Magnetic fabrics in Portuguese Variscan granites: structural markers of the Variscan orogeny

2020 ◽  
Author(s):  
Helena Sant Ovaia ◽  
Ana Gonçalves ◽  
Claudia Cruz ◽  
Fernando Noronha
Keyword(s):  
Stress Field ◽  
Shear Zones ◽  
Ductile Deformation ◽  
Magnetic Fabric ◽  
Variscan Orogeny ◽  
Maximum Compression ◽  
Compression Direction ◽  
Magnetic Fabrics ◽  
Central Portugal ◽  
Magnetic Lineations

<p>This work focuses on the magnetic fabric of 20 variscan granitic massifs from northern and central Portugal and considers the Anisotropy of Magnetic Susceptibility (AMS) results obtained in about 750 sampling sites. In the northern and central Portugal, three main ductile deformation phases were recognized and described: D<sub>1</sub>, D<sub>2</sub> and D<sub>3</sub>, being the variscan magmatism events mainly related to D<sub>3</sub> phase. D<sub>3</sub> produced wide amplitude folds with NW-SE subhorizontal axial plane and subvertical dextral and sinistral ductile shear zones, forming obtuse angles with the maximum compression direction, σ1, NE-SW oriented. The post-D<sub>3</sub> brittle phase was responsible for the development of conjugate faults (NNW-SSE, NNE-SSW and ENE-WSW), related to a N-S maximum compression. The studied granites were subdivided according to U-Pb dating, field observations and considering the chronology of their emplacement relative to the D<sub>3</sub> phase of Variscan orogeny. Therefore, the studied granites are subdivided into: (1) syn-D<sub>3</sub> two-mica granites, ca. 311 Ma; (2) late-D<sub>3</sub> monzogranites, biotite-rich and two-mica granites, ca. 300 Ma; (3) post-D<sub>3</sub> monzogranites and biotite-rich granites, ca. 299 – 297 Ma. Magnetic fabric gives two types of directional data, magnetic foliations and magnetic lineations, which provide important information regarding the orientation of the magmatic flow, feeder zone location, relationship between the magma emplacement and tectonics and, also, the stress field. The data obtained for the magnetic fabric, based on AMS technique, allowed concluding: (i) syn-D<sub>3</sub> granites show magnetic foliations and lineations consistent with the syn-D<sub>3</sub> variscan structures ca. N110°-120°E, related to a NE-SW maximum stress field . The foliations are, mainly, subvertical (> 60º), which may indicate a high thickness of the granitic body and deep rooting; on the other hand, the magnetic lineations exhibit variables plunges. (ii) Late-D<sub>3</sub> granites are characterized by foliations and lineations, dominantly NNW-SSE to NNE-SSW oriented. The foliations are subvertical dips (> 60º) and the lineations have, generally, soft plunges. (iii) Post-D<sub>3</sub> granites have, in general, magnetic foliations and lineations associated with important regional post-D<sub>3</sub> brittle structures, which display NNE-SSW and ENE-WSW trending. The subhorizontal fabric may suggest a small thickness of the granitic bodies. In all granite sets under study there is a dominance of weakly dipping lineations (slope <60º), indicating that the feeding zones are deep, which supports the idea of an emplacement at high structural levels.</p><p>Acknowledgments: The authors thank Department of Geosciences, Environment and Spatial Planning at Faculty of Sciences of the University of Porto and the Earth Sciences Institute (Porto Pole, Project COMPETE 2020 (UID/GEO/04683/2013), reference POCI-01-0145-FEDER-007690).</p>

scholarly journals Magnetic fabric of Siwalik sediments (Nepal): implications to time-space evolution of stress field

2008 ◽  
Vol 38 ◽  
pp. 39-48
Author(s):  
Pitambar Gautam
Keyword(s):  
Stress Field ◽  
Bedding Plane ◽  
Horizontal Stress ◽  
Magnetic Polarity ◽  
Magnetic Fabric ◽  
Compression Direction ◽  
Time Space ◽  
Depositional Age ◽  
Tectonic Rotation ◽  
Magnetic Lineations

