1,721,115 research outputs found
Progressive eclogitization under fluid-present conditions of pre-Alpine mafic granulites in the Austroalpine Mt Emilius Klippe (Italian western Alps)
No full textMetabasites from the Austroalpine Mt Emilius Klippe preserve different stages of the evolution from pre-Alpine granulite- to Alpine eclogite-facies assemblages. The pre-Alpine protolith includes layered garnet I—clinopyroxene I—plagioclase—hornblende granulites and gabbros. The eclogitic paragenesis comprises garnet II—clinopyroxene II—epidote—Ca-amphibole ± chlorite. Eclogitic deformation is composite and involves two main episodes (D1A, D1B). D1A produces a layer-parallel foliation and incomplete transformation of granulites and is concluded by an episode of brittle deformation leading to the formation of eclogitic veins. D1B gives rise to widespread S—L-tectonites and mylonites. The eclogitic veins and surrounding haloes, enriched in fluids during veining, control the nucleation of D1B shear zones. Deformation is accomplished through ductile flow of reaction-weakened plagioclase and hornblende sites and cataclasis of garnet I and clinopyroxene I. Both meso- and microscale observations indicate that fluids play a primary role during eclogitization in both enhancing deformation and the kinetics of metamorphic reactions. Fluids influence plastic flow rates in matrix aggregates and promote fracturing of hard minerals. The extent of metamorphic re-equilibration is closely related to the density of the fluid pathways
Control of the geometry of precursor brittle structures on the type of ductile shear zone in the Adamello tonalites, Southern Alps (Italy)
No full textAmphibolite facies ductile shear zones developed during the immediate post-intrusive cooling history of the Adamello tonalites (Southern Alps, Italy). Shear zones include: (i) thin (a few mm's thick) fault-like shear zones that accommodate shear strain values up to several 100’s (the dominant type); (ii) mylonitic horizons (dm's thick) in sharp contact with the undeformed wall rock; (iii) continuous shear zones with sigmoidal-shaped S or composite S–C′ foliations. A transition between the different types occurs along strike over short distances. Discontinuous shear zones are also localized on pegmatite dykes and quartz veins, and both (i) and (ii) are discontinuously outlined by quartz layers. Detailed surface mapping reveals that most fault-like shear zones are arranged en-échelon, mainly forming contractional steps. Markers crosscut by shear zones allow the displacement to be measured at several positions along shear zones and this reveals very steep displacement gradients close to the shear zone tips. Differential displacement is mainly accommodated at contractional steps by the development of foliated domains. Geochemical analyses of major and trace elements show that there is no compositional change along strain gradients. The overall features are consistent with nucleation of shear zones on former sets of en-échelon joints, in many cases intruded by pegmatite dykes or filled with quartz. Reactivation of joints produced strongly localized shear zones, whereas broader foliated zones evolved from the contractional jogs between adjacent stepped joint terminations during progressive shearing. These jogs were progressively involved in the accommodation of shear displacement and overall shear zone development
The control of precursor brittle fracture and fluid-rock interaction on the development of single and paired ductile shear zones
No full textDuctile shear zones can occur as relatively isolated single structures, as arrays, or as characteristic paired zones. In continuous glaciated exposures of metagranodiorites from the Tauern window (Eastern Alps), the control of initial dilatant brittle fracture and associated fluid–rock interaction on the geometry of subsequent ductile shear zones can be unequivocally established. Shearing occurred under amphibolite facies conditions. Fractures in weakly deformed metagranodiorites are often less than 1 mm thick but extend for tens of metres. Many are healed joints without shear offset. Others show minor (mm–cm), discrete dextral offset. Such brittle faults commonly display a low-angle en-échelon arrangement, with displacement transferred between discrete fracture segments by ductile compressive bridges. The geometry of more strongly reactivated zones depends on the degree and heterogeneity of fluid–rock interaction, which is related to fluid infiltration and veining along the primary fractures. With little fluid–rock interaction, reactivation produces single heterogeneous ductile shear zones centred on and immediately flanking the pre-existing fracture. With increased fluid–rock interaction, a bleached halo is developed symmetrically to either side of a central epidote–quartz (±garnet±calcite) vein. Ductile shear zones commonly flank this bleached zone, to develop a characteristic paired pattern. Strain is partitioned, localizing in the central fracture/vein and the flanking shear zones. Paired zones may anastomose in accordance with changes in the width of the central bleached zone, but are always symmetrically spaced with regard to the central fracture/vein. With increasing deformation, the ductile shear zones broaden into the adjacent metagranodiorite but not into the bleached zone, which remains preserved as a low strain region. Paired shear zones can also develop to either side of aplite dykes. Examples of characteristic paired shear zones, usually with a clear central vein, are found in many areas ranging from greenschist to eclogite facies, suggesting that the mechanism of their formation is quite general
