and uplift – textural observations and geochemical evolution of hydrous minerals” Paper 3 comprises a detailed study of alteration processes in amphibolites. The study focuses on the chemistry of amphiboles as a function P, T and the composition of the co-existing fluids. Amphibole and biotite undergo a co-genetic evolution both becoming more Mg-rich during cooling and exhumation. The chemical changes in biotite and amphibole reflects the interaction with the brines present throughout cooling and uplift. A relationship between element depletion in amphibolites and formation of Fe-Cu sulphides is observed. As earlier stated the papers are strongly tied together. In conjunction with each other they deduce the metamorphic and metasomatic evolution of the area and put the observed quartz recrystallisation and purification into a physiochemical framework. The locality numbering varies between the papers because samples from different localities are included in each paper, accordingly a locality termed”1” in one of the papers may have a different number in the next paper. This was done because the papers are supposed to be independent research papers and the numbering logically relate to the context of the single papers and not the thesis as a whole

In situ evidence of earthquakes near the crust mantle boundary initiated by mantle co2 fluxing and reaction-driven strain softening

This study aims to understand the process behind the worldwide connection between deep crustal/upper mantle earthquakes and CO2 emissions along faults in rift zones. We do this by studying CO2-induced mineral reactions that facilitate strain localization in peridotites from an ancient rift zone in the Seiland Igneous Province (SIP), North Norway. Strain localization in association with hydration processes is well documented in all types of tectonic settings and has major implications for rheological behavior in active plate margin processes. The implications of CO2-bearing fluids are less studied, though experiments have shown how CO2 can influence the flow laws of olivine by imposing a brittle and more localized type of deformation. This study documents narrow shear zones observed within ultramafic rocks from the Seiland Igneous Province (SIP) comprising large volumes (>20,000 km3) of mafic, ultramafic, silicic and alkaline melts that were emplaced into the lower continental crust (25–30 km) between 570 and 560 Ma under an extensional regime. The extensional shear zones are mm cm-scale and contain extremely fine-grained material with a distinct shape preferred orientation (SPO), but weak to absent crystallographic preferred orientation. The shear zones offset dykes across numerous micro-faults that are documented in areas close to a major fault zone cutting through the area. Within the shear zones, olivine and clinopyroxene react to form orthopyroxene and dolomite at approximately 11 kb and 850 °C according to the reaction: 2 Olivine + Clinopyroxene + 2 CO2 = Dolomite + 2 Orthopyroxene This reaction formed coronas of orthopyroxene and dolomite between olivine and clinopyroxene in the shear zones. In addition, large olivine grains proximal to the shear zones show a microfabric with subgrain walls decorated by rounded grains of dolomite and more irregular and elongated grains of orthopyroxene. Clinopyroxene grains are separated from the enstatite and dolomite by at least hundreds of microns, suggesting material transport within the shear zone. The shear zones thus provide a unique insight into the interplay between CO2-metasomatism and reaction accommodated strain softening. Carbonation-driven cracking and mineral reaction also serves to reduce grain size, making grain boundary sliding an efficient process, further enhancing the rheological contrast between the shear zone and the host rock. The sudden decrease in rock strength could lead to rapid deformation and triggered pseudotachylite formation during earthquake events in the near proximity of the micro-shear zones. Our observations match the relations between CO2 emissions and earthquakes observed in present rift environments such as the East African rift and in New Zealand, and underline the importance of active shear zones as fluid conduits in the lower crust and upper mantle

Compositional Features and Swelling Potential of Two Weak Rock Types Affecting Their Slake Durability

Weak and weathered rocks are well known for their sensitivity to changes in moisture content. Degrading behavior is common in weak rocks with moisture-sensitive mineral components and present numerous stability problems. The slake durability is a measure of the resistance to weakening and disintegration of rock materials which quantitatively distinguishes durable from non-durable rock materials. Several rock material parameters interact on the process of disintegration when exposed to cyclic moisture changes, whereby the content of clay is believed to play a major role. This manuscript evaluates the overall material composition of flysch and serpentinite rocks cored from the wall of the shotcrete-lined headrace tunnel of a hydropower project, including minerals, structure, porosity, the presence of micro-discontinuities, and swelling potential, and links these properties to the slake durability. Further, the different methods used to assess compositional features affecting the durability of weak rocks are evaluated and discussed. The manuscript argues that the mineralogical composition and microstructures present in the intact rock and the content of moisture-sensitive constituents, as swelling clays, control the long-term durability of weak rock material. It is demonstrated that XRD assessments are not sufficient to detect the content of brucite and swelling components, and that methods as thin section and SEM analyses should be carried out in the assessment of weak and weathered rock mass