Magnetic fabric data based on the anisotropy of magnetic susceptibility (AMS) of the sediments constituting the Siwalik sections (Kamali R., Amilia- Tui Road, Surai R., Tinau R., and Rato R.) in Nepal have been analysed for the variability of magnetic lineation and the implications to the time-space evolution of the stress field in this region during the last 16 myrs. This invo lved compilation of (i) the magnetic polarity data that constrain the depositional age of the Nepalese Siwaliks to ca. 16 to 1 Ma, and (ii) the declination of characteristic magnetic remanence to reveal the relative tectonic rotations (17° CCW at Butwal to 9° CW at Amilia). The magnetic fabric, defined mainly by alignment of paramagnetic minerals, corresponds to an oblate ellipsoid with foliation parallel to bedding plane, implying a sedimentary-compactional origin. The magnetic lineations show well defined clusters (confined in or close to the bedding plane). Being subparallel to the fold axes/bedding strikes/thrust fronts, these lineations are assumed to originate from a secondary mild deformation process related to the compression tectonics in the Siwalik foredeep and therefore correspond to the active direction of the minimum principal horizontal stress active during foredeep deposition. Hence, the direction of compression is orthogonal to the mean lineation. The compression direction in the palaeogeographic coordinates can be obtained by introducing an additional correction for the tectonic rotation about the vertical, using the palaeomagnetic declination. Available AMS-based lineations, corrected for rotation about vertical using palaeomagnetic declinations, reveal that the compression direction in the Himalayan foreland remained in general N to NNE with significant deviations in its far western part, in particular around the Amilia- Tui section where the direction was NS8°E.

Magnetic fabric in brittle faults and ductile shear-zones: Examples from cataclasites from the Iberian Peninsula

2020 ◽  
Author(s):  
Marcos Marcén ◽  
Antonio Casas-Sainz ◽  
Teresa Román-Berdiel ◽  
Belén Oliva-Urcia ◽  
Ruth Soto ◽  
...  
Keyword(s):  
Shear Zones ◽  
Fault Zones ◽  
Magnetic Fabric ◽  
Fault Rock ◽  
Fault Rocks ◽  
Magnetic Lineation ◽  
Magnetic Fabrics ◽  
Transport Direction ◽  
Wide Range ◽  
Magnetic Lineations

<p>Shear zones, or their counterparts in near-surface conditions, the brittle fault zones, constitute crustal-scale, narrow, planar domains where deformation is strongly localized. The variation with depth of deformation conditions (P-T), rheology and strain rates entails a wide range of fault rock types, characterized by different petrofabrics and classically grouped into mylonitic (fault rocks undergoing crystalline plasticity) and cataclasitic (fault rocks undergoing frictional deformation) series. Magnetic fabric methods (most frequently anisotropy of magnetic susceptibility, AMS) have been established as a useful tool to determine fault rock petrofabrics in shear/fault zones, being interpreted as kinematic indicators with a considerable degree of success. However, mylonites and cataclasites show remarkable differences in magnetic carriers, shape and orientation of the fabric ellipsoid. Here, we present a study of ten brittle fault zones (one of them at the plastic-brittle transition) located in various locations in the Iberian Plate, with an aim  to interpret patterns of AMS in cataclasites.</p><p>Reviewing AMS studies dealing with SC mylonites, three fundamental features can be drawn: i) the presence of composite magnetic fabrics with shape and lattice-preferred orientations, ii) the fabric is carried predominately by ferromagnetic minerals and iii) surprisingly in composite fabrics, the absolute predominance of magnetic lineations parallel to (shear) transport direction (88% of the reviewed sites), independently of fabrics being defined by paramagnetic or ferromagnetic carriers. Based on our study, magnetic fabrics in cataclasites: i) are mainly carried by paramagnetic minerals and ii) show a strong variability in magnetic lineation orientations, which in relation with SC deformational structures, are either parallel to transport direction (44% of sites) or parallel to the intersection lineation between shear (C) and foliation (S) planes (41%). Furthermore, changes between the two end-members can be frequently observed in the same fault zone. Sub-fabric determinations (LT-AMS; AIRM and AARM) also indicate that the type of magnetic lineation cannot be consistently related with a specific mineralogy (i.e. paramagnetic vs ferromagnetic minerals).</p><p>The wide range of deformation conditions and fault rocks covered in our study allowed us to analyse the factors that control these different magnetic lineation orientations, especially in brittle contexts. Plastic deformation results into a mineral stretching parallel to transport direction which can be directly correlated with the development of transport-parallel magnetic lineation. In brittle fault zones, the degree of shear deformation can be directly correlated with the type of magnetic lineation. The fault cores, where strain and slip are localized, show a predominance of transport-parallel magnetic lineations, most probably related with the development of lineated petrofabrics. Furthermore, the minor development of shear-related petrofabrics enhance the frequency of intersection-parallel magnetic lineations, also contributing the presence of inherited, host rock petrofabrics in the fault rocks.</p>