Brittle precursors of plastic deformation in a granite: an example from the Mont Blanc massif (Helvetic, western Alps)
No full textIn the Mont Blanc Helvetic massif, granites record mesoscale Alpine structures, which include joints, veins, cataclastic to mylonitic shear zones and foliated granites. A detailed structural analysis indicates that brittle deformation predates plastic strain. Joints never pass through, and veins are offset by, cataclastic shear zones and mylonites. The mylonites progressively develop by plastic reactivation of cataclastic shear zones during greenschist facies metamorphic conditions. Plastic deformation is first localized in the brittle discontinuities and the fine-grained matrix of cataclasites. Then it involves the granite within brittle shear zones, and this is initially accomplished mainly by flow of reaction-softened aggregates of sericite, widely replacing the strain-supporting magmatic plagioclase. The brittle-to-plastic evolution has resulted in highly localized discontinuous plastic shear zones with high lateral continuity, and these characteristics are derived from reactivation of, and focusing along, pre-existing brittle discontinuities. In addition, mylonites may inherit high angles of intersection, and may contain granite porphyroclasts. These features may allow the inference of a precursor brittle deformation where the plastic overprint has completely erased the initial brittle fabrics
Why calcite can be stronger than quartz
No full textIn the Neves area (Eastern Alps), calcite forms asymmetric centimeter-scale single-crystal porphyroclasts in quartz mylonites developed during hydrous amphibolite facies metamorphism at ~550°C. Under these conditions, coarse calcite was clearly stronger than the surrounding polycrystalline, dynamically recrystallized, quartz matrix. Experimental results indicate that coarse calcite is less strain rate sensitive than wet quartzite, consistent with an inversion in strength on extrapolation to natural strain rates. For this to occur, wet quartzite must be weak, flowing at differential stress of <10 MPa. The lack of high-temperature twins (showing bulging or recrystallization) in calcite clasts is consistent with such low stresses during shear zone development under near peak metamorphic conditions. The maximum effective viscosity ratio of coarse calcite to quartzite for these conditions is probably not large (<10). However, numerical modeling shows that ratios of around 2 are sufficient to maintain near rigid calcite clast behavior for power law rheology with stress exponents appropriate to quartz (n ~ 3–4) and coarse calcite (n ≥ 6). The inversion in relative strength reflects the difference in influence of water on the crystal plastic flow of calcite and quartz: water has a dramatic effect for quartz but little or no effect for calcite. Quartz-rich rocks under hydrous amphibolite facies conditions in the middle to lower crust are therefore relatively weak (in fact, weaker than coarse calcite) and flow at much lower stresses than dry quartz-rich rocks at similar crustal levels
Brittle-viscous deformation cycles in the dry lower continental crust
No full textMany rheological models of the lithosphere (based on “strength envelopes”) predict a weak aseismic lower
crust below the strong brittle upper crust. An alternative view, based on the distribution of crustal seismicity,
is that the lower crust could also be strong and seismic. It has been suggested that a strong, seismogenic lower
crust results from the dry conditions of granulite facies rocks, which inhibit crystal plastic flow. This study
investigates exhumed networks of shear zones from Nusfjord (Lofoten, northern Norway) to understand initiation
and localization of viscous shearing in the dry lower crust. In the study area, different sets of ultramylonitic shear zones are hosted in the massive coarse-grained anorthosite. Metamorphic conditions of 720 ◦C, 0.9 GPa have been estimated for ductile deformation using amphibole-
plagioclase geothermobarometry. Field evidence indicates that ductile shearing exploited pseudotachylyte veins
and the associated damage zone of extensive fracturing. Undeformed pseudotachylyte veins locally overprint
mylonitic pseudotachylytes suggesting that frictional melting occurred at the same metamorphic conditions of
mylonitization. The deep crustal origin of the pseudotachylytes is also indicated by (1) the presence of microlites
of labradoritic plagioclase and clinopyroxene, and of dendritic garnet, and (2) the recrystallization of clinopyrox-
ene in the damage zone flanking the pseudotachylyte veins. Therefore the association of pseudotachylytes and
mylonites records brittle-viscous deformation cycles under lower crustal conditions.
The ultramylonites show phase mixing, fine grain size (5-20 μm) and equant shape of all minerals. Nucleation
of amphibole in triple junctions and dilatant sites is common. EBSD analysis indicates that the minerals in the
matrix are internally strain free and do not show a crystallographic preferred orientation. Taken together, these
observations suggest that diffusion creep and grain boundary sliding were the main deformation mechanisms in
the ultramylonites. Nucleation of hornblende indicates synkinematic fluid infiltration. Ongoing measurements of
intracrystalline water content along gradients from the pristine anorthosite to the ultramylonite will shed light on
the effect of water infiltration on the deformation mechanisms of plagioclase and clinopyroxene.