Magnetic fabric in carbonatic rocks from thrust shear zones

2021 ◽  
Author(s):  
Sara Satolli ◽  
Claudio Robustelli Test ◽  
Elena Zanella ◽  
Dorota Staneczek ◽  
Fernando Calamita ◽  
...  
Keyword(s):  
Cluster Analysis ◽  
Pure Shear ◽  
Shear Zones ◽  
Magnetic Fabric ◽  
Structural Deformation ◽  
Axis Orientation ◽  
Main Thrust ◽  
Magnetic Lineation ◽  
Magnetic Fabrics ◽  
And Cluster Analysis

<p><strong> </strong></p><p>The aim of this study is to investigate how structural deformation in shear zones is documented by the anisotropy of magnetic susceptibility (AMS). The study area is located in the Pliocene outer thrust of the Northern Apennines, which involved Cretaceous to Neogene calcareous and marly rocks. Here, brittle-ductile tectonites show different characteristics along two differently oriented thrust ramps: the NNE-SSW-trending oblique thrust ramp is characterized by the presence of S tectonites, while the NW-SE-trending frontal ramp is characterized by the presence of SC tectonites.</p><p>Samples for AMS fabric investigation were collected on shear zones from three sectors of the belt, at different distance from the main thrust to detect possible magnetic fabric variations. The three study area are characterized by different combinations of simple and pure shear, thus different degree of non-coaxiality, which has been quantified through the vorticity number W<sub>k</sub>.</p><p>Specimens were measured with an AGICO KLY-3 Kappabridge at the CIMaN-ALP Laboratory (Italy) on 15 different directions mode. Only measurements with all three F-statistics of the anisotropy tests higher than 5 were accepted as reliable. Moreover, outliers characterized by ± 2σ difference with respect to the mean value of AMS scalar parameters were excluded from further analysis. In order to distinguish groups of specimens affected by different sedimentary or tectonic processes, we identified clusters of AMS scalar parameters; when clusters were not defined by these parameters, we applied a combination of contouring and cluster analysis on each principal axis to identify different subfabrics.</p><p>The magnetic fabric revealed straightforward correlations with structural data and specific changes of AMS axis orientation depending upon the increasing of deformation (lower vorticity number) and proximity to the main thrust. Similar evolution was detected in different deformation regimes. Overall, the magnetic fabric is more sensitive to the simple shear deformation, as the magnetic lineation tends to parallelize mostly with the computed slip vector; however in pure-shear dominated regimes, the magnetic lineation becomes parallel to the transport direction when the deformation is really intense (sites at less than 15-30 cm from the thrust plane).</p><p>The applied combination of density diagrams and cluster analysis on AMS data successfully allowed discriminating subfabrics related to different events, and shows a great potential to unravel mixed sedimentary and/or tectonic features in magnetic fabrics.</p>

scholarly journals Magnetic properties of pseudotachylytes, Jämtland, central Sweden

2019 ◽  
Author(s):  
Hagen Bender ◽  
Bjarne S. G. Almqvist ◽  
Amanda Bergman ◽  
Uwe Ring
Keyword(s):  
Magnetic Susceptibility ◽  
Shear Zones ◽  
Anisotropy Of Magnetic Susceptibility ◽  
Fault Zones ◽  
Small Sample ◽  
Magnetic Fabric ◽  
Normal Faults ◽  
Magnetic Fabrics ◽  
Nappe Complex ◽  
Fault Kinematics

Abstract. Nappe assembly in the Köli Nappe Complex, Jämtland, Sweden, has been associated with in- and out-of-sequence thrusting. Kinematic data from shear zones bounding the Köli Nappe Complex are compatible with this model, but direct evidence from fault zones internally subdividing the nappe complex does not exist. We studied a series of pseudotachylyte exposures in these fault zones for deciphering the role seismic faulting played in the assembly of the Caledonian nappe pile. To constrain the fault kinematics, microstructural and magnetic fabrics of pseudotachylyte in foliation-parallel fault veins have been investigated. Because the pseudotachylyte veins are thin, we focused on small (c. 0.2 cm3) samples for measuring the anisotropy of magnetic susceptibility. The results show inverse proportionality between specimen size and anisotropy of magnetic susceptibility degree, which is most likely an analytical artifact related to instrument sensitivity and small sample dimensions. This finding implies magnetic anisotropy results acquired from small specimens demand cautious interpretation. However, analysis of structural and magnetic fabric data indicates that seismic faulting occurred during exhumation into the upper crust but yield no kinematic in-formation. Structural field data suggest that seismic faulting was postdated by brittle E–W extensional deformation along steep normal faults. Therefore, it is likely that the pseudotachylytes formed late during out-of-sequence thrusting of the Köli Nappe Complex over the Seve Nappe Complex.

Reduced- or ilmenite-type granites versus oxidized- or magnetite-type granites: occurrence in Iberian Variscan Belt

2021 ◽  
Author(s):  
Helena Sant Ovaia ◽  
Cláudia Cruz ◽  
Ana Gonçalves ◽  
Fernando Noronha
Keyword(s):  
Redox Conditions ◽  
Ductile Deformation ◽  
Variscan Orogeny ◽  
Paramagnetic Behavior ◽  
Magma Sources ◽  
Iron Leaching ◽  
Central Portugal ◽  
Type Granite ◽  
Magma System ◽  
Compositional Diversity

<p>The magnetic susceptibility (K<sub>m</sub>) of granites is an important characteristic and it is mainly controlled by the presence of certain oxide minerals like magnetite and/or ilmenite, as well as ferromagnesian phyllosilicates such as biotite. The abundance of magnetite or ilmenite can be explained by different redox conditions in the magma chamber and distinct magma sources. The presence of magnetite or ilmenite as accessory minerals represents oxidized- or magnetite-type granites and reduced- or ilmenite-type granites, respectively.</p><p>This work focuses on the K<sub>m</sub> of 20 Variscan granitic massifs from northern and central Portugal and considers the results obtained in about 750 sampling sites, in order to deduce the redox conditions in the magma system. These granites are essentially two mica mesocrustal and biotite-rich basicrustal/infracrustal in origin and their emplacement was related to Variscan orogeny. In the northern and central Portugal, three main ductile deformation Variscan phases were recognized and described: D<sub>1</sub>, D<sub>2</sub> and D<sub>3</sub>. The studied granites were subdivided in three main groups according to U-Pb dating, field observations and emplacement relative to the D<sub>3 </sub>phase. Therefore, the studied granites are subdivided as following: (1) syn-D<sub>3</sub> two-mica (mosc=biot) granites, ca. 311 Ma; (2) late-D<sub>3</sub> monzogranites, biotite-rich and two-mica granites (biot>mosc), ca. 300 Ma; (3) post-D<sub>3</sub> monzogranites and biotite-rich granites, < 299 Ma.</p><p>The evaluation of the K<sub>m</sub> variation of the different granite groups shows that, as granites become progressively younger, the K<sub>m</sub> parameter tends to increase as a result of the increasing in the mantellic contribution to the genesis of the magmas. Syn-D<sub>3</sub> granites display K<sub>m</sub> between 17.8 μSI and 186 μSI. This variation is due to the high textural and compositional diversity, including two-mica granites with different relative proportions of muscovite and biotite. Late-D<sub>3</sub> granites are represented by two-mica and biotite-rich granites with calcium plagioclase, with several degrees of post-magmatic alteration, implying iron leaching processes. These processes also promote the crystallization of secondary muscovite, which implied a decrease in the K<sub>m</sub> values, and also a wide dispersion of K<sub>m</sub> values ranging between 7.3 μSI and 276 μSI. Post-D<sub>3</sub> granites are mostly represented by biotite-rich granites with calcium plagioclase close to I-type granites. This granite group is divided into two subgroups: (i) post-D<sub>3</sub> ilmenite-type granites with K<sub>m</sub> values of ca. 113 μSI, typical of biotite-rich granites; and (ii) post-D<sub>3</sub> magnetite-type granites with K<sub>m</sub> values between 2078 μSI and 11676 μSI representing magnetite-type granites. In N and Central Portugal, these magnetite-type granites can occur in homogeneous plutons or in composite plutons constituted by ferromagnetic and paramagnetic facies.</p><p>As a conclusion, mostly of the granites from northern and central Portugal exhibit average K<sub>m</sub> values below 1000 μSI and are characterized by a paramagnetic behavior corresponding to reduced- or ilmenite-type granites. Among the all studied granites, only one pluton showed to be a truly oxidized- or magnetite-type granite, with K<sub>m</sub> of the order of 11676 μSI.</p><p><strong>Acknowledgements: </strong>This work was funded by the Fundação para a Ciência e a Tecnologia (FCT) under UIDB/04683/2020 project.</p>