In summary, this study indicates that brittle (coseismic) fracturing was essential to induce grain size reduction
and fluid infiltration in the dry and strong lower crust. These processes promoted weakening by activating grain
size sensitive creep in the fine-grained hydrated material and resulted in the ductile shear zones localized to
the brittle precursors. In the absence of intense fracturing dry granulites would not undergo deformation and
metamorphism, and would survive metastably in the course of Wilson cycles. This has obvious implications for
long-term continental dynamics and for strain localization at plate boundaries, and will need to be included in
future geodynamic models
Effects of recrystallization and strain on Ti re-equilibration in quartz in a cooling pluton
No full textSince a couple of years the trace amount of Ti in quartz (Ti-in-quartz or TitaniQ) has been used to constrain the deformation temperature in quartzitic rocks. Independently of how precise the estimate of deformation temperature could be, a basic question still remains controversial of how effective is dynamic recrystallization to reset the Ti in quartz in mylonites. The study of a heterogeneous ductile shear zone developed during post-magmatic cooling of a titanite-bearing granodiorite allows the effect of strain and recrystallization on Ti re-equilibration in quartz to be assessed. The different strain facies show a heterogeneous distribution of Ti content (measured by SIMS) which correlates well with cathodoluminescence (CL) intensity. In the granodiorite protolith CL-bright Ti-rich (20-38 ppm) quartz shows CL-dark Ti-poor haloes (Ti as low as 6-8 ppm) surrounding euhedral titanite. Grain-scale heterogeneities include Ti depleted (CL-darker) grain boundaries (Ti 4-6 ppm). In the protomylonite quartz shows a variable degree of recrystallization associated with strain gradients along S-C foliations anastomosing around feldspar porphyroclasts. Original CL-dark haloes surrounding titanite were passively stretched into the foliation; away from these haloes recrystallized quartz appears mainly bright in CL and retained high Ti contents as in the protolith. Quartz-filled pressure shadows, appended to disrupted feldspar porphyroclasts, show dark CL indicative of very low Ti content (1-3 ppm). In the mylonites and ultramylonites quartz forms totally recrystallized layers that are dominantly dark in CL but show internally a “subtle” CL layering subparallel to foliation reflecting variations of Ti in the range of 3 to 12 ppm. EBSD analysis of quartz indicates that prism was the dominant crystallographic slip system, associated with subgrain formation and subgrain rotation recrystallization, at all stages of deformation. This indicates together with dynamic recrystallization of K-feldspar and plagioclase (Oligoclase: An 16-20%) deformation conditions at ∼ 500 ◦C. We conclude that under, the dominant conditions of deformation at ∼ 500 ◦C: (i) Ti content is strongly dependent on microstructure; (ii) high strain and complete recrystallization by subgrain rotation produced only incomplete homogenization of Ti, (iii) water-assisted synkinematic precipitation of new quartz in pressure shadows dramatically changed the Ti content of quartz to very low values. These observations pose serious limitations to the use of the Ti-in-quartz thermo-barometer to constrain ambient conditions of ductile deformation
The mylonites of the Austroalpine Dent Blanche nappe along the northwestern side of the Valpelline Valley (Italian Western Alps)
No full textAlpine deformation in the Dent Blanche nsppe (Austroalpine, Western Alps) comprises three main folding phases (D1-D3). Regional schistosity is composite. Development of penetrative mylonitic foliation is associated with both D1 and D2 deformations (S1m, S2m). The northwestern side of the Valpelline Valley has been selected for this study of mylonites. Magmatic (“Arolla Serie” Auct.) and Pre-alpine high-T granulite-amphibolite facies metamorphic (“Valpelline Serie” Auct.) protoliths were largely converted into fine-grained S-L tectonites. The mylonitic foliation S1m is preserved in areas of D2 low-strain where D2 structures are open to isoclinal folds with axial-plane crenulation cleavage. D2 fold axes are orthogonal to L1 mineral stretching lineation. New mylonitic foliation S2m develops along D2 high-strain zones. Mylonitic lineation L2 is parallel to L1 and trends NW-SE. D3 gives rise to open folding, kinking with flat to steeply-dipping axial planes and subhorizontal axes around NE-SW. D3 does not produce a new penetrative foliation. Alpine evolution includes an early relatively high-P phase (aegirinaugite, garnet, blue amphiboles, phengite, kyanite, zoisite-clinozoisite) followed by white mica-chlorite-actinolite-albite-epidote assemblages typical of Meso-alpine greenschist facies metamorphism. Relatively high-P assemblages are stable on S1. D2 and D3 develop under greenschist facies conditions. Kinematic analysis on microscopic indicators in the mylonites records a top-to-NW sense of tectonic transport.The parallelism between L1 and L2 suggests roughly constant bulk kinematic regime through the Eo-alpine (late Eo-alpine?)/Meso-alpine evolution