Deformation and magnetic fabric of the Capinha granite (Fundão, Central Portugal): ascent and emplacement mechanisms during the late-Variscan crustal thinning

2020 ◽  
Author(s):  
Ana Gonçalves ◽  
Helena Sant'Ovaia ◽  
Fernando Noronha
Keyword(s):  
Magnetic Susceptibility ◽  
Major Axis ◽  
Magnetic Fabric ◽  
Variscan Orogeny ◽  
Magnetic Lineation ◽  
Metasedimentary Rocks ◽  
Magnetic Fabrics ◽  
Crustal Thinning ◽  
Deformation Patterns ◽  
Magnetic Foliation

<p>The Capinha area is located in the Central Iberian zone and is characterized by several Variscan granites intruded in the Neoproterozoic–Cambrian metasedimentary rocks. The main goal of the study is to identify the deformation patterns and provide crucial information to investigate the evolution of the magnetic fabrics in a post-Variscan granite emplaced during the crustal thinning, at the end of the Variscan orogeny. In order to achieve these purposes, fieldwork, petrography, microstructures and anisotropy of magnetic susceptibility (AMS) analysis were undertaken. The AMS was measured in 160 oriented cores, collected from 20 sampling sites homogeneously distributed, allowing the quantification of scalar (magnetic susceptibility, K; paramagnetic anisotropy, P<sub>para</sub>; magnetic ellipsoid shape, T) and directional data (magnetic lineation, //K<sub>1</sub>; magnetic foliation, perpendicular to K<sub>3</sub>). The Capinha granite (CG), exposed over an area of about 7 km<sup>2</sup>, is a small circular circumscribed outcrop in the NE-SW contact between the regional Belmonte–Caria granite (301.1±2.2 Ma) and the metasedimentary sequences. The CG is cut by two main fracturing systems: N30º-40ºE and N110º-120ºE, both subvertical. The contact is sharp, intrusive and discordant with the general trending of the D<sub>1</sub> and D<sub>3</sub> Variscan structures registered in the metasedimentary rocks. The CG is homogeneous in the whole area and consists of a fine- to medium-grained, muscovite-biotite leucogranite. The CG exhibit a paramagnetic behaviour with a K mean of 73 µSI, belonging to the ilmenite-type granites. At several scales, the CG does not show any magmatic flow or ductile deformation patterns displaying P<sub>para</sub> of about 1.6%, which corresponds to dominant magmatic to submagmatic microstructures. The P<sub>para</sub> highest values are concentrated in the NE border associated to prolate ellipsoids (linear fabric). Based on the interpretation of the magnetic fabric, is possible to observe that the orientation of the magnetic foliation is variable ranging from NNW-SSE to NNE-SSW. Generally, the magnetic foliations are sub-horizontal, being the vertical dips observed in the NE border, near the intersection of the N100º-120ºE and the N30º-40ºE fractures. The arrangement of the magnetic foliations follow concentric trajectories, with the symmetry axe parallel to the major axis of the outcrop (roughly NNE-SSW). The magnetic lineations are mainly sub-horizontal NNE-SSW parallel to the granite major axis; although, in the SW border the lineations tend to be parallelized to the contact. The magnetic lineation arrangement develops linear trajectories converging to the NE zone, where the dip is strong. The common gently magnetic fabric suggests the roof of the CG intrusion. During the late stages of the Variscan orogeny (D<sub>3</sub>, 321-300 Ma), ductile extensional detachments promoted the thinning of a previously thickened crust, providing the opening of pre-existing structures and the production of new ones. These structures act as conduits for a passive magma ascending and emplacement at shallow levels. Therefore, it is suggested that the CG magma ascent and emplaced in the intersection of pre-existing fractures, located in the NE zone, and flowed to the SW, developing a small asymmetric laccolith, poorly eroded, with a tongue-shaped body.</p>

Unravelling heterogeneities in magnetic fabric record of the strain: a combined AMS and ApARM data analysis applied to intraplate shear zones.

2021 ◽  
Author(s):  
Claudio Robustelli Test ◽  
Elena Zanella ◽  
Andrea Festa ◽  
Francesca Remitti
Keyword(s):  
Cluster Analysis ◽  
Hanging Wall ◽  
Shear Zones ◽  
Magnetic Fabric ◽  
Plate Boundaries ◽  
Thrust Faults ◽  
Localized Deformation ◽  
Main Thrust ◽  
Shear Sense ◽  
And Cluster Analysis

<p>Deciphering the stress and strain distribution across plate boundary shear zones is critical to understanding the physical processes involved in the nucleation of megathrust faults and its behaviour. Plate boundaries at shallow depth represent complex and highly deformed zones showing structures from both distributed and localized deformation.</p><p>As magnetic minerals are sensitive to stress regime, the investigation of the magnetic fabric has proven to be an effective tool in studying faulting processes at intraplate shear zones.</p><p>Anisotropy of magnetic susceptibility (AMS) provides insights into the preferred orientation of mineral grains and the qualitative relationships between petrofabrics and deformation intensity.</p><p>We present an approach of combined Contoured Diagram and Cluster Analysis to isolate the contribution of coexisting petrofabrics to the total AMS and evaluating the significance of magnetic fabric clusters.</p><p>Our results reveal distinct subfabrics with reasonably straightforward correlations with structural data. Specific AMS pattern may be associated to the intensity of the reworking related to tectonic shearing and the structural position within the shear zone (i.e., the proximity to the main thrust faults).</p><p>Close to the main thrust the magnetic fabric is dominantly oblate with magnetic foliation consistent to the S-C fabric and/or mélange foliation and the magnetic lineation parallel to the shear sense.</p><p>Away from the thrust faults the degree of anisotropy as well as the ellipsoids oblateness gradually diminishes. Thus, the presence of subfabrics related to previous tectonic events or less intense deformation (i.e. intersection lineation fabric) became dominant. The discrimination of subfabrics also allowed to unravel the presence of minor thrust plane and qualitatively evaluate the heterogeneous registration of strain (i.e. distributed versus localized deformation).</p><p>An abrupt change in magnetic ellipsoid shape and parameters is also observed below the basal décollements showing purely sedimentary magnetic fabric or previous deformation history with minor to absent evidences of shearing in the hanging wall.</p><p>Then, the integration with anisotropy of magnetic remanence experiments in different coercivity windows (ApARM) allow to separate the contribution of different ferromagnetic subpopulation of grains, constraining the significance of the different magnetic pattern/clusters detected through the AMS analysis.</p><p>In conclusion, our results show the potential of a combination of density diagrams and cluster analysis validated by ApARM experiments in distinguishing the superposition of deformation events, unravelling strain partitioning/concentration and thus to better understand the geodynamic evolution of subduction-accretion complexes.</p>