Early-Oligocene low-angle normal faulting in the Eastern Alps
No full textDuring Early-Oligocenic cooling of the Rieserferner pluton (RFP, Eastern Alps) through amphibolite-greenschist facies temperature conditions, a set of shallowly (15-25 ◦ ) E-dipping joints, epidote and quartz veins developed. These joint and veins were exploited as top-to-the-East normal ductile shear zones, then overprinted by a set of steeper brittle-ductile shear zones with the same orientation and kinematics. Strong fluid-rock interaction, feldspar destabilization and chlorite+white mica+calcite veins characterize these brittle-ductile mylonites. As a whole, the orientation, kinematics and meso-microstructural evolution of these shear zones are similar to that described for the Katschberg normal fault (KNF, Genser and Neubauer, 1989), a regional scale low-angle normal fault (LANF). Therefore, some genetic relationship between the two might be outlined.
The characteristics, timing and the comparison of this set of structures with the KNF allow us to investigate both regional geology topics and mechanical processes related to LANF, in particular:
1) The KNF is miocenic in age (23 Ma). Comparing field, microstructural and published thermo-chronological data, we infer that structures in the RFP developed between 30 and 26 Ma, defining, therefore, the first occurrence of exhumation tectonics in the future Tauern Window region. In addition, diffuse extensional structures might be responsible for the “regional E-down tilting” inferred from thermochronological data (Steenken et al., 2002).
2) Given the limited amount of accommodated strain, these structures might represent the incipient stages of deformation and nucleation (on brittle precursors) of a regional-scale LANF. Nucleation probably occurred at very low-angle, given the small amount of supposed “regional E-down tilting” (5 ◦ , Steenken et al., 2002) and the unlikely occurrence of rolling-hinge model rotations. The transition from shallow- to high-angle dips probably represents the evolution in geometry of fault system during footwall uplift and decreasing confining pressure. Veins and fluid-rock interaction suggest the occurrence of high pore-fluid pressure.
3) Different weakening mechanisms contributed to slip along misoriented planes: (i) reaction weakening processes in mylonites exploiting epidote veins; (ii) inherently weak quartz with respect to the host granitoid during quartz vein shearing; (iii) feldspar-to-mica reaction during later stage brittle-ductile mylonite development. The results of rheological models about shear zone deformation will be discussed focussing to the strength evolution and to the LANF-slip/seismicity conundrum
High-temperature fracturing and ductile deformation during cooling of a pluton: The Lake Edison granodiorite (Sierra Nevada batholith, California)
No full textIn the Bear Creek area of the Sierra Nevada batholith, California, the high temperature post-magmatic deformation structures of the Lake Edison granodiorite include steeply dipping orthogneiss foliations, joints, and ductile shear zones that nucleated on joints and leucocratic dykes. Exploitation of segmented joints resulted in sharply bounded, thin shear zones and in large slip gradients near the shear zone tips causing the deformation of the host rock at contractional domains. The orthogneiss foliation intensifies towards the contact with the younger Mono Creek granite and locally defines the dextral Rosy Finch Shear Zone (RFSZ), a major kilometre-wide zone crosscutting the pluton contacts. Joints predominantly strike at N70-90°E over most of the Lake Edison pluton and are exploited as sinistral shear zones, both within and outside the RFSZ. In a narrow (∼ 250 m thick) zone at the contact with the younger Mono Creek granite, within the RFSZ, the Lake Edison granodiorite includes different sets of dextral and sinistral shear zones/joints (the latter corresponding to the set that dominates over the rest of the Lake Edison pluton). These shear zones/joints potentially fit with a composite Y-R-R′ shear fracture pattern associated with the RFSZ, or with a pattern consisting of Y-R-shear fractures and rotated T′ mode I extensional fractures. The mineral assemblage of shear zones, and the microstructure and texture of quartz mylonites indicate that ductile deformation occurred above 500 °C. Joints and ductile shearing alternated and developed coevally. The existing kinematic models do not fully capture the structural complexity of the area or the spatial distribution of the deformation and magmatic structures. Future models should account more completely for the character of ductile and brittle deformation as these plutons were emplaced and cooled
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