Deformation mechanisms in naturally-deformed blueschist facies metabasalts: constraints from exhumed subduction complexes in Greece and California

2021 ◽  
Author(s):  
Carolyn Tewksbury-Christle ◽  
Alissa Kotowski ◽  
Whitney Behr
Keyword(s):  
Electron Backscatter Diffraction ◽  
Shear Zones ◽  
Ductile Deformation ◽  
Interface Shear ◽  
Dislocation Glide ◽  
Dislocation Activity ◽  
Metasedimentary Rocks ◽  
Temperature Conditions ◽  
Low Viscosity ◽  
Lower Temperature

<p>The strength, or viscosity, of the subduction interface is a key parameter in subduction dynamics, influencing both long-term subduction plate speeds and short-term transient deformation styles. Fossil subduction interfaces exhumed from downdip of the megathrust record ductile deformation accommodated by diverse lithologies, including metasedimentary and metamafic rocks. Existing flow laws for quartz-rich rocks predict relatively low viscosities, in contrast to high viscosities predicted for basalt and eclogite, but the rheological properties of blueschists representative of metamorphosed oceanic crust of the down-going slab are poorly constrained. Two key questions remain: 1) are there significant viscosity contrasts between blueschists and quartz- or mica-rich metasedimentary rocks, and 2) what are the microscale mechanisms for creep in naturally deformed blueschists and how do they vary with pressure and temperature? To address these questions, we characterized deformation in natural samples from the Condrey Mountain Schist (CMS) in northern California, USA, and the Cycladic Blueschist Unit (CBU) on Syros Island, Cyclades, Greece, using outcrop-scale structural observations, optical microscopy, and Electron Backscatter Diffraction. The CMS and CBU record pressure-temperature conditions of 0.8-1.1 GPa, 350-450°C and 1.4-1.8 GPa, 450-550°C, respectively. </p><p>In the field, blueschists form m- to km-scale lenses that are interfolded with quartz schists, ultramafics, and, in the CBU, eclogites and marbles. At the outcrop scale in both localities, quartz-rich schists and blueschists each exhibit strong foliations and lineations and planar contacts at lithological boundaries. At the thin section scale, the prograde foliation and mineral lineation in blueschists are commonly defined by Na-amphiboles elongated in the lineation direction. Crystallographic preferred orientations in Na-amphibole in all samples have c-axes parallel to lineation and a-axes predominantly defining point-maxima perpendicular to the foliation, suggesting some component of dislocation activity for all temperature conditions in our sample suite. Microtextures in lower temperature CMS samples suggest strain accommodation primarily by dislocation glide and kinking in Na-amphibole, with extremely high-aspect-ratio grains and limited evidence for climb-controlled dynamic recrystallization. Some higher temperature CBU samples show large porphyroclasts with apparent ‘core-and-mantle’-type recrystallization textures and subgrain orientation analyses consistent with the (hk0)[001] slip systems. In contrast, epidote grains accommodate less strain than Na-amphibole, via some combination of rigid rotation, brittle boudinage, and minor intracrystalline plasticity.</p><p>Observations of evenly-distributed strain, despite lithological heterogeneity, suggest low viscosity contrasts and comparable bulk strengths of quartz schists and blueschists. Our microstructural observations suggest that Na-amphibole was the weakest phase and accommodated the majority of strain in mafic blueschists. Dislocation activity, and not just rigid-body-rotation or diffusional processes, accommodated some component of strain and possibly transitioned with increasing temperature from glide- to climb-controlled. Although effective viscosities appear to be similar, subduction interface shear zones dominated by blueschists may exhibit a power-law rheology consistent with dislocation activity, in contrast to the common inference of Newtonian creep in metasediments. Complementary experimental work on CMS and CBU rocks will also be presented at this meeting (see Tokle et al. and Hufford et al.).</p>